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12. Glossary

The single alphabetical reference for the terms of art this manual uses: the power-systems vocabulary of the solver, the GB/regulatory vocabulary of the frequency and evidence work, and the GridSim-specific coinages. Each entry gives the definition at the level the code supports, the canonical value where the term carries one, and the grounding symbol or file.

Family tags: [PS] general power-systems usage; [GB] a GB grid, regulatory or NESO term; [GS] a GridSim-specific coinage, class name or in-code convention.

Canonical figures used throughout (a figure that reappears with a different value is a bug, not a variant):

Figure Value Ground
case9 total real loss 4.641 MW tests/GridSim.Tests/PowerFlowTests.cs
case9 convergence 4 iterations to 1.65e-14 pu (gridsim case9 --once; ch. 11 rounds it to 1.6e-14) README.md (Solve a network); technical ch. 11
WSCC inertia constants H = 23.64 / 6.40 / 3.01 s technical ch. 11
WSCC total inertia 7.465 GVA.s technical ch. 11; Case9InertiaTests.cs
gb-full size 3,539 buses / 5,918 branches (raw 4,240) ch. 2, Loading networks
gb-spine size 316 buses (GSP level) ch. 2, Loading networks; GDA NetworkModel/emit_spine_case.py
Residual worst mismatch 1.4e-7 MVA on the six classic cases technical ch. 11 (tolerance summary, residual row)
RoCoF thresholds legacy 0.125 Hz/s, modern 1.0 Hz/s Core/Dynamics/FrequencyLimits.cs
LFDD stage 1 / working set-point 48.8 Hz / 49.2 Hz FrequencyLimits.cs, StabilityMetrics.cs
Horizon 169 hours (7 days + 1 h) Core/Diagnostics/ForwardInferenceGuard.cs:HorizonHours
Bridge clock-skew allowance none (exact 169 h horizon) Core/Net/GridBridgeClient.cs:HandleState
Solve tolerance / iteration cap 1e-8 pu / 30 Core/PowerFlow/PowerFlowOptions.cs
Dense/sparse Newton threshold always sparse on the CPU (0 buses); 200 buses is the estimator observability / GPU / solve-export crossover Core/PowerFlow/SolverSelector.cs:DefaultSparseThreshold, DenseSparseCrossoverBuses
Newton step damping / Vm floor 0.4 / 0.2 pu Core/PowerFlow/NewtonShell.cs:MaxUndampedStep, VmFloorPu
Test suite the count dotnet test reports (it changes with every release; not a canonical figure) dotnet test GridSim.slnx -c Release --no-build

A

Admittance [PS]. The reciprocal of impedance, Y = 1/Z (per-unit in GridSim). A branch's series admittance is ys = 1/(R + jX); real part conductance, imaginary part susceptance. See Y-bus and 02-data-model-and-per-unit.

ALoMCP [GB]. Accelerated Loss of Mains Change Programme - the GB programme that retuned distributed-generation loss-of-mains relays from the legacy 0.125 Hz/s RoCoF setting to 1.0 Hz/s by around 2022. GridSim carries both thresholds and the MECHANICAL tab shows margin against each. See RoCoF (dual threshold).

Assessment [GS]. A FrequencyAssessment (Core/Dynamics/FrequencyLimits.cs): the graded verdict on a frequency excursion: RoCoF, nadir, settle, per-limit breach flags and a worst-first summary string. See 05-frequency-dynamics.

Avoided cost [GS]. The pound figure the evidence engine attaches to a counterfactual: recorded LFDD MW shed x Value of Lost Load x outage duration (Core/Evidence/AvoidedCost.cs). Computed only when the record actually shed load and the modelled device would have held the nadir above LFDD. The VoLL default (GBP 6,000/MWh) is a curated placeholder. See VoLL, 07-evidence-engine.

B

Backward-forward sweep (BFS) [PS]. The distribution-class solver (Core/PowerFlow/BackwardForwardSweep.cs) for radial and weakly-meshed feeders; SolverSelector routes radial networks (one slack and buses-1 in-service branches) to it. It carries line charging, shunts and non-unity / phase-shifting taps exactly, draws ZIP loads at the swept voltage, and honours the flat-start, Q-limit and cancellation options as the Newton engines do (R036). See 03-power-flow.

Bad data (identification) [PS]. The largest-normalised-residual loop: after a converged fit that fails chi-square, the measurement with the largest normalised residual is removed and the estimate re-run, pass by pass, with critical measurements protected (Core/Estimation/BadDataAnalysis.cs). See Critical measurement, Prior-backfill, 06-state-estimation.

Base MVA [PS]. The system-wide power base every per-unit conversion is taken against (GridModel.BaseMva, 100 MVA for the WSCC and GB cases). See Per-unit.

