ABB RED615 Technical Manual page 171

1YHT530004D05 D
615 series
Technical Manual
• Selective overcurrent and short circuit protection of feeders in distribution and
subtransmission systems
• Backup overcurrent and short circuit protection of power transformers and
generators
• Overcurrent and short circuit protection of various devices connected to the
power system, for example shunt capacitor banks, shunt reactors and motors
• General backup protection
PH3xPTOC is used for single-phase, two-phase and three-phase non-directional
overcurrent and short circuit protection. Typically, overcurrent protection is used
for 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 72. The low-set stage PH3LPTOC operates time-selectively both
Section 4
Protection functions
165