Virtual Impedance Control Research Articles

Distributed Control With Virtual Capacitance for the Voltage Restorations, State of Charge Balancing, and Load Allocations of Heterogeneous Energy Storages in a DC Datacenter Microgrid

This paper proposes a distributed coordination control strategy for load sharing and energy balancing between heterogeneous energy storages. These control objectives are satisfied through a two-level control structure. At the primary level, the decentralized virtual impedance control, without the requirement of communication links, allocates the low frequency component of the loads to batteries, while the high frequency component is allocated to ultracapacitors. Distributed control strategy, introduced at the secondary level over a sparse communication network, achieves battery state of charge balancing and regulation of the local bus voltages. Two sets of data are exchanged via the communication links, the local bus voltages, and state of charges of batteries. The distributed controller for the restoration of local bus voltages implements an average consensus protocol, while the controller for energy balancing uses a cooperative protocol. In addition, the ultracapacitor voltages are locally restored at a slower time-scale. The proposed control strategy is resilient to communication link failures and features plug-and-play capability. Presented results demonstrate performance of the proposed control strategy for an islanded 380 VDC datacenter with variable loads. Different operating conditions are verified through the RTDS Technologies real-time digital power system simulator using switching converter models and nonlinear battery models.

Distributed secondary control based on cluster consensus of inhibitory coupling with power limit for isolated multi‐microgrid

Multiple nearby isolated microgrids (MGs) can be interconnected to become multi-MGs (MMGs) and meet larger bulk power demands, thus improving reliability/security of MMGs and reducing the power supply investment costs. However, a multi-layer control structure and complex communication network are inevitable, which increase communication delays and complicate the control structure. To solve this problem, this article introduces a distributed secondary control (DSC) based on cluster consensus of inhibitory coupling with a power limit scheme. Compared with an inter-cluster communication coupling structure, the communication channels of the proposed scheme are minimised to simplify network topology, decrease the communication delays and control dimensions. The proposed control strategy reaches multi-valued consensus states for accurate active/reactive power sharing in different MGs. Furthermore, active/reactive power overloading in a MG cluster is discussed under an inhibitory coupling configuration. Based on the power limit scheme, each distributed generator (DG) of the overloaded MG is controlled to generate power within the rated power range for high local autonomy. In addition, an adaptive virtual impedance control is introduced to decrease voltage deviations between DGs. The analytical results are verified by a simulation in MATLAB/Simulink.

Modeling and Stability Analysis of Inverter-Based Microgrid Under Harmonic Conditions

Microgrids (MGs) operate under harmonic conditions due to the integration of nonlinear loads. The autonomous harmonic compensation control of inverter-interfaced DG has been proposed to successfully mitigate the harmonics. However, the small-signal analysis of harmonic compensation controls has not been investigated in the microgrid with multiple inverters. This paper develops the modeling and analysis of the inverter-based MGs under harmonic conditions. The concept of dynamic phasor (DP) is used to describe the fundamental and harmonic components of an ac waveform via dc variables. The developed model consists of droop-controlled distributed generators (DGs), diodes rectifiers (working as nonlinear load) and resistance loads. Virtual impedance control is considered in the droop-controlled DGs for the autonomous harmonic compensation. Based on the developed DP model, the dynamic behavior of the microgrid is investigated via small-signal analysis. It is observed that the virtual impedance for harmonic compensation brings inter-inverter oscillations on harmonic domain. Participation and eigenlocus analysis are performed to investigate the influence of parameter tuning of harmonic compensation on microgrid stability. Numerical simulations are carried out to validate the effectiveness of the proposed modeling method and the analysis results.

