مدلسازی و تجزیه و تحلیل سیستم های پویای بی ثباتی در یک پست، خازن جفت تهیه یک موتور القایی

A capacitor-coupled substation (CCS) is a relatively inexpensive way of supplying power to communities living near high voltage power lines. However, when a CCS is used to supply a large induction motor (IM), self-excitation can occur, resulting in significant sub-synchronous voltage, current and speed oscillations. Useful insight can be obtained by analysing a CCS-induction motor (CCS-IM) system using analysis methods from control systems theory. In this paper, a dynamic model of self-excitation is formed and compensation techniques are analysed using Root Locus. The model is validated by comparing it with experimental results from a laboratory installation.

A capacitor coupled substation (CCS), also known as a capacitive divider substation, is a relatively inexpensive way of supplying power to communities living near high voltage power lines because the cost of a capacitor bank and tuning reactor is substantially less than that of a conventional electromagnetic transformer. However, when a CCS is used to supply power to induction motors, particularly large induction motors with high inertia, instabilities may be experienced. These instabilities are typically characterized by sub-synchronous voltage, current and speed oscillations, and are termed “sub-synchronous resonance” (SSR). A South African electricity utility installed a CCS to supply a 1.3 MVA induction motor (IM) in the vicinity of a 275 kV line. Severe voltage and current oscillations were experienced at low motor speeds (∼60% of operating speed) but the CCS worked well with a resistive load [1], indicating that instabilities were due to an interaction between the induction motor and the CCS. Analysis of results from various technical reports of CCS-IM systems did not show the severe current spikes that occur twice every cycle that are typical signs of ferroresonance [2] and [3]. In the CCS installation supplying the 1.3 MW induction motor, a ferroresonance filter was unsuccessful in preventing SSR [3], while removing the filter did allow typical ferroresonance to occur. It is therefore unlikely that the SSR was caused or initiated by ferroresonance, which must not be overlooked when dealing with such systems and for which there are several tried and tested techniques for prevention [2], [4] and [5]. This paper summarizes the CCS circuit, a previous analysis of self-excitation, the power injection remedy and an overview of the s-plane used in Root Locus methods. A dynamic model is developed, and Root Locus plots of the dynamic model are used to illustrate instability in various scenarios. A physical representation of the CCS-IM system was developed in the laboratory, and experimental results are compared with predictions from the dynamic model, leading to conclusions and recommendations. The influence of the transmission network on the CCS-IM system was excluded in the modelling to allow a greater emphasis to be placed on the analysis of the SSR phenomenon.

Based on the analysis and laboratory testing, the following conclusions are made: • A control systems, dynamic analysis of the CCS-IM system provided quick insight into the system's stability. • Treating the CCS-IM as a fictitious control system showed that its dynamics were ultimately set by the open-loop pole positions, which means that a passive controller is not an adequate solution to CCS-IM instabilities. Energy must be injected or removed from the system to eliminate the possibility of SSR. • Accurate equivalent circuit parameters are required to predict the instabilities experienced in a practical installation. Comparing the dynamic analysis predictions with physical results has shown that the method is valid although not complete. The dynamic analysis model did not take into account all the possible influences, such as the effect of the transmission network supplying the CCS. Such differences do not have a significant effect on the underlying structure of the dynamic analysis model, making it a powerful tool for the analysis of such systems. The application of CCS technology to supply power to mostly resistive loads will not encounter the stability problems associated with a CCS-IM system, and the approach remains a useful one for supplying most communities near high voltage lines. Further work is needed on potential instability analysis and the active stability control before the CCS can be deployed for supplying large induction motor loads.