Application Notes P54x/EN AP/La4
MiCOM P543, P544, P545 & P546
(AP) 6-83
AP
6. CURRENT TRANSFORMER REQUIREMENTS
6.1 Recommended CT classes (British and IEC)
Class X current transformers with a knee point voltage greater or equal than that calculated
can be used.
Class 5P protection CTs can be used, noting that the knee point voltage equivalent these
offer can be approximated from:
Vk = (VA x ALF)/In + (RCT x ALF x In)
Where:
VA = Voltampere burden rating
ALF = Accuracy limit factor
In = CT nominal secondary current
6.2 Current differential requirements
For a
ccuracy, class X or class 5P current transformers (CTs) are strongly recommended.
The knee point voltage of the CTs should comply with the minimum requirements of the
formulae shown below.
Vk K. In (Rct + 2 RL)
Where:
Vk = Required IEC knee point voltage
K = Dimensioning factor
In = CT nominal secondary current
Rct = CT resistance
RL = One-way lead impedance from CT to relay
K is a constant depending on:
If = Maximum value of through fault current for stability (multiple of In)
X/R = Primary system X/R ratio
K is determined as follows:
For relays set at Is1 = 20%, Is2 = 2 In, k1 = 30%, k2 = 150%:
K must be the highest of:
K 40 + (0.07 x (If x X/R))
Or
K 65
This is valid for (If x X/R) 1000
For higher (If x X/R) up to 1600:
K = 107
For relays set at Is1 = 20%, Is2 = 2 In, k1 = 30%, k2 = 100%:
K must be the highest of:
K 40 + (0.35 x (If x X/R))
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