Improving Accuracy and Computational Efficiency of the Load Flow Computation of an Active/Passive Distribution Network

Verma, Rishabh and Sarkar, Vaskar (2019) Improving Accuracy and Computational Efficiency of the Load Flow Computation of an Active/Passive Distribution Network. PhD thesis, Indian institute of technology Hyderabad.

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Abstract

Over the last couple of decades, there has been a growing trend to make a paradigm shift from the passive distribution network to the active distribution network. With the rapid enlargement of network and installation of distributed generation (DG) units into distribution network, new technical challenges have arisen for load flow computation. The available techniques for the active distribution load flow calculation have limited scope of application and, sometimes, suffer from computational complexity. The complexity level of the distribution system power flow calculation is higher because of the issues of phase imbalance and high R/X ratios of feeder lines. The phase-imbalance increases computational complexity, whereas, the high R/X ratio makes time-consuming derivative based solver such as Newton-Raphson inviable for such large system. The motivation behind this work is to propose distinct mathematical approach for accurate modeling of network components, and loads to reduce computational time with improve accuracy. The applicability of an existing technique remains limited either by DG control modes, or by transformer configurations. The objective of this work is basically to develop an active distribution load flow (ADLF) algorithm with the following features. • Improved computational efficiency. • Applicability to any feeder network. • Accurate modeling of loads. • Applicability to different mode of operations of distributed generators (DGs). Typically, distributed generators are power-electronically interfaced sources that can be operated either in the current-balanced or in the voltage-balanced mode. The integration of DGs to the feeder network enables the distribution system to have bidirectional power exchange with the transmission grid. Which, also improve the voltage profile of the distribution network by providing additional sources of reactive power compensation. The contribution of the first work is to carry out the load flow analysis of a distribution network in the case of the dominant presence of induction motor loads. For a given operating condition, the load representation of an induction motor on the distribution network is made by analyzing its exact equivalent circuit. Thus, the induction motor is precisely represented as a voltage and frequency dependent load. The necessity of representing an induction motor by means of its precise load model is verified through a detailed case study. The convergence of the load flow solution with the precise modeling of induction motor loads is ensured by carrying out the load flow analysis over a complex distribution network containing several loops and distributed generations. The specific contribution of the second work is to improve the accuracy of the results obtained from the load flow analysis of a distribution network via forwardbackward sweeps. Specific attention is paid to the two-port modeling of a transformer with precise consideration for the zero sequence components of its port voltages. The zero sequence voltages at transformer ports are often ignored in the conventional load flow analyses. A new two-port network model is derived, which is generalized enough for the accurate representation of a transformer in the cascaded connection. Based upon the novel two-port representation made, a new set of iteration rules is established to carry out the forward-backward sweeps for solving the load flow results. All possible transformer configurations are taken into account. It is shown that the load flow analysis technique proposed is suitable for both active and passive distribution networks. The accuracy analysis of the load flow results is also carried out. For a given load flow result, by assessing the nodal current imbalances are evaluated based upon the admittance matrix representation of the network. Extensive case studies are performed to demonstrate the utility of the proposed load flow analysis technique. The contribution of the third work is to develop a computationally efficient and generalised algorithm for the load flow calculation in an active distribution network. The available techniques for the active distribution load flow calculation have limited scope of application and, sometimes, suffer from computational complexity. The applicability of an existing technique remains limited either by DG control modes or by transformer configurations. In this chapter, the load flow calculation is carried out by using the concept of Gauss-Zbus iterations, wherein the DG buses are modeled via the technique of power/current compensation. The specific distinctness of the proposed Gauss-Zbus formulation lies in overcoming the limitations imposed by DG control modes for the chosen DG bus modeling as well as in having optimized computational performance. The entire load flow calculation is carried out in the symmetrical component domain by decoupling all the sequence networks. Furthermore, a generalised network modeling is carried out to define decoupled and tap-invariant sequence networks along with maintaining the integrity of the zero sequence network under any transformer configurations.The computational efficiency and accuracy of the methodology proposed are verified through extensive case studies. The contribution of the fourth work is to identify and eliminate unnecessary itvii eration loops in the load flow analysis of an active distribution network so as to improve its overall computational efficiency. The number of iteration loops is minimized through the integrated modeling of a distributed generator (DG) and the associated coupling transformer. The DG bus is not preserved in the load flow calculation and the aforementioned DG-transformer assembly is represented in the form of a voltage dependent negative load at the point of connection to the main distribution network. Thus, the iteration stage that is involved in indirectly preserving the DG in the form of a voltage source or negative constant power load can be got rid of. This, in turn, eliminates the need for multiple rounds of forward-backward sweep iterations to determine the bus voltages. The power characteristics of the DG-transformer assembly are thoroughly investigated through a carefully performed case study so as to assess the potential convergence performance of the proposed.

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