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Author Topic: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications  (Read 2436 times)

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Offline IanB

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #25 on: September 24, 2017, 11:22:27 AM »
Similar to the previous post, I was thinking of testing continuity by injecting a small test current across the contacts from an isolated and floating supply. The system would be set up in such a way that the test current can be distinguished from the normal system current, either by polarity or by frequency.

However, that system is not fail safe, since a fault in the detection circuit would give a false impression that the contactor is open.

The engineer in me would prefer a mechanical system. When the contactor is open something must move and provide a positive indication that the contactor is in the open position. If the contactor can "stick" closed and yet still enable a mechanical indication of open, then I start to have my doubts about the design of the contactor. Since I am unfamiliar with this field, I do not know what such contactors look like or how they are constructed.
I'm not an EE--what am I doing here?
 

Offline DrGeoff

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #26 on: September 24, 2017, 11:37:52 AM »
The ideal solution would be some kind of mechanical detection for closed contacts, since you cannot rely on any system voltage or current being present with the contacts welded closed (other isolators or fault detection circuits may have tripped).
Was it really supposed to do that?
 

Offline Spaghetti_142

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #27 on: September 24, 2017, 12:04:08 PM »
Diode's?
 

Offline Spaghetti_142

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #28 on: September 24, 2017, 12:14:18 PM »
I agree way to much juice could be drawn, called ARC FLASH. 75ft away if testing.
 

Online Brumby

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #29 on: September 24, 2017, 12:46:16 PM »
This is an idea that's been floating around in the back of my head - and something similar has been mentioned.

A non-contact system that relies on the difference in capacitance between the system when the contactor is open versus closed.  The "capacitance responsive circuit" could be anything that takes your fancy.

The capacitive plates could possibly be something as simple as foil wrapped around the input and output power cables.  Set your capacitance by how big they are.

Certainly there would need to be some characterisation of such a circuit to identify open and closed contactor conditions.  There would also need to be some protection from transients - and if the detection circuit were to inject an AC signal, then perhaps some ferrite beads either side of the detection system to prevent escape of the injected signal and protect it from spurious signals being picked up.  With this approach, frequency of operation is a key consideration.


There's probably a lot of issues that I don't have credentials to address ... but it's just an idea...
« Last Edit: September 24, 2017, 12:48:27 PM by Brumby »
 

Offline DrGeoff

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #30 on: September 24, 2017, 01:10:14 PM »
Why not ask Andy (photonic induction)? He has a lot of experience with power systems.
Was it really supposed to do that?
 

Offline oldway

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #31 on: September 24, 2017, 05:51:52 PM »
Don't forget that there is always some inductance in the circuit (wiring) and that you can expect an overvoltage (U=dI/dt) when you are interrupting current.
The simple circuit I designed is well protected against such overvoltages.
That's the reason why I choosed to use 6 wirewounds resistors 5W in serie with the reed relay.

To be reliable, the circuit must be as simple as possible.....that's what I tried to do.
Max leakage current is 21.7 mA at 800V which is < 30mA.
Primary safety must be done by protection against contact with live conductive circuit, then a max leakage current < 30mA is correct as secondary safety.

NB: If battery is earthed, you can't use 2 contactors....the earth connection can't be interrupted.
« Last Edit: September 24, 2017, 06:30:41 PM by oldway »
 

Offline IanMacdonald

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #32 on: September 24, 2017, 06:28:42 PM »
Think it's also generally the case that contactors are not considered safety isolation devices. There are manual isolators available that have forced contact break mechanics and are designed for this type of situation. Though, the safest way to work on any high power equipment is plug out of socket.

Short of someone spilling patent snake oil on the cable, it stays disconnected.  ;)
 

Offline oldway

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #33 on: September 24, 2017, 06:35:49 PM »
Think it's also generally the case that contactors are not considered safety isolation devices. There are manual isolators available that have forced contact break mechanics and are designed for this type of situation. Though, the safest way to work on any high power equipment is plug out of socket.

Short of someone spilling patent snake oil on the cable, it stays disconnected.  ;)
Yes indeed, I already wrote this.

