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## Meter Testing - Traditional & New Techniques

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### Meter Testing - Traditional & New Techniques

Presented by

T Jeffrey Wolters

GE Meter, Somersworth, NH

The following people designed, developed or provided the substance for the material presented:

Dave Elmore, Don Bullock (ret), Les Rosenau, Mike Coit, Warren Germer, Joe Provost

Some “K” Definitions

- Kh - Watt-hours per (equivalent) disk revolution
- Ke - Energy value per pulse (used commonly for PI)
- K - Register programming watt-hour constant (for EV, Kh/12 = K)
- Kt - Test constant: Watt-hours (or VArH) per pulse
- Kr - Dial Multiplier

Inside the kV, K values have no real meaning or bearing on energy calculations. All energy values are supplied from the DSP in engineering units.

GE kV Meter Test LED

Test LED

GE kV Meter Functional Diagram

- DSP “blinks” theOptocom LED every Kt watt-hours (or VArh)
- Through programming, one can set the calibration pulse rate (Kt) to just about any practical value. This can:
- allow one to adjust testing speed (within the confines of other, more restrictive parameters);
- standardize pulse values regardless of meter type.
- Remember the kV DSP outputs (and the register stores) actual energy values rather than pulses which can be converted to energy values.

Kt Value Selections

- The kV meter offers a great deal of flexibility in specifying Kt (test pulse) and PI (Pulse Initiator) values.
- To obtain the most accurate results, it is important to understand more about how the kV calculates its values.
- There is a number stored in the kV’s firmware which is responsible for making the math work based on meter class.

Rev3 & below Rev 04 & above

Meter Class kV divisor kV divisor

20 0.0006 0.0005

200 0.006 0.005

320 0.009 0.0075

Kt Value Selections

Rev3 & below Rev 04 & above

Meter Class kV divisor kV divisor

20 0.0006 0.0005

200 0.006 0.005

320 0.009 0.0075

- The use of Kt (and PI) values that are evenly divisible by these numbers will prevent small errors from being introduced due to truncation of repeating or long decimal values.
- Most traditional Kt (and PI) value selections provide error-free operation, or contain insignificant errors.

Kt Value Selections

- To calculate the magnitude of any possible error, use the formula above.

Example: Use a Kt value of 1.0 on a CL 20 meter, Firmware 3

1-((Integer(1/0.0006))/(1/0.0006))x100

= 1-(1666/1666.666)x100

=0.04% error

Kt Value Selections

- Now Use a Kt value of 1.0 on a CL 20 meter, Firmware 4

1-((Integer(1/0.0005))/(1/0.0005))x100

= 1-(2000/2000)x100

=0.00% error

Some Factors Affecting Measurement Uncertainty

- Total meter performance is a function of many variables.
- The methods we use to verify the meter’s performance are also subject to many variables.
- In general, any variable which affects the meter and the standard differently will lead to UNCERTAINTY.

Some Factors Affecting Measurement Uncertainty

In digital meters, the more obvious factors affecting uncertainty include:

- Repeatability limits of the test equipment being used;
- “Random” systematic errors including response time and characteristics of the meter calibration LED circuit and the test equipment optotransistor circuit used in the pickup device;
- Digitizing error (remember we reduced this by careful selection of sampling rate);
- Inherent measurement differences between filtered and unfiltered methods of measuring AC power;
- Temperature, frequency, stray magnetic fields, current and voltage non-linearities...

Inherent measurement differences between filtered and unfiltered methods of measuring AC power

Take two meters having equal accuracy:

The first, a heavily filtered meter (e.g., RadianTM standard). It integrates average power with respect to time;

The second, an unfiltered meter (e.g., GE kV meter). It integrates instantaneous power with respect to time.

Measurement Uncertainty ExampleMeasurement Uncertainty Example

Inherent measurement differences between filtered and unfiltered methods of measuring AC power

- Compare the meters’ power calculation during the time interval from a to b in the figure.
- The unfiltered meter will record energy proportional to the horizontally shaded area plus the cross-shaded area.
- The filtered meter will record energy proportional to the cross-shaded area.

Measurement Uncertainty Example

Inherent measurement differences between filtered and unfiltered methods of measuring AC power

- In this example, the Instantaneous power measurement is obviously larger than the Average power measurement. For other small time differences (a and b), Average power could be larger or the same.
- As test time increases, the effect of this source of uncertainty disappears. Also, if you could test over an exact integral number of cycles, this uncertainty would not exist.

GE kV FlashCalibration

- Flash Calibration Objective
- Calibrate voltage gain, voltage phase, current gain and current phase
- Accomplish multiple measurements simultaneously
- Minimize calibration time
- Minimize number of conditions necessary to assure overall meter accuracy

GE kV FlashCalibration

- How Flash Calibration Works
- Meter is put into Flash Cal Mode via Optocom
- Meter “blinks” Optocom LED and starts accumulation
- After specified time, meter stops accumulation and “blinks” LED
- Accumulated quantities are read and compared to standard meters
- New correction factors are calculated and programmed into the meter as required
- Rerun until all results are acceptable

GE kV FlashCalibration

- How Flash Calibration Works (cont)
- Factory has independent, per-phase equipment
- Meter Flash Calibrated at FL and LAG
- Meter verified using both Flash Calibrate mode AND pulse mode
- Total Flash Cal time is just a few seconds
- How can we do that? ... Remember, if you choose an amount of time that is an exact integral number of power cycles, there is no uncertainty error between instantaneous and average power measurements.

9S vs 8S Testing Conditions

Why doesn’t the 9S kV meter seem to test correctly on my calibration panel set up for an 8S meter?

- The socket terminal connections are the same.
- The actual (in)SERVICE phasors presented will be read by either meter. In fact, since the 9S meter is a Blondel solution, it will properly meter unbalanced conditions, where the 8S will not.
- The problem is that most test panels do not (by default) apply the voltages the way they would be presented in service. They treat the 8S as a two voltage element device, applying one voltage A-B and the other C-N. For the 8S, this is good enough to test, but for the 9S kV, there is no real A-N voltage reference since the A-B voltage is subjected to a “voltage divider” between kV’s A-N and B-N voltage sensing resistors.
- In service, this is not a problem because the line to neutral volts are all independently regulated by the distribution transformers.

VN-C

IC

-IB

IC

VB-A

VN-A

VN-B

IB

IA

IB

IA

SERVICE

TEST PANEL

9S vs 8S Testing Conditions- Some test panels can be configured to present a more realistic 4 wire delta input to the meter under test.
- This can also be used on 8S meters.

kV Test Voltages

- Typically, apply Test Voltage (TV nameplate). 120v.
- Other voltage(s) may be used. Wide range meter is 120-480v Line to Line (not Line to Neutral).
- WHEN TESTING, DO NOT APPLY 480 L-N.
- IN POLYPHASE, THIS WILL EXCEED THE 575V LIMIT ON THE POWER SUPPLY ;
- WITH REVENUE GUARD, THIS WILL EXCEED THE 575V LIMIT ON THE POWER SUPPLY AND THE REVENUE GUARD OPTION BOARD.
- Be aware of test board power supply limitations
- some boards were not designed with newer meter switch-mode power supplies in mind.
- Test board error magnitudes typically increase with test voltage increase.

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