Improved vector control methods for brushless double fed induction generator during inductive load and fault conditions

A Brushless Double-Fed Induction Generator (BDFIG) has shown tremendous success in wind turbines due to its robust brushless design, less maintenance, smooth operation, and variable speed characteristics. These generators are composed of two back-to-back voltage source converters, a Grid Side Converter (GSC) and a Rotor Side Converter (RSC). Existing control techniques use a “trial and error” method that results in a poor dynamic response in machine parameters during the absence of load. The RSC control is used for reactive current control during the inductive load insertion. However, it is more suitable for stabilizing steady-state behaviour, but it suffers from slow response and introduces a double fundamental frequency component to the Point of Common Coupling (PCC) voltage. In addition, generally, a Low Voltage Ride Through (LVRT) fault is detected using a hysteresis comparison of the power winding voltage. The LVRT capability is provided by using fixed reference values to control the winding current. This approach results in an erroneous response, sub-optimal control of voltage drops at PCC, and false alarms during transient conditions. This thesis aims to solve the mentioned issues by using an improved vector control method. Internal Model Control (IMC) based Proportional-Integral (PI) gains calculation is used for GSC and RSC. These are controlled to enhance the transient response and power quality during no-load, inductive load, and fault conditions. Firstly, a GSC-based vector control method is proposed to suppress the PCC voltage fluctuations when a large inductive load is suddenly connected. The proposed technique is based on an analytical model of the transient behaviour of the voltage drop at the PCC. To block a double fundamental frequency component as a result of reactive current compensation, a notch filter is designed. Secondly, an RSC-based vector control method is proposed using an analytical model of the voltage drop caused by a short circuit. Moreover, using a fuzzy logic controller, the proposed technique employs the voltage frequency in addition to the power winding voltage magnitude to detect LVRT conditions. The analytical model helps in reducing the power winding voltage drop while the fuzzy logic controller leads to better response and faster detection of faults. However, the reference value for reactive current compensation is analysed using an analytical model of the voltage drop at the PCC in the event of a short-circuit fault. The results obtained from MATLAB/Simulink show that the GSC-based vector control method technique can effectively reduce about 10% voltage drop at PCCs. Total Harmonics Distortion (THD) is improved to 22.3% by notch filter in comparison with an existing technique such as instantaneous reactive power theory. The RSC-based vector control method can achieve up to 11% voltage drop reduction and improve the THD by 12% compared to recent synchronous control and flux tracking methods

LVRT and HVRT control strategies of doubly- fed induction generator

The Doubly Fed Induction Generator (DFIG) has a high sensitivity to the Grid Faults (GFs), which can cause many problems on the power quality and the production continuity. Actually, the grid connection requirements impose strict laws to respect to Low Voltage Ride Through (LVRT), High Voltage Ride Through (HVRT), and grid support capacities following the Grid Codes (GCs). In fact, when detecting voltage fault, Wind Turbines (WTs) should stay in connection with the grid in order to hold a safe and stable operation. The main objective of this work is to propose LVRT and HVRT strategies able to retain WTs connected to the grid during severe grid voltage faults. The proposed approach is a hybrid method combining two methods (active and passive methods): The first aim is to develop the control of DFIG, while the second is applied for severe voltage faults using hardware protection circuits

Techniques for Ensuring Fault Ride-Through Capability of Grid Connected DFIG-Based Wind Turbine Systems: A Review

Renewable energy sources (RES) are being integrated to electrical grid to complement the conventional sources to meet up with global electrical energy demand. Among other RES, Wind Energy Conversion Systems (WECS) with Doubly Fed Induction Generator (DFIG) have gained global electricity market competitiveness because of the flexible regulation of active and reactive power, higher power quality, variable speed operation, four quadrant converter operation and better dynamic performance. Grid connected DFIG-based WECS are prone to disturbances in the network because of direct connection of stator windings to grid. The ability of the Wind Turbine (WT) to remain connected during grid faults is termed the Fault Ride-Through (FRT) capability. The grid code requirement for integrating the DFIG-based WTs to power networks specified that they must remain connected and support the grid stability during grid disturbances of up to 1500 ms. The use of compensation devices offers the best FRT compliance thereby protecting the DFIG and the converters from voltage fluctuations and over currents during the grid fault. The paper presents a review of techniques employed in ensuring FRT compliance. The article also proposes the state-of-the-art techniques for compensating voltage sag/swell and limiting the fault short-circuit current. Keywords: Renewable energy sources, DFIG, wind turbine system, fault ride-through, grid codes, dual-functional DV

Low-voltage ride-through techniques for DFIG-based wind turbines: State-of-the-art review and future trends

This paper deals with low-voltage ride-through (LVRT) capability of wind turbines (WTs) and in particular those driven by a doubly-fed induction generator (DFIG). This is one of the biggest challenges facing massive deployment of wind farms. With increasing penetration of WTs in the grid, grid connection codes in most countries require that WTs should remain connected to the grid to maintain the reliability during and after a short-term fault. This results in LVRT with only 15% remaining voltage at the point of common coupling (PCC), possibly even less. In addition, it is required for WTs to contribute to system stability during and after fault clearance. To fulfill the LVRT requirement for DFIG-based WTs, there are two problems to be addressed, namely, rotor inrush current that may exceed the converter limit and the dc-link overvoltage. Further, it is required to limit the DFIG transient response oscillations during the voltage sag to increase the gear lifetime and generator reliability. There is a rich literature addressing countermeasures for LVRT capability enhancement in DFIGs; this paper is therefore intended as a comprehensive state-of-the-art review of solutions to the LVRT issue. Moreover, attempts are made to highlight future issues so as to index some emerging solutions

