Mansour Mohseni

Impacts of Symmetrical and Asymmetrical Voltage Sags on DFIG-Based Wind Turbines Considering Phase-Angle Jump, Voltage Recovery, and Sag Parameters

This paper presents a new analysis into the impacts of various symmetrical and asymmetrical voltage sags on doubly fed induction generator (DFIG)-based wind turbines. Fault ride-through requirements are usually defined by the grid codes at the point of common coupling (PCC) of wind farms to the power network. However, depending on the network characteristics and constraints, the voltage sag conditions experienced at the wind generator terminals can be significantly different from the conditions at the PCC. Therefore, it is very important to identify the voltage sags that can practically affect the operation of wind generators. Extensive simulation studies are carried out in MATLAB/Simulink to investigate the transient overshoots and ripples that appear in the rotor current and dc-link voltage when the DFIG is subjected to various types of (a)symmetrical faults. For the first time, the impacts of phase-angle jump and operational constraints of circuit breakers are examined. Furthermore, the influences of sag parameters including type, initial point-on-wave instant, depth, and impedance angle are investigated. Complementary theoretical analyses are also presented to support the validity of observations made in the simulation studies.

Enhanced Hysteresis-Based Current Regulators in Vector Control of DFIG Wind Turbines

This paper proposes enhanced hysteresis-based current regulators in the field-oriented vector control of doubly fed induction generator (DFIG) wind turbines. The proposed control scheme is synchronized with the virtual grid-flux space vector, readily extractable by a quadrature phase-locked loop (QPLL) system. Identical equidistant-band vector-based hysteresis current regulators (VBHCRs) are then used to control the output currents of the rotor-side and grid-side converters. The proposed hysteresis-based technique has excellent steady-state performance and reveals several advantages in comparison with the commonly used proportional-integral (PI) current regulator, including very fast transient response, simple control structure, and intrinsic robustness to the machine parameters variations. Moreover, the fixed hysteresis bands in VBHCRs are replaced with equidistant bands to limit the instantaneous variations of the switching frequency and reduce the maximum switching frequencies experienced in the converters. Extensive simulation studies are carried out for a 1.5 MW DFIG-based wind turbine to examine the operation of the proposed vector control scheme under changing wind speed and compare its transient and steady-state performances with the conventional PI current regulators.

A New Vector-Based Hysteresis Current Control Scheme for Three-Phase PWM Voltage-Source Inverters

This paper presents a new vector-based hysteresis current controller (HCC) for three-phase pulsewidth modulation (PWM) voltage-source inverters (VSI). The HCC is intrinsically robust to the load parameters variations, exhibits very fast transient performance, and is suitable for simple implementations. Despite these advantages, the conventional HCC has a major drawback when applied to the three-phase PWM-VSI: interphases dependency leads to very high-switching frequencies in the inverter. This paper starts with a review on the vector-based HCCs reported in the literature to address this problem. Then, a new vector-based method is proposed using multilevel hysteresis comparators integrated with a switching table. The proposed method works with the inverter current vector represented in the stationary α- β frame and produces a coordinated switching pattern. The current error is kept inside a square tolerance region without significant increase in the complexity of the hardware implementation. Simulation results show that the proposed vector-based method can retain the advantages of the conventional HCC. However, the steady-state performance of the proposed current regulator is significantly improved by reducing the switching frequency and minimizing oscillations of the inverter current vector. The proposed current controller is finally compared with other reported vector-based methods and its advantages are illustrated.

Transient Control of DFIG-Based Wind Power Plants in Compliance With the Australian Grid Code

Australian Grid Code has recently enforced stringent regulations on the transient response of large wind power plants (WPPs). The new grid code requires wind generators to ride-through severe low- and high-voltage conditions, provides reactive power support during the fault period, and exhibits fast power recovery after the supply voltage restoration. This paper proposes a new control scheme for doubly fed induction generator (DFIG)-based WPPs to fulfill these requirements in one inclusive approach. New design strategies for the outer power control loops of DFIG are suggested and their corresponding P-Q capability curves are rigorously studied. It is shown that safely overloaded converters can enhance the reactive power capability of DFIGs during the fault periods. Moreover, for the inner current control loops, the conventional PI current regulators are replaced with enhanced hysteresis-based current regulators. This current regulator, with very fast transient response, increases low- and high-voltage ride-through capabilities of the DFIG, as requested by the Australian Grid Code. Finally, time-domain simulation studies are conducted to evaluate the capability of the proposed control scheme to fulfill the Australian regulations and examine its positive impacts on the transient response of the adjacent fixed-speed WPP.

