| Modular multilevel converter(MMC)is very suitable for high-voltage and high-power flexible direct current transmission.High Voltage Direct Current Based on MMC(MMC-HVDC)has been regarded as the development trend of DC transmission technology in the next generation.Recently,MMC-HVDC is getting valued by more and more scientists and engineers all over the world.Therefore,the thesis focus on several key issues in control strategy and operation of MMC-HVDC Transmission System.The main research and innovations are as follows:(1)The characteristic of the circulating current of MMC is investigated to reveal the main harmonic components through formula derivation.To restrain the second harmonic of the circulating current,a method for calculating the bridge arm inductance is presented.The effect of bridge arm inductance on circulating current and system dynamic response are tested by the simulation,which provides a theoretical basis for the precise design of bridge arm inductance.In the thesis,a dedicated circulating current suppressing controller is designed to minimized the high order harmonic,which have simple structure and less computation complexity.The circulating current suppressing controller can also be applied to single-phase MMC system.Simulation results prove that the circulating current suppressing method has good performance.(2)Traditional sorting and balancing strategy easily occurred unnecessary operations on the sub-modules,which cause a significant increase in system power consumption.Based on the analysis of the control strategy characteristics,the thesis put forward three optimized sorting balancing strategies of sub-module capacitor voltage,sorting cycle diffusion balancing,pretreated capacitor voltage sampling value balancing and sub-module switch state changing compared balancing?The proposed strategies are compared from the operational complexity,the sub-module capacitor voltage amplitude and the average switching frequency.Simulation results shows the sub-module capacitor voltage can be well balanced and the DC and AC characteristics is not affected?A the same time,the average switching frequency is significantly reduced,which reduces the power consumption of the system.(3)For the MMC-HVDC system with the large number of sub-modules,the conventional sorting balancing strategy brings a heavy burden of the sorting operation to the central processor,and it requires fast data exchange between controllers.To reduce the sorting operation,a grouping sorting balancing strategy based on the classification of the positive and negative bias of the submodule capacitor voltage is proposed,which makes the sub-module capacitor voltage balanced and consistency.Meanwhile,this method has the advantage of low computation and hardware requirement.A optimized sorting balancing strategy with multi-layer grouping based on the prime factorization method is proposed to further reduce the sorting operation.The simulation experiment results show that the optimization method greatly reduces the number of sorting and has good balancing effect.The research results have important engineering value in the field of the high-voltage high-power MMC.(4)The trigger signal of the sub-module switch device has unpredictability owing to the sorting selection process randomness in the conventional sorting balancing method,which is not conducive to the realization and stability control of the high-voltage high-power MMC-HVDC system.To solve this problem,the thesis presents a sub-module capacitor voltage balacing method based on the fundamental frequency switching frequency without measuring the capacitor voltages or using any other forms of feedback control.Moreover,the frequency and pulse width of all the sub-module switch devices are constant,which makes MMC-HVDC system easy to realize and control.The simulation results show the proposed balancing method and the method optimized with sub-module switching signal exchanging are effective in both steady and dynamic running conditions of the MMC system.(5)Coordination control strategy is the key to ensure the stability control and reliable operation of Multi-Terminal Direct Current(MTDC)system.The control principles of the master-slave single point control,the DC voltage deviation control,the DC voltage control with droop characteristic and the active power DC voltage hybrid control are analyzed in detail,and the effect of the different coordinated control methods on the operation of the MTDC system is researched.The corresponding controller is designed for different coordination control strategies respectively based on the MTDC system.Finally,the performance of the controllers for the proposed coordinated control strategies is verified.(6)AC side faults at the power output terminal of the MMC-HVDC system with large scale wind farms result in DC side overvoltage,which affect the AC grid side stability of non-fault terminal.The thesis proposes a new modular dynamic braking resistor circuit to consume the unbalanced power difference caused by AC side faults,the constant DC voltage can be maintained and fault isolation can also be achieved.The power consumption of the method is flexible and controllable,and it can be applied to all kinds of AC faults.This study analyzes the control strategy of modular dynamic braking resistor circuit and the parameter design method of braking sub-modular.A capacitor voltage balance strategy of inter-group braking sub-module combined with the balancing strategy of in-group sorting method is proposed based on the analysis of the working characteristics of modular dynamic braking resistor circuit.The fault characteristic analysis and the effectiveness of the proposed capacitor voltage balancing control strategy are verified by simulation experiments.For the DC single-pole-to-ground fault of MMC-HVDC system,the thesis analyzed the fault characteristics and simulated based on the method of large resistance grounding in DC side,and put forward the corresponding coordination principle of fault control and protection. |
PDF Full Text Request