Development and Control of Generator-Converter Topology for Direct-Drive Wind Turbines

In this chapter, a new topology for Direct-Drive Wind Turbines (DDWTs) with a low-voltage generator design is presented in order to eliminate the required dc-bus capacitors or dc-link inductors. In the presented topology, the grid-side converter is replaced by a boost Current Source Inverter (CSI) therefore removing the need for the dc-bus electrolytic capacitors which results in increasing the system lifetime. In the developed topology, the synchronous inductance of the generator is utilized. This facilitates the elimination of the intrinsically required dc-link inductor in the CSI which further contributes to a reduction in the overall system weight and size. The boost CSI is capable of converting a low dc voltage to a higher line-to-line voltage. This results in the implementation of a low-voltage generator for DDWTs. The feasibility of the presented low-voltage generator is investigated through Finite Element (FE) computations. In this chapter, a modified 1.5 MW low-voltage generator for the proposed topology is compared with an existing 1.5 MW Permanent Magnet (PM) synchronous generator for DDWTs. The feasibility of the presented topology of generator-converter for DDWTs is verified through simulations and laboratory tests. Furthermore, the controls developed for the developed wind turbine topology is also presented in this chapter

A Discrete-Time Direct-Torque Control for Direct-Drive PMSG-Based Wind Energy Conversion Systems

This paper proposes a novel flux space vector-based direct-torque control (DTC) scheme for permanent magnet synchronous generators (PMSGs) used in variable-speed direct drive wind energy conversion systems (WECSs). The discrete time control law, which is derived from the perspective of flux space vectors and load angle, predicts the desired stator flux vector for the next time-step with the torque and stator flux information only. The space-vector modulation (SVM) is then employed to generate the reference voltage vector, leading to a fixed switching frequency as well as lower flux and torque ripples when compared to the conventional DTC. Compared with other SVM-based DTC methods in the literature, the proposed DTC scheme eliminates the use of PI regulators and is less dependent on machine parameters, e.g., stator inductances and permanent magnet flux linkage, while the main advantages of the DTC, e.g., fast dynamic response and no need of coordinate transform, are preserved. The proposed DTC scheme is applicable for both nonsalient-pole and salient-pole PMSGs. The overall control scheme is simple to implement and is robust to parameter uncertainties and variations of the PMSGs. The effectiveness of the proposed discrete-time DTC scheme is verified by simulation and experimental results on a 180 W salient-pole PMSG and a 2.4-kW nonsalient-pole PMSG used in variable-speed direct-drive WECSs

Speed sensorless and MPPT control of IPM synchronous generator for wind energy conversion system

The popularity of renewable energy has experienced significant growth recently due to the foreseeable exhaustion of conventional fossil fuel power generation methods and increasing realization of the adverse effects that conventional fossil fuel power generation has on the environment. Among the renewable energy sources, wind power generation is rapidly becoming competitive with conventional fossil fuel sources. The wind turbines in the market have a variety of innovative concepts, with proven technology for both generators and power electronics interfaces. Recently, variable-speed permanent magnet synchronous generator (PMSG) based wind energy conversion systems (WECS) is becoming more attractive in comparison to the fixed-speed WECS. In the variable-speed generation system, the wind turbine can be operated at maximum power operating points over a wide speed range by adjusting the shaft speed optimally. This thesis presents both wind and rotor speed sensorless control for the direct-drive interior permanent magnet synchronous generator (IPMSG) with maximum power point tracking (MPPT) algorithm. The proposed method, without requiring the knowledge of wind speed, air density or turbine parameters, generates optimum speed command for speed control loop of vector controlled machine side converter. The MPPT algorithm based on perturbation and observation uses only estimated active power as its input to track peak output power points in accordance with wind speed change and incorporates proposed sensorless control to transfer maximum dc-link power from generator. In this work for the IPMSG, the rotor position and speed are estimated based on model reference adaptive system. Additionally, it incorporates flux weakening controller (FWC) for wide operating speed range at various wind speed and other disturbances. Matlab/Simulink based simulation model of the proposed sensorless MPPT control of IPMSG based WECS is built to verify the effectiveness of the system. The MPPT controller has been tested for variable wind speed conditions. The performance of the proposed WECS is also compared with the conventional control of WECS system. The proposed IPMSG based WECS incorporating the MPPT and sensorless algorithms is successfully implemented in real-time using the digital signal processor (DSP) board DS1104 for a laboratory 5 hp machine. A 5 hp DC motor is used as wind turbine to drive the IPMSG. The speed tracking performance and maximum power transfer capability of the proposed WECS are verified by both simulation and experimental results at different speed conditions

