Current Transformers on neutral for electrical meter

[meternerd]

Mechanical meters make use of induced currents in the aluminum disk to provide a force on the disk which is proportional to the product of the instantaneous current and the instantaneous voltage. With the resistance supplied proportional to the speed of the disk, you now have the disk rotating at a speed proportional to the kW, not the kVA.
and without any additional mechanical devices in the dial chain, the disk will rotate backwards during reverse power flow (such as from local generation.)
And what the utility sells is energy produced by its generators, and a higher kVA delivered to the customer does not cost more fuel. It just requires larger distribution lines.

True....watts are what does work, but power is bought and sold based on both MWh and MVARh (added costs when PF is below specified values). That helps offset the construction and equipment costs associated with higher than optimal currents due to low PF. All of our equipment is sized by KVA rating, not KW. Somebody has to pay for the larger wire, transformers, etc. when PF will be less than unity. Those costs are ultimately passed on to the consumer. If billing was based on KVA, the worse the PF, the more you'd pay. That's how utilities have to buy their power. It's an incentive to keep PF under control. Saves money on infrastructure during construction and power costs for purchasers (like us) later. Generation fuel costs are only part of the overall pricing. Adding capacitor banks, static VAR systems, etc. isn't free. Ain't this stuff fun? Bet we've either lost or bored almost everybody. Including me. I think we've killed this dead horse. Bye.

 

[mivey]

The error is small, but it will always be in favor of POCO.

This assumes the higher current is on the lower voltage leg.

 

[mivey]

One thing to think about is that residential meters measure KWh. Utilities supply KVAh. Any power factor less that unity results in more current being supplied than what is measured. Probably makes up more than the difference. In your example, if you have a service that's 120V @ 30A and 110V @ 50A, (for an hour) the meter would not register 9.2 KWh only if PF was unity. Actual would be 3.6 KVAh + 5.5KVAh = 9.1KVAh. But at a .989 PF, the KWh would be 9.1 KWH. Actual PF is probably lower than that on most services. But, too early in the morning to think about this stuff. Gotta go get another cup of coffee.

Get some more coffee, we don't sum kVA values.

 

[mivey]

Most utilities I know of bill KWh and add charges for KVARh and KW peak demand.

They are separating the energy and demand charges, as they should.

The KWh rate is usually lower than for non-demand customers.

Because the non-demand customers pay for demand costs (and usually some of the customer costs as well) using the energy billing determinates.

Since ours is a small utility with very little large commercial load, we don't bill for KVARh.

It is not really the kvarh you are after but the capacity charge. However, the kvarh will give you some loss data.

Our system PF is pretty close to unity, so we pay no penalties to our suppliers for poor PF, thus no charge to the customer.

But you have loss cost. Even without the wholesale fee, you should still consider the incremental cost to encourage high load factors and to keep from crossing the line where you incur wholesale fees.

We do add PF correction equipment if the PF is too bad at any particular location.

Local costs in lieu of wholesale fees.

It's always been my opinion that the industry should bill based on KVA, since that's really what we sell. That would eliminate all the other junk as far as PF. But it's been KWh since metering was invented, and they aren't about to change.

What we really sell is energy. The demand is a separate fee that is a function of other ancillary charges, mostly how fast and how efficiently we deliver the energy.

 

[mivey]

True....watts are what does work, but power is bought and sold based on both MWh and MVARh (added costs when PF is below specified values).

There are three basic cost components: customer costs (mostly use agnostic stuff), demand costs, and energy costs. You have the correct idea that billing using energy determinates alone can inaccurately bill for some of the costs but need to realize that kVA is not a good basis by itself. The more accurate rate will have multiple components and not be based on just one or the other.

 

[GoldDigger]

This assumes the higher current is on the lower voltage leg.

If, as I stated, the voltage difference is entirely the result of current related voltage drop in the service wires, then the higher current must be on the leg with the lower voltage.

 

[mivey]

If, as I stated, the voltage difference is entirely the result of current related voltage drop in the service wires, then the higher current must be on the leg with the lower voltage.

Yes. The point is that there are assumptions about the load that may not always make the error in the POCO favor. Unequal power factors on the legs is another reason, even if the higher current is on the lower voltage leg (i.e. let the higher current leg have a lower power factor).

For example:

118v, 100a at 0.80pf
and
120v, 95a at 0.95pf

 

[GoldDigger]

All that I am saying is that there are only two types of situation, those where the error will randomly favor either customer or POCO (which will average out to zero) and those that can only favor POCO and will not average to zero.

