This paper presents a switching strategy for a grid-connected voltage source inverter (VSI) using sliding mode control (SMC). The conventional SMC suffers from sliding losses due to the cross-coupling among controllers by the neutral point voltage expression. This issue can be addressed by using the switching technique to obtain the voltage and currents in a new reference frame through a non-linear transformation. In this frame, a decoupled non-linear model of the VSI is derived, and two sliding mode controllers are designed using hysteresis band comparators. The switching strategy not only avoids sliding losses but also increases the control range of the converter by introducing a third harmonic by the control signal. Simulation results using Matlab Simulink are provided to validate the robustness of the proposed controller.

This paper presents a model predictive control (MPC) for a grid-connected voltage source inverter (VSI) with current limitation. The control algorithm is performed in the dq frame to obtain the amplitude of the inverter currents. First, a state observer with a reduced model of the converter is used to estimate the inverter current and perform active damping. Then, an MPC algorithm is used to regulate the currents to a desired reference but with the advantage that these currents can be limited to a maximum value by introducing a constraint in the cost function instead of clipping the current of the inverter above a maximum current. This controller has been tested experimentally under a current reference step change and without a physical damping resistor.

The growing number of electric vehicles (EVs) connected to the grid are a promising potential to support the overall system stability, known as vehicle to grid concept. During grid faults these vehicles can be seen as distributed dispatchable sources that can collaborate to the system restoration. In the case of islanded microgrids under a fault, electric vehicles can provide its stored energy in order to imitate the low voltage ride through protocols (LVRT) that are required to distributed generation systems connected to the main grid. The islanded microgrid tested here is composed of grid-forming voltage sources supplying different loads, some of them EVs. The proposal in this work is that, during a voltage sag caused by an overcurrent in the system, EVs must inject active power to support the voltage, instead of supplying reactive power as LVRT require. The theoretical reasoning of this new approach will be supported by a complete system modelling in the complex valued domain. Selected experimental results demonstrate the effectiveness of the control proposal.

In industrial microgrids with medium power rating photovoltaic systems, the low-voltage ride-through capability should be incorporated to help keep power system stability. The goal of this paper is to present a control strategy that allows taking advantage of the maximum capability of the inverters to support the most dropped phase in a photovoltaic-based industrial microgrid during voltage sags. Depending on the grid code requirements, the rated current value could be injected by the local inverters during these disturbances. The currents can be injected through the positive and negative sequences depending on the sag characteristics and the power rating of the local inverters. Also, to prevent overcurrent, the controller can apply active power curtailment. Lastly, selected simulation results are presented to validate the performance of the control scheme.

Industrial microgrids require control strategies that guarantee high reliability and power quality. In this paper a multi-layer active power and frequency control strategy for industrial microgrids is presented. It is composed by a three-layer hierarchical scheme. The first control layer is based on the droop method. The secondary control layer is based on an event-driven communication scheme that launches a protocol which controls the secondary terms. The tertiary control layer calculates an optimal economical dispatch. The proposed control guarantees an excellent performance in terms of frequency restoration and power sharing. Experimental results obtained on a laboratory microgrid are presented to validate the performance of the proposed control scheme.

It is well-know that in systems composed of several digital processors, clock signals of individual units differ from each other due to clock drifts. This paper investigates the effect of clock drifts from voltage source inverters (VSI) in industrial microgrids. Particularly, a primary/secondary droop-free distributed control scheme is considered in order to evaluate its robustness against clock drifts. The study is carried out by fully experimental means using a laboratory microgrid.

Grid codes are continuously evolving to improve the ride-through services during voltage sags. The main reason for this evolution is the increasing flexible capabilities of distribution generation inveters. The problem of selecting an appropriate strategy during grid faults is an open research topic that depends on many aspects. Among the possible solutions, this paper solves an optimization problem based on the maximization of the voltage support during grid faults. The control strategy is based on raising as much as possible the positive sequence voltage. This strategy allows to reduce the risk of disconnection by under-voltage that can led to a blackout. The problem is related to two main issues: the amount of injected current and the grid impedance. Regarding the injected current, it should be desirable to develop the references so as to inject the rated current of the inverter. Therefore the voltage support could be improved while keeping the inverter within a safety operation mode. Regarding the impedance, the strategy for inductive or resistive grids should be different. For a combination of both impedances, an optimal solution should exist in order to maximize the voltage increment. This solution is related to the amount of active and reactive current injected by the inverter. The work proposes a voltage support control strategy intended to increase as much as possible the positive sequence voltage and to inject the maximum rated current of the grid-connected distributed generation inverter. The problem is mathematically solved for any type of grid impedance, even resistive, inductive or a combination of both. Simulation results are presented to corroborate the theoretical solution.