A Virtual Impedance Control Strategy for Improving the Stability and Dynamic Performance of VSC–HVDC Operation in Bidirectional Power Flow Mode

It is a common practice that one converter controls DC voltage and the other controls power in two-terminal voltage source converter (VSC)–based high voltage DC (HVDC) systems for AC gird interconnection. The maximum transmission power from a DC-voltage-controlled converter to a power-controlled converter is less than that of the opposite transmission direction. In order to increase the transmission power from a DC-voltage-controlled converter to a power-controlled converter, an improved virtual impedance control strategy is proposed in this paper. Based on the proposed control strategy, the DC impedance model of the VSC–HVDC system is built, including the output impedance of two converters and DC cable impedance. The stability of the system with an improved virtual impedance control is analyzed in Nyquist stability criterion. The proposed control strategy can improve the transmission capacity of the system by changing the DC output impedance of the DC voltage-controlled converter. The effectiveness of the proposed control strategy is verified by simulation. The simulation results show that the proposed control strategy has better dynamic performance than traditional control strategies.

Virtual Synchronous Generator Grid Connected Control Method Based on Virtual Impedance

The virtual synchronous generator technology is used to simulate the working principle of the synchronous generator, which provides the inertia and damping support for the power grid. At present, the measurement of the phase, amplitude and frequency of the virtual syn-chronous generator is used to measure the output voltage of the distributed generation system. To solve this problem, this paper proposes a virtual synchronous machine grid virtual impedance control method based on the virtual impedance calculation of virtual current in the virtual synchronous machine from the network state and the grid between the assumption of power exchange, puts forward the pre synchronization strategy based on virtual impedance, and gives the simulation results. The results show that compared with the existing methods, the proposed method is more practical and more smooth.

Development of a 10-kV regional compensation dynamic voltage restorer with multiple functions

ABSTRACTThe first 10-kV regional compensation dynamic voltage restorer (DVR) has been installed in a 110-kV substation in Shenzhen power grid, to protect plenty of sensitive loads in a wide area simultaneously from the disturbances caused by voltage sags. The location selection and performance indicators of the DVR are determined based on the voltage sag statistic results in recent years of Shenzhen power grid. The implementation schemes of main circuit elements, such as the energy storage unit, inverter and LC filter are presented. To further improve the DVR’s cost-effectiveness, a novel virtual impedance control strategy is proposed to provide the DVR with a function of a Fault Current Limiter (FCL) or a Series Compensator (SC) besides the function of voltage sag compensation. Simulation and field experimental results are provided to show the correctness and effectiveness of the main circuit design and control system development of the 10-kV DVR.

Stability Analysis of PV Plant-Tied MVdc Railway Electrification System

As ac electric railways have various problems (e.g., power quality issues, complex catenary networks, limited supply region, and passing neutral section), the medium-voltage direct current (MVdc) railway electrification system (RES) may be a potential alternative electric railway mode and can be freely accessed to the renewable energy systems. However, some instability issues may be aroused due to the interaction between flexible MVdc RES and electronic-based dc–dc equipment. In this paper, a comprehensive small-signal impedance model, combining a photovoltaic (PV) plant and an MVdc RES, is performed, while available electric variables and control parameters are taken into consideration. Then, a novel passivity-based stability criterion (PBSC) is introduced to determine the instability issue about associate influential factors, including proportional and integral parameters of traction substation and PV converters, the power of train converters, and the dc catenary length. A virtual impedance control scheme is designed to stabilize the system. The experimental results demonstrate that the PBSC can identify the instability in the MVdc RES. The results also match well and support the conclusions obtained.

DAVIC: A New Distributed Adaptive Virtual Impedance Control for Parallel-Connected Voltage Source Inverters in Modular UPS System

In this paper, an average active power sharing control strategy based on the distributed concept for the parallel operation of voltage source inverters is proposed to be applied to the modular uninterruptible power supply (UPS) systems. The presented method is named distributed adaptive virtual impedance control (DAVIC), which is coordinated with the droop control method. Low bandwidth CAN-based communication is used for the requirement of data sharing of the proposed method in the real modular UPS system. Unlike the conventional virtual impedance control techniques, the virtual impedance of a converter module is adjusted automatically by using global information when DAVIC is applied, further to tune the output impedance of the power modules. The adaptive virtual impedance is calculated by using the difference between the active power of a local module and the average active power of all the modules in a modular UPS. The DAVIC overcomes the drawback of the conventional virtual impedance control since an accurate value of the real output impedances of different converter modules is not required. Simulations using PLECS and experiments on a real commercial modular UPS are developed to verify the effectiveness of the proposed control methodology. These results shown a superior power sharing performance is obtained when using the proposed method.

