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Weak-grid, SSO & impedance-based stability studies

Converter controls on a weak or series-compensated grid can produce oscillations that barely die out, often below 50 Hz. We find them with impedance scans and a Nyquist check, then prove them in the time domain.

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Nyquist · loop gainIllustrative
1ReImmargin−1
Nyquist plot of the loop gain. The nearer the curve passes to −1, the less damping there is.
When it's needed

When you need it.

  1. A converter plant connects to a weak grid or near series compensation.
  2. The area is dense with converters, so control interactions are more likely.
  3. The operator wants impedance or stability screening for the connection.
  4. Oscillations have been seen on site and the cause needs finding.
Method

How we run it.

  1. 01

    Scan

    Get converter and grid impedance across frequency, in dq or sequence domain.

  2. 02

    Screen

    Use a Nyquist check to find marginal or unstable conditions.

  3. 03

    Confirm

    Inject at the flagged frequencies in EMT to prove the mode is real.

  4. 04

    Bound

    Vary grid strength, dispatch and unit count to bound the risk.

  5. 05

    Fix

    Recommend changes where damping is poor, and check they work.

Tools and scenarios

What we use and what we test.

Tools

  • Impedance scan toolsFrequency domain
  • PSCAD / EMTDCTime domain
  • Vendor control modelsConverters

Scenarios we run

  • A range of short-circuit ratios and series-compensation levels
  • One converter, and several together
  • Different control modes and setting groups
  • Damping before and after the fix
Deliverables

What you get.

  • ReportImpedance and stability report with the modes and margins found
  • EvidenceNyquist plots and the EMT waveforms that confirm them
  • FixesRecommended changes, each checked in the model
  • MethodMethod, assumptions and model sources, written down

How we check it

Screening and time-domain runs are checked against each other, so a flagged mode is shown, not guessed. The conditions that decide stability are stated plainly for the reviewer.

Selected work

Related case studies.

Case 03 · SSO and compliance40 MW battery: SSO screening and NESO complianceImpedance scan, Nyquist, voltage injection, FRT and FFCI from one model.Case 04 · RMS to EMTNetwork reduction for large RMS modelsFive tools that get a wide-area RMS model ready for EMT.
FAQ

Questions engineers ask.

What is subsynchronous oscillation in inverter-based systems?
A lightly-damped or growing oscillation, usually below 50/60 Hz, caused by interaction between converter controls and the network impedance. It is different from classic torsional SSR on synchronous machines, though the screening ideas overlap.
Why use impedance methods first?
An impedance scan plus Nyquist assessment efficiently locates the frequencies and conditions where damping is marginal, so EMT effort can be focused where it matters rather than run blindly.
Can settings fix an SSO problem?
Often, yes. Retuning converter control loops or adding supplementary damping can restore margin. We verify any change in the model before recommending it.
Next step

Have a study like this coming up?

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Related: Consultancy hub · Control interaction studies · 40 MW battery case study · PSCAD / EMT studies