28-12-2012, 06:07 PM
Differential Protection for Power Transformers with RET 670
Differential Protection.pdf (Size: 132.47 KB / Downloads: 150)
Introduction
Differential protection for power transformers has been used for decades. In order to correctly
apply transformer differential protection proper compensation for:
! power transformer phase shift (i.e. vector group compensation)
! CT secondary currents magnitude difference on different sides of the protected
transformer (i.e. ratio compensation)
! zero sequence current elimination (i.e. zero sequence current reduction)
shall be done. In the past this was performed with help of interposing CTs or special
connection of main CTs (i.e. delta connected CTs). With numerical technology all these
compensations are done in relay software.
This document will demonstrate how this compensation shall be done for RET 670.
RET 670 design principles
RET 670 is fully numerical device. Thus it is capable to provide differential protection for all
standard three-phase power transformers without any interposing CTs. It has been designed
with assumption that all main CTs will be star/wye connected. For such applications it is then
only necessary to enter directly CT rated data and power transformer data as they are given
on the power transformer nameplate and differential protection will automatically balance
itself. However RET 670 can as well be used in applications where some of main CTs are
connected in delta. In such cases the ratio for main CT connected in delta shall be
intentionally set for sqrt(3)=1.732 times smaller than actual ratio of individual phase CTs (e.g.
instead of 800/5 set 462/5). At the same time the power transformer vector group shall be set
as Yy0 because the RET 670 shall not internally provide any phase angle shift
compensation. The necessary phase angle shift compensation will be provided externally by
delta connected main CT. All other settings should have the same values irrespective of main
CT connections. It shall be noted that irrespective of the main CT connections (i.e. star/wye
or delta) on-line reading and automatic compensation for actual on-load tap-changer position
can be used in RET 670.
Application Examples with RET 670
Three application examples will be given here. For each example two differential protection
solutions will be presented:
! First solution will be with all main CTs star/wye connected
! Second solution will be with delta connected main CT on Y (i.e. star/wye) connected
sides of the protected power transformer
For each differential protection solution the following settings will be given:
1. Input CT channels on the RET 670 TRM modules
2. General settings for the transformer differential protection where specific data
about protected power transformer shall be entered
Finally the setting for the differential protection characteristic will be given which can be
identical for all presented applications.
Summary and conclusions
RET 670 can be used for differential protection of three-phase power transformers with main
CTs either wye or delta connected. However the relay has been designed with assumption
that all main CTS are wye connected. However RET 670 can be used in applications where
main CTs are delta connected. For such applications the following shall be kept in mind:
1) Ratio for delta connected CTs shall be set sqrt(3)=1.732 times smaller then actual
individual phase CT ratio
2) Power transformer vector group shall be typically set as Yy0 because the
compensation for power transformer actual phase shift is provided by external delta
CT connection
3) Zero sequence current is eliminated by main CT delta connection. Thus on sides
where CTs are connected in delta the zero sequence current elimination shall be set
to Off in RET 670