Researches On Capacitor Voltage Balancing Control And Fault-Tolerant Operation Strategy Of Modular Multilevel Converters

With the increasing demand that the new power system imposes on the power electronic equipment in the voltage level and power capacity,multilevel converters have been one of the research focuses in electrical engineering.Among massive converter topologies,modular multilevel converter(MMC)is the most popular multilevel converter in high-voltage and high-power projects such as high-voltage direct current transmission systems and multi-terminal direct current power grids due to the modular structure,easy scalability and high-quality output.However,in order to withstand the higher voltage and transmit more power,each arm in the high-voltage MMC contains hundreds of submodules(SMs)and operates at a low frequency,which increases the complexity of the MMC control system and reduces the balance among capacitor voltages.Therefore,optimizing the performance of the high-voltage MMC control algorithm is an important measure to ensure the reliable operation of MMC.In addition,the control system of a high-voltage MMC is complicated,and the communication failures between the control layers will lead to the disorder of capacitor voltages.It is necessary to study the corresponding solution for voltage balancing.Besides the large number,the power devices and capacitors inside the SMs are also vulnerable and easy to damage compared with other devices such as the arm inductor.To improve the reliability of MMC.it is necessary to detect the states of power devices and capacitors,and study the subsequent protection measures.Based on the above targets,this paper takes high-voltage MMC as the research object and proposes the capacitor voltage balancing optimized algorithm under low frequency operation,fault tolerance for the voltage balancing under communication failures,the power device fault diagnosis strategy,the capacitor condition monitoring method and the SM fault-tolerant scheme,respectively.Furthermore,the feasibilities of the proposed methods are validated through the simulation and the experimental results.The specific research contents of this paper are as follows:(1)The capacitor voltage balancing algorithm for the high-voltage MMC is proposed to solve the contradiction between the low-frequency operation of highvoltage MMC and the balancing performance of capacitor voltages.At first,the relationship among the switching frequency of MMC,the state switching times of SMs and the capacitor voltage deviation is deduced.The relationship denotes the state switching times of SMs are more,the switching frequency of MMC is higher and the capacitor voltage deviations are smaller.Then,the voltage deviations in the positive and negative intervals of the arm current are further analyzed.Based on this,a switching times reassignment based reduced switching frequency capacitor voltage balancing algorithm is proposed.The method adjusts the switching times in different intervals of a line cycle.By that,the switching times in the interval with a large voltage deviation are increased and the switching times in the interval with a small voltage deviation is reduced.Based on the reassignment of switching times,the deviation among capacitor voltages is reduced without rising the switching frequency.and the balancing performance of capacitor voltages is improved.(2)Two capacitor voltage balancing algorithms without arm current messages are respectively improved and proposed to realize the fault-tolerant operation of the voltage balancing under the communication interruption or optical fibers failure of arm currents.To eliminate the influence of the measurement noise,the existing currentless voltage balancing algorithm employs a filter.However,the performance of the currentless voltage balancing algorithm based on the sum of the capacitor voltages is disturbed by the delay of the filter.Then,this paper adds a phase compensation to improve the defect of the mentioned method.In addition,a currentless voltage balancing algorithm based on the average of the minimum capacitor voltage is also proposed.The above two algorithms are both immune to the burrs in sampled capacitor voltages and can respectively utilize the sum of all the capacitor voltages and the average of the minimum capacitor voltage in an arm precisely to replace the arm current.Thus,both the improved and proposed methods can be independent on the arm currents and realize the superior balancing of the capacitor voltages without the arm current message,which effectively elevates the reliability of MMC under communication failure.(3)A fault diagnosis strategy for power devices in an SM is proposed to accurately identify and locate the faulty arm.faulty SM and faulty device.Firstly,the fault characteristics of the power devices are analyzed,which derives the abnormal features of the capacitor voltage and the SM output voltage under the fault condition.In order to detect the fault feature,the repetitive signal generator is used to repeatedly output the deviation between the maximum and minimum capacitor voltages in the normal state.This deviation is employed as the reference signal of the detection algorithm.The faulty arm can be detected by comparing the measured voltage deviation with the reference voltage deviation,and then an auxiliary criterion is designed to clear the disturbance caused by the change in the operation of MMC for the detection.After the faulty arm is confirmed,the fault localization algorithm will further initiate in the faulty arm.The fault localization algorithm confirms the faulty SM based on the abnormality of the faulty capacitor voltage.Afterward,the faulty device in the faulty SM is located by the changing trend of the faulty capacitor voltage.The proposed fault diagnosis algorithm not only reinforces the sensitivity for the faults.which reduces the dependence on the value of the fault capacitor voltage and the difficulty in the selection of the threshold,but also can realize the detections and localizations of three different faults.(4)A hierarchic capacitor condition monitoring strategy is proposed to realize the online monitoring of abnormal capacitors and provide the initiation criterion for protection measures.Firstly,the analysis indicates that the reduction of capacitance makes the SMs switch the states more frequently.Owing to this feature,a switching times based abnormal capacitor identification method is proposed.To confirm whether the detected abnormal capacitor needs to be replaced or not.the proportional relationship between the capacitor voltage,capacitance in the reference SM and the capacitor voltage,capacitance in the SM to be measured is employed to further calculate the capacitance accurately.Moreover,the operation mode and interval of the capacitance calculation algorithm are elaborately designed and selected according to the features of the reduced frequency voltage balancing algorithm in the high-voltage MMC,which reduces the realization difficulty of the capacitance calculation and lowers the influence on the voltage balancing.In the hierarchic structure of the condition monitoring strategy,the abnormal capacitor identification algorithm shrinks the operation range of the capacitance calculation algorithm to avoid the calculation for the healthy capacitance and the capacitance calculation algorithm omits the complex integration,which effectively reduces the calculation burden in the control system of MMC.This capacitor condition monitoring strategy is simple efficient and easy to implement in the high-voltage MMC.(5)In view of the power device fault and capacitor abnormality in the SM.a new fault-tolerant SM topology and the related fault-tolerant method are proposed.In the fault-tolerant SM,an operating SM and a spare SM are connected by an intermediate link.Based on the above structure,the operation cases of the proposed SM in the normal operation mode and two fault-tolerant operation modes are analyzed in detail and the control schemes of the different operation modes are proposed.That not only ensures the stable operation of MMC in the normal condition,but also realizes the fault-tolerant operation of MMC in the SM fault condition.In the normal operation of MMC,the operating SM in the fault-tolerant SM can charge the capacitor in the spare SM.However,the spare SM does not take part in the normal operation of MMC,which eliminates the loss and fault risk of the traditional hot-reserved SM in normal operation.Since the spare SM has been well charged,it can initiate seamlessly in the fault-tolerant condition.This feature overcomes the tedious charging interference of the traditional cold-reserved SM before starting.The fault-tolerant SM can keep the symmetry of the arms in the same phase leg all the time.Thus,the fault tolerance can be easily realized by changing the switch signals of the proposed SM.which avoids adding extra control algorithms on the basis of the hot-reserved SM to eliminate the odd-order circulating currents.The fault tolerance scheme is effectively simplified.