1.Control Centre Operation of Power Systems

Introduction

o The power system of today has evolved from its early stage of point

to point transmission system into a complex interconnected system.

This has made the control and operation of the system quite a

sophisticated task. Hence, in order to monitor and control such a vast

network, human expertise coupled with computer assistance became

inevitable.

o A power control center is thus formed where information from the

entire system is collected and analyzed. The analyzed results are then

used for taking proper decisions regarding the operation of the entire

system, such as allocating generation level at each generation stations,

managing load dispatch and protecting vital installations.

o The main function of the control center is:

(a)To collect the information, such as voltage, current, frequency,

transformer tap settings, etc, through various communication

channels.

(b)To coordinate the power transferred/shared among various

loads.

(c)To coordinate the response of network elements in both normal

and emergency conditions .

o Power system control involves:

(1) Ensuring power quality: The system’s parameters such as voltage

and frequency are to be maintained within the specified limits.

(2)Economic operation: The energy should be supplied with minimum

loss and cost.

(3)Ensuring adequate supply of active and reactive power: In order to

meet the continually changing load demand for active and reactive

power, adequate “spinning” reserve of active and reactive power should

be maintained and appropriately controlled at all times.

Note : The power control center is so designed that system operation is

possible without the power control center.

Subsystems of a power system

Note: simplify the above diagram as done in sir notes.

The subsystem of a power system consists of the following:

(1) Generating unit controls: It consists of prime mover, speed governing

system, generator, and excitation system.The function of the speed

governing system is to sense changes in speed of the prime mover and

accordingly increase/decrease decrease the speed of the prime mover in

order to regulate the frequency. The function of the excitation system is

to control the reactive power generation. It senses the generator’s

voltage and accordingly adjusts the excitation to regulate the voltage.

(2) System generation and control: In a system there should be adequate

generation to meet all kinds of load demand. The primary function of the

system generation and control is to balance the mismatch between the

load demand and the generation so that the desired frequency and power

interchange with neighbouring systems(i.e, tie line flows) is maintained.

(3) Transmission controls: Transmission controls include power and

voltage control devices, such as static var compensators, synchronous

condensers, switched capacitors, tap changing transformers, phase-

shifting transformers, and HVDC transmission controls.

Operating states of a system

The various operating states of a system are as follows:

(1)Normal state: All the system variables are within the specified levels

and the system is in a secure state. Moreover, the system will remain in

stable condition even after a contingency.

(2) Alert state: In this state, the system is said to be conditionally stable

as long as there is no contingency. The stable system may become

unstable after a contingency.

(3) Emergency state: The system enters the emergency state if a

sufficiently severe disturbance occurs when the system is in the alert

state. The voltages at several buses may be low and the generators may

be overloaded. The system in the emergency state can be restored to the

alert state by initiating actions such as fault clearing, excitation control,

generation tripping, etc.

(4)In extremis state: The system enters the in extremis state when the

system cannot be restored to the alert state. This state can result in

cascading outages and possibly a shutdown of a major portion of the

system.

(5)Restorative state: In the restorative state, some control actions will be

initiated to reconnect all the facilities and restore system load. Attempts

will be made to restore the system back either to either the alert state or

the normal state.

Hierarchial representation of a power system

At the lower level controllers such as plant controllers such as power

plant controllers and transmission plant controllers directly control the

the operation of devices such as boilers,steam valve,transformer tap

changers,etc.there is usually some form of overall plant controller that

coordinates the controls of closely linked elements.The plant controllers

are supervised by system control centre ,which in turn is supervised by

the pool control centre.

SCADA systems provide information to indicate the system status. State

estimation programs filter the monitored data and provide an accurate

picture of the system’s condition. The human operator plays an

important role at various levels in this hierarchy. The primary function

of the operator is to monitor system performance and manage resources

so as to ensure economical operation while maintaining the required

quality and reliability of power supply.

SCADA

Follow sir notes, for more information refer your kusic T.book chapter1.

Automatic generation control

In an interconnected power system, automatic generation control and

economic load dispatch are two significant areas of concern for

generation control.

Automatic generation control (AGC): Automatic generation control is an

on-line computer control that maintains the system frequency at or very

close to the specified nominal value (50 Hz) as well as the net

interchange of power(tie-line interchange) between the control areas.

