Inverters In Islanded Microgrids Research Articles

A Communication-Less Secondary Control Method for Parallel Grid-Forming Inverters in Islanded Microgrids Based on Mode Switching

A Communication-Less Secondary Control Method for Parallel Grid-Forming Inverters in Islanded Microgrids Based on Mode Switching

Accurate Small-Signal Terminal Characteristic Model and SISO Stability Analysis Approach for Parallel Grid-Forming Inverters in Islanded Microgrids

This paper derives a small-signal terminal characteristic model for the grid-forming inverter with droop-control in the synchronous reference ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> -) frame, accurately covering the dynamics of the power controller, the voltage and current control loops and all circuit components, where a new set of terminal characteristics is proposed to characterize the small-signal dynamics of fundamental frequency introduced by the fundamental frequency-active power droop scheme. Based on the terminal characteristics of individual inverters, the stability of parallel grid-forming inverters can be predicted by the generalized Nyquist criterion (GNC). Furthermore, to simplify the stability analysis process and reduce calculation complexity, a single-input single-output (SISO) stability analysis approach is proposed by exploring the interaction among the droop-control of parallel inverters through analyzing mathematical feature of proposed terminal characteristic model of individual inverters as well as the return ratio of the parallel inverters, which uses <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d-d</i> channel element of system return ratio matrix and Nyquist criterion to predict the stability. Finally, simulation and experimental results validate the effectiveness of proposed small-signal model and SISO stability analysis approach.

Improving voltage imbalance in inverter‐based islanded microgrids during line‐to‐line short circuits

AbstractLine‐to‐line short circuits are the transient disturbances that cause the highest currents as well as the largest voltage imbalances in inverter‐based microgrids. This paper presents a control scheme for grid‐forming inverters in islanded microgrids that limits the current to a safe value during these types of short circuits while providing a lower voltage imbalance compared to state‐of‐the‐art current limiting techniques. This control scheme is based on reducing both the amplitude of the reference voltage with the maximum amplitude of the reference current and the instantaneous reference voltage with only the inductive component of the virtual impedance. The combination of these two control actions provides low voltage imbalance, as the theoretical and experimental studies reveal. This voltage imbalance improvement is the main contribution of this paper and is maintained when the microgrid supplies linear and non‐linear loads. In the theoretical study, the paper includes control design guidelines to satisfy static and dynamic specifications. The theoretical predictions are validated by means of experimental results measured in a laboratory microgrid. A practical performance comparison of current limiting techniques based on experimental tests is also reported.

A new virtual oscillator control for synchronization of single-phase parallel inverters in islanded microgrid

ABSTRACT This paper describes the concept of virtual oscillator control (VOC) for parallel-connected single-phase inverters (SPIs). The principal idea is to introduce a series of weakly coupled oscillators (Deadzone oscillator-based VOC and Van der Pol oscillator-based VOC) that can be used for the regulation of single-phase power inverters in an islanded microgrid (MG). Its dynamic equations are used to provide the frequency and amplitude references of the inverters. In these traditional methods, there is always the presence of a 3−order harmonic in the output voltage, which causes a significant amount of 3−order harmonic current in the system. The non-linear dynamical equations of the oscillator are analyzed and its non-linear current source (NCS) is made simpler in order to develop new VOC for SPIs that can effectively get rid of the 3−order harmonic component in the oscillator’s output voltage. Finally, an extensive comparison of Deadzone-based VOC (Dz-VOC), Van der pol-based VOC (VdP-VOC), and new VOC-based controllers with linear and nonlinear loads is presented. The new VOC-based controller minimizes the 3-order harmonic component in the output voltage and achieves a quicker response compared to the conventional VdP-VOC-based controller. Simulation results of the Dz-VOC, VdP-VOC, and proposed new VOC-based control methods with different loads (Resistive, Linear RLC, non-linear) were compared and analyzed in detail. The total harmonic distortion (THD) of the current in Dz-VOC, VdP-VOC, and new VOC are 1.98%, 1.11%, and 0.21%, respectively. The 3rd harmonic is dominant in both Dz and VdP VOCs, while in the new VOC the 3rd harmonic is very less and below 0.1%. The new VOC is also showing good current sharing and fast voltage synchronization (within 0.2 s). Hardware experimentation is also carried out to analyze the efficacy of the proposed new VOC controlled SPI in standalone MG. The results clearly depict that the new VOC control strategy is quite efficient in handling the output voltage harmonics and situations of different loadings in the standalone MG.

