Power System Blackout

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Power system blackout

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CHAPTER1: INTRODUCTION

In power system, blackouts have been the mos t problem occur in the interconnected grid which results in

large loss. Power system blackout is one of the major challenges o f electric utilities in the world such as

blackout of Greece, Italy, North America, Sweden and Denmark. However, in recent years t hisphenomenon has considerably increased base on the f requency and the severity of this problem. This

may be due to t he consequence of new regulations and restriction rest ricted by power system

deregulation. In f act, it is not possible to completely prevent t hese power def iciency occur. However,

proper monitoring, contro l and protection schemes, f requency and the severity of this phenomenon

occur may be reduced. Some kind of operat ion cont ributes to the recently occurred blackouts such as

Incorrect o f protective syst ems, vo ltage instability, and lack of under f requency load shedding. Under

f requency load shedding (UFLS) scheme is proposed to enhance the reliability o f power systems t o

against system f ailure and fault occur. This scheme is classif ied as pro tection scheme or system

protection scheme. System protection scheme is def ined as protection st rategies designed to detect a

particular condition which is known to cause failures to t he power system and prepare some kind of

predetermined action counteract the o bserved condition in a cont rolled manner. System protectionscheme is specially designed to detect abnormal conditions at the same time take predetermined

corrective action, other than the isolation of f aulted element, to preserve system integrity and provide

acceptable system performance. The objective of using the protection scheme is to increase power

system reliability especially in term of security during extreme cont ingencies and to improve power

system operation as well. Under f requency load shedding (UFLS) scheme is one of the most commonly

used as a protection schemes. This kind of protection scheme is employed due to the effectiveness to

counter power system failure regardless what kind of disturbance it is applied. Under f requency load

shedding (UFLS) scheme is conventionally designed to preserve the balance power in the island during

f ault occur. Under f requency load shedding (UFLS) is a very important approach to prevent f requency

decline. It should have capability not only to shed load under dif f erent operating modes when local

systems are connected to the main systems, but also capable to maintain the f requency stability when

local systems are islands. The f requency decline, caused by the power imbalance between generation

and demand is considered as serious problems which lead to the excess load. The frequency decline may

cause a permanent damage to the turbine blades and plant themselves lead to f ailures because at low

f requency their auxiliary are not be able to maintain normal output when the f requency is about 10 to

15% below normal. The primary method to bring back to the nominal f requency level is to shed amount o f

load. In power systems protect ion scheme, the f requencies are widely used as a set ting in UFLS design.

Under f requency load shedding (UFLS) must be perf ormed quickly to arrest power system f requency

decline by decreasing power system load t o match available generating capacity. Extreme frequency

decline can occur within seconds. An automatic under f requency load shedding (UFLS) scheme is applied

to restore t he system f requency to an acceptable level f ollowing a major system emergency which cancause a generation def iciency. In conventional under f requency protection design the only measured

parameter o f the system involved in decision making is f requency. Convent ional UFLS schemes are

designed to prevent extreme frequency decline because of disturbance which lead to imbalance between

generat ion and demand. Excess ive frequency decline may cause damage to the equipments o f power
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system particularly turbine blades in power plant at frequency below 47.5Hz. During normal operation of

power system the amount of generation is equal to the amount of demand. Whenever the f aults occur,

either the amount of generation is decreases o r the amount of load is suddenly increases, the balance

of power is violated and the frequency falls at the predetermined threshold the portion of the load will be

shed in a f ew steps to equalize the amount of demand and generation to prevent the system collapse.

The loads to be shed in this system are shed constant load f eeders and are not selected adaptively. In

the other hand, these systems always drop the same load regardless of the location o f disturbance. In

this system loads are classif ied in three groups of non vital, semi vital and vital loads. The system

usually shed non vital load. Sometime in severe condition semi vital loads may also be shed. Although thiskind of protection scheme is easy to implement, but it is suf f er in term of adaptability when various kind

of f ault is experience. In the other hand, regardless o f the severity o f the disturbance, sett ing of the

under f requency load shedding scheme is the same or constant. This kind of scheme may be introduced

over shedding or under shedding f or small disturbance or large disturbance respectively. In this pro ject

rate of change of f requency is proposed to enhance the adaptability of under f requency relays. By using

this load shedding method, fast reactions could be obtained f or major system f ailures. Adaptive under

f requency load shedding (AUFLS) scheme can prevent complete system blackouts in the case of large or

small dist urbance. In other word, adapt ive under f requency load shedding (AUFLS) scheme is specially

designed to counter any kind of in coming disturbance applied. Rate of change of f requency, df /dt is the

indicator to detect the magnitude of disturbance or f aults. The rate o f change of f requency df/dt is an

instantaneous indicato r of power imbalance and is present ly used with the f requency function to provide

a more selective or faster operation. To make the rate of change of frequency df/dt as power deficiency

indicator additional inf ormation about t he system is required such as voltage, spinning reserve, load and

etc. Such information may be communicated to the relay. There are three types o f the under f requency

relays available f or load shedding scheme purpose. They are electromechanical relays, so lid-s tate relays

and digital relays. The purpose of the under f requency relay is f or monito ring the frequency of an

alternating current power line and protect the system by giving signal when ever the f requency drop

below predetermine value for a specif ic length interval. The f requency of grid system must be maintain at

50Hz, if the f requency drop below nominal value the protection scheme must be initiated in order to

maintain its generator on line even at low frequency. Frequency drop in power line may take several steps

to measure. One of the methods is by using oscillato r, the f requency which is substantially higher thanthe nominal f requency being measured, and count ing the number of pulses f rom the oscillator during the

period. When the line f requency decrease, the period will correspondingly increase, and therefore a high

number of oscillator output pulse will be obtained. It is no desired to detect under f requency condition

which result in load shedding when an under f requency condition does no t exist in fact, primarily because

of the inconvenience may cause customer unsatisf ied. It is also because of the dif f iculty and duration

taken to restore power to those customers. The objective of the invention of under f requency relay is t o

provide method of detecting an under f requency condition on an alternating current power line including

the s tep o f sensing by means of electrical circuit. Aft er the power def icient has been cleared, the amount

of the power generation and the power demand is approximately the same. Theref ore t he f requency of

the system will bring back to the acceptable level because t he power imbalance is proportional to the

f requency deviation. Thus, when the f requency is synchronous with the f requency of the grid system,

the islanded locat ions will connected back to the grid in case of the f ault has been cleared.

1.1 OBJECTIVE:

AIM 1: To des ign adaptive under f requency load shedding scheme for small Dist ributed Generat ion.

AIM 2: To develop model to predict the rate of f requency (df /dt) decay during system disturbance.

1.2 SCOPE OF THESIS

The scopes of work are:

1. Gathering all suitable method f or load shedding scheme f rom reliable resources such as journal or

internet.

2. Chose one suitable metho d which is can adapt any disturbance the network applied. Study and


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understand well the method and design the algorithm.

3. Write program into C++ programming language based on the written algorithm.

4. Modeling simulation network by using Power System CAD (PSCAD) so f tware. Interf acing C code to

the PSCAD software.

