Tore Undeland

Low Voltage Ride Through of Wind Farms With Cage Generators: STATCOM Versus SVC

This paper analyzes the extent to which the low voltage ride through (LVRT) capability of wind farms using squirrel cage generators can be enhanced by the use of a STATCOM, compared to the thyristor controlled static var compensator (SVC). The transient stability margin is proposed as the indicator of LVRT capability. A simplified analytical approach based on torque-slip characteristics is first proposed to quantify the effect of the STATCOM and the SVC on the transient stability margin. Results from experiments with a STATCOM and a 7.5 kW induction machine emulating a wind turbine are used to validate the suggested analytical approach. Further verifications based on detailed time-domain simulations are also provided. Calculations, simulations and measurements confirm how the increased STATCOM rating can provide an increased transient stability margin and thus enhanced LVRT capability. Compared to the SVC, the STATCOM gives a larger contribution to the transient margin as indicated by both calculations and simulations. The inaccuracies introduced by neglecting the flux transients in the suggested approach are discussed and found reasonable for an estimation method when considering the simplicity compared to detailed time-domain simulation studies. A method for estimating the required rating of different compensation devices to ensure stability after a fault is suggested based on the same approach.

Extending the Life of Gear Box in Wind Generators by Smoothing Transient Torque With STATCOM

Gearboxes for wind turbines must ensure high reliability over a period of 20 years, withstanding cumulative and transient loads. One main challenge to this is represented by electromagnetic torque transients caused by grid faults and disturbances, which will result in significant stresses and fatigue of the gearbox. Possibilities for limiting the torque transients in fixed-speed wind generators have not been previously reported. This paper presents a technique by which the transient torques during recovery after a grid fault can be smoothed in a wind farm with induction generators directly connected to the grid. A model-based control technique using the quasi-stationary equivalent circuit of the system is suggested for controlling the torque with a static synchronous compensator (STATCOM). The basis of the approach consists of controlling the induction generator terminal voltage by the injection/absorption of reactive current using the STATCOM. By controlling the terminal voltage as a function of the generator speed during the recovery process, the electromagnetic torque of the generator is indirectly controlled, in order to reduce the drive train mechanical stresses caused by the characteristics of the induction machine when decelerating through the maximum torque region. The control concept is shown by time-domain simulations, where the smoothing effect of the proposed technique on a wind turbine is seen during the recovery after a three-phase-to-ground-fault condition. The influence of the shaft stiffness in a multimass drive train model is discussed, and the performance of the control concept in the case of parallel connection of several turbines is investigated to discuss the applicability in a wind farm.

Restructuring of first courses in power electronics and electric drives that integrates digital control

Since 1994, the University of Minnesota has been undertaking a long overdue restructuring of power electronics and electric machines/drives courses. This restructuring allows digital control to be integrated into first courses, thereby teaching students what they need to learn, making these courses appealing, and providing a seamless continuity to advanced courses. By a concise presentation in just two undergraduate courses, this restructuring motivates students to take related courses in programmable logic controllers, microcontrollers and digital signal processor applications. This ensures a first-rate education that is meaningful in the workplace as well as in graduate education leading to a research and development oriented career. This restructuring has several components to it. Outdated topics that waste time and mislead students are deleted. To integrate control in the first courses, unique approaches are developed to convey information more effectively. In the first course in power electronics, a building block is identified in commonly used power converter topologies in order to unify their analysis. In the field of electric drives, the use of space vectors is introduced on a physical basis to describe operation of ac machines in steady state in the first course, and to discuss their optimum control under dynamic conditions in the advanced course. Appropriate simulation software and software-reconfigurable hardware laboratories using a DSP-based rapid prototyping tool are used to support the analytical discussion.

STATCOM-Based Indirect Torque Control of Induction Machines During Voltage Recovery After Grid Faults

This paper proposes a control method for limiting the torque of grid-connected cage induction machines during the recovery process after grid faults, by using a static synchronous compensator (STATCOM) connected at the machine terminals. When a STATCOM is used for transient stability improvement, common practice is to design the control system to keep reactive current at maximum level until the voltage has returned to its initial value. This will result in high torques during the recovery process after grid faults. The control method proposed in this paper is intended to limit such torque transients by temporarily defining a new voltage reference for the STATCOM control system. As torque is controlled through the voltage reference of the STATCOM, the method is labeled indirect torque control (ITC). The presented concept is a model-based approach derived from a quasi-static equivalent circuit of the induction machine, the STATCOM and a The¿venin representation of the power system. For illustration and verification, time-domain simulations of a wind power generation system with a STATCOM at the terminals of an induction generator, are provided. As the objective of limiting the torque of the induction machine is achieved, the derivation of the concept proves to be reasonable. The approach is presented in its most general form, oriented to torque limitation of induction machines both in generating and motoring mode, and is not restricted to the presented example.

