Lieven Degroote

A Control Strategy for Islanded Microgrids With DC-Link Voltage Control

New opportunities for optimally integrating the increasing number of distributed-generation (DG) units in the power system rise with the introduction of the microgrid. Most DG units are connected to the microgrid via a power-electronic inverter with dc link. Therefore, new control methods for these inverters need to be developed in order to exploit the DG units as effectively as possible in case of an islanded microgrid. In the literature, most control strategies are based on the conventional transmission grid control or depend on a communication infrastructure. In this paper, on the other hand, an alternative control strategy is proposed based on the specific characteristics of islanded low-voltage microgrids. The microgrid power is balanced by using a control strategy that modifies the set value of the rms microgrid voltage at the inverter ac side as a function of the dc-link voltage. In case a certain voltage, which is determined by a constant-power band, is surpassed, this control strategy is combined with P/V -droop control. This droop controller changes the output power of the DG unit and its possible storage devices as a function of the grid voltage. In this way, voltage-limit violation is avoided. The constant-power band depends on the characteristics of the generator to avoid frequent changes of the power of certain DG units. In this paper, it is concluded that the new control method shows good results in power sharing, transient issues, and stability. This is achieved without interunit communication, which is beneficial concerning reliability issues, and an optimized integration of the renewable energy sources in the microgrid is obtained.

Active Load Control in Islanded Microgrids Based on the Grid Voltage

In the islanded operating condition, the microgrid has to maintain the power balance independently of a main grid. Because of the specific characteristics of the microgrid, such as the resistive lines and the high degree of power-electronically interfaced generators, new power control methods for the generators have been introduced. For the active power control in this paper, a variant of the conventional droop P/f control strategy is used, namely the voltage-droop controller. However, because of the small size of the microgrid and the high share of renewables with an intermittent character, new means of flexibility in power balancing are required to ensure stable operation. Therefore, a novel active load control strategy is presented in this paper. The aim is to render a proof of concept for this control strategy in an islanded microgrid. The active load control is triggered by the microgrid voltage level. The latter is enabled by using the voltage-droop control strategy and its specific properties. It is concluded that the combination of the voltage-droop control strategy with the presented demand dispatch allows reliable power supply without interunit communication for the primary control, leads to a more efficient usage of the renewable energy and can even lead to an increased share of renewables in the islanded microgrid.

Neutral-point shifting and voltage unbalance due to single-phase DG units in low voltage distribution networks

When distributed generation (DG) units are connected to a low voltage (LV) single-phase distribution network the voltage profile will change. The connection of single-phase DG units to three-phase distribution networks will not only alter the voltage profile in the connected phase, but also the two other phase voltages will be influenced due to neutral-point shifting and voltage unbalance. The purpose of this paper is to examine the influence of single-phase DG units on the phase voltages, in different circumstances. This will be investigated by performing several simulations. In order to make these detailed simulations, an unbalanced multiphase harmonic power flow method, considering the neutral wire, will be proposed. The presented model uses the iterative forward/backward method. Furthermore, the network is solved by using symmetrical components and the prevalent convergence problems will be tackled in this paper.

Power balancing in islanded microgrids by using a dc-bus voltage reference

Recently, there has been a considerable increase of distributed generators (DGs) connected to the distribution network. Therefore, the concept of the microgrid has been developed. A microgrid is a cluster of DGs, loads and power storage devices that is typically connected to the distribution network via a single point of connection. Most DGs are connected to the microgrid via a power-electronic 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. In this paper, a control strategy based on the specific characteristics of the distribution grid is proposed. The microgrid power is balanced by modifying the set-value of the grid voltage amplitude at the inverter ac-side according to changes of the dc-bus voltage of the DGs. In this way, straight-forward power balancing and voltage control are achieved and frequent changes of the generated power are avoided.

The influence of grid-connected three-phase inverters on voltage unbalance

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. To decrease this voltage unbalance problem three-phase four-wire inverter-connected distributed generation units may be used. In this paper, the influence of the control strategy of three-phase inverter-connected distributed generation (DG) units on voltage unbalance is studied. One of the control strategies is a damping control method for a three-phase four-wire inverter for DG units which behaves resistively towards voltage disturbances independently of the input power. The other control methods are the classical sinusoidal control strategy and the resistive control strategy. The simulation results show the positive effect of the damping control strategy on voltage unbalance.

Profits of power-quality improvement by residential distributed generation

Improvement of the power quality in the utility grid can be obtained by connecting damping power-electronic devices. Regarding the increased presence of converter-based distributed generation units in the power system, an active front end for distributed generation units is extended with a mitigating function. However, in order to encourage the connection of mitigating devices to the power system, damping behaviour should be financially rewarded. In this paper, the financial benefits of connecting a damping distributed generation unit to the present power system are verified.

Nonlinear transformer model in the frequency domain and with symmetrical components

Purpose – The aim is to develop a nonlinear transformer model to achieve an accurate model to obtain the frequency components of the magnetizing current based on the harmonic voltages at the primary and secondary side. So, it can easily be implemented in a harmonic load‐flow program.Design/methodology/approach – The transformer model is based on the harmonic balance method. The electric and magnetic equations of the transformer are derived from the electric and magnetic equivalent circuits.Findings – The transformer model can be easily implemented in a harmonic load‐flow program. The accuracy of the model has been shown by comparing it with a finite element simulation. The transformer model can be used with asymmetrical supply voltages, because different saturation levels of the phases can occur. There is a coupling between the phases which can be concluded out of the asymmetrical currents in the transformer under symmetrical supply voltages.Research limitations/implications – The transformer model does n...

Influence of converter-based distributed generators on the harmonic line losses

Distributed generation (DG) units alter the power flow in distribution networks, which results in changing line losses. When converter-connected DG units are used, not only the fundamental losses will alter, but also the harmonic losses, caused by the effect of the applied converters on the power quality. When damping converters, which are designed to improve the power quality, are used, the harmonic losses are diminished. This paper deals with the influence of damping converter-connected DG units on the harmonic line losses and makes a comparison between the commonly used converters and the damping converter.

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.