The microgrid concept has been emerged into the power system to provide reliable, renewable, and cheaper electricity for the rising global demand. When the microgrids are introduced, there will be several concerns, such as active and reactive powers’ sharing, load management, connecting to the main grid, and voltage and current deviations. Recently, with the fast deployment of distributed generation, methods to mitigate microgrid harmonics caused by nonlinear loads and power electronic devices have become one of the main focus areas. Consequently, many research works are devoted to this area, introducing different harmonic mitigation methods suitable for the microgrids. Hence, the main goal of this article is to clearly present a comprehensive review of harmonic mitigation methods from a hierarchical control viewpoint. The control strategies proposed to mitigate harmonics are classified into three groups: primary, secondary, and tertiary. Furthermore, this overview draws a sketch on the global trends in harmonic mitigation methods of an ac microgrid directly applicable to today’s smart grid applications.

/pdf/review-of-harmonic-mitigation-methods-in-microgrid-from-a-1mzcgwimsn.pdf

Review of Harmonic Mitigation Methods in Microgrid: From a Hierarchical Control Perspective

Nonlinear loads (NLLs) in three-phase systems are known to produce current harmonics at −5, 7, −11, 13,… times the fundamental frequency. However, harmonics of the same frequencies are induced in microgrid voltage thereby reducing the power quality. Dedicated equipment, such as active power filters, can be used to compensate the microgrid harmonics. Alternatively, each distributed generation (DG) unit present in the microgrid can also be used for the same. The use of the virtual admittance concept combined with a proportional integral with resonant controller (PI-RES) control structure has previously been proposed as a harmonic compensation sharing strategy when multiple DGs operate in parallel. The drawback of this methodology is that a large number of RES controllers might be required to compensate for all the harmonic components induced by NLLs, increasing the tuning complexity as well as the execution time. This paper proposes the combined use of virtual admittance control loop and repetitive controller (RC) for harmonic compensation. The main advantage of the proposed method is that only one RC is required to compensate for all the harmonic components, significantly reducing the computational burden and the design complexity. The dynamic performance of the whole system is tested under variable NLL.

/pdf/design-of-a-cooperative-voltage-harmonic-compensation-ixelhp4m40.pdf

Design of a Cooperative Voltage Harmonic Compensation Strategy for Islanded Microgrids Combining Virtual Admittance and Repetitive Controller

Unbalance or asymmetry in the distribution network is a well-known power quality issue. In the modern active distribution system, with the increasing penetration of renewables, this phenomenon becomes more pronounced. In the context of microgrids (MGs), several works have been proposed for the management and mitigation of the unbalance, for both the sharing of unbalanced load and maintaining the voltage quality in the islanded mode and for the control of distributed generators in the grid-connected mode during unbalanced conditions. This study comprehensively reviews, summarises, and classifies the various strategies of the unbalance mitigation techniques for the islanded and grid-connected modes of operation for three-phase MGs and presents the possible challenges and avenues for future investigations on the topic.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-pel.2019.1080

Unbalance mitigation strategies in microgrids

Power generation through the renewable energy sources has become more viable and economical than the fossil fuel based power plants. By integrating small scale distributed energy resources, microgrids are being introduced as an alternative approach in generating electrical power at distribution voltage level. The power electronic interface provides the necessary flexibility, security and reliability of operation between micro-sources and the distribution system. The presence of non-linear and the unbalanced loads in the distribution system causes power quality issues in the Microgrid system. This paper explores and reviews different control strategies developed in the literature for the power quality enhancement in microgrids. Also comparisons of different control methods are presented with suggestions for future research.

https://ijrer.com/index.php/ijrer/article/download/6643/pdf

A Review: Control Strategies for Power Quality Improvement in Microgrid

Non-linear loads (NLLs) in three-phase systems are known to produce current harmonics at −5, 7, −11, 13… times the fundamental frequency; harmonics of the same frequencies are induced in microgrid voltage, reducing therefore the power quality. Dedicated equipment like active power filters can be used to compensate the microgrid harmonics; alternatively, each distributed generation (DG) unit present in the microgrid can be potentially used to compensate for those harmonics. The use of the virtual admittance concept combined with a PI-RES control structure has been recently proposed as a harmonic compensation sharing strategy when multiple DGs operate in parallel. The drawback of this methodology is that a large number of RES controllers might be required to compensate for all harmonic components induced by NLLs. This paper proposes the combined use of virtual admittance control loop and repetitive controller (RC) for harmonic compensation. The main advantage of the proposed method is that only one RC is required to compensate for all the harmonic components, significantly reducing the computational burden and the design complexity.

/pdf/design-of-a-cooperative-voltage-harmonic-compensation-4jg3cll9r7.pdf

Design of a cooperative voltage harmonic compensation strategy for islanded microgrids combining virtual admittance and repetitive controller

This paper investigates the inverter output voltage regulation method for islanded mode of microgrids. When DG (distributed generation) based inverters operate in islanded mode, the inverter can supply the full local load demand. Hence, the microgrid can be defined as a voltage source and regulate the output voltage. If an inverter supplies the power to the nonlinear and unbalanced loads, it generates the high order harmonics to the output voltage. Therefore, in this paper, P+multi resonant control which has large gain on the narrow band of specific frequency has been considered additionally for the performance improvement of output voltage regulation instead of the simple fundamental component control. Furthermore, the controller design method which satisfies the stability and performance of the system depending on the control gain parameters has been suggested after considering the effect on system. Improved THD performance of P+multi resonant control is verified through the PSiM simulation. It is also compared and analyzed with the fundamental component PI method and the PI method with harmonics compensation.

Output voltage regulation based on P plus resonant control in islanded mode of microgrids

This paper describes the output voltage regulation for the islanded mode of microgrid based on inverter with LCL filter. When a microgrid is transferred to the islanded mode, it has to supply the full local load demand without supporting of grid and to operate as a voltage source with an appropriate voltage regulation. Here, if the local loads are unbalanced and nonlinear, then it makes the output voltage distorted and so the performance of power quality will be degraded. In this paper, P+ multiple resonant controller which makes the control gain very high on the specific frequency range has been selected instead of PI controller for the compensation of voltage distortion due to the unbalanced and nonlinear load. The effect of voltage regulation method using P+ multiple resonant controller under the nonlinear and unbalanced condition has been verified through the PSIM simulation results. And then it also has been compared with the conventional PI and the advanced PI for harmonic compensation.

Jaesig Kim

Papers

Output voltage regulation based on P plus resonant control in islanded mode of microgrids

P+ multiple resonant control for output voltage regulation of microgrid with unbalanced and nonlinear loads