NESO grid-code compliance · Inverter-based resources

NESO compliance evidence for inverter-based connections.

Every inverter-based resource connecting in Great Britain — BESS, solar PV, hybrid sites and inverter OEMs — must give NESO credible compliance evidence: a validated EMT model that genuinely represents the plant, plus plant-level studies covering fault ride-through, fast fault current injection and oscillation behaviour. Velon builds that evidence in PSCAD/EMTDC — FRT, FFCI, SSO and the wider GB Grid Code requirements — and takes it through NESO scrutiny. The delivered 40 MW BESS project below is one worked example.

Scope a compliance package→ Why this matters now↓

EMT

Validated PSCAD models for submission

FRT

Fault ride-through evidence

FFCI

Fast fault current injection

SSO

Oscillation & impedance assessment

1–500 Hz

MIMO dq0 scan bandwidth

01 · The regulatory driver

Why NESO compliance is now a design-critical workstream.

For inverter-connected projects, compliance is no longer a document exercise bolted on at the end. It is an engineering workstream that shapes model delivery, study scope and programme.

NESO assesses inverter-based connections against the GB Grid Code and the requirements set out in the relevant Bilateral Agreement. Demonstrating compliance is not about producing paperwork — it requires a model that behaves like the real plant, studies that cover the operating envelope NESO specifies, and a clear chain of evidence linking the connection requirements to the simulations and the submitted results. Where any of those break down, the submission stalls in review.

In practice, four things have to hold together: plant model fidelity (the EMT model is a true representation, with all controllers that influence behaviour — including the Power Plant Controller and its communication delays — represented), operating-mode coverage (export and import, minimum and maximum dispatch, the required control modes and short-circuit levels), fault-performance evidence (ride-through and fast fault current injection across fault types), and oscillation assessment (small-signal stability across the required bandwidth). Each must be traceable back to a stated requirement.

Plant model fidelity

A detailed EMT model that is the same one submitted for compliance, with all influencing controllers — inner loops, PLL, voltage/Q and active-power control, and the PPC with its cycling and communication delays — represented accurately.

Operating-mode & fault coverage

Studies run across the operating envelope NESO specifies — control mode, short-circuit level, minimum and maximum active power, import and export — and fault performance evidenced across the relevant fault types and durations.

Traceable evidence

A clear chain from connection requirement, to simulation case, to submitted result — well-labelled, captioned and auditable, so a NESO reviewer can assess each case and trace it back to the requirement it addresses.

Anchored in NESO's published requirements

NESO IBR oscillation guidance (V2, 2025) — small-signal SSO studies for IBR connections
GB Grid Code — fault ride-through and fast fault current injection requirements
ENA EREC G99 — connection of generation to distribution networks
Bilateral Agreement — scheme-specific connection conditions

02 · What we deliver

A full inverter-compliance capability set.

The studies and engineering tasks Velon can deliver for an inverter-based connection — from model integration through to supporting NESO's review of the submission.

EMT model review & integration

Reviewing vendor EMT models and integrating them into the full plant and network representation for study.

PSCAD model validation

Checking model behaviour, initialisation and numerical stability so it is fit for compliance study and submission.

RMS-to-EMT alignment

Reconciling phasor-domain (RMS) and EMT representations so steady-state and dynamic behaviour agree.

Fault ride-through (FRT) studies

Voltage ride-through performance across fault types, durations and retained-voltage conditions.

Fast fault current injection (FFCI)

Reactive-current injection behaviour during faults, evidenced at the connection point.

SSO assessment

Sub-synchronous oscillation studies per the NESO IBR oscillation guidance, time-domain and frequency-domain.

MIMO impedance scanning

dq0 impedance/admittance characterisation using the MHI Full Impedance/Admittance Scanning (3 Phase) tool.

Nyquist stability review

Minor-loop gain eigenvalue loci assessed against the critical-point criterion, with gain margins.

Operating-mode studies

Coverage across control mode, short-circuit level and the required dispatch points.

