Differential Protection
for Arbitrary Three-Phase
Power Transformers
Zoran Gajić
Doctoral Dissertation
Department of Industrial Electrical Engineering and Automation
Lund University- SWEDEN
Abstract
This thesis describes how to provide standardized, current based,
differential protection for any three-phase power transformer, including
phase-shifting transformers with variable phase angle shift and
transformers of all construction types and internal on-load tap-changer
configurations. The use of standard transformer differential protection for
such applications is considered impossible in the protective relaying
standards and practices currently applied.
The first part of the thesis provides the background for different types of
power transformers and the differential protection schemes currently
applied. After that a complete mathematical proof for the new, universal
transformer differential protection principle, based on theory of
symmetrical components, is derived. It is demonstrated that it is possible
to make numerical differential protection relays which can properly
calculate differential currents for any power transformer, regardless of
whether it is of fixed or variable phase angle shift construction and
whether current magnitude variations are caused by on-load tapchanger(
s).
It is shown how to correctly calculate differential currents by
simultaneously providing on-line compensation for current magnitude
variations, on-line compensation for arbitrary phase angle shift variations
and settable zero-sequence current reduction on any power transformer
side. By using this method differential protection for arbitrary power
transformers will be ideally balanced for all symmetrical and nonsymmetrical
through-load conditions and external faults. The method is
independent of individual transformer winding connection details (i.e. star,
delta or zigzag), but dependent on having the correct information about
actual on-load tap-changer(s) position if they are built-in within the
protected power transformer.
The implementation and practical use of this new universal principle is
quite simple, as all necessary transformer data is commonly available on
the protected power transformer rating plate. Practical application of the
universal method for the differential protection of standard transformers,
special transformers and phase shifting transformer is presented. Detailed
testing of this new universal differential protection method is given and it
is based on actual field recordings captured by numerical relays in existing
phase-shifting transformer installations and on simulations from the Real
Time Digital Simulator for a practical dual-core, symmetrical phaseshifting
transformer. The implementation of the universal transformer
differential method for analogue and
numerical transformer differential
relays is also described.
LINK ORIGINAL EN LA WEB
http://www.iea.lth.se/publications/Theses/LTH-IEA-1055.pdf
