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CHAPTER- 9

HYDRO GENERATOR, CHARACTERISTICS AND PERFORMANCE 9.1 GENERAL

The electric generator converts the mechanical energy of the turbine into electrical energy. The two major components of the generator are the rotor and the stator. The rotor is the rotating assembly to which the mechanical torque of the turbine shaft is applied. By magnetizing or “exciting” the rotor, a voltage is induced in the stationary component, the stator. The principal control mechanism of the generator is the exciter-regulator which sets and stabilizes the output voltage. The speed of the generator is determined by the turbine selection, except when geared with a speed increaser. In general, for a fixed value of power, a decrease in speed will increase the physical size and cost of the generator. The location and orientation of the generator is influenced by factors such as turbine type and turbine orientation. For example, the generator for a bulb type turbine is located within the bulb itself. A horizontal generator is usually required for small turbine e.g. tube turbine and a vertical shaft generator with a thrust bearing is appropriate for vertical turbine installations. Conventional cooling on a generator is accomplished by passing air through the stator and rotor coils. Fan blades on the rotating rotor assist in the air flow. For larger generator (above 5 MVA capacity) and depending on the temperature rise limitations of the winding insulation of the machine, the cooling is assisted by passing air through surface air coolers, which have circulated water as the cooling medium.

Large Generators interconnected with the grid should meet grid standards issued by Central Electricity Authority (CEA) (relevant extracts are enclosed as annexure-1).

9.2 Hydro Generators Early Designs 9.2.1 Large Hydro

Large salient pole hydro generators specified for installation up to 1970 were constrained by following considerations. Insulation Systems for Stator and Rotor was Class B insulation with organic binding material which permitted lower temperature rises. Material for rotor rim punching etc. required limiting the diameter of the rotor so as to permit operation at runaway speed. Bearing arrangements: Top thrust and guide bearing supported on heavy brackets, capable of supporting total generator weight was provided with a bottom guide bearing to all hydro generators including slow speed large generator which constitutes majority of large hydro generators. This resulted in high cost of machine and building. Shaft mounted excitation systems were slow and unable to meet the requirements of quick response required from large generators feeding large modern grid systems. Stability requirements for long distance transmission lines required to feed distant load centre/grids was achieved by manipulating reactances, excitation response ratio and flywheel effect. This resulted in larges size of the machine. Grids were small and there was no stringent requirement for voltage and frequency variation. Typical section arrangement for Bhakra Left bank machines (100 MVA; 166.7 RPM) with top thrust and guide bearing and bottom guide bearing is shown in figure 9.1 and the capability curve is shown in figure 9.2.

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9.2.2 Small Hydro

Small hydro were a typically installed to feed remote areas and worked in isolated mode. The hydro turbines (slow speed) were directly coupled to high cost slow speed generators. Hydro stations were manually operated. The development of load was very poor. The small hydro became highly uneconomical to operate because of low load factors, high installation cost and very high running cost.

9.3 Modern Large Hydro Generator

Hydraulic turbines driven generators for hydro plant above 5 MW are salient pole synchronous alternating current machines. Large salient pole generators are relatively slow speed machines in the range 80-375 rpm with large number of rotor poles. These generators are specifically designed. These salient pole hydro generators interconnected with large grids have undergone considerable changes over time which has resulted in reducing size of hydro generators considerably from the electrical and mechanical point of view. Development in the following areas is most prominent. i) Insulation system for stator and rotor winding ii) Improved material iii) Ventilation and cooling system iv) Advanced manufacturing technology v) Formation of large grids requires special design consideration for operation and stability.

Fig. 9.1: Bhakra Left Bank Power House (100 MVA, 90 MW, 11 kV,0.9 pf, 3 phase, 50 Hz, 166.4 RPM, 36 Poles Vertical Wheel Water Generator

Commissioned in 1960) Source: Notes completed for uprating the unit as member uprating committee)

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Fig. 9.2: Bhakra Left Bank Generators – Capability Curve (Source: Notes compiled for uprating the units as member uprating committee)

9.3.1 Design Criteria 9.3.1.1 Site Operating Conditions (as per IEC: 60034, IEEE C-50-12 & IS: 4722)

Rated operation condition be specified as follows: If site operating conditions are deviating from these values, correction may be applied. Maximum Ambient Temperature Steady State duty: Salient-pole open ventilated air-cooled synchronous generators operate successfully when and where the temperature of the cooling air does not exceed 400C.

Salient-pole totally enclosed water to air cooled (water) synchronous generators operate successfully when and where the secondary coolant temperature at the inlet to the machine or heat exchanger do not exceed 250C. If the cooling air temperature (ambient) exceeds 400C, or cooling water temperature exceeds 250C then maximum allowable temperature based on temperature rise on reference temperature (400/250C) of the insulation class be specified instead of temperature. The minimum temperature of the air at the operating site is – 150C, the machine being installed and in operation or at rest be de-energized. Note: If temperatures different from above are expected. The manufacturer should be informed of actual site conditions. Generators: Generators should operate successfully at rated MVA, frequency, power factor, and terminal voltage. Generators at other service conditions should be specified with the standards of performance established at rated conditions.

Altitude: Height above sea level not exceeding 1000 m. For machines intended for operation on a site where the altitude is in excess of 1000 m. should be specifically brought out.

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9.3.1.2 Number of starts and application of load: Anticipated no. of starts and maximum MVA, power, and reactive power loading rate of change are requirements for the manufacturer to take into account in the machine design. The method of starting must be identified in the case of peaking stations.

9.3.1.3 Variation from rated voltage and frequency: Generators should be thermally capable of continuous operation within the capability of their reactive capability curves over the ranges of ± 5 % in voltage and ± 2 % in frequency. Voltage and Frequency Limits for Generators (as per IEC: 60034)

Normal Emergency Voltage limits ± 5% ± 5% to ± 8% Frequency limit ± 2% + 2% to + 3%;

– 2% to – 5%

a) As the operating point moves away from rated values of voltage and frequency, the temperature rise of total temperatures of components may progressively increase. Continuous operation at outputs near the limits of the generator’s reactive capability curve (figure 9.3) may cause insulation to age thermally at approximately two times to six times its normal rate.

b) Generators will also be capable of operation within the confines of their reactive capability curves within the ranges of + 3 % to -5 % in frequency with further reduction of insulation life.

c) To minimize the reduction of the generator’s lifetime due to the effect of temperature and temperature differentials, operation outside the above limits should be limited in extent, duration, and frequency of occurrence. The output should be reduced or other corrective measures taken as soon as practicable.

d) The boundaries as defined result in the magnetic circuits of the generator to be over fluxed under fluxed by no more than 5%.

e) The machine may be unstable or margins of stability may be reduced under some of the operating conditions mentioned in ‘a’ above. Excitation margins may also be reduced under these operating conditions.

f) As the operating frequency moves away from the rated frequency, effects outside the generator may become important and need to be considered. For example, the turbine manufacturer will specify ranges of frequency and corresponding periods during which the turbine can operate, and the ability of the auxiliary equipment to operate over a range of voltage and frequency should be considered.

g) Operation over a still wider range of voltage and frequency, if required, should be subject to agreement between the purchaser and the manufacturer and need to be specifically brought out in tender specification.

SYSTEM STABILITYLIMIT

LINE CHARGING LIMIT

MINIMUMEXCITATIONLIMIT

UN

DER

EXC

ITED

(LEA

DIN

G)

MEG

AVA

RS

OVE

REX

CIT

ED(L

AG

GIN

G)

CAVITATIONLIMIT

LIMITED BY FIELD HEATING

POWER FACTOR

RATED MVA LIMITED BYSTATOR HEATING

MEGAWATTS

SHAFT STRESS ORHYDRAULIC LIMIT

Fig. 9.3: Typical Hydro-Generator capability Curve (Typical)

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9.3.2 Transient and Emergency Duty Requirements

A generator confirming to these guidelines will be suitable for withstanding exposure to transient event and emergency duty imposed on a generator because of power system faults. Sudden short circuit at the generator terminals: A generator should be capable of withstanding, without injury, a 30 second, 3 phase short circuit at its terminals when operating at rated MVA and power factor and at 5% over voltage, with fixed excitation. The machine shall also be capable of withstanding, without injury, any other short circuit at its terminals of 30s duration or less in accordance with IEEE C 50. 12-2005. Generator circuit breaker needs to be selected accordingly. Synchronizing

a. Generators be designed to be fit for service without inspection or repair after synchronizing that is

within the limits given below:

i) Breaker closing angle ±10% ii) Generator voltage relative to system 0% to +5% iii) Frequency difference ±0.067 Hz

Additional information on synchronizing practices can be found in IEEE std. C37. 102TM- 1995. b. Faulty synchronizing is that which is outside the limits given above. Under some system conditions,

faulty synchronizing can cause intense, short duration currents and torques that exceed those experienced during sudden short circuits. These currents and torques may cause damage to the generator.

c. Generators should be designed so that they are capable of coasting down from synchronous speed to a stop after being immediately tripped off-line following a faulty synchronization. Any generator that has been subject to a faulty synchronization should be inspected for damage and repaired as necessary before being judged fit for service after the incident. Any loosening for stator winding bracing and blocking and any deformation of coupling bolts, couplings, and rotor shafts should be corrected before returning the generator to service. Even if repairs are made after a severe out-of-phase synchronization, it should also be expected that repetition of less severe faulty synchronizations might lead to further deterioration of the components.

d. It should be noted that the most severe faulty synchronizations, such as 1800 or 1200 out-of-phase synchronizing to a system with low system reactance to the infinite bus, might require partial or total rewind of the stator, or extensive or replacement of the rotor, or both.

Check synchronizing relay and auto synchronizing equipment need to be set accordingly. Normally synchronizing closing angle is kept ±7%.

