Integrated electromechanical wind turbine control for power system operation and load reduction

With the penetration level of wind power in electric power networks increasing rapidly all over the world, modern wind turbines are challenged to provide the same grid services as conventional synchronous power plants. The dynamic interaction between wind turbines and grid has to be assessed first before replacing large amount of conventional power plants by wind power. Over the last few years many power system operators have revised their grid codes and established more demanding requirements for wind power connection. In the past, when wind turbines were small, they were allowed to simply disconnect during a grid fault/disturbance. However, as wind turbine size has increased considerably, their fault ride-through capability has to be improved if the penetration of wind power is to be further increased. Wind turbine design and control need to be improved to optimize the compatibility of wind power and the grid. Among the various requirements that wind turbines have to meet, fault ride-through is of great importance and a very challenging one. Grid faults cause transients not only in the electrical system, but also in the wind turbine mechanical system. The dynamic performance of wind turbines is determined by both mechanical and electrical systems. From the mechanical point of view, the grid disturbance adds extra loads on wind turbine components. Severe grid faults may even lead to wind turbine emergency shut-down. From the electrical point of view, wind farms may lose power generation during a grid fault, which deteriorates the fault impact and slows down the fault recovery. Advanced control and active damping is required to improve wind turbine operation and assist it to remain connected during a grid fault. The novelty of this research is the study of the interaction between mechanical and electrical systems of the wind turbine. The detailed modelling of both the wind turbine mechanical and electrical dynamics not only helps to identify possible problems that wind turbines encounter during grid faults, but also allows adopting a combined approach to design the wind turbine controller. This thesis aims at improving the wind turbine fault ride-through capability and the ability of wind turbine to provide network support during grid disturbances. The main contents are as follows: The detailed model of wind turbine and grid including wind turbine mechanical model, wind turbine controller, synchronous and induction generator model, doubly fed induction generator (DFIG) controller and a generic network model are presented; A wind turbine fault ride-through strategy considering structural loads alleviation is proposed; A controller for asymmetrical fault ride-through of DFIG wind turbines is presented; The effect of having Power System Stabilizer (PSS) on wind turbine is investigated. A multi-band PSS controller for DFIG wind turbine is demonstrated.With the penetration level of wind power in electric power networks increasing rapidly all over the world, modern wind turbines are challenged to provide the same grid services as conventional synchronous power plants. The dynamic interaction between wind turbines and grid has to be assessed first before replacing large amount of conventional power plants by wind power. Over the last few years many power system operators have revised their grid codes and established more demanding requirements for wind power connection. In the past, when wind turbines were small, they were allowed to simply disconnect during a grid fault/disturbance. However, as wind turbine size has increased considerably, their fault ride-through capability has to be improved if the penetration of wind power is to be further increased. Wind turbine design and control need to be improved to optimize the compatibility of wind power and the grid. Among the various requirements that wind turbines have to meet, fault ride-through is of great importance and a very challenging one. Grid faults cause transients not only in the electrical system, but also in the wind turbine mechanical system. The dynamic performance of wind turbines is determined by both mechanical and electrical systems. From the mechanical point of view, the grid disturbance adds extra loads on wind turbine components. Severe grid faults may even lead to wind turbine emergency shut-down. From the electrical point of view, wind farms may lose power generation during a grid fault, which deteriorates the fault impact and slows down the fault recovery. Advanced control and active damping is required to improve wind turbine operation and assist it to remain connected during a grid fault. The novelty of this research is the study of the interaction between mechanical and electrical systems of the wind turbine. The detailed modelling of both the wind turbine mechanical and electrical dynamics not only helps to identify possible problems that wind turbines encounter during grid faults, but also allows adopting a combined approach to design the wind turbine controller. This thesis aims at improving the wind turbine fault ride-through capability and the ability of wind turbine to provide network support during grid disturbances. The main contents are as follows: The detailed model of wind turbine and grid including wind turbine mechanical model, wind turbine controller, synchronous and induction generator model, doubly fed induction generator (DFIG) controller and a generic network model are presented; A wind turbine fault ride-through strategy considering structural loads alleviation is proposed; A controller for asymmetrical fault ride-through of DFIG wind turbines is presented; The effect of having Power System Stabilizer (PSS) on wind turbine is investigated. A multi-band PSS controller for DFIG wind turbine is demonstrated

A multi-port power conversion system for the more electric aircraft

In more electric aircraft (MEA) weight reduction and energy efficiency constitute the key figures. Additionally, the safety and continuity of operation of its electrical power distribution system (EPDS) is of critical importance. These sets of desired features are in disagreement with each other, because higher redundancy, needed to guarantee the safety of operation, implies additional weight. In fact, EPDS is usually divided into isolated sections, which need to be sized for the worst-case scenario. Several concepts of EPDS have been investigated, aiming at enabling the power exchange among separate sections, which allows better optimization for power and weight of the whole system. In this paper, an approach based on the widespread use of multi-port power converters for both DC/DC and DC/AC stages is proposed. System integration of these two is proposed as a multiport power conversion system (MPCS), which allows a ring power distribution while galvanic isolation is still maintained, even in fault conditions. Thus, redundancy of MEA is established by no significant weight increase. A machine design analysis shows how the segmented machine could offer superior performance to the traditional one with same weight. Simulation and experimental verifications show the system feasibility in both normal and fault operations

Consideration of the effects of symmetrical and asymmetrical voltage dips in the control and operation of a grid-connected doubly-fed induction generator.

