Public Low Voltage Research Articles

Impact of flat-top voltage waveform distortion on harmonic current emission and summation of electronic household appliances

Electronic household appliances are non-linear loads and emit harmonics into the low voltage networks. Usually, these loads are simply modelled by constant current source models, which only represent the harmonic emission of the appliances for a single supply voltage distortion, mostly sinusoidal conditions. Measurements have shown that the harmonic currents emitted by electronic devices can significantly depend on the circuit topology and the existing supply voltage distortion. This paper studies the impact of supply voltage distortion, which can be typically found in public low voltage networks (so-called flat-top voltage waveform), on the harmonic current emission of individual devices with different circuit topologies as well as the impact of supply voltage distortion on the harmonic summation of these devices.

Impact of a High Penetration of Electric Vehicles and Photovoltaic Inverters on Power Quality in an Urban Residential Grid Part I – Unbalance

This paper presents a measurement-based assessment of the impact of a high penetration of plug-in electric vehicles with on-board charger (EVC) and photovoltaic inverters (PVI) on Power Quality in a public low voltage (LV) network. The paper consists of two parts. This first part is focused on the impact on unbalance, the second part is related to harmonics. The results are based on extensive measurements in an urban residential grid with almost 100 households (HH), 43 PVIs and 36 EVCs. The impact of PVI and EVC on unbalance at different levels of aggregation is described in detail. At first the individual impact of a single PVI and EVC is discussed for different operating conditions. In particular, a sunny day, a cloudy day and hours during the night are used for PVI, while periods of charging and periods of no charging are considered for EVC. Next sections analyse the interaction between PVI, EVC and residual load at household level as well as the overall impact of PVIs and EVCs in the low voltage grid. The analysis considers current unbalance and voltage unbalance.

Framework to assess the stable operation of commercially available single-phase inverters for photovoltaic applications in public low voltage networks

This paper presents a framework to assess the grid compatibility of single-phase inverters connected to public low voltage network, which is characterized by a stable operation at reasonable emission. The assessment is based on considering emission, immunity and stability. The state of the art for commercially available devices relies only on the classical, impedance-based stability analysis based on the Nyquist criterion. However, experiences have shown that this is not sufficient for a statement towards the stable operation of an inverter, since the stable operation can also be affected by other factors than the network impedance, e.g. a background voltage distortion. Exemplary laboratory measurements of two commercially available single-phase PV inverters are presented to validate the statement by showing that despite the impedance-based stability criterion is met, inverters can still trip in presence of a high background voltage distortion. The paper concludes that the immunity of the inverter with regard to the voltage distortion at the point of connection (PoC) is additionally of importance for its stable operation.

Measurement-Based Identification of DC-Link Capacitance of Single-Phase Power Electronic Devices for Grey-Box Modeling

This article presents a non-destructive measurement-based identification of the dc-link capacitance in single-phase power electronic devices for modeling purposes. These devices are typically found in public low voltage networks for low power applications. A large number of manufacturers of these devices with individual approaches to the design of the components of the power electronic devices challenges the modeling and thus the analysis of their interaction with other devices and the low voltage network. Since the manufacturers typically do not disclose their design, only little information based on the assumptions can be made with regard to an individual device. This article provides a new method to identify the dc-link capacitance of single-phase power electronic devices based on analyzing the ripple part of the dc-link voltage, which contributes to the present research activities toward realistic modeling of such devices for power systems studies. The analytical background is provided and a simulative validation is performed. Finally, a laboratory application to a commercially available single-phase inverter for photovoltaic systems is demonstrated in terms of real measurement.

Measurement-Based Black-Box Harmonic Stability Analysis of Commercial Single-Phase Inverters in Public Low-Voltage Networks

This paper presents a measurement-based stability analysis of commercially available single-phase inverters in public low-voltage networks. In practice, manufacturers typically do not disclose the parameters of the inverter design, although interactions with the low-voltage network need to be assessed and predicted. State-of-the-art modeling methods require knowledge about the internal parameters. The method proposed in the paper is based on measurements in the laboratory and does not require detailed knowledge about the specific inverter design for the identification of the black-box linear time-periodic representation. The gained information is used for the black-box stability analysis in the frequency range up to 2 kHz, which covers the bandwidth of the control of the inverters. The method is validated for a commercially available photovoltaic inverter in the laboratory. An instability that leads to a shutdown of the inverter is demonstrated, while the critical frequency range is predicted accurately.

A Novel Extended Noninvasive Harmonic Resonance Detection Technique for Public Low-Voltage Networks

Harmonic resonance is increasingly observed in public low-voltage (LV) networks of central European countries. Adverse effects, such as harmonic disturbance amplification, can be caused by harmonic resonance, which necessitates and emphasizes the importance of their continuous monitoring and detection. This article proposes a noninvasive harmonic resonance detection technique for public LV networks that rely only on the measurement of voltage and current at the LV busbar. The proposed technique is formulated in three stages, each identifying a different characteristic of harmonic resonance: the presence of harmonic resonance, resonant harmonic order and bandwidth, and resonant intensity. The performance of each stage is first verified by extensive simulation. Then, using measurement data at three different LV networks, it is validated that the proposed technique can reliably track the three different characteristics of harmonic resonances accurately and continuously.

Automatic Identification of Different Types of Consumer Configurations by Using Harmonic Current Measurements

Power quality (PQ) is an increasing concern in the distribution networks of modern industrialized countries. The PQ monitoring activities of distribution system operators (DSO), and consequently the amount of PQ measurement data, continuously increase, and consequently new and automated tools are required for efficient PQ analysis. Time characteristics of PQ parameters (e.g., harmonics) usually show characteristic daily and weekly cycles, mainly caused by the usage behaviour of electric devices. In this paper, methods are proposed for the classification of harmonic emission profiles for typical consumer configurations in public low voltage (LV) networks using a binary decision tree in combination with support vector machines. The performance of the classification was evaluated based on 40 different measurement sites in German public LV grids. This method can support network operators in the identification of consumer configurations and the early detection of fundamental changes in harmonic emission behaviour. This enables, for example, assistance in resolving customer complaints or supporting network planning by managing PQ levels using typical harmonic emission profiles.

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.

Optimal utilisation of small-scale embedded generators in a developing country – A case study in Malaysia

Building integrated photovoltaic (BIPV) systems are likely to become a dominant type of small-scale embedded generator (SSEG) on public low voltage (LV) distribution network in Malaysia due to the enormous amount of initiatives and efforts taken by the government to promote the use of BIPV. The growth of BIPV systems on LV distribution networks has the potential to alter the direction of power flow across the distribution networks, hence imposing several serious technical issues relating to power quality, distribution system efficiency and possible equipment overloading. Therefore, the utility companies need to study the following technical issues: (i) voltage regulation, (ii) voltage rise, (iii) voltage unbalance, (iv) network power losses, and (v) cable and transformer thermal limits. This paper describes research carried out to investigate and quantify the impacts of BIPV on LV networks with particular reference to developing countries striving to increase the utilisation of renewable energy. This paper presents and discusses the impacts of BIPV systems on two types of LV distribution network: a commercial LV distribution network and a residential LV distribution network in the state of Selangor, Malaysia. The results of these studies are compared with those from European networks to identify how the differences in the electrical network characteristics influence the allowable penetration of small-scale embedded generators.

A Practical Implementation Of A Distributed Control Approach For Microgrids

Abstract Public low voltage feeders containing a mixture of several micro-sources, distributed energy storage units (ESUs) and controllable loads, which appear to the upstream distribution network as controllable entities, are known as MicroGrids. Through intelligent co-ordination of micro-generators and ESUs, coupled with demand side management techniques, MicroGrids have the potential to offer significant improvements in the commercial value and environrnental impact of installed micro-generators. Furthermore, using appropriate active control techniques, MicroGrids could potentially overcome the low voltage distribution network constraints associated with high levels of micro-generation. The reseazch described in this paper builds upon previous research carried out at Durham University, which proposed a preliminary distributed control approach for MicroGrids. The fast steps in this approach have now been implemented using agent technology on the laboratory based Experimental MicroGrid at Durham Universi...