06-05-2011, 02:57 PM
1. Introduction
Transformers are a critical and expensive component of the
power system. Due to the long lead time for repair of and
replacement of transformers, a major goal of transformer
protection is limiting the damage to a faulted transformer.
Some protection functions, such as overexcitation protection
and temperature-based protection may aid this goal by
identifying operating conditions that may cause transformer
failure. The comprehensive transformer protection provided
by multiple function protective relays is appropriate for critical
transformers of all applications.
2. Transformer Protection Overview
The type of protection for the transformers varies depending
on the application and the importance of the transformer.
Transformers are protected primarily against faults and
overloads. The type of protection used should minimize the
time of disconnection for faults within the transformer and to
reduce the risk of catastrophic failure to simplify eventual repair.
Any extended operation of the transformer under abnormal
condition such as faults or overloads compromises the life of
the transformer, which means adequate protection should be
provided for quicker isolation of the transformer under such
conditions.
3. Transformer Failures
Failures in transformers can be classified into
• winding failures due to short circuits (turn-turn faults,
phase-phase faults, phase-ground, open winding)
• core faults (core insulation failure, shorted laminations)
• terminal failures (open leads, loose connections, short
circuits)
• on-load tap changer failures (mechanical, electrical, short
circuit, overheating)
• abnormal operating conditions (overfluxing,
overloading, overvoltage)
• external faults
4. Innovative GE Multilin Solutions to Transformer Protection Applications
4.1 Differential Characteristic
The major operating challenge to transformer differential
protection is maintaining security during CT saturation
for external faults while maintaining sensitivity to detect
low magnitude internal faults. CT saturation reduces the
secondary output current from the CT, and causes a false
differential current to appear to the relay. GE Multilin
differential relays meet this challenge in the following
ways:
• the restraint current is based on the maximum
measured winding current, as opposed to the
traditional magnitude sum of the currents. This
ensures ideal restraint for the actual fault condition,
balancing sensitivity and security.
• the differential element uses a dual slope-dual
breakpoint characteristic. The differential element
can be set to account for both DC and AC saturation
of the CTs, ensuring security, while maintaining
sensitivity.
Available in the T60, T35
Download full report
http://geindustrialpm/journals/Transformer_Protection.pdf