In this paper, an optimal daily management for a solar-based industrial microgrid is proposed by analyzing two different load profiles. For each profile, the first step is to find the optimum size of the battery bank in order to identify the best daily configuration. The second step is to solve a multi-objective optimization problem, which takes into account economic evaluation indices (such as the initial investment, replacement, maintenance cost and the cost related to selling or purchasing energy to or from the grid) as well as the loss of power-supply probability index for evaluating system reliability. To solve the multi-objective optimization problem, first the weighted-sum method is used to convert it into a single objective optimization problem. Second, the new formulation is solved using an enhanced version of the bacterial foraging algorithm.

In islanded microgrids, parallel operation of voltage source inverters is commonly enabled by primary droop control to achieve power sharing and secondary control that removes the frequency deviation by mimicking at some extend the operation of an integral controller. Often the main task of the distinct secondary control approaches is to compute a corrective term considering the frequency error that is then used by the droop mechanism for frequency compensation. This paper considers the secondary control approach where local integral controllers that compute the frequency corrective term are implemented at each inverter. And considering that integrals are performed in different nodes, the paper shows that global time between nodes achieved through clock synchronization is mandatory for accurate power sharing and frequency restoration.

This paper presents an improved variable hysteresis-band current-control in natural frame for a three-phase shunt active power filter. 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 and also reducing the THD of the current delivered to the grid. 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.

This paper presents an active damping method in natural frame for a three-phase voltage source inverter with LCL filter. The proposed method is based on the pole placement technique via Ackermann's formula. This approach is used to obtain the proper sliding surface vector coefficients to emulate a virtual resistor in series with the capacitor filter. Besides a well-known method in the literature have been used to obtain three decoupled controllers in natural frame. The stability is theoretically studied and experimental results shows the validity of this proposal.

This paper present a sliding mode control operating at fixed switching frequency for a three-phase unity power factor rectifier using the vector operation technique. The proposed method allows to find a large-signal converter dynamic model in order to transform a tree-phase system into two decoupled subsystems. With this technique only two sliding-mode controllers are needed and the neutral point voltage influence is removed from the controllers dynamics. A fast output-voltage control is used which avoids output-voltage variations when a sudden change in the load appears. Besides a variable hysteresis band control is used in order to fix the switching frequency of the sliding-mode controllers. Experimental results are provided using a fully-digital control system in order to validate the theoretical predictions.

Three-phase inverters are the mostly used converters for connecting distributed generation sources to the main grid. Conventionally, these renewable sources must inject only its generated active power. Recently, reactive power injection has been included in some national grid codes. In voltage regulation, the conventional PI control can not be used. The whole system tends to instability when PI controllers work distributively for voltage regulation in different connection points along the grid. The slope voltage controller seems to be the best option to avoid these instabilities. The parameters design of the slope voltage controller is normally carried out by trial and error methodology due to lack of a small-signal model of the complete system. With this widely used methodology, the dynamic behavior can not be determined by the designer. This work develops a closed-loop small signal model of a grid-connected distributed generation source with voltage regulation capabilities. The derived mathematical model predicts accurately the system behavior in the frequency range of interest. This model can be used in future research for tuning the controller parameters with the aim of achieve predefined design objectives. The model has been validated through simulation results.

The increasing deployment of distributed generation (DG) such as photovoltaic panels (PV) connected to low-voltage (LV) grids is becoming a common trend in urban areas. The advances of information and communication technology (ICT) facilitates the collaborative operation of DG systems to achieve collective benefits. The fusion of these two trends creates a new scenario where reactive power control methods can offer additional features and benefits beyond the conventional voltage regulation provided by the droop method. Taking advantage of this new scenario, this paper formulates the application of reactive power control as an optimization problem where simple and ideal settings are imposed by design in order to facilitate the exploration search as well as to avoid over-constraining the optimization space. By appropriately using the reactive power capacity of inverters, the desired collective benefit is to minimize power losses while individual voltages at each inverter should be kept within the statutory limits. The simulated solution of the optimization problem is applied to a real-inspired PV-LV grid subject to an over-voltage situation, which may typically occur during periods of high production but low consumption. Simulation results reveal unexpected optimal settings for reactive power control set-points at each inverter, which calls for a final discussion to review the applicability of the optimization approach.

A topic of interest within the microgrid (MG) community is to achieve accurate sharing of active and reactive power among voltage source inverters (VSI) working in parallel while keeping the voltage frequency and amplitude stable, and close to a given reference. This problem has been mainly solved by the so-called frequency and voltage droop controllers that are local control loops implemented in each VSI. However, since they introduce a frequency and amplitude deviation, a secondary integral-like control loop using a communication system is usually deployed to enable each VSI to cancel the deviations. This paper evaluates two secondary control implementations of the frequency restoration with respect to properties of the communication system. Networked control applications are subject to known problems that are inherent to the use of a communication system such as message losses or varying sampling/transmission intervals, to name a few. The performance analysis reveals that control policies should be designed accounting also for the properties of the communication system. Otherwise, unexpected results violating the control specifications may appear.

Unbalanced voltages in three-phase power systems is a common perturbation propagated along the grid. Distributed Generation plants have gained widespread attention due to their capability to improve power quality in a distributed manner, including voltage unbalance mitigation. A conventional control strategy to command power plants during balanced grid voltages, is the use of power factor to inject/absorb reactive power depending on grid conditions. Advanced control strategies during unbalanced grid voltages can be selected prioritizing positive or negative sequence active and reactive power. This selection determines the voltage support service and therefore can improve the voltage profile, i.e. phase-voltages can be properly supported and voltage imbalance simultaneously corrected. Based on the definition of effective power factor in IEEE 1459 Standard, the reactive power needed for any control strategy is obtained and the resulting effects are described.

This paper proposes a new active damping method based on virtual resistors for three-phase grid connected voltage source inverters (VSI) with LCL filter. The use of this filters is a standard solution to provide the proper attenuation to the sideband voltage harmonics of the modulation signal. However these grid filters can introduce an unstable dynamics if a passive or active damping is not properly introduced. The passive damping solution causes a decrease of the overall efficiency due to the associated losses and reduces the filter effectiveness but in contrast no more extra sensors are needed. Active damping methods are more efficient because they don't introduce losses but suffers from control and system complexity because in some cases a high number of sensors are needed. The proposed strategy employs a nonlinear sliding mode control (SMC) scheme in natural frame in combination with a Kalman filter to emulate a resistor in the LCL filter which damps the system actively without the use of extra sensors. Two current controllers are designed in order to control the active and reactive power independently and a voltage controller is designed to control the neutral point which is fixed with a third harmonic voltage in order to increase the control dynamic range.

This paper presents a novel power sharing control strategy for islanded mode microgrids (MG). The goal is to enable renewable power sources connected to the point of common coupling (PCC) through three-phase three-wire inverters to cooperatively feed linear and non-linear local loads. The strategy contribution is two-fold. First, a centralized controller is designed to facilitate the power sharing capability of all inverters by means of data communications regardless of load changes and inverter faults. Second, a communication protocol is designed following an event-driven paradigm in such a way that the consumed bandwidth is reduced. Selected simulation results show the main features of the power sharing control strategy.

Penetration of distributed power generation poses new problems in energy distribution. For example, photovoltaic generation systems may difficult the voltage management in distribution lines and voltage rise may occur. To overcome this problem, several voltage control strategies are available and they can be classified as autonomous or distributed regarding whether communication is available between dispersed generators. Through a preliminary case study, this paper investigates the impact that the communication infrastructure has for a given distributed voltage control strategy. Communication issues like transmission rate, delays, and losses are analyzed with respect to the voltage control application performance.

Grid faults are one of the most severe perturbations to deal with in grid connected systems. To support the grid under voltage-sags (dips), different reactive power control strategies are needed depending on the type of grid fault. One of the possible objectives when supporting the grid voltage under type C voltage sags could be to avoid undervoltage in the faulted phases and over-voltage in the non-faulted phase. Therefore, the fault can ride-through the perturbation and the system disconnection can be avoided. This may be accomplished by injecting positive sequence reactive power to raise the voltages and negative sequence reactive power to equalize them. In this work, a detailed mathematical description is introduced to limit the voltage in the phase that do not suffers the voltage-sag just below over-voltage limit, while increasing as much as possible the other two faulted phases. This voltage support service is limited by the maximum rated current. Selected simulation results are provided to understand the proposed reactive power control during type C voltage-sags.