Speak EV - Electric Car Forums banner

1 - 20 of 39 Posts

·
Co-author of CanZE. Q210 nov 2013
Joined
·
4,405 Posts
Discussion Starter #1
I am looking for people who could assist in shedding more technical light on the required grounding and when/how Zoe dismisses it as inadequate. The next introduction may contain errors. I'd stand happily corrected, it is not my field of expertise, but I am intrigued how this works / should work. Please join in with facts, knowledge, proposed measurement procedures, etcetera.

INTRODUCTION

All somewhat more serious electrical installations require grounding. In many (not all) countries, ground is tied closely to N (neutral). For discussions sake, we'll dismiss the countries where this is not the case. Ground resistance is, AFAIK set in norms. If I am not mistaken, maximum ground resistance is somewhere in the neighbourhood of several hundred ohms and might differ per country. I'd love to hear an official (sourced) value. Resistance however, is always measured between two points. For a decent measurement, one needs a reference ground (better two or three) and there should be no current injected to ground in close vicinity of the measurement. The rule of thumb is at minimum a radius of 5 times the size of the grounding probe. It's actually a tad of a black art and the equipment is pretty specific. Sometimes it's called a reverse megger ;-)

ZOE

Zoe is picky about the ground resistance, but by definition does not have a guaranteed-good reference. For starters, it's sitting on 4 rubber wheels :) The only way Zoe can do this measurement is by approximation using the N wire as reference. However, this imposes several challenges.
  • N is simply not the same as ground reference
  • Even is she's not charging, downstream there might be serious currents pulling N voltage away from ground
  • If she IS charging, in a single phase scenario, her own N feeding wire is pulling 10 amps minimum and up to 32 amps leading to significant difference between N and GND. On my granny charger, fixed at 12 amps, it's enough to let a LED with a 180 kOhm resistor in series (!!) glow a bit.
  • If she's charging on 3 phases, N in an ideal world is not carrying any current, but the world is not ideal. Average current will be much lower compared to 1 phase, but spiking might be higher.
SO WHAT?

I am intrigued how Zoe does her "grounding is (not) OK" measurements. We've seen several hypotheses on this forum, but I'd rather get some facts. If anyone is in the know, please share. Until then, here are a few of these hypotheses and I'd be curious what the technically inclined think and what would be the best way to confirm or dismiss these. Once we've established a baseline, we (I) might be able to build an emulator and identify iffy chargers and iffy Zoe's.

HYPOTHESES (plural)

  1. voltage measurement after the chargepoint contactor has closed but before the Zoe closes hers.
  2. same, but injecting a small, fixed current between N and GND
  3. same, but to exclude all sorts of nasty power grid influence, use relatively high frequency AC (high, is in e.g. 1kHz)
  4. same as 1, but now during charging (and using wider margins of course)
  5. same as 4, but using different margins for 3 phase charging
And then of course probably filter out harmonics.
 

·
Registered
Joined
·
109 Posts
I am looking for people who could assist in shedding more technical light on the required grounding and when/how Zoe dismisses it as inadequate. The next introduction may contain errors. I'd stand happily corrected, it is not my field of expertise, but I am intrigued how this works / should work. Please join in with facts, knowledge, proposed measurement procedures, etcetera.


HYPOTHESES (plural)

  1. voltage measurement after the chargepoint contactor has closed but before the Zoe closes hers.
  2. same, but injecting a small, fixed current between N and GND
  3. same, but to exclude all sorts of nasty power grid influence, use relatively high frequency AC (high, is in e.g. 1kHz)
  4. same as 1, but now during charging (and using wider margins of course)
  5. same as 4, but using different margins for 3 phase charging
And then of course probably filter out harmonics.
I would say:

1 Easy to implement so very likely.
2 I wouldn't do that: Very difficult to do safely and reliably.
3 Difficult and I don't hear anything that might indicate it.
4 Possible using charger's switching frequency to test. Maybe on off section of switching.
5 Cannot see the need for different margins. I believe, if Zoe's 3 phase input is configured as a star, it is effectively three single phase inputs in parallel. Also doubt Zoe knows whether she is charging on single or three phase.

Probably could test as follows:

1 Disconnect E and feed from a Variac: Caution: Very dangerous effectively live working.
2 Place a high power low value resistor between N and E and measure voltage across it.
3 As 2 but using an oscilloscope to measure voltage.
4 As 3.
5 As 3.

