ABB RELION 620 Series Technical Manual page 254

Section 4
Protection functions
248
clearing two-phase and three-phase short circuits. Therefore, it can be chosen how
many phases, at minimum, must have currents above the start level for the function to
operate.
Many applications require several steps using different current start levels and time
delays. PH3xPTOC consists of three protection stages:
• Low PH3LPTOC
• High PH3HPTOC
• Instantaneous PH3IPTOC.
PH3LPTOC is used for overcurrent protection. The function contains several types of
time delay characteristics. PH3HPTOC and PH3IPTOC are used for the fast clearing
of very high overcurrent situations.
Transformer overcurrent protection
The purpose of the transformer overcurrent protection is to operate as the main
protection when differential protection is not used. It can also be used as a coarse
backup protection for differential protection in the faults inside the zone of protection,
that is, faults occurring in incoming or outgoing feeders, in the region of transformer
terminals and in the tank cover. This means that the magnitude range of the fault
current can be very wide. The range varies from 6xIn to several hundred times In,
depending on the impedance of the transformer and the source impedance of the
feeding network. From this point of view, it is clear that the operation must be both
very fast and selective, which is usually achieved by using coarse current settings.
The purpose is also to protect the transformer from short circuits occurring outside the
protection zone, that is, from through-faults. Transformer overcurrent protection also
provides protection for the LV-side busbars. In this case, the magnitude of the fault
current is typically lower than 12xIn, depending on the fault location and transformer
impedance. Consequently, the protection must operate as fast as possible, taking into
account the selectivity requirements, switching-in currents and the thermal and
mechanical withstand of the transformer and outgoing feeders.
Traditionally, overcurrent protection of the transformer has been arranged as shown in
Figure 111. The low-set stage PH3LPTOC operates time-selectively both in
transformer and LV-side busbar faults. The high-set stage PH3HPTOC operates
instantaneously, making use of current selectivity only in the transformer HV-side
faults. If there is a possibility that the fault current can also be fed from the LV-side up
to the HV-side, the transformer must also be equipped with an LV-side overcurrent
protection. Inrush current detectors are used in startup situations to multiply the
current start value setting in each particular IED where the inrush current can occur.
The overcurrent- and contact-based circuit breaker failure protection CCBRBRF is
used to confirm the protection scheme in case of circuit breaker malfunction.
1MRS757644 F
620 series
Technical Manual