Dynamic Modelling and Performance Analysis of Energy Storage Systems for Frequency Regulation in Bulk Power Systems

Abstract

Renewable Energy Sources (RESs) provide a feasible alternative to supply electrical loads without the unfavorable environmental impacts of fossil fuels. However, despite the significant environmental benefits of RESs, several operational challenges associated with their high levels of penetration in power systems need to be addressed. Extensive research has shown that Energy Storage Systems (ESSs) facilitate increased penetration levels of RESs by providing flexibility to the system, especially considering the technical maturity and decreasing cost of these technologies; hence, penetration of ESS, such as batteries and flywheels is likely to grow significantly in the coming years. Indeed, services that have been traditionally procured from synchronous generators such as Frequency Regulation (FR) are already being provided by ESSs. However, appropriate frequency control must be considered to take advantage of the fast response capability of ESS facilities, while coordinating their response with the bulk conventional generators currently used for FR. Some characteristics of the bulk power grids, regulation signals, and the State of Charge (SoC) management of the ESSs need be considered for the design of proper FR controls.

In this thesis, a FR model is proposed of a large interconnected power system including ESSs such as Battery Energy Storage Systems (BESSs) and Flywheel Energy Storage Systems (FESSs), considering all relevant stages in the frequency control process. The model, which considers Communication Delays (CDs) in the transmission of signals in the FR control loop, is developed from the viewpoint of an Independent System Operator (ISO), using the Ontario Power System (OPS) as case study. To this effect, empirically-based and generic SoC models for FESS and BESS considering the charging and discharging process characteristics are proposed. The system, ESSs, and SoC components are modelled in detail from a FR perspective and validated using real system and ESSs data, and a practical transient stability model of the North American Eastern Interconnection (NAEI) in Dynamic Security Assessment Tools (DSATools™) platform. The proposed model is validated with and considers all main stages of the FR control process, including CDs and the SoC management model of the ESS facilities, ensuring a realistic closed-loop response. Simulation studies show that the proposed model accurately represents the FR process of a large interconnected power system including ESSs, and can be used for accurate FR studies. The impact of CDs and SoC management of ESS facilities on the Area Control Error (ACE), and the computational efficiency of the proposed FR model are studied and discussed.

A novel H2 filter design is proposed to optimally split the FR signal between conventional and fast regulating ESS assets, considering typical CDs. The design approach includes filtering the FR signal by producing a slowly-varying component or Traditional Regulation Signal (RegA) to be provided to the slow regulating resources (i.e., Traditional Generators (TGs)), while the remaining fast component or Dynamic Regulation Signal (RegD) is provided to the fast response ESS facilities (FESS and BESS) to take advantage of their fast response characteristics. The design of the H2 filter is formulated as an optimal control design problem, and the proposed filter is integrated into the previously validated FR model with ESSs to form an Integrated Model, which includes a Proposed Set-Point (PSP) calculation and an anti-windup strategy. The PSP allows FR capacity from ESSs to be comparable to TGs FR capacity while keeping the system stable, which is not the case in the current FR process for the OPS. The proposed anti-windup strategy is added to avoid saturation when both TGs and ESSs reach their limits, or TGs reach their limits while the ESS facilities are not able to follow the PSP signals because of their SoC limits. Thus, the proposed filter sends RegA and RegD signals considering the SoC of fast response resources and capacity limits of ESSs and TGs, and depend on the conditions of the system, working in a coordinated manner. The FR performance with the H2 filter signals, RegA and RegD, is also compared with the existing FR process in the OPS, focusing on studying the impact of CDs and limited regulation capacity, and the effect of the PSP calculation and anti-windup strategy. The results show that the H2 filter design and signal splitting strategy improves the FR process performance significantly, in terms of reducing the ACE, and thus reduce the need for regulation capacity.

Finally, a detailed methodology is developed to obtain Marginal Rate of Technical Substitution (MRTS) curves for the Independent Electricity System Operator (IESO). The IESO’s MRTS curves consider different ESSs and discharging times (i.e., 15 min for FESS, and 15 min, 1 h, 2 h, and 4h for BESS), scenarios (i.e., peak hours, non-peak hours, morning ramp hours, and evening ramp hours), and seasons. The criteria agreed upon with the IESO for the generation of heat maps and MRTS is also presented. Furthermore, the procedure to select the representative typical days per season to be used in the generation of the MRTS curves is explained in detail, and an example of how to interpret one of the MRTS curves is explained. Heat maps and MRTS curves are proposed as analysis tools to allow ISOs to select the desired performance metric, and the combination of RegA and RegD resources that would allow to achieve it while still reducing the total regulation capacity. Although this methodology is applied to the IESO, it could be applied to other ISOs with appropriate modifications.