Qirong Jiang

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.

Investigation of SSR in Practical DFIG-Based Wind Farms Connected to a Series-Compensated Power System

Subsynchronous resonance (SSR) was observed in wind farms located in North China. These wind farms prevailingly consist of doubly-fed induction generators (DFIGs) and are connected to series-compensated transmission lines. The observed resonant frequency is about 6 ~ 8 Hz, which is much lower than that of the reported SSR occurred in Texas. The frequency varies during the occurrence and this phenomenon is observed for the first time. The output power is usually within a certain range, when SSR occurs. Based on the practical system, an equivalent simulation system has been established, in which wind farms are modeled as many identical low rating DFIGs. Then, the SSR event is reproduced by simulations. Analysis results indicate that SSR happens even when the equivalent transmission system compensation level seen from wind farms is only 6.67%. Eigenvalue analysis shows that this phenomenon is an electrical resonance, and could be affected considerably by wind speed, number and control of DFIGs. The number of in-service DFIGs has a nonlinear impact on the damping of SSR. An equivalent electric circuit is deduced to intuitively explain why SSR happens and how the above factors affect it. Considering its features, this phenomenon is recognized as DFIG control participated induction generator effect.

Voltage-Source Control of PV Inverter in a CERTS Microgrid

Microgrids are highly compatible with photovoltaic (PV) sources because of their ability to internally aggregate and balance multiple renewable sources. Traditional grid-connected PV inverter control configurations are basically current sourced and cannot easily control ac voltage or frequency. The PV inverter using the Consortium for Electric Reliability Technology Solutions (CERTS) concepts can control ac voltage and frequency but have a major problem with load transients. During a load transient, the PV microsource becomes overloaded with the possibility of collapsing the dc bus voltage resulting in an ac voltage drop. This paper presents a PV inverter control strategy which enables PV to behave as a voltage source and is capable of maintaining dc bus voltage stability during load transient. With this PV inverter control configuration, it is shown that the PV microsource can operate as a voltage source in the CERTS microgrid.

Mitigation of Multimodal Subsynchronous Resonance Via Controlled Injection of Supersynchronous and Subsynchronous Currents

This paper presents a novel approach to analyze the mechanism of torsional interaction (TI) and its solution for series-capacitor-compensated power systems. The relationship between the oscillation of the generator shaft and the consequent electromagnetic torque is deduced by a time-domain analysis. It is revealed that the subsynchronous currents caused by torsional oscillations provide negative damping electromagnetic torque and are the cause of TI. A family of subsynchronous dampers (SSDs) is proposed, which is based on the controlled injection of supersynchronous and subsynchronous damping currents into the generator stator. The design procedure of subsynchronous damping controller (SSDC) of series SSD is elaborated. Eigenvalue analysis and simulations for the IEEE first benchmark model (FBM) have verified the effectiveness of the proposed SSDs in solving the multimodal subsynchronous resonance problem.

Development and Field Experiments of a Generator Terminal Subsynchronous Damper

This paper presents a novel controlling device, named generator terminal subsynchronous damper (GTSSD), to mitigate subsynchronous resonance (SSR) in the power system. The proposed GTSSD consists of a multimodal complementary current calculator (MCCC) and a complementary current generator (CCG). The CCG is composed of a current tracking controller and a power-electronic converter. The rotor speed deviation of the generator is fed back to the MCCC to calculate the current references of the CCG. The outputs of CCG consist of both sub- and supersynchronous complementary current, part of which flows into the generator and creates damping torque on the rotor. The relationship between the control parameters of the GTSSD and the resultant damping improvement is derived. A 10-MVA GTSSD prototype has been developed and tested in an actual series-compensated power system. The experimental results fully demonstrate its ability to enhance torsional damping and to depress subsynchronous oscillation, proving that the GTSSD provides a new and effective approach to solve SSR problems.

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.