87T Transformer Differential Protection
Overview of Functions
The
Transformer Differential Protection
function (87T) is primarily intended to detect faults in 2
and 3 winding transformers. The function can also be applied to other plant items e.g. auto-transformers,
reactors, short lines and motors.
Structure of the Function
The transformer differential protection function element has group dependent settings. These settings can be
different in each settings group – group numbers (Gn) 1 to 4.
This function corresponds to the logical node A81Hb2PHARn, A81Hb5PHARn, A87BdPDIFn and A87HsPDIFn in
IEC 61850.
The function monitors the primary system using 2 or 3 sets of the 3 phase current inputs e.g. CT1/2/3, CT5/6/7
and CT9/10/11. The fundamental frequency component is measured from processing of the measured current
samples.
The
Transformer Differential Protection
function consists of 2 elements 87T-BD and 87T-HS.
Differential protection is based on the comparison of currents at each end of the protected zone. Detection of
differential current indicates a fault within the protected zone. The calculation of the difference is determined
through the current direction definition. The current direction is defined as positive current flows towards the
protected object. The current difference results from the vector addition of the currents.
[dw_7SR5_function87TStructureOfTheFunction1, 1, en_US]
Figure 5-48
Basic Principle of Differential Protection – Example for 2 Inputs
The application of differential protection to transformers must consider:
•
The current magnitude change introduced by the transformer HV/LV turns ratio. The current ratio may
also be variable due to the presence of an
On-Load-Tap-Changer
(OLTC).
•
The transformer connections which may introduce a phase change between the currents flowing into
each winding of the transformer.
•
Magnetizing inrush current which flows in only 1 winding of the transformer when energized.
5.22
5.22.1
5.22.2
Protection and Automation Functions
5.22 87T Transformer Differential Protection
Reyrolle 7SR5, Transformer Protection Device, Device Manual
307
C53000-G7040-C015-1, Edition 11.2019