Comparing technical connection requirements for large wind power plants

This paper presents a comparative study of the gird code regulations stipulated by international transmission system operators (TSOs) for large wind power plants. A brief overview of the current global wind power capacity is first presented, followed by comparing the most common regulations that are included in majority of grid codes, namely low and high voltage ride-through, active and reactive power responses during and after faults, extended range of frequency variations, active power control (frequency regulation), and reactive power control (voltage regulation). This paper also presents a discussion on the global harmonization of grid codes as well as future trends of the regulations. Finally, the ability of different wind generator technologies to comply with the international grid code requirements is investigated. This study assists TSOs to establish their connection requirements for wind power plants or to compare their existing regulations with other countries. This work also enables wind turbine manufacturers to obtain a more precise understanding from the latest international regulations.

Low voltage ride-through of DFIG wind turbines complying with Western-Power grid code in Australia

Western Power has recently imposed stringent low voltage ride-through (LVRT) requirements on wind power plants that seek connection to the Western Australian transmission system. Doubly-fed induction generators (DFIGs) are the dominant technology used in wind generation systems. However, this type of wind generator is very sensitive to fault conditions and without efficient “ride-through” strategy, continuous operation of DFIG may fail due to destructive overcurrents in the rotor winding or large overvoltages in the dc-link capacitor. This paper introduces a hybrid current control scheme, implemented in the rotor-side converter of DFIG, to enhance its LVRT capability in compliance with the Western Power grid code. The proposed control scheme is constituted of two switching strategies: standard PI current controllers for normal operating conditions and vector-based hysteresis current controllers for DFIG protection during severe fault conditions. Simulation studies are carried out to examine the effectiveness of the proposed LVRT strategy under symmetrical and asymmetrical voltage sags. It is shown that this hybrid control scheme can constrain the rotor current and dc-link voltage within the safety limits of DFIG and as a result, the wind generator meets the strict regulations defined by Western Power.

A novel current controller for three-phase voltage-source inverters

A novel vector-based hysteresis current controller for three-phase PWM voltage-source inverters is proposed in this paper. This current control scheme implements two sets of hysteresis comparators in three-phase abc and stationary α-β frames integrated with a switching table. 2-level three-phase comparators are employed as region detector to confine the optimal voltage space vectors to be applied. Then, the appropriate voltage space vector at each instant is determined using two 3-level comparators with narrow hysteresis band working in α-β coordinated frame. The proposed control scheme utilizes the advantages of both space vector modulation and hysteresis current controllers. Simulation studies are carried out to demonstrate the improvements achieved by the proposed method.

Emergency control of DFIG-based wind turbines to meet new European Grid Code requirements

New regulatory requirements outlined in the European Grid Code dictate fault ride-through and reactive power support requirements for wind energy conversion systems. This paper introduces a simple emergency control scheme for DFIG-based wind turbines to comply with the grid code requirements. The proposed control design uses an enhanced phase locked loop (PLL) system to extract the synchronization signal from the three-phase grid voltage. The proposed PLL will also present an on-line estimation of the grid voltage magnitude, which is needed to detect the fault and provide the reactive power support accordingly. To increase the fault ride through capacity of DFIG, the standard PI current regulator in the rotor-side converter of DFIG is replaced with a vector-based hysteresis current regulator. The main advantages of the proposed current regulator are its very fast transient response and simple control structure. Simulation studies are carried out to demonstrate the effectiveness of the proposed emergency controller to comply with the new European Grid Code requirements under various network disturbances.

International regulations on the transient response of large wind power plants

This paper presents a comprehensive study on the latest requirements imposed by international transmission system operators (TSOs) on the transient response of large wind power plants (WPPs). The grid codes of Australia, Canada, Denmark, Germany, Ireland, Nordic Countries, Spain, UK and USA are covered. Regulations on the transient response of large WPPs can be broadly divided into three groups, including fault ride-through curves, active and reactive power responses during the fault period, and extended range of voltage-frequency variations. This paper also presents a discussion on the ability of different wind generator technologies to fulfill the international regulations. The presented study assists new TSOs to establish their connection requirements for large WPPs or to compare their existing regulations with other TSOs around the world. This work also enables wind turbine manufacturers and wind farm developers to obtain a more precise and consistent understanding from the latest international regulations imposed on large WPPs.

Review on Australian grid codes for wind power integration in comparison with international standards

This paper compares Australian grid codes for wind power integration with the international standards. First, the wind status in Australia is investigated to provide general background of wind power utilization all over the country. Next, technical requirements of connecting wind turbine in Australia are discussed including a comparison with international grid codes. Finally, a brief investigation is provided on issues in complying with grid requirements including harmonization of grid codes and their compliance certificate. In addition, practiced solutions from perspectives of wind related manufacturers to grid compliance issues will also be presented. The latest update of the grid codes provided in this paper will help the Australian transmission system operators in examining recent regulations in comparison to international practices while also giving some insights to the wind turbine manufacturers, developers and operators in improving their services and/or products in complying with the existing standards.