IPEC/HVSL On-Line Partial Discharge (PD) Sensors

IPEC/HVSL High Frequency Current Transfomers - 2 options

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 it's 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'.

The most important measure of the HFCT performance in terms of on-line PD testing, is the Transfer Impedance, Tr (measured in mV/mA). For this reason all HFCT's supplied by IPEC/HVSL are tested and calibrated before delivery to the customer, with the Transfer Impedance of the sensor provided on a calibration sheet. 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 PDGold© software in the OSM-Longshot PD spot Tester 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 IPEC/HVSL 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 (100kHz to 20 MHz). This is illustrated below (Tr vs Frequency Plot shown is for the HFCT 100/50 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 metalclad 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 localisation of the PD site using Time-Of-Flight (TOF) measurements of signals when 2 or more sensors are used in conjunction. The IPEC/HVSL 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.

The IPEC/HVSL Ultrasonic PD Probe is a very high sensitivity, low noise acoustic pickup designed for the detection of acoustic energy emitted into air by partial discharge.

The IPEC/HVSL Ultrasonic PD 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 probe head has four powerful magnets to hold the device firmly to steel, metalclad 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).

This is a single channel, Battery-Powered Power Pack, Amplifier and Decoupler which is used in conjunction with the IPEC/HVSL Ultrasonic Probe. The amplifier provides two levels of amplification, a standard amplification of 60dB (decibels) and an increasd 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 OSM-Longshot PD Spot Tester Unit or an oscilloscope.

The IPEC/HVSL PD sensors listed above have been designed for use only on the earthed, outer surfaces of metal-clad equipment and the earth/neutral connections of cables/switchgear. Under no circumstances should these PD sensors be connected to the high voltage terminals of HV Plant under test.

Caution: Always check with the Plant Owner's Safety Rules regarding the installation of the HFCT 's around the earth strap or cable core of live equipment as restrictions may apply.

General Safety Precautions

Before using the OSM-Longshot Partial Discharge Test Unit, Sensors and Accessories in the substation or switchyard it is important for the user to read and understand the following general safety information.

The equipment user should obey the plant owner's High Voltage safety rules at all times. The following safety rules which are particular to the OSM-Longshot equipment should be applied in addition to the existing safety rules, which are required by the plant owner. General safety rules for the use of the OSM-Longshot are as follows:

• Avoid working alone.
• Only accredited, competent personnel with appropriate Health and Safety Training should use the equipment.
• Do not use the equipment if it is damaged, or its safety is impaired in any way.
• Inspect and test all ground leads and signal cables for continuity.
• The User should have read and understood the OSM-Longshot User Manual before commencing testing.
• Always select the appropriate sensor and sensor connection for the application. The PD sensors supplied with equipment have been designed for use only on the earthed, outer surfaces of metal-clad equipment and the earth/neutral connections of cables/switchgear.
• 'WARNING' - Under no circumstances should the OSM-Longshot unit or PD sensors be connected to the high voltage terminals of HV Plant under test.
• It is advisable to test the equipment and instruments immediately before and after performing site tests to ensure that the site results have been correctly recorded.
• The User should wear sensible clothing when in the substation.

Access To the Substation - The Test Engineer shall only enter the customer's switchyard or substation after they have received permission from an Authorised Person appointed by the Plant Owner. If in doubt on any mater the Plant Owner's High Voltage Safety rules should be consulted by the Test Engineer(s) who will carry out the testing.

Visual Inspection of Substation - On entry to the test area a visual inspection of the area should be carried out by the Test Engineer(s). This should enable the operator to locate a power source for the Test Equipment, to identify any potential hazards in the area and to assess both the type of equipment to be tested and the method of doing so i.e. Switchgear using TEV probes or Cables using HFCT sensors. Only when this visual inspection has been completed can the equipment be set-up for testing.

Before the commencement of any testing it is important to ensure that the test equipment and sensors to be used are in good working order, BNC leads should be matched for length if any time-of-flight testing is to be carried out, ensure all connectors and extension leads are present and in good order.

Once the Spot Tester has been set up and connected to a suitable power supply then the unit should be switched on (using the ON/OFF push- button at bottom left of the front panel). The unit's PC will take around 2 minutes to boot-up and then the Desktop Windows Menu will be displayed. Data Input to the Software Forms is done with the supplied keyboard and mouse.

The PD sensors supplied with equipment have been designed for use only on the earthed , outer surfaces of metal-clad equipment and the earth/neutral connections of cables/switchgear. Under no circumstances should the OSM-Longshot Unit or PD sensors be connected to the high voltage terminals of HV Plant under test.

Caution: Always check with the Plant Owner's Safety Rules regarding the installation of the HFCT 's around the earth strap or cable core of live equipment as restrictions may apply.

Photo of Sensor Attachment to Cable Earths/Switchgear

The PD sensors required for the test should then be placed on the equipment and/or cable to be tested, in general terms an HFCT is used for cable testing and is placed on the earth strap/earth bar of the cable (or around the cable core after the earth strap has been 'taken off').

The TEV sensors are used for the detection of 'Local PD' in Switchgear and other Metalclad equipment. The TEV sensor should placed on the outer surface of switchgear, normally next to the cable box or cable termination. A Photograph and diagram of an example of the connection of these sensors is shown.

It should be noted that, in order to obtain measurable PD signals from the cable under test the cable must have an insulated gland between itself and the switchgear (sometimes referred to as 'zonal protection' between cable and switchgear).

Diagram of Sensor Connection to OSM-Longshot unit

The PD sensors are connected to the channel input points on the front of the OSM-Longshot Spot Tester using the 50 Ohm/50Hz matching/filter connectors (except for the Airborne acoustic sensor which requires its own power supply which has a built-in 50 Ohm termination). Up to 4x PD sensors may be employed at the same time on the 4-channel Longshot unit which can be useful when observing 3-phase activity (this is used when it is possible to access the individual earths of each phase of the circuit, although this is not always the case). The OSM-Lonsghot unit allows each channel to be displayed either individually or together on screen as required by the operator.