Manual on PROTECTION OF GENERATORS, GENERATOR TRANSFORMERS AND 220 kV AND 400 kV NETWORKS

PUBLICATION NO. 274 (Revised)

Editors C.V.J. VARMA P.K. LAL

,SF _"I- ----__

S, >, \-;c ,5,,L, ;; - 7. ! ,>.-, -

LIST OF CONTENTS

Section 1 Section 2 Section 3 Section 4 Section 5 Section 6 Section 7 Sectiorl 8 Section 9 Section 10 Section 11 Section 12 Section 13

Foreword Introduction Generator and Generator Transformer Protection Line Protection Auto Reclosure Transformer Protection Reactor Protection Bus Bar Protection Local Breaker Back-up (Breaker fail) Protection Fault Locator, Disturbance Recording and Event Logging Equipment Guidelines for Protection System Engineering CT, CVT Locations Commissioning Tests & Maintenance Recommendations Test Equipments Reference Standards

Page iii

1 2

19 23 26 29 3 1 33 35 38 41

Section 1 GENERATOR AND GENERATOR TRANSFORMER

PROTECTION

1.0 GENERAL

1 . 1 Generators are designed to run at a high load factor for a large number of years and permit certain incidences of abnormal working conditions. The machine and its auxiliaries are supervised by monitoring devices to keep the incidences of abnormal working conditions down to a minimum. Despite the monitoring, electrical and mechanical faults may occur, and the generators must be provided with protective relays, which, in case of a fault, quickly initiate a disconnection of the machine from the system and, if necessary, initiate a complete shut down of the machine.

1.2 Recommendations contained herein for electrical protective systems of generator and generator transformer are intended to be used for generators of 10 MVA and above. It describes the requirements of various protections, special comments to help in determining application of these protections, for units of various types and sizes, setting criteria and tripping principles.

The protection requirements of machines used in pumped storage schemes need further attention and the recommendations given below do not cover these details.

1.2.1 Generator can be connected to the primary transmission system in following two ways:

1. Unit Scheme In this scheme no switchgear is provided between the generator and generator transformer which are treated as a unit. A unit auxiliary transformer is tapped off at the interconnection, for the supply of power to the auxiliary plant.

11. Generator Circuit Breaker Scheme In this scheme, a generator circuit breaker is provided between the generator and generator transformer: 1.3 In view of generator unit being a complex system, various electrical hazards require

consideration. These are given below: - Stator insulation failure - Overvoltage - Rotor faults - Loss of synchronism - Over/under frequency - Overload - Unbalanced loading - Loss of excitation - Reverse power - Inadvertent energisation of generator

1.4 Following a r e the various protections recommended for the generator and generator transformer protection.

Type of Fault ANSI Device No. Protection Functions

Generator differential Overall differential Minimum impedance (alternatively Over current/under voltage) Negative sequence Overload 95% stator earth fault 100% stator earth fault Loss of excitation Pole slip Low forward powerheverse power (Double protection for large generators) Minimum frequency Over voltage or over current Overfluxing volt/Hz Over voltage Dead machine PT fuse failure

GENERATOR STATOR Short Circuits

Asymmetry Stator overload Earth fault stator

Loss of excitation Out of step ~oni tor ing

Blade fatigue Inter turn fault Mag. Circuits Higher voltage Accidential energisation Monitoring GENERATOR ROTOR

87 G 87 GT 21 G

51/27 G 46 G 51 G

6 4 G I 6 4 G2 40 G 98 G

32 G/37 G

81 G 95 G 99 G 59 G

27/50 G 6 0 G

Rotor ground 6 4 F I Rotor earth fault GENERATOR TRANSFORMER Short circuits

Ground fault

Overhang

87 GT 51 GT 87 T

51 NGT 87 NT 87 HV

Overall differential Overcurrent Transformer differential Earth fault over-current Restricted earth fault HV winding cum overhang differential

UNIT AUXILIARY TRANSFORMER Short circuit

Ground fault

87 UAT 51 UAT

51 NUAT 6 4 UAT

Transformer differential Over-current Residual over-current Restricted earth fault

The transformer guards like Buchholtz protection, Winding temperature high, Oil temperature high, 2tc. are also important and shall be provided as per transformer manufacturer's recommendations. 1.5 Recommendations regarding selection of the generator protections for units of different

types and sizes are summarised in the table given below:

RECOMMENDED PROTECTIONS FOR GENERATORS

2.0 DETAILED REQUIREMENTS 2.1 Generator Stator

2.1.1 Generator Differential Protection (87 G)

Functions <

Differential 95% Stator E/F 100% Stator E/F Interturn Faults Bacltup Impedance Voltage Controlled O/C Negative Sequence Field Failure Reverse Power

Pole Slipping Overload Over voltage Under frequency Dead Machine Rotor Earth Fault Overfluxing

This is a unit type protection, instantaneous in operation, covering the stator winding for phase to phase faults. The generator differential relay is not sensitive to single phase to earth faults due to the high neutral earthing .resistance arrangement.

Large (> 100 MVA)

Y

Y

Y Y

Y

N

Y

Y

Y

Y Y

Y Y

Y Y Y

Small (< 10 MVA)

Y

Y

N Y N

Y

Y

Y

Y N N Y

Y

N Y N

Hydro Small (< 10 MVA)

Y Y

N Y

N

Y Y

Y Y

N Y Y Y

N Y N

Turbines Medium (10-100

MVA) Y

Y

Y/N Y

Y

N Y

Y

Y N Y

Y Y

N Y Y

Steam/Gas Medium (10-100

MVA) Y

Y

Y/N Y Y

N Y

Y

Y N N Y

Y

N Y Y

Turbines Large (> 100 MVA)

Y

Y

Y Y Y

N Y

Y Y

Y N Y

Y

Y

Y Y

As this protection operates for generator internal faults, opening the generator breaker in class-A eliminates the system in-feed to the fault (if the unit is synchronised). For all machines of ratings 10 MVA and above, this protection shall be provided. Requirements:

(i) Be triple pole type with individual phase indication (ii) Have operating time less than 30 milliseconds at 2 times setting

(iii) Be high or low impedance principle based (iv) Operating current 5 to 10% of nominal current (v) High stability against maximum through fault condition, CT saturation, harmonics and

DC transients (vi) Be provided with suitable voltage dependent resistors across the relay to limit the voltage

to safe level, in case of high impedance type relay

2.1.2 Generator-Transformer differential protection (87 GT) This is a unit type protection with coverage from the generator terminals up to the HV breaker and the generator transformer HV terminals.

It will detect phase faults on both sides of the generator transformer and single phase to earth faults of the HV side only (the earth fault current on the LV side is very small, due to the high neutral earthing resistance). Generator transformer differential relays have harmonic restraint circuits to prevent incorrect operation when the generator and unit transformers are energised from the system. This shall be provided for all machines of rating 10 MVA above. The protection need not .' , include Unit Auxiliary Transformer (UAT) in its zone and UAT should be covered by a separate !'

'

protection.

In case of breaker-and-half switching scheme, 'the CT's associated with main and tie breakers shall be connected to separate bias winding and these shall not be paralleled in order to avoid false operation due to dissimilar CT transient response. Requirements:

(iii)

(vi) (vii) (viii)

Be triple pole type with individual phase indication Have through fault restraint features for external faults with an adjustable or multi-bias setting Have,magnetizing inrush restraint features and also be stable for inrush under normal over fluxing conditions, magnetising inrush proof feature shall not be achieved through any intentional time delay e.g. use of timers to block relay operation Shall have unrestrained instantaneous highset overcurrent unit operation which is unaffected by inrush Have one bias winding per phase and per CT input (However UAT CT need not have separate input and may be paralled with the Generator CT) Have an adjustable operating current Have an operating time not greater than 30 milli seconds at 5 times setting Have facility for ratio and phase angle correction either through auxiliary transformer or through in built provisions.

2.1.3 Backup Protections for Short Circuits This shall be provided for all machines of 10 MVA and above. Voltage restrained over current relay may be used in place of minimum impedance relay in case of machines with rating less than 10 MVA.

2.1.3.1 Backup impedance protection (21 G) This operates for phase faults in the unit, in the HV yard or in the adjacent transmission lines, with a suitable delay, for cases when the corresponding main protection fails to operate. The impedance measured by the relay is influenced by the relay connection, the fault type and generator and system source impedance for faults on the high voltage side of the delta-star connected generator transformer. A circular characteristic the center of which corresponds to connection position of VT should be used for this reason. The impedance relay shall have fuse-fail interlock. Requirements

(i) Be triple pole type (ii) Be of single step under impedance type

(iii) Shall have two adjustable definite time delay relays of 0.5-5 seconds (iv) Be suitable for measuring two and three phase faults within a set distance from the

point of installation (v) Shall be able to operate for fault currents down to 0.3 In.

