Yingdong Wei

A Novel Phase-Locked Loop Based on Frequency Detector and Initial Phase Angle Detector

This paper proposes a new three-phase software phase-locked loop (PLL) that operates fast and accurately in unbalanced, polluted, and frequency deviating circumstances. The proposed PLL consists of a frequency detector and an initial phase angle detector. In the synchronous reference frame, the initial phase angle detector tracks a ramp phase angle, which is generated from frequency deviation of the inputs, with steady error. This detection error is utilized to estimate the actual grid frequency. Frequency adaptive moving average filters (MAFs) are applied in this new PLL to eliminate noises, harmonics, and negative sequence components. In this paper, the effect of discrete sampling on the MAFs is analyzed, and a linear interpolation is employed to enhance the performances of the MAFs. The stability of the proposed PLL is also analyzed, a sufficient stability condition is identified, and the design procedures of the control parameters are also presented. Simulations and experiments verify the performances of the novel PLL.

STATCOM Operation Scheme of the CDSM-MMC During a Pole-to-Pole DC Fault

In this paper, a static synchronous compensator (STATCOM) operation scheme of the modular multilevel converter adopting clamp double submodules (CDSM-MMC) during a pole-to-pole dc fault is proposed. By analyzing the current paths of a CDSM, this paper proposes three new operation states of the CDSM, which allow the CDSM to provide bipolar voltages when the arm current that flows through it is negative. Through control of the CDSMs to operate in these new states along with the blocked state, the arms of the CDSM-MMC can operate alternately in two different modes, that is, the conducting mode and blocked mode even when the dc-link voltage drops to zero. Thus, the CDSM-MMC is capable of working as a STATCOM during a dc fault. Moreover, a control strategy of the dc-fault STATCOM operation mode is designed to control the reactive power provided by the CDSM-MMC, to limit the dc fault current, and to reserve and balance the energies stored in capacitors. Simulation results conducted in PSCAD/EMTDC demonstrate the validity of the proposed STATCOM operation scheme and control strategy of the CDSM-MMC during a pole-to-pole dc fault.

Study on Capacitor Voltage Balancing Control of Modular Multilevel Converter at Low Frequency

 Abstract—This paper describes a control method and the operating performances of Modular Multilevel Converter (MMC) for high-voltage motor drive. The magnitude of capacitor voltage ripples increases when operating frequency decreases. To deal with the significant voltage fluctuation under low frequency conditions, theoretical analysis is presented in this paper, and a new capacitor voltage balancing control strategy is proposed, which is based on carrier phase-shifted sinusoidal pulse width modulation (CPS-SPWM). The simulation results show that MMC works well and capacitor voltages are balanced with the control strategy at low frequency.

A novel control strategy for optimization of power capacity based on railway power static conditioner

The railway static power conditioner (RPC) is used to compensate negative phase sequence (NPS) current, reactive current and harmonic current in electric railway. This paper, based on the decoupling of NPS current and reactive current, presents a novel control strategy which makes the PRC achieve flexible compensation targets for three-phase voltage unbalance and power factor drop in accordance with the corresponding power quality standards in railway. An optimum algorithm based on reactive current priority compensation control strategy is proposed to realize the optimal utilization of the power capacity of PRC. In the following part of the paper, the theoretical analysis is provided and the optimum compensation performance is verified by the simulation results in various track loads conditions.

A new dynamic strategy for improved ride-through capability of wind turbine generator

As the penetration of large-scale grid-connected wind farms increasing year by year, low voltage ride-through (LVRT) capability are required by many countries for power quality and fail-safe operation at grid fault. A new dynamic strategy using dynamic voltage restorer (DVR) is proposed in this paper to improve LVRT capability of wind turbine generators (WTGs). During the voltage dip, LVRT-DVR will be connected in series on the connection between grid and the generator by opening the static switch cabinet. And the power balance between grid and generator is achieved by using the dynamic braking resistor of LVRT-DVR. In order to increase the response speed of LVRT-DVR, anti-voltage technology is used to make the SCRs in static switch cabinet shutting off immediately. PSCAD/EMTDC simulations are carried out to prove the effectiveness of the proposed technique and some related conclusions and comments are summarized.

