Michael Jordan

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.

Converter topology analysis for aircraft application

This paper deals with an aircraft weight optimization through an analysis of different load-converter-architectures. A major effort in the development of new aircraft is the reduction of the weight. This results from a direct relationship between aircraft mass and efficiency. At a first step the global onboard converter architecture is analyzed. In addition to the conventional architectures two DC-grid architectures are shown. In the second step a weight reduction through converter optimization is presented.

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.

Influence of small Constant-Power-Loads on the power supply system of an aircraft

Today most electrical consumers in an aircraft grid are electronic loads. Compared to resistive or motoric loads these loads have constant power consumption, which is independent of the supply voltage. This characteristic may have a negative effect on the stability of a power supply system. In addition real Constant-Power-Loads can have some other peculiarities. Depending of the topology or the supplied voltage type (AC or DC) the dynamic characteristic differs. This paper deals with a characterization of 4 types of Constant-Power-Loads in an actual and future aircraft grid. The investigations are focused to the dynamic characterization of different loads. Theoretical investigations are combined with measurements in an artificial aircraft grid with realistic supply voltage variations.

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.

Implementation of multiplex-switches for power feeder balancing in aircraft

Multiple approaches are available for designing more flexible and energy-efficient electrical grids on board aircraft in order to lower the weight of aeroplanes. Here, two novel concepts in aviation, dealing with intelligent switching nodes based on power semiconductors, are introduced. The balancing concepts (phase balancing and power feeder balancing) reduce the return currents and allow reduction of the power feeder diameter by symmetrising the load of all phases of the power feeders. The focus of this paper is examination of the feasibility and practical realization of multiplex-switches for implementing the two concepts. That is why a prototype multiplex-switch has been developed and tested. The implementation of the multiplex-switch, which is usable for phase and feeder balancing, is explained in detail in this paper. Different transistor types, possible realisations of bidirectional switches and the suitability of Flexible AC Transmission System (FACTS) for load balancing have been discussed. Additionally, experimental results of the prototype multiplex-switch are shown and discussed. Different switching strategies for shifting loads between different feeders have been implemented in the multiplex-switches and have been tested with different load types on a test bench.

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.

Influence of grid-connected solar inverters and mains monitoring systems on the spectral grid impedance

The impact of harmonic distortion on the power quality, caused by solar inverters, is strongly affected by the spectral characteristic of the grid impedance at the point of common coupling (PCC). The grid impedance at a PCC can be modelled as a parallel connection of the equivalent impedances of the grid structure and the solar inverter system. Yet, it is challenging to determine the exact frequency characteristics of these parameters. It requires detailed spectral models, which are often not available especially for the grid. In this contribution, both parameters are measured and evaluated in the frequency range from DC up to 20 kHz. In order to analyse the impact, the impedance is measured on a test grid structure and a PCC. In particular, this paper analyses the influence of the number of parallel-connected inverters as well as the operation point and the integrated mains monitoring system on the resulting spectral grid impedance.

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.