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MINISTRY OF SCIENCE AND TECHNOLOGY
DEPARTMENT OF TECHNICAL AND VOLCATIONAL EDUCATION
DEPARTMENT OF ELECTRICAL POWER ENGINEERING
FINAL EXAMINATION
B.E (EP), 2006
EP 05022 POWER SYSTEM PROTECTION
Date: 25.10.2006(Wednesday) Time:8:30am-11:30am************************************************************************
Attempt any Six questions
1. Consider again the portion of a 138kV transmission system shown in figure. Lines1-2,
2-3 and 2-4 are respectively 64, 64 and 96 km long. The positive-sequence impedance
of the transmission lines is 0.05 + j0.5 ohm per kilometer. The maximum load carried
by line 1-2 is 50MVA. Design a three-zone step distance relaying system to the extent
of determining for R 12 the zone settings which are the impedance values in terms of
CT and CVT secondary quantities.
2. Discuss good practice in Transmission-line relay protection.
3. Derive and sketch in the complex impedance plane, the impedance seen by each of
the three phase distance relays for a phase b to phase c fault on the transmission line
by using the symmetrical components. Neglecting the mutual effects, charging
currents of the transmission lines and load current. Indicate on the same diagram, the
operating characteristics of an impedance relay, a mho relay and a reactance relay set
to protect the whole length of the line.
Zone 3
Zone 2
Zone 1
1 2 3
4B 12 P1 P2B 21
B 23
B 24
P3
P4
B 42
B 32
4. Derive the voltage and current equations at the relay location, and the impedance seen
by each of the three phase distance relays for a phase a to ground fault on the power
transmission line by the use of symmetrical components. Neglecting the mutual
effects, charging currents of the transmission lines and load current.
5. Explain the following protection-system.
(a) Stator-overheating protection
(b) Overvoltage protection
6. Discuss the protection of a three winding transformer with a two winding percentage
differential relay with necessary sketch.
7. Describe the following bus-protection.
(a) Current differential relaying with percentage differential relays.
(b) Current differential relaying with over voltage relays.
8. Name differential types of static relays. Discuss the use of electronic relays and
transistor relays.
Answers:
1.
Line 1-2 3.2 + j32
Line 2-3 3.2 + j 32
Line 2-4 4.8 + j 48
Max: Load crt, (ILoad) max =
(ILoad)max =
C.T Ratio = 200:5
Vp =
CVT Ratio =
Zp = ?
Line 1-2 0.11 + j 1.1 secondary
Line 2-3 0.11 + j 1.1 secondary
Line 2-4 0.16 + j 1.6 secondary
Assume p.f = cos = 0.8 sin = 0.6
Z Load =
= (10.2 + j 7.68)
Zone 1 of R12 = 0.8 (0.11 + j 1.1) = 0.088 + j0.88
Zone 2 of R12 = 1.2 (0.11 + j1.1) = 0.132 + j1.32
Zone 3 of R12 = (0.11 + j1.1) + 1.2 (0.16 + j1.6)
= 0.302 + j 3.02
2. Good practice in transmission line relay protection
On important high voltage lines, high speed fault clearing is generally necessary. For
reason of system stability and for other reasons. On many such lines high speed distance
relays are used which give high speed clearing of both ends of the line for fault in the middle
eight-tenth of the line length and sequencial clearing of faults in the two ends zone each one-
tenth of the line in length. The ground relays are usually of either the slow speed and the high
speed overcurrent types. Fortunately, from the standpoint of stability, high speed clearing is
not so necessary for one line to ground faults as for phase fault. If the stability situation is so
crital that high speed clearing of ground faults and end zone phase faults is required, then
carrier pilot relaying is generally employed.
On long or heavy load lines, the distance relay should be of a kind not too susceptible
to trip on swings for which the system will recover.
On short but important lines, pilot wire relaying is appropriated. It use has ground
greatly since relays were developed that give protection against all internal fault by means of
a pilot channel consisting of only two small wires.
On unimportant lines, slow speed overcurrent relays are ordinary used.
