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A traditional distance protection relay uses a lumped parameter model of the transmission line. For a long 1000-kV UHV line with large distributed capacitance, such a simplified line model is inadequate and can cause malfunction of the relay due to large errors in the impedance measurement. This paper presents a new fine model for a distance relay and the procedure of reliable fault impedance measurement and relay setting. It describes the proposed application of the relay developed to a 645-km 1000-kV UHV transmission line including shunt reactor compensation, which is currently being constructed in China. Tests are carried out on an RTDS (real time dynamic system) to evaluate the relay performance, showing significant improvement compared to a more traditional distance relay. The new distance relay is shown to satisfy the requirement of 1000-kV UHV transmission line protection. Index Terms--Distance relay, protection, transmission line equations, 1000-kV UHV transmission line.

As the conventional mho relay usually takes the bus voltage as a polarizing voltage, its operating characteristic is fixed and cannot adapt to fault resistance. To enhance the capability against fault resistance, based on the compensated voltage changing with fault resistance, a kind of adaptive impedance relay with composite polarizing voltage is presented in this paper. On the other hand, the operating boundary of the adaptive relay may enter into an 'inphase' area along with the increase of fault resistance. To prevent the adaptive relay from maloperation in the 'inphase' area, a new additional blocking component is also proposed. Both the adaptive relay and the blocking component were all analyzed in the voltage plane and the impedance plane. Theoretical analyses show that both of them have circular characteristics with greater resistive tolerance. The performance of the relay has been demonstrated on a two-source 500-km line 750-kV system. Index Terms--Adaptive relaying, extremely high voltage (EHV) transmission lines, impedance measurement, mho relay, power system faults, power system relaying, power transmission faults, power transmission protection, protective relaying, resistance.

In order to accurately measure the impedance, the residual current for impedance measurement in ground distance relay is compensated in a fixed manner. It has been shown that the above compensation manner may have negative impact on the phase selection in mho elements in a single-pole trip scheme. A new residual current compensation method based on the magnitude of current of each phase for mho elements is presented in this paper. The performance of this new method is analyzed under all kinds of fault conditions. Simulation studies are carried out to investigate the new compensation method based fault component impedance relay in a typical transmission system. The relay has shown satisfactory performances under various fault conditions especially for the close-in ground fault and cross-country fault. This new compensation method can also be used in phase selector based on impedance measurement. Index Terms--Distance relay, impedance measurement, phase selection, protective relaying.