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Abstract-Grid

Grid faults are one of the most severe perturbations in power systems. During these extreme disturbances, the reliability of the grid is compromised and the risk of a power outage is increased. To prevent this issue, distributed generation inverters can help the grid by supporting the grid voltages. Voltage support mainly depends on two constraints: the amount of injected current and the grid impedance. This paper proposes a voltage support control scheme that joins these two features. Hence, the control strategy injects the maximum rated current of the inverter. Thus, the inverter takes advantage of the distributed capacities and operates safely during voltage sags. Also, the controller selects the appropriate power references depending on the resistive-inductive grid impedance. Therefore the grid can be better supported since the voltage at the point of common coupling is improved. Several voltage objectives, which cannot be achieved together, are developed and discussed in detail. These objectives are threefold: a) to maximize the positive sequence voltage, b) to minimize the negative sequence voltage, and c) to maximize the difference between positive and negative sequence voltages. A mathematical optimal solution is obtained for each objective function. This solution is characterized by a safe peak current injection, and by the optimization of the voltage profile in any type of grid connection. Therefore, the proposed control scheme includes advanced features for voltage support during voltage sags, that are applicable to different power facilities in different types of networks. Due to system limitations, a suboptimal solution is also considered, analyzed and discussed for each of the optimization problems. Experimental results are presented to validate the theoretical solutions.

Abstract-This paper presents an improved variable hysteresisband current-control in natural frame for a three-phase unity power rectifier. The proposed control algorithm is based on three decoupled sliding-mode controllers combined with three independent Kalman filters. The use of Kalman filters instead of a non-adaptive state observer improves the quality of the estimated signals in presence of noise, increasing the immunity of the control loop in noisy environments. To reduce drastically the computational load in the Kalman algorithm, a reduced bilinear model is derived which allows to use a Kalman filter algorithm instead of an extended Kalman filter. A fast output-voltage control is also presented which avoids output-voltage variations when a sudden change in the load or a voltage sag appears. Moreover, a fixed switching frequency algorithm is proposed which uses a variable hysteresis-band in combination with a switching decision algorithm, ensuring a switching spectrum concentrated around the desired switching frequency. The overall control proposal has been fully integrated into a digital signal processor. Selected experimental results are introduced to validate the theoretical contributions of this paper.

Abstract-This paper presents a new concept in active damping techniques using a reduced model of a LCLfiltered grid connected inverter. The presence of the LCL filter complicates the design of the inverter control scheme, particularly when uncertainties in the system parameters, especially in the grid inductance, are considered. The proposed control algorithm is addressed to overcome such difficulties using a reduced model of the inverter in a state observer. In this proposal, two of the three state variables of the system are obviated from the physical inverter model and only the inverter side current is considered. Therefore, the inverter side current can be estimated emulating the case of an inverter with only one inductor, thus eliminating the resonance problem produced by the LCL filter. Besides, in the case of a distorted grid, the method allows to estimate the voltages at the point of common coupling free of noise and distortion without using any PLL-based synchronization algorithm. This proposal provides the following features to the closed-loop system: 1) robust and simple active damping control under system parameters deviation, 2) robustness against grid voltage unbalance and distortion, and 3) an important reduction in the computational load of the control algorithm which allows to increase the switching frequency. To complete the control scheme, a theoretical stability analysis is developed considering the effect of the observer, the system discretization and the system parameters deviation. Experimental and comparative evaluation results are presented to validate the effectiveness of the proposed control scheme.

Abstract-This paper presents a robust model-based control in natural frame for a three-phase shunt active power filter. For the proposed control method a linear converter model is deduced. Then, this model is used in a Kalman filter in order to estimate the system state-space variables. Even though the states estimation do not match the variables of the real system, it has allowed to design three sliding mode controllers providing the following features to the closed loop system: a) robustness due to the fact that control specifications are met independently of any variation in the system parameters; b) noise immunity, since a Kalman filter is applied; c) a lower THD of the current delivered by the grid compared with the standard solution using measured variables; d) the fundamental component of the voltage at point of common coupling is estimated even in the case of a distorted grid; and e) a reduction in the number of sensors. Thanks to this solution the sliding surfaces for each controller are independent. This decoupling property of the three controllers allows using a fixed switching frequency algorithm that ensures a perfect current control. Finally, experimental results validate the proposed control strategy and illustrate all its interesting features.

Abstract-This paper presents a variable structure control in natural frame for a three-phase gird-connected voltage source inverter with LCL filter. The proposed control method is based on modifying the converter model in natural reference frame, preserving the low frequency state space variables dynamics. Using this model in a Kalman filter, the system state-space variables are estimated allowing to design three independent current sliding-mode controllers. The main closed-loop features of the proposed method are: 1) robustness against grid inductance variations because the proposed model is independent of the grid inductance, 2) the power losses are reduced since physical damping resistors are avoided, 3) the control bandwidth can be increased due to the combination of a variable hysteresis comparator with the Kalman filter, and 4) the grid-side current is directly controlled providing high robustness against harmonics in the grid. To complete the control scheme, a theoretical stability analysis is developed. Finally, selected experimental results validate the proposed control strategy and permit illustrating all its appealing features.

This paper presents a Continuous Control Set Model Predictive Control with a receding horizon for a three-phase voltage source inverter with an LCL filter. In this proposal, a reduced model of the converter with an embedded integrator and a Kalman filter are used to obtain the inverter-side currents without oscillation. Then, a cost function makes use of these currents to generate the optimum duty cycles for the space vector modulator. With the proposed method, active damping performances, a zero state error and a reduction of the computational burden are achieved. Compared to the Finite Control Set Model Predictive Control, the proposed method operates with a fixed switching frequency. Simulations and experimental results show that this proposal works correctly even in the case of grid harmonics and voltage sags.