Page 57 - Energize October 2021

TECHNICAL

MV distribution cable testing

by Mike Rycroft, Energize features writer

Underground cables are the most common method of distribution in built-up environments such as urban
areas, as well as in space-restricted situations. Although buried cables are protected from physical damage
and the effects of weather, aging of the cable, installation and handling errors can lead to internal damage
and failure or faults, and regular testing is necessary to avoid unplanned failures and cable outage.

rrespective of type, all underground cables deteriorate with time and subjects the cable to possible breakdown, and is generally known as
can develop faults and breakdowns. Failures of cable systems are the HiPot or high potential test. Classic methods use a DC test voltage,
Idisruptive, expensive and hazardous and result in loss of vital service. but systems using a very low frequency source with a specially shaped
Cables can deteriorate and fail from any combination of electrical, waveform have now become the accepted standard.
mechanical and thermal factors. Failure is taken as the breakdown of insulation under the test
The life of a cable is dominated by the aging of cable insulation conditions. If the test cable has a defect severe enough at the test
system. In medium-voltage power cables, there are factors which could voltage, an electrical tree will initiate and grow in the insulation to the
accelerate aging and degradation, such as internal ohmic heating and point of failure. Inception and growth time are functions of several
partial discharges. In addition, medium-voltage cables are susceptible to factors including test voltage, source frequency and amplitude and the
aging mechanisms, such as the formation of water trees. geometry of the defect. 1
Voltage test levels and testing time durations that will allow the
Cable testing defect to grow to breakdown have been established for the most
Cable testing is essential both at installation and over the lifetime of the commonly used test voltage sources: the DC source, the cosine-
cable. Regular tests will ensure that weaknesses that develop will be rectangular and the sinusoidal wave shapes. However, the time to
identified before cable failure occurs, which allows corrective action to failure will vary according to the type of insulation.
be taken. Four phases of testing are considered necessary:
• Installation test: Conducted after the cable is installed but before any DC HiPot test
accessories (joints/splices and terminations) are installed. These tests The DC dielectric test measures leakage current, while applying a DC
are intended to detect any manufacturing, transport and installation voltage in steps to a voltage at or above the cable operating level for
damage that may have occurred to the cable. a period of time. This measurement aids in determining the insulation
• Acceptance test: Performed after the installation of all cable and system’s ability to withstand over-voltages such as lightning strikes and
accessories, but before energising the cable with system voltage. Its switching surges. If the leakage current exceeds the maximum specified
purpose is to detect shipping and installation damage in both the for the cable, the test is considered to have failed. DC testing has the
cable and cable accessories. disadvantage that damage to the cable may result. DC testing cannot
• Maintenance test: Performed throughout the service life of the reveal some of the faults in more advanced cable systems.
cable system. Its purpose is to assess the condition and check the
serviceability of the cable system so that suitable maintenance
procedures can be initiated.
• After repair test: It is important to carry out tests on the cable after
repairs or alterations to the cable system, to ensure the operational
integrity of the cable, and avoid a disaster when re-energising. These
will immediately show up any faults or weaknesses in the repair or
alteration work.

The objective of any diagnostic test is to identify problems that may
exist within a cable in a non-destructive way, and test procedures and
equipment are designed to avoid damage to the cable. Figure 1: DC HiPot testing procedure

Two types of cable test are used: AC testing
• Voltage or dielectric withstand test Mainly used for acceptance and maintenance testing, AC testing is
• Partial discharge (PD) analysis carried out to assess the condition of the insulation, similar to the DC
HiPot test. It is carried out using an AC voltage higher than the operating
Voltage (dielectric) withstand test voltage, normally two or three times the RMS operating voltage. It is
Voltage withstand tests are conducted with the cable in an off-line non-destructive and does not introduce additional weaknesses into the
condition. The purpose of the test is to verify the integrity of the cable, cable. The voltage is applied for an extended period of time, normally
and is essentially a pass/fail test. The test involves applying a voltage 60 minutes, to allow development of water trees. The result is either
higher than the rated voltage (U 0) for an extended period of time. This pass or fail. The cable passes if it holds the prescribed test voltage for the

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