Research On Control Strategies For The Inverters In Power Conditioning System

With the rapid development of the energy storage technology, energy storage system,with power conditioning system as the interface device, can accept rational management ofthe power sector to play the role of “load shifting” in the power system; When the grid isout of electric power, the energy storage system can provide stable and reliable powersupply for local loads. Inverters is the most critical power electronic conversion devices inpower conditioning system. In this paper, reaserches on control strategies for invertersoperating in various modes is clearly performed.When power conditioning system works in grid-connected mode, LCL type filter isalways used in grid-connected inverter to reduce the system volume and improve thesuppression characteristic of high-frequency harmonics. In this paper, current controltechnologies are researched for LCL type grid-connected inverters. To ensure the stabilityof the system in different conditions, A single-loop control method is analysed and designedconsidering the stability margin principle, meanwhile, A PIR resonant controller is used toimprove the quality of the current waveform. To damp the filter resonance, a grid currentand capacitive current double-loop control method is often used, but this method isproposed through block diagram transformation in continuous domain, the sampling andcalculation time delay, especially the one-beat-delay link caused during the PWM signalgeneration process, is not considered, it may cause system instability. A state observerbased double-loop ontrol method is proposed to compensate the one beat delay link, andsystem stability is highly improved.Power conditioning system also needs to work in stand-alone mode. In this paper, Thecapacitor voltage and inductor current double-loop control method is adopted. The activedamping principle is analysed by the impedance equivalent method, and then, the controllerparameters are designed in digital domain. The system achieves a good dynamic andsteady-state characteristics when the load is linear. But when the load is non-linear, the loadcurrent is distorted obviously，and the inverter is difficult to achieve a desired power qualityfor a large output impedance is regulated under the double-loop control strategy. To solve this problem, a repetitive controller based double-loop control method is adopted in thispaper. At the base of the double loop controller, the repetive controller and it’s compensatoris designed in detail. The total harmonic distortion of the output voltage waveform isgreatly reduced.In large-scale energy storage systems, in order to facilitate the expansion of systemscapacity, the power conditioning systems is often used in the parallel operation pattern.When the paralleled power conditioning systems work in stand-alone mode, the invertersmust face the current equipartition problem. In this paper, the coupling relationship betweenthe active/reactive power and d/q axis voltage is analysed in detail, considering the outputimpedance and the line impedance. Under the double-loop control scheme, the outputimpedence is deduced, and according to the nature of the impedance, the coupling betweenthe active/reactive power and d/q axis voltage is decoupled. Based on the introduction ofthe active/reactive power sharing control-loop, the active and reactive power is perfectlyshared.