Fida Hasan Md Rafi

Design of a single stage transformerless VSI in a smart microgrid for PV-STATCOM/ESS operations

In recent years, the microgrid has emerged as a promising concept and gained significant attention along with different renewable energy sources in the distribution network. Proper power coordination and strategic controller selection can extend the horizon of applications of the traditional microgrid system. This paper demonstrates the multi-operations from a single stage transformerless voltage source inverter (VSI) in the ac-dc microgrid. The designed VSI coordinates active power from the Photovoltaic (PV) system, acts as a rectifier for the dc loads, performs VAR compensation as a static synchronous compensator (STATCOM) and reduces voltage and power oscillation as STATCOM/ESS (energy storage system) operation under divergent operating conditions, such as, sudden radiation changing, random load changing, different faults effects etc. The developed microgrid model represents the commercial smart microgrid built at Griffith University, Nathan campus, which consists of PV, battery energy storage system (BESS) and single stage transformer-less STATCOM. Performance analysis of the smart microgrid model is carried out in power system computer aided design (PSCAD) /electromagnetic transient dc (EMTDC) software environment. The results prove that the designed VSI can coordinate individual power profiles distinctively maintaining system stability and the excess PV generated power contributes similarly as an ESS for STATCOM/ESS operations in the microgrid.

Reactive power management of a AC/DC microgrid system using a smart PV inverter

This paper analyses the effect of a smart voltage source inverter (VSI) embedded in a photovoltaic (PV) unit on the voltage stability/reactive power balancing of a microgrid. The conventional unity power factor PV-VSI is modified to operate for both active and reactive power management operations. The PV integrated AC/DC microgrid system is designed with a central smart VSI in PSCAD/EMTDC software environment and implemented in the Energex 11 kV distribution network, Australia. The results show superior dynamic performance and robust transient recovery from the smart VSI integrated microgrid for real life radiation variations, random loads variations, faults and intentional external disturbances.

Improved Neutral Current Compensation With a Four-Leg PV Smart VSI in a LV Residential Network

Loads in low voltage (LV) residential areas are mainly single-phase types which are supplied from a delta/wye connected distribution transformer with a grounded neutral conductor creating a three-phase (3P) four-wire (4W) distribution system. The unbalanced single-phase load distributions in traditional 3P-4W LV networks cause significant neutral currents which can result in the overloading of the neutral conductor and electrical safety concerns for customers. In this paper, the dependency of the load generated neutral current on the distribution line variable zero sequence R/X ratios is developed, and a new decentralized robust active neutral compensation method is proposed using a multifunctional transformerless 3P four-leg (4L) photovoltaic (PV) smart voltage source inverter (SVSI). System stability is verified from the bode analysis and the improved neutral compensation is evaluated from the circuitry model analysis. Actual single-phase customer loads with real sun irradiance and temperature profiles are used with an urban 3P-4W LV network model from Australia and implemented in simulation using the PSCAD/EMTDC software to verify the efficacy of the proposed controller in real world distribution networks. Different transient faults analyses and point of common coupling dynamic voltage regulation are also performed and faster fault recovery, better neutral compensation, and optimized voltage profile are achieved from the designed PV-SVSI.

Impact of controlling zero sequence current in a three-phase four-wire LV network with PV units

The dynamic impacts of controlling zero sequence current in a three-phase four-wire low voltage (LV) distribution network with a four-leg voltage source inverter (VSI) and PV installations are investigated in this paper. The PV VSI is designed to control active power at unity power factor (p.f) with additional neutral and zero sequence current controls. Inherently, most of the three-phase four-wire LV distribution networks exhibit voltage unbalance characteristics due to the connection of single- and three-phase nonlinear loads as well as occurrence of asymmetrical faults in the networks. The control over zero sequence current can improve the inherent voltage unbalance in the LV networks. The transient characteristics of the unbalanced network are investigated utilizing a widely used software environment, PSCAD/EMTDC, for the Energex 11 kV/420 V distribution network in Brisbane, Australia. The performance of the designed four-leg VSI is compared with the traditional three-leg VSI experiencing different asymmetrical faults. The results show that, with the additional degree of freedom from the four-leg VSI, faster fault recovery and lower MVA requirement can be achieved for unbalanced LV networks.

EV charging in a commercial hybrid AC/DC microgrid: Configuration, control and impact analysis

The progressive deployment of electric vehicles (EVs) and the increased penetration of direct current (DC) and alternating current (AC) type renewable resources pave the path towards hybrid AC/DC microgrid operations. Considering the potential future participation of EVs in a hybrid AC/DC microgrid environment, this paper presents a generalized synopsis on EV charging in a commercial hybrid AC/DC microgrid. Constituents of this paper encompass the charger configurations, vehicle-to-grid (V2G) control strategies of individual chargers and their effects on the microgrid operations. Impacts of three types of EV chargers are considered in this paper which include three-phase AC and DC fast charging and single-phase residential charging. All configurations and their control strategies are developed in MATLAB/SIMULINK environment. Initial simulations are carried out considering real commercial load and solar irradiation data which shows stable microgird operations. Later the impacts of various types of EV charging on the AC and DC bus voltages and frequency of the commercial hybrid AC/DC microgrid are analyzed. The obtained results highlight the implications of bulk coordinated EV charging to reduce adverse operational impacts and network investment.

Implementation of independent improved neutral current controller using four leg PV-VSI

An independent improved neutral current controller is implemented in a three-phase (3p) four-wire (4w) low voltage (LV) distribution network with a four-leg voltage source inverter (VSI) and photovoltaic (PV) installations in this paper. The PV VSI is designed to control active power at unity power factor (p.f) with additional improvement in the neutral current controller using the load generated neutral current as a direct reference. As, inherently, most of the three-phase four-wire LV distribution networks exhibit voltage unbalance characteristics due to divergent load connection types such as single- and three-phase loads, the independent control over neutral current at the point of common coupling (PCC) becomes imperative especially for decentralized controllers. The improved performance of the neutral current controller both at customers installation and at the distribution transformer (DT) terminal is investigated by implementing the designed PV-VSI with actual Australian (Energex) LV network model in PSCAD/EMTDC software environment. The performance of the designed four-leg VSI is compared with the traditional active neutral compensator with actual single phase customer loads. The results show that, with the proposed independent neutral current controller, improved neutral compensation can be achieved for unbalanced LV networks for whole day operations.

PV microgrid islanded operation analysis with the designed smart VSI

This paper analyses the islanded operations of the photovoltaic (PV) microgrid system with a designed smart voltage source inverter (VSI) for extreme and real life operations, for example, under random load variations, external severe faults, and stochastic radiation variations. The designed VSI for the PV system is modified to effectively supply the real and reactive powers both in grid-tied and islanded modes. In this proposed design, the VSI controller requires minimal modification for reference transfer from grid-tied to islanding mode and vice versa. The designed microgrid system is analyzed in PSCAD/EMTDC software environment and implemented in the real distribution network from Energex. The designed VSI shows robust fault recovery and faster oscillation damping even during islanded condition as well as transfers reference smoothly between operating modes without causing severe delays or disturbances in the distribution network.