IEEE C57.12.58-2017 pdf download

IEEE C57.12.58-2017 pdf download.IEEE Guide for Conducting a Transient Voltage Analysis of a Dry-Type Transformer Coil.
A recommended range for front capacitors is equal to 1 nF to 100 nF in two decades. The recommended range for front resistors is from 10 to 250 i. It is not recommended that these be in decades, as too many resistors in series may result in high inductance and high-frequency oscillations on the output wave form. (Ceramic resistors may not have this problem.)
The calculated value of front resistance serves as a starting point in adjusting for rise time. Although separate equivalent circuits for the front end adjustments have been assumed for starting values of capacitance and resistance, there is some interaction between the individual adjustments.
In practice, it is necessary to adjust one parameter first, prefi.rably the tail, then the front. It is possible that there may be one or two iterations in these adjustments.
6. Instrumentation
6.1 General
Transient waveforms generated within the test coil are displayed on an oscilloscope or other transient recording system. Of particular concern are measurements of peak amplitude and time-to-peak. The equipment should have sutlicient response for the fastest transients expected, and, at the same time, it should have a capability of measuring differential voltages within the test coil. Both are accomplished by using an oscilloscope with a high-speed medium-gain-differential preamplifier. A recording range consistent with the output voltage of the recurrent surge generator and minimum expected signal is required. The typical range is from 10 mV to 150 V. Probes should be selected with reduction of input capacitance as a major criterion. The use of low-input capacitance probes may minimize errors due to additional stray capacitance. 12.5 pF paralleled by 10 M1 is a typical figure for lOx probes.
6.2 Frequency response
There may he high-frequency oscillations within the coil when a wave with a steep Iront is applied across the tenninals of the test coil. Some of these oscillations may exceed the rise time of the input wave. The highest frequency oscillations are generally from turn-to-turn, with the decreasing resonant frequencies seen layer-to-layer or section-to-section. The choice of an oscilloscope or other measuring device is related to the shortest rise time expected and the bandwidth to be measured. The typical relationship between rise time and bandwidth is shown in Equation (12) as follows:
Final differential compensation is made using the full output voltage of the recurrent surge generator, with the differential amplifier set at the scale at which the differential voltage is to be observed. This requires a preliminary observation of the differential voltage output. The balancing procedure may have to be repeated each time the gain of the amplifier is changed. Afier adjusting the probes for minimum common-mode voltage, a detectable voltage called “tare” may exist. If tare voltage exceeds 4% of the expected differential voltage, it should be considered as a part of the measurement. The tare voltage may be positive or negative, and it should he subtracted algebraically from the signal voltage. The actual value of the tare used should be the voltage occurring at the same instant that the peak of the measured voltage OCCUrS.
6.4 Grounding practice
Ground currents tend to cause common-mode problems, particularly in the situation in which high amplifier gains are required. Good grounding practice reduces the problem. The probe grounds should not be isolated from each other, nor should the grounds at the probe tips be connected to the coil ground. Rather, the grounds at the tips should be connected to each other with a short strap. This is illustrated in Figure X. It is best to keep the probes close to each other, even at the tip, in order to minimize a ground loop. Likewise, test leads should be kept close together. If it is necessary to use a long ground connection, ferrite beads should be placed in the ground interconnection to increase the losses and squelch high-frequency oscillations.
7. Test sample
7.1 Sample description
The test sample shall he a ftill-size model of the transtbrrner coil being studied, The test sample preferably
should be a production coil using the same materials as the coil being studied.
7.2 Description of transient phenomena
There are three major causes of high stress during transient phenomena as follows:
a) The nonlinear initial impulse distribution that causes high stresses near the line end of the coil
b) Resonance as the initial distribution oscillates to a final linear distribution (this can result in high stresses almost anywhere inside the coil)
c) The possibility that an idle portion of the winding, such as an unused tap. may rise to voltages in excess of the applied impulse
7.3 Test leads
Test leads should be installed in appropriate places in the coil if the test points are not accessible to a probe. Layer-wound coils are an example of coils requiring test leads. The test leads should be placed at the various locations of the coil by soldering, brazing, or mechanically wrapping a wire to the conductor when the coil is wound. The wire should be brought to the closest end of the coil, leaving the minimum extension [not exceeding 25.4 mm (I in)] outside the coil for access to the test probes. The test-lead conductor should not be larger than 0.518 mm2 (AWG 20). The size and length of the test lead may modify the capacitive distribution inside the coil, thereby modifying the transient voltage distribution. In addition, no two test leads being used for measurement should complete a turn around the core. This practice results in less induced voltage error.