A shielded resistor for voltage transformer testing
Francis B. Silsbee · Scientific Papers of the Bureau of Standards · 1925
Voltage transformers are widely used in measuring electric power and energy, and since their accuracy enters directly into the charges for this commodity the need for testing them is apparent.This paper describes the equipment and method used at the Bureau of Standards for testing such transformers over the range from 220 to 30,000 volts.The null method employed is capable of a precision of 0.01 per cent in ratio and 0.3 minute in phase angle at 60 cycles.The 500,000-ohm working resistor connected across the high voltage is subdivided into sections of 20,000 ohms, each of which is inclosed in a metal shield which in turn is connected to a tap at the proper point of an auxiliary guard circuit.The charging currents from these shields to ground, therefore, flow in the guard circuit only and do not directly affect the working circuit.A theoretical discussion of subdivided shielded resistors of this type and also of the uniformly shielded type shows that the error arising from capacitaiice to ground, though greatly reduced by the use of the guard circuit, is not completely eliminated and increases in importance very rapidly as the working voltage of the resistor is increased.