EE2351 POWER SYSTEM OPERATION AND CONTROL UNIT I THE POWER SYSTEM AN OVERVIEW AND MODELLING PART A

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1 EE2351 POWER SYSTEM OPERATION AND CONTROL UNIT I THE POWER SYSTEM AN OVERVIEW AND MODELLING PART A 1. What are the advantages of an inter connected system? The advantages of an inter-connected system are as follows: a. Improved economy. Reliaility 2. What are the main responsiilities of a power system? It is to maintain the generation of electrical power at adequate level. Also the generated power has to e transmitted to the various load centers in response to the changing demands in proper form and quality, in accordance with the individual customer s specification. 3. Define Per-unit System The per unit value of any quantity is defined as, Actual Quantity Quantity in p.u= Base value of the quantity 4. Write the advantages of per-unit system. a. Per-unit impedance of the transformer is same whether referred to primary or secondary side of the transformer.. Manufacturers usually specify the impedance of a piece of apparatus in percent or per-unit on the ase of the name plate rating. 5. Write the formula for changing p.u. value from old ase to new ase. New p.u. value in any quantity = Old p.u. value in same quantity x 2 2 Base KV, old Base MVA, new KV, new MVA, old Base Base 6. Draw the standard symols for two winding transformer, three winding transformer, power circuit reaker etc. a. Two winding transformer. Three winding transformer

2 c. Power circuit reaker 7. Write the formula for finding ase impedance from ase capacity and ase operating voltage. Normally in power system capacity of the power system components are expressed in KVA or MVA & the operating voltage are expressed in KV or MV. Base impedance z KV 2 MVA in where, KV ase operating voltage in KV, MVA ase capacity in MVA. 8. What is meant y impedance diagram? The impedance diagram is the equivalent circuit of power system in which are various components of power system are represented y their approximate or simplified equivalent circuits. 9. What is meant y reactance diagram? It is simplified equivalent circuit of power system in which the various components are represented y their reactance. 10. What are the approximations made while forming impedance diagram? a. The neutral reactance are neglected.. The shunt ranches in equivalent circuit of induction motor and transformer are neglected. 11. What are the factors that need to e omitted for an impedance diagram to reduce it to a reactance diagram? a. The neutral reactance are neglected.. Shunt ranches in the equivalent circuits of transformer are neglected.

3 c. The resistances are neglected. d. All static loads and induction motors are neglected. 12. What are the approximations are made while forming reactance diagram? a. The neutral reactance are neglected.. Shunt ranches in the equivalent circuits of transformer are neglected. c. The resistances are neglected. d. All static loads and induction motors are neglected. 13. The ase KV and ase MVA if a 3 transmission line is 33 KV and 10 MVA respectively. Calculated the ase current and ase impedance. a. Base impedance z 2 2 KV / ph. MVA 10. Base current l MVA A 3KV A generator rated at 30 MVA, 11 kv has a reactance of 20 %. Calculate its p.u. reactance for a ase of 50 MVA & 10 kv. New p.u. reactance of generator (xp.u. new) Here Xp.u. old =20% 0.2p.u.; KV, new =10KV; MVA, new =50 MVA Xp.u. new = pu Write the equations for transforming ase KV on LV side to HV side of transformer. a. Base KV on HT side = HT Voltage rating Base KV on LT side L Voltage rating T. Base KV on LT side = LT Voltage rating Base KV on HT side L Voltage rating T

