Questions & Answers of Power Systems

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,

1. Magnetic amplifier
2. Rotating amplifier
3. Modern electronic amplifier.

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.

Give the characteristics of line compensators?

The characteristics of line compensators are,

1. Ferranti effect is minimized.
2. Under excited operation of synchronous generator is not required.

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).

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.

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.

Give two kinds of capacitors used in shunt compensator?

The two kinds of capacitors used in shunt compensator are,

1. Static Var Compensator (SVC) : These are banks of capacitors ( sometimes inductors also for use under light load conditions).

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.

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.

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.

Give some of the Static compensators schemes. a. Saturated reactor

1. Thyristor- Controlled Reactor (TCR) c. Thyristor Switched capacitor (TSC)
2. Combined TCR and TSC compensator.

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.

Write the types of tape changing transformers. a. Off- load tap changing transformers.

1. Tap changing under load transformers.

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.

Define economic dispatch problem?

The objective of economic dispatch problem is to minimize the operating cost of active power generation.

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.

Define base point?

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

Define participation factor?

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

Define hydrothermal scheduling problem?

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

Define Uncommitment?

Commitment of minmum generator to meet the required demand.

Define spinning reserve?

It is the term describe the total amount of generation availability from all units synchronized on the system.

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.

What are the thermal unit constraint?

Minimum up time, minimum down time crew constraints.

Define minimum up time?

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

Define min.down time?

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

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.

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.

What are the techniques for the solution of the unit commitment problem?

Priority list method dynamic programming Lagrange relation

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.

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.

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

Dynamic programming composite hydraulic simulation methods statistical production cost.

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.

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.

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.

What are the functions of control center?

System monitoring contingency analysis security constrained optimal power flow.

What is the function of system monitoring?

System monitoring provides upto date information about the power system.

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.

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.

Define alert mode?

The occurrence of all possible outages the system does not remain in the secure is called alert mode.

What are the distribution factors?

Line outage distribution factor, generation outage distribution factor.

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.

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

What are linear sensitivity factors?

Generation shift factors line outage distribution factors.

What is the uses of line distribution factor?

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

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.