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The digital real-time simulation-based function and dynamic performance tests of the modular multi-level converter (MMC) control and protection systems have become an important method to ensure the safety and reliability of the MMC-HVDC projects, of which the real-time simulation model of the MMC is one of the key factors affecting the test effect. Currently, the MMC models are generally implemented in the central processing unit (CPU) and field programmable gates array (FPGA) with multi-rate time steps (large-small time step simulation). In case of fault simulations, the loss of accuracy caused by communication delay between CPU and FPGA can be significant. In order to further improve the test performance, the FPGA based small time step model of MMC has been studied. Comparative simulation has been carried out under AC and DC fault conditions to verify the proposed method. The results show that the real-time simulation accuracy of the MMC can be effectively improved with the new method.

The dual active bridge (DAB) DC-DC converter is the core link of a power electronic transformer (PET). With the development of PETs toward medium- and high-voltage applications, more DAB modules are adopted. However, conventional Electromagnetic Transient (EMT) simulation for a multi-switch circuit is quite time consuming. In this paper digest, a modified constant admittance method has been developed for modeling of an isolated DAB. By reducing the expense of admittance matrix reconstruction and related iterative operation at switching instant, the simulation efficiency has been improved significantly, and the new model becomes more suitable for a real-time simulation algorithm. Meanwhile, time domain simulations are given, which show that the proposed method can keep the same accuracy as the conventional model.

Conventional Electromagnetic Transient (EMT) simulations of a Modular Multilevel Converter based HVDC (MMC-HVDC) is quite time consuming. Therefore, the sub modules are often modeled with equivalent transformation methods. In this paper, two equivalent models with much higher efficiency have been presented for a specific MMC topology, i.e. the diode clamped half bridge topology, which convenient the researches on MMC-HVDC with overhead lines. Details of the modeling technologies have been explained. Simulations have been carried out, focusing on computational efficiency, steady and transient performances. Compared to conventional EMT model, the proposed models are accurate enough, while with much higher efficiency.

Nearest Level Modulation (NLM) is a kind of staircase wave modulation suitable for MMC with massive submodules. There are several implementations of NLM for different application scenarios and operation conditions. In order to discuss the feasibility of different implementations and the inner stability of MMC, a state-space system is established firstly to represent MMC internal dynamics. Based on these results, Lyapunov theory is introduced for stability analysis. Optimized designs to improve stability are also discussed. The analysis of this paper shows that the inner stability of MMC depends on whether measured value or reference value of submodules voltage and dc-bus voltage are taken in NLM calculation. By designing common-mode voltage controller carefully, or low-pass filtering measured dc-bus voltage, inner stability of MMC can be effectively improved. Simulation results in PSCAD/EMTDC verify the validity of stability analysis of this paper.

This paper presents a robust current control design method to improve the performance of Voltage Source Converter under unbalanced grid voltage conditions. Unbalanced grid voltage would increase the low order harmonic distortion of the converter's input current, power and DC voltage. Current compensation strategies are derived in both synchronous and stationary coordinates based on instantaneous power theory. The control object is to suppress the oscillations in current or power. The current controller is developed in the unified frame of H∞ control theory. Proper weight functions are chosen for each coordinate. Simulation results revels the feasibility of the proposed control method.

Considering the problem of low order current harmonics (5th, 7th and so on) caused by the distortion or unbalanced grid voltage conditions and the variations of generator itself parameters, an improved control strategy for harmonics restraint of double-fed wind power system is proposed in this paper. To eliminate the rotor current harmonics, restrain the stator current harmonics indirectly, and thus improve power quality of wind turbines, one control strategy of proportional integral (PI) regulator paralleled with a harmonic resonant (R) controller in the synchronous reference frame is simulated, which has not effect on the control of fundamental current, and need not the harmonic separation filter and the sequence decomposition in multi reference frames. Simulation analysis is studied based on Matlab/Simulink for the proof of the control strategy effectiveness, which shows that the control strategy proposed in this paper could improve power quality effectively and it is easy to implement digital control.