T and R Test Equipment designed for the wind power industry

Wind Farm Application: Detection of High Voltages Using a Capacitive Type HVD

Written by Gary Biggs (from the T & R Test Equipment Group, T & R High Voltage Instruments Ltd)

When working upon any high voltage system, ensuring the system is ‘dead’ and therefore safe to touch has always been an important aspect of safety.

There are two methods of checking if the high Voltage system is live. The first method is using a resistive device. This kind of device is attached to the system under test with the other end of the device attached to an earth point. The unit passes the current to earth through a string of resistors to lower the voltage and therefore limit the current; then the device will either have an indicator to show live or many will have a meter to display the numerical value of the kV present within the system. The voltage present operates the tester, so batteries are not required. This method is limited though; if the kV is too high it would be dangerous and therefore these types of devices can generally only be utilised on systems up to 33kV.

The second method is using a capacitive type of device. These capacitive devices do not need to be earthed; they work by means of electrostatic induction. When the detector tip is placed in contact (or proximity to) the kV source there is capacitance between the target source and the detector. These High Voltage Detectors therefore need to be battery powered to work and cannot indicate a measured value; indeed, these would only be used to prove live or dead on HV systems. They have versions tuned to the kV being tested from 3kV, all the way to 275kV. The key aspect of these detectors is the threshold voltage; if it is too low, false live could be shown. Also conversely, if the threshold is set too high, then the unit may not indicate the live status which would be very dangerous.

Both live line resistive testing and capacitive - High Voltage detectors are now the mainstay of safety for many high voltage electrical engineers around the world. Everitt Edgecumbe & Co Ltd were one of the UK’s first manufacturers of both types of instruments, later to be recognised under the 'Metrohm' branding, becoming High Voltage Instruments Ltd. and today a part of the T&R Group of companies. We’re still producing HV testing equipment today!

The one aspect of both these devices is that a connection needs to be made to a live conductor, which act in and of itself carries danger.

Testing for dead on transformers has normally involved the exposure of the conductor at the cable termination box. This in turn could be dangerous as you are then exposing the conductor.

Wind Farm Transformer Testing

What if you could test without exposing the live conductor? The principle of a HVD means that it is possible; this technology underpins the capacitive test Point variant of the High Voltage Detector.

Two versions were developed, one for 11kV systems and one for 33kV systems. They are referred to as the HVD35/2A and the HVD47/2B. They have been designed to safely indicate the status of both 11kV and 33kV transformers prior to work being carried out, without exposing the live conductors and therefore in a much safer manner.

These testers work the same as a conventional HVDs from T & R High Voltage Instruments Ltd but are 'tuned' to this application. They are supplied as kits with handles and each variant comes with the correct tip to test at the transformer test point... without exposing the live conductor!

These testers can therefore improve safety when testing transformers and indeed are the preferred method for many HV organisations when having to check if a transformer has been made 'dead' prior to work commencing.


High Voltage HVD35


HVD35/2A / HVD47/2B Data Sheet

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