The operation of the power system is slowly changing owing to the integration of the distributed energy resources in the distribution networks. Reference to the huge economic and technical benefits of the distributed energy resources, a number of support schemes including feed-in-tariff, subsidy, and tax reduction has been introduced.
Irrespective of the benefits of the distributed energy resources, they also pose a challenge to transmission system operator and distribution system operator to coordinate the power network in a coordinated format. It is paramount to coordinate the access of resources as well as data management between distribution and transmission system operators. This is in a bid to realize the potential flexibilities from the distributed energy resources. These flexibilities entail balancing demand and supply, voltage control, and network congestion management.
Another important issue of coordinating the economic dispatch of the distribution and transmission system operators if the problem of optimal power flow. The optimal power flow is a crucial aspect when it comes to the optimization of operations of power system. The objective functions of the optimal power flow include operation and investment cost, power losses, stability margin of the power system, and renewable energy spillage.
Zhao Yuan and Mohammad Reza Hesamzadeh from KTH Royal Institute of Technology in Sweden proposed a hierarchical approach to effectively coordinate the dispatch decision of transmission and distribution operator systems for energy and reserves taking into account extensive integration of distributed energy sources. The proposed mechanism functions by communicating the generalized bid function from distribution system operator to transmission system operator. The proposed study demonstrated that the hierarchical coordination mechanism could deal with several network congestion scenarios. Their research work is published in peer-reviewed journal, Applied Energy.
The authors formulated the coordination problem for dispatching reserve and energy. Benders decomposition was the mathematical foundation of the proposed coordination approach. The authors defined the generalized bid function to estimate the dispatch cost of the distribution network through a series of affine functions. The generalized bid function was communicated from the distribution system operator to transmission system operator. The authors implemented a convex AC optimal power flow model, therefore the convergence of hierarchical coordination was guaranteed. They then proposed a grid computing structure in General Algebraic Modeling System. This was in a bid to accelerate the computation.
The outcomes of the distribution system operator dispatch were packaged in the form of generalized bid functions and communicated to the first level of hierarchy; the transmission system operator. Once the economic dispatch of the first level was solved, the results were communicated back to the distribution operator in the second hierarchy.
The authors simulated varying scenarios of possible network congestions in distribution and transmission networks. The outcomes indicated that the simulation of the hierarchical coordination approach could realize very close outcomes as opposed to the centralized dispatch within three iterations.
By formulating the problem of scheduling generation reserves, the researchers were able to demonstrate that the proposed hierarchical coordination method was capable of coordination reserves and energy at the same time.
The proposed hierarchical coordination method by Yuan and Hesamzadeh for distribution and transmission system operators provides a path to guarantee the market efficiency for both distribution and transmission networks.
Zhao Yuan and Mohammad Reza Hesamzadeh. Hierarchical coordination of TSO-DSO economic dispatch considering large-scale integration of distributed energy resources. Applied Energy, volume 195 (2017), pages 600–615.
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