ADVANCED VOLTAGE AND CURRENT TRANSDUCERS BASIC INFORMATION

Advanced state-of-the-art loss measuring systems utilize a number of voltage and current sensors that have very low or zero phase angle error. Modern Voltage sensors utilize standard compressed gas capacitors connected with various active feedback circuits to minize the phase angle error of the voltage.

Although the compressed gas capacitors are known for stability and extremely low loss, the electronics associated with the divider must be designed to limit drift to acceptable levels in order to meet the accuracy requirements of the standards.

Also, sensing of the current for accurate scaling for transformer loss testing can be done by utilizing one of the following concepts:

a) Zero flux passive design current transformers

b) Two-stage current transformers

c) Amplifier-aided two stage current transformers.

These current transformers operate on the principle of reducing the flux in the active core of the CT to or near zero; thereby reducing the phase angle error associated with the flux into CT core. The use of high accuracy solid state transducers combined with digital readout can improve overall measurement accuracies due to the following factors:

a) Random error due to the limited resolution of analog instruments is virtually eliminated by the use of digital instruments.

b) Technology, such as solid state time division multiplier techniques for measurement of power, can improve accuracy over conventional electrodynamometer type wattmeters.

The accuracy is also improved because of reduced burden on the instrument transformers and reduction in internal phase shifts. Compensation for lead losses can be designed into these devices.

c) Judicious use of electronic circuits, aided by operational amplifiers, can ensure operation of transducers in their optimal operating ranges. This minimizes the error that is dependent upon the input magnitude as a percent of full scale.

d) Computing circuits for summing and averaging of three-phase measurements can be included in the system design to minimize calculation errors. Errors due to incorrect signs and errors due to selfheating are also minimized by these circuits.