Opportunity Load

Re: Opportunity Load

Here is one thread where this was discussed with some possible solutions:

Grid tie inverter that does NOT feed into the grid

-Bill

Re: Opportunity Load

igor1960 wrote: »

Bill, the problem is that if we use ON/OFF frequency on the heater, then Electric Company might see negative current on SmartMeter, as SmartMeters ADC from what I understand runs at least at 4khz (or maybe even more). So, what I'm thinking is just a dimmer like this: http://www.electricbargainstores.com/product-p/insteon-2476dbk.htm or that: http://www.smarthome.com/2477DH/SwitchLinc-INSTEON-Remote-Control-Dimmer-Dual-Band-High-Wattage-White/p.aspx. I strongly feel heating element is "dimmable"(????)

I believe those dimmers would function along these lines:

http://www.edn.com/design/led/4368149/Efficient-method-for-interfacing-TRIAC-dimmers-and-LEDs-4368149

Attachment not found.

So, if you are trying to modulate the heater current to zero out the current to the "smart meter"--It will not work if the meter is measuring PF (i.e., the 4 kHz digital sampling) as meter will "see" the back driven power during part of the cycle...

That is always the problem with trying to modulate loads to "hide" the GT Inverter from a smart meter. In the US, the utility may just drive down the street (or use google maps) to look for "illegal solar power installs". (in California, even Off Grid solar power system are "illegal" in places connected to utility power--without permits).

Bill, I'm confused here: want to measure current+direction on the main panel and not on GT inverter. As you are saying direction might not be that easy to find without voltage -- then probably the best way is to use Neutral/Ground voltage drop/rise to figure out NET current for the whole house, but that would probably require shunt. I was looking for simple non contact solutions.

Yep--Without knowing the voltage (amplitude and timing/phasing) relative to the current (these are vector quantities)--You cannot just connect a current probe and figure out the direction of power transfer.

Think of peddling a tricycle (peddles fixed to wheel)--Looking from the outside you see legs going up and down. But without knowing the "phasing" of the force vs peddle position, the force could be propelling the trike forward (in-phase, 0 degrees), +/- 90 degrees out of phase (not supplying any forward or reverse average torque), or 180 degrees out of phase and slowing the trike down.

Regarding contact (shunt, or measuring voltage drop across a length of neutral wire), on non-contact (Current Transformer or Hall Effect Probe). All three methods are pretty much equivalent--Just different methods with differing positive/negative attributes (cost, isolation, sensitive, drift, etc.).

In general, AC currents are very easy to accurately measure with CT and Hall devices. Shunts are usually used when measuring current with a DC component.

Still a little bit confuse, why transducer is not changing signal direction on change of AC current, shouldn't magnetic field become opposite?

With a CT transformer, it is outputting a scaled current. Drop it across a precision resistor, then you have a voltage (that is proportional and in-phase to the monitored current) that is easy to use an Analog to Digital converter to get your digital data stream. Of course, this only "works" with AC current with (typical) a zero DC current bias/frequency below the cutoff frequency of the CT.

A precision resistor (typically with Kelvin Contacts)--Again, a voltage drop proportional to current. Bring the voltage sense leads to a millivolt A to D of some sort--Again another digital data stream... However, this can do DC because of the direct coupling.

Hall effect transistors can measure AC or DC fields--But they tend to drift. Since the DC average of AC current is zero--They can be "rezerod" by software averaging (I think--Not my area of expertise). With DC Fields, not so easy (there may be "DC friendly" Hall sensors--again, not my field).

-Bill

Re: Opportunity Load

igor1960 wrote: »

Bill, again confused here. Assuming LOAD="heating element", how this is different from LOAD="incandescent bulbs"? Are you saying that there is fundamental difference between them?

For a first order approximation, a filament lamp is a "heating element". For a second order approximation, a tungsten filament is not a bad "current source/sink"... I.e., as you raise the voltage (hotter filament), the resistance goes up, and the current stays, roughly the same. Drop the voltage, the filament cools, resistance goes down, current stays roughly the same.

So, depending on what your "element" is--You may have some tweak the feedback loop to remain stable.

And yes SmartMeter (Itron) is measuring PF, by integrating A*V as well as integrating A*V90 (reactive), where V90 is previous 90 degrees early Voltage. Then finding Active by subtracting AV-AV90...

A modern "smart meter" can measure many things, and if the sampling frequency is 4 kHz, then the Nyquest frequency is 2 kHz or out to the 40th harmonic (if 50 Hz, or 33rd harmonic if 60 Hz power).

I guess--I keep coming back to the original question... What is it you are trying to do? Trick a "generic" meter (smart or dumb) to not run backwards and cause grief with the billing computers? Or trying to follow the "intent" of the law that there be "NO EXPORT" at fundamental or higher harmonics of power (and any new&improved smart meter which will measure harmonics out to 2 kHz and report back to the utility that there is a sub-cycle net export of power/very poor power factor/very poor derivative of power factor/power quality/etc.).?

For a utility, they are in the business of selling fundamental 60/50 Hz power. Harmonics are pretty much a 100% loss for the utility (usually shows up as overheating motors and circulating current/overheated transformers).

Also, utilities really want to charge for kVAH, not kWH. In the US, pretty much all residences are charged for kWH, and businesses (especially larger power users) are charged a poor power factor penalty (basically a kVAH surcharge).

I could put a "perfect inductor" into a wall outlet and have it pass 10 amps @ 120 VAC.... The kWH meter would see current lagging by 90 degrees and (rightly) charge me zero kWH for that pure inductive load (ignoring the real part of the resistance of the wiring).

And if it was business--The the utility would see this as a Peak 10a*120v=1,200 VA load... And charge the business based on that load (sort of).

From the utility's point of view, the wiring, local distribution transformer, back to the transmission lines and back to the power plant--They see the 10 amp inductive load and have to provide 10 amps of current. For the house, they get zero revenue. For the business--The revenue can be so high, that the business will "do something" about the poor power factor (i.e., add capacitors to bring the PF close to 0.95 PF--such as done with large induction motors).

And why the government mandated CFL lamps are no near as good at saving our infrastructure costs as the government "pretends".

I buy a 120 watt filament lamp... I pay for the 100 Watts it takes (1 amp * ~120 volts = 120 VA = 100 Watts).

I buy a 30 watt CFL lamp... I pay for the 30 Watts it uses, but look at the base of the lamp and see that (many) are 0.5 PF... That meas the VA rating for the 30 watt lamp is:

P=V*A*PF
VA = P/PF = 30 watts / 0.5 = 50 VA

So while I reduced my bill to 25%, the utility only saw its RMS current on the line reduced by 50%. For a random CFL light--Not a big deal... Change all the bulbs on a large lighting circuit (or in a whole city), the issue can become quite a problem.

Harmonics and such with three phase power became a big issue when personal computers (and their power supplies) became heavily used in businesses... The poor power factor (and failure to cancel neutral phase currents) in a large office park would cause building wiring to overheat--Even though the wiring should have been heavy enough to manage the loads.

There is just a huge amount of issues with AC power where secondary effects can cause problems==From revenue to fires. For a few plug and play GT inverter systems--It is not a big deal. If it becomes popular, then a whole bunch of people/companies/regulators will rain down on P&P/No backfeed installations.

-Bill