Three-phase Four-wire Microgrid Research Articles

Power Sharing in Three-Level NPC Inverter Based Three-Phase Four-Wire Islanding Microgrids With Unbalanced Loads

The droop-based control schemes are widely used for power-sharing and control of the isolated microgrids. The conventional droop control scheme with inner current and voltage controllers can work effectively under ideal voltage. However, the existence of unbalance tends its performance to worsen. This paper aims to propose an enhanced droop control method to improve the power-sharing, and maintain the distributed generators’ (DGs) terminal voltages and the system frequency under the balanced as well as unbalanced loads. The enhancements of the proposed control scheme include the droop controller, inner current and voltage controller, and virtual impedance loop. The proposed virtual impedance deals with the accurate sharing of powers in the case of unequal line impedance and the control of zero sequence current. Moreover, to enable the proposed control to compensate for the unbalance, three-level neutral point clamped (NPC) inverters are used to form a three-phase four-wire microgrid. With this control scheme, the voltage unbalance factors (VUFs) of the DGs’ terminal voltages are reduced from 4% to less than 1%, negative sequence reactive powers, which are also an indication of unbalance, have been reduced significantly, and the system frequency is maintained within the standard permissible limit of 2.5%. Furthermore, a local load controller is also proposed to control the reactive loading of the DGs when they have local loads supplied from their terminals. The investigations have been carried out under simulation as well as lab-based environments to validate the efficacy of the proposed control method.

Distributed Control Strategy of Single-Phase Battery Systems for Compensation of Unbalanced Active Powers in a Three-Phase Four-Wire Microgrid

Unbalanced active powers can affect power quality and system reliability due to high penetration and uneven allocation of single-phase photovoltaic (PV) rooftop systems and load demands in a three-phase four-wire microgrid. This paper proposes a distributed control strategy to alleviate the unbalanced active powers using distributed single-phase battery storage systems. In order to balance the unbalanced active powers at the point of common coupling (PCC) in a distributed manner, the agents (households’ single-phase battery storage systems) must have information on the active powers and phases. Inspired by supervised learning, a clustering approach was developed to use labels in order to match the three-phase active powers at the PCC with the agents’ phases. This enables the agent to select the correct active power data from the three-phase active powers. Then, a distributed power balancing control strategy is applied by all agents to compensate the unbalanced active powers. Each agent calculates the average grid power based on information received from its neighbours so that all agents can then cooperatively operate in either charging or discharging modes to achieve the compensation. As an advantage, the proposed distributed control strategy offers the battery owners flexibility to participate in the strategy. Case studies comparing performance of local, centralized, and the proposed distributed strategy on a modified IEEE-13-bus test system with real household PV powers and load demands are provided.

Distributed Control Strategy for Low-Voltage Three-Phase Four-Wire Microgrids: Consensus Power-Based Control

Most of the distributed control strategies for grid-connected power converters are droop-based approaches composed of converters driven in voltage-control mode, based on local and shared data with adjacent units. They are usually combined with consensus protocols to deal with the trade-off between power sharing accuracy and voltage/frequency regulation. To achieve the desired results these control systems usually incorporate other techniques and need to take into account details of primary control dynamic. Additionally, power flow control and current unbalance compensation at the PCC are rarely addressed in such approaches. Contrariwise, the centralized control strategy power-based control has been successful in achieving these functionalities. It is oriented to a set point selection to the whole system, considering converters driven in current-control mode and a central converter in voltage-control mode. However, the dependence on centralized communication network in this method still requires improvement. Thereby, the complementary features of both strategies are combined herein in the consensus power-based control, based on a master/slave peer-to-peer integration using sparse communication. This model-free approach provides all aforementioned benefits to the grid without any other technique. Implementation complexity and costs are decreased, while the flexibility and reliability are enhanced. All these achievements are demonstrated by simulation results under different operational conditions and compared to previous works.

Control and Implementation of a Multifunctional Solar PV-BES-DEGS Based Microgrid

A three-phase four-wire microgrid comprising of a solar photovoltaic (PV) array—battery energy storage—a diesel engine generator set (DEGS) is presented in this article. Here an enhanced adaptive filter (EAF) control and an incremental conductance (INC) maximum power point tracking algorithm are used to improve the power quality and to extract the maximum power from the PV array. The EAF control provides higher disturbance rejection capability over other controls. The EAF control is applied to the voltage source converter (VSC) to improve the power quality, such as compensation of harmonics, reactive power, and load unbalance. The INC control is used to harvest the PV array maximum power. For controlling the voltage output of the DEGS, an electronic automatic voltage regulator is provided at the synchronous generator field winding. The neutral current compensation is achieved by controlling the fourth-leg of VSC. This microgrid is simulated using MATLAB/Simulink tool. The robustness of the EAF control strategy is tested on a developed system prototype in the laboratory.

