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Coherency Control in Power Systems

Rodrigo Bernal, Ignacio Ponce, Federico Milano

TL;DR

This work addresses damping of inter-area oscillations in grids with heterogeneous generation by introducing a complex-frequency (CF) based coherency objective for inverter-based resources (IBRs). A grid-following IBR controller generates references by applying a complex gain to a measured external current, enforcing equality of the injected currents' CFs to a remote reference, and a coherency parameter $C$ to interpolate between coherent and independent dynamics, with $C$-dependent mixing of conventional and coherency-driven power. The approach is technology-agnostic, relies only on terminal measurements, and is demonstrated on two-area Kundur and IEEE 39-bus systems, showing improved damping when using the real part of CF in addition to the imaginary part. Practical considerations, including communication delays and measurement noise, reveal trade-offs between robustness and propagation of disturbances, highlighting the need for filtering strategies and careful implementation; future work includes integrating current/voltage limits, GFM deployment, WAC coordination, and weighted-reference schemes.

Abstract

This paper proposes a coherency control strategy for Inverter-Based Resources (IBRs) to establish coherence among power system devices. Using the equivalence of the Complex Frequency (CF) of the injected currents as the definition for coherency among devices, the control enforces an output current with a proportional magnitude and a constant phase shift relative to a reference. This formulation makes the control technology-agnostic, enabling coherency with any type of resource. Case studies based on the two-area and IEEE 39-bus systems demonstrate the controller's potential to improve damping and overall dynamic behavior. The paper further evaluates practical implementation aspects including delay/noise sensitivity and the trade-off between oscillation mitigation and disturbance propagation. This work establishes coherency as a viable direct control objective for IBRs in modern power systems.

Coherency Control in Power Systems

TL;DR

This work addresses damping of inter-area oscillations in grids with heterogeneous generation by introducing a complex-frequency (CF) based coherency objective for inverter-based resources (IBRs). A grid-following IBR controller generates references by applying a complex gain to a measured external current, enforcing equality of the injected currents' CFs to a remote reference, and a coherency parameter to interpolate between coherent and independent dynamics, with -dependent mixing of conventional and coherency-driven power. The approach is technology-agnostic, relies only on terminal measurements, and is demonstrated on two-area Kundur and IEEE 39-bus systems, showing improved damping when using the real part of CF in addition to the imaginary part. Practical considerations, including communication delays and measurement noise, reveal trade-offs between robustness and propagation of disturbances, highlighting the need for filtering strategies and careful implementation; future work includes integrating current/voltage limits, GFM deployment, WAC coordination, and weighted-reference schemes.

Abstract

This paper proposes a coherency control strategy for Inverter-Based Resources (IBRs) to establish coherence among power system devices. Using the equivalence of the Complex Frequency (CF) of the injected currents as the definition for coherency among devices, the control enforces an output current with a proportional magnitude and a constant phase shift relative to a reference. This formulation makes the control technology-agnostic, enabling coherency with any type of resource. Case studies based on the two-area and IEEE 39-bus systems demonstrate the controller's potential to improve damping and overall dynamic behavior. The paper further evaluates practical implementation aspects including delay/noise sensitivity and the trade-off between oscillation mitigation and disturbance propagation. This work establishes coherency as a viable direct control objective for IBRs in modern power systems.

Paper Structure

This paper contains 15 sections, 5 equations, 23 figures.

Table of Contents

  1. Introduction
  2. Motivation
  3. Literature review
  4. Contribution
  5. Paper organization
  6. Coherency Framework
  7. Complex frequency
  8. Ideal instantaneous coherency
  9. Case Studies
  10. Two-area Kundur System
  11. Dynamic Effect of Increasing Coherency Among Devices
  12. Sensitivity to Delays and Noise
  13. Conventional vs Complex Coherency
  14. 39-Bus system
  15. Conclusion and Future Work

Figures (23)

  • Figure 1: Simplified diagram of coherency control.
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  • ...and 18 more figures