Virtual Impedance Current Limiting for Inverters in Microgrids With Synchronous Generators

This paper focuses on current limiting for voltage controlled inverters during overloads caused by poor transient load sharing between inverters and synchronous generators in islanded microgrids. The use of simple current reference saturation limiters can cause instability when the voltage regulator loses control after the current reference saturates. The use of virtual impedance for current limiting is shown to provide stable current limiting when operating in parallel with synchronous generators. Small signal analysis is used to set the virtual impedance magnitude and X/R ratio, and validation is provided by simulation and experimental results.

Overload mitigation for grid-forming inverters in islanded microgrids with synchronous generators

Stability in island microgrids is crucial for efficient power distribution among distributed generation (DG) inverters. Conventional droop control, while effective in power sharing, poses challenges with voltage stability due to frequency and voltage deviations resulting from changing load power. Such deviations can lead to system instability, impacting power flows within each inverter. Therefore, this paper introduces a proposed droop control approach that effectively tackles the issues of frequency and voltage deviation, aiming to restore them to their rated values and significantly enhance transient response in power flows among inverters. The novel method incorporates integrating controllers for frequency and voltage, coupled with the utilization of virtual impedances. These virtual impedances, comprising virtual positive/negative-sequence impedance (VPI/VNI) loops at the fundamental frequency and a virtual harmonic impedance (VHI) loop at harmonic frequencies, play a crucial role in overcoming mismatched line impedance conditions, ultimately improving overall system performance. Simulation results demonstrate the effectiveness and outstanding performance of inverters operating in parallel within an island AC microgrid. The proposed approach ensures stable voltage and frequency levels in all operational states, regardless of varying load conditions.

Grid-forming (GFM) inverters are recognized as a viable solution to increase the penetration of renewable energy in bulk power systems. However, they are physically different from synchronous generators in terms of overcurrent capability. To protect the power semiconductor devices and support the power grid under severe symmetrical disturbances, the GFM control systems should be able to achieve the following requirements: current magnitude limitation, fault current contribution, and fault recovery capability. Various current-limiting control methods are reported in the literature to fulfill these goals, including current limiters, virtual impedance, and voltage limiters. This paper presents an overview of those methods. Emerging challenges that need to be addressed, including temporary overcurrent, unspecified output current vector angle, undesired current saturation, and transient overvoltage, are pointed out. Comparative simulations are conducted to demonstrate the performance of different methods under grid voltage drops and phase jumps. Finally, open issues of current-limiting control methods for GFM inverters, including transient stability assessment, voltage source behavior under overcurrent conditions, and windup of voltage controllers, are shared.

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 P-f 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.). © 1972-2012 IEEE.

An islanded microgrid (MG) including low-inertia converter-based distributed generations (DGs) is subjected to instability. The virtual inertia concept was proposed to alleviate the stability issues by imitating the synchronous generators behavior. This paper spotlights on the optimization of virtual synchronous generator (VSG) parameters and virtual impedances (VI) in islanded MGs using particle swarm optimization (PSO). A small-signal model for MG is developed at first. The permissible ranges of virtual inertia (J) and virtual damping (D) based on MG small-signal stability are scrutinized afterwards. Moreover, VI are considered to lower the reactive power mismatch between converters. Finally, considering the permitted intervals for these parameters, an optimization method and objective function are defined to calculate VSG parameters and VI in the islanded MG. The proposed optimization method enhances the small-signal stability of the MG, decreases the current overshoot and minimizes reactive power mismatches. Simulation results drawn by the “VSG + VI” control include three scenarios. The effectiveness of the proposed “VSG + VI” control method in comparison with “droop” control, “droop + VI”, “non-optimal VSG + VI”, and “VSG ” is verified through simulation studies.

This paper explores the impact of the output impedance on the power-sharing stability between parallelized microgrid inverters (MGIs) operating with virtual synchronous generator (VSG) method. In these systems, a virtual output impedance is usually added to the control loop of each MGI to improve the power sharing, regardless of line impedance unbalances. The time derivative of the MGI output current is usually used to implement the virtual impedance, which makes the system highly sensitive to the output current with high slew rate. To solve this, the virtual impedance is designed based on a cascaded second order general integrator (CSOGI) scheme, which is less sensitive to the output current noise, avoids to perform the time derivative function, achieves better output voltage total harmonic distortion. The proposed strategy is demonstrated in detail and validated with a two 100kVA-MGIs parallel system under linear and nonlinear loads.

By applying virtual synchronous generator (VSG) method in high penetration level microgrids, current limiting issue under fault condition has become an important challenge. The main objective of this paper is to propose a novel current limiting strategy which has the capability of voltage tracking error reduction to avoid voltage controller saturation. This purpose is obtained by adaptive virtual impedance-based reference voltage correction during fault condition. Furthermore, positive and negative peak detection-based current limiting method implemented in double loop control system prepares soft transition ability. Moreover, this current limiting method generates the adaptive term of virtual impedance part. In contrast to conventional VSG methods, the proposed approach provides high quality voltage and current for microgrid in both grid-connected and islanded modes during fault conditions. The offline and hardware in the loop (HIL) simulation results confirm that the peak current of the inverter is restricted to the threshold value under symmetrical and asymmetrical faults for two operating modes of microgrid.

