Xinchun Lin

A novel solid state fault current limiter for DC power distribution network

DC power distribution network is specifically used in industrial manufactures and aircraft/ship power distribution since it can provide flexible interface to various power converters with high efficiency and less conversion stage. The protection for DC power distribution system is more difficult compared with those AC systems since theres no natural zero crossing point for the current. Low shortcut impedance will cause high current rising rate, which can bring heat, arc and electrical force damage to the DC power distribution grid in a quite short time that the DC breakers hardly act. This paper proposed a solid state fault current limiter (FCL) using a combined power switch. Solid state FCL can achieve fast response to the fault current in the DC power grid, without special requirement of the cooling system. Using a combined power switch instead of single fully controlled power electronics component can reduce the steady power loss while normal working states. When fault states occurs, the power electronics control technique can strictly limits the fault current beneath a certain desired range. In that way, the fault damage can be limited. Experiment results are taken to prove this design.

An active islanding detection method for grid-connected converters

Islanding, detection is a mandatory function for gird-connected converters. The popular slip mode frequency shift and auto phase shift active islanding detection methods are investigated and an improved slip mode frequency shift (IM-SMS) strategy is proposed in this paper. In the proposed method, additional phase shift is introduced to help stimulating the islanding detection function and the algorithm is simplified as well. When the utility grid is disconnected, the algorithm keeps the frequency of the converter output voltage deviating until the frequency protection relay is triggered. The working principle of the method is introduced and the guidance of parameters selection is also provided. The islanding detection performance is evaluated through theoretical analysis, digital simulation and experiment. The IM-SMS method exhibits features of simplicity, easy implementation and high reliability and is expected to be an effective active islanding method.

Waveform compensation of PWM inverter in UPS

This paper proposes a new control scheme based on a two-layer control structure to improve both the transient and steady-state responses of a PWM inverter in UPS. The experiment results show that this control scheme can achieve both fast dynamic responses and low harmonics distortion.

Analysis and design of the DSP-based fully digital-controlled UPS

This paper proposes a new control scheme based on two parallel controllers to improve both the steady and transient state responses of close-loop regulated pulse-width-modulated (PWM) inverter for UPS. The controller consists of the repetitive and instantaneous controllers. The repetitive controller is designed to eliminate periodic disturbance and the instantaneous controller is synthesized for transient state response improvement. Design procedure is given for synthesizing the repetitive and instantaneous controllers. The proposed control scheme has been realized using a single-chip digital signal processor (DSP) TMS320F240 from Texas instruments. A 7.5 kVA UPS has been constructed to verify the proposed control scheme. Simulation and experimental results show that the DSP-based fully digital-controlled UPS can achieve both low total harmonics distortion and good dynamic response.

Harmonic suppression and resonance damping for shunt APF with selective closed-loop regulation of PCC voltage

Due to nonlinear loads and shunt power-factor-correction capacitor in power distribution system, harmonic distortion and harmonic resonance could be caused, which would seriously deteriorate the power quality. Shunt active power filters (SAPFs) only compensate the harmonic currents or suppress harmonic resonance, which may not be sufficient to meet a good power quality. This paper investigates harmonic suppression and resonance damping combined control for SAPF with selective closed-loop regulation of PCC voltage. Firstly, a recursive discrete Fourier transform (RDFT) algorithm is utilized to extract the harmonic suppression current reference from nonlinear load. Then, a novel selective closed-loop regulation of PCC voltage is proposed to damp harmonic resonance. Meanwhile, a zero steady-state error control strategy composed of ordinary PI controller and advanced repetitive controller in parallel under synchronous rotation frame is adopted to improve the output current tracking accuracy of SAPF. Finally, a lot of simulation and laboratory test results are provided to verify the effectiveness of the suggested control tactics.