Setting Recommendations The impedance relay shall have reach setting to cover the longest HV outgoing line or 70% of rated generator load impedance. - Time relay of step 1 - 0.5 sec. - Time relay of step 2 shall be set to coordinate with third zone time of distance relay or

back up o/c relay of the outgoing line

2.1.3.2 Overcurrent/Undervoltage protection (51/27) G Requirements

(i) Be triple pole type (ii) Shall be able to operate when the fault current from the generator terminals becomes

low due to excitation system characteristic (iii) Shall have under voltage criteria !iv) Shall reset to de-energised position if under-voltage criteria disappears

2.1.4 Negative Phase Sequence Protection (46G) The negative phase sequence protection safeguards the generator rotor against over heating caused by the induced double frequency (100 Hz) currents when negative phase sequence currents are present in the stator. The negative phase sequence current can appear due to unbalanced single phase loads or transmission line unsymmetrical faults. This shall be provided for all machines of ratings 10 MVA and above. Requirements:

(i) Be triple pole type

(ii) Have an alarm unit and a trip unit (iii) Have a continuously adjustable negative sequence current setting (iv) Have alarm unit range covering negative phase sequence current of 5-10% of generator

rated current continuously adjustable (v) Have a definite time setting range of 1-10 secs. associated with an alarm unit (vi) Have a trip unit with a variable time current characteristics matching with the generator

I22t characteristic Setting Recommendations The relays should be set to the NPS capability of the generator. The NPS capability of the machine varies considerably from one machine to another. Alarm unit shall be set at 50% of continuous withstand capability value of the machine and time delay for alarm can be 3 Secs.

2 .1 .5 Generator Overload Protection (5 1G) Overload relay is used as an additional check of the stator winding temperature. This may be provided for hydro units where there are high head variations and connected for alarm. It may also be connected for run back. In case of thermal set there is no chance of thermal overloading as the machine is provided with number of limiters and therefore is not recommended. I

Requirements: (i) Be single pole type

(ii) Be of definite time over-current type (iii) Have a continuously adjustable setting range of 50-200% - (iv). Have a drop-off/pick-up ratio greater than 95% (v) Have an adjustable time setting range of 2.5 to 2 5 sec.

2.1.6 Generato; Stator Earth Fault Protection The high neutral earthing resistance arrangement limits the generator earth fault current to less than 1 0 amperes, thus minimising damage to the core laminations. Although a single phase to earth fault is not critical, it requires clearance within a short time, due to the following: - It may develop into a phase to phase fault (due to presence of ionised air). - If a second earth fault occurs the current is no longer limited by the earthing resistor. - Fire may result from the earth fault arc.

Two different types of stator earth fault relays are recommended both installed in the secondary circuit of the generator.

2.1.6.1 0-95% stator earth fault protection (64G1) This protection zone is limited to approximately 95% of the stator winding due to the danger of false tripping. This shall be voltage relay monitoring the voltage developed across the earthing resistor by the neutral return current. This is normally used as back up protection. It also covers the generator bus, low voltage winding of the unit transformer and the high voltage winding of the UAT. When connected to open delta winding of generator PT, the protection shall be blocked for PT fuse failure.

For faults within 10% of the generator neutral, the resulting current is not enough to operate the relay. This shall be provided for all machines of ratings 10 MVA and above. Requirements:

(i) Single pole type (ii) Shall have independently adjustable voltage and time setting

(iii) Suitable to protect 95% of stator winding (iv) Be suitable for operation from broken-delta voltage transformers or neutral grounding

transformer secondary (v) Shall be insensitive to 3rd harmonic voltage

Setting Recommendations Pick up voltage of the relay - 5% of maximum neutral voltage Time delay - 0.3 to 0.5 secs.

2.1.6.2 100% stator earth fault protection (64G2) This protects the whole stator winding and the generator neutral. The relay generally operates on the principle of low frequency signal injection into the secondary of the earthing transformer, detecting the corresponding current if an earth fault occurs. The relay is set in terms of insulation resistance. This is normally used as the main protection. Alternatively, a protection based on change in magnitude/distribution of 3rd harmonic voltage caused by an earth fault is used. When provided, this shall have voltage check or - current check unit as applicable, to prevent faulty operation of the relay at generator stand still or during the machine running down period. This protection shall always be provided for machine above 100 MVA. The option is left to the utility depending upon the importance of the machine for machines of smaller sizes. Low frequency current injection based relays are recommended for machines of 200 MVA and above.

, The 95-100% relay if separately available, may be connected for alarm and operator can take the machine out. Requirements:

(i) Be suitable to protect 100% of stator winding (ii) Be insensitive to external faults, transients and inherent harmonic currents.

(iii) Shall be based on low frequency current injection principle/alternatively shall operate on the principle of detecting change in the magnitude/distribution of 3rd harmonic voltage caused by an earth fault.

(iv) Have continuously adjustable time delay range 1-10 seconds (v) If based on injection principle - shall continuously monitor ground circuit - shall continuously monitor injection signal and injection equipment - shall be in service at standstill, startup and stop

(vi) If based on 3rd harmonic voltage principle - shall have under voltage or over current check

Setting Recommendations 100% stator earth fault relay (Injection Principle) Pick up level of the relay = 500 ohms time delay - 2 seconds (Greater than 3rd zone of distance relay) 100% stator earth fault relay (3rd harmonics principle) - Setting 0.45 V (should be checked w.r.t. 3rd harmonic voltage

generated by the machine) - Time delay 2.0 sec. - Voltage check unit 80% of rated voltage - Current check unit 20% of rated current

2.1.7 Loss of Excitation Protection (40G) A conlplete loss of excitation may occur as a result of unintentional opening of the field breaker, an open circuit or a short circuit of the main field or a fault in AVR. When a generator with sufficient active load loses the field current, it goes out of synchronism and starts -to run asynchronously at a speed higher than the system absorbing reactive power for the excitation from the system. Under these conditions the stator end regions and part of the rotor get over heated. This is recommended for machines of all sizes above 10 MVA. Requirements:

(i) Have mho characteristic lying in 3rd and 4th quadrant of impedance diagram with adjustable reach and off set. Alternatively, this protection shall be based on directional current unit with setting range to match generator capability curve

(ii) Shall have an under voltage relay and/or over current relay as an additional check (iii) Shall have timer with adjustable range of 1-10 seconds to distinguish loss of excitation

from power swings Setting Recommendations:

(i) For off set mho type relay - Diameter of mho circle - ( xd - x'a/2 - Off set of the mho circle from origin - X'd/2 - Time delay = 1.0 sec. - Under voltage relay = 70%

(ii) For directional current type relay - Direction current relay is set to match with the generator capability curve in 4th quadrant - Time delay - 0.5 to 1 sec. - Under voltage - 70% - Over current - 110-1 15%

2.1.8 Pole Slipping Protection (98G) The loss of field protection shall be supplemented by an additional out of step function which detects all pole slips.

Pole slipping of generators with respect to the system, can be caused by a number of conditions leading to an increase in rotor angular position beyond the generator transient stability limits. Some of the causes of pole slipping are:

(a) Large network disturbances (b) Faults on the electrical network close to the generator (c) Weak tie between the network and the generator (tripping of transmission lines) (d) Loss of generator field (field winding or excitation supply failure) (e) Operating the generator in an excessive under excited mode

This is recommended for machines of 100 MVA and above. For hydro machines utilities can decide depending on machine parameters.

I Requirements: (i) Shall be capable of detecting a power swing which can lead to instability in addition to

being able to detect an actual pole slip (ii) By varying size of the characteristic it shall be possible to ensure that a trip command is

given to the circuit breakers in such a way that separation of the poles occurs at a controlled angle at any time.

Setting Recommendations (a) If the source of oscillation lies within a generatorhransformer unit, the machine has to

be isolated from the network after the first'slip. Forward reach of relay characteristics shall cover generator/generator transformer. Tripping in this zone shall be in the first pole slip. The reach of this zone is 0.7 T.

(b) If the source of oscillation lies outside the unit in the network, the generator should not be switched off or atleast not until several pole slips have occurred.

2.1.9 Low forward power/reverse power interlock relays (32G/37G) I 1

The low forward power interlock is recommended for thermal machines and reverse power protection may be used for hydro machines to protect against motoring.