Quote
For voltages as high as 700V, the only valid safety is a visible disconnection of both poles (disconnector) and a circuit / load earthing.
All subway cars have such a device, it is mandatory.
But elima is a "safety expert" he said, he does not need to discuss about safety concern.... :-DD
 

Offline elima

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #34 on: September 24, 2017, 07:09:24 PM »
Thank you all for the replies. The systems I work on consist of low voltage DC and high voltage DC. The low voltage DC controls of course the high voltage system with a very complicated circuitry and lots of safety measures taken. Never said that I am a safety expert and whether I am or I am not, I can always learn from others.
Attached is a very simplified schematic of the HV system side with the contactors that I am talking about so that people can have a slightly better understanding of the needed circuitry.
Mechanical contacts have proven to be reliable this far but they can also fail. I am looking into improving the systems from several points as per the requirements. In some systems expenses and weight are not an issue while in others they can be. Designing such circuitry can be used to increase the diagnostic coverage of the systems with aux. contacts and thus make them more safe. I would basically like to have an electronic circuit backup for the mechanical contacts as well in systems with mechanical contacts.
Having a test current controlled from the LV system(isolated..) is the preferred approach as I would like to have the system as safe as possible. AC impedance measurement might be the right approach but I am still open for other suggestions. I have seen myself few years back such circuit with few transistors, diodes and isolated DC-DC.
For the safety experts here: with the electric cars of these days in which voltages are higher than 400Vdc, where does the end-user have those visible disconnections for both poles? Enlighten us.
 
 

Offline oldway

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #35 on: September 24, 2017, 07:31:53 PM »
Quote
For the safety experts here: with the electric cars of these days in which voltages are higher than 400Vdc, where does the end-user have those visible disconnections for both poles? Enlighten us.
I never worked with this technology, only on DC drives up to 550Vedo 5000A for metalurgy , choppers (subway), high power No-breaks, SMPS, thyristorized industrial battery chargers up to 330V 600A, rectifiers up to 3000Vdc 1500A and electrical distribution cabinet up to 120Kv....
But I imagine that both polarities of the battery are fused and that the visible desconnections are done by removing both fuses.

Your design has an obvious safety failure as only one polarity of the battery is fused.
« Last Edit: September 24, 2017, 08:12:03 PM by oldway »
 

Offline SeanB

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #36 on: September 24, 2017, 08:24:54 PM »
Electric cars do have disclosed and labelled disconnect loops in multiple places, and any one of them being removed or cut disables the high voltage circuits. these are generally known to the fire services and first responders who attend to the accidents.
 

Offline sibeen

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #37 on: September 24, 2017, 09:29:17 PM »

Why does the battery bank "float" with respect to ground? This is unusual and not permitted with 125VDC battery banks I've worked with in buildings.


One of the issues with an international forum, rules may be different :)

I've worked with floating systems up to about 900VDC in Australia. In fact, as personal safety went I always felt way more assured with that style of system. Could lick the battery terminal at any battery without an issue.

As this is the beginners page I will now add, "kiddies, do not try this at home".
 

Offline IanMacdonald

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #38 on: September 25, 2017, 01:05:09 AM »
There are pro's and con's of earthed neutrals and floating supplies. 

Earthed neutrals increase the risk of a shock if you touch the live whilst earthed. Which is why isolated supplies are best for opened equipment on the electronics test bench.

On equipment in use, floating supplies create uncertainty as to whether a voltage (to earth) is to be expected on any given cable, and that may depend on whether a parasitic earth has arisen somewhere else in the circuit. An earthed neutral avoids that uncertainty. Of course that is only true on single phase supplies.

The other issue with floating supplies is the need for double pole switching and/or double pole fusing. A circuit in which only one conductor needs to be fused and switched is simpler and less likely to be subject to unexpected fault conditions such as one pole of the double pole switch failing, leaving an inoperative but live piece of equipment. Early UK supplies had neutral fuses, and it was found that they caused accidents rather than preventing them.
 

Offline tpowell1830

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #39 on: September 25, 2017, 01:51:24 AM »
Thank you all for the replies. The systems I work on consist of low voltage DC and high voltage DC. The low voltage DC controls of course the high voltage system with a very complicated circuitry and lots of safety measures taken. Never said that I am a safety expert and whether I am or I am not, I can always learn from others.
Attached is a very simplified schematic of the HV system side with the contactors that I am talking about so that people can have a slightly better understanding of the needed circuitry.
Mechanical contacts have proven to be reliable this far but they can also fail. I am looking into improving the systems from several points as per the requirements. In some systems expenses and weight are not an issue while in others they can be. Designing such circuitry can be used to increase the diagnostic coverage of the systems with aux. contacts and thus make them more safe. I would basically like to have an electronic circuit backup for the mechanical contacts as well in systems with mechanical contacts.
Having a test current controlled from the LV system(isolated..) is the preferred approach as I would like to have the system as safe as possible. AC impedance measurement might be the right approach but I am still open for other suggestions. I have seen myself few years back such circuit with few transistors, diodes and isolated DC-DC.
For the safety experts here: with the electric cars of these days in which voltages are higher than 400Vdc, where does the end-user have those visible disconnections for both poles? Enlighten us.

