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Ancillary Services Provision by Cross-Voltage-Level Power Flow Control using Flexibility Regions

Christian Holger Nerowski, Zongjun Li, Christian Rehtanz

TL;DR

The paper addresses stability and ancillary service provisioning challenges arising from high distributed renewable generation by proposing a framework based on FPUs, FORs, and FRs for Cross-Voltage-Level Power Flow Control. It contrasts AC OPF-based mapping of FOR/FR boundaries with accelerated techniques—Piecewise Linearization and DistFlow relaxation—to reduce computation time while preserving accuracy. A practical ADN-TSO/DSO coordination framework is outlined, and the approach is validated on two medium-voltage grids, illustrating runtime-performance tradeoffs and FR evolution over time. The results support the feasibility of real-time ancillary services through ADNs and lay groundwork for scaled implementations, including Power-Hardware-in-the-Loop verification.

Abstract

The large-scale integration of distributed renewable energy sources into the electricity grid requires the investigation of new methods to ensure stability. For example, Active Distribution Networks (ADNs) can be used at (sub-) transmission levels for emergency operation, provided robust and efficient control is available. This paper investigates the use of Feasible Operating Regions (FORs) and Flexibility Regions (FRs) for Cross-Voltage-Level Power Flow Control (CPFC). The enhancement of network stability due to the provision of ancillary services is illustrated, as is the need for strengthened cooperation between Transmission (TSOs) and Distribution System Operators (DSOs). Optimal power flow methods are considered, focusing on computational advances through PieceWise Linearization (PWL) and convex relaxation techniques aiming to speed up runtime while keeping high accuracy. To illustrate the algorithms' benefits and drawbacks, they are analyzed using exemplary medium voltage grids.

Ancillary Services Provision by Cross-Voltage-Level Power Flow Control using Flexibility Regions

TL;DR

The paper addresses stability and ancillary service provisioning challenges arising from high distributed renewable generation by proposing a framework based on FPUs, FORs, and FRs for Cross-Voltage-Level Power Flow Control. It contrasts AC OPF-based mapping of FOR/FR boundaries with accelerated techniques—Piecewise Linearization and DistFlow relaxation—to reduce computation time while preserving accuracy. A practical ADN-TSO/DSO coordination framework is outlined, and the approach is validated on two medium-voltage grids, illustrating runtime-performance tradeoffs and FR evolution over time. The results support the feasibility of real-time ancillary services through ADNs and lay groundwork for scaled implementations, including Power-Hardware-in-the-Loop verification.

Abstract

The large-scale integration of distributed renewable energy sources into the electricity grid requires the investigation of new methods to ensure stability. For example, Active Distribution Networks (ADNs) can be used at (sub-) transmission levels for emergency operation, provided robust and efficient control is available. This paper investigates the use of Feasible Operating Regions (FORs) and Flexibility Regions (FRs) for Cross-Voltage-Level Power Flow Control (CPFC). The enhancement of network stability due to the provision of ancillary services is illustrated, as is the need for strengthened cooperation between Transmission (TSOs) and Distribution System Operators (DSOs). Optimal power flow methods are considered, focusing on computational advances through PieceWise Linearization (PWL) and convex relaxation techniques aiming to speed up runtime while keeping high accuracy. To illustrate the algorithms' benefits and drawbacks, they are analyzed using exemplary medium voltage grids.
Paper Structure (10 sections, 12 equations, 7 figures, 1 table)

This paper contains 10 sections, 12 equations, 7 figures, 1 table.

Table of Contents

  1. Introduction
  2. Modeling Flexibility Providing Units
  3. Determination of Feasible Operating Regions and Flexibility Regions
  4. Setup
  5. Non-linear OPF exploration
  6. Accelerated calculation methods
  7. Piecewise linearization
  8. The DistFlow model
  9. Test System Results
  10. Conclusion and Outlook

Figures (7)

  • Figure 1: Classification of Flexibility Providing Units according to Contreras.2018b
  • Figure 2: A Network's Feasible Operating Regions and Flexibility Regions
  • Figure 3: Conceptual Framework of Coordination Processes
  • Figure 4: Test grids
  • Figure 5: CIGRE's Feasible Operating Regions using various Algorithms
  • ...and 2 more figures