3.
Phase a phase network for a phase b to phase c fault
Z1 = (Z1×Zy1) / (Zx1 + ZY1)
Z2 = (Zx2× ZY2) / (Zx2 + ZY2)
I1 = E1/(Z1+Z2+Rf) = E1 / (2Z1+ Rf) (Z1=Z2)
I1= 1/K
K= (2Z1+Rf)/E1
V1= V2 + I1 Rf/2 – I2 Rf/2
= -I2 Z2 + I1 Rf/2 + I1 Rf/2
= I1 Z1 + I1 Rf
= I1 (Z1 + Rf)
Ia1 = I1 × (ZY1) / (Zx1+ ZY1)
Ia1 = I1 C1 ( C1= (ZY1) / (Zx1+ ZY1) )
Ia1 = (1/K) ×C1
K Ia1 = C1
Ia2 = I2 × (ZY2) / (Zx2+ ZY2)
= I2 × C2 ( C2 = (ZY2) / (Zx2+ ZY2) )
= - I1 C1 ( I1= - I2 , C1 = C2 )
= -(1/K) C1
K Ia2 = - C1
There is zero sequence component for phase to phase fault
K Ia0 = 0
The actual phase current in relay location are
K Ia = K Ia1 + K Ia2 + K Ia0 = C1 - C1 =0
K Ib = a2 K Ia1 + aK Ia2 + K Ia0 = ( a2 - a) C1
K Ic = a K Ia1 + a2K Ia2 + K Ia0 = ( a - a2 ) C1
If delta connected CT’s are involved
K(Ia - Ib) = KIa - KIb = - (a2 - a) C1
K(Ib - Ic) = KIb - KIc = (a2 - a - a + a2 ) C1
K(Ic - Ia) = KIc - KIa = (a - a2) C1
Va1 = V1 + Ia1 Z1 ́
= I1(Z1 + Rf) + I1C1 Z1 ́
= 1/K(C1 Z1 ́ + Z1 + Rf )
K Va1 = C1 Z1 ́ + Z1 + Rf
Va2 = V2 + Ia2 Z2́
= - I2 Z2 + I2 C2 Z2́
= I1Z1 – I1 C1 Z2́
= 1/K(-C1 Z1 ́ + Z1)
K Va2 = - C1 Z1 ́ + Z1
There is no zero sequence component for phase to phase fault
K Va0 = 0
The actual phase to neutral fault on relay location are
K Va = K Va1 + K Va2 + K Va0 = C1 Z1 ́ + Z1 + Rf - C1 Z1 ́ + Z1 = 2 Z1 +Rf
K Vb = a2K Va1 + aK Va2 + K Va0 = a2 (C1 Z1 ́ + Z1 + Rf ) + a (-C1 Z1 ́ + Z1)
= ( a2 – a) C1 Z1 ́+ ( a2
+ a) Z1 + a2 Rf
= ( a2 – a) C1 Z1 ́ - Z1 + a2 Rf
K Vc = aK Va1 + a2K Va2 + K Va0
= a (C1 Z1 ́ + Z1 + Rf ) + a2 (-C1 Z1 ́ + Z1)
= ( a – a2) C1 Z1 ́+ ( a2 + a) Z1 + a Rf
= ( a – a2) C1 Z1 ́ - Z1 + a Rf
The Phase to phase voltage are
K(Va - Vb ) = K Va - K Vb = (2Z1 + Rf ) – [ (a2-a) C1 Z1 ́- Z1 + a2 Rf]
= - (a2-a) C1 Z1 ́ + 3Z1 + (1- a2) Rf
K(Vb- Vc) = KVb- K Vc = [(a2-a) C1 Z1 ́- Z1+a2 Rf]-[(a2-a) C1 Z1 ́ - Z1+a2) Rf]
= 2(a2-a) C1 Z1 ́+ (a2-a) Rf
K(Vc - Va) = K Vc - K Va = [(a-a2) C1 Z1 ́- Z1+a Rf] –(2 Z1+ Rf)
= (a-a2) C1 Z1 ́- 3Z1 + (a-1) Rf
The impedance measured by the relays are
Zab = K(Va - Vb ) / K(Ia - Ib ) = [ - (a2-a) C1 Z1 ́ + 3Z1 + (1- a2) Rf] / [-(a2-a) C1]
= Z1 ́ + -
= Z1 ́ - j Zx1 -
Zbc = =
4.
