I don't follow. If you add a digital back end to a rotating disk meter you can add any of these features. So what features can't you add to a rotating disk meter with a digital back end? Is the advantage that the digital meters are less expensive?
Rick
And what happens to that with 0.1% numbers...it becomes vanishingly small...
Three watts seems pretty extreme to me. This is something that should be doable for well under a quarter of a watt before power supply inefficiencies are considered. I have no idea how they deal with that however. In fact, the more efficient, switched power supplies will have poor PF, lol! I believe I've read that there will be a requirement to manage PF even on low wattage power supplies.
Rick
That doesn't mean it doesn't cost to operate the disk power meter. Most homes always have some load, even if it just a couple of clocks running so it is never zero. The power meter will drop voltage and depending on which side of the current coil you measure the voltage, you are paying for the power of the meter. Actually, I guess you only pay for part of the power used in a disk type meter. If the voltage coil is on the upstream side of the current coil, you pay for the power lost in the current coil. If the voltage coil is on the downstream side you pay for the power lost in the voltage coil.
I only care about one or the other since you can't measure both at the same time without a third sensor.
Rick
The problem with harmonics is that the AC power meters are not very good at measuring the power in the harmonics. In the power biz, failure to pay for power is a serious problem. If PG&E were to deliver square wave instead of sine wave power, 1/3 of the power would be in the harmonics.
That's usually the official excuse when the insurance company wants to pass the blame for a fire to the building owner or electrical contractor and thus avoid paying for the fire damage. The thermal circuit breaks should|would have tripped long before any danger of fire.
In the US, the neutral wire is sized to carry the maximum current of ONE of the two phases. This way, if one phase is loaded to maximum, and the other has no load, the system would still be safe. With such a neutral, I don't see how PF changes can create exessive current in the neutral wire in a 2 phase residential system.
Things are not so convenient with 3 phase power, where the maximum neutral current is 1.73 times the current in each phase. That can cause problems, but certainly will not burn down the building. The article below suggests that for 3 phase power, the neutral is sized at
125% of the current in each phase, but lacking the time to dig through the NEC code book, I'm not certain that's the correct number.More on sizing the neutral wire:
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
But there's no power in the current harmonics.
Not many people put breakers in the neutral wires.
The classic wire sizing assumes that current from the three phases will cancel into the neutral. When the current is dominated by higher harmonics, namely big current spikes at the peak of each sine wave, they add up; they don't cancel because they are not simultaneous in time.
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
Not much good for delta loads ;)
BPC is the only residential submetering that I've done, about 1720 apartments, which keep having bad luck: 9/11, then this flood.
Submetering is a low-margin business, so I don't do it any more.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Generally one wouldn't meter current (it would require a second pair of CTs or shunts) and maybe not voltage, if the added register is battery powered. The ones I've seen were basically a disk rotation counter with LCD, battery powered, with one of those opticon readout things. I guess one could do a disk meter with all the goodies behind it.
If anybody knows of disk-based meters with all the fancy features, please say so.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Arrgh. It is the same phase in 120/240 V systems, NOT phases. Just opposite ends of a single center tapped winding. Opposite polarity is correct, and other hot ( / leg) is acceptable. I get twitchy on this because i have to correct sparkies and dumball "engineers" on this all the time.
120/208 V "single phase systems" are different. Other leg still applies.
That is just plain wrong. Do the arithmetic. Oh wait, you are allergic to that.
?-)
There are a few legally mandated cases for doing so, and failure to do so when mandated is a serious issue; get your ass sued off serious.
?-)
You
the
amore.
Certainly both of these and providing for co-generation (e.g. solar) is much easier by just going all digital direct in the first place (also cheaper which the utilities DO care about).
?-)
the
business
Actually low watt PF corrected power supplies can be efficient, it is just a different design. What is really difficult is near zero idle power and short pulses of a fractional to a few watts. MPS430 based designs are famous for this. Many sleepable uCs and uPs have this issue.
?-)
If we're considering sine waves with odd harmonics, why would it matter if one talks of opposite polarity or 180 degrees phase difference?
As John pointed out, if the load consists of rectifiers without PF correction, the currents do not cancel in the neutral wire. In fact, the current may end up to be the *sum* of the currents in both hot wires.
Jeroen Belleman
What if the users' equipment was a resistive heater load ? The square wave wouldn't power that with all its might and use all the energy to make heat ?
(assume a low-inductance resistor)
boB
My stipulation was
which falls out of the trig. Yet another reason why an electronic power meter doesn't need a super-high sampling rate.
Of course, if the AC line were a square wave, things would be different.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
That even applies to 120/240 center-tapped systems where the 120 volt loads are 0 and 180 degrees. If the loads were half-wave diode rectified, and each line was running at its rated max RMS current, the neutral could be overloaded. Again, if the line currents don't overlap in time they add into the neutral, not balance as would happen with sinewave loads.
You could get the same effect with a resistive load on one side and a capacitive load on the other. The orthogonal neutral currents wouldn't cancel.
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
While I very much doubt that the harmonics will actually burn down the house, the (possibly urban legends) that the harmonics have burned the neutral conductor in a cable, causing hazardous voltages appears to be believable.
At higher distribution levels, it is assumed that the neutral current will cancel out in a three phase system and thus, the neutral connector might quite a lot weaker than the phase connector. In a ground cable, the armoring around the phase conductors might act as the neutral conductor. In a pole mounted cable system, three aluminum phase connectors plus a steel cable servicing as the mechanical strength as well as the neutral condition (assuming neutral conductor canceling out).
However, if this canceling assumption is not true with electronics load, the third harmonic current is far larger than initially assumed, causing burn through of weak neutral conductors.
It's more a problem in offices, with lots of PCs.
PFC power supplies fix this problem.
The AC in my office is pretty flat on top, so there must be lots of ugly rectifier-capacitor loads somewhere nearby.
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
That's what I am asking. I haven't heard anyone say the digital meters are cheaper than the disk meters. Is that true?
The issue of solar is moot. When you add your solar collectors, if you want to feed into the grid you have to coordinate with the utility and they give you a special meter. My house *still* has the same disk meter it had 50 years ago.
Rick
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