It is important that the faulty power transformer can be disconnected as fast as
possible. As the differential protection is a unit protection, it can be designed for
fast tripping, thus providing selective disconnection of the faulty railway power
transformer. The differential protection should never operate on faults outside the
protective zone. A transformer differential protection compares the current flowing
into the transformer with the current leaving the transformer.
The differential current should be zero during normal load or external faults if the
turn-ratio and the phase shift are correctly compensated. However, there are several
phenomena other than internal faults that will cause unwanted and false differential
currents. The main reasons for unwanted differential currents are:
•
different characteristics, loads and operating conditions of the current
transformers
•
zero sequence currents that flow only on one side of the power transformer
•
normal magnetizing currents
•
magnetizing inrush currents
•
overexcitation magnetizing currents
6.2.3
Setting guidelines
GUID-4A513657-5E32-433F-9148-5C6A0828647D v1
The parameters for the transformer differential protection T1PPDIF function are set
via the local HMI or Protection and Control Manager PCM600.
Common base IED values for primary current (
IBase
), primary voltage (
UBase
)
and primary power (
SBase
) are set in the global base values for settings function
GBASVAL.
GlobalBaseSel
: This is used to select GBASVAL function for reference of base
values.
Operation
: This is used for transformer differential protection
Off
/
On
.
6.2.3.1
Restrained and unrestrained differential protection
GUID-B65931F7-5592-4494-93C0-1762287C5299 v1
To make a differential IED as sensitive and stable as possible, restrained
differential protections have been developed and are now adopted as the general
practice in the protection of power transformers. The protection should be provided
with a proportional bias, which makes the protection operate for a certain
percentage differential current related to the current flowing through the
transformer. This stabilizes the protection under through-fault conditions while still
permitting the relay to have good basic sensitivity.
The bias current should be defined as the highest power transformer current
because this will reflect the difficulties met by the current transformers much
better. The differential protection function uses the highest differential current
contribution from the two sides as bias current. The principle with the operate-bias
characteristic is to increase the pick-up level when the current transformers have
difficult operating conditions.
1MRK 506 375-UEN A
Section 6
Differential protection
Railway application RER670 2.2 IEC
79
Application manual
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