Voltage Dependent Overcurrent (51V); Cold Load Settings (51C) - Siemens 7SR21 Manual

2.2

Voltage dependent overcurrent (51V)

Reduced voltage can indicate a fault on the system, it can be used to make the 51 elements more sensitive.
Typically Voltage Dependent Over-current (51V) is applied to:
Transformer Incomers: Where the impedance of the transformer limits fault current the measured
voltage level can be used to discriminate between load and fault current.
Long lines: Where the impedance of the line limits fault current the measured voltage level can be used
to discriminate between load and fault current.
Generator circuits: When a Generator is subjected to a short circuit close to its terminals the short-
circuit current follows a complex profile. After the initial "sub-transient" value, generally in the order of 7
to 10 times full load current, it falls rapidly (around 10 to 20ms) to the "transient" value. This is still about
5 to 7 times full load and would be sufficient to operate the protection's over-current elements. However
the effect on armature reactance of the highly inductive short-circuit current is to increase significantly
the internal impedance to the synchronous reactance value. If the Automatic Voltage Regulation (AVR)
system does not respond to increase the excitation, the fault current will decay over the next few
seconds to a value below the full load current. This is termed the steady state fault current, determined
by the Generator's synchronous reactance (and pre-fault excitation). It will be insufficient to operate the
protection's over-current elements and the fault will not be detected. Even if AVR is active, problems may
still be encountered. The AVR will have a declared minimum sustained fault current and this must be
above the protection over-current settings. Close-in short circuit faults may also cause the AVR to reach
its safety limits for supplying maximum excitation boost, in the order of several seconds, and this will
result in AVR internal protection devices such as diode fuses to start operating. The generator excitation
will then collapse, and the situation will be the same as when no AVR was present. The fault may again
not be detected.
Current grading remains important since a significant voltage reduction may be seen for faults on other parts of
the system. An inverse time operating characteristic must therefore be used.
The VDO Level - the voltage setting below which the more sensitive operating curve applies - must be set low
enough to discriminate between short-circuits and temporary voltage dips due to overloads. However, it must also
be high enough to cover a range of voltage drops for different circuit configurations, from around 0.6Vn to almost
zero. Typically it will be set in the range 0.6 to 0.8Vn.
2.3

Cold Load Settings (51c)

Once a Circuit-Breaker has been open for a period of time ed, higher than normal levels of load current may flow
following CB re-closure e.g. heating or refrigeration plant. The size and duration of this current is dependent upon
the type of load and the time that the CB is open.
The feature allows the relay to use alternative Shaped Overcurrent (51c) settings when a Cold Load condition is
identified. The cold load current and time multiplier settings will normally be set higher than those of the normal
overcurrent settings.
The relay will revert to its usual settings (51-n) after elapse of the cold load period. This is determined either by a
user set delay, or by the current in all 3-phases falling below a set level (usually related to normal load levels) for
a user set period.
©2010 Siemens Protection Devices Limited
7SR21 & 7SR22 Argus Applications Guide
Chapter 7 Page 13 of 39