Export & import conditions

For storage, both discharging (export) and charging (import) operating conditions assessed.

PPC representation

Plant-level controller behaviour, including cycling time and communication delays, captured in the model.

Compliance reports & submission support

Grid-code evidence reports, and support through NESO or DNO technical review and reviewer queries.

03 · Worked example · 40 MW BESS

A delivered compliance package for a 40 MW battery plant.

This Large BEGA connection brought together oscillation and fault-performance compliance in one coherent evidence package. The plant: a 40 MW BESS — twelve inverter units on a 33 kV collector network — modelled in detail in PSCAD/EMTDC v5. Client confidential.

40 MW

BESS · 12 inverter units · 33 kV collector network

SSO + FRT/FFCI

Oscillation and fault-performance evidence combined

PSCAD v5

Detailed EMT model · 10 µs time step

The scheme connects under a Large BEGA agreement, which under the NESO IBR oscillation guidance requires both a time-domain voltage-magnitude injection study (Section 3.2.1) and an active MIMO frequency scan (Section 3.3). Alongside the oscillation work, the same validated EMT model was used to evidence fault ride-through and fast fault current injection against the GB Grid Code — so the developer received a single, internally consistent compliance package rather than disconnected studies from separate models.

Select a subsystem below to see what was represented in the model and why it matters. The full PSCAD architecture follows.

Phase-locked loop (PLL)

The PLL is the inverter's primary synchronisation mechanism — it tracks the grid voltage angle to provide a rotating reference frame for the current controller. Its bandwidth and damping directly set the lowest-frequency interaction mode of the plant: a PLL that is too fast can excite SSO in weak grid conditions, while one that is too slow loses synchronism during voltage disturbances. The PLL was modelled as implemented in the inverter firmware, including its filter stages and bandwidth setting.

Synchronous reference frame PLL Low-pass filter PI regulator Frequency tracking Bandwidth-critical for SSO

Voltage / reactive-power control

The voltage control loop regulates terminal voltage by adjusting reactive power output, feeding a reactive-current reference into the inner current controller. It introduces its own phase shift and bandwidth into the plant's impedance characteristic — particularly relevant in the 1–20 Hz region where the outer voltage loop operates. The baseline scenario for NESO assessment is voltage-control mode; reactive-power control and power-factor control modes were also assessed where applicable.

V/Q droop controller Reactive-current reference Voltage-control mode baseline Qmax / Qmin limits

Active-power control loop

The active-power loop regulates DC-link power exchange and translates a power setpoint into an active-current reference for the inner controller. For a BESS, this loop governs both charging and discharging modes, which have different loop gains and time constants — explaining why NESO requires both export and import conditions to be assessed. The control bandwidth here interacts with inductive network elements to create resonant features in the mid-band impedance.

P/I setpoint controller Export (+40 MW) mode Import (−40 MW) mode DC-link dynamics included

Power plant controller (PPC)

The PPC is the supervisory layer that operates at the plant level — receiving setpoints from the market or operator, computing active and reactive power dispatch targets for the site, and distributing them to individual BESS units through the 33 kV collector network. NESO guidance explicitly requires the PPC and its associated communication cycles and delays to be represented in the EMT model. For this plant, the PPC cycling time and its interaction with individual unit controllers were captured accurately as they affect the low-frequency (sub-5 Hz) oscillatory response.

Plant-level supervisory Communication cycle delays Setpoint distribution NESO-required in model

Collector network & transformers

The 33 kV collector network distributes active and reactive power from the twelve BESS units to the Point of Connection via dedicated cable feeders and six three-winding step-down transformers. The cable impedances and transformer leakage reactances form the frequency-dependent network between plant and grid — they are part of the impedance seen by the scan tool and must be represented accurately. For runtime efficiency, the model was partitioned into seven PSCAD sub-projects using the Parallel Network Interface (PNI) feature, with transformer impedance reconciliation verified to preserve the electrical characteristics at the PoC.

