Ziping Wu

Comprehensive modeling and analysis of Permanent Magnet Synchronous Generator-Wind Turbine system with enhanced Low Voltage Ride Through Capability

With a growing penetration of Permanent Magnet Synchronous Wind Turbine Generations (PMSG-WT) into the modern power system, a comprehensive modeling and analysis of PMSG-WT is required to investigate its dynamic stability and interaction between large wind farm and power grids. In this work, a complete detailed MW-class variable speed Wind Turbine System based on a Permanent Magnet Synchronous Generator and a full-scale IGBT Voltage Source Converter is developed for PSCAD/EMTDC simulation study. This control scheme comprises both the mostly applied Maximum Point Power Tracking Operation and Double PWM active/reactive power independent control strategy. Moreover, a DC-link over-voltage protection scheme is designed and implemented in this model. A two mass drive train model is integrated into this WT model to achieve a reasonably accurate simulation on the transient stability of PMSG-WT. The feasibility of the established PMSG-WT physical model plus the effectiveness of proposed control and protection scheme are evaluated through a series of simulation studies under both variable wind speed conditions and a three-phase grid disturbance. Simulation results demonstrate that the PMSG-WT model possesses desirable capabilities of operation at the maximum power point as well as enhanced Low Voltage Ride Through Function (LVRT).

A coordinated primary frequency regulation from Permanent Magnet Synchronous Wind Turbine Generation

As a growing number of Permanent Magnet Synchronous Wind Turbine Generations (PMSG-WT) are integrated into the modern power grid, there has been a great concern for system operators who expects PMSG-WT to play an active role in the primary frequency regulation when frequency event occurs. However, PMSG-WTs are still unable to provide any additional active power in response to the system frequency change. In this work, a control scheme of coordinated frequency regulation for DFIG is redesigned and implemented into a PMSG-WT model by synthesizing the inertial control, over-rotor speed control and pitch angle control. Inertial control emulates the total inertia response of both wind turbine and generator to provide a short-term frequency support. According to the de-loaded optimum power extraction curve, the over-rotor speed and pitch angle controls are coordinated to maintain the reserved power and meanwhile assist the primary frequency regulation in a long-term period. The coordinated control strategy of PMSG-WT comprises the low, medium and high wind speed modes. By this means, the comprehensive control method will promote the PMSG-based wind farms to effectively participate in the system frequency regulation under a full scope of wind speed conditions.

Coordinated Control Strategy of Battery Energy Storage System and PMSG-WTG to Enhance System Frequency Regulation Capability

A novel inertial control method based on the torque limit control (TLC) is proposed in this study for the purpose of maximizing the temporary inertial response of permanent magnet synchronous generator-wind turbine generator (PMSG-WTG) over a wide range of variable wind speed conditions. To eliminate the secondary frequency drop issue during the rotor speed restoration, a small-scale battery energy storage system (BESS) is utilized by adopting the coordinated control strategy between BESS and PMSG-WTG. For the sake of minimizing the excessive use of BESS energy, this control strategy can smoothly discontinue the participation of BESS once the system frequency restores to the specified value. The simulation results conclude that the overall system frequency regulation performance can be significantly improved through such coordination control of BESS, PMSG-WTG, and conventional generators for the enhanced inertial response under different wind power penetration levels as well as variable wind speed conditions. Furthermore, the potential impact of TLC on the mechanical structures of wind turbine throughout the inertial response is investigated by using the CART2-PMSG integrated model.

Inertial response of wind power plants: A comparison of frequency-based inertial control and stepwise inertial control

The frequency regulation capability of a wind power plant plays an important role in enhancing frequency reliability especially in an isolated power system with high wind power penetration levels. A comparison of two types of inertial control methods, namely frequency-based inertial control (FBIC) and stepwise inertial control (SIC), is presented in this paper. Comprehensive case studies are carried out to reveal features of the different inertial control methods, simulated in a modified Western System Coordination Council (WSCC) nine-bus power grid using real-time digital simulator (RTDS) platform. The simulation results provide an insight into the inertial control methods under various scenarios.

