An Enhanced Microgrid Load Demand Sharing Strategy

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.

Microgrids consist of multiple parallel-connected distributed generation (DG) units with coordinated control strategies, which are able to operate in both grid-connected and islanded modes. Microgrids are attracting considerable attention since they can alleviate the stress of main transmission systems, reduce feeder losses, and improve system power quality. When the islanded microgrids are concerned, it is important to maintain system stability and achieve load power sharing among the multiple parallel-connected DG units. However, the poor active and reactive power sharing problems due to the influence of impedance mismatch of the DG feeders and the different ratings of the DG units are inevitable when the conventional droop control scheme is adopted. Therefore, the adaptive/improved droop control, network-based control methods, and cost-based droop schemes are compared and summarized in this paper for active power sharing. Moreover, nonlinear and unbalanced loads could further affect the reactive power sharing when regulating the active power, and it is difficult to share the reactive power accurately only by using the enhanced virtual impedance method. Therefore, the hierarchical control strategies are utilized as supplements of the conventional droop controls and virtual impedance methods. The improved hierarchical control approaches such as the algorithms based on graph theory, multi-agent system, the gain scheduling method, and predictive control have been proposed to achieve proper reactive power sharing for islanded microgrids and eliminate the effect of the communication delays on hierarchical control. Finally, the future research trends on islanded microgrids are also discussed in this paper.

With conventional droop control, the droop relationship between the voltage and reactive power is not purely linear because of the filter reactance. This paper focuses on the theoretical analysis to account for this characteristic and presents a precise power flow method for conventional droop control. The proposed method is universal, which can handle not only conventional droop control, but also other control strategies, such as robust droop control, constant power control, and constant voltage–frequency control. It can also handle frequency-dependent active and reactive power loads and is adapted for islanded and grid-connected systems. The proposed method extends the applicability of conventional power flow methods to microgrids so that the framework of the method is generic; any conventional power flow algorithm can be adapted to this framework. Compared with the time-domain simulation method, the proposed method is accurate, simple, and easy to implement for industrial applications.

Droop control is a common method in the universal microgrid applications. Conventional droop control is unpractical for low‐voltage microgrid, where the line impedance among distributed generation units (DGs) is mainly resistive to generate the active and reactive power of DG is coupled. Besides, accurate reactive power sharing is not achieved due to the voltage deviation of DGs caused by disparate line impedance in microgrid. An improved droop control based on the virtual power source (VPS) and composite virtual impedance, which is constituted by a negative resistance and a negative inductance, is proposed for low‐voltage microgrid. The virtual negative resistance counteracts the line resistance to decouple power. The reactive power sharing accuracy based on the virtual negative inductance and VPSs control increases through analysing the line and virtual voltage drop. Finally, the fractional‐order PID controller is adopted to control the inverter voltage. The optimal controller parameters are obtained by the differential evolution algorithm. To verify the viability and availability of improved control strategy, simulations are carried out on MATLAB/Simulink.

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.

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 proposes new droop control methods for load sharing in a rural area with distributed generation. Highly resistive lines, typical of rural low voltage networks, always create a big challenge for conventional droop control. To overcome the conflict between higher feedback gain for better power sharing and system stability in angle droop, two control methods have been proposed. The first method considers no communication among the distributed generators (DGs) and regulates the converter output voltage and angle ensuring proper sharing of load in a system having strong coupling between real and reactive power due to high line resistance. The second method, based on a smattering of communication, modifies the reference output voltage angle of the DGs depending on the active and reactive power flow in the lines connected to point of common coupling (PCC). It is shown that with the second proposed control method, an economical and minimum communication system can achieve significant improvement in load sharing. The difference in error margin between proposed control schemes and a more costly high bandwidth communication system is small and the later may not be justified considering the increase in cost. The proposed control shows stable operation of the system for a range of operating conditions while ensuring satisfactory load sharing.

In this paper, an improved load sharing strategy is proposed for distributed generation units (DGs) connected in a microgrid. Conventional frequency and voltage droop control result in unacceptable active and reactive power sharing. The proposed method formulates a suitable algorithm for load sharing in the islanded microgrid. The feeder power loss and the line impedance voltage drops are minimized so as to regulate the voltage at the point of common coupling (PCC) at its nominal value. The desired DG output voltages are calculated and a linear relationship is obtained between the shared active and reactive powers and the DG output voltages. A master DG controller sets the frequency which is followed by other DG units. The reference powers for the DG units are adjusted so as to maintain the rated PCC voltage. The proposed strategy is verified taking into account the DG ratings, unequal line impedance drops, feeder losses, change in system impedance and effect of DG local loads and formulates an improved power sharing strategy that also facilitates PCC voltage regulation under variable loading conditions. Simulation and experimental results are presented to verify the effectiveness of the proposed method.

In conventional droop control, accurate proportional load sharing could not be achieved among parallel operated inverters due to the mismatch of output impedance and system disturbances. In this paper, an uncertainty and disturbance estimator (UDE)-based robust droop control strategy is proposed for accurate proportional load sharing, particularly reactive power sharing, among parallel operated L-inverters. The reactive power dynamics is derived from power delivery equation passed by a low-pass filter, while the reactive power reference is designed with the feedback of load voltage. The reactive power control is developed based on UDE-based method. The model nonlinearity and uncertainty (e.g., power angle), and system operation disturbances (e.g., change of output impedance, and load change), can be estimated and compensated by this UDE-based robust droop control. Experimental validation is provided to show the effectiveness of the proposed method.

