Easier Short-Circuit and Switching Conditions in Bus-Node Substations

摘要

The "Bus-Node"-concept (BN-concept) for air-insulated substations (AIS) is characterized by separated phase layout and by high-voltage cable connections to the feeder bays. This new topology leads to extremely compact AIS-layouts with largely reduced footprints at significantly lower costs. A complete 3D-model is used to verify and optimize a 245 kV double bus-node step-down (HV/MV) substation. Due to the separated phase layout of BN-AIS, electromechanical forces on conductors, insulators and high-voltage apparatus caused by short-circuits are restricted mainly to phase-to-ground faults. Compared to conventional AIS, the probability of this type of fault in BN-AIS is 5 times lower. The non-existence of the most severe mechanical stresses of 2- and 3-phase faults in BN-AIS rises the availability, lowers the damage risk and decreases the mechanical stresses and loads. The less demanding shape of the transient recovery voltage (TRV) reduces substantially the electrical short-circuit stresses of circuit breakers in bus-node substations. Theoretical considerations and simulations of switching transients in BN-AIS show that the capacitance of the high-voltage cable connections between feeder bays and overhead lines or transformers, respectively, (a) reduce TRV peak amplitudes in the case of single-phase faults by a factor of 1.3, and (b) smooth the initial shape of the TRV. Lower TRV peak amplitudes and absence of initial transient recovery voltage (ITRV) facilitate substantially the thermal mode of current interruption in the circuit breakers. This improvement applies to all fault-conditions including terminal faults, short-line faults and faults with currents less than the rated breaking current (test duties T10 to T60). This leads to higher safety margins in the circuit breakers in view of the requirements of IEC Standard 62271-100. The much shorter loop lengths in BN-AIS compared to AIS with conventional bus-bar arrangements substantially lowers the rated bus-transfer voltage. Low rated bus-transfer voltages are inherent to BN-AIS and they reduce the electrical stress of disconnectors substantially during bus-transfer current switching. This implies practically no wear and tear at the contacts of the bus-node disconnectors.