A Method of Single-terminal MMC Single-pole Grounding Protection for DC Distribution Network Based on Multi-class Similarity Analysis

Based on the analysis model of single-pole to ground fault occurred in single-ended MMC DC distribution network, the characteristics of single-pole to ground fault are studied. The similarity of positive and negative current between the fault feeder and the non-fault feeder, the similarity of fault current between the fault feeder and the non-fault feeder, and the similarity of positive and negative current sum between the two feeders are analyzed. Based on the differences among the three types of similarity, a new method of single-pole grounding protection for DC distribution network with MMC based on multi-type similarity analysis is proposed. The simulation results show that this method can accurately determine the fault feeder, and has the characteristics of strong anti-interference ability, simple calculation and easy implementation. It can effectively solve the problem of single-pole grounding protection of DC distribution network and effectively improve the safety and reliability of power supply of DC distribution network.


Introduction
Modular multilevel converter because of its advantages such as good output characteristics, easy to expand and modular structure, the DC distribution network based on MMC has gradually become a research hotspot [1]. Bipolar short-circuit fault and monopole grounding fault are DC side faults. The influence of monopole grounding fault on the system can be reduced by selecting a reasonable neutral grounding mode of transformer. Therefore, the existing research on MMC-based protection schemes for DC distribution network is mostly focused on bipolar short circuit fault, while the research on unipolar ground fault protection scheme is less [2]. At present, there are mainly directional pilot protection methods based on transient wavelet transform for single-pole grounding fault, but these methods have the shortcomings of high sampling frequency or depending on communication, with a certain delay [3].
The similarities of positive and negative currents between fault line and non fault line, fault Figure 1 is the schematic diagram of single ended MMC DC distribution network. Among them, the AC measurement adopts the transformer with wiring . Figure 1. Structure diagram of single-ended MMC DC distribution network Because the DC transmission line has the distributed parameter characteristic which changes along with the frequency, the line voltage will have a damping oscillation process before entering the stable state when the DC transmission line is single-pole grounded [6]. The constant change of line voltage in the process of oscillation leads to the changing attenuation of oscillation current flowing in and out of the distributed capacitance of the line to the ground.

Fault Characteristic Analysis
When the feeder L1 is single-pole grounded, the capacitive current distribution of each line to ground is shown in Figure 2.  (1) The magnitude of the transient capacitance at the head end of other non fault line is equal to the current of the line itself to the ground, and the direction is from the bus to the line.
Through the analysis, it can be seen that the transient capacitive current of the fault line is equal to the sum of the transient capacitive current of all other lines, while the other feeders are equal to their own transient capacitive currents [7]. The direction of the measured current on the side of the tie line close to the fault feeder is positive, and the direction of the measured current on the other end of the tie line is negative. Similarly, when a fault occurs on a tie line, the currents at both ends of the fault tie line are negative; All feeder are positive and equal to their own capacitive currents; And one end of other tie line close to that fault tie line is positive, and the other end is negative. According to the above characteristics, the fault location and the fault feeder can be judged [8].

A New Single-pole Grounding Protection Method Based on Multi-class Similarity Analysis.
When the expression (1) is satisfied, it is determined that a single-pole ground fault has occurred.
Where in : ､ ､ and are respectively the rated values of the positive pole-to- ground voltage, the negative pole-to-ground voltage, the inter-electrode voltage and the inter-electrode voltage.
To reduce the calculation of the protection, the data length N of the current criterion is determined by Equation (2). For the equation (2), the calculation is started from t=1, and the calculation is stopped when the equation (2) is satisfied, wherein the value of t is the length N of the data window [9].
(3) 1 1 Where: T is the number of sampling points in 5ms.
is the mutual correlation coefficient of fault pole current of the feeder th and the is the sum of the positive and negative pole currents of the th and th feeder

Simulation and Verification
As shown in the structure diagram of the single-ended MMC DC distribution network in Figure 1, the rated voltage of the DC side is + -10kV; The sampling frequency is 24 kHz; There are four tie lines L1 ~ L4 in this power network, the length of which are 5km, 8km and 3km respectively [10]. When the voltage drop of positive pole (or negative pole) to ground exceeds 20% of the rated voltage to ground, it is considered that a single pole grounding fault has occurred [11]. The calculation results of the sum of the mutual similarity coefficients based on the similarity of the feeder positive and negative pole currents, the sum of the mutual similarity coefficients based on the feeder fault pole currents, and the sums of the positive and negative pole corresponding current sampling values are respectively shown in Tables 1 to 4:  Table 4. Calculation conclusion of comprehensive relative criterion for each feeder

Conclusion
In this paper, an integrated single-pole grounding protection method based on the similarity coefficient method is proposed for MMC two-terminal DC distribution network. The method is feasible to be apply to various MMC-based direct-current distribution networks; The method can judge the fault line and is not influenced by the connection mode of the line; As long as the data acquisition accuracy meets the requirements, the fault point transition resistance basically has no effect on the method. However, this method can only be used to identify single-pole grounding fault, and the inter-pole short circuit fault needs further study.