Tag Archive for: Point of Common Coupling

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Project study: Resonance suppression through a special active filter

Problem description

After installing a second photovoltaic system on a low-voltage grid with a high proportion of inverters, frequent faults were detected in the first and larger solar system. The inverters of this system, but also of the newer PV system, frequently switched off with the message “grid fault”.

During a grid quality measurement carried out by the local power supply company, high voltage distortions and exceeding of the limit values according to EN 50160 as well as EN 61000-2-2 could be determined at times. However, no cause was given or determined.

Since a CHP unit with a higher output is to be connected to this network and further disturbances were to be expected, we were commissioned with a network and harmonic analysis. The goal here is always the clear determination of the cause and the subsequent planning of suitable solutions.

Approach and analysis

First, we obtained an overview of the overall situation of the voltage supply by measuring with a special network analyzer. Already after connecting the first measurement to the transformer infeed, very high voltage distortions were detected (see the following picture of the voltage and current waveform).

Voltage distortions

Since this is a customer-owned grid, we applied the limit values of EN 61000-2-4 class 2 for the evaluation of the voltage quality. Of course, these limits were also exceeded, in some cases very significantly in the case of individual harmonics and interharmonics.

Two causes are already evident in the current and voltage waveforms. Firstly, the grid is almost exclusively loaded with 6-pulse converters (typical for frequency converters). And secondly, a clear resonance oscillation is recognisable. The latter can usually be seen as the main cause of fault messages from sensitive or particularly protected electrical equipment, such as solar inverters.

Parallel resonance arises as an interaction between transformer inductance and capacitive components in the low-voltage grid. In most cases, these are distributed over several systems and therefore cannot be specifically eliminated as the cause of the problem. During further measurements in this network, we were therefore able to identify several feeders that are involved in the resonance. On the one hand, these were mainly lighting systems and, on the other hand, the disturbed photovoltaic systems. In both cases, the use of unchoked or only slightly choked capacitors as main or filter components is typical. Thus, the disturbed system components are also involved in the disturbance itself. In addition, there was also a frequent shift of the resonance frequency, the cause of which was the switching on and off of the components involved.

The solution

Therefore, there were only two possible solutions. The first way would be a comprehensive restructuring of the network and therefore very time-consuming and expensive. The second way turned out to be feasible in the short term and was preferred by the customer.

As a short-term solution, a special active filter was installed and parameterized by us. Selection, installation, and commissioning were carried out in close cooperation with our customer and the supplier of the filter. The objective was to eliminate the resonance oscillation as the cause of the inverter’s fault messages and also the greatest risk of faults in the CHP to be installed later.

The results are clear:

At the time of commissioning, the resonance was not so pronounced. Therefore, the voltage in the image without the filter is also less distorted than in the first image. Nevertheless, the installed filter also caused a permanent and reliable suppression of the resonance oscillation in the course of the verification measurement over two weeks.

The remaining voltage distortions in the right picture are due to the frequency inverters and contain only slight exceedances of the limit values according to EN 61000-2-4. These do not lead to disturbances at the solar inverters but can be reduced to below the limit values by an additional filter if necessary.

All the systems, including the CHP unit that has since been put into operation, have been working trouble-free since the filter was put into operation.

Imprint:
PQ Professionals GmbH
Landsberger Street 4
04157 Leipzig
represented by the Managing Director Dipl.-Ing. (FH) Frank Strobel

 

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Interaction between Grid-Connected PV systems & LED Lamps: Directions for Further Research on Harmonics and Supraharmonics

This paper discusses different approaches to investigate the interaction through harmonics, interharmonics, supraharmonics, and light flicker, between photovoltaic (PV) inverters and LED lamps in low-voltage installations. Single grid connected power generators and electronic loads like LED lamps can be easily characterized in terms of harmonics in a given range of frequency. This subject is relatively well understood, and specific standards for measuring and restricting emissions are already established to ensure a low probability of interference. However, when connected together, source and load exhibit behavior that requires further study and understanding. This work presents a discussion serving as a guide for future work on analysis of losses and other impacts of the disturbances regarding this specific load and source interaction. The following are taken into account: the nonlinearity of LED loads and PV converters; the technologies and methods used in control; and the changes in power flow caused by load and power production variations. Index Terms  – electric power systems, power quality, harmonics, supraharmonics, solar power.

From PQTBlog

Published by:

  • Tatiano Busatto, Fahim Abid, Anders Larsson and Math H. J. Bollen, Electric Power Engineering, Luleå University of Technology, Skellefteå 931 87, Sweden, @mail: tatiano.busatto@ltu.se
  • Gaurav Singh, Department of Electrical and Computer Engineering, Clemson University, Clemson, South Carolina 29634, USA, @mail: gauravs@clemson.edu 

Conference Paper: 16-19 Oct. 2016, Belo Horizonte, Brazil.

Published in 2016 17th International Conference on Harmonics and Quality of Power (ICHQP)

Introduction

With the constant development, the inclusion of new energy sources and consumption devices becoming increasingly complex, a broader understanding is required of the interaction between these elements and the electrical system. In this context, the use of distributed energy Read more

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The use of suitable current transformer sensors pays off

Accuracy of the measurement questioned.

Measuring instruments are usually classified according to standards and accuracy. Accuracy is an important indicator for being able to usefully build on a solid measurement result in the analysis and its resulting measures. However, it can be observed that although the measuring devices used correspond to a required accuracy class, the necessary sensors are often less in focus.

It can be seen that although class A measuring devices used in power quality applications correspond to a data sheet accuracy of 0.1% for U/I and 0.2S at the energy meter, upstream current transformers are often designed significantly worse (e.g. 0.5% or worse). And this is apart from the fact that not only accuracy plays a role in power quality measurements, but also the inevitable compatibility against harmonics – one of the modern and growing main players in power quality. Read more

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Tier 1-4: Increasing secure 24/7 operation in data centres

Certified power quality monitoring and fire protection enhancement

Preamble

The volume of digital data continues to increase rapidly and steadily. Not least caused by cryptocurrencies, such as Bitcoin (XBT), Ether (ETH), Litecoin (LTC), etc. or, for example, by the blockchain process, a quasi-type database that works on various networked servers. The increasing demand for data exchange also increases the need for data centres, which are planned, built and maintained on a large scale globally. However, data centres in the electrical energy context are subject to complex challenges that can influence (legally) secure 24/ operation.

The basic requirements

When planning the energy supply of a data centre, many demands must be taken into account:

  • Secure location in terms of energy supply and environmental conditions
  • High energy efficiency to minimise operating costs
  • Maximum availability (Uptime Insitute Tier 1-4) through use of redundancies, e.g. UPS, generators
  • High security (fire protection, access, defence against cyber attacks)
  • System stability and reliability of the operating resources used
  • Scalability

The problem description

Various studies show that power quality problems cost billions each year. Read more

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System for power quality, energy management and pump automation

Actual situation

In a large urban city, the technology for supplying fresh water is getting old. Although the pumping stations and the infrastructure are in good condition, the maintenance and repair costs are continuously and extremely increasing. Electrical monitoring of the entire system is only available to a limited extent, and if so, only partially. Read more

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PQ Easy Report – Conformity reports at the touch of a button

 

With the “PQ-Easy Report”, statistical reports can be generated directly from the measuring instrument, either according to standard or also user-specific. And this completely autonomously and detached from proprietary software solutions that make life difficult for many users as well as the IT department.

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