Enhanced Virtual Synchronous Generator Control for Parallel Inverters in Microgrids
TL;DR: An enhanced VSG control is proposed, with which oscillation damping and proper transient active power sharing are achieved by adjusting the virtual stator reactance based on state-space analyses and communication-less accurate reactive power sharing is achieved based on inversed voltage droop control feature and common ac bus voltage estimation.
Abstract: Virtual synchronous generator (VSG) control is a promising communication-less control method in a microgrid for its inertia support feature. However, active power oscillation and improper transient active power sharing are observed when basic VSG control is applied. Moreover, the problem of reactive power sharing error, inherited from conventional droop control, should also be addressed to obtain desirable stable state performance. In this paper, an enhanced VSG control is proposed, with which oscillation damping and proper transient active power sharing are achieved by adjusting the virtual stator reactance based on state-space analyses. Furthermore, communication-less accurate reactive power sharing is achieved based on inversed voltage droop control feature ( V–Q droop control) and common ac bus voltage estimation. Simulation and experimental results verify the improvement introduced by the proposed enhanced VSG control strategy.
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TL;DR: In this article, the adaptive/improved droop control, network-based control methods, and cost-based droop schemes are compared and summarized for active power sharing for islanded microgrids.
Abstract: 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
593 citations
Additional excerpts
...The droop controls are the well-developed control methods without communication lines and can be used to achieve the power sharing by imitating steady state characteristics of the synchronous generator (SG) in islanded MGs [10]– [13]....
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TL;DR: In this article, the authors present a literature review of the current state-of-the-art of virtual inertia implementation techniques and explore potential research directions and challenges, and discuss several research needs, especially for systems level integration of VINs.
Abstract: The modern power system is progressing from a synchronous machine-based system towards an inverter-dominated system, with large-scale penetration of renewable energy sources (RESs) like wind and photovoltaics. RES units today represent a major share of the generation, and the traditional approach of integrating them as grid following units can lead to frequency instability. Many researchers have pointed towards using inverters with virtual inertia control algorithms so that they appear as synchronous generators to the grid, maintaining and enhancing system stability. This paper presents a literature review of the current state-of-the-art of virtual inertia implementation techniques, and explores potential research directions and challenges. The major virtual inertia topologies are compared and classified. Through literature review and simulations of some selected topologies it has been shown that similar inertial response can be achieved by relating the parameters of these topologies through time constants and inertia constants, although the exact frequency dynamics may vary slightly. The suitability of a topology depends on system control architecture and desired level of detail in replication of the dynamics of synchronous generators. A discussion on the challenges and research directions points out several research needs, especially for systems level integration of virtual inertia systems.
416 citations
Cites methods from "Enhanced Virtual Synchronous Genera..."
...Similarly, in [85] another technique of adjusting the “virtual stator reactance” of the virtual inertia unit has also been proposed to reduce such active power oscillations....
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TL;DR: The GDC's controller parameter design is more intuitive and flexible, and this paper provides a distinct design process, and the effectiveness of the proposed control method is validated by the simulation and experimental results.
Abstract: In this paper, a generalized droop control (GDC) is proposed for a grid-supporting inverter based on a comparison between traditional droop control and virtual synchronous generator (VSG) control Both the traditional droop control and VSG control have their own advantages, but neither traditional droop control nor VSG control can meet the demand for different dynamic characteristics in grid-connected (GC) and stand-alone (SA) modes at the same time Rather than using a proportional controller with a low-pass filter, as in a traditional droop control, or fully mimicking the conventional synchronous generator parameters in a VSG control, the active power control loop of the GDC can be designed flexibly to adapt to different requirements With a well-designed controller, the GDC can achieve satisfactory control performance; unlike a traditional droop control, it can provide virtual inertia and damping properties in SA mode; unlike a VSG control, the output active power of an inverter with GDC can follow changing references quickly and accurately, without large overshoot or oscillation in the GC mode Moreover, given specific controller parameters, the GDC can function as both a traditional droop control and a VSG control The GDC's controller parameter design is more intuitive and flexible, and this paper provides a distinct design process Finally, the effectiveness of the proposed control method is validated by the simulation and experimental results
312 citations
Cites background from "Enhanced Virtual Synchronous Genera..."
