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How DC chargers work

Posted: Wed, 15 Aug 2018, 10:45
by brendon_m
I've been trying to work out how a DC fast charger works, as in internally not how they charge charge a car.
I've tried googling it but my Google fu seems to be failing me and I just keep coming up with "they charge with dc and not ac and it's fast..."

Do they just pull a heap from the grid under demand? Surely not because you would need massive cables to pull 150kw instantly or more with these new standards, wouldn't you?
Or do they have a high capacity battery inside on location? I don't think they are big enough?
So a bucket load of capacitors?
Or a combination?

Re: How DC chargers work

Posted: Wed, 15 Aug 2018, 11:11
by Rusdy
Hi Brendon, 150kW is not hard to get (although not trivial). From Tritium user manual (albeit, 100kW station only), it recommends 50mm2 cable (150A feed).

Bunch of no-gooders broke the world record in Kwinana sometime ago, requiring 800A feed (half Mega-Watt). Too bad, I can't find the detailed info how much it pulled from the grid. I remembered it mentioned somewhere in this forum, but I can't trace it :oops:

Re: How DC chargers work

Posted: Wed, 15 Aug 2018, 11:54
by MDK
brendon_m wrote:
Wed, 15 Aug 2018, 10:45
I've been trying to work out how a DC fast charger works, as in internally not how they charge charge a car.
They rectify AC into DC, use the DC to drive IGBTs at a high frequency to create AC and then rectify that back into DC at the desired voltage.

And as Rusdy says they tend to just have a very large connection to the grid, though Tesla is considering using powerpacks alongside their superchargers, and @jonescg has been chomping at the bit to wire a large battery in parallel to a 50kW charger (eg in Fremantle where the grid can't handle it)
Rusdy wrote:
Wed, 15 Aug 2018, 11:11
Bunch of no-gooders broke the world record in Kwinana sometime ago, requiring 800A feed (half Mega-Watt). Too bad, I can't find the detailed info how much it pulled from the grid. I remembered it mentioned somewhere in this forum, but I can't trace it :oops:
There were two 400A three-phase feeds (one per warehouse) and we got pretty close to 400A/phase in one warehouse, and ~330A/phase in the other.

Kwinana-feed-sm.jpg
Kwinana-feed-sm.jpg (526.18 KiB) Viewed 1936 times
As you can see near the bottom of the image we pulled ~287kW in one warehouse and ~247kW in the other.

Balancing a bunch of single-phase chargers across six phases was an interesting challenge!

Re: How DC chargers work

Posted: Wed, 15 Aug 2018, 12:14
by brendon_m
OK, so they do just lay large cables in and pull straight from the grid. That's what I was trying to work out but I couldn't find it.
At 150kw I can see it working fine but the new 350kw standard surely will push it, and more so if there are multiple stations (like at a servo on a highway might have in the future). And I was reading the other day about a 800kw standard. That's a lot of electrons...

Re: How DC chargers work

Posted: Wed, 15 Aug 2018, 15:18
by Rusdy
brendon_m wrote:
Wed, 15 Aug 2018, 12:14
... And I was reading the other day about a 800kw standard. That's a lot of electrons...
Indeedy. I'm very interested to see whether DC microgrid will come into play due to this reason as well. Even Siemens already mentioning MVDC network for AEMO. Why not even go further down to LV network?

Re: How DC chargers work

Posted: Wed, 15 Aug 2018, 15:25
by brendon_m
Simply put, DC connection has higher capital costs but lower line losses.
I'm not sure I trust that articles author

Re: How DC chargers work

Posted: Wed, 15 Aug 2018, 15:32
by Rusdy
brendon_m wrote:
Wed, 15 Aug 2018, 15:25
Simply put, DC connection has higher capital costs but lower line losses.
I'm not sure I trust that articles author
Hi Brendon, benefit in using DC transmission is already well documented. For example, here.

Specific to the lower line losses argument, this is due to DC doesn't suffer from reactance losses. The legacy benefit of AC in transmitting higher voltage (hence less loss) is no longer true (since DC technology now can do that). Although, DC equipment is more expensive to date.

Re: How DC chargers work

Posted: Wed, 15 Aug 2018, 16:02
by brendon_m
The things you learn.
However I wonder if the benefits of 3 phase out weigh the reactance losses

Re: How DC chargers work

Posted: Wed, 15 Aug 2018, 16:31
by jonescg
This is the basic layout of DC fast charger:
DCFC circuit.jpg
DCFC circuit.jpg (71.26 KiB) Viewed 1893 times
1. Input precharge circuit to protect switching gear from high inrush currents.
2. Input full bridge rectifier
3. Input capacitors for smoothing the DC.
4. IGBT array for generating high frequency AC
5. Isolating transformer (can also be used to step voltages up or down)
6. Output rectifier to convert HF AC back to DC
7. Snubber filter
8. Output chokes
9. Output filter capacitor
10. Reverse protection diode
11. Prehcarge circuit for inrush suppression when connecting the battery to the charger.

Re: How DC chargers work

Posted: Tue, 21 Aug 2018, 09:58
by LouB
Are these proposed 350kW, and possibly 800kW charging stations, predicated on the general adoption of solid state and/or ultracapacitor batteries? Wouldn't current Lithium-ion batteries be seriously degraded by the heat generated (unless charging rate is drastically controlled)?

Cheers
Lou

Re: How DC chargers work

Posted: Tue, 21 Aug 2018, 10:01
by jonescg
Nope - just size them accordingly. 350 kW at 700 volts is 500 amps. As long as the battery was bigger than 500 Ah, that's only a 1 C discharge. Battery buffers for fast chargers makes even more sense when considering the instantaneous loads on the grid.

Re: How DC chargers work

Posted: Tue, 21 Aug 2018, 12:03
by LouB
Thanks jonescg. Your figures make sense.
I just asked because of reports like this:

Tesla explains why it limits Supercharging speed after high numbers of DC charges

Link: https://electrek.co/2017/05/07/tesla-li ... r-charges/

Cheers
Lou