99

X = Z1 x 1.25 (C.T. Ratio)

-------------------- x ----------------

Cos (theta-delta) (P.T. Ratio)

b) Assuming single line operation from the feeding post (FP) upto the sectioning post

(SP) & double line operation from SP to the adjoining FP, calculate the OHE impedance;

say this is Z calculate Y from the following relation:2

Y = Z x 1.25 (C.T. Ratio)

Cos (delta-delta) (P.T. Ratio)

a) a) Assuming an overload of 50% of the traction transformer and that the entire

current is fed through one feeder circuit breaker, critical impedance setting of the

relay (to allow line operation at maximum loads) is calculated from the following

relation:

Critical impedance setting = 24,000V CT Ratio

------------- -------------

1.5x rated full load PT ratio

current of transformer

in Ampere x cos ( theeta-fyi )

Where Theeta= maximum torque angle of relay

Fyi = Load angle may be taken as 40 degree

Say, critical impedance setting is Z. For the relay not to operate under the said overload

conditions, the impedance setting of the relay must be lower than Z.

b) b) The impedance setting of the Mho relay should be a minimum of Y and a

maximum of X. The setting should also not exceed Z. In other words, when the

value of Z lies between X & Y, the relay may be set to Z. If Z is higher than Z,. then

the relay may be set to Z. If Z is less than Y, then the relay should be set to Y and it

should be understood that the assumed overload will not be permitted by the relay.

Note that the values X, Y and Z are calculated along the maximum torque angle line

of the relay.

iii. iii. The relay settings may be calculated by following the procedure given

at (I) or (ii) above for the two sides of the feeding post separately. However, the

distance protection relays of both the feeders should be set to higher of the two

calculated values. This is to ensure that when one feeder breaker is taken out for

maintenance, the relay should be able to see the faults on either side of feeding

post.

2.2 2.2 Instantaneous over-current protection

This relay provides primary protection to the catenary on earth faults in the vicinity of the

feeding post. The current setting of the relay may correspond to about 200% of the

continuous current rating of the traction transformer. Assuming that a factor of 1.25 will

account for the CT and relay errors and relay transient over reach, the relay will allow

loads of about 200/1.25 i.e. 160% of the rated load current.

23. 23. Wrong Phase coupling protection using offset MHO type

YCG 14( English – Electric Make)

The impedance setting of the English – Electric make YCG-14 relay is given by K 1 K 2

(K3 + K4 ), where K1 & K2 are plug board settings and K & KK are potentinmeter

setting ( Refer Relay Catalogue). The maximum torque angle of the relay is 125 and

forward off set is about 10% of K K .. The WPC relay at that substation where the 25

kV voltage is lagging with respect to the voltage at the substation with which it has been

wrongly coupled will operate.

The impedance setting of the WPC relays at TSS – 1 may be determined graphically

following the procedure

INDIAN RAILWAYS-AC TRACTION MANUAL [100]

h. The impedance settings of wrong phase coupling relay for two sides of the TSS-1

may be calculated individually following the above procedure. The higher of the two

values may be adopted for both the WPC relays at TSS-1.\

1. 1. Traction sub-station Transformer protection:

Low Voltage side ( 25 kV side)

1. Restricted earth fault relay

The current setting of this relay may correspond to 10% of the rated current of traction

power transformer.

ii. ii. IDMT over current relay

The function of this relay is to act as back up protection to the feeder protection relays.

The reach of the relay should be as much as possible. However, to permit overloading of

the traction power transformer of the traction substation, the current setting of the relay

may be selected to correspond to 150% of the rated current of the traction power

transformer.

The time-multiplier setting of the IDMT relay may be selected such that the relay

operation time is 0.4 to 0.5 sec. For an earth fault on the 25 kV bus.

1.2 1.2 High Voltage side ( 220KV/132KV/110KV/66KV side)

I. I. Restricted earth fault relay

The current setting of the relay may correspond to 10% of the rated current of traction

power transformer.

ii. ii. IDMT over current relay with instantaneous over current element

The instantaneous element may be set to correspond to a current of 1.25 times the fault

current, for an earth fault on the 25 kV bus at the traction substation. The purpose of

such setting is to avoid operation of this relay for 25 kV bus faults at the traction

substation.

The current setting of the IDMT relay may be such that it has maximum reach but

permits overloading of traction transformer. The setting may, therefore, be selected to

correspond to 150% of the rated current of traction power transformer. To have time

grading with the IDMT relay on the 25 kV side, the time multiplier setting may be

selected such that the relay operating time is 0.8 to 0.9 sec. For earth fault on the 25 kV

bus at the traction sub-station.

iii. Biased differential relay (type DDT)

The three settings of the relay may be selected as follows:

a) The percentage bias setting should be so chosen that the relay remains inoperative

on differential currents resulting from (1) tap changing on traction transformer, (2)

mismatch in CT ratios and (3) difference in CT saturation levels under through-fault

conditions. Percentage mismatch resulting from factors (1) and (2) may be calculated

from actual data and an allowance of 7.5 to 15% may be made for factor (3).

b) The operating current setting may be taken as 40%. If mal-operations of the relay

are observed on through faults and magnetizing inrush (switching in of power

transformer), a higher setting may be considered.

c) The time multiplier setting may be taken as 1. If mal-operations of the relay are

noted on magnetizing in rush but not on through faults, then the time multiplier

setting may be increased.

d) INDIAN RAILWAYS – AC TRACTION MANUAL –VOL II PART II

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