A novel Thevenin-based voltage droop control improving reactive power sharing with structures to identify Thevenin parameters

In this research project, a low-cost local voltage compensation strategy is proposed that evenly utilises the capability of the customers’ inverters spreading over a branched suburban utility network. The improved utilisation is based on a straightforward two-element equivalent of the network locally seen by each inverter. The network is simultaneously probed by all inverters, each one tries to identify its two-element perspective. Receiving an appropriate local response is challenged by the interference created by simultaneous probing and demand variation and also inverters’ compensating nature. Identification structures are designed to suppress all challenges while probing remains effectively invisible to the customers

Improved reactive power sharing among photovoltaic inverters using Thevenin's impedance based approach

Unity power factor injection from rooftop photovoltaics (PVs) in residential distribution systems are creating voltage magnitude problems. One way to deal with this is to ensure reactive power sharing from PV inverters. However, the conventional droop based strategies lead to uneven reactive power contribution penalizing the PV inverters towards the end of the radial feeder. Instead, this paper proposes Thévenin's impedance based approach for higher degree of fairness in reactive power sharing among inverters. An iterative method is used to determine the Thévenin's equivalent impedance and hence the amount of reactive current injection from the inverter. The convergence property is verified using basics of circuit theory such as Thévenin's theorem and its dual Norton's theorem. Simulation results shows the efficacy of the proposed approach in a radial feeder with two inverters

Improved reactive power sharing among customers' inverters using online Thevenin estimates

A growing number of inverter interfaced rooftop photovoltaic and battery systems are changing suburban distribution systems. The potential of reactive power support from this equipment is attracting attention as a way of addressing the systems' voltage magnitude problems. However, if the voltage is controlled by a conventional droop-based strategy, common radial configuration at suburban feeders can cause an uneven allocation of the reactive power compensation among the inverters. A droop control strategy is developed in this paper using two parameters of the steady-state Thévenin equivalent model of the system seen by each inverter. The Thévenin based droop control permits even allocation of the reactive power compensation. A sufficiently invisible low-level probing is utilized by each inverter to robustly identify Thévenin reactance and Thévenin source in steady state. The inverter's output power is adjusted by the two identified parameters. Demand changes are addressed. Interference by the neighbor inverters' simultaneous probing and compensation is also considered. Simulations results on the IEEE 33-bus and a large-scale system show the efficacy of the proposed strategy.</p

Simultaneous Local Identification of Thévenin Equivalent Impedances in a Distribution System

Advances in grid analytics are enabling significant operation improvement by identification of Thévenin equivalent system. Thévenin impedance is particularly useful for stability analysis in systems facing influx of distributed energy resources (DERs) and control of inverters interfacing DERs. In this paper, a connection point Thévenin impedance is identified by each customer, simultaneously across the feeder. First, the potential of existing power exchange of loads and DERs is explored towards this simultaneous identification and the limitation is shown based on data from a Queensland's suburban feeder consisting of overhead lines. Then, probabilistic and deterministic probing-based identification methods are proposed, drawing on the inverters' programmability. The probabilistic methods are based on the normal and the uniform distributions. The deterministic method utilizes the Walsh codes. All proposed methods use a one-step change in the customer's current magnitude, voltage magnitude, and phase angle. Efficacy of the proposal is demonstrated through simulations on IEEE 33-bus system amended by the empirical data. Analysis of equally strong methods shows Walsh coded probing gives more accurate Thévenin estimate but potentially makes identification longer and more visible to the customers. Uniformly distributed probing offers the best balance on the preceding criteria. Practical insights into the proposal are further provided