Lei Wanjun

A novel PLL for grid synchronization of power electronic converters in unbalanced and variable-frequency environment

Grid synchronization is one of the most important issues of the distributed power generation system (DPGS) connected to utility network. This paper proposes a novel phase-locked loop (PLL) for synchronization of power electronic converters in unbalanced and variable-frequency environments. The method utilizes double rotating reference frames to transform the input signal and detects the positive fundamental component and negative sequence component simultaneously. Its based on a synthesis circuit which generates the orthogonal and in-phase signals to decouple input signal. This method detects the phase without distortion even in variable-frequency environment comparing with existing synchronization methods. Structural simplicity of the method greatly simplifies its implementation in digital software and/or hardware environments. This paper gives a description and derivation of the method. Its excellent performance is verified by simulation.

Research on DC capacitor voltage self-balancing space vector modulation strategy of five-level NPC converter

DC capacitor voltage balance is one of the key issues in the control of five-level NPC (neutral point clamped) converter. In this paper, it analyses the influence of space vectors to the DC voltage balance in five-level NPC converter SVM (space vector modulation) strategy and present a novel DC capacitor voltage self-balancing SVM strategy. The novel strategy divides the whole five-level space vector diagram into low modulation region (mSV≤0.5) and high modulation region (mSV>0.5). In low modulation region, DC capacitor voltage balance is achieved by combining objective function method with reference voltage decomposition method. In high modulation area, DC capacitor voltage balance is realized by applying an optimal balancing vector selection (OBVS) method. The characteristics of the novel strategy are smaller amount of calculation, much simpler realization and better control effect. Simulation and experimental results verify the correctness of the proposed method.

A novel control algorithm of shunt active power filter based on voltage detection

In the system with multi-sources and multi-loads, the shunt active power(SAPF) filter could not work effectively by the reason of difficulty to detect the all the load current or source current by an APF. Aiming to solve the problem, this paper presents a novel control algorithm of APF based on PCC voltage detection without current detection. The paper analyzes the relationship between the PCC voltage and the load harmonic current, and focuses on the research about the control algorithm. Simulations for the shunt active power filter are implemented and the experimental results confirm the theoretical analysis.

Harmonic current source based on fractional order FIR repetitive control

Harmonic current sources (HCS) that can provide variable-frequency and amplitude output current are widely used in automatic testing of the electrical equipments. Digital repetitive control (RC) can achieve zero steady-state error tracking of any periodic signal with known integer period, hence, its suitable for harmonic control, but it usually fails to provide precise tracking of variable frequency periodical waveform with fractional time delay. In this paper, a fraction order delay FIR filter based on Lagrange interpolation is used to approximate the factional delay items in RC. The design criteria is summarized in a simple form, and the stability of Fractional order RC is also analyzed in this paper, experiments is conducted to verify the effectiveness of the proposed FORC strategy.

Grouping-sorting optimized model predictive control of modular multilevel converter with reduced computational load

Modular Multilevel Converter (MMC) is a promising topology for high voltage/power AC/DC and DC/AC applications. Model Predictive Control (MPC) is an advanced and simple method for power converters control. However, its computational load becomes unacceptable when MMC voltage level increases. Although an optimized MPC proposed for the MMC decreased the number of the possible actuations of each phase from C2NN to N+1 for N+1 levels MMC, which is still difficult for application as the number of submodule for each arm reached hundreds. The Grouping-Sorting algorithm optimized MPC (GSOMPC) is proposed in the paper. The submodule capacitor voltages within the group and among groups are sorted according to the arm current polarity, the proposed GSOMPC decreased the number of the possible actuations of each phase from N+1 to 2X+M+3 (N=M×X) for N+1 levels MMC. The simulation and experimental results based on 25-level MMC verify the correctness and effectiveness of the GSOMPC strategy.

Research on hybrid dynamic capacitor — A novel approach to VAR compensation and resonance suppression

In this paper, a control method named hybrid dynamic capacitor is proposed to provide dynamic VAR compensation and suppress resonance between power capacitor and system inductance in power distribution network. This paper explains the theory of hybrid dynamic capacitor and this method benefits by reducing the initial cost, switching loss and less harmonic current. Finally, simulation and experiment results are given to verify the viability and effectiveness of the hybrid dynamic capacitor and its compensation characteristic.

A novel neutral point voltage automatic balancing carrier-based modulation strategy of three-level NPC converter

A novel carrier-based modulation strategy of NPC three-level converter which achieves neutral point voltage balance under the whole modulation index and power factor is proposed in this paper. It divides the traditional single modulation wave of each phase in SPWM strategy into two modulation waves, which are compared respectively with the carrier to decide the output switch state of the converter. It analyzes the internal relations of the modulation waves between the new strategy and traditional SPWM strategy, and the 64 solutions of the modulation waves of the new strategy are deduced. On the basis of analysis and derivation, the new strategy is simple and easy to be implemented in digital system. Through the analysis of the DC voltage utilization ratio, the device switching losses and the output phase voltage THD characteristics, the new strategy has the disadvantage of larger switch losses and higher harmonic content. Finally, the analysis of this paper is verified by experiment.

Inter-harmonic resonance suppression with hybrid parallel power filters

Inter-harmonics currents cause parallel resonance between line impedance and passive filter, which puts the hybrid parallel power filter operating in dangerous condition. A novel feed-forward control method is proposed to suppress the resonance by detecting resonant frequency current in load current. The feed-forward control coefficient is independent with resonant frequency, which simplifies the current detection. Both simulation and experiments verified the proposed method.

Development of a resonance-free power quality controller for middle voltage distribution system

In this paper, a resonance-free power quality controller (RFPQC) for middle voltage distribution system is described. The RFPQC system discussed in paper exhibits the following advantages: (a) The RFPQC has good filtering performance to eliminate most of the harmonic current generated by nonlinear load in characteristic frequency; (b) The RFPQC can damp parallel resonance between PF and grid impedance driving by load current; (c) The RFPQC also can damp series resonance between PF and grid impedance driving by grid voltage; (d) The harmonic current source in RFPQC has very small power rating comparing with nonlinear load. In this paper, the topology and control strategy has been analyzed in detail. The simulation and experiment result certify the RFPQC system has propose performance.