An energized GSU can apply voltage to an unloaded low-voltage bus. Describing generator terminals as “open circuit” does not say whether they are isolated from that bus or connected to it without carrying load. Those are different electrical states. Keep the selected winding arrangement within the general power-transformer specification framework [1].
Mark the open device on the single-line diagram. If it is the generator breaker, distinguish its transformer-side terminals from its machine-side terminals. If the generator remains connected and another device is open, the machine terminals may receive the GSU’s reverse-transformed voltage.
Record the boundary explicitly
| Connection | Voltage assessment | Evidence required |
|---|---|---|
| GSU energized; LV conductors connected to machine | Steady voltage follows ratio and regulation | Tap, HV voltage and complete conductor path |
| Generator breaker open between GSU and machine | GSU-side bus remains energized; machine side assessed separately | Breaker location and alternate source paths |
| Instrument transformer on GSU-side bus | Its primary is energized with that bus | Actual measuring-point location |
| Transient across an open boundary | Capacitive coupling can produce a different transient response | Stray capacitances and relevant switching waveform |
Separate ratio voltage from transient coupling
At no load and approximately sinusoidal steady state, VLV ≈ VHV/a, where a is the selected line-voltage ratio for the stated vector group. This estimates the voltage on a connected LV winding and bus. It must not be extended across an open generator breaker as though the breaker were conducting.
An isolated conductor can receive transient voltage through capacitance. Its response depends on capacitances, grounding, attached instruments and the switching waveform. A static ratio calculation does not quantify that exposure. Use the relevant electromagnetic-transient model when it forms part of the insulation duty.
Calculate voltage only up to the open device
Assume a 50 Hz grid at 400 kV line-to-line and a selected GSU ratio of 20.0. The connected LV bus is approximately 400/20 = 20.0 kV at no load; at 420 kV grid voltage it becomes 21.0 kV, 5% higher. Neither calculation assigns voltage to the machine side of an open generator breaker. Mark the 21.0 kV on the transformer-side bus and assess the separated machine section from its own sources and coupling before using any meter indication to support isolation.
Read the indication at its true location
A zero generator-terminal meter and a live GSU-side bus can coexist. Likewise, an indication from a bus voltage transformer may be incorrectly described as machine voltage in an operating screen. Confirm the primary point and the data tag before using either indication to support isolation.
Keep arresters and instrument transformers on the state drawing because they remain part of the energized section. Electrical isolation and work authorization require the plant’s approved procedure; an unloaded circuit or stopped machine does not establish a dead circuit. The drawing should leave no uncertainty over which side of the open device each claim refers to.
Verify the selected terminal ratio with a test method matched to the winding connection [2].
References
- IEC, IEC 60076-1:2011, Power transformers – Part 1: General.
- Ohlen, M., and Werelius, P., A Guide to Transformer Ratio Measurements, Megger Sweden, 2010-01-18, Definitions and TTR measurements.