BiCGSTAB [PS]. ILU0-preconditioned biconjugate-gradient-stabilised Krylov solver (Core/Numerics/Sparse.cs). Only a fallback: the Newton step uses it when the direct sparse LU reports the Jacobian singular. A GPU sparse solve that gives up is retried on the exact CPU LU first (1.95.009). See 04-sparse-and-gpu-numerics.

BM - Balancing Mechanism [GB]. The near-real-time market through which NESO accepts BIDs (turn down) and OFFERs (turn up). BM acceptances stream over the bridge (BalancingActionDto with fuelType and synchronous); the RP1 tab grades each by its effect on intrinsic inertia.

Boundary [GB]. An ETYS transmission boundary - a defined cut across the GB network (e.g. B6, Scotland-England) with a transfer limit. Modelled as Boundary/BoundaryBranch in Core; carried by case boundaries.json, by the bridge topology's boundaries block, and per-period by the estimated_state lake's boundary_json. See 09-gda-integration.

Bus / bus type [PS]. A network node (Bus record, Core/Model/Model.cs) classified Slack, Pv or Pq - the classification that decides which of a bus's four electrical quantities are specified and which are solved. See 02-data-model-and-per-unit.

C

Chi-square (chi-square) consistency test [PS]. The global fit test on a converged estimate: the objective J is compared against the chi-square 95 % threshold for the fit's degrees of freedom (Core/Estimation/ChiSquare.cs). PASS means the residuals are consistent with the declared sigmas; FAIL triggers bad-data identification. NaN without redundancy. See 06-state-estimation.

Chi-square pill [GS]. The per-frame chi-square verdict shown on the WPF bridge status bar when replaying source=estimated - driven by the state message's estimate.chiPassed (src/GridSim.Wpf/MainWindow.xaml.cs, bridge status pill; BridgeProtocol.EstimateBlock.ChiPassed). The quality indicator a measurement-fitted frame carries and a demand-scaled re-solve cannot.

Comp-range [GS]. The switchable reactive range (capacitive + inductive MVAr) of a site's compensation plant, from the ETYS Appendix B table (GDA NetworkModel/build_reactive_compensation.py). A switched MSC/reactor/ SVC's state is unobservable from settlement data, so a compensated bus's Q pseudo-measurement carries the range as extra sigma (uniform-range standard deviation, range/sqrt12), tagged pseudo-demand+comp-range (build_estimator_measurements.py:_comp_range_by_bus). Declaring the real uncertainty ended the systematic QInj rejections at compensated sites.

Convergence [PS]. Max mismatch below tolerance (default 1e-8 pu). The full Jacobian gives quadratic convergence: case9 in 4 iterations to 1.65e-14 pu (gridsim case9 --once). A non-converged or non-finite solve is reported as a failure (PowerFlowStatus.MaxIterations / NonFinite), never as converged. See Convergence contract.

Convergence contract [GS]. ConvergenceContract.RequireConverged (Core/PowerFlow/ConvergenceContract.cs), the first statement of every study that starts from a solved state - fault analysis, protection assessment and cascade, the loss-contingency sweeps, small-signal assembly, subtransient initialisation and the RMS transient studies. A non-converged state throws NonConvergedStateException naming the study, the status, the mismatch and the iteration count, so it can never flow into a result reported as success (1.95.010).

Counterfactual [GS]. A "what the device would have done" result - ProvenanceTag.Counterfactual. Never an input to another replay, and gated: no counterfactual until the recorded baseline is reproduced with no device. See Reproduced-baseline gate.

Critical measurement [PS]. A measurement that is the sole observer of some state direction: its residual is structurally zero, its error undetectable, and it is protected from bad-data removal (flagged critical in the export; normalised residual NaN). se_residual_check.py --leverage arbitrates the flags independently.

Crosswalk (BMU -> GSP) [GS]. The bounded table binding every Elexon BM unit to its best GSP identity with an explicit method + confidence per row (GDA NetworkModel/build_bmu_crosswalk.py -> resolved/bmu_crosswalk.parquet). Ladder: name-match 0.9 (bmUnitName only) -> strong TEC-register 0.75 (curated station codes + >=2-token matches) -> OSUKED-nearest 0.7 -> weak TEC 0.75 -> interconnector 0.6 -> gsp-group-only 0.5 -> unmapped 0. See 09-gda-integration.

CSR - Compressed Sparse Row [PS]. The sparse-matrix storage of the large solves (CsrMatrix, Core/Numerics/Sparse.cs); memory O(nnz). See 04-sparse-and-gpu-numerics.

CuratedPlaceholder [GS]. The ProvenanceTag for a figure standing in for data not yet sourced, always accompanied by a [VERIFY] text marker. The tag makes a number's provisional status travel with it; preserving these markers is required. See [VERIFY] convention, 08-provenance-and-invariants.

