Yih-Der Lee

Development of power quality analysis platform for INER Microgrid

The deployment of ac microgrids including distributed energy resources (DERs) can reduce the investment cost of building new fossil-fuel based power plants, facilitate the carbon-oxide reduction, provide reactive power compensation and frequency regulation, and enhance the system stability. However, some undesired effects such as imbalance, voltage fluctuation, and harmonics accompanied with the operations of DERs and nonlinear loads may affect the power quality (PQ) within the microgrid system. It is thus crucial to maintain the power quality within the microgrid to a satisfactory level. This paper presents the development of a graphical user interface-based analysis platform to monitor the power quality of the INER AC microgrid.

Modelling and simulation for INER AC microgrid control

A microgrid integrates renewable energy and other forms of distributed energy resources (DER) to serve its loads. The microgrid can increase the DER penetration and improve power quality and reliability through appropriate management. Under the normal operation, a microgrid is connected to the utility grid, and each DG provides a preset power to the network through active/reactive power control (PQ control), whereas the utility grid is responsible for supplying/absorbing any power discrepancy. When the microgrid is disconnected from the utility grid, at least one unit switches to droop control from PQ control in order to regulate the voltage and frequency in the microgrid and continue to supply power to local load. This paper adopts Matlab/Simulink to model and simulate an actual ac microgrid with proposed control strategies. Several operation conditions including grid connected mode, islanding mode and load variation are simulated. Results show that the proposed control strategies for the microgrid are effective.

A study of passive harmonic filter planning for an AC microgrid

A microgrid usually consists of small-scale of thermal generation sources and other distributed energy resources (DER) such as photovoltaic, wind power, and fuel cells to serve its loads. The microgrid can increase DER penetrations and operation reliability through appropriate control schemes and energy management. When the inverter-based DERs supply electric energy to nonlinear loads in the microgrid, harmonic currents are produced and resonances in the grid may occur and causes negative impacts on some critical loads or other equipment, as well as extra power losses. To mitigate the undesired harmonic effects and improve the microgrid performance, this paper presents a study of planning single-tuned passive harmonic filters for the microgrid. Simulation results show that the designed filters can effectively mitigate the harmonic currents in the microgrid.

A simplified and automated fault current estimation approach for grid-connected low-voltage AC microgrids

For a grid-connected AC microgrid (MG), when a fault occurs at the microgrid, adaptive overcurrent (OC) or directional overcurrent (DOC) relays must be activated as fast as possible to protect all distributed generation (DG) units and loads. Fault currents inside the grid-connected AC microgrid can be significantly varied because fault current contributions from the main grid and DG units are different depending on various fault locations, fault types, and high penetration of inverter-based distributed generators (IBDGs) and rotating-based distributed generators (RBDGs) in the microgrid. A traditional fault analysis method cannot be applicable for AC microgrids with the presence of both rotating-based distributed generators and inverter-based distributed generators. Therefore, this paper proposes a simplified and automated fault current estimation approach for grid-connected AC microgrids, which is effective to quickly and accurately calculate fault current contributions from IBDGs and RBDGs and the grid fault current contribution to any faulted sections in the microgrid. This simplified and automated fault current calculation approach is mainly focused on grid-connected and small-sized low-voltage (LV) AC microgrids with the support of communication system. Under the grid-connected operation mode of the MG, fault tripping current thresholds of adaptive OC/DOC relays can be properly adjusted by the proposed fault analysis method. In particular, relying on fault current distribution coefficients of IBDGs, RBDGs, and the utility grid, pick-up currents of the adaptive OC/DOC relays in the LV AC microgrid are instantly and accurately self-adjusted according to different MG configurations as well as the operation status of DG units during the grid-connected mode.

Neural network-based approach for determining optimal reference compensation current of shunt active power filter

With the increasing use of non-linear loads, the harmonic problems have become of great concerns. One of the solutions to harmonics is the application of shunt active power filters (APFs). The APF reference compensation strategy proposed in this paper adopts the backpropagation neural network (BPNN) to train the control algorithm and calculate the reference compensation current under steady state and varying load levels. Simulation results obtained by using MATLAB/Simulink show that the proposed BPNN-based APF control strategy can effectively mitigate harmonic currents generated by the nonlinear load.

Ancillary voltage control for a distribution feeder by using energy storage system in microgrid

This paper presents ancillary voltage control for a distribution feeder by using energy storage system (ESS) in microgrid. The microgrid consists of photovoltaic, wind power, and distribution generator. The active and reactive power control, voltage and frequency droop control for the power converter of ESS are proposed to reduce microgrid impact on the distribution feeder. The voltage variation along the feeder due to change of renewable power generation is derived by executing power flow analysis. The actual voltage fluctuation on the microgrid is measured, which is used to control active and reactive power output of the power converter via energy management system in the microgrid SCADA platform. Three operation scenarios of the microgrid with ESS are tested to verify the effectiveness of the proposed control method. It is found that the power output of ESS can be well regulated to reduce voltage fluctuation on the microgid, which is complied with grid connect code to achieve better voltage control performance for the distribution feeder.

Transient Responses and Appropriate Fault Protection Solutions of Uni-grounded AC Microgrids

This paper simulates transient situations of a uni-grounded low-voltage (LV) AC microgrid through various fault tests and operation transition tests between grid-connected and islanded modes of the uni-grounded microgrid. Based on transient simulation results, available fault protection methods are proposed for main and back-up protection of a uni-grounded AC microgrid. As a result, main contributions of the paper are: (i) analysing transient responses of a uni-grounded LVAC microgrid through line-to-line faults, line-to-ground faults, three-phase fault and microgrid operation transition tests; and (ii) proposing available fault protection methods for uni-grounded microgrids, such as: non-directional or directional overcurrent protection, under/over voltage protection, differential protection, voltage-restrained overcurrent protection, and other protection principles not based on phase currents and voltages (e.g. total harmonic distortion detection of currents and voltages, using sequence components of current and voltage, 3I0 or 3V0 components).

Available Fault Protection Methods of Ungrounded AC Microgrids Evaluated by Transient Simulation Results

This paper evaluates fault protection methods of ungrounded low-voltage (LV) AC microgrids (MGs) based on transient simulation results of a typical ungrounded LVAC microgrid. By considering operation characteristics of ungrounded MGs and a literature review on existing MG fault protection solutions in recent years, possible fault protection methods are proposed for an ungrounded AC MG. Transient simulation results of an ungrounded AC MG are obtained by line-to-line (LL) and line-to-ground (LG) faults, and operation transition tests of the microgrid between autonomous and grid-connected operation modes. Based on the simulation results, advantages and disadvantages of each ungrounded microgrid protection solution are highlighted. In order to get the optimal fault protection, combinations among some or all of possible fault protection solutions of an ungrounded LVAC microgrid are found out. As a result, main contributions of the paper contain: (i) proposing and analysing available fault protection solutions of ungrounded LVAC MGs, (ii) doing the transient simulations of a typical ungrounded microgrid under different disturbance cases, and (iii) suggesting the necessary combinations among proposed fault protection solutions of ungrounded MGs.