Voltage Control Element For Overcurrent Protection - NR Electric PCS-9611 Instruction Manual

3.3.3 Voltage Control Element for Overcurrent Protection

If the current detected by a local relay for a remote fault condition is below its overcurrent setting,
a voltage controlled overcurrent (VCO) element may be used to increase the relay sensitivity to
such faults. In this case, a reduction in system voltage will occur; this may then be used to reduce
the pick up level of the overcurrent protection. The VCO function can be selectively enabled on
the first four stages of the main overcurrent element, which was described in Section 3.3.1. When
the VCO is enabled, the overcurrent setting can be modified just to be in excess of the maximum
value of the load current.
Overcurrent Element
Note that the voltage dependent overcurrent relays are more often applied in practical protection
applications in order to give adequate overcurrent relay sensitivity for close up fault conditions.
The fault characteristic of this protection must then coordinate with any of the downstream
overcurrent relays that are responsive to the current decrement condition. It therefore follows that
if this relay is to be applied on an outgoing feeder from a generator station, the use of voltage
controlled overcurrent protection in the feeder relay may allow better coordination with the VCO
relay on the generator.
For the operation accuracy of the VCO protection, it is necessary to take the status of the voltage
transformer into account. If the voltage transformer has a fault, the numerical relay will issue a
[VTS.Alm] signal and block all the elements that relate to the voltage measurement.
The logic diagram of the voltage control overcurrent protection is shown in Figure 3.3-2. Each
stage of the overcurrent protection can be set with voltage control by its relevant independent
setting respectively. The voltage control signal also can be from other device through the binary
input [BI_VCE_CrossCtrl] (preconfigured through the PCS-Explorer).
The detailed logic diagram for the voltage control element of phase A for the stage 1 overcurrent
protection is shown as below. The logic diagrams for voltage control elements of phase B and
phase C can be gotten on the analogy of this.
[50/51P.En_VTS_Blk]
[VTS.En]
[VTS.Alm]
Uab < [50/51P.Upp_VCE]
Uca < [50/51P.Upp_VCE]
U2 > [50/51P.U2_VCE]
[BI_En_VT]
[BI_VCE_CrossCtrl]
[50/51P.En_VCE_CrossCtrl]
[50/51P1.En_VCE]
Figure 3.3-5 Logic diagram of the OC1 phase A voltage control element
PCS-9611 Feeder Relay
Ia>
Ib>
Ic>
&
Date: 2014-04-08
Voltage for Controlling
Uab< or Uca< or U2>
Ubc< or Uab< or U2>
Uca< or Ubc< or U2>
&
50/51P1.VCE_A
3 Operation Theory
3-9