Research On Detection And Compensation Control Strategy For UPQC

With the accelerated process of industrialization and information technology,grid structure and load composition become increasingly complex, Voltage andcurrent problems often come out in pairs in the distribution system. How to ensureand improve power quality becomes one of the key challenges in the field whichshould be solved. Unified Power Quality Conditioner (UPQC), which is from userpower technology area, is a power quality controlling device with comprehensiveability to compensate. Based on power electronics technology, it’s able to restoredynamic voltage, compensate the harmonic currents and reactive power, meet thedemand in distribution system to curb quality problems of the voltage and current, andit has got broad prospects for development and practical value.Fristly, basised on none neutral line three-phase three-wire system UPQCtopology, The mathematical model described by the switching function and duty ratiois formulated in the three-phase stationary coordinate system. In order to facilitate thecontroller design, the low-frequency model in the synchronous rotating coordinatesystem is established, which is based on state-space average theory. Similarities anddifferences of sinusoidal conditions, non-sinusoidal conditions, and instantaneouspower theory are analyzed comparatively. Based on instantaneous power theory, theUPQC internal power flow is calculated to obtain the internal active and reactivepower flow diagrams.Secondly, aiming at the defects of hardware and software phase-locked loop, akind of simplified detection algorithm for UPQC compensations without PLL ispresented on the base of virtual cosine function table and the low-pass filter, whichcan calculate the conpensations accurately under grid distortion and unbalance withless conputations for DSP. The article detailed analysised the influence of thehardware delay to the compensation detection results, and hardware delaycompensation is processed from the perspective of the correction synchronousrotating reference. Meanwhile, for the calculation delay of the LPF, the solution isgiven which adds an internal feedback loop to improve the dynamic response.Thirdly, considering the coupling phenomenon in the system mathematicalmodel, voltage and current compensation unit (VCU and CCU) adopted fulldecoupled and imperfect coupled PI control under the two-phase stationary coordinatesystem and two-phase synchronous rotating coordinate system. Aiming to the defects of PI controllor and taking the coordination control between VCU and CCU intoconsideration, sliding mode control which with strong robustness is adopted to makecompensations can track the reference accurately by taking the grid voltage and loadcurrent as the indeterminacy. Because of the different characteristics of VCU andCCU, it is neccessary to convert the VCU to the decoupled inverse system. Thendifferent sliding surfaces are selected respectively, and corresponding sliding modecontrollers are designed, meanwhile their stability condition is defined too. Theresearch result shows that UPQC has excellent compensation effect and goodrobustness based on the sliding mode variable structure control method.Finally, in order to further expand the functionof traditional UPQC functions andmake it has characteristics similar to the UPS, a kind of new power qualityconditioner composed of traddtional UPQC and ultricapacitor energy storage systemis presented. Under the different conditions of power grid outage and non-outage fault,its operation mode is divided into compensation mode and UPS mode. Comparingwith conventional UPQC, it has two characteristics: First, the voltage control of theDC-LINK is accomplished by the ultracapacitor energy storage system. Second, whenthe power grid outage, the power stored in ultracapacitor invert to AC power andsupply the load in short term in order to avoid or reduce the economic losses.