How to Replace an Unsuitable Current Transformer
Jun 22, 2026| I. Preliminary Preparations (Power Outage Required)
1. Obtain a power outage work permit, verify the voltage in the maintenance area, confirm that all primary and secondary circuits are de-energized, and implement proper grounding protection measures.
2. Conduct pre-testing of the new current transformer: Confirm that parameters such as transformation ratio, polarity, accuracy, and insulation meet requirements. The transformer can only be brought into the site after passing the test; replacement is prohibited if it fails.
3. Prepare hoisting and insulated tools, confirm that the crane, boom lift, and other equipment are in normal working order, and ensure proper site safety isolation.
II. Steps for Removing the Old Current Transformer
1. Short-circuit grounding of the secondary circuit: Open all current circuit sliders in the terminal box and short-circuit ground to prevent high voltage from being generated by open circuits.
2. Record and remove secondary wiring: First, open the secondary junction box, take photos of the original wiring method for record-keeping, and then remove the terminals one by one in sequence; after removal, wrap the secondary cable ends with insulating tape to prevent damage to the insulation during hoisting.
3. Remove primary leads and grounding: Following the sequence A-phase → B-phase → C-phase, remove the leads from the cross-circuit busbar and the disconnector switch side, and finally remove the connecting bolts between the current transformer and the grounding busbar.
4. Lift and remove the old current transformer: Secure the slings to the current transformer's lifting points, hook it to the hook to apply force to the slings, and completely remove the base fixing bolts; slowly lift, with a designated person assisting in pulling out the secondary cable. Secure guy ropes to both sides of the current transformer to control its direction and prevent accidental contact with surrounding equipment; smoothly place the old current transformer onto a transport vehicle and pull it out of the equipment maintenance area.
III. New Current Transformer Installation Steps: The installation sequence is the reverse of removal, following the sequence C-phase → B-phase → A-phase to avoid installation collision damage:
1. Lifting and Positioning: Secure guy ropes to both sides of the current transformer to control its vertical direction, lift slowly, and during the descent, slowly thread the secondary cable into the secondary junction box. After positioning, tighten the base fixing bolts.
2. Restore Wiring: Restore the secondary wiring according to the photos taken before dismantling, grind the contact surfaces, and ensure bolts are securely tightened; restore the primary busbar, disconnector leads, and grounding device in the order of C phase → B phase → A phase.
3. Oil Level Adjustment: The oil level of the oil-immersed transformer must be adjusted to the acceptable range according to standards to ensure insulation performance meets requirements.
IV. Post-Replacement Testing and Acceptance
1. Static Waiting: After replacing the oil-immersed transformer, it must be allowed to stand for 24 hours to allow the oil level to stabilize and air bubbles to dissipate.
2. Handover Testing: Complete the withstand voltage test, oil sampling test, insulation resistance measurement, transformation ratio error measurement, and dielectric loss measurement in sequence. It can only be put into operation after all tests are passed.
3. Site Cleanup: Remove temporary safety measures, inventory tools, restore the maintenance area, complete the work permit termination procedures, and prepare for power restoration.
Replacement Precautions
1. Replacement must be done with the power off. Live work is prohibited. Before removing the secondary side, it must be short-circuited and grounded to prevent high voltage from being generated by an open circuit.
2. New current transformers must meet the following requirements: same transformation ratio, same polarity, matching voltage level, similar volt-ampere characteristics, and pass testing before replacement.
3. When replacing a group of current transformers, in addition to meeting the above requirements, it is also necessary to readjust the protection settings and recalculate the instrument multiplier to avoid protection and metering errors.
