Signal flow graph modelling of interleaved buck converters

This paper presents a systematic development of unified signal flow graph model for an interleaved buck converter system operating in continuous inductor current mode. From this signal flow graph small, large‐signal and steady‐state models are developed, which are useful to study the converter dynamic and steady‐state behaviour. Converter performance expressions like steady‐state voltage gain, efficiency expressions and other small‐signal characteristic transfer functions are derived. Development of unified signal flow graph is explained for a 3‐cell interleaved converter system. Derivation of large, small‐signal and steady‐state models from the unified signal flow graph is demonstrated by considering a 2‐cell interleaved buck converter system. Large signal model was programmed in TUTSIM simulator, and the large‐signal responses against supply, load disturbances were predicted. Signal flow graph analysis results are validated with PSIM simulations. Further, the mathematical models obtained from the signal flow graph modelling are in agreement with those obtained from the state‐space averaging technique. Copyright © 2003 John Wiley & Sons, Ltd.     

Control strategy for a distributed DC power system with renewable energy

This paper deals with a DC-micro-grid with renewable energy. The proposed method is composed of a gearless wind power generation system, a battery, and DC loads in a DC distribution system. The battery helps to avoid the DC over-voltages by absorbing the power of the permanent magnet synchronous generator (PMSG) during line-fault. In addition, the control schemes presented in this paper including the maximum power point tracking (MPPT) control and a pitch angle control for the gearless wind turbine generator. By means of the proposed method, high-reliable power can be supplied to the DC distribution system during the line-fault and stable power supply from the PMSG can be achieved after line-fault clearing. The effectiveness of the proposed method is examined in a MATLAB/Simulink^® environment.     

A new control methodology of wind turbine generators for frequency control of power system in isolated island

A wind turbine generator (WTG) system's output is not constant and fluctuates depending on wind conditions. Fluctuating power causes frequency deviations and adverse effects to an isolated power system when large output power from WTG systems is penetrated in the power system. This paper presents an output power control methodology of a WTG for frequency control using a load power estimator. The load power is estimated by a disturbance observer, and the output power command of the WTG is determined according to the estimated load. Besides, the WTG can also be controlled during wind turbulence since the output power command is determined by considering wind conditions. The reduction of the power system frequency deviation by using the WTG can be achieved by the proposed method. The effectiveness of the proposed method is validated by numerical simulations. Copyright © 2010 John Wiley & Sons, Ltd.     

Modelling–based approach for digital control design for nonlinear WECS in the power system

The case has been established that the wind power plant must be treated as an integral part of the electric system, thereby constituting the wind energy conversion system. Recent advancement in size and technology of wind turbines requires sophisticated control systems to effectively optimize energy conversion and enhance grid integration. As a first step toward controller design, modelling has become a prerequisite. This paper explores controller design based on modelling the wind speed as a stochastic process, and the wind turbine as a multi‐mass system with a soft shaft linking the turbine with the doubly fed induction generator. A control strategy incorporating a linear quadratic Gaussian (LQG) that relies on state estimation for full‐state feedback is proposed to augment a linear controller for generator torque control. The control objectives are to reduce stresses on the drivetrain and to ensure operation geared toward optimal power conversion. This study focuses on above‐rated wind speeds, and the LQG's main purpose is to add damping to the drivetrain, thereby minimizing cyclic fatigue, while a pitch control mechanism prevents rotor overspeed, thereby maintaining rated power. Simulations show the efficacy of the proposed paradigm in meeting the control objectives. Copyright © 2009 John Wiley & Sons, Ltd.     

Output power dispatch control for a wind farm in a small power system

Nowadays, a wind turbine generator (WTG) is required to provide control capabilities as the output power of WTG fluctuates. Under this scenario, this paper proposes an output power control method of a wind farm (WF) connected to a small power system using pitch angle control. In this control approach, the WF output power control is achieved by two control levels: central and local. In the central control, the WF output power command is determined by considering the frequency deviations and wind speeds using a fuzzy function. Then, the local output power commands for each of the WTGs are based on the proposed dispatch control. In the proposed dispatch control, the output commands of each WTG are determined by considering wind conditions for each of the WTGs. The simulation results by using an actual detailed model for the wind power system show the effectiveness of the proposed method. Copyright © 2010 John Wiley & Sons, Ltd.     

