Calculating General Settings - ABB REC650 Applications Manual

1MRK 511 286-UUS A
3.1.6.1
Application manual
The residual overcurrent protection is more difficult to set as the ground-fault current is
highly dependent on the network configuration of the power system. In order to achieve
settings that assure selective fault clearance, a large number of calculations have to be
made with different fault locations, different switching states in the system and different
ground-fault types. Below one example of setting of residual overcurrent protection for a
line in a meshed solidly grounded system is given.
If there is no generation at the low voltage side of the generator the transformer can only
feed ground-fault currents as long as any of the non faulted lines are still in operation. If
there is generation connected to the low voltage side of the transformer the transformer
can only feed 145 kV ground-faults.
The residual overcurrent protection has the following purpose:
• Fast and sensitive protection for ground-faults on the 145 kV busbar
• Backup protection for ground-faults in the 145 kV transformer winding
• Backup protection for ground-faults on the outgoing 145 kV lines
• Sensitive detection of high resistive ground-faults and series faults in the 145 kV
network
The reach of the residual overcurrent line protection is dependent on the operation state
and the fault type. Therefore the setting must be based on fault calculations made for
different faults, fault points and switching states in the network. Although it is possible to
make hand calculations of the different faults it is recommended to use computer based
fault calculations.
The following principle for the residual overcurrent protection is proposed:
• Step 1 (Pickup1) with a high current setting and a short delay (about 0.4 s). Step 1 is
a non-directional function. This step gives a fast trip for busbar ground-faults and
some ground-faults on the lines.
• Step 2 (Pickup2) with a current setting, if possible, that enables detection of ground-
faults on the 145 kV lines out from the substation. Step 2 is a non-directional function.
The function has a delay to enable selectivity with respect to the line protections.
• Step 4 (Pickup4) with a current setting that enables detection of high resistive
ground-faults and series faults in the network. Step 3 is a non-directional function.
The function has a longer delay to enable selectivity.

Calculating general settings

The (HV) winding data should be related to Global base 1.
Section 3
REC650 setting examples
51