We are running 2 GE 6B machines doing Black Start testing and were wondering if there are levels that the DC will get to that would trip the units? Through the logic I see nothing, the inverter needs to see a "DC threshold". Has anyone come across a value that GE uses as "safe"?

We alarm with a "DC Undervoltage" alarm when the charger is turned off but we have determined that this is just an issue of how they termed the actual alarm. Also if anyone has a black start procedure that they could share I would appreciate it. Ours works fine but my concern is isolating 480 volt devices as the unit gets to full speed. Thanks.

 

If you're starting a unit "black" you'll be happy if it starts in an emergency, and won't really want it trip on low DC voltage.

In general, most Speedtronic turbine control power supplies have internal protection that shuts the power supply down when the DC power inputs drops below approximately 90 VDC or if it gets above approximately 140 VDC.

But, that's about it. In an emergency, you want every bit of margin you can get. And, if the system will ride through a low voltage condition and allow the unit to reach synchronous speed and then produce power, then the world is a good place!

The alarm should normally read something like 'DC Battery Charger Trouble' or 'DC Battery Charger Undervoltage', not just 'DC Undervoltage', if it's the alarm that's coming from the battery charger that's annunciated when the charger output is low, such as when there's no 480 VAC input.

I don't really understand your concern about "isolating" 480 VAC devices. A unit designed to be black-started will usually have some means to power one cooling water pump and one cooling water fan, to try to provide some L.O. cooling during starting, especially if the unit was running before AC was lost and tripped and coasted down without cooling water.

If the L.O. was hot it's even questionable sometimes if the unit will reach synchronous speed without tripping on high L.O. temperature even with one pump and one fan running. That's why black start machines usually block the high-high L.O. temp trip during starting so that hopefully it will "ride through" the condition as the cooling water pump and fan are run up to speed.

Most sites designed for black start will have a means of powering AC motors (vent fans, exhaust frame blowers, etc.) through a separate tap off the main step-up transformer as soon as the generator breaker is closed.