Hitachi Relion 670 Series Applications Manual page 81

1MRK506375-UEN Rev. N
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
6.2.4 Setting guidelines
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: To select GBASVAL function for reference of base values.
Operation: For transformer differential protection Off/On.
6.2.4.1 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 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.
The usual practice for transformer protection is to set the bias characteristic to a value of at least
twice the value of the expected spill current under through-fault conditions. These criteria can vary
considerably from application to application.
The default settings for the operating characteristic with IdMin = 30% of the railway power
transformer rated current can be recommended in normal applications. If the conditions are known in
more detail, a higher or a lower sensitivity can be chosen. In such cases, the selection of the suitable
characteristic should be based on the class of the current transformers, the short-circuit power of the
systems, and so on.
The second section of the restrain characteristic has an increased slope in order to deal with the
increased differential current during heavy loading of the transformer and external fault currents.
The third section of the restrain characteristic further decreases the sensitivity of the restrained
differential function in order to cope with CT saturation during heavy through faults.
The unrestrained operation level has a default value of IdUnre = 800% of the railway power
transformer rated current, which is typically acceptable for most of the standard applications. The
overall operating characteristic of the transformer differential protection is shown in Figure 32:
Railway application RER670
Application manual
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Section 6
Differential protection
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