Functions
2.5 Single-Phase Overcurrent Protection
Figure 2-45
Connection diagram of the ground fault differential protection according to the high-impedance
principle
Even with an unfavorable external circuit, the maximum voltage peaks should not exceed 2 kV for safety reasons.
If performance makes it necessary to switch several varistors in parallel, preference should by given to types
with a flat characteristic to avoid asymmetrical loading. therefore recommend the following types from METROSIL:
600A/S1/S256 (k = 450, β = 0.25)
600A/S1/S1088 (k = 900, β = 0.25)
The pickup value (0.1 A or 0.05 A in the example) is set in address 2706 50 1Ph-1 PICKUP in the device.
The 50-2 element is not required (address 2703 50 1Ph-2 PICKUP = ∞ ).
The trip command of the protection can be delayed via address 2707 50 1Ph-1 DELAY. Normally, such delay
is set to 0.
If a higher number of CTs is connected in parallel, e.g. as busbar protection with several feeders, the magnetizing currents of the transformers connected in parallel cannot be neglected anymore. In this case, the magnetizing currents at half the knee-point voltage (corresponds to the setting value) have to be summed up. These
magnetizing currents reduce the current through the resistor R. Therefore the actual pickup value will be correspondingly higher.
Application as Tank Leakage Protection
The use as tank leakage protection requires that a sensitive input transformer is available at the device input
IN/INS. In this case, only the pickup value for single phase overcurrent protection is set at the 7SJ62/64 device
for the current at input IN/INS.
The tank leakage protection is a sensitive overcurrent protection which detects the leakage current between
the isolated transformer tank and ground. Its sensitivity is set in address 2706 50 1Ph-1 PICKUP. The 50-2
element is not required (address 2703 50 1Ph-2 PICKUP = ∞ ).
The trip command of the element can be delayed in address 2707 50 1Ph-1 DELAY. It is normally set to 0.
Note
In the following settings, addresses 2703 and 2706 are valid for a highly sensitive current measuring input independently of the nominal current.
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-4, Release date 09.2009
139