Short-time volts/hertz variations: The manufacturer should provide a curve of safe short-time volts/hertz capability. Identify the level of over flux above which the machine should never be operated, to avoid possible machine failure. Unless otherwise specified, the curve apply for time intervals of less than 10 min.

9.3.3 Rotor Surface Heating

Continuous phase current unbalance: Generator above 5 MVA should be capable of withstanding, without injury, the effects of a continuous phase current unbalance corresponding to a negative current of the values in table 9.1, provided the rated MVA is not exceeded the maximum expressed as a percentage of rated stator current.

Table –9.1 Continuous negative sequence current capability

Type of generator or generator/motor Permissible I2 (%)

Non-connected amortisseur winding 5 Connected amortisseur winding 10

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These values also express the negative-sequence current capability at reduced generator MVA capabilities, as a percentage of the stator current corresponding to the reduced capability.

Continuous performance with non connected amortisseur windings is not readily predictable. Therefore, if unbalanced conditions are anticipated, machines with connected amortisseur windings should be specified. Negative sequence relays (phase unbalance) be set accordingly.

9.3.4 Types of Generators and Configuration (Vertical or Horizontal)

Vertical shaft generators are generally used. There are two types of vertical shaft hydro generators distinguished by bearing arrangements.

Umbrella type generators: These generators have combined bottom thrust and guide bearings and confined to low operating speeds (up to 200 rpm) are the least expensive generator design. In semi umbrella type generators a top guide bearing is added. Umbrella/Semi Umbrella design is being increasingly used for slow speed vertical generator.

Conventional generators: Prior to introduction of umbrella and semi umbrella designs conventional design comprised of top-mounted thrust and guide bearing supported on heavy brackets, capable of supporting total weight of generator. All thrust bearing supported brackets take care the weight of generator rotating parts. Turbine rotation parts and axial component of water thrust acting on turbine runner. A bottom guide bearing combined with turbine shaft is usually provided. This conventional design is used for high speeds (up to 1000 rpm) generators. Some medium size low flow turbine and tube turbine generators are horizontal shaft. Direct driven bulb turbine generators are also horizontal shaft generators located in the bulb. Pelton turbine coupled generators are mostly horizontal shaft.

9.3.5 Capacity and Rating

kW Rating: kW capacity is fixed by turbine rated output. In a variable head power plant the turbine output may vary depending upon available head. In general the generator is rated for turbine output at rated head. In peaking power plant higher generator kW rating could be specified to take care of possible higher turbine output. Economic analysis is required for this purpose as the cost will increase and generator capacity remains unutilized when heads are low.

The kilowatt rating of the generator should be compatible with the kW rating of the turbine. The most common turbine types are Francis, fixed blade propeller, and adjustable blade propeller (Kaplan). Each turbine type has different operating characteristics and imposes a different set of generator design criteria to correctly match the generator to the turbine. For any turbine type, however, the generator should have sufficient continuous capacity to handle the maximum kW available from the turbine at 100-percent gate without the generator exceeding its rated nameplate temperature rise. In determining generator capacity, any possible future changes to the project, such as raising the forebay (draw down) level and increasing turbine output capability, should be considered. Typical hydro generator capability curve is shown in figure 9.3.

kVA Rating and power factor: kVA and power factor is fixed by consideration of interconnected transmission system and location of the power plant with respect to load centre. These requirements include a consideration of the anticipated load, the electrical location of the plant relative to the power system load centers, the transmission lines, substations, and distribution facilities involved. A load flow study for different operating condition would indicate operating power factor, which could be specified.

(Turbine output in MW) x (Generator efficiency) Generator MVA = Generator power factor

9.3.6 Electrical Characteristics

Electrical Characteristics e.g. voltage, short circuit ratio, reactance, line charging capacity etc. must conform to the interconnected transmission system. Large water wheel generators are custom designed to

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match hydraulic turbine prime over. Deviation from normal generator design parameters to meet system stability needs can have a significant effect on cost. The system stability and other needs can be met by modern static excitation high response systems and it is a practice to specify normal characteristics for generators and achieve stability requirements if any by adjusting excitation system parameter (ceiling voltage/exciter response).

9.3.6.1 Generator Terminal Voltage Generator terminal voltage should be as high as economically feasible. Standard voltage of 11 kV or higher is generally specified for hydro generator.

9.3.6.2 Insulation and Temperature Rise

Modern hydro units are subjected to a wide variety of operating conditions but specifications are prepared with the intention of achieving a winding life expectancy of 35 years or more under anticipated operating conditions. Present practice is to specify class F insulation system for the stator and rotor winding with class B temperature rise over the ambient. Ambient temperature rise should be determined carefully from the temperature of the cooling water etc.

If may be noted that as per IS the temperature rise specified over an ambient of 400C. Accordingly maximum temperature for the insulation class under site conditions should be specified.

The class F system is known as thermo setting insulation system. The epoxy resins systems may be divided into the following two major classes.

i) Resin rich system ii) Resin poor system Resin poor technology needs sophisticated resin storage, transfer and impregnation plants. Most of large machines commissioned in India have class F insulation resin rich. It is generally believed that stator insulation degradation occurs due to slot discharge as well as due to ionization. Slot discharge is a result of poor contact between the conducting surface on generator coil and stator iron and built up of high surface potential. Discharges of this nature can be very severe because of the charges current involved and cause serious damage to the insulation. At some stage slot discharges appear to or begin to increase between the surface of the insulation of the winding and laminated slot wall and provide one of the deteriorating factor. Phenomenon of slot discharges is known since long and efforts have been made to evolve suitable methods for monitoring the state of erosion of insulation in an assembled machine. With adoption of epoxy mica insulation and use of increased current densities resulting in higher electro-dynamic forces, there is an increase in the intensity of these slots discharges. Slot discharges have been reported in water wheel generators. Thermosetting insulation systems materials are hard and do not readily conform to the stator slot surface, so special techniques and careful installation procedures must be used in applying these materials to avoid possible slot discharges. Special coil fabrication techniques, installation, acceptance and maintenance procedure are required to ensure long, trouble-free winding life.

All components of stator and rotor insulation should be of class F insulation with class B temperature rise.

9.3.6.3 Short Circuit Ratio

The short circuit ratio of a generator is the ratio of field current required to produce rated open circuit voltage to the field current required to produce rated stator current when the generator terminals are short circuited and is the reciprocal of saturated synchronous reactance. Normal short circuit ratios are given below. Higher than normal short circuit ratio will increase cost and decrease efficiency.

Generator Power Factor Normal Short Circuit Ratio

0.8 1.0 0.9 1.10

0.95 1.17

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In general, the requirement for other than nominal short-circuit ratios can be determined only from a stability study of the system on which the generator is to operate. If the stability study shows that generators at the electrical location of the plant in the power system are likely to experience instability problems during system disturbances, then higher short-circuit ratio values may be determined from the model studies and specified.

Refer Para 9.4 for more detailed discussion. 9.3.6.4 Line Charging and Synchronous Condensing Capacity

This is the capacity required to charge an unloaded line. Line charging capacity of a generator having normal characteristics can be assumed to equal 0.75 of its normal rating multiplied by its short circuit ratio. If the generator is to be designed to operate as synchronous condenser. The capacity when operating over excited as condensers can be as follows:

Power Factor Condenser Capacity

0.80 65% 0.90 55% 0.95 45%

9.3.6.5 Reactance

The eight different reactances of a salient-pole generator are of interest in machine design, machine testing, and in system stability model studies. Lower than normal reactances of the generator and step-up transformer for system stability will increase cost and is not recommended.

Both rated voltage values of transient and subtransient reactances should be used in computations for determining momentary rating and the interrupting ratings of circuit breakers. Typical values of transient reactances for water wheel generators up to 25 MA are given below. Guaranteed values of transient reactances will be approximately 10% higher.

Rated Sub-transient Reactance - dx ′′ MVA Rating Speed RPM 100 150 300 10 - 25 0.27 0.26 0.25

9.3.6.6 Damper Winding

A short circuit grid copper conductor in face of each of the salient poles is required to prevent pulling out of step the generator interconnected to large grid. Two types of damper windings may be connected with each other, except through contact with the rotor metal. In the second, the pole face windings are connected at the top and bottom to the adjacent damper windings.

The damper winding is of major importance to the stable operation of the generator. While the generator is operating in exact synchronism with the power system, rotating field and rotor speed exactly matched, there is no current in the damper winding and it essentially has no effect on the generator operation. If there is a small disturbance in the power system, and the frequency tends to change slightly, the rotor speed and the rotating field speed will be slightly different. This may result in oscillation, which can result in generator pulling out of step with possible consequential damage.

The damper winding is of importance in all power systems, but more important to systems that tend toward instability, i. e. systems with large loads distant from generation resources, and large intertie loads.

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In all cases, connected damper windings are recommended. If the windings are not interconnected, the current path between adjacent windings is through the field pole and the rotor rim. This tends to be a high impedance path, and reduce the effectiveness of the winding, as well as resulting in heating in the current path. Lack of interconnection leads to uneven heating of the damper windings, their deterioration, and ultimately damage to the damper bars.

The damper winding also indirectly aids in reducing generator voltage swings under some faults conditions. It does this by contributing to the reduction of the ratio of the quadrature reactance and the direct axis reactance, dq XX ′′′′ / . This ratio can be as great as 2.5 for a salient pole generator with no damper winding, and can be as low as 1.1 if the salient pole generator has a fully interconnected winding practice is to provide dq XX ′′′′ / L 1.3.

9.3.6.7 Efficiency

As high an efficiency as possible which can be guaranteed by manufacturer should be specified. Calculated values should be obtained from the manufacturer.

