Study of improved load sharing methodologies for distributed generation units connected in a microgrid

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.

Interconnecting microgrids in LV power system presents appealing features such as self-healing or power quality. When networked microgrids are not connected to a strong utility grid, their Point of Common Coupling (PCC) voltage and their power reserves vary with the operating point. An external droop control architecture is proposed that allows active and reactive power sharing among the different microgrids, thereby stabilizing the system frequency and PCC voltage, and the maximum achievable droop gains are supplied. Next, the design of appropriate primary controllers for the Distributed Energy Resources inside each microgrid allows to achieve a specified aggregated external droop controller at the connection point with little communication requirements. This methodology is applied to a modified CIGRE benchmark and shows good results while keeping a standard decentralized control architecture.

In future low voltage grids, with multiple inverter interfaced sources connected, voltage regulation may become a necessary task. The potential exists for inverter interfaced sources to be deployed to regulate the voltage at the point of common coupling (PCC) of each inverter interfaced sources. The PCC voltage regulation is attainable with inverter interfaced sources by dynamically controlling the amount of reactive power injected to the power distribution grid by individual systems. In the current research, a closed-loop controller is proposed to regulate the PCC voltage of a solar photovoltaic (PV) system that is connected to a single-phase power distribution feeder (with R to X ratio greater than 1). The plant model of the PCC voltage controller of the PV system is derived considering both reactance and resistance of the network to which the PV system is connected. Three different compensators are evaluated to identify a suitable compensator for the closed-loop PCC voltage controller to regulate the PCC voltage at a given reference voltage. Simulation studies and experimental verification confirm that the theoretical approach taken to derive the control plant model of the PCC voltage controller is accurate and the procedure that is followed to design the controller is robust. The control design procedures illustrated in the current research leads to a PCC voltage control system with acceptable dynamic and steady state performance.

Brushless doubly-fed induction generators have higher reliability, making them an attractive choice for not only offshore applications but also for remote locations. These generators are composed of two back-to-back voltage source converters, a grid side converter and a rotor side converter. Existing techniques use the rotor side converter for reactive current control; however, it is more suitable for stabilizing steady state behavior. In order to stabilize the voltage fluctuations at the point of common coupling (PCC) due to sudden inductive load introduction, the grid side converter may be a better choice due to faster response and higher control bandwidth. Therefore, this paper proposes a control scheme for the grid side converter to suppress the PCC voltage fluctuations when a large inductive load is suddenly connected. The proposed technique is based on an analytical model of the transient behavior of the voltage drop at the PCC. The analysis shows that reactive current control using the grid side converter introduces a double fundamental frequency component to the PCC voltage. To block this harmonic, we designed a notch filter. The simulation results in Matlab/Simulink show that the proposed technique can not only significantly reduce the voltage drop but also results in an 82% reduction in voltage distortion at the PCC.

This study presents a new method for detecting an islanding event in a microgrid with embedded converter‐based distributed generation (DG). Unlike other schemes, the proposed scheme injects negligible perturbations into the microgrid after the generation of an alert signal. The proposed scheme then uses another index called superimposed impedance, ΔZ. The ΔZ is characterised by a low steady‐state magnitude during the grid‐connected mode and a high magnitude during the islanded mode. Furthermore, for a fault at the point of common coupling (PCC), the magnitudes of ΔZ and PCC voltage are both very low – except during the initial transient period, where |ΔZ | momentarily crosses the threshold. Therefore, an islanding event can be detected if the magnitude of ΔZ is high for some specified time. Moreover, a fault event will not be misdirected as an islanding event because the steady‐state magnitudes of both the ΔZ and PCC voltage are very low in the case of a fault. The robustness of the proposed detection scheme is evaluated against different islanding conditions and also, for a fault at the PCC, first by using MATLAB‐based simulations and later by using a laboratory‐based experimental setup. A comparison with a recently published detection scheme shows the superiority of the proposed schemes.

ABSTRACTThis paper presents an enhanced power sharing scheme (EPSS) for parallelly connected distributed generators (DGs) in an islanded microgrid (MG) using a constant switching frequency‐model predictive control (CSF‐MPC). Unlike conventional droop control, wherein only accurate real power sharing among DGs is possible, the EPSS enables accurate sharing of reactive and harmonic powers even under mismatched line impedances. Additionally, the voltage unbalance and distortion at the point of common coupling (PCC) are mitigated. The EPSS is implemented using a reference voltage that consists of (1) the fundamental droop control; (2) the virtual impedance control; (3) the unbalance compensation control; and (4) the harmonic compensation control. The performance of the EPSS is tested on a MG consisting of two DGs of both similar and dissimilar capacities under a step changes in loads. The EPSS is implemented using CSF‐MPC, which offers superior performance compared with conventional proportional plus integral and proportional resonant controllers. The simulation and hardware‐in‐loop results confirm the effectiveness of the EPSS in addressing power sharing issues and mitigating both distortion and unbalance of PCC voltage.

