Fei Liu

Solid-State Circuit Breaker Snubber Design for Transient Overvoltage Suppression at Bus Fault Interruption in Low-Voltage DC Microgrid

Under a short-circuit fault in low-voltage dc microgrid, solid-state circuit breaker (SSCB) assumes the responsibility of the quick and effective isolation of the faulted area, while its own safety and reliability depends on the overvoltage suppression ability of its snubber. For SSCB snubber design, however, traditional method suited for converter switch snubber cannot be directly applied, because SSCB snubber stresses more on its overvoltage suppression and fault current withstanding instead of snubber loss reduction. Therefore, this paper strives to build a snubber design method specialized for SSCB overvoltage suppression at bus fault interruption. A comprehensive and target-focused set of measurements is proposed and applied to the comparison of three RCD snubber candidates: charge–discharge type, discharge-suppressing type I, and discharge-suppressing type II. The set of measurements includes six indicators with peak SSCB voltage and peak bus current as the main indexes. After parameters are determined, the indicators of the three snubbers are compared, through which discharge-suppressing type I is selected as the most appropriate SSCB snubber. Experiments of the three snubbers have also been conducted in a ±200-V/dc system to verify the correctness of comparison results and the validity of discharge-suppressing type I for SSCB overvoltage suppression at dc-bus fault.

Research on seamless transfer from CC to CV modes for IPT EV charging system based on double-sided LCC compensation network

In this paper, based on a double-sided LCC compensation network, the leakage inductance equivalent model of the loosely coupled transformer (LCT) is built and the constant current (CC) output with load independent is analyzed in an inductive power transfer (IPT) system. This paper proposes four resonance conditions for achieving constant voltage (CV) output in various load conditions for the compensation network. The Zero Voltage Switching (ZVS) operation for the primary side H-bridge converter can be achieved in CC mode and CV mode to minimize the switching loss. A seamless transfer control strategy is also investigated to realize transmission from CC mode to CV mode for IPT EV (electric vehicle) charging system. An IPT EV system with 3.3 kW transmission power is built. The theoretical analyses are confirmed by simulation and experiment results.

Power recovery and cost reduction oriented optimization of regenerative cells embedded in cascaded multilevel converter

The novel hybrid cascaded multilevel converter, with partial regenerative cells supplied by bidirectional rectifiers, is cost effective to recover the regenerative energy. To apply it in industrial fields containing various motors with loads, a systematic scheme designing the power cells is indispensable. This paper reveals that the maximum power factor angle of the motor is determined by the deceleration velocity, and that the controllable domain of the converter is determined by the sum of the DC voltage of the regenerative cells. From these analyses derives a step-by-step designing scheme. Furthermore, the scheme is optimized to reduce the regenerative cells, so as to be more cost effective. The results of simulation and experiment demonstrate that the optimized scheme can enable the converter to cost effectively recover the regenerative energy.

Analysis and design of voltage control and coordinate strategy in DC microgrid under multiple operation conditions

Considering multiple operation condition in DC microgrid, a bus voltage control strategy with terminal coordination is proposed in this paper, which could stabilize the bus voltage in extreme operation condition through receiving the operation information and issuing commands to every terminal. The equivalent circuit of DC microgrid is established and analysed, which can provide instruction on voltage control. During control strategy designing, the operation modes and switching criteria under multiple conditions are made. Accordingly, terminals coordinate control and flow chart of selection are introduced in this paper. Finally, DC microgrid model with loop topology with four terminal ports is built in MATLAB/Simulink to validate this strategy, and experiment results will be added in final paper.

Feedback decoupling and distortion correction based reactive compensation control for single-phase inverter

This paper analyzes the characteristic of singlephase converter system and establishes the active power and reactive power model in d-q axis, which is the same with the there-phase system in d-q axis and could employ decoupling feedback control for reactive compensation and active power control. For realizing d-q transformation in single-phase system, a new orthogonal decomposition method with sliding window filter (SWF) is presented. While, for the sake of SWF, the coupling between active power and reactive power could not be eliminated completely, which leads to poor dynamic process in control system and significant distortion in compensation current. For overcoming these shortages, this paper presents a distortion correction based control strategy, which not only could improve the control system dynamic characteristics, but also eliminate compensation currents distortion. At last, simulations with different control strategies demonstrate the validity and superiority of proposed control strategy.