6 × 3-winding transformers 8.4 MVA · 33/0.66/0.66 kV D0y11y11 vector group PNI partitioning (7 sub-projects) Impedance reconciliation verified

Grid Thévenin equivalent

Per NESO guidance, the AC grid is represented as a Thévenin equivalent source and impedance — characterising the strength of the network at the connection point. The minimum short-circuit level (weakest grid) was used as the baseline for all tests, as this is where SSO risk is highest: a weaker grid means the plant-to-grid impedance ratio is less favourable. The grid impedance defines the other half of the Nyquist assessment — Z₁ in the minor-loop gain L(jω) = Z₁(jω)·Y₂(jω).

SCL: 849 MVA X/R ratio: 21.69 Minimum SCL baseline Thévenin source

PoC measurement & scan interface

The Point of Connection measurement block captures the quantities required for SSO reporting: active power, reactive power, voltage magnitude and phase, and frequency at the 33 kV busbar. During the impedance scan, voltage and current signals at the PoC are processed via Fourier transformation to extract the frequency-indexed impedance data — both plant-side and grid-side — stored in PSCAD's FSOUT format for post-processing into dq0 impedance matrices, Bode plots and Nyquist loci.

P · Q · V · angle · f at PoC FFT-based impedance extraction FSOUT data format dq0 frame · 3×3 matrix

PSCAD model structure BESS SSO assessment — twelve inverter units organised across six three-winding transformers, feeding a 33 kV PoC via a collector network, with Thévenin grid equivalent and MIMO DFScan tool injecting at the PoC. PNI partitioning used for runtime efficiency; PPC supervisory layer represented throughout.

04 · Fault performance · FRT & FFCI

Fault ride-through and fast fault current injection.

The same EMT model carries a configurable fault module used for both FRT and FFCI studies — letting the plant's fault behaviour be evidenced against the GB Grid Code in the same environment as the oscillation work.

For fault performance, the plant is assessed as a Type C Power Park Module under Engineering Recommendation G99 Issue 2 (10 March 2025) — a connection point below 110 kV with registered capacity between 10 and 50 MW. The fault module is configurable for fault type, fault impedance, X/R ratio, fault initiation time and fault duration, so each required scenario can be reproduced precisely, and further scenarios defined by adjusting the same parameters.

Four fault types were studied — three-phase, phase-to-phase, two-phase-to-earth and single-phase-to-earth — each applied for a 140 ms duration with 10% retained voltage at the Point of Interconnection (10% in the faulted phase or phases for the unbalanced faults). All cases were run for both export and import operation, at +40 MW and −40 MW respectively, with the plant at its maximum leading power factor — absorbing its maximum reactive-power capability from the grid — and the POI voltage held at 1 p.u.

Configurable: fault type fault impedance X/R ratio fault initiation time fault duration Run at ±40 MW max leading PF POI 1 p.u.

3PH

Three-phase fault

Duration140 msRetained V10%Modes±40 MW

PH-PH

Phase-to-phase fault

Duration140 msRetained V10%Modes±40 MW

2PH-E

Two-phase-to-earth fault

Duration140 msRetained V10% faultedModes±40 MW

1PH-E

Single-phase-to-earth fault

Duration140 msRetained V10% faultedModes±40 MW

A measurement setup at the POI captures the quantities the evidence rests on: active and reactive power, line-to-line and line-to-earth voltages, and the positive-sequence active and reactive current. FRT evidences that the plant rides through the disturbance and recovers; FFCI evidences the positive-sequence reactive current injected during the fault — the quantities a reviewer looks for in the fault-performance part of the submission.

05 · Oscillation methodology · SSO

Two complementary lenses on the same stability question.

NESO requires both for good reason: the time-domain test shows the plant behaves stably under real disturbances; the frequency-domain scan quantifies the margins. Both were delivered for the 40 MW BESS, in export, minimum export and import modes.

