Fixed Speed Wind Turbine Generator Research Articles

Impact of Superconducting Magnetic Energy Storage Unit on Doubly Fed Induction Generator Performance During Various Levels of Grid Faults

Since 2004, Doubly Fed Induction Generator (DFIG) has become the most admired wind turbine generator type that is widely installed worldwide. The popularity of DFIG is due to its capability to extract more energy than fixed speed wind turbine generator type. Moreover, a DFIG requires only one-third of the full converter size used by type-4 wind turbine generator which in turn reduces the total cost of DFIG installation. The main weakness of DFIG is its vulnerability to grid faults such as voltage dip and swell. Grid faults may have adverse impacts on the overall performance of the DFIG including the voltage at the point of common coupling that, at certain fault levels, may violate the permissible margins of the grid codes established by the transmission line operators. This paper investigates the application of Superconducting Magnetic Energy Storage (SMES) unit to improve the fault ride-through capability of a DFIG during grid faults. Results show that the connection of a SMES unit at the point of common coupling between the DFIG and the grid can enhance the overall performance of the DFIG during such faults events.

Two-port network model of fixed-speed wind turbine generator for distribution system load flow analysis

Load flow analysis has always been used in determining the steady-state operation of an electric power or distribution system. For conventional power system without wind turbine generator, the method for load flow analysis has been well established. However, for modern system embedded with wind turbine generator, the investigation of analysis method is still an active research area. This paper proposed a new method to integrate fixed-speed wind turbine generator into distribution system load flow analysis. The proposed method is derived based on two-port network theory where the parameters of induction generator of the wind turbine generator are embedded in general constants of the two-port network. The proposed method has been tested and verified using a representative electric distribution system.

SSR Damping in Fixed-Speed Wind Farms Using Series FACTS Controllers

Subsynchronous resonance (SSR) damping in fixed-speed wind turbine generator systems (FSWTGS) by using two series flexible ac transmission system (FACTS) devices, the thyristor-controlled series capacitor (TCSC), and gate-controlled series capacitor (GCSC) are studied in this paper. The former is a commercially available series FACTS device, and the latter is the second generation of series FACTS devices using gate turnoff (GTO) or other gate-commuted switches. The GCSC is characterized by a fixed capacitor in parallel with a pair of antiparallel gate-commuted switches enabling rapid control of series impedance of a transmission line. It is shown that the SSR damping with a GCSC is limited to changing the resonance frequency, in comparison with a fixed capacitor, which may not be adequate to damp out the SSR. Therefore, a supplementary SSR damping controller (SSRDC) is designed for the GCSC. Moreover, it is proven that the GCSC equipped with a well-designed SSRDC can effectively damp the SSR in FSWTGS. In order to verify the effectiveness of the GCSC in SSR damping, its performance is compared with the TCSC, which is an existing series FACTS device. In addition, time-frequency analysis (TFA) is employed in order to evaluate and compare the SSR time-varying frequency characteristics of the GCSC and TCSC. The IEEE first benchmark model on SSR is adapted with an integrated FSWTGS to perform studies, and extensive simulations are carried out using PSCAD/EMTDC to validate the result.

A Combined Approach of Using an SDBR and a STATCOM to Enhance the Stability of a Wind Farm

This paper presents a method to enhance the stability of a grid-connected wind farm composed of a fixed-speed wind turbine generator system (WTGS) using a combination of a small series dynamic braking resistor (SDBR) and static synchronous compensator (STATCOM). The SDBR and STATCOM have active and reactive power control abilities, respectively, and a combination of these units paves the way to stabilize well the fixed-speed wind farm. In this paper, a centralized control scheme of using an SDBR and a STATCOM together is focused, which can be easily integrated with a wind farm. Different types of symmetrical and unsymmetrical faults are considered to evaluate the transient performance of the proposed control scheme, applicable to a grid-connected wind farm. The effect of a multimass drive train of a fixed-speed WTGS in fault analysis, along with its importance in determining the size of the SDBR to augment the transient stability of a wind farm, is investigated. Extensive simulation analyses are performed to determine the approximate sizes of both SDBR and STATCOM units. Dynamic analysis is performed using real wind speed data. A salient feature of this work is that the effectiveness of the proposed system to minimize the blade-shaft torsional oscillation of a fixed-speed WTGS is also analyzed. Simulation results show that a combination of a small SDBR and STATCOM is an effective means to stabilize the wind farm composed of a fixed-speed WTGS.

