AI-generated illustration for this transformer WTI guide; not a HEAKOR product photograph or an approved wiring, CT-matching or WTI configuration.
A transformer WTI using a thermal-image method does not directly measure a sensor embedded in the winding of an oil-immersed transformer. In this arrangement, the indicated temperature combines an oil-temperature measurement with a load-related thermal representation. Before accepting “WTI included,” agree which winding is represented, the current-transformer input, the required temperature gradient and its supporting basis, and the selected instrument configuration. Keep the approved calibration record separate from the alarm, trip and remote-indication requirements.
This guide is for EPC engineers and procurement teams specifying oil-immersed substation transformers for IEC-based industrial projects. It concerns purchasing records and design review, not field calibration, secondary-circuit work or protection settings.
Establish what the WTI actually represents
Name the measurement method before comparing instruments. A thermal-image WTI, an electronic thermal model and a directly installed winding-temperature sensor are not interchangeable descriptions. Ask the supplier to identify the proposed method and show its inputs on the instrument schedule.
In a conventional thermal-image arrangement, a temperature-sensing element measures oil temperature and a current-related heating arrangement adds a representation of winding heating. The details vary by instrument: the heating element, matching arrangement and temperature sensor are not necessarily assembled in the same location in every design.
The useful buying question is therefore not simply “Does the dial show winding temperature?” It is “What produces this indication, and which transformer conditions does it represent?” A local dial or remote display is not proof of direct winding measurement.
Tie the current input to a named winding and rating
The WTI input must be traceable to the transformer duty used to configure it. Request the winding designation, relevant current source, CT ratio or selected tap, and expected secondary current at the stated reference load. Have the supplier identify the exact receiving instrument and any intermediate matching equipment.
The thermal-indication CT duty should not disappear inside a general protection-CT schedule. Equally, a nominal secondary-current label does not establish compatibility with every WTI input. The selected device, its permitted input and the complete proposed arrangement need an explicit review.
For the wider location, core and terminal schedule, use the bushing CT procurement guide. This WTI review adds a different question: does that current input support the intended thermal representation?
Obtain the gradient basis, not an unexplained setting
Ask the transformer supplier to state the temperature gradient required by the selected WTI arrangement and the evidence supporting it. Identify the associated winding, reference loading and cooling condition. Record whether the basis is an approved design calculation, applicable temperature-rise test information, or another accepted project document.
Do not substitute a generic temperature difference from another transformer, or an average-winding temperature-rise value, without the responsible engineer confirming that it is the correct input for this device and method. A number without its definition and conditions is not a usable approval record.
Where the transformer has several cooling ratings, ask whether one configuration represents the specified duty or whether the offered system needs distinct gradients or another approved treatment. Multiple-gradient functions exist in some instruments; they are not universal features or automatic requirements for every transformer.
An illustrative purchasing gap is a WTI setting sheet that lists a gradient but identifies neither the winding nor the reference current. Procurement should return that sheet for clarification, not infer the missing basis. This is an example of a document gap, not a reported HEAKOR project.
Make the thermal-image acceptance record explicit
Use one compact record to connect the thermal basis with the supplied hardware. The following is a suggested review structure, not a standard-mandated form.
Transformer and WTI identity
Question that must be closed: Which unit, instrument model and option does this record cover?
Represented winding and current reference
Question that must be closed: Which winding and loading condition drive the indication?
CT and matching arrangement
Question that must be closed: Which selected ratio and intermediate components are included?
Gradient and supporting document
Question that must be closed: What thermal quantity is being represented, under which conditions?
Accepted configuration and verification record
Question that must be closed: Which settings, procedure reference and results belong to this assembly?
Revision and change status
Question that must be closed: Who approved the record, and which later changes require review?
Agree who supplies the thermal basis, who configures the instrument, who performs the specified verification and who accepts the resulting record. The thermometer's calibration evidence and the complete thermal-image configuration answer different questions; retain both where required by the purchase specification.
Keep indication, control and remote transmission distinct
A completed thermal-image record does not by itself close the protection or controls interface. Separately identify the requested cooling commands, alarms, trip contacts and remote temperature values. Obtain the contact ratings and output options for the actual device, then have the responsible designer approve their use.
For a remote value, state what temperature it represents, its units and scaling, its destination, and the selected output arrangement. Confirm any required power supply and integration equipment. A self-powered mechanical indication must not be taken as proof that every optional remote function also operates without auxiliary power.
Use the liquid-filled transformer auxiliary-power guide to close the supply boundary. Keep the WTI thermal-basis record here rather than duplicating the full plant control or feeder schedule.
Read standards and tender examples within their scope
Use the project specification to define acceptance; do not turn another buyer's accessory package into a universal rule. IEC 60076-22-1:2019 addresses protective devices on liquid-immersed transformers and includes common and device-specific requirements. A reference to the standard alone does not identify the equipment offered for a particular order.
For example, a June 2021 utility transformer specification, section 6.12 separately addresses the image coil, auxiliary CTs where required, local indication and remote-indication equipment. This demonstrates why the package needs definition; its ratings and quantities are not recommendations for every industrial project.
Frequently asked questions
Does a WTI replace direct winding-temperature measurement?
Not automatically. A thermal-image WTI represents winding temperature indirectly. If the project requires directly installed sensors, identify that measurement system and its coverage separately.
Can a replacement WTI reuse the old thermal settings?
Do not assume so. Review the replacement's measurement method, current input, matching arrangement and configuration against the transformer records before accepting retained settings.
Who chooses the alarm and trip temperatures?
The responsible technical parties must approve them for the transformer and project. Procurement should obtain the settings and approval record, not select thresholds from an unrelated catalogue or tender.
What if the thermal basis is not available at quotation stage?
Mark it as an open deliverable with an owner and submission milestone. Obtain the engineer's decision on what can proceed conditionally; do not describe an incomplete configuration as approved.
Prepare a focused HEAKOR enquiry
Start with the substation transformer enquiry category. Provide the destination, transformer duty, winding voltages, frequency, cooling arrangement, requested temperature-indication method and relevant project specification. Identify any existing CT schedule or thermal-basis document and mark missing inputs clearly.
Submit Your Project Specification and request a response identifying the proposed WTI configuration, required documents, exclusions and unresolved interfaces. The offered instruments, thermal basis, verification records and supply scope require written confirmation for the specific HEAKOR proposal.