1000 Vdc / 1000A Over voltage clamp? - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: October 06, 2017, 09:16:05 pm
content: Several of my clients are using large DC supplies as battery simulators to allow them to conduct high power electric motor tests without using heavy and potentially dangerous (esp. at prototype level) batteries. These DC supplies are typically up to 1000Vdc and 1000A, and are two quadrant devices (source/sink). When operating in SINK mode, if the DC supply is disabled, which can occur when it tries to protect itself from over voltage or over current, the DC voltage obviously then shoots up as there is suddenly know where for the current to go too! This overvoltage event is obviously bad for the device being tested, as that device generally includes some sort of semiconductor switch (IGBT/Mosfet).So, it got me wondering, how would i make a overvoltage clamp or crowbar device, that attempts to sink the fault currents and keeps the dc bus below the danger level? Large watercooled resistors and a switching device are the obvious answer, but i wonder if there is any other options that could be used at these power levels?

author: Molenaar
author_rank: Regular Contributor
timestamp: October 06, 2017, 10:03:20 pm
content: Would a thyratron to shunt the load work?

author: jbb
author_rank: Super Contributor
timestamp: October 07, 2017, 01:55:21 am
content: Quote from: max_torque on October 06, 2017, 09:16:05 pmOK, this is a very good question. It's not immediately obvious what to do. Great big resistors are attractive but expensive.CrowbarsThese basically short the DC bus down to near zero. A suitably sized thyristor is generally used (perhaps with a little series inductor and parallel RC snubber to keep turn on dI/dt and turn-off dV/dt within acceptable limits, check your data sheet).The problem here is the DC link will then not accept any energy, so it will be dissipated elsewhere.If your back-energy source is an induction machine, this can work really well, because you will remove the magnetising VArs from the machine (i.e. de-excite it) and all the currents will fall off quite rapidly.If your back-energy source is a permanent magnet machine (or another battery bank or whatever), you will effectively short it and experience significant current flow until everything comes to a halt.Load DumpsI.e. great big resistors. These can be a good solution. You already have 2 quadrant supplies, so you don't need the resistors to dump load continuously, just in fault conditions. That means you just need a resistor which can handle the jandal for a short period - maybe seconds? - while your test jig trips other stuff.It looks like you'd probably want something in the 1000V / 1000A = 1 Ohm range. A good way to make a tough-as-old-boots resistor like that is to take a thick sheet of stainless steel, and get a local engineering shop to water-cut it into a serpentine (zig-zag) resistor. The resistor will heat up adiabatically (i.e. all the energy goes to temperature rise and very little radiates or conducts away) and you can dimension it to absorb a set amount of energy.Here's an example:Vmax = 1000VImax = 1000ATdump = 2secTinitial = 100 deg C (note: this means you don't have to wait forever after a trip to go again)Tfinal = 400 deg CSo, we get 1MW * 2 sec = 2 MJ. Temperature rise is 300 deg C. Therefore we need to soak up 6667 Joules / deg C.For SAE304 stainless, the specific heat is around 480 J / kg . deg C. Therefore you'd need around 13.9kg to soak up the heat energy.Terminations would be heavy bolts and quality crimps.Note:the specific heat of SAE304 might drop off as it gets hot - you'll need to verify this!

author: David Hess
author_rank: Super Contributor
timestamp: October 07, 2017, 02:49:42 am
content: I assume that you do not want to simply short out the excessive voltage and that once the power supply disconnects itself, an orderly shutdown will be done and the problem is dumping the excessive power from the motor controller. That leaves something like:1. Use several diodes and battery banks in parallel to act as a load with the internal resistance of the battery banks serving to ballast the diodes. When the voltage rises above the battery bank voltage, the diodes conduct so operation is completely passive. I would consider this if I had access to free battery banks which seems likely in this case.2. Use several SCRs which trip under over-voltage conditions or when the power supply indicates a fault to connect banks of power resistors. This would be safer than the battery bank idea but perhaps not as reliable. jbb covered making your own resistors.

author: max_torque
author_rank: Super Contributor
timestamp: October 07, 2017, 02:17:02 pm
content: I don't have to protect for continuous sinking in fault conditions, just as mentioned, deal with the mechanical inertia of the rotating system, being transferred as it is braked to a rest into the dump device. I'll have to go an lookup with the max energy stored can be (assuming operation at peak speed at the point of shut down). When the dc supply goes into fault it disconnects from the DC bus, so is effectively off, at this point, i will want to limit the bus voltage to no higher than the normal operating voltge. i.e. all the energy coming in from the decelerating DUT is shunted into the dump.I think using resistors is a definite possibility, and several could be used in parallel to allow the control system to apply differing loads depending on the overshoot voltage being experienced. One option would be using stainless steel tubes full of glycol with a small pump system to help with heat rejection. Probably the bigger issue is what to use as the power switching element. Some form of mechanical contractor would probably be fast enough (say <50ms response) but could suffer from welding issues, whereas a semiconductor switch would be easily fast enough, but more vulnerable to high voltage spikes and high current flows etc!

