Design and testing of a new protection relay for variable speed DFI motor generators

This paper describes the design and testing of a new and innovative protection relay for protecting large variable speed Double Fed Induction (DFI) machines for pumped storage plants. The testing encompasses testing of the new protection with Non-Conventional Instrument Transformers (NCITs), Rogowski CTs and resistive voltage dividers, on a small DFI machine at EPFL as well as RTDS testing of the new protection on a large DFI machine, covering a range of stator and rotor fault conditions. For these machines low frequency currents are supplied to the rotor by means of VSC (Voltage Source Converter) power electronic converters from the AC system. ALSTOM is developing new protection to provide variable speed motor generators with a fully independent protection system that guarantees the plant´s safety in all conditions. Currently, the market offers no protection relay to provide protection for extremely low frequency currents and voltages in variable speed rotors. The new protection relay will measure and monitor these signals, taking into account the start, stop and operation in both pump and generator modes. Cutting-edge “Digital Substation” technology, which incorporates NCITs with IEC 61850-9-2 LE process bus communications to the protection relays, is used to measure the low-frequency current and voltage signals. The main difficulties with the relay development are: (1) the dynamic variation of frequency; (2) the extremely low frequency; (3) the effect of the converter modulating the rotor voltage (PWM). New technology solutions have been developed, tested and implemented in a new relay to meet the challenging requirements to protect variable speed machines. ALSTOM offers a unique and complete protection package covering the entire variable speed machine (both stator and rotor) incorporating the latest NCIT and IEC 61850 technology.