An Advanced Droop Control Strategy for Reactive Power Assessment in Islanded Microgrids

Inverter based islanded MicroGrids (MGs) are very fascinating but challenging ways to integrate Distributed Energy Resources (DERs) in electric networks. Active and reactive power regulation is a major issue due to the necessity of optimally exploiting the sources characteristics and controlling voltage and frequency. Focusing the attention on the reactive power control, the main needs are to regulate voltage at suitable values and ensure a proper reactive power sharing among the energy sources. The conventional control for reactive power presents relevant drawbacks and so an Advanced Droop Control strategy (ADC) is presented to approach primary voltage control and reactive power sharing among Grid-Supporting inverters in islanded MGs. The ADC strategy is presented defining its control law and the control scheme with a stability analysis. Then, an analytical procedure is developed to set the controller parameters. Following, a comparison between the ADC and droop control is performed with dedicated simulations on a common benchmark Microgrid in order to highlight the advantages of the proposed control strategy. Finally, a comparison with other advanced control techniques is proposed in order to show that ADC is able to guarantee improved performance in terms of the combined regulation of voltage and reactive power.

A New Current Limiting and Overload Protection Scheme for Distributed Inverters in Microgrids Under Grid Faults

Unlike a synchronous generator that could withstand a large overcurrent, an inverter-based distributed generation (DG) has low thermal inertia, and the inverter is likely damaged by overcurrents during grid faults. In this article, a new strategy, namely positive- and negative-sequence limiting with stability enhanced <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</i> - <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> droop control (PNSL-SEPFC), is proposed to limit the output currents and active power of droop-controlled inverters in islanded microgrids. This strategy is easy to implement in the inverter controller and does not require any fault detection. Inverter stability is analyzed mathematically, which gives guidelines to design the parameters of the PNSL-SEPFC strategy. PSCAD/EMTDC simulation based on a four-DG microgrid shows that the proposed PNSL-SEPFC can limit inverter output currents and powers with better performance under both symmetrical and asymmetrical faults. Furthermore, hardware experiments demonstrate that the proposed PNSL-SEPFC can ensure the inverters riding through grid faults safely and stably. (A video of experimental waveforms is attached.)

Reactive power correction using virtual synchronous generator technique for droop controlled voltage source inverters in islanded microgrid

Reactive power correction using virtual synchronous generator technique for droop controlled voltage source inverters in islanded microgrid

Design an Accurate Power Control Strategy of Parallel Connected Inverters in Islanded Microgrids

When the overall loads change periodically, the accurate real power and reactive power sharing considered is essential for the operation of an islanded microgrid. In this work, an accurate power control strategy of parallel-connected inverters in an AC microgrid in islanded mode has been designed. The examined microgrid involves four inverters linked to the load via different system impedance conditions is performed. Also, the control strategy for islanded microgrid operation with detailed switching models is simulated by using PSCAD/EMTDC software, and all dynamic and steady-state real and reactive power sharing is explained. Moreover, the real power sharing is achieved using only the decentralized control (droop control), while the reactive power sharing control approach is proposed as a central controller (secondary controller) in order to improve the precision of fundamental reactive power sharing, gives an efficient dynamic performance, and reduce the circulating current. The central controller in the microgrid requires communications. It is used as an external loop to accomplish identical reactive power sharing according to reactive power load especially when the mismatch in voltage drops through the feeders is occurred. The control approach will still work with traditional droop control when the central controller is a failure. The obtained results show that the proposed approach is immune to the time delay in the central controller. Additionally, the active power and the reactive power is shared accurately.

