Standard cubicle configurations for a medium voltage metal-enclosed switchgear

MV metal-enclosed switchgear

This technical article will shed some light on the standard design of medium voltage metal-enclosed switchgear cubicles in terms of enclosure configurations as well as the characteristics of busbar system. Article explains the following cubicles types: incomer feeder, direct incomer, bus coupler, bus riser and metering.

Metal-enclosed, medium voltage switchgear cubicles and associated apparatus, rated from 1 kV to 52 kV, are covered by IEC 62271-200 (this standard supersedes IEC 60298).

MV cubicle design and construction is determined by several key operating factors and classifications:

• Rated voltage U_r (kV). Determines the minimum insulation level requirements
• Rated current I_r (A)
• Rated frequency ƒ_r (Hz)
• Short circuit power S_SC (MVA). Determines elements of mechanical cubicle design and selection of integrated switchgear apparatus
• Accessibility to cubicle compartments
• Continuation of service with main compartment open
• Necessary isolation and segregation of live parts
• Level of internal arc withstand

Please note that in addition to basic cubicle types, there are other types such as fuse-switch cubicle for transformer protection or motor control cubicles with contactor…

Table of content:

1. MV enclosures:
   1. Incomer feeder cubicle
   2. Direct incomer cubicle
   3. Bus coupler cubicle
   4. Bus riser cubicle
   5. Metering cubicle
2. Busbar system and its characteristics

1. Medium voltage switchgear cubicles

1.1 Incomer Feeder Cubicle

Diagram:

Where:

1. Withdrawable circuit breaker,
2. Current transformer set,
3. Earth switch and
4. Voltage transformer (fused and withdrawable)

As the name implies, this cubicle configuration serves two purposes, as an incomer cubicle and as a feeder cubicle.

If used as an incomer cubicle, it switches the incoming main supply onto the common horizontal busbar system of a metal-enclosed switchgear arrangement

If used as s a feeder cubicle, it switches the main supply from the common horizontal busbar system of a metal-enclosed switchgear arrangement onto a specific feeder circuit.

Current transformers are fitted to interface with a protection relay for circuit breaker trip operation. Depending on the required function, voltage transformers can be supplied. These can be 3-phase or single phase, either fixed or withdrawable style.

A variety of low voltage equipment is used, which is mounted in its own segregated compartment, situated at the top-front of the enclosure assembly.

Front, side and rear view:

Go back to Content Table ↑

1.2 Direct Incomer Cubicle

Diagram:

Where:

1. Current transformer set
2. Earth switch
3. Voltage transformer (fused and withdrawable)

A direct incomer cubicle connects the incoming main supply onto the common horizontal busbar system of a metal enclosed switchgear arrangement, without any primary switching device.

An earth switch is typically provided at the cable termination end of the circuit for isolation during shutdown and maintenance.

Current and voltage transformers can be supplied as optional items, along with a variety of low voltage equipment, which is mounted in its own segregated compartment situated at the top-front of the enclosure assembly.

Front, side and rear view:

Go back to Content Table ↑

1.3 Bus Coupler Cubicle

Diagram:

Where:

1. Circuit breaker
2. Current transformer set
3. Earth switch

A bus coupler cubicle connects two adjacent horizontal busbar systems together using a main circuit breaker (normally a withdrawable type), which is housed in its own compartment of the cubicle. The horizontal busbar system of metal-enclosed switchgear is usually situated towards the top of the cubicle enclosure.

A main earth switch, current and voltage transformers and low voltage equipment can all be supplied as optional extras.

Front, side and rear view:

Go back to Content Table ↑

1.4 Bus Riser Cubicle

Diagram:

Where: (1) – Voltage transformer (fused and withdrawable)

A bus riser cubicle contains a vertical 3-phase bus which connects the output of a bus coupler cubicle at the bottom of the enclosure, to a horizontal busbar system at the top of the enclosure.

Voltage transformers, along with low voltage equipment, can be supplied as optional extras.

Front, side and rear view:

Go back to Content Table ↑

1.5 Metering Cubicle

Diagram:

Where:

1. Earth switch
2. Voltage transformer (fused and withdrawable)

A metering cubicle contains a primary horizontal busbar system with a bus tap-off that drops vertically to the bottom of the enclosure. The vertical bus is connected to voltage transformers, which can be of the fixed or withdrawable type.

Sometimes a main earth switch is supplied. Metering equipment is often contained within the segregated low voltage compartment, located at the top-front of the enclosure.

Front, side and rear view:

Go back to Content Table ↑

2. Busbar Systems

Medium voltage busbar systems consist of two general arrangements. The main switchgear distribution bus has three busbar sets (one set per phase) which run horizontally through all the cubicles in a line-up. These distribution busbars run through a dedicated chamber within each metal-enclosed cubicle.

Segregation of busbar chambers, between adjacent cubicles, is provided by using insulated through-bushings. Inside the horizontal busbar chamber of each cubicle, a vertical feeder busbar system can be tapped off the main horizontal system, for incomer, feeder, bus-coupler, bus-riser, metering or motor starter circuit.

2.1 Busbar Ratings

The nominal current rating (I_r) of an incomer busbar system usually matches the rating of the main busbar system it is feeding. Likewise, bus-coupler and bus-riser systems have the same current rating as the main busbar system they are connecting. A feeder circuit busbar system has a nominal current rating to match the expected load.

The nominal current rating is determined by the cross sectional area, shape and configuration of the individual phase bars.

The short-time withstand current rating (I_k) of the busbar system must be greater than the highest expected symmetrical fault current at the point of installation. This rating is for a short-time withstand period of 1 or 3 seconds (t_k). All busbar systems installed in the same switchgear line-up usually have the same short-time withstand current/time rating.

The nominal voltage rating (U_r) of a busbar system must be greater than the installation’s operating voltage. This voltage rating determines the minimum phase-to-phase and phase-to-earth busbar clearances.

The nominal frequency rating (ƒ_r) of a busbar system must match the installation’s operating frequency.

NOTE! – The nominal current must be derated for high ambient temperatures (usually above 40 °C). The nominal voltage and insulation ratings of a busbar system must be adjusted for altitudes over 1000 metres.

2.2 Design

Busbar system design must consider:

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

2.3 Voltage ratings and clearance

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

Table 1 – Typical voltage ratings and minimum clearances for busbar systems and insulator standoffs

Source: derived from IEC 62271-1

2.4 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.

Table 2 – Typical current ratings and nominal dimensions for medium voltage busbar systems

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.

NOTE! – Dimensions should be used as a guideline only and may vary. The dimensions stated in this table are based on bare copper at ambient temperature of 40 °C, maximum permissible temperature rise of 50 °C, operating at 50 Hz.

2.5 Temperature rise

During short circuit conditions the busbar will rise in temperature, depending on the level of short circuit current and time duration. This temperature rise must not exceed the thermal limits of any equipment in contact with the busbar.

Table 3 – Maximum permissible temperature rise for bolt-connected devices, including busbars

Source: derived from IEC 62271-1

NOTE: – When engaging parts with different coatings, or where one part is of bare material, the permissible temperature and temperature rise shall be those of the surface material having the lowest permitted value.

2.6 Electrodynamic withstand

During short circuit conditions, the peak current associated with the first loop of the fault current produces electrodynamic forces which stress the busbar and insulator standoff supports.

Stress on the busbars must not exceed the limits of the material used. Bending forces must not exceed the mechanical limits of the insulator standoffs.

Where:

• d – Distance between phases (cm)
• l – Distance between insulators on a single phase (cm)
• F₁ – Force on busbar centre of gravity (daN)
• I_p – Peak value of short circuit current (kA)
• H – Insulator height
• h – Distance from head of insulator to busbar centre of gravity
• F – Force on head of insulator stand-off (daN)

NOTE: 1 daN (dekanewton) is equal to 10 newtons.

2.7 Resonant frequency

The busbar system must be checked for potential resonance under normal operating conditions and fault conditions. This is done by calculating the natural resonant frequency of the system, which must meet the following criteria:

• 50 Hz supply: not within the ranges 48 Hz to 52 Hz and 96 Hz to 104 Hz
• 60 Hz supply: not within the ranges 58 Hz to 62 Hz and 116 Hz to 124 Hz

2.8 Calculation requirements

Busbar systems are subjected to thermal and electrodynamic stresses under normal operating conditions, but more so under short circuit fault conditions.

To check the safety of a busbar system:

1. Check that the current rating of the busbar system (I_r) exceeds the expected nominal current. Main factors affecting the busbar rating are busbar material and configuration, ambient temperature and maximum permissible temperature rise.
2. Check the maximum expected temperature rise of the busbar during a short circuit fault. In the event of short circuit current flow (I_th), the surface temperature of a busbar must not exceed the thermal limits of any material coming in contact with it (ie insulator standoffs).
3. Check the maximum expected electrodynamic forces imparted on the busbars and insulator standoffs, due to the peak short circuit fault current (I_dyn). Do not exceed the mechanical limitations of the material.
4. Check that the busbar system will not resonate under normal operating and fault conditions.

Go back to Content Table ↑

Sources:

1. Medium Voltage Application Guide by Aucom
2. IEC 62271-200 – High-voltage switchgear and controlgear – Part 200: AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV