What Tests and Maintenance Are Required for Voltage Transformers?

Voltage transformers reduce the voltage information required for metering devices and protection relays in high-voltage systems to a safe and standard level. Therefore, a small ratio error, fuse problem, secondary discontinuity or phase relationship error in a VT or potential transformer circuit does not only produce incorrect measurement; it also directly affects many functions such as directional protection, undervoltage protection, synchronism check and power quality monitoring. For this reason, the tests and maintenance required for voltage transformers should be considered not as an auxiliary task, but as a basic part of the protection and metering infrastructure. For related context, see What Is a Voltage Transformer? What Does It Do, How Does It Work and What Types Are There?.

When planning voltage transformer maintenance, the first step is always safety and visual inspection. After the equipment is de-energized, the resin surface should be reviewed for cracks, contamination, moisture traces, carbonization, partial discharge marks, loose connections, terminal damage, accuracy of secondary terminal labels and the physical condition of the primary fuse compartment if present. In voltage transformers used in MV cubicles, even small surface deterioration can affect insulation behavior over time, so maintenance based only on whether the device operates is not sufficient. For related context, see What Is a Transformer? What Does It Do and What Types Are There?.

Cleaning is also one of the indispensable steps of maintenance. However, the purpose here is not only to remove dust from the equipment, but to keep the insulation surface healthy. Dirt accumulated on resin or porcelain surfaces can facilitate leakage current paths when combined with moisture. Therefore, controlled cleaning should be performed during maintenance, harsh practices that may damage the surface coating should be avoided and terminal areas should also be checked for looseness or oxidation. For related context, see What Tests and Maintenance Are Required for Current Transformers?.

Checking the primary fuse and disconnector arrangement on voltage transformers is also especially important. Because in many fields, the reason a VT circuit does not operate 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 cubicles, the condition of primary fuses, fuse contact surfaces and connection continuity must be evaluated. Since relay inputs and metering circuits may be affected at the same time when voltage information is lost, this point should not be neglected. For related context, see What Is High Voltage Operation Responsibility? Why Is It Necessary and What Does It Cover?.

In secondary circuit maintenance, continuity, terminal structure, cable markings, protection relay and meter connections and grounding arrangement should be examined together. The voltage transformer secondary should be grounded appropriately for safety; however, incorrect connections that could create a short-circuit risk in the secondary circuit must definitely be avoided. Just as an open circuit is critical for current transformers, an accidentally created secondary short circuit is equally dangerous and fault-prone for voltage transformers. Therefore, the secondary circuit logic and the input structure of connected devices must be verified before maintenance.

One of the most basic headings of electrical tests is the ratio test. The voltage transformer ratio test checks whether the primary-secondary conversion complies with the nameplate value. In addition, seeing only the nominal ratio is not enough; phase shift and accuracy behavior should also be considered in metering and protection applications. Because relays often make decisions not only according to voltage magnitude, but also according to the direction of the voltage and its relationship with other phases. Therefore, phase verification, polarity check and, if necessary, assessment suitable for the accuracy class should be considered together with the ratio test.

Burden, in other words secondary load assessment, is a subject that is often overlooked in voltage transformers but is extremely important. The real behavior of the VT changes depending not only on its own nameplate information, but also on the connected cable distance, metering devices, relay input impedance and intermediate terminals. When the secondary load is higher than necessary, voltage error and phase deviation may increase. Therefore, burden control must be included in the maintenance program especially in old panels, systems where new devices have been added or projects where the secondary cable route has been changed.

In inductive voltage transformers, the winding resistance test can also be applied when necessary. This test helps identify problems such as open circuit, loose internal connection or deterioration in the winding. It is not mandatory as a routine test in every maintenance activity, but it is a strong diagnostic tool for voltage transformers where comparison with a reference value is needed, suspicious differences between phases are seen or behavior inconsistent with ratio and accuracy results is observed.

Insulation tests are one of the most critical parts of voltage transformer maintenance. Insulation health between primary-earth, primary-secondary and secondary-earth provides important information about aging, moisture ingress, surface deterioration and internal insulation problems. Especially in resin-insulated MV voltage transformers, cracks, impact-related micro damage or environmental contamination can turn into larger insulation problems over time. Therefore, insulation resistance measurement, dielectric checks performed with suitable test procedures and trend comparison with previous measurements significantly increase maintenance quality.

If a capacitive voltage transformer, namely CVT, is used in the system, the maintenance approach should be expanded slightly. Since the health of the capacitive divider is critical in CVTs, capacitance and dissipation factor measurements should also be evaluated. Deterioration in capacitive layers, moisture effects or partial damage may not be understood by the naked eye at the first stage. However, change in the measured capacitance value or increase in dissipation factor can provide early warning about aging or insulation deterioration. Therefore, these tests should be an important part of the maintenance program in high-voltage facilities with CVTs.

One of the most valuable verifications after commissioning or major maintenance/revision is field verification of the VT circuit. At this stage, primary injection or VT circuit voltage measurement is used to check whether the voltage information reaching the relay and metering device is actually correct. Especially in directional protection, synchronism check and applications sensitive to phase relationship, polarity check, phase-angle check and grounding check should be handled together. Even if the device itself is healthy, a small mistake in field wiring can invalidate the entire protection logic.

In voltage transformer maintenance, it is not enough to measure only the values of the device itself; it must be considered together with the system to which it is connected. The metering 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 healthy, the system may continue operating with incorrect information in the field because of wrong phase connection, disconnected secondary cable or mixed terminal markings.

Recording maintenance and test results is also a separate requirement. Nameplate ratio, 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 appear not suddenly, but through slowly developing deviations. When trend analysis is performed, insulation weakening, secondary load increase or deterioration in phase accuracy can be detected earlier.

In summary, the tests and maintenance required for voltage transformers consist of visual inspection, cleaning, primary fuse and connection inspection, secondary continuity and grounding check, ratio and polarity tests, burden assessment, insulation measurements, winding resistance test when required and capacitance and dissipation factor tracking in CVT applications. After revision and commissioning, primary injection, VT circuit voltage check and phase angle verification must definitely be performed. If MV/HV voltage transformers, protection relays and metering circuits in your facility need to be evaluated together, HV/MV testing, maintenance and repair, LV/MV/HV project design and consultancy for general system design and HV operation responsibility services in operation processes can be planned together.