Preference cone based multi-objective evolutionary algorithm applied to optimal management of distributed energy resources in microgrids

The existence of unbalanced loads and high penetration of single-phase distributed energy resources connected unevenly throughout the phases of a microgrid causes unbalanced currents through the microgrid’s point of common coupling and then degradation of power factor. This paper proposes a multi-objective optimization model applied to optimization the use of distributed energy resources in an energy system. The centralized control strategy is divided into three levels integrated into a master controller. The primary level performs the local functions of distributed inverters. The secondary level is split into two layers: the first layer performed by the power-based control is responsible for sharing active/reactive power among distributed units, as well as dispatching power to the upstream network using a low-speed communication link; and the second layer employs a preference cone based multi-objective evolutionary algorithm based on decomposition for the solution of the proposed multi-objective optimization formulation to maximize the active power injection by single-phase units, and minimize the currents unbalance into the main grid. The preference cone based multi-objective evolutionary algorithm has obtained solutions with good repeatability, convergence and distribution leading the microgrid to operate at its optimal point while maintaining its stability. Finally, the tertiary level defines the constrains of active and reactive power based on the utility status. This paper focuses on the secondary level of control and the proposed method is assessed by computational simulations considering a three-phase four-wire microgrid operating in both islanded and grid-connected modes under realistic operational conditions in terms of load, generation and grid variations.

Cooperative Regulation of Imbalances in Three-Phase Four-Wire Microgrids Using Single-Phase Droop Control and Secondary Control Algorithms

Collaborative control of power converters operating in microgrids with unbalanced single-phase loads is difficult to achieve, considering that the voltages and currents have positive-, negative-, and zero-sequence components. In this paper, a new control scheme for collaborative control of four-leg microgrids is proposed. The main advantage of the proposed methodology is simplicity, because the sharing of the powers produced by the positive-, negative-, and zero-sequence voltage and currents is simple to achieve using the easy to implement and well-known droop control algorithms, i.e., as those based on $P$ – $\omega$ and $Q$ – $v$ droop control. The proposed droop algorithms do not require high bandwidth communication channels and the application of virtual impedances, whose design usually demands extensive simulation work, is not required. Three secondary control systems are also analyzed, discussed, and implemented in this paper to regulate the frequency, voltage, and phase at the point of common coupling (PCC), to achieve a balanced 50-Hz three-phase voltage supply in the PCC during steady-state operation. For these secondary control systems, single-phase phase-locked loop based on quadrature signal generators are implemented. Small signal modeling and design are discussed in this paper. A microgrid prototype of $\approx$ 5 kW, implemented using two power converters of 3 kW (each), is used to experimentally validate the proposed algorithms.

Optimal Multiobjective Control of Low-Voltage AC Microgrids: Power Flow Regulation and Compensation of Reactive Power and Unbalance

The presence of single-phase distributed generators unevenly injecting active power in three-phase microgrids may create undesired upstream current unbalance. Consequently, voltage asymmetry and even active power curtailment may occur in such networks with negative economic impact. Thus, this paper proposes an optimal multiobjective approach to regulate the active and reactive power delivered by distributed generators driven by a three-layer hierarchical control technique in low-voltage microgrids. This method does not require previous knowledge of network parameters. The multiobjective algorithm is implemented in the secondary level achieving optimal dispatch in terms of maximizing the active power generation, as well as minimizing the reactive power circulation and current unbalance. By the existence of a utility interface three-phase converter placed at the point-of-common-coupling, the proposed control can regulate the power circulating among the microgrid phases, and the microgrid structure can withstand grid-connected and islanded operating modes. The path for interphase power circulation through the DC-link of the utility interface allows the multiobjective algorithm to achieve better results in terms of generation and compensation compared to the system without utility interface. The proposed method is assessed herein by computational simulations in a three-phase four-wire microgrid under realistic operational conditions.

Centralized Control of Distributed Single-Phase Inverters Arbitrarily Connected to Three-Phase Four-Wire Microgrids

This paper proposes an effective technique to control the power flow among different phases of a three-phase four-wire distribution power system by means of single-phase converters arbitrarily connected among the phases. The aim is to enhance the power quality at the point-of-common-coupling of a microgrid, improve voltage profile through the lines, and reduce the overall distribution losses. The technique is based on a master/slave organization where the distributed single-phase converters act as slave units driven by a centralized master controller. Active, reactive, and unbalance power terms are processed by the master controller and shared proportionally among distributed energy resources to achieve the compensation target at the point-of-common-coupling. The proposed control technique is evaluated in simulation considering the model of a real urban power distribution grid under non-sinusoidal and asymmetrical voltage conditions. The main results, concerning both steady-state and transient conditions, are finally reported and discussed.