Droop Control of Converter-Interfaced Microsources in Rural Distributed Generation

This paper proposes a point of common coupling (PCC) voltage compensation method for islanding microgrids using an improved power sharing control scheme among distributed generators (DGs) without communication. The PCC voltage compensation algorithm is implemented in the droop control scheme to reduce the PCC voltage deviation produced by the droop controller itself and the voltage drop on the line impedance. The control scheme of each individual DG unit is designed to use only locally measured feedback variables and an obtained line impedance to calculate the PCC voltage. Therefore, traditional voltage measurement devices installed at the PCC as well as communication between the PCC and the DGs are not required. The proposed control scheme can maintain the PCC voltage amplitude within an allowed range even to some extent assuming inaccurate line impedance parameters. In addition, it can achieve proper power sharing in islanding microgrids. Experimental results obtained under accurate and inaccurate line impedances are presented to show the performance of the proposed control scheme in islanding microgrids.

An improved centralized/decentralized accurate reactive power sharing method in AC microgrids

In recent researches on inverter-based distributed generators, disadvantages of traditional grid-connected current control, such as no grid-forming ability and lack of inertia, have been pointed out. As a result, novel control methods like droop control and virtual synchronous generator (VSG) have been proposed. In both methods, droop characteristics are used to control active and reactive power, and the only difference between them is that VSG has virtual inertia with the emulation of swing equation, whereas droop control has no inertia. In this paper, dynamic characteristics of both control methods are studied, in both stand-alone mode and synchronous-generator-connected mode, to understand the differences caused by swing equation. Small-signal models are built to compare transient responses of frequency during a small loading transition, and state-space models are built to analyze oscillation of output active power. Effects of delays in both controls are also studied, and an inertial droop control method is proposed based on the comparison. The results are verified by simulations and experiments. It is suggested that VSG control and proposed inertial droop control inherits the advantages of droop control, and in addition, provides inertia support for the system.

This paper proposes a control strategy for sharing load requirements in a low voltage islanded microgrid with converter interfaced distributed energy resources. In high voltage lines with large X/R ratio, real and reactive power are decoupled and thus conventional droop control can effectively share the power needs of the load. However, the same strategy cannot be applicable in dominantly resistive remote microgrids where, X/R ratio is very small. In order to ensure accurate sharing of load power, this paper adopts cut in contrast of conventional angle droop control, where active power sharing is achieved by controlling voltage and reactive power by angle droop method. In addition, virtual resistive output impedance is utilized to effectively decouple real and reactive power. Moreover, this strategy includes influence of line impedance in load power sharing and offers an added advantage of effective overall system damping. Third harmonic injection based pulse width modulation scheme is used for each distributed generators(DGs), which provides better utilization of DC link voltage and suppress neutral currents under unbalanced conditions.

The primary function of microgrid (MG) is to share the power accurately among different distributed generation (DG) units according to the load demands. The droop control methods are normally used for appropriate power sharing. However, these control techniques suffer badly due to the feeder impedances and uncontrollable power coupling issues. Many techniques have been proposed to share power accurately, most of them rely on communication link. In this work a localized/communication free signal generating mechanism is proposed. The mechanism injects the transient reactive power in frequency droop characteristics by observing the changes in microgrid. This injection disturbs the actual active power sharing which is helpful for the computation of reactive power sharing errors. These errors are minimized by utilizing the proportional integral (PI) controller. The PI controller modifies the y-intercept and the slop of Q-E (Reactive Power –Voltage) droop characteristic for the correction of reactive power sharing error. Moreover, the parameters of droop and PI controller of inner control loop are optimized by Particle Swarm Optimization (PSO) to obtain high precision and reliability. The proposed technique has the following salient features:1. Accurately shares the reactive power without the information of feeder line impedances2. This accurate reactive power sharing scheme has been started locally without the need of communication signal from the central controller. Thus, there is no delay in monitoring the notable changes in microgrid caused by DGs. 3. The careful selection of PI & droop controller parameters through PSO makes the system more stable as compared to the conventional methods in terms of transients and steady state responses.

