What are the Tests and Maintenances That Should Be Done on Voltage Transformers?

Voltage transformers reduce the voltage information required for measuring devices and protection relays in high voltage systems to a safe and standard level. Therefore, a small ratio error, fuse problem, secondary disconnection or phase relationship error in the VT or potential transformer circuit will not only produce incorrect measurements; It also directly affects many functions such as directional protection, undervoltage protection, synchronous control and energy quality monitoring. Therefore, the tests and maintenance that must be done on voltage transformers should be considered as a basic part of the protection and measurement infrastructure, not as an auxiliary task.

The first step when planning voltage transformer maintenance is always safety and visual inspection. After the equipment is de-energized, the resin surface should be checked for cracks, contamination, traces of moisture, carbonization, traces of partial discharge, loose connections, terminal damage, accuracy of secondary end labels and the physical condition of the primary fuse compartment, if any. Since even small surface deteriorations in voltage transformers used in MV cubicles can affect the insulation behavior over time, it is not sufficient to carry out maintenance only by wondering whether the device is working.

Cleaning is one of the indispensable steps of maintenance. However, the purpose here is not only to clean the equipment from dust, but also to keep the insulation surface healthy. Dirt accumulated on the resin or porcelain surface, combined with moisture, can facilitate stray current paths. For this reason, controlled cleaning should be performed during maintenance, harsh applications that would damage the surface coating should be avoided, and terminal areas should be checked for looseness or oxidation.

Control of the primary fuse and disconnector arrangement in voltage transformers is also of particular importance. Because in many areas, the reason why the VT circuit does not work is not directly the transformer itself, but a blown fuse on the primary side, incorrect fuse selection or a contact problem in the fuse holder. Especially in metal-clad MV switchgear, the condition of the primary fuses, fuse contact surfaces and connection continuity must be evaluated. This point should not be neglected, as relay inputs and measurement circuits may be affected at the same time when voltage information is cut off.

In secondary circuit maintenance, continuity, terminal structure, cable markings, protection relay and meter connections and grounding scheme should be examined together. The voltage transformer secondary must be grounded appropriately for safety purposes, but faulty connections that will create a risk of short circuit in the secondary circuit must be strictly avoided. Just as open circuit is critical in current transformers, accidental secondary short circuit in voltage transformers is equally dangerous and prone to failure. For this reason, the secondary circuit logic and the input structure of the connected devices must be verified before maintenance.

One of the most basic topics of electrical tests is the ratio test. The voltage transformer ratio test checks whether the primary-secondary conversion complies with the label value. Besides, it is not enough to see only the nominal rate; Phase shift and accuracy behavior must also be taken into account in measurement and protection applications. Because relays often make decisions based not only on the voltage magnitude but also on the direction of the voltage and its relationship with other phases. Therefore, phase verification, polarity control and, if necessary, evaluation in accordance with the accuracy class should be considered together with the ratio test.

Burden, or secondary load evaluation, is an often overlooked but extremely important topic in voltage transformers. The actual behavior of VT depends not only on its label information; It varies depending on the connected cable distance, measuring devices, relay input impedance and intermediate terminals. When the secondary load is excessive, voltage error and phase deviation may increase. Therefore, burden control must be included in the maintenance program, especially in old panels, systems with new devices or projects where the secondary cable route has been changed.

Winding resistance test can also be applied to inductive voltage transformers when necessary. This test; It helps to understand problems such as open circuit, loose internal connection or winding deterioration. It is not necessary to perform routine maintenance at every maintenance, but it is a powerful diagnostic tool in voltage transformers that need to be compared with the reference value, have suspicious differences between phases, or behave incompatibly with the ratio and accuracy results.

Insulation tests are one of the most critical parts of voltage transformer maintenance. Insulation health between primary-soil, primary-secondary and secondary-soil; It provides important information about aging, moisture absorption, surface deterioration and internal insulation problems. Especially in resin MV voltage transformers, cracks, impact-related micro damage or environmental pollution can turn into bigger insulation problems over time. Therefore, insulation resistance measurement, dielectric checks with appropriate test procedures and trend comparison with previous measurements significantly improve the quality of maintenance.

If a capacitive voltage transformer, that is, CVT, is used in the system, the maintenance approach should be expanded a little more. Since the health of the capacitive divider is critical in CVTs, capacitance and dissipation factor measurements should be evaluated separately. Deterioration, moisture effect or partial damage to the capacitive layers may not be noticeable to the naked eye at first. However, a change in the measured capacitance value or an increase in the dissipation factor can give an early warning about aging or insulation deterioration. Therefore, in high voltage plants with CVT, these tests should be an important part of the maintenance program.

One of the most valuable verifications to be made after commissioning or after major maintenance/overhaul is the field verification of the VT circuit. At this stage, it is checked whether the voltage information coming to the relay and the measuring device is actually correct by primary injection or VT circuit voltage measurement. Polarity check, phase-angle check and grounding check should be handled together, especially in applications sensitive to directional protection, synchronous control and phase relationship. Even if the device is solid on its own, a small mistake in the field wiring can invalidate the entire protection logic.

In voltage transformer maintenance, it is not enough to just measure the device's own values; It is necessary to think together with the system to which it is connected. Measurement circuit, relay input, fuse structure, terminal transitions, secondary grounding point and phase sequence should be evaluated as a single maintenance package. Otherwise, even if the VT appears to be intact, the system may continue to operate with incorrect information in the field due to incorrect phase connection, broken secondary cable or confused terminal signals.

Recording maintenance and test results is also a necessity. Label rate, accuracy class, secondary voltage value, burden information, fuse type, test dates, measured values, comparisons with previous results and detected nonconformities should be archived regularly. Because problems in voltage transformers often occur not suddenly, but with slow deviations. When trend analysis is performed, insulation weakening, secondary load increase or phase accuracy deterioration can be detected earlier.

In summary, the tests and maintenance that must be done on voltage transformers are; It consists of visual control, cleaning, primary fuse and connection inspection, secondary continuity and grounding control, ratio and polarity tests, burden evaluation, insulation measurements, winding resistance test when necessary, and capacitance and dissipation factor monitoring in CVT applications. After revision and commissioning, primary injection, VT circuit voltage control and phase angle verification must be performed. If you want to evaluate MV/HV voltage transformers, protection relays and measurement circuits together in your facility. HV/MV testing, maintenance and repairfor overall system design LV/MV/HV project design and consultancy and business processes SA operation responsibility services can be planned together.