5. Testing adaptive under f requency load shedding scheme in the simulation modeling.

6. The result is studied f ro dif f erent power imbalance.

CHAPTER2: LITERATURE REVIEW

2.1 OVERVIEW

In our interconnected system all the power demand and power supply is in the balance mode or it is called

in synchronism. They are all synchronous with the grid system in term of generator speed or f requency

to maintain its stability. All synchronous generato rs are in the same speed or f requency, and not one o f

them run at slower or faster to ensure the generator run at rated speed. Before disturbances occur, the

f requency is maintained at 50Hz so that the prime movers will drive generato r at constant speed. Since

the f requency is directly proportional to the speed of the generator, hence when the f requency

decreases the generator speed will decreases as well. When there is a disturbance because of f ault or

f ailures occur there is instability within the system. The relay grading will set the t ime to discriminate the

fault location to protect the equipment and to ensure that there is no total blackout occurs. If we were

not discriminating the saf e location it may cause large loss and damage the equipment. Disturbance is

classif ied as large, medium and small so that we can know what type of disturbance occur to take

predetermines action. The f ault location is being iso lated that location is called islanding. Islanding is

condition where the distributed generation (DG) and local bus is disconnected f rom the grid due to f ault

or f ailure. When ever islanding occurs, the interconnected grid will not f eed the energy to the islanded

location unt il the entire f ault has been cleared. During islanding, the distributed generation (DG) needs to

supply suf f icient power to the local bus because there are loads connected at the bus as well to ensure

the security o f the load af ter islanding is guaranteed. Whenever islanding occur, the distributed

generation (DG) has to run at ref erence speed to saf e the local load. Aft er islanding, the f requency maydrop and the speed of the generato r is reduced as well. To maintain at nominal f requency the prime

mover have to drive the generato r much f aster. When the distributed generation (DG) is not capable to

supply enough power to the load the power imbalance is es tablish which is t he power of demand is

greater than the power of supply. To ensure the power is always available, the spinning reserve is

activated. In the case of the spinning reserve take to longer time to supply power or it can not capable to

maintaining supply, thus under f requency load shedding scheme is initiated to prevent f requency decline

or power imbalance. In the extreme contingency the loads have to shed to prevent f rom to tal cascaded

blackout occur. Frequency decline is detected by under f requency relay. Under f requency relay is very

sensitive to the change of the f requency due to t he power imbalance between generation and

consumption. In this pro ject, the rate of f requency change is used to detect the power deficiency. The

rate of f requency decay is small if there is large imbalance between the supply and demand or it is called

large disturbance. In the o ther word, if there is small disturbance occur then the rate of f requency decay

is large. The rate of change of f requency is inversely proport ional to the power imbalance. The larger the

power imbalance the smaller the rate of f requency change. If the disturbance is large the rate of the

f requency decline is small which indicate t hat t he f requency will drop f aster and proper action must be

taken quickly. Under f requency relay must be performs quickly to arrest power system of f requency

change (decline) by decreasing power system load to match with the available generating capacity.

Adaptive under f requency load shedding is used f or purpose to drop the load in a proper manner when

imbalance power established regardless t he amount o f the power def iciency it is experience. The value

of the rate of f requency change is a powerful indicator to achieved proper load shedding scheme. This

method will shedding load based o n the magnitude o f disturbance without introducing any undershedding or over shedding which lead to the improper load shedding and may cause large loss. This kind

of protect ion scheme is called Adaptive Under f requency Load Shedding Scheme (AUFLS). This scheme is

usually designed to pro tect power system from total blackout which is the load to be shed is

programmed to specified which load will be safe the most . The load must be safe the most is usually


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related to the government o f f ice. This method is compared with the conventional load shedding which is

shed the same location which can cause the importance of the load. Usually, the loads to be shed are

chosen f rom the loads which have low degree of importance and the loads are not concentrated at any

specific area. For t he purpose o f preserving the island, the amount of the load and supply must be equal.

The power demand and supply is calculated to determine the amount o f load to shed and the value will

be used to select which load will be shed based on the importance of the load. The less importance of

the load, the f irst load will be shed and vice versa. The value of the rate o f change of f requency is

inf luence the value of the load to be shed. Because the large disturbance the large load is to shed. Aft er

load is reduced the to tal load is also reduced then the demand is approximately equal to the powersupply. Therefore the f requency is bringing back to the nominal f requency, but s lightly below the nominal

level. The power of the total load might be loss in a small quantity. Aft er the f ault has been cleared the

islanding location and the grid will reconnect in the case the islanding location and the grid is

synchronous in term of frequency.

2.2 ELECTRICAL GRID SYSTEM

When we say about t he power industry, "grid" is a term used f or an electricity network which may support

all or some of the f ollowing three obvious operations:

1. Electricity generation

2. Electric power transmission

3. Electricity distribution

1. Electricity generation.

The main component in power generation o f power system is synchronous generato r or also known as

alternator. Synchronous generator comprises of two rotating field, which is from rotor driven at

synchronous speed and excited by dc current and the other f ield is produced by three phase armature

currents in the stator windings. The dc current is provided by the excitation systems for the rotor

windings. In the older units, t he dc current exciters are providing by dc generato rs with rotating rectif iers,known as brushless excitat ion systems. The generator vo ltage and reactive power f low is contro lled by

the generator excitation system. Ac generators can generate high power at high voltage, typically 30KV

due to lack of the commutator. Typically in power plant the size of generators can be varied from 50MW

to 1500MW. The prime movers will produce mechanical power to move the t urbine blade. The prime

movers usually come fro m hydraulic turbines at waterf alls, s team turbines whose energy comes f rom the

burning of coal, gas and nuclear f uel, gas t urbines, or occasionally internal combust ion engines burning

oil. Steam turbines usually operate at 3600 rpm and 1800 rpm f or 2 and 4 poles respectively, the

generators to which they are coupled are cylindrical rotor. Hydraulic turbines, part icularly those o perating

with a low pressure, operate at low speed. Their generators are typically a salient type of rotor with many

poles. In the grid system, several synchronous generato rs are operat ing in parallel switch other

generator t o provide the total power needed. They are connected at the common po int known as BUS.

2. Elect ric power transmission

The purpose of an overhead transmission network is to transfer electric energy from generating unit at

various locations to the dist ribution system which ultimately supplies the load. Transmission lines also

interconnect neighboring utilities which permits not only economic dispatch of power between regions

during normal conditions, but also the transf er of power regions during emergencies. Standard

transmission voltages are established in the United States by the American National Standards Inst itute

(ANSI). Transmiss ion vo ltage lines operating at more than 60KV are standardizing at 69KV, 115KV,

138KV, 161KV, 230KV, 345KV, 500KV and 765KV line to line. Transmission voltages above 230KV isconsidered to as extra high voltage (EHV). High voltage transmission lines are terminated in

substations, which are called high voltage substations, receiving substations or primary substations.

Switching stat ions are f unction f or switching circuit in and out of service. The voltage is stepped down to

a value more suitable f or t he next part o f the journey toward the load at t he primary substat ions. Very

large industrial customers may be served f rom the t ransmission syst em. Sub transmission network is


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referred as the port ion of the transmission system that connects the high voltage substations through

step-down transformers to the distribution substations. There is no clear delineation between

transmission and sub t ransmission voltage levels. The sub t ransmission voltage level usually ranges

f rom 69KV to 138KV. Some large industrial customers may be served f rom the sub transmissions f or

maintaining the transmission line voltage.

3. Electricity distribution

The distribution system is those parts which connect the distribution substations to consumers service entrance equipment o r in the other word part of supply electricity which is dealing with the customer.

The primary distribution lines are usually in the range of 4 to 4.5KV and supply the load in a well def ined

geographical area. Some small industrial customers usually served directly f rom the primary feeders. The

secondary distribution network stepped down the voltage f or ut ilization commercial and residential

consumers. Lines and cable are usually not exceeding a f ew hundred feet in length then deliver power to

the individual consumers. The secondary distribution serves most of the consumers at a level 240/120V,

single phase, three wire, 208Y/120V, three phases, four wire, 480Y/227V. The typical power for home is

derived f rom a transf ormer which stepped down the primary feeder voltage to240/120V using a three

wire line. Distribution systems are bot h overhead and underground. The growth of underground

distribution systems has been extremely rapid and as much as 70% o f new residential construction is

served underground.

In an electricity grid system, electricity demand and supply must be balance at all times, any s ignif icant

imbalance power may cause grid instability or severe voltage f luctuat ions, and cause f ailures within the

grid system. Total generation capacity is therefore s ized to correspond to to tal peak demand with some

margin of error and allowance f or contingencies (such as plants being of f - line during peak demand

periods o r outages). Operato rs will generally plan to use the least expensive generating capacity (in

terms o f marginal cost ) at any given period, and use additional capacity f rom more expensive plants as

demand increases. Demand response in most cases is targeted at reducing peak demand to reduce the

risk of potential disturbances or f ailure, avoid additional capital cost requirements f or additional plant,

and avoid use o f more expensive and/or less ef f icient operating plant. Consumers o f electricity will also

pay lower prices if generation capacity that would have been used is f rom a higher-cos t source of power

generation.

