study guide 14

ECE 4310: Energy System IIStudy GuideWinter 2014

1 Real Power and Frequency Control

• Hydro turbines have a peculiar response due to the fact that the flow ratein the high pressure pipes cannot be changed quickly. This is becauseof the large inertia of water in the pipe. Hydro turbines need specializedgovernors to handle this effect. Learn how to derive (given the turbinetransfer function) the response of hydro and steam turbines to a stepchange in valve position.

• Reheat type turbines respond slower than non-reheat type turbines dueto the large time constant associated with the re-heater. Learn how towrite down the transfer function, given the percentage power from eachstage of the turbine.

• A disturbance can be (a) a load change, (b) generation loss, or (c) lossof a tie-line between two areas.

• Primary frequency control alone cannot bring back the frequency to nom-inal value after a disturbance. Learn how to derive an expression forsteady state frequency error (a simple derivation with s=0 for steady stateis enough). Learn how to calculate:

– frequency error.

– turbine power output at steady state subsequent to a disturbance.

– new load at steady state.

• When there is no secondary frequency control, the impact of a distur-bance is shared by the two areas in proportion to the rating of the areaif the droop settings are equal. Prove this and learn how to calculate thecontribution coming from the ‘helping area’.

• A smaller droop setting improves the contribution to frequency regula-tion. −∆f

R .

• Secondary frequency control is required to eliminate the frequency error.

• When two areas are connected through an ac tie-line the frequencies inthe two areas will be the same at steady state. Explain why.

• Integral feedback of only the frequency error will not eliminate tie-linepower deviation. Therefore tie-line bias control is necessary. In tie-linebias control, the feedback signal is ACE = ∆Ptie +B∆f . (Lab #2).

• For the two areas to share the burden of frequency regulation, both areasmust have secondary frequency control.

• After a sudden change in load (or generation) the initial rate of changeof frequency depends mainly on the inertia of the system. The effect ofD is seen later, followed by the effect of the turbines. Learn how to showthis.

• When the tie-line is lost due to a fault the two areas are independent andsteady state frequencies can be different.

2 Reactive Power and Voltage Control

• Synchronous generators can supply or absorb reactive power dependingon whether it is over or under excited. Learn how to calculate E and δfrom given terminal conditions (V, θ, P and Q). See the example done inthe class.


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• Increasing E does not increase real power (real power can be increasedonly by increasing the turbine power). With an increased E, the samereal power will be delivered at a reduced δ. See the example done in theclass.

• Power transfer capability of a transmission line can be improved by sup-plying reactive power either at the end of the line or somewhere in themiddle. Learn how to calculate the maximum power limit. Learn how tocalculate the maximum power limit with reactive power supplied at theload end of the line.

• Learn the functional block diagram of an excitation system. What is thefunction of (a) Voltage regulator, (b) Terminal voltage transducer and loadcompensator, (c) Exciter, (d) Excitation System Stabilizer, and (e) PowerSystem Stabilizer.

• There are three types of excitation systems. DC - a dc generator sup-plies the dc voltage to the main field winding. The voltage applied to themain field winding is controlled by controlling the field current of the dcgenerator. AC- and ac generator supplies the main field winding througha rectifier bridge. The voltage applied to the main field winding is con-trolled by controlling the firing angle of the main bridge or the auxiliarybridge. ST - a controlled rectifier bridge supplied the voltage to the mainfield winding, The power source to the rectifier bridge can be the genera-tor itself or an outside power supply that feeds the power station. Voltagefed to the field winding is controlled by controlling the firing angle.

• High gain is good from steady state point of view. High gain could makethe excitation system unstable. In this case a stabilizing circuit is re-quired. Learn how to calculate the steady state voltage error for a givenGain K. Learn how to explain the role of the stabilizing circuit using theNyquist diagram (Lab #3).

3 Economic Load Dispatch

• When one or more units are switched on to a system, the incrementalfuel costs must be equal for optimum operation. Learn how to prove:λ1 = λ2 = · · · = λn.


