Abstract
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.
Highlights
IntroductionMicrogrids are small-scale power systems with local distributed energy resources (DERs) for production, consumption, and storage, which can work connected to the main grid or islanded [1]
Microgrids are small-scale power systems with local distributed energy resources (DERs) for production, consumption, and storage, which can work connected to the main grid or islanded [1].Power quality issues are among the most challenging topics of controlling microgrids, especially in the islanded mode
The LV microgrid is assumed to operate in islanded mode and is comprised of a three-feeder distribution system with two electronically-coupled four-wire dispatchable distributed generation (DG) units
Summary
Microgrids are small-scale power systems with local distributed energy resources (DERs) for production, consumption, and storage, which can work connected to the main grid or islanded [1]. Power quality issues are among the most challenging topics of controlling microgrids, especially in the islanded mode. Two of the leading causes of power quality problems in microgrids are nonlinear and unbalanced loads. Nonlinear loads, such as switching power supplies, introduce harmonics into system voltage and current, and single-phase loads can cause voltage and current imbalances between the three phases of the network. The recent proliferation of nonlinear and single-phase loads in microgrids has created difficult challenges in maintaining the power quality of these networks [2].
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