( )
v1 = v2 + v0 + 3RFI1
= I2z2 + I0z0 + 3RFI1
= I1z1 + I1z0 + 3RFI1
= I1 (z1 + z0 +3RF)
Ia1 = I1 ,
kIa1 = c1
Iaz = I2 ,
kIa1 = c1
Ia0 = I0
kIa0 = c0
Actual phase to phase current for relay location are
kIa = kIa1 + kIa2 + kIa0 = 2c1 + c0
kIb = a2kIa1 + akIa2 + kIa0 = (a2 +a) c1 + c0 = -c1 + c0
kIc = akIa1 + a2kIa2 + kIa0 = (a2 + a2) c1 +c0 = -c1 + c0
If delta connected CT's are involved
k(Ia – Ib) = kIa – kIb = -c1 + c0 + c1 – c0 = 3c1
k (Ib - Ic) = kIb – kIc = -c1 + c0 + c1 – c0
k (Ic – Ia) = kIc – kIa = -c1 + c0 - 2c1 – c0 = -3c1
va1 = v1 + Ia1z1'
= I1 (z1 + z0 + 3RF ) + I1c1 z1'
= I1 (c1z1' + z1 + z0 + 3RF)
= (c1z1' + z1 + z0 + 3RF)
kva1 = c1z1' + z1 + z0 + 3RF
va2 = -v2 + Iazz2'
= -I1z2 + I2c2z2'
= -I1z1 + I1c1z1'
= I1 (c1z1'- z1)
= (c1z1'-z1)
kva2 = c1z1'-z1
va0 = -v0 + Ia0z0'
= -I0z0 + I0c0z0'
= -I1z0 + I1c0z0'
= I1 (c0z0 '- z0)
kva0 = c0z0'-z0
The actual phase to neutral phase voltage for relay location are
kva = kva1 + kva2 + kva0
= c1z1' + z1 + z0 + 3RF + c1z1' – z1 + c0z0' – z0
= 2c1z1' + c0z0 ' + 3RF
kvb = a2kva1 + akva2 + kva0
= a2(c1z1' + z1 + z0 + 3RF)+ a(c1z1' – z1) + c0z0' – z0
= (a2 + a) c1z1' + c0z0 ' + (a2 – a) z1 + (a2 – 1) z0 + a23RF
= -c1z1' + c0z0' + (a2 - a) z1 + (a2 – 1)z0 + 3a2RF
kvc = akva1 + akva2 + kva0
= a (c1z1' + z1 + z0 + 3RF) + a2 (c1z1' – z1) + c0z0' – z0
= (a + a2) c1z1' + c0z0 ' + (a2 – a) z1 + (a2 – 1) z0 + a23RF
= -c1z1' + c0z0' + (a - a2) z1 + (a – 1)z0 + 3aRF
The phase to phase voltage are
k(va – vb) = 2c1z1' + c0z0 ' + 3RF + c1z1' - c0z0' - (a2 - a) z1 - (a2 – 1)z0 - 3a2RF
= 3c1z1' – (a2 – a) z1 – (a2 – 1) z0 + (1- a2) 3RF
k (vb – vc) = -c1z1' + c0z0' + (a2 - a) z1 + (a2 – 1)z0 + 3a2RF + c1z1' - c0z0' - (a - a2) z1 –
(a - 1)z0 - 3aRF
= ( a2 – a – a + a2) z1 + ( a2 - 1 - a + 1) z0 + (a2 - a) RF
= 2 ( a2 – a) z1 + (a2 – a) z0 + (a2 – a) RF
k (vc – va) = -c1z1' + c0z0' + (a - a2) z1 + (a – 1)z0 + 3aRF - 2c1z1' - c0z0 ' - 3RF
= -c0z0' – 3RF
= -3c1z1' + (a – a2)z1 + (a – 1) z0 + (a – 1) 3RF
The impedance measured by the relays are
Zab =
Zbc =
zca =
=
=
5.(a) Stator overheating protection
Figure: Stator overheating relaying with resistance temperature detector
General stator overheating is caused by overloading or by failure of the cooling
system. It can be detected quite easily overheating because of short circuit lamination is very
localized it can be detected before serious damage is done.
