11KV Busbar Sizing

[SK07]

Arrangement of an Existing 11KV Switchgear receiving power from 132KV substation through cables is as follows;

Upstream 132KV Substation - 4 Nos. of 132/11 KV 30 MVA Transformers feeding power to 11KV Switchgear at downstream. The configuration is 3 + 1 hot standby, which means all 4 Transformers are energized at all times and connected to downstream 11KV Switchgear through cables (less than 100 Meters distance).

Downstream 11KV Substation - 4 Nos. of Incomers (3 working + 1 standby) rated 11KV 2000 Amperes with Bus couplers (3 Nos.) 11KV 3150 Amperes.
All Incomers (4 Nos.) and Bus couplers (3 Nos.) are switched 'ON' at all times. Busbar rating is 3150 Amperes.

The question is on Busbar rating 3150 Amperes, which does not seem to have been sized as per standard engineering practice.

Also, can we reduce the busbar rating to 2500 Amperes.

Please provide your feedback on the adequacy of busbar rating of 3150 Amperes selected for above 11KV Switchgear and possibility of reducing to 2500 Amperes for cost saving.

What ideally could be the right busbar size and what can be the concerns in the above mentioned scenario.

 

[che12345]

"... Up stream 32/11 KV 30 MVA Transformers feeding power Downstream 11KV Substation ...4No. breaker each rated 11KV 2000 Amperes with Bus couplers (3 Nos.) 11KV 3150 Amperes. All Incomers (4 Nos.) and Bus couplers (3 Nos.) are switched 'ON' at all times. Busbar rating is 3150 Amperes...The question is on Busbar rating 3150 Amperes, which does not seem to have been sized as per standard engineering practice.."
I have the following opinion for your consideration.
1. Trafo 30 MAV 11kV, breaker 2kA , no issue.
2. 3 No. of 2kA breaker connected to BB and Coupler rated 3150A may NOT be of any issue if the total load is well established/certain < 3150A, which is the standard size. The next higher rating would be certainly of higher cost.
3. Consider that 30MVA trafo can only supply around 1.5kA each, the current that may go through the couplers <3150A, with the loads equally distributed on each section of the BB. In view of the cost, 3150A BB and couplers is "ideal" at this stage, with not much room for future expansion.
Che Kuan Yau (Singapore)

 

[Parchie]

@SK07,
You haven't posted the temperature at which your installation is designed for. Can you specify if it's for 30 degrees, 50 or 65 degrees C?

 

[SK07]

@ Parchie

50 degree C

 

[che12345]

@SK07,
" You haven't posted the temperature at which your installation is designed for. Can you specify if it's for 30 degrees, 50 or 65 degrees C?... 50 degree C ".
1. Please do NOT answer for the fun if it. Ambient temperature 50 [sup]o[/sup]C is certainly UNUSALLY high. You may have to ventilate the switch room to < 40 [sup]o[/sup]C , otherwise all the gears including busbar would have to be DERATED ! Please reconfirm that the ambient temperature and the location is <1000m altitude?
Che Kuan Yau (Singapore)

 

[SK07]

@che12345

The project site location is in Middle East. The substations are air conditioned.

The Specifications mentions ''Equipment located inside buildings shall be rated for continuous operation throughout the range of specified ambient temperatures, without regard for the air-conditioning. All electrical equipment shall continue operation without failure under the temperature from -3 degree C to 50 degree C'.

Ground elevations above the lowest spring tide (LST) vary between 5M and 150 M above mean sea level.

 

[che12345]

"..... #1. All electrical equipment shall continue operation without failure under the temperature from -3 degree C to 50 degree C....#2.Ground elevations above the lowest spring tide (LST) vary between 5M and 150 M above mean sea level".
1. Fine. The condition is in the contracture document, therefore should be comply with. WARNING: Consult the OEM to have their written current derating factors for the gear and the busbars; respectively. Attention: IEC or IEEE gears ratings are based on <50 [sup]o[/sup]C.
2. Ground elevations 5m and 150m , no issue.
Che Kuan Yau (Singapore)

 

[Parchie]

@SK07,
Your 3150A busbar rating @ 50 deg C rating is an overkill, IMO (if there are no future expansions being contemplated). Your expected full load is just half of that.
If the install is still on the drawing board, I recommend you downgrade to 2300A (1/2" x 4") or 2750A (1/2" x 5"). The rule of thumb is to maintain a 1000A per square inch current density for busbars under 3000A. Using 2300A gives you a current density of 961A/ in2 though. Skin effect ratios are just close to the 3150A bussing which IIRC is 1.34.(skin effect ratio of 2300A is 1.25 while skin effect ratio of 2750A busbar is 1.30)
If the project is already in-place, just live with it. There's no harm in having equipment sized larger than what is needed.

 

[SK07]

@Parchie
Reference my original message, the total working capacity of Substation is 30MVA x 3 Nos = 90 MVA = 4725 Amperes. Power Factor is maintained at 0.95.

4th Transformer of 30 MVA is hot standby. The Switchgear has 4 Bus sections with 4 Incomers (3 working + 1 hot standby).

Now, if we go by standard engineering practice based on total Transformer Capacity of 90 MVA, the bus bar rating should have been minimum 4725 Amperes + derating factor due to temerature of 50 degree as applicable.

As mentioned the existing busbar rating is 3150 Amperes (@ 50 degree C). This is based on that each transformer / incomer (total 3 working, excluding hot stand by) will contribute maximum 1575 Amperes. Total for 3 Transformers / Incomers will be 1575 x 3 = 4725 Amperes.

All Incomers & Buscouplers are CLOSED under normal operating conditions and each bus section designed for 30MVA will share load of total 90MVA (equal to the rating of 3 Nos. of working Transformers).

Now, the questions as mentioned in above original message;
1. Can we reduce the busbar rating to 2500 Amperes for future substations.
2. Your feedback on the adequacy of busbar rating of 3150 Amperes selected for above existing 11KV Switchgear since as mentioned as standard practice it should have been minimum 4725 Amperes.

Let us leave temperature derating for manufacturer to work on that and if required increase the size of busbar & related circuit breakers from the minimum required that we decide based on Transformer rating.