Dry-type transformer parallel operation needs a compatibility review before an additional unit is ordered. Give the engineer and prospective supplier the existing transformer's records, the proposed new-unit data and the intended operating arrangements. Matching nominal voltage and kVA does not establish acceptable load sharing or system performance. The purchasing decision should rest on a documented review of the specific pair, with outstanding conditions resolved before technical release.
This guide is for EPC engineers and industrial procurement teams considering an additional three-phase MV/LV dry-type transformer for an IEC-specified project. It provides a buyer-side evidence workflow, not connection instructions, protection settings or permission to energize.
Define what the additional transformer must achieve
State the required operating outcome before asking for a matching transformer. Is the project seeking extra continuous capacity, a separately supplied bus section or an operating arrangement that includes parallel service? Ask the electrical designer to identify the proposed normal, maintenance and contingency modes on a revision-controlled single-line diagram.
Record which transformers and sources are involved in each mode. Include any proposed transfer interval that could place the units in parallel, even if continuous parallel operation is not intended. Procurement should obtain the approved operating description rather than infer it from the presence of a bus coupler.
Keep three decisions separate: whether the proposed transformer design permits the intended use, whether the two units are compatible, and whether the complete installation is acceptable. A positive answer to only one does not complete the review.
Dry-type transformer parallel operation: evidence checklist
Request a paired technical schedule with separate columns for the existing unit, proposed unit and engineering conclusion. Use identifiable records rather than transcribing values from an unverified catalogue match. The following is a suggested purchasing tool, not a mandatory IEC form.
| Review item | Existing-unit evidence | New-unit response and decision |
|---|---|---|
| Identity and duty | Nameplate, asset identifier, rating, phase, frequency and relevant operating history | Proposed design reference and duty; identify the exact existing unit being assessed |
| Ratio and taps | Rated winding voltages, connection drawing, tap schedule and available ratio results | Proposed ratios and tapping data; engineer to identify the permitted combinations |
| Phase relationships | Vector group, terminal identification and approved system drawings | Connection evidence and explicit engineering acceptance of the phase relationship |
| Impedance | Available measured results with rating base, tap and reference conditions | Guaranteed data, tolerances and additional study inputs requested by the engineer |
| System interfaces | Bus arrangement, LV links, earthing and protection-study records | Updated system-review scope, responsible designer and unresolved inputs |
| Loading and release | Load records, cooling basis and relevant service limitations | Accepted loading by operating mode, evidence references and approval conditions |
Distinguish an old factory result from a present operating observation. Record alterations or repairs that could affect the validity of the original documents. If records disagree, preserve both versions and ask the responsible technical party to resolve the difference.
Compare ratios and impedance on an explicit basis
The technical comparison must identify actual ratios, phase relationships and impedance characteristics. Published MV transformer application guidance identifies phase-angle relationship, voltage ratio, tapping range and relative short-circuit impedance as parallel-operation considerations. It also highlights how impedance varies across the tapping range.
Ask the reviewer to explain the significance of each mismatch. A tap number is a label; the schedule should show the corresponding voltage or ratio. For impedance, request the electrical base, tap and reference conditions used for the comparison, together with any resistance/reactance or X/R information the study requires.
Do not write a universal percentage mismatch limit into the RFQ from a web checklist. Have the project engineer define the acceptable range and verification basis, then obtain the proposed supplier's written response. A value acceptable for one operating arrangement may not settle another.
Ask for usable capacity, not an added nameplate total
Require an engineering conclusion on the permitted loading of each transformer and the resulting usable capacity. Unequal impedance can limit how much combined load is usable before one unit reaches its limit. The result must address the actual pair and its operating conditions.
Include the connections between transformers and the common bus in the study inputs. The MV/LV installation reference includes LV-link impedance among the characteristics to consider. Request the installed cable or busway information from the system designer rather than assuming that matching transformer data makes both paths equivalent.
Ask the responsible engineer to identify the required updates to fault-duty, protection and earthing assessments. Record the applicable switchgear and circuit ratings and the study revision supporting acceptance. Procurement should track completion of these assessments, not select settings or decide how the bus will be operated.
Separate product evidence from parallel-service approval
IEC 60076-8:1997 is an application guide whose published scope includes parallel operation, system fault currents and the selection of rated and tapping quantities. That scope identifies a relevant engineering reference; it does not approve a particular transformer pair.
IEC 60076-11:2018 addresses dry-type power transformers with highest voltage for equipment up to 72.5 kV and at least one winding operating above 1.1 kV, subject to specified exclusions. These are standard-scope boundaries, not HEAKOR product ratings. Confirm the contractual editions, destination requirements and required evidence for the offered design.
Ask the supplier to state whether its specific proposed design is intended for the requested parallel duty. Keep that response alongside the system engineer's review. Neither a general IEC statement nor a routine test report alone establishes permission to operate the installation in parallel.
Make technical release conditional on named evidence
Use an issue register with a question, owner, required record, due milestone and acceptance decision. Link every conclusion to the same equipment and drawing revisions. A preliminary quotation can proceed with stated assumptions, but unresolved compatibility must remain visible.
Before technical release, request a consolidated decision identifying the accepted transformer pair, operating modes, loading limits, document revisions and remaining conditions. Agree who reassesses changes to the offered design or existing system. Keep factory acceptance, shipment release and site energization as separate milestones.
Use the project technical-document centre to prepare the document request. Confirm availability for the proposed equipment rather than assuming every report or study is included in the supply.
Frequently asked questions
What if the existing transformer test report is missing?
Keep the affected data unresolved and ask the engineer what alternative evidence or assessment is acceptable. A photograph of the nameplate can support identification but cannot reconstruct missing test results.
Can transformers from different manufacturers be considered?
Brand identity alone should not decide compatibility. Request design-specific responses and an engineering assessment of the actual pair; do not assume either automatic approval or automatic rejection.
Does a brief transfer overlap need to be disclosed?
Yes. Include every intended parallel interval in the operating description for engineering review. A short duration does not authorize procurement to bypass that review.
Does parallel operation automatically provide redundancy?
No. Ask the designer to define which loads remain supplied in each required outage case. An increase in combined capacity is not, by itself, an accepted continuity-of-supply design.
Who approves the final compatibility conclusion?
Name the responsible project engineering authority and any required owner or network approvals. Define the supplier's equipment-data responsibility separately from system-study and commissioning responsibilities.
Submit the parallel-operation requirement with your enquiry
For a HEAKOR dry-type transformer enquiry, include the existing-unit records, proposed operating modes, required additional duty, voltages, phase, frequency, destination and site conditions. Add the standards, study-data needs, approval responsibilities, quantity and schedule.
Submit Your Project Specification and ask for a proposal identifying the offered configuration, available supporting documents, deviations and open questions. Parallel-service suitability, documentation and supply scope require written confirmation for the specific proposal and project engineering approval.