Abstract
DC line fault is one of the key problems that must be solved in a flexible HVDC system. During quite a long time between the existing main protection and backup protection of the HVDC line, there is no line protection method to detect the fault, which may lead the protection at the AC side to act before the backup protection of the DC line. To solve the problem, a novel two-step distance protection for flexible HVDC lines is proposed in this manuscript. Firstly, based on the uniform distributed parameter model, the equivalent lumped parameter model of the HVDC transmission line at low frequency is analyzed. Secondly, according to the time domain differential equation and the least squares algorithm, novel distance protection based on the iterative calculation of fault distance is proposed, which can eliminate the influence of distributed capacitive current and improve the precision of calculation. To improve the rapidity and reliability of the distance protection, low pass filters with two different cut-off frequencies are used to process the electrical quantities. Finally, simulation results show that the proposed distance protection can respond to metallic poleto-ground faults and pole-to-pole faults rapidly and reliably.
Highlights
The flexible HVDC power transmission technology based on the modular multilevel converter (MMC) has a wide application prospect in the fields of large-scale distributed power grid interconnection, asynchronous grid interconnection, and urban HVDC transmission and distribution[1,2,3]
The statistical data show that the number of forced outages caused by HVDC transmission line fault accounted for 36.8% of the total number from 2006 to 2012[5]
This paper proposes a novel two-step distance protection scheme based on the iterative calculation of fault distance, which can eliminate the influence of distributed capacitive current and improve the precision of calculation
Summary
The flexible HVDC power transmission technology based on the modular multilevel converter (MMC) has a wide application prospect in the fields of large-scale distributed power grid interconnection, asynchronous grid interconnection, and urban HVDC transmission and distribution[1,2,3]. To avoid the influence of distributed capacitive current, the current differential protection often adopts a large time delay, which will be too long for the flexible HVDC systems. In [7], the fault distance is calculated by solving the differential equations based on the distributed parameter line model. The above two methods are based on distributed parameter line models, which require high sampling frequency and a large amount of calculating. In [9], a distance protection algorithm based on the RL line model is proposed, which can detect the internal fault through the polarity of the ratio of the compensation voltage at the setpoint and the measured voltage. Simulation results show that the proposed distance protection can rapidly and reliably respond to metallic pole-to-ground faults and pole-to-pole faults
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