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.

Energy Management and Coordinated Control Strategy of PV/HESS AC Microgrid During Islanded Operation

An energy management control strategy is proposed for an islanded AC microgrid with the hybrid energy storage system, including the battery and the supercapacitor (SC). According to the state of charge of the battery, the photovoltaic system can work in either maximum power point tracking mode or load power tracking mode to prevent the battery from over charging. Similarly, the load shedding control is adopted to prevent the battery from over discharging. A virtual impedance control strategy is proposed to achieve effective power sharing in hybrid energy storage systems, where the battery provides steady state power and the SC only supports transient power fluctuations. The terminal voltage of the SC can be restored to the initial value automatically by introducing a high-pass filter in the voltage control loop. The AC bus voltage remains constant using the voltage secondary controller to compensate the voltage drop caused by the virtual impedance control strategy. The simulation results under typical working conditions verify the correctness and effectiveness of the proposed control strategy.

Enhanced control and parameter analysis for virtual synchronous generators without PLL

Currently, virtual synchronous generator (VSG) technology is proposed to be a solution to stability controlling of micro-source inverter. However, the deviation from phase locked loop (PLL) influences the reference frequency calculating which is important to the VSG control system. This paper focuses on the virtual inertia controller in the VSG control system, an enhanced controller is achieved for avoiding the effect of PLL on the VSG dynamics. For analysing dynamic characteristics of the enhanced VSG control system, the linearised small-signal model of VSG in isolated operation is well established, including electrical system, enhanced virtual inertia controller, voltage and current controller, virtual impedance, and droop controller. By small-signal stability analysis, the influence rules of the main parameters (load, line, virtual inertia and proportional-Integral gain) on VSG system impacts are provided, which helps identifying possible feedback signals for controller design to improve the system stability. Results obtained from time-domain small-signal model are compared with those from the electrical simulation to verify its accuracy.

Adaptive complex virtual impedance control scheme for accurate reactive power sharing of inverter interfaced autonomous microgrids

The mismatched feeder impedance of the voltage-sourced converter (VSC) units may result in reactive power sharing error in the islanded microgrids. This study presents a reactive power sharing strategy for single-phase inverter-based microgrids that eliminates the circulating currents and reduces the coupling between real and reactive powers. The proposed controller is based on an adaptive complex virtual impedance that aims to equalise the E - Q characteristic output of VSCs. The resistive term regulates the virtual voltage drop using the proposed ΔR v - P droop characteristic, while the inductive one controls the phase angle of VSC current and increases the X/R ratio of the microgrid. The proposed strategy does not require a central controller and knowledge of the microgrid lines and loads data and is applicable for the various microgrid topologies. Furthermore, it employs the unidirectional low bandwidth communication link and is robust against the delay and failure of communication channels. The proposed method is implemented using the conventional droop control scheme and presents the medium plug and play capability. Both simulation and experimental case studies using various microgrid topologies are conducted to demonstrate the merits of the proposed scheme.