Thanks for posting the information. Those are totally enclosed KILOVAC relays made by Tyco, very nice. No visible moving parts. May be difficult to find the components (appropriate rating of diode and resistor) but a snubber circuit across the contacts would help, reducing the chances of welded contacts. As far as sensing whether it is open or not is a challenge. I believe you said in an earlier post that there may not be current running through the contacts even if relay is closed? A few coils of wire around the lead wire would sense current, but if you have no current, that will not work. A voltage divider across the contacts (high value resistance) would be a check, but if there is no current load, again may not work and is not isolated. You could make it isolated by firing a opto-isolator with the voltage/current from voltage divider and that would tell you the contactor is open.

The diagram helped, some of the letters are mirrored but I was able to read them, curious?
PEACE===>T
 

Offline elima

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #40 on: September 25, 2017, 02:40:48 AM »
In my application, the end-user has an access to a high voltage disconnect which basically cuts one pole of the battery. Same is done in electric vehicles. There is a high voltage disconnect connector that basically cuts the loop of one pole wiring. If there are multiple battery containers, each should have a high voltage disconnect. For the people that will work with the high voltage system, there are internal interconnects that can be removed and they basically divide the battery into smaller stacks with voltages below 60Vdc for each stack and each stack has multiple small fuses.
I have been working with such systems for 3 years now and never had any welded contactor but this doesn't stop me from trying to improve my system.
To avoid problems with earthing/grounding, high accuracy insulation measurement device is used between the HV and LV system and can easily detect any problem. In case of a problem, the HV system is cut immediately and latched to that state.
How would a second fuse help in this application?

Thanks.

 

Offline floobydust

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #41 on: September 25, 2017, 05:48:28 AM »
I have a few ideas, roughly drawn here. I'm assuming a power system for a nuclear submarine or similar lol.

The basic principle is a balanced bridge.
Any one contactor failing closed will imbalance the bridge and energize the opto-coupler(s) LED. Could add a CCS to the LED portion to take the 2:1 voltage range expected.
I target for about 1mA of LED current, to keep heat low.

Another building block is the isolated HV switch, if you must have zero quiescent current. Use a photo-voltaic opto to drive a HV IGBT or MOSFET acting as a switch, with a series-diode to prevent backfeed through the body diode. Yes it's a DC system and reverse polarity never exists in theory- but in reality I find faults/arcs make -ve spikes due to stray inductance and cause drama. Add the switch in series with battery side resistor i.e. R1 to give zero quiescent current.

In your FMEDA you get good diagnostic coverage if you don't close both relays simultaneously. Switch one and contactor fault should be asserted (=self-test).
A bit of a hassle to test each polarity. Or use one opto and a bridge rectifier driving the opto's LED, but you lose the ability to tell which contactor is stuck.


Ground-fault detection exists somewhere in your system? I see that on floating systems, so this circuit might affect.
Floating-ground systems are problematic for common-mode EMI due to stray capacitance and leakage currents to chassis/earth ground.
 

Offline elima

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #42 on: September 25, 2017, 07:12:34 AM »
@floobydust. Liked your idea but the main working principle limits a bit the use of such circuitry in my application.
The idea is to be able to detect the status of each contactor without relying on other parts of the circuitry.
The way I do it now with the auxiliary contacts is feeding a constant isolated signal (lets say 3.3V) with a very short pulse(just to eliminate the problems of signal stuck to high..).
There is a specific order in closing/opening the contactors and none of these operations happen at the same time.
For example:
1. Voltage/current across the HV bus is checked, should be ZERO(or very very close). States of all contactors are checked using auxiliary contacts. If everything is ok, discharge contactor which is normally closed is opened, voltage/current across the HV DC bus bar is checked again to verify that everything is ok, next.
2. Check that the rest of contactors are in the state they are supposed to be using the auxiliary contacts. Everything is ok? next.
3. HV- contactor that is normally opened is closed, verified with auxiliary contacts that everything is ok including the rest of the contactors, voltage/current measurement on HV bus..all good? next.
4. Testing circuit that ensures that the precharge resistor is still connected and has the right value is used. If all seems ok, precharge contactor is closed.
5. Precharge process in monitored voltage/current/time, when the precharge is over, the whole precharge circuitry is bypassed with the HV+ contactor.
and so on...
Perhaps there are some more inner checks that I am forgetting now, but the idea is that diagnostic coverage is quite high and basically everything is "doubled" in that sense. Haven't had a single failure during my 3 years working on this project and it has been daily used. Having another circuit for detection will improve the diagnostic coverage and also offer different ways for checking the contactors.