...Since then, different VSG control methods have been proposed to enhance system frequency stability, such as the virtual synchronous machine (VISMA)-Method2 [19], the VSG [20]–[25], the synchronverter [26], [27], and modified droop control [17]....
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TL;DR: A comprehensive review of inertia enhancement methods covering both proven techniques and emerging ones and the effect of inertia on frequency control is presented and it is concluded that advances in semiconductors and control promise to make power electronics an enabling technology for inertia control in future power systems.
Abstract: Inertia plays a vital role in maintaining the frequency stability of power systems. However, the increase of power electronics-based renewable generation can dramatically reduce the inertia levels of modern power systems. This issue has already challenged the control and stability of small-scale power systems. It will soon be faced by larger power systems as the trend of large-scale renewable integration continues. In view of the urgent demand for addressing the inertia concern, this paper presents a comprehensive review of inertia enhancement methods covering both proven techniques and emerging ones and also studies the effect of inertia on frequency control. Among those proven techniques, the inertia emulation by wind turbines has successfully demonstrated its effectiveness and will receive widespread adoptions. For the emerging techniques, the virtual inertia generated by the dc-link capacitors of power converters has a great potential due to its low cost. The same concept of inertia emulation can also be applied to ultracapacitors. In addition, batteries will serve as an alternative inertia supplier, and the relevant technical challenges as well as the solutions are discussed in this paper. In future power systems where most of the generators and loads are connected via power electronics, virtual synchronous machines will gradually take over the responsibility of inertia support. In general, it is concluded that advances in semiconductors and control promise to make power electronics an enabling technology for inertia control in future power systems.
261 citations
Cites background from "Enhanced Virtual Synchronous Genera..."
...models can further be simplified [84]–[87], [89]–[91], [93]....
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TL;DR: This study reviews the various control techniques and technologies that offset a decrease in inertia and discusses the inertia emulation control techniques available for inverters, wind turbines, photovoltaic systems, and microgrid.
Abstract: The utilization of power electronic inverters in power grids has increased tremendously, along with advancements in renewable energy sources. The usage of power electronic inverters results in the decoupling of sources from loads, leading to a decrease in the inertia of power systems. This decrease results in a high rate of change of frequency and frequency deviations under power imbalance that substantially affect the frequency stability of the system. This study focuses on the requirements of inertia and the corresponding issues that challenge the various country grid operators during the large-scale integration of renewable energy sources. This study reviews the various control techniques and technologies that offset a decrease in inertia and discusses the inertia emulation control techniques available for inverters, wind turbines, photovoltaic systems, and microgrid. This study attempts to explore future research directions and may assist researchers in choosing an appropriate topology, depending on requirements.
184 citations
References
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01 Nov 2009
TL;DR: The hierarchical control derived from ISA-95 and electrical dispatching standards to endow smartness and flexibility to MGs is presented and results are provided to show the feasibility of the proposed approach.
Abstract: DC and AC Microgrids are key elements to integrate renewable and distributed energy resources as well as distributed energy storage systems. In the last years, efforts toward the standardization of these Microgrids have been made. In this sense, this paper present the hierarchical control derived from ISA-95 and electrical dispatching standards to endow smartness and flexibility to microgrids. The hierarchical control proposed consist of three levels: i) the primary control is based on the droop method, including an output impedance virtual loop; ii) the secondary control allows restoring the deviations produced by the primary control; and iii) the tertiary control manage the power flow between the microgrid and the external electrical distribution system. Results from a hierarchical-controlled microgrid are provided to show the feasibility of the proposed approach.
4,145 citations
"Enhanced Virtual Synchronous Genera..." refers background or methods in this paper
...and tertiary control, it is still recommended to realize the basic functions of a microgrid in the primary control level without communication [2], [3]....
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...Meanwhile, the P–ω and Q–V droop controls are still valid in LV microgrid by adding inductive virtual impedance [2], [3], [9]....
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...In a microgrid, in order to share the active and reactive power according to the ratings of DGs without communication, k∗ p = (kpω0)/Sbase, k∗ q = (kqE0)/Sbase, P∗0 = P0/Sbase and Q∗0 = Q0/Sbase should be designed equally for each DG in default [2]....
