IEEE C57.12.58-1991 (R2002) pdf free.IEEE Guide for Conducting a TransientVoltage Analysis of a Dry-Type Transformer Coil.
IEEE C57.12.58 Since voltages between points on the test coil are measured on a differential basis, and the voltage from each point toground may be grealer tha the difTerential voltage, lhere exists the polential ol a comon-mode rejeclion problen.
A portion of the common-mode voltage will be injected into the system, either in the probes or in the differentialamplifier. The degree of rejection of this common-mode voltage is called the common-mode rejection ratio. Bydefinition, it is the ratio of the common-mode voltage to the differential voltage appearing at the ouput of theoscilloscope or other measuring means when both probe leads are connected to the common-mode voltage. Thecommon-mode rejection ratio(CMRR) may be expressed as a ratio either directly or in decibels.
CMRR is a function of both the balance in the amplifier and probe compensation.The capability of an amplifier isstated usually in the specification sheet. The CMRR generally decreases as frequency increases. To check thisparameter, first the equivalent 3 dB frequency for the shortest rise time should be determined, then the CMRR vs.frequency chart in the specification sheet should be consulted. To obtain the best CMRR possible, it is important tohave a gain and frequency balance available. Gain balance should be available on the differential amplifier or on theprobe housing.Frequency compensation match is achieved by adjustments on the probes. It is important that theprobes be checked individually for frequency compensation and gain before the differential adjustment: are made.The individual adjustments are made by applying a calibrated (rise time s0.01 ns) square wave to the probe tips.Final differential compensation is mace using the full output voltage of the recurrent surge generator，with thedifferential amplifier sct at thc scalc at which the differential voltagc is to be obscrved.This will requirc a prcliminaryobservation of the differential voltage output. The balancing procedure may have to be repeated each time the gain ofthe amplifier is changed.Afier adjusting the probes for minimum common-mode voltage, a detectable voltage called“tare”may cxist. If tare voltage exceeds 4% of the expected differential voltage, it should be considered as a part of themeasurement. The tare voltage may be positive or negative, and it should be subtracted algebraically from the signalvoltage. The actual value of the tare used should be the voltage occurring at the same instant that the peak of themeasured voltage occurs.IEEE C57.12.58 pdf download.