This paper presents a control strategy based on fuzzy logic to inject efficiently the active power from a three-phase grid-connected photovoltaic (PV) system into the local grid with supporting regulation for the frequency of grid voltage. In which the control strategy consists of three main modules as follows. Firstly, a simulation module based on the mathematical model of a PV panel is utilized to predict the maximum total power from PV arrays; the second one is a frequency regulation module used to compute a proper reference value for the output active power; and the last is a coordinated main controller for power-electronic converters and battery charger to deliver active power to the grid exactly according to the reference value computed beforehand. Especially in the frequency regulation module, a unique fuzzy logic controller (FLC) is designed to help determine accurately the reference value of active power. Besides, a control method for state-of-charge (SOC) of battery bank is also introduced. Simulations show the suggested control strategy has good performances in supplying suitably the active power to grid with regulating the grid frequency in acceptable ranges, even when the solar radiation or AC-system load suddenly changes. Also, effectiveness in regulating grid frequency of the proposed control strategy is compared with the conventional strategy using full maximum power point tracking (MPPT) mode.
This paper discusses two techniques based on the feedback linearization (FBL) method to control the active and reactive output powers of three-phase grid-connected photovoltaic (PV) inverters. The first control scheme is an application of the direct FBL approach. The other is an appropriate combination of the FBL and fuzzy logic (FBL-FL), and is the main proposed method of this study. Wherein, a unique fuzzy logic controller (FLC) is designed to enhance effectiveness of the linear control method used in the direct FBL. In detail, its major objectives are to improve the transient response and reduce steady-state oscillations in the output powers. In this research, the illustrative PV inverter utilizes a three-level DC-AC converter, an R-L filter and a 250 V/10 kV wyewye transformer to inject the energy, obtained from PV array with a nominal power of 100 kW, into the 10 kV/60Hz three-phase grid. Numerical simulations in MATLAB and PSIM illustrate that the two FBL-based structures perform very well in independently regulating the active and reactive output powers to the reference values, even within the parametric uncertainties and the unbalanced grid voltage condition. Moreover, comparisons of simulation results, obtained from the traditional proportional-integral (PI) control and the two FBL-based structures, show advantages of the proposed FBL-FL hybrid technique in terms of fast response, small overshoot, acceptable steady-state fluctuation and high robustness.
This paper presents a comprehensive analysis technique about harmonic resonance issues in megawatt gridconnected wind farms where the doubly fed induction generator (DFIG) is used. Wherein, firstly, the background theories of the series resonance, parallel resonance and their impacts on the electric system are described. Moreover, a suitable equivalent circuit of typical grid-connected DFIG wind farms for analyzing impedance in the frequency domain is proposed. Then, mathematical formulas are introduced detail to determine exactly orders and amplifications of the voltage and current distortions on both the medium-voltage (MV) bus and lowvoltage (LV) bus. Lastly, results obtained from the proposed equivalent circuit and impedance analysis technique are compared to other results achieved with the detailed wind farm simulated in MATLAB/SimPowerSystems. As well, many different operation cases of the illustrative grid-connected DFIG wind farm, including parametric variations, are also examined to evaluate the accuracy and effectiveness of the proposed analysis technique.
Physical value of active power, in Watts Value expressed in per-unit system of Ratio value expressed in percent Index of local agent in a PV farm Present step of sampling time in control Superscripts * Reference value Meas Measured actual value Pre Predicted value Min Minimum value Max Maximum value TrS Value computed in the transient state StS Value computed at the steady state Diff Difference value This study proposes a two-level coordinated control strategy with fuzzy logic for appropriately adjusting the total active power supplied to a grid by large-scale photovoltaic (PV) farms in order to regulate grid frequency. For a solar farm, the strategy includes a central coordinating controller and many local controllers at PV power assemblies, treated as agents. In detail, the central controller uses a frequency regulation module based on a new automatic-tuning fuzzy-logic controller scheme to compute the appropriate reference values according to the total power needed. Then, the individual reference value for each local controller is determined. Each local controller governs all power-electronic converters installed at the PV agent to inject power into the grid according to the individual reference value received. Additionally, each local controller uses an algorithm to manage the state-of-charge of the battery bank installed at the agent so that it remains in the safe range of 20-80% while operating and close to the desired idle value of 50% at the steady state. Besides, a special control mode is developed and integrated into the overall strategy to aid rapid recovery of the grid frequency under emergency conditions. Numerical simulations demonstrate that the suggested strategy has the good response in terms of injecting an appropriate amount of power into the grid to regulate the frequency deviation into acceptable ranges of ±0.2 (Hz) in the transient state and ±0.05 (Hz) at the steady state, even when the weather conditions (solar radiation, air temperature), AC system load, and important control parameters of the grid suddenly change. Furthermore, the effectiveness in improving the grid-frequency stabilization by using the proposed strategy is validated within a four-area power system, where four PV farms are connected and the operating parameters of the grids at the areas are fairly different.
This study is the first research to present a detailed assessment of core loss, copper loss and magnetic flux density of the interior permanent magnet synchronous motor (IPMSM) with pulse-amplitude modulation (PAM) inverter under different excitation angles in both no-load and load conditions. The PAM method automatically controls the amplitude of changeable DClink voltage, and the excitation angle for switches in the inverter is varied from 120° to 180° according to a 12-step switching pattern designed particularly for the PAM-based inverter of IPMSM. For reference purpose, the pulse-width modulation excitation with a very high voltage modulation index is performed. Various conditions with changes in speed and torque are examined. Experimental results show that harmonic components produced by the PAM inverter in the IPMSM current, voltage and magnetic flux density have large effects on the motor core and copper losses; physics-based explanations and insights are also presented. The PAM under 135° excitation angle has an excellent performance in reducing IPMSM core and copper losses since it can significantly decrease the harmonic components in motor current, voltage and magnetic flux density. Furthermore, simulations using the finite element method are conducted to validate the experimental results of IPMSM core loss with the PAM excitations. Nomenclature PAM 120°PAM inverter under 120° excitation angle PAM 135°PAM inverter under 135° excitation angle PAM 150°PAM inverter under 150° excitation angle PAM 165°PAM inverter under 165° excitation angle PAM 180°PAM inverter under 180° excitation angle PWM 10 kHz PWM inverter with 10 kHz carrier frequency
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