Hauke Langkowski

Grid impedance determination — relevancy for grid integration of renewable energy systems

As the power of renewable energy plants is increasing, their impact on the power quality becomes more and more important. The occurring grid feedback is related to the grid impedance. Therefore the exact measurement of the grid impedance is necessary. In this paper simulations of a windfarm grid integration scenario are presented that show the advantages of a grid impedance measurement in context of harmonics. In addition methods to determine the frequency dependent grid impedances are outlined and the influence of the virtual-starpoint on the calculations is examined. Based on the examinations a novel method to identify the real grid impedances is proposed.

Frequency dependent grid-impedance determination with pulse-width-modulation-signals

The power quality in electrical grids is increasingly influenced by generators and loads with power electronic grid coupling. Even comparatively small harmonic currents caused by these units can lead to unacceptable high voltage harmonics at the point of common coupling (PCC) due to highly resistive resonance points of the grid impedance. Therefore, the effective grid perturbation is defined by the frequency dependent grid impedance at the PCC. In this contribution an approach is presented and analyzed to measure the frequency and time dependent grid impedance. The measured grid impedance is an important parameter for filter and controller design and can help to evaluate power quality problems of generators and loads in advance.

Harmonic network impedance identification with pulse response analysis

The network impedance at a point of common coupling (PCC) of public mains is an essential parameter for design and control of power electronic systems connected to the PCC. Identifying its spectral characteristic is challenging, because it requires a measureable excitation of the network in the desired frequency range. Further on, the network impedance varies over time. Therefore, the identification process should be fast and repeatable. In this paper a system is presented, which allows a direct excitation of the network impedance on the medium-voltage level with defined current pulses. Short current pulse signals are evaluated and optimized in a simulation model in order to realize a precise and fast determination of the network impedance with this system.

Novel grid impedance measurement setups in electrical power systems

This contribution introduces methods and novel practical setups for time and frequency dependent impedance identification in distribution grids. These methods and measurement devices were developed at the Helmut-Schmidt-University department of electrical power systems and were successfully tested in real grids up to 20 kV.

Online network impedance identification with wave-package and inter-harmonic signals

In power system analysis the main supply at a point of common coupling (PCC) is often presented by its Thévenin or Norton equivalent. In most cases these models adequately define the electrical characteristics of a PCC and are commonly used for power flow evaluations, fault estimations, as well as stability and power quality assessments. To determine the equivalent impedance of a PCC through analytical calculation or simulation precisely may be challenging due to the complexity and time varying nature of power supply systems. Especially in medium- and low-voltage distribution networks with a high share of fluctuating consumers and generators a measurement may be required to exactly capture the equivalent impedance over time. This contribution presents a method, which can be used to identify varying equivalent parameters of an AC power system with high accuracy and time resolution. To identify the network impedance the system is excited with short sequences of current signals while measuring the corresponding voltage and current characteristics. The method is evaluated in a simulation model. Further on, impedance measurements on the low-voltage level are presented and discussed.

Grid impedance identification considering the influence of coupling impedances

In this paper the identification of grid connection points is analyzed. These points, also referred to as PCCs (Points of Common Coupling), are used to connect electrical equipment to the grid. The method described in this paper allows to set up a mathematical model for any PCC that reflects its behavior. The grid and input coupling impedances of an equivalent circuit can be determined. The obtained grid impedance can be used to assess the grid feedback, for example of wind-energy converters that feed harmonic currents into the grid.

Concept analysis of an electrical fuel cell integration in modern aircraft

The reduction of pollutant emissions and hence the increasing of the eco efficiency of future aircraft is one of the major challenges for the aviation industry. Executing the More Electric Aircraft concept can be one approach to achieve these high targets. One idea in this context is to substitute the traditional Auxiliary Power Unit by a Multifunctional Fuel Cell System. It is necessary to transform the load-dependent fuel cell stack output voltage to the intended electrical onboard grid voltage level. To increase the efficiency and reduce the system weight an optimized electrical integration is presented. Measurement results on a test bench confirm the theoretical concepts.

Determination of the capacity of grid connection points by grid impedance measurement

This contribution deals with the determination of the capacity of grid connection points in order to connect potentially more e.g. wind energy converters (WEC) to the grid. At first the determination of the short-circuit power at the fundamental frequency is outlined. Then the frequency characteristic of the grid impedance is analyzed. The focus lies on the identification of the grid impedance by measurement with a novel measurement container for the medium-voltage level. This grid impedance measurement container can be used to assess the capacity of points of common coupling (PCC) and also the harmonic emission of WEC.

Solid state switches for applications at medium- and high-voltage-level

In this article applications of solid state switches at medium- and high-voltage-level are presented. The switches have been developed to excite grid connection point with pulsed current signals in order to identify the frequency characteristic of the internal network impedance. Further on these switches can be used as switching nodes to change different grid structures as well as remote controlled load switches within a smart grid. Both switches are designed to control DC voltages. With modification however for example in an anti-serial setup they can also be applied at AC power grids.

Grid impedance identification using SiC-Jfet switch-module

The grid impedance identification is important to evaluate the effect on the power quality by renewable energy sources using inverters. It can also be used for improved design of existing grid-side filters and the control parameters for the inverters. One approach to determine the grid impedance is based on the switching of an ohmic load. The resulting transients caused by this switching can be used to calculate the frequency dependent grid impedance. In this paper the development of a switch using SiC-Jfets is presented. With this switch high switching frequencies and adjustable slopes within certain limits can be generated which is important for a precise identification of the grid impedance. Measurements of the first 6 kV prototype and the design for the required gate driver are presented.