Mitigation of inrush current in dynamic voltage restorer injection transformers

The dynamic voltage restorer (DVR) is among the most cost-effective solution against voltage sags. By using the transformer coupled topology, DVR could take the advantage of electrical isolation and voltage boost. However, at the start of compensation, the injection transformer may experience a flux linkage that is up to twice its steady-state value. The flux linkage will be driven to the level of magnetic saturation and severe inrush current occurs. Oversizing the transformer is a common approach to avoid the inrush current, but this would dramatically increase the cost, size and weight. This paper proposes a new technique for mitigating the inrush current. By taking use of the relationship between injection voltage and flux linkage, this method is characterized by interrupt compensation around the peak or trough of injection voltage to make the interruption time shortest. Detailed explanations of this technique are presented, and the effectiveness of this scheme is verified by laboratory test results.

A Coordinated Strategy Providing Zero-Sequence Circulating Current Suppression and Neutral-Point Potential Balancing in Two Parallel Three-Level Converters

In two parallel three-level T-type converters (3LT2Cs) with the sharing dc bus and ac bus directly, the neutral-point (NP) potential balancing and the zero-sequence circulating currents (ZSCCs) have to be considered to ensure normal operation of parallel system. In the conventional control scheme, the distribution factor of small vectors with space vector pulse width modulation (SVPWM) strategies (or the injected zero-sequence voltage with SPWM strategies) has to carry out two tasks simultaneously: NP potential balancing and ZSCCs suppression. However, the common challenge is how to coordinately control ZSCCs and NP potential due to only one controllable degree of freedom for two different targets. In this paper, the conventional scheme is investigated comprehensively. Theoretical analysis shows that the control authority over ZSCCs and NP potential is limited. Furthermore, a coordinated control strategy is proposed as follows: the distribution factor of one converter is in charge of the common NP potential balancing with the auxiliary NP bus, while the distribution factor of the other converter is in charge of ZSCCs regardless of NP potential. Based on the strategy, the distribution factor of every individual converter only needs to control one target, achieving the independent control of NP potential and ZSCCs. Theoretical analysis shows that NP potential balancing and ZSCCs suppression can be achieved simultaneously in two parallel 3LT2Cs. Finally, the effectiveness of the proposed strategy is verified by experimental results.

A repetitive control scheme for neutral-point low-frequency fluctuation suppression of three-level neutral-point-clamped converter

The Neutral-Point Balancing is always a problem for Three-Level Neutral-Point-Clamped Converter. The problem can be solved by the zero-sequence voltage injected in the modulation-reference signal for Carrier-Based Pulse Width Modulation (CB-PWM) strategy or by allocating the time of the small vector for the Nearest Three Space Vector Pulse Width Modulation (NTSV PWM). This paper firstly deduces the precise model of the Neutral-Point (NP) voltage in frequency domain based on CB-PWM. At the same time, a novel plug-in repetitive controller and PI controller are used to calculate the zero-sequence voltage injected. The proposed method can suppress low-frequency NP fluctuation which contains mainly third order harmonic in steady state and gain a smooth regulation in dynamic process compared with the conventional method. In addition, a Dual Sampling and Dual PWM duty ratio Update (DSDPU) scheme is used to expand the bandwidth of Neutral-Point (NP) balancing controller. The proposed concept will help extend in the NTSV PWM. Performance of the presented strategy for neutral-point balancing is also experimentally verified.

State-feedback-with-PI control for NPC three-level inverter

A discrete state-feedback-with-PI control scheme is proposed for NPC three-level inverter. The discrepancy between two approaches to design the controller, one in continuous domain, the other in discrete plane, is discussed. By means of continuous transformation, a load current feed-forward decoupling strategy (LCFF) is successfully proposed in this paper to make feed-forward possible for the discrete state-space controller. Based on transfer function analysis and comparison of discretized and continuous system, comprehensive verification is provided to verify the effectiveness of the derivation of the LCFF path. Simultaneously, the robustness analysis of the proposed strategy to the output impedance is also performed. Finally, experimental results are provided to verify the feasibility and validity of the proposed strategy.