7.16.2 Application
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A breakdown of the insulation between phase conductors or a phase conductor and earth
results in a short-circuit or an earth fault. Such faults can result in large fault currents and may
cause severe damage to the power system primary equipment.
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:
• 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
• One undervoltage step with the following built-in feature:
• Definite time delay
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).
7.16.2.1 Base quantities
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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.
7.16.2.2 Application possibilities
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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.
Section 7 1MRK 511 358-UEN A
Current protection
170
Application manual
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