D

Design basis (3,600 MW) [GS/GB]. The correlated-cluster scenario the stability panels show - Dogger Bank A+B+C (CredibleLossBasis.DesignBasisLossMw, Core/Presentation/CredibleLossBasis.cs), labelled "correlated cluster (not secured)": a simultaneous event that NESO's Frequency Risk and Control Report 2025 does not secure. It is shown beside 1,800 MW (CredibleLossBasis.FictionLossMw), the FRCR 2025 future largest loss and the NETS SQSS Infrequent Infeed Loss Risk, which is the official figure.

Dispatch [PS]. Assignment of real-power output to generators. Core/Dispatch/Dispatcher.cs applies default GB capacity factors by fuel and balances the slack.

DNO [GB]. Distribution Network Operator (NGED, UKPN, NPg, SPEN, SSEN, ENWL). Their LTDS datasets are the raw material of the GDA case assembly. See LTDS, 09-gda-integration.

Dynamic MIF [GS]. The live intrinsic-energy floor a current credible loss needs to hold a RoCoF limit: GVA.s = f0*deltaP/(2*RoCoF_limit) (StabilityMetrics.DynamicMifGvaS). See MIF.

E

E - stored energy [PS]. Kinetic energy in a machine's rotating mass at synchronous speed, E = H*MBase (MW.s). Summed over the fleet it is the system inertia.

Era-mismatch [GS]. The documented reason the ETYS circuit overlay is opt-in: ETYS 2025 describes today's network, so applying its real circuits to a historical day injects reinforcements that did not exist then - the 2019 event day regressed from 46/46 chi-square-clean periods to 32/46 under the overlay. Until the overlay is era-aware, the validated synthetic+curated spine is the default (GDA NetworkModel/spine/build_spine.py:apply_etys_circuits). See Spine overlay.

Estimated source [GS]. The bridge stream source source:"estimated": the measurement-conditioned WLS lake (Derived/estimated_state) replayed frame by frame, each frame carrying its own estimate quality block (J, chi-square verdict, dof, removals). Contrast presolved (demand-scaled deterministic re-solve lake) and live (static solve + per-period scalars). See 09-gda-integration.

ETYS [GB]. Electricity Ten Year Statement - NESO's published network and boundary data (boundaries + transfer limits; Appendix B circuit parameters). Grounds the boundary bars, the spine boundary limits and the opt-in circuit overlay.

Evidence dossier [GS]. The evidence verb's umbrella output, schema gridsim-evidence/1: recorded vs simulated vs counterfactual columns, inertia risk curve, fleet comparison and avoided cost, gated on a reproduced baseline. See 07-evidence-engine, 10-file-and-wire-formats.

F

FFR - Fast Frequency Response [GB]. Sub-second active-power injection from converter-connected resources. In the swing model FFR is a separate power injection, never folded into stored energy E; across a staggered fleet it is non-additive. See Fleet dynamics.

Fleet dynamics [GS]. The result that a fleet of small units is not one big device: intrinsic inertia is N-additive but staggered fast response is not, so an equal-budget fleet reaches a deeper nadir (Core/Fleet/FleetDynamics.cs).

Fold map [GS]. fold_map.json - the gb-full-bus -> owning-spine-bus map (plus spine-bus -> PES group) produced by the gb-spine fold's simultaneous BFS and written to both case directories; resolves any bus to its PES group for per-group demand scaling (GDA NetworkModel/emit_spine_case.py:_fold_map).

Forward-inference guard [GS]. The structural invariant that GridSim only processes data at least 169 hours (7 days + 1 h) old: ForwardInferenceGuard.HorizonHours = 169.0 (Core/Diagnostics/ForwardInferenceGuard.cs; widened from 25 h in July 2026). Every dated input - event onsets, measurement takenUtc stamps, bridge state ticks - passes through RejectFuture, which throws ForwardInferenceException on anything newer. Enforced by construction, not policy. See Horizon, 08-provenance-and-invariants.

Frame [GS]. A SimFrame(Model, Result, Hour, Tick, FrequencyHz, GenerationMw, DemandMw, ...) - the unit the UIs bind to. SimFrame.Model carries post-solve tap ratios. Live simulation, presolved replay and the bridge all produce frames.

G

gb-full [GS]. The materialised GB network - 3,539 buses / 5,918 branches, assembled from normalised DNO LTDS tables plus the transmission spine (GDA NetworkModel/assemble/run.py), reduced from a raw 4,240 buses by busbar-section deduplication. Solves to a machine-epsilon residual; the sparse estimation target.

gb-spine [GS]. The 316-bus GSP-level reduction of gb-full (emit_spine_case.py): one bus per NESO spine node, demand/generation folded on by the BFS owner map, sub-GSP impedances dropped by design