Standard Differential protection relays (IEDs) may have problem when applied on a Converter Transformer, which can have the phase shift different from standard 30 degrees (e.g. 7.5 degrees). Applying differential protection to such special transformer is difficult, but not impossible as demonstrated in a paper from Zoran Gajić of ABB Sweden.
Converter transformers, used to supply MV drives or feed into a power electronics devices (FACTS), don’t follow the usual transformer pattern of having a phase shift angle which is multiple of 30 degrees. The low voltage windings can be phase shifted for almost any angle, but most commonly used have (7.5 degrees).
That causes problems for standard transformer differential protection IED, which (in that example) will get a constantly-false differential current of 26 per cent of the through load current. To prevent false operation the differential protection pickup will have to be set to twice that, resulting in a system with very low sensitivity which is very important to be able to detect low level internal faults, such as winding turn to turn faults.
In the paper, presented at Cigre Colloquium in 2013, Gajić provides three practical example circuits how to make such installations, along with the captured readings from the differential protection IED. Interposing current transformers are used to compensate for the non-standard phase angle shift, allowing the protection system to operate with much greater sensitivity.
The examples show how this technique can be applied to converter transformers at 18, 24 & 36 pulse, and how a standard differential protection IED can protect such special transformers.
The technique; using interposing current transformers to align the individual winding phase currents, demonstrably lets standard differential protection IED to be applied to converter transformers, and the paper concludes citing practical experience that such systems are already proving reliable in the field.
Read more here:
Practical Experience with Differential Protection for Converter Transformers