RTE - Transmission Line Protection (Issues and Solutions)

Author: Volker Leitloff, France

RTE - Transmission Line Protection (Issues and Solutions)

RTE is the French Transmission System Operator. It operates a network comprising approximately 100 000 km of lines and 2450 substations. Almost half of the lines correspond to the transmission level (400 kV and 225 kV) including the interconnections to the neighbor countries, the other half belong to the regional sub-transmission level (90 kV and 63 kV). Today, RTE operates approximately 16,000 line protection relays, 15% of which are digital.

RTE has elaborated a set of documents used as reference for the protection of all network components. The protections to be used are defined depending on the voltage level, the component to be protected and its characteristics (underground or overhead lines, busbar, transformers, etc.) and the importance of this component in the network. One of the main principles applied to the protections by RTE is that a protection should only clear faults related to short circuits or other equipment failures. That means that protections must neither trip under overload conditions nor due to power swing. There are specific automatons dedicated to detect these conditions and to trip, if required, in a controlled and preset way that limits the consequences to the network In this context, a "short line" is defined as a line for which the zone 1 of distance protections cannot be set to 80% of the line length, requiring thus a blocking scheme with the associated telecommunication equipment.

The main problems RTE is confronted with at the moment as far as line protection is concerned arise from the fact that several regions have a highly meshed network, leading to particular constraints in the coordination of the protections of several substations. The installation of capacitor banks, transmission lines with high load capability, phase shifting transformers, SVC's and multi-terminal lines have been adding constraints over the past decade.

For transmission lines (400 kV and 225 kV) RTE uses 2 main protections (Distance and / or Line differential), each from a different vendor. The power supply of these protections relies on the same battery and charger. The circuit breakers have in some particular cases a redundant trip coil and single pole tripping. RTE also uses an elaborated reclose scheme. For this voltage level, RTE uses a permissive tripping scheme of the distance protections. For the HV voltage level, only three-phase tripping and reclosing is normally used.

On the sub-transmission level, line bays are equipped with one main and one backup protection. Except the lines where blocking schemes have to be applied (short lines) or for cables (current differential, sometimes transfer trips), there is usually no communication between the relays at the ends of the line.

The new equipments appearing in the network (those mentioned above and probably others to come) increase both the need for selective tripping and the difficulties to obtain it. The growing complexity of setting parameters, the proper administration of hard- and software versions of protections and of the associated setting parameters are included in the challenges the protection engineers will have to face in the coming decade.

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