Transformer Question No. 17 - Converter transformer

#1
Sun, December 21, 2008 - 13:27
For rectifier transformer sizing, many terms such as the harmonic losses effects and internal impedance which influence on the transformer rating shall be considered by designer.
The traditional IEEE method for rating a rectifier transformer has always been the root-mean-square (rms) kVA drawn from the primary line. This is still the method used to develop all of the tables and figures given in ANSI/IEEE C57.18.10, Clause 10. However, the IEC converter transformer standards define the kVA by the fundamental kVA drawn from the primary line. The rms-rated kVA method is based on the rms equivalent of a rectangular current wave shape based on the dc rated load commutated with zero commutating angle. The fundamental kVA method is based on the rms equivalent of the fundamental component of the line current. According to IEC, it is only proper to rate the transformer at the fundamental frequency. Transformer rating and test data will then correspond accurately.
The traditional IEEE rms-kVA rating method will not be exactly accurate at test. However, it does represent more accurately what a user sees as meter readings on the primary side of the transformer. Users feel strongly that this is a better method, and this is what their loading is based on. ANSI/IEEE C57.18.10 allows for both kVA methods. It is important for a user to understand the difference between these two methods so that the user can specify which rating is wanted.

-Harmonic Loss Effects

The term harmonic-loss factor, was developed by IEEE and IEC as a method to define the summation of harmonic terms that can be used as a multiplier on winding eddy-current losses and other stray losses. These items are separated into two factors, winding eddy-current harmonic-loss factor, and the other-stray-loss harmonic-loss factor.
The new IEEE Recommended Practice for establishing transformer capability when Supplying Non-Sinusoidal Load Currents, ANSI/IEEE C57.100, gives a very good explanation of these terms and comparisons to the UL definition of K-factor. The Transformers Committee of the IEEE Power Engineering Society has accepted the term harmonicloss factor as more mathematically and physically correct than the term K-factor. K-factor is used in UL standards, which are safety standards. IEEE standards are engineering standards.
As is evident, the primary difference is that the other stray losses are only increased by a harmonic exponent factor of 0.8. Bus-bar, eddy-current losses are also increased by a harmonic exponent factor of 0.8. Winding eddy-current losses are increased by a harmonic exponent factor of 2. The factor of 0.8 or less has been verified by studies by manufacturers in the IEC development and has been accepted in ANSI/IEEE C57.18.10. Other stray losses occur in core clamping structures, tank walls, or enclosure walls. On the other hand, current-carrying conductors are more susceptible to heating effects due to the skin effect of the materials. Either the harmonic spectrum or the harmonic-loss factor must be supplied by the specifying engineer to the transformer manufacturer.

- Commutating Impedance

Commutating impedance is defined as one-half the total impedance in the commutating circuit expressed in ohms referred to the total secondary winding. It is often expressed as percent impedance on a secondary kVA base. For wye, star, and multiple-wye circuits, this is the same as derived in ohms on a phase-toneutral voltage basis. With diametric and zigzag circuits, it must be expressed as one-half the total due to both halves being mutually coupled on the same core leg or phase. This is not to be confused with the short-circuit impedance, i.e., the impedance with all secondary windings shorted. Care must be taken when expressing these values to be careful of the kVA base used in each. The commutating impedance is the impedance with one secondary winding shorted, and it is usually expressed on its own kVA base, although it can also be expressed on the primary kVA base if desired. Care must be taken when specifying these values to the transformer manufacturer. The impedance value, whether it is commutating impedance or short-circuit impedance, and kVA base are extremely important.Use ANSI/IEEE C57.18.10 as a reference for commutating impedance. The tables of circuits in this reference are also useful.