An online partial discharge survey is a non-destructive electrical measurement performed on de-energized high-voltage equipment such as transformers, switchgear, or cable systems to detect incipient insulation defects that produce localized electrical discharge inside the insulation without creating a full short circuit to ground. The survey uses calibrated sensors and instrumentation to capture the magnitude, timing, and frequency content of partial discharge (PD) pulses across the equipment's insulation system, reporting results that reveal the presence, severity, and location of voids, cavities, contamination, or surface irregularities within or on the insulation. NETA ATS-2025 Section 11 lists it as an acceptance-testing method for equipment where partial discharge monitoring is specified, either as part of the standard test battery or as a supplemental assessment when baseline data is required for future comparison.
Partial discharge is a precursor to insulation breakdown and catastrophic failure. A localized electrical stress that exceeds the insulation's withstand capability at that point begins to erode the insulation material through ionization, heat, and chemical degradation, a process that accelerates over time and eventually produces a full flashover or fault. Detecting partial discharge activity during acceptance testing before the equipment is placed in service provides a baseline against which future maintenance or degradation trends can be tracked, and identifies whether the equipment as manufactured or delivered meets the insulation-integrity requirements of the application. An acceptance survey also confirms whether the insulation system can be safely operated within its rated duty cycle or whether a defect has already developed that warrants repair or replacement before energization.
Acceptance: performed on new or reconditioned equipment before it is first energized and placed into permanent service, typically as part of the initial electrical test sequence. The survey is conducted at the equipment's rated frequency and voltage (or at a test voltage if no-load or reduced-load testing is performed), and results are recorded as a formal baseline. Future condition-monitoring surveys on the same equipment are scheduled at periodic intervals or after triggering events such as a through-fault, overload, or service interruption, and are compared against the acceptance-test baseline to assess whether partial discharge activity has emerged or changed.
With the equipment de-energized, isolated, and grounded per the site's safety procedures, the survey instrumentation (a partial-discharge detection system or online PD monitor with calibrated input amplifiers, filters, and signal-acquisition hardware) is connected to accessible points on or near the insulation under test - such as bushing test taps, surge arresters, or dedicated monitoring ports - following the equipment manufacturer's guidance and NETA ATS-2025 Section 11 requirements. The test voltage or operating voltage is applied (or ramped to the specified level in the case of acceptance testing), and the detector measures the magnitude and frequency of partial discharge pulses across the test bandwidth, recording the peak apparent charge (pC), repetition rate, and spectral content. The measurement is conducted for a defined duration sufficient to capture a representative sample of PD activity, and the results are logged with the test date, facility location, equipment identification, ambient conditions, and any observed discrepancies.
For each piece of equipment surveyed: the facility and site location, equipment identification (manufacturer, model, serial number, nameplate ratings), survey date and time, test voltage applied or operating voltage at time of survey, the PD detection instrument make, model, serial number, and calibration status, measured partial discharge magnitude (in apparent charge units such as pC or nC), repetition rate (PD events per second or per minute), and frequency or phase-angle distribution if the instrument captured spectral data. Any discrepancies observed during the survey - such as unusually high PD activity at certain voltage levels, phase-angle clustering that suggests a localized defect, or PD that emerges intermittently - are documented alongside recommended actions for further investigation, repair, or acceptance.
NETA ATS-2025 Section 11 (Test Results) and Section 11.2 (Survey Report) establish that the acceptance survey report must present the measured partial discharge results alongside a statement of whether they are acceptable, unacceptable, or require further investigation. Acceptance is determined by comparing the baseline PD activity (magnitude, distribution, and rate) against criteria that reflect the equipment type, voltage class, and insulation system design - higher voltage equipment generally tolerates higher PD levels than lower voltage systems because the insulation thickness is greater, but the presence of PD activity that is concentrated at a specific location or that grows with voltage rise indicates a localized defect that warrants repair or rejection regardless of overall magnitude. The survey report documents any discrepancies discovered during testing and recommends whether the equipment should be accepted, accepted with conditions pending further monitoring, or rejected from service until the insulation defect is corrected. See the purchased NETA ATS-2025 standard for the complete evaluation framework and equipment-type-specific guidance.
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