POWER MEASUREMENT ACCURACY IN WATTMETERS

POWER MEASUREMENT ACCURACY IN WATTMETERS
Some factors considered in wattmeter accuracy equipment?

Electronic wattmeter of 0.1% or better accuracy may be based on a pulse-area principle. Voltages proportional to the applied voltage and to the current (derived from resistors or transformers) govern the height and width of a rectangular pulse so that the area is proportional to the instantaneous power. This is repeated many times during a cycle, and its average represents active power. Average power also can be measured by a system which samples instantaneous voltage and current repeatedly, predetermined intervals within a cycle.

Laboratory-standard wattmeters use an electro-dynamic mechanism and are in the 0.1% accuracy class for dc and for ac up to 133Hz. This accuracy can be maintained up to 1 kHz or more. Such instruments are shielded from the effects of external magnetic fields by enclosing the coil system in a laminated iron cylinder. Instruments having current ranges to 10A and voltage ranges to 300V are generally self-contained. Higher ranges are realized with the aid of precision instrument transformers.

Portable Wattmeters are generally of the electro-dynamic type. The current element consists of two fixed coils connected in series with the load to be measured. The voltage element is a moving coil supported on jewel bearings or suspended by taut bands between the fixed field coils. The moving coil is connected in series with a largely non-inductive resistor across the load circuit. The coils are mounted in a laminated iron shield to minimize coupling with external magnetic fields. Switch board wattmeter have the same coil structure but are of broader accuracy class and do not have the temperature compensation, knife-edge pointers, and anti-parallax mirrors required for the better-class portable instruments.

Line connections should be such that the moving –coil end of the voltage circuit and the current coils are on the same side of the circuit being measured to minimize potential differences between the fixed and moving coils. When used with instrument transformers, the moving coil end of the voltage circuit should be connected to the ground terminal of the voltage transformer, and an electrostatic tie (a resistance of a few thousand ohms) should be connected between this terminal and one of the current terminals. Otherwise, there may be sufficient electrostatic forces between the fixed and moving coils to cause an error, or if their voltage difference is large, insulation between the windings may be broken down.

Correction for wattmeter power consumption may be important when the power measured is small. When the wattmeter is connected directly to the circuit (without the interposition of instrument transformers), the instrument reading will include the power consumed in the element connected next to the load being measured. If the instrument loss cannot be neglected, it is better to connect the voltage circuit next to the load and include its power consumption rather than that of the current circuit, since it is generally more nearly constant and is more easily calculated.

The inductance error of a wattmeter may be important at low power factor. At power factors near unity, the non-inductive series resistance in the voltage circuit is large enough to make the effect of the moving-coil inductance negligible at power frequencies, but with low power factor, the phase angle of the voltage circuit may have to be considered.