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Analysis of Dynamic Behaviour of Electric Power Systems with Large Amount of Wind Power

This paper presents a new operational strategy for a small scale wind farm which is composed of both fixed and variable speed wind turbine generator systems (WTGS). Fixed speed wind generators suffer greatly from meeting the requirements of new wind farm grid code, because they are largely dependent on reactive power. Integration of flexible ac transmission systems (FACTS) devices is a solution to overcome that problem, though it definitely increases the overall cost. Therefore, in this paper, we focuses on a new wind farm topology, where series or parallel connected fixed speed WTGSs are installed with variable speed wind turbine (VSWT) driven permanent magnet synchronous generators (PMSG). VSWT-PMSG uses a fully controlled frequency converter for grid interfacing and it has abilities to control its reactive power as well as to provide maximum power to the grid. Suitable control strategy is developed in this paper for the multilevel frequency converter of VSWT-PMSG. A real grid code defined in the power system is considered to analyze the low voltage ride through (LVRT) characteristic of both fixed and variable speed WTGSs. Moreover, dynamic performance of the system is also evaluated using real wind speed data. Simulation results clearly show that the proposed topology can be a cost effective solution to augment the LVRT requirement as well as to minimize voltage fluctuation of both fixed and variable speed WTGSs.

A Variable Speed Wind Turbine Control Strategy to Meet Wind Farm Grid Code Requirements

Fast acting static synchronous compensator (STATCOM), a representative of FACTS family, is a promising technology being extensively used as the state-of-the-art dynamic shunt compensator for reactive power control in transmission and distribution system. Over the last couple of decades, researchers and engineers have made path-breaking research on this technology and by virtue of which, many STATCOM controllers based on the self-commutating solid-state voltage-source converter (VSC) have been developed and commercially put in operation to control system dynamics under stressed conditions. Because of its many attributes, STATCOM has emerged as a qualitatively superior controller relative to the line commutating static VAR compensator (SVC). This controller is called with different terminologies as STATic COMpensator advanced static VAR compensator, advanced static VAR generator or static VAR generator, STATic CONdenser, synchronous solid-state VAR compensator, VSC-based SVC or self-commutated SVC or static synchronous compensator (SSC or S2C). The development of STATCOM controller employing various solid-state converter topologies, magnetics configurations, control algorithms, switching techniques and so on, has been well reported in literature with its versatile applications in power system. A review on the state-of-the-art STATCOM technology and further research potential are presented classifying more than 300 research publications.

Static synchronous compensators (STATCOM): a review

Voltage stability is a key issue to achieve the uninterrupted operation of wind farms equipped with doubly fed induction generators (DFIGs) during grid faults. This paper investigates the application of a static synchronous compensator (STATCOM) to assist with the uninterrupted operation of a wind turbine driving a DFIG, which is connected to a power network, during grid faults. The control schemes of the DFIG rotor- and grid-side converters and the STATCOM are suitably designed and coordinated. The system is implemented in real-time on a real time digital simulator. Results show that the STATCOM improves the transient voltage stability and therefore helps the wind turbine generator system to remain in service during grid faults.

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Real-Time Implementation of a STATCOM on a Wind Farm Equipped with Doubly Fed Induction Generators

This paper uses data collected at Central and South West Services Fort Davis wind farm (USA) to develop a neural network based prediction of power produced by each turbine. The power generated by electric wind turbines changes rapidly because of the continuous fluctuation of wind speed and direction. It is important for the power industry to have the capability to perform this prediction for diagnostic purposes-lower-than-expected wind power may be an early indicator of a need for maintenance. In this paper, characteristics of wind power generation are first evaluated in order to establish the relative importance for the neural network. A four input neural network is developed and its performance is shown to be superior to the single parameter traditional model approach.

Using neural networks to estimate wind turbine power generation

Effective reactive power control in distribution networks can have an important influence on the level of embedded generation that is acceptable and on the costs of its connection. A comprehensive study was undertaken to investigate how STATCOMs could be used with fixed-speed wind turbines, which use induction generators, to improve both the steady-state and dynamic impact of a wind farm on the network. An optimal power flow model based on loss minimisation was developed and used to show that operation of a wind farm at unity power factor is unlikely to allow maximum penetration of wind energy into a weak distribution circuit. The results of the electromagnetic simulations showed that the use of a STATCOM improves the steady-state stability limits of the network. They also showed that the use of a STATCOM with an appropriately designed control strategy can prevent damaging overvoltages that may occur under islanding conditions. The results also demonstrated that a STATCOM can be used to mitigate voltage fluctuations at blade passing frequency, which may occur if the rotors of a number of wind turbines fall into synchronism.

Application of STATCOMs to wind farms

Contemporary fixed-speed wind turbines use induction generators rated at up to 500 kW, although turbines with ratings of over 1 MW are under development. A large wind farm may consist of up to 100 such turbines. The output power of each turbine fluctuates with its local incident wind and the position of the blades, so causing voltage disturbances on the public-supply network. A simple model has been developed to represent the induction generators, local power-factor-correction capacitors and network of a wind farm. This allows the variations in network voltage due to the changing aerodynamic torque of each turbine to be determined conveniently. The simple model has been compared to higher-order models and good agreement observed.

Simple wind farm dynamic model

Accurate models for voltage flicker prediction are required so that large wind turbines may be connected, to weak utility networks, in the confidence that excessive flicker levels will not occur. These models must work satisfactorily with different wind turbine designs so that risks to manufacturers, intending to adopt a particular type of turbine, are minimised. This paper presents a simple model for identifying critical busbars, where the levels of flicker may be high. It also describes a more complex model for accurately predicting flicker induced by operating wind turbines. The models are very flexible and were developed specifically to be coupled to an existing dynamic model of an entire wind turbine. The various models representing static and dynamic loads are described and their impact on flicker are investigated. In addition, the effects of the X/R ratio and fault level of the network are discussed. Various wind conditions and different network parameters have been simulated. Results are presented from simulations carried out using measured wind turbine power time series and data from operating wind farms.

Z. Saad-Saoud

Papers

Application of STATCOMs to wind farms

Simple wind farm dynamic model

Models for predicting flicker induced by large wind turbines