Areva MiCOM P125, Техническое руководство

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Application Guide P12y/EN AP/D32
MiCOM P125/P126 & P127 Page 33/74
3.4.1 Setting Guidelines
The voltage setting applied to the protection elements is dependent upon the magnitude of
residual voltage that is expected to occur during an earth fault condition.
This in turn is dependent upon the method of system earthing employed. It must also be
ensured that the relay is set above any standing level of residual voltage that is present on
the system.
The protection element has one programmable element with delay time tUe>>>>.
The setting range and the functionality limits for the residual over-voltage are described in
the TD chapter, the setting menu is described in the User Guide chapter.
3.5 (27) Undervoltage Protection (P127)
Under-voltage conditions may occur on a power system for a variety of reasons, some of
which are outlined below:
Increased system loading
Generally, some corrective action would be taken by voltage regulating equipment such as
AVRs or On Load Tap Changers, in order to bring the system voltage back to its nominal
value. If this voltage regulating equipment is unsuccessful in restoring healthy system
voltage, then tripping by means of an under-voltage relay will be required following a suitable
time delay.
Faults occurring on the power system result in a reduction in voltage of the phases involved
in a fault. The proportion by which the voltage decreases is directly dependent upon the type
of fault, method of system earthing and it’s location with respect to the relay installation point.
Consequently, co-ordination with other voltage and current-based protection devices is
essential in order to achieve sufficient discrimination.
Complete loss of bus-bar voltage
This may occur due to fault conditions present on the incomer or busbar itself, resulting in
total isolation of the incoming power supply. For this condition, it may be a requirement for
each of the feeders to be isolated, so that when supply voltage is restored, the load is not
connected. Hence, the automatic tripping of a CB on a feeder upon detection of complete
loss of voltage on the busbar may be required. This may be achieved by a three-phase
undervoltage protection element.
3.5.1 Setting Guidelines
In the majority of applications, undervoltage protection is not required to operate during
system earth fault conditions. If this is the case, the protection element should be selected in
the menu to operate from a phase-to-phase voltage measurement, as this quantity is less
affected by the decrease of a single-phase voltage due to an earth fault.
The voltage threshold setting for the undervoltage protection element should be set at some
value below the voltage decreases which may be expected under normal system operating
conditions. This threshold is dependent upon the system in question but typical healthy
system voltage decreases may be in the order of -10% of nominal value.
The undervoltage protection element has two programmable thresholds with delay timers:
tU<, tU<<
After a trip command the thresholds will be reset when all phase voltages have risen above
105% of setting values.
The undervoltage protection element, which can be set as OR or AND logic, operates by
comparing each UAB, UBC and UCA voltage input with the U<, U<< thresholds.
The relay continuously monitors the phase-to-phase voltages.
NOTE: If the relay is connected in the mode 3 Vpn to control the phase to
neutral voltages, the thresholds must be multiplied by
3
.