Voltage regulation using a permanent magnet synchronous condenser with a series compensator

Wind power plant (WPP) is often operated at unity power factor, and the utility host where the WPP connected prefers to regulate the voltage. While this may not be an issue in a stiff grid, the connection to a weak grid can be problematic. This paper explores the advantages of having voltage regulation capability via reactive power control. Another issue in wind power generation is that not all turbines are able to control its reactive power due to technical reason or contractual obligations. A synchronous condenser (SC) using a permanent magnet synchronous generator (PMSG) is proposed for providing necessary reactive power for regulating voltage at a weak grid connection. A PMSG has the advantage of higher efficiency and reliability. Because of its lack of a field winding, a PMSG is typically controlled by a full-power converter, which can be costly. In the proposed system, the reactive power of the SC is controlled by a serially connected compensator operating in a closed-loop configuration. The compensator also damps the PMSGs tendency to oscillate. The compensators VA rating is only a fraction of the rating of the SC and the PMSG. In this initial investigation, the proposed scheme is shown to be effective by computer simulations.

Frequency support of PMSG-WTG based on improved inertial control

With increasing integrations of large-scale systems based on permanent magnet synchronous generator wind turbine generators (PMSG-WTGs), the overall inertial response of a power system will tend to deteriorate as a result of the decoupling of rotor speed and grid frequency through the power converter as well as the scheduled retirement of conventional synchronous generators. Thus, PMSG-WTGs can provide value to an electric grid by contributing to the systems inertial response by utilizing the inherent kinetic energy stored in their rotating masses and fast power control. In this work, an improved inertial control method based on the maximum power point tracking operation curve is introduced to enhance the overall frequency support capability of PMSG-WTGs in the case of large supply-demand imbalances. Moreover, this method is implemented in the CART2-PMSG integrated model in MATLAB/Simulink to investigate its impact on the wind turbines structural loads during the inertial response process. Simulation results indicate that the proposed method can effectively reduce the frequency nadir, arrest the rate of change of frequency and mitigate the secondary frequency drop while imposing no negative impact on the major mechanical components of the wind turbine.

Assessment of system frequency support effect of PMSG-WTG using torque-limit-based inertial control

To release the “hidden inertia” of variable-speed wind turbines for temporary frequency support, a method of torque-limit based inertial control is proposed in this paper. This method aims to improve the frequency support capability considering the maximum torque restriction of a permanent magnet synchronous generator. The advantages of the proposed method are improved frequency nadir (FN) in the event of an under-frequency disturbance; and avoidance of over-deceleration and a second frequency dip during the inertial response. The system frequency response is different, with different slope values in the power-speed plane when the inertial response is performed. The proposed method is evaluated in a modified three-machine, nine-bus system. The simulation results show that there is a trade-off between the recovery time and FN, such that a gradual slope tends to improve the FN and restrict the rate of change of frequency aggressively while causing an extension of the recovery time. These results provide insight into how to properly design such kinds of inertial control strategies for practical applications.

A serially-connected compensator for eliminating the unbalanced three-phase voltage impact on wind turbine generators

Untransposed transmission lines, unbalanced tap changer operations, and unbalanced loading in weak distribution lines can cause unbalanced-voltage conditions. The resulting unbalanced voltage at the point of interconnection affects proper gird integration and reduces the lifetime of wind turbines due to power oscillations, torque pulsations, mechanical stresses, energy losses, and uneven and overheating of the generator stator winding. This work investigates the dynamic impact of unbalanced voltage on the mechanical and electrical components of integrated Fatigue, Aerodynamics, Structures, and Turbulence (FAST) wind turbine generation systems (WTGs) of Type 1 (squirrel-cage induction generator) and Type 3 (doubly-fed induction generator). To alleviate this impact, a serially-connected compensator for a three-phase power line is proposed to balance the wind turbine-side voltage. Dynamic simulation studies are conducted in MATLAB/Simulink to compare the responses of these two types of wind turbine models under normal and unbalanced-voltage operation conditions and demonstrate the effectiveness of the proposed compensator.

Permanent magnet synchronous condenser with solid state excitation

A synchronous condenser consists of a free-spinning wound-field synchronous generator and a field excitation controller. In this paper, we propose a synchronous generator that employs a permanent magnet synchronous generator (PMSG) instead of a wound-field machine. PMSGs have the advantages of higher efficiency and reliability. In the proposed configuration, the reactive power control is achieved by a voltage source converter connected in series with the PMSG and the grid. The converter varies the phase voltage of the PMSG so as to create the same effect of over or under excitation in a wound-field machine. The converter output voltage level controls the amount and the direction of the produced reactive power and the voltages phase is kept in-phase with the grid voltage except a slight phase can be introduced so that some power can be drawn from the grid for maintaining the DC bus voltage level of the converter. Since the output voltage of the converter is only a fraction of the line voltage, its VA rating is only a fraction of the rating of the PMSG. The proposed scheme is shown to be effective by computer simulation.