For a generator of any given speed and power factor rating, design efficiencies are reduced by the following:

i. Higher Short-Circuit Ratio ii. Higher WR2 iii. Above-Normal Thrust

9.3.6.8 Total Harmonic Distortion (THD)

Limits: When tested on open circuit and at rated speed and voltage, the total harmonic distortion (THD) of the line-to-line terminal voltage, as measured according to the methods laid down in IS should not exceed 5%.

9.3.6.9 Phase Sequence

Phase sequence defines the rotor in which the phase voltages reach their positive maximum at the terminals of the machine, and shall be agreed upon the manufacturer and purchaser. Typically this is given as a three letter sequence, R, C, L, (right, center, left) or L, C, R (left, center, right), as defined by an observer looking at the terminals from outside the machine. In the case of terminals on the top or bottom of the machine, the sequence is defined looking from the end of the machine nearest the terminals toward the centerline of the machine.

Care must be exercised to ensure that the defined phase sequence of the machine is consistent with that of the connected equipment, particularly in situations where the plant layout requires otherwise identical machines to have different phase sequence.

9.3.7 Mechanical Characteristics 9.3.7.1 Direction of Rotation

The direction of the rotation of the generator should suit the prime mover requirements. 9.3.7.2 Large Generator Stator

The stator frame is designed for rigidly and strength to allow it to support the clamping forces needed to retain the stator punching in the correct core geometry. Strength is needed for the core to resist deformation under fault conditions and system disturbances. Also, the core is subjected to magnetic forces that tend to deform it as the rotor field rotates. In some large size machines, this flexing has been known to cause the core to contact the rotor during operation. In one instance, the core deformed and contacted the rotor, the

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machine was tripped by a ground fault, and intense heating caused local stator tooth iron melting, which damaged the stator winding ground insulation. In machines with split phase windings where the split phase currents are monitored for machine protection, the variation in the air gap causes a corresponding variation in the split phase currents. If the variations are significant, the machine will trip by differential relay action, or the differential relays will have to be desensitized to prevent tripping. Desensitizing the relays will work, but it reduces their effectiveness in protecting the machine from internal faults.

9.3.7.3 Rotor Assembly Critical Speeds

A rotor dynamic analysis of the entire shaft system should be performed. This analysis should include the prime mover, generator, and any other rotating components. This analysis should include lateral and torsional shaft system response to the various excitations that are possible within the operational duties allowed by the standards. When the turbine generator is purchased as a set, it would be typical that the manufacturer should perform this analysis. When shaft components are purchased from different manufacturers, the purchaser should arrange to have this analysis. Critical speeds of the generator rotor assembly should not cause unsatisfactory operation within the speed range corresponding to the frequency range agreed in accordance with 9.3.1.4. The generator rotor assembly shall also operate satisfactorily for a reasonable period of time at speeds between standstill and rated speed agreed upon by the prime mover and generator designers. The turbine generator set shaft vibration at operating speed should be within limits specified by ISO: 7919-5 for machine sets in hydraulic power generating and pumping plants.

9.3.7.4 Bearings

The allowable hydraulic thrust provided in standard generator design is satisfactory for use with a Francis runner, but a Kaplan runner requires provision for higher-than-normal thrust loads. It is important that the generator manufacturer have full and accurate information regarding the turbine.

Specifications for generators above 10 MW, and for generators in unmanned plants, should require provisions for automatically pumping oil under high pressure between the shoes and the runner plate of the thrust bearing just prior to and during machine startup, and when stopping the machine.

9.3.7.5 Noise Level and Vibration

Under all operating conditions, the noise level of generator should be less than 85-95 dB (A) at a distance of 1 meter radialy & 1.5 m from floor of operating. In order to prevent undue and harmful vibrations, all motors should be statically and dynamically balanced in accordance with IEEE std. C50.12-2005. Test procedure for verification should be based on ISO 3746. Acoustic treatment may be necessary to achieve decreasing sound pressure levels at 90 db.

9.3.7.6 Over speed withstand

It is general practice in India to specify all hydro generators to be designed for full turbine runaway conditions (IS: 4722-2001, table 3 Clause 1). The stresses during design runaway speed should not exceed two-thirds of the yield point.

American practice as per Army Corps Engineers Design Manual is as follows;

Generators below 360 rpm and 50,000 kVA and smaller are normally designed for 100% over speed. 9.3.7.7 Flywheel Effect

The flywheel effect (WR2) of a machine is expressed as the weight of the rotating parts multiplied by the square of the radius of gyration. The WR2 of the generator can be increased by adding weight in the rim of the rotor or by increasing the rotor diameter. Increasing the WR2 increases the generator cost, size and weight, and lowers the efficiency. The need for above-normal WR2 should be analyzed from two standpoints, the effect on power system stability, and the effect on speed regulation of the unit. Speed regulation and governor calculation are discussed in the guidelines for selection of turbines and governors.

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Electrical system stability considerations may in special cases require a high WR2 is only one of several adjustable factors affecting system stability, all factors in the system design should be considered in arriving at the minimum overall cost. Sufficient WR2 must be provided to prevent hunting and afford stability in operation under sudden load changes. The index of the relative stability of generators used in electrical system calculations is the inertia constant, H, which is expressed in terms of stored energy per kVA of capacity. It is computed as:

H = kVA

skW • =

kVArWR 622 10min)/)((231.0 −×

The inertia constant will range from 2 to 4 for slow-speed (under 200 rpm) water wheel generators. Transient hydraulic studies of system requirements furnish the best information concerning the optimum inertia constant, but if data from studies are not available, the necessary WR2 can be computed or may be estimated from knowledge of the behavior of other units on the system.

Flywheel effect is expressed as moment of inertia (GD2) (in India) as compared to flywheel effect WR2 (US/English) GD2 = weight x Diameter2 and WR2 = weight x Radius2 (lb.ft2).

Accordingly, WR2 = 4

2GD

Conversion factor for WR2 (USA) in lb.ft2 and GD2 (India) kg. m2 is as follows: GD2 = 22 .6.9.5 ftlbxftlbx ≈=× The flywheel effect of the generator can be increased by adding weight in the rim of the rotor or by increasing the rotor diameter. Increasing the GD2 increases the generator cost, size and weight, and lowers the efficiency. The need for above-normal WR2/ GD2 is analyzed from two standpoints, the effect on power system stability, and the effect on speed regulation of the unit. Speed regulation and governor calculation are discussed in guidelines for turbine selection. Power system stability considerations do not arise in small hydro generators under considerations.

Mechanical characteristics of the generator are based on the hydraulic turbine data to which the generator will be coupled. Characteristics regarding speed, flywheel effect have been discussed in guidelines of turbine selection.

9.3.7.8 Cooling

Losses in a generator appear as heat which is dissipated through radiation and ventilation. The generator rotor is normally constructed to function as an axial flow blower, or is equipped with fan blades, to circulate air through the windings. Small-generators up to 5 MW may be partially enclosed, and heated generator air is discharged into the generator hall, or ducted to the outside. Adequate ventilation of the generator hall preferably thermostatically should be provided in this case.

Water to air coolers normally is provided for all modern hydro generators rated greater than 5 MVA. The coolers are situated around the outside periphery of the stator core. Generators equipped with water-to-air coolers can be designed with smaller physical dimensions, reducing the cost of the generator. Automatic regulation of the cooling water flow in direct relation to the generator loading results in more uniform machine operating temperatures, increasing the insulation life of the stator windings. Cooling of the generator can be more easily controlled with such a system, and the stator windings and ventilating slots in the core kept cleaner, reducing the rate of deterioration of the stator winding insulation system. The closed systems also permits the addition of automatic fire protection systems, attenuates generator noise, and reduce heat gains that must be accommodated by the powerhouse HVAC system.

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Normally, generators should be furnished with one cooler than the number required for operation at rated MVA. This allows one cooler to be removed for maintenance without affecting the unit output. The generator cooling water normally is supplied from the penstock via a pressure reducing station or pumped from the tailrace. In either case, automatic self-cleaning filters must be provided in the cooling water supply lines to avoid frequent fouling or plugging of the water-to-air coolers.

9.3.7.9 Fire Extinguishing System All hydroelectric generators greater than 25 MVA should be furnished with either a water deluge or carbon dioxide (CO2) fire extinguishing system, to minimize the damage caused by a fire inside the machine. Generators 25 MVA or below should be evaluated individually to ensure installation of a cost effective system. When total thermo setting insulation system is adopted water sprinkler system may be used. This system is safer for operating staff.

9.3.7.10 High Pressure Oil Injection System (For Thrust Bearing)

Modern Thrust Bearings have a high-pressure oil lift system, which injects oil into each shoe pad of the bearing during starting and stopping of the unit. This helps establish a hydrodynamic oil film between the rotating (thrust collar) and stationary (bearing shoes or pads) members of the thrust bearing. The presence of the hydrodynamic oil film minimizes bearing wear. Consequently, during a unit start the high pressure oil of lift system is turned on before the unit starts to rotate. It is usually shut off after the unit speed exceeds 75 to 90% of rated speed because, at the higher rotational speeds, the hydrodynamic oil film is self sustaining. On unit shutdown, the high pressure oil lift system is turned on when the unit speed decreases to the 75 to 90% range and maintained until the unit is at stand still. The maximum wear on the bearing pads occurs at slow speeds, when, due to hydrodynamic effects, the oil film is not maintained over the entire bearing surface.

