APPENDIX V
GUIDELINES FOR RELAY SETTINGS AT TRACTION SUBSTATIONS AND
SECTIONING POSTS
The following guidelines may be followed for calculating the settings of various protective relays at traction substations and sectioning posts:
1. Ohm Impedances
The following values of OHE impedances may be used for the purpose of calculating relay settings:
i. Single track OHE without return conductor 
0.41/ 70° Ohm/km 
ii. Double track OHE without RC 
0.24/ 70° Ohm/km 
iii. Single track OHE with RC 
0.70/ 70°Ohm/km 
iv. Double track OHE with RC 
0.43/ 70° Ohm/km 
v. Add booster transformer impedance at the rate of 0.15 Ohm per booster transformer, where these are provided.
2. Feeding post: Feeder protection
2.1 Distance protection using 'Mho' relay. Type YCG 14.
At present, electromechanical type 'Mho' relays are used for protection against catenarytoearth faults. The maximum torque angle of this relay is 75°. Its impedance setting may be decided as follows:
i. In the case of singleline sections, the impedance setting of the relay may correspond to 1.25 times the impedance of the OHE from the feeding post up to the adjoining feeding post. The factor 1.25 used here is to cater for errors in the relays, CT & PT. Following relations may be used for calculating the relay settings.
II. In the case of double line sections, the following procedure may be followed:
a) Calculate the single line impedance of the OHE from the feeding post to the adjoining feeding post; say this is Z_{1 }. Calculate X from the following relations:
b) Assuming single line operation from the feeding post (FP) up to the sectioning post (SP) & doubleline operation from SP to the adjoining FP, calculate the OHE impedance; say this is Z_{2}Calculate Y from the following relation: 2
c) 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:
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.
d) The impedance setting of the Mho relay should be a minimum of Yand 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 X, then the relay may be set to X. 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. 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 shall be able to see the faults on either side of feeding post.
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 overreach, the relay will allow loads of about 200/1.25, i.e. 160% of the rated load current.
2.3 Wrong phase coupling protection using offset MHO type YCG14 (English  Electric make).
The impedance setting of the English Electric make YCG14 relay is given by K_{1}K_{2}(K_{3}+K_{4}), where K_{1} & K_{2} are plug board settings and K_{3} & K_{4}are potentiometer setting (Refer Relay Catalogue). The maximum torque angle of the relay is 125 and forward offset is about 10% of K_{1}K_{2}.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 TSS1 may be determined graphically following the procedure
given below (refer Fig. A 5.01):
a. Draw lines A'A, AB & BB' as shown.
where,
A is the origin of RX diagram
A'A = Source impedance at TSS1 as seen on the 25 kV side.
The impedance angle may be taken as 85° (This includes transformer and transmission line impedances).
AB = Minimum OHE impedance between TSS1 and TSS2. The impedance angle may be taken as 70° .(For a doubleline section, this corresponds to the doubleline impedance of the section, whereas for a single line section this corresponds to the singleline impedance of the section).
BB' = Source impedance at TSS2 as seen on the 25 kV side. The impedance angle may be taken as 85°. (This includes transformer and transmission line impedances).
b. Join A' with B'. Construct a rightangle bisector to A' B' and locate points P& Q on the bisector line such that the angles subtended at these points by A'B' are 120° and 60° respectively. P & Q are the WPC points for CaseI( refer Fig. A 5.01).
c. Draw lines AC & CC', where
AC = Maximum OHE impedance between TSSland TSS3. The impedance angle may be taken as 70° . (This corresponds to the singleline OHE impedance between TSS1 and TSS3 irrespective of singleline /double line section).
CC = Source impedance at TSS3 as seen on the 25kV side. The impedance angle may be taken 85°. (This includes transformer and transmission line impedances).
d. Join A' with C'. Construct a rightangle bisector to the line A'C'. Locate points R & S the rightangle bisector such that the angles subtended by A'C are 120° and 60° respectively. R and S are the WPC points for CaseII (refer Fig. A 5.01).
e. Draw line AO at an angle of 125° with the R axis. This line is the maximum torque angle line for the relay.
f. Draw a circle with centre on the maximum torque angle line such that the points P,Q,R & S are just enclosed by the circle. The circle cuts the maximum torque angle line at D and E. The offset 'AE' will be equal to 0.1 x K_{1} x K_{2} x PT ratio/CT ratio. As K_{1} and K_{2} are not known exactly, guess may be made initially. Step (f) may be repeated after going through step (g). .
g. Measure AD, say it is 'Z_{1} '.
