Max Domagk

Assessment of Prevailing Harmonic Current Emission in Public Low-Voltage Networks

The total harmonic current emission in a public low-voltage (LV) network is mainly determined by the large number of electronic devices, which use different circuit topologies with different harmonic emission characteristics. Consequently, the total harmonic current measured at the LV busbar can provide valuable qualitative information, for example, about dominating circuit topologies. This paper describes and applies a methodology to characterize the harmonic current emission of an LV network by a prevailing phasor. Using measurements from 130 German public LV networks, the “prevailing” location of 3rd and 5th harmonic current is analyzed with respect to different types of consumers. The results are compared with initial measurements received from other countries. Based on the prevailing phasor, a simplified method for an approximate estimation of the impact (cancellation potential) of a particular LV network or a new equipment technology (e.g., electric-vehicle charging) on the harmonic levels in a public distribution network is proposed.

Seasonal variations in long-term measurements of power quality parameters

Power quality levels in public low voltage grids are influenced by many factors which can either be assigned to the electrical environment (connected consumers, connected generation, network characteristics) or to the non-electrical environment (e.g. climatic conditions) at the measurement site. Type and amount of connected consumers (consumer topology) are expected to have a very high impact on power quality (PQ) levels. The usage behaviour of equipment by customers usually varies over the year. Subsequently the levels of PQ parameters like harmonics may show a seasonal variation. The aim of the paper is the identification and quantification of these seasonal variations. It starts with a systematic overview of major factors impacting Power Quality levels and continues with the classification of the typical types of their variation (short-term, medium-term and long-term). Next a two-stage approach based on time series decomposition is introduced in order to quantify seasonal variations by a set of indices. Finally several existing long-term measurements of public LV grids lasting more than one year are selected for a comprehensive illustration of the method. The sites represent different consumer topologies, like shops, offices and residential areas. Seasonal variations are quantified in detail for these sites and selected current quality parameters.

Trend identification in power quality measurements

Power quality (PQ) levels in public low voltage grids are influenced by many factors like type of connected customers, level of distributed generation or climatic conditions. In particular, type and number of the connected electronic equipment have a significant impact on PQ levels. Therefore, the introduction of new device technologies on a large scale, like the transition from incandescent to LED lamps, might result in long-term changes to the levels of PQ parameters (e.g. harmonics). Major aims of the paper are the identification and quantification of long-term trends in time series of continuous PQ parameters which can support network operators with the early detection of fundamental changes in PQ levels. This information can e.g. support the asset management or network planning in managing PQ levels using optimized costs. The paper begins with a systematic overview of major factors with impact on PQ levels and continues with a classification of their typical variation behavior (short-term, medium-term and long-term). The analysis of the long-term behavior (trend) starts with the extraction of a smoothed trend component based on time series decomposition. This trend component is used to quantify global trends (looking on the measurement duration as a whole) and local trends (looking on individual segments of the whole time series). Finally, the application of both methods is illustrated for selected voltage and current quality parameters using a set of three year measurements from German LV grids with different consumer configuration.

Survey on international practice of calculating harmonic current emission limits

The limitation of harmonic currents emitted by individual equipment and customer installations is essential in order to maintain harmonic voltages below compatibility or planning levels and consequently to ensure Electromagnetic Compatibility (EMC). At present a large variety of methods exists worldwide for calculating harmonic emission limits, majorly expressed as harmonic currents. As starting point for the revision of the methods currently used in Germany, a systematic search on available methods has been performed for more than 70 countries from all over the world. Finally 18 individual methods have been selected for a detailed qualitative comparison based on a set of characteristics, like voltage level, frequency range, allocation principle or network topology. The paper summarizes the results of the survey and provides an initial quantitative comparison of the selected methods based on two particular examples.

Assessment of Power quality performance in distribution networks part I - Measurement campaign and initial analysis

Power quality (PQ) is a raising concern in distribution grids of modern industrialized countries. The PQ monitoring activities of distribution system operators (DSO) and consequently the amount of PQ measurement data increases continuously. To keep the routine assessment of PQ levels efficient, new and automated tools for validating, analysing and visualizing these data are required. The paper illustrates the challenges of processing long-term PQ monitoring campaigns by using measurement data at 8 different sites in public low voltage (LV) networks for more than one year (64 weeks). The paper is divided into two parts. The first part describes the measurement campaign, the pre-processing of the data and an easy and flexible way for routine compliance assessment. The second part is dedicated to the comparison of two different global PQ indices for assessing an average site performance as basis for e.g. benchmarking purposes.

