Quick answer: Select a grounding transformer from the required neutral behaviour: system voltage, connection, earth-fault current, fault duration, zero-sequence impedance, neutral resistor/reactor interface and any continuous auxiliary load.
Information required before selection
The grounding study and protection philosophy are more important than a generic kVA value.
- System voltage, frequency and grounding method
- Zigzag/star-delta connection requirement and vector group
- Permitted earth-fault current and duration
- Required zero-sequence impedance and tolerance
- Neutral grounding resistor/reactor rating and interface
- Continuous secondary/auxiliary load if any
- Ambient, enclosure, cooling, terminals, CTs and protection accessories
Technical selection framework
| Review area | What the buyer should confirm | Why it changes the supply |
|---|---|---|
| Connection | Confirm zigzag or star-delta arrangement and accessible neutral. | Connection determines how the artificial neutral is created. |
| Fault duty | State neutral current and withstand duration. | Short-time thermal duty cannot be inferred from continuous kVA. |
| Zero-sequence impedance | Provide the required system value and tolerance. | It controls earth-fault current together with the NGR/reactor and network. |
| Auxiliary winding | Separate continuous load from grounding duty. | A unit serving station load needs a combined thermal design. |
| Protection | Define neutral CT, temperature devices, surge protection and relays. | Grounding duty must be coordinated with the protection scheme. |
Recommended review sequence
- Obtain the grounding study.
- Define connection and neutral duty.
- Calculate/confirm impedance and NGR coordination.
- Add continuous auxiliary load if required.
- Issue guaranteed particulars and test requirements.
Common RFQ mistakes
- Specifying only 200kVA without neutral-current duty.
- Confusing positive-sequence and zero-sequence impedance.
- Providing contradictory impedance values in different schedules.
- Omitting fault duration or NGR resistance/current.
What a quotation package should contain
The supplier should return connection diagram, continuous and short-time ratings, neutral-current duty, zero-sequence impedance, NGR interface, temperature/protection accessories, tests and deviation schedule.
Need equipment matched to your project?
Project-based example
In one transformer procurement review, a grounding-transformer schedule contained conflicting zero-sequence impedance values. Clarifying the governing requirement before final design prevented suppliers from quoting fundamentally different units.
Standards note: IEC 60076 requirements may apply, but grounding-transformer duty must come from the project grounding and protection study. Do not substitute a catalogue kVA for calculated neutral duty.