Reliability Model Development for Wind Turbine Drivetrain with Brushless Doubly-Fed Induction Machine as Generator

Brushless doubly-fed induction machines (BDFIM) are attractive generators to be used in wind turbines due to the absence of brushes and slip rings. Furthermore, the BDFIM is a medium-speed generator and hence only requires one or two-stage gearbox. This feature simplifies the gearbox system and therefore improve reliability and reduce maintenance costs for the wind turbine. Although the design and operation of the BDFIM has been widely studied in the literature, there are only few studies on reliability assessment of the machine as a wind turbine generator. This paper proposes a comprehensive reliability model for two wind turbine drivetrain configurations: One with doubly-fed induction generator, and the other when the BDFIM is employed as the generator. The model is capable of evaluating the failure rate and repair rate indexes for the both configurations. Real field survey data from a 90 MW wind farm as well as calculated reliability data are then utilised to determine the reliability index values for the two drivetrain configurations in order to compare their reliability performance

Numerical Analysis of Stator Magnetic Wedge Effects on Equivalent Circuit Parameters of Brushless Doubly Fed Machines

This paper studies the effects of magnetic wedges used for closing stator open slots on the Brushless Doubly Fed Machines' (BDFM) equivalent circuit parameters. The BDFM is an attractive generator solution for offshore wind power and can replace doubly-fed slip-ring induction generators. It is shown in this paper that the use of magnetic wedges, commonly used in large induction machines, reduces the stator windings magnetising currents, reflected in the values of magnetising inductances. But they also increase the leakage flux of the stator windings and hence change the series inductance in the equivalent circuit. The series inductance significantly affects the machine performance as well as the rating of its converter. 2-D Finite element analysis of a 250 kW experimental BDFM is used to investigate the effects of magnetic wedges on the machine's magnetic field distribution and how these can alter the machine's parameters values. Experimental tests have also been carried out to validate the analysis

Emerging Multiport Electrical Machines and Systems: Past Developments, Current Challenges, and Future Prospects

Distinct from the conventional machines with only one electrical and one mechanical port, electrical machines featuring multiple electrical/mechanical ports (the so-called multiport electrical machines) provide a compact, flexible, and highly efficient manner to convert and/or transfer energies among different ports. This paper attempts to make a comprehensive overview of the existing multiport topologies, from fundamental characteristics to advanced modeling, analysis, and control, with particular emphasis on the extensively investigated brushless doubly fed machines for highly reliable wind turbines and power split devices for hybrid electric vehicles. A qualitative review approach is mainly adopted, but strong efforts are also made to quantitatively highlight the electromagnetic and control performance. Research challenges are identified, and future trends are discussed

Experimental and finite element studies of a 250kW brushless doubly fed induction generator

This paper studies, using experimentally verified Finite Element analysis, various performance measures of the Brushless Doubly Fed Induction Generator (BDFIG) when magnetic wedges are used for closing stator open slots. The BDFIG is an attractive generator solution for offshore wind power and can replace doubly-fed slip-ring induction generators. It is shown in this paper that the use of magnetic wedges, commonly used in large induction machines, reduces the stator windings magnetising currents in a BDFIG, reflected in the values of magnetising inductances. They also increase the leakage flux in the stator slots, leading to a larger series inductance in the equivalent circuit. The series inductance significantly affects the BDFIG performance as well as the rating of its converter. The effects of magnetic wedges on BDFIG air gap flux, synchronoustorque and stator tooth top saturation is also investigated. 2-D finite element analysis of an experimental 250 kW BDFIG is used in the study, verified by experimental measurements

Vibration anlysis of brushless doubly fed machines in the presence of rotor eccentricity

In this work, an analytical study has been performed on the Brushless Doubly Fed Machine's (BDFM) vibration due to the interaction of its fundamental magnetic fields, exerting bending forces in the back iron. The effects of rotor eccentricity on exacerbating the machine's vibration have been considered by assessing the stator back iron displacement function in the presence of rotor eccentricity. Finite element analysis is carried out for a 250 kW BDFM built in frame size D400 to validate the analytical methods. The stator back iron displacement is determined for an ideally-constructed machine as well as when the rotor has static and dynamic eccentricity. In addition, the prototype BDFM was tested at different operating conditions in order to examine its noise and vibration levels. A set of measurements was conducted to assess the main vibration component frequencies developed by the machine at different rotor speeds. It is shown that the main vibration components are created by bending set-up in the back iron, rotor eccentricity, and the components with time and space harmonic natures. The results obtained from finite element analysis and experimentally agree with the analytical theory of BDFM vibration

Analytical Study of Rotor Eccentricity Effects on Brushless Doubly Fed Machines Vibration

The Brushless Doubly Fed Machine (BDFM) with high reliability and robust structure demonstrates commercial and technical advantages both as a generator and motor for variable speed applications. As a generator it is particularly attractive to be used in offshore wind turbines where reliability improvement and maintenance cost reduction are the key factors in market growth. As a motor it may be utilized for adjustable speed drives. In this work, an analytical study has been performed on the BDFM’s vibration due to the interaction of its fundamental magnetic fields, exerting bending forces in the back iron. The effects of rotor eccentricity on exacerbating the machine’s vibration have been considered by assessing the stator back iron displacement function in the presence of rotor eccentricity. A prototype 250 kW BDFM built in frame size D400 was tested at different operating conditions in order to examine its noise and vibration levels. A set of measurements was conducted to assess the main vibration component frequencies developed by the machine at different rotor speeds. It is shown that the main vibration components are created by bending set-up in the back iron, rotor eccentricity, and the components with time and space harmonic natures. The results obtained from experimental tests agree with the analytical theory of BDFM vibration