A Novel Power Sharing Scheme of Controlling Parallel-Operated Inverters in Islanded Microgrids

Virtual impedances are often used in droop-controlled paralleled-voltage-sourced inverters (VSIs) to achieve proper power sharing. However, different and unknown output impedances of VSIs will affect the power decoupling and power sharing performances. To solve this problem, this article proposes a novel controller for VSIs by using a disturbance observer (DOB). The DOB is adopted to observe the output currents of VSIs without any extra current sensors. On the one hand, the observed currents are fed forward to the output of voltage compensators, thereby the output impedances of VSIs are eliminated. On the other hand, the observed currents are introduced into the virtual complex impedance loops, which not only reconstruct the impedances of VSIs but also realize the proper power sharing. Besides, to improve the reactive power sharing accuracy, a synchronization control is proposed to strictly synchronize the VSIs only by using local measurements. Each VSI operates at a constant frequency, so the inherent frequency deviation of the traditional droop control is eliminated. A universal parameter design method is also presented based on the detailed discussion of system stability and voltage quality. Finally, simulation and experimental results clearly validate the proposed control scheme and the parameter design method.

Secondary‐control‐based harmonics compensation scheme for voltage‐ and current‐controlled inverters in islanded microgrids

In this study, a coordinated control scheme is proposed for sharing harmonics compensation effort among voltage and current controlled mode (VCM and CCM) inverters in islanded microgrids. In this method, the voltage harmonics compensation of sensitive bus (SB) is achieved by using secondary control as well as virtual impedance and admittance loops in primary control of VCM and CCM units. The limited capacity of the inverter is taken into account for harmonics compensation. Photovoltaic (PV) systems are considered as CCM units. The harmonics compensation is mainly performed by VCM inverters. However, in order to prevent these units from overloading, the PV interfacing inverters (CCM units) are called to collaborate in harmonics compensation whenever needed. The results of simulation study in Matlab/Simulink show the effectiveness of this method in coordination of CCM and VCM units.

Harmonic and unbalanced voltage compensation with VOC‐based three‐phase four‐leg inverters in islanded microgrids

This study proposes a novel method that combines sliding mode control (SMC) and virtual oscillator control (VOC) to eliminate voltage harmonics and unbalance caused by non-linear and unbalanced loads in a three-phase low-voltage microgrid. The conventional VOC is modified to fit the three-phase four-leg inverter systems and a novel SMC control is designed using the high-pass filter technique and linearised super twist algorithm. The proposed control method can keep the voltage harmonics and unbalance at the point of common coupling within the IEEE standard limits, under heavily non-linear and unbalanced load conditions. The validity of the proposed method is verified by both offline simulations in Matlab/Simulink and Control Hardware-in-the Loop simulations using an OPAL-RT platform.

Local Frequency Restoration for Droop-Controlled Parallel Inverters in Islanded Microgrids

In islanded microgrids, voltage source inverters working in parallel are expected to provide regulation of the local frequency while granting active power sharing. This paper presents a local control approach at each inverter based on an event-driven operation of a parameter-varying filter. It ensures perfect active power sharing and controllable accuracy for frequency restoration without requiring the exchange of control data between inverters over the communication network. This paper includes stability analysis and design guidelines for the control parameters using a modeling approach that considers the interaction between inverters. Selected experimental results on a three-inverter laboratory microgrid corroborate the effectiveness of the proposed control scheme, and outlines its advantages with respect to previous similar schemes and the performance cost that implies not using communications.

Improved secondary control for optimal total harmonic distortion compensation of parallel connected DGs in islanded microgrids

This study proposes a two-layer hierarchical control to actualize optimal total harmonic distortion (THD) compensation in different buses of parallel-connected inverters in islanded microgrids which had not been studied so far. The proposed secondary layer is used to realize THD compensation of sensitive load bus (SLB) and make distributed generators (DGs) distribute the compensating efforts between them according to their rated capacity. It is noteworthy that improving THD at the SLB can lead to an increase in THD at local buses and/or DG terminals. Although the THD limitations of these buses are not as strict as the THD limitation of SLB, it is necessary to control them within their allowed range. This important problem is not well studied in the literature. A novel complementary part is designed and added to the secondary control to tune the compensation portion of each DG while the THD limitations in DG terminals and local buses are considered. The proposed method actualizes a multi-level voltage quality control in multi-bus islanded microgrids with parallel DGs through a simple yet effective solution. Furthermore, considering the DGs peak current limitation is added to the controller and a method for calculating this peak value is proposed.

