Batteryless current clamps?
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From: John Fields on 20 Nov 2009 08:09 On Thu, 19 Nov 2009 21:29:10 -0500, daestrom <daestrom(a)twcny.rr.com> wrote: >If you make a 'tap' upstream of the revenue meter, even with just >transformer action, you're stealing. Revenue meters (kilowatt-hour >meters) have always had terminal voltage as one of their inputs. An >illegal tap upstream may affect the voltage at the service entrance some >small amount, but the metering will reduce the billed kWh accordingly. >So regardless of the exact voltage supplied by the utility (it often >varies slightly throughout the day), the amount of energy delivered at >the service entrance is what is billed for. Power drawn off before the >meter isn't measured and is 'stolen'. > >Of course if you just 'wrap some turns around the power line' without >orienting the coil properly in relation to the line, you're not going to >get any power because transformer action won't work when your turns of >wire are parallel to the power line's magnetic field (i.e. 'wrapped >around' the power line). And I think that was John Field's point. --- Indeed. Thank you. :-) JF
From: John Larkin on 20 Nov 2009 10:12 On Fri, 20 Nov 2009 12:54:00 +0000, ChrisQ <meru(a)devnull.com> wrote: >John Larkin wrote: > >> >> The coolest current transformer is a second-harmonic DCCT, accurate to >> parts-per-million from DC to many kilohertz. >> > >Enquiring minds etc :-). I thought at first that sounded like a >variation on the old fluxgate compass idea from ww2 and earlier, where >the earth's magnetic field varies the saturation of a toroidal ring and >the second harmonic amplitude recovered in 3 levels in 120 degree >segments, but it's quite different. Here's a good article on the principle: > The DCCT essentially uses a fluxgate - two of them, usually - to detect the cancellation of two currents, one the customer's current and the other a local feedback current. The null detection can be insanely sensitive, just as a fluxgate compass is insanely sensitive. John
From: Tzortzakakis Dimitrios on 20 Nov 2009 10:28 ? "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> ?????? ??? ?????? news:dnsag51jfmub8nppj2kfj3nt07l3jv6djg(a)4ax.com... > On Thu, 19 Nov 2009 07:29:26 -0800, Fester Bestertester <fbt(a)fbt.net> > wrote: > >>So, for a millivolt output probe, this might be as simple as 2 windings >>(or a >>tapped single winding) with a range switch to select the winding? > > Current transformers are usually dumped into a load resistor aka > burden resistor, to convert their output current into voltage. I'm > sure the Fluke clamp-on has an internal burden resistor, and they may > switch that to change ranges. > > Without a burden resistor, the output voltage will be proportional to > frequency and very dependent on core reluctance, which would be fatal > for a clamp-on meter with a hinge and a non-repeatable air gap. > > Coreless Rogowsky coils are used unloaded, but need a downstream > integrator to accurately measure current. > > http://en.wikipedia.org/wiki/Rogowski_coil > > The coolest current transformer is a second-harmonic DCCT, accurate to > parts-per-million from DC to many kilohertz. > > http://www.gmw.com/electric_current/Danfysik/866_867/867.html Anyway, current transformers must always be operated with the secondary shorted. In the generating facilities in Kozani, West Macedonia, where 400 kV current transformers were involved, the operators of the plant had a special indicator whether the secondary was shorted. -- Tzortzakakis Dimitrios major in electrical engineering mechanized infantry reservist hordad AT otenet DOT gr
From: daestrom on 20 Nov 2009 10:53 Tzortzakakis Dimitrios wrote: > ? "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> ?????? ??? > ?????? news:dnsag51jfmub8nppj2kfj3nt07l3jv6djg(a)4ax.com... >> On Thu, 19 Nov 2009 07:29:26 -0800, Fester Bestertester <fbt(a)fbt.net> >> wrote: >> >>> So, for a millivolt output probe, this might be as simple as 2 windings >>> (or a >>> tapped single winding) with a range switch to select the winding? >> Current transformers are usually dumped into a load resistor aka >> burden resistor, to convert their output current into voltage. I'm >> sure the Fluke clamp-on has an internal burden resistor, and they may >> switch that to change ranges. >> >> Without a burden resistor, the output voltage will be proportional to >> frequency and very dependent on core reluctance, which would be fatal >> for a clamp-on meter with a hinge and a non-repeatable air gap. >> >> Coreless Rogowsky coils are used unloaded, but need a