 
 

Offline floobydust

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #43 on: September 25, 2017, 07:36:43 AM »
I'm not clear what "...without relying on other parts of the circuitry" means.

You have to end up relying on some hardware to work. The only way around this is self-test which I see you have in your start-up sequence. Whether it's mechanical aux. contacts or opto-couplers reporting back to your controller,  self-test gives 100% coverage on that part of the safety function and eliminates the need for redundancy in the FMEDA as I recall.

 

Offline joeqsmith

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #44 on: September 25, 2017, 07:41:28 AM »
I'm not sure why you can't use a contactor with the circuit built-in for this but maybe you could mount a proximity detector to the outside.

If you could fire the solenoid, you could try and detect the back EMF when the contact bounces.  Some fuel systems work like this to detect the injector response time. Tthere may be reasons for not doing that (like the other contactor has failed). 

If we assume when the contacts are welded, it changes the location of the moving contact.  I wonder what the inductance of the solenoid is with the contactor closed vs open.  I just tried it with a contactor I had laying around and indeed, pick the right frequency, the inductance changed more than 50%.   You would just switch out your normal driver when making the measurement.  Maybe a train mode to work with different contactors. 
How electrically robust is your meter?? http://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
 

Offline elima

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #45 on: September 25, 2017, 08:28:41 AM »
@floobydust. The solution offered relies on the states of both contactors and not one. I would prefer a test circuit powered from the LV system(isolated) that is able to check each and each contactor individually without the need of closing/opening other contactors as I do now with the aux. contacts.

I am quite impressed with the level of knowledge on this forum. Having self-tests with high Diagnostic Coverage(99%) eliminates the need for redundancy in the FMEDA. Having redundancy doesn't harm though.
I have already mentioned that I am trying to offer different solutions for different requirements. In some cases I need to cut with expenses and weight and in some cases the requirements are quite strict.

I will try to get forward with AC impedance measurement. Will see where this leads me unless someone comes with a brilliant idea(which can be very simple).

 

Offline AlfBaz

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #46 on: September 25, 2017, 09:01:17 AM »
Here's an idea I had. Isolation can be as simple as an opto connected to the outputs.
You can use multiple resistors in series to get the appropriate voltage ratings and the whole thing can be potted in fire retardant high voltage epoxy.

You end up with a 6 lead device. 2wires connected to the load side, 2 to the feed side the other 4 to your detection cct


 

Offline DrGeoff

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #47 on: September 25, 2017, 09:17:52 AM »
Here's an idea I had. Isolation can be as simple as an opto connected to the outputs.
You can use multiple resistors in series to get the appropriate voltage ratings and the whole thing can be potted in fire retardant high voltage epoxy.

You end up with a 6 lead device. 2wires connected to the load side, 2 to the feed side the other 4 to your detection cct

Under a fault condition there may be no voltage present.
Was it really supposed to do that?
 

Offline AlfBaz

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #48 on: September 25, 2017, 10:12:56 AM »
Here's an idea I had. Isolation can be as simple as an opto connected to the outputs.
You can use multiple resistors in series to get the appropriate voltage ratings and the whole thing can be potted in fire retardant high voltage epoxy.

You end up with a 6 lead device. 2wires connected to the load side, 2 to the feed side the other 4 to your detection cct

Under a fault condition there may be no voltage present.
Under a fault condition one would think/hope the appropriate fault clearing mechanism would do ITS job (aka fuses)
 

Offline Neomys Sapiens

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Re: Circuit for detecting open/closed relay/contactor in 400-800Vdc applications
« Reply #49 on: September 25, 2017, 10:53:54 AM »
Maybe there is still some misunderstanding about the specific fault condition to be detected and avoided:
it results from being previously switched on (both rails:complete circuit) and occurs on switching OFF.
The aim is to detect this condition in order to avoid any subsequent switching ON, because that would be unsafe.

So, when the sought condition appears, no current will flow and no fuse will blow.
 


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