Woodward highprotec MRI4, User Manual

Page: 441 / 727

Protective Elements
51V - Voltage Restraint Overcurrent*
* =available only for devices that offer voltage measurement.
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.
441 MRI4 DOK-HB-MRI4-2E
Pickup%
VRestraint max
25% VRestraint max
25%
100%
V