ABB Relion 670 Series Applications Manual page 210

Section 8
Current protection
A typical application of the voltage-restrained time overcurrent protection is in the generator
protection system, where it is used as backup protection. If a phase-to-phase fault affects a
generator, the fault current amplitude is a function of time, and it depends on generator
characteristic (reactances and time constants), its load conditions (immediately before the
fault) and excitation system performance and characteristic. So the fault current amplitude
may decay with time. A voltage-restrained overcurrent relay can be set in order to remain in
the picked-up state in spite of the current decay, and perform a backup trip in case of failure
of the main protection.
The IED can be provided with a voltage-restrained time overcurrent protection (VRPVOC). The
VRPVOC function is always connected to three-phase current and three-phase voltage input in
the configuration tool, but it will always measure the maximum phase current and the
minimum phase-to-phase voltage.
VRPVOC function module has two independent protection each consisting of:
• One overcurrent step with the following built-in features:
•
•
• One undervoltage step with the following built-in feature:
•
The undervoltage function can be enabled or disabled. Sometimes in order to obtain the
desired application functionality it is necessary to provide interaction between the two
protection elements within the VRPVOC function by appropriate IED configuration (for
example, overcurrent protection with under-voltage seal-in). Sometimes in order to obtain the
desired application functionality it is necessary to provide interaction between the two
protection elements within the D2PTOC function by appropriate IED configuration (for
example, overcurrent protection with under-voltage seal-in).
8.16.2.1 Base quantities
GlobalBaseSel defines the particular Global Base Values Group where the base quantities of
the function are set. In that Global Base Values Group:
IBase shall be entered as rated phase current of the protected object in primary amperes.
UBase shall be entered as rated phase-to-phase voltage of the protected object in primary kV.
8.16.2.2 Application possibilities
VRPVOC function can be used in one of the following applications:
• voltage controlled over-current
• voltage restrained over-current
In both applications a seal-in of the overcurrent function at under-voltage can be included by
configuration.
8.16.2.3 Undervoltage seal-in
In the case of a generator with a static excitation system, which receives its power from the
generator terminals, the magnitude of a sustained phase short-circuit current depends on the
generator terminal voltage. In case of a nearby multi-phase fault, the generator terminal
voltage may drop to quite low level, for example, less than 25%, and the generator fault current
may consequently fall below the pickup level of the overcurrent protection. The short-circuit
204
Selectable definite time delay or Inverse Time IDMT characteristic
Voltage restrained/controlled feature is available in order to modify the start level of
the overcurrent stage in proportion to the magnitude of the measured voltage
Definite time delay
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