PD Testing Equipment for Commissioning: Building the Right Test Kit
1. VLF excitation source (the PD driver)
PD must be excited with AC stress close to service conditions. For extruded cable, that means a VLF test set at 0.1 Hz.
What to look for:
- Output voltage and current matched to the cable class you commission (MV distribution through HV transmission)
- Waveform type — cosine-rectangular, sinusoidal, or bipolar pulse. The waveform affects PD inception behaviour and should be compatible with your PD measurement method
- Stable, low-noise output — a source with excessive switching noise makes small-PD measurement impossible; this is the single most common equipment-driven limitation in field PD testing
- Integrated control with the voltage sequence needed for commissioning (rise, hold, PD measurement, controlled discharge)
What to avoid: assuming a DC set can double as a PD source. DC cannot excite PD in extruded insulation the way AC does — a separate VLF capability is required. For the underlying physics, see VLF vs DC hipot: choosing the right cable commissioning test.
2. Calibrated PD measurement unit
This is the instrument that reads apparent charge in picocoulombs (pC). It must comply with the requirements of IEC 60270 for conventional PD measurement.
What to look for:
- Measured quantity: apparent charge (pC) with a defined measurement bandwidth
- Sensitivity appropriate to your acceptance criteria — resolution in the single-digit pC range is needed to demonstrate that a new cable is effectively PD-free
- Noise separation: time-window gating, and ideally phase-resolved PD (PRPD) display so the operator can distinguish PD from interference by pattern, not just magnitude
- PD mapping / location: time-of-flight reflectometry that reports PD magnitude at a distance along the cable, or per joint. This is what turns a pC number into an actionable location
- Data output: the ability to export test records for the asset file and to compare against commissioning baselines
What to avoid: instruments that report only a single aggregate number with no pattern and no location. They can flag a problem but cannot help you find it.
3. Coupling capacitor and PD coupler
The coupling network is how the measurement unit "sees" the cable. Its value and quality directly set the sensitivity of the whole system.
What to look for:
- Appropriate capacitance for the cable length being tested (long cables need more coupling capacitance to maintain sensitivity)
- Low internal PD level — a coupling capacitor that discharges internally at test voltage creates a false floor and hides real defects
- Robust, weather-resistant construction: commissioning tests happen in cable trenches and substations, not laboratories
What to avoid: reusing a coupling capacitor of unknown internal PD level. A contaminated or damaged coupling capacitor is an invisible source of false readings.
4. PD calibrator
Without a calibrator, a PD instrument measures volts. With it, you measure charge.
What to look for:
- A known, traceable pulse output (e.g. 100 pC / 1 nC steps) injected at the test object
- Direct-injection capability at the cable end being tested
- A calibration certificate with valid traceability to a national standard
The procedure: inject the calibrator pulse, read the instrument, and record the scale factor for that circuit. Repeat after any change to the test circuit (cable length, terminations, coupling). Log the calibration factor in the test report. Skipping this step is the single most common field error — see common mistakes in on-site PD testing.
5. Optional but valuable: sensors for online and hybrid testing
For follow-up work and for circuits that cannot be taken out of service, HFCT (high-frequency current transformer) sensors and portable PD survey instruments extend the same methodology to online measurement.
What to look for: HFCT sensors that clamp around the cable's earth/neutral path with sufficient bandwidth to capture PD pulses, and the ability to correlate an online spot-check with offline commissioning data on the same circuit.
Recommended commissioning PD test kit
Specification questions to ask any supplier
- What is the internal PD level of your coupling capacitor and VLF set?
- Does the measurement comply with IEC 60270, and what is the stated measurement bandwidth?
- What is the lowest PD magnitude the system can resolve reliably in a field environment?
- Can the system locate PD along the cable, or only measure total apparent charge?
- What is the noise separation method — gating, PRPD pattern analysis, or both?
- Can test records be exported as commissioning baselines for comparison at each later test?
- What VLF waveform does the set produce, and how does it interact with the PD measurement?
FAQ
Can I skip the PD calibrator if my instrument is "factory calibrated"? No. Factory calibration validates the instrument; circuit calibration validates your measurement setup on that cable. Cable capacitance changes the transfer function, so the scale factor must be established on site for every test circuit.
Is a VLF withstand test enough without PD measurement? They answer different questions. A withstand test shows the insulation survived a stress; PD measurement shows whether a defect is actively discharging. For commissioning, both are useful — the PD result is what gives you a baseline to compare against later.
Do I need PD mapping for a short cable? For short runs the value is lower, because the number of possible joint locations is small. For anything with multiple joints, or where the source could be in the cable body rather than a termination, mapping is what makes the result actionable.
What PD level should trigger action at commissioning? Follow the acceptance criteria in the applicable standard and your own specification. As a practical rule: any repeatable PD activity attributable to a joint or termination should be investigated before energization, while the joint is still accessible. For the measurement sequence itself, see PD testing for commissioning.
Related reading
- PD Testing for Commissioning — the measurement sequence and acceptance criteria
- Common Mistakes in On-Site PD Testing — the ten errors that produce false results
- Cable Commissioning Tests: The Complete Checklist — where PD fits in the full pre-energization sequence
- VLF vs DC Hipot — why VLF, not DC