Supervisory Control of Full Converter Wind Generation Systems to Meet International Grid Codes

This thesis proposes a new supervisory control scheme for full converter wind generators (FCWGs) in compliance with the latest international grid codes. Intermittent behaviour of wind turbines and maximum converter capacity are taken into account in determining the reactive power injection to the grid following severe disturbance. Detailed simulations show that the proposed controller can improve the fault-ride-through capability of FCWGs while also providing support to the network as required by the grid codes

Direct Torque Control for Silicon Carbide Motor Drives

Direct torque control (DTC) is an extensively used control method for motor drives due to its unique advantages, e.g., the fast dynamic response and the robustness against motor parameters variations, uncertainties, and external disturbances. Using higher switching frequency is generally required by DTC to reduce the torque ripples and decrease stator current total harmonic distortion (THD), which however can lower the drive efficiency. Through the use of the emerging silicon carbide (SiC) devices, which have lower switching losses compared to their silicon counterparts, it is feasible to achieve high efficiency and low torque ripple simultaneously for DTC drives. To overcome the above challenges, a SiC T-type neutral point clamped (NPC) inverter is studied in this work to significantly reduce the torque and flux ripples which also effectively reduce the stator current ripples, while retaining the fast-dynamic response as the conventional DTC. The unbalanced DC-link is an intrinsic issue of the T-type inverter, which may also lead to higher torque ripple. To address this issue, a novel DTC algorithm, which only utilizes the real voltage space vectors and the virtual space vectors (VSVs) that do not contribute to the neutral point current, is proposed to achieve inherent dc-link capacitor voltage balancing without using any DC-link voltage controls or additional DC-link capacitor voltages and/or neutral point current sensors. Both dynamic performance and efficiency are critical for the interior permanent-magnet (IPM) motor drives for transportation applications. It is critical to determine the optimal reference stator flux linkage to improve the efficiency further of DTC drives and maintain the stability of the drive system, which usually obtained by tuning offline and storing in a look-up table or calculated online using machine models and parameters. In this work, the relationship between the stator flux linkage and the magnitude of stator current is analyzed mathematically. Then, based on this relationship, a perturb and observe (P&O) method is proposed to determine the optimal flux for the motor which does not need any prior knowledge of the machine parameters and offline tuning. However, due to the fixed amplitude of the injected signal the P&O algorithm suffers from large oscillations at the steady state conditions. To mitigate the drawback of the P&O method, an adaptive high frequency signal injection based extremum seeking control (ESC) algorithm is proposed to determine the optimal reference flux in real-time, leading to a maximum torque per ampere (MTPA) like approach for DTC drives. The stability analysis and key parameters selection for the proposed ESC algorithm are studied. The proposed method can effectively reduce the motor copper loss and at the same time eliminate the time consuming offline tuning effort. Furthermore, since the ESC is a model-free approach, it is robust against motor parameters variations, which is desirable for IPM motors

Medium Voltage Generation System with Five-level NPC Converters for Kite Tidal Power