Assessment of Power Quality performance in distribution networks part II - Performance Indices and ranking of network buses

Power Quality has become one of the main measures of distribution network operation evaluation. Utilities and customers monitor the different PQ aspects for benchmarking and compatibility check. This is a second part of a paper that investigates a long term PQ measurement campaign. In this part, global PQ indices are applied to combine a number of PQ phenomena indices into one number that represents the PQ performance of a site. Two global PQ indices are applied on 64-week PQ measurements of 8 sites. The indices are compared in terms of the final rank of sites. Different levels of flexibility are introduced to the indices to consider a variable importance or priority of the phenomena and sites.

Validation of aggregated harmonic current source models based on different customer type configurations

Harmonic voltages are important voltage quality parameters defined in EN 50160. For harmonic voltage studies in electricity networks, harmonic emission of loads is often modelled as harmonic current source. In this paper harmonic current sources were parameterised on the basis of measure-ments of total harmonic current emission of several different low voltage networks. Measurements from low voltage net-works with different typical customer configurations were used: residential, offices and PV. A real medium voltage electricity distribution network of an Austrian distribution system operator, with significant consumption of residential and office customers, was chosen for this analysis. In order to automate the harmonic load flow calculations in DIgSILENT PowerFactory for every 10-minute interval of one week, a script in DIgSILENT Programming Language - DPL was developed. Harmonic voltage results from the harmonic load flow simulation are compared with the results of harmonic voltage measurements from power quality monitoring system installed in this network. The goal of this paper is to assess the suitability of the approach, where only background harmonics and key harmonic current sources are modelled. Since the approach provided good results, it can be used in future work as a basis for optimising the number and locations of power quality monitors in electricity distribution networks.

Uncertainty Evaluation for the Impact of Measurement Accuracy on Power Quality Parameters

Power quality (PQ) analysis requires to calculate a lot of different parameters (e.g. unbalance, THD, harmonic powers, harmonic impedances, …) based on measured voltages and currents (fundamental and harmonics). Respective standards and data sheets of measurement equipment define the measurement accuracies only for these measured quantities. The resulting uncertainty of parameters derived from these measurement quantities are usually not available, but are crucial for a wide variety of applications, like the assessment of prevailing harmonic phasor or the fundamental negative sequence unbalance. This paper studies the influence of the accuracy of voltage and current measurements (fundamental and harmonics) on a set of PQ parameters that are derived from them. The analysis is performed in two ways: analytical (if possible) and probabilistic based on Monte Carlo simulations. Even if the different aspects of uncertainty propagation are known and has already been studied for several of the mentioned parameters, this paper is intended to provide a summary for a as complete as possible set of PQ-parameters taking also practical accuracy characteristics of todays PQ instruments into account. The paper finishes with a classification of the impact of the measurement accuracy on the resulting uncertainty of the discussed PQ parameters and indicates conditions where the calculated PQ parameters may suffer from insufficient accuracy.

Probabilistic comparison of methods for calculating harmonic current emission limits

The limitation of harmonic currents emitted by individual equipment and customer installations is essential in order to maintain harmonic voltages below compatibility or planning levels and consequently to ensure Electromagnetic Compatibility (EMC). At present, a large variety of methods exists worldwide for calculating harmonic emission limits, majorly expressed as harmonic currents. A systematic search on available methods has been performed for more than 70 countries from all over the world. Finally 16 individual methods for low voltage (LV), medium voltage (MV) and high voltage (HV) networks have been selected for a detailed quantitative comparison based on a probabilistic approach. This paper compares harmonic emission limits of the different methods for individual customer installation and is focused on generating installations. The first part provides details about the framework of comparison; the second part summarizes the results for the different voltage levels.

Identification of consumer topologies in low voltage grids by time series analysis of harmonic currents

Power quality levels in public Low Voltage (LV) grids are mainly influenced by the network, connected consumers, the connected generating installations and climatic conditions. The connected consumers (consumer topology) are expected to have a high influence on power quality (PQ) levels. Continuous phenomena like harmonics are closely linked to a one-day-cycle, which means a more or less periodic behavior of the continuous PQ parameters. Consumer topologies like office buildings or residential areas differ in their harmonic emitting behavior. Therefore, time series analysis is a suitable tool to derive factors which allow distinguishing between different consumer topologies. The paper starts with an explanation of relevant factors influencing power quality. It furthermore describes the extensive measurement campaign, set up in order to get the necessary power quality data for the identification of the correlations. Afterwards, the paper introduces a method based on the additive component model to distinguish between different consumer topologies. Finally, the suitability of the method is presented by several example applications.