Demand response may also be used to increase demand during periods of high supply and/or low

demand. Some types o f generating plant must be run at close to f ull load (such as nuclear), while other

types may yield at negligible marginal cost (such as wind and solar). Since there is usually limited load to

store energy, demand response may attempt t o increase load during these periods to maintain grid

stability. For example, in the province of Ontario in September 2006, there was a short period of time

when electricity prices were negative for certain users. Energy sto rage such as Pumped-sto rage

hydroelectricity is a way to increase load during periods of low demand f or use during later periods. Use

of demand response to increase load is less common, but may be essential or ef f icient in systemswhere there are large amounts of generating capacity t hat cannot be easily cycled down.

Some grids may use pricing system that are no t real-time, but easier to implement (users pay higher

prices at the day and lower prices at night, f or example) to provide some of the benef its o f the demand

response mechanism with less demanding technological requirements. For example, in 2006 Ontario

began implementing a "Smart Meter" program that implements "Time-of -Use" (TOU) pricing, which tiers

pricing according to on-peak, mid-peak and of f -peak schedules. During the winter, on-peak is def ined as

morning and early evening, mid-peak as mid-day to late af ternoon and of f -peak as night- time; during the

summer, the o n-peak and mid-peak periods are reversed, ref lecting air conditioning as the driver o f

summer demand. In 2007, prices during the of f -peak were C$0.034 per KWh and C$0.097 during the on-

peak demand period, o r just less than three t imes as expensive. As of 2007, few utilities had the metersand systems capability to implement T OU pricing, however, and most customers are not expected to get

smart meters until 2008-2010. Eventually, the TOU pricing (or real-time pricing) is expected to be

mandatory for most customers in the province.


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2.3 DISTRIBUTED GENERATION

Dist ributed generation (DG) is anticipated to become more important in the f uture generation system.

The current literature, however, does no t use a cons istent def inition of DG. The relevant issues and

aims at providing a general def inition f or distributed power generation in competitive electricity markets.

In general, DG can be def ined as electric power generation within distribution networks or on t he

customer side of network. In addition, the terms distributed resources, dist ribution capacity and

distributed utility are discussed. Network and connection issues of the generation are presented.

Dist ributed generation, also called on-site generation, dispersed generation, embedded generation,

decentralized generation, decentralized energy or distributed energy, generates electricity f rom many

small energy resources.

Dist ributed Generation are considered to be important in improving the security of energy supplies by

decreasing dependency on imported fossils f uels and in reducing the emissions of greenhouse gases.

Dist ributed Generation usually referred to t he local generation o f electricity in the case of cogeneration

system, heat f or indust rial processes o r space heating. Basically, distributed generation takes place

close to the po int where the energy is actually demanded or it is called local generation. Dist ributed

Generation is no t centrally planned and mos tly conducted or produced by independent power producers

or consumers and it is not centrally dispatched. Usually Dist ributed Generation is produce smaller than50MW. Distributed Generation is connected to the electricity distribution network or grid. All renewable

energy system mos tly is also distributed generation systems. Distributed Energy Resources usually

referred to the dist ributed electricity generation and electricity storage. Generally, distributed generation

will used a port ion of the electricity for local use and the rest will be fed into the grid. The heat, on the

other hand, is usually used locally due to the cost ly transport and may cause large loss . Dist ributed

Generation is usually used for domestic, commercial and indust rial purpose. The purpose of the

distributed generation is central rather than distributed is due to the economy of scale, efficiency fuel

capability and lif etime. Theref ore, by increasing size of production unit increases t he ef f iciency and

decreases the cos t per MW. However, in term of economy advantage, the small units are benef iting f rom

cont inuing technological developments, awhile large units are already f ully developed. The o ther reason

to keep building large power plants in fuel capability. Coal is not economically suitable f or DG in fact it is

mos t abundant f ossil f uel with s teady suppliers all over the world and a stable price. Additionally, large

power plants will remain the prime source of electricity within 25 50 years lif etime. Currently, indust rial

countries generate most of their electricity in large centralized facilities, such as f ossil fuel (coal, gas

powered) nuclear or hydropower plants. These plants have excellent economies of scale, but usually

transmit electricity long distances and can af f ect to t he environment. Mos t plants are built this way due

to a number of economic, health & saf ety, logist ical, environmental, geographical and geological factors .

For example, coal power plants are built away f rom cities t o prevent their heavy air pollution f rom

af f ecting the populace, in addition such plants are of ten built near collieries to minimize the cost o f

transport ing coal. Hydroelectric plants are by their nature limited to operat ing at s ites with suf f icient

water flow. Most power plants are of ten considered to be too f ar away fo r their waste heat to be usedf or heating buildings. Low po llution is a main advantage of combined cycle plants that burn natural gas.

The low pollution permits t he plants to be near enough to a city to be used, perhaps even in the same

building. This also reduces the size and number of power lines that must be constructed. Typical

distributed power sources in a Feedin Tarif f (FIT) scheme have low maintenance, low pollution and high

ef f iciencies. In the past, these traits required dedicated operating engineers, and large, complex plants to

pay their salary and reduce pollution. However, modern embedded systems can provide these traits with

automated operation and a renewable, such as sunlight, wind and geothermal. This reduces the sizes of

the power plant that can show a prof it. With everything interconnected, and open competit ion occurring

in a f ree market economy, it starts to make sense to allow and even encourage dist ributed generation

(DG). Smaller generators , usually not owned by the ut ility, can be brought online to help supply the need

f or power. The smaller generation f acility might be a home-o wner with excess power f rom his so lar panel

or wind turbine. It might be a small of f ice with a diesel generator. These resources can be brought on-line

either at the ut ility's behest o r by owner of the generation in an ef f ort to sell electricity. Many small

generators are allowed to sell electricity back to t he grid fo r the same price they would pay to buy it.


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2.3.1 SYNCHRONOUS GENERATORS

Large scale of power is produced by three phase synchronous generators which are also known as

alternators. Synchronous generator is either driven by steam turbines, hydro turbines, or gas turbines.

The stationary part or so called stator is placed armature windings. The armature winding is specif ically

design for generation of balanced three-phase voltages and are arranged to develop the same number

of magnetic poles as the f ield winding which is in the roto r. The f ield which relatively draw small power

(0.2 3 percent o f the machine rating) for its excitat ion is placed on rot or. The ro to r is also occupied by

one or more damper winding as a short circuit winding. The prime mover drive the rot or at constantspeed and its f ield circuit is excited by direct current. Slip rings and brushes will be providing the

excitation by means o f dc generato r also known as exciters mounted on t he same shaf t as the roto r of

the synchronous machine. However, for modern excitation systems, they usually use ac generator with

rotating rectif iers which is also known as brushless excitat ion. The generato r voltages and reactive

power flow is controlled by the generator excitation to keep maintain. The rotor of the synchronous

generator may either kind of cylindrical or salient construction. The cylindrical type is also known as

round ro to r, has one distributed winding and a uniform air gap. Synchronous generator is driven by steam

turbines are design for high speed at 3600 rpm or at 1800 rpm for two and for po le machines

respectively. The ro to r o f these generators has a relatively large axial length and small diameter to limit

the centrifugal f orces. About 70% of the large synchronous generators are cylindrical rotor t ype raging

f rom 150 to 1500MVA. The salient type of rotor has concentrated windings on the poles and non unif orm

air gaps. The synchronous generator usually driven by hydraulic turbines in hydroelectric power plant.