Output power leveling of wind turbine generators using pitch angle control for all operating regions in wind farm

Effective utilization of renewable energies such as wind energy instead of fossil fuels is desirable. Wind energy is not constant and windmill output is proportional to the cube of the wind speed, which causes the generated power of wind turbine generators (WTGs) to fluctuate. In order to reduce the output power fluctuation of wind farms, this paper presents an output power leveling control strategy for a wind farm based on both the average wind farm output power and the standard deviation of the wind farm output power, a cooperative control strategy for WTGs, and pitch angle control using a generalized predictive controller (GPC) in all WTG operating regions. Simulation results using an actual detailed model for wind farm systems show the effectiveness of the proposed method. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 158(4): 31– 41, 2007; Published online in Wiley InterScience ( www.interscience. wiley.com ). DOI 10.1002/eej.20448     

A hybrid power system using alternative energy facilities in isolated island

A hybrid power system uses many wind turbine generators in isolated small islands. The output power of wind turbine generators is mostly fluctuating and has an effect on system frequency. In order to solve this problem, we propose a new power system using renewable energy in small, isolated islands. The system can supply high-quality power using an aqua electrolyzer, fuel cell, renewable energy, and diesel generator. The generated hydrogen by an aqua electrolyzer is used as fuel for a fuel cell. The simulation results are given to demonstrate the availability of the proposed system in this paper.     

An integrated control method for a wind farm to reduce frequency deviations in a small power system

Output power of wind turbine generator (WTG) is not constant and fluctuates due to wind speed changes. To reduce the adverse effects of the power system introducing WTGs, there are several published reports on output power control of WTGs detailing various researches based on pitch angle control, variable speed wind turbines, energy storage systems, and so on. In this context, this paper presents an integrated control method for a WF to reduce frequency deviations in a small power system. In this study, the WF achieves the frequency control with two control schemes: load estimation and short-term ahead wind speed prediction. For load estimation in the small power system, a minimal-order observer is used as disturbance observer. The estimated load is utilized to determine the output power command of the WF. To regulate the output power command of the WF according to wind speed changing, short-term ahead wind speed is predicted by using least-squares method. The predicted wind speed adjusts the output power command of the WF as a multiplying factor with fuzzy reasoning. By means of the proposed method, the WF can operate according to the wind and load conditions. In the WF system, each output power of the WTGs is controlled by regulating each pitch angle. For increasing acquisition power of the WF, a dispatch control method also is proposed. In the pitch angle control system of each WTG, generalized predictive control (GPC) is applied to enhance the control performance. Effectiveness of the proposed method is verified by the numerical simulations.     

Signal flow graph modelling and analysis of interleaved DC?DC parallel converters

This paper presents a systematic development of a unified signal flow graph model for an interleaved DC–DC parallel converter system operating in continuous current mode. This signal flow graph approach provides a means to translate directly the switching converter to its graphic model, from which the steady-state and dynamic behaviour of the converter can be studied easily. The development of a unified signal flow graph is explained for a three-cell interleaved parallel converter system. Derivation of large-signal, small-signal and steady-state models from a unified signal flow graph is demonstrated by considering a two-cell interleaved converter system operating in complementary activation mode. Converter performance expressions such as steady-state voltage gain, efficiency expressions and small-signal characteristic transfer functions are also derived. A large-signal model was programmed in a TUTSIM simulator, and the large-signal responses against supply and load disturbances were predicted. Signal flow graph analysis results are validated with PSIM simulations. Experimental observations are provided to validate the signal flow graph modelling method. Further, the mathematical models obtained from the signal flow graph modelling are in agreement with those obtained from the state-space averaging technique.     

Next Day Peak Load Forecasting Using Neural Network With Adaptive Learning Algorithm Based On Similarity

In this article we propose the adaptive learning algorithm of neural network with respect to a rapid temperature change of forecasted day. The proposed adaptive learning algorithm is used to shift the learning range of previous year of forecasted day. Therefore, the proposed neural network can be trained by using learning data, including the maximum temperature to be forecasted. The suitability of the proposed approach is illustrated through an application to actual load data of Okinawa Electric Power Company in Japan.