CERTIFIED NETWORK QUALITY ANALYSIS

Means of legal certainty for EN50160

Including differential current monitor and certified consumption values

Initial situation

The topic of power quality (also known as PQ) now has many facets. In this way, not only the actual phenomena of power quality according to the standardisation of electromagnetic compatibility (EMC) according to IEC 61000-x-x are considered. Rather, other important parameters are being added all the time to support security of supply, and in some cases even legal certainty on the consumer side. Increasingly, this also includes consumption values, which are relevant for reducing CO2 emissions to a more climate-friendly level. In combination and as a minimum, however, all of the above criteria require a solid measurement as a basis.

Problems

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). In addition, the question arises under which criteria the energy measurement is referenced in order to comply with the above-mentioned accuracy class 0.2S at all. 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.

Fault messages in the residual current monitor

Often, for operational reasons, no disconnecting residual current monitoring systems (RCD) may be used for fire and system protection in fixed installations. These are usually connections with measurable operating fault current, e.g. in UPS networks, data centres, production facilities, airports, etc. Here, a residual current monitor (RCM) can be sensibly placed as a protective device, provided that
no RCD is prescribed as personal protection.

But here, too, it can be observed more and more that RCM systems are vulnerable. As a rule, repeated harmonics interfere with the measurement, triggering false alarms. For example, reporting a boundary violation even though there is no fault. This causes uncertainty for the operator and also leads to unnecessary effort in error analysis. It is worse, however, if disregarding or mismeasuring results in actual damage in a real fault case, e.g. the triggering of an extinguishing device.

Approaches

Certified power quality analysis

In order to present, evaluate and document the data of a safe measurement in the field of power quality in a legally secure manner, e.g. a statistical evaluation according to EN50160, it is recommended to use metrologically certified analysis devices. At Camille Bauer Metrawatt AG, at the Swiss factory in Wohlen/AG with the attached academy for further training, such stationary as well as mobile end devices are developed and produced. In the process, the devices were certified to Class A by the Swiss Federal Institute of Metrology METAS in Bern.

24/7 Power Quality

Current transformer up to 20kHz

For a legally compliant overall context, care should be taken to also bring suitable sensors into use. For this purpose, current transformers of the XCTB, XKBU and XKBR series for power quality analysis are advantageous in the stationary area. This generation of transducers guarantees interference-free operation even in the harmonic band up to 20kHz. This means that the metrologically certified instrument measurement procedure of Camille Bauer Metrawatt AG according to IEC 61000-4-30 Ed. 3, Class A from the sensor to the end device of the LINAX PQ3000, LINAX PQ5000 and LINAX PQ5000-RACK series is systemically and permanently guaranteed.

Optimisation of the residual current monitor

To contribute to the optimisation of the residual current monitor, residual currents as well as earth conductor currents can be measured and evaluated in the same application with the LINAX PQ series. For this purpose, residual current transformers of the DACT and KBU series are used. In order to avoid faulty measurements in the presence of harmonics, the measuring system is limited to 1 kHz of the monitored frequency spectrum. In addition, advantages such as the number of freely selectable monitoring channels (0/2/4/6/8), selection for differential and/or earth conductor current monitoring, conductor break monitoring, freely selectable limit values, earth conductor current measurement also with standard current transformers, logging of violations of the alarm and pre-warning thresholds, direct alarming via digital outputs, etc. help to operate a stable system. Particularly noteworthy is the flexibility in the selection of the channels that can be monitored, which allows rapid fault localisation to be carried out in case of real need.

Certified active energy

Finally, to verificate the accuracy of the active energy meters, the meter class 0.2S on the LINAX PQ devices were again assessed, tested and certified by the Swiss Federal Institute of Metrology METAS according to IEC 62053-22. This means that there are no longer any application restrictions, as is the case with meters where class 0.2S is only given when using /5A converters. This makes the measurement data more independent, increases the level of precision and helps to optimally assess and optimise the system in terms of energy.