Yi Qi, Yi Zhang

July 2026

Observation of Oscillations in IBRs

with GFM/GFL controllers

using Real-Time Simulation

Analysis

Used in this work

• Small signal -
• Analytical (linearized) state space equations (‘s’ domain)
• Experimental frequency scan (‘f’ domain)
• Small step-change time domain simulations (‘t’ domain)

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• Large signal -
• Time domain simulations (non-linear characteristics)

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Supported by real-time digital simulator software package

Frequency scan & analysis tool (FSAT) ^*

• Analytical (from s domain)

• Node admittance matrix
• Passive linear network ^**
• Similar as PSCAD/EMTDC

• ‘Black Box’
• Power electronics
• Anything with linearity/equilibrium

• Experimental (from t/z domain) ^***

* Yi Zhang, et.al, ”Analysis of Wideband Oscillations in Power Systems Using Real Time Digital Simulator”, RTDS China User Workshop – Frequency scan and stability analysis, Oct 2024, Beijing.

** Arunprasanth Sakthivel, Manual of frequency scan component, RTDS manual, 2020.

*** Y. Qi, H. Ding, Y. Zhang, X. Shi and A. M. Gole, "Identification of Sub-Synchronous Interaction in MMC Systems using Frequency Scanning," 2020 IEEE Power & Energy Society General Meeting (PESGM), Montreal, QC, Canada, 2020, pp. 1-5.

IBR-GFM

FSAT

Upgrading available GFL-IBRs to GFM-IBRs enables the stable operation in weak ac systems. ^*

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* D. Li, Y. Qi, H. Ding, A. Sakthivel, Y. Zhang, “A Method of Determining Ratio of GFM Converters Using Impedance Scan”, 2025 IEEE Energy Conversion Conference and Expo, Pennsylvania, USA, Oct 2025, pp 1-6.

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^{Reproduced from [*]}

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IBR-GFM

FSAT

• GFM systems face challenges connecting into strong ac networks.
• Large inertia constant results in lower oscillation mode.
• GFM inherits issue (control blocks) from synchronous generators.

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* L. Liu, Y. Qi, C. Jegues, S. Elimban, Y. Zhang, "Enhancing Tuning and Testing of Grid-Forming IBRs with Real-Time Simulators," CIGRE 2025 International Symposium, Montreal, Sep 2025, 1-12.

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^{Reproduced from [*]}

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^{Reproduced from [*]}

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Unified Grid Forming-Following (UGFF)

General ideas

GFM

Control

GFL

Control

Input

V/I

K

1-K

+

Modulation waveforms

*Y. Qi, J. Dadkhah, Y. Zhang, A. M. Gole, L. Luo, K. Dey, “ Implementation and Evaluation of a Unified Grid Forming-Following Controller for BESS-IBR Systems ”, 2026 IEEE Power & Energy Society General Meeting (PESGM), Montreal, pp 1-5. (26PESGM0144)

UGFF

Control blocks

• No switching between different integrators (resulting in transient step change)
• FFC = 1 🡪 Same as GFM
• FFC = 0 🡪 Same as GFL
• FFC = 0.5 🡪 Hybrid (or other values)

^{Reproduced from 26PESGM0144}

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UGFF

FFC=1

FFC=0

Concerns?

Then use FFC = 1 or FFC = 0

^{Reproduced from 26PESGM0144}

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UGFF – IBR/BESS

• IBR/BESS system
• Can be applied into any ac-dc converters (with energy storage)

^{Reproduced from 26PESGM0144}

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UGFF

Seamless transition

• GFL 🡪 GFM 🡪 GFL

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• There is no switching between integrators compared to GFM/GFL seamless transition algorithms, and do not need to apply additional logic to reset them.

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• Provide online dynamic adjustment capability of operating mode

^{Reproduced from 26PESGM0144}

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UGFF

Strong system behaviors (SCR=10)

0.0 GFL

1.0 GFM

0.5 Hybrid

^{Reproduced from 26PESGM0144}

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UGFF

Weak system behaviors (SCR=2)

0.0 GFL

1.0 GFM

0.5 Hybrid

^{Reproduced from 26PESGM0144}

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UGFF

Transient recovery time

In general, FFC around 0.5 presents fast recovery for both strong and weak ac systems after a PCC fault.

^{Reproduced from 26PESGM0144}

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UGFF

‘s’, ‘t’, ‘f’ validation

Between ‘s’ and ‘t’

Between ‘s’ and ‘f’ (FSAT)

*Y. Qi, Y. Zhang, A. M. Gole, “Stability Analysis and Control of a Unified Grid Forming-Following Controller in a Battery Energy Storage System”, submitted to IEEE Transactions on Smart Grid.

^{Reproduced from [*]}

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^{Reproduced from [*]}

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UGFF

Adaptive control - example

• System becomes unstable after transients (SCR increases)
• The voltage level is still normal (slowly diverging, between [0.88 1.1])
• FFT applied to the active power to monitor all the frequencies
• 5Hz component exceeding 0.05pu triggers FFC adjust
• System recovers to equilibrium

^{Reproduced from [*]}

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*Y. Qi, Y. Zhang, A. M. Gole, “Stability Analysis and Control of a Unified Grid Forming-Following Controller in a Battery Energy Storage System”, submitted to IEEE Transactions on Smart Grid.

Discussions

• GFL – weak ac system 🡪 stability concerns; GFM – strong ac system 🡪 potential oscillations
• Unified Grid Forming-Following (UGFF) controller has a bottom-line behavior – equal to a pure GFM or GFL controller
• Hybrid (FFC=0.5) scheme provides satisfied post-fault results for both strong / weak systems
• Advantages & potential applications:
• Upgrade the current GFL-IBRs with GFM capabilities with seamless transitions between modes
• Adaptive control of FFC to enhance transient behaviors
• Can potentially mitigate low frequency oscillations in future GFM grid

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• Case example & models will be included in RTDS Example case directory

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