Modeling And Analog Simulation Technology Research Of Modular Multilevel Converter

With the development of the power electronic technology, the pulse width modulation (PWM) power converters are used more and more widely in the AC/DC power supplies, inverters, new energy generation, AC/DC transmission and other power conversion system. Particularly, the modular multilevel converter (MMC) gets more and more attention in HVDC systems because of its modular structure. In applications, with the further increase of the modern power system voltage level, the requirement of the power system simulation technology is becoming higher. Although real-time simulator can solve this problem, the existing digital real-time simulator requires the use of the multi-core high-speed computer or FPGA with real-time algorithm, the system structure is complex, expensive, and there is also the problem of limited simulation step.Therefore, this paper proposes a new technique, namely the analog simulation circuit of PWM power converter. Due to the PWM power converter includes at least one half-bridge power switch circuit, reactor, capacitor, resistance and power supply, etc. in the structure, hence, the analog simulation circuit presented in this paper is a low-voltage circuit, which is using electronic analog switch circuit and a first-order inertia circuit, proportional integration circuit, scaling circuit, an adder circuit, the inverter circuit and the DC/AC signal generator circuit constituted by an operational amplifier circuits, etc. to simulate equivalent instead of the power switching circuits, reactors, capacitors and resistors and other components in the PWM power converter actual circuit. And we call this main loop simulation platform based on operational amplifiers and other analog devices for the hardware-in-the-loop simulation system.Compared with the traditional simulation technology and existing real-time simulator, this method does not require a computer and other advanced equipment, the advantages of this method are high simulation precision, low cost, easy to implement and unlimited simulation step size, etc.; The platform can also be used to debug the pre-control strategies and algorithms, after the control strategies validated, only need minor modification or no changes if the design is exact, then the control strategies can be used to control the real strong electric main circuit of the system (Since the analog simulation circuit only simulates the main circuit, the analog circuit and the actual circuit can use the same control system). With this method, we can quickly verify the feasibility of the algorithm, shorten the development cycles, and also can avoid the PWM power switch burned out caused by the algorithm error, save development funds.Firstly, this paper describes the analog simulation circuit and its implementation methods of PWM power converter. Then with half-bridge PWM buck converter as an example for detailed explanation. Finally, a simulation experiment in PSIM simulation software is used to verify the correctness of the analog simulation circuit.Secondly, this paper according to the working principle of the three-phase MMC and its equivalent circuit gives the parameter values and integrated control strategy of the master device; Then the analog simulation circuits of the three-phase MMC rectifier, inverter, back to back are founded, and simulation experiments in PSIM simulation software are used to verify respectively the correctness of them.Finally, hardware design and hardware experiments are conducted. On the one hand, with half-bridge PWM buck converter as an example, its hardware-in-the-loop simulation system and its simulation model in RT-LAB real-time simulator are set up, at the same time the experimental results are given respectively and compared, the experimental results show the advantage of the analog simulation circuit compared to the real-time simulator; On the other hand, a hardware-in-the-loop simulation system experiment platform of the three-phase MMC is founded, and we can debug the control strategy on this platform, after the control strategies validated, record waveforms, without any changes use the same control strategy directly to control the real main circuit of MMC and record experimental waveforms, experimental results show that the hardware-in-the-loop simulation system experimental platform is an ideal platform for control algorithm debugging, with this method, we can avoid the PWM power switch burned out caused by the algorithm error, reduce costs, shorten cycle.