With a growing contribution from distributed power generation the distribution network will more and more start resembling a transmission network, where the direction of the power flow changes with varying operating conditions. This network topology development poses new challenges for protection systems.

Plain overcurrent protection technology will no longer be able to provide absolute selectivity, ie complete assurance that the protection system will not be activated by faults outside the protected zone. Inverse time overcurrent protection or directional overcurrent protection can provide the required selectivity but the relatively long operating times remain a disadvantage. Line differential protection offers a solution. It works by comparing the magnitude or phase and magnitude of the currents at the ends of the lines in the protected section. Making use of the I/Os, logic circuits and the horizontal and vertical communication capabilities of line differential protection relays opens up the possibilities of much smarter protection solutions for distributed systems.

The line differential protection concept is particularly well suited to the protection of ring-type and meshed networks. The settings of the protection system can be kept unchanged even though the network configuration and connections change. This makes the line differential protection concept an ideal solution for providing fast and absolutely selective power system protection.

At the same time the operating principle of the differential protection scheme provides good sensitivity. In power systems with distributed generation fault current magnitudes may vary considerably with the prevailing network configuration, but this will not affect the operation of a line differential protection system.

ABB’s new line differential protection system, based on the RED615 phase-segregated two-end line differential protection and control IED (intelligent electronic device), is also fitted with a high-set instantaneous differential current stage. Feeders of distribution networks may occasionally include branch lines inside the protected line section. If the magnitude of the load current in a branch line is modest and the branch line ends with a transformer, the whole branch line can be regarded as belonging to the zone of protection of the line section. The instantaneous stage enables fast tripping of the entire protection zone, provided that the start value of the stage has been set higher than the fault current obtained from faults behind (downstream of) the transformer of the branch line. Choosing settings is facilitated by an in-rush current detection function, which blocks the operation of the high-set instantaneous stage when the line section including a transformer is energised. The low-set differential current stage can also be set to operate selectively with the protection systems behind the transformer of the branch line, by properly delaying the operation of the low-set differential current stage.

Inter-substation communication link

Line differential protection systems clearly require an auxiliary connection to provide a communication channel between the two line ends. Fibre-optic cables have particular advantages in this application as they are non-conducting and therefore no attention needs to be paid to possible differences in potential between the two line ends in a fault situation.

The auxiliary connection can also be used for the transfer of information needed by the distribution automation system. Being fast, the straight connection between two points is well suited for preventing islanding. When a distributed power generation unit is located on the protected line section the generation unit continues to feed fault current to the fault spot even after the main supply of the line section has been switched off. The fastest way to protect the consumers is to transfer a trip command to the opposite end of the line as the main supply is tripped. To enable trip signal transfer the RED615 line differential protection IEDs are fitted with the necessary I/Os and logics. The auxiliary connection can also be used for the transfer of other data, such as status information on switching devices.

The system employs IEC 61850 based communication.