Dynamic voltage balancing of series connected IGBTs using slope regulating and voltage clamping

Series connection of IGBTs is applied to increase the voltage rating of converters. The critical issue is to balance the dynamic voltage of each device in the series string. This paper investigates an auxiliary circuit as a solution involving slope regulating and voltage clamping. It adjusts the Miller capacitance and gate current with additional capacitors and feedback current from collector to gate, by which the gate voltage and then the collector-emitter voltage are controlled. Consequently, the differences between block voltages of IGBTs are restrained. A 15kV test rig is established and experiments are carried out successfully based on up to sixteen 1.7kV/800A IGBTs in series connection. It is proved that the proposed circuit has no limitation to the number of series connected devices and is simple, reliable and low-cost.

Study on Railway Unified Power Quality Controller based on STATCOM technology

First of all, the paper introduces great importance and significant advantages of electrified railway for our nation. And then negative effects to the grids power quality brought by electrified railway are emphasized, followed by typical solutions such as the dynamic compensation strategy, among which compensator based on STATCOM technology shows the best general performance. Finally, the paper presents a two-phase STATCOM device- Railway Unified Power Quality Controller (RUPQC)s topology, and its excellent performance is shown and verified through simulation.

A Novel Hybrid-Arm Bipolar MMC Topology With DC Fault Ride-Through Capability

This paper proposes a novel bipolar modular multilevel converter (MMC) topology with dc fault ride-through capability. The proposed bipolar MMC topology is a hybrid of two types of arms: the half-bridge submodule (HBSM)-based arms (HBSM-arms) which are connected to the ground pole, and the unipolar-voltage full-bridge submodule (UFBSM)-based arms (UFBSM-arms) which are connected to the positive or negative pole. During a pole-to-ground or pole-to-pole dc fault, the hybrid-arm design offers the proposed UFBSM-based hybrid-arm bipolar MMC (UHA-BMMC) the capability to provide additional reactive power to support the ac grid while blocking the dc fault current. The topology, operation principle, dc fault mechanisms, and dc fault ride-through scheme of the UHAB-MMC are presented in this paper. Compared with other possible bipolar MMC topologies, the UHA-BMMC provides good dc fault ride-through capability with a small cost of power-electronic devices. The validity and features of the proposed UHA-BMMC are verified by simulation results conducted in PSCAD/EMTDC.

Novel high speed railway uninterruptible flexible connector based on modular multilevel converter structure

Phase sequence commutating method among traction feeders results in a neutral section without power supply between two feeders in traction substation of electrified railway. Electric locomotives suffer from a no-power time of more than 100 millisecond when going through the neutral section, during which the power failure and restoration process causes serious over-voltage and over-current problems. A novel railway uninterruptible flexible connector based on two-phase modular multilevel converter structure(MMC-UFC) is proposed in this paper, which can completely eliminate power supply dead zone. System utilization and operational principle are analyzed. Mathematical model of MMC-UFC is built to derive the difference between MMC-UFC. Double close-loop control strategies are given and close-loop control of both upper arm current and lower arm current for MMC converter is proposed to ensure the quality of output current. PSCAD/EMTDC simulations and experiment are carried out to prove the effectiveness of proposed MMC-UFC and its control system.

An optimal control strategy for power capacity based on railway power static conditioner

The negative phase sequence (NPS) current, reactive current (VAR) and harmonic current in electrified railway power supply system can be compensated by the railway static power conditioner (RPC). Based on the analysis of NPS current and reactive current decoupling, two control strategies for different objectives priority of RPC applied with asymmetrical transformers is proposed. It makes the PRC achieve the goal of a flexible and independent compensation in accordance with the corresponding power quality standards in railway for three-phase voltage unbalance and power factor drop in traction substation. Then, an optimum algorithm based on power factor priority compensation control strategy is proposed to realize the optimal utilization of the power capacity of PRC.