In the absence of energy storage, photovoltaics (PV) also need to provide inertia support to the power electronic power system. Based on the DC bus voltage droop control to provide the inertia of the converter, two improved virtual inertia control strategies are proposed. From the perspective of DC bus voltage U dc and system virtual inertia coefficient H v under load disturbance, the advantages and disadvantages of three virtual inertia control strategies and the influence of control parameters on system inertia are analyzed. The conventional droop control is a full-band response load disturbance, and the inertia coefficient H dp increases with the droop coefficient $K$ dp increases, but the DC bus voltage cannot be restored to the initial set value. The improved low-pass filter control (ILPFC) only responds to the load disturbance in the low frequency band. In fact, a delay link is introduced on the basis of the conventional droop control to slow down the frequency response speed, and increasing the K1 or ω1 can also increase the system inertia effect. The improved high-pass filter control (IHPFC) only responds to load disturbances in the high frequency band, overcomes the problem that the conventional droop control cannot restore the DC bus voltage to the initial set value. Meanwhile, increasing Kh or decreasing ω $h$ can also improve the system inertia support. Finally, the effectiveness of the control strategy and the correctness of the analysis conclusion are verified by simulation.

The interline power flow controller (IPFC & UPFC) is one of the latest generation flexible AC transmission systems (FACTS) controller used to control power flows of multiple transmission lines. The main objective of this paper is detailed study about a new real and reactive power coordination controller for a interline power flow controller (IPFC & UPFC). The basic control for the IPFC is such that the series converter of the UPFC controls the transmission line real/reactive power flow and the shunt converter of the (IPFC & UPFC) controls the bus voltage/shunt reactive power and the DC link capacitor voltage. Because of the common link, any inverter within the (IPFC & UPFC) is able to transfer real power to any other and thereby facilitate real power transfer among the lines of the transmission system. Since each inverter is able to provide reactive compensation, the (IPFC & UPFC) is able to carry out an overall real and reactive power compensation of the total transmission system. This capability makes it possible to equalize both real and reactive power flow between the lines, transfer power from overloaded to under loaded lines, compensate against reactive voltage drops and corresponding reactive line power and to increase the effectiveness of the compensating system against dynamic disturbances. A simulation in MATLAB has been done in order to extend conventional algorithm based on this model.

Local control of Distributed Energy Resources (DER) in microgrids usually relies on droop algorithm in order to ensure stability to the system in islanded operation. Conventional and reverse droop control are applied respectively to Voltage Control Mode (VCM) inverters and to Current Control Mode inverters (CCM). In low-voltage microgrid, traditional P-ω and Q-V droop characteristic is subject to the power coupling and could lead to unstable operating conditions due to dependence of load on the voltage and on the frequency. Therefore P-V and Q-ω droop characteristic, named resistive droop, is used in simulations and in experimental tests to show its effectiveness for a stable and decoupled regulation of islanded LV microgrid.

This paper proposes a new real and reactive power coordination controller for a unified power flow controller (UPFC). The basic control for the UPFC is such that the series converter of the UPFC controls the transmission line real/reactive power flow and the shunt converter of the UPFC controls the UPFC bus voltage/shunt reactive power and the DC link capacitor voltage. In steady state, the real power demand of the series converter is supplied by the shunt converter of the UPFC. To avoid instability/loss of DC link capacitor voltage during transient conditions, a new real power coordination controller has been designed. The need for reactive power coordination controller for UPFC arises from the fact that excessive bus voltage (the bus to which the shunt converter is connected) excursions occur during reactive power transfers. A new reactive power coordination controller has been designed to limit excessive voltage excursions during reactive power transfers. PSCAD-EMTDC simulation results have been presented to show the improvement in the performance of the UPFC control with the proposed real power and reactive power coordination controller.

Improving power sharing in islanded networked microgrids using fuzzy-based consensus control

Parallel operation of inverter modules is the solution to increase the reliability, efficiency, and redundancy of inverters in microgrids. Load sharing among inverters in distributed generators (DGs) is a key issue. This study investigates the feasibility of power-sharing among parallel DGs using a dual control strategy in islanded mode of a microgrid. PQ control and droop control techniques are established to control the microgrid operation. P-f and Q-E droop control is used to attain real and reactive power sharing. The frequency variation caused by load change is an issue in droop control strategy whereas the tracking error of inverter power in PQ control is also a challenge. To address these issues, two DGs are interfaced with two parallel inverters in an islanded AC microgrid. PQ control is investigated for controlling the output real and reactive power of the DGs by assigning their references. The inverter under enhanced droop control implements power reallocation to restore the frequency among the distributed generators with predefined droop characteristics. A dual control strategy is proposed for the AC microgrid under islanded operation without communication link. Simulation studies are carried out using MATLAB/SIMULINK and the results show the validity and effective power-sharing performance of the system while maintaining a stable operation when the microgrid is in islanding mode.