Tag Archive for: Power quality

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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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Ensuring grid quality on a small scale

Ensure power quality! These claims are becoming stronger and louder. This is confirmed by utilities, industrial companies, but also by many of the electrical specialists who are increasingly confronted with the topic on the part of their customers. The question often arises as to which measuring device should be used, with which expertise and with which budget.

Ensure the quality of the network for industrial needs. This is now offered by the LINAX PQ1000 series power analyzer according to IEC61000-4-30 class S. The measuring instrument is specially designed for the area of “Demand Side Power Quality” (DSPQ). There you will find the process for securing the power quality on the consumer side (according to the PoCC as per IEC TR 63191).

But why class S and not class A

Standards

Measuring instruments according to IEC 61000-4-30 Class A generally provide measured values that are comparable across measuring instruments and manufacturers. In case of legal cases, class A is mandatory and is particularly relevant for distribution system operators.

IEC 61000-4-30 class S power quality analysers are intended for basic / advanced power quality analysis and provide useful monitoring data. Instruments that meet Class S performance requirements are used for statistical power quality surveys and other applications and measurement services. There are no potential disputes there. Thus, comparable measurements are also not mandatory. The performance requirements for Class S are less stringent than for Class A. Among other things, this also results in a lower price. They are often used in industrial and supply engineering at the IPC(according to IEC [TR] 63191 this is the network distribution according to the Point of Common Coupling (PoCC)). Even in data centers, these are strongly recommended according to EN50600-2-2:2019-08 [Kapitel 6.2.3 Spannungsqualität] within the infrastructure.

Ensuring power quality with certification even for class S

A very important criterion for correct and repeatable measurement of power quality is compliance with standards for the measurement procedure. These are not to be confused with power quality compliance standards. For this reason, class S measuring devices should also be certified. With the LINAX PQ1000, this is ensured on the basis of the big brothers LINAX PQ3000 & PQ5000 by METAS, the Swiss Federal Institute of Metrology. Swiss precision.

Ensuring network quality with the highest standards of cyber security

Cyber Security

The topic of cyber security is also becoming more and more important due to the constantly growing networking. Especially in the areas of power distribution, whether in public or private networks. Due to the threat situation, effective cyber security is essential. To this end, the LINAX PQ1000offers many of the same effective protective features as its larger siblings. These include:

Designs of the LINAX PQ1000

LINAX PQ1000 all views

The PQI, according to Definition according to IEC 62586-1/2 for the analysis of power quality in power supply systems also called Power Quality Instrument (PQI), is available in various options. With the common 96x96mm form factor, the meter fits well anywhere. Whether for panel mounting with TFT display or for DIN rail mounting with or without TFT display. All variants are possible and offer high flexibility. Added to this is the simplest operation and communication via integrated web browser. Without additional software, operation, parameterization and monitoring are made child’s play.

More tutorials (e.g. on the topics UPS, PQ analysis, PQEasy reporting, data export, etc.) can be found here.

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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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