Be fun testing.
 

·
Co-author of CanZE. Q210 nov 2013
Joined
·
4,405 Posts
Discussion Starter #3 (Edited)
Thank you, and I don't disagree.

Another option would be to put said resistor in the GND wire and test with a scope if there are any signals that might indicate some sort of active measurement.

I agree connecting GND to N (car side, leaving the grid's GND disconnected), while not "wise", creates the absolute base reference.

Edit:
1. Fully agree and likely, though what would "0 volt mean"? Dangerous to assume "all is A-OK"
2. So I have a weak spot for 2 :)
3. OK
4. I doubt that to be honest. Assumed routing is: AC filter -> Recifier -> Big caps -> MOSFET switchers. Would be hard to both try to get rid of the harmonics and use them at the same time.
5. See next comment
 

·
Co-author of CanZE. Q210 nov 2013
Joined
·
4,405 Posts
Discussion Starter #4
I disagree on the three phases not being influential though. Also in a star, with resistive loads, the current through N will be zero. Add 3 sinuses 120 degrees out of phase to see why. It's the "with resistive loads", aka assuming neat sinusses that's killing the premise though. IMHO.
 

·
Registered
Joined
·
109 Posts
Yeah, but we know the motor windings are used in the switching converter so load is not perfectly resistive although I assume EMC requirements necessitate PFC. I'd have thought assuming 0V N-E as OK would be a reasonable assumption give car is isolated from actual earth due to rubber tyres and fed through an RCD. So charger becomes

Filter > Rectifier > Capacitor> Motor Winding > Switching Transistor > Battery

Shouldn't be difficult to set capacitors correctly to suppress harmonics.
I was thinking that the quenching diode current could be used in the test.
 

·
Banned
Joined
·
789 Posts
Zoe isn't picky! She is just safer than all other ev's. Given she charges at so many different rates that other ev's can't, I'd say it's a good thing. This is pretty insignificant anyway, my zoe has done 30000 miles and she has bci'd once and that was a dodgy chargedisaster post.
 

·
Co-author of CanZE. Q210 nov 2013
Joined
·
4,405 Posts
Discussion Starter #7
@ActEVist You are right and we tend to take this feature for granted too much I agree. Having said that, let us try to stay on topic.

@Paul Prosser How do you see the diodes used in the test? Any idea how could we confirm that, and then emulate?
 

·
Registered
Joined
·
2,653 Posts
Because we do not know how the Zoe monitors things, then very difficult to discuss. But the electrics exist in the real world and the three phase system is never really balanced. With 32 Amps drawn and say 32 mA trip that is 1/1000 of the current in each phase. So the tolerances in the car need to be very well designed. but what if a single phase car plugs in as well ? Locally we have three phases but one is loaded with 64 Amps and the other 2 with 32 Amps. How far away is the local substation ? What other loads are un balancing the three phases ? What cable has been used ? The real world has so many variable factors that influence things that are hard to nail down.

Richard
 
  • Like
Reactions: yoh-there and emc

·
Co-author of CanZE. Q210 nov 2013
Joined
·
4,405 Posts
Discussion Starter #9
@halcyonrichard Thank you! Absolutely true, though we shouldn't confuse GFI trip current (which is a true and dangerous error, and for which 32 mA is a reasonable value, and which is NOT measured through the grounding wire), with neutral imbalance for which 32mA is absolutely unthinkably tight. I think we agree on this.
 

·
Banned
Joined
·
789 Posts
All of this sounds very technical and relies on the success of the installation by companies that cut corners and scrimp on costs! One network comes to mind and the director is still blaming zoe as a car and not his poor quality equipment and dreadful installations.
 

·
Registered
Joined
·
109 Posts
@yoh-there Sorry I think you misunderstand. When the converter switches off each cycle, the inductive current must be quenched through a diode to N. This will give a pulse which MAY be used by Zoe to detect faults and could give us a handle on strategy. Thinking aloud.
 

·
Co-author of CanZE. Q210 nov 2013
Joined
·
4,405 Posts
Discussion Starter #13
@Paul Prosser The entire switching is done after rectifying and I am reasonably confident this is bridge rectified to a plus and minus rails, then switched / "inductified" :whistle: / quenched. What I am saying is I don't see how the car could work this back to N. But indeed maybe I am not getting what you're trying to say?