A minimize communication control method is proposed for photovoltaic (PV) grid-connected systems with cascaded H-bridge inverters. In the system used in this paper, PV panel in each unit is directly connected to the input port of the local H-bridge, then the output ports of these H-bridge inverters are cascaded with each other, each PV unit works at its maximum power point (MPP) to guarantee system’s maximum power generation efficiency. With communication, the inverter that connect to the point of common coupling (PCC) is controlled as a current source to keep synchronous operation with the utility grid by regulating its output current in phase with the grid voltage. The other cascaded inverters are controlled as voltage resources and keeping self-synchronization with unity power factor (PF) by the proposed control method without any communication. Finally, the common link current drives all cascaded inverters keep self-synchronization with the grid and provide maximum power with high quality to the grid. Compare with other control methods, the proposed control method only requires one communication link, the communication cost is effectively reduced, the reliability and power quality of the system are greatly improved. Simulation test verifies the correctness and validity of the proposed control strategy.

In this study, a novel droop control method for ac microgrids is proposed to enhance the performance of power regulation, which is composed of three parts. The angle droop and the frequency droop are adopted to control the active power in coordination, while the modified voltage droop is used to control the reactive power. There are several improvements in this proposed triple droop control method compared with the conventional droop control. First, the dynamics of active power regulation are enhanced because of the angle droop. Second, the stability of the system is upgraded and the droop coefficients can vary in greater range. Third, the accuracy of reactive power regulation is improved, especially in the grid‐connected mode, the DGs can output the specified reactive power with the modified voltage droop, which benefits the economic dispatch of the microgrid. Through these three combined droop control strategies, the performance of the whole system can be improved a lot whether in the grid‐connected mode or the islanded mode. Furthermore, the stability of the proposed triple droop control is analysed, which shows that the stability of the system is upgraded. The effectiveness of the proposed control method is verified by the hardware‐in‐loop tests.

This paper describes a control method for distributed generation (DG) units to implement active power sharing and frequency recovery simultaneously in an islanded microgrid. Conventional active power–frequency ( P – f ) droop control is used for the DG controller, and the frequency deviation is recovered by the DG itself via self-frequency recovery control, without requiring secondary frequency control. Because the electrical distance (impedance) from each DG unit to a point where the load demand changes differs among DG units, the instantaneous frequency deviations may differ between DG units. These differences are fed into the integrators of the self-frequency recovery control and may result in errors in active power sharing. To solve this problem and share active power more accurately, a compensation control method is developed for active power sharing, which considers the droop coefficients of each of the DG units. Simulation results show that the proposed control method is effective.

The escalating demand of reactive power consumption by the load in the grid connected system usually causes the following problems 1) Voltage sag and swell at the point of common coupling (PCC) 2) Harmonic distortion of the grid current 4) low power factor etc. More over, to meet the rising reactive power load demand, distributed generation (DG) is a viable option. For that, usually current regulated PWM voltage-source inverters (VSI) are used to inter face DG with utility grid in order to ensure the grid stability. In this paper, reactive power compensation for inverter interfaced DG system based on hysteresis with proportional integral (PI) controller is proposed along with independent control of active and reactive power of the grid. The hysteresis with PI controller can largely reduce the current harmonics at PCC considerably which ensures lower total harmonic distortion (THD) during reactive power load demand increases. The performance indices include THD of the grid current, fast current tracking during steady state and transient conditions. The studied system is modeled and simulated in the MATLAB Simulink environment. Further, an experimental work has been carried out in the laboratory set-up in the discrete-domain via TMS320F2812 digital signal processor (DSP) and we could find there is an effective validation with simulated results.

Grid-connected inverters based on droop control have been widely used in distributed generation (DG) systems, as they can ensure smooth transition between grid-connected mode and islanded mode. However, the traditional active power-frequency (P-ω) and reactive power-amplitude (Q-E) droop control method is vulnerable to the grid voltage distortion, because of the following two reasons. Firstly, the inverter under droop control behaves like a controlled voltage source connected in parallel with utility grid, and the grid current is not directly controlled. Secondly, the line impedance interlinking the inverter and the grid is usually very small, so a slight distortion in grid voltage can lead to serious distortion in grid current. To suppress grid current distortion, this paper proposes to employ harmonic virtual impedance (HVI), which is effective only at harmonic frequency, to increase the equivalent impedance between inverter and grid. Compared with the existing control methods, the proposed HVI control scheme does not need to detect grid voltage and it is easy to be implemented. Comprehensive simulation and experimental results are provided to validate the effectiveness of the proposed method.

