The subsynchronous oscillation has been a dynamic stability problem that threatens the safe and stable operation of power systems, and serious subsynchronous oscillation events have occurred frequently in recent years around the world. Therefore, it is important to promptly extracting the oscillation components and precisely remove the oscillation source by monitoring the wide-frequency measurement data online. Most of conventional subsynchronous component extraction methods, however, have sluggish calculation times and low calculation accuracy, making it challenging to meet the oscillation monitoring demands of a wide-frequency measurement system. To address these issues, this paper suggests a method of calculating instantaneous power using the voltage and current from the wide-frequency measurement system, realizing the extraction of the frequency of the subsynchronous oscillation and the identification of the oscillation source.
Based on the instantaneous power from the measured data, the analytical formulas for the multimodal subsynchronous power of a doubly-fed induction generator grid-connected system were derived in this article. The major components of the subsynchronous power were extracted in the three-phase symmetric and asymmetric situations. Afterwards, using the features of the alternating power component in the main component of subsynchronous power, it was suggested to develop a frequency extraction method based on the whole-cycle instantaneous power difference. This method is faster and more accurate than the traditional method, avoiding the picket fence effect and spectrum leakage issues. Additionally, a method of the identification for subsynchronous oscillation source utilizing half-cycle instantaneous power difference was proposed based on the subsynchronous frequency extraction. This method can locate the subsynchronous oscillation source for the centralized gridconnected system of multiple wind farms and support for cutting machines.
To validate the suggested method, a doubly-fed induction generator was simulated in PSCAD by incorporating it into an infinite grid model using series complementation. The frequency extraction was carried out for the of three-phase symmetric and asymmetric situations in the subsynchronous frequency extraction example. When comparing the FFT analysis results, it shows that the instantaneous power-based frequency extraction results have good accuracy. For every 5 s increase in the studied time series, the computing time of the approach in this study grows by roughly 0.3 ms while the FFT method increases by about 2 ms, the difference between which is an order of magnitude. The result demonstrates how the suggested approach has clear advantages in the processing performance. Meantime, the approach reduces the possibility of spectrum leakage caused by discontinuous sampling endpoints in FFT analysis, and permits variable selection of truncation points for measurement data. The oscillation source identification was done for two scenarios in the multi-machine synchronous oscillation source identification examples: single oscillation source and multi-oscillation source. The simulations demonstrate that the voltage and current at the subsynchronous frequency of the generator where the oscillation source is located are larger than those at other generators. As a result, the corresponding half-cycle instantaneous power difference at the subsynchronous frequency has a larger amplitude. The findings can be used to accurately identify the oscillation source generators in the centralized grid-connected system of multiple wind farms. Furthermore, the magnitude of the absolute mean value of the half-cycle instantaneous power difference can be used to rank the significance of the oscillation sources and to rationally plan emergency response actions in the event of subsynchronous oscillations.
The simulated analysis leads to the following conclusions: (1) The primary elements of the instantaneous power in the symmetric and asymmetric three-phase circuits are a constant power component and an alternating power component of subsynchronous frequency when the subsynchronous oscillation occurs in the wind power grid-connected system. (2) The frequency corresponding to the smallest absolute mean value of the whole-cycle instantaneous power difference is the subsynchronous oscillation frequency, based on which this paper suggests a method for extracting the subsynchronous oscillation frequency. The approach can avoid the picket fence effect problem and offers improved calculation accuracy and speed compared to FFT. (3) The oscillation source, the wind generator, in the centralized grid-connected system of multiple wind farms can be detected because the half-cycle instantaneous power difference of the oscillation source generator at the subsynchronous sampling period is the biggest.
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