Rion Takahashi

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

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.

Operation and Control of HVDC-Connected Offshore Wind Farm

This paper presents operation and control strategies of an offshore wind farm interconnected to a high-voltage dc (HVDC) system. The offshore wind farm composed of variable speed wind turbines driving permanent magnet synchronous generators is considered in this study, based on dc-bus concept. The HVDC transmission system based on a three-level neutral point clamped voltage source converter (VSC) is used for the interconnection between the offshore wind farm and onshore grid. Detailed modeling and control strategies are developed for the individual component of the overall system. A simple fuzzy logic controller is adopted in the offshore VSC station, which is one of the salient features of this study. Real wind speed data is used in the simulation study to obtain realistic response. Both dynamic and transient analyses of the proposed system are carried out using the laboratory standard power system software package, PSCAD/EMTDC.

Integration of an Energy Capacitor System With a Variable-Speed Wind Generator

This paper presents a system using an energy capacitor system (ECS) to smoothen the output power fluctuation of a variable-speed wind farm. The variable-speed wind turbine driving a permanent-magnet synchronous generator is considered to be connected to the ac network through a fully controlled frequency converter. The detailed modeling and control strategy of the frequency converter as well as variable-speed operation of a wind turbine generator system are demonstrated. Afterward, a suitable and economical topology of ECS composed of a current-controlled voltage-source inverter, dc-dc buck/boost converter, and an electric double layer capacitor (EDLC) bank is presented, including their control strategies. Exponential moving average is used to generate the real input power reference of ECS. Another novel feature of this paper is the incorporation of a fuzzy-logic-controlled reference signal adjuster in the control of the dc-dc buck/boost converter, in which the stored energy of the EDLC bank is utilized in an efficient way. Due to this controller, the energy storage capacity of the EDLC bank can be reduced in size, thus resulting in reduction of the overall cost of the ECS unit as well as decrease in irrepressible operations during high and low energy levels of the EDLC bank. Finally, extensive simulation results are presented that validate the effectiveness of the proposed system to smoothen the output power fluctuation of the variable-speed wind farm.

Wind Farms Fault Ride Through Using DFIG With New Protection Scheme

This paper proposes a control strategy of doubly fed induction generators (DFIGs) with new protection schemes for enhancing fault ride through capability of wind farms composed of DFIGs and induction generators (IGs). Since the DFIGs will be stressed or overloaded in the process of stabilizing the wind farm during a grid fault, it is paramount to consider a protection scheme for the DFIG, in order to protect its power converters. Two schemes, the dc-link chopper-controlled braking resistor with the supplementary rotor current (SRC) control of the rotor side converter of the DFIG and series dynamic braking resistor (SDBR) connected to the stator of the DFIG, are proposed and compared. Merits and drawbacks of both schemes are highlighted as well. The simulation results in power system computer aided design and electromagnetic transient including DC (PSCAD/EMTDC) show that the two proposed schemes can eliminate the need for an expensive crowbar switch in the rotor circuit, because both could limit the rotor current of the DFIG within its nominal value during a grid fault. Finally, considering the overall system performance, the latter is recommended.

Transient Stability Augmentation of Power System Including Wind Farms by Using ECS

This paper presents a new method to enhance the transient stability of multimachine power system including wind farms, when a severe network disturbance occurs in the power system. For this purpose, the energy capacitor system (ECS) composed of power electronic devices and electric double layer capacitor (EDLC) is proposed. The control scheme of ECS is based on a sinusoidal PWM voltage source converter (VSC) and fuzzy logic controlled dc-dc buck/boost converter using insulated gate bipolar transistors (IGBT). Two wind farms are considered to be connected to the power system. Two-mass drive train model of wind turbine generator system (WTGS) is used in the analyses as the drive train modeling has great influence on the dynamic characteristics of WTGS. Real wind speed data are used in the analyses to obtain realistic responses. Different types of symmetrical and unsymmetrical faults are considered as the network disturbance. Simulation results clearly show that the proposed ECS can enhance the transient stability of wind generators in multimachine power system as well as their low voltage ride through (LVRT) capability.

