Consultancy · study

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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Frequency scan · series-compensated lineIllustrative

Series compensation

Resonanceof the network– Hzsub-synchronous

Net resistanceat resonance– Ωat least 0 Ω

Dampingof the mode–%at least 5%

When it's needed

When you need it.

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

Results

The Nyquist check, before and after.

Choose a case to see the result.

Nyquist · loop gainIllustrative

Encircles −1: unstable

Phase margin 37°

Before the fix the curve encircles −1: the oscillation grows.After the fix the curve passes well clear of −1, with 37° of phase margin.

Method

How we run it.

01
Scan
Get converter and grid impedance across frequency, in dq or sequence domain.
02
Screen
Use a Nyquist check to find marginal or unstable conditions.
03
Confirm
Inject at the flagged frequencies in EMT to prove the mode is real.
04
Bound
Vary grid strength, dispatch and unit count to bound the risk.
05
Fix
Recommend changes where damping is poor, and check they work.

Tools and scenarios

Every scenario through every study.

Impedance scan toolsFrequency domain
PSCADTime domain
Vendor control modelsConverters

Frequency scanNyquist checkEMT time domainDamping check

Series compensation levels

Weak grid

Line outages

Low output

Nearby converters

HVDC nearby

Series compensation levels · Frequency scanAt 35% compensation the resistance turned negative near 30 Hz. A damping controller restores it.Line outages · Nyquist checkWith one line out the Nyquist plot passed close to −1. Retuned current control restores the margin.Nearby converters · Damping checkTwo plants shared a 7 Hz mode. Coordinated gains raise its damping above 5%.

22 checks · 3 led to a change · every case scripted and rerunnable

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 modelsThree 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

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