Incorporation of fixed speed wind turbine generators in load flow analysis of distribution systems

Small scale wind farms are increasingly being integrated into distribution systems. This paper proposes a simple model of a fixed speed wind farm and incorporates that model into a standard load flow program of a distribution system. For each wind farm, the proposed model augments the size of the network by two buses that allow including all parameters of the generator. For a given wind speed, the mechanical power of the wind turbine is first determined from manufacturer's supplied data and then it is incorporated into one of the new buses as a negative load. This would allow evaluating the load flow results of the system using any standard load flow program without modifying the source codes of the program. The effectiveness of the proposed method is then evaluated on a simple system as well as on two distribution systems consisting of 33 and 69 buses.

Low voltage ride-through enhancement of fixed-speed wind farms using series FACTS controllers

This paper studies the application and control of three series flexible AC transmission system (FACTS) devices namely, the gate-controlled series capacitor (GCSC), the thyristor controlled series capacitor (TCSC), and series braking resistor (SBR), in order to enhance the low voltage ride-through (LVRT) in fixed-speed wind turbine generator systems (FSWTGS). Modeling and simulations are carried out in order to investigate and compare the performance of these devices, considering (1) successful reclosing of circuit breakers (CBs), (2) unsuccessful reclosing of CBs, and (3) connection of a dynamic load to the point of common coupling (PCC). Simulation results exhibit significantly enhanced transient stability of FSWTGS due to the employment of the GCSC. Furthermore, the GCSC is competitive with the TCSC and the SBR, and it requires less power rating to stabilize the wind generator system. Therefore, the proposed GCSC can be considered an effective tool to improve the LVRT of FSWTGS.

Comparative Study between Two Protection Schemes for DFIG-based Wind Generator Fault Ride Through

Fixed speed wind turbine generators system that uses induction generator as a wind generator has the stability problem similar to a synchronous generator. On the other hand, doubly fed induction generator (DFIG) has the flexibility to control its real and reactive powers independently while being operated in variable speed mode. This paper focuses on a scheme where IG is stabilized by using DFIG during grid fault. In that case, DFIG will be heavily stressed and a remedy should be found out to protect the frequency converter as well as to allow the independent control of real and reactive powers without loosing the synchronism. For that purpose, a crowbar protection switch or DC-link protecting device can be considered. This paper presents a comparative study between two protective schemes, a crowbar circuit connected across the rotor of the DFIG and a protective device connected in the DC-link circuit of the frequency converter. Simulation analysis by using PSCAD/EMTDC shows that both schemes could effectively protect the DFIG, but the latter scheme is superior to the former, because of less circuitry involved.

Wind Farm Stabilization by using DFIG with Current Controlled Voltage Source Converters Taking Grid Codes into Consideration

Recent wind farm grid codes require wind generators to ride through voltage sags, which means that normal power production should be re-initiated once the nominal grid voltage is recovered. However, fixed speed wind turbine generator system using induction generator (IG) has the stability problem similar to the step-out phenomenon of a synchronous generator. On the other hand, doubly fed induction generator (DFIG) can control its real and reactive powers independently while being operated in variable speed mode. This paper proposes a new control strategy using DFIGs for stabilizing a wind farm composed of DFIGs and IGs, without incorporating additional FACTS devices. A new current controlled voltage source converter (CC-VSC) scheme is proposed to control the converters of DFIG and the performance is verified by comparing the results with those of voltage controlled voltage source converter (VC-VSC) scheme. Another salient feature of this study is to reduce the number of proportionate integral (PI) controllers used in the rotor side converter without degrading dynamic and transient performances. Moreover, DC-link protection scheme during grid fault can be omitted in the proposed scheme which reduces overall cost of the system. Extensive simulation analyses by using PSCAD/EMTDC are carried out to clarify the effectiveness of the proposed CC-VSC based control scheme of DFIGs.

