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Topic Title: EV RE-charger
Topic Summary: How to connect an electric vehicle to a syngas fuelled generator?
Created On: 29 August 2012 11:08 AM
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 29 August 2012 11:08 AM
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Caradog

Posts: 2
Joined: 27 December 2008

Hello Renewable Energy experts,
I want to install an electric vehicle charging point to be supplied from our demonstration syngas- fuelled engine/generator. It is currently outputting up to 20Kw to a 3phase resistor load bank and I could do with some advice on a cost-effective solution:
Should I convert the 3phase output from the engine/generator to a single phase output - (I've never done this before but assume its just a case of changing tapping wires), or should I just take one of the three phases to the ev-charge-point (230V/16A required), leaving the resistor bank connected?
I suppose the alternative would be to use a 3phase to single phase converter but suspect that this could become very expensive (supplier recommendations)?
 30 August 2012 11:39 AM
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aroscoe

Posts: 91
Joined: 18 October 2002


What line-line voltage is your existing generator?

If it is 400-440V line-line (RMS) at 50Hz then in theory you COULD connect a single phase-neutral (assuming you have an accessible star point) to your single-phase EV charger, giving 230-240V RMS, with approximately 6-7kW (~28A) capacity on a single 230V single-phase outlet. This will operate.

However, this wouldn't be recommended in the longterm since essentially you will be applying 100% negative phase sequence currents to your poor generator, which will result in a power (torque) ripple from 0 to double the EV charger power, 100 times per second (twice the electrical frequency). Its likely you could damage the generator/diesel bearings etc over time. The electrical voltage unbalance will also be non-ideal. But, it may not be a problem for a small generator - you could ask the manufacturer.

It would be much better for you if you could either get a three-phase charger, or, if you could arrange for balanced loads on the three phases. There isn't much point leaving your resistor bank connected since it will just waste energy, and won't reduce the power/torque ripple, you actually need to try and add equal loads on the other two phases (ideally at the same power factor) to match the existing single phase load.

Andrew

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Dr. Andrew Roscoe

http://personal.strath.ac.uk/andrew.j.roscoe
 30 August 2012 03:28 PM
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ArthurHall

Posts: 736
Joined: 25 July 2008

I dont know what it does to harmonics or the negative and zero sequence currents, but I have used a Scott connected transformer in the past to spread a single phase load across three phases.
 30 August 2012 05:13 PM
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aroscoe

Posts: 91
Joined: 18 October 2002

Arthur,
Interesting ... I had to look that one up. Unfortunately it won't help in this case since the single phase EV charger fundamentally demands power on a single phase (i.e. 100Hz cyclic) power basis, and the Scott 3/2 phase transformer can only produce a balanced three-phase loading if the two-phase loading is also balanced. if you only load one of those 2 phases the power ripple still flows right back to the three-phase winding again. There's no equivalent 3/1 or 2/1 phase transformer - its not possible to make a transformer do this due to the laws of energy conservation. Given a three-phase supply and a single phase load, the only way to mitigate the unbalance (power ripple) is to add some form of energy storage device. It would be possible to make a piece of power electronics to do this, with storage in a DC bus capacitor, but as Caradog says this will be an expensive bit of kit.
It is worth finding out from the manufacturer if your generator/engine will just be able to stand the single-phase (and torque ripple) loading at your 3-4kW demand. Especially if this is a demonstrator and won't be running all the time, maybe you just go with the single-phase connection. If you're wanting to run this 24/7 though, you probably need to be more wary of longterm damage.
If not, a three-phase charger is probably the best bet. A three-phase charger doesn't need to have a much bigger component count (or cost) than the single-phase version (in fact the DC bus capacitor requirement is less due to the same power ripple arguments above). Of course, if you have already bought your single-phase charger, this argument doesn't work unless your supplier is prepared to do you a swap of some kind.
Andrew

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Dr. Andrew Roscoe

http://personal.strath.ac.uk/andrew.j.roscoe
 31 August 2012 02:50 PM
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Caradog

Posts: 2
Joined: 27 December 2008

Thanks Andrew and Arthur,
The generator is used for demonstration only (so not connected to the grid). We thought it would be good to show an environmentally friendly use for our synthesised gas by charging an electric vehicle i.e. leaving 3 phases connected to our load bank, achieving 16amps/phase load then switching one of the phases to charge our EV. Now you mention it, I guess the current would drop as the battery charge increased - which would further exacerbate any phase imbalance). Oh well.
Thanks and Regards
 31 August 2012 03:27 PM
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aroscoe

Posts: 91
Joined: 18 October 2002


Hey, don't give up. I wouldn't want to be the one to derail your project since you've obviously got quite a bit invested in it already. Everything I said about unbalance is true, but at the end of the day you are only proposing to put (at worst) a 3-4kW (16A) single phase loading onto a 20kW 3-phase generator. It isn't optimum on a long-term basis due to the unbalance, but it will work, and over short periods will not destroy your genset. I have been wondering how much portable power generators can cope with (e.g. Aggreko) but I can't find the answer obviously on their website.

You say something interesting now as well. What you could do is have your loadbank on, say, 10kW loading (3-4kW per phase) and then switch out just one phase of it and replace with the single-phase EV charger. The power draw of the charger MAY change during the cycle if it is a Lead-acid charger, but it may not change very much at all if it is a constant-current NiMH or Li-Ion type device.

If this is a demonstrator and you have the kit already, I would be tempted to just go ahead and do it. When/if you want to scale it up to more EVs or higher duty, then you should probably go for three phase chargers if you can, or a grid-connected system (which will "soak up" the unbalance for you).



Andrew

-------------------------
Dr. Andrew Roscoe

http://personal.strath.ac.uk/andrew.j.roscoe
IET » Energy » EV RE-charger

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