A test board for watt-hour meter standards
1
A Test Board for Watt-Hour Meter Standards F. J. L V I T S Y ASSOCIATE MEMBER AIEE T HE Central Laboratory of the New England Power Service Company maintains the watt-hour standards for the New England Electric System. Up to recently, calibration techniques were not satisfactory since not only field watt-hour meter standards but reference watt-hour meter standards showed errors, minor in so far as com- mercial yardsticks were concerned, but annoying from the laboratory point of view. To minimize these errors, test procedures and equipment were reviewed. The aim was to stabilize all conditions which might influence the refer- ence standards, to eliminate in so far as possible the errors due to the human element, and thus reduce inaccuracies. Using the best ideas which could be found among manu- facturers and utilities, a new test board was designed and constructed. This board employs selected watt-hour meter standards operating under as near to ideal conditions as possible, and not only provides a simple and accurate means for the test of standards brought in from the field, but also permits immediate calibration of reference standards. By employing three reference watt-hour meter standards, a quick intercomparison of these standards can be made. Friction was lessened by the removal of registers. With the elimination of the registers, a major problem was the calibration of the reference standards. It was possible to use the photoelectric pickups installed in the standards to obtain a count of full revolutions only. A method previously developed uses this count of full revolutions in conjunction with a chronograph, giving precise time meas- urement, to compute watt-hours with time as the variable. In spite of the accuracy of the chronograph it was be- lieved that a definite time interval could be fixed more precisely than an indeterminate time interval could be measured, and did not introduce a human error in such time measurement. Thus it was decided, for calibration of the reference standards, to retain the 1-minute interval and to measure both partial and whole revolutions of the disks by employing an additional photoelectric pickup in each standard and an electronic counter. To actuate the photoelectric pickup, 200 evenly spaced teeth approximately 1/16 inch deep were cut in the edge of the disks in each reference standard. Since the aperture between adjacent teeth would not transmit sufficient light to actuate the phototube, a metal plate, in which a 1/2- inch by 1 /l 6-inch lattice had been cut, was mounted over a segment of the meter disk. The lattice openings match the slits in the disk to form a shutter. The photoelectric system has the light source above the shutter and the phototube below. As the disk rotates the light intensity on the phototube varies from a small value when the Digest of paper 55-150, "A New Test Board for Portable Watt-Hour Meter Standards,'* recommended by the AIEE Committee on Instruments and Measurements and ap- proved by the AIEE Committee on Technical Operations for presentation at the AIEE Winter General Meeting, New York, . Y., January 31-February 4, 1955. Scheduled for publication in AIEE Communication and Electronics, 1955. F. J. Levitsky is with the New England Power Service Company, Worcester, Mass. teeth in the disk are directly under the slots in the plate to a maximum value when the slots in the disk are in line with the slots in the plate, thereby giving 200 output pulses per revolution. The magnitudes of these pulses are still too small to be used directly, so a 2-stage amplifier is mounted on the pickup chassis. The amplified pulses are then fed to a high-speed electronic counter. This counter consists of a wave shaper, amplifier, and four decade counting units which give a direct reading of the number of pulses. The rated maximum input of the decade count- ing units is 40,000 pulses per second, while the maximum input rate in this circuit is 55 pulses per second. To calibrate the reference standards, which run con- tinuously, the power is held constant and the clock relay contacts operate the start and stop circuit of the decade counter. A total reading on this counter will give, when divided by 200, whole revolutions and partial revolutions of the reference standards to the nearest 0.005 revolution. The accuracy of the decade-counting units is absolute ± 1 count. Errors in starting and stopping have been minimized by the use of a high-speed start-stop relay. Current and voltage for the field standards are obtained with uniload equipment. Through the use of precision transformers, the reference standards operate with constant magnitude voltage and current. From test results of the precision transformers made at the National Bureau of Standards, calculation of combined transformer errors to be expected at the commonly used calibration points shows a maximum of 0.03 per cent at unity power factor and 0.04 per cent at 50 per cent lagging power factor. These errors are considered insignificant and no transformer corrections are applied in routine calibrations. As outlined above, absolute calibration of the reference standards is accomplished by holding the wattage constant and measuring revolutions and partial revolutions for a fixed time interval. No single test result on the reference standards is used, but the average of a number of tests. The stability of the reference standards is considered excellent. Calibration data posted to the nearest 0.05 per cent show no change in the average calibration during the first 3 months of use. A high degree of consistency among reference watt- hour meter standards has been achieved. Calibrations of these standards indicate a stability considerably better than had been shown by previous experience. In many ways the equipment is similar to that used by other laboratories; but by this technique using two photoelectric systems, it is possible to calibrate any one reference standard by laboratory indicating instruments, to intercompare any two of the three reference standards, to cross-check accuracy of the mechanical and electronic counters, and to calibrate with the requireddegree of accuracy, the standards brought into the laboratory. JULY 1955 Levitsky—Watt-Hour Meter Standards 605
Transcript of A test board for watt-hour meter standards
A Test Board for Watt-Hour Meter Standards
F. J. L Ε V I T S Κ Y A S S O C I A T E M E M B E R A I E E
TH E Central Laboratory of the New England Power Service Company maintains the watt-hour standards
for the New England Electric System. U p to recently, calibration techniques were not satisfactory since not only field watt-hour meter standards but reference watt-hour meter standards showed errors, minor in so far as commercial yardsticks were concerned, but annoying from the laboratory point of view. T o minimize these errors, test procedures and equipment were reviewed. The aim was to stabilize all conditions which might influence the reference standards, to eliminate in so far as possible the errors due to the human element, and thus reduce inaccuracies.
