In order to reduce superheat degree of the extraction steam of steam turbine(large turbine) and decrease the irreversible loss in heat exchange process

a back pressure extraction feed pump steam turbine(BEST) system with small generator and controlled by converter is proposed and applied. While driving the boiler feedwater pump

the excessive power of this steam turbine(small turbine) is absorbed and converted by the supporting small generator

the generated electric energy is used by the equipment in the plant

so as to effectively reduce the auxiliary power consumption rate. In practical application

it is found that the residual power of the BEST small turbine is too large

which triggers the “electric load overrun” protection action

and the steam inlet regulating valve of the small turbine is turned down

resulting in a large fluctuation of the boiler feedwater flow

and the unit trips due to the “low feedwater flow” action of the boiler main protection. By analyzing the operation curves

it is considered that the back pressure of the small turbine is abnormally low and the steam enthalpy drop is too large

which leads to the residual power of BEST small turbine exceeding the design value and the “load overrun” of small generator.By optimizing and adjusting the operation scheme of the steam extraction and regenerative system

reasonable back pressure of the small turbine is gradually established

so that the residual power of the small turbine at each speed is in normal range

and the problem of electric load overrun of the BEST feedwater pump system is reliably solved

which effectively ensures the subsequent commissioning of the unit. Relevant commissioning and adjustment experiences can be used as a reference for units of the same type.