Now, the desired impedance setting of the relay at TSS1, say Z_{2}
The factor 1.25 used here is to cater for errors in the CT, PT & relay. Values of K_{1}, K_{2} , K_{3} , & K_{4} may be suitably selected to get the impedance setting Z . Forward offset AE may be checked to be around 0.1 x K_{1} x K_{2} x PT ratio/CT ratio. If the offset is different, the circle drawn at (f) may be redrawn to satisfy this requirement.
DETERMINATION OF THE SETTING OF WRONG PHASE COUPLING RELAY
FIG. A 5.01
h. The impedance settings of wrong phase coupling relay for two sides of the TSS1 may be calculated individually following the above procedure. The higher of the two values may be adopted for both the WPC relays at TSS1.
3. Traction substation: Transformer protection:
Low Voltage side (25 kV side)
I. Restricted earth fault relay
The current setting of this relay may correspond to 10% of the rated current of traction power transformer.
II. IDMT over current relay.
The function of this relay is to act as backup 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 timemultiplier 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.
3.2 High Voltage side (220kv/132kV/110kV/66kV side)
i. Restricted earth fault relay
The current setting of the relay may correspond to 10% of the rated current of traction power transformer.
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 substation.
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 throughfault 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 maloperations of the relay are observed on through faults and magnetising inrush (switching in of power transformer), a higher setting may be considered.
c) The time multiplier setting may be taken as 1. If maloperations of the relay are noted on magnetising in rush but not on through faults, then the time multiplier setting may be increased.
4. Sectioning posts
4.1 Undervoltage protection .
in case of extended feed, the bridging interrupter at the SP is in closed condition. An under voltage relay is provided to open this interrupter when the OHE voltage falls below a satisfactory operating value.
The undervoltage relay may be set to operate between 19 and 20 kV depending on local conditions.
SAMPLE CALCULATIONS FOR RELAY SETTINGS
EXAMPLE1
The following traction substation details are assumed here:
1.1 Traction Power Transformer:
2. Feeding post : Feeder protection:
2.1 Distance protection using Mho, relay, YCC14.
The OHE section details assumed here are given in Fig. A 5.02.
FIG. A5.02
The calculations given below are for the setting of CB1. The values of Z_{1} X, Z_{2}, Y, Z given in the guidelines are calculated as:
A BT spacing of 2.66 km is assumed above, giving 6 BTs in each line. The impedance angle of the BT is assumed here the same as the OHE, I.e. 70°
Since the value of Z lies between X & Y. the calculated setting for CB1 = 17.18 Ohm. Similarly, calculate the setting for CB2. and . say the setting calculated for CB2 should be = 18.0 Ohm.
Now following the guidelines given in para 2.1 (iii). both the Mho' relay of CB1 and CB2 should be set at 18.0 Ohms.
2.2 Instantaneous over current protections.
The current setting has to correspond to 200% of the continuous current rating of the traction transformer, i.e. 1000 Amps, on the primary side and 10 Amps, on the secondary side of CT. Hence the relay has to be set to 10 Amps. If the rated current of the relay is 5 Amps., this corresponds to 200% setting of the relay.
2.3 Wrong phase coupling protection using off set MHO Relay type
YCG14 (Fig.A5.03)
Assumption made in the sample calculation;
i) 3ph.Fault level of TSS1 & TSS2 is 1000MVA and TSS 3 is 1500MVA.
ii) Traction transformer impedance is 12% at 13.5 MVA, 27 kV Base.
iii) Double line section without B.T.& R.C.
iv) Distance between TSS1 & TSS2 Is 60 km.
v) Distance between TSS2 & TSS3 is 60 km.
vi) OHE impedance angle = 70°
vii) Source impedance (Transmission line & Traction Transformer) angle is 85
viii) Max. Torque Angle of the YCG14 WPC relay: 125 .
ix) C.T. Ratio = 500 A/5A
x) P.T. Ratio = 25000V/110V.
3Phase Fault level of TSS1 & TSS2 on 132 kV side. = 1000MVA
Hence,
AE = 2.7 / 125° Ohm.
Draw the circle diagram as per the guidelines as shown in Fig. A 5.04.
FIG. A5.04
