What are the Tests and Maintenances That Should Be Done in OLTCs?

OLTCs are one of the most critical subsystems that enable the transformer to regulate voltage by changing taps when under load. For this reason, the tests and maintenance that must be done in OLTCs should not be seen only as an auxiliary maintenance job. The main purpose is to ensure that the transformer can safely maintain the voltage setting, that the step transitions work properly, and that signs of wear or deterioration are noticed before a fault occurs. Because while the main body of the transformer operates largely static, OLTC is a dynamic system that performs mechanical movement and switching.

The first step in maintenance is always safety. Before working on an OLTC, the transformer and associated auxiliary systems must be secured in accordance with the manufacturer's procedure. The local-remote position, energy, mechanical locks and protective circuits, if any, of the motor-drive mechanism should be checked. Especially when working on the OLTC and motor-drive system, the risk of incomplete step change, incorrect position information or involuntary movement should be taken into consideration. Therefore, OLTC maintenance cannot be treated like ordinary mechanical equipment maintenance.

Visual inspection is the basis of maintenance. The motor-drive cabinet, position indicator, auxiliary contact boxes, cable entries, seal structures, signs of rust-corrosion, possibility of oil leakage, protective relay connections and general surface condition should be carefully examined. Cover sealing and condensation risk are also important in motor-drive mechanisms operating outdoors. Because condensation may cause problems in auxiliary contacts, terminals and control circuits over time.

The motor-drive mechanism is one of the most critical parts of OLTC maintenance. The task of this mechanism is to perform the selected step change with the correct movement and in the correct order. Therefore, the condition of the motor, gear system, shaft connections, mechanical adjustment, position indicator, operating counter and auxiliary contacts should be checked regularly. Manufacturer sources emphasize the control of the motor-drive mechanism and its lubrication at necessary points. Because mechanical hardening or misalignment that occurs here may cause the step change to be completed incompletely or incorrectly.:contentReference[oaicite:1]{index=1}

The number of runs is very important in OLTC maintenance. Because the lifespan of this system often depends not only on the calendar year but also on how many tap-changes it makes. In some manufacturer manuals, the maintenance interval is defined by a certain number of operations or a certain year limit. Therefore, it is not enough to just look at the annual calendar when preparing a maintenance plan; OLTC meter value should also be monitored regularly. In transformers with intensive regulation, the number of operations may increase very quickly and this may bring forward the need for maintenance.:contentReference[oaicite:2]{index=2}

In conventional fatty diverter type OLTCs, the diverter switch section requires special attention. This section manages the switching under current during step transition and is naturally one of the parts exposed to the most electrical stress. Contact wear, oil contamination, carbonization and deterioration in transition elements can affect performance over time. For this reason, in some maintenance manuals, diverter switch maintenance, oil cleaning and detailed inspection of the relevant compartment are listed among the main maintenance items.:contentReference[oaicite:3]{index=3}

The Selector section should also be reviewed. Not every OLTC design is the same; In some structures, the selector and switching section work together, in others they work separately. But the general principle is the same: the selective mechanism must reliably determine the correct tap point. Mechanical misalignment, worn contacts or partial transition problems can cause incorrect positioning of the tap change. Therefore, the maintenance approach should focus not only on the external cabinet but also on the internal working order of the OLTC.

Oil health is an important maintenance issue in oil-filled OLTC types. It should be noted that main transformer oil and OLTC oil may not always behave under the same conditions. Arcing and switching stress occurring in the tap changer compartment can cause faster contamination or aging of the relevant oil. Therefore, especially in designs with separate oil compartments, oil level, oil condition, pollution indicators and, if necessary, oil analysis should be evaluated. The correct operation of protective elements such as the pressure relay and the minimum oil level should also be checked.:contentReference[oaicite:4]{index=4}

The maintenance approach may be different for OLTCs with vacuum technology. Maintenance intervals are much longer on some modern vacuum OLTC designs, and for some models certain subsystems may be offered as maintenance-free. However, this does not mean that no checks will be made. The motor-drive system, position accuracy, auxiliary contacts, meters and general operating behavior should also be monitored. So even if technology changes, the need for surveillance does not disappear; Only the content of care changes.:contentReference[oaicite:5]{index=5}

Protective elements and auxiliary circuits are also integral to OLTC maintenance. Pressure relay, oil level alarm, position indicator, tap position signals, engine protective switch and circuits in the control cabinet should be checked regularly. Because many OLTC faults can occur not only in the switching part but also in the auxiliary control chain. It is especially important that the motor-drive and OLTC position information are consistent. Position mismatch means serious business risk.:contentReference[oaicite:6]{index=6}

One of the most powerful diagnostic methods that can be performed in OLTC maintenance is dynamic resistance measurement. Dynamic Resistance Measurement, or DRM, is used to examine the electrical behavior of the diverter/switching section during step transition. Thanks to this method, defects such as interruption, resistance change, contact wear, commutation resistance problem or mechanical synchronization disorder that occur during transition can be understood more clearly. Classical static winding resistance measurement does not always show this transition behavior; Therefore, DRM has a special place in OLTC evaluation.:contentReference[oaicite:7]{index=7}

Dynamic behavior can be monitored not only electrically but also mechanically. In recent years, vibro-acoustic measurement approaches have come to the fore in OLTC condition assessment. An idea about mechanical problems can be obtained by evaluating the vibration and acoustic signature that occurs during stage transition. Although this method does not tell everything on its own, it offers deeper diagnosis when used with DRM. It is one of the important tools that strengthen the condition-based maintenance approach, especially in critical transformers.:contentReference[oaicite:8]{index=8}

Field observations should not be underestimated in OLTC maintenance. Symptoms such as unusual noise during tap change, slow operation, deviation of voltage behavior from expected even though the position appears to be changing, overheating in the driver's cabin or auxiliary circuit alarm are early signs that require detailed examination. Especially if the protection device or Buchholz-like elements have shown an event, the OLTC and the transformer should be evaluated together before the device is re-energized, as stated in the manufacturer's manuals.:contentReference[oaicite:9]{index=9}

It is not correct to give a single fixed period for the maintenance period. Because OLTC technology, model structure, operation intensity and monitoring infrastructure may differ. While 50,000 or 100,000 operations and certain year intervals may be critical on some conventional types, much longer intervals are possible on some vacuum designs. Therefore, the correct approach is to take the manufacturer's maintenance manual as basis, follow the number of operations and evaluate condition monitoring data if necessary.:contentReference[oaicite:10]{index=10}

At the end of the maintenance, all results should be recorded. The number of operations, engine-drive observations, position accuracy, oil condition, protective element tests, DRM or vibro-acoustic findings, adjustments made and replaced parts should be recorded in the maintenance history. Because OLTC failures often seem sudden, but behind them are symptoms that have been growing for a long time. If trend monitoring is done, contact wear, mechanical fatigue or auxiliary circuit weaknesses can be detected before failure occurs. In summary, the tests and maintenance that must be done in OLTCs are; It consists of visual control, motor-drive and position system inspection, diverter/selector side inspection, oil and protective element monitoring, maintenance plan according to the number of operations, and when necessary, advanced diagnostic methods such as DRM and vibro-acoustics. If OLTC transformers, voltage regulation, transformer maintenance plan and MV/HV operation safety will be evaluated together in your facility. transformer maintenance and testing, HV/MV testing, maintenance and repair and LV/MV/HV project design and consultancy services can support the technical decision process.