Study On A Novel Voltage Source Converter And Its Application In High Voltage Direct Current Transmission

With the development of the fully controllable power electronic devices, the technology of HVDC based on voltage source converter(VSC) (HVDC Light) improves the ability of traditional line-commutated HVDC. In theory a multi-terminal connection is propitious to be applied in grid connection of offshore renewable energy systems.However, the application is limited in project. VSC has the ability of four quadrant operation and reversible power flow on the a.c. side, and thus so many fully controllable power electronic devices are used. What's more, these switches need to deal the power of fundamental and harmonic at the same time. The switching frequency needs to be high enough to reduce harmonic power, which increases the switching losses. That increases the cost of the system and limits the technology's application.This article introduced a new model of Buck voltage source converter (BVSC) at first time, which consists of a dual buck type converter and a thyristor bridge inverter operated at the line frequency. A new model needs only two high frequency devices and can get a good output waveform. Then a new model of dual-frequency-control Buck voltage source converter (DBVSC) is introduced at first time, which adds a dual buck type converter on the BVSC. In the dual Buck converters, the former BVSC works on high frequency and deal with harmonic power only, and the added BVSC is operated at a much lower switching frequency and deals with fundamental power. DBVSC can improve the output waveform of converters and reduce the losses of system, so the system can improve the level of power and thus is fit for the application of bulk power. Much less switching devices are used in the two new structures compared with the existent structures. This article mainly discusses following topics:1) Analyzing the principle of the new model of BVSC, deducing circuit structures in different switching states from equivalent circuits in the condition that the inductors are placed in different locations. Applying one-cycle-control in the new model of BVSC at first time, and concluding the control function based on one-cycle-control under the following conditions for unity power factor and for controllable active power and reactive power on the ouput a.c.side.2) Analyzing the working principle of the DBVSC and getting the circuit models of devided coupling.The added switches are controlled so that the majority of the phase currents will be deverted to the added dual buck stage which is operated at a much lowerswitching frequency, the current through the high frequency device depends on the variation of the inductor currents in the two stages.The formulas for the current averages through the high frequency devices and lower frequency devices are concluded. Employing the method of one-cycle-control in DBVSC, and getting a control function under the following two conditions: The added dual Buck converters are able to realize unity power factor and controllable active power and reactive power on the ouput a.c.side.3) Analyzing the reason of capacitor voltage unbalance at the input d.c. side, presenting mathematic relation between the capacitor voltage offset and midline current at the input d.c. side, and introducing a simple but very effective solution to keep the voltage of both capacitors balanced.4) Realizing the claim of control with simple the control circuit, and verifying the validity of the theory analyzing by simulation and experience.