Extraction of open-loop frequency response of power apparatus using transient data from multiple naturally occurring disturbances

Abstract

Frequency response analysis (FRA) is a mature technique to characterize the behaviour of apparatus like transformers, and is commonly used for condition monitoring and diagnostics. FRA is also used for sub-synchronous resonance/control interaction analysis of rotating machines and power electronic converters. The frequency responses (transfer functions) can be experimentally obtained from tests at the apparatus terminals. These are generally done offline, i.e. when the equipment is out of service. Online testing is attractive as it can be done frequently and periodically, but it requires auxiliary equipment for injecting probing signals into an energized high voltage system. The alternative online approach uses high bandwidth measurements of “naturally” occurring external transients (due to faults, switching and other disturbances) to obtain the frequency response. The key challenge in online FRA of multi-port apparatus is in the extraction of the “open-loop” transfer function of the apparatus, while connected to the rest of the system, essentially in a “closed-loop” configuration. This paper proposes a technique to address this challenge by utilizing data from multiple independent events, and presents proof-of-concept simulations. The technique can be applied not only to passive components like transformers and transmission lines, but also to power electronic systems and rotating machines.