9.3.7.11 Dynamic Braking System

Modern hydroelectric generators use electrical dynamic braking in addition to the mechanical friction braking system. The electrical dynamic braking system minimizes the wear on the mechanical brake ring and brake pads, prolonging their life. In addition, electrical dynamic braking reduces the duration over which the mechanical brakes are applied during a unit stop sequence, thereby minimizing the amount of brake dust produced by the mechanical brake system. The extended life and reduced brake dust are especially significant for units that are started and stopped several times a day; or units; such as those connected to pelton turbines, which operate at very high rpms. Electrical dynamic braking is initiated at a higher unit rotational speed, normally 50% of rated speed; and maintained until the mechanical friction brakes are applied. When dynamic braking is utilized in conjunction with the mechanical brakes, the mechanical brakes normally are applied at ten to 15% of rated speed. Modern hydroelectric generators, especially pumped storage units, also utilize a brake dust vacuum system to capture most of the brake dust produced when the mechanical friction brakes are applied.

9.4 Characteristics of Large Hydro Generators of Dehar Power Plant – case study

Studies for fixing capacity and unit size are outlined in chapter 2 Para 2.4. Considerations involved for fixing, Type and principal electrical and mechanical characteristics of 174 MVA Dehar Generators on Beas -Satluj Link project are given below:

9.4.1 Introduction

Dehar Power Plant of Beas Sutlej link Project commissioned in 1976 to 1980, has six (four first stage and two second stage) semi-umbrella type vertical generators of 0.95 pf of 165 MW each. A longitudinal section of the scheme is shown in Figure 9.4. The 300 rpm generators are coupled to 282 m (925 ft) rated head Francis turbines having a turbine runaway speed of 516 rpm. Unit size of 174 MVA selected for the power plant was the largest unit size in the country at that time and is perhaps a machine with highest speed

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with semi umbrella construction. The power plant is to be interconnected with the Northern Regional Grid of India in the first stage by a 280 km long single circuit 420 kV line at Panipat and by a 60 km long double circuit 245 kV line at Ganguwal. A second 420 kV line was added alongwith two second stage units. 420 kV was being introduced in the region for the first time.

The main interconnected transmission system to which Dehar power plant was to be interconnected (1st stage) is shown in Figure 9.5and interconnection (1st stage) of the power plant with the grid is shown in Figure 9.6. Considerations of economy dictated the size and type of generating unit to be installed and transmission outlets to be provided at a hydro generating site. Further in the initial stage of development of an EHV system problems of stability of hydro generators are liable to be critical because of weak system, large transmission angle and operation of generators at leading power factor. There is also risk of self-excitation and problem of voltage stability due to capacitive loading of long EHV lines. Further, smaller inertia constants of modern machines of advanced design may also jeopardize the stability of turbine governor gear. These problems are brought out and the system and other considerations involved in fixing the economical size, type and principal electrical and mechanical characteristics of Dehar hydro generators and excitation system keeping in view advancement in material technology, availability of modern fast acting static excitation equipment and methods used for analysis of system problems are discussed. Dehar Power Station is located in the Himalayan region of India. This region and many other hilly areas where hydro stations are located are seismic zones. Provisions made in the generator for safeguarding against seismic forces have also been outlined.

Fig. 9.4: Longitudinal Section

(Source: Paper by Author – 2nd world Congress, International Water Resources Association 1979)

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Fig. 9.5: Dehar EHV System and Interconnected Northern regional Grid single line diagram 2200 kV and above (Source: Paper by Author – 2nd world Congress, International Water Resources Association 1979)

9.4.2 Megawatt Rating and Number of generating Units

Substantial economies in the cost of equipment and civil structure are obtained by the installation of lesser machines of bigger size especially in a high head power plant. Further, higher efficiency is associated with larger generating units. Limitation on the size of the unit is placed on the one hand by water turbine and on the other hand by system considerations. With the availability of better techniques and materials, hydraulic turbine design has undergone rapid advancement and very large sized medium and high head Francis turbine hydraulic turbines have been made or proposed, e.g., 8,20,000 HP hydraulic turbines for third power plant at Grand Coulee in USA. Accordingly the major constraint on unit size for medium and high head Francis turbine driven generators is system characteristics and other limitations. As the system grows bigger, larger size generating units can be installed. In general maximum economic unit size that can be installed in a system can be found out by evaluating the increase in system spare generating capacity required as a result of increase in unit size. Evaluation of system spare generating capacity for various unit sizes is dependent upon size and characteristics of the power plants in the system and spare capacity already available in the grid. Other considerations involved are part load operation and transportation of heavy and big single piece packages, The economic unit size at Dehar Power Plant was fixed keeping in view the maximum available working capacity of 583 MW as determined by water and power studies, the system spare generating capacity required to be provided at the power plant to meet forced outages in the system and the capacity outage required for scheduled maintenance of power units. Seasonal variation of load and available generating capacities was taken into consideration for fixing spare capacity. The spare generating capacity required for forced outages was worked out by probability methods.

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Transportation was also a consideration in fixing the maximum size of the units. Details are given in chapter 2 (Para 2.5). Accordingly the optimum number and kilowatt rating of generating units was fixed as four units of 165 MW each of the first stage. Two additional second stage units of equal capacity were proposed in the power house to take care of peaking requirements of future thermal power plants in the grid.

9.4.3 Power Factor and MVA Rating

The required MVA rating of generators is determined from the system reactive power requirements. Growth of system network and a better understanding of its behaviour has resulted in a definite trend towards specifying higher power factor rating especially for remotely located hydro generators, interconnected with the grid by EHV lines, so that the improvement in performance associated with the operation of generating unit nearer to its rated power factor can be realized. In case of Dehar Power Plant detailed load flow studies were carried out so as to find out the VARS (reactive power) fed into the system from Dehar generators for various interconnection alternatives. The reactive flow, megawatt load and consequently the actual power factor at which the machines operate in the study were found out. In the study total VARS of the system load were balanced by VARS from generators, capacitors already installed in the system and additional capacitors at load end adjusted so as to ensure tail end voltages at proper level. A typical study for an equivalent reduced network is shown in Figure 9.7. For various schedules of generation and transmission alternatives and line outage the reactive flow and power factor of operation at Dehar machines varies from 0.96 to 0.98. Consequently the power factor of the machines was specified as 0.95. Procurement of higher power factor machines resulted in saving in the cost of generators.

9.4.4 Type of Generators

Savings in the cost of generators, overhead traveling crane capacities and civil structures can be made by adopting umbrella type of construction with a combined thrust and guide bearing below the rotor. Major conditions required to be fulfilled before umbrella type of construction can be adopted for large sized high speed hydro generators may be summarized as below: a) The ratio of core length to rotor diameter be kept as low as possible and at the same time ensuring

that a reasonably high output co-efficient is obtained consistent with the required winding temperature rises and transient reactances.

b) The required flywheel effect is incorporated in the rotor rim and poles with the stresses in the rotor at

turbine runaway speed not exceeding two-third of the yield point of material. c) The rotor overhang above the guide bearing to be reduced sufficiently, to ensure that the calculated

critical speed of the combined generator and turbine shaft system is higher than the runaway speed by an adequate margin.

d) The radial width of air gap to be as high as possible so as to minimize the unbalanced magnetic pull

on the rotor and thereby reducing the over-turning moment on the bearing. (e) Ample and adequate design of bearing and easy accessibility to the thrust bearing. In order to satisfy the above conditions for umbrella construction for Dehar generators, it was

necessary that the length of the rotor core (and hence rotor overhang above the guide bearing) be reduced. It is, therefore, obvious that umbrella generator construction would require large diameter rotors with ratio of core length to core diameter less than about 0.29 so that critical speeds are well above turbine runaway speeds. Large diameter rotors would mean higher speeds and consequently higher stresses. Quality of steel available for rotor fabrication was, therefore, one of the main factors which hitherto restricted umbrella construction to smaller sized medium and low speed Francis and Propeller or Kaplan turbine generators. Similarly the limit curve for hydro-generator of above 100 MVA rating did not exceed 200 rpm in Japan. It was now possible to adopt umbrella type

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construction for large high speed generators due to the advancement in material technology with special reference to higher tensile sheet steel for rotor rim punchings so as to obtain a minimum factor of safety of 1.5 on the yield point at turbine runaway speed.

Fig. 9.6: Interconnection of Dehar power plant with grid (Source: Paper by Author – 2nd world Congress, International Water Resources Association 1979)

Fig. 9.7: Reduced equivalent system load flow study (Max. hydro)- maximum load condition

(Source: Paper by Author – 2nd world Congress, International Water Resources Association 1979)

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For Dehar Power plant 173.8 MVA 300 rpm generators, high tensile sheet steel with a yield strength of 56 kg/sq mm (36 tons/sq in.) was used which permits its operation on a turbine runaway speed of 510 rpm at a peripheral speed of 176 m/sec for its large diameter (6.8 m) rotors with the specified factors of safety. With the use of this high tensile sheet steel, it was possible to reduce the length of the core (and hence the rotor overhang above the guide bearing) sufficiently to satisfy the conditions for umbrella construction. A second guide bearing at the top, although not considered necessary by suppliers of generators, was got provided so as to provide better stability for the unit taking into consideration seismic zone of location. Manufacturers intimated that the cost of the umbrella machines is about 12 to 15 percent lower than that of conventional top thrust bearing and two guide bearing arrangement. Semi-umbrella generator actually provided was about 10 percent cheaper. The calculated first critical speed for the combined generator and shaft system with the proposed semi-umbrella arrangement for the generator was about 20 percent above turbine runaway speed.

