Comprehensive Guide to Acquiring Used Transformers

When to Opt for Purchasing Used Transformers Over New Investments

Used transformers should be considered when project timelines are critical, and equipment capacity aligns with the current load.

Used transformers are feasible when spare capacity aligns with short-term forecasts without exceeding safe operating thresholds. From a field perspective, physical condition, maintenance records, insulating oil samples, and insulation test results should be checked before finalizing the plan.

• Project timelines — the primary advantage of acquiring used transformers in projects needing fast electrification.
• Load capacity compatibility — typically used if not exceeding 80-90% of continuous rated capacity.
• Comprehensive operational and maintenance documentation — mitigates breakdown risks and facilitates acceptance.
• Physical condition & testing — oil, windings, and insulation must meet standards.
• Operational objectives — suitable for temporary or phase extensions; not recommended for critical 24/7 operations.

Used transformers save initial costs compared to new ones but pose higher risks if the equipment is beyond its service life or has a poor operational history. During maintenance or surveys at plants, prioritize equipment dismantled from official grids to reduce origination risks.

Operational warning: purchasing is discouraged if high reliability is needed for continuous 24/7 production; EVN acceptance may be more stringent, affecting initiation timelines. After field surveys and tests, compile a detailed condition assessment report for decisions on refurbishment or subsequent replacement.

Indicators of Reusability for Old Transformers

Old transformers should only be reused if the insulating oil, insulation resistance, phase balance, and mechanical structure meet field inspection criteria; if any key benchmarks fail, discard is advised. During maintenance and plant surveys, prioritize observations of oil leaks and casing deformations before conducting in-depth measurements.

Key inspections and field signs include:

• Insulating oil check: dark color, impurities, or leakage — field-wise, leaks through the breather signal seal failure or casing corrosion.
• Insulation resistance measurement: typically, resistance below 100 MΩ at 20°C indicates serious insulation failure; necessary to measure at winding terminals.
• Dissolved Gas Analysis (DGA): detecting high H2, CH4 indicates partial discharges or overheating in insulation.
• Ratio and phase balance checks: no-load voltage ratio deviation >2% between phases usually signals core or winding faults.
• Noise and vibration: noise >60 dB when unloaded or severe vibrations indicate loose core bolts or mechanical damage.
• Insulation paper strength: brittle, degraded paper or tensile strength <20% of the original value often requires full solid insulation replacement.

Field operation warnings: continuous oil temperature over 95°C accelerates cellulose insulation degradation, and failed short-circuit endurance tests can indicate weak mechanical structure prone to damage upon reinstallation. Burns on casing or burnt smell signify safety risks, requiring immediate discard even if some measures are within acceptable limits.

The final decision should be based on a combination of measurement results, field observations, and operational history; full field surveys and standard tests, such as TCVN 11824:2017, alongside safety checks per QCVN 01:2020/BCT, should be conducted before accepting reuse or proceeding with discard.

Checklist for Surveying and Technical Evaluation before Purchase

Finalizing a purchase should only occur if field surveys and basic tests indicate that the transformers, RMUs, cables, and grounding meet safe operational requirements; if severe signs manifest, stop the transaction.

Field-wise, surveys must encompass insulating oil checks (DGA analysis), moisture and burnt odor, oil leaks, winding and core condition assessments, alongside RMU or control cabinet evaluations (circuit breakers, relays, temperature sensors). During plant surveys, insulation resistance between windings and cores, winding resistance measurement, transformer ratio checks by no-load method, and auxiliary equipment capabilities like cooling fans, oil pumps, and gas release valves should be verified.

With cables and grounding systems, measuring grounding resistance at connection points is mandatory; if the resistance exceeds safety limits or cable insulation degrades, recalculating installation risks at new positions is necessary. Maintenance surveys and checks always involve technical documentation: recent testing certificates, maintenance reports, and repair histories should be matched against actual conditions.

Key decision criteria and operational warnings:

• Halt transactions if signs of electrical failure, oil breakdown gases at alarm levels, severe oil contamination, or missing testing records are found.
• During surveys, if grounding resistance exceeds safety parameters, delay closing until grounding systems are enhanced before transportation.
• If repair costs estimates exceed purchase device value, recommend halting or renegotiating terms.

