Consultancy · study

Weak-grid, SSO & impedance-based stability studies

Subsynchronous oscillations (SSO) and other converter-driven instabilities appear as lightly-damped modes, often below the fundamental frequency, where converter controls meet weak or series-compensated networks. We assess them with impedance-based methods and confirm findings in the time domain.

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When it's needed

When this study is needed.

  • Inverter-based plant connects to a weak grid or near series compensation.
  • Converter-dense regions raise the risk of subsynchronous control interactions.
  • An operator requires impedance or stability screening for the connection.
  • Oscillations have been observed and their origin must be diagnosed.
Method

How we approach it.

  1. Derive the converter and network impedance characteristics across frequency (dq or sequence domain).
  2. Apply a Nyquist-style stability assessment to locate marginal or unstable conditions.
  3. Confirm flagged modes with time-domain EMT injection at the relevant frequencies.
  4. Sweep grid strength, dispatch and the number of units to bound the risk.
  5. Recommend and verify mitigations where a mode is poorly damped.
Software

Tools we use.

  • Impedance-scan / frequency-domain analysis tooling
  • PSCAD / EMTDC for time-domain confirmation
  • Vendor converter control models
Scenarios

Scenarios & operating points.

  • Range of short-circuit ratios and series-compensation levels
  • Single- and multi-converter configurations
  • Different control modes and setting groups
  • Pre- and post-mitigation damping comparison
Deliverables

What you receive.

  • Impedance / stability report with identified modes and margins
  • Nyquist evidence and confirming EMT waveforms
  • Mitigation recommendations with verification
  • Documented method, assumptions and model sources
Quality & validation

Evidence that stands up to review.

Screening and time-domain confirmation are cross-checked so a flagged mode is demonstrated, not merely inferred, and the conditions that govern stability are stated explicitly for review.

Selected work

Comparable experience.

Case-study write-ups for this service are in preparation. To discuss directly relevant project experience, brief us — we can talk it through, under NDA where appropriate.

FAQ

Questions engineers ask.

What is subsynchronous oscillation in inverter-based systems?
A lightly-damped or growing oscillation, usually below 50/60 Hz, arising from interaction between converter controls and the network impedance — distinct 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

Brief us on a study.

Send a one-paragraph scope — the system, the codes that apply, and the question you need answered. We typically reply within two working days.

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Related: Consultancy hub · Control interaction studies · BESS & inverter compliance · PSCAD / EMT studies

Direct
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consulting@velonenergy.com