Offshore power generation has emerged as a prominent source of energy and the installed capacity of new plants has been steadily increasing in recent years. Tidal power specifically is a promising renewable energy source which has not been highly exploited yet, despite its distinctive advantages of being predictable and independent of weather conditions. The main objective of this Licentiate thesis is to analyze and propose solutions for two common problems in offshore power production, which are the power variations due to the non-steady speed profile of the water speed flowing through the turbine and the efficient transportation of the produced power to the shore.The tidal power application utilized in this thesis is the subsea kite, which is a recently developed tidal energy conversion technology that can increase the generated power compared to a traditional static tidal turbine. A turbine is mounted on a submerged kite and the kite moves inside the sea following a predefined trajectory and generating electric power from the tidal currents. The speed and torque of the turbine varies periodically due to the periodic movement of the kite in the sea and, therefore, the control of the generator needs to be able to handle this variable generated power. The kite studied in this thesis has rated active power of 500 kW.In the first part of the thesis, the power generation system of the subsea kite is modelled and the profile of the generated power is extracted given a specific tidal current and turbine geometry. The control of the power converters is described and tested for the specific profile of the generated power. The speed of the generator is controlled by a properly designed Maximum Power Point Tracking algorithm, which ensures that the generator extracts the maximum power from the tidal stream. Experimental verification of the model of this innovative system is also conducted on a laboratory 35 kVA emulator of the tidal power generator.The second part of the thesis deals with the design of a medium voltage generator\ua0drive. The use of medium voltage in the power generation system is highly advantageous for the tidal kite application, since it can reduce the current flowing through the undersea cables connecting the tidal plant to the local grid. Therefore, the size of the cables can be reduced. The drive proposed here uses two 5-level Neutral Point Clamped (NPC) converters connected back-to-back. The 5-level NPC converters can operate with high voltage, while using multiple low-voltagerated power switches. Contrarily, the typical 2-Level converters have limited voltage capability, since they would require more expensive high-voltage-rated power switches. The increased operating voltage of the power conversion system results to lower current and losses in the cables. Another advantage of the NPC converter is the low harmonics at the ac side, which reduces the requirements for passive grid filters. However, the voltage balancing of the dc-link capacitors in this converter topology is a challenge which has not been effectively solved in previous studies. Therefore, a novel voltage balancing strategy is proposed here that uses advanced Space-Vector-Modulation techniques and hardware-based voltage balancing schemes with reduced number of components and lower power losses. Finally, a laboratory prototype of the NPC-converter-based power conversion system is developed with rated power 50 kVA. SiC MOSFETs are used on theconverters to further increase the system’s efficiency and voltage capability.This thesis presents the model, control and laboratory emulator of a kite-based tidal power generator. The experimental set-up can be utilized for conducting research on other renewable sources, such as wind power, that have similar performance. Also, the developed multilevel drive is suitable for various applications where medium voltage grid-connected drives are used and particularly in distributed renewable power generation