2.3.2 POWER SYSTEM DYNAMIC:

The tendency of power system to develop restoring forces equal to or greater than the disturbing

f orces to maintain the state of equilibrium is known as stability. If the f orces tending to hold the

machines in synchronism with one another are suf f icient to overcome the disturbing f orces, the system

is said to remain stable (to stay in synchronism). The s tability problem is considered with t he behavior o f

the synchronous generator af ter disturbances occur. In the easy way, st ability problems generally divided

into two major categories which is steady state stability and transient s tability. Steady state stability canbe def ined as the ability o f the power system to regain synchronism af ter small and low disturbances,

such as gradual power changes. It is convenient to assume that the disturbances causing the changes

disappear. The mot ion is f ree, and stability is assured if the system is returned to its o riginal state. Such

a behavior can be determined in a linear system. It is assumed that the linear automatics control, such as

voltage regulato r and governor is not active. An extension o f steady state s tudy is dynamic study. The

dynamic study is concerned with small disturbance last ing for along t ime with the inclusion of automatic

control devices. The t ransient stability studies deal with the ef f ect of large, sudden disturbance such as

sudden occurrence of fault, the sudden outage of a line or the sudden application or removal of loads.

Transient stability studies are needed to ensure that system can withstand the t ransient condition

f ollowing a major disturbance. Frequently, such studies are conducted when new generating and

transmitting f acilities are planned. The s tudies are helpful in determining such things as the nature of therelaying system needed, critical clearing t ime of circuit breakers, vo ltage level of , and t ransf er capability

between systems.

SWING EQUATION:

Under normal operat ing conditions, the relative pos ition of the roto r axis and the resultant magnetic f ield

axis is f ixed. The angle between the two is known as power angle or torque angle. Whenever

disturbances occur rotor will accelerate o r decelerate with respect to t he synchronously rotating air gap

(mmf ), and a relative motion begins. The equation describing this relative motion begins. The equation

describing this relative mot ion is known as the swing equation. If , af ter this oscillato ry period, the rotor

locks back into synchronous speed, the generator will maintaining its stability. In the case o f the

disturbance does not involve any net change in power, the roto r return to its o riginal pos ition. if the

disturbance is created by a change in generation, load, or in network conditions, the roto r comes to a

new operating power angle relative to the synchronously revolving f ield.


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Consider a synchronous generato r developing an electromagnetic Te and running at the synchronous

speed wsm. If Tm is the driving mechanical torque, then under steady state operation with losses

neglected we have

Tm=Te (1)

A departure f rom steady state due to the disturbance result in an accelerating (T m>Te) or decelerating

(Tm

Ta=Tm-Te (2)

If J is the combined moment o f inertia of the prime mover and generator, neglecting f rictional and

damping torques, f rom laws o f rotation we have

J(dw/dt) = Tm Te = Ta (3)

Where

J is the moment of inertia(kg-m2)

w Is the angular velocity(rad/s)

dw/dt is the angular acceleration(rad/s2)

Ta is the resulting torque(N-m)

2.3.3 AUTOMATIC GENERATION CONTROL (AGC)

If the load on the system is increased, the turbine speed drops before the governor can adjust the input

of the s team to a new load. Whenever the change in the value of the speed diminishes, t he error s ignal

becomes smaller and the position of the governor f ly balls gets closer to the point required to maintain a

constant speed. However, the constant speed will not be the set po int, and there will be an of f set. One

way to resto re the speed or f requency to its nominal value is to add an integrato r. The integral unit

monito rs the average error over a period o f time and will overcome the of f set . Because of its ability to

return a system to its set po int, integral action is known as rest action. As the system load changes

cont inuous ly, the generation is adjusted automatically to restore t he f requency to the nominal value. This

scheme is also known as automatic generation control (AGC). In an interconnected system cons isting of

several pools, the role of the AGC is to divide the loads among systems, station, and generators so as

to achieve maximum economy and correctly controlled interchanges of tie-line power while maintaining a

reasonably uniform f requency. During large transient disturbances and emergencies, AGC is bypassed

and load shedding is applied. The generato r excitat ion syst em maintains generato r and contro ls the

reactive power f low. The generato r excitation of older systems may be provided through the s lip rings

and brushes by means o f dc generator mounted on the same shaf t as the rotor o f the synchronousmachine. However, modern excitat ion system usually use ac generators with rotating rectif iers, and are

known as brushless excitat ion. A change in the real power demand af f ects essent ially the f requency,

whereas a change in the reactive power af f ects mainly the voltage magnitude. The interaction between

voltage and f requency contro ls is generally weak enough to justif y their analysis separately. The source

of the reactive power is come f rom generato rs, capacitors and reactors . The generator reactive powers

are contro lled by excitat ion. Other supplementary methods o f improving the voltage prof ile on electric

transmission systems are transformer load-tap changers, switch capacitors, step voltage regulators,

and stat ic var contro l equipment. The primary means of generator reactive power contro l is the generator

excitation contro l using automatic voltage regulato r (AVR). The f unction of the AVR is to hold the

terminal voltage magnitude of a synchronous generator at a specif ied level. An increase in the reactive

power load o f the generato r is accompanied by a drop in the t erminal voltage magnitude. The voltage

magnitude is sensed through a potential transf ormer on one phase. This voltage is rectif ied and

compared to a dc set point s ignal. The amplif ied error signal contro ls the exciter f ield and increases the

exciter terminal voltage. Thus the generator f ield current increased, which result in increase in the

generated emf . The reactive power generat ion is increased to a new equilibrium, raising the terminal


9/23

voltage to the desired value. The f actors which contribute power generation at minimum cost are

operat ing ef f iciencies, f uel cos t and transmission losses. A program called dispatch was developed to

f ind the opt imal dispatch of generation f or an interconnected power system. The opt imal dispatch may

be considered within the f ramework of Load Frequency Control (LFC). Digital computer is included in the

control loop which scans the unit generation and tie- line f lows in direct digital control systems. These

setting are compared with the opt imal sett ings which are derived f rom solution of the optimal dispatch

program. If the actual set tings are deviate f rom opt imal values, the computer will generates the

raise/lower pulses which are not sent to the individual units . The ot her program will also be considered in

the t ie-line power cont racts between the areas. Parallel with t he development o f modern cont rol theory,several concept are included in the automatic generato r cont rol (AGC) which is go beyond the simple tie

line bias cont rol. The basic approach is the use o f more extended mathematical models. The

automatic generato r control (AGC) can also be used to include the representat ions o f the dynamic area

and the complete system as well. Other concepts of modern contro l theory are also can be employed,

such as s tate est imation and optimal contro l with linear regulato r utilizing constant f eedback gains. In

addition to the structures which aim at the contro l of deterministic signal and disturbances, there are

scheme which use stochast ic cont rol concepts . The generator excitation system should be maintaining

voltage and controlling f low of reactive power. For older system generator excitation may be provided

through slip rings and brushes by means o f dc generato rs mounted on the same shaf t as the ro tor o f

the synchronous machine, but f or modern generator excitation systems usually use ac generators with

rotat ing rectif iers and are known as brushless excitation. Basically, when ever power demands change, it

may result in f requency change, whereas a change in reactive power will result a change in the vo ltage

magnitude essent ially. The relationship between voltage and f requency controls is generally weak

enough to justif y their analysis separately. Reactive power is draws f rom generator, capacito r and

reactors . Field excitat ion will contro l the generato r reactive power. In addition methods of improving the

voltage profile on electric transmission systems are transformer loadtap changers, switched

capacitors, step regulators and stat ic var control equipment. The generator excitation contro l primarily

using automatic voltage regulator (AVR) f or generato r reactive power contro l. The purpose of the

automatic voltage regulato r (AVR) is to ho ld the terminal voltage magnitude of a synchronous generator

at a specified level. If the reactive power loads o f the generato r increase the terminal voltage magnitude

will decrease. The potential transf ormer will detect the voltage magnitude. Terminal voltage will rectif y andcompared to the dc set point s ignal. The exciter f ield will contro lled by the amplif ied error signals to

increase the exciter terminal voltage. Therefore, t he generator f ield current is increased and directly

increase the generated emf . The reactive power generat ion is established in a new equilibrium, and

increase the terminal voltage to the desired value.