Transient Stability Analysis of Wind Generator System with the Consideration of Multi-Mass Shaft Model

In order to investigate the impacts of the integration of wind farms into utilities networks, it is necessary to analyze the transient characteristic of wind power generation system (WPGS). In most of the cases, it is seen that the simple one-mass lumped model is considered for transient stability analysis of wind generator. But for the sake of exact analysis of transient stability of WPGS, it is needed to consider multi-mass shaft system model. In this study a detailed analysis has been done with the consideration of two-mass shaft system model of wind generator system when a severe network disturbance occurs in power system. Moreover, an online logical pitch controller has been proposed, which can enhance the transient performance of WPGS as well as maintain output power of wind generator at rated level when wind speed is above the rated speed

Frequency control of isolated power system with wind farm by using Flywheel Energy Storage System

Due to the recent expansion of renewable energy applications, wind energy conversion is receiving much interest all over the world. However, output fluctuations of wind generators cause network frequency variations in power systems. This can decrease the power quality and then cause a restriction of wind farm installation, especially in an isolated power system, for example, power system in a small island. Therefore the problem of output fluctuation needs to be solved for expanding further the wind energy conversion. This paper proposes an application of flywheel energy storage system (FESS) to an isolated power system with a wind farm for improving the network frequency quality. The validity of the proposed method is evaluated by computer simulation analyses using PSCAD/EMTDC.

Wind Generator Output Power Smoothing and Terminal Voltage Regulation by Using STATCOM/ESS

In this work, it is reported that STATCOM/BESS (battery energy storage system) topology can significantly decrease voltage and power fluctuations of grid connected fixed speed wind generators. One major problem in wind generator output power smoothing is setting of the reference output power. Constant output power reference is not a good choice because there can be some cases where wind speed is very low and then sufficient power cannot be obtained. In that case, energy storage device can solve the problem but large energy capacity may be needed. This paper proposes exponential moving average (EMA) to generate the reference output power, and thus the energy capacity of BESS unit can be small. The objective of the proposed STATCOM/BESS topology is to smooth the wind farm output by absorbing or providing real power. Moreover, its reactive power output can also be controlled to keep the wind farm terminal voltage constant. Control methodology of STATCOM/BESS topology suitable for the two objectives stated above is developed in this paper. Real wind speed data is used in the simulation analyses, which validates the effectiveness of the proposed control strategy. Simulation results clearly show that the proposed STATCOM/BESS unit can smooth well the wind generator output power and also maintain the terminal voltage at rated level.

Stabilization of Grid Connected Wind Generator by STATCOM

Recently voltage-source or current-source inverter based various FACTS devices have been used for flexible power flow control, secure loading and damping of power system oscillation. Some of those are used also to improve transient and dynamic stability of wind power generation system (WPGS). In this paper, we propose the static reactive compensator (STATCOM) based on voltage source converter (VSC) PWM technique to stabilize grid connected squirrel cage wind generator system. A simple control strategy of STATCOM is adopted where only measurement of rms voltage at the wind generator terminal is needed, i.e. there is no need of reactive power measurement. Fuzzy logic controller is used as the control methodology of STATCOM, rather than conventional PI controller. The voltage sag and swell improvement of WPGS is compared with both fuzzy and PI controller. Moreover, the steady state capacitor value used with induction generator is reduced by certain percentage when STATCOM is used with WPGS. Comprehensive results are presented to assess the performance of the STATCOM connected with WPGS, where the simulations have been done by PSCAD/EMTDC.

Output Power Smoothing and Hydrogen Production by Using Variable Speed Wind Generators

This paper presents a combination system of wind energy conversion and hydrogen production. Hydrogen is expected as an alternative energy source in the future, and this is the best way to produce it from renewable energy like wind energy. On the other hand, the output of a wind generator, in general, fluctuates greatly due to wind speed variations, and thus the output fluctuations can have a serious influence on the power system operation. In the proposed system, a variable speed wind generator is adopted, and an electrolyzer is installed in parallel with it for hydrogen production. Output power from the wind generator is smoothed and supplied to the power system as well as to the electrolyzer based on the cooperative control method. The performance of the proposed system is evaluated by simulation analyses, in which simulations are performed by using PSCAD/EMTDC.