




#48552  02/16/05 03:04 PM
Re: Panel Board Overheating

Member
Registered: 01/20/05
Posts: 354

I switched from EE to computers in college. But that still got me some electrical background (and studying for a ham radio license added a bit more). Then working in large data centers (one with as many as 2000 computers) with many switching power supplies, I've learned (and understand why) harmonics can also be an issue with single phase power (and is underrated as an issue with three phase power).
I'll try to explain it with this analogy.
Suppose you have a single phase 100 amp 240 volt circuit with 19200 watts of resistive load (lights, heaters, whatever). That load would pull 80 amps. Now suppose the wiring feeding it has 1/100 ohm on each phase, for a total of 1/50 ohm. That wiring will have a 1.6 volt drop, dissipating 128 watts over its distance. As long as the load is on 100% of the time, that's 128 watts dissipated 100% of the time.
Now let's change the load around. Instead of the load being on 100% of the time, let's make it 50% of the time (for example it might be a large flashing billboard going on an off 1/2 second at a time). Now that 128 watts is dissipated only 50% of the time. The wiring will not get as hot.
But now here's where it gets interesting. You wouldn't do this (right?) but lets suppose the load is doubled to 38400 watts (remember, this is a 50% duty cycle). The current is now doubled to 160 amps, and the voltage drop jumps to 3.2 volts. That 3.2 volts across 1/50 ohm dissipates 512 watts ... when the load is on. With a 50% duty cycle, that averages to 256 watts dissipated.
The total energy used is about the same. But the power being dissipated is four times as much, half the time. So on average it is double the power being dissipated. Remember "I squared R". The wire (or thermal element) gets twice as hot.
If you were going to run TWO 19200 watt billboards in 50% flashing mode on that 100 amp circuit, you'd want to alternate between them instead of having them both on at the same time, right?
Don't forget that the thermal element in a breaker (or fuse) heats up by dissipating a small amount of power the same way. The above system should eventually trip the 100 amp breaker since the load would be the equivalent of 113.137 amps continuous. Fortunately the thermal element does emulate the heating in the wiring, so it should provide the correct protection when the breaker capacity is correct for the wiring. You just have to match the capacity with the load to make it a usable setup.
Switching power supplies do just this kind of thing. They turn on briefly during each AC half cycle, and pull a heavy amount of current for a brief instant, topping off the capacitors, then switch back off. Most do this at about the same point in the AC cycle as all the rest, so the current spikes tend to all happen together when you have many such loads on the same circuit.
Computers can be some of the worst offenders for this. I don't know to what degree high efficiency lighting will do this, since that's not what I have worked with. However, I am starting to get into some of that for my personal research on the effects of nonincandescent lighting on people with Autism. Apparently, electronic ballasts will be new culprits in the harmonics problem. Those that convert to DC first (this would be needed to eliminate the 120 Hz flicker) would likely be a switching power supply. And they are certainly going to be as cheap as the manufacturer can get them UL listed. And those that don't convert to DC and just chop up the AC cycle to limit the total current can still be a problem.
So yes, harmonics can be a problem, even on single phase. It's just different than what people people are aware of on three phase (but this problem can combine with the neutral sequencing problem to make three phase even worse when the switching power supply current duty cycles are shorter than 33%, which is often the case).

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#48554  02/16/05 04:13 PM
Re: Panel Board Overheating

Member
Registered: 11/10/00
Posts: 2209
Loc: IL

pdh, Can you document a problem that has actually occurred in the field in either a single or three phase system that was conclusively caused by harmonics? It seems that almost all of the information on the problems caused by harmonics comes from people (companies) with a vested economic interest in solving the problem. Also how is the heating problem that you described in your post a "harmonic" problem? The above system should eventually trip the 100 amp breaker since the load would be the equivalent of 113.137 amps continuous. It is very possible that 113 amps will never trip a 100A breaker. The trip curve for a major brand of breakers show that a 113A load on their 100A breaker will trip between 400 seconds and never. Don Don
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Don(resqcapt19)

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