downstream >> integrator to accurately measure current. >> >> http://en.wikipedia.org/wiki/Rogowski_coil >> >> The coolest current transformer is a second-harmonic DCCT, accurate to >> parts-per-million from DC to many kilohertz. >> >> http://www.gmw.com/electric_current/Danfysik/866_867/867.html > Anyway, current transformers must always be operated with the secondary > shorted. In the generating facilities in Kozani, West Macedonia, where 400 > kV current transformers were involved, the operators of the plant had a > special indicator whether the secondary was shorted. > Some old switchboard CT's I worked on in the Navy had very thin insulator between two spring clips. Whenever we wanted to remove a meter for cal, we slip the insulator out so the two clips would short together, shorting the CT. Then we could open circuit the meter and remove it from the panel. I don't remember exactly what the blade was made of, but it's surface wasn't perfectly smooth like polished material, more porous like unglazed ceramic (of course it wasn't any metal). The reason they built the insulator so thin was that if one accidentally open-circuited the CT without removing the wafer first, the high voltage developed by the CT would just 'punch thru' the wafer and safely short the CT. Then all you had to do to repair things was make sure you closed the circuit and replace the wafer-thin insulator blade. Was kind of surprised when I moved to commercial power systems that they didn't use something similar. Just has to have a breakdown voltage that is low enough to avoid damaging the CT. daestrom
From: John Larkin on 20 Nov 2009 11:44
On Fri, 20 Nov 2009 10:53:45 -0500, daestrom <daestrom(a)twcny.rr.com> wrote: >Tzortzakakis Dimitrios wrote: >> ? "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> ?????? ??? >> ?????? news:dnsag51jfmub8nppj2kfj3nt07l3jv6djg(a)4ax.com... >>> On Thu, 19 Nov 2009 07:29:26 -0800, Fester Bestertester <fbt(a)fbt.net> >>> wrote: >>> >>>> So, for a millivolt output probe, this might be as simple as 2 windings >>>> (or a >>>> tapped single winding) with a range switch to select the winding? >>> Current transformers are usually dumped into a load resistor aka >>> burden resistor, to convert their output current into voltage. I'm >>> sure the Fluke clamp-on has an internal burden resistor, and they may >>> switch that to change ranges. >>> >>> Without a burden resistor, the output voltage will be proportional to >>> frequency and very dependent on core reluctance, which would be fatal >>> for a clamp-on meter with a hinge and a non-repeatable air gap. >>> >>> Coreless Rogowsky coils are used unloaded, but need a downstream >>> integrator to accurately measure current. >>> >>> http://en.wikipedia.org/wiki/Rogowski_coil >>> >>> The coolest current transformer is a second-harmonic DCCT, accurate to >>> parts-per-million from DC to many kilohertz. >>> >>> http://www.gmw.com/electric_current/Danfysik/866_867/867.html >> Anyway, current transformers must always be operated with the secondary >> shorted. In the generating facilities in Kozani, West Macedonia, where 400 >> kV current transformers were involved, the operators of the plant had a >> special indicator whether the secondary was shorted. >> > >Some old switchboard CT's I worked on in the Navy had very thin >insulator between two spring clips. Whenever we wanted to remove a >meter for cal, we slip the insulator out so the two clips would short >together, shorting the CT. Then we could open circuit the meter and >remove it from the panel. I don't remember exactly what the blade was >made of, but it's surface wasn't perfectly smooth like polished >material, more porous like unglazed ceramic (of course it wasn't any metal). > >The reason they built the insulator so thin was that if one accidentally >open-circuited the CT without removing the wafer first, the high voltage >developed by the CT would just 'punch thru' the wafer and safely short >the CT. Then all you had to do to repair things was make sure you >closed the circuit and replace the wafer-thin insulator blade. > >Was kind of surprised when I moved to commercial power systems that they >didn't use something similar. Just has to have a breakdown voltage that >is low enough to avoid damaging the CT. > >daestrom Lower-power CT, like residential-metering size, 100 amps or so, will generally tolerate being unloaded. They will saturate and make two not-too-huge voltage spikes per cycle and not get very warm. The nasty part is that, once the burden is reconnected, they are very likely to wind up magnetized, which will mess up low-current accuracy. John |