Using the best ideas which could be found among manufacturers and utilities, a new test board was designed and constructed. This board employs selected watt-hour meter standards operating under as near to ideal conditions as possible, and not only provides a simple and accurate means for the test of standards brought in from the field, but also permits immediate calibration of reference standards.
By employing three reference watt-hour meter standards, a quick intercomparison of these standards can be made. Friction was lessened by the removal of registers.
With the elimination of the registers, a major problem was the calibration of the reference standards. I t was possible to use the photoelectric pickups installed in the standards to obtain a count of full revolutions only. A method previously developed uses this count of full revolutions in conjunction with a chronograph, giving precise time measurement, to compute watt-hours with time as the variable.
In spite of the accuracy of the chronograph it was believed that a definite time interval could be fixed more precisely than an indeterminate time interval could be measured, and did not introduce a human error in such time measurement. Thus it was decided, for calibration of the reference standards, to retain the 1-minute interval and to measure both partial and whole revolutions of the disks by employing an additional photoelectric pickup in each standard and an electronic counter.
To actuate the photoelectric pickup, 200 evenly spaced teeth approximately 1/16 inch deep were cut in the edge of the disks in each reference standard. Since the aperture between adjacent teeth would not transmit sufficient light to actuate the phototube, a metal plate, in which a 1/2-inch by 1 / l 6-inch lattice had been cut, was mounted over a segment of the meter disk. The lattice openings match the slits in the disk to form a shutter. The photoelectric system has the light source above the shutter and the phototube below. As the disk rotates the light intensity on the phototube varies from a small value when the Digest of paper 55-150, "A New Test Board for Portable Watt-Hour Meter Standards,'* recommended by the AIEE Committee on Instruments and Measurements and approved by the AIEE Committee on Technical Operations for presentation at the AIEE Winter General Meeting, New York, Ν. Y., January 31-February 4, 1955. Scheduled for publication in AIEE Communication and Electronics, 1955. F. J. Levitsky is with the New England Power Service Company, Worcester, Mass.
teeth in the disk are directly under the slots in the plate to a maximum value when the slots in the disk are in line with the slots in the plate, thereby giving 200 output pulses per revolution. The magnitudes of these pulses are still too small to be used directly, so a 2-stage amplifier is mounted on the pickup chassis. T h e amplified pulses are then fed to a high-speed electronic counter. This counter consists of a wave shaper, amplifier, and four decade counting units which give a direct reading of the number of pulses. The rated maximum input of the decade counting units is 40,000 pulses per second, while the maximum input rate in this circuit is 55 pulses per second.
To calibrate the reference standards, which run continuously, the power is held constant and the clock relay contacts operate the start and stop circuit of the decade counter. A total reading on this counter will give, when divided by 200, whole revolutions and partial revolutions of the reference standards to the nearest 0.005 revolution.
T h e accuracy of the decade-counting units is absolute ± 1 count. Errors in starting and stopping have been minimized by the use of a high-speed start-stop relay.
Current and voltage for the field standards are obtained with uniload equipment. Through the use of precision transformers, the reference standards operate with constant magnitude voltage and current. From test results of the precision transformers made at the National Bureau of Standards, calculation of combined transformer errors to be expected at the commonly used calibration points shows a maximum of 0.03 per cent at unity power factor and 0.04 per cent at 50 per cent lagging power factor. These errors are considered insignificant and no transformer corrections are applied in routine calibrations.
As outlined above, absolute calibration of the reference standards is accomplished by holding the wattage constant and measuring revolutions and partial revolutions for a fixed time interval. No single test result on the reference standards is used, but the average of a number of tests.
The stability of the reference standards is considered excellent. Calibration data posted to the nearest 0.05 per cent show no change in the average calibration during the first 3 months of use.
A high degree of consistency among reference watt-hour meter standards has been achieved. Calibrations of these standards indicate a stability considerably better than had been shown by previous experience. In many ways the equipment is similar to that used by other laboratories; but by this technique using two photoelectric systems, it is possible to calibrate any one reference standard by laboratory indicating instruments, to intercompare any two of the three reference standards, to cross-check accuracy of the mechanical and electronic counters, and to calibrate with the requireddegree of accuracy, the standards brought into the laboratory.
JULY 1955 Levitsky—Watt-Hour Meter Standards 605
