FEASIBILITY STUDY OF TWO PHASES OPERATION OF A DOUBLE-CIRCUIT TRANSMISSION LINE WITHOUT ACTIVE COMPENSATION FOR THE DISSYMMETRY

摘要

In the event of a permanent earth fault on a transmission line the common practice is to switch off the two unfaulted phases as well, although they could temporarily transport power during heavy load conditions. This paper investigates the possibility to operate a double-circuit transmission line with one circuit having a phase unavailable without active compensation for the unbalance. This two phases operation mode can be considered as an extension of the single pole autoreclosure scheme.rnSimulations and field tests in the 220 kV Swiss power system show some aspects of this unusual operation mode. It turns out that in a powerful network, the temporary (30...60 minutes) two phases operation mode of the faulted line does not necessarily produce dangerous unbalance and leads to economical and operational advantages when managing overload conditions.rnThe asymmetrical operation mode also has drawbacks, which must be carefully considered for each individual network configuration. Its feasibility depends onrn1. Physical limitations: The unbalance produced by the single pole opening results in zero and negative sequence current flows that can stress or overload equipments such as generators, transformers.rn2. Technological limitations: Many protection and control devices are designed for symmetrical normal conditions and could be disturbed by the asymmetrical operation.rnThe paper begins with both these essential aspects of the two phases operation. Then it presents field tests results made on a 220 kV double-circuit transmission line, that enables to check the previous simulations of the feasibility study. Finally, the paper discusses the benefits and drawbacks of an effective application for the proposed scheme and concludes with some practical recommendations and comments for other operators.rnThe following conclusions can be drawn from the investigations and field tests presented in this paper:rn1. The two phases operation of one circuit of a double transmission line is not so critical as one could have initially thought and can be effectively applied in the network configuration presented. The scheme takes advantage of the parallel circuits providing compensation for the asymmetry.rn2. Simulations are reliable tools to assess the feasibility of the scheme and must be performed for each individual network configuration.rn3. The consequences for all the equipment should be investigated carefully.rn4. The scheme is mainly applicable in powerful and well interconnected systems.rn5. Two phases operation could represent an economical alternative to dynamically manage overload situations, increase the flexibility of the power system and help in contingency situations.rn6. The two phases operation scheme could be implemented in practice via the protection relay.