The problems of the distributed generation integration into the main network related to the dynamic interaction of the main network generators with large inertia with distributed generation which have a small inertia, are considered. As a result of this interaction, there are dangerous weakly stable oscillations, which can lead to the breakdown of consumers technological equipment, or electrical equipment in the main network. Modal analysis is the main method of the dangerous oscillations monitoring in the world. The method involves calculating the eigenvalues (modes) of the system's dynamics matrix and determining the eigenvalue with the maximum real part. The subgramians method allows to calculate the current energy value of the weakly stable modes accumulated in their group, which is the basis of a new technology for the stability margins estimating based on energy criteria. The main attention is paid to modal analysis and the method of gramians, their comparison and description of the hybrid method of small signal stability analysis are given.