Voltage-Sensor-Less Synchronization to Unbalanced Grids by Frequency-Adaptive Virtual Flux Estimation

This paper proposes a simple method for inherently frequency-adaptive virtual flux (VF) estimation intended for voltage-sensor-less grid synchronization and control of voltage source converters. The frequency-adaptive characteristics are obtained by using a second-order generalized integrator configured as a quadrature signal generator (SOGI-QSG) with the grid frequency as an explicit input variable. It is also shown how the properties of VF estimation based on SOGI-QSGs can be utilized to achieve a simple and effective synchronization technique that integrates frequency-adaptive bandpass filtering, VF estimation, and symmetrical component sequence separation into one operation. This new method avoids cascaded delays of VF estimation and sequence separation and is labeled as dual SOGI-based VF (DSOGI-VF) estimation since it is based on two parallel SOGI-QSGs. The properties and performance of the DSOGI-VF estimation are analyzed in comparison to conventional configurations of VF estimation and sequence separation, demonstrating that it is a simpler structure with improved dynamic response. In fact, similar response time as for grid synchronization based on voltage measurements is achieved. The operation and performance of the proposed DSOGI-VF estimation method have been verified by laboratory experiments in a small-scale converter setup.

Virtual-Flux-Based Voltage-Sensor-Less Power Control for Unbalanced Grid Conditions

This paper presents a virtual flux-based method for voltage-sensor-less power control of voltage source converters under unbalanced grid voltage conditions. The voltage-sensor-less grid synchronization is achieved by a method for virtual flux estimation with inherent sequence separation in the stationary reference frame. The estimated positive and negative sequence (PNS) virtual flux components are used as basis for calculating current references corresponding to the following objectives for control of active and reactive powers under unbalanced conditions: 1) balanced positive sequence currents, 2) elimination of double-frequency active power oscillations, and 3) elimination of double-frequency reactive power oscillations. For simple implementation and flexible operation, the derived current references are synthesized into one generalized equation where the control objectives can be selected by real coefficients. Since the converter has a limited current capability, a simple, generalized, method for current limitation is also presented with the purpose of maintaining the intended power flow characteristics during unbalanced grid faults. The proposed strategies for virtual flux-based voltage-sensor-less operation have been investigated by simulations and laboratory experiments, verifying the expected performance of active and reactive power control with different objectives.

Main grid frequency support strategy for VSC-HVDC connected wind farms with variable speed wind turbines

A wind farm (WF) connected by voltage source converter-based HVDC (VSC-HVDC) results in a completely ‘inertia-less’ in offshore AC grid . AC frequency is determined by control of the VSC-HVDC on the WF side, independently of the main grid frequency. Moreover, the rotational speed of a variable speed wind turbine is independent of the AC frequency of the offshore AC grid. These two aforementioned features result in ‘inertia-less’ system, that is absence of mechanical energy storage to accomodate for abrupt changes in generation and load. An ‘inertia-less’ system does not have a load-frequency response similar to traditional AC grid with inertia, hence making it difficult for the wind turbines to contribute to frequency support of the main grid. This paper develops an artificial coupling of the wind farm grid and the main grid frequencies, for main grid frequency support by the wind farm. Artificial frequency coupling is achieved by use of droop controllers for the DC voltage control. A case of study, using PSCAD, demonstrates the effectiveness of artificial frequency coupling for the main grid frequency support.

Offshore Wind Farm Grid Integration by VSC Technology With LCC-Based HVDC Transmission

High-voltage dc (HVDC) transmission based on the line-commutated converter (LCC) is the most established and widespread technology around the world. However, HVDC based on the voltage source converter (VSC) has emerged as the best option to integrate renewable energy sources, e.g., offshore wind farms. This work investigates the feasibility of using a conventional LCC-HVDC transmission in combination with a VSC to integrate offshore wind farms. Such integration results in a hybrid HVDC connection, i.e., the connection of a VSC with an LCC through a dc cable. The operational features of a model of three-terminal hybrid HVDC, two LCC stations and one VSC station, is investigated using PSCAD/EMTDC. The simulations include an aggregate model to emulate the wind farm. The corresponding control strategies are proposed for each terminal and verified under various conditions including wind speed variations and ac faults.

Power electronics as grid interface for actively controlled wave energy converters

Off the coasts of Europe, the potential for power generation from offshore renewable energy is huge and the technology for offshore wind power is already becoming available. Achievement of a cost-effective technology is the main concern for commercial development of wave energy converters (WEC). It is widely recognized that some kind of active control of the primary conversion is needed in order to increase the electricity production and hence improve the economical payback. Moreover, if high penetration levels are going to be reached with offshore renewables, barriers to interconnection will be encountered. These barriers need to be identified and the technologies that will mitigate the impacts on the power system stability need to be investigated. Power electronics will be the enabling interface that will permit wave farms to act as one large power plant. This paper presents known approaches for maximizing production from wave power through active control, and discusses how these can be implemented by power electronics controllers used as interface with the power network. Three alternatives of power electronics interface are presented through three case studies for active control of WEC, power conditioning, and stability. Simulation results indicate the role of the power electronic interfaces in the active control of the wave energy converters and in network interactions.

Synchronous Reference Frame Hysteresis Current Control for Grid Converter Applications

This paper investigates the operation of space-vector-based hysteresis current control in the synchronous reference frame (SRF) for grid-connected voltage source converters. The space-vector-based approach allows for the systematic application of zero-voltage vectors and prevents high switching frequencies caused by phase interaction. The average switching frequency can be minimized by always selecting the zero-voltage vector that is closest to the previous switching state. Implementation in the SRF ensures a simple structure based on two three-level hysteresis comparators and a switching table that fits well into the traditional structure of vector-oriented current control. For operation in a grid-connected converter with an LCL filter, the current references to the hysteresis controllers are given by a dc-link voltage controller and an active damping mechanism for suppressing filter oscillations. Results from simulations and laboratory experiments are presented to illustrate the features of the investigated approach compared to traditional phase-current hysteresis controllers.