Modular multilevel converter (MMC) may cause multi-band oscillation during grid-connected operation

while converters with different control strategies

such as grid-following (GFL) and grid-forming (GFM)

have a significant impact on system stability. This paper focuses on the analysis of the stability performance of two converters in multi-band

and establishes a comparative analysis framework of the system

quickly scanning key parameters and identifying modal transfer laws. Firstly

the detailed small-signal models of GFL and GFM MMC with unified reference

unified MMC main circuit parameters

and unified link delay are established. Then

the oscillation mode characteristics of GFL and GFM MMC in low frequency

sub/super synchronization

medium frequency and high frequency range are compared and studied by using the method of ‘eigenvalue analysis+root locus'. Finally

the key difference between the two in multi-band stability is revealed: the GFL MMC is mainly dominated by the power outer loop control in the low frequency band. The sub/super-synchronous section mainly depends on the circulating current suppression and current inner loop control

while the role of the phase-locked loop is relatively weak. Although the GFM-type MMC can enhance the system inertia

its power loop and virtual impedance control may excite sub/super-synchronization and intermediate frequency oscillation

and the adjustment of virtual impedance shows an obvious frequency band coupling effect. It is found that the current inner loop controller shows high sensitivity in multi-band in both control strategies. In addition

it is also found that the reduction of power grid strength contributes to high-frequency stability but weakens medium-frequency stability; excessive control delay will induce medium- and high-frequency negative damping. The research in this paper can provide an important theoretical basis for the optimization of MMC control parameters for different operating scenarios.