Harmonic Virtual Impedance Design for Optimal Management of Power Quality in Microgrids
Abstract : Power quality is an important concern for practical microgrid (MG) applications due to the widespread use of non-linear loads, and it is characterised by the implicit trade-off between voltage quality in the MG nodes and harmonic current sharing between distributed generators (DGs). Active and passive filters can be used to address this problem. Nevertheless, modification of the control system of the DGs represents a cheaper and more practical solution. To this end, virtual impedances can be included in the controller. However, the power quality trade-off is difficult to adjust, especially in the presence of electrical distribution lines. In this paper, an optimisation algorithm for setting the harmonic virtual impedances of the DGs in a multibus MG is presented. This optimisation algorithm can be configured for any degree of harmonic current sharing between the DGs, while improving the voltage quality is set as the main objective. Compared to other works presented in the literature, power quality can be adequately managed without relying on time-critical communications since only the values of the harmonic virtual impedances are modified. Theoretical developments were validated by simulation in a benchmark model, and in a laboratory environment.
? Most of the existing literature for primary and secondary control in microgrids is based on classical PI-PWM controllers.
? These kinds of controllers do not achieve good results in the transient response, which is highly dependent of the tuning of the corresponding parameters of the controllers.
? The interconnected mode can be considered as a hybrid mode between the connected and the islanded mode, since a main grid which imposes the references in voltage and frequency does not exist, but there can be energy exchange between the interconnected microgrids.
? This paper presents a method for minimizing the voltage distortion, while simultaneously ensuring good sharing between the DGs.
? This is done by formulating an optimization problem that minimizes the virtual impedances of the converters, where the physical feeder impedances are included as constraints.
? Now, the virtual impedances are set according to the proposed control, based on the feeder estimation and the optimization problems.
? The waveforms are more sinusoidal than in the case without any virtual impedance.
• The main objective of the proposed method is to achieve a low SF operation. The cost function is formulated to reduce the SF, and a fuzzy logic control (FLC) technique is employed to dynamically choose the weighting factors.
• The article presented in Reference proposes a novel flexible reference current generation technique by using a tuning parameter to reduce the active power oscillation flexibly.
• The proposed methodology is based on considering a desired operating point for the system state (converter current reference), an associated predefined SHE voltage pattern is obtained as a required steady-state control input reference.
? In, a design methodology for setting the virtual impedance is presented, in order to achieve P Q decoupling for effective droop control, as well as achieving sufficient stability margins and transient performance.
? Here, negative resistances are employed for improving the performance.
? However, in this work, the same virtual impedance is applied to all components of the load current.
? In order to test the performance and accuracy of the proposed sharing method, the case without any virtual impedance is used as a base case.
? Hence, in this mode of operation, the DGs control their voltages to be purely sinusoidal.
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