On-Line PD Testing and PD Mapping (PD site location) of Medium Voltage and High Voltage Cables
Power Quality Engineering Sdn Bhd utilizes the advance Partial Discharge Test System and PD Event Recognisation Software to carry out On-Line (in-service) PD Testing of Medium Voltage and High Voltage Cables. On-line PD Testing is carried out using a combination of split-core, inductive HFCT sensors (connected around the earth strap or the core of the cable) and capacitive TEV sensors (attached to the cable termination or cable box).
The Partial Discharge Event Recognisation Software automatically differentiates between the captured waveforms of Partial Discharge pulses and other electrical 'noise' such as thyristor switching and RF noise to provide an instant On-line diagnosis of Partial Discharge activity.
Online Partial Discharge Testing of HV Cable Sealing Ends
Power Quality Engineering using advance On-line Partial Discharge Testing to perform Partial Discharge measurement for outdoor HV Cable Sealing Ends. The main requirement for these tests is how to discriminate between the highly dangerous, internal Partial Discharge activity and the relatively benign surface discharges and corona which are present on outdoor plant.
TEV and HFCT Sensor attachment to the HV Cable below the earthed Sealing End 'Bell'
On-Line Partial Discharge 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.
Photo of Sensor Attachment to Cable Earths/Switchgear
The Partial Discharge sensors are connected to the channel input points on the front of the Partial Discharge Tester using the 50 Ohm/50Hz matching/filter connectors. The Partial Discharge System allows each channel to be displayed either individually or together on screen as required by the operator.
Factors affecting HV Cable Partial Discharge Testing
In the case of testing HV cables there is normally a large amount of external Radio Frequency (RF) noise interference which is picked up by the Partial Discharge sensor. These noise levels are particularly high when the Partial Discharge measurements are made in outdoor switching yards where the pylons and overhead HV connections act as antennae for radio signals in the air. In these instances any Partial Discharge activity tends to be mixed together with a lot of noise pulses and thus when making a traditional ‘peak and count’ measurement , the noise will be counted as Partial Discharge and a false reading can lead to erroneous conclusions from the testing. Therefore, one of the main challenges to carrying out On-line Partial Discharge testing of HV cables and Cable Sealing Ends in outdoor environments is the identification and location of any internal Partial Discharge pulses (these being potentially fatal), whilst simultaneously rejecting RF noise and corona.
Corona & Surface Discharge Interference
When Partial Discharge like discharge signals are measured in an outdoor, open environment the source of the discharge is not always from within the HV plant item under test. The sources of such external signals can be non-critical corona or surface discharges on the cable sealing end under test or other HV equipment in vicinity; and discharges on insulator surfaces when they are wet.
RF Interference
RF interference is a particular problem when the Partial Discharge measurements are made in outdoor switching yards where the pylons and overhead connections act as antennae for radio signals in the air. In these instances any Partial Discharge activity tends to be mixed together with a lot of noise pulses and thus the traditional ‘peak and count’ measurements are not sufficient.
Analysis of Interfering Discharge Signals
When discharge signals are detected it is crucial to establish whether the source is local or from afar. One useful criterion is to use multiple sensors and use time of- flight analysis. It has been found with the TEV sensors that signals that have travelled a significant distance have a greater ‘preamble’ and a longer pulse rise time before the peak than those close by to the sensor. The effect is similar to the sound of thunder in a lightning strike. The further away the thunder, the more the wave shape is spread out and the less like an impulse it then becomes which leads to a ‘rumbling’, lower-frequency sound. Conversely, if the lightning strike is nearby, then the sound of the thunder is much more like an impulse and is of short duration, louder and of higher frequency. Also noteworthy is that the wave shapes can be different for different origins. This can be for several reasons:-
- Pulses are close to the sensor, and produce the largest amplitude signal at the start of the pulse.
- Partial Discharge signals which have travelled some distance, often have a peak amplitude in their middle
- Partial Discharge sites which originate in cables often have a monopolar pulse shapes.
- Signals which have travelled down cables lose some of their HF components and have longer rise times and lower frequency content.
- Partial Discharges which originate on the surfaces of insulators often exhibit slower pulses than internal Partial Discharges.
- Pulse shapes can often be fixed by the geometry of the HV plant which causes them. Hence large outdoor sealing ends represent different circuits than small components in confined enclosures, and thus produce different pulse types.
On-Line Partial Discharge Cable Mapping (Partial Discharge Site Location)
For the accurate location of Partial Discharge sites along the length of MV & HV cables Power Quality Engineering utilize the advance Partial Discharge Tester with Partial Discharge Event Recognizer software and HFCT Sensors. This technique utilizes the principle of Time-Of-Flight (TOF) measurement of the Partial Discharge pulses, in power-line carrier mode, along the cable. The Partial Discharge pulse emanating from within the cable insulation passes along both the earth screen and the core of the cable in this way. By measuring the time difference between a directly received pulse and a reflected pulse (which has been reflected from the far end of the cable) it is possible to locate the position of the Partial Discharge site on the cable to less than 1% of the cable length.
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 over voltages, 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 Partial Discharge survey is relatively inexpensive compared with off-line testing, allowing annual surveys to be performed economically at most facilities.
Contact our application engineers for further details click here.