A topology of the multi-port DC circuit breaker for multi-terminal DC system fault protection

Hybrid DC circuit breaker, a combination of disconnector and multiple power electronic devices, has gained popularity for its low-loss at normal operation and its quick and effective circuit breaking during fault protection. However, its bulkiness threatens to limit its application in multi-terminal DC system, where the terminal is usually connected to more than one transmission lines and hence the circuit breaker tends to appear in clusters. In this context, a multi-port DC circuit breaker topology is proposed here. The circuit breakers at one terminal is integrated into one. Through this, it is able to cut down by half the number of power electronic devices needed, greatly reducing the size and cost for circuit breaking. The effectiveness of the proposed topology is verified by simulation results.

A segmented power distribution control system based on hybrid regenerative cascaded multilevel converter

In order to realize motors regenerated energy storage and reutilization, a segmented power distribution control system is proposed in this paper based on hybrid regenerative cascaded multilevel converter. The energy storage cells and ordinary cells are connected together to form a hybrid cascaded topology, which reduces the number of active front ends and secondary windings of phase-shifting transformer. According to motors different operation modes, a novel three-segmented control strategy based on active and reactive power control is proposed to realize the energy flow among the ordinary cells, storage cells and the motor, which can achieve the reutilization of regenerated energy and stabilization of DC voltages. A hybrid cascaded multilevel converter with two ordinary cells and two energy storage cells in each phase is taken as an example conducted on a 380V scale-down laboratory prototype. Experimental results demonstrate the effectiveness of the proposed topology and its control strategy.

Performance analysis of RCD and MOV snubber circuits in low-voltage DC microgrid system

In order to protect solid-state circuit breaker (SSCB) from transient impacts under short circuit fault, an effective and economical snubber design is quite important. Traditional RCD snubbers are widely used because of their good voltage-clamping ability and short fault clearing time. Meanwhile, metal oxide varistor (MOV) also emerges as a suitable solution due to its non-linear U-I characteristic and simplicity. To optimize snubber design, it is imperative to analyze and compare the performance of these two snubber candidates. Therefore, a set of measurements is selected to focus on the performance analysis of RCD snubber and MOV, which contains five indicators: peak SSCB voltage, peak bus current, fault clearing time, snubber reliability and cost. A ±200 V Low-voltage DC (LVDC) system experiment is conducted for comparison. The results show that improved RCD snubber, Discharge-Suppressing Type I has smaller overvoltage but its cost is much higher than that of MOV. MOV stands out for shorter fault clearing time, however, its transient fluctuation is larger than that of RCD.

A Regenerative Hexagonal-Cascaded Multilevel Converter for Two-Motor Asynchronous Drive

This paper presents a hexagonal-cascaded multilevel converter for two-motor asynchronous drive application. By adding an ac input to the hexagonal configuration, this multilevel converter becomes a tri-port system with one input port and two output ports, which can drive two motors with asynchronous operation by sending different control commands. The detailed analysis of power distribution is presented, and it is found that the six branches are equivalent to two groups of three-phase voltage sources, which can share the power of two motors. The regenerative energy from one generating motor can be directly supplied to the other one in motoring mode. Consequently, the dc-link voltage overshoot can be suppressed, and the energy storage requirement of capacitors can be reduced as well. A two-motor drive system has been tested in MATLAB/Simulink and a downscaled laboratory prototype. Simulation and experimental results are provided to demonstrate the effectiveness of the proposed motor drive system and control strategies.

Combined optimization of SSCB snubber and freewheeling path for surgeless and quick bus fault interruption in low-voltage DC microgrid

When a bus fault occurs in a low-voltage DC microgrid, solid-state circuit breakers (SSCB) on either end of the DC bus assume the responsibility of the isolation of the faulted section, while the freewheeling path attached between SSCB and the bus takes on the task of fault energy absorption and fault current damping. However, during fault interruption, the snubber attached in parallel with SSCB for overvoltage suppression will influence the fault energy absorption performance of freewheeling path, while the resistance of the freewheeling path will also influence the overvoltage suppression capability of the SSCB snubber. Hence, this paper analyzed the interaction between the two and proposed a combined design so as to optimize the fault interruption performance for minimum SSCB overvoltage, and shortest fault clearing time, with less cost and size. Preliminary experiments are conducted on a ±200V/DC prototype, and the experimental results demonstrated the effectiveness of the design.