1

When the steam flow through turbine is interrupted by closing the ESVs or the governor valves, the remaining energy stored in the set is delivered to the system and the machine enters into a motoring condition drawing power from the system to supply its losses while keeping the turbo alternator shaft at synchronous speed. - The low forward power relay detects that the unit is motoring and must therefore be

shutdown - Tripping for mechanical faults and abnormal conditions

For faults in the turbine or boiler, the turbine protection closes the ESVs. For abnormal conditions, the generator protection closes the ESVs. When the generator develops low forward power, the relay after a short time delay trips the generator breaker. A protection field suppression signal may also be required if the AVR does not have a built-in facility to reduce the generator field current as the speed decreases, to avoid overfluxing conditions. Requirements:

(i) Be single phase power measuring type

(ii) Have a' power setting of approximately 0.5% - 1% of rated active power of generator unit (iii) Have independent time delay relay with setting range of 1-10 seconds and 0-20 seconds

respectively on pick up (iv) Have one more common timer with a pick up setting range 5-50 seconds for annunciation

that the Turbo generator set has started motoring (v) Have suitable arrangement for preventing the operation of this protection during start

up and synchronising of the unit (vi) Shall be provided with possibility of angle correction to facilitate measurement of power

accurately Setting Recommendations: - low forward power relay pickup < 0.05 x Pn - timer t l - 2 seconds, t2 - 2 seconds

2.1.10 Under-frequency protection (81G) The under frequency protection - Prevents the steam turbine and generator from exceeding the permissible operating

time at reduced frequencies - Ensures that the generating unit is separated from the network at a preset value of

frequency that is less than the final stage of system load shedding - Prevents the AVR from exciting the machine at reduced speeds when some protective

relays may not perform at all - Prevent over fluxing of the generator. The over fluxing relay is used to protect against

small overfluxing for long periods while the over voltage and under frequency relays also protect against large over fluxing for short times

The stator under frequency relay measures the frequency of the stator terminal voltage Though under frequency tripping is recornmended by turbine manufactures, care should be taken by grid operating personnel in ensuring that machines are not run at lower frequencies and instead resort to means like load shedding in the event of overload. Requirements:

(i) Have one alarm stage and two tripping stages (ii) Shall have setting of range of 4 5 Hz - 55 Hz with a least count of 0.1 Hz for each stage

(iii) Timer for alarm stage have a range of 0 . 5 to 5 second with a least count of 0.5 second. Timers for each tripping stage shall have range of 1 to 1 0 seconds with a least count of 0 .1 second

(iv) Shall have undervoltage blocking Setting Recommendations - Stator under frequency relay pick up level = 48.5 Hz - Time delay for alarm - 2 Sec.

For time delay setting of tripping stages recommendations of turbine manufacturers may be followed.

2.1.11 Inter turn fault protection (95G) It is generally considered difficult to obtain reliable protection against short circuit of one turn if the stator winding has large number of turns per phase.

Inter turn fault protection is recommended only for machines where there is a split winding and all the six terminals are brought out on the neutral side. For generator with split neutrals, conventional inter-turn fault protective scheme comprises a time delayed low set over-current relay which senses the current flowing in the connection between the neutrals of the stator winding. Alternatively a split phase differential protection may be used. Requirements

(i) Over current relay with time delay (ii) Over current relay shall have built in filters to reject higher harmonics (iii) Range of over current shall be selected depending on maximum spill current for external

fault Setting Recommendation Overcurrent relay shall be set to maximum unbalanced current in case of external fault. Time delay 0.2 - 0 .4 seconds

2.1.12 Generator Transformer Over Fluxing Protection (99 GT) Overfluxing protection is provided to safeguard the generator, generator transformer and unit auxiliary transformer against operation at flux densities which may cause accumulative damage to the core. From the fundamental equation V = 4.44 x f x n x 4, the level of flux is proportional to the ratio of terminal voltage to frequency (v/f). This ratio is monitored by the protective relay. An over fluxing condition is more likely to occur while the generator is separated from the system and the speed is allowed to drop, but it can also happen with the machine on load if the tap changer of the generator transformer (HV side) is on a low tap position and the excitation of the generator is manually increased. In this case the increased generator terminal voltage knay cause over fluxing tripping at nominal frequency. The over fluxing protection operates with a time delay after which the tripping functions are executed. This protection must b e provided for generator-transformers of size 1 0 MVA and above. Requirements

(i) Shall be phase to phase connected. (ii) Shall operate on the principle of measurement of voltage io frequency ratio.

(iii) Have inverse time characteristics compatible with generator transformer over fluxing withstand capability for tripping.

(iv) Provide an independent alarm with a definite time delay of value of V/f between 100% to 130% of rated value.

(v) Have a high resetting ratio of 98% or better. Setting Recommendations: The overfluxing capability of the transformer must be checked and the characteristic matched accordingly for both alarm and trip.

2.1.13 Generator Over Voltage Protection (59G) An over voltage on the terminals of the generator can damage the insulation of the generator, bus ducting, breakers, generator transformer and auxiliary equipment such as voltage transformers, excitation transformer etc. This should be provided for machines of all sizes, hydro and thermal Requirements:

(i) Be single pole type/or triple pole type (ii) Shall have two separately adjustable stages

(iii) Have a continuously adjustable setting range of 100 - 140% of rated voltage (iv) Have a drop off to pick up ratio greater than 95% (v) Have a continuously variable time delay setting range of 0.5 - 5 seconds for one relay

and 2 to 20 seconds for the other relay. Setting Recommendations: Stage 1 - Over voltage relay pick up - 1.15 x Vn

Timers t 1 - 1 0 seconds Stage 2 - Over voltage relay pick up - 1.3 x Vn

Timer t2 - 0.5 seconds

2.1.14 Dead machine protection (27/50G) Despite existence of interlocking schemes, a number of generators have been inadvertently energized while at stand still or on turning gear. The generator and rotor may get damaged beyond repair under this condition. Other protective relays like loss of excitation, back up impedance, reverse power would operate with delays and this is not admissible. The dead machine protection permits fast tripping. This should be installed in switchyard panel rather than in generator panel to ensure that protection is available during maintenance periods when the generator protection can be rendered inoperative by switching off the DC supply to the panel. This protection is recommended for all-machines of size 100 MVA and above. The protection is connected to trip generator breakers, generator transformer breaker and the HV Bus. Requirements:

(i) Shall consist of 3 high speed over current relays of range (0.02 - 20 In) to initiate instantaneous tripping if generator terminal voltage is below set value.

(ii) Shall have under voltage relays of range (0.2-1 Un) to permit operation of over current relays when voltage is low

(iii) Shall have timers with adjustable range (0-605) to avoid operation of protection for nearby fault when the machine is in service.

(iv) Shall be secure against voltage transients at closing Setting Recommendations:

For Weak System Overcurrent relay 1-2 pu Under voltage relay 20% to 40% rated voltage Activation of relay 20 Seconds after dead time

For Strong System 3-4 pu

50% to 70% of rated voltage 20 Seconds after dead time

2.1.15 Generator VT fuse failure monitoring (60G) This has to be provided for all the machines since it is required for blocking of relays which can mal-operate in the event of PT fuse blowing in primary side or secondary side. Requirements:

(i) Be triple pole type (ii) Be able to detect fuse failure in both primary and secondary side of VT

(iii) Have a fixed setting of 70% of rated voltage and have a time delay of 4 0 to 5 0 milli- seconds on pick up

(iv) Be of voltage balance or equivalent type (v) Have sufficient contacts to block tripping of those relays which are voltage dependent

and give alarm

2.1.16 Rotor ground fault protection This protection shall be provided for machines of all sizes. It is recommended that the protection is connected only for alarm and the operator may take the machine out at the earliest opportunity after the first earth fault has occurred. The tripping logic should also include unit tripping in case field circuit breaker opens when the machine is running. Requirements:

(i) Shall be based on DC injection principle or low frequency AC injection principle (ii) Shall be single stage or two stages

(iii) Shall have built-in time delay to prevent unwanted operation of the relay Alternatively A sensitive voltage function operating on bridge measuring basis with auxiliary equipment. This shall have two levels, one for alarm and one for trip. Set t ing ranges : Alarm s tage 1 0 0 ohm t o 2 5 k o h m , Time delay 2 - 6 0 Seconds Trip stage 100. ohm to 2 5 K ohm, Time delay 2-60 Seconds Setting Recommendations: - alarm level - 2 5 K ohm - pick up level - 5 K ohm - timer t l - 1 seconds - timer t 2 - 5 seconds

2.2 Generator-Transformer Protection For short circuit protection, transformer-differential relay and over-current relay connected to different groups are recommended. For ground faults, earth fault o/c relays and restricted earth fault relay connected to different groups are recommended. In case a overhang protection is required, the same may be combined with REF protection on HV side of generator transformer.

2.2.1 Generator Transformer Oifferential Protection (87 GT/87 T) Requirements:

(i) Be triple pole with individual phase indication

(ii) Have unrestricted instantaneous high set over current units which shall not operate during inrush

(iii) Have an adjustable or multi bias setting (iv) Have second harmonic or other inrush proof features and also shall be stable under

normal over fluxing conditions. Magnetising inrush proof feature shall not be achieved through any intentional time delay e.g. use of timers to block relay operation or using disc operated relays.

(v) Have one bias winding per phase and per C.T. input (vi) Have an adjustable operating current

(vii) Have an operating time not greater than 30 milli seconds at 5 times of setting (viii) Shall have facility for ratio and phase angle correction either through auxiliary transformer

or through in built provisions.

2.2.2 Generator Transformer Backup Overcurrent Protection (5 1 GT) Requirements:

(i) Be triple pole type (ii) Be of definite time over current type

(iii) Have an adjustable setting range of 50-200% of rated current and 0.5 - 5 seconds time delay

2.2.3 Generator Transformer Back Up Earth Fault Protection (51 NGT) This relay monitors the current in the generator transformer neutral. It can detect faults in the transformer HV side or in the adjacent network. Requirements:

(i) Be of single pole type (ii) Be of definite time characteristic

(iii) Have an adjustable setting range of 10 to 100% of rated current (iv) Have a timer setting range of 0.5 to 5 seconds

2.2.4 HV winding cum overhang differential protection (87 HV/87 NT) This is a unit type protection which operates for earth faults on the generator transformer HV side and also covers a large portion of the HV winding and the HV terminals upto the HV current transformers. Requirements:

(i) Be triple pole type (Single pole if used as Restricted E/F Protection) (ii) Have operating time less than 30 milliseconds at 2 times setting

(iii) Be high impedance or low impedance type (iv) Operating current shall be 0.1 - 0.4 In (v) High stability against maximum through fault condition, CT saturation, harmonics and

DC transients (vi) Be provided with suitable non linear resistors across the relay to limit the peak voltage

to 1000 volts, in case of high impedance type (vii) Be provided with faulty phase identification

2.3 Unit Auxiliary Transformer Protections For short circuit protection, unit auxiliary transformer differential relay, overload relay for alarm

and over current relay connected to different groups are recommended. However a utility may carry out an application check and decide not to have a separate differential relay for UAT. For ground faults residual O/c relay and Restricted E/F relays connected to different groups shall be provided. The type of connection may be either current based or voltage based depending on the grounding system.