Masters Degree. University of KwaZulu-Natal, Durban.Grid integration of Type 3 Wind Energy Conversion Systems (WECS) based on the doubly-fed induction generator (DFIG) is a great challenge since the direct connection of the stator side to the grid makes it susceptible to loss of control and rotor over-voltages during voltage dips. Without effective countermeasures in place, this can result in the fatal failure of the power electronic converter. This thesis presents a selective study of the behaviour of Type 3 WECS based on DFIG under symmetrical and asymmetrical voltage dips. As part of the mitigation strategies, a crowbar protection scheme, demagnetizing current control and dual vector control are studied in this thesis. The main findings are drawn from a MATLAB/Simulink simulation model of a 2 MW, 690 V DFIG. This software platform offers built-in power electronic device models; therefore, the study is mainly focused on the control aspects of the DFIG. The fault ride-through (FRT) capability of the 0.8 kW DFIG test bench is also analyzed in this research. It is deduced that it is possible to control a DFIG WECS during voltage dips that are less than 32 % in depth by solely using the traditional dual vector control technique. Voltage dips greater than 32 % result in the saturation of the power electronic converter and loss of control. As part of the mitigation strategies developing in this study, it was found that the combined control of the demagnetizing current and the injection of a backwards rotating flux produced excellent results

Grid Voltage Unbalance and The Integration of DFIG’s

Double-fed induction generators (DFIG’s) became the predominant generator installed for wind generation applications in the mid 1990’s. Issues pertaining to the operation and control of DFIG’s subsequently became apparent, particularly in weak areas of the grid network. Ironically weak areas of the grid tend to be where the average wind speed is high and the usual location of wind farms. One of the issues that emerged was the quality of the voltage in the network at the point of common coupling (PCC) with the DFIG’s. An important issue is the question of voltage unbalance at the PCC. As part of this work, research was undertaken into the issue of voltage unbalance in a distribution network. Investigative studies were undertaken on a small wind farm connected to the Irish distribution network. The results obtained were then analysed and conclusions drawn, with recording of daily, weekly and seasonal variation of voltage unbalance. The behaviour of DFIG’s to varying levels of network voltage unbalance at the wind farm was analysed, and it was observed that the DFIG’s had difficulty remaining connected to the distribution network when voltage unbalance exceeded certain threshold levels. The behaviour of DFIG’s to the effects of grid network voltage unbalance is further investigated in this work. A literature review was undertaken of the effects that utility network voltage unbalance has on DFIG’s. Emerging from this research, the suitability of appropriate control schemes to alleviate the problems caused by grid voltage unbalance were investigated. Control techniques to improve performance of a DFIG during conditions of asymmetrical grid voltage including measures to control the rotorside and grid-side converters in a DFIG, were designed and then implemented in Matlab/Simulink and results showed improved behaviour. A synchronous generator system was similarly investigated and improvements shown. This research also includes development of a laboratory based DFIG test system. A DSP based digital microcontroller and interfacing hardware has been developed for a 5kVA DFIG laboratory based system. The system comprises of a machine set; a dc machine with common shaft coupling to a three-phase wound rotor induction machine. The dc machine emulates a wind turbine, and drives the induction machine in response to required speed. A converter has been constructed to control the rotor power of the induction machine. Interfacing schemes for the required feedback signals including voltage and current transducers and speed measurement were designed to enable control of both the rotor-side and grid-side converters of the DFIG. Grid/stator voltage oriented control is implemented to control both the rotor side and grid side converters respectively. An additional feature is the implementation of a single DSP controller, configured to control both the rotor side and grid side converters simultaneously. Initially the DFIG test rig was tested as a standalone system, with a load bank connected to the stator terminals of the induction machine. Testing of the DFIG was also conducted with the test rig connected directly to the grid, and the system operated in subsynchronous and super-synchronous modes of operation. Hardware and software solutions were implemented to reasonable success. The laboratory based test rig has been designed for operation as a rotor converter for a DFIG; however the converter can also be configured to operate as a system for a synchronous generator, or for operation as a machine drive. Further research may allow the rig to be used as a DFIG/UPQC (unified power quality controller) test bed

Control Strategy for Five-Phase Dual-Stator Winding Induction Starter/Generator System

This paper presents an integrated control strategy for a starter/generator (S/G) system based on five-phase dual-stator winding induction machine (FPDWIM). The FPDWIM has a cage-type rotor and two sets of stator windings. One is a five-phase control winding (CW) and the other is a five-phase power winding (PW). In the starting mode, the FPDWIM works as a motor. The CW provides both active power and reactive power to drive the engine. In the generating mode, the CW mainly handles reactive power while the PW outputs active power. To achieve the integration of the starting and generating controls, indirect CW-flux-oriented control (ICWFOC) is proposed to operate in both starting and generating modes. In starting mode, the CW current and flux are controlled to output a constant starting torque; while in generating mode, both CW and PW DC bus voltages are regulated. In this way, the principles and structures of the control strategies in both modes are compatible, resulting in a simpler implementation and improved performance. With the proposed control strategy, the system can complete the starting-generating operation with a smoother transition process. Simulation and experimental results are compared to validate the proposed control strategy