I would expect your country/state to have codes of practice that govern tariff metering that will give overall metering system accuracy and individual component accuracy.
The 5S meter may be used for measuring three wire three phase systems.
The form 5s (or 5A) uses two potential coils and two current coils.
It may be used to meter three phase circuits with the assumption that the voltages and phase angles are equal. There are techniques to accurately measure all three phase currents but the voltage of the third phase is inferred from the two measured phases.
The form 5S may be used to meter four wire delta systems and four wire wye systems subject to possible voltage and phase angle errors.
The form 9S has three current coils and three potential coils and will accurately measure all system types.
Bill
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"Why not the best?"
Jimmy Carter
ANSI has 0.5% and 0.2% categories for both meter types. 0.2% is becoming more common with the move to digital.
Both use CTs. ANSI CTs are 0.3% at 100% rating and 0.6% at 10%. If VTs are used, they are 0.3%.
I've seen where the error percentages are simply added to get a worst possible case. I believe it is more reasonable to use root of the sum of the squares approach.
The 9S is Blondel compliant for 4 wire three phase and the 5S is Blondel compliant for 3 wire three phase. The 5S, however, is commonly also used in 4 wire three phase applications. If we compare similar accuracy 5S and 9S on a 4 wire three phase, they would be similar provided the voltages are perfectly balanced. For unbalanced voltages, the 9S will be more accurate.
The common 2S on most of our houses is also non-Blondel compliant, using two elements to meter three wires. This also depends on voltage balance to be accurate.
This comparison of errors is concerned with external errors in addition to the stated limits of error referenced by stevenal.
The external errors in the 2S metering are generally small and are not related to system voltage drop ahead of the service transformer.
When a current unbalance is of such a magnitude as to cause a significant voltage unbalance between the two sides of the line at the meter, the meter will over charge.
Consider a case where the normal voltages to neutral at the meter drop from 120:240 Volts to 116:121 Volts. (A voltage rise is common with unbalanced loads on a 120:240 Volt circuit. When ever the current unbalance exceeds 2:1, the voltage to neutral on the lighter loaded side will rise.
The meter will still meter line to line loads properly.
However, the meter sees a line to line voltage of 237 volts. The meter infers that the line to neutral voltages are 118.5 Volts and meters accordingly.
So the heavier loaded side will be metered as if the voltage is 118.5 Volts rather than 116 Volts and will be about 2.2% high in this example.
The lighter loaded side will be metered at 118.5 Volts rather than 121 volts and will be about 2.07% low in this example.
The total error introduced by unbalanced loading in this example less than 2%, is in the favor of the utility and is related to the I2R loss on the neutral which is a result of the customer practice of unbalanced loading.
I (and most people) consider this to be an acceptable and partially justified error.
However the 5S meter will compensate for these errors on a single phase circuit or a circuit comprised of two phases of a three phase system.
The 19S-3 is a self contained meter that will accurately measure single phase three wire circuits.
The external errors resulting from the use of a 5S meter on a system with unbalanced voltages may be much greater.
I have encountered quite a few distribution circuits with voltage unbalances near or greater than 5%.
Also there is generally a greater dollar value involved with installations that justify three phase CT metering.
Bill
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"Why not the best?"
Jimmy Carter
To quantify the external 5S error when applied to 4 wire 3 phase, according to Westinghouse a 1% voltage unbalance at 50% pf gives a half a percent error. The same unbalance at unity pf results in no error.
I had some issues where the primary voltage unbalance was a LOT more than 1%. We had phase angle errors as well. We changed to Form 9S meters.
Can you give me a link to Westinghouse information, Steven.
Thanks.
Bill
Bill
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"Why not the best?"
Jimmy Carter
stevenal: I don't have that book, but it sounds like I should get it. any chance that you could list a few more examples from the Westinghouse book with different voltage imbalances and power factors?
I've looked on the used market, and have not been able to find it. The owner of the book on my desk is retiring soon, so maybe I'll inherit it.
Looks like they considered only balanced three phase loading with unbalanced line to neutral voltage of 1% while varying the pf. Besides the two points given before, the only addition is that as pf approaches 0 the percentage error increases without limit.
Can someone please post a referene where the mathematics of this is described?
I tried to work it out myself for a system with different impedances on phases A, B and C, and hence different voltages on A and B and C as well, and the only way i could get to an answer was that I needed to measure all four currents. The only way it's working out with 3 CTs is by assuming the voltages are the same.