9.4.5 Generator Flywheel Effect and Stability of turbine Governor System

Large modern hydro generators have smaller inertia constant and may face problems concerning stability of turbine governing system. This is due to the behaviour of the turbine water, which because of its inertia gives rise to water hammer in pressure pipes when control devices are operated. This is in general characterized by the hydraulic acceleration time constants. In isolated operation, when frequency of the whole system is determined by turbine governor the water hammer affects the speed governing and instability appears as hunting or frequency swinging. For interconnected operation with a large system the frequency is essentially held constant by the later. The water hammer then effects the power fed to the system and stability problem only arises when the power is controlled in a closed loop, i.e., in case of those hydro generators which take part in frequency regulation. The stability of turbine governor gear is greatly affected by the ratio of the mechanical acceleration time constant due to the hydraulic acceleration time constant of the water masses and by the gain of the governor. A reduction of the above ratio has a destabilizing effect and necessitates a reduction of the governor gain, which adversely affects frequency stabilization. Accordingly a minimum flywheel effect for rotating parts of a hydro unit is necessary which can normally only be provided in the generator. Alternatively mechanical acceleration time constant could be reduced by the provision of a pressure relief valve or a surge tank, etc., but it is generally very costly. An empirical criteria for the speed regulating ability of a hydro generating unit could be based on the speed rise of the unit which may take place on the rejection of the entire rated load of the unit operating independently. For the power units operating in large interconnected systems and which are required to regulate system frequency, the percentage speed rise index as computed above was considered not to exceed 45 percent. For smaller systems smaller speed rise be provided (Refer Chapter 4).

Fig. 9.8: Longitudinal section from intake to Dehar Power Plant

(Source: Paper by Author – 2nd world Congress, International Water Resources Association 1979)

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For Dehar Power Plant, the hydraulic pressure water system connecting the balancing storage with the power unit consisting of water intake, pressure tunnel, differential surge tank and penstock is shown in Figure 9.8. Limiting the maximum pressure rise in the penstocks to 35 percent the estimated maximum speed rise of the unit upon rejection of full load worked out to about 45 percent with a governor closing time of 9.1 seconds at rated head of 282 m (925 ft) with the normal flywheel effect of the rotating parts of the generator (i.e., fixed on temperature rise considerations only). In the first stage of operation the speed rise was found to be not more than 43 percent. It was accordingly considered that normal flywheel effect is adequate for regulating frequency of the system.

9.4.6 Generator Parameters and Electrical Stability

The generator parameters which have a bearing on stability are the flywheel effect, transient reactance and short circuit ratio. In the initial stage of development of 420 kV EHV system as at Dehar problems of stability are liable to be critical because of weak system, lower short circuit level, operation at leading power factor, and need for economy in providing transmission outlets and fixing size and parameters of generating units. Preliminary transient stability studies on network analyzer (using constant voltage behind transient reactance) for Dehar EHV system also indicated that only marginal stability would be obtained. In the early stage of design of Dehar Power Plant it was considered that specifying generators with normal characteristics and achieving requirements of stability by optimizing parameters of other factors involved especially those of excitation system would be economically cheaper alternative. In a study of the British System also it was shown that changing generator parameters have comparatively much less effect on the stability margins. Accordingly normal generator parameters as given in the appendix were specified for the generator. The detailed stability studies carried out are given in chapter 10 in Para 10.12.

9.4.7 Line Charging Capacity and Voltage Stability

Remotely located hydro generators used to charge long unloaded EHV lines whose charging kVA is more than the line charging capacity of the machine, the machine may become self excited and voltage rise beyond control. The condition for self excitation is that xc < xd where, xc is capacitive load reactance and xd the synchronous direct axis reactance. The capacity required for charging one single 420 kV unloaded line E2/xc up to Panipat (receiving end) was about 150 MVARs at rated voltage. In second stage when a second 420 kV line of equivalent length is installed, the line charging capacity required to charge both the unloaded lines simultaneously at rated voltage would be about 300 MVARs. The line charging capacity available at rated voltage from Dehar generator as intimated by suppliers of the equipment was as follows:

(i) 70 percent rated MVA, i.e., 121.8 MVAR line charging is possible with a minimum positive

excitation of 10 percent.

(ii) Up to 87 percent of rated MVA, i.e., 139 MVAR line charging capacity is possible with a minimum positive excitation of 1 percent.

(iii) Up to 100 percent of rated MVAR, i.e., 173.8 line charging capacity can be obtained with approximately 5 percent negative excitation and maximum line charging capacity that can be obtained with negative excitation of 10 percent is 110 percent of rated MVA (191 MVAR) according to BSS.

(iv) Further increase in line charging capacities is possible only by increasing the size of the machine. In the case of (ii) and (iii) hand control of excitation is not possible and full reliance has to be placed on continuous operation of quick acting automatic voltage regulators. It is neither economically feasible nor desirable to increase the size of the machine for the purpose of increasing the line charging capacities. Accordingly taking into consideration operating conditions in the first stage of operation it was decided to provide for a line charging capacity of 191 MVARs at rated voltage for the generators by providing negative excitation on the generators.

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Critical operating condition causing voltage instability may also be caused by the disconnection of load at the receiving end. The phenomenon occurs due to capacitive loading on the machine which is further adversely affected by the speed rise of generator. Self excitation and voltage instability may occur if. Xc ≤ n2 (Xq + XT) Where, Xc is capacitive load reactance, Xq is quadrature axis synchronous reactance and n is the maximum relative over speed occurring on load rejection. This condition on the Dehar generator was proposed to be obviated by providing a permanently connected 400 kV EHV shunt reactor (75 MVA) at the receiving end of the line as per detailed studies carried out.

9.4.8 Damper winding

Principal function of a damper winding is its capacity to prevent excessive over-voltages in the event of line to line faults with capacitive loads, thereby reducing over-voltage stress on the equipment. Taking into consideration remote location and long interconnecting transmission lines fully connected damper windings with the ratio of quadrature and direct axis reactances Xn

q/ Xnd not exceeding 1.2 was specified.

9.4.9 Generator Characteristic and Excitation System

Generators with normal characteristics having been specified and preliminary studies having indicated only marginal stability, it was decided that high speed static excitation equipment be used to improve stability margins so as to achieve overall most economical arrangement of equipment. Detailed studies were carried out to determine optimum characteristics of the static excitation equipment and discussed in chapter 10.

9.4.10 Seismic Considerations

Dehar Power Plant falls in seismic zone. Following provisions in the hydro generator design at Dehar were proposed in consultation with the manufacturers of equipment and taking into consideration the seismic and geological conditions at site and the report of the Koyna Earthquake Experts Committee constituted by Government of India with the help of UNESCO.

Mechanical Strength Dehar generators be designed to withstand safely the maximum earthquake acceleration force both in the vertical and horizontal direction expected at Dehar acting at the centre of machine.

Natural Frequency Natural frequency of the machine be kept well away (higher) from the magnetic frequency of 100 Hz (twice the generator frequency). This natural frequency will be far removed from the earthquake frequency and be checked for adequate margin against the predominant frequency of earthquake and critical speed of rotating system.

Generator stator support The generator stator and lower thrust and guide bearing foundations comprise a number of sole plates. The sole plates be tied to foundation laterally in addition to normal vertical direction by foundation bolts.

Guide Bearing Design Guide bearings to be of segmental type and the guide bearing parts be strengthened to withstand full earthquake force. Manufacturers further recommended to tie up the top bracket laterally with the barrel (generator enclosure) by means of steel girders. This would also mean that the concrete barrel in turn would have to be strengthened.

Vibration Detection of Generators Installation of vibration detectors or eccentricity meters on turbines and generators were recommended to be installed for initiating shutdown and alarm in case the vibrations due to earthquake exceed a predetermined value. This device may also be used in detecting any unusual vibrations of a unit due to hydraulic conditions affecting the turbine.

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Mercury Contacts Severe shaking due to earthquake is liable to result in false tripping for initiating shutdown of a unit if mercury contacts are used. This can be avoided by either specifying anti-vibration type mercury switches or if found necessary by adding timing relays.

9.4.10 Conclusions

(1) Substantial economies in the cost of equipment and structure at Dehar Power Plant were obtained by adopting large unit size keeping in view size of the grid and its influence on system spare capacity.

(2) Cost of generators was reduced by adopting umbrella design of construction which is now possible for large high speed hydro generators due to the development of high tensile steel for rotor rim punchings.

(3) Procurement of natural high power factor generators after detailed studies resulted in further savings in the cost.

(4) Normal flywheel effect of the rotating parts of the generator at the frequency regulating station at Dehar was considered sufficient for stability of turbine governor system because of the large interconnected system.

(5) Special parameters of remote generators feeding EHV networks for ensuring electrical stability can be met by fast response static excitation systems.

(6) Fast acting static excitation systems can provide necessary stability margins. Such systems, however, require stabilizing feed back signals for achieving post fault stability. Detailed studies should be carried out.

(7) Self-excitation and voltage instability of remote generators interconnected with the grid by long EHV lines can be prevented by increasing line charging capacity of machine by resort to negative excitation and/or by employing permanently connected EHV shunt reactors.

(8) Provisions can be made in the design of generators and its foundations to provide safeguards against seismic forces at small costs.

9.4.11 Main Parameters of Dehar Generators Short Circuit Ratio = 1.06 Transient Reactance Direct Axis = 0.2 Flywheel Effect = 39.5 x 106 lb ft2

Xnq/Xn

d not greater than = 1.2 9.5 Small Hydro Generator Up to & Below 5 MW 9.5.1 General

Standardized or upgraded mass-produced machine should be used where possible conforming to IS: 4722. Most “off-the-shelf” or mass-produced machines are designed for lower over speed values (typically 1,25 to 1,50 times rated speed) than are experienced with hydraulic turbines. Therefore, such generator designs should be checked for turbine runaway conditions.

Accordingly cylindrical rotor synchronous may be considered up to 3 MW capacity.

