Type of Document Dissertation Author Wu, Zhichao URN etd-09032010-110552 Title Single-stage Grid-interactive Inverter for Distributed Energy and Integrated Storage with Enhanced Power Flow Control Degree Doctor of Philosophy Department Electrical and Computer Engineering, Department of Advisory Committee
Advisor Name Title Hui Li Committee Chair Simon Y. Foo Committee Member Juan Carlos Ordonez University Representative Uwe H. Meyer-Baese University Representative Keywords
- multilevel inverter
- distributed generation
- grid-interactive inverter
- energy storage
Date of Defense 2010-08-31 Availability unrestricted AbstractNowadays, relatively small-scale distributed generation (DG) technologies are developing rapidly in an effort to both protect the environment and to address the bottleneck of centralized power systems. DG tends to utilize diverse renewable energy sources (RES) such as solar power, wind power, etc. which lessen the emission of greenhouse gases and other pollutants. Except for its islanded operation in some remote areas, DG tends to interconnect with existing utilities owing to benefits in supporting voltage, improving power quality, trading surplus power and serving as backup power.
A grid-interactive inverter is crucial to any DG interconnection system, and there are two types of grid-interactive converters: those with energy storage (ES) and those without. Compared to those without ES, inverters with ES can achieve more flexible real/reactive power management and energy management. This allows them to tackle nondeterministic RES output, slow RES dynamics and fluctuating loads. And also this capability is significant as increasing numbers of DG units begin penetrating utilities. Generally speaking, today’s grid-interactive inverters still need greater improvement in functionality, efficiency, cost, size and weight.
This paper proposed a grid-interactive inverter for small-scale residential application consisting of one bulk converter and two conditioning converters in cascade that features single stage, multilevel structure, transformerless conversion, and integrated ES. Single stage offers a simpler configuration favorable to cost, efficiency and inverter reliability. Multilevel structure contributes its well-known benefits in efficiency, power quality, device stress, EMI, etc. The transformerless conversion waives the need for dedicated high frequency (HF) and bulky line frequency (LF) transformers, which is advantageous to inverter efficiency, cost, size and weight. And integrated ES equips the inverter with flexible power and energy management functionality interacting with grid. Based on the proposed inverter for typical home application (3.5KW), circuit parameters are designed in detail especially for LCL filter.
To design a control system and analyze system characteristics in grid-connected mode, dynamic average model (DAM) of the proposed inverter was derived in three forms: (1) equation sets in input side, inverter side and output side; (2) equivalent circuit; and (3) transfer function (TF) diagram. As detailed in this paper, ES (common capacitor or supercapacitor) interfaced with conditioning converters only supply reactive power, so the capacitor balance voltage control (CVBC) was developed to keep the auxiliary DC bus voltage constant. To track power flow accurately, high performance grid current control was achieved using a dual loop strategy. In the inner filter capacitor current feedback loop, proportional (P) regulator actively damps LCL filter resonance to gain sufficient stability margin. In the outer grid current feedback loop, proportional + resonant (PR) controller contributes to a near-zero steady-state error.
A dedicated reactive power allocation (RPA) strategy was developed for the proposed inverter to supply wide range reactive power ranging from inductive rating to capacitive rating, which can mitigate grid voltage fluctuation, undervoltage and overvoltage, and also can adjust power factor to any expected value according to grid-side demand. Combining controllable real power with wide range reactive power, the proposed inverter can broadly operate with pure real power, pure reactive power, and the mix of both. Phasor analysis was introduced to illustrate the principle of RPA. And to make RPA effective in wide power region, the proper reactive power allocation coefficient (RPAC) was theoretically derived to ensure unsaturated duty cycle in the bulk converter and conditioning converters. Accordingly, power area effective to RPA was analyzed with different RPAC selections. Five typical cases were analyzed to clarify this strategy.
Co-simulation using PSIM plus Matlab/Simulink is executed to prove the viability of proposed technique. Besides, results from RTDS platform validate the performance of the whole scheme. Finally, a 3.5 KW prototype is built to verify the feasibility of proposed technology.
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