King Saud University College of Engineering Electrical Engineering Dept.

EE449 Power System ProtectionFinal Exam 1st Semester 1426-1427

Time Allowed: 3 Hours

Question#1 Select the correct answer (only ONE) for the following questions

1.1 The generator side windings of the step-up transformer in a power plant are usually connected into DELTA to

a) prevent the generator from the zero sequence current for all faults. b) prevent the line from the zero sequence current for all faults. c) prevent the line from the negative sequence current for faults occurring on the line side. d) prevent the generator from the zero sequence current for faults occurring on the generator side. e) prevent the generator from the zero sequence current for faults occurring on the line side.

1.2 The dependability requirement for a protective relay means that a) the relay must not trip for a non-fault condition. b) the relay must not trip for a fault condition. c) the relay must trip for a fault condition.

d) the relay must trip for a non-fault condition. e) the protection system must disconnect only the

faulted zone. 1.3 The contacts of a relay are always connected in series with

a) the CB contacts and the DC supply. b) the relay contacts and the CT. c) the CB trip coil and the DC supply.

d) the CB trip coil and the CT. e) the CB contacts, the CT and the DC supply.

1.4 The abbreviation CCVT stands for

a) Continuous Current Voltage Transformer b) Continuous Capacitor Voltage Transformer c) Continuous Capacitor Voltage Transformer

d) Capacitor to Capacitor Voltage Transformer e) Coupling Capacitor Voltage Transformer

1.5 A digital relay means

a) An induction type relay b) A relay made from electronic components c) A microprocessor-based relay

d) the relay has a digital display e) the relay status will be ON or OFF

1.6 Consider a CT having a ratio of 400:5. The primary current is 6400A, while the secondary current

is 64 A. Thus, the percentage CT error is

a) 5% b) 10% c) 20%

d) 25% e) 40%

1.7 The fuse is responsible for a) Detecting faults then interrupting the circuit

b) Sensing voltage and/or current.

c) Detecting faults and sending a trip signal to the circuit breaker.

d) Interrupting the circuit under load condition.

1.8 To overcome the problem of inrush current for differential protection of transformers, we use

a) A percentage differential relay b) Different relay tap settings

c) CT's connected in Delta/Wye d) A second-harmonic filter

1.9 The recloser is responsible for a) Breaking (OFF) and making (ON) under no load condition

b) Breaking (OFF) and making (ON) under fault condition

c) Breaking (OFF) and making (ON) under loading condition

d) Breaking (OFF) under fault condition and making (ON) under loading condition

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1.10 The impact of the phenomenon of inrush current on the differential protection of transformers, is

a) Decreasing security b) Decreasing selectivity c) Increasing dependability

d) Decreasing speed e) Increasing security

1.11 The partitioning of the system into zones will a) Improve security b) Improve simplicity c) Improve dependability

d) Decrease speed e) Improve selectivity

1.12 The use of backup protection will a) Improve dependability b) Improve selectivity c) Improve security

d) Decrease speed e) Improve simplicity

1.13 The digital relay subsystem which is responsible for reducing the level and filtering of the input current and/or voltage is the

a) Data-Acquisition subsystem b) signal conditioning subsystem

c) digital processing subsystem d) Power supply subsystem

1.14 The abbreviation TDS means

a) Time Dial Setting b) Total Digital Setting

c) Tap Dial Setting d) Time Dial Supply

1.15 A/D converter in a digital relay will

a) reduce the values of input currents b) convert the current signal into voltage

c) convert the continuous signal into discrete d) convert the input signal into numbers

1.16 The reach of Zone-1 of a distance relay is usually set to only 80% of the line to

a) avoid selectivity violation b) improve security

c) ensure dependability d) increase speed

1.17 The coordination of overcurrent relays in a radial system must start with

a) The relay in down -stream b) The relay in up-stream

c) The relay in the mid-point of the feeder d) The relay connected to the most important load

1.18 The standard rating for CT secondary is a) 5A for American standard and 1 A for European standard b) 5A for European standard and 1 A for American standard

c) 1 A for all standards d) 5 A for all standards

1.19 To prevent the differential relay from false trip, it should incorporate

a) two operating coils b) one operating coil

c) one restraining coils d) two restraining coil

1.20 In overcurrent relays, the relation between pickup current and current tap setting is

a) they are the same b) pickup current > current tap setting

c) pickup current < current tap setting d) pickup current = 1.5 current tap setting

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King Saud University College of Engineering Electrical Engineering Dept.

EE449 Power System ProtectionFinal Exam 1st Semester 1426-1427

Time Allowed: 3 Hours

.

Question#2The 132 kV power system shown in Fig.1 is subjected to a LLG fault at the mid point of line-

23. Calculate the three-phase voltages (kV) and currents (A) seen by breaker B23 during fault.

Consider the transformers phase shift in your calculation.

1 3

G1

Fig-1

X1 = 0.6 X2 = 0.6 X0 = 2.0

G2

All reactances are given in pu referred to 200 MVA base

X1= 0.08 X1= 0.2 X2= 0.15 X0= 0.05

X1= 0.15 X2= 0.12 X0= 0.04 Xn= 0.08

X1= 0.1

42 B23

Question#3

A) The burden of a 200:5, 10C400 CT is 2 Ohms. What will be the percentage error of the CT

if the primary current is 3000 A.

B) Fig.2 shows a radial distribution system protected by fuses, recloser and overcurrent relays.

There are two types of fuses 65T and 100T. The recloser is set at 560A trip rating for phase

and 280A for ground with two operations: fast and delayed. The overcurrent relay settings

are 9A current tap setting (CTS) for phase relay with TDS=2 and 4A CTS for ground relay

with TDS=1. The time current curves for these components are described in the attached

Fig-3.

100T

Load 1

1

65T

Load 2

2

100T

Load 3

3

R

4

controlled by Ground OC Relays and Phase OC Relays

CB

Fig.2

Describe the operating sequence of the protective devices for the following faults:

i- Line-Line permanent fault at Load#2, IF=1125A.

ii- Line-Ground self-clearing fault at bus#3, IF=1700A.

iii- Three-phase permanent fault at Load#1, IF=1000A.

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Question#4

A) Draw the wiring diagram of three-phase directional overcurrent relays showing the CT/VT,

the relays coils, the relays contacts and the breaker trip coil.

B) A three-phase distance protection unit is used to protect a transmission line whose

impedance is 6 + j60 . This protection unit is composed of SIX distance relays; 3 relays for phase faults (Rab, Rbc, Rca) and 3 relays for ground faults (Rag, Rbg, Rcg). Each of the 6

relays is a 3-zone mho relay with: zone-1 80% instantaneous, zone-2 120% 0.3 s delay,

zone-3 240% 0.6 s delay. The compensation factor for ground relays is m=2.0.

When a B-C fault occurs on the line, the currents and voltages seen by the distance

protection unit are: Ia = 0, Ib = -Ic=152.73 A /-174.92o, Vag = 7.967 kV /0o, Vbg = 7.355 kV

/236.58o, Vcg = 7.282 kV /122.53o

i) Calculate the impedance seen by each of the six relays.

ii) What will be the response of each of the six relays (Block / Trip (delay=?)).

iii) Comment on the performance of these distance relays.

Question#5

A) Give a brief description of the different communication channels used for pilot protection of

lines.

B) Discuss the principle of operation of a directional comparison blocking scheme pilot

protection.

C) Use sample and first derivative method to determine the rms value of the current i(t), where,

i(t) = 5 sin (2fo t) A Assume that fo = 60Hz and the sampling frequency = 1200 Hz. Compare the calculated rms

value with the expected value.

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