Special Design Features as per IEC 1116 conforming to IS: 4722 for these generators is as follows:

i) Designed to mechanically withstand continuous operation at runaway speed.

ii) These generators should be factory assembled that are shipped to the field as two integral component parts, rotor and stator. So that assembled work at site is minimize.

iii) Class F insulation with class B temperature rise

iv) Self lubricated journal type maintenance -free pedestal bearing

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v) Open ventilation

vi) Fully assembled and dynamically balanced

Standard BHEL generators confirming to the IEC standards are given in table 9.2. 9.5.2 Type of Generators

There are basically two types of alternating current generator: synchronous and asynchronous (or induction) generators. The choice of the type to be used depends on the characteristics of the grid to which the generator will be connected and also on the generator’s operational requirements.

Synchronous generators are used in the case of stand alone schemes (isolated networks). In case of weak grids where the unit may have significant influence on the network synchronous generator are used. Salient pole machines or cylindrical rotor machines are specified. For grid connected schemes both types of generator can be used. In case grid is weak; Induction generators may be used if there are two units, one of the unit can be synchronous so that in case of grid failure; supply could still be maintained. Unit size be limited to 250 kW. In case of stronger grids induction generators up to a 2000 kW or even higher have been used.

Before making a decision on the type of generator to be used, it is important to take the following points into consideration:

- A synchronous generator can regulate the grid voltage and supply reactive power to the network. It

can therefore be connected to any type of network. - An induction generator has a simpler operation, requiring only the use of a tachometer to couple it to

the grid as the machine is coupled to the grid there is a transient voltage drop, and once coupled to the grid the generator absorbs reactive power from it. Where the power factor needs to be improved, a capacitor bank will be necessary. The efficiency of an asynchronous generator is generally lower than that of a synchronous one.

UNDP/World Bank Energy Sector Management Assistance Programme (ESMAP) funded a large number of mini hydro developments on irrigation dams and canal drop in India with Induction generators. Induction (asynchronous) generators, (essentially induction motors) which are driven at slightly above synchronous speed, were specified for all schemes in the range 350 kW to 3500 kW. The primary function of the irrigation based mini-hydro schemes was to provide energy to the remote sections of the grid. Hence, induction generators, which require no separate excitation source since they draw magnetizing current from the grid, were considered to be appropriate. The operating speed of induction generators was specified. The difference between the rotating speeds of the turbine and generator were used to establish the specification for speed increasing mechanisms. The goal was to keep the speed of the turbines as high as possible and to minimize the gearbox ratio by maintaining the lowest feasible speed for the generators (See also chapter 13).

Two typical schemes 1500 kW (2 x 750 kW) at Narangwal in Punjab and proposed Lower Bhawani Project in Tamilnadu – ALT –II 7000 KW (2 x 3.5 MW) (as per ESMAP Report) each with induction generators and capacitor bank installation is shown in figure 9.9& figure 9.10.

Table 9.2 STANDARD SHP GENERATORS MANUFACTURED BY M/S BHEL INDIA Ltd. (*)

A. SHP S. No.

Rating in kW

Speed in RPM 300 333.3 375 426 500 600 750 1000 1500

1. 500 230M20 230M20 183M20 183M20 145M20 145M20 145M20 145M20 132M25 2. 1000 230M25 230M25 183M25 183M25 183M25 145M50 145M38 145M38 132M50 3. 1500 230M35 230M35 183M50 183M50 183M50 145M75 145M57 145M57 132M50 4. 2000 230M45 230M45 183M70 183M70 183M60 145M75 145M57 145M75 132M50

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5. 2500 230M70 230M70 230M60 183M70 183M60 203M70 145M75 145M75 132M100 6. 3000 230M70 230M70 230M50 254M50 254M40 203M70 203M50 145M100 132M100 7. 3500 230M90 230M70 230M60 254M50 254M50 203M70 203M50 145M100 132M100 8. 4000 230M90 230M90 230M80 254M65 254M50 203M95 203M70 145M100 - 9. 4500 230M90 230M90 230M80 230M65 254M50 203M95 203M70 - - 10. 5000 230M90 230M90 230M80 230M65 254M50 203M95 203M70 - - B. Mini Micro: generators 200-500 kW; speed 300 to 1500 RPM; power factor 0.67 lag.; Voltage 415 to 11

kV

(*) A. M. Gupta BHEL, Bhopal - Small Hydro Generators – International course on technology selection for small hydro power development at Alternate Hydro Energy Centre (AHEC) during Feb. 18-28, 2003

Merits and demerits of synchronous and induction generator is given in table 9.3.

Table 9.3: Merits & Demerits Synchronous V/S Induction Generators

S. No. Item Syn. Generator Ind. Generator

1 Rotor construction Salient pole type Squirrel cage type

2 Excitation Required Not required

3 Isolated operation Possible Not possible

4 Stability To be maintained by excitation control No problem

5 Maintenance More because of excitation & control equipments

Less because of squirrel case rotor

6 Efficiency High Low

7 Inertia High Low

8 Cost High Low

9 Power factor Adjustable by excitation control Not adjustable determined by load

10 Suitability for highly fluctuating loads

Ideal Not suitable

11 Loads Highly capacitive Only inductive

12 Voltage variation Possible Not possible Climatic conditions (ambient temperature, altitude, humidity) can affect the choice of the class of insulation level and temperature rises. The cooling system of the generator should be evaluated. In the case where heat from the generator is expelled into the powerhouse sufficient power house ventilation should be provided.

If necessary, a braking system (either air or oil operated) should be considered.

9.5.3 Selection and Mechanical Characteristics

Small hydro up to 5 MW is generally category-2 generators. These generators are factory assembled that are shipped to the field as two integral component parts, rotor and stator.

9.5.3.1 Vertical/Horizontal Configuration

With all turbines, a vertical or horizontal configuration is possible. The orientation becomes a function of the turbine selection and of the power plant structural and equipment costs for a specific layout. As an example, the Francis vertical unit will require a deeper excavation and higher power plant structure. A horizontal machine will increase the width of the power plant structure yet decrease the excavation and overall height of the unit. It becomes apparent that generator orientation and setting are governed by compatibility with turbine selection and an analysis of overall plant costs.

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1 2 3 4

415V 4000A

G1 G2750 Kw

ACB2000A

ACB2500A

MAIN TRANSFORMER2500kVA 415/11kV

INDOOR VCB 630A

400A

OUTDOOR VCB 630A

400A

GRID

CAPACITOR

5 6 7 8

CAPACITOR BANK-1

1 CAPACITOR FEEDER 1 100kVAR2 CAPACITOR FEEDER 2 100kVAR3 CAPACITOR FEEDER 3 100kVAR4 CAPACITOR FEEDER 4 100kVAR

CAPACITOR BANK-2

5 CAPACITOR FEEDER 5 100kVAR6 CAPACITOR FEEDER 6 100kVAR7 CAPACITOR FEEDER 7 100kVAR8 CAPACITOR FEEDER 8 100kVAR

415V 400kVAR 415V 400kVAR

415V 400A

OL RELAY

ACB1250A

Fig, 9.9: Narangwal SHP with Induction Generators and capacitor Bank

Fig.9.10: Lower Bhavani Project ALT – II (ESMAP Study)

(Source: ESMAP)

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9.5.3.2 Speed (rpm): The speed of a generator is established by the turbine speed. The hydraulic turbines should determine the turbine speed for maximum efficiency corresponding to an even number of generator poles. Generator dimensions and weights vary inversely with the speed. For a fixed value of power a decrease in speed will increase the physical size and cost of generators. Low head turbine can be connected either directly to the generator or through a speed increaser. The speed increaser would allow the use of a higher speed generator, typically 600, 750 or 1000 (1500) r/min, instead of a generator operating at turbine speed. The choice to utilize a speed increaser is an economic decision. Speed increasers lower the overall plant efficiency by about 1% for a single gear increaser and about 2% for double gear increaser. (The manufacturer can supply exact data regarding the efficiency of speed increasers). This loss of efficiency and the cost of the speed increaser must be compared to the reduction in cost for the smaller generator. It is recommended that speed increaser option should not be used for unit sizes above 5 MW capacity.

9.5.3.3 Dimension

Three factors affect the size of generator. These are orientation, kVA requirements and speed. The turbine choice dictates all three of these factors for the generator. The size of the generator for a fixed kVA varies inversely with unit speed. This is due to the requirements for more rotor field poles to achieve synchronous speed at lower rpm.

9.5.3.4 Over speed Withstand

In the interest of safety, units with synchronous generators are designed to withstand continuous runaway conditions.

9.5.3.5 Guide and Thrust Bearings The shaft system is designed to minimize the number of bearings. It is essential to study the turbine and generator bearings as systems. The choice is between journal, ball or roller bearings, attention should be given to their ability to withstand vibrations, eddy currents and runaway conditions. If the unit size is small and for reasons of simplicity, the use of self-lubricating bearings should be preferred.

9.3.5.6 Braking System If necessary braking system (mostly oil operated) is used. 9.5.4 Ratings and Electrical Characteristics 9.5.4.1 kW Rating: The kilowatt rating of the generator should be compatible with the kW rating of the turbine.

The most common turbine types are Francis, fixed blade propeller, and adjustable blade propeller (Kaplan). Each turbine type has different operating characteristics and imposes a different set of generator design criteria to correctly match the generator to the turbine. For any turbine type, however, the generator should have sufficient continuous capacity to handle the maximum kW available from the turbine at 100-percent gate without the generator exceeding its rated nameplate temperature rise. In determining generator capacity, any possible future changes to the project, such as raising the forebay (draw down) level and increasing turbine output capability, should be considered. In a variable head power plant the turbine output may vary depending upon available head. In general the generator is rated for turbine output at rated head.

9.5.4.2 kVA Rating and power factor: kVA and power factor is fixed by consideration of location of the power

plant with respect to load centre. These requirements include a consideration of the anticipated load, the electrical location of the plant relative to the power system load centers, the transmission lines, substations, and distribution facilities involved.