The practice is to embed resistance temperature coil or thermocouple in the slot with
the stator winding of generator larger than about 500 to 1500 KVA. Enough of these dectors
are located at different places in the winding. Several of the detectors that give highest
temperature indication are selected for use with a temperature indicator or recorder usually
having alarm contacts.
One form of dector operated relaying equipment using a wheatstone bridge circuit and
a directional relay. In another form of equipment, the stator current is used to energize the
bridge.
The relay is arranged with heating and heat storage element so as to heat up an cool
down as soon as the same rating as the machine in response to the same variation in the
current. A thermostatic element closes contacts at a selected temperature and it will not
operate for failure of the cooling system.
The temperature detector operated devices are preferred because they response more
nearly to the actual temperature of stator.
In unattended station, temperature relays are arranged to reduce the load or shunt
down the unit if it overheats, but in an attended station the relay, if used merely sounds an
alarm.
5.(b) Overvoltage protection
The overvoltage protection is recommended for all hydroelectric or gas turbine
generator that are subject to overspeed and consequence overvoltage on loss of load. It is no
generally used with steam turbine generator.
This protection is often provided by the voltage regulating equipment if it is not
should be provided by an ac overcurrent relay. This relay should have a time delay unit with
pickup at about 130% to 150% of rated voltage. Both the relay units should be compensated
against the effect of variable frequency. The relay should be energized from a potential
transformer, other than the one used for the automatic voltage regulator. Its operation should
prefereably first cause additional resistance to be inserted in the generator or exciter field
circuit. Then, if overvoltage persists the main generator breaker and generator or exciter field
breaker should be tripped.
6. The protection of a three winding transformer with a two winding percentage differential
relay
Unless there is a source of generation bank of only one side of a power transformer a
two winding percentage differential relay should not be used to protect a three winding
transformer. As shown in fig(a) when a two winding relay is used the secondary on two sides
of generation bank of one of these sides the condition shown by the arrow of fig(a) may be
sufficient unbalance between CT currents, either because of mismatch or error or both to
cause the differential relay to operate undesirably. The relay should not have the benefit of
through current restraint which is the basis for using the percentage differential principle.
Instead only the unbalance current would flow all of the operating coil and half of the
restraining coil. In effect this consistitude a 200% unbalance and it is only necessary that
unbalance current be above the relays minimum pickup for the relay to operate.
Of course, if two sides where CT are paralleled in fig(a0 supply load and do not
connect to a source generation. A two winding relay may be used with impurity.
As shown in fig(b) if a three winding relay is used, there will always be through
current restraint to restrict the relay against undesired operation.
A further a advantage of a three winding relay with a three winding transformer is that
where relay types are involved having taps for matching the CT secondary current it is often
unnecessary to use any auxiliary contacts. Thus a three winding relay may even be used with
advantage whenever a two winding relay might suffice. There is no disadvantage other than a
slight increase in cost in using a three winding relay on a two winding transformer no harm is
done if on of the restraint circuit is left unconnected.
Figure: (a) A misapplication of two winding transformer differential relay
Figure: (b) illustrate advantage of a three winding relay with a three winding transformer
7.(a) Current differential relaying with percentage differential relays
As in differential relaying for generator and transformer the principle of percentage
differential relay is a great improvement over current relay in a differential CT circuit. The
problem of providing enough restraint circuit has been largely sloved by so called
multirestraint relay. By judicious grouping of circuit and by the use of two relays per phase
where necessary sufficient restraining circuit is generally provided. Further improvement in
sensitivity is also provided by the variable percentage characteristics like that decried in
connection with generator protection with this characteristics one should make sure that very
high internal fault current will not cause sufficient restraint to prevent tripping.