Test 3.2.1 · Time domain

Voltage-magnitude oscillation injection

Sinusoidal perturbations at 1% of nominal voltage are injected one frequency at a time from 1 Hz to 100 Hz in 1 Hz steps, using a Schroeder multisine approach with up to five simultaneous frequencies per set to reduce run time. The PoC voltage is recorded with and without the plant. FFT extraction of the injected component gives the amplification ratio: a ratio below 1.0 means the plant damps the oscillation at that frequency.

✓ Both export and import modes assessed · no growing oscillations found

Test 3.3 · Frequency domain

Active MIMO impedance scan (DFScan)

The MHI Full Impedance/Admittance Scanning (3 Phase) tool injects positive and negative sequence signals at 0.5% magnitude to characterise the full 3×3 dq0 impedance matrix from 1–500 Hz (1 Hz steps to 100 Hz, 10 Hz steps beyond). Both scheme-side and grid-side impedances are extracted. The minor-loop gain L(jω) = Z₁(jω)·Y₂(jω) is formed, and its eigenvalue loci are plotted as Bode and Nyquist curves to give the definitive stability verdict.

✓ No encirclement of [−1, 0] — stable in all operating conditions

Voltage injection — PoC response with and without plant

With plant (BESS in service) Without plant (grid baseline)

Illustrative of the method. The "with plant" trace exhibits a bounded oscillatory response during the injection window that decays cleanly to the pre-injection level once the disturbance is removed — no evidence of growing oscillations or loss of synchronism. The "without plant" baseline remains flat.

Test matrix

Select a test case to see what it reveals.

06 · Stability verdict

The Nyquist criterion: the definitive stability check.

A MIMO system is deemed stable if the eigenloci of the minor-loop gain L(jω) do not encircle the critical point [−1, 0] in the complex plane. This is the test NESO requires for the active frequency scan.

The minor-loop gain is formed as L(jω) = Z₁(jω)·Y₂(jω), with Z₁ the grid-side impedance matrix and Y₂ the scheme-side admittance matrix — both extracted from the MIMO scan. The three eigenvalues of L(jω) are traced across the scanned 1–500 Hz bandwidth for each operating condition, giving the MIMO generalisation of the classical Nyquist criterion. The dominant coupling concentrated in a narrow band around the nominal-frequency region; a dedicated 0–3 Hz scan verified trajectory proximity to the critical point where PPC cycling introduces additional phase shift.

Result: across maximum export, minimum export and maximum import, the eigenvalue loci do not encircle [−1, 0]. Negative-real-axis crossings were identified in the mid-band but well separated from the critical point, confirming adequate gain margin and stable, well-damped behaviour.

Nyquist stability plot Illustrative of the MIMO eigenvalue approach. All three eigenloci (Eig1, Eig2, Eig3) pass through the negative real axis well to the left of the critical point — none encircle [−1, 0]. The system is assessed as stable across the tested bandwidth.

07 · Compliance mapping

Mapped to the NESO IBR oscillation guidance — Large BEGA.

For the 40 MW BESS, the study matrix for a Large BEGA connection requires Tests 3.2.1 and 3.3; fault performance is evidenced separately against the GB Grid Code. Expand each item to see how it is evidenced.

NESO IBR Guidance V2 · Section 2 (Study Types)

The assessment was performed in PSCAD/EMTDC v5 using the same detailed EMT model submitted to NESO for compliance. The model was verified to run at a fixed 10 µs time step, consistent with the requirements for BESS studies. All PSCAD project files and FSOUT output data are retained for audit and can be provided to NESO on request.

NESO IBR Guidance V2 · Section 2 (Study Types)

All control loops that influence the oscillatory response are represented: the inner current controller, voltage/reactive-power control, active-power control, PLL synchronisation, and the plant-level Power Plant Controller (PPC) including its cycling time and communication delays. The PPC is a plant-level SCADA function coordinating all twelve inverter units — omitting it or simplifying its delays would distort the low-frequency (sub-5 Hz) impedance characteristics.