9.5.4.3 Frequency and Number of Phases: In India standard frequency is 50 cycles, 3 phase power supply.

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9.5.4.4 Generator Terminal Voltage: Generator terminal voltage is kept as high as economically feasible. Generators of less than 5000 kVA are designed for 6.6 kV, 3.3 kV or 415 volts depending upon requirement of generator WR2 or generator reactance. Economical terminal voltage for small hydro generators recommended by CBI & P (publication no. 280 – 2001) and AHEC/MNRE guidelines are as follows:

CBI & P Publication No. 280 (Economical) AHEC/MNRE Guidelines

Up to 750 kVA 415 volts Up to 400 kW (or kVA) 415 volts

751 – 2500 kVA 3.3 kV 401 – 2500 kW (or kVA) 3.3 kV

2501 – 5000 kVA 6.6 kV 2501 – 5000 kW (or kVA) 6.6 kV

Above 5000 kVA 11 kV Above 5000 kW (or kVA) 11 kV Preferred voltage rating of generator as per IEC 60034-1 is as follows: 3.3 kV - Above 150 kW (or kVA) 6.6 kV - Above 800 kW (or kVA) 11 kV - Above 2500 kW (or kVA) 9.5.4.5 Stator Winding Connection: Star, stator winding connection are providing for both grounded or

ungrounded operation and six terminal (3 on line side and 3 on neutral side) are brought out, except for small generators when only one neutral is brought for ground connections.

9.5.4.6 Excitation Voltage: Rated generator rotor voltage is specified by the manufacturer, based on the rotor winding resistance and the excitation current required for full load operation at rated voltage and power factor, including suitable margin. Ceiling voltage is as agreed upon by the manufacturer and purchaser. Standard voltage of excitation system are 62.5, 125, 150, 250 V DC.

9.5.5 Insulation and Temperature Rise 9.5.5.1 Synchronous Generators

a) Stator:

Class F insulation level and Class B temperature rises are recommended. The American practice is to provide Class H insulation with a temperature of not more than 80oC.

b) Rotor :

The insulation level should normally be Class-F and temperature rises Class-B. 9.5.5.2 Asynchronous (Induction) Generator

a) Stator

Class F insulation level and Class B temperature rises are recommended.

b) Rotor

Squirrel cage construction, Class F insulation and Class B temperature rises are recommended. These units should be designed to withstand continuous runaway conditions.

9.5.6 Typical Characteristics

Typical Technical Particulars and characteristics of some SHP generators as installed in India are given in table 9.4.

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Table 9.4

SHP Projects Remarks Awapan

(2 x 250 kW) Sebari SHP (2 x 500 kW)

Kitpi SHP 2 x 1500 KW)

Type Salient pole Cylindrical pole Cylindrical rotor Operation Isolated/grid connected Grid connected Grid connected Grid connected Configuration Horizontal Vertical Horizontal Sebari SHP has siphon

intake Rating (kW) 250 500 1500 Voltage (volts) 415 415 3.3 kV Rated power factor 0.8 Lag 0.85 0.8 Speed rpm 1500 750 600 Runaway Speed/withstand in minutes 2700/15 min 2175/30 min 1080/15 min Frequency (Hz) 50 50 50 SCR 1.0 0.8 1.03 Insulation Class F H F Temp. rise Class B Class B Class B Stator by res. 800C 1000C 800C Rotor by res. 800C 1100C 800C Line charging Noise 90 dB Vibration IS: 12065 Waveform IS: 4722 Efficiencies at rated pf. 93.40 % 95.10% 94.5% Excitation Brushless (thyristor) static AVR yes Automatic voltage

regulator APFR yes Automatic power factor

regulator

9.6 Micro Hydro

Synchronous generators are generally used. Generators may be selected in accordance with quality standard issued by AHEC extracts enclosed as Annexure 2. These generators are self excited and factory assembled and classified as category-1 generator in American Practice. They are shipped to site completely assembled depending on the rpm selected, unit speed/weights and method of transportation to site. In case of isolated units, small capacity Induction generators with variable capacitor bank may be used up to a capacity of about 50 kW especially if there is no or insignificant Induction motor load i.e. less than about 20%.

9.7 Generator Efficiencies

The efficiency of an electrical generator is defined as the ratio of output power to input power. Typical Efficiency values for some generators are given in table 9.4. There are five major losses associated with an electrical generator. Various test procedures are used to determine the magnitude of each loss. Two classes of losses are fixed and therefore independent of load. These losses are (1) windage and friction (2) core loss. The variable losses are (3) field copper loss, (4) armature copper loss and (5) stray loss or load loss.

Windage and friction loss is affected by the size and shape of rotating parts, fan design, bearing design and the nature of the enclosure. Core loss is associated with power needed to magnetize the steel core parts of the rotor and stator. Field copper loss represents the power losses through the dc resistance of the field. Similarly, the armature copper loss is calculated from the dc resistance of the armature winding. Stray loss for load loss is related to armature current and its associated flux. Typical values for efficiency range from 91 to 98%. This efficiency value is representing throughout the whole loading range of a particular machine; i.e., the efficiency is approximately the same at ¼ load or at ¾ load. Typical efficiencies of some SHP generators as installed is given in table 9.4.

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CHAPTER 9: ANNEXURE-I

CENTRAL ELECTRICITY AUTHORITY (GRID STANDARDS) REGULATIONS-2006 (ABSTRACTS)

1. Short Title, Commencement and Interpretation

a) These regulations may be called the Central Electricity Authority (Grid Standards) Regulations, 2006 framed as per provisions under section 34, Section 73(d) and section 177(2) (a) of the Electricity Act, 2003.

b) These regulations shall come into force on the date of their publication in the official Gazette.

c) Grid Standards for Operation and Maintenance of Transmission Lines as prescribed by the Authority are given in the "Schedule" appended to these Regulations.

d) These regulations shall be reviewed by the Authority in consultation with all the stake holders as and when considered necessary.

2. Definitions

In these Regulations, unless the context otherwise requires;

a) "Act" means the Electricity Act, 2003.

b) “Appropriate Load Despatch Centre" means the National Load Despatch Centre (NLDC), Regional Load Despatch Centre (RLDC) or State Load Despatch Centre (SLDC) or Area Load Despatch Centre as the case may be.

c) “Area Load Despatch Centre" means the centre as established by the state for load Despatch & control in a particular area of the state.

d) “Bulk consumer" means a consumer who avails supply at voltage of 33 kV or above.

e) Disaster management is the mitigation of the impact of a major breakdown on the system and bringing about restoration in the shortest possible time.

f) “Emergency Restoration System" A system comprising transmission towers/ structures of modular construction complete with associated components viz. insulators, hardware fittings, accessories, foundation plates, guys, anchors, installation tools etc. to facilitate quick restoration of damaged/failed transmission line towers/ sections.

g) “Islanding Scheme" is a scheme for separation of the grid into two or more independent systems as a last resort with a view to save healthy portion of the grid at the time of the grid disturbance.

h) “Standards" means "Grid Standards for Operation and Maintenance of Transmission Lines" set forth in the Schedule appended to these Regulations.

i) “Transient stability" means the ability of all the elements in the network to remain in synchronism following abrupt change in operating conditions like tripping of a feeder, tripping of generating unit, sudden application of a load and network switching etc.

j) “User" means a person such as a Generating Company including captive generating plant or Transmission Licensee other than the Central Transmission Utility (CTU) and State Transmission Utility (STU), Distribution Licensee or Bulk Consumer whose electrical plant is connected to the Grid at voltage level 33kV and above.

k) "Voltage Unbalance" is defined as the deviation between highest and lowest line voltage divided by Average line Voltage of the three phases.

The words and expressions used and not defined in these Regulations but defined in the Electricity Act, 2003 shall have the meaning assigned to them in the said Act.

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Schedule Grid Standards for Operation and Maintenance of Transmission Lines

1. Frequency

The standard frequency of system operation is 50 Hz and all efforts shall be made to operate at frequency close to nominal as possible. The frequency shall not be allowed to go beyond the range 49.0 to 50.5 Hz, except during the transient period accompanying tripping or connection of load.

2. Voltage

The steady state voltage shall be maintained within/the, limits given below:

Nominal System Voltage kV rms Voltage Variation 765 +/- 3% 400 +/- 3% 220 +/- 5%

132 and below +/- 10%

Temporary over voltage due to sudden load rejection shall be within the limits specified below:

Nominal System Voltage kVrms Phase to Neutral Voltage kV peak 765 914 400 514 220 283 132 170

For the voltage level below 132 kV, the voltage variation limits as given in (2) above shall be decided by the State Commission in the respective State Grid Code.

3. Voltage Unbalance

The maximum permissible values of voltage unbalance shall be as under: Nominal System Voltage kVrms Voltage Unbalance %

765 and 400 1.5% 220 2% 132 3%

i) Bulk consumers shall ensure balanced load during operation.

ii) Low Voltage Single phase loads shall be balanced periodically at the distribution transformer by

Distribution licensees. 4. Protection Standards

i) The Transmission Licensee and Users shall provide standard protection systems having the required reliability, selectivity, speed and sensitivity to isolate the faulty equipment and protect all components from any type of faults, within the specified fault clearance time. Protection coordination shall be done by the RFC.

ii) Fault Clearance Time:

The maximum fault clearance times are as given below:

Nominal System Voltage kVrms Maximum Time ( in milliseconds) 765 and 400 100 220 and 132 160

iii) In the event of non clearance of the fault by a circuit breaker within the time limit prescribed in 4 (ii) above the Breaker Fail Protection shall initiate tripping of all other breakers in the concerned bus-section to clear the fault in next 200 milliseconds.