Low voltage ride through capability enhancement of wind turbine generator system during network disturbance

The energy capacitor system (ECS), composed of power electronic devices and electric double layer capacitor to enhance the low voltage ride through (LVRT) capability of fixed speed wind turbine generator system (WTGS) during network disturbance, is discussed. Control scheme of ECS is based on a sinusoidal pulse width modulation voltage source converter and DC–DC buck/boost converter composed of insulated gate bipolar transistors. Two-mass drive train model of WTGS is adopted because the drive train system modelling has great influence on the characteristics of wind generator system during network fault. Extensive analysis of symmetrical fault is performed with different voltage dip magnitudes and different time durations. Permanent fault because of unsuccessful reclosing is also analysed, which is one of the salient features of this study. A real grid code defined in the power system is considered and LVRT characteristic of WTGS is analysed. Finally, it is concluded that ECS (20 MW) can significantly enhance the LVRT capability of grid connected WTGS (50 MW) during network disturbance, where simulations have been carried out by using PSCAD/EMTDC.

Integration of space vector pulse width modulation controlled STATCOM with wind farm connected to multimachine power system

In this work, dynamic and transient characteristics of a multimachine power system connected with two wind farms composed of fixed speed wind turbine generator systems (WTGS) are analyzed. At each wind farm the terminal one space vector pulse width modulation controlled voltage source converter based static synchronous compensator (STATCOM) is considered to be connected. The capacitor bank capacity of fixed speed wind generator is reduced by certain percentages when a STATCOM is integrated at a wind farm terminal. As wind speed is always fluctuating, the terminal voltage of a fixed speed wind generator also fluctuates randomly, which has an adverse effect on the rest of the power system. It is reported that the STATCOM with a reduced capacitor bank can decrease the voltage fluctuations of the multimachine power system as well as wind generator terminals. Moreover, it is shown that a STATCOM can also enhance the transient stability of induction and synchronous generators when a network disturbance occurs in the power system. Since shaft system modeling of a wind turbine has a significant effect on the transient stability analysis of WTGS, a two-mass shaft model is adopted in this study. Both of the symmetrical and the unsymmetrical faults are analyzed in light of the real wind farm grid code. The transient performance of a STATCOM on the network is evaluated by the transient stability index based on the total kinetic energy of generators. For dynamic performance evaluation, real wind speed data are used in the simulation. Finally, it is concluded that the STATCOM can enhance both dynamic and transient stability of wind farms connected with a multimachine power system.

Optimal Power Flow under Variable Wind Generation

In this paper Optimal Power Flow (OPF) problem is solved for a power system, which includes wind power as an additional generation input under variable load conditions. The fixed-speed Wind Turbine Generator (WTG) option is considered in this study: Simulations have been performed for various sizes of wind farm attached to the IEEE 30-bus systems using the DIgSILENT Power Factory simulation software and all simulations used the DIgSILENT package. The main conclusion is that a fixed-speed wind farm generator is effective in minimising the losses and the total generation cost as well. IEEE 30-bus system is considered to demonstrate the suitability of the proposed approach for solving the OPF problem.

Increasing wind farm capacity

The issue of wind farm capacity is addressed through study of a simple but generic system. Wind-farm size is defined in terms of its rating relative to system fault level at the wind-farm terminals. The voltage rise for a wind farm whose capacity is 20% of system fault rating is determined for various utility network reactance-to-resistance ratios (X/R). The focus is on a network with an X/R ratio of unity, typical of 33 kV networks in the UK. It is shown that capacitive compensation for a fixed-speed wind turbine generator can contribute to the voltage rise problem. A typical compensation scheme, with extra capacitance being switched in as active power increases, results in the maximum allowed voltage rise for a wind-farm capacity of 13% of system fault rating. A modified scheme, with capacitive compensation being reduced at greater active power outputs, enables the capacity to be increased to 20% of system fault rating. The modified scheme leads to slightly increased network losses. It has the advantage that the reduced capacitive compensation is below the level required for self excitation. An analogous study for a wind farm based on doubly-fed induction generators results in the same capacity limitation, but with slightly reduced losses.