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
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
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 = (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
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
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
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
5. Define hydrothermal scheduling problem?
The objective is to minimize the thermal generation cost with the constraints of
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
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
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
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
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
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.
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
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
15. Explain the mathematical technique for hydro thermal scheduling problem?
16. Explain about system equivalency?
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