Research On Fault Diagnosis And Tolerant Control Of Switching Devices In Distribution Static Synchronous Compensator

For the purpose of improving power quality,the Distribution Static Synchronous Compensator(DSTATCOM)is widely used as the power compensation equipment for the low-voltage distribution network.It has the ability to compensate reactive power and balance the three-phase load,which has been successfully applied to practical application in recent years.The core power unit of DSTATCOM is generally composed of controllable semiconductor power devices such as IGBTs.With the development and application of multi-level technology,the number of semiconductors in the DSTATCOM system also increases with the complexity of the topology.Once the switching device fails,it will directly affect the stability and reliability of the entire DSTATCOM system.Therefore,it is necessary to equip the DSTATCOM system with fault diagnosis and fault-tolerant control mechanism of the switching devices.In this paper,the open circuit(OC)fault diagnosis and fault-tolerant control of switching devices in DSTATCOM are studied.The main strategies presented in this paper are somewhat versatile,that is,they may also be applicable to other applications with similar topologies.First,the single-switch OC fault currents of DSTATCOM are analyzed in detail.From the internal perspective of DSTATCOM,the fault current paths and the three-phase output pole voltages in OC fault conditions are presented for both two-level,NPC three-leve and T-Type three-level topologies.The differences of fault current paths and output pole voltages are also compared in detail.Based on the above fault current analysis,zero current phenomenon of OC fault and single switch mathematical model of OC fault are presented.The mathematical models of OC fault of two-level,NPC three-level and T-Type three-level topologies are compared.Based on the above mathematical models,a fault current quantitative analysis method based on current rate is presented.The cause of the fault current distortion and the law of the fault current changes after the open circuit fault are explained.In particular,the reasons why the fault current value drops to zero,the coming moment of zero current,and the absolute duration length of zero current are proved.The conclusions may provide new ideas for exploring OC fault strategies of converters for reseachers.Then,two novel single switch OC fault diagnosis methods are proposed in this paper.The first one is a novel OC fault diagnosis method for three-phase four-wire three-level DSTATCOM based on voltage error correlation.This method is based on observing the output pole voltage error of DSTATCOM through two kinds of algorithms.One is voltage error analytical algorithm,which derives the four output voltage error analytic expressions through pulse state,current value and dc bus voltage,respectively,assuming that each switch of certain phase comes to OC fault.The other algorithm is the current circuit equation algorithm,which calculates the real-time output voltage error through basic circuit theory.Correlation is introduced to measure the similarity of the output voltage errors between the 2 algorithms and the OC fault is located by the maximum of the correlations.FPGA has been chosen to implement the proposed method due to its fast prototyping.Simulation and experimental results are presented to show the performance of the proposed OC fault diagnosis method.The specific fault detection process is only designed for the NPC three-level topology,and it is also applicable to other two topologies designed in this paper.This fault location strategy does not require additional sensors and is independent from load changes and transients.It is suitable for unbalanced load conditions.The response time of the strategy is within two supply cycles.The disadvantage is that a control unit with fast computing speed such as an FPGA or a CPLD is required.The second OC fault diagnosis method proposed in this paper is implemented by decomposing the circuit model into positive and negative-sequence model and zero-sequence model,and locating the OC fault based on the law of positive and negative-sequence voltage error offsets and zero-sequence voltage error offsets.Due to the zero-sequence currents in the four-wire systems,the three-phase currents become more complex on both amplitudes and phases in the unbalanced load conditions,so that the existing open circuit(OC)fault diagnosis methods of the switching devices cannot be directly used.The proposed diagnosis method base on T Type topology is suitable for both unbalanced load and unbalanced input voltage conditions.The equivalent offset of the ?-? plane corresponds to the offset of the positive and negative voltage errors,and the equivalent offset of the 0-axis is equivalent to the offset of the zero-sequence voltage error.Simulation and experimental results are presented to verify the correctness of the proposed strategy.No additional sensors are required for the fault location strategy and it is also appliable for NPC three-level topologies as well as two-level topologies.Secondly,an online risk estimation strategy for OC fault of DSTATCOM is proposed.The exsiting literature is mainly focused on the measures after the occurrence of the OC fault.In the normal conditions,is it possible to predict the impact of each IGBT OC failure on the system? How to assess its impact online and quantify the risk value? Based on this consideration,this paper puts forward an effective online OC fault risk assessment mechanism,taking the three-phase four-wire NPC three-level DSTATCOM as the research object.Then a novel strategy is introduced to optimize the positioning accuracy and the ability of anti-jamming and load mutation by using the obtained risk assessment coefficients to adjust the weight of the fault location decision function.The feasibility of this evaluation strategy is verified by simulation and experiment.Then this paper proposes a novel OC fault tolerance strategy for four wire DSTATCOM.The differences of fault-tolerant control between four-wire and three-wire systems are firstly analyzed and compared.An effective fault tolerance strategy for inner OC faults applied to four-wire T-Type converter systems is presented.The selection of fault-tolerant pulse combination and the calculation method of pulse action time in the new 3D orthogonal coordinate system are introduced in detail.Meanwhile,considering the possible imbalance of the midpoint potential of the busbar of the four-wire system,a control algorithm for actively controlling the balance of the upper and lower busbars is proposed.The effects of the proposed fault-tolerant control strategy are verified by simulation and experiment.Finally,in order to verify the main conclusions of this paper,experimental prototypes of DSTATCOM based on two-level,NPC three-level and T-Type three-level topologies with capacity of 100 kVar are designed(see appendixes).The hardware and software implementation introduced in detail,and the relevant experimental waveforms under normal working conditions are given.