This type of relaying equipment is available with operating time of the order of 3 to 6
cycles. It is not suitable when high speed operation is required.
As in current differential relay with overcurrent relay the problem of calculating the
CT error is very difficult. The use of percentage restraint and variable percentage
characteristics make the relay quite insensitive to the effect of CT error. Nevertheless, it is
recommended that each application be referred to the manufacture together with all the
necessary data.
A disadvantage of this type of equipment is that the CT secondary leads must run to
relay panel.
7.(b) Current differential relaying with over voltage relays
Figure: bus protection using current differential relaying with over voltage relays
A type of high speed relaying equipment employing current differential relaying with
over voltage relay also eliminated the problem of current transformer saturation with this
equipment conventional exactly as for current differential relay over voltage rather than over
current relay are used.
In this equipment the impedance of over voltage relays coil is made to appear to the
circuit as resistance by virtue of a fall wave rectifier. The efficiency of this equipment is not
lowered as it would be if a series reactor were used.
The capacitance and inductance as shown in series with the rectifier circuit are in
series resonance at fundamental system frequency. The purpose of this is to make the relay
responsive to only the fundamental component of the CT secondary current so as to improve
the relay selectivity. This has the disadvantage of slowing the voltage relay response slightly,
but this is not serious in view of the high speed operation of an overcurrent relay element
now to be described.
As in fig an overcurrent relay unit in series with the voltage limiter provides high
speed operation for bus fault involving high magnitude current. As the overcurrent unit is
relied on only for high magnitude current it’s pickup can easily be made high enough to
avoide operation for external fault.
For the most possible result, all the CT should have the same rating and should be a
type like a bushing CT with a distributed secondary winding that gives little or no secondary
leakage reactance.
8. Different type of static relays
i. Electronic Relays
ii. Transistor Relays
iii. Transducer Relays
iv. Rectifier Bridge Relays
v. Hall effect Relays
vi. Gauss effect Relays
(i) Electronic Relays
- The two basic arrangement one as an amplitude comparator and another as a phase
comparator as shown in fig:
- In the former case, two a.c quantities to be compared are rectified and applied in
opposition in the control grid ckt of an electronic tube, so that operation occurs when one
quantity exceed the other by an amount depending on the bias.
- In the latter case, one a.c quantity can be connceted to the control grid of an
electronic tube; the another a.c quantity to the screen grid, so that operation occurs when two
quantities are in phase.
Advantages of an electronic relay
i. Low burden on C.Ts & P.Ts, since the operation power is from an auxiliary d.c
supply.
ii. Absence of mechanical inertia and bouncing contain
iii. Fast operation
iv. Low maintenance
Disadvantages of an electronic relay
i. Presence of incandescent filament and necessary low vtg power supply to heat them.
ii. Short life of the electronic valves.
iii. High power consumption
iv. Requirement of high tension supply
v. High cost of simple relay such as overcrt relays.
Transistor Relays
- The characteristics of modern transistor are such that they can replace the functional
element which are used in electromechanical relay to give necessary characteristics.
- Two basic arrangement of relays based on transistor comparators are shown in fig:
- In either of these ckt, crt of constant magnitude flow in the collector ckt only when
the input a.c quantities are simultaneously negative, a relay in the collector ckt will
pick up when the overlap angle exceeds a certain value.
Advantages of transistor relay
i. Quick response, long life, high resistance to shock & vibration
ii. Quick reset action
iii. No bearing friction or contact troubles
iv. Ease of providing amplification enable greater sensitivity to be obtained.
v. The low energy level required in measuring ckt.
vi. Use of printed to avoid wiring error and to facilitate rationalization of batch
production
Limitations
i. Variation of characteristics with temperature and age.
ii. Dependence of reliability on large number of small components and their electrical
connection.
iii. Low short time overload capacity.
