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.

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 currents, 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.

The solution: measuring systems with qualified sensors

To improve the accuracy of a safe power quality measurement, current transformers of the XCTB, XKBU and XKBR series are recommended for power quality analysis. The new generation of transducers guarantees interference-free use even in the harmonic band up to 20kHz. Therefore, 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.

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 prewarning thresholds, direct alarming via digital outputs, etc. help to operate a stable system. Particularly noteworthy is the flexibility in the selection of monitorable channels, which allows rapid fault localisation to be carried out in case of real need.