4 16. What is meant y single line diagram? It is a diagrammatic representation of power system in which the components are represented y their symols and the interconnection etween them are shown y a single straight line (even though the system are 3). 17. What are the components of power system? The components of power system are generators power transformers, transmission line, sustation transformers, Distriution transformers and loads. 18. Draw the symols used to represent fuse, machine or rotating machine, air circuit reaker and three phase star neutral ungrounded in a power system. Symols a. Machine or rotating armature. Fuse c. Air circuit reaker d. Three phase star neutral un grounded 19. What are the quantities whose ase values are required to represent the power system y reactance diagram? The ase values of voltages, current power and impedance are required to represent the power system y reactance diagram. 20. What is the need for ase values? The components or various sections of power system may operate at different voltage & power levels. It will e convenient for analysis purpose if the voltage, power current and impedance rating of components of power system are expressed with reference to a common value called ase value. 21. Draw the equivalent circuit of a 3 generator. Let Xs = Synchronous reactance /phase. Ra = Armature resistance / phase. Eg = induced emf/ph. Ig = Current per phase.

5 22. Draw the single phase equivalent circuit of a three-winding transformer. 1. What is meant y us? UNIT II POWER FLOW ANALYSIS PART A The meeting point of various components in a power system is called a us. The us is a conductor made of copper of aluminum having negligile resistance. 2. What is meant y us impedance matrix? The matrix consisting of driving print impedances and transfer impedances of the network of a power systems is called us impedance matrix. 3. What is meant y us admittance matrix?

6 The matrix consisting of the self & mutual admittances of the network of a power system is called us admittance matrix. 4. Write the diagonal & off-diagonal elements of us admittance matrix. The diagonal elements of us admittance matrix are called self-admittances of the uses and off diagonal elements are called mutual admittances of the uses. 5. Write the diagonal & off diagonal elements of us impedance matrix. The diagonal elements of us impedance matrix are called driving point impedances of the uses and off-diagonal elements of us impedance matrix are called transfer impedances of the uses. 6. Form the us admittance matrix for the system whose reactance diagram is shown elow. Y us jo.1 jo.25 jo.5 jo.25 jo jo.25 jo.5 jo.25 jo.5 jo.2 -j16 j6 = j6 -j11 7. What are the methods availale for forming us impedance matrix? There are two methods availale. They are, a. From the us admittance matrix & then take inverse to get us impedance matrix.

7 . Directly form the us impedance matrix from the reactance diagram. This method utilizes the techniques of modification on existing us impedance matrix due to addition of new us. 8. Write the equation to find the elements of new us admittance matrix after eliminating nth row & column in a n n us admittance matrix. The element yjk of new us admittance matrix is given y, YY in nk Yjki new =Y jk Y nn for j= 1,2,,(n-1) & K=1,2,3,..(n-1) where Yjk,YinYnn are elements of original or given us admittance matrix of order (n n). 9. Write the four ways of adding an impedance to an existing system. So as to modify us admittance matrix. To modify a us impedance matrix, a ranch of impedance Z can e added to original system in the following four different ways, a. Adding a ranch of element Z from a new us p to the reference us.. Adding a ranch of impedance Z from a new us- p to an existing us q. c. Adding a ranch of impedance Z from an existing us q to the reference us. d. Adding a ranch of impedance Z etween two existing uses h&q. 10. Determine Zus when Yus = j1 jz j2 -js Adjoint of Y 1 us Z us =Y us = Determinant of Y us -j1 j2 Determinant of Y us = j2 -j5 -j5 j2 Adjoint of Y us = T -j2 -j1 -j5 j2 = -j2 -j1 = -j1 -j5 - j2 =-5+4=-1 2

8 Zus=-1 j5 j2 -j2 -j1 j5 j2 = j2 j1 11. A 50 MVA, 30 kv, 3-, 60 Hz synchronous generator has a synchronous reactance of 9 /ph and a negligile resistance. The generator is delivering rated power at a 0.8 p.f. lagging at the rated terminal voltage to an infinite us. Determine the excitation voltage per phase E and the power angle. The 3- apparent power is S3 = 50*Cos = 50 < MVA =40 MW +j30 MVAR The rated current is I a 3 S3 * X A 3 V * The excitation voltage per phase is E = (j9)(962.25< ) = < V and KV/ph & power angle is Draw the per-phase model or three-phase transformer. where, Ze1 & Ze2 are the equivalent impedances ased on the line to neutral connections.