The common practice is to carry out generation control on a

decentralized basis. That is, each individual area tries to maintain its

scheduled interchange of power.

Economic load dispatch (ELD): It is also an on-line computer control,

whose function is to assign the generation level to each of the generators

in order to share the system load in the most economical manner.

Several constraints (e.g., pollution control, etc) are to be satisfied while

carrying out economic load dispatch.

Area control error

To maintain the net interchange of power of an area with its

neighbouring areas, an AGC uses real power flow measurements of all

tie lines emanating from the area and subtracts the scheduled

interchange value from it. This error value, along with a gain B(called

frequency bias) as a multiplier on the frequency deviation, f, is called the

area control error(ACE) and is given by

ACE= ∑PK−¿PS¿ + 10B (f act−f 0) MW

where

K= particular tie line

GA= MW tie line flow out of the area (defined as positive)

PS= Scheduled MW interchange

f 0= Scheduled base frequency

B =Frequency bias in MW/0.1 Hz

A positive ACE represents a flow out of the area. Fig(a) shows the use

of ACE signal as an input to AGC to control generation.

Wheeling power

Area D

Area BArea A

Ps=0

Ps=0

Ps=+P ACE=0

Ps=-PACE=0Area C

Fig(a) Transfer of power from area A to area C

Consider the four area power pool shown in fig(a).If the power is

transferred from area A to area C, area A would introduce a scheduled

interchange, Ps= +p, into its own ACE so that power flows out of area A

until its AGC forces ACE to become zero. Simultaneously, area C would

also introduce a scheduled interchange, Ps=-p, into its own ACE so that

power flows into area C until its AGC forces ACE to zero. Notice that

the areas B and D participate in this transfer of power, but power is not

consumed in them. This is because input and output tie line powers for

areas B and D are equal and opposite, hence ACE=0 in both the cases.

Thus, we see that some power is transmitted via the areas B and D,

but not consumed by them. This power which is transmitted via an area

but not consumed by it, is known as wheeling power.

Objectives and functions of AGC:

Objectives:

(1)To maintain overall system frequency at or very close to the specified

nominal value(50 Hz).The maximum permissible change in power

frequency is ± 0.5 Hz.

(2) To maintain the correct value of net interchange of power between

control areas.

(3)To maintain each unit’s generation at the most economic value.

Functions:

(1)The AGC brings about net interchange of power between areas on a

scheduled basis.

(2)The AGC senses the area control error(ACE), which is the measure of

the error in net interchange of power of an area with its neighbouring

areas from the desired/scheduled net interchange value,and accordingly

sends control signals to generating units to increase/decrease generation

so as to obtain the desired or scheduled flow value. This is done until

ACE becomes zero.

(3)The ACE also contains the change in frequency term, i.e. the

deviation of the system frequency from the scheduled frequency. Thus,

AGC also regulates the system frequency by sending control signals to

the generating units.

Tie-line

In interconnected power systems, the neighbouring power systems are

interconnected by one or more transmission lines called the tie lines, as

shown in fig(a).

Area KArea K

Area 1 Area 2

Tie-line

Fig.(a) Typical interconnected power systems

Tie lines are employed due to the following reasons:

Tie lines allow exchange of power between the areas on a

scheduled basis as forced by the AGC.

Tie lines allow areas experiencing disturbances to draw on other

areas for help.

Tie lines provide a long- distance transmission link for the sale and

transfer of power.

Operation without central computers(or AGC)

The speed governing system built for the turbine-generator set and

the excitation system for the generator allows the operation of power

systems without a central computer.

Any change in load will force generators within neighbouring areas

to share load.

Fig.(a) A simple interconnected system

Consider a simple interconnected system as shown in fig(a).Assume

that the breaker is open and there is no tie line flow between areas A

and D. Let the area A overall generation-frequency characteristic br

represented by curve GG in fig.(b).

Frequency(Hz)

GGeneration(MW)

49.5

50.0

50.5

0G

A

fig(b)generation- freq. characteristic curve

The generation frequency characteristic curve is described by the

equation

GA=G0+ 10β1(f act-f 0)

Similarly, the overall load- frequency curve for area A can be

represented by curve LL in fig(c).

