medium voltage aerial bundled cable

Author: Iloka Benneth Chiemelie
Published: 22/July/2016

Diagnostic tools

High Voltage Inc. (2013) presented a clear analysis of some of the most common and reliable way for diagnosing MV ABC. They noted that the diagnostic field testing is now a realty in the case of cables, and the technologies for the test is now prevalent with proves that they can be used for testing MV ABCs. It was also noted that any VLF model from VHI can be used to perform the test which will be described with relation to the methods discussed below.

VLF withstand testing – the most basic of this test is to perform a go/no-go withstand test which is used to expose the defects that are not capable of holding the test voltage. If the cable is not able to hold approximately 1.5-3 time of the normal voltage, then the capable can be considered to be defective. This is as illustrated below.

Tangent Delta / Dissipation Factor / Loss Angle Testing - When a non-destructive diagnosis test is preferred in the place of the withstand test, there are no proven options that can be used as the best way forward. The first technique and the most common in this case is the Tangent data. This test is adopted across the globe and used to provide an end to end condition analysis. This test functions by making use of the VLF is the source of voltage and a separate divided for measurement, which raises the voltage while measuring the Tan Delta of the cable.

Partial discharge texting – another tool that is used for diagnosing MV ABC is partial discharge testing and it is failure new, but proven and accepted. This test is used to determine the individual locations of electrical discharge from which the operator will then determine whether the level of PD by the MV ABC is acceptable or something that should be worried about. Any PD in the insulation at levels that are close to or slight above the operating voltage creates the needs for worrying and as such it is unacceptable.

Advantages and disadvantages

	VLF withstand testing	Tangent Delta	PD testing
Advantages	It is very useful following installation, repair, or to insure critical cables are sound. In situations where the user can tolerate a failure during the test, it is the simplest and most certain way to test a cable. Any defect severe enough to be driven to partial discharge is allowed to fail. Lesser defects and good insulation are unaffected. It is the ultimate diagnostic test.	Test many cables and rate them as Highly or Moderately Degraded, or Good. This data is used to help prioritize cable replacement, injection, and/or to determine what other tests may be of value. TD testing is easily performed and interpreted	PD testing is used to find the individual locations of electrical discharge.
Disadvantages	It is only good for basic analysis and not used for complex diagnosis.	It cannot be used to find the individual location of the electrical discharge.	The most expensive and difficult to perform.

Online and offline measurement

VLF withstand – it is used to measure how far a capable can go without voltage

TD – it is used to measure the overall health of a cable

PD – it is used to measure the individual location of the cable’s electrical discharge.

Limitations

From the above discussion, it is clear that these tools are limited in their application in the sense that they have specific areas where they can be applied and as such they are limited in application and none of the tools is capable of performing the functions of testing cable voltage, electrical discharge and overall health together. On that ground, it is clear that they cannot function independently.

Reference

High Voltage Inc. (2013) “High Voltage, Inc. offers VLF test equipment for all medium and high voltage cables up to 220 kV.” Available at: http://www.hvinc.com/downloads/Cable_Testing_Products.pdf [Accessed on: 27/08/2013].