Unbalanced Current Sharing Control in Islanded Low Voltage Microgrids

This paper reports a new control strategy to improve sharing of unbalanced currents in islanded LV microgrids. This technique provides fast and effective sharing of positive-, negative- and zero-sequence currents, and is the first example of zero-sequence current sharing in the literature. The controllers are designed in the stationary frame. The control structure consists of four loops: (1) the current controller; (2) the voltage controller; (3) the droop controller and the (4) negative and zero sequence current controllers. The output current is considered unknown for the controller and is added to the control system as a disturbance. The proposed controller features a high gain in fundamental and harmonic frequencies, hence a good voltage quality is obtained in the presence of unbalanced and nonlinear loads. To this aim, a proportional-resonant (PR) controller is adopted as the current controller. By using a multi-resonant controller as current controller, a unified control structure is obtained which is suitable for both grid-connected and islanded modes. The voltage controller is designed using a resonant controller so that the voltage can have low VUF and THD in the presence of unbalanced and nonlinear loads. Furthermore, in this paper, the droop method is applied to the control structure to share real and reactive powers. Simulation studies show that the conventional droop method cannot share the oscillatory part of the output power that is due to the presence of unbalanced loads in the microgrid. This paper relies on using zero and negative sequence virtual impedance controller to share the oscillatory part of output power. By using zero-sequence virtual impedance controller (ZSVIC) and negative-sequence virtual impedance controller (NSVIC), the zero and negative sequence currents in the microgrid are controlled and shared effectively. By compensating zero- and negative-sequence currents locally, the flow of these currents in the microgrid is minimized, and the overall power quality of the islanded LV microgrid is improved.

A controllable inductive power filtering system: modeling, analysis and control design

This paper presents a controllable inductive power filtering (CIPF) system with the novel filtering mechanism. Due to the zero-impedance design of the power transformer, the harmonic magnetic flux between the secondary load winding and the customized filtering winding can be balanced each other. Hence, the contaminated area of harmonics is reduced. The addition of the active filter contributes to further improve the filtering performance with the task of harmonic isolation between the power grid and the load. First, the three-phase unified equivalent circuit model and the mathematical model are given out at the harmonic domain. Based on the deduced transfer matrix, the filtering evaluation criterion of CIPF system is established, and the filtering principle is revealed, as well. According to the special operational law, an improved virtual impedance control strategy is designed to prevent the potential harmonic amplification, and the relationship between the phase/magnitude of the virtual impedance and the filtering performance is discussed. Finally, the experimental results based on the laboratory scaled prototype show the validity of the CIPF system with improved virtual impedance control.

Stabilization of a Cascaded AC–DC–DC System With Small Bus Capacitance Based on Small Signal Analysis

In this paper, the source converter (voltage source rectifier) of a cascaded system is modeled based on small signal analysis, and the closed-loop transfer functions of the converter are obtained. Then, the output impedance of the source converter is calculated. The analysis of the output impedance indicates that the smaller the bus capacitance, the worse the stability of the cascaded system, and the smaller the power converted by the cascaded system. Three kinds of virtual impedance control methods are proposed to reduce the output impedance of the source converter; all of them improve the stability and increase the maximum power of the cascaded system. The parameters of the three kinds of methods are designed and their effects are compared. Simulation and experimental results show a good agreement with the theoretical analysis, which proves the validity of the virtual impedance control methods.

Resonance Propagation and Elimination in Integrated and Islanded Microgrids

In this paper, a micro grid resonance propagation model is investigated. To actively mitigate the resonance using DG units, an enhanced DG unit control scheme that uses the concept of virtual impedance is proposed. It can be seen that a conventional voltage-controlled DG unit with an LC filter has a short-circuit feature at the chosen harmonic frequencies, whereas a current-controlled DG unit presents an open-circuit characteristic. The application of underground cables and shunt capacitor banks may introduce power distribution system resonances. This paper additionally focuses on developing a voltage-controlled DG unit-based active harmonic damping technique for grid-connected and islanding micro grid systems. An improved virtual impedance control method with a virtual damping resistor and a nonlinear virtual capacitor is proposed. The nonlinear virtual capacitor is used to compensate the harmonic dip on the grid-side inductor of a DG unit LCL filter. The virtual resistance is principally answerable for micro grid resonance damping. The effectiveness of the proposed damping method is examined using each a single DG unit and multiple parallel DG units<strong>.</strong>