Modeling and Control of Diesel-Hydrokinetic Microgrids

A large number of decentralized communities in Canada and particularly in Québec rely on diesel power generation. The cost of electricity and environmental concerns suggest that hydrokinetic energy is a potential for power generation. Hydrokinetic energy conversion systems (HKECSs) are clean, reliable alternatives, and more beneficial than other renewable energy sources and conventional hydropower generation. However, due to the stochastic nature of river speed and variable load patterns of decentralized communities, the use of a hybrid diesel- hydrokinetic (D-HK) microgrid system has advantages. A large or medium penetration level has a negative effect on the short-term (transient) and long-term (steady-state) performance of such a hybrid system if the HKECS is controlled based on conventional control schemes. The conventional control scheme of the HKECS is the maximum power point tracking (MPPT). In the long-term conditions, the diesel generator set (genset) can operate at a reduced load where the role of the HKECS is to reduce the electrical load on the diesel genset (light loading). In the short-term, the frequency of the microgrid can vary due to the variable nature of water speed and load patterns. This can lead to power quality problems like a high rate of change of frequency or power, frequency fluctuations, etc. Moreover, these problems are magnified in storage-less DHK microgrids where a conventional energy storage system is not available to mitigate power as well as frequency deviations by controlling active power. Therefore, developing sophisticated control strategies for the HKECS to mitigate problems as mentioned above are necessary. Another challenging issue is a hardware-in-the-loop (HIL) platform for testing and developing a D-HK microgrid. A dispatchable power controller for a fixed-pitch cross-flow turbine-based HKECS operating in the low rotational speed (stall) region is presented in this thesis. It delivers a given power requested by an operator provided that the water speed is high enough. If not, it delivers as much as possible, operating with an MPPT algorithm while meeting the basic operating limits (i.e., generator voltage and rotor speed, rated power, and maximum water speed), shutting down automatically if necessary. A supervisory control scheme provides a smooth transition between modes of operation as the water speed and reference power from the operator vary. The performance of the proposed dispatchable power controller and supervisory control algorithm is verified experimentally with an electromechanical-based hydrokinetic turbine (HKT) emulator. The permanent magnet synchronous generator (PMSG) is preferred in small HKECSs. So, a converter-based PMSG emulator as a testbed for designing, analyzing, and testing of the generator’s power electronic interface and its control system is developed. A 6-switch voltage source converter (VSC) is used as a power amplifier to mimic the behaviour of the PMSG supplying linear and non-linear loads. Technical challenges of the PMSG emulator are considered, and proper solutions are suggested. Finally, an active power sharing control strategy for a storage-less D-HK microgrid with medium and high penetration of hydrokinetic power to mitigate: 1) the effect of the grid frequency fluctuation due to instantaneous variation in the water speed/load, and 2) light loading operation of the diesel engine is proposed. A supplementary control loop that includes virtual inertia and frequency droop control is added to the conventional control system of HKECS in order to provide load power sharing and frequency support control. The proposed strategy is experimentally verified with diesel engine and HKT emulators controlled via a dSPACE® rapid control prototyping system. The transient and steady-state performance of the system including grid frequency and power balancing control are presented

Control of a fractional-slot, concentrated-wound interior permanent magnet generator for direct-drive wind generation applications

This thesis assesses improvements to two types of control for a novel interior permanent magnet (PM) synchronous generator with fractional-slot, concentrated-wound stator designed for direct-drive wind energy conversion. The two control techniques assessed are a) field oriented control using a back-to-back converter arrangement and b) a current controller with a rectifier-connected boost converter. These were chosen to understand the potential and the limitations of the generator and its control. Modifications to the control techniques are proposed to improve the generator efficiency, the dynamic performance in the flux-weakening range and the torque ripple performance. The adequacy of the distributed-wound PM synchronous machine model for steady-state and dynamic control of this generator was experimentally validated under field oriented control using a back-to-back converter connected to the grid. The effectiveness of the existing current trajectory controls on the efficiency of the new generator was evaluated. A new flux-prioritized maximum torque per ampere technique which is independent of speed-dependent predefined trajectories was introduced, and a similar efficiency improvement was gained as the conventional loss minimization method in the partial load range. Thus, the control model validation and efficiency imrpovement of the new generator are the primary contributions. The dynamic performance of the generator, directly driven by a non-pitchable wind turbine emulator was investigated from cut-in speed to cut-out speed using maximum power point tracking and then constant power control above rated speed. A significant contribution was done in the power control above base wind speed that was achieved by utilizing the extended flux-weakening capability of the machine with its wide constant power-speed range. High torque ripple was observed when operated with a rectifier and boost converter using boost converter inductor current control. A new direct torque control technique using a machine rotor position based torque estimator was proposed to minimize this torque ripple. Eventhough the reduced torque ripple is still higher than that with back-to-back converter, the achieved ripple reduction is significant. The control of generator speed under each method is also demonstrated. Although the new method gives a faster speed dynamics than the conventional method, it shows slower speed response than that of back-to-back converter control. However, the significance of the study using a diode rectifier-connected boost converter control is highlighted with the achieved torque ripple minimization and performance enhancement of the generator. This study is expected to open new investigations in flux-weakening control of the PM generators using rectifier-connected boost converter. In this thesis, back to back converter control is demonstrated in order to optimally control the novel generator under the field oriented control, energy efficient current control and power control together with voltage control operating above rated speed. Torque ripple minimization of the generator is also presented when used with a diode rectifier-connected boost converter control

