Research On Thermal Balance Analysis And Control Method Of MMC In Solid State Transformer

The modular multi-level converter has the modular structure,easy to installation and maintenance,simple manufacturing process and low switching frequency,which simplifies the difficulty of using MMC for HVDC transmission.And because the equivalent inductance and all capacitors of each arm of the MMC are connected in series,the handling of faults is relatively fast and simple,making it widely used in flexible direct current transmission.At present,there are a lot of systematic researches on MMC's equalizing circulation and other control methods,but there are few studies on the junction temperature of MMC sub-module(SM)components and on the causes of junction temperature unbalance.Through the simulation of the capacitor voltage balance method,the thesis is found that this method will lead to the unbalance of junction temperature between SMs.From the perspective of SM parameter dispersion,the reason for the analysis of the junction temperature unbalance between SMs was proposed.A method of improving the voltage balance of the capacitor for the thermal balance control reduces the junction temperature difference between the SMs.The main work of the thesis is as follows:Firstly,the topology of MMC and the operation mode of SMs were introduced.The calculation method of MMC loss was studied,and the thermal model of MMC was established.According to the established thermal model,the working condition junction temperature of each component in the SM was obtained by iterating the loss and junction temperature.Secondly,from the two aspects of the capacitor voltage balance method and the SM parameter dispersion used in the modulation strategy,the causes of the SM thermal unbalance were analyzed.Under the NLM and CPS strategies,the thesis simulated and analyzed the SM junction temperature characteristic of the voltage balance method.The simulation results show that compared with the energy-balance method in CPS modulation,the equivalent switching frequency of each component in the SM is not fixed in NLM,and the junction temperature of each component is obviously inconsistent.Then,a heat balance control method was given for the unbalance of the junction temperature of the SM in NLM.When the junction temperature deviation of the component exceeds the corresponding reference value to adopt the thermal balance control method.It is judged whether the submodule needs to be inserted or bypassed according to the difference between the number of submodules inserted in this cycle and the last cycle.And then,on the basis of the operation mode of the SM,the junction temperature of thecomponent and the SM capacitor voltage are integrated by the weight parameter,and then the integrated values are sorted by the direction of the bridge arm current.Finally,it is decided whether the capacitor voltage of the inserted SM is more than the maximum set value of the capacitor voltage to ensure the stability of the output voltage of the SM.Finally,it is verified by simulation that this thermal balance control method can reduce the difference between the maximum and the minimum value of the components in the SM by providing a more uniform switch configuration.It also can reduce the fluctuation of the junction temperature of the components in a single SM.