Abstract: For the safe operation of the power system, all the generation units require certain standby generation capacity during certain power grid abnormal conditions, especially for the power network with increasing generation capacity of the renewable energy. From the operation point of view of the power grid, if the standby generation capacity is close or at the same geographical position to the abnormal fault or outage units, the impact on the power grid will be controlled to a minimum level. When the power grid is operating in high loading and the load increasing of the nearby units is difficult in grid abnormal condition, such as large scale power loss of the local grid from wind energy, the overpower capability of the generation units in grid abnormal area will be very important for the grid. The objectives of the related work including: (1). to study the feasibility of unit overpowering using the high pressure feedwater heater bypass system of the generation unit, under abnormal grid condition; (2). to analyze the potential of the overpowering capacity for a certain existing generation unit. First, the proposed method of unit overpowering is described under power grid abnormal conditions. The proposed method is based on the change of the operation mode of the turbine high pressure feedwater heater system. Under power grid abnormal condition, the high pressure feedwater heater bypass valve is opened, and the feedwater flow through the high pressure feedwater heater and the resulting extraction steam flow required to the heaters are decreased, thus the steam flow through the steam turbine and power output of the unit are increased. A theoretical analysis method for evaluation of the existing generation unit is put forward. Based on the design parameter of the full load condition, an overpower factor of the unit under high pressure feedwater heater bypass condition can be calculated. This overpower factor is illustrated as a function of the feedwater heater outlet mixed temperature drop or feedwater heater bypass flow. This factor can also be used for evaluation of the unit for similar purposes under other operation conditions. An example of the unit overpowering scheme is provided for a 1000MW generation unit. Based on the analysis results, it is shown that under the condition of grid abnormal operation condition the feedwater flow to the HP heater bypass line can be increased about 5%-35%, and a 0.77%-5.09% of the unit rated output increase can be expected under full load condition. Typically the high pressure feedwater heater bypass is opened and 20% of the feedwater is bypassed, and nearly 3% of the unit output can be increased under full load condition. The theoretical analysis results also give some operation limitation for future site application. It is also shown that under the HP heater bypass operation condition the final temperature dropping is nearly linear to the bypass line flow fraction. Therefore, controlling of the final feedwater temperature can be taken as an indirect indication of the heater bypass flow. Similar function and logic is already tested and put into operation on site. Test results on site verified the validity and accuracy of the analysis method proposed.