Thomas Wijnhoven

LV distribution network feeders in Belgium and power quality issues due to increasing PV penetration levels

In this paper, the impact of residential distributed energy resources (DER) on the power quality is investigated in four feeder types of the electrical LV distribution network in Flanders, Belgium. The investigated power quality issues are over-voltage, under-voltage and unbalance. The results of the simulations are discussed in detail. The paper leads to an estimation of the compliance to the power quality standard EN 50160, and a summary of issues in the distribution grids when increasing the amount of DER.

Enhanced Dynamic Voltage Control of Type 4 Wind Turbines During Unbalanced Grid Faults

The fully rated converter of type 4 wind turbines is capable of providing dynamic voltage control during grid faults by injecting controlled reactive currents. This paper describes three different dynamic voltage control options during unbalanced grid faults: 1) the positive sequence voltage control with only a positive sequence reactive current injection and suppression of the negative sequence current; 2) the positive sequence voltage control with limitation of the positive sequence reactive current injection and suppression of the negative sequence current; and 3) the positive and negative sequence voltage control with both a positive and a negative sequence reactive current injection. These different control options are compared in simulations of a wind power plant connected to a meshed power system, including synchronous generators. It is shown that both the positive sequence voltage control with limitation and the positive and negative sequence voltage control can overcome the voltage rise and voltage distortion that can occur with pure positive sequence voltage control without limitation. Both of these options have a distinct fault response, where the positive and negative sequence voltage control results in a fault response that resembles the fault response of a synchronous generator with higher fault current contributions in the faulted phases.

Control aspects of the dynamic negative sequence current injection of type 4 wind turbines

State-of-the-art distributed generation with a fully rated converter only injects positive sequence current into unbalanced faults. Recently, both publications and draft standards have suggested that the injection of negative sequence reactive current into unbalanced faults can have benefits for the power system. This paper discusses a selection of control aspects that are important for negative sequence current injection: negative sequence angle detection, converter current limitation, and voltage recovery after fault clearing. The impact of these control aspects, and negative sequence current injection in general, on the power system is also considered in the discussion.

Inverter modelling techniques for protection studies

Traditional fault calculation techniques are not capable of accurately modelling the short-circuit behaviour of inverter interfaced distributed generation (IIDG). As the share of IIDG continues to rise, more accurate fault simulations are required. This paper gives an overview of fault calculation techniques described in literature. All techniques are evaluated from the viewpoint of protection studies in medium and low voltage grids with a high share of IIDG. The averaged value modelling technique is selected. The merits of this technique are illustrated in a Simulink/PLECS simulation environment, where averaged value models are compared to full time-domain switched models in terms of simulation time and accuracy.

Fault-tolerant topology of a grid-connected PV inverter coupled by a Scott transformer

A grid-connected photovoltaic(PV) power plant is mainly based on power electronics equipments which are considered as the most vulnerable parts in a PV system. In order to increase the reliability of modular grid-connected PV panel, a solution by using a Scott transformer is presented to reduce the number of switches and to continuously operate the PV system in case of switch-failures of the power converter. The three-phase type PV inverter is analyzed in the normal and fault-operation. The simulation has shown that fault tolerance can be achieved with the proposed system configuration to give a redundancy of power switches in an Integrated power electronic module. Index Terms-Power converter, Scott transformer, redundant components, fault-tolerant, grid-connected inverter, grid-tie, reference frame, Clarke transformation.

Reliability analysis of grid concepts

Traditional techniques that evaluate the reliability of electrical grids lack the capability of providing an objective basis to assess the reliability of industrial grids. Therefore, there is an increasing demand towards probabilistic techniques that are able to create a more solid basis regarding future grid investments and to justify implementation decisions made in the grid. In this paper, the probabilistic method of the discrete absorbing Markov chain is proposed together with the matrix multiplication method to gain insight in both the reliability and the average cost of production loss due to faults in an industrial electrical grid. The merits of this technique are illustrated through two case studies in which parts of the electrical grid of the chemical production site of BASF Antwerp are simulated. From these case studies, both the weakest parts of the grid are identified as well as the average cost of production loss after one year.

Flexible fault current contribution with inverter interfaced distributed generation

This paper describes a control scheme to flexibly control the positive and negative sequence fault current contribution of inverter interfaced distributed generation (IIDG). This flexible control can be beneficial to limit the impact of distributed generation (DG) on existing grids. This way, a large scale integration of DG becomes possible in existing grids. A double synchronous reference frame (DSRF) control strategy is chosen. The phase locked loop (PLL) and current controller are based on available techniques and are described briefly. The performance of the control system is illustrated in a PSCAD simulation environment. Both symmetrical and asymmetrical faults are applied on a generic European medium voltage grid. Only the setpoints during faults have to be changed to obtain a specific fault response, creating the required flexibility.

Response of an AC - DC hybrid transmission system to faults in the AC network

Existing transmission corridors in power systems have to be efficiently utilized because new transmission systems are expensive and the realization is complex and time consuming. Upgrading existing AC transmission systems by adding DC circuits is a potential solution for increasing the transmission capacity of a corridor. It results in an AC and DC hybrid transmission system in which the converter stations at the ends must participate in the steady-state and dynamic control strategy of the interconnected power system. In this paper the control structure of the High Voltage Direct Current converter is introduced and the dynamic response of an AC and DC hybrid transmission system during AC grid faults in the interconnected power system is investigated.

Influence of voltage support by converter based distributed generation on the short-circuit power

This paper describes the influence of replacing centralised generation by distributed generation (DG) with a full converter (e.g. type 4 wind turbines, solar power) from a short-circuit power and a voltage support point of view. To allow investigating the influence the settings of the voltage support of the DG units, an iterative approach based on linear network calculations is used. Inherently, the short-circuit capability of these DG units is limited. However, it is shown that DG units can contribute to the short-circuit power of higher voltage levels to compensate (partially) for the short-circuit reduction that arises when DG units replace centralised generation. Additionally, it is verified that any increase of the short-circuit power is concentrated on the local grids where the DG units are connected. Other local grids are influenced far less. This allows to tune the voltage support settings based on the requirements of the high voltage grid, but to take into account constraints in the short-circuit power of the local grid.