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5. Criteria for System Security

The following minimum security criterion shall be followed for operation and maintenance planning of the elements of the grid:

The Grid System shall be capable of withstanding one of the following contingencies without experiencing loss of stability: (a) Outage of one single largest generating unit of the system or (b) Outage of a 132 kV Double circuit line or (c) Outage of a 220 kV Double circuit line or (d) Outage of a 400 kV Single circuit line or (e) Outage of a 400 kV Single circuit line with series compensation or (f) Outage of 765 kV Single circuit line without series compensation or (g) Outage of one pole of HVDC Bipolar line or (h) Outage of an Interconnecting Transformer

6. Transient Stability

Under any one of the following contingencies the system shall remain stable and sustain integrity (i.e., no generator shall lose synchronism and no part shall get isolated from the rest of the system): a) Tripping of a single largest generating unit or b) Transient ground fault in one phase of a 765 kV Single Circuit Line close to the bus or c) A sustained single phase to ground fault in 400 kV single circuit line followed by 3 pole opening of

the faulted line or d) A sustained fault in one circuit of a 400 kV Double Circuit Line when both circuits were in service

in the pre-contingency period or e) A transient single phase to ground fault in one circuit of a 400 kV Double Circuit Line when the

second circuit is already under outage or f) A three-phase sustained fault in a 220 kV or 132 kV or g) A sustained fault in one pole of HVDC bipolar in a HVDC Converter Station.

7. Harmonics

The voltage wave-form quality shall be maintained at all points in the Grid. i) In this Standard Harmonic Content limits are stipulated as follows:

Total Harmonic Distortion = VTHD (expressed as percentage)

100xVVn

V2

1

240n

2nTHD ∑

=

=

=

'I1 refers to fundamental frequency (50 Hz) 'n' refers to the harmonic of n*1 order (corresponding frequency is 50 x n Hz)

ii) Maximum Limits of total Harmonic Distortion

System Voltage Total Harmonic Distortion

Individual Harmonic of any particular Frequency

kVrms % % 765 1.5 1.0 400 2.0 1.5 220 2.5 2.0 132 3.0 2.0

This Standard shall come into force not later than five years from the date of the publication in the official Gazette.

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CAPTER 9: ANNEXURE -2

Generators: Extracts from Micro Hydro Standards issued by AHEC 1. Generator Special Requirement

Description Category (Installed Capacity in kW) Category A

(Up to 10 kW)

Category B (Above10kW and up

to 50 kW)

Category C (Above 50 kW and up to 100

kW) Generator

Types

Synchronous/ Induction - Single Phase/ 3 phase

Synchronous/ Induction 3 Phase

Synchronous 3 Phase

Terminal Voltage, frequency 240 V, 1 –phase, 50 Hz

415 V 3 phase, 50 Hz

415 V, 3 phase, 50 Hz

Make and Runaway withstand

Standard / Special generators designed to withstand against continuous runaway condition.

Insulation and Temperature Rise

Class F/H insulation and Class B Temperature rise

Notes

1. For efficiency of turbine, the performance curves of similar turbines manufactured by the bidder (tested by independent institution) will be provided.

2. Generator will conform to IS: 4722 (2001) and single-phase induction generators to IS: 996 (1979).

3. Electric load controllers shall be type tested by an independent institution for adequacy, for

performance, surge protection, waveform deviation, electromagnetic interference, emissions of radio noise.

4. Micro hydro for power generation category B & C should have the following provisions:-

(i) Parallel operation in local grids whenever available. (ii) Parallel operation with main grid whenever extended.

5. Micro hydropower generating station category B & C having more than 1 unit shall have

following additional provisions:- (i) Parallel operation between units at the station (ii) The Governor/Load Controller, AVR should have adequate provision for adjusting the

Speed Droop and Voltage Droop for facilitating the Parallel Operation of the Units. 2. Synchronous Generator and Induction Motors as Generators

i. Brand. The brand and power rating of the generator or motor should be approved by the

manufacturer of the turbines and by the purchaser.

ii. Nameplate. The original manufacturer’s nameplate for the generator or motor must be retained. New nameplates can be added but must not replace the originals.

iii. Over-rating. The power rating given on the original nameplate must be at least 10% more than the scheme rated power.

iv. Generator voltage. The “power house voltage” is the voltage at the generator terminals with powerhouse-consumer isolation switch in off position. This must be between the nominal national voltage (415 V) and +10% of 415 V.

v. Generator rotational speeds to be selected shall be 1500 rpm (+slip) or lower. In cases of direct coupling 750 rpm or 1000 rpm generators should be preferred.

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3. Synchronous Generator

i. Frequency. The operating frequency should be between 47.5and 52.5 Hz.

ii. Pf. The power factor rating should be 0.8 when an Electronic Load Controller (ELC) is in use except where all loads and the ELC present a unity power factor.

iii. Brushless generators shall be supplied with regulator (AVR). The unit proposed for interconnection with grid shall have in addition automatic power factor Regulator (APFR) with automatic change over from AVR to APFR when grid interconnection circuit breaker.

iv. The generator shall be capable of continuous withstand against runaway speed. 4. Induction Motor

i. Frequency. The frequency should be between 50 and 52.5 Hz. The frequency should be within this range under all operating conditions, including minimum and maximum power output, zero consumer load and worst-case consumer load power factor.

ii. The induction generator must be over-voltage protected to avoid excessive currents to flow through the excitation capacitors and induction machine. A protection system is required that disconnects all or some of the capacitors, to limit the currents flowing to below the limits for the induction machine windings and the capacitors. Provide MCBs of suitable current rating in the series with excitation capacitors.

iii. The generator shall be capable of continuous withstand against runaway speed. 5. Turbine/generator base-frame

i. The turbine and generator should be mounted on a single steel fabrication, the base frame, which shall be set into or fixed to the powerhouse floor (separate fixings shall be avoided in order to avoid tension stresses occurring in the concrete floor). This shall be fabricated from angle iron or channel section. A base frame may be omitted if the turbine and generator are close-coupled, that is, their own frames are rigidly connected to each other’s.

ii. The turbine and generator should be fixed securely to the base frame in a workshop before installation to achieve correct positioning. This should avoid problems with bearing alignment and belt tensions during operation. The runner must be easily removable on site.

6. Bearings

i. On larger machines, bearings must be selected and maintained to provide a service life of 10 years. On smaller machines a service life of 5 years is acceptable.

ii. Bearings must be properly aligned, either by use of self-aligning types or by adjusting of the bearing housings. There must never be more than two bearings on one shaft. Poor alignment will cause bearing failure and will be evident in the first year of operation of the turbine. The mandatory one-year warranty should be given to ensure that the manufacturer covers bearing failure costs, and to ensure that correct alignment is established.

iii. With the bearing housing open, the bearing housing should be one third full of clean grease.

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References A. National and International Standards and Codes

Latest edition of the following standards are applicable. IEC-34-2A-1972 - Rotating Electrical Machines

Methods for determining losses and efficiency of electrical machinery from tests (excluding machines for traction vehicles

IEC-34-1: 1983 – Rotating Electrical Machines, Rating and Performance IEC-85-1987 - Classification of materials for the insulation of electrical

machines IEC-34-5-1991 – Classification of degrees of protection provided by enclosures for rotating

electrical machines (IP Code) IEC-1116: 1992 – Electro-Mechanical Equipment Guide for Small Hydro-electric Installation IS-4722 –1992 Rotating electrical machines IS-325 –1996 Three phase induction motor IS-8789 –1996 Values of performance characteristics for three phase induction

motors IS: 12824-204 Type of duty and rating assigned to electrical machines

IEEE: C50-12 IEEE std. for salient pole 50 Hz and 60 Hz synchronous generator Micro hydro Std. AHEC IIT Roorkee SHP std. Standards, manual and guidelines by AHEC, IIT Roorkee B. Other References

FLOYD, G. D. and SILLS, H.R. : “The evolution of the modern water wheel generators in Canada” AIEE Transaction, April 1958, pp. 6-61. THAPAR, O. D. : “ Unit Size at Dehar Power Plant” CBI & P 44th Annual Session, Novernber 1972, Publication No. 114, pp. 101-108 HALL, H.E. and SHANKSHAFT, G,: Developments in the stability characteristics of power systems of England and wales.” 32.05, CIGRE 1970 Session, 24 August, 2 September. THAPAR, O.D.: “Stability for developing EHV system” CBI & P. forty third annual research session, June 1973, publication no. 121, pp. 22-30. ELLINGSEN, HANS VIDAR and OBERLANDOR JANOS: “Stability problems with EHV transmission systems with special reference to large generators.” Siemen’s publication “Extra high voltage AC transmission” pp. 18-26 AGGARWAL, R. P. and SINHA, V. P. : Over-voltages study of EHV system under Beas project part-I- dynamic overvoltage due to sudden load rejection” Report of study development of electrical engineering, Indian Institute of Technology , Kanpur ATEZZE, M. SCHUMM; NISHIMURA, F.; NARAYANA, RAO, H.V.; DESHMUKH, B. V.; IYENGAR. B.R.R. and KAR, K.W. : “Koyna Earthquake of 11 December 1967 – Report of Experts Committee.” UNESCO Sl. No. 1489 BMS –LD set, September 1969. THAPAR O. D. Characteristics of large hydro generators of Dehar Power Plant- Proceedings 2nd world congress, International water resources Association, New Delhi, December 1975, page 17-24. David M. Cleman – 1999– Hydro plant electrical system HCL publication

UNDP/World Bank Energy Sector Management Assistance Programme (ESMAP) – Mini Hydro Development on irrigation Dams and canal Drops Pre-investment Study Volume I & II: 1991.