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. In this paper, an OC fault diagnosis strategy applied to four-wire T-type converter systems is proposed, which 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. The proposed diagnosis method is suitable for both unbalanced load and unbalanced input voltage conditions. Meanwhile, the differences of fault-tolerant control between four-wire and three-wire systems are analyzed and compared. An effective fault tolerance strategy for inner OC faults applied to four-wire T-type converter systems is presented, which owns the ability to suppress DC bus imbalance. Simulation and experimental results are presented to verify the correctness of the proposed strategy.

At present, virtual flux voltage oriented control strategy is one of the widely used control strategies without grid voltage sensor. To solve the dc bias resulting from voltage vector integration in vector calculation of virtual flux and further to avoid the steady state error, a virtual flux observer with negative feedback resonant filter is presented in this paper based on virtual flux principle. A delay compensation algorithm is also proposed to solve the virtual flux delay. In addition, the control system diagram of PWM rectifier without grid voltage sensor is proposed. Then, simulation algorithm and experimental platform are established to simulate and test the rectifier respectively to verify the correctness and feasibility of the proposed algorithm.

This paper presents a novel approach to adapt the conventional direct power control (DPC) methods that are applied to ac/dc converters with an inductive capacitive inductive (LCL) filter. These new solutions improved the conventional DPC with an active damping algorithm via multi‐variable constraint adjustment, which is based on an augmented look‐up table and multi‐variable comparators. The novel look‐up table proposed for the active damping algorithm of the LCL filter allows an optimal voltage vector assignment to be chosen using differences in converter‐side current and capacitor voltage. This method is also compared with the DPC method with active damping via the addition of a virtual damping resistor and a harmonic rejection loop. The steady‐state (total harmonic distortion of the grid current) and dynamic performances (response time on step changes of the set current), as well as the immunity under distorted grid voltage conditions of the proposed system, are verified through simulation results and experimental measurements.

IGBT Open-Circuit Fault Diagnosis for 3-Phase 4-Wire 3-Level Active Power Filters based on Voltage Error Correlation Active Power Filter;Correlation;Fault Diagnosis;IGBT OC Fault;NPC 3-Level; A novel open-circuit fault diagnosis method for 3-phase 4-wire 3-level active power filters based on voltage error correlation is proposed in this paper. This method is based on observing the output pole voltage error of the active power filter through two kinds of algorithms. One algorithm is a voltage error analytical algorithm, which derives four output voltage error analytic expressions through the pulse state, current value and dc bus voltage, respectively, assuming that all of the IGBTs of a certain phase come to an OC fault. The other algorithm is a current circuit equation algorithm, which calculates the real-time output voltage error through basic circuit theory. A correlation is introduced to measure the similarity of the output voltage errors between the two algorithms, and OC faults are located by the maximum of the correlations. A 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.