2.3.4 RESPONDS OF THE GENERATOR DURING ISLANDING

Island operation occur whenever one or more dist ributed generation (DG) cont inues to energize a part o f

the grid aft er the connection to the rest of the system has been lost . Islanding operation can be in either

intentional or unintentional. Intentional islanding is the purposef ul sectionalized of the utility system

during widespread disturbances to create powerisland. These islands can be designed to maintain acont inuous supply of power during disturbances of the main distribution system. When the disturbances

are comes on a dist ributed utility system, the grid sectionalized by itself . This protection is really a

system protection o f last resort . This scheme are supposes that the integrity of the system cannot be

maintained in spite o f the automatic load shedding, f or every possible emergency. Instead of allowing the

system to disintegrate by the tripping of generators and transmission lines as the disturbance develops,

the islanding scheme itself sectionalizes the whole system into sustainable small systems each

consisting of a group of generating stations and a group of load that can be supplied by these

generating stat ions. In ef f ect each group becomes a sustainable island and hence the name islanding

scheme. The dist ributed generation can then supply the load power demand of the islands created until

reconnection with t he main utility systems occurs. As the demand f or more reliable and secure power

systems with greater power quality increases, the concepts of distributed generation (DG) have become

more popular. This popularity of DG concepts has developed simultaneously with the decrease in

manufacturing costs asso ciated with clean and alternative technologies, like f uel cells, biomass, micro-

turbine, and so lar cell systems. Although the cost s associated with these technologies have cont inued to

decrease more work is essential to make these technologies readily available. To make these distributed


10/23

energy resource (DER) technologies more economically viable and energy ef f icient, powerelectronics

based conversion systems need to be developed for proper conditioning of the energy to be delivered to

the current three phase power system. These power conversion systems (PCS) allow f or increased

reliability, security and f ewer downtimes by incorporating intentional islands into the ut ility grid without

having to add or replace the exist ing transmission system.

When dist ributed generation (DG) and their local load (the island) are iso lated f rom a larger grid. If the

distributed generation or so called synchronous generator and its prime mover ( a turbine or a

reciprocating engine) and the load o f this small "island" that can be iso lated f rom the grid and poweredby this generator and it's prime mover is 25 MW. So, when this island and it's generato r are isolated f rom

the larger grid some load has to be shed (automatically disconnected) in order not to exceed the rating

of the generator's prime mover. In this example, a to tal of 5 MW of load would have to be disconnected

from the isolated island in order not to exceed the rating of the generators' prime mover if it were to be

operated at rated f requency when isolated f rom a largergrid.

In order f or a prime mover and it's generato r to energize a load at a constant f requency the prime mover

needs to be operated in that way that it is cont rolling f requency in response to changes in load. This is

usually called Isochronous Governor mode, o r Isochronous Speed Control mode. And a prime mover and

its generator can only produce power at rated frequency up to the rating of the synchronous generator's

prime mover.

When the generator connected to a larger grid in parallel with other generato rs and their prime movers

the normal mode of operation for the prime mover governor (control system) is Droop Speed Control

mode, this is because so me other "entity" is contro lling the f requency of the grid. when connected to a

larger grid in parallel with other generato rs and their prime movers a generato r can only produce power

up to the rating of its prime mover.

So, when a prime mover and its generato r are suddenly disconnected f rom a larger grid and are to be

provide power to a local load (the island) independent of the grid, the prime mover's governor is usually

switched to Isochronous Speed Control mode. If the load of this small island exceeds the rating of the

generator's prime mover then some of the load must automatically be disconnected, and this is ref erredto as load shedding.

Usually, there is a contact on the breaker that connects the island to the larger grid that serves to tell the

prime mover governor to switch from Droop Speed Cont rol mode to Isochronous Speed Contro l mode

*and* to initiate load shedding to reduce the load below the generator prime mover's rating.

Depending on how fast the load is shed when the grid tie breaker opens and also depending on how fast

the prime mover's governor (control system) can react to the change in load, what usually happens if the

island load is initially greater than the prime mover's rat ing is that the f requency will decrease. Once the

load of the island has been reduced to at least t he rating of the generator prime mover and the prime

mover's governor has responded to the change in load, then the f requency should return to rated andremain at rated as long as the load of the island does not exceed the rating of the generator's prime

mover.

As long as the island load is not allowed to exceed the rating of the generator's prime mover it should be

able to respond to any change in load and s till maintain rated f requency. That is, the prime mover's

control system (the governor) should be able to respond to any change in load up the rating of the prime

mover and still be able to maintain frequency.

The generator produces power at a f requency that is proportional to t he speed at which it's ro to r is

being turned by the prime mover (the turbine or reciprocating engine). When operating a small island of

isolated load, the amount of load must be less t han the rated power of the prime mover driving thesynchronous generator, or else t he f requency will not remain at rated value.

It's not the generator which controlling frequency or the amount of load, but it's the control system (the

governor) o f the generator's prime mover which is contro lling the f requency (when operating in

Isochronous Speed Contro l mode) o r the load (when operating in Droop Speed Contro l mode in parallel


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with o ther generators and their prime movers). A generator is just a device for converting torque (f rom a

prime mover) into amperes. Those amperes can then be transmitted o ver wires to motors which convert

the amperes into t orque. (Lighting is a way to convert amperes into heat, heat so ho t that it produces

light.) So, the load of a generator is proportional to the amount of torque being produced by it's prime

mover. And that to rque is a f unction of the energy which f lowing into the prime mover (usually fuel or

steam or water or wind).

2.3.5 DROOP SPEED CONTROL

The speed of the synchronous generator is a f unction of f requency or generator speed. Because of its

name synchronous generator indicate that no generator can go faster or slower than the speed that is

dictated by the f requency. All are in synchronism to maintaining grid stability. In other word, they are all

connected together and their roto rs are locked into synchronism with each other (magnetically), the

prime movers which are mechanically tied to the generators can not change their speed either.

Synchronous generator is connected to a grid with other generato rs which driven by their prime mover

respectively. In the large interconnected grid, the f requency of the generato r is cont rolled by the

f requency of the grid, the f requency must be constant and in synchronism with the other generato r.

Therefore, the speed of the prime mover is in a f unction of the f requency indicating that the prime mover

is inject constant ly, hence the speed of the prime mover is f ixed as well. In the grid, all synchronous

generators must be run at the same speed. Not o ne of them runs at f aster or slower than other

generator. If the synchronous generators run at 50.0001Hz, all generators must be run at same speed.

To make the prime mover (which is providing the to rque input that the generato r is converting to amps

that is being converted to torque by motors which are also connected to the grid) stably control its

power output while connected in parallel with the other generators and prime movers on t he grid, the

control system employ st raight proport ional control also known as droop control o r droop speed contro l.

The generato r is sharing load during cont rolling stably power output when connected to a grid in parallel

with the other generato rs. Droo p speed contro l will consider both the prime movers speed reference and

actual speed which is in the f unction of the grid f requency. The power output is directly proport ional to

the speed reference, hence to increase the power output , the speed ref erences is increased. The speed

actually can not change the increased error between reference and actual speed is converted to increasef uel flow which lead to increased f uel f low. Therefore, the speed will increase due to extra to rque as well.