A large number of wind turbine generators in the large-scale offshore wind farm system poses a challenge to the wind farm control system due to the computational burden in the central control methods or the complexity of the communication network in the decentralized control strategies. A hybrid control method based on the distributed consensus control and the central model predictive control is proposed in this study to overcome the problem. Typically, the wind turbine generators in the large-scale offshore wind farm system are clustered into several groups. The consensus-based distributed reactive power coordination control is proposed to each cluster and the centralized predictive voltage control is used to manage the total reactive powers of all clusters and regulate the voltage at the point of common coupling. The gradient-descent algorithm for the optimal design of the consensus-based cluster control is presented firstly. Based on the convergence property of the consensus control, the equivalent model of the total reactive power response of each cluster is identified, which is used for the design of the centralized predictive voltage control. Eigenvalue analysis of the proposed predictive control strategy is carried out to verify the stability of the distributed and predictive control systems. The robustness of the proposed predictive controller is evaluated in the conditions of significant model errors due to the communication delay in each cluster. A comparison study with the full distributed control based on consensus algorithm is presented to demonstrate the effectiveness of the proposed control method. The feasibility of the proposed predictive controller is evaluated by the control-hardware-in-the-loop simulation using OPAL-RT Technologies. An additional comparison study in term of computation time with the central control method is also carried out. Real-time simulation results show the superior performance of the proposed hybrid method compared to the full distributed consensus controller or the central control strategies.

In today’s world, where the grid structure becomes smarter day by day, microgrid, one of the fundamental methods for the integration of renewable energy, are taking firm steps to be building blocks of the modern electrical grid. Along with its advantages, the microgrid brings extra challenges which attracts the attention of many researchers in these days. This paper focuses on addressing two of the main challenges: voltage stability and reactive power sharing in islanded mode. Contents of this paper include: first, elaboration of general hierarchical control structures of inverter-based distributed generators (DGs), second, review of primary droop control methods and analysis of their drawbacks, third, review and comparison of current secondary reactive power control methods to find the gap of more practical secondary control method. In the paper, a centralized secondary controller based on intermittence level, effective-impedance to load and severity of voltage deviation is proposed to overcome the disadvantage of current secondary reactive power control methods. The proposed method takes practical aspects into consideration and prioritizes DGs in reactive power balancing and voltage stability. Performance of proposed method is illustrated by a simulation study.

The unequal impedances of the interconnecting cables between paralleled inverters in the island mode of microgrids cause inaccurate reactive power sharing when the traditional droop control is used. Many studies in the literature adopt low speed communications between the inverters and the central control unit to overcome this problem. However, the losses of this communication link can be very detrimental to the performance of the controller. This paper proposes an improved reactive power-sharing control method. It employs infrequent measurements of the voltage at the point of common coupling (PCC) to estimate the output impedance between the inverters and the PCC and then readjust the voltage droop controller gains accordingly. The controller then reverts to being a traditional droop controller using the newly calculated gains. This increases the immunity of the controller against any losses in the communication links between the central control unit and the inverters. The capability of the proposed control method has been demonstrated by simulation and experimental results using a laboratory scale microgrid.

The integration of single-phase microgrids (MG) and unbalanced loads to three-phase MGs results in power quality issues at the point of common coupling (PCC). These issues include unbalanced voltage, unequal power flow, and inaccurate reactive power sharing. In this article, an energy storage system (ESS) has been proposed with novel control methods to help mitigate unbalance issues in these islanded multimicrogrids (MMGs). It presents two novel control methods: modified reverse droop control and multifunction control to compensate the reactive power of the MMG and generate the needed active and reactive power references of the ESS. The developed control methods mitigate PCC voltage and power unbalance of the MMG and also achieve the load power sharing proportionally among different rated photovoltaics (PV) inverters. In addition, the system stability with the proposed control is also presented under different scenarios. The proposed multifunction controller operation is validated using Opal-RT real-time simulator and experimentally using control hardware-in-the-loop. The results demonstrate that the PCC unbalanced voltage and power are reduced significantly by 97.8%, and the proper reactive power sharing is achieved among the different PV inverters in the proposed system.

Nonsingular Fixed-time Sliding Mode Control (NFSMC) method is one of the nonlinear control methods which is robust against uncertainties and external disturbances. This paper presents an NFSMC method to synchronize two similar Willamowski–Rossler reaction chaotic systems. The proposed control method is robust against uncertainties and external disturbances; also, it is an accurate and fast control method. The proposed control method is compared with a Nonsingular Terminal Sliding Mode Control. The comparison results show that the proposed control method provides a better control solution for this system.