**EE2401 POWER SYSTEM OPERATION AND CONTROL**

**SHORT QUESTIONS AND ANSWERS**

**UNIT-I - INTRODUCTION**

**1. What is load curve?**

The
curve drawn between the variations of load on the power station with reference

to
time is known as load curve. There are three types, Daily load curve, Monthly load

curve,
Yearly load curve

**2. What is daily load curve?**

The
curve drawn between the variations of load with reference to varioustime

period
of day is known as daily load curve.

**3. What is monthly load curve?**

It
is obtained from daily load curve. Average value of the power at a month for a

different
time periods are calculated and plotted in the graph which is known as monthly

load
curve.

**4. What is yearly load curve?**

It
is obtained from monthly load curve which is used to find annual load factor.

**5. What is connected load?**

It
is the sum of continuous ratings of all the equipments connected to supply

systems.

**6. What is Maximum demand?**

It
is the greatest demand of load on the power station during a given period.

**7. What is Demand factor?**

It
is the ratio of maximum demand to connected load.

Demand
factor= (max demand)/ (connected load)

**8. What is Average demand?**

The
average of loads occurring on the power station in a given period (day or month
or year) is known as average demand.

Daily
avg demand = (no of units generated per day)/ (24 hours)

Monthly
avg demand = (no of units generated in month)/ (no of hours in a month)

Yearly
avg demand = (no of units generated in a year)/ (no of hours in a year)

**9. What is Load factor?**

The
ratio of average load to the maximum demand during a given period is known

as
load factor.

Load
factor = (average load)/ (maximum demand)

**10. What is Diversity factor?**

The
ratio of the sum of individual maximum demand on power station is known as

diversity
factor.

Diversity
factor = (sum of individual maximum demand)/(maximum demand).

**11. What is Capacity factor?**

This
is the ratio of actual energy produced
to the maximum possible energy that

could
have been produced during a given period.

Capacity
factor= (actual energy produced)/ (maximum energy that have been produced)

**12. What is Plant use factor?**

It
is the ratio of units generated to the product of plant capacity and the number
of

hours
for which the plant was in operation.

Units
generated per annum= average load * hours in a year

**13. What is Load duration curve?**

When
the load elements of a load curve are arranged in the order of descending

magnitudes
the curve then obtained is called load duration curve.

**UNIT-II – REAL POWER FREQUENCY CONTROL**

**1. What is the major control loops used in large generators?**

The
major control loops used in large generators are

1.
Automatic voltage regulator (AVR)

2.
Automatic load frequency control (ALFC).

**2. What is the use of secondary loop?**

A
slower secondary loop maintains the fine adjustment of the frequency, and also

by
reset action maintains proper MW interchange with other pool members. This loop
is

insensitive
to rapid load and frequency changes but focuses instead on drift like changes

which
take place over periods of minutes.

**3. What is the adv of AVR loop over ALFC?**

AVR
loop is much faster than the ALFC loop and therefore there is a tendency, for

the VR dynamics to settle down before they can
make themselves felt in the slower load

frequency
control channel.

**4. What is the diff. between large and small signal analysis?**

Large
signal analysis is used where voltage and power may undergo sudden changes

of
magnitude that may approach 100 percent of operating values. Usually this type
of

analysis
leads to differential equations of non-linear type. Small signal anaysis is
used when

variable
excursions are relatively small, typically at most a few percent of normal
operating

values.

**5. What is the exciter?**

The
exciter is the main component in AVR loop. It delivers the DC power to the

generator
field. It must have adequate power capacity and sufficient speed of response
(rise

time
less than 0.1 sec).

**6. What is the function of AVR?**

The
basic role of the AVR is to provide constancy of the generator terminal voltage

during
normal, small and slow changes in the load.

**7. Explain about static AVR loop?**

In a
static AVR loop, the execution power is obtained directly from the generator

terminals
or from the station service bus. The AC power is rectified by thyristor bridges
and

fed
into the main generator field via slip rings. Static exciters are very fast and
contribute to

proved
transient stability.

**8. Write the static performance of AVR loop?**

The
AVR loop must regulate the terminal |V| to within required static accuracy

limit.
Have sufficient speed of response. Be stable.