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TL;DR: The idea of operating an inverter to mimic a synchronous generator (SG) is motivated and developed, and the inverters that are operated in this way are called synchronverters.
Abstract: In this paper, the idea of operating an inverter to mimic a synchronous generator (SG) is motivated and developed. We call the inverters that are operated in this way synchronverters. Using synchronverters, the well-established theory/algorithms used to control SGs can still be used in power systems where a significant proportion of the generating capacity is inverter-based. We describe the dynamics, implementation, and operation of synchronverters. The real and reactive power delivered by synchronverters connected in parallel and operated as generators can be automatically shared using the well-known frequency- and voltage-drooping mechanisms. Synchronverters can be easily operated also in island mode, and hence, they provide an ideal solution for microgrids or smart grids. Both simulation and experimental results are given to verify the idea.
2,115 citations
"Enhanced Virtual Synchronous Genera..." refers background in this paper
...parallel operation, droop characteristics are also emulated in some VSG control schemes [12], [14], [20], [21]....
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...proposed in recent literatures, such as virtual synchronous generator (VSG) [10]–[18], virtual synchronous machine [19], and synchronverter [20], [21]....
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28 Sep 1991
TL;DR: In this article, a control scheme for parallel-connected inverters in a standalone AC supply system is presented, which uses feedback of only those variables that can be measured locally at the inverter and does not need communication of control signals between the inverters.
Abstract: A scheme for controlling parallel-connected inverters in a standalone AC supply system is presented. This scheme is suitable for control of inverters in distributed source environments such as in isolated AC systems, large and distributed uninterruptible power supply (UPS) systems, photovoltaic systems connected to AC grids, and low-voltage DC power transmission meshes. A key feature of the control scheme is that it uses feedback of only those variables that can be measured locally at the inverter and does not need communication of control signals between the inverters. This is essential for the operation of large AC systems, where distances between inverters make communication impractical. It is also important in high-reliability UPS systems where system operation can be maintained in the face of a communication breakdown. Real and reactive power sharing between inverters can be achieved by controlling two independent quantities: the power angle and the fundamental inverter voltage magnitude. Simulation results are presented. >
1,550 citations
TL;DR: This paper reviews the status of hierarchical control strategies applied to microgrids and discusses the future trends.
Abstract: Advanced control strategies are vital components for realization of microgrids. This paper reviews the status of hierarchical control strategies applied to microgrids and discusses the future trends. This hierarchical control structure consists of primary, secondary, and tertiary levels, and is a versatile tool in managing stationary and dynamic performance of microgrids while incorporating economical aspects. Various control approaches are compared and their respective advantages are highlighted. In addition, the coordination among different control hierarchies is discussed.
1,234 citations
"Enhanced Virtual Synchronous Genera..." refers background or methods in this paper
...and tertiary control, it is still recommended to realize the basic functions of a microgrid in the primary control level without communication [2], [3]....
[...]
...Meanwhile, the P–ω and Q–V droop controls are still valid in LV microgrid by adding inductive virtual impedance [2], [3], [9]....
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TL;DR: In this article, a power control strategy for a low-voltage microgrid is proposed, where the mainly resistive line impedance, the unequal impedance among distributed generation (DG) units, and the microgrid load locations make the conventional frequency and voltage droop method unpractical.
Abstract: In this paper, a power control strategy is proposed for a low-voltage microgrid, where the mainly resistive line impedance, the unequal impedance among distributed generation (DG) units, and the microgrid load locations make the conventional frequency and voltage droop method unpractical. The proposed power control strategy contains a virtual inductor at the interfacing inverter output and an accurate power control and sharing algorithm with consideration of both impedance voltage drop effect and DG local load effect. Specifically, the virtual inductance can effectively prevent the coupling between the real and reactive powers by introducing a predominantly inductive impedance even in a low-voltage network with resistive line impedances. On the other hand, based on the predominantly inductive impedance, the proposed accurate reactive power sharing algorithm functions by estimating the impedance voltage drops and significantly improves the reactive power control and sharing accuracy. Finally, considering the different locations of loads in a multibus microgrid, the reactive power control accuracy is further improved by employing an online estimated reactive power offset to compensate the effects of DG local load power demands. The proposed power control strategy has been tested in simulation and experimentally on a low-voltage microgrid prototype.
1,060 citations