ET online
June 2016
Beyond the bridge

Beyond the bridge

01 June 2016

Diego Robalino - Senior applications engineer

In the past, measuring the capacitance and dissipation factor of a test specimen at manufacturing facilities was carried out using high voltage sources and Schering bridges. Even though Schering bridges are capable of producing good results in many applications, they are time consuming and not always user friendly. Today, automatic-balancing bridges provide an innovative solution with very high accuracy and an easy interface.

The Megger CDAX605 high-precision capacitance and dissipation factor test set is the most suitable option for high voltage C & DF measurement in factories. With the CDAX605, working voltage is limited only by the characteristics of the reference capacitor or resistor. This novel test set produces accurate results at frequencies up to 400 Hz, and is practical for use in a laboratory, as part of a factory production test facility, or in the field.

As with a conventional bridge, the CDAX605 compares the test specimen against a reference capacitor or resistor but unlike a bridge, it does not require manual tuning. Instead, it uses the automatic current comparative method to measure capacitance and dissipation factor.

Sophisticated self-calibration procedures using an internal ratio-arm bridge allow very high levels of accuracy to be achieved for example, dissipation factor results are within ± 0.05% + 0.002% for measurements made at 50 or 60 Hz, with measurement currents between 15 μA and 300 mA. Absolute voltages and currents are measured with an accuracy of ± 0.1%.

A major benefit of the CDAX605 is that it can compare currents irrespective of their phase difference. Because of this unique feature, capacitive measurements can, in practice, be made using a reference resistor. This is particularly useful when measuring the ratio of coupling capacitor voltage transformers (CCVTs), since a simple and relatively inexpensive resistive divider is all that’s needed as the reference element, rather than a costly gas-insulated reference capacitor.

Another unique characteristic of the CDAX605 is that it has one input channel for the reference capacitor or resistor, but two input channels for the specimens under test. Having two independent input channels allows for two test specimens to be connected and tested at the same time and, particularly in production line test applications where high throughput is important, the resulting saving in test time is remarkable. 

The automatic operation of the instrument, which has no front panel balancing knobs, is another invaluable timesaving feature.

The CDAX605 makes no sacrifices in the area of versatility. It can accommodate all standard test configurations, including UST, GST-GND and GSTguard, and it offers a wide measurement range, typically covering capacitance from 0 to 100 μF and dissipation factor from 0 to 10, as well as phase angle measurement from -180º to +180º.

The instrument is used in conjunction with an external PC and it is supplied with purpose-developed software that provides a convenient and notably intuitive user interface, as well as comprehensive data logging capabilities. Support is also included for a LabView interface that further enhances the versatility of the instrument.

It is likely that most purchasers will already have a suitable voltage source, together with reference capacitors and resistors. This means that they have all the additional equipment needed to form a complete dissipation factor and capacitance measuring system, with minimum expenditure.
The benefits of Megger’s innovative CDAX605 high-precision capacitance and dissipation factor test set have already proved convincing for many leading organizations in the electrical power sector. GE-Alstom, for example, has the instruments in its HV test laboratories, as has Trench Canada, a major supplier of CCVT equipment.

With the new CDAX605, HV measurements of capacitance and dissipation factor can now be performed quickly and conveniently. The CDAX 605 delivers accurate results more efficiently and reliably than older instruments – or, to put it another way, this new test set undoubtedly goes well beyond the bridge!

To find out more about the CDAX605 click here.

Tags: automatic, balancing, bridges, capacitance, dissipation, factor, high, measuring, Schering, test, voltage