Transformer differential protection requires more than a vector-group designation to define the complete connection handover. Before closing the equipment interface, obtain the winding and terminal identification, system phase connections, CT location and polarity information, and the mapping from CT circuits to the selected relay inputs. The protection engineer needs a consistent picture of the installation, not isolated drawings from different suppliers. Assign responsibility for reconciling these documents and approving subsequent changes.
AI-generated document-review illustration, not an actual HEAKOR project, protection design or commissioning record.
This guide addresses three-phase liquid-immersed substation transformers for IEC-based industrial projects, including Vietnamese factory projects where the project specification uses that basis. It makes no claim about Vietnamese market demand, national approval or a mandatory local protection scheme. It is a document-procurement guide, not a relay-setting or commissioning procedure.
Why is the nameplate only a starting point?
The relay measures the currents presented by the complete connection arrangement. Transformer winding connections matter, but the surrounding CT connections, polarity and phase mapping also matter. The primary technical paper Beyond the Nameplate: Selecting Transformer Compensation Settings for Secure Differential Protection, in its April 2019 revision, particularly Section III, explains why considering the transformer nameplate alone can give an incomplete picture.
Do not turn that observation into a generic compensation rule. Relay-specific conventions and the actual installation must be reviewed by the protection engineer. This article intentionally provides no compensation matrix, pickup setting, slope, restraint level or injection-test sequence.
Request a connected document set, not a larger pile of files
Every required connection should be traceable across the supplier boundaries. Use an interface register to identify the documents, owners, revisions and unresolved points. The following is a suggested procurement structure; the protection engineer should define the final evidence requirements.
Evidence: Transformer connection record
What the buyer asks to identify: Winding arrangement, vector/phasor diagram, terminal markings and relevant nameplate data.
Typical document owner to confirm: Transformer supplier.
Evidence: System-to-transformer connections
What the buyer asks to identify: System phase identification and the intended connections to each transformer terminal.
Typical document owner to confirm: Project electrical designer.
Evidence: CT arrangement
What the buyer asks to identify: Physical locations, polarity marks, selected ratios, assigned cores and secondary circuit identity.
Typical document owner to confirm: Equipment suppliers and protection designer.
Evidence: CT-to-relay mapping
What the buyer asks to identify: Terminal references, cable/core identification and receiving relay input mapping.
Typical document owner to confirm: Panel designer and installation package owner.
Evidence: Protection review record
What the buyer asks to identify: Selected relay/documentation basis, accepted drawing set, review outcome and unresolved assumptions.
Typical document owner to confirm: Responsible protection engineer.
These roles are proposed allocation fields, not promises that the transformer supplier performs all protection work. Ask the project to name the actual organizations. Where a CT is in another equipment package, its information still needs to join the same interface record.
Separate buying CTs from approving the differential scheme
Accepting a CT supply schedule does not settle the full protection application. HEAKOR's bushing CT supply-boundary guide addresses per-core duties, evidence and terminal endpoints. This article starts where those equipment records must be reconciled with system connections and relay inputs.
For example, a transformer supplier may have clearly identified a CT core and a cabinet terminal, while the panel package still uses a different core identifier in its drawing. Procurement should open a reconciliation item and obtain a common, accepted reference. It should not decide which drawing is electrically correct or authorize field reconnection.
A useful clarification response states which reference changes, who approves it, which downstream files are affected and whether the release milestone changes. “Noted” does not close an interface conflict unless the accepted document set is identifiable.
Set review milestones before drawings become construction inputs
Agree when the protection engineer receives the information and what must be resolved before the next project stage. During tender, request the document list, proposed scope and any unavailable inputs. During design review, reconcile transformer, CT, system and relay records. Before installation or commissioning, require the project’s approved issue set and a controlled route for changes.
Keep factory document acceptance, protection-design acceptance and site commissioning acceptance distinct. The Relay-Assisted Commissioning technical paper (2005/2006) treats physical connections, settings and system verification as related but separate checks. Its historical relay algorithms are not a universal acceptance method and are not reproduced here.
The purchasing purpose is to make sure the correct specialist has the required information at the agreed time. Do not substitute a blog checklist for the project protection study, relay manufacturer's instructions or site safety process.
Control changes that look small on a purchase order
A revised electrical interface needs an explicit technical disposition. Flag changes to terminal identification, CT core allocation, selected ratio, wiring endpoints, relay input allocation or relay model. Ask the protection engineer which calculations, drawings, settings records and test plans require revision.
Use a short change register: proposed change, originating document, affected packages, required reviewer, decision and accepted revision. This is an original document-control recommendation, not a new electrical standard. Its value is keeping an equipment substitution from bypassing the interface review.
Frequently asked questions
Does the transformer supplier have to supply the protection relay?
Not necessarily. State whether the order covers only the transformer, specified CTs and wiring, or a wider protection package. Any relay or engineering service must be explicitly included in the offer.
Can I copy settings from another transformer with the same vector group?
Do not treat that similarity as approval. Have the responsible protection engineer review the actual transformer, CT circuits, system connections and relay documentation.
What if a drawing and nameplate disagree?
Keep the discrepancy open and ask the responsible supplier and project engineer for a controlled resolution. Do not silently choose one value or rename terminals in a downstream document.
Does a factory test report prove the final relay installation is correct?
No. A report covers its stated test scope and equipment condition. The installed protection arrangement needs the separately agreed design and commissioning acceptance evidence.
Define the HEAKOR document request
For an oil-immersed substation transformer enquiry, include MVA or kVA, winding voltages, frequency, destination, project specification and requested connection documents. Add the CT schedule where applicable, protection-package owner and required submission dates. Do not assume 50 Hz or a particular vector group without the project requirement.
Request Drawings and Technical Documents with the interface register. Ask the offer to identify available transformer-side documents, exclusions and items needing engineering confirmation. HEAKOR's document centre is an enquiry starting point; it does not establish that a particular protection drawing, relay service or report is already available.