Create detailed survey reports, listing all faults and retesting requirements along with repair estimates as pricing or transaction cancellation basis.

Procedure for Dismantling, Relocating, and On-Site Testing

Dismantling and relocating old transformers must start with complete power disconnection and verification of de-energization before any steps are initiated.

During maintenance, examine casing, oil, bolts, and grounding systems for pre-existing damages; typically, these defects directly impact fault risks during lifting and transport. Pre-dismantling requires deploying protective measures, setting warning signs, and preparing appropriate measurement equipment to confirm de-energization state.

Once the initial checks are completed, relocating the transformer is a heavy mechanical task, necessitating cranes, specialized vehicles, and proper load ratchet equipment; if the equipment contains mineral oil, hazardous goods transport regulations and environmental protection must be followed. During plant surveys, pay attention to lift hooks, casing and frame stress points, avoiding deformations that could compromise insulation.

In terms of testing, after re-positioning, at least the following measurements are required: insulation resistance, transformer ratio checks, oil (if applicable) inspection, and grounding resistance; depending on the specific model and operational conditions, additional deep tests according to applicable standards may be added. In plant practice, detailed recording of measurement values is needed for future comparison.

Operational warning: if oil leaks, severe insulation damage, or structural deformities are found during checks, halt relocation and execute professional treatment or environmental protection. Subsequent decisions often require additional field surveys and repair feasibility assessments before further transportation.

Fieldwork should conclude with complete documentation of dismantling, transportation, and testing to facilitate post-installation inspections; re-inspections per current standards are mandatory before energizing and commissioning the equipment for operation.

Documentation, Procedures, and Acceptance Conditions Pre-Reconnection

Documentation for acceptance prior to reconnection must comprehensively include main and auxiliary device test reports, medium-voltage connection drawings, and confirmations from the power unit.

Field-wise, standard required trials include insulation checks, winding resistance measurement, insulating oil analysis, and load testing per equipment capability; testing reports must be prepared by authorized entities. During plant surveys, exterior condition checks to confirm no oil leaks, intact casing, and compliant grounding system parameters are necessary before proceeding with acceptance paperwork.

Acceptance documentation lists and on-site checkpoints can be summarized as follows.

Throughout documentation preparation, consider practical operational decisions: if equipment fails to meet original specifications, defer file submission for power restoration; handle, repair, or replace before inviting EVN for inspections. Additionally, adjusting protection systems, relays, and grid synchronization is a step that cannot be overlooked during acceptance.

Per electrical distribution systems and safety regulations, documentation submitted to power units must include connection diagrams, device handover reports, and testing reports from authorized providers; coordinating submission for temporary or full re-energization needs practical inspection date alignment. Record all acceptance files thoroughly for future audits and maintenance operations.

Cost Determining Factors and Selecting the Right Handling Unit

Costs for acquiring and refurbishing used transformers are determined by rated capacity, current technical conditions, and the scope of required tests.

Field-wise, distinguish clear cost elements before finalizing purchase prices: initial survey costs, comprehensive electrical tests, transport/dismantling, refurbishment actions (oil changes, winding repairs, mechanical improvements), and costs for certification/acceptance documentation. During maintenance or plant surveys, mandatory inspections include: insulation resistance measurements, dissolved gas analysis in oil, moisture and acid levels in insulating oil.

Criteria for selecting suitable handling units should prioritize survey capabilities, on-site testing abilities, and the capacity for issuing inspection certificates. In plant practice, verify experience with popular brands (THIBIDI, SHIHLIN, ABB, KP Electric) and capacity for mandatory field experimentation.

• Capability requirements: technical surveys, testing (insulation resistance, DGA), certification issuing.
• Purchase contracts: define technical acceptance criteria, post-refurbishment warranty responsibilities, and hidden defect clauses.
• Implementation timeline: testing and refurbishment typically extend 4-8 weeks depending on complexity, and must factor into project scheduling.

Operational warning: initial purchase price doesn’t reflect costs arising from testing and hidden defect rectification; during maintenance, safety plans must be in place when relocating or integrating equipment into the grid. Decision-wise, refurbishment is often cheaper than new purchases but only when detailed surveys and costings validate feasibility.

Next steps: comprehensive field surveys and complete testing sets are required to convert the above factors into clear cost estimates and contract clauses.