Power Generation: Low Voltage or Medium Then Stepdown?

Interesting question, but some additional description would help. For instance, assume that the powerhouse is near the center of the property. Then it would be about 400 meters to near the corners, which is admittedly a considerable distance. I suspect this is what motivates the the medium-voltage option. But do the main loads need to be that far away from the generators? Some site plan details could shed light on this.

Medium-voltage switchgear will probably require some specialized personnel to run and maintain it, to say nothing of the cost of some large stepdown transformers. In this respect, my first guess would be to place the power plant close to the largest loads, stick with low voltage (380), and upsize the conductors to the smaller but more distant loads.

Another possibility, if medium voltage is still a viable candidate, is also to use medium-voltage motors, with smaller transformers for the remaining lesser loads. I apologize for being a bit vague, but that's the level of info so far. Another way to deal with this is to design in nearly full detail both a medium-voltage and a low-voltage network, and then compare the initial and operating costs.

As to the number of generators, I think you need assess the relevant profiles at less than full load. For example, if there is a significant time that you might need 75% of full capacity, you should choose at least 4 generators, so that almost always 3 would be available.

Dear Tornado

Thanks

The loads are 400Mtr(70%) and 300Mtr(25%). I have a sketch but dodnt know how to attach, sorry

Hari

Hi Hari,

This type of question is very suitable and relevant for this forum.You have 2 very important known parameters to start with :- 1). Capacity= 3.8MVA, 2).Area of site 600 X 600 sq M.

The points to consider are:

A).Costs and availability.It is always more economical in money and time to select commonly available voltage and capacity ranges of main equipment.MV alternators are more expensive for 1 to1.5MVA range and you will need MV switchgears to handle and distribute MV supply.Prime mover depends on its HP/KW output.Future spares and emergency replacement for LV will be cheaper and faster.

B).Location of load and power source.If noise (can be greatly redused by sound proofing of room),heat and vibration is not too big a problem,you should locate the generators very near to the points of major power consumtion.LV generation becomes suitable and will save a lot of expensive copper conductors,busducts or cables,as well as reduse voltage drop problem.

However,this is not a problem with MV power supply where you can centralised all the generating capacity in one far location and distribute the power to each load centre with cheaper and smaller 185 or 240 sq mm 3-Core copper or aluminium xlpe MV cables but the extra cost is required by stepdown transformers.An added advantage is you can have MV ring circuit or interconected supply between the substations at lower costs to secure the supply back-ups.

C).Grouping of the of the types of load ( essential and non-essential supplies ).This will affect the distribution and the security of supply to the particular load.

D).Simple/singular larger loads area (such as Administration block) can be supplied by one LV genarator and spread out medium size loads 500KVA to 750KVA can be from MV supply.

I am assuming that you are at the priliminary,budgetary/costing,planing stage.In this square site,let say you are having a very evenly distributed load centres all over the complete area which normally is not the case.You can locate 4 nos of LV (380V) 1000KVA to 1250KVA each (some 25% extra capacity for maintenance shut down ) diesel-generating stations at the 4 corners of this site.When you have the more detail and accurate data of where and how big the loads are located,then you can adjust/shift the location and change the quanta of each station to fit into the exact requirement.The costing will remain nearly the same.

In short,use supply from the power provider as far as possible.Use the same voltage if you are generating your own MV supply.Select LV generation as first priority.Need not look at 3.8MVA as one single lump of capacity.

Best regards,

Khor

Dear Khor

This very detailed information is highly appreciated. We are at a planning stage. Could this be little bit more detailed in terms of investment and operating cost comparing LV Vs MV.

I expect in future we may get the Main power source from a goverment public grid also. But in my experience (Saudi Arabia) the generation cost in private is cheaper about 30%. So We will still choose to generate own and keep the Grid power standby.

Help us more, please

Hari

Dear Hari,

I have just read your reply and I am quite surprised to know that it is cheaper to generate your own power rather than from the supply authority in Saudi Arabia.I have never been to Saudi.

