On-Line Inspection and Testing

On-line inspection and testing is generally limited to visual, external and physical examination in order to ensure in which the plant is in a safe condition. Infra-red detectors must be used periodically for inspection of overhead lines and open terminal, substation bus-bars for hot spots caused through faulty terminals. Further, live line washing methods are also available for cleaning overhead lines or open terminal substation insulators. Purified water along with a high resistance value is used in a fine spray fitted from well-earthed nozzle. Functional testing and trip schemes needs special switching arrangements initially to reconfigure the power system network.

Switchgear site tests during operational maintenance stage vary from utility to utility depending upon the quality of upkeep of the equipment and environmental conditions of the site. These commonly include the following checks and tests:

1. General checks involves inspection and checking of

• A tightness of terminal connection, piping junctions and bolted joints;
• Painting and corrosion protection;
• Cleanliness;
• Cracking and chipping of bushings;
• Foundation bolts; and
• Lubrication of contacts and moving parts of the circuit breakers.

2. Electrical circuit checks include checking of

• Insulation check;
• Dielectric strength of an insulating oil;
• Level of the oil;
• Quality of sf gas/ insulating medium such as humidity content, filling pressure or density except for sealed apparatus;
• Leakage of oil, etc.

3. Mechanical tests include

• Inspection of operating circuits (hydraulic, pneumatic, spring charged) and consumption during operation;
• Verification of correct rated operating sequence (recharging, etc).

4. Time checks include checking and adjustment of

• Track alignment and interlocking mechanism;
• Closing and opening times;
• Control and operation of auxiliary circuits;
• Recharging time of os mechanism after specified sequence; and
• Other specific operations.

5. Electric tests include

• Dielectric tests; and
• Testing of the resistance of main circuit.

If the substation is constantly attended, the rounds of switchgear are commonly planned for every shift so in which all the equipment will be looked at least once a day. Equipment is also inspected instantly after a trip out.Substation switchgear needs regular cleaning in accordance along with its design, type of insulation, the degree of pollution of the atmosphere or ambient air, etc. The frequency of cleaning depends upon the type of layout of the apparatus and insulators. Therefore, cleaning must be done during every preventive maintenance activity.Even by the vacuum switchgear does not need elaborate maintenance like the oil insulated switchgear, it is still essential to make periodic routine inspection. The absence of ionized gas and carbon during interruption removes the main source of insulation contamination.