Power cables fail at joints and terminations, and partial discharge measurement is the standard way to find those defects before they fail. But cable PD is measured in several different ways, and the choice of method determines what the result tells you: a snapshot of the whole cable during a test outage, a continuous trend at one point, or a localised acoustic confirmation. This article compares the three main techniques.
Oscillating wave testing (OWTS/DAC)
An oscillating wave test stresses the whole cable with a damped alternating voltage wave and measures the PD produced, including its location along the cable. The HUWOR HMJ9031 operates with a DAC frequency of 45-1000 Hz.
Strengths:
- Measures and locates PD along the entire cable in one test
- Alternating stress resembles service conditions without damaging healthy insulation
- Provides a quantified record for commissioning and periodic diagnosis
Limitations:
- Requires the cable to be taken out of service and tested
- One snapshot at the time of the test, not a continuous trend
Best for: commissioning new cable, verifying repaired sections, and periodic diagnosis during planned outages.
HFCT sensors (continuous monitoring)
An HFCT sensor clamps around the cable screen, the earth strap or a termination and measures the high-frequency current pulses of the discharge continuously. The HUWOR HMJ1010-HCT covers a frequency range of 0.5-30 MHz and is installed at transformer grounding terminals and high-voltage cable joints.
Strengths:
- Continuous online monitoring of the points where cables enter or leave equipment
- Broadband and robust; can be installed without an outage
- Supports trend analysis and alarm on growth
Limitations:
- A single-point measurement – it only sees discharges that flow through the monitored conductor
- No direct location of the source along the cable
Best for: continuous protection of critical terminations and joints, and long-term trend of cable condition.
Acoustic techniques
Ultrasonic sensors detect the airborne or structure-borne sound of the discharge. A handheld ultrasonic detector can localise the source by moving along the cable route and the joint bays.
Strengths:
- True localisation – the operator hears and sees where the discharge is
- Immune to electrical noise on the cable
- Works on energised cable without any connection
Limitations:
- Sound attenuates through soil and cable layers – buried cables are hard to survey acoustically
- Not quantitative for buried installations; sensitivity depends on the path
Best for: joint bays, cable terminations and above-ground sections where the operator can reach the surface near the cable.
Combining the three
The three methods answer different questions, and a complete cable maintenance programme uses all of them:
- HFCT sensors monitor the critical terminations and joints continuously, flagging a rising trend.
- When the trend rises, an OWTS test during the next outage locates the discharge along the cable and measures its severity.
- The acoustic instrument confirms the location on the accessible sections and supports the jointing crew at the repair.
This combination keeps the cost proportionate: continuous coverage where it matters, whole-cable diagnosis when it is needed, and localisation at the moment of action.
Summary
OWTS locates and quantifies PD along the whole cable during a test, HFCT provides continuous online trend at terminations and joints, and acoustic methods confirm and localise on accessible sections. HUWOR supplies all three product lines – the HMJ9031 OWTS tester, the HMJ1010-HCT HFCT sensor and ultrasonic handheld detectors – with OEM/ODM support. Contact the HUWOR sales team for a cable testing plan.