EE342 - Electrical Power Lab

College of Engineering, Qassim University, Buraidah, Saudi Arabia Electrical Power Lab. (EE344), 2014-15, Term 2

Experiment # 6 Analysis of Power System FaultsObjectives To analyze the power system during a fault.

To investigate the effect of : (1) the type of a fault; (2) the location of a faultBackgroundGeneralFault analysis forms an important part of power system analysis. The analysis aims to determine the bus voltages, and line currents during various types of system faults. Faults on power systems are divided into three-phase balanced faults and unbalanced faults. Different types of unbalanced faults are single line-to-ground fault, line-to-line fault, double line-to-ground fault (Fig. 1). The information obtained from fault analysis are used to determine the rating of the protective switchgears (i.e. breakers). Also, fault analysis are used to select and set the protective relays and their coordination.

Fig. 1 Power System Fault TypesThe magnitude of the fault currents depends on the internal impedance of the generators plus the impedance of the intervening circuit. For the purpose of fault studies, the generator behavior can be divided into three periods: the subtransient period (2 cycles- Xd), the transient period (30 cycles- Xd), and finally the steady state period (Xd). Various method has been devised for the solution of power systems during faults. Thevenins method and the bus impedance method have been used for a long time. The bus impedance method is the systematic and the most applicable method. Symmetrical components theory is used to transform the three-phase unbalanced system into three balanced systems (sequences): the positive system (sequence), the negative system (sequence) and the zero system (sequence). The positive system is a three-phase system with a phase shift of 120 degrees, the negative system is a three-phase system with a phase shift of 240 degrees and the zero system is a three-phase system with a phase shift of 0 degrees. Therefore, a power system during the unbalanced fault is transformed into three balanced systems, each one is composed of the equivalent impedances of each element in the power system. For example, the positive system is composed of the positive equivalent of each element in the system. Interconnection of System SequencesAlso, the three sequence systems are interconnected together to interpret each type of fault, Fig. 2. In L-G fault, the three sequence systems are connected in series to represent the fault. For the case of L-L fault, the positive and negative systems are connected in parallel. While for the L-L-G fault, the three sequence systems are connected in parallel to cope with the physical fault type.

Fig. 2 Power System Sequence InterconnectionsEarthing and Transformer Connection

The neutral earthing impedance used for generators and for transformers as well affect their equivalents in the zero system. In addition, the connection of the transformer windings themselves affects the transformers zero equivalents. Delta and star with or without earthing are the only connections of the transformer windings. In conclusion, the transformer and earthing connections affect the fault currents flowing in the system elements. Power System Sequence Equivalents

Each element in the power system has its sequence equivalent. The generator has a positive sequence equivalent consists of an emf (E) in series with its +ve impedance (Z1). It has a ve sequence impedance equivalent (Z2). The zero sequence equivalent of the generator is an impedance (Z0) in series with 3 time the earthing impedance (3* Zn). The transmission line has its sequence equivalents as: Z1 = Z2, Z0 = (2.5-3) Z1. The transformer has equal sequence impedance equivalents as : Z1 = Z2 = Z0, where the zero equivalent depends on the type of transformer connection. Solution MethodThe most common and systematic method for solving the power system during all fault types is the bus impedance method.Experimental Procedure1- Effect of Fault location1- Consider the power system shown in Fig. 3. All system data are given on Table A (p.u).

2- The power system +ve sequence equivalent is shown in Fig. 4.

3- Also, the zero sequence equivalent for the power system is shown in Fig. 5, considering a generator earthe impedance of j0.25/3 p.u.

4- Run the power system fault analysis program for the given system, using the Matlab software, for a three-phase fault at bus 3 and with a fault impedance of j0.1 p.u.

5- Record the faulted bus voltages and the fault currents in Table 1.

6- Repeat for the other fault locations (bus 2 & bus 1) and record your results in Table 1.

2- Effect of Fault Type

1- Run the power system fault analysis program for the given system, using the Matlab software, for a three-phase fault at bus 3 and with a fault impedance of j0.1 p.u. (step #4 in part #1).2- Record the faulted bus voltages and the fault currents in Table 2.

3- Repeat for other fault types and record your results in Table 2.

Fig. 3 Power System Under Study

Fig. 4 Equivalent Positive SystemFig. 5 Equivalent Zero System

Table A: Power System p.u Data (base MVA=100)

ItemkV RatingSystem reactances

X1 X2 X0

G 1200.150.150.05

G 2200.150.150.05

T 120 / 2200.100.100.10

T 220 /2200.100.100.10

L 1-22200.1250.1250.30

L 1-32200.150.150.35

L 2-32200.250.250.7125

Observation TablesTable 1: Effect of Fault Location: Symmetrical fault at different busFault

LocationFaulted Bus VoltagesFault CurrentsSeverity

Rank

VaVbVcIaIbIc

Bus #1

Bus #2

Bus #3

Table 2: Effect of Fault Type at bus # 1Fault

TypeFaulted Bus VoltagesFault CurrentsSeverity

Rank

VaVbVcIaIbIc

L-L-L

L-L-G

L-L

L-G

Check list1- Comment on the fault location and its effect on the fault currents

2- Discuss the effect of fault type on the bus faulted voltages and fault currents.

3- Identify the serious fault type on the power system.4- Attach the MATLAB solution for the given system.L - L

3-phase

F

Zero system

N

F

- ve system

N

F

+ve system

N

F

-ve System

N

F

+ve System

N

F

+ve system

N

F

Zero system

N

F

-ve system

N

F

+ve system

N

L - G

L L- G

L - L

3 - phase

L - G

L L - G

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