Areva MiCOM P441, Техническое руководство

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Application Notes P44x/EN AP/E33
MiCOM P441/P442 & P444 Page 91/220
2.19.1 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 element should be selected in the menu
to operate from a phase to phase voltage measurement, as this quantity is less affected by
single phase voltage depressions due to earth faults.
The voltage threshold setting for the undervoltage protection should be set at some value
below the voltage excursions which may be expected under normal system operating
conditions. This threshold is dependent upon the system in question but typical healthy
system voltage excursions may be in the order of -10% of nominal value.
Similar comments apply with regard to a time setting for this element, i.e. the required time
delay is dependent upon the time for which the system is able to withstand a depressed
voltage.
2.20 Overvoltage protection
Undervoltage conditions may occur on a power system for a variety of reasons, some of
which are outlined below:-
•
Under conditions of load rejection, the supply voltage will increase in magnitude. This
situation would normally be rectified by voltage regulating equipment such as AVRs or
on-load tap changers. However, failure of this equipment to bring the system voltage
back within prescribed limits leaves the system with an overvoltage condition which
must be cleared in order to preserve the life of the system insulation. Hence,
overvoltage protection which is suitably time delayed to allow for normal regulator
action, may be applied.
•
During earth fault conditions on a power system there may be an increase in the
healthy phase voltages. Ideally, the system should be designed to withstand such
overvoltages for a defined period of time.
As previously stated, both the over and undervoltage protection functions can be found in the
relay menu “Volt Protection”. The following table shows the overvoltage section of this menu
along with the available setting ranges and factory defaults.
Menu text Default setting Setting range Step size
Min Max
Group 1
Volt protection
V> Measur't Mode Phase-Neutral Phase-phase or Phase-neutral
V>1 Function DT Disabled, DT pr IDMT
V>1 Voltage Set 75V 60V 185V 1V
V>1 Time Delay 10s 0s 100s 0.01s
V>1 TMS 1 0.5 100 0.5
V>2 Status Enabled Disabled or Enabled
V>2 Voltage Set 90V 60V 185V 1V
V>2 Time Delay 0.5s 0s 100s 0.01s
As can be seen, the setting cells for the overvoltage protection are identical to those
previously described for the undervoltage protection. The IDMT characteristic available on
the first stage is defined by the following formula:
t = K / (M - 1)
Where:
K = Time Multiplier Setting
T = Operating Time in Seconds
M = Measured Voltage / relay Setting Voltage (V>)