The generators convert that extra torque to the extra current. This operation is done very smoothly and

all the prime movers and their generators behave nicely and work together to provide the load. If the load

on t he generato r is to be increased, then the turbine speed reference is increase again, the error

between the actual speed and the speed ref erence increases again, which increases t he f uel f low directly

increases the torque and the amps. Droop speed contro l is directly proportional control in the strictest,

purest sense of the word. There is no integral action or reset to increase the fuel to make the actual

speed be equal to t he reference speed. The actual speed can not achieve reference speed because it is

physically not possible. Therefore, droop speed control take this disadvantages to stably control fuel in

proportion error. The error can occur either one of two reasons which is a change in the speed reference

or a change in actual speed. When the grid f requency or t he generator speed change, the control systemautomatically responds to the change because the error changes and adjusts the fuel to try to

compensate f or t he change in actual speed relative to the speed reference. The erro rs between speed

reference and actual speed for all generators within large inf inite grid are f airly constant. When the

f requency change, the error also change because the f requency is proport ional to the generato r speed

hence it will change error o f speed as well. If the f requency in the grid decreases, then the error

increases t his will lead increases in f uel of all the machines because they are all connected together to

the grid. Each prime movers governor will respond to a change of f requency as a f unction of the amount

of droop that the cont rol system has been programmed. A 1% change in the f requency of the machine

with 5% droop will result a 20% change in load, nominally, supposing the machine was running at 80% of

load or less to begin with. A unit with 4% droop will respond with a 25% change in load, nominally, againpresuming the machine was running at 75% or less than rated load to begin with. the prime mover can not

increase its output power f urther if the machine is operate at rated power output on droop speed control

whenever the f requency is decrease. The prime movers always operat ing at rated power output when

interconnected in the grid system. Even though they are in droop speed control mode they can not add

additional load b increasin their ower out ut when the rid f re uenc decreases. Whenever a machine


12/23

which is to have 5% droop speed control will normally reach rated output when the speed ref erence

reaches approximately 105% of rated speed. A machine with 4% droop will reach its rated power when

the speed reference is 104%. The power produced by the generator heavy duty gas turbine operating on

most fuels is generally directly proportional to the fuel flow rate. The rate of fuel injected is directly

proportional to the f uel stroke ref erence (FSR), which is a reference for t he opening of the f uel contro l

valve and/or the fuel flow rate through the control valve. The purpose of droop speed control primarily

is to allow a prime mover and its generato r to smoothly and stably share in supplying the load of a grid

while operating in parallel with other prime movers and their generators. The secondary purpose o f

droop speed control is t o help to maintain grid f requency when it varies f rom desired. When the gridf requency decreases because of the power imbalance (generally the amount of load is greater than

generation), hence droop speed control increases the output to try to help support the local load at

particular time. The amount o f generation must be balance with the amount o f the load.

Droop speed control is referred to as proportional contro l. The amount of power produced is directly

proportional to t he error between the turbine speed reference and actual speed. This is explaining that

how the fuel is cont rolled during parallel operat ion with other prime movers and generator connected to a

grid supplying a load. Prime movers mostly uses something similar, it is not only for s table contro l of f uel

f low f rom no load to rated load based on simple parameters, but it is also usef ul when trying to maintain

load on a grid when the grid f requency is not at the rated output. The speed error will decrease when

ever the grid f requency increases above rated. Droop speed contro l does not try to make the actual

speed of generator equal to t he speed reference. It is based on the f act that there will be an error

because under normal circumstances the grid f requency is stable at 100% and therefore, the actual

turbine speed is s table at 100%, and the amount o f f uel flow is proport ional to the error o f speed which

is dif f erent o f actual speed and reference speed. A machine which have 4% droop set ting in generally

new and clean conditions being operated at ambient temperatures less than nameplate rated will usually

reach exhaust temperature contro l or Base Load at T NR greater t han 104% and the load will be greater

than nameplate rated value. Generally f or every 1% change in TNR the unit power output will increase by

approximately 25% of rated power output and this value is corresponding to the 4% droop speed

control. In the case of a machine is not new or in other word in dirty compressor, high inlet f ilter

dif f erential pressure and increased to lerances in the axial compressor and/or turbine sections, will not beas ef f icient and the change in load f or t he change in TNR will be slightly depending on the severity of the

condit ion less than specif ied values. A machine with 4% droop speed cont rol might have only 23%

increases power output and 1% change in speed error. Therefore, with unclean condition the machine will

not be able to achieve optimal result. Droo p speed cont rol is only part of the change in the power output

relative to the speed error, and that portion is related to rated power output not actual power output

under less than rated conditions. The desired rated power output o f the machine is only achieve if the

machine in new and clean condition. The main purpose of the droop speed contro l is to contro l prime

mover governor to allow a prime mover and its generator to smoothly and stably share in supplying the

load of a grid wile operating in parallel with other prime movers and their generators .

2.4 ESTIMATION MAGNITUDE OF DISTURBANCES (EMD)

In this pro ject t he auxiliary indicator to determine the magnitude of disturbance is rate of f requency

change, df/dt. The power imbalance is proportional to the rate of frequency change, df/dt. Therefore we

can recognize what kind of the disturbance the system experience to predict the action to be taken. If the

disturbance experience is large , the rate of change of f requency is smaller and if the disturbance applied

is small, the rate o f f requency change is large. In the other word, the power imbalance or dist urbance is

inversely proport ional to the rate of f requency change, df /dt. Under f requency relay is very sensitive to

the f requency deviation. If the power in the system is not equal which is imbalance between power

demand and power supply, the frequency will decline proportionally with the power imbalance. This

f requency decline will establish slope, t hese s lope will be using as indicator either the disturbance is

large, medium or small. From this point of view, we can use the value of rate o f f requency change as

threshold to shed the load. To shed loads t he value of rate of change of f requency is included in several

range. Theref ore, we can classif ied what suitable range of rate of f requency change for small, medium

and large disturbance. In this project the instantaneous value of df /dt is taken as auxiliary indicato r. After

the disturbance has been recognize, the appropriate load will be shed simultaneously. Therefore, t he


13/23

f requency is bring back to the acceptable level rapidly without introducing under shedding or over

shedding.

Large Disturbance: this diagram shows that the sharp drop of the f requency indicate that the islanding

experience large disturbance.

Medium Disturbance: this diagram shows that the f requency drop is s lightly faster and it is classif ied as

medium disturbance because the f requency does no t f all as f ast as in large disturbance.

Small Disturbance: this diagram shows that the f requency drop is s lower than the medium disturbanceand this kind of disturbance is classif ied as small disturbance.

The rate of the f requency change is calculated by determine the value of the slope of f requency drop,

the equation is shown below.

The short time interval, will give more accurate value. In the other word t2 and t1 must be in the short

duration to get accurate value.

s1 is representing the small disturbance which is the slope is bigger because the f requency is f all slowly.

s2 is representing the s lightly greater f rom small disturbance, which is t he slope is slightly smaller thanthe small disturbance.

s3 is represent ing the medium disturbance which is the slope is quite small because the f requency is

drop quite f aster.

s4 is represent ing the s lightly greater f rom medium disturbance, which is the slope is s lightly smaller than

the medium disturbance.

s5 is representing the large disturbance which is the slop is very small because the f requency is drop

rapidly.

CHAPTER3: METHODOLOGY

3.1 INTRODUCTION

ADAPTIVE UNDER FREQUENCY LOAD SHEDDING SCHEME

The method that employed in this project is adaptive LD df /dt characteristic scheme. In this method the

amount of load to be shed is a f unction o f df /dt variable. The load to be shed in based on the value of

the df/dt. Because rate of change of frequency of the system will determine the magnitude of the

disturbance, hence load to be shed is proportional to the value of the df /dt. Adaptive LDdf /dt is very

powerful characterist ic to achieve proper load shedding. Whenever large dist urbance occur, this scheme

will shed large load and in the case of small disturbance occur, small load will be shed and the same

cases f or medium disturbance. In other word, this load shedding scheme will introduce proper load

shedding and enhancing the power system reliability without intro ducing large loss. This t ype of scheme

is designed several times f or dif f erent levels o f df /dt. The adaptive Under Frequency Load Shedding

Scheme is specially designed to encounter f or any level of disturbance, with its related df /dt, minimum

frequency of the system does not fall to the below a certain value. This method is designed for each

value of df /dt a suitable value of LD is calculated where LD is the amount of the load to be shed.

3.2 ADAPTIVE LD -df /dt CHARACTERISTIC SCHEME.

1. DETERMINATION OF THE EXPECTED OVERLOAD:

The expected overload will determine the amount o f the pro tection is to be provided. The value is

get f rom the following formula.

2. DETERMINATION OF THE NUBMBER OF LOAD SHEDDING STEPS:


14/23

In the adapt ive under f requency load shedding scheme, the load will be shedding simultaneo usly

based on the magnitude of the disturbance and it will result f aster recovery of f requency decline.

Compared to the conventional load shedding, the take a few step to shed load and it will result

under shedding or over shedding because they shed constantly regardless what kind of the

disturbance it is applied. Hence it will result s lower f requency recovery. In adaptive under f requency

load shedding scheme, the main powerful too l is the rate o f f requency change df/dt variable, so

the load to be shed is programmed based on the several range of the df /dt value.

