Studies On Theory Of Power Flow And Optimization In Power Transmission-Distribution Grid

With the environment deterioration and non-renewable energy exhaustion, applying the energy conservation policy and making good use of clean energy are becoming extremely urgent. Electrical energy conversion is dominant in the use of energy, while the energy source composition in the power system will surely be significantly changed. In this context, electrical power transmission and distribution is about to develop gradually in an indeterminate, decentralized and distributed way. As a result, the theory of power flow and optimal power flow are facing new problems. It is theoretically significant and practically valuable to do its intensive study.Based on the results of previous studies, the author of this thesis has done some depth-study of the complex issues mentioned above with the idea that the power flow in the transmission and distribution network and its optimization must employ the global analysis concept. Moreover, with the power flow as the core issue, this thesis expands the exploration of the theory and practice in the decomposition and coordination between each sub-problem and as well as each sub-problem itself and the expected results has been achieved. The main innovative results are as following:1. To reduce the trouble in coping with newly added elements and element retrievals in different sparse storage format during the calculation of AC power flow correction equations, a closed format power flow algorithm is proposed, utilizing the incidence among the factorization process, the numbering of nodes and the sparse storage during Newton power flow calculation in polar coordinates. The proposed algorithm is expressed by correction equations of branch increment models and directly related to Jacobian matrix in additional form, so the element to be updated is located by branch node number and admittance matrix is not needed. According to the incidence between node numbers and forward-backward substitution operation, changes of factorization table structure in elimination process are described with the network topology changes, so that the newly added elements could be predicted and their operation would be easier to be made. Besides that, this algorithm uses triangle retrieve storage format and make automatic location of forward substitution operation and automatic releasing of backward operation, so that the consumption of retrieval information in the process of calculation is smart redued.2. In allusion to the influence of DC transmission lines on the format and convergence of AC power flow calculation, a power flow algorithm for AC-DC power grid based on the equivalence elements of DC transmission line is proposed. According to the communication among different operation modes of DC transmission lines in mathematics, this algorithm translates various modes into one equivalent mode by introducing the concept of the equivalence modes of DC transmission line, in order to realize the unification of DC transmission line models. When considering DC transmission line in power flow calculation, this algorithm only need to add the incremental model of its equivalent model, so that sparse storage structure and computational format are not changed which means that the algorithm has unified and fixed rules and AC power flow algorithm based on AC transmission line modes can be continue used.3. For the power flow calculation problem of power transmission-distribution network containing numerous distributed generations. Analysis about the interpenetration characteristics between transmission and distribution network is given. The characteristics are:in the active power aspect, the distribution network shows either the characteristics of receiving end or the power source part; in the reactive power aspect, either transmission grid will support the distribution network or the distribution network may show its voltage support initiative; in between, the active and the reactive power may be either weakly coupled or in strongly coupled state. Each of the latter part of the characteristics mentioned above demonstrates the necessity of the implementation of a unified analysis of the transmission and distribution network. On this basis, under the active and reactive power balance constraints, taking the transmission grid as the main problem and some distribution network as the sub-problem, power flow calculation model which displays the correlation between distribution grid and transmission grid is created. And at the same time, transmission and distribution network power flow decomposition and coordination algorithm with cut-off point voltage as covariant is proposed. In practice, it shows that the algorithm is simple and effective. Besides, under feasible conditions, the algorithm is with great convergence, which could satisfy the requirements of the computation of the transmission and distribution network.4. For distribution networks including wind power, solar power and energy storage, the dynamic optimal power flow model is proposed and its mechanism is analyzed in order to describe the active behavior of distributed generations. Due to the coupling between real and reactive power and the connection in time, distribution network power flow need to be dynamicly optimized so that its efficiency can be fully expressed. On this basis, this thesis describes in detail the characteristics and constraints of different kinds of distributed generations, and proposes the corresponding dynamic optimal power flow model and its solution strategy. Numerical examples show that this model can fully utilize the regulation capability of distributed generations and realize the optimal configuration of power sources.5. In allusion to the transmission-distribution network optimization problem with extensive distributed generations, this thesis proposes a whole network optimal power flow model and studies the effect rule between transmission and distribution network. Firstly, objective function, constraints and variables of transmission and distribution networks are decomposed with the replication of boundary variables. In allusion to the inseparability caused by equality constraints of boundary variables, auxiliary problem principle is applied and the augmented Lagrangian function is transformed into a series of auxiliary problems that can be solved in parallel, so that the coordinated transmission-distribution network optimal power flow model is proposed on the basis of transmission and distribution optimization sub-problems. Under that coordination mechanism, the two networks only need to treat with its own sub-problem and virtual generators costing parameters can be corrected with a little amount of boundary information exchange in order to realize the coordination of different areas and the optimal configuration of power sources. Numerical examples show that this algorithm is simple in computer implementation and has good convergence; therefore, it can be very useful theoretical reserve and of great guiding significance for coordinated transmission-distribution network optimal problems in the future.