Formulae

A)  Thermal Rating

Thermal ratings of some common conductors used in transmission system is tabulated below

Table-1

SrNo	Conductor Size/Name	Thermal Rating in AMP	Remarks
1.	0.2 Panther	487 Amp.	Used for 132kV lines
2.	0.4 Zebra	737 Amp.	Used for 220kV lines

B)  Deciding relay PSM (Plug Setting Multiplier)

For Overcurrent relay

For E/F Relay

NOTE:

1)   For O/C PSM values mentioned above is approximate value and may change slightly depending upon site situation. However as a general rule O/C PSM adopted shall be such that full capacity utilization of the equipment being protected shall be possible.

2)   If CT ratio is very high as compared to expected fault current or full load current of equipment being protected then E/F PSM may reduce to 10% too. Generally it shall be 30% of full load current

C) Deciding desired time of operation                                                       

Time of operation for O/C E/F relay for protection of EHV line shall depend on location of relay in power system and mode of operation of the line. Following guidelines may be adopted for deciding time of relay operation.

 

Note: Adopted relay operational time shall be validated for operational time of other relays in system

D) Deciding TMS (Time Multiplier Setting)

CT Secondary fault current =

 

Where value for required time of operation is 350 ms or 500 ms as per cases mentioned in part C.

NOTE: Valid for NI3 characteristics and 1 Amp Relay.

E)  Deciding MTA

For protection of EHV lines generally directional overcurrent and earth fault relays are used. In case of directional relay it is necessary to set correct MTA for over current as well as earth fault relay. Maximum Torque Angle depends upon factors such as source impedance and method adopted for earthing of generators and transformers. In transmission system generally solidly earthed transformers are used. Hence it is common to use following settings for MTA

a.   For Overcurrent Relay MTA = +45⁰ or 45⁰ leading

b.   For Earth Fault Relay MTA = -45⁰ or 45⁰ lagging