# How to protect a power transformer during ground faults,

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```							How to protect a power transformer during ground faults, that may be
left undetected by the main percent differential function of the T60
relay?

The T60 relay is equipped with a number of RGF (Restricted Ground Fault)
protection elements that provide restricted ground fault protection.
The RGF element uses maximum phase current as a restraining signal and differential
neutral current as an operating signal. Pickup and slope are settings of a single-slope
differential characteristic of the RGF protection function.

For setting calculations example, consider a transformer with the following nameplate
data:

10MVA, 33kV /11kV,
10% Impedance,
Delta/Wye 30°,
winding neutral grounded through resistance of Rg = 6.3 ohms,
phase and neutral CT ratios = 600:1

•   Rated Load Current = I Rated = 10MVA / 3 x11kV = 525 Amps
•   Maximum Phase-to-Ground Fault Current = I gf (max) = 11kV / 3 x6.3 = 1000 Amps
•   For a winding fault point at 5% from the neutral: I fault = 0.05 xI gf (max) = 50 A
•   The phase currents I ph increases due to the fault are negligible, and therefore
3I 0 = 0 .

Therefore:
• Pickup setting = I gd = 3I 0 − I g = (0 − I fault ) / CTratio = 0.08 xCT
•   Slope = I gd / I phase(max) = (50 / 525) x100% = 9.5% ; Select Slope setting = 9%
•   Time delay: dependent on downstream protection coordination (100ms typical)

NOTE: The RGF protection may pickup during external faults with saturation on either
phase CTs or the ground CT. It is recommended to add appropriate time delay to cope
with CT saturation on external faults.

--- end of public record ----
Author: Lubo Sevov
Date: June 25, 2001
Products: T60
Keywords: T60, ground fault protection, transformer winding faults, setting calculations
Reviewed by: Bogdan Kasztenny
Reviewed on: June 27, 2001
Approved by: Bogdan Kasztenny
Approved on: June 27, 2001

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