What are the Tests and Maintenances That Should Be Done on MV Cells?

MV switchgears are critical switchgear equipment used for safe switching, protection, measurement and distribution of medium voltage electrical energy. For this reason, the tests and maintenance that must be done on MV cubicles are not just about opening the cell cover and looking at it visually. The main purpose is to verify that the breaker, disconnector, earth separator, busbar system, cable headers, current transformers, voltage transformers, protection relays and auxiliary circuits work together safely. Because a small connection looseness, insulation weakness or mechanical failure in the MV switchgear can turn into a serious outage and security risk over time.

The first step in maintenance is always safety. Before working on the MV cubicle, the cell must be de-energized, the relevant supply points must be safely separated, the absence of voltage must be verified with an appropriate method and the necessary grounding procedures must be carried out. Any intervention at medium voltage level must be carried out with the correct maneuver sequence and locking procedure. A separator or earth separator maneuver performed in the wrong order during cell maintenance can pose a serious risk to personnel and equipment.

Visual inspection is the basis of MV cubicle maintenance. Switchgear body, covers, locks, observation windows, busbar compartment, cable compartment, breaker compartment, relay compartment and terminal connections should be carefully examined. Symptoms such as rust, corrosion, moisture marks, dust accumulation, burning smell, tarnish, loose parts, cracked insulator, deformed cable head or loose connection should be taken seriously. Most of the MV cubicle failures show their first signs during visual inspection.

Inner cell cleaning should be done regularly. Dust, moisture, metal shavings, insect residue or foreign matter can weaken the insulation security. Especially the cable head area, insulator surfaces, terminal areas and busbar connection points should be kept clean and dry. Appropriate insulating material and the correct method should be used during cleaning, and the equipment should not be damaged by random chemical or pressurized applications. The risk of surface leakage and partial discharge may increase in an unclean MV cubicle.

Mechanical maneuver control is one of the most important parts of maintenance. The breaker, disconnector, load disconnector and earth separator must operate correctly in open and closed positions. If there is a hardening of the mechanism, spring installation problem, mechanical jamming, incomplete maneuver, position indicator incompatibility or unusual sound, a detailed examination is required. If the mechanical part of the MV switchgear is not healthy, the expected opening or closing may not occur in the event of a fault, even if the electrical protection system works correctly.

The interlock system must be tested. MV switchgears have a locking mechanism that provides a safe maneuvering sequence between the breaker, disconnector, earth separator and door locks. The purpose of this system is to prevent accidental access to the live section and incorrect maneuvering. Door interlocks, earth separator interlocks, breaker position conditions and mechanical-electrical interlocks must be verified during maintenance. A cell whose interlock system is disabled or malfunctioning cannot be considered safe.

Breaker tests must be performed separately in MV cubicles with breakers. The opening and closing times of the breaker, spring winding mechanism, motor feeding, opening coil, closing coil, auxiliary contacts and mechanical working counter should be checked. Timing measurements can be made with suitable test devices. If there is an abnormal deviation in the on-off time, time difference between phases or difference in coil current, the mechanical or electrical condition of the breaker should be evaluated in detail.

Contact transition resistance measurement is an important test for breaker and some main current paths. Increased contact resistance at the main contacts or connection points may cause heating under load and serious failure in the future. Contact transition resistance should be low and balanced between phases. A significantly higher value in one phase compared to the others may indicate a loose connection, contaminated contact, wear or mechanical contact problem. This test helps catch thermal failures before they occur.

Insulation resistance measurement is one of the basic tests frequently used in MV cubicle maintenance. Phase-phase and phase-ground insulation status can be checked in the busbar, cable, breaker and other insulated parts of the switchgear. It may warn about low insulation value, moisture, contamination, superficial leakage, damaged insulator or cable termination problem. However, insulation measurement alone may not be sufficient for a definitive decision; The results should be evaluated together with historical values, environmental conditions and visual findings.

Cable terminations require special attention. MV cable terminations should be examined for surface cracks, marks, darkening, signs of oil or moisture, shield connection errors, loose connections and improper assembly findings. Faulty cable head may lead to partial discharge, heating and insulation failure over time. Phase clearances, cable supports, shield grounding and header stress control zones in the cable compartment must be carefully checked. A significant portion of MV cubicle failures may start from the cable connection area.

Busbar connections and insulators should be examined in detail during maintenance. If there is loosening of the busbar bolts, cracks, contamination, surface marks or color change in the insulators, this may be a symptom before failure. There should be no foreign objects in the busbar compartment, the connection points should be checked for tightness and, if necessary, torque control should be performed. Since the busbar system is the main energy route of MV cubicles, any problem that occurs here may affect the entire cell group.

Current transformer and voltage transformer circuits should also be checked. CT ratio, polarity, secondary connection continuity, short-circuiting terminals, grounding point and protection-measurement core separation must be verified. On the VT side, fuses, secondary connections, phase sequence, connection order and measurement-protection feeds should be examined if there is an open triangle circuit. Fault in the CT or VT circuit may cause incorrect measurement of the protection relay and erroneous tripping or non-tripping.

Protection relay tests are an indispensable part of MV cubicle maintenance. Overcurrent, ground fault, directional protection, voltage or other relevant functions must be verified by secondary injection testing. The pickup values, time curve, binary inputs, trip outputs and alarm contacts of the relay should be checked. Seeing the value on the relay screen is not enough; It should be tested that the relay gives the opening command to the breaker from the correct output in the real fault scenario.

The trip circuit and control circuit should be tested separately. Breaker opening coil, closing coil, spring winding motor, control switch, local-remote selection, auxiliary relays, DC supply and fuses should be checked. Even if the protection relay operates correctly, the breaker may not trip if the trip circuit is faulty. For this reason, protection relay test and trip circuit test should be performed together in MV cubicle maintenance. Real safety is ensured when the relay's decision reaches the breaker.

Thermal camera control is a very valuable maintenance method in MV cubicles. In the thermal examination under load, cable headers, busbar connections, breaker connections, fuse holders, disconnector contact areas and panel connection points can be monitored. A significant temperature difference between phases may indicate a loose connection or increased contact resistance. Thermal control is a powerful method for detecting unnoticeable heating problems at an early stage.

Partial discharge control becomes especially important in critical MV cubicles and aged systems. If there are signs of partial discharge on insulator surfaces, cable terminations, busbar compartments and intra-cell insulation systems, this may turn into insulation perforation in the future. Partial discharge is not always visible; Sometimes it is noticed by sound, smell, surface traces or special measurement methods. Therefore, in critical facilities, MV cubicle isolation health can be evaluated with more advanced diagnostic methods.

In switchgears with fuses and load disconnectors, the fuse and disconnector mechanisms should be examined separately. Fuse holders, fuse holders, striker pin mechanism, load disconnector contacts and mechanical connections should be checked. If a fuse has been replaced, the correct type, correct rating and correct installation must be verified. Wrong fuse selection or incorrect placement can cause serious problems in transformer protection.

In RMU and gas-insulated cells, additional checks appropriate to the cell type should be made. The gas pressure gauge, alarm contacts, mechanical indicators, joystick mechanism, cable compartment, grounding scheme and signs of leakage should be examined. Random interventions should not be made in gas insulated systems, and the manufacturer's procedures should be followed. In this type of cells, a problem that seems simple from the outside may indicate a more critical situation in the internal structure.

At the end of the maintenance, all results should be recorded. Insulation values, contact transition resistances, breaker on-off times, relay test results, thermal camera findings, cable head observations, mechanical maneuver status and settings should be reported. If a comparison is made with old measurements, the tendency to deteriorate will be noticed earlier. The healthiest approach to MV cubicle maintenance is not a one-time check, but regular recording and trend monitoring.

In summary, the tests and maintenance that must be done on MV cubicles are; It consists of safe working preparation, visual control, cleaning, mechanical maneuver test, interlock verification, breaker timing test, contact transition resistance measurement, insulation resistance measurement, cable header and busbar control, CT-VT circuit control, protection relay secondary injection test, trip circuit verification, thermal camera inspection and partial discharge evaluation when necessary. If these processes are carried out regularly, it is possible to ensure safe operation of MV cubicles, reduce the risk of failure and protect the energy continuity of the facility.