Woodward HighPROtec MCDLV4, Руководство пользователя, Страница: 704 / 1275

Protective Elements
51V - Voltage Restraint Overcurrent
For activating this function, the parameter »
VRestraint« has to be set to active in the parameter set of the
corresponding overcurrent element I[x].
The
51V protection function restrains operation which reduces pickup levels. This allows the User to lower the
pickup value of the
51V protection function with the corresponding phase input voltage (phase-to-phase or phase-
to-ground, depending on the setting of »
Measuring Channel« within the current protection module). When the
minimum fault phase current is close to the load current, it may make the phase time overcurrent protection
coordination difficult. In this case, an undervoltage function may be used to alleviate this situation. When the
voltage is low, the phase time overcurrent pickup threshold may be set low accordingly, so that the phase time
overcurrent protection may achieve adequate sensitivity and better coordination. The device uses a simple linear
model to determine the effective pickup by characterizing the relationship between the voltage and the phase time
overcurrent pickup threshold.
Once the voltage restraint protection function is activated, the effective phase time overcurrent pickup threshold will
be the calculated Pickup% times the phase time overcurrent pickup setting. The effective pickup threshold must be
within the setting range allowed and, if it is less, the minimum pickup value will be used.
That means:
Vmin = 0.25*Vmax;
•
Pickup%min = 25%;
•
Pickup% = 25%, if V <= Vmin;
•
Pickup% = 1/Vmax*(V - Vmin) + 25%, if Vmin < V < Vmax;
•
Pickup% = 100%, if V >= Vmax;
The tripping curves (characteristic) will not be influenced by the voltage restraint function.
If the voltage transformer supervision is activated, the voltage restraint overcurrent protection element is blocked in
case of m.c.b. trip to avoid false trippings.
704 MCDLV4 DOK-HB-MCDLV4-2E
Pickup%
VRestraint max
25% VRestraint max
25%
100%
V