IEEE Std C57.138:1998 pdf free download

IEEE Std C57.138:1998 pdf free download.IEEE Recommended Practice for Routine Impulse Test for Distribution Transformers.
A large majority of distribution transformers presently produced have a fairly high impedance at the primary terminals when excited by impulse frequencies. As a resuk the load presented to the impulse generator when performing routinc impulse tcsts on distribution transformers often has only a small effect on (he voltage wave shape produced by a well-designed impulse generator, Some transformers however will have a large effect, especially on an impulse generator whose design was not carefully considered.
To provide a more consistent “time to half value” one can use larger stage capacitors and lower value parallel or tail resistors, At the same time, a reduction in the value of front or series resistors will reduce the wave shape distorting etTects of variations in load current. He aware however that reducing the value of front resistors will also reduce front time and may increase oscillations near the wave crest.
As load capacitance changes. so does the impulse front time, often making it difficult to obtain the specified value. The front time is generally determined by the impulse circuit inductances and the RC time constant formed by the front resistors and the circuit capacitances. The two capacitances having the greatest effect are the preload or wave shaping capacitor (C2 in Figure I). and the effective load capacitance. Since the preload capacitor is in parallel with the load, using a preload capacitance that is niuch greater than the capacitance of the load reduces the sensitivity of front time to changes in load capacitance. The stage capacitance is in series with the load and jreload capacitances and is typically much larger than the preload capacitance. As such, the choice of stage capacitance usually has little effect on front time. Lead inductances and internal generator inductances can, when large enough. have a significant effect on front time. For this and other reasons it is always a good idea to keep all series inductances as low as possible through the use of low- inductive components and short testing leads.
Using good grounding practices also improves the consistency of impulse wave shape and amplitude. For a general discussion on recommended grounding techniques. see section 5 of IEEE Std C57.98-1993.