L

Frequency(Hz)

49.5

50.0

50.5

LA

0(f )

Load(MW)

The load-frequency characteristic curve is described by the equation

LA=L0+10β2(f act-f 0)

Effect of load increase in area A

The operating frequency is determined by the point of intersection (Io) of

the GG and LL curves,shown at 50 Hz in fig(d).

fig(d)

GG=gen freq. plot

LL=Load freq. plot

CC=combined area characteristic

Now, assume that there is load increase in area A,which shifts LL to the

new position L’L’. The intersection of L’L’ and GG gives the new

operating point(I1) at 49.9 Hz.

In order to restore the operating frequency to 50 Hz,GG is shifted to the

new position G’G’. The intersection of L’L’ and G’ G’ gives the new

operating point (I2) at 50 Hz. The resulting combined characteristic is

now shown by line C’C’.

The combined characteristic is given by

GA−LA=G0+ 10β1(f act-f 0¿−L0−¿10β2(f act-f 0)

This can be rewritten in terms of increments

ΔA= (GA−G0) + (L0−LA)= 10β1(f act-f 0) −¿10β2(f act-f 0)

=10BA X A ¿¿-f 0)

=10BA X AΔf

Where BA=¿Natural egultion characteristic of area A

X A=¿ Generating capacity of area A

or Δf = ΔA /(10BA X A ¿

Effect of tie line flow

Consider the interconnected system shown above, with breaker T closed.

Suppose the area D experiences a disturbance due to which the systems

freq. drops from 50 Hz to 49.9 Hz. Now, the power generation no longer

matches with the load demand in area A and the difference between

them is defined by the difference between intercepts I2 and

I1,respectively, of the GG and LL curve with the 49.9 HZ line, as shown

in fig(a).

If ΔLArepresents the decrease in load in area A and ΔGA represents the

increase in generation in area A, then the tie-line flow between A and D

is

ΔT L=ΔGA- ΔLA MW

If area A has an AGC that applies frequency bias B, GG will be shifted

to the new position G’G’, resulting in a large tie line flow ΔT L' .

Derivation of ΔT L and Δf

Consider the interconnected system shown above, with breaker T closed.

Let area D experiences a disturbance ΔD.Now,the freq. change due to

disturbance ΔD and tie line flow from A to D, is given by

Δf = ( ΔD−¿ ΔT L¿/(10BD XD) ………..(1)

(ΔT L is taken as negative since it flows into the area D)

Also, with respect to area A, we have

Δf = ΔT L /¿10BA X A) ………..(2)

(sinceΔA = 0. Here, ΔT L is taken as positive

it flows out of the area A)

Since the frequency is same for both the areas,

Δf=( ΔD−¿ ΔT L¿/(10BD XD) = ΔT L /¿10BA X A) …….(3)

Tie line flow, ΔT L =¿10BA X A) ΔD /(10BA X A + 10BD XD ) ……(4)

Substituting eqn(4) in eqn.(2), we get

Δf = ΔD /¿(10BA X A + 10BD XD ) ……….(5)

Parallel operation of generators

Parallel operation of two dissimilar units

Fig(b)Parallel operation of two dissimilar units

Consider the case of two units of different capacity and regulation

characteristics operating in parallel,as shown in fig(b).The regulations R1

and R2 are

R1= Δf(p.u)/ΔP1(p.u) = (Δf/50)/( ΔP1/P1rated) pu

Δf=50R1( ΔP2/P2rated) ……(1)

R2 =(Δf/50)/( ΔP1/P1rated) ……(2)

The sharing ratio of the two units is obtained by dividing eqn (1) by (2)

(ΔP1/ ΔP2) = (R2/R1) x (P1rated/P2rated) ……(3)

If the initial load is P1.0 + P2.0, then a change in load will be,

ΔL= ΔP1+ ΔP2 = (Δf x P1 rated/R1)+ (Δf x P2 rated/R2) ……(4)

Thus, the equilvalent regulation of the paralleled system is,

Ŕsystem = Δf/ ΔL= 1/ [(P1rated/R1)+(P2rated/R2)] 1/MW

In terms of p.u.,

Rsystem= Ŕsystem(P1 rated + P2 rated) p.u