NESO IBR Guidance V2 · Section 3.2.1 · Large BEGA: Required

Voltage-magnitude perturbations were injected at 1% of nominal, sweeping 1–100 Hz in 1 Hz steps with a Schroeder multisine approach (five frequencies per injection set). Both multi-tone and single-tone (one-by-one) injection runs were performed for export, minimum export and import conditions. The ratio of PoC voltage with plant to without plant confirms that the BESS damps rather than amplifies oscillations across the tested bandwidth.

NESO IBR Guidance V2 · Section 3.3 · Large BEGA: Required

MIMO impedance scanning was performed using the MHI Full Impedance/Admittance Scanning (3 Phase) tool, which is NESO's recommended toolset. Positive and negative sequence injections at 0.5% magnitude were applied from 1–500 Hz (1 Hz increments to 100 Hz, 10 Hz increments beyond). The 1–100 Hz scan ran for approximately 620 s and the 100–500 Hz extension for approximately 275 s. Both scheme-side and grid-side 3×3 dq0 impedance matrices were extracted and stored in FSOUT format.

NESO IBR Guidance V2 · Model fidelity requirement

To achieve practical run times with the twelve-unit detailed model, the network was partitioned into seven PSCAD sub-projects using the Parallel Network Interface (PNI) feature. PNI interfaces require non-negligible impedance — so fictitious line impedances were implemented by allocating a defined portion of existing transformer leakage reactance to the interface, leaving total series impedance at the PoC unchanged. A formal impedance reconciliation table demonstrates zero net change for all winding combinations, satisfying the NESO requirement that the EMT model is a true representation of the plant's electromagnetic response.

NESO IBR Guidance V2 · Section 2 (Test Scenario Configurations)

Studies were run at maximum export (+40 MW), minimum export (0 MW at PoC) and maximum import (−40 MW) in voltage-control mode at minimum short-circuit level — all conditions specified by NESO guidance for Large BEGA assessments. The minimum export condition confirms impedance behaviour under reduced dispatch. All conditions use the same detailed EMT model with PPC active.

NESO IBR Guidance V2 · Section 3.3 Acceptable Response

The minor-loop gain L(jω) = Z₁(jω)·Y₂(jω) was formed using Z₁ as the grid impedance, consistent with the NESO-recommended convention. Three eigenvalue loci (Eig1, Eig2, Eig3) were computed across frequency for each operating condition. Nyquist plots confirm that none of the eigenloci encircle the critical point [−1, 0] in any operating condition. Negative real-axis crossings were identified at specific mid-band frequencies but with adequate separation from the critical point. Dedicated 0–3 Hz verification confirmed trajectory behaviour in the low-frequency region.

Fault performance & traceability

Alongside this oscillation evidence, fault ride-through and fast fault current injection were evidenced against the GB Grid Code using the same validated model — so the SSO and fault-performance results form one internally consistent package rather than outputs from separate models. Every case is traceable back to the requirement it addresses. NESO retains the right to request further studies if field measurements reveal unexpected behaviour; the structured FSOUT dataset and PSCAD project archive make it straightforward to extend the assessment.

08 · Deliverables

What lands in your compliance package.

A practical, procurement-friendly set of deliverables — everything needed to submit, and to respond when a reviewer comes back with questions.

PSCAD project archiveThe runnable EMT model and project files used for the studies.

Model integration notesHow the vendor model was integrated into the plant and network.

Assumption registerDocumented assumptions behind the model and study set-up.

Study scenario matrixThe full set of cases, operating modes and conditions run.

FRT & FFCI resultsFault ride-through and fast fault current injection outputs per case.

SSO impedance scan resultsdq0 impedance datasets (FSOUT) for scheme and grid sides.

Nyquist plotsEigenvalue loci with the critical-point assessment.

Bode plotsEigenvalue magnitude and phase versus frequency.

POI measurement plotsP, Q, voltage, angle and frequency at the connection point.

Compliance reportThe evidence report mapping results to NESO requirements.

Response to reviewer commentsSupport answering NESO or DNO technical queries on the submission.

Model handover notesGuidance so the model can be re-run and maintained later.