author: max_torque
author_rank: Super Contributor
timestamp: October 07, 2017, 06:30:49 pm
content: Thinking about it, a thyristor switch driving some current limiting resistance (to stop ridiculous currents flowing due to any capacitance on the dc bus (inverters have a fair bit in them) and as the system only triggers under fault conditions, the inability to stop the thyristors conducting is not an issue. Could use a small micro to read the DC link voltage, perhaps with two separate input channels (redundancy) and if both exceed the setpoint, the system fires the thyristors, dumping the bus.Could also easily include some IR temp measurement of the load resistors (black painted stainless steel sheets as suggested) and have the system only allow a "re-arm" once there temp is below a sensible value?

author: Marco
author_rank: Super Contributor
timestamp: October 07, 2017, 08:10:48 pm
content: Quote from: max_torque on October 07, 2017, 02:17:02 pm1000 amps adds up to a lot of voltage at 50 msec without a metric ton of capacitance, so I kinda doubt it. You'll have to dump it somewhere else for that time (silicon, MOV or aforementioned metric ton of capacitance). Thyristors won't turn off, so even if you use multiple parallel paths for increasing currents it's a semi-crowbar, not a clamp.

author: max_torque
author_rank: Super Contributor
timestamp: October 07, 2017, 09:20:44 pm
content: I think i'd put the thyristors in series with mechanical contactors (Kilovac type etc) . To arm the system the mech contactors are put in, then the system monitors the bus. if it overvolts, the system triggers the Thyristors. If the bus stays high for an excessive period, and there is a danger of the resistors overheating (shouldn't be, because the control system will disable the DC supply and motor drive etc), then the system could open the contactor stop the current flow. Normally the bus would discharge to the voltage at which the thyristor current becomes un-selfsustaining.Ideally i'd design it to be modular, say with 3 parallel resistors and thyristors, so the current drawn from the bus can be modulated (coarsely). Being resistive the power sunk will be the square of the average voltage.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 07, 2017, 09:29:00 pm
content: Are there size constraints?

author: jbb
author_rank: Super Contributor
timestamp: October 07, 2017, 10:48:27 pm
content: Quote from: max_torque on October 07, 2017, 02:17:02 pmI'm not sure how glycol would react to the applied voltage. Certainly normal water cooling is out because the tubing will be on unknown potential (voltage differences of >1V cause electrolysis). DeIonised (DI) water is an option, but it's actually quite hard to deal with (eats brass and some grades of aluminium, needs an ion exchange bed to keep it deionised, the pain goes on...)If you haven't got experience with liquid cooling systems, I recommend sticking to big air cooled components as they will likely be lighter and cheaper.Quote from: max_torque on October 07, 2017, 02:17:02 pmIt depends on the exact system dimensions, but you might be able to trigger the dump load before the power supply itself trips. That way you still have some bus voltage control.Quote from: max_torque on October 07, 2017, 02:17:02 pmThe simplest and toughest is probably an SCR which discharges the whole bus. You can build a trigger unit which will derive its power from the DC rail (that way you can't forget it). If you don't want to take the bus all the way to ground, it's hard to turn thyristors off, so options such as max_torque's mechanical series switch are well worth considering (note: substantial DC breaking capacity required, check your ratings carefully). Also, you can get fancy with active electronic turn off circuits (e.g. a -20V rail with big capacitor bank and a second thyristor + inductor to force commutation. Welcome back to the 1970s)If you want better control - but slightly reduced robustness - I would suggest a 1700V rated IGBT module. If properly handled they can be very tough - for example, with a proper gate driver they can survive a dead short situation. Make sure you have a flyback diode to clamp the back EMF at turn off (your leads and resistor banks will have some inductance).

author: Marco
author_rank: Super Contributor
timestamp: October 08, 2017, 01:17:12 am
content: IMO you should precharge "a capacitor" to some voltage above the supply voltage and clamp the rail to that, then have ~25 20Ohm dump resistors switched in/out by MOSFET's at slightly different voltages above the clamp voltage with some hysteresis.Fault redundant and is an actual clamp unlike almost everything else mentioned here.

author: SeanB
author_rank: Super Contributor
timestamp: October 08, 2017, 03:40:43 pm
content: Something like this, seen at the local scrapyard, removed during a lift upgrade. Just add in some high voltage IGBT drivers and some voltage sensing to drive them, and a lot of DC bus capacitors to smooth out the transient.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 08, 2017, 07:08:00 pm
content: A VFD braking resistor, of course. This should be something you can fairly easy get a few of. With specifications.You'd still need an IGBT to control the power, but sure semikron has some you can use, they even have drivers.