**9. What is the dis.adv of high loop gain? How is to be eliminated?**

High
loop gain is needed for static accuracy but this causes undesirable dynamic

response,
possibly instability. By adding series AND/OR feedback stability compensation

to
the AVR loop, this conflicting situation can be resolved.

**10. What are the effects of generator loading in AVR loop?**

Added
load does not change the basic features of the AVR loop, it will however

affect
the values of both gain factor Kf and the field constant. High loading will
make the

generator
work at higher magnetic saturation levels. This means smaller changes in |E|
for

incremental
increases in if, translating into the reduction of KF. The field time constant
will

likewise
decreases as generator loading closing the armature current paths. This

circumstance
permits the formation of transient stator currents the existence of which
yields

a
lower effective field induction.

**11. What are the functions of ALFC?**

The
basic role of ALFC‟s is to maintain desired MW
output of a generator unit

and
assist in controlling the frequency of large interconnection. The ALFC also
helps to

keep
the net interchange of power between pool members at predetermined values.
Control

should
be applied in such a fashion that highly differing response characteristics of
units of

various
types are recognized. Also unnecessary power output changes should be kept at a

minimum
in order to reduce wear of control valves.OM SAKTHI

**12. Specify the dis.adv of ALFC loop?**

The
ALFC loop will main control only during normal changes in load and

frequency.
It is typically unable to provide adequate control during emergency situations,

when
large MW imbalances occur.

**13. How is the real power in a power system controlled?**

The
real power in a power system is being controlled by controlling the driving

torque
of the individual turbines of the system.

**14. What is the need for large mechanical forces in speed-governing system?**

Very
large mechanical forces are needed to position the main valve against the high

stream
pressure and these forces are obtained via several stages of hydraulic
amplifiers

**UNIT-III - REACTIVE POWER -VOLTAGE CONTROL**

**1. What are the sources of reactive power? How it is controlled?**

The
sources of reactive power are generators, capacitors, and reactors.

These
are controlled by field excitation.

Give
some excitation system amplifier.

The
excitation system amplifiers are,

a)
Magnetic amplifier

b)
Rotating amplifier

c)
Modern electronic amplifier.

**2. When is feedback stability compensation used?**

High
loop gain is needed for static accuracy but this causes undesirable dynamic

response,
possibly instability. This conflicting situation is resolved by adding feedback

stabling
compensation to the AVR loop.

**3. Give the characteristics of line compensators?**

The
characteristics of line compensators are,

a.
Ferranti effect is minimized.

b.
Under excited operation of synchronous generator is not required.

**4. What is known as bank of capacitors? How it is adjusted?**

When
a number of capacitors are connected in parallel to get the desired

capacitance,
it is known as bank of capacitors. These can be adjusted in steps by switching

(mechanical).

**5. What is the disadvantage of switched capacitors are employed for compensation?**

When
switched capacitors are employed for compensation, these should be

disconnected
immediately under light load conditions to avoid excessive voltage rise and

Ferro
resonance in presence of transformers.OM SATHI

**6. What are the effects of capacitor in series compensation circuit?**

The
effects of capacitor in series compensation circuit are,

Voltage
drop in the line reduces.

Prevents
voltage collapse.

Steady
state power transfer increases.

Transient
stability limit increases.

**7. Give two kinds of capacitors used in shunt compensator?**

The
two kinds of capacitors used in shunt compensator are, a. Static Var

Compensator
(SVC) : These are banks of capacitors ( sometimes inductors also for use

under
light load conditions).

**8. What is synchronous condenser?**

It
is a synchronous motor running at no-load and having excitation adjustable over
a

wide
range. It feeds positive VARs into the line under overexcited conditions and
negative

VARs
when under excited.

**9. Write about Static VAR Compensator (SVC).**

These
comprise capacitor bank fixed or switched or fixed capacitor bank and

switched
reactor bank in parallel. These compensators draw reactive power from the line

thereby
regulating voltage, improve stability (steady state and dynamic), control

overvoltage
and reduce voltage and current unbalances. In HVDC application these

compensators
provide the required reactive power and damp out sub harmonic oscillations.

**10. What is Static VAR Switches or Systems?**

Static
VAR compensators use switching for var control. These are also called static

VAR
switches or systems. It means that terminology wise SVC=SVS. And we will use

these
interchangeably.

**11. Give some of the Static compensators schemes.**

a.
Saturated reactor

b.
Thyristor- Controlled Reactor (TCR)

c.
Thyristor Switched capacitor (TSC)

d.
Combined TCR and TSC compensator.

