Abstract
In this paper, we investigate a stability of voltage oscillation modes in longitudinal power systems equipped with static var compensators (SVC). These modes are usually stable, so they do not appear without external force. The conventional harmonic resonance is a phenomenon in which an oscillation mode is excited with a harmonic source. Hence, it is solved by removing the source with a harmonic filter. For this reason, a SVC is equipped with several filters. However, there is a possibility where an oscillation mode itself becomes unstable. If a mode is unstable, it can have a large amplitude without any harmonic source. Since it needs no harmonic source, its frequency is not limited to a harmonic frequency, but can have any value. The harmonic instability in direct-current transmission systems, is an example of such phenomena. PLL(phase locked loop) is considered to be effective to suppress the harmonic instability by SVC. However, no theoretical reason has been shown, yet. This paper clarifies the effectiveness of PLL based on a stability condition for the voltage oscillation modes. Frequency responses of a thyristor controlled reactor (TCR), a component of SVC, are largely influenced by existence of PLL. If PLL exists, the stability condition is always satisfied, and all modes are stable. Lastly, we perform numerical simulations to show the validity of our investigation.