09 · How we work

The compliance evidence workflow.

A repeatable route from connection requirements to a submitted, defensible compliance package.

Review connection requirements

Work through the Bilateral Agreement, Grid Code obligations and applicable NESO guidance to define exactly which studies and operating conditions the connection needs.

Receive vendor EMT model

Take in the inverter manufacturer's EMT model — including confidential or black-box models — and confirm what it contains and what it omits.

Integrate the plant model

Build the model into the full plant: collector network, transformers, PPC and connection point, ready for representative studies.

Validate operating modes

Confirm correct steady-state and dynamic behaviour across the control modes and dispatch points the assessment requires.

Configure network equivalents

Represent the AC grid as the required Thévenin or equivalent network, at the short-circuit levels NESO specifies.

Run FRT & FFCI cases

Execute the fault ride-through and fast fault current injection cases across fault types, durations and operating modes.

Run SSO & impedance scans

Perform the time-domain injection studies and the MIMO dq0 impedance scan across the required bandwidth.

Review pass/fail evidence

Assess results against the acceptable-response criteria, identify any margin concerns, and recommend mitigation where needed.

Decision gate

Prepare compliance report

Assemble the evidence report and appendices, traceable case by case back to the connection requirements.

Support NESO / DNO queries

Respond to technical review comments and iterate the studies or reporting until the submission is accepted.

10 · For OEMs & developers

When the model is the thing slowing you down.

Compliance delays usually trace back to the model — not the plant. If the EMT model is confidential, black-box, unstable, slow to run or awkward to integrate, that is where we work.

Model behaviour

Debugging & controller behaviour

When a model will not initialise, runs unstably, or behaves in ways the controller logic does not obviously explain, we work through the control structure — PLL, voltage/Q, active-power and inner loops — to find and resolve the cause.

Plant-level interaction

PPC delays & plant interactions

Plant-level effects — PPC cycling and communication delays, interactions between units across the collector network — are a common source of low-frequency behaviour. We represent them properly rather than simplifying them away.

Confidential models

Black-box & confidential models

Black-box vendor models are the norm. The impedance-based approach lets us characterise and assess plant behaviour at the connection point without needing access to internal controller parameters.

Runtime

Slow-running models

Detailed multi-unit models are heavy to run. Network partitioning techniques such as PSCAD's PNI can make a study set practical to execute, with the electrical characteristics at the connection point preserved and verified.

Measurement

POI measurement

Clear, correct measurement at the Point of Interconnection — P, Q, voltage, angle, frequency and sequence currents — is what the evidence rests on. We make sure the right quantities are captured the right way.

Evidence

Evidence traceability

A reviewer needs to trace every result back to a requirement. We structure the studies and reporting so the chain from requirement to case to result holds up under scrutiny.

Work with us

Tell us about your connection.

Whichever side of the table you are on, the starting point is the same: scheme rating, connection voltage, plant type, compliance deadline and model availability. Send those and we will scope the work.

Inverter OEMs

Model compliance support

Your inverter model needs to pass NESO scrutiny across multiple projects. We validate, debug and evidence it so it stands up in review — and stays reusable.

Send usscheme rating · connection voltage · plant type · compliance deadline · model availability

Get model compliance support→

BESS & PV developers

Preparing for NESO submission

You need a defensible compliance package for a specific connection. We deliver the EMT model, FRT/FFCI, SSO and the evidence report, and support the review.

Send usscheme rating · connection voltage · plant type · compliance deadline · model availability

Scope your submission→

EPCs & consultants

PSCAD study support

You need additional PSCAD study capacity or a specific compliance study delivered. We work as an extension of your team on the EMT and stability workstream.

Send usscheme rating · connection voltage · plant type · compliance deadline · model availability

Discuss PSCAD support→

Consultancy enquiries

consulting@velonenergy.com

NESO inverter compliance, EMT modelling, FRT/FFCI and SSO studies, MIMO impedance scanning and grid-code support for BESS and PV developers, inverter OEMs, DNOs and TSOs.