A voltage unbalance compensation strategy based on virtual impedance for DGs in island microgrid without sequence component separation

Grid-forming (GFM) converter is a promising interface for integrating renewable energy sources. During grid faults, GFM converters inherently contribute to reactive power support, while current limiting can significantly affect the reactive power characteristics and pose challenges for grid code compliance. In this study, the impacts of virtual impedance (VI)-based current limiting on reactive power characteristics of GFM control are investigated. It reveals that the VI may cause phase offset and deterioration of the natural and instant reactive power support capability, and the subsequent reactive power response faces a coupling with the active power control due to the current magnitude limitation. Leveraging these characteristics, an enhanced fault ride-through (FRT) strategy is proposed. Firstly, the VI impedance angle is recommended to match the equivalent connection impedance, which inherits the natural power support capability of GFM converters and improves instant voltage support capability. Secondly, an FRT strategy based on dynamic active power limitation is proposed for fulfilling the grid code and enhancing the subsequent reactive power support. The effectiveness of the proposed strategy is validated through comparative time-domain simulations.

An inverter control strategy which can suppress harmonic and circulation is proposed for the paralleled multi-inverter system with non-linear loads in islanded microgrid in this paper. The proposed strategy does not require any active or reactive power calculations. Instead, a phase-locked loop (PLL) is used. The virtual impedance is set to contain the virtual negative impedance, the fundamental virtual inductance and the harmonic virtual resistance. The virtual negative impedance is set to offset the resistance component of the line impedance and reduce the system voltage drop caused by line resistance. The fundamental virtual inductance and harmonic virtual resistance is set to be put into the voltage feedback loop in fundamental domain and harmonic domain, respectively. Simulation and experimental results verify the validity of the control strategy. The control strategy improves the quality of output voltage and output current of each inverter and suppresses the circulation.

When inverters in voltage control mode are paired with synchronous generators they exhibit poor transient load sharing in islanded operation. The inverter initially picks up the majority of every load step, and the generator's output slowly increases until they reach their steady state load sharing given by droop settings. This excessive transient power output from the inverter constrains its rating compared to the largest anticipated load step, and could negatively impact battery life in battery energy storage inverters. This paper examines the transient load sharing characteristics between synchronous generators and inverters in voltage control, i.e. grid-supporting-grid-forming control, and investigates methods to improve it. An equivalent circuit is proposed to describe the initial power sharing ratio, and the effects of increased inverter droop slope and increased governor integral gain are investigated. An inverter control strategy of emulating a synchronous generator is shown to provide good transient load sharing. Validation is provided by simulation and experimental results for real and reactive loads.

Distributed power generators in islanded microgrid usually adopt droop control strategy or virtual synchronous generator (VSG) control strategy to simulate Q-U droop characteristic of synchronous generators for voltage support ability enhancement and reliability improvement. However, because the distributed power generators' capacities and locations are random, they cannot share reactive load proportional to their rated capacity, resulting in overload protection, and even system instability. In order to improve reactive power sharing precision, voltage control precision, and system stability simultaneously, an improved reactive power sharing algorithm combining virtual impedance and virtual capacitor is proposed in this paper. First, the three existing open-loop reactive power sharing strategies are summarized and the effect of their parameters on reactive power sharing error, steady-state voltage deviation, and system stability are analyzed. Afterwards, based on the analysis results, a reasonable configuration of virtual impedance and virtual capacitor values are put forward to realize the control targets of reducing steady-state reactive power sharing error, improving voltage control precision and system stability meanwhile. Finally, simulation and experimental platforms are built to verify the effectiveness of the proposed strategy.

Most microgrid converters are controlled by droop method, which could ensure microgrid converter to realize load distribution under low-speed communication. Because the microgrid line impedance is resistive, which makes the output active power and reactive power coupling each other and reduces the stability of microgrid. Introducing virtual impedance can adapt droop control for resistive lines. However, virtual impedance may lead to great voltage drop and harmonic amplification. The traditional droop control strategy were analyzed, and a droop control strategy based on voltage and current double-loop for multi-inverters parallel operation in microgrid was proposed by introduction of virtual negative resistance . By offsetting part of line resistance by the virtual negative resistance, equivalent performance can be achieved with smaller virtual impedance, which improves voltage quality. Matlab simulation confirms the effectiveness of the proposed method.

Virtual impedance, as a fault current limitation unit, is commonly employed in grid-connected converters with virtual synchronous generator (VSG) control. However, it has been reported in the existing literature that virtual impedance can deteriorate transient stability, and a detailed analysis of the impact of virtual impedance on transient performance is absent. In this paper, a power transfer model of a VSG-based converter considering virtual impedance is newly established. Based on the model, the negative effects of virtual impedance on dynamic performance and transient stability are revealed by using the phasor portrait and attraction region. Specifically speaking, virtual impedance can reduce the maximum transmission power, enlarge power angle overshoot, and shrink transient stability margin. Furthermore, an adaptive transient control scheme is introduced. This method can significantly improve transient performance via control parameters optimization. Detailed optimization of control parameters is presented. Finally, the effectiveness of the theoretical analysis and the proposed scheme are verified by simulations.