The determination of the true current harmonic distortion produced by a nonlinear load is problematic due to the fact that the voltage waveform provided by the power supply at the Point of Common Coupling (PCC) rarely is free of harmonic distortion. This paper proposes a method based on Artificial Neural Networks (ANN) in order to realize the estimation of the harmonic content presented in the nonlinear load current waveform supposing a pure sinusoidal power supply. However, this estimation is made when the nonlinear load is in fact connected to a distorted power supply. For this, a dynamic ANN with time delays is utilized for modeling the nonlinear load admittance in a pre-established condition of PCC voltage with little or none harmonic distortion. Once achieved a satisfactory training, now for a PCC voltage with any level of harmonic distortion, the ANN is utilized for predicting the nonlinear load current waveform by a power supply estimation that preserves similarities with that applied during its training. One of the advantages of the method is that only the instantaneous values of voltage and current are necessary for modeling the loads. It does not need the knowledge of any load or power system parameter.

As the non‐linear loads increase along the low‐voltage distribution network (LVDN), voltage harmonic components will appear at the point of common coupling (PCC). To improve the voltage power quality of PCC, an active PCC voltage power quality restoring strategy based on droop controlled grid‐connecting microgrid (DCGCM) is proposed. The load current of LVDN and the grid‐connecting current of DCGCM are sampled and are calculated through an additional PCC voltage restoring controller adopted in the secondary level of system. Then, the generated voltage harmonic reference offset is sent to the primary level and tracked by voltage controlled inverter. In this way, the required harmonic current is injected into the LVDN to supply the nonlinear loads at the cost of slight voltage distortion of DCGCM's output voltage. Therefore, the voltage power quality of PCC can be restored. At last, the simulation results from SIMULINK/MATLAB have been presented to verify the validity of the proposed control strategy.

This paper proposes a point of common coupling (PCC) voltage compensation algorithm using a current limitation strategy for use in distributed generation (DG). The proposed strategy maintains the PCC voltage by prioritizing currents when an output current reference is larger than the current capacity of the power condition system (PCS) of the DG. With this strategy, the DG outputs the active current, reactive current, and the negative sequence current. The DG uses the reactive current for maintaining the PCC voltage within a normal range; the negative sequence current is used for reducing the PCC voltage unbalance. The proposed method was verified using PSIM simulation and experimental results.

Summary If sensitive loads are connected to the point of common coupling (PCC), compensation should be carried out in order to reduce PCC voltage harmonics. In this case, compensation is performed by the interface converters of distributed generation (DG) units. In this paper, a novel method is proposed for voltage harmonics compensation in islanded microgrids. The proposed method is capable of adjusting compensation percentage in proportion to the existing level of distortion at PCC. Also, this controller provides a proper sharing of compensation workload, considering the nominal capacity of DGs, which will improve the sharing of nonlinear load. Also, to decrease the asymmetry among phase impedances of microgrid, a novel structure is proposed to generate virtual impedance. At fundamental frequency, the proposed structure for the virtual impedance improves the control of the fundamental component of the power, and at harmonic frequencies, it acts to adaptively improve nonlinear load sharing among DG units. In the structures of the proposed harmonics compensator and the proposed virtual impedance, a self-tuning filter is used for separating the fundamental component from the harmonic components. This self-tuning filter decreases the number of the phase-locked loops. The simulation results in MATLAB/SIMULINK environment show the efficiency of the proposed approach in improving load sharing and decreasing voltage harmonics. Copyright © 2016 John Wiley & Sons, Ltd.

In this work, a novel control of single‐phase solar photovoltaic (PV) fed unified active power filter (PV‐UAPF) is proposed for use in low‐voltage distribution system. The shunt voltage source converter (VSC) of PV‐UAPF mitigates current power quality issues such as current harmonics, reactive current etc. while the series VSC of PV‐UAPF protects the sensitive load against point of common coupling (PCC) voltage disturbances. The shunt VSC control is based on extraction of fundamental load active current using recursive least squares with a variable forgetting factor. The proposed method improves the dynamic response of the system without sacrificing the steady‐state performance. The series VSC is controlled using a proportional‐resonant controller and it injects voltage in/out phase with PCC voltage during voltage sags/swells, respectively. The PV array which is integrated at the DC link of the PV‐UAPF through a reverse blocking diode, provides the load power demand and also supplies remaining power into PCC. The system performance is evaluated using Matlab‐Simulink software and is then experimentally verified on a prototype developed in the laboratory under steady‐state and dynamic conditions such as irradiation variation, PCC voltage fluctuations and load disturbances to validate the performance of the system.