9 13. Draw the autotransformer model circuit. 14. Draw the three-winding transformer model circuit. UNIT III FAULT ANALYSIS-BALANCED FAULT PART A 1. What is meant y a fault? A fault in a circuit is any failure which interferes with the normal flow of current. The faults are associated with anormal change in current, voltage and frequency of the power system. The faults may cause damage to the equipments if it is allowed to persist for a long time. Hence every part of a system has een protected y means of relays and circuit reakers to sense the faults and to isolate the faulty part from the healthy part in the event of fault. 2. Why does fault occur in a power system?

10 The faults occur in a power system due to insulation failure of equipments, flashover of lines initiated y a lightning stroke, due to permanent damage to conductors and towers or due to accidental faulty operations. 3. How are the faults classified? In one method of classification, the faults are classified as shunt and series faults. The shunt faults are due to short circuits in conductors and series faults are due to open conductors. In another method of classification, the faults are classified into symmetrical and unsymmetrical faults. In symmetrical faults the fault currents are equal in all the phases and can e analyzed on per phase asis. In unsymmetrical faults the fault currents are unalanced and so they can analyze only using symmetrical components. 4. List out the various types of shunt and series faults. The various types of shunt faults are, 1. Line-to-ground fault 2. Line-to-line fault 3. Doule line-to-ground fault 4. Three phase fault The various types of series faults are 1. One open conductor fault 2. Two open conductor fault 5. List out the various symmetrical and unsymmetrical faults. The three phase fault is the only symmetrical fault. All other types of faults are unsymmetrical faults. The various unsymmetrical faults are, 1. Line to ground fault 2. Line to line fault 3. Doule line to ground fault 4. One or two open conductor faults 6. What is meant y symmetrical and unsymmetrical fault?

11 The fault is called symmetrical fault if the fault current is equal in all the phases. The fault is called unsymmetrical fault if the fault current is not equal in all the phases. 7. List out the various methods of reducing short circuit current. The following are the two methods of reducing the short circuit current: 1. By providing neutral reactance 2. By introducing a large value of shunt reactance etween uses. 8. Write the main differences in representation of power system for load flow and short circuit studies. 1. For load flow studies oth the resistances and reactances are considered whereas for fault analysis the resistances are neglected. 2. For load flow studies the us admittance matrix is useful whereas for short circuit studies us impedance matrix is used. 3. The load flow study is performed to determine the exact voltages and currents whereas in short circuit studies the voltages can e safely assumed as 1 pu and the prefault current can e neglected. 9. Write the relative frequency of occurrence of various types of faults. Type of fault 1. 3 phase fault 2. Doule line-to-ground fault 3. Line-to-line fault 4. single line-to-ground fault Relative frequency of occurrence 5 % 10 % 15 % 70 % 10. What is meant y fault calculations? The fault condition of a power system can e divided into su-transient, transient and steady state periods. The currents in the various parts of the system and in the fault are different in these periods. The estimation of these currents for various types of faults at various locations in the system are commonly referred to as fault calculations. 11. What is the need for short circuit studies or fault analysis? The short circuit studies are essential in order to design or develop the protective schemes for various parts of the system. The protective schemes consists of current &

12 voltage sensing devices, protective relays and circuit reakers. The selection (or proper choice) of these devices mainly depends on various currents that may flow in the fault conditions. 12. What is meant y douling effect? If a symmetrical fault occurs when the voltage wave is going through zero than the maximum momentary short circuit current will e doule the value of maximum symmetrical short circuit current. This effect is called douling effect. UNIT IV FAULT ANALYSIS - UNBALANCED FAULT 1. Write the fault in which positive, negative and zero sequence component currents are equal. In single line-to-ground fault the positive, negative and zero sequence component currents are equal. 2. Write the fault in which positive and negative sequence component currents together is equal to zero sequence current in magnitude. Doule line-to-ground fault 3. Write the various unsymmetrical faults in a power system. The unsymmetrical faults in a power system are, (i) Single line-to-ground fault (ii) Line to line fault (iii) Doule line to ground fault (iv) Open conductor fault 4. Define Negative Sequence Impedance The negative sequence impedance of an equipment is the impedance offered y the equipment to the flow of negative sequence current. 5. Write the faults which do not have zero sequence currents flowing.

