dividing obtained current value from the metrosil curve with √2. Use the maximum value from
the metrosil curve given in Figure 38.
6.1.4.5 Alarm level operation
M16850-196 v6
The 1Ph High impedance differential protection HZPDIF function has a separate alarm level,
which can be used to give alarm for problems with an involved current transformer circuit. The
setting level is normally selected to be around 10% of the operating voltage
U>Trip
.
As seen in the setting examples above the sensitivity of HZPDIF function is normally high,
which means that the function will in many cases operate also for short circuits or open
current transformer secondary circuits. However the stabilizing resistor can be selected to
achieve sensitivity higher than normal load current and/or separate criteria can be added to
the operation, like a check zone. This can be either another IED, with the same HZPDIF
function, or be a check about the fault condition, which is performed by an earth overcurrent
function or neutral point voltage function.
For such cases where operation is not expected during normal service the alarm output should
be used to activate an external shorting of the differential circuit avoiding continuous high
voltage in the circuit. A time delay of a few seconds is used before the shorting and alarm are
activated. Auxiliary relays with contacts that can withstand high voltage shall be used, like
RXMVB types.
The metrosil operating characteristic is given in the following figure.
IEC05000749 V1 EN-US
Figure 38: Current voltage characteristics for the non-linear resistors, in the range 10-200 V, the
average range of current is: 0.01–10 mA
1MRK 511 358-UEN A Section 6
Differential protection
99
Application manual
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