It seems I should simply start by doing some measurements over a very small resistor in the N and GND wires with a scope, to get some sort of baseline idea, scratch a few hypotheses?
 

·
Registered
Joined
·
109 Posts
@yoh-there You're probably right there. I was sort of brainstorming and thinking there wouldn't be room for large enough capacitors to completely smooth the rectified mains, so hf current pulses would appear on the mains input etc.
 

·
Co-author of CanZE. Q210 nov 2013
Joined
·
4,405 Posts
Discussion Starter #15
I agree. Rectifying in itself only uses the "top of the sine", so it starts there already with the harmonics.. And the switching frequency certainly flows back to the mains: listen to my PV inverter when she charges!! Thinking out loud: for a 10 volt ripple after the rectifier, when the charger takes 16 A, you'd need a cap in the order of 0.16 F if I'm not misteken. That's beefy, read, unrealistic indeed.
 

·
Registered
Joined
·
109 Posts
And from that I would infer that current could be measured in the N line with a current transformer and compared to the possibly phase shifted voltage ripple on the E maybe. Seems a lot of complication but who knows.
 

·
Co-author of CanZE. Q210 nov 2013
Joined
·
4,405 Posts
Discussion Starter #17
I have already build (almost finished) a device that can measure at the same time, charging voltage and current, so that would enable me to view (and share) harmonics, switched power supply feedback and the supposed insane inrush currents (which I highly doubt, but again, that will prove one or the other). I also borrowed a 4 channel scope, so there is progress. Those intended measurements have near to nothing to do with the grounding subject, though I might be able to re-use some of the components.
 

·
Registered
Joined
·
2,653 Posts
Hi,
I have been reading the Fastned story on line. They had problems with the ZOE stopping charging. May have nothing to do with earthing but may cause confusion. From :- ps://issuu.com/jessicachristina/docs/fastned_story_2_en
Fastned ZOE.jpg
 
  • Like
Reactions: yoh-there

·
Co-author of CanZE. Q210 nov 2013
Joined
·
4,405 Posts
Discussion Starter #20
They've had two problems with Zoe's that might help you guys in the UK debug a bit.

1) Zoe sometimes stops charging for a few while kicking in the active battery cooling through the Clima. The old firmware in FastNed's ABB chargers interpreted this as an end-of-charge and basically waved the car goodbye. This is non-standard behavior (by the charger) and they've fixed it with a firmware update that ends the session when the pilot signals's resistance is removed, read, plug removed, and since then, all is nice and dandy.

2) Currently there is a very edge case problem that ONLY applies when all 4 of the following conditions are met
  • only on certain older Q210 models
  • only on 43kW chargepoints
  • only on low SOC and high temperature (so there is actually a realistic 43 kW charging session going on)
  • only on stations that have their own distribution transformer (about 10 out of their 50+ stations)
FastNed has been very forthcoming with the issue, and has switched half of the AC chargers of the stations affected (read, with their private distro trafo) to 22kW, both chargers clearly stickered. They've also done measurement sessions together with ABB and Renault on site *). We've NOT been made privvy of what the technical outcome was, other than that the charger (not the car, and not the transformer protection system) was ending the session. Word in the street was it was "Renault's problem to fix". I do suspect a nasty harmonics or switching frequency coming back through the supply, but nobody is talking, and I have seen way too many "Zoe is at fault" for what was actually non standard compliant behaviour of the chargepoint (i.e. item 1) above) to take this as gospel, though I am sure many happily would *). Let me repeat this is a 43kW issue in specific circumstances and by definition does not apply to the R240.

*) note Renault's swift and correct involvement in the issue. They have an interest to get to the bottom of things like this. I have heard (meaning it has the same reliability level as "Zoe is at fault" BTW) they flew in specialists from France to look into this, together with ABB.

Thank you for bringing this up @halcyonrichard , but it is not related to Grounding.

@ActEVist No doubt and good for you!! Things are running along absolutely smoothly here and as I said, both issues are/were real, but 1) is fixed quite a while ago and 2) is very, very much an edge case.

Back to topic please :)
 
1 - 20 of 39 Posts
Top