What are the common constants used in electricity metering?
Register constant Kr – is the factor by which the register reading must be multiplied in order to provide proper consideration of the register or gear ratio and of the instrument-transformer ratios to obtain the registration in the desired units.
Register ratio Rr – is the number of revolutions of the first gear of the register, for one revolution of the first dial pointer.
Watthour constant Kh – is the registration expressed in watthours corresponding to one revolution of the rotor. (When a meter is used with instrument transformers, the watthour constant is expressed in terms of primary watthours. For a secondary test of such a meter, the constant is the primary watthour constant, divided by the product of the nominal ratios of transformation.)
Test current of a watthour meter is the current marked on the nameplate by the manufacturer (identified as TA on meters manufactured since 1960) and is the current in amperes which is used as the basis for adjusting and determining the percentage registration of a watthour meter at heavy and light loads.
Percent registration = (Kh X R X 3600 X 100) / (PS)
where P is true watts. This is the basic formula for watthour meters in terms of true watt reference.
The following definitions of various meter constants are taken from the Code for Electricity Metering, 6th edition, ANSI C12.
Register constant Kr – is the factor by which the register reading must be multiplied in order to provide proper consideration of the register or gear ratio and of the instrument-transformer ratios to obtain the registration in the desired units.
Register ratio Rr – is the number of revolutions of the first gear of the register, for one revolution of the first dial pointer.
Watthour constant Kh – is the registration expressed in watthours corresponding to one revolution of the rotor. (When a meter is used with instrument transformers, the watthour constant is expressed in terms of primary watthours. For a secondary test of such a meter, the constant is the primary watthour constant, divided by the product of the nominal ratios of transformation.)
Test current of a watthour meter is the current marked on the nameplate by the manufacturer (identified as TA on meters manufactured since 1960) and is the current in amperes which is used as the basis for adjusting and determining the percentage registration of a watthour meter at heavy and light loads.
Percent Registration of a meter is the ratio of the actual registration of the meter to the true value of the quantity measured in a given time, expressed as a percentage. Percentage registration is also sometimes referred to as the accuracy or percentage accuracy of a meter.
The value of one revolution having been established by the manufacturer in the design of the meter, meter watt-hours = Kh X R, where Kh is the watthour constant and R is the number of revolutions of rotor in S seconds. The corresponding power in meter watts is Pm = (3600 X R X Kh)/S. Hence, multiplying by 100 to convert to terms of percentage registration (accuracy)
Percent registration = (Kh X R X 3600 X 100) / (PS)
where P is true watts. This is the basic formula for watthour meters in terms of true watt reference.
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