The dynamic characteristics of the power grids in the future are discussed. Starting from the basic principle of AC power grid operation，this paper describes three technical challenges faced by power grids with high proportion of non-synchronous machine sources. The three technical challenges are： the new form of synchronous stability-generalized synchronous stability； the inapplicability of electromechanical transient analysis method in analyzing the generalized synchronous stability； and the wide-band resonance instability caused by negative resistance of power electronic equipment. It is pointed out that the non-synchronous machine source must rely on the control means to realize the synchronization with other sources in the power grid； and the decisive factor is the phase-locked loop （PLL）. The lock failure of the PLL means that the non-synchronous machine source is out of step with other sources in the power grid. Therefore，the concept of the generalized synchronous stability is proposed. It is difficult to simulate the PLL characteristics accurately by the electromechanical transient analysis method based on the positive sequence fundamental phasor. Therefore，the countermeasure to study the generalized synchronous stability of the high proportion non-synchronous machine power grids by the electromagnetic transient analysis method is proposed. It is pointed out that the essence of the problem of the wide-band resonance instability caused by negative resistance of power electronic equipment is the attenuation characteristics of the free components with natural resonance frequency in response to power grid disturbances，and the s-domain nodal admittance matrix method is proposed to study the problem.