The solar photovoltaic (PV)‐based microgrid is one of the most ideal renewable energy resources. This paper presents a utility grid intertie multi‐PV‐inverter‐based microgrid (MG) control for the solar rooftop application. The main and ancillary voltage source converters (VSCs) DC links are assimilated with PV arrays of independent maximum power point tracking (MPPT) algorithms for the economical and efficacious single‐stage configuration. At the point of common coupling (PCC), parallel VSCs arrangement increases the MG's power rating with distinctive local loads. In normal circumstances, the current control methodology is employed by the main VSC under the grid‐interactive mode. Conversely, in some grid contingency conditions under the isolated mode, its control transfers to the voltage control, which regulates the frequency and the voltage at PCC and acts like a grid‐forming inverter. The ancillary VSC operation is implemented with the current control algorithm. The main VSC supplies the active power towards the grid and accomplishes the load requirement, whereas the ancillary VSC retains its load demand in a grid‐interactive mode and also regulates the load requirement in an islanded mode of operation. The utilization of feed‐forward components for PV arrays powers in the VSCs current control techniques improves the MG dynamic performance. The seamless mode transfer performance of the main VSC is implemented via a power electronics switch. The MATLAB software is used to model a microgrid and its performance is analysed via RT‐LAB in real time through the OPAL‐RT (OP4510) controller for various scenarios, that is, varying solar insolation, load alterations and unbalanced non‐linear loads. The utilization of the modified Kwong's algorithm‐based current control enhances the power quality such as harmonics current elimination and improvement in the power factor. The voltage profile of PCC voltages is enhanced via using a modified Kwong's algorithm‐based filter in the voltage controller. The modified Kwong's algorithm has fast convergence, reduces misadjustments and gives very good performance compared to the least‐mean‐square (LMS) and Kwong's approaches. The total harmonic distortion (THD) of grid currents and PCC voltages THD in an isolated mode is found according to the limits of the IEEE‐519 standard.

In this paper an energy management central control is integrated with a feedback from the source power rather than renewable energy power feedback in conventional schemes. The proposed energy management scheme reacts faster to the load changes as compared to traditional algorithms. The energy management control generates a trip signal for tripping the circuit breakers of renewable sources battery and fuel cell. The other two renewable sources (PVA and wind farm) are always connected to the micro grid. In order to reduce the THD caused by the power electronic devices an active power filter is connected at the Point of common coupling (PCC). Active power filter control structure is introduced with feedback control from PCC voltage and current. Due to the feedback from PCC the active power filter only operates during harmonic generation which don’t happen in passive filters. All the comparisons and modelling are designed in MATLAB Simulink environment with graphical representation with respect to time.

In this paper, a new passive islanding detection method for grid-connected photovoltaic inverters is proposed. The proposed method is based on extracting signatures from the high frequency components of voltage at the point of common coupling (PCC), which occur after an islanding condition. These high frequency components contain signature information capable of identifying the islanding condition. The PCC voltage is measured and its high frequency sub-band contents are extracted using wavelet packet transform (WPT). This characterization helps to provide enough information to efficiently detect and discriminate the effect of load switching disturbances and grid voltage distortions from islanding conditions. The effectiveness of the proposed method is tested in simulation of a grid-connected inverter system. The simulation results, carried out by Matlab/Simulink are used to validate the performance of the proposed method. The results demonstrate that the proposed method can detected within one cycle (20 ms), even with small non-detection zone and can discriminate between islanding and other events in the system, as load switching disturbances and grid voltage distortions.

This paper presents a real-time simulation of a photovoltaic-based distribution static compensator (PV-DSTATCOM) using the Typhoon HIL 604 simulator, demonstrating its effectiveness in reactive power compensation, harmonic mitigation, and active power injection. The system seamlessly transitions between pure DSTATCOM mode, operating during night-time or when PV generation is zero, and PV-DSTATCOM mode, where it injects PV power while enhancing the power quality. A voltage source converter (VSC) regulated by an indirect current control strategy ensures improved power quality at the point of common coupling (PCC). Real-time simulation results under both balanced and unbalanced PCC voltage conditions validate the ability of the PV-DSTATCOM to mitigate power quality disturbances. Notably, the PV-DSTATCOM effectively reduces total harmonic distortion (THD), maintaining levels between 3.03%–5.58% under balanced PCC voltage and 3.95%–5.93% under unbalanced PCC voltage conditions, generally adhering to or remaining near the acceptable limit of IEEE-519 standards across varying solar irradiance levels and non-linear load conditions. Additionally, it maintains a near-unity power factor operation at PCC and stabilizes the DC-link voltage at 900 V (± 5%), ensuring reliable operation.