TRANSFORMER STANDARD SOUND LEVEL BASIC INFORMATION AND TUTORIALS

ANSI/IEEE C57.12.90 specifies the method for measuring the average sound level of a transformer. The measured sound level is the arithmetic average of a number of readings taken around the periphery of the unit. For transformers with a tank height of less than 8 ft, measurements are taken at one-half tank height.

For taller transformers, measurements are taken at one-third and two-thirds tank height. Readings are taken at 3-ft intervals around the string periphery of the transformer, with the microphone located 1 ft from the string periphery and 6 ft from fan-cooled surfaces.

The ambient must be at least 5 and preferably 10 dB below that of the unit being measured. There should be no acoustically reflecting surface, other than ground, within 10 ft of the transformer. The A weighting network is used for all standard transformer measurements regardless of sound level.

NEMA Publication TR 1 contains tables of standard sound levels. For oil-filled transformers, from, 1000 to 100,000 kVA, self-cooled (400,000 kVA, forced-oil-cooled) standard levels are given approximately by

Equation L = 10 log E K

where E equivalent two-winding, self-cooled kVA (for forced-oil-forced-air-cooled units, use 0.6  kVA), K constant, from Table 10-3, and L = decibel sound level.

Example. A transformer rated 50,000 kVA self-cooled, 66,667 kVA forced-air-cooled, 83,333 kVA forced-oil-forced-air-cooled, at 825 kV BIL, would have standard sound levels of 78, 80, and 81 dB on its respective ratings.


Public Response to Transformer Sound.
The basic objective of a transformer noise specification is to avoid annoyance. In a particular application, the NEMA Standard level may or may not be suitable, but in order to determine whether it is, some criteria must be available.

One such criterion is that of audibility in the presence of background noise. A sound which is just barely audible should cause no complaint.

Studies of the human ear indicate that it behaves like a narrowband analyzer, comparing the energy of a single frequency tone with the total energy of the ambient sound in a critical band of frequencies centered on that of the pure tone. If the energy in the single-frequency tone does not exceed the energy in the critical band of the ambient sound, it will not be significantly audible.

This requirement should be considered separately for each of the frequencies generated by the transformer core. The width of the ear-critical band is about 40 Hz for the principal transformer harmonics. The ambient sound energy in this band is 40 times the energy in a 1-Hz-wide band.

The sound level for a 1-Hz bandwidth is known as the “spectrum level” and is used as a reference. The sound level of the 40-Hz band is 16 dB (10 log 40) greater than the sound level of the 1-Hz band. Thus, a pure tone must be raised 16 dB above the ambient spectrum level to be barely audible.

The transformer sound should be measured at the standard NEMA positions with a narrow-band analyzer. If only the 120- and 240-Hz components are significant, an octave-band analyzer can be used, since the 75- to 150-Hz and 150- to 300-Hz octave bands each contain only one transformer frequency. The attenuation to the position of the observer can be determined.

The ambient sound should be measured at the observer’s position. For each transformer frequency component, the ambient spectrum level should be determined. An octave-band reading of ambient sound can be converted to spectrum level by the equation

S = B - 10 log C

where B decibels octave-band reading, C hertz octave bandwidth, and S decibels spectrum level.

THREE (3) PHASE TO THREE (3) PHASE CONVERSION BASIC INFORMATION AND TUTORIALS

The delta-delta, the delta-Y, and the Y-Y connections are the most generally used; they are illustrated in figure below. The Y-delta and delta-delta connections may be used as step-up transformers for moderate voltages.

  

                      Standard 3-phase/3-phase transformer

 systems.

The Y-delta has the advantage of providing a good grounding point on the Y-connected side which does not shift with unbalanced load and has the further advantage of being free from third-harmonic voltages and currents; the delta-delta has the advantage of permitting operation in V in case of damage to one of the units.

Delta connections are not the best for transmission at very high voltage; they may, however, be associated at some point with other connections that provide  means for properly grounding the high voltage system; but it is better, on the whole, to avoid mixed systems of connections. The delta-Y step-up and Y-delta step-down connections are without question the best for high voltage transmission systems.

They are economical in cost, and provide a stable neutral whereby the high-voltage system may be directly grounded or grounded through resistance of such value as to damp the system critically and prevent the possibility of oscillation.

The Y-Y connection (or Y-connected autotransformer) may be used to interconnect two delta systems and provide suitable neutrals for grounding both of them. A Y-connected autotransformer may be used to interconnect two Y systems which already have neutral grounds, for reasons of economy.

In either case, a delta-connected tertiary winding is frequently provided for one or more of the following purposes. In stabilization of the neutral, if a Y-connected transformer (or autotransformer) with a delta connected tertiary is connected to an ungrounded delta system (or poorly grounded Y system), stability of the system neutral is increased.

That is, a single-phase short-circuit to ground on the transmission line will cause less drop in voltage on the short-circuited phase and less rise in voltage on the other two phases. A 3-phase three-leg Y-connected transformer without delta tertiary furnishes very little stabilization of the neutral, and the delta tertiary is generally needed.

Other Y connections offer no stabilization of the neutral without a delta tertiary. With increased neutral stabilization, the fault current in the neutral on single-phase short circuit is increased, and this may be needed for improved relay protection of the system.

Third-harmonic components of exciting current find a relatively low impedance path in a delta tertiary on a Y-connected transformer, and less of the third-harmonic exciting current appears in the connected transmission lines, where it might cause interference with communication circuits. Failure to provide a path for third-harmonic current in Y-connected 3-phase shell-type transformers or banks of single-phase transformers will result in excessive third-harmonic voltage from line to neutral.

The bank of a 3-phase, three-legged core-type Y-connected transformer acts as a delta winding with high impedance to the other windings. As a consequence, there is very little third-harmonic line-to-neutral voltage and a separate delta tertiary is not needed to reduce it. An external load can be supplied from a delta tertiary. This may include synchronous or static capacitors to improve system operating conditions.