Not that ABB was working along – engineers from two Slovenian utilities, ELES & Istrabenz Gorenje, worked with researchers from ABB Sweden and ABB Slovenia to implement a protection scheme in two Phase Shifting Transformers (rated 600MVA, 400kV, ±40º with 64 tap positions). In the process the team discovered that IEEE’s 1999 recommendations could be misleading, and documented the changes necessary to use them with modern equipment.
The designed protection scheme has been in operation since 2010, and proven itself effective and reliable, despite deviating from the IEEE recommendations.
The problem with the published guide is the example on which it builds – the IEEE based its calculations on phase shift transformers (PST) rated at 450MVA, 345kV, 753A, 60Hz, ±25º with in total 32 tap positions, but modern PSTs have twice that number of taps and are constructed differently, both of which have a significant impact.
The number of taps shouldn’t change the original scheme, as tap information is not an input to the calculations, but the team established that the IEEE had got this wrong – the additional taps had a significant impact on the construction of the PST and thus the parameters needed to configure it effectively. Not only does 64-taps require a different approach to voltage reversal (using an advance-retard switch rather than an on-load tap-changer), but it also mandates blocking of the secondary differential-protection relay during the switching time, as it will be entirely de-balanced.
The details are laid out in research paper, which was presented at the CIGRÉ Colloquium in Brazil, and as it concludes the existing recommendations from the IEEE can be used, but only with careful consideration of how the evolution of PST hardware will impact the deployment. ABB’s example should serve to demonstrate how such an allowance can be made, at least until the IEEE decides to update the published recommendations.
Read the technical paper here