3. DETERMINATION OF THE AMOUNT OF LOAD TO BE SHED:

The f irst s tep is to calculate the amount of load to be shed to maintain f requency above minimum

permissible f requency f or maximum expected overload. Total amount of load to be shed is

calculated by the f ollowing equation:

Where

LD = to tal load that must be shed

L = expected overload

f = minimum permissible frequency

d = load reduction f acto r

f n = nominal f requency (50Hz)

The value of the load must be shed is included in the several range of the rate o f the f requency

change. Because LD value is proportional to t he value of the rate of f requency change. The

disturbance is large if t he value of the rate of f requency change is large and directly the load to be

shed is also large. Theref ore, the value of large load t o be shed is included in the large value of

df /dt interval. Adaptive under f requency load shedding scheme will be shed the load simultaneously

or lump sum based of the magnitude of the disturbance. Compare to the conventional underf requency load shedding scheme, the load to be shed is divide into several percentage and shed

f ollowing several steps. This method will lead to the under shedding or over shedding.

LD df /dt CHARACTERISTIC SCHEME:

This characterist ic scheme is very important part in adaptive under f requency load shedding

scheme. This part will distinguish between the conventional and adaptive under frequency load

shedding scheme. In this method the amount of the to be shed is in the function of the df/dt. The

characterist ic of LD df/dt is shown below.

From the diagram, this method indicates that the load to be shed is in a function of df /dt. If thesmallest the value of the df /dt the larger the load is to be shed. For example, Df2 is large

disturbance and its value is smaller, the load to be shed is LD3 which indicate that the largest load

is to be shed. This correlation is explaining that when the large disturbance occur, the f requency

decline is very f ast which result the smaller value of df /dt and this indicate that the larger value of

load must be shed to prevent excess f requency decline. In the case of Df1, the load to be shed is

LD2, this indicate when medium magnitude o f disturbance occur, the medium value of load is to be

shed. The same case f or the small disturbance.

3.2.1 ALGORITHM

1. START

2. Insert values of

Generation power, pg

Frequency, f


15/23

Power load1, pload1

Power load2, pload2

Power load3, pload3

Power load4, pload4

Power load5, pload5

Power load6, pload6

Power load7, pload7

Power load8, pload8

Power load9, pload9

3. Calculate value of to tal power load, pt

Total power load, pt = power load4, pload4 + power load5, pload5 + power load6, pload6 + power

load7, pload7

+ power load8, pload8 + power load9, pload9

Or

Total power load, pt =pload4+pload5+pload6+pload7+pload8+pload9

Pload1 = pload4+pload5

Pload2=pload6 +pload7

Pload3 =pload8+pload9

4. Calculate expected overload, L

or

5. Calculate d and LD

6. Calculate rate of f requency change, df /dt

7. Set several range of rate of change of f requency change, df /dt

df /dt=s1=-0.0001 fo r small disturbance which is large value of rate f requency drop

df /dt=s2 =-0.002 f or slightly bigger than smaller disturbance which is s lightly less than s1

f /dt=s3=-0.004 for medium disturbance which is medium value of rate of f requency drop

df /dt=s4=-0.005 f or slightly large than medium disturbance which is slightly smaller than s3

df /dt=s5=-0.007 for large disturbance which is give smaller value of rate f requency change

The rate of f requency change is divided into several range

if s2< df/dt< p="">

if s3< df/dt< p="">

if s4< df/dt< p="">

if s5< df/dt< p="">

if df /dt>s5

8. The value of the load to be shed, LD is compared to the load which is selected to shed. If the load


16/23

to be shed is less or equal to the value of the selected load, the corresponding breaker of the

load will be tripped.

If LD


17/23

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HVDC, SVC, and other FACTS cont rollers

Wind source, turbines and governors

PSCAD, and its simulation engine EMTDC, have enjoyed close to 30 years of development,

inspired by ideas and suggest ions by its ever strengthening, worldwide user base. This

development philosophy has helped to establish PSCAD as one of the most powerf ul and intuitive

CAD so f tware packages available.

3.3.2 INTERFACING C CODE TO THE PSCAD SOFTWARE

In this project C language programming is used to give better calculation result. PSCAD itself isusing FORTRAN as a compiler, so that C code have to interface with PSCAD with several method.

1. PASTE C CODE FILE TO PROJECT

- First, we have to copy C code file fromCINTERFACE example, and paste it inside our project.

C code is written inside this C code f ile. Any programmed we want to write is written inside the C

code f ile f or C language only.

The ways we write C code inside the C code f ile is slightly diff erent f or make it readable by the

PSCAD so f tware. The header must be include * whether it is type of double or f loat or

integer. Anything is not come from PSCAD must beattach with the*.

NOTE: what ever variable we want t o used inside the pro gram, must be declare in the header. If not

it may cause error.

The variable is attach with * to make the readable by the PSCAD so f tware.


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2. CREATE NEW COMPONENT

-New component is created to determine how many input and output we desire to do. Usually the

lef t hand side is input and right hand side is the output.

3. CALL C SUBROUT INE

- The way we call c subroutine is by call c code f rom the c code f ile through the new component

script.

Write header in the c code f ile like f ollowing header

And call c code through the new component script

Therefore, anything program writt en in the c code f ile will be read by calling through new

component script. The flow of operation is shown below.

3.3.3 TAKE VALUE FROM CONTINUOUS DATA.

In the PSCAD sof tware the value input we want to use is must be sto red or taken to ensure the

right value is use f or programming C. the way to store data is explain below.

1) Create new component

The new component f or sto re the data is diff erentiating with the new component f or interf acing

with C Code. The input is set accordingly with the input o f the C interf ace component, this is

because the sto red data is using for t he input of c code interface. And obviously the output of this

new component is the same with the input of the c interface component.

2) Write at what time we want to read the input at new component script.

This f igure shows that the value for L4 will be read from 9 second onward because the time is set

greater or equal to 9 second (GE is in Fort ran mean greater or equal). In the case we want thevalue at instantaneous time, we can set the t ime is equal to ==. And this data will be send t o

the input o f the c code interf ace. The data we want to stored is address f rom the generated

FORTRAN compiler like shown below.

The address of the data is t aken fro m this diagram. Cautious must be taken in the writing program

to store data FORTRAN is used.

The whole process of interf acing and storing data is shown above; this f low process is easily to

understand and save time to understand. For the summary of this part, the data is catch or s to red

f rom the meter by call the address of the variable and set the t ime in the FORTAN script when we

want t o start sto ring the data. This data is used f or t he input of the calculation in the c codeprogramming to run the process and af ter that the output is send to t he channel and can be

displayed in the graph or control panel.

3.4 SIMULATION MODELLING

In this project, the network modeling is in single phase diagram. The Grid system comprises of

3BUS, BUS1 BUS2 and BUS3 is f eed 33KV, 11KV and 11KV respect ively. The load is connected to

the BUS3 which is 11KV f eeder. Six load is connected to t he same BUS to get power supply. The

dist ributed generat ion (DG) is rated at 4.51MVA or 1.0 p.u. When ever fault occur at the grid

system, BREAKER1 is t rip and the load is islanded to prevent f rom the total blackout .

3.4.1 MEASURING DATA

Figure above shows t hat the power generation, voltage, f requecy, to rque, power load and breaker

1 is monito red and measured. Those variable are measure their data through the channel and can


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be displayed by using graph f rame or contro l panel. Next s tep is take the data or storing the data

to be use.

3.4.2 STORING DATA

Figure above is block diagram of new component created to t ake value f rom the cont inuous data.

the data is taken from the meter which read the variable data. From f igure, pload1 to pload9,

w2(f requency) and Pdg1(power generation) is input to the new component f or s to ring the input

data. the output o f the new component is the data which is has been sto red and ready to be use inprogramming.