2.3.1 Unit Transformer Differential Protection (87 UAT) This is a unit type protection covering the unit transformer and the cable connection to the unit board. This protection operates for phase faults, but not for single phase to earth faults as this current is limited by the neutral earthing resistance. The unit transformer differential relay initiates a general trip and unit shutdown. Requirements:

(i) (ii)

(iii) (iv)

(4 (vi)

(vii) (viii)

2.3.2 (i) (ii) (iii)

Be triple pole with individual phase indication Have unrestricted instantaneous high set over current unit which shall not operate during inrush Have an adjustable or multi bias setting Have second harmonic or other inrush proof features and also should be stable under normal over fluxing conditions, magnetising inrush proof feature shall not be achieved through any intentional time delay e.g. use of timers to block relay operation or using disc operated relays. Have one bias winding per phase and per C.T. input Have an adjustable operating current Have an operating time not greater than 3 0 milli seconds at 5 times of setting Shall have facility for ratio and phase angle correction either through auxiliary transformer or through in built provisions.

Unit Transformer back up over current protection (51 UAT) Relay Be triple pole type Be of definite time over current type Have an adjustable setting range of 50-200% of rated current and 0 .5 to 5 seconds time delay

2.3.3 Unit Transformer back-up earth fault protection (51 NUAT) (i) Be single pole type

(ii) Be of definite time over current type (iii) Have a setting rage of 10-100% of rated current and time setting range of 0.5-5 seconds

2.3.4 Unit transformer restricted earth fault protection (64 UAT) (i) Be single pole type

(ii) Have operating time less than 3 0 milli seconds at 2 times setting (iii) Be high impedance or low impedance type (iv) Operating current shall be 0.1 - 0 .4 In (v) High stability against maximum through fault condition, CT saturation, harmonics and

DC transients

(vi) Be provided with suitable non-linear resistors across the relay to limit the peak .voltage to 1000 volts, in case of high impedance type

(vii) Be provided with fault indication

3. ENGINEERING RECOMMENDATIONS 3.1 Redundancy The protection functions shall be subdivided into two groups each being independent and capable of providing uninterrupted protection even in the event of one of the protection groups failing. Given below is recommendation for dividing the protections in two groups:

GROUP A Generator Generator differential Minimum impedance (or alternatively over current/under voltage) Negative sequence protection Over load protection 100% stator earth-fault Rotor earth fault Reverse Power Over voltage Inter-turn fault

GROUP B

Over-all differential

95% stator earth fault Loss of excitation Pole slip Under frequency

Reverse power Over fluxing

( 1 ~ e n e i a t o r Transformer 1 I Transformer differential Earth fault over current

Over current Restricted earth fault HV winding cum overhang differential

I I Unit Auxiliary Transformer 1 I I I Transformer differential I Over current I 1 1 Ground fault over current I Restricted earth fault I 3.2 Tripping Principles In general, it is best if generator protection relays initiate non-sequential trip mode (Class-A) for unit isolation. However sequential tripping (Class B) provides a better means of tripping a steam turbine generator on some abnormal operating conditions where delayed tripping of the generator will not result in increased damage to the turbine, generator or other electrical equipment. The reason for sequentially tripping a steam turbine generator is to avoid the over- speed condition that results when the generator main breaker is tripped while steam is applied to the turbine.

Proper control logic is critical to the design of a sequential tripping scheme and requires some sort of mechanical "Turbine Tripped" indication (i.e. steam stop valve limit switches, trip oil system pressure switches etc.) which is supervised by an electrical reverse power relay. This relay is normally set to detect very low power levels and incorporates a brief time delay of the order of several seconds for added security. It has been recommended that generator protective relays, initiate non-sequential trip mode for isolation of the unit due t o electrical faults. Only devices protecting unit from 'an abnormal mechanical operating condition or an abnormal (not faulted) electrical condition or normal shut down should initiate a sequential trip.

Section 2 LINE PROTECTION

1 . 0 GENERAL The line protection relays are required to protect the line and clear all type of faults on it within shortest possible time with reliability, selectivity and sensitivity. The line protection relays shall be suitable for use with capacitive voltage transformers having passive damping and transient

. response as per IEC 186. Recommendation for provisions of line protection at different voltage levels are as follows:

There should be two independent high speed main protection schemes called Main-I and Main- I1 with at least one of them being carrier aided non-switched three zone distance protection. The other protection may be a phase segregated current differential (this may require digital communication) phase comparison, directional comparison type or a carrier aided non-switched distance protection. Further, if Main-I and Main-I1 are both distance protection schemes, then they should be preferably of different types. However, they need not necessarily be of different make. Both the protections should be suitable for single and three phase tripping. In addition to above following shall also be provided:

(i) Two stage over-voltage protection. However in such cases where system has grown sufficiently or in case of short lines, utilities on their discretion may decide not to provide this protection.

(ii) Auto reclose relay suitable for 1 ph/3 ph (with deadline charging and synchro check facility) reclosure.

(iii) Sensitive IDMT directional E/F relay

1.2 220 kV Lines There should be atleast one carrier aided non-switched three zone distance protection scheme. In addition to this another non-switched/switched distance scheme or directional over current and earth fault relays should be provided as back up. Main protection should be suitable for single and three phase tripping. Additionally, auto-reclose relay suitable for 1 ph/3 ph (with dead line charging and synchro-check facility) reclosure shall be provided. In case of both'line protections being Distance Protections, IDMT type E/F relay shall also be provided additionally.

2 . 0 REQUIREMENTS 2.1 Distance Protection Distance Protection scheme shall have the following attributes:

(i) Three independent zones (ii) Non-switched type with separate measurements for a!l phase to phase and phase to

ground faults. (iii) Capable of single and three phase tripping (iv) Directional (mho or quadrilateral or other suitably shaped) characteristics for zone 1,

zone 2 and zone 3

(v) (vi) (vii)

Capable of operation for close-up three phase faults and switch on to faults Adjustable characteristic angle to match line angle wherever applicable Accuracy of better than or equal to 5% of set value for reach measurement in zone 1 and better than or equal to 10% of set value for zone 2 and zone 3

(viii) Accuracy of better than or equal to 5% of set value for time measurement of zone 2 and zone 3

(ix) Variable residual compensation (x) Shall include power swing detection feature for selectively blocking, as required. (xi) Shall include suitable fuse-failure protection to monitor all types of fuse failure and block

the protection. (xii) Maximum operating time under given set of condition shall be as follows:

SIR 4 15 Relay setting 10 Ohms 2 Ohms Fault location (as % of relay setting) 60% 60% Trip duty Contacts per phase* 4 4 Fault resistance Zero Ohms Zero Ohms

Max. Operating Times 45 ms 45 ms for 3 phase faults Including trip relays (if any) for all faults 60 ms for other faults and with CVT

* These trip duty contacts can be provided either in-built in the distance relay or through additional relay. Making capacity of trip contracts shall be 3 0 A for 0.25 S with an inductive load of L/R > 10 ms

2.2 Directional Comparison Protection

(iii)

(vii) (viii)

Shall be a unit system of protection Shall be directional comparison type suitable for operation with one signalling channel, e.g., PLCC Shall have high speed fault detection based on principles like detection of post fault travelling wave, superimposed components, etc. Shall have high sensitivity for all types of faults Shall be suitable for 1 and 3 phase tripping Shall not be affected by heavy load transfer, power swings, CT saturation, CVT transients, VT fuse failure, line charging currents, distorted primary currents and voltages, external switching activities, sudden power reversal, zero sequence, mutual coupling, shunt reactor "in-zone" switching etc. and lightning strokes Shall have features like switch on to fault, weak end infeed, echo system Shall have feature to cover nearby fault at high speed in the event of channel fail

2.3 Phase Segregated Line Differential Protection (i) The relay shall be all digital multi microprocessor based, designed for use with modern

digital telecommunication system multiplexer conforming to ITU-T (CCITT) specifications and Fibre Optic medium.