There are, IMO, no situations which will systematically favor the customer.

 

[meternerd]

Get some more coffee, we don't sum kVA values.

Why? VA does not take into account phase angle between voltage and current, so I though for single phase, it was VA = V x A and Watts = V x A x PF. Not so? Residential metering is Line to Line, so wouldn't angle between each leg be 180 deg? Been a LONG time since I had to remember this stuff. Makes my head hurt! Not too old to learn, though.

 

[meternerd]

All that I am saying is that there are only two types of situation, those where the error will randomly favor either customer or POCO (which will average out to zero) and those that can only favor POCO and will not average to zero.

There are, IMO, no situations which will systematically favor the customer.

Actually, there are. We set 50kW as peak the demand amount before we go to a demand type of billing. At the demand rate, the kWh charge is a lot lower than the non-demand rate. It's usually made up for with the addition of the peak demand charge. Peak demand is measured in 15 minute intervals and the peak demand is reset every month. Demand history is reviewed once a year. So...if a customer has a high demand only during a few months a year (such as a gravel pit which cuts way back in the winter), they get the lower energy charge all year long. If the demand is just above the 50kW, the demand charges are not nearly offsetting the reduced energy rate. The customer makes out. I know this is a billing issue and not a metering issue, but most POCO's really do try to be fair. The customer tells us what their expected demand will be and we size the service accordingly. Bigger the service, the higher the installation charges and the monthly "facility fee". It doesn't take long to find out if they were "sandbagging" with lower numbers when the transformers start smoking. Maybe I'm just being "sensitive".:happyyes:

 

[GoldDigger]

Actually, there are.

That is "no situations resulting from not measuring the line to neutral voltages at the meter." Please read what I say in context.

 

[ronaldrc]

That is "no situations resulting from not measuring the line to neutral voltages at the meter." Please read what I say in context.

I think a simpler way of looking at why you don't need to monitor the neutral with a CT.
is that any current that flows through the neutral has to has to flow in either line #1 or
line #2 and has already been added.

For Three phase using Two CTs. instead of Three, the same might be true because any current that has to flow through the phase leg without the CT. has to flow through one of the other two phase legs with CTs.
But this might be debatable.

 

[kwired]

I think a simpler way of looking at why you don't need to monitor the neutral with a CT.
is that any current that flows through the neutral has to has to flow in either line #1 or
line #2 and has already been added.

For Three phase using Two CTs. instead of Three, the same might be true because any current that has to flow through the phase leg without the CT. has to flow through one of the other two phase legs with CTs.
But this might be debatable.

I am by no means a metering expert but with thee phase and two CT's like you mentioned, what is metered it you have current flowing from the non CT phase to a neutral? Or would this kind of metering only be used for three wire delta system?

 

[GoldDigger]

You always need one fewer CT than the number of conductors (not the formal CCC count used for ampacity adjustment).
The issue with the 120/240 single phase meter is not with the current measurement, but with the fact that only one voltage is sensed, with the neutral not even connected to the meter circuit.

 

[ronaldrc]

The reason I said three phase was debatable was because if you have a neutral you might
need three?

 

[GoldDigger]

The reason I said three phase was debatable was because if you have a neutral you might
need three?

You would need three if the loads are such that there is any neutral current. And since POCO has no control over the loads, they would have to use three if they provide a neutral.

 

[ronaldrc]

You would need three if the loads are such that there is any neutral current. And since POCO has no control over the loads, they would have to use three if they provide a neutral.

That's the way I see it to, but with the extra ct.they are measuring each leg twice.
I 'm not a math person but could you correct that by dividing your KW by 1.73 probably not because your also
dividing your neutral current to, its a no win for the customer.

 

[meternerd]

Read this if you want to get really, really nerdy....

http://powermeasurements.net/library/Presentations/NCMS 2013 - Non-Blondel Metering.pdf

 

[ronaldrc]

Read this if you want to get really nerdy....

http://www.meteringforlinemen.com/Blondel_s_Theorem.html

Thanks that backs my thought of this being a hard thing for the engineers address anyway.

I an't smart enough to be a nerd.

 

[GoldDigger]

That's the way I see it to, but with the extra ct.they are measuring each leg twice.
I 'm not a math person but could you correct that by dividing your KW by 1.73 probably not because your also
dividing your neutral current to, its a no win for the customer.

As long as the meter on a wye takes the vector (dot) product of each line current with the corresponding line (to ground) voltage and adds the three there is no need for any additional correction factor.