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Thread: help on transformer size

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    help on transformer size

    Hi Guys

    Been a while not being here .I need help asap.I have 1 pump motor 1,6km (9 KW) from the nearest electrical point , 2 more 180m (3 KW) and 90m (2 KW ) away in diferrent directions from the 1st one ( 9KW ). What type of cabling do i use and what kva transformers do i use ?

    I appreciate your help guys .Dave, Andy, Murdock ?

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    Full Member MullerR's Avatar
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    Hi Jaboel.

    Does the installation look something like the attached drawing?Click image for larger version. 

Name:	TRFR and Motors Distance.jpg 
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    Diamond Member AndyD's Avatar
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    I'm a bit confused, you've got some odd sized motors that don't conform to standard kilowatt ratings???? What is your proposed network going to consist of and how/where does the transformer fit in? How are the pumps configured, will they be switching together or are they individually controlled? Some more info would be good.
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    The 1.6KM site I would use a step up 11KV and step down transformer of no more than 15KVA to save on cabling over the 1.6KM. The other 2 sites standard cable to handle the voltage drop over the distance probably would be a cheaper option.

    I suggest you contact a transformer manufacturer (Transelectron) based in Germiston who will be able to advise more on your requirement.
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
    Solar and LED lighting solutions - www.microsolve.co.za

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    Diamond Member AndyD's Avatar
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    Once you get over 1KV you would need to get special permission from the supply authority, it falls outside of the SANS1042 regs. By all means do the figures but once you're at 1KV or above you're into MV cabling, MV rated enclosures and switchgear etc so it might not be cheaper than the larger cable with LV. Maybe look at a soft starter for the pump as well to keep the start/inrush current and volt-drops within sensible limits.

    I've used asymmetrical transformers before to supply a well pump that was over 1km away and was another 140m down a hole. At the supply end we stepped up to 990v and used standard XLPE SWA cabling which was rated to 1KV, at the pump end we stepped back down to 400v. We ratioed the step up and step down transformers slightly differently to compensate for some of the volt drop as well. From memory we saved 20-25% on the materials cost by doing this but if you've not done this type of install before you might want to factor in for some school fees.
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    Quote Originally Posted by AndyD View Post
    I'm a bit confused, you've got some odd sized motors that don't conform to standard kilowatt ratings???? What is your proposed network going to consist of and how/where does the transformer fit in? How are the pumps configured, will they be switching together or are they individually controlled? Some more info would be good.
    Hi Andy

    The network will consist of 3 pumps , 7,5kw ,2,5kw and 1,5kw . All these pumps are seperately controled with DOL 3phase starters.
    All of these pumps will pump the water into a dam . Since the distance are so far from the nearest consumption point , i will be needing
    a step up and step down transformer . Its on a farm . Nearest motor 7,5kw are 1600m away from the closest electrical point above ground level.
    The 2nd motor 2,5kw are 180m away and the 3rd 1,5kw motor 90m away from the 7,5kw motor above ground . My question is , can i get away
    with a 1000v step up and 1000v step down transformer? I have been looking to a save costs by using a overhead 35mm ABC bundle conductor
    between the 2 transformers and 6mm or can i get away with a 4mmm armoured cable to the 3 motors. the 7,5kw will be 10m ,the 2,5kw 180m
    and the 1,5kw 90m away from my step down transformer . Compensate the distance down the borehole estimated on max 100m .

    Please advice Guys ?

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    Hi Just

    A 11kv are a bit expensive .

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    Hi Muller

    It looks like that . Look at my response to Andy . I need you guys to assist me with the cheapest possible but the correct exceptable method .
    I appreciate your help .

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    Diamond Member Justloadit's Avatar
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    Which ever way you are going to take, it is no simple walk in the park.
    You have to do the maths for each method that can be used to achieve your requirements.

    Careful consideration has to be taken into account for each method including external factors which may influence the cost of the installation.
    You have to almost walk the terrain, and look for possible points which may influence the installation which may bring in added cost.

    If you are going overhead supply, you have to almost feel the ground where you are going to dig the holes to plant the poles.
    You have to visualize where the cable will be fed from pole to pole, and see if there are any obstructions.
    Maybe look for an contractor who does this kind of work on a regular basis, and has the experience to give you a quote.

    Another option is to approach ESKOM to install a 400V point where you require it, and from there feed the motors.

    Maybe solar may be a solution.
    Just remember the most cost effective solar solution would only pump water when there is sunshine.
    For 7.5KW you would need around 42 (250W) solar panels. Brochures tend to under spec the number of panels required. You have to add approximately 40% to the panel capacity for winter water pumping, and to allow pumping from earlier/later in the day.
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
    Solar and LED lighting solutions - www.microsolve.co.za

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    Diamond Member AndyD's Avatar
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    Quote Originally Posted by Jaboel View Post
    ......Nearest motor 7,5kw are 1600m away from the closest electrical point above ground level.
    The 2nd motor 2,5kw are 180m away and the 3rd 1,5kw motor 90m away from the 7,5kw motor above ground
    So schematically it looks like this.

    Click image for larger version. 

Name:	Pump Scheme1.jpg 
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    As Justloadit suggests a solar installation might be viable given the distances involved and the expense of the other options ie enormous cable sizes or MV transformers etc. There's many factors to consider and the only way to arrive at the best solution is to basically go through the entire design process for each option (LV, MV and PV) and QS them to cost them out.

    I'd hazard a wild guess that if the projected running duration of all the pumps is low (a few minutes per hour) then maybe a 2 or 3kW solar PV array with battery storage and an inverter to drive the pump might be the way to go. You could then also run a smaller supplementary supply from the closest DB and make the PV and storage installation grid tied for extra flexibility. If pump run times are long then I'd be leaning toward DB power and possible MV transformers.

    I envy you, it's an interesting project you can get your teeth into. Unfortunately you can only get limited assistance from the forum because even to begin with a design you need intimate knowledge of the entire system and it's application as well as contingencies or scale-ability requirements future expansion. Projects like this are usually destined to be a raging success or doomed to be an expensive failure in the very early design stages where the big decisions are made so this is the critical point where you need good solid advice from a PV expert and an MV distribution expert with relevant experience. I'd suggest you liaise with the client and get these advisors on board.
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