HVPD High Frequency Current Transformers

Type HFCT 220/150

The HFCT 220/150 is a large-sized portable, 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 220/150 has a 140mm 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.

Specifications of HVPD High Frequency Current Transformer Type HFCT 220/150 Characteristic

Characteristic	HFCT 220/150
Transfer Impedance, Tr	2.8 mV/mA ±5%
Frequency Response	100 kHz - 10 MHz
Droop Time	2.5 µs±5%
Typical Risetime Response	Better than 35 ns
Internal Clearance (Diameter)	140 mm
External Diameter	237 mm
Recommended Load Impedance	50 Ω
Material/Finish	HDPE Black finish
Output Connector	BNC Male
Max 50Hz Current	300 A (with PVC spacers)
Weight	1.8 kG

HFCT's for On-Line PD testing - A Technical Summary

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 HVPD 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 HVPD 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 Ω ) / (Input Current)

The Transfer Impedance is a measure of the sensitivity of the HFCT at a given set of conditions. The HVPD HFCT's have been designed to have a flat Transfer Impedance, Tr, in the range of 2.7 to 3.2mV/mA across the full frequency band of interest when detecting PD in HV Cables and Switchgear (100kHz to 12 MHz). This is illustrated below (Tr vs Frequency Plot shown is for the HFCT 220/150 Sensor).

Model	Rise Time (nSec)	Fall Time (uSec)	Low Freq. -3dB Point (kHz)	High Freq. -3dB Point (MHz)	Transfer Impedance (at 1 MHz) (mV/mA)
220/150	33	2.41	~100	~9.5	2.6

***Important Notice*** - Placement of Sensors onto HV Plant

The HVPD 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.

HVPD Longshot and PD Sensors - On-Line PD Testing Work Procedures

General Safety Precautions

Before using the HVPD 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 HVPD 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 HVPD 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 HVPD 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 HVPD 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.

On-Line PD Testing - Equipment Set-Up

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.

***Important*** - Placement of Sensors onto HV Plant

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 HVPD 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.

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 Metal clad 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).

The PD sensors are connected to the channel input points on the front of the HVPD 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 HVPD Longshot unit allows each channel to be displayed either individually or together on screen as required by the operator.

Read about other HVPD Sensors.