Control principles of micro-source inverters used in microgrid
© The Author(s) 2016
Received: 12 May 2016
Accepted: 16 May 2016
Published: 27 June 2016
Since micro-sources are mostly interfaced to microgrid by power inverters, this paper gives an insight of the control methods of the micro-source inverters by reviewing some recent documents. Firstly, the basic principles of different inverter control methods are illustrated by analyzing the electrical circuits and control loops. Then, the main problems and some typical improved schemes of the ωU-droop grid-supporting inverter are presented. In results and discussion part, the comparison of different kinds of inverters is presented and some notable research points is discussed. It is concluded that the most promising control method should be the ωU-droop control, and it is meaningful to study the performance improvement methods under realistic operation conditions in the future work.
Recently, with the increased concern on environment and intensified global energy crisis, the traditional centralized power supply has shown many disadvantages.Meanwhile, the high-efficiency, less-polluting distributed generation (DG) has received increasing attentions [1, 2]. Microgrids [3–5], which comprise micro-sources, energy storage devices, loads, and control and protection system, are the most effective carrier of DGs. When a microgrid is connects to the utility grid, it behaves like a controlled load or generator, which removes the power quality and safety problems caused by DGs’ direct connection. Microgrids can also operate in islanded mode, thus increase system reliability and availability of the power supply.
Proper control is a precondition for microgrids’ stable and efficient operation. The detailed control requirements come from different aspects, such as voltage and frequency regulation, power flow optimization etc. Since these requirements are of different importance and time scale, a three-level microgrid control structure is proposed in . As the foundation of microgrid control system, the primary control is aimed at maintaining the basic operation of the microgrid without communication, which has become a hot research topic recently. Since most micro-sources utilize inverters to convert electrical energy, the primary control is essentially the management of power inverters. Micro-source inverters are required to work in a coordinated manner based only on local measurements and the control strategies decide the roles of each micro-source. According to the principle of master–slave control, the micro-source inverters can be divided into grid-feeding, grid-forming, and PQ-droop grid-supporting inverters. From the perspective of peer control, the ωU-droop grid-supporting invertershelp to realize microgrids’ plug and play function. Although being widely discussed in the technical literatures, it still lacks a sufficient practical control method andexisting control technologies need to be further studied and improved. This paper describes the control principles of several typical micro-source inverters and compares their characteristics so as to provide a fundamental understanding of microgrids’ primary control.
T c(s) needs be designed in a way to ensure G c(s) have sufficient bandwidth. Meanwhile, the gain and phase shift of G c(s) around fundamental frequency should be close to 0 dB and 0 degree respectively. Therefore, the output current of the GFD inverter can track their references quickly and accurately.
For unbalanced operation cases, the GFD inverters need simultaneously controlthe positive and negative sequence currents [8, 9]. Under such condition, using PR controller  in αβ reference frame might be a better choice as a single PR controller can regulate both the positive and negative sequence currents, and the control effect is similar to that of using two PI controllers in double positive/negative dq reference frames.
According to the above analysis, the GFM inverters can also precisely control their inductor current by a properly designed inner current loop. The impact of the grid current on capacitor voltage is removed by current feedforward and thus, u is fully controlled by adjusting i.
the line impedance of a low-voltage microgrid has a large resistive component, thus P-ω and Q-U droop control is no longer suitable.
the voltages at the PCs of each inverter are not completely equal, thus the GS inverters cannot share reactive power precisely.
Decoupling transformation methodAs depicted in Fig. 6, the voltage at the PC of theωU-droop GS inverter is denoted by U∠δ, and the voltage at the microgrid bus is denoted by E∠0. ZL is the line impedance between the inverter’s filter capacitor and the microgrid bus with an impedance angle of θ.
Virtual impedance method
where G u(s) is the voltage closed-loop transfer function of the ωU-droop GS inverter, and Z V is virtual impedance.
Reactive power sharing method based on communication
To improve the reactive power sharing accuracy, a common method is to revise the GS inverters’ droop control parameters, including no-load voltage and droop coefficient. The following analysis takes the inductive line (cosθ ≈ 0,sinθ ≈ 1) as examples. According to Eq. (11), the relation between the output reactive power and the voltage of the GS inverter’s PC is shown as:
In the Q-U plane, the intersection of the operational curve described by Eq. (20) and the reactive power droop curve is the GS inverter’s stable operating point .
In this method, the output reactive power of each GS inverter is independent to the line impedance Z L. By delivering the voltage information of the microgrid bus to each GS inverter, accurate reactive power sharing can be realized. This method doesn’t require a central controller to participate, avoiding the usage of complicated algorithms. Besides, the additional parameter, K u, can be used to adjust the dynamic response of reactive power control.
Results and Discussion
As can be seen from the above sections, the GFD inverter behaves as constant power source and it participates neither in voltage regulation nor in load variations sharing. The GFM inverter behaves as constant voltage source and it is responsible not only for maintaining the microgrid’s voltage and frequency, but also for keeping power balance. Load sharing among the GFM inverters is a function of the impedances between the inverters and microgrid bus. The PQ-droop and ωU-droop GS inverters can be regarded as the upgraded version of the GFD and GFM inverters, and they behave as controlled power source and controlled voltage source, respectively. When the microgrid operation conditions change, they can adaptively adjust the output power or voltage to achieve a more flexible load sharing. Currently the most promising control method is the ωU-droop control, because it can make the system autonomy and achieve seamless mode switching. When the microgrid is operated in islanded mode, any addition or reduction of a single ωU-droop GS inverter do not influence the configuration of the original system. When the microgrid operated in grid-connected mode, the ωU-droop GS inverter can output the specified power by modifying its no-load voltage and frequency. However, this autonomous control method is not widely applied among numerous experimental microgrids, because there still exist many practical problems, such as the dynamic response speed, the impact of control parameters on system stability, the capability to deal with unbalanced and non-linear loads, and control strategies under fault conditions. In addition, it can be seen from the above analysis that the performance of the ωU-droop GS inverter operating with no communication is inferior. In order to enhance the accuracy of reactive load sharing, it is worthwhile to study the design of the control algorithms with reduced communication requirements.
This paper illustrates the control principles of micro-source inverters, including grid-feeding, grid-forming, and grid-supporting inverters. The PQ-droop and ωU-droop grid-supporting inverters can be regarded as the upgraded version of grid-feeding and grid-forming inverters with a more flexible load sharing capability. Since the conventional ωU-droop control exists some shortages, several improved methods of ωU-droop based grid-supporting inverters are presented. The comparison of various inverters is carried out and the valuable research points are also discussed.
This work was supported in part by Nation Natural Science Foundation of China (51407128) and the key technologies research project on distribution network reconfiguration of State Grid Hunan Electric Power Company (5216A1300JV).
The authors declare that they have no competing interests.
WG and LM conceived and designed the study. WG wrote the paper. All authors read and approved the final manuscript.
About the authors
W. M. Guo was born in 1989 in Hunan, China. He received his B.S. degrees in electrical engineering from Tongji University in 2011, where he is currently working towards a Ph.D. degree. His current research interests are microgrid protection and control.
L. H. Mu was born in 1963 in Jiangsu, China. He is currently a full professor in the Department of Electrical Engineering, Tongji University, Shanghai, China. His current research interests include protective relaying of power system, microgrid and power quality.
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