Views: 0 Author: Site Editor Publish Time: 2026-09-30 Origin: Site
Zisheng Electric treats transformer bushing CT factory release as an interface decision between transformer manufacturing and the EPC protection system. A bushing current transformer may be securely installed and still create a site problem if its core identity, ratio, polarity, secondary terminals or documentation do not agree with the approved protection design. Once a large transformer is shipped, tracing an embedded CT error can mean dismantling covers, rechecking wiring and delaying protection commissioning.
The factory review should follow one unit from approved drawings through assembly, test and packing. It should leave the site team with enough evidence to wire and prove each CT circuit without assuming that a generic catalog diagram matches the installed transformer. This guide covers that release package; it does not specify relay settings or claim that one acceptance value suits every project.
Begin with the purchase specification, protection single-line, CT schedule, bushing arrangement and terminal drawings. Identify which CTs are supplied inside the transformer package and which sit in external switchgear or neutral equipment. For each transformer-mounted CT, record the winding and physical location, core number, intended protection or metering duty, ratio or taps, accuracy requirement, secondary terminals and final marshalling destination. A statement such as “three CTs on HV” is too imprecise when several independent cores serve different relays.
IEC 61869-2:2012 addresses requirements for newly manufactured inductive current transformers used with measuring instruments and protective devices. The project must specify the adopted edition and its CT performance requirements; the published standard description is not a substitute for the actual contractual test schedule. IEC 60076-1:2011 addresses general power-transformer requirements. The factory acceptance record should show how the CT package and transformer package meet their respective specified interfaces, rather than implying that a power-transformer test proves every CT characteristic.
Confirm the design authority. Protection engineering defines the required CT duty and zone; transformer design places the CT and routes its leads; factory quality records construction and test; the EPC or purchaser accepts the interface. A change in CT location can move a differential protection boundary even if the ratio remains unchanged.
Confirm each CT core and terminal route against approved drawings before final assembly.
Issue one revision-controlled CT schedule tied to the transformer serial number. The schedule should identify every core and any multi-ratio connection, including the exact terminals used for the ordered ratio. Check the schedule against the approved protection study and relay input burden. Do not copy a ratio from an earlier tender when the final protection design changed. Conversely, do not alter the factory CT simply because a relay configuration screen offers a convenient setting.
Record the arrangement in a drawing that shows primary current direction and the adopted polarity convention. The physical orientation of a toroidal bushing CT, its P1/P2 reference where applicable, and the S1/S2 terminal identification must support the protection scheme. A reversed secondary can be corrected in wiring only after the design authority checks the complete zone and all parallel cores. It should not be left as an undocumented site workaround.
Where a CT has multiple cores, the schedule must distinguish them by location and service. A protection core and a metering core can share a bushing but have different accuracy, burden and testing requirements. An unused core still needs an approved termination and safe secondary treatment. Factory release should never depend on a vague note to “short unused CTs on site.”
The broader factory traceability guide explains why unit identity and drawing revision belong on every inspection record. Apply the same discipline to the bushing CTs before they are hidden by final assembly.
The matrix below connects each engineering question to proof and release action. The accepted values belong in the approved project schedule; none should be inferred from the transformer voltage class alone.
Review item | Compare against | Why it matters | Release action if inconsistent |
|---|---|---|---|
Core location and zone | Protection single-line, bushing GA and CT layout | Defines which lead and winding faults fall inside each protection zone | Hold for protection-design review |
Core identity and ratio | Approved CT schedule, nameplate and ratio-test report | Controls relay scaling and metering accuracy | Reconcile the installed core and selected taps |
Polarity and terminals | Orientation drawing, terminal diagram and polarity test | Prevents differential or directional logic errors | Correct wiring or approved drawing, then retest |
Accuracy and burden | Protection study, CT design data and test evidence | Determines performance under specified measuring or fault conditions | Obtain engineering disposition; do not relabel |
Secondary wiring | Terminal plan, cable schedule and continuity results | Ensures the tested core reaches the intended panel | Trace and retest every affected circuit |
Shipment boundary | Packing list, removable accessory plan and site wiring scope | Prevents open or ambiguous secondary circuits after transport | Define temporary shorting and final connection owner |
Add a column for observed value, test instrument, report number, drawing revision and closeout signature. The point is to make exceptions actionable. “CT OK” on a checklist does not tell the site electrician which core is connected to which terminal or whether the measured ratio used the specified tap.
At an appropriate factory hold point, check the number of CT cores, their orientation, mechanical support, lead routing and clearances against approved assembly drawings. Confirm that CT leads are protected from sharp edges, pinch points and later bushing installation work. The inspector should photograph identifiers and surrounding location in a way that can be tied to the transformer serial number. A close-up of a loose CT on a bench is weaker evidence than a unit-specific installation record.
