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

Current transformers convert the high current in the primary circuit into a safe and standard value on the secondary side for the correct operation of measurement systems and protection relays. Therefore, a small ratio error, polarity problem or secondary circuit break in the current transformer; It not only produces inaccurate measurements, but may also cause the protection system to trip late, trip unnecessarily, or not work at all. For this reason, the tests and maintenance that must be done on current transformers are not only to keep the equipment clean; It is also critical for facility safety, energy quality and business continuity.

Current transformer maintenance should generally be addressed under three main headings: visual and mechanical checks, electrical tests and circuit verification studies. In the first stage, cracks in the resin body, surface contamination, traces of moisture, carbonization, signs of overheating, loose connections, damaged cable lugs and terminal markings are checked. Maintenance should not only consist of testing, as dust, dirt and surface contamination can affect the insulation performance in the long term, especially in measurement transformers operating in MV cubicles.

One of the most basic operations to be carried out during periodic maintenance is cleaning. However, instead of random cleaning, a controlled approach that will not damage the insulation of the equipment is required. The current transformer manufacturer's maintenance instructions clearly state that excessive dust and dirt accumulation should be removed with a soft brush. Since harsh interventions can cause unwanted damage to the resin surface or secondary cables, the method used by maintenance personnel is at least as important as the maintenance itself.

The most critical safety issue of maintenance in current transformers is the secondary circuit. The secondary of a working current transformer should not be left open. Unused secondary windings should be short-circuited and properly grounded. In addition, it is generally desired to apply grounding from a single point in the secondary circuit. Failure to comply with these rules may cause dangerous voltage to occur at the secondary terminals and damage the transformer. Therefore, secondary bridges, short circuit blocks, test terminals and grounding connections must be checked before maintenance.

One of the first steps in electrical testing is the current transformer ratio test. The ratio test checks whether the primary-secondary conversion matches the label value. Wrong ratio selection or winding problem, especially in facilities operating with ratios such as 100/5, 200/5, 300/5 or 600/1; It affects many results, from meter reading to relay setting. Ratio control should be reviewed not only during initial commissioning, but also after secondary circuit changes, relay replacements or cell revisions.

Polarity test is one of the basic verifications that should not be neglected in current transformers. Failure to connect P1-P2 and S1-S2 directions correctly may cause serious errors, especially in differential protection, directional protection and energy measurement applications. Therefore, polarity testing should be evaluated together with ratio testing; If necessary, field cabling should also be included in the same control chain. It is not enough for the device to be robust, the connection logic between the CT and the relay or measuring device must also be established correctly.

In current transformer tests, burden, that is, secondary load evaluation, should be considered as a separate heading. Because the real performance of a CT is revealed not only with the ratio written on the label, but also with the cable length, relay input, meter circuit and terminal transitions to which it is connected. It is emphasized in the manufacturer's technical documentation that current transformer behavior under special load conditions should be evaluated similar to the real load condition. Therefore, as the secondary circuit load on the field increases, a more careful testing approach is required in terms of ratio and phase error.

In cases where more detailed evaluation is required, winding resistance test is applied. This test is an important tool in detecting problems such as open circuit, short circuit or broken connection in the winding circuit. Diagnostic documentation states that measuring DC winding resistance is an established method for identifying shorted circuits and open circuit problems. This test is a powerful diagnostic tool, especially if one core behaves differently from the others, problems occur at certain stages in multi-ratio CTs, or abnormal values ​​are obtained in the secondary circuit.

Excitation test and knee point evaluation are also of great importance in current transformers used for protection purposes. The excitation characteristic shows when the CT will approach saturation and how much reliable information it will carry to the relay in case of fault currents. Modern CT testing systems; It can analyze parameters such as ratio and phase accuracy, winding resistance, excitation characteristics, knee point, burden impedance, remanence and residual magnetism together. Especially in systems with differential protection, high impedance protection or sensitive fault analysis, it is often not enough to be satisfied with just ratio testing.

Insulation tests are used to understand current transformer problems such as aging, moisture absorption or surface deterioration. In diagnostic resources, diagnostic measurements for measurement transformers are divided into two main groups: electromagnetic circuit and isolation system; On the insulation side, it is stated that insulation tests, capacitance and dissipation/power factor measurements and partial discharge analyzes are used in advanced applications. Insulation health should also be monitored in critical facilities, especially in resin MV current transformers, as cracks, impacts or production-related gaps may cause PD behavior in the future.

One of the most valuable verifications after commissioning and major overhaul is the primary injection test. This test includes not only CT; It is used to verify that the entire connection is established correctly, from CT to relay, from relay to trip chain. Some manufacturer sources clearly state that primary injection can be used to verify that all trip system connections have been made correctly. So, while secondary injection tests the relay itself, primary injection provides a more comprehensive look at verifying the entirety of the actual current path in the field.

In addition, controlling the secondary loop resistance is also very important in practice. Because in the secondary circuit, elements such as cable, joint, test block, terminal and relay input directly affect the total load. In the commissioning documentation, CT secondary loop resistance measurement is recommended to verify that the secondary circuit DC resistance is within appropriate limits for the connected equipment. This check should be done carefully, especially in systems with long secondary cables, multiple intermediate terminals and relay/counter combinations.

In current transformer maintenance, testing alone is not enough; The results also need to be recorded. Label rate, class, burden, test date, measured values, differences with previous tests, connection changes made and any nonconformities should be tracked in a single maintenance file. Because deterioration in CTs often occurs not suddenly, but with slowly developing deviations. When trend monitoring is done, ratio shifts, insulation weakening or unexpected resistance increases in the secondary circuit are noticed earlier.

In summary, the tests and maintenance that must be done on current transformers are; It consists of visual inspection, cleaning, secondary safety checks, ratio and polarity testing, burden assessment, winding resistance, excitation-knee point, isolation checks and primary injection verification when necessary. Which test will be done and how often; It should be determined according to the voltage level of the facility, the criticality of the equipment, environmental conditions and the structure of the protection system. If you want to evaluate current transformers, protection relays and measurement circuits together in your MV/HV facility HV/MV testing, maintenance and repair, for general field suitability LV/MV/HV project design and consultancyOn the business side SA operation responsibility services can be planned in an integrated manner.