What are the common causes of errors in watt-hour metering?
At rated voltage and frequency Percent Registration
Unity power factor, 100% current 99.7 to 100.3
Unity power factor, 10% current 99.7 to 100.3
Unity power factor, 50% current 99.7 to 100.3
50% lagging power factor, 100% current 99.3 to 100.7
% registration = (actual registration kWh x 100) / ( true kWh)
The above accuracy limits apply to each element of a polyphase meter and to the combination of all elements when tested on a single phase with the current coils in series.
Meter Error Sources
Aside from the inherent errors due to variations in temperature, frequency, etc., which are factors of design, the most common causes of error within a meter are listed below and may be detected by inspection and corrected.
Typical Causes
• Dirt (on the disk; in the airgaps).
• Magnetic particles (in the permanent-magnet air gaps).
• Gummy oil and/or dirt in bearings.
• Broken jewels.
• Disk rubbing in air gap.
• Improper mesh of gears or dirty-gearing.
• Improperly adjusted bearings.
• Vibration of the meter mounting.
• Creeping.
Other Causes
• External magnetic fields which may add to, or subtract from, the normal meter magnetic flux.
• Overloads and short circuits. The effect of overloads and short circuits may be to alter the magnetization of the brake magnets, to magnetize adjacent masses of iron, and in general to disarrange the parts.
• Short-circuited turns in meter coils.
The following ranges, based on reclamation standard, should be the main basis in which the meters should be adjusted. Meter accuracy is very important in watt-hour meter since these meters should be a precise instrument that should capable of adjusting within very close limits and with regular servicing maintaining its accuracy indefinitely;
At rated voltage and frequency Percent Registration
Unity power factor, 100% current 99.7 to 100.3
Unity power factor, 10% current 99.7 to 100.3
Unity power factor, 50% current 99.7 to 100.3
50% lagging power factor, 100% current 99.3 to 100.7
Unlike the conventional way, the term “percent error” is never used in the recording of meter accuracy rather the term “percent registration” is considered.
% registration = (actual registration kWh x 100) / ( true kWh)
The above accuracy limits apply to each element of a polyphase meter and to the combination of all elements when tested on a single phase with the current coils in series.
Meter Error Sources
Aside from the inherent errors due to variations in temperature, frequency, etc., which are factors of design, the most common causes of error within a meter are listed below and may be detected by inspection and corrected.
Typical Causes
• Dirt (on the disk; in the airgaps).
• Magnetic particles (in the permanent-magnet air gaps).
• Gummy oil and/or dirt in bearings.
• Broken jewels.
• Disk rubbing in air gap.
• Improper mesh of gears or dirty-gearing.
• Improperly adjusted bearings.
• Vibration of the meter mounting.
• Creeping.
With the exception of the last two above, it will be noted that all defects listed introduce friction and will cause the meter to register "slow."
Other Causes
• External magnetic fields which may add to, or subtract from, the normal meter magnetic flux.
• Overloads and short circuits. The effect of overloads and short circuits may be to alter the magnetization of the brake magnets, to magnetize adjacent masses of iron, and in general to disarrange the parts.
• Short-circuited turns in meter coils.
External Causes
Some of the sources of error which may occur outside the meter itself are:
• Instrument transformer phase-angle and ratio errors.
• Improper connections such ascross-phasing and reversed polarity.
• Broken or high-resistance connections and short circuits in meter wiring and test blocks, blown potential fuses, short-circuiting switches inadvertently closed or left closed, etc.
• Improperly calibrated or poorly maintained rotating standards.
Some of the sources of error which may occur outside the meter itself are:
• Instrument transformer phase-angle and ratio errors.
• Improper connections such ascross-phasing and reversed polarity.
• Broken or high-resistance connections and short circuits in meter wiring and test blocks, blown potential fuses, short-circuiting switches inadvertently closed or left closed, etc.
• Improperly calibrated or poorly maintained rotating standards.
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