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Forced resonance mechanism is one of many reasons of power system low frequency oscillations. According to this reason, the interactions between thermal system and power system are considered and the sources of resonance mechanism low frequency oscillations in power system are searched from thermal system in this paper. The custom modeling method is adopted in simulation, including unit mathematical model, coordinated control system and turbine digital electro-hydraulic control system model etc. The theoretic foundation of power system primary frequency adjustment is discussed, and control scheme for the primary frequency adjustment control on coordinated control and turbine digital electro-hydraulic control system side. Then the relationships between turbo-pressure pulsation and power system forced oscillations are studied, the reasons that caused pressure pulsation are explored in particular. The simulation results indicate that power system low frequency oscillations of resonance mechanism might be caused by the pulsation of turbine's main steam pressure, when its pulsating frequency is consistent with or close to power system natural oscillations frequency. In thermal system, the introduction of a large number of control strategies is one of the reasons that caused the turbine pressure pulsation. The conclusion can provide reference to research into the causes of power system low frequency oscillations.