On-Line Partial Discharge (PD) Sensors
High Frequency Current Transformers
1. Type HFCT 100/50
The HFCT 100/50 is a medium-sized, split-core, high frequency current transformer for earth screen/earth strap connection and is suitable for on-line PD testing and monitoring of HV cables. The HFCT 100/50 has a 50mm diameter internal 'window' and has been designed so that it can be easily connected and then disconnected around the earth strap of the cable circuit.
2. Type HFCT CAE 140/100
The CAE 140/100 is a large-size, split-core, high frequency current transformer for earth screen/earth strap connection and/or connection around the insulated core of the cable (this is effective only after the point where the earth screen has been 'taken off'). The CAE 140/100 HFCT has a 100mm diameter internal 'window' and has been designed with flexibility in mind with its primary purpose being to measure PD in HV cables.
Other applications of the sensor have included the measurement of PD activity in HV 3-phase Motors and Generators through connection around the earth straps of the machine. Whilst the CAE 140/100 has a similar specification as the HFCT 100/50 it has the significant, added advantage of having built-in electrical screening to reduce interference from external electrical and RF 'noise'.
3. TEV Sensor
On-line PD measurements of air insulated and solid insulated switchgear have been made for around 30 years using TEV (Transient Earth Voltage) sensors connected to the earthed metal clad equipment (these are sometimes referred to as 'Capacitive Couplers'). The TEV sensor is a small Radio Frequency (RF) Aerial which can detect the high frequency PD pulses emanating from 'local discharge' from within switchgear, bushings and other plant. These pulses tend to have pulse widths of a few tens of nanoseconds, and act as a good medium for non-intrusive PD detection in switchgear etc.
Detection of these pulses can also be used for localization of the PD site using Time-Of-Flight (TOF) measurements of signals when 2 or more sensors are used in conjunction. The TEV Sensor (see above right) is recommended for this application. The Sensor is magnetically latched to the earthed, metal-casing of the switchgear panel and can therefore be used for on-line testing.
4. Airborne Acoustic Sensor - Ultrasonic Partial Discharge Probe
The Ultrasonic Partial Discharge Probe is a very high sensitivity, low noise acoustic pickup designed for the detection of acoustic energy emitted into air by partial discharge.
The Ultrasonic Partial Discharge Probe has a built-in amplifier circuit which is powered along its BNC cable using the Airborne Probe Power Pack (see right). The Probe is particularly useful in detecting surface discharge on cable terminations, sealing ends and within air-insulated switchgear (line-of-sight required). As the pick-up of the probe is directionally sensitive it is possible to locate the source of airborne/surface discharge through a 'point-and-measure' approach. Furthermore the use of 2x Acoustic Probes can also be very useful when Time-Of-Flight (TOF) measurements are carried out to locate the source of airborne discharges.
The Ultrasonic Partial Discharge probe head has four powerful magnets to hold the device firmly to steel, metal clad switchgear and other plant. A camera screw thread is also built into the Probe to provide mounting on standard camera-type tripods as shown above (tripod supplied as an optional extra).
5. Ultrasonic Probe Amplifier
This is a single channel, Battery-Powered Power Pack, Amplifier and Decoupler which is used in conjunction with the Ultrasonic Probe. The amplifier provides two levels of amplification, a standard amplification of 60dB (decibels) and an increased amplification of 80dB if the user requires more sensitivity.
The unit is battery-powered for portability and provides the power source for the Ultrasonic probe which is powered along the BNC Co-axial lead supplied with the unit. The amplifier unit also provides a de-coupling network and low-frequency (50Hz) filter to enable direct connection to the Partial Discharge Spot Tester Unit or an oscilloscope.
HFCT's for On-Line PD testing - A Technical Summary
The most important measure of the HFCT performance in terms of on-line Partial Discharge testing is the Transfer Impedance, Tr (measured in mV/mA). With a known Tr, the HFCT can then be used to make measurements of PD on HV cables in picoCoulombs (pC's) on-line (the PD Event Recognisation software allows the user to program in the Tr of the HFCT for this purpose).
The Transfer Impedance for an HFCT is defined as:
Transfer Impedance = (Output of the HFCT onto 50 W ) / (Input Current)
The Transfer Impedance is a measure of the sensitivity of the HFCT at a given set of conditions. The HFCT's have been designed to have a flat Transfer Impedance, Tr, in the range of 4.2 to 4.8mV/mA across the full frequency band of interest when detecting PD in HV Cables and Switchgear (100 kHz to 20 MHz).
The Benefits of On-Line Partial Discharge Testing are:
- It is truly a predictive test, indicating insulation degradation in advance of the failure (the detection of 'incipient' faults).
- It is a non-intrusive test, requiring no interruption of service and is performed under normal operating voltage and load.
- It is a nondestructive test i.e. it does not test to failure or adversely affect the equipment under test.
- It does not use any overvoltages, thereby not exposing the tested equipment to higher voltage stresses than those encountered under normal operating conditions.
- Trending can be accomplished by storing results to allow comparison with future tests.
- In many instances the site of the partial discharge occurrence can be located within the test object, so the localized problem can be repaired.
- The cost to perform an On-line PD survey is relatively inexpensive compared with off-line testing, allowing annual surveys to be performed economically at most facilities.
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