**12. What is tap changing transformers?**

All
power transformers and many distribution transformers have taps in one or more

windings
for changing the turn’s ratio.
It is called tap changing transformers.

**13. Write the types of tape changing transformers.**

a.
Off- load tap changing transformers.

b.
Tap changing under load transformers.

**14. What is the use of off-load tap changer and TCUL ?**

The
off- load tap changers are used when it is expected that the ratio will need to
be

changed
only infrequently, because of load growth or some seasonal change. TCUL is used

when
changes in ratio may be frequent or when it is undesirably to de-energize the

transformer
to change the tap.

**UNIT-IV – COMMITMENT AND ECONOMIC DISPATCH**

**1. Define economic dispatch problem?**

The
objective of economic dispatch problem
is to minimize the operating cost of

active
power generation.

**2. Define incremental cost?**

The
rate of change of fuel cost with active power generation is called incremental

cost.
Write the load balance equation? Pg-pd-pl=0.

**3. Define base point?**

The
present operating point of the system is called base point.

**4. Define participation factor?**

The
change in generation required to meet power demand is called as participation

factor.

**5. Define hydrothermal scheduling problem?**

The
objective is to minimize the thermal generation cost with the constraints of

water
availability.

**6. Define Uncommitment?**

Commitment
of minmum generator to meet the required demand.

**7. Define spinning reserve?**

It
is the term describe the total amount of generation availability from all units

synchronized
on the system.

**8. What is meant by scheduled reserve?**

These
include quick start diesel turbine units as well as most hydro units and

pumped
storage hydro units that can be brought online, synchronized and brought up to
full

capacity
quickly.

**9. What are the thermal unit constraint?**

Minimum
up time, minimum down time crew constraints.OM SATHI

**10. Define minimum up time?**

Once
the unit is running, it should not be turned off immediately.

**11. Define min.down time?**

Once
the unit is decommited, there is a
minimum time before it can be

recommended.

**12. Define crew constraints?**

If a
plant consist of two (or) more units, all the units cannot be turned on at the
same

time
since there are not enough crew members to attend both units while starting up.

**13. What are the two approaches to treat a thermal unit to operating temperature?**

The
first allow the unit boiler to cool down and then heat backup to operating

temperature
in time for a scheduled turn on. The second requires that sufficient energy be

input
to the boiler to just maintain operating temperature.

**14. What are the techniques for the solution of the unit commitment problem?**

Priority
list method dynamic programming Lagrange relation

**15. What are the assumptions made in dynamic programming problem?**

A
state consists of an array of units with specified units operating and the rest
of the

time.
The startup cost of a unit is independent of the time it has been offline.
There are no

costs
for shutting down the units.

**16. Define long range hydro scheduling problem?**

The
problem involves the long range of water availability and scheduling of

reservoir
water releases. For an interval of time that depends on the reservoir
capacities.

**17. What are the optimization technique for long range hydro scheduling problem?**

Dynamic
programming composite hydraulic simulation methods statistical

production
cost.

**18. Define short range hydro scheduling problem?**

It
involves the hour by hour scheduling of all generators on a system to achieve

minimum
production condition for the given time period.

**19. Define system blackout problem?**

If
any event occurs on a system that leaves it operating with limits violated, the

event
may be followed by a series of further actions that switch other equipment out
of

service.
If the process of cascading failures continues, the entire system of it may

completely
collapse. This is referred as system blackout.OM SAKTHI

**20. What is meant by cascading outages?**

If
one of the remaining lines is now too heavily loaded, it may open due to relay

action,
thereby causing even more load on the remaining lines. This type of process is
often

termed
as cascading outage.

**UNIT-V – COMPUTER CONTROL OF POWER SYSTEMS**

**1. What are the functions of control center?**

System
monitoring contingency analysis security constrained optimal power flow.

**2. What is the function of system monitoring?**

System
monitoring provides upto date information about the power system.

**3. Define scada system?**

It
stands for supervisory control and data acquisition system, allows a few
operators

to
monitor the generation and high voltage transmission systems and to take action
to

correct
overloads.

**4. What are the states of power system?**

Normal
state alert mode contingency mode emergency mode. Define normal mode?