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• Learn how to determine the optimum dispatch using the graphical method.

• What is meant by the marginal cost when there are several generatorssupplying a load. Learn how to calculate it.

• Learn the iterative algorithms – λ iteration and Gradient search (Lab #4and Assignment #2).

• Show that for optimum operation, λiLi is the same for all machines. Li =1

1−∂PL/∂Pi. Learn how to incorporate losses into optimum dispatch.

• Learn how to stack generation offers and demand bids to form sup-ply and demand curves – and determine the MCP and the dispatch ofbids/offers (example shown in the class).

• Extend the above to include a transmission line with a limited capacity(example shown in the class).

4 Unit Commitment

• What is the difference between an ELD and a UC problem?

• Give examples of constraints that link consecutive time intervals.

• How does a priority list method differ from searching all possible combi-nations

• Give examples of optimization techniques suitable for solving the UCproblem

5 Protection

• Criteria for desiging protection systems; primary and back up protection,protection zones.

• Duties of circuit breakers; types of circuit breakers.

• Compute maximum and minimum fault currents for a radial power sys-tem.


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• Choose CT ratios.

• Determine the current settings of relays.

• Determine the time setting of Inverse Time Overcurrent relays when thecharacteristics are given either as a set of curves or as an equation.

• Derive the operating region of (a) impedance, (b) mho, and (c) offset mhocharacteristics.

• Learn how to calculate the impedance seen by a distance relay.

• Explain the concept of differential protection.

• What is spill current, what is the problem with it, and what are the solu-tions to eliminate the problem.

• How does the saturation of CTs affect the performance of (a) overcurrent,and (b) differential protection systems.

• Explain the three-zone distance protection scheme – Impedance andmho characteristics; Zone 1, Zone 2, and Zone 3 settings; timing dia-gram; effect of fault resistance; effect of heavy loading.

• What is the difference between electromechanical, static and numerical(digital) relays.

6 Power System Reliability and Its Assessment

This section provides a brief introduction to the basic concepts, models andmethodologies associated with power system reliability assessment using prob-abilistic techniques. Particularly the discussions emphasize on HL-I adequacyevaluation ( or generating capacity adequacy evaluation or resource adequacyevaluation):

• Functional zones of an electric power system can be divided into threehierarchical levels for reliability analysis. Reliability techniques can bedivided into the two general categories of probabilistic and deterministicmethods. The fundamental approaches used in a probabilistic evaluationcan be generally described as being either direct analytical evaluation or


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Monte Carlo simulation. The three basic probabilistic indices are proba-bility, expected frequency and expected duration of an adverse event.

• The main objective in HL-I assessment is the evaluation of the systemreserve required to satisfy the system demand. HL-I adequacy evalua-tion involves the development of a generation model, the developmentof a load model and the combination of the two models to produce arisk model. The analytical generation model is based on a capacity out-age probability table (COPT) which can be created by different ways forexample using the state enumeration approach or recursive algorithm.The load model used in the analytical approach is usually either the dailypeak load variation curve (DPLVC) or a load duration curve (LDC).

• Single area reliability analysis concerns calculating various adequacy in-dices by combining the COPT with different load models without consid-ering any potential assistance from other areas. Interconnected systemreliability analysis, however, considers potential assistances. The inter-connection assistance from the assisting system can be modeled as anequivalent unit. The equivalent unit model can be combined with theassisted system COPT to form the modified COPT and the reliabilityindices can be calculated by convolving the modified COPT with appro-priate system load model.

• The generation and the load model can be combined using Monte Carlosimulation. The Monte Carlo method estimates system indices by sim-ulating the actual process and random behavior of the system. Thesetechniques can be broadly divided into two categories of non-sequential(state sampling) and sequential (state duration sampling). In state sam-pling, the generation model can be obtained by sampling all the compo-nent states and load is usually represented by a multi-state model. Instate duration sampling, the system capacity model is obtained by com-bining the operating cycles of units in the system and the load is usuallyrepresented by a chronological hourly variation profile.


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study guide 14

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