Power quality improvements through power electronic interfaced distributed generation

In low-voltage distribution networks a large amount of single-phase nonlinear loads are connected. This leads to the combined presence of power system unbalance and harmonic distortion. The research presented in this paper focusses on these steady-state power quality problems. It uses a harmonic load flow program, implemented in symmetrical components, to investigate the influence of several single-phase inverter control strategies used to connect any kind of primary energy source to the grid. The influence of these single-phase distributed generation units in the three-phase four-wire distribution network is discussed by means of two recently formulated indicators that combine the power system unbalance and the existing harmonics

A voltage-source inverter for microgrid applications with an inner current control loop and an outer voltage control loop

Distributed generation (DG) units are commonly inter-faced to the grid by using voltage-source inverters (VSI’s). Extension of the control of these inverters allows to improve the power quality if the main power grid is disturbed or disconnected. In this paper, a control technique is developed for a VSI working in island mode. The control technique is designed in the time domain, combining an inner current control loop with an outer voltage control loop. Voltage regulation under various linear and non-linear load disturbances is studied

Overview of three-phase inverter topologies for distributed generation purposes

The increasing presence of single-phase distributed generators and unbalanced loads in the electric power system may lead to unbalance of the three phase voltages, resulting in increased losses and heating. Distribution network operators are seeking to install larger DG units (viz. $>5$kVA in Belgium) by means of three-phase connections instead of single-phase to reduce voltage unbalance. There are several possible topologies to connect the DG units to the three-phase distribution network. These topologies can be divided into three groups: the three-phase three-wire inverters, the three-phase four-wire inverters and the multilevel inverters. In this paper, an overview of the aforementioned topologies is given

Voltage Dips and Converter-Connected Distributed Generation Units

The interaction between converter-connected distributed generation units and voltage dips will become increasingly important. This paper focusses on the relation between the behaviour of converters during voltage dips and their current control strategy. A comparison is made between a recently proposed control strategy with programmable damping resistance and the classical sinewave control algorithm. The first-mentioned control structure will prove to yield an improved voltage dip immunity. Experimental tests on a single-phase full-bridge bidirectional converter are carried out and validate the aforementioned postulations. Moreover, the retained voltage at the point of connection of the DG unit will increase thanks to the implementation of the damping control strategy

Grid voltage control in islanded microgrids with inverter-interfaced power sources

The increased amount of distributed generation units connected to the distribution network has led to the development of the microgrid, which can operate in grid-connected or islanded mode. In the islanded microgrid, the mostly inverter-interfaced power sources are responsible for both voltage and power control. Therefore, adequate control algorithms for the microgrid inverters have to be developed. In this paper, an active power / grid voltage droop control strategy is applied for the power control. It is introduced as an outer control loop that determines the set-value of the inner grid voltage control loop. The grid voltage control is performed with different control strategies that are compared in steady-state and in worst-case circumstances. It is shown that the PI and fuzzy logic controller show the best overall performance in the microgrid application

Voltage control in islanded microgrids by means of a linear-quadratic regulator

The increased public interest in renewables and the development of the distributed generation (DG) technology are leading to an increasing amount of generators connected to the distribution network. New opportunities for the coordinated operation of these DGs rise with the development of the microgrid, a cluster of DGs, loads and power storage devices. The grid elements are mainly connected to the microgrid via a voltagesource inverter with dc-bus. In order to exploit the DGs in an islanded microgrid effectively, new control methods for these inverters have to be developed. Usually, an islanded microgrid is controlled by two control loops with different bandwidths. The grid voltage is controlled in the inner voltage control loop and its set value of amplitude and frequency is determined by an outer control loop. In this paper, the inner voltage control loop is studied. For the voltage control, a linear-quadratic regulator is proposed and the performance of this control method is studied