Medium Voltage Application Guide

SWITCHGEAR
Exercise 3: Protection CT2 for transformer feeder circuit:
Step1: Calculate transformer TXR2 nominal primary current, In (A)
n
The primary current for TXR2 is 105 A
Step 2: Calculated maximum expected short circuit current at CT2 installation, Isc (A)
Ignoring any power cable or busbar impedances:
I sc
I n
The maximum expected short circuit current at CT2 is 2100 A
Step 3: Select protection CT2 ratings
Primary rated current
Use a rating of 150A
Secondary rated current
Use a rating of 1 A
Short-time withstand rating,
Use a rating of 10 kA
Primary circuit voltage
Use a ratings of 12 kV
I pr = (1.0-1.25) x I n
= (1.0-1.25) x 105
I sr
I th ≥ I sc
U p ≥ U
Real output power: typically > 1 VA for digital type protection relay.
Use 2.5 VA (this allows 1.5 VA for cable burden, etc)
Step 4: Calculate protection class 5PX
The instantaneous trip current level of protection relay OC2 is set to 10 x I n
I_trip = 10 x 105
= 1050 A (primary current)
(Note: In most digital protection relays, the trip current levels are set with respect to the secondary current. In this
case
SEC
The instantaneous trip current level for the CT secondary is 7 A
The trip current level should fall between 100 to 50% of the accuracy limit factor (ALF).
Using an ALF of 10 (5P10), the trip current level of 1050 A falls within the range of 100% to 50% ALF so a 5P10
protection class CT is suitable.
)
)
Use protection class 5P10
710-12280-00A Medium Voltage Application Guide Page 117