I will try to answer your question as much as I can very soon and I will try to give you some figures and prices on the capital costs of the switchgears,transformers,cable etc. based on a design that I think you will need.Is this an Industrial Park that consists of a number of small factories (where loads cannot be confirmed untill very much later) or single manufacturing/industrial or housing developement? Are there any critical and special part of the supply where it cannot be stopped for a certain period of time such as plastic injection machines,smelting,heaters/arc furnance or chemical processing units,may be operation room?

I have to go out now and will continue later.

Regards,

Khor.

Thanks again dear Khor

The project is new, at an area away from town - a poultry processing plant. Majority load is refrigeration 70%. The total load will be applied during process hours say approximate 16 hours a day and during off hours a minimum of 25% load will be there as cold stores, water treatment etc, are non stop.

For your information, 1 ltr diesel in saudi Arabia only cost Rs 2.6 (Indian). If used HFO as fuel then it is half price only. Fuel is so cheap there.

1 Mtr cable 3X 120 Sq M costs 1Mtr = Rs 6250 approx (Medium voltage. Also LV cable costs about 80% of above).

I think your recomendation will favor LV generation close to the Load points!

Very interested to hear from you

Await your support

Haridas

Hari,

First, welcome to CR4, and thanks for a great question.

At 380V, your load requires more than 5700A. The conductors, circuit breaker and distribution panel must all be rated for 6kA. That may be very expensive.

My first option would be two 2MW generators rated 380V. 3000A from each is more manageable for distribution to your loads, and still avoids the costs and concerns of MV equipment. If your load is concentrated in more than one place, you could install the generators in separate locations, then "cluster" the loads around each generator to reduce the cable lengths and associated losses. Also, if you have multiple refrigeration units, processing lines and auxiliary equipment, power half your process capacity from each generator, and you could still operate at reduced capacity if one generator was out of service.

Regards,

Tony

Thanks Tony

Very good suggestion here also

I am looking for a rough estimate on MV Vs LV to convince the Director Board now who are primarily believing on the MV generation and step down etc old traditional thinkers.

Appreciate if some cost figures available with any one

Thanks Hari

This is really a great deal, of extended help

Thanks lot

I have problem to see the Sketch not visible even I clicked "show Picture"

If you dont mind could it be emailed to harivdas@hotmail.com

Thanks a lot Dear Khor

After a brief study of the proposal, excuse me I am end up with few doubts. Would you please help to clear me?

1. Why cant us Generate 6.8 KV and distribute (Ring Main), then step down at each load point?

2. How can we dedicate an incomer MV for future Grid connectivity?

3. In your suggested scheme we generate 380V, Step up, Distribute, and Step down then to load. If we do directly generation 380V and distribute also directly to the loads, as another scenario. Which will be cheaper? Any guess

Sorry to bother again and again I wish to be convinced

Regards

In my design,I have given priority to uniform/modular equipment to be used and also common range of commercially available rating/specification thus obtainable at competitive prices and quicker delivery from many manufacturers.The added advantage is also the need to keep only one set of one type of spare parts rather than a set each of several types.The extra cost for this part of the design is the two step-up transformers which may be lower than the extra costs of 2 or 3 units of 6.8KV alternators.Please check out the difference in cost for 380V alternator and the 6.8KV alternator for a total of 3 MVA involved.

1).You can generate 6.8KV directly and distribute the power and then step-down at load areas.That is perhaps using 2 X 1500 KVA ,6.8KV diesel gensets.The 6.8KV from each generator can be supplied directly to the main VCB board via 2 VCB incomer panels.These 2 VCB should be equiped with synchronisation of the two sources of MV supplies to have a "common pool of supply" for varous loads,L1,2,3...

You still require MV generator boards to control and protect the genset.

2).You can put another incomer-3 VCB panel with a second VCB busbar coupler to receive future Grid supply when it is available.However,These two panels must be electrically and mechanically interlocked with incomer 1 and incomer 2 ( treated as a single group ).If combine supply of generated as well as grid supply is required,then incomer-3 must have synchronising components to syn with either incomer-1 or 2.

3). Answering your 3rd question,my guess is that there will be not too much of a difference as mentioned in my answer no 1.

4). There are several cheaper designs and alternatives which you can explore,but it is not the same thing.

Best regards,

Khor.

Thanks dear Khor

Let me try to get some cost figures, will feed you back soon

Regards

Hari