Development of fast multi-system simulation models for permanent magnet synchronous motor and generator drive systems

This research project investigates the development and validation of alternative simulation models for voltage source inverter fed permanent magnet synchronous machine drive systems which can rapidly and accurately analyse and evaluate the performance of PM machine drives and associated control system designs. Traditionally simulations have been conducted using switching models and state space average value methods. The simulation of switching models is time consuming and that of state space averaging involves complex mathematical transformation to d-q axis, with additional circuitry and this limits their application in a time critical design process. Even if the complex calculations of state space are overcome, the proposed model can still achieve better results. This thesis presents the development of fast multi system simulation models for permanent magnet synchronous motor and generator drive systems. The fast simulation model: Average Voltage Estimation Model (AVEM) was developed for two-level, three phase VSI-fed PMSM drive systems and two-level three phase full-scale back-back VSI incorporated in a PMSG wind energy conversion system. The method uses the principle of control strategy and switching function to derive the average phase voltage in one switching period and then uses the average voltages to drive piecewise-linear voltage sources across the terminals of the permanent magnet synchronous machine and three phase system. A voltage source inverter loss model was also developed and incorporated into the AVEM to simulate the drive system power flow and its performance evaluated. The average voltage estimation model is also used to estimate and simulate the energy output of the variable speed PMSG wind energy conversion system. Practical implementation of this technique is achieved using a DSP based controller and validation made through comparison of the DSP AVEM energy estimation method with calculated energy. The study also presents the development of detailed VSI switching models for a variable speed PMSM and a PMSG wind energy conversion system which serve as benchmarks for the proposed AVEM models. A detailed description of both models will be presented. Since models require a control strategy: these control strategies were also developed using the carrier-based sinusoidal (SPWM) and implemented with PI regulators. In the permanent magnet synchronous generator wind energy conversion system application, the SPWM is applied to control the speed of the generator side converter to track maximum power as wind speed varies using the developed passive MPPT control technique and controls the AC load side converter to maintained constant DC link voltage. The sinusoidal PWM control provides a simplified control suitable for the variable speed PMSM drive system and the PMSG wind energy conversion system. Lastly, this thesis presents a detailed development of an experimental test rig. The test rig is developed to provide flexibility for the validation and comparison of the results of both simulation models against real practical implementations for PMSM drive system and PMSG wind energy conversions system. Several simulation case studies were performed using the PORTUNUS simulation package to validate and analyse the steady state accuracy of the proposed average voltage estimation model and control system against the switching model. Experiments were also carried out to validate the results of the simulation models. The simulation models results are presented and compared with experimental results. Suitable steady state performance analysis of two-level, three phase voltage source inverter fed permanent magnet synchronous motor and two-level three phase full scale back-back voltage source inverter with permanent magnet synchronous generator drive simulation and experimental performance are also carried out. The results show good agreement of the proposed average voltage estimation model with the switching model and experimental data, and where necessary the reasons for differences are discussed. The simulation of the AVEM is approximately 50 times faster than the switching model. The limitation of the proposed model is also discussed; mainly it cannot be used for the study and analysis of the internal dynamics of the voltage source inverter. The results from the proposed modelling method utilising the average voltage estimation confirm that this method can be used as an alternative to the detailed switching model for fast simulation and steady state analysis of PM machine drive systems given the advantages of speed, simplicity and ease of implementation. Note that the proposed model is only used for steady state performance analysis; however, in future its application can be extended to transient analysis. In addition, the model is not about maximium power point tracking techniques but it can accommodate maximium power point tracking techniques. It should also be highlighted that exactly the same digital control block is used in both the switching and AVEM models thus allowing a true comparison of controller behaviour. The model developed in this research project has application beyond PMSM drive system and PMSG wind energy conversion system. It can be applied to modelling, simulation and control of other electrical machine drives such as induction machines, switched reluctance machines and three-phase VSI-fed systems