Coordination and Control of Distributed Energy Resources: Modeling and Analysis Techniques

Abstract

Coordinated control of distributed energy resources (DERs), such as flexible loads, storage devices and solar photovoltaic inverters, can provide valuable services to the electricity grid by reducing peak demand, balancing renewables and avoiding voltage excursions. Aggregate control of thermostatically controlled loads (TCLs), such as air-conditioners, water-heaters and refrigerators, offers a promising way of accommodating significant DERs in power systems. This dissertation focuses primarily on modeling and analysis techniques for ensembles of TCLs. It also develops techniques for efficiently aggregating DER-based flexibility.

A wide-variety of load ensemble control techniques have been developed in the literature – with strategies including probabilistic switching signals, TCL set-point variation, and price-based signals. However, synchronization of TCL temperatures, oscillations in aggregate demand and bifurcations have been observed, which can lead to detrimental power- and voltage-related issues in the electricity grid. A detailed investigation is undertaken of a market-based transactive energy coordination (TEC) scheme, where TCL users submit bids for their energy demand and an aggregator clears the market to allocate energy among users. This study confirms the presence of such issues. To avoid these unintended consequences of load control, a Markov-chain-based state-transition model has been developed to capture the aggregate TCL dynamics under TEC. Using the state-transition model, a model predictive control scheme has been formulated to attain near-optimal control policies that maximize social welfare while limiting the possibilities of TCL synchronization and power oscillations. To further investigate unintended behavior arising from the control of load ensembles, a generalized hybrid dynamical system representation is developed to accurately capture the interactions between the continuous dynamics of loads and discrete control actions. This representation can capture diverse control-update intervals, from fifteen-minute intervals for economic dispatch problems to 2-10 seconds for frequency regulation services. Using this hybrid representation and modal analysis, it is shown that synchronizing behavior in TCLs can be identified under a wide range of control schemes, such as probabilistic and priority-based switching, and TEC. A number of practical constraints, such as limited availability of TCLs for control and/or limited TCL parameter information, are considered to quantify performance bounds of load control schemes.

To compute the aggregate flexibility available from spatially distributed DERs, special convex sets known as homothets and zonotopes are employed. First, aggregation algorithms are developed assuming DERs are located at a single node of the network. The setting is then extended to spatially distributed resources by incorporating the network and power flow constraints. It is shown that network parameters and voltage limits often limit the flexibility that can be transferred from one node to its upstream or downstream neighbors. This flexibility model lends itself to several applications, including optimal power flow in distribution networks and efficient coordination of transmission and distribution systems.