Circulating Current Control Strategy Based on Equivalent Feeder for Parallel Inverters in Islanded Microgrid

Among parallel inverters in islanded microgrid, circulating current originated by output impedance mismatches and output voltage differences cannot be neglected. Conventional droop control and fixed virtual impedance control are sensitive when the load changes. In this paper, the concept of equivalent feeder is proposed to cover the mismatch of external feeder impedance. The differences of external inductor, local load, and transmission line among parallel inverters are considered. Based on equivalent feeder impedance calculation, an aggregated control strategy of improved droop control and adaptive virtual impedance control is introduced. To improve power sharing accuracy, the equivalent feeder voltage drop of each inverter is calculated and compensated to the reference voltage of droop control. Conventional virtual impedance control is defective to cope with sudden load perturbations, especially when local loads exist. In this regard, the adaptive virtual complex impedance control can adjust to the equivalent feeder impedance, which provides accurate and quick response to load changes and line impedance mismatches. Simulation results show that the proposed control strategy can reduce circulating current effectively as well as improve power sharing accuracy.

A Modified Droop Controller for Parallel Operation of Single-Phase Inverters in Islanded Microgrid

Conventional droop control is a basic control strategy for power sharing in islanded microgrid applications. This strategy has several limitations, such as low transient response, frequency and voltage deviations. This paper presents a modified droop control method for the proper operation of parallel-connected inverters in islanded microgrid. The proposed method is able to improve transient response and achieved higher output power without voltage and frequency deviations by introducing a power derivative term into a conventional droop method. A new method of average power computation is proposed which significantly reduces the delay encountered in conventional approaches and provides the power signal without DC components. The simulation was developed in MATLAB/Simulink to verify the effectiveness of the modified droop control scheme. Based on the results, it can be concluded that the modified droop control strategy improves the performance of parallel-connected power electronic inverters in an islanded microgrid as compared to conventional droop control strategy.

Uncovering Droop Control Laws Embedded Within the Nonlinear Dynamics of Van der Pol Oscillators

This paper examines the dynamics of power-electronic inverters in islanded microgrids that are controlled to emulate the dynamics of Van der Pol oscillators. The general strategy of controlling inverters to emulate the behavior of nonlinear oscillators presents a compelling time-domain alternative to ubiquitous droop control methods which presume the existence of a quasistationary sinusoidal steady state and operate on phasor quantities. We present two main results in this paper. First, by leveraging the method of periodic averaging, we demonstrate that droop laws are intrinsically embedded within a slower time scale in the nonlinear dynamics of Van der Pol oscillators. Second, we establish the global convergence of amplitude and phase dynamics in a resistive network interconnecting inverters controlled as Van der Pol oscillators. Furthermore, under a set of nonrestrictive decoupling approximations, we derive sufficient conditions for local exponential stability of desirable equilibria of the linearized amplitude and phase dynamics.

Compensation of Voltage Harmonics for LCL-filtered Inverters in Islanded Microgrids

Compensation of Voltage Harmonics for LCL-filtered Inverters in Islanded Microgrids

Improved Reactive Power Sharing for Parallel-operated Inverters in Islanded Microgrids

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.

Optimal Control of Droop Controlled Inverters in Islanded Microgrids

For an islanded microgrid modeled by a Kuramoto oscillators nonlinear model, we design the distributed optimal controller using the maximum principle optimization theory. We first quantify synchrony in terms of phases and droop coefficients at the inverters in the microgrid and then we maximize it. We prove that the solution of the distributed optimal control problem exists and we find it. We evaluate performance in a simulation case.

Synchronization and power sharing for droop-controlled inverters in islanded microgrids

Motivated by the recent and growing interest in smart grid technology, we study the operation of DC/AC inverters in an inductive microgrid. We show that a network of loads and DC/AC inverters equipped with power-frequency droop controllers can be cast as a Kuramoto model of phase-coupled oscillators. This novel description, together with results from the theory of coupled oscillators, allows us to characterize the behavior of the network of inverters and loads. Specifically, we provide a necessary and sufficient condition for the existence of a synchronized solution that is unique and locally exponentially stable. We present a selection of controller gains leading to a desirable sharing of power among the inverters, and specify the set of loads which can be serviced without violating given actuation constraints. Moreover, we propose a distributed integral controller based on averaging algorithms, which dynamically regulates the system frequency in the presence of a time-varying load. Remarkably, this distributed-averaging integral controller has the additional property that it preserves the power sharing properties of the primary droop controller. Our results hold for any acyclic network topology, and hold without assumptions on identical line admittances or voltage magnitudes.