Index Terms—Euclidean distance, entropy weight method, flexible direct current power grid, pilot protection, waveform similarity.

Introduction

To integrate and accommodate large-scale renewable generations such as wind power and solar power, multi-terminal flexible DC grid (MTFDC) based on modular multi-level converter (MMC)-HVDC has been widely used as a promised solution due to its independent power regulation and flexible operation capabilities, small switching loss and strong expansibility [1-5]. However, the stable and reliable operation of MTFDC is heavily dependent upon the fast and reliable DC line protection [6]. The protection scheme of the DC line can be categorized as two types: single-ended quantity protection and double-ended quantity protection according to the extracted electrical quantity of converter station from one-end or both-end of the DC line [7-23].
Typical single-ended quantity protection schemes on the MTFDC grid mainly include boundary protection, transient voltage protection and voltage traveling wave protection [7-13], and so on. In [7-8], the protection principles proposed are based on the change rate of voltage measured at the line side of DC inductors. However, the reliability under high-resistance faults cannot be guaranteed and the required sampling frequency is much high. In [9], a protection method using the change rate of the DC inductor voltage is proposed. In [10], a single-ended transient-voltage-based protection strategy for a flexible DC grid was proposed. The proposed method has higher reliability and stronger ability to endure high transition resistance, but the high transition resistance is only 300Ω. In [11], the method based on the high-frequency components of transient voltages on DC lines is proposed, which has fast operation speed and good robustness. However, an accurate theoretical basis is not analyzed thoroughly. In [12], according to the second-order difference of backward voltage traveling wave, a protection scheme is proposed. The high-resistance problem is overcome, but the required time window is more than 3ms. In [13], the method based on transient high-frequency energy of line current is proposed. Lumped parameter-based DC line model cannot reflect the propagation process of traveling waves on long transmission lines. According to the above analysis, the single-end protection can identify internal fault and external fault quickly without communication only by using the one-end data of the DC line, but it also has some disadvantages such as weak resistance to high transition resistance and low sensitivity.
As for double-ending protection, many achievements mainly focus on traveling waves, line boundary elements, transient impedance[14-25], etc. In [14], a protection scheme is proposed, which is based on the time difference between the initial forward and backward traveling waves reaching the relays on both ends of the DC line. In [15-16], the protection methods can be implemented by the voltages of DC inductors on both ends of the DC line. In [17], the protection principles are both based on the ratio of the transient voltages (ROTV) at both sides of the DC inductor. To ensure the reliability under high-resistance faults, the ROTVs at both ends of the DC line are required. In [18], a protection method based on the transient measured impedance is proposed. In [19], short-time energy is used to identify DC single-pole ground fault in a multi-terminal flexible DC system. In [20-21], the ratio of electrical fault quantity at both ends of the line is selected to identify internal and external faults. In [21], protection based on incremental reactive power coefficients (IRPCs) is proposed. The protection principles can be carried out by the both-end data of the DC line, which can meet the absolute selectivity of fault identification. However, due to the communication delay, some algorithms require strict synchronization of double-terminal data and high error measurement ability, which limits the rapidity of protection. Besides, a large short-circuit current will be generated in an ultrashort time after DC fault occurs in multi-terminal power transmission systems, which can damage the semiconductor devices and even lead to the blocking of the converter station if the fault cannot be cleared quickly[22]. In [24], a new protection scheme for flexible distribution system based on cosine similarity of DC current waveform is proposed. The proposed protection scheme can identify internal faults in less than 0.3 millisecond with good reliability and high sensitivity. And it is robust to distributed capacitance, and not requires strict communication synchronization. In [25], an adaptive reclosing scheme based on the transient current waveform similarity matching is proposed. The protective reach of reclosing criterion proposed can cover the entire transmission line, with high sensitivity and well applicability.
In [26], a new scheme of traveling wave longitudinal protection based on measuring wave impedance Euclidean distance is put forward, the protection principle can identify internal and external faults with reliably, sensitively and quickly, and has a strong ability to withstand transition resistance and noise interference. In [27], load curve clustering method based on Euclidean Dynamic Time Warping Distance and Entropy Weight is introduced, which can be used in power system protection principle.
In view of the above reasons, in order to improve the ability of the modular multi-level converter to quickly clear and isolate faults in the multi-terminal flexible direct current grid, it is necessary to study a new type of protection methods for DC grid.
This paper analyzes the characteristics of current faults in the multi-terminal flexible direct current grid, introduces the comprehensive distance of current fault waveform, and proposes a line protection method based on the similarity of current fault waveform. The proposed method has good resistance to high resistance, and its rapidity and adaptability are further improved.
The remainder of this paper is organized as follows. In Section 2, the topology of the four-terminal MMC flexible DC grid is introduced; In Section 3, the current fault characteristics are analyzed; In Section 4, the measurement methods of the current fault component waveform similarity are introduced; In Section 5, the protection criteria and protection principal flow chart of are introduced; In Section 6, simulation verifications are carried out in MATLAB; And conclusions are given in Section 7.
Topological Structure of Modular Multi-terminal Flexible DC Grid with Multi-level Converter
This paper analyzes the characteristics of Zhangbei four-terminal ring flexible DC power grid. The system is a four-terminal symmetrical bipolar MMC flexible DC transmission line with a DC voltage level of ±500kV, and the length of each line is marked according to the actual length. Topology and fault location are shown in Fig.1. A series of small inductors are added to the DC side of each station to construct the boundary conditions. According to the actual situation, the parameter is 0.3H.