Medium-voltage cables are expected to last decades, but the weakest point of a cable network is never the cable itself – it is the insulation at joints, terminations and damaged sections. Finding the defective joint before it fails means stressing the insulation and listening for partial discharge, without damaging the healthy parts of the cable in the process. Oscillating wave testing (OWTS, also known as damped alternating current, DAC) is the method designed for exactly this job. This article explains the technique and the HUWOR cable oscillating wave partial discharge tester (model HMJ9031).
Why damped AC instead of DC or 50 Hz
Traditional cable testing used DC overvoltage, which has largely been abandoned for extruded cables because a DC test can create space charges that damage XLPE insulation and because it does not reproduce the alternating stress that produces discharge in service. A full 50 Hz withstand test requires a huge test set and does not localise the defect. The oscillating wave test occupies the middle ground:
- The cable is charged to a test voltage and then discharged through an inductor, producing a damped alternating voltage wave at a frequency in the range that excites partial discharge.
- The stress is alternating, like service conditions, but the duration is short, so healthy insulation is not damaged.
- The PD signals are measured and their location is calculated from the time of arrival, giving the repair team the joint or section that needs attention.
According to the published specification, the HMJ9031 operates with a DAC frequency of 45-1000 Hz, measured under the reference conditions stated in the datasheet.
What the HMJ9031 provides
The HUWOR HMJ9031 cable oscillating wave partial discharge tester is designed for on-site cable insulation diagnosis, fault location and withstand testing. In one test session it delivers:
- PD measurement under alternating stress, with the discharge magnitude and pattern recorded
- Localisation of the discharge along the cable, so the crew goes to the right joint instead of re-terminating the whole cable
- Withstand verification for newly installed or repaired cable sections before they are returned to service
The instrument is used on power cable commissioning sites and during maintenance outages, where it gives an objective record of cable condition without removing the cable from its route.
Interpreting the results
An oscillating wave test produces a PD pattern and a discharge location map. A healthy cable shows no significant discharge at the test level; a cable with a defective joint shows a discharge peak at a specific distance. The magnitude of the discharge, together with its position and the test voltage at which it appears, tells the engineer whether the defect needs urgent attention or can be monitored until the next planned outage.
In practice, the strongest value of OWTS is the positive result: proving that a cable section is sound before it is returned to service, and proving that a suspected joint is the actual problem before it is dug up.
OWTS and continuous monitoring together
Continuous PD monitoring (for example with HFCT sensors at terminations) tracks the trend of discharge activity during operation. OWTS adds the periodic, high-sensitivity diagnosis during outages. The two tools answer different questions – the monitoring system asks whether the discharge is growing, while the OWTS answers where exactly it is and how severe it is – and together they cover the full cable maintenance cycle.
Summary
Oscillating wave testing finds cable insulation defects under realistic alternating stress, localises them along the route, and verifies new or repaired sections – all without removing the cable. The HMJ9031 provides OWTS/DAC testing with a 45-1000 Hz damped AC frequency for commissioning and maintenance use. HUWOR supplies the tester with training and OEM/ODM support. Contact the HUWOR sales team for specifications and a test procedure template.