Overcurrent Relay Setting Calculation
1. Pickup Current Setting
Time Overcurrent relay using definite Characteristics
For example: CT Ratio: 800/1A
Rated Voltage: 132kV
For Phase element
Plug Setting:
Considering maximum current through the circuit: 800A
I am taking here 90% =0.9*In =0.9
Current Setting =720/800
Correspondence secondary current IN =0.9
Hence pick up setting =0.9A
2. Time Multiplier Setting (TMS) OR (TDS)
Formula: TMS = (Operating Time × (PSM^0.02 – 1)) ÷ 0.14
Where PSM = Plug Setting Multiplier = Fault Current ÷ Pickup Current
For coordination with downstream relay:
• For Numerical Relay TMS typically ranges from 0.1 to 1.0 (=0.5 sec)
• For Electro Mechanical Relay TDS Typically ranges from 0.1 to 1.0
• Higher TMS or TDS = longer operating time
So Plug Setting: =0.9A
TMS Setting: =0.5 sec
Instantaneous Setting: =Blocked