Reliability Unit Commitment In ERCOT Nodal Market

Abstract

The Electric Reliability Council of Texas (ERCOT) is the independent system operator (ISO) that ensures a reliable electric grid and efficient electricity markets in the ERCOT region. ERCOT has successfully transited from a zonal market to an advanced nodal market since Dec. 2010. In the new ERCOT nodal wholesale market, a reliability unit commitment (RUC) process has been designed and implemented to ensure transmission system reliability and security. The main objective of RUC is to ensure that enough resource capacity, in addition to ancillary service capacity, is committed in the right locations to reliably serve the forecasted load in the ERCOT system. The "make-whole" payment mechanism has been employed by ERCOT for RUC settlement to ensure all generating resources committed by RUC are adequately compensated for their operation costs.The RUC is implemented in a security constrained unit commitment (SCUC) framework that minimizes the total operation costs based on generator three-part supply offers subject to various system and resource security constraints. The SCUC is comprised of two main functions: network constrained unit commitment (NCUC) and network security monitor (NSM). An efficient NCUC-NSM iteration clearing process has been proposed to solve the SCUC problem. Some enhanced features have been implemented in the SCUC engine to handle special resource scheduling such as combined cycle resources, split generation resources and self-committed resources. The mixed integer programming (MIP) methodology has been adopted to solve the SCUC problem due to its robustness over other unit commitment algorithms.The combined cycle unit (CCU) contributes a significant share of ERCOT total installed capacity. How to accurately and efficiently model the CCU is one of the key factors for a successful ERCOT nodal market. A robust CCU modeling is proposed for the SCUC in two different ways to facilitate market operations and ensure the system reliability. The configuration-based model is more adequate for bid/offer processing and dispatch scheduling and therefore it is adopted in the NCUC. On the other hand, the physical unit modeling is more adequate for the power flow and network security analysis and therefore it is adopted in the NSM.Currently there are eight phase shifters in the ERCOT system. These phase shifters are primarily intended for relieving transmission overloads caused by variations in wind generation. To improve dispatch efficiency and accuracy, a phase shifter optimization model has been proposed to automatically determine the tap positions of the phase shifters in the RUC optimization.During the design phase of the nodal RUC project, a prototype RUC program with the proposed combined cycle unit (CCU) modeling has been developed to verify the effectiveness of the CCU modeling. The prototype RUC program has been tested on a revised PJM 5-bus system and the results are very promising. Because of the positive testing results, the proposed CCU modeling has been adopted for the RUC project and eventually implemented for the production RUC by the vendor. The testing results from the production RUC have demonstrated that the proposed RUC system is very robust and can improve dispatch efficiency and system reliability as well as ensuring more effective congestion management.