An inductive voltage transformer is energized by the voltage across its primary winding, often phase to earth. During a network disturbance, that voltage can rise even when the line-to-line system voltage changes little. The specified overvoltage capability must therefore identify the winding reference, magnitude and duration. [1] [2]
ABB’s application guide discusses voltage factor together with network grounding and operating duration. Its historical example values should not be copied into every purchase specification. The applicable requirement belongs to the actual system and agreed equipment specification. [1]
Begin with the winding voltage and frequency
For sinusoidal induced voltage, Erms ≈ 4.44 f N Φpeak. Erms is in volts, frequency f in hertz, turn count N is dimensionless and peak flux Φpeak is in webers. Applied primary voltage approximates induced voltage only when winding drops are suitably small.
At unchanged turns and frequency, higher winding voltage demands higher flux. Lower frequency can increase flux demand even without higher voltage. Distorted voltage requires a waveform-based flux calculation; the sinusoidal factor should not be used as if every waveform had the same relationship between RMS voltage and peak flux.
| Operating envelope input | Magnetic interpretation | Thermal implication |
|---|---|---|
| Primary winding voltage | Flux demand from the actual terminal pair | Changed excitation and core loss |
| Frequency or voltage waveform | Relevant volt-second accumulation | Loss behaviour may differ from rated sinusoidal service |
| Overvoltage duration | Time spent in the stressed magnetic condition | Heat accumulation before clearing or recovery |
| Network grounding and fault state | Which phase-earth voltages can occur | Which VTs experience the duty |
| Secondary burdens | Winding-current and voltage-drop contribution | Additional copper loss during the event |
Separate flux capability from temperature capability
A design may avoid severe saturation yet produce increased loss during sustained overvoltage. Conversely, a short magnetic excursion may end before bulk temperature reaches a limiting value. Assess the specified duration and prior thermal condition instead of treating either a flux margin or a steady-state heat run as a complete answer.
The relevant starting temperature may follow normal load and ambient conditions. Repeated disturbances can prevent complete cooling. Include that history if the proposed duty expects repeated operation.
Translate network states into equipment duties
Use a network study to identify the actual primary-voltage envelope and clearing times. System grounding affects phase-earth voltage during earth faults, but the response also depends on the network and protection. A country name or a broad grounding label is insufficient to define every VT duty.
Assume sinusoidal mean induction 1.20 T at 50 Hz. Voltage 1.20 pu and frequency 0.95 pu give B = 1.516 T, +26.32%, at unchanged active turns and negligible drops. This is only a magnetic-data screen. For an independent incremental thermal model, assume extra loss 8 W, C = 100 J/K and G = 0.50 W/K: τ = C/G = 200 s and extra rise is (8/0.5)(1−e⁻ᵗ/²⁰⁰) [3]. At 10/300 s it is 0.780/12.430 K. From assumed 60 °C initial temperature, a fictional 70 °C cap passes the short duty and fails the longer. Obtain actual voltage-duration material/loss data before substituting these illustrative thermal inputs.
Finally, match the magnetic and thermal evidence to the exact winding, burden combination and voltage-duration requirement. The result should state which envelope is supported and which abnormal waveform or prolonged fault condition lies outside it. That is a more precise specification than an overvoltage multiplier without a clock or voltage reference.
References
- ABB, Instrument Transformers Application Guide, publication 1HSM 9543 40-00en, Edition 4, February 2015, sections 3.4 and 4.9–4.12.
- ABB, Instrument Transformers: Technical Information and Application Guide, 1VAP420003-TG, revision A, December 2004.
- U.S. Department of Energy, DOE-HDBK-1012/2-92, Thermodynamics, Heat Transfer, and Fluid Flow, Volume 2, 1992.