(ii) (iii) (iv)

(vii) (viii)

(xii) (xiii)

Each phase current shall be separately evaluated at both ends for both amplitude and phase. Shall be suitable for single phase tripping and autoreclosing. The message transmitted by the relay to other end shall include information on currents, supervision information, CT saturation detection, synchronisation of terminals etc. The measurement shall be stabilised phase by phase for CT saturation. The communication delay shall be continuously measured and automatically compensated for in the differential measurement. Communication telegram shall have error detection and correction feature. Suitable programmable evaluation algorithm will be provided to ensure proper security and dependability of the message. The connection equipment from the relay to the communication shall be a 64 kbps Channel. An optional electrical or optical port shall be provided to directly connect the signal to auxiliary channel of OLTE (optical line terminal equipment) by passing the multiplexer or for redundancy purpose. The relay shall have communication port for remote monitoring, programmaing and control. The direct intertrip signal also shall be transmitted as part of telegram. The sampling frequency for analog signals shall be minimum 2 kHz. Filtering and measuring techniques shall be used to ensure correct performance during all operating and transient conditions.

2.4 Phase C~mparison Protection (i)

(ii) (iii) (iv) (4 (vi) (vii)

(viii)

Shall be Current phase comparison type. Shall be suitable for operation with one signal channel. Shall have high sensitivity for all types of faults. Shall be capable of single and three pole tripping. Shall have facility for blocking/permissive trip modes Shall have facility for direct transfer tripping Shall have comprehensive alarm and test facilities Shall not be affected by heavy load transfer, power swings, CT saturation,'CT phase errors, Propagation delays, Capacitance current etc., as is typical of unit protections.

2.5 Over Voltage Protecti~n The relay shall have following features: - Have a high drop off to pickup ratio - Have adjustable setting range for voltage and time - Have two stages - Low set stage shall monitor any one phase to phase voltage and shall have associated

timer - High set stage shall monitor all three phase to phase voltage and shall have associated

timer

3.0 Special Comments (i) If found necessary, at certain locations, out of step tripping relays shall be provided for

islanding the system during disturbances

(ii) For short line application distance relays should have shaped characteristics for ground faults and be used in permissive over reach mode with weak end infeed logic. Further, if it is a double circuit line, current reversal logic should also be available.

(iii) O/V relay for 400 kV lines shall be connected to trip concerned line breaker(s), start LBB, block auto reclosure and send direct trip command.

(iv) The directional earth fault relay recommended along with the distance relay should be seen as a supplement to it and not as a back up. It helps to detect very high resistance fault which distance relay cannot.

(v) HVDC Systems connected to AC networks with low short circuit levels can influence AC line protections in it vicinity. This aspect needs to be looked into on case to case basis.

4.0 SETTING CRITERIA 4.1 Reach Settings of Distance Protection

(i) Zone-I: to be set to cover 80-85% of protected line length (ii) Zone-11: to be set to cover minimum 120% of length of principle line section. However,

in case of D/C lines 150% coverage must be provided to take care of, under reaching due to mutual coupling effect but, care is to be taken that it does not reach into next lower voltage level.

(iii) Zone-111: For 400 kV lines Zone-111 to be set to cover 120% of principle section plus , adjacent longest section subject to a reach restriction so that it does not reach into next

lower voltage level. For 220 kV lines, Zone-111 reach may be provided to cover adjacent longest section if there is no provision of LBB or all protection are connected to single DC source at remote and substation.

(iv) Resistive reach: Resistive reach should be set to give maximum coverage subject to check of possibility against load point encroachment considering minimum expected voltage and maximum load. Also attention has to be given to any limitations indicated by manufacturer in respect of resistive setting vis-a-vis reactance setting.

' I I , I

4.2 Time Setting of Distance Protection , 1 1 I I

I I ] A Zone-I1 timing of 0.3 second is recommended. However, if a long line is followed by a short ' , / I line, then a higher setting (say 0.5 sec) may be adopted on long line to avoid indiscriminate , tripping through Zone-I1 operation on both lines.

Zone-I11 timer should be set so as to provide discrimination with the operating time of relays provided in subsequent sections with which Zone-111 reach of relay being set overlaps.

4.3 O/V Protection 4.3.1 Low set stage may be set at 110% with a typical time delay of 5 seconds. However, a

time grading of 1 second may be provided between relays of different lines at a station. Longest time delay should be checked with expected operating time of overfluxing relay of the transformer to ensure disconnection of fine before tripping of transformer.

4.3.2 High set stage may be set at 150% with a time delay of 100 m.second.

4.4 Power Swing Blocking Function Associated with Distance Relays Decisions pertaining to allowing which Zone to trip and which to block should be taken based on system studies oil case to case basis.

Section 3 AUTO-RECLOSING

1.0 GENERAL The auto-reclosing of power lines has become a generally accepted practice. Reports from different parts of the world show that in certain networks in regions subject to a high lightning intensity only about 5 per cent of the faults are permanent. Auto-reclosing therefore provides significant advantages. Outage times will be short compared to where station personnel have to re-energise the lines after a fault. Additionally, in interconnected networks auto-reclosing helps in maintaining system stability. Following different arrangements of auto-reclosing are possible:

High speed single pole reclosing (HSAR) High speed three pole reclosing (HTAR) Delayed three pole reclosing (DAR) DAR with dead line (DL) and synchronism check(SC) HTAR with DL and SC HTAR with parallel line check (PC)

In case of HSAR and HTAR without check, the two ends can be reclosed at about the same instant. In case of DAR and HTAR with DL and SC the C.B. at one end recloses first after the set dead time and with a check of dead line condition. After a successful line re-energization from one end the voltage, phase angle and possibly frequency on the line and station side are compared. At acceptable synchronous in-phase conditions the C.B. at the second end is reclosed.

1.1 Recommendations for provisions of auto-reclosing. Presently 1 phase high speed auto-reclosure (HSAR) at 400 kV and 220 kV level is widely practised including on lines emanating from Generating Stations and the same is recommended for adoption. If 3-phase auto-reclosure is adopted in future the application of the same on lines emanating from generating stations should be studied and decision taken on case to case basis.

2.0 REQUIREMENTS It shall have the following attributes:

(i) (ii)

(iii) (iv) (4 (vi) (vii)

(viii) (ix)

Have single phase and/or three phase reclosing facilities. Have a continuously variable single phase dead time. Have continuously variable three phase dead time. Have continuously variable reclaim time. Incorporate a facility of selecting single phase/three phase/single and three phase auto- reclose and non-auto reclosure modes. Have facilities for selecting check synchronising or dead line charging features. Be of high speed single shot type Suitable relays for SC and DLC should be included in the overall auto-reclose scheme. Should allow sequential reclosing of breakers in one and half breaker or double breaker arrangement.

3.0 SPECIAL COMMENTS 3.1 Fast simultaneous tripping of the breakers at both ends of a faulty line is essential for

successful auto-reclosing. Therefore, availability of protection signalling equipment is a pre-requisite.

3.2 Starting and Blocking of Auto-reclose Relays: Some protections start auto-reclosing and others block. Protections which start A/R are Main- I and Main-11 line protections. Protections which block A/R are: - Breaker Fail Relay - Line Reactor Protections - O/V Protection - Received Direct Transfer trip signals - Busbar Protection - Zone 2/3 of Distance Protection - Carrier Fail Conditions - Circuit Breaker Problems.

When a reclosing relay receives start and block A/R impulse simultaneously, block signal dominates. Similarly, if it receives 'start' for 1-phase fault immediately followed by multi-phase fault the later one dominates over the previous one. .3.3 Following comn~ents are for multi-breaker arrangements of one and half breaker or

double breaker arrangement 3.3.1 In a mu.lti-C.B. arrangement one C.B. can be taken out of operation and the line still be

kept in service. After a line fault only those C.Bs which were closed before the fault. shall be reclosed.

3.3.2 In multi-C.B. arrangement it is desirable to have a priority arrangement so as to avoici closing of both the breakers in case of a permanent fault. This will help in avoiding unnecessary wear and tear.

A natural priority is that the C.B. near the busbar is reclosed first. In case of faults on two lines on both sides of a tie C.B. the tie C.B. is reclosed after the outer C.Bs. The outer C.Bs. do not need a prioritin3 with respect to each other.

3.3.3 In multi-breaker arrangement it is necessary to trip two C.Bs. to clear a line fault and also auto reclose these two. Basically two types of arrangement for C.B. associated relays are possible i.e.

- C.B. Oriented - Line Oriented

With C.B. oriented arrangement co-operation between C.Bs, Synchrocheck relay etc. is straight forward and autoreclose mode can be selected separately for each breaker. With line oriented arrangement interconnections between line relays and reclosure relay is simpler, but cooperation with circuit breakers schemes becomes complicated. For the above reasons C.B. oriented arrangement is recommended.

3.4 In case of bus bar configuration arrangement having a transfer breaker, a separate auto- reclosure relay for transfer breaker is recommended.

4.0 SETTING CRITERIA 4.1 Dead Time Auto-reclosing requires a dead time which exceeds the de-ionising time. The time required for the de-ionising of the fault path depends on several factors including the arcing time, fault duration, wind conditions, circuit voltage, capacitive coupling to adjacent conductors, etc. The circuit voltage is the factor having the predominating influence on the de-ionising time. Single phase dead time of 1.0 sec. is recommended for both 400 kV and 220 kV system.