13 In line to line faults, the zero sequence currents do not flow. 6. Write the faults involving ground. The faults involving ground are, (i) (ii) (iii) Single line to ground fault Doule line to ground fault Three phase fault 7. Define Positive Sequence Impedance The positive sequence impedance of an equipment is defined as the impedance offered y the equipment to the flow of positive sequence currents. 8. In what type of fault the positive sequence components of current is equal in magnitude ut opposite in phase to negative sequence components of current. Line-to-line fault 9. In which fault, the negative and zero sequence currents are asent? In three phase fault, the negative and zero sequence currents are asent. 10. Write the oundary condition in single line-to-ground fault. Boundary condition: Va = 0; I = Ic = 0 UNIT V STABILITY ANALYSIS PART A 1. In a 3-machine system having ratings S1, S2 and S3 and inertia constants M1, M2 and M3, what is the inertia constants M and H of the equivalent system. M M S M S M S eq S S S where, S1,S2,S3 = MVA ratings of machines 1,2,3, respectively

14 S = Base MVA or MVA rating of system. H eq fm S eq 2. Define Synchronizing Coefficient Synchronizing coefficient is defined as the stiffness in synchronous machine. The system is stale if the synchronizing coefficient is positive. It is positive when 0 0 /2. 3. Write the swing equation in a power system. 2 H d P 2 m P f dt e Since M in p.u.=h/f, we can write, 2 d M P 2 m P dt e H = Inertia constant in MW-s/MVA F = frequency in Hz M = inertia constant in p.u. Pm = Mechanical power input to the system (neglecting mechanical losses) in p.u. Pe = Electrical power output of the system (neglecting electrical losses) in p.u. 4. Define Swing Curve Swing curve is defined as the plot or graph etween the power angle and time t. It is usually plotted for a transient state to study the nature of variation in for a sudden large disturance. From the nature of variations of the staility of a system for any disturance can e determined. 5. Write the simplified power angle equation and the expression for Pmax. The simplified power angle equation for a generator feeding energy to infinite us is

15 given y, Pe = Pmax sin where, P max E V X E = Magnitude of internal emf of generator. V = Magnitude of infinite us voltage. X = Transfer reactance etween generator and infinite us. = Power angle or torque angle. 6. Define Power Angle The power angle (or torque angle) is defined as the angular displacement of the rotor from synchronously rotating reference frame. 7. For the swing equation operation? 2 d M P What will e the value of 2 a dt Pa during steady state During steady state operation, there is no acceleration or deceleration on the rotor. Therefore, Pa =0. (Under this condition is a constant). 8. Write the two ways y which transient staility study can e made in a system where one machine is swinging with respect to an infinite us. For a single machine infinite us system, the following two methods can e employed for transient staility studies. i) Equal area criterion ii) Point y point method 9. Define Critical Clearing Time and Critical Clearing Angle Critical clearing angle,cc is defined as the maximum allowale change in the power angle efore clearing the fault, without loss of synchronism. The time corresponding to this angle is called critical clearing time, tcc. Critical clearing time, tcc is defined as the maximum time delay that can e

16 allowed to clear a fault without loss of synchronism. 10. List out the methods of improving the transient staility limit of a power system. The following are the methods used to improve the transient staility of a system: i) Increase of system voltage and use of AVR (Automatic Voltage Regulation) ii) Use of high speed excitation systems iii) Reduction in system transfer reactance iv) Use of high speed reclosing reakers