A proposal to test the transformer feeder with damped AC needs a clear object and purpose. Is it assessing cable insulation, locating partial-discharge activity or verifying a newly repaired accessory? The excitation method alone does not answer these questions. [1]
Damped AC produces a decaying oscillatory voltage. Its frequency, decay and applied sequence form part of the diagnostic condition. The installed circuit’s capacitance and the test system influence that waveform, so retain the actual excitation record rather than just the name DAC.
Draw the tested circuit boundary
Identify cable sections, joints and terminations included in the assessment. Identify connected apparatus that could change the circuit or be within an unsuitable test. The transformer, surge protection and measurement equipment require explicit decisions in the approved plan; the word feeder cannot settle their inclusion.
| Boundary item | Plan must establish | Meaning for interpretation |
|---|---|---|
| Cable sections | Insulation types and lengths included | Defines the dielectric population |
| Joints and terminations | Included identities and construction | Defines possible local PD sources |
| Connected transformer/apparatus | Approved included or isolated state | Avoids unintended test exposure |
| Excitation waveform | Measured oscillation and decay | Defines the observed PD condition |
| Detection system | Sensitivity and location basis | Defines what can be detected or located |
| Assessment purpose | Condition, acceptance or fault investigation | Determines the evidence needed |
Keep excitation and diagnosis distinct
Applying the voltage is one part of the test. Measuring and interpreting discharge activity is another. A reported absence of PD is meaningful only within the detection sensitivity, waveform and tested boundary. It is not a complete statement about every possible deterioration mechanism.
Preserve location uncertainty
For traveling-wave location, cable propagation and reflections need a model. Mixed sections and accessories can affect the signal. An apparent distance should be compared with a credible route and joint map, not assigned directly to the nearest convenient component.
When event voltage is reconstructed from an excitation trace, time alignment and voltage-channel uncertainty enter the result. Record those contributions alongside the actual waveform and pulse timestamp. [2]
Retain the measured DAC decay
Assumptions: the DAC voltage is u(t) = 20 exp(−t/5 ms) cos(2π×200 Hz×t) kV, with t = 0 at the initial positive peak. At 5 ms, the envelope and instantaneous voltage are both 20e⁻¹ = 7.36 kV because one full oscillation has elapsed. At 2.5 ms, the voltage is −20e⁻⁰·⁵ = −12.13 kV. If an event is time-stamped at 5 ms, report its 7.36 kV instantaneous condition; assigning the initial 20 kV peak would overstate it by a factor of 2.72. Retain measured phase, frequency and decay with the event. These assumed values define a waveform example and prescribe no test level. [1][2]
References
- IEEE, IEEE 400.4-2015, IEEE Guide for Field Testing of Shielded Power Cable Systems Rated 5 kV and Above with Damped Alternating Current (DAC) Voltage (historical edition).
- B. N. Taylor and C. E. Kuyatt, NIST Technical Note 1297, Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results, 1994, sections 5 and 6.