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Cable Testing : Circuit Breakers : Co-ordination Studies : Low Voltage : Maintenance :
MV Switchgear : MV Testing : Substation Structure : Transformers

MV Switchgear

A switchgear inspection and maintenance program is an important factor in system reliability. Although the ultimate long term performance of the switchgear depends on the reliability of its insulating components and an inspections should be primarily directed to at the insulation details, it is recommended to inspect the condition of all switchgear elements.

There are many things to look for when inspecting switchgear. Some of them can be checked before de-energizing the switchgear equipment:
• sound caused by electrical discharges or sparkling or noises of vibration caused by electrical resonance
• visible blue or purple corona halos
• ozone odor produced by corona or overheating of insulation materials

After de-energizing and before any cleaning of the switchgear components interior of the switchgear should be checked for physical damage of structural elements, loose hardware and signs of the moisture. Only after all of these preliminary works complete cleaning, inspecting and testing should be performed at switchgear components. Inspection and maintenance operations on the switchgear can be divided into two broad areas; the circuit breaker and cell equipment. The switchgear cell is usually divided into couple separate compartments to comply with the requirements of switchgear construction. The number and type of the compartments primarily depends on the switchgear construction type. A very brief explanation of both switchgear configuration types is given below.

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Metal-Clad Switchgear

Metal-Clad switchgear type usually has the next compartments:
• circuit breaker compartment
• main busbar compartment.
• cable connection compartment,
• low voltage compartment.

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Circuit Breaker Compartment

The circuit breaker compartment contains the circuit breaker mounted on a withdrawable carriage which permits the breaker to be taken out of the enclosure. The front door of the compartment is provided with a viewing window to observe the status of the circuit breaker without opening the door. The door also has a shutter through which the racking handle can be inserted to rack the circuit breaker in or out.

 

What should be inspected on a circuit breaker during a regular maintenance program?

Contacts

The functioning of the circuit breaker depends on the contacts. When closed practically entire load current passes through them. They also must withstand complete overload or short-circuit current. If the resistance of the contacts becomes high they will overheat which can cause damage or destroying of the complete breaker. Due to this regular maintenance of those are important and it should include cleaning and smoothing.

Arc Interrupters
The arc interrupters have a role to disperse and cool the arc flame. Regular maintenance of arc interrupters includes cleaning, checking for physical damage and performing an a-c high potential test across the arc chute.

Operating Mechanisms
The operating mechanisms function is to open and close the breaker contacts. It consists of a series linkages connected to a power mechanism, either solenoid or a spring devices. Inspection should include cleaning, Checking for wear and hand closing the breaker to make sure all parts operate freely.

 

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Auxiliary Devices
Auxiliary equipment includes control relays and switches and protective relays. During regular maintenance program they should be cleaned and tested for operation. Also, protective relays should be checked for setting to assure they are coordinated with other relays in the system.

Main Busbar Compartment
The busbars provide the path for current to flow from the utility lines to the transformer. Regular inspection can easily detect a pending problem due to cracked or broken components. All buses and their support insulators should be cleaned, checked for spacing and inspected for tightness. Further to this, the megger test should be performed on the bus insulators. All bus control wiring, instrument transformer leads, mountings, and all other devices also should be inspected for tightness. A part of this compartment can also be lightning arresters.

Lightning Arresters
Basically, the lightning arrester provides the first level of protection to the electrical system from voltage spikes. As power quality becomes increasingly of concern, proper care and maintenance of the lightning arresters become more apparent. Undetected problems or failed arresters may result in the loss of sensitive electronic equipment due to lightning strikes. As a standard practice, full maintenance program should verify the integrity of the lightning arresters insulation body by megohmmeter test.

 

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Cable compartment
Cable compartment is usually located at the rear of the cubicle. Connections can be bottom or top entry cables type or top entry bus duct type. Also, this compartment can be equipped with the pressure relief vents.
A regular maintenance consists of cleaning high voltage insulation and cable terminations, checking for physical damage and a megohmmeter and high-potential testing.

Low Voltage Compartment
Low voltage compartment is completely segregated from the high voltage compartment. This compartment can contain protection devices, meters, instruments, control switches, pilot lamps etc. which are usually mounted on the hinged door. Fuses, auxiliary relays, terminal blocks etc. are mounted inside this compartment.

Metal Enclosed Switchgear
In case of the metal enclosed switchgear configuration the compartments are:
• load break switch compartment and main bus compartment
• power fuse and cable compartment
• low voltage compartment if required.

Loadbreak Switch Compartment
A loadbreak switch compartment usually contains the load break switch and main bus connection. The compartment door is interlocked with the power fuse compartment door and equipped with a window to view the position of the switch blades.

 

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Power Fuse and Cable Compartment
A power fuse and cable compartment is normally combined. The power fuse compartment door is interlocked with the switch handle and the load break switch compartment door so that this door cannot be opened if the switch is closed and will not permit the load break switch compartment door to open if the door is closed.

Low Voltage Compartment
A low voltage compartment also can be part of the metal enclosed switchgear. The compartment can contain mainly terminal blocks, control fuses, thermostats and occasionally current transformer test blocks and ammeters.

 

Cable Testing : Circuit Breakers : Co-ordination Studies : Low Voltage : Maintenance :
MV Switchgear : MV Testing : Substation Structure : Transformers