Research on control strategy for AC side asymmetric fault of MMC-HVDC transmission system

. When the Modular Multilevel Converter (MMC) fails on the AC side, the operating characteristics of MMC will be damaged. If the system wants to retain stable for a long time in the process of operation, it is obliged to design the control strategy in case of asymmetric fault. In this article, the control strategy based on feedback linearization principle is adopted to diminish the negative sequence current caused by breakdown, and a DC voltage controller is devised to suppress the fluctuation of the double frequency component under asymmetric fault conditions. The validity of the control strategy requires to be verified, the running results on PSCAD platform indicate this method is effective and feasible.


Introduction
Due to the increasing demand for electricity, the traditional power supply mode is difficult to meet people's greater demand for power resources. The power transmission system which has advantage of capacity, highly safety and efficiency. These merits determine that it will be used in the power grid. The development of flexible DC transmission technology [1][2] solves the power supply problems under special conditions, such as island power supply and offshore wind farm access to the grid, which makes largescale and long-distance transmission possible. The modular multilevel converter is composed of several sub modules in series [3][4]. The voltage can be adjusted by turning on and off the electronic device. Because of its superior performance, it has been widely used in many conditions. A control method is employed to make the mmc-hvdc system operate normally in [5][6], so that the system can reach stability faster. The circulating current in the arm of MMC during operation is analyzed and a control method is designed to suppress the circulating current component in [7][8][9]. A method is employed to reduce the fluctuation of current and power in [10].The sequence components on the AC side are segregated into diverse sections, and the change process of AC and DC side current is analyzed in [11]. At the same time, the corresponding negative sequence current control method is proposed to reduce the fluctuation range of module voltage and improve the stability of system operation. The second order generalized integrator can speedily separate the sequence components into separate sections. [12]. A reduced order resonant regulator is adopted to quickly separate the positive and negative sequence components in [13][14]. Aiming at the issues of AC side fault in MMC system, the feedback linearization method is adopted to project the inner loop current circuit, which can restrain the negative sequence current to a certain extent. At the same time, a DC voltage zero sequence compensator is designed to reduce the fluctuation of DC side voltage and current and improves the operation characteristics of the system when asymmetric fault occurs.

Analysis of MMC equivalent circuit
The schematic wiring diagram of modular multi electric equal value is drawn here. The bridge arm is linked with lots of modules. pj i and nj i are the currents flowing through the upper and lower bridge arms respectively. pj U and nj U represent the voltage of each cell separately. sj u is AC side voltage and sj i is AC side current. dc U is DC side voltage and dc I is the corresponding current. 0 R and 0 L are bridge arm resistance and reactance separately.The bridge arm reactor is able to suppress the current as a result of instantaneous voltage imbalance, but adding the filter reactor will reduce the circulating current effect and increase the complexity of the system. MMC module has a high degree of integration. In the process of operation, the on-off signal is applied to the controller to control the on-off of the electronic devices linked in the system. The number of sub modules has a certain effect of the waveform of output voltage. In actual use, the number of sub modules can be appropriately increased to improve the accuracy.

Mathematical model of MMC under asymmetric condition
When the MMC AC side system has asymmetric fault, it will engender negative sequence current. This ingredient will give arise to the asymmetry of system electrical parameters. Therefore, it is obliged to take relevant control strategies. The appropriate model contributes to analyse the system. The asymmetric components can be represented by the equivalent models of positive sequence and negative sequence. The figure below shows the equivalent diagram under two order components.

Fig. 2. Equivalent circuit diagram of sequence components
Through the analysis of the simplified modular multilevel equivalent circuit, the mathematical expressions of grid side and converter side of the system are easily obtained.
Through a series of coordinate transformation of equation (1), we can get the expression of positive and negative sequence voltage mathematical model in d and q coordinate system as follows.

Mathematical model analysis and positive and negative sequence current controller design
The transformed positive and negative sequence components are independent of each other. The sequence component current controller can be received by analysis, and the mathematical formula of low frequency transient can be obtained by rewriting equations (2) and (3).
In the decoupling process of the above formula, some suitable additional equations are joined to simplify the whole process. The equation is below.
In equations (6) and (7),  and  are the parameters of PI controller. By introducing equations (7) and (6) into equations (5) and (4) According to the analysis of the decoupling mathematical model, the block diagram of the inner loop positive and negative sequence current controller is designed which appears in the figure below. i  are set to 0. sdref i  and sqref i  are determined according to the active control circuit and reactive control circuit. Under the above control, the negative sequence current is eliminated with modulation algorithm.

DC voltage control strategy
The The compensation component of the zero sequence voltage can be obtained by passing the generated zero sequence voltage through the zero sequence compensator. The compensation component is applied to the voltage reference command, and the corresponding trigger pulse is generated by the modulation algorithm. From the above analysis, the structure diagram of DC voltage fluctuation controller can be drawn.

Simulation validation
The validity of the designed control strategy requires to be tested when asymmetric fault occurs in the system. It is obliged to build MMC-HVDC on PSCAD / EMTDC platform. The rectifier side is dominated by constant DC voltage and reactive power, while the inverter side is regulated by constant active power and reactive power. The major system parameters are listed down below. It is assumed that the single-phase ground fault arises at the AC side of the system in 3S.The fault is cleared after 0.15s, and the fault phase is selected as phase A. Figure 6 shows that the phase A voltage basically drops to 0, and the voltage returns to normal after clearing the fault. Figure 7 shows the output current waveform of the AC side of the power grid. Under the action of the current controller, the negative sequence current is controlled, and the three-phase current remains symmetrical. Figure 8 and Figure 9 are the simulation diagrams of DC side voltage and current after the controller is put into operation. The voltage and current fluctuations decrease after the controller is put into operation, which shows that the designed controller can improve the operation characteristics of the system and improve the stability of the system in the case of asymmetric fault.

Conclusion
Asymmetric fault will destroy the stability of MMC system. This paper designs a strategy under asymmetric fault to improve this situation. Through the design of the corresponding current controller, the negative sequence current component is reduced, and then the voltage compensation method is adopted to decrease the frequency doubling fluctuation of the DC side electrical component. This strategy has a certain engineering application value. This work is supported by the National Natural Science Foundation of China (51567016).