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IamSmooth

Fri Feb 26 2010, 04:14PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I'm trying to find a location to purchase a heavy duty relay. The control voltage would be DC. It could be anything from a few volts to a couple of hundred. It has to handle a DC load. I looked on Ebay and found some but they handle AC loads. I am also not sure if I would want a solid state or mechanical. I plan to keep looking.

Any suggestions for a source to get these or a good brand?

GeordieBoy

Fri Feb 26 2010, 05:00PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

When you are doing your searching you might find it useful to know that heavy duty relays are often reffered to as "contactors".

It is also a lot harder for a mechanical switch to interrupt a heavy DC current than it is to break an AC one. AC has natural current zeros which will help to prevent an arc being drawn out as the switch opens. With DC there is nothing to stop arcing if the voltage and current are in the right range. So, for HVDC you might want a solid-state switch or something like a vacuum or SF6 relay if the voltage is in the kV's.

Providing a voltage and current spec might yield a few more responses on here...

-Richie,

Electroholic

Fri Feb 26 2010, 05:04PM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

did you say the control voltage should have a range of volts to 100s of volts?

IamSmooth

Fri Feb 26 2010, 06:32PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I probably started this thread backwards. THis is the problem.

I made, with the help of many here, a shunt controller for my wind turbine. It has worked well so far. When the grid goes down it diverted the current to a bank of resistors. Recently, I have blown two IGBTs on it. This is why I was thinking of a mechanical relay.

I am not sure what the conditions are when it is blowing, which makes reproducing it harder on the bench. I am not set up like an industrial workshop that is planning on making a few thousand of these.

I am thinking what is happening is the unit is diverting power while the grid is still active because the winds recenly have been very intense, and my grid has not gone down.

The unit, as seen near the end of my web page on the turbine project, gets a variable DC voltage from 0 up to +300v. The shunt is set to divert at 250vdc. The power goes to an Aurora grid-tie inverter.

The current at 250v is about 8-10A. The IGBT is 65A/1200v. It is suppose to switch on hard. I have a large capacitor on the chips to maintain operating voltage in case the wind suddenly stopped, and the IGBT is to then get turned off.

I don't know if the IGBT gate is getting over or under-voltaged, but I have 18v zener clamps. Maybe the IGBT is getting turned off too softly. Maybe the shunt is hitting +250v when the grid is on and this is affecting the IGBT. I am not sure, but I would appreciate your thoughts.

So, I figured if I had a mechanical relay that is activated by sufficient voltage on an electromagnet I would be set. I could possibly open it when it is still three-phase.

Henry H

Fri Feb 26 2010, 06:39PM

Registered Member #2298 Joined: Sat Aug 15 2009, 08:16PM
Location: ex UK, now Santa Cruz, CA
Posts: 35

Sounds like you could be getting a gate undervoltage. That would be bad, because it puts the IGBT into a linear region where a lot of power is dissipated in the transistor. I would suggest a battery backup for your gate supply and control electronics. Arrange the control so that once it has tripped once it stays tripped (IGBT on) until either you reset it or a timeout occurs, this will avoid rapid uncontrolled oscillation that can also cause high losses in the IGBT.
This sort of shunt application is not a very demanding one for the IGBT. With the right control / gate drive scheme it should work just fine.

You could also use an SCR.

Relay/contactor may or may not be fast enough to protect whatever you're trying to protect, depending on what the source of the overload is.

MinorityCarrier

Fri Feb 26 2010, 06:57PM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

I suppose it's too late to suggest you install a windmill speed governor, something that changes the pitch angle of the blades to prevent overspeed. Dumping good power into resistors seems like a waste and hardship on your system.

radiotech

Fri Feb 26 2010, 07:14PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

The mechanical irelays in power systems are overvoltage (OVR) and undervoltage(UVR) and can be calibrated. Ebay might have some as modern systems tend to use solid state.

IamSmooth

Fri Feb 26 2010, 08:38PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I thought about a governor, but I did not want to get into get involved fabricating this. At the time I was still learning how to weld, and I don't have a machine shop.

If it is undervoltage, how is it possible if I have the zener clamps?

Henry H

Sat Feb 27 2010, 01:50AM

Registered Member #2298 Joined: Sat Aug 15 2009, 08:16PM
Location: ex UK, now Santa Cruz, CA
Posts: 35

Zener protects against gate overvoltage, not undervoltage.
If your gate drive power supply starts to droop out, and you don't have explicit undervoltage protection, eventually the gate voltage will fall into the linear region (approximately between +3V and +12V, but it depends on the particular IGBT). It only needs to be there for a few milliseconds for the IGBT to burn out.

This is not the only possible cause of failure. If the IGBT were to directly short a low-impedance high-current source (e.g. generator output or bus capacitors), it would die even with Vgs = +15 or +20. Your shunt resistors should prevent that happening, though.

IamSmooth

Sat Feb 27 2010, 03:50AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Undervoltage is something I considered which is why I have a 39,000uf capacitor in there to give a little boost if the turbine power suddenly falls. Scenario: high winds and the shunt hits the threshold and goes on hard with a gate drive voltage of 18v. Suddenly the wind dies and the power drops. I have a small hysterisis on the schmitt so I don't oscillate. Now the power to the gate drive falls; before I can turn the gate to 0v the voltage droops despite my capacitor.

If this is the cause of the IGBT burn-outs, what is my solution? A battery in the system to maintain the voltage to the ICs? I would prefer a battery-free solution.

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