Inspect the insulation and separation of secondary wiring according to the approved design. Secondary conductors should be routed so that maintenance personnel can distinguish CT circuits from auxiliary contacts and heater wiring. The exact wire size, terminal type and screen treatment belong in the project specification. If an assembly change moves a CT or cable route, trigger a controlled drawing review before the tank is closed.
Trace each lead to the marshalling box. The marshalling cabinet inspection guide covers physical wiring, labels and access; the bushing CT review adds core identity and polarity to that cabinet boundary. Check that shorting links or test blocks, if specified, are supplied in the agreed location and can be operated by authorized personnel without disturbing another circuit.
Record ratio and polarity by core, tap and unit serial number.
The test procedure should state the connections, source method, selected taps, ambient and instrument details where relevant. Witness a ratio and polarity result for each identified core and applicable tap. Compare the measured result with the approved CT schedule and the manufacturer's stated criterion for the specified class. Record terminal designations on the test sheet. A ratio result without core ID can be attached to the wrong winding; a correct ratio with reversed polarity can still cause a protection problem.
For a multi-ratio CT, prove the ordered connection and label the unused taps clearly. Do not assume a field technician will infer the intended ratio from wire color. A controlled secondary injection or point-to-point check may be needed to verify the whole path after the final panel is connected. The factory test proves the supplied package boundary; the site test proves the integrated protection circuit.
Review the specified accuracy and performance evidence from the CT manufacturer or approved test laboratory. A protective CT may require different evidence from a revenue-metering CT. Confirm the governing standard, performance designation, rated burden, frequency and ordered ratio. Avoid inventing a universal pass threshold; the contract and agreed test method determine acceptance. If the CT design is changed after type evidence was submitted, ask engineering to identify whether that evidence still applies.
Insulation checks on secondary circuits should follow the approved procedure and connected equipment limitations. Disconnect or protect sensitive electronics as the vendor requires. Record test points and circuit boundaries; a single megohmmeter number for “all CTs” can hide a crossed or omitted core. After testing, restore the approved shorting or terminal state and record it. Never leave an energized primary with an open CT secondary.
Classify a discrepancy before correcting it. A wrong terminal label may be corrected with controlled labeling and a repeat point-to-point check if the physical circuit and design agree. A ratio mismatch may reflect a wrong tap, a wrong supplied core or a stale schedule; each requires a different remedy. A polarity mismatch demands protection-engineering review of the complete zone. An absent core or wrong physical location can require significant rework and must not be accepted through a cosmetic drawing change.
Use a technical query or nonconformance record with the original observation, root cause, affected equipment serial number, approved disposition and retest evidence. If field wiring is deliberately changed from the original drawing, revise the terminal schedule and protection documents before shipment. The buyer should know exactly which revision represents the as-built unit. The technical bid evaluation guide provides the procurement context for keeping design deviations visible through delivery.
Do not use the relay setting to conceal an undocumented CT error. Software compensation may be technically available, but the accepted settings, drawings and test records must agree. A later maintenance crew should not have to discover a factory wiring correction by reverse-engineering the relay file.
Reconcile as-built CT records with the protection schedule before shipment.
The release package should contain the final CT schedule, bushing and core layout, marked primary current direction, secondary terminal drawing, CT nameplate or identification records, ratio and polarity reports, relevant performance evidence, cable and marshalling schedule, approved deviations and open-item closure. Include clear photographs of the installed arrangement before concealment and of the accessible terminals after assembly. Tie all records to the transformer serial number.
For transport, state which terminations remain wired, which accessories are detached, and how secondary leads are preserved and identified. The packing list should link any separate terminal box or bushing component to the same unit. At site, the EPC commissioning procedure should trace each core through cable, test block, relay input and trip scheme. Factory evidence reduces uncertainty but does not replace final site point-to-point and protection tests.
Before factory release, obtain signatures from transformer quality, protection design and the purchaser's appointed witness according to the contract. A signed test sheet without an approved CT schedule is incomplete; an approved drawing without unit-specific measured results is also incomplete. The acceptance decision should state release, conditional release with a named closeout, or hold. No unresolved protection-critical item should be hidden in a generic shipping note.
For a 110 kV–115 kV power transformer or 132 kV–138 kV power transformer, Zisheng Electric can coordinate the bushing CT schedule with the approved transformer design and the EPC protection engineer. Send the drawings, data sheets, load list, technical specification, single-line diagram, grid parameters, environmental conditions and installation-site conditions. Include the protection zones, relay input requirements, CT schedule and terminal philosophy. A documented transformer bushing CT factory release makes the manufactured unit and the protection package agree before dispatch. Our engineering team will review the requirements and respond to project inquiries within 24 hours.
