Setting Guidelines; Restrained And Unrestrained Differential Protection - ABB RET650 Applications Manual

Section 6
Differential protection
6.1.3
6.1.3.1
76
It is important that the faulty transformer 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 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. A correct analysis of fault
conditions by the differential protection must take into consideration changes due
to the voltage, current and phase angle caused by the protected transformer.
Traditional transformer differential protection functions required auxiliary
transformers for correction of the phase shift and ratio. The numerical
microprocessor based differential algorithm as implemented in the IED
compensates for both the turn-ratio and the phase shift internally in the software.
No auxiliary current transformers are necessary.
The differential current should theoretically be zero during normal load or external
faults if the turn-ratio and the phase shift are correctly compensated. However,
there are several different phenomena other than internal faults that will cause
unwanted and false differential currents. The main reasons for unwanted
differential currents may be:
• mismatch due to varying tap changer positions
• different characteristics, loads and operating conditions of the current
transformers
• zero sequence currents that only flow on one side of the power transformer
• normal magnetizing currents
• magnetizing inrush currents
• overexcitation magnetizing currents
The three winding transformer differential protection function can
be also used in variety of application for two winding transformer
protection. In such application, it is suggested to set the tertiary
winding (W3) rated current and rated voltage to the exact same
rating as secondary winding (W2).

Setting guidelines

The parameters for the Transformer differential protection function are set via the
local HMI or Protection and Control IED Manager (PCM600).

Restrained and unrestrained differential protection

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
1MRK 504 169-UEN A
Transformer protection RET650 2.2 IEC
Application manual
IP14936-1 v2
M15266-4 v5
M15266-289 v10