Using Utility-Scale Microgrids in Deregulated Wholesale Power Markets

The objective of this dissertation is to make microgrids, especially utility-scale ones, play a more important role in power system operations. Equipped with power electronic interfaces to power systems, microgrids were initially used to improve power quality by providing power services locally. As a new kind of energy resource in future electric power grids, microgrids are expected to contribute more to power system operations. Both technical and operational challenges should be conquered to realize this vision. The technical challenge concerns power quality in the form of voltage magnitude and frequency synchronization. The operational challenge involves the impact of utility-scale microgrids on deregulated power market operations.

Before widely using microgrids in a power system, stability analysis is necessary to ensure high power quality at each microgrid's connection point. It is assumed that droop control mechanisms are used as the primary controllers at each microgrid's point of connection. Conditions for voltage stability and frequency synchronization are derived for a power distribution network with microgrids. In voltage stability analysis, uncontrollable voltage drops, i.e. voltage collapses, are identified using purely algebraic power flow equations. Using only local information, static voltage stability is monitored by an index showing how close the power system is to voltage collapse. In a power system that is far from voltage collapse, frequency stability analysis examines whether generators in the power system synchronize. The frequency synchronization conditions are in the form of constraints on controller gains, network parameters, and local loads. In the planning stage of connecting microgrids to a power system, these conditions provide a frequency stability guarantee.

In addition to presenting a stability analysis based on primary droop control, this dissertation designs secondary controllers to improve voltage and transient stability at each microgrid's connection point. Microgrids, therefore, become valuable resources that can be integrated into a power system with minor modifications to existing control algorithms. These distributed secondary controllers improve voltage stability and transient stability at the connection point of each individual microgrid. Furthermore, the proposed secondary controllers for microgrids do not deteriorate frequency synchronization of the overall power network. The proposed secondary controllers therefore enable microgrids to serve as a valuable resource enhancing power security.

To meet the operational challenge, this dissertation analyzes the impact of utility-scale microgrids on power markets and proposes a management method that performs better than the traditional approach. As newly introduced energy resources, microgrids and renewable energy resources have a great impact on deregulated power market operations. This impact is reflected in power market performance that is measured in three aspects, including i) average prices for power buyers, ii) power output from these new energy resources, and iii) market total surpluses. Using the traditional approach to ensure a valid market clearing point, market operators manage this impact by restricting power supply offers from microgrids and renewable energy resources. Because available low-price power supply offers are discarded by market operators, the traditional approach leads to poor power market performance. To achieve better performance, a new method is proposed to manage the impact of microgrids and renewable energy resources. The proposed method generates lower average electricity prices, greater power output from new energy resources and larger total surpluses in power market operations. In addition, implementing this proposed method requires little change to the existing information exchange between traders and market operators. As a result, our proposed method effectively manages utility-scale microgrids so they can serve a greater role in meeting future electrical energy demand.

In summary, the main contributions of this dissertation are solutions to technical and operational problems to facilitate greater penetration of utility-scale microgrids in power system operations. From the technical aspect, this dissertation analyzes stability and designs secondary controllers to ensure power quality at each microgrid's connection point. With proposed secondary controllers, each utility-scale microgrid is able to maintain high power quality so that the technical challenge is met. To solve operational problems, this dissertation investigates the impact of utility-scale microgrids on power market operations. To achieve more desirable market operation results, this dissertation proposes a new method that promotes greater participation of utility-scale microgrids in the current power markets. By solving both technical and operational problems, utility-scale microgrids can contribute more to power system operations in deregulated wholesale power markets.