Abstract
The ultra-low frequency oscillation (ULFO) problem reappears in China recently. Different from the classical low frequency oscillation (LFO), the generators in ULFO have the same mode shapes, namely oscillating in the same pace. To reveal the reason why ULFO generators do not oscillate inversely as classical LFO does, this paper investigates the mechanism of ULFO based on improved Philips-Heffron model of two-machine system. Through this model, the phase angle relationship between different generators is obtained. Based on the bode diagram, it is found that the phase angle difference of the two machines is around zero in ultra-low frequency band however the generator parameters change. On the contrary, the situation is different in classical low frequency band, which reveals the mechanism that generators in ULFO always oscillate in the same pace. Finally, the analysis above is validated through a 2-area 4-machine system and a 6-machine system established in PSASP software.
Highlights
In recent years, the ultra-low frequency oscillation (ULFO) reappears in Chinese power system
The ULFO is not well known compared to classical low frequency oscillation (LFO), it can be traced as early as 1966, at which time a ULFO of 2.5 to 3.5 cycles per minute was firstly reported in United States Northwest Pacific system [1]
ULFO are the oscillations in power systems with frequency below 0.1 Hz
Summary
The ultra-low frequency oscillation (ULFO) reappears in Chinese power system. To reveal the ULFO mechanism in multi-machine system, based on the equivalence model and simulations of the multimachine system, the large phase lag of hydro unites was regarded as the main cause of ultra-low frequency oscillation [17], [19]. The effect of nonlinear components and the dead zone on ultra-low frequency oscillation were analyzed [20], [21] All these researches did not explain the most important characteristic of ULFO, that is all the generators participating in ULFO basically have the similar phases. The detailed derivation procedure and expressions of the coefficients in (11) and (12) can be found in supporting document
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