Distributed Finite-Time Convergence Control of an Islanded Low-Voltage AC Microgrid

The control of an islanded AC microgrid with high penetration level of renewable generations is complicated and challenging due to its low inertia and fast changing operating conditions. To address the drawbacks of traditional droop control with low-voltage AC microgrid, an adaptive virtual impedance control is proposed to improve the active/reactive power sharing accuracy. To deal with the frequency and voltage deviation caused by the primary control while satisfying the fast response requirements and various users' preferences, a flexible distributed control strategy is proposed as the secondary control. The proposed control strategy can restore the frequency and voltage magnitude to the desired values in a finite-time manner, using only local and neighboring information. The finite-time convergence of the proposed distributed control algorithm is guaranteed through rigorous analysis. Simulation studies validate the effectiveness and scalability of the proposed distributed control strategy.

Distributed virtual negative-sequence impedance control for accurate imbalance power sharing in islanded microgrids

Abstract Distributed generations (DGs) can be used to enhance the power quality in islanded microgrids. However, if the DG line impedances are mismatched, the imbalance power for voltage unbalance compensation at the point of common coupling (PCC) cannot be accurately shared among the DGs. In order to overcome this problem, this paper proposes a distributed virtual negative-sequence impedance control for realizing the accurate imbalance power sharing. The consensus algorithm is used to adaptively regulate the virtual negative-sequence impedance. The control approach does not require the knowledge of the line impedance and central controller. Simulation and experimental results are shown to demonstrate the effectiveness of the proposed method for achieving the accurate imbalance power sharing to compensate the voltage unbalance at the PCC.

Hybrid AC/DC System Harmonics Control Through Grid Interfacing Converters With Low Switching Frequency

The nonlinear loads in distribution grid may generate harmful low-order harmonics, polluting the grid and deteriorating the voltage quality particularly when the grid is weak. The grid interfacing converters in the distribution system, such as the distributed generation (DG) interfacing converters or hybrid ac/dc grid interlinking converters, can participate in distribution grid harmonic control. In this paper, two virtual-impedance-based harmonics control methods are developed for grid interfacing voltage-source inverters (VSIs) to improve the power quality of the distribution grid. As the control parameters are designed based on the virtual impedance theory, clear physical meanings are given to explain their impacts on the system. Also, the proposed methods do not rely on the closed loop feedback control and, therefore, are very suitable for VSIs with low switching frequency (such as those for high-power DGs or interlinking converters for hybrid ac/dc systems), whose closed-loop control feedback bandwidth may be limited for harmonic regulation. The influence of system delay and feedback control loop is considered and modeled in the design procedure; thus more accurate virtual impedance control is achieved for low-switching-frequency VSIs. Moreover, a comprehensive comparison of the two methods, including stability and harmonic compensation performance, are given. Their effectiveness is verified by experiment results.

Fully Distributed Hierarchical Control of Parallel Grid-Supporting Inverters in Islanded AC Microgrids

In this paper, a fully distributed hierarchical control strategy is proposed for operating networked grid-supporting inverters (GSIs) in islanded ac microgrids (MGs). The primary control level implements frequency and voltage control of an ac MG through a cascaded structure, consisting of a droop control loop, a virtual impedance control loop, a mixed ${H_2}/{H_\infty }$ -based voltage control loop, and a sliding-mode-control-based current loop. Compared to conventional proportional-plus-integral-based cascaded control, the proposed cascaded control does not require a precise model for the GSI system. The proposed secondary control level implements distributed-consensus-based economic automatic generation control and distributed automatic voltage control, which integrates the conventional secondary control and tertiary control into a single control level by bridging a gap between traditional secondary control and tertiary control. Simulation results demonstrate the effectiveness of the proposed hierarchical control strategy.