3.4.3 BREAKER CONTROL

Figure 3.4.30 above shows the block of control created f rom new component. This diagram is

illustrate how the breaker is contro l f rom the C Code. The input f rom the continuous data is

process inside the block and give the output in the right hand side. The program will determine

which breaker will be trip depending on the what kind of disturbance it is applied. The OR gate is

f unction to hold the state o f breaker af ter load shedding, theref ore the load has been shed is

maintain shed.

3.4.4 GENERATOR SPEED CONTROLLER

Figure 3.4.40 above is to rque-speed contro ller. The current f requency is compared with the speed

reference which is 1p.u or 50Hz. This comparison will produce error if the speed or f requency

deviate f rom the nominal speed, and this error will send to t he droop speed cont rol to produce

more power to bring back the f requency to the nominal. In this pro ject the droop contro l is 5%

which is generate excess 20% o f the rated generation. And the power generated is compared with

the rated speed reference to ensure that t he speed or f requency is between in the acceptable

level. And f inally this s ignal is send to adjust the t orque directly change the speed. This contro ller

will run in its droop mode when ever the BREAKER1 is trip which is indicate the network is islanded

and speed will decrease. In the normal condition the speed error is does not exist and thegeneration is constant.

3.4.5 DERIVATIVE OF FREQUENCY (df/dt)

Figure 4.4.50 shows how the derivative of f requency is obtain f rom the continuous data. Frequency

is input and the FORTRAN comment line is set to derivative and the output is derivative of

f requency or the other word is rate o f f requency change, df/dt. The rate of change is set to 0.001

second which mean the time interval is set to 0.001s. And this time interval will give the accurate

answer of the frequency derivative.

CHAPTER4:

4.1 CASE 1(For 0.9 p.u of power generation)

DISCUSSION

In this case po wer generat ion is 4.09MVA and the power load is 5.25MW. The power generated is

0.9 f rom the rated power. There is imbalance of power exist and some load has to be shed to

match with the power generated. The expected overload, L is calculated and the value is 0.2934

which is indicate that the loss of 23 percent of to tal generation. The load to be shed is 1.1623MW

which indicate that this amount of power must be shed to bring back the f requency to nominal.This amount is the minimum number of load must be shed. From the f requency graph, there is

slope exist and it is indicate that there is power imbalance. For this case the rate of f requency

change measured is -0.0003 Hz/s which is mean the f requency is drop -0.0003Hz in a second. This

value is not critical yet and hence it is consider as a small disturbance. Theref ore, t his value of

f requency change is f all at t he f irst interval which is between -0.001 and -0.0001. if the load


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shedding is not initiate the f requency drop at 45Hz which cause turbine blade damage. And by

initiate load shedding scheme the f requency drop at 49.6Hz which is in the saf e mode. The

appropriate load to be shed is properly chosen. Because the priority load can not be shed, in this

modeling network, the priority load is load 6 which is the larger load in this network. To choose load

to be shed, the power imbalance is calculated and we got 1.19MW. This number of load must be

shed either equal or slightly equal. For this case the closer load is power load1 which have 1.2MW.

Theref ore, power load1, load4 and load5 are shed by tripping breaker 2, 5, and 6 respect ively. All

the loads to be shed are shed simultaneously by tripping the breaker at the same time set ting.

Therefore, t he f requency will recover f aster and bring back to the acceptable level. So, the remainpower load is 4.05MW which is approximately equal to the po wer generation which is 4.09MW.

Hence, the f requency is bringing back to the nominal value. Network diagram shows that the

corresponding load is shed. The breaker is trip by give signal logic 1 to the breaker. Figure of 4.10,

4.11 and 4.12 shows the behavior o f f requency when without load shedding, with load shedding

and comparison between them respectively. The f requency without load shedding f inish at slightly

below load shedding f requency because the f requency can not bring back to nominal due to

generator t rip. Load shedding succeed to bring the f requency rise at 49.6Hz to prevent f rom to tal

blackout. Figure 4.13 illustrate simulation model after load shedding.

4.2 CASE 2(. For 0.8 Power Generation)

DISCUSSION


0.8 f rom the rated power. There is imbalance of power exist and some load has to be shed to



which indicate that this amount of power must be shed to bring back the f requency to nominal.

This amount is the minimum number of load must be shed. From the f requency graph, there is


change measured is -0.00217 Hz/s which is mean the f requency is drop -0.00217Hz in a second.This value is quite critical and hence it is consider as a slightly greater than small disturbance.

Therefore, this value of f requency change is f all at the second interval which is between -0.004

and -0.002. if the load shedding is not initiated the f requency is drop at 42Hz which cause the

turbine blade damage. And by initiate load shedding scheme the f requency will drop at 48.9Hz which

is in the saf e mode. The appropriate load to be shed is properly chosen. Because the priority load

can not be shed, in this modeling network, the priority load is load 6 which is the larger load in this

network. To choose load to be shed, the power imbalance is calculated and we got 1.642MW. This

number of load must be shed either equal or s lightly equal. For t his case the closer load is total of

power load1 and power load8 which have 1.2MW and 0.45MW respectively. Theref ore, power load1,

load4, load5 and load8 are shed by tripping breaker 2, 5, 6 and 8 respectively. All the loads to be

shed are shed s imultaneously by tripping the breaker at the same time set ting. Theref ore, the

f requency will recover f aster and bring back to the acceptable level. So, the remain power load is

3.6MW which is approximately equal to the power generat ion which is 3.608MW. Hence, the

f requency is bringing back to the nominal value. Network diagram shows that the corresponding

load is shed. The breaker is t rip by give signal logic 1 to the breaker. Figure 4.20, 4.21 and 4.22

shows the behavior o f f requency when without load shedding, load shedding and comparison

between them respectively. Load shedding brings the f requency rise at 48.9Hz and go back to the

nominal level. The f requency is not exactly 50Hz but slightly below that. Figure 4.23 illustrate

simulation model af ter load shedding.

4.3 CASE 3(For 0.7 Power Generation)

DISCUSSION


0.7p.u f rom the rated power. There is imbalance of power exist and some load has to be shed to


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This amount is the minimum number of load must be shed. From the f requency graph, there is


change measured is -0.00409 Hz/s which is mean the f requency is drop -0.00409Hz in a second.

This value is critical and hence it is consider as a medium disturbance. Theref ore, t his value of

f requency change is f all at the t hird interval which is between -0.005 and -0.004. if the load

shedding is not initiated the f requency is drop at 38Hz which cause the turbine blade damage. Andby initiate load shedding scheme the f requency will drop at 48.2Hz which is in the safe mode. The

appropriate load to be shed is properly chosen. Because the priority load can not be shed, in this

modeling network, the priority load is load 6 which is the larger load in this network. To choose load

to be shed, the power imbalance is calculated and we got 2.093MW. This number of load must be

shed either equal or slightly equal. For t his case the closer load is total of power load1 and power

load3 which have 1.2MW and 1.05MW respectively. Theref ore, power load1, load4, load5, load3,

load8 and load9 are shed by tripping breaker 2, 4, 5, 6, 9 and 10 respectively. All the loads to be

shed are shed s imultaneously by tripping the breaker at the same time set ting. Theref ore, the

f requency will recover f aster and bring back to the acceptable level. So, the remain power load is

3.0MW which is approximately equal to the power generat ion which is 3.157MW. Hence, the

f requency is bringing back to the nominal value. Network diagram shows that the corresponding

load is shed. The breaker is t rip by give signal logic 1 to the breaker. Figure 4.30, 4.31, 4.32 shows

the behavior of f requency when without load shedding, load shedding and comparison between

them respectively. For this case, frequency with load shedding is slightly odd, because the gap

between load shed and power generation is large which 0.157MW is. so the behavior o f graph is

inf luence by the response of the generator. Fort unately the f requency is still can be bring back to

acceptable level. Figure 4.33 illustrate s imulation model af ter load shedding.

4.4 CASE 4(For 0.6 Power Generation)

DISCUSSION


0.6p.u f rom the rated power. There is imbalance of power exist and some load has to be shed to




This amount is the minimum number of l

Power System Blackout

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Transcript of Power System Blackout