1 Introduction
A Phase shifting transformer is a generic name for a wide
range of applications. The most common types are rectifier
transformers, converter transformers, harmonic mitigating
transformers, zigzag transformers and quadrature boosters.
Though the applications are totally different on the mentioned
transformer types, one feature – unconventional phase
shifting was the key design change that deviates from the
classical power and auto transformers. The classical three
phase vector group combinations using delta and star
connections were further modified by different internal
connection arrangements to achieve these unconventional
phase shift values like +/-7.50, +/-150, +/-220, and so on. The
connection types to achieve these phase shift variations are
polygon or extended delta connection arrangements. These
connection arrangements become complex when included
with tap change arrangements on the primary winding.
Classical transformer testing involves ratio testing, excitation
current testing, winding resistance testing, core balance
testing, vector group check, Tan delta testing for Insulation,
oil BDV tests, Dissolved Gas Analysis, and so on. Advanced
tests like Frequency Response Analysis and Frequency
Dielectric Spectrum for moisture analysis are the few other
tests done by high powered transformer owners. Testing the
ratio and verifying the vector group involves different test
methods and is conventionally done by applying a single
phase low voltage on primary and measuring the secondary
voltages. For verifying vector groups, the like-phase short
circuit between primary and secondary windings was the
usual method along with manual calculations to verify. This
method will be too complex when it comes to these special
transformers and methods like three phase voltage source
method and expensive power analyser usage becomes
mandatory. Testing these transformers on-site during
commissioning and periodic maintenance can be a laborious
process and could be performed by different methods by
different organisations. Standard single phase ratio testers
were the solution for many years to perform ratio testing in a
simplistic manner at site. The demand on these ratio testers
increased to offer features like excitation current values, three
phase and multi-tap values, phase angle values, automatic
vector group recognition and so on. The phase shift
measuring feature was in big demand by special transformer
maintenance teams. The focus turned onto the phase shift and
its importance in these transformers.