The
system is in secure even the occurrence of all possible outages has been
simulated the

system
remain secure is called normal mode.

**5. Define alert mode?**

The
occurrence of all possible outages the system does not remain in the secure is

called
alert mode.

**6. What are the distribution factors?**

Line
outage distribution factor, generation outage distribution factor.

**7. Define state estimation?**

State
estimation is the process of assigning a value to an unknown system state

variable
based on measurements from that system according to some criteria.

**8. Define max. likelihood criterion?**

The
objective is to maximize the probability that estimate the state variable x, is
the

true
value of the state variable vector (i.e, to maximize the P(x)=x).

**9. Define weighted least-squares criterion?**

The
objective is to minimize the sum of the squares of the weighted deviations of

the
estimated measurements z, from the actual measurement.OM SATHI

**10. Define minimum variance criterion?**

The
objective is to minimize the expected value of the squares of the deviations of

the
estimated components of the state variable vector from the corresponding
components

of
the true state variable vector.

**11. Define must run constraint?**

Some
units are given a must run status during certain times of the year for reason
of

voltage
support on the transmission network.

**12. Define fuel constraints?**

A
system in which some units have limited fuel or else have constraints that
require

them
to burn aspecified amount of fuel in a given time.

**13. What are the assumptions made in priority list method?**

No
load cost are zero unit input-output characteristics are linear between zero
output

and
full load there are no other restrictions startup cost are affixed amount.

**14. State the adv of forward DP approach?**

If
the start up cost of a unit is a function of the unit is a function of the time
it has

been
offline, then a forward dynamic program approach is more suitable since the
previous

history
of the unit can be computed at each stage.

**15. State the dis.adv of dynamic programming method?**

It
has the necessity of forcing the dynamic programming solution to search over a

small
number of commitment states to reduce the number of combinations that must be

tested
in each period.

**16. What are the known values in short term hydro scheduling problem?**

The
load, hydraulic inflows & uit availabilities are assumed known. What is
meant

by
telemetry system? The states of the system were measured and transmitted to a
control

center
by means of telemetry system.

**17. What are the functions of security constraints optimal power flow?**

In
this function, contingency analysis is combined with an optimal power flow

which
seeks to make changes to the optimal dispatch of generation. As well as other

adjustments,
so that when a security analysis is run, no contingency result in violations.

**18. Define the state of optimal dispatch?**

This
is the state that the power system is in prior to any contingency. It is
optimal

with
respect to economic operation but may not be secure.OM SAKTHI

**19. Define post contingency?**

This
is the state of the power system after a contingency has occurred. Define

secure
dispatch? This is state of the power system with with no contingency
outages,but

with
correction to the operating parameters to account for security violations.

**20. What are the priorities for operation of modern power system**?

Operate
the system in such a way that power is delivered reliably. Within the

constraints
placed on the system operation by reliability considerations, the system will
be

operated
most economically.

**21. What is meant by linear sensitivity factor?**

Many
outages become very difficult to solve if it is desired to present the results

quickly.
Easiest way to provide quick calculation of possible overloads is linear
sensitivity

factors.

**22. What are linear sensitivity factors?**

Generation
shift factors line outage distribution factors.

**23. What is the uses of line distribution factor?**

It
is used to apply to the testing for overloads when transmission circuits are
lost.

**24. What is meant by external equvalencing?**

In
order to simplify the calculations and memory storage the system is sub divided

into
3 sub systems called as external equvalencing.

**16 MARKS**

1.
Explain the method availabilities for providing economic operation of power
system.

2.
Write short notes on load v curve load duration curve energy curve.

3.
Explain about spinning reserve, hot reserve, cold reserve.

4.
Explain the solution technology for solving priority list method by dynamic

programming
method.

5.
Explain about load forcasting & weather sensitive load model.

6.
Explain the static state estimation of power system.

7.
Explain the algorithm for system when operating non steady state condition.

8.
How to detect and identify the bad data?

9.
Derive the equation for loss coefficients?

10.
Explain about base point and participation factor?

11.
Explain the solution technique for solving hydro thermal scheduling problem?

12.
Explain the operating states of power system?

13.
Explain the preventive action taken for emergency and restorative control?

14. Write
short note on long range hydro scheduling problem short range hydro scheduling

problem.

15.
Explain the mathematical technique for hydro thermal scheduling problem?

16.
Explain about system equivalency?