4.2 Reclaim Time The reclaim time is the time during which a new start of the auto-reclosing equipment is blocked. If reclosing shot has been carried out and the line is energised and a new fault occurs before the reclaim time has elapsed, the auto-reclosing equipment is blocked and a signal for definite tripping of the breaker is obtained. After the reclaim time has elapsed, the auto-reclosing equipment returns to the starting position and a new reclosing sequence can occur. The reclaim time must not be set to such a low value that the intended operating cycle of the breaker is exceeded, when two fault incidents occur close together. If the breaker is closed manually, the auto-reclosing equipment is blocked and cannot start again until the reclaim time has elapsed. For the breaker to be used for auto-reclosing, it is essential that it has the operating mechanism and breaking capacity necessary for it to be able to perform the auto-reclosing sequences required.

4.3 Circuit Breaker Requirement According to IEC Publication 56.2, a breaker must be capable of withstanding the following operating cycle with full rated breaking current:

The recommended operating cycle at 400 kV and 220 kV is as per the IEC standard. Therefore, reclaim time of 2 5 Sec. is recommended.

Section 4 TRANSFORMER PROTECTION

1 . 0 GENERAL A Power Transformer is a very valuable and vital link in a Power Transmission system. Reliable, secure and fast protection system for the transformer is essential to minimise the damage in case of an internal fault with suitable back up protection scheme to take care of uncleared system faults.

Faults occur in Transformer due to insulation breakdown, ageing of insulation, overheating due to over excitation, oil contamination and leakage or reduced cooling. To give an early warning and to minimise the damage in case of a fault it is necessary to equip it with monitors and Protective relays. Recommendations for provision of protective and monitoring equipment for transformers of 400 kV and 220 kV class are as follows: 1.1 Following are the various protections recommended for the transformer protection:

Transformer differential protection Overfluxing protection Restricted earth-fault protection Back up directional O/C + E/F protection on HV side Back up directional O/C + E/F protection on LV side Protection and monitors built in to Transformer (Buchholz relay, Winding and Oil Temperature Indicators, Oil Level Indicator and Pressure Relief Device) Protection for Tertiary winding Overload Alarm

2 . 0 REQUIREMENTS

(i) (ii)

(4 (vi) (vii) (viji)

Differential Protection Triple pole with individual phase indication Have unrestrained instantaneous high set over-current units which should not operate during inrush. Have an adjustable or multi bias setting Have second harmonic or other inrush proof features and also should be stable under normal overfluxing conditions, Magnetising inrush proof feature shall not be achieved through any intentional time delay e.g. use of timers to block relay operation or using disc operated relays. Have one bias winding per phase and per C.T. input Have an adjustable operating current Have an operating time not greater than 30 milli seconds at 5 times of setting The scheme shall have facility for ratio and phase angle correction either through auxiliary transformer or through in-built provisions.

2 . 2 Overfluxing Protection (i) Overfluxing protection shall be phase to phase connected.

26

(ii) Operate on the principle of measurement of voltage to frequency ratio. (iii) Have inverse time characteristics compatible with transformer overfluxing. (iv) Provide an independent alarm with a definite time delay at value of v/f between 100%

to 130% of rated value. (v) Have a high resetting ratio of 98% or better.

2.3 REF Protection (i) Shall be single pole

(ii) Have an operating current sensitivity of at least 10% of nominal current (iii) Be tuned to the system frequency (iv) Have a suitable non-linear resistor to limit the peak voltage during in-zone faults in case

of high impedance type (v) Shall be high or low impedance principle type.

2.4 Back-up overcurrent protection relay (on HV side and MV side) (i) Be 3 pole type

(ii) Have IDMT characteristic (directional on MV side) (iii) Have a variable setting range of 50-200% of rated cilrrent (iv) Have a characteristic angle, 30(/45( degrees lead (v) Shall include high unit having low transient over-reach and variable setting range typically

500-2000% of rated current (vi) Include hand reset indicators per phase

2.5 Back up earth-fault protection relay (i) Shall be single pole type

(ii) Have IDMT characteristic (directional on MV side) (iii) Have a variable setting range of typically 20-80% of rated current (iv) Shall have a characteristic angle of 45/60 degree lag (v) Shall include high set instantaneous unit having low transient over-reach and variable

setting range of typically 200-800% of rated current (vi) Shall include hand reset indicators

2.6 0;erload Alarm relay (i) Shall be of single pole type

(ii) Shall be of definite time overcurrent type (iii) Shall have a continuously variable current range of 50-200% of rated current and

continuously variable timer setting range of 1-10 sec. (iv) Shall have a drop off to pick up ratio of 95% or better.

2.7 Tertiary Winding Protection There are variations in the practices adopted for bringing out the tertiary terminals of ICTs. Therefore, depending upon the type of connections, adequate protection scheme may be adopted in consultation with the manufacturer of the transformer.

3.0 Special Comments 3.1 Duplication of transformer protection is not considered necessary but the protection

and monitors shall be divided in two groups viz. Gr A and Gr B at 400 kV. At 2 2 0 k ~ level this is recommended depending on the importance of the substation and where it is decided to go for two groups of protection with two station batteries.

Given below is one possible way of grouping these protections: Group A Group B Transformer biased R.E.F. Protection differential relay Buchholz Protection Back up Protection (HV) Back up Protectior, (MV) Overfluxing Protection (HV) Overfluxing Protection (MV) Oil temperature high tripping Overload protection (Alarm only) winding

temperature high tripping Pressurn relief tripping OLTC Buchholz tripping Delta winding protection Oil level high/low tripping

Group A and B protections shall be connected to separate DC source/separately fused supplies. DC sources shall be supervised. Both Gr A and Gr B protections shall give out tripping impulses to liV, MV and LV (if applicable), circuit breakers. 3.2 The transformer overfluxing protection has been recommended on both sides for

interconnecting transformers. This is to cover all possible operating conditions, e.g. the transformer may remain energised from either side. For other transformers overfluxing relay shall be provided on the untapped winding of the ~ransformer.

3.3 In case of breaker and half switching schemes, the differential protection C.Ts. associated with Main and Tie breakers should be connected to separate bias windings and these should not be paralleled in order to avoid false operation due to dissimilar C.T. transient response.

3.4 Whenever separate phase-wise C.Ts are available on neutral side of transformer, triple pole high impedance relay may be provided instead of single pole R.E.F. relay.

4.0 SETTING CRITERIA 4.1 The current setting of the back up O/C relay shall be set above the expected maximum

load current so as to allow possible overload on account of loss of one of the parallel transformers.

4.2 Overload relay shall be set at 110% of rated current with delay of 5 seconds. This shall be connected to give only alarm and not for tripping.

Section 5 REACTOR PROTECTION

1.0 GENERAL Shunt Reactors are used in EHV systems to limit the overvoltages due to capacitive VAR Generation in Long Transmission Lines. The shunt reactors are normally connected

(a) Through isolators to a line (b) Through circuit breakers to a busbar (c) Through circuit breakers to the tertiary of a Interconnecting transformer.

Faults occur in shunt reactors due to insulation breakdown, ageing of insulation, overheating due to overexcitation, oil contamination and leakage. To give an early warning and to minimize the damage in case of a fault it is necessary to equip it with monitors and protective relays. Recommendations for provision of protection and monitoring equipment for Reactors are as follows:

(a) Reactor differential Protection (b) Reactor REF Protection (c) Reactor- backup protection (Impedance type or definite time o/c & E/F)

( d ) Protections and monitors built into reactor (buchholz, winding temperature, oil temperature,. pressure relief, oil level, Fire protection)

2.0 REQUIREMENTS 2.1. Differentia1 Protection

(i) Shall be triple pole type (ii) Have an operating current sensitivity of atleast 10% of nominal current.

(iii) Be tuned to system frequency (iv) Have operating time not greater than 30 ms at 5 times of setting (v) Have suitable non-linear resistors to limit peak voltage during in-zone faults in case of

high impedance type (vi) Shall be high or low impedance type.

2.2 Restricted earthfauIt protection (i) Shall be 'single pole

(ii) Have an operating current sensitively of atleast 10% of nominal current (iii) Be tuned to system frequency (iv) Have suitabhnon-linear resistors to limit peak voltage during in-zone faults (v) Shall be high or low impedance type

2.3 Backup protection ReIay Either

(i) Shall be triple pole type (ii) Shall be single step polarised 'mho' or impedance distance relay suitable for measuring

phase to ground and phase to phase faults.

(iii) Shall grounds a characteristic angle between 60-80 degrees (iv) Shall have an adjustable definite time delay relay with setting range of 0.2. to 2.0 Sec. (v) Shall have a suitable range for covering 60% of reactor impedance

(i) Shall be a single stage definite time 3 pole, overcurrent relay with adjustable current and time.

(ii) Shall be connected for 2 O/C and 1 E/F connection and shall be non-directional with high reset ratio and low transient overreach.

