Study on Power Low Frequency Oscillation Caused by Primary Frequency Modulation

Power low-frequency oscillation is a complex dynamic behavior of the power system, which is related to the safe and stable operation of the power grid. This paper studies the mechanism of the unit's low-frequency oscillation during a frequency modulation action of the unit. It is pointed out that during a frequency modulation action, when the projection of the mechanical torque vector of the unit on the axis of the speed offset is positive, it means that the mechanical torque characteristic is negative damping. If the negative damping component is too large and exceeds the positive damping component after power system stabilizer compensation, the unit will show negative damping characteristics as a whole, which will cause adjustment power oscillation. Based on the historical data of the synchronous phasor measurement unit in the low-frequency oscillation event of the coal-fired unit, it is judged that the low-frequency oscillation is a forced oscillation caused by the primary frequency modulation of the unit. Through mechanism analysis, the unit coordinated control system exits, the digtal electric hydraulic control system is put into the closed loop mode of the power loop, and the logic setting of the primary frequency modulation command switching is unreasonable, resulting in a substantial increase in valve position output commands. It is equivalent to that the forward open loop gain of the speed channel is too large, which causes the negative damping component of the mechanical torque to be too large, which causes the regulation oscillation. Through simulation demonstration, it is determined that during the primary frequency modulation, the unreasonable logic setting and parameter setting of the primary frequency modulation control loop are the main reasons for the low frequency oscillation of the coal-fired unit power. Finally, the main measures to eliminate the unit oscillation caused by the primary frequency regulation of the unit and the suggestions for stable operation of the unit are put forward.