Balanced loading of DC Bus? 1

[bobo480]

I am looking suggestions on how to achieve balanced loading of a 700 VDC bus without incurring large power losses using bleed resistors. The buss consists of two 450 VDC rated capacitor banks conected in series. The loads are to be connected across each capacitor half-section. The loads are designed to operate at 300-400 VDC. Approximate loading is 40KW.

 

[VEBill]

"using bleed resistors"

Very high power isn't my area, but people will probably want to know - how matched are the loads?

But the entire approach sounds dangerous. If one of your loads suddenly disappears, then it's going to get interesting.

 

[bobo480]

The two loads rated 350 VDC have their own DC Bus capacitors, and loading is fairly symetrical. Test show that the DC 1/2 Bus voltages quickly drift & one half assumes more load than the other (voltage imbalance).

 

[VEBill]

Steady-state matching is one thing, but during start-up, the dynamic matching might also be an issue. My gut tells me that if there were any negative slopes then you might create an oscillator.

Thinking outside the boundaries of the problem you've stated, where does the 700 VDC come from? Is there any simple change that could be made there to halve the voltage and double the current? Some power supply architectures can be 'halved' very easily, like the 110/220 VAC switch on some power supplies.

Other than that, I foresee active voltage regulators in your future.

 

[bobo480]

The DC supply is basically a 480 VAC VFD operating in the regenerative mode. The problem is the AC Grid-Tie inverters (Load) are rated only to 570 VDC max. Splitting the DC bus would allow safe operation within the device ratings as long as one-half of the DC bus does not short. The only solution I see at this point is switching to a 230 VAC Inverter, though I would prefer using hardware already in my inventory.