3.0 SPECIAL COMMENTS 3.1 No duplication of reactor protections needs to be done but the protections and monitors

shall be divided in two group viz. Gr. A and Group B. Given below is one possible way of grouping these protections

Group A Group B Reactor differential relay Buchholz trip Reactor back up relay Reactor R. E/F relay Oil temperature trip Winding temperature trip Pressure relief trip Oil level high/low trip Fire protection trip

Gr. A and Gr. B. protection shall be connected to separate DC Source/separately fused supplies and DC sources shall be supervised. Both Gr, A and Gr. B protections shall give out trip impulses to main breaker and also block auto reclosing where they are directly connected to the line they should also trip remote end CB. 3.2 It may be noted that the connection of Restricted Earth Fault protection on the neutral

side shall be from residually connected bushing CTs (in case of bus reactor) or from the ground side CT in the neutral grounding reactor (for line shunt reactor). The latter is to ensure that the protection covers the neutral earthing reactor as well in the protected zone.

3.3 The impedance or overcurrent backup protection may not be able to detect inter-turn fault in the reactor, for which the buchholz may be the only answer, unless the number of turns involved is very high. Manufacturers of reactor and relays may be consulted in this regard.

4.0 SETTING CRITERIA The magnitude and nature of the switching-in currents should be considered when determining settings of reactor protections.

4.1 Typical settings for o/c relays are: Current Setting - 1.3 x Rated current Time setting - 1 sec.

4.2 Typical setting for impedance type of relays are - Reach - 60% of Reactor Impedance Time setting - 1 sec.

Section 6 BUS BAR PROTECTION

1.0 GENERAL Bus bar protection is required to be provided for high speed sensitive clearance of bus bar faults by tripping all the circuit breakers connected to faulty bus. Recommendations for providing bus bar protection at different voltage levels are as follows:-

(i) Bus bar protection must be provided in all new 400 kV and 220 kV substations as well as generating station switchyards.

(ii) For existing substations, provision of bus bar protection is considered a must at 400 KV level and at 220 KV level it is essential at substations having multiple feed. In case of radially fed 220 KV substations, having more than one bus it is desirable to have bus bar protection but is not a must.

2.0 REQUIREMENTS Bus bar protection shall have following features.

(i) (ii)

(iii) (iv) (4

(vii) (viii) (ix)

It shall be of 3 phase type and operate selectively for each bus bar section. It shall operate on differential principle and provide independent zones of protection for each bus. It shall provide zone indication. It shall be stable for through faulf conditions upto maximum 40 kA fault level. For applications where bus differential protection sensitivity has to be set below load current, as may be a case with use of concrete structures, it is recommended that a separate check zone is provided otherwise provision of separate check zone is not essential. Check zone, if provided, shall be of high impedance type. It shall incorporate continuous supervision for C,T. secondaries against any possible open circuits. In case of detection of any open circuiting of C.T. secondaries, after a time delay, the affected zone of protection shall be rendered inoperative and an alarm will be initiated. It shall include D.C. supply supervision Include adequate number of high speed tripping relays. Whenever C.T. switching is involved the scheme shall include necessary C.T. switching relays and have provision for C.T. switching incomplete alarm. It shall include IN/OUT switching facility for each zone.

3.0 SPECIAL COMMENTS 3.1 The D.C. supply for bus bar protection shall be from an independent feeder. 3.2 Faults lying between C.B and C.T. shall be cleared from one side by opening of C.B on

busbar protection operation. However clearing of fault from other side shall be through breaker failure protectionhack up protection.

3.3 3 Phase trip relays shall be provided for each circuit breaker which shall also initiate B.F.P. of concerned breaker.

3.4 Length of secondary leads should be kept as minimum as possible. Where lead runs ar excessive, an increase in wire size or use of parallel conductors are means to reduc lead resistance.

3.5 In case of existing substations where current transformers are of different ratios, biase type differential protection is recommended for use.

4.0 SETTING CRITERIA 4.1 C.T wire supervision relays should be set with a sensitivity such that they can detect C.?

secondary open circuit even in case of least loaded feeder. 4.2 Bus bar differential protection should have overall sensitivity above heaviest loaded feede

current unless a separate check zone has been provided. In cases where fault current are expected to be low as brought out in 2 (v) above, the protection should be sensitivi enough to take care of such expected low fault current.

4.3 In case of voltage operated high impedance type protection, the voltage setting shoulc be above expected voltage developed across the relay during maximum through faul current condition. In case of current operated relays for stability under through faul condition, external resistance is to be set such that voltage developed across relay an( resistance combination is below the voltage required for forcing required relay operating current.

Section 7 LOCAL BREAKER BACK-UP PROTECTION

(BREAKER FAILURE PROTECTION)

1.0 GENERAL In the event of any circuit breaker failing to trip on receipt of trip command from protection relays, all circuit breakers connected to the bus section to which the faulty circuit breaker is connected are required to be tripped with minimum possible delay through LBB protection. This protection also provides coverage for faults between C.B and C.T. which are not cleared by other protections. Recommendations for providing LBB protection at different voltage levels are as follows:

(i) In all new 400 kV and 220 kV substations as well as generating stations switchyards, it must be provided for each circuit breaker.

(ii) For existing switchyards, it is considered a must at 400 kV level and also at 220 kV switchyards having multiple feed. In case of radjally fed 220 kV substations, provision of LBB protection is desirable but not essential.

2.0 REQUIREMENTS LBB protection shall have following features:

(i) Have short operation and drop off times. (ii) Have three phase current elements with facility for phase wise initiation.

(iii) Have current setting range such that these can be set at minimum 200 mA for line and 50 mA for generators (for 1A C.T. Secondary).

(iv) Have one common associated timer with adjustable setting. 3.0 SPECIAL COMMENTS

(i) The relay is separate for each breaker and is to be connected in the secondary circuit of the CTs associated with that particular breaker. This CT secondary may be a separate core, if available. Otherwise it shall be clubbed with Main-I or Main-I1 protection core.

(ii) For line breakers, direct tripping of remote end breaker(s) should be arranged on operation of LBB protection. For transformer breakers, direct tripping of breaker(s) on the other side of the transformer should be arranged on operation of LBB protection.

(iii) For lines employing single phase auto-reclosing, the LBB relays should be started on a single phase basis from the trip relays. This is to avoid load currents in the healthy phases, after single phase tripping, leading to unwanted operation of the breaker failure protection, since the current setting is normally lower than the load current.

(iv) It is considered a good practice to have DC circuits of Gr.A and Gr.B protections and LBB relay independent. A separately fused supply should be taken for LBB relay in this case.

(v) LBB cannot operate without proper initiation. It is good practice to provide redundant trip output and breaker fail input where other forms of redundancy does not exist. One way of doing this is by providing separate aux. relay in parallel with trip unit and using contacts of these for LBB initiation.

(vi) Separation should be maintained between protective relay and CB trip coil DC circuit so that short circuit or blown fuse in the CB circuit will not prevent the protective relay from energising the LBB scheme.

(vii) In addition to other fault sensing relays the LBB relay should be initiated by Busbar protection, since failure of CB to clear a bus fault would result in the loss of entire station if LBB relay is not initiated.

(viii) Whenever used in combination with busbar protection scheme, tripping logic of the same shall be used for LBB protection also.

(ix) For breaker-fail relaying for low energy faults like buchholz operation, special considerations may have to be given to ensure proper scheme operation by using C.B. contact logic in addition to current detectors.

4.0 SETTING CRITERIA (i) Current level detectors should be set as sensitive as the main protections. A general

setting of 0.2 A is commonly practiced for lines and transformers. However, in case of existing schemes associated with lines having single phase autoreclosure and where phase wise initiation is not available, it is recommended that 2ph + 1 E/F element may be used with phase element set above maximum expected load current while E/F element may be set sensitively.

(ii) Current level detector for generators may be set at 50 mA (for 1A C.T. secondaries). (iii) Timer setting should be set considering breaker interrupting time, current detector reset

time and a margin. Generally a timer setting of 200 ms has been found to be adequate.

Sect ion 8 DISTURBANCE RECORDING AND FAULT

LOCATION EQUIPMENT

1.0 GENERAL 1.1 Disturbance Recorder - Provides better understanding of the behaviour of Power network after a disturbance - Gives useful information to improve existing equipment and in planning/designing new

installations Disturbance recorder shall be microprocessor based and shall be used to record the graphic form of instantaneous values of voltage and current in all three phases, open delta voltage and neutral current, open or closed positions of relay contacts and breaker during the system disturbances. Disturbance recorders are recommended for all the 400 kV lines. At 220 kv level also they are recommended for all interconnecting lines. In other cases utilities may decide depending on their need. It is also recommended that all the disturbance recorders in the station are synchronised with GPS

1.2 Fault Locator Distance to fault locator is recommended to be provided as a standard for all 400 kV and 220 kV lines on both ends. However for short lines of length upto 2.0 kms, fault locator can be provided at one end only.

1.3 Event Logger The Event logger is used to record the state of switchyard equipment and relays and occurrences of alarms. The equipment also records events recorded by disturbance recorder, as also changes in digital inputs, i.e operation and resetting of relay contact and switching of primary plant within the substation. In case all required events can be accommodated in disturbance recorder no separate event logging equipment is recommended.

2.0 . REQUIREMENTS 2 . 1 Disturbance Recorder Recording capacity - Record minimum eight analogue inputs (8) and minimum 16 binary signals per bay or

circuit. Memory capacity - Minimum 5 sec of total recording time

Recording times - Minimum prefault recording time of 100 ms

- Minimum Post fault recording time of 1000 ms

Trigger - Any of digital signals can be programmed to act as trigger - Analogue channels should have programmable threshold levels for triggers. Selectior

of over or under levels should be possible. Time Tagging - Has built in real time clock and calendar to time tag recorded disturbances. Drift of thc

in built clock shall not be more then 0 .5 sec per day. Sampling Rate - 1 KHz

Recording band width , - 5-250 Hz

Voltage Channel - Dynamic range 0.01-2 x Nominal Voltage - Resolution 0.1%

Current Channel - Dynamic range with full DC offset - .01-60 x Nominal Current

Printer -- Printer shall be suitable for taking print on plane paper

2.2 Fault Locator - Be on line type - Have built in display unit - The display shall be directly in percent of line length or kilometers without requiring an\

further calculation - Be suitable for applicable breaker operating time - Have an accuracy of 3% or better - Shall take care of

(a) presence of remote end infeed (b) predominant DC. component in fault current (c) high fault arc resistance (d) Severe CVT transient

- Shall have mutual zero sequence compensation if fault locator is to be used on double circuit line.

3.0 SPECIAL COMMENTS 3.1 Start function to disturbance recorder is to be provided by change in state of one or

more of the events connected and/or by any external triggering so that recording of events during a fault or system disturbance can be obtained. List of typical signals recommended to be recorded is given below:

(i) Recommended Analogue Signals From CT

From Line VT

From Aux. VT v o

(ii) Recommended Digital Signals(Typica1) - Main 1 Carrier receive - Main 1 Trip - Line O N Stage I/Stage I1 - Reactor Fault Trip - Stub Protection Optd. - Main I1 Trip - Main I1 Carrier Receive - ~ i r e c t Trip CH A/B - CB I Status A PH - CB I Status B PH - CB I Status C PH - CB I1 Status A PH - CB I1 Status B PH - CB I1 Status C PH -. Bus bar trip - MainI'Tie CB LBB Optd.

Note : These may need modification depending upon Protections chosen and the contact availability for certain functions.

3.2 If disturbance recorder function or fault locator functions are available as integral part of any of main protection, then separate stand alone units for this functions are not required. However both DR and Fault locator function, preferably should not be in the same unit.

3.3 In case of DR being part of main protection, it should be possible to connect external binary inputs.

3.4 Stand alone DR which can cater to more than one bay/circuit can also be used. However this should have minimum 8 analogue and 16 binary inputs per bay/circuit.

Section 9 GUIDELINES FOR PROTECTION SYSTEM ENGINEERING

1.0 GENERAL Some broad guidelines for Engineering of Protection System in addition to engineering recommendations made in various sections, are given below and could be refined if felt necessary by the Utility according to its specific needs and practices.

2.1 Gr. A and Gr. B Protection Wherever two sets of DC sources are available, to obtain redundancy and to be able to take protection out for maintenance, while equipment is in service, the relays are electrically and physically segregated into two groups. Grouping is done to the extent possible in such a way that each group can independently carry out protective functions with near equal redundancy. Interconnection between these two groups shall not generally be attempted. However if found absolutely necessary such interconnection shall be kept to the bare minimum. Even in cases when only one set of battery source is available segeration of protections and trip circuits in two groups may be considered by giving DC supplies through separate fuses.

2.2 Trip Unit In case of segregation being made as indicated above, scheme associated with each circuit breaker is provided with two sets of trip units, one in Gr A and one in Gr.B. Each set consists of one unit for 1/3 phase tripping and another for 3 phase tripping only. 1/3 phase tripping unit and 3 phase tripping units are initiated by functions as described below. However, in case the protection relay itself is having sufficient number of trip duty contacts, then separate trip units will not be necessary. 2.2.1 1 ph & 3 ph trip (Protections which,start auto recloser) - Main I Line Protection - Main I1 Line Protection

These units shall be applicable for line circuits only and shall be with self reset type contacts 2.2.2 3 ph trip (Protections which block auto reclosure) - Direct trip receive - Line reactor protection - Bus bar protection - Transformer protection - Overvoltage protection - Back up protections

These units shall be hand reset type 2.2.3 Direct trip of remote end breaker In line with the present 400 KV practice it is recommended that direct trip signal is sent to remote end under following conditions.

(i) When LBB relay operates (ii) Reactor protection and O/V protection

2.3 DC Distribution Since the availability of even the best engineered protection depends upon the integrity of the DC auxiliary supply, considerable care has to be given to the design of a system which will retain the reliability of the protection system as a whole, under all conditions. Secondly from an operational point of view, it is essential that it is possible to work on any part of the protection system with full security, while not disturbing the rest of the protective system. Arising out of these considerations, following guidelines for designing a suitable DC distribution system have been set forth. 2.3.1 For 400 kV stations there shall be two separate battery systems available for protection,

control and tripping/closing operations. For 220 kV stations where only one set of battery source is available segregation of protection and trip circuits in two groups may be considered by giving DC supplies through separate fuses.

2.3.2 Distribution of DC supply shall be done bay wise to feed the following (a) Protection (b) CB functions (c) Isolator/earth switch functions (d) Annunciation/Indications (e) Monitoring functions(Disturbance Recorder, Fault Locator, Event Logger etc.) when these

are not integral part of protection

2.3.3 (a) Protection Function For each group of protection(ie Group A and Group B relays) separate DC sources are recommended.

(b) CB Functions Trip coil 1&2 shall be fed from separate sources. Closing coil can be from either of these two sources.

(c) Isolator/Earth switch These associated with any one circuit shall be fed from one of the two DC sources. In the case of a 1 & 1/2 CB arrangement, the Isolator/Earth switch associated with the tie CB can be fed from either Source 1 or 2.

(d) Annunciation & Indication For each bay, these functions can be fed from either one of the 2 sources. Each function shall be fed however through separate feeds.

(e) Monitoring Functions These shall be grouped in 3 groups (i) Disturbance Recorders (ii)- Fault Locators (iii) Event Loggers

All the three groups shall be fed through separate teeds from either of the two sources. This is to enable the full availability of these equipment irrespective of that of the protections.

2.3.4 General Notes: 1. A s to how the separate D.C. feeds are to be taken from the sources, there are two

alternatives. (a) Through separate feeds from the board to each circuit panel (b) Through a limited no of feeds to a separate panel of the relay or control board

(Exclusively meant for marshalling the DC circuits for the various relay panels). The choice between (a) or (b) is left to the utilities as per their individual practices.

2. Sub-fusing of the DC circuits shall be done with care since blowing of the fuse will have to be monitored. Also such sub-fusing shall be kept to the minimum and can be augmented with isolating links as required.

3. Selection of sources in the event of one supply to a function failing is to be done with care, because if the fault lies downstream one may lose both supplies.

4. As a rule every DC supply going through fuse should be supervised with a no-volt relay. For trip circuits where separate trip circuit supervision relays are provided it is not necessary to provide separate DC supervision relays.

5. MCBs of adequate break-up capacity can also be used instead of fuses.

2.4 Cabling It is recommended that:

(i) Separate cables are used for AC & DC circuit (ii) Separate cables are used for DC 1 & DC 2 circuits (iii) For different cores of CT & CVT separate cables shall be used.

Section 10 LOCATION OF CTS AND VTS IN SUB-STATIONS

1.0 GENERAL Instrument transformers (CTs and VTs) are used to obtain measured quantities of current and voltage in appropriate form for use in control, protection and measuring equipment such as Energy meters, indicating instruments, protective relays, fault locators, fault recorders, synchronizers. These are installed in different bays such as line, transformer, bus coupler bays and also at the busbar.

2.0 LOCATION OF CTS AND VTS IN DIFFERENT SUB-STATION ARRANGEMENTS

2.1 Given below are some examples of different bus configurations showing suitable location of CTs and VTs.

i WWW 3-Phase Bus Sectionolizer

Fig. 1 : Double busbar arrangement

I www 1 www Tr v

Fig. 2 : Double Main with Transfer bus arrangement

1 3-Phase -

Fig. 3 : Double breaker arrangement

3-Phase, 3~1 /ba

Fig. 7 : Typical Protective Relaying Scheme for one and a half breaker busbar arrangement

2.2 Current Transformers Some explanatory notes and comments with reference to above are given below regarding location of CTs. 2.2.1 Double Bus Arrangement (Ref. Fig.4) The CTs shall be placed near the circuit breakers (CBs) and on the line side. The detection zones of line relays and busbar relays start at the CTs. It is advantageous if these two points are close to each other. In the improbable case of a fault between the CT and CB the busbar protection will detect and clear the fault. 2.2.2 Double Main and Transfer Bus Arrangement (Ref :Fig. 5) It is advantageous to locate the CTs on the line side of the disconnectors for Line and Transformer bays. In this way the protective relay connected to the CT will remain connected to the line or Transformer when it is switched over to the transfer busbar. A separate CT is required to be provided in the Transfer bus coupler bay to obtain selective tripping for faults on Transfer bus. 2.2.3 Bus Coupler and Bus Sectionalizer. Bays (Ref. Fig.4 and Fig.5) A set of CT is necessary to enable different busbar protection zones to be formed. The protection can be arranged to give complete fault clearing with a short time-delay (L.B.B. time) for faults between CB an