Comparison of 6.6 KV System vs 11 KV System

Economy and merits are two-edged swords.

You have to to the cost benefit analysis based on the load, losses, cost of the transmission infrastructure, switchgears etc for both the systems.

For example at our place we have 6.6KV lines since quite a few of out motors are on 6.6KV and the cabling are through underground cables.

Though this could have been 11KV too had we didn't have the motors in so many numbers. At individual workshops of course we have to again step down to the 230/415V levels.

The final is the overall cost (and that includes the $s lost through losses)

11KV system will call for less No. of power cables as compared to 6.6 KV system.However, if you have any 6.6 KV motors to be fed ,then you have to go for 6.6 KV system.If your motors are of capacity higher than 1000 KW,it will be economical to go for 11 KV system.

if your plant already running & there is no expasion then there is no thinking of changing to 11kv. you can think only if line faults are more(for overhead lines) or supply is interrupting frequently then switching to 11kv can be considered.

if there is expansion then only it can be considered with following consideration.

if you are having 6.6kv equipment(like motors,fans and so more) then it will not be advisable to switch to 11kv

consider line losses, & protection equipment

Dear Sir,

Plant capacity of transformer is 35 KVA. It is not clear what is the user voltage. However it is preferrable to step down from 11 KV for various units of plants.This is due to maintaining voltage at optimum level for starting voltage drop out in machines.

Dear sir,

my clarification is same 35MVA power demand. some plants are running with 6.6KV & some plants are running with 11KV. volatge drop is not an issue in our plants.(both are HV voltage only.)

regards,

N.Ganesan

Dear Sir,

Now it is clear that plants are running with tensions of both 6.6 KV and

11 KV. If changes or renewals are to be done in plants then it is better to have

one system of 6.6 KV.Technoeconomically seeing the cost of breakers, starters,

cables as well control of equipments 6.6 KV may be a better choice. Cost

benefit analysis may be done for the changes/renewals.It is better to have one

system of supply.

Wishing you best,

If you have an existing 6.6KV system then stay with that, there's nothing worse that having two distribution voltages running side by side.

On the LV side I had the misfortune to work on a plant with 550/440/380V all in close proximity. It was total chaos!

This is the new plant.we are analysing the merits & demerits of both the system.

Pl. reply few more question.

1.Captive power plant and its capacity if any.

2.No. of substation and there capacity.

3.All Voltage level under consideration (for MV and LV)

4. Share of power among Grid and in plant generation if any.

5. Mode of operation with grid.

Dear Sir,

1. we are getting power from EB only.

2.4 to 5 substations with capcatity of 4-5 MVA/each.

3.220KV/11-6.6KV/433V.

4.No

5.No

Dear Ganeshakce-electrical,

Before I reply one more question

1 Maximum rating of Motor and its starting method.

Dear sir,

4000KW mill application HT motor. starting through LRS.

regards,

N.Ganesan

The choice which I would make would be 132KV, 6,6KV and 415V voltage level

You would be needing Transformer with OLTC 132/6.6 to regulate bus voltage in you plant. The bus current at 6.6 KV with 35 MVA load would by 3100 Amps. Getting Switch board of such rating is not an issue.

At substation level you could have 6.6 KV for MV motors including big motor and 0.415V for LV Motor up to say 200 KW.

If you select 11KV and .415 V you are left with two choices

1. Use 11 KV MV motor for small ratings which is uneconomical choice

or

2. Use LV motor rating as high as 500 KW less reliable.

dear sir,

thanks for your valuable reply.

but my transformer shall be 40 MVA. in this case my full load current is around 3500 A. so i shall be selected to 4000A breaker.in this case 4000A VCB is available in market or not.

regards,

N.Ganesan

You may visit website of SWGR manufacturer like abb.com and check as far as I know it is not a problem

Dear Mr. Ganesan,

If rating of the largest drive motor is 4000 KW,then I would recommend you to go for 11 KV system in view of less No. of power cables both for the motor as well as the incomer from the power transformer to the 11 KV main switch board.In that case, availability of suitable VCBs will not be a problem.Accordingly you may go for 11 KV/415 V ,1000 KVA or 2000 KVA distribution transformers for feeding LT loads which can cater to LT motors upto 250-300 KW capacity.

In case you also have a number of motors of 300-1000 KW capacity ( such as pumps, fans, conveyers etc ) , you may go for 11 KV/6.6 KV ,4-8 MVA transformers alongwith 6.6 KV switch board at the concerned load centre.These transformers can be fed from the 11 KV main switch board.

The above kind of arrangements are working in large integrated steel plants.

As yours is green field plant, I would advice to go for 11kV and reasons are as below:- (1.1) Technical merits- (a) reduced fault level (b) lesser sizing for cables (as in cement plants there are considerable number of HT drives for ball mill, cement mill, crusher, etc, hence it will give you good benefit of it in cables, cable trays and accessories, erection steel, termination, etc) (c) lesser space requirement ref Panel GA of siemens and ABB hence civil requirement can also optimised. (1.2) Technical demerits-(a) motor frame size may increases to next frame, mostly (also depending on rating) (b) transformer dimensions will be higher. (2.1) economical view- now a days there is hardly any difference in 11 and 6.6 kV switchgear. Hardly 10-15% difference max. If you are ready to pay for 6.6 kV then just some premium you may have 11 kV system. But please note that, due to increased voltage level, insulation requirement is more hence, design also changes in case of transformer and motors. Cable cost also increases in some minor percentage. We may conclude that, commercially there is certainly price increase in 11kV system by about 15% overall than 6.6kV system.

I think it will come down the "bean counters" to decide. Whatever works out cheapest!

As an engineer you can talk until you're blue in the face, the accountant will say what goes unfortunately.