In view of the research situation of backpressure extraction steam turbine technology for ultrasupercritical power generation units

the off-design condition calculation model and exergy analysis model of the unit was established

and the influences of steam mass flow

temperature

pressure

steam turbine back pressure and the BEST heat extraction stage on the thermal performance of the unit were analyzed. The exergy efficiency of unit increased first and then decreased with main steam flow rate

reaching the maximum value(52.42%) when main steam flow rate was 750 kg/s. Exergy efficiency of unit increased with the increase of main steam temperature and pressure but decreased with the increase of turbine back pressure. The thermal performance of unit was more affected by back pressure at low load. In addition

the BEST series has a significant effect on the thermal performance of the unit

and the BEST match degree with the system design scheme is achieved when the series is 4

and the thermal performance is optimal. The calculation method and BEST series selection scheme can provide reference for the scheme design and operation optimization of ultra-supercritical units.