MEDIUM VOLTAGE SWITCHGEAR SELECTION AND DESIGN GUIDE - SECOND CHAPTER: Switchgear Compartments

There are many different types of enclosure designs for medium voltage switchgear use. However, the most commonly accepted and used style is metal-enclosed, with segregated and insulated apparatus compartments.

There are four compartments are exist in metal clad switchgears :

• Busbar compartment
• Cable compartment
• Switching compartment
• Low voltage compartment

Busbar compartment

The busbar compartment houses the main busbar system, which is connected to the fixed upper isolating contacts of the main switchgear apparatus by means of branch connections. The main busbars are made of high conductivity copper. The busbar compartment of each panel is isolated from the busbar compartments of the neighbouring compartments.

Single or double busbar configuration is used depending on the current rating

• Version with rated normal current 1250 A, 2500 A, 3150 A or 3600 A (conductor bar connections Cu-Ag as per IEC 62 271-1)

• Busbars made of bare flat copper, bolted from panel to panel Pressure relief to the rear into the pressure relief duct

Voltage ratings and clearance

IEC 62271-1 gives typical voltage ratings for busbar systems and insulator standoffs.

Busbar system design must consider:

• Adequate minimum required clearance between phases and phase to earth
• Selection of adequate busbar insulator standoffs
• Bolting arrangements for continuous busbar connections
• Thermal effects on busbar and insulator standoffs under normal and fault conditions
• Electrodynamic forces applied to busbars and insulator standoffs under fault conditions
• Avoidance of mechanical resonance under normal operating and fault conditions

•Current ratings and dimensions

The nominal current rating of a busbar is determined by the type of material, shape and cross sectional area of the bar and the maximum permissible temperature rise of the material. If the busbar is carrying AC current, the operating frequency has a slight effect on the busbar rating due to magnetic skin effect. A busbar system has a short-time withstand current rating. The temperature rise in the event of a short circuit condition must not exceed the thermal limits of busbar standoffs.

1. Source: current rating information is derived from IEC 62271-1

• Most medium voltage switchgear including busbar systems have short-time withstand ratings of 16 kA, 20 kA,25 kA or 31.5 kA for 3 seconds.

Cable compartment

The cable compartment houses some of the following components:

1. Post insulator
2. Feeder earthing switch (option)
3. voltage transformer (option)
4. Earthing busbar
5. Operating shaft for feeder earthing switch
6. High-voltage cables
7. Ring-core current transformer

Circuit Breaker Compartment

The switching compartment houses the bushing insulators containing fixed contacts for the connection of the switching apparatus to the busbar and cable compartment. The bushings are single-pole type and are made of castresin. They are covered by metallic shutters.The metallic shutters operate automatically during movement of the switching apparatus from the test position(racked-out) to the service position (racked-in) and vice versa. Shutters may be locked if required.The position of the switching apparatus can be seen from the front of the panel through an inspection window.

1. Rating plate
2. Contact pole
3. Vacuum interrupter
4. Truck
5. Mechanical interlock for truck operation
6. Operation for racking the switching device truck
7. Fixing levers to lock the truck in the panel
8. Front plate of 3AH5 circuit-breaker
9. Low-voltage plug connector

1. Auxiliary switch S1
2. ON pushbutton
3. OFF pushbutton
4. Switch position indicator
5. 1st shunt release Y2
6. Operating shaft for the vacuum interrupters
7. Closing solenoid Y9
8. Closing spring charged/ not charged indicator
9. Operating cycle counter
10. Closing spring
11. Position switch
12. Gear for charging the closing spring manually
13. Rating plate

Low-voltage compartment

The low voltage compartment provides safe isolation from any medium voltage equipment. This is used for installation of low voltage control equipment, including DIN rail mounted terminal blocks. Equipment can be panel mounted on the LV compartment door for customer interfacing.

References :

-Schneider Electric Guide

-ABB Guides

-Siemens Guides

Also Read these Articles :

- MV Circuit Breakers

-Medium Voltage Contactors

- MEDIUM VOLTAGE SWITCHGEAR SELECTION AND DESIGN GUIDE : FIRST CHAPTER