Performance of distance protection based on the traditional fault line impedance calculation method is adversely affected by fault resistance, which may cause distance protection to operate incorrectly, threatening the safe operation of power system. To improve immunity to the fault resistance, this paper presents a novel fault line impedance calculation method. The fault line impedance, supplementary impedance, and measured impedance are rotated simultaneously in the complex plane until the supplementary impedance coincides with the positive direction of the real axis. Following the geometrical relationship between the rotated fault line impedance and the rotated measured impedance in the complex plane, the fault distance and fault line impedance are solved. The proposed method is immune to the fault resistance and insensitive to power angle and fault location variations. Furthermore, the proposed method can work well under various types of faults, e.g., AG, BC, BCG, and ABC. The simulation results show that the proposed method can calculate the actual fault distance accurately and identify in-zone and out-of-zone faults correctly.

Circulating current can be decomposed into double line-frequency positive-, negative- and zero-sequence components under unbalanced voltage conditions. First, in order to decrease peak/rms value of arm current, double line-frequency circulating current should be eliminated. Second, ac-side current that consists of positive- and negative-sequence components under unbalanced voltage conditions should be regulated to track the reference signal. Furthermore, fundamental frequency circulating current caused by asymmetry of the upper and lower arm should be also removed. To effectively control ac-side current and eliminate all circulating currents, an arm current control strategy based on a decoupler and proportional-integral controller plus the vector proportional integral (VPI) controller is proposed in this paper. The proposed control strategy works well under balanced and unbalanced voltage conditions as well as asymmetry of the upper and lower arm. In addition, this paper proposes a design procedure for the VPI controller to reduce the uncertainty of this process. A three-phase modular multilevel converter simulation model is set up on PSCAD/EMTDC to verify the effectiveness of the proposed control strategy.

The exclusive fault characteristics of photovoltaic (PV) power plants and the existence of fault resistance cause a high probability of failure in operation and unwanted operation of distance relay with conventional mho or quadrilateral characteristic. Different from the fixed operation zones of conventional mho and quadrilateral characteristics, the adaptive mho characteristic presented herein can adaptively adjust its operation zone to solve the aforementioned issues. The basic principle of adaptive mho characteristic-based distance protection is described and the uniform expression of adaptive setting impedance is derived. Subsequently, taking the fault characteristics of PV power plant into consideration, the method of solving the phase angle of fault current is presented, which is the key to calculating adaptive setting impedance. The performances of the proposed protection scheme are evaluated by extensive digital simulations. The proposed protection scheme exhibits satisfactory robustness against the fault resistance and excellent adaptability to different grid codes. It can not only discriminate the internal and external faults occurring in the lines emanating from the PV power plant but also correctly identify the fault direction.

Fault resistance causes conventional distance protection to operate incorrectly, threatening the safe operation of power system. The immunity to fault resistance is improved by utilizing fault-point voltage, measured current, and setting impedance to construct the virtual measured voltage. Then, a new distance protection scheme, known as protection scheme I, is presented to identify in-zone and out-of-zone faults by comparing the amplitudes of the original and virtual measured voltages. Then, the original and virtual measured voltages are improved by a special voltage phasor in phase with the fault-point voltage phasor. Based on the improved original and virtual measured voltages, another distance protection scheme is presented, namely protection scheme II. The two proposed protection schemes are immune to fault resistance and load current and operate well under all types of faults. However, protection scheme II has higher sensitivity to fault detection compared with protection scheme I. Simulation results verify the correctness and effectiveness of the two proposed schemes.

The exclusive fault behaviors of MMC-HVDC stations bring great challenges to the correct operations of distance relays. This paper presents a new zone I distance relaying scheme that can accurately calculate fault distance in the case of integration of MMC-HVDC system. The two calculation formulas of fault distance are derived from mathematical perspective and geometrical relationship of impedances, respectively. Phase angle of fault current is the key to calculate fault distance. In power system without MMC-HVDC stations, the existing method for solving this phase angle is based on two premises, that is, the positive- and negative-sequence current distribution factors are equal and real numbers. However, owing to the integration of MMC-HVDC system, aforementioned premises are both not true. Consequently, the existing method cannot obtain accurate phase angle of fault current. To solve this problem, the three solutions corresponding to three different types of asymmetrical faults are provided. The proposed scheme has the advantages of satisfactory immunity against fault resistance, applicable to various asymmetrical faults, and performing well under rectifier as well as inverter modes. The performance of the proposed scheme is evaluated by simulation results in PSCAD/EMTDC and compared with prior-art schemes.