What do you need to measure?

Power quality is assessed based on a number of power quality parameters. Today’s instruments usually handle the most common parameters such as voltage, current, power, harmonics, voltage dips, flicker and transients. Often it is difficult to say in advance what type of disturbance gives rise to a fault condition. The more different types of disturbances an instrument handles, the more likely it is that one can identify the disturbance, or alternatively to exclude certain types of disturbances.

Some recommendations for permanent power quality measurements

When investing in systems for continuous power quality monitoring it is important to consider the number of measuring points that should be included in the system. In terms of administration and management you do not want a system with 5, 10 or 100’s of meters to take more time to deal with than a system that only includes 1 or 2 meters.

As the system grows it needs to be scalable, i.e. it should be possible to analyze multiple meters simultaneously, to centrally configure and administer meters in groups and to automatically download data fom meters. It should be easy to get an overview and see how disturbances come up and spread throughout the grid.

If you intend to work with power quality statistical and/or in the long term, for example with immunity levels, you should review the options available to analyze data over several years. One condition is such that the system supports some kind of database where you collect data and then analyze it as a whole without a broken timeline.

If you were limited to an analysis of only a month or so, it would be very difficult to see trends that occur over time. For continuous monitoring, remote communication is an essential part of the system. Choice of communication is mainly controlled by the communication options available on the measuring point. If Ethernet is available, it is to prefer. Otherwise you have to choose equipment that supports other appropriate communication, such as dial-up telephone calls (PSTN modem), wireless modem communications (eg GPRS), etc.

Some recommendations for portable instruments

If one makes many short measurements (measurements of a few hours) and works a lot with troubleshooting, an instrument with display may be preferable for a simpler direct analysis. However, if measurements are made outdoors or in freezing temperatures, the display will be exposed to extreme temperatures, to impacts and to scratches – and therefore instruments without displays are to be preferred in such cases.

If the instrument is intended to be used in tough environments with humidity and dust, or in environments where the temperature varies serverly, an IP-65 rated instrument is to be recommended. If you want to perform very long measurements and need to be able to have constant access to your data, there are instruments that use GSM communication, which lets you both configure the meter and retrieve data remotely.

Portable vs. fixed monitoring

In some respects, there is a clear distinction between portable and fixed power quality equipment. The purpose of permanent monitoring is to be able to register the disturbances as they occur and then to be able to explain these events.

Another advantage of continuous monitoring is that you can work more with prevention and that you can detect impending problems before they escalate and lead to very far-reaching consequences. Portable instruments are used more for fault corrections/troubleshooting after a disturbance has occurred. With portable measurements, one is always “too late” out there, the disturbance has already occurred! Depending on the nature of the fault/disturbance it might be necessary to measure during both shorter and longer measurement periods.

If you perform measurements that are a few months or longer, permanently installed measuring devices are to recommend. From a security perspective, permanently installed equipment is to be preferred, but also because in some cases they have a better handling of long measurement periods and more alternative solutions for remote communication. With short-term measurements (hourly or weekly measurements) and measurements from a troubleshooting perspective, a portable instrument with its flexibility is preferable.

How important is accuracy?

There are large differences between different types of measuring instruments as far as accuracy is concerned. Therefore it is important before procurement of instruments to be clear about whether normative or indicative measurements need to be performed.

Power quality instruments can be compared to the development of breathalyzers. There is a variety of indicative breathalyzers of very varying quality, while the normative precision meters that the police uses constitutes an entirely separate segment where there are tough demands on measurement accuracy, measurement methods and regular calibration.

In case you perform measurements where the absolute values are of importance in disputes, inspections and measurements for the design of protection etc., it is a prerequisite to use equipment that fully complies with IEC 61000-4-30 Class A. However, if you perform measurements for troubleshooting where the absolute levels are less important, you can get far with the indicative standard instruments available on the market.

What is Class A?

IEC 61000-4-30 Class A is an international standard which regulates how an instrument measures and computes various power quality parameters so that the values measured are comparable between different instruments and different brands.

In the market there are a number of instruments that claim to comply with IEC 61000-4-30 Class A. However, it has been shown in connection with tests and comparative measurements that only a few of these instruments fully comply with IEC 61000-4-30 , Class A. Two parameters proved particularly difficult to measure according to the standard: flicker and harmonics.

To take flicker as an example, it refers to disturbances within the eye’s sensitivity range, which requires that high-frequency disturbances and signals are filtered out. This requires that the instrument is provided with (active) anti-aliasing filter to make sure that anti-aliasing phenomena do not occur and to make sure that no incorrect flicker values are measured.

If a measuring instrument is accurate in accordance with Class A it is assumed that the instrument is equipped with an anti-aliasing filter whose cutoff frequency is at 3KHz and signals and noise above that are filtered out. When purchasing an instrument it is of great importance to look very critically at the measurement performance specified in the technical details. For larger purchases it is recommended that you take the help of a test lab, certification institute or university to check the actual measurement performance of the instrument.

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