Transformer Voltage Regulation: NPTA915
The NPTA915 regulates output voltage but also provides backup protection for transformers; it is generally intended to be combined with an NPT916 in a transformer protection system.   It is modular and customisable with up to three I/O or communication cards.

Do you also need differential protection for your transformer?  Consider using the NPT916 as well.

ANSI functions for NPTA915: 
87N / 49T / 50 / 51 / 50N / 51N (S)/ 50H / 51H / 68H / 46 / 46R / 46L / 59 / 27 / 59N / 59P / 27P / 47 / 24 / 50BF / 52BF / 60 / 74TC / 90

This relay is part of the NP900 range, offering multi-function protection and communication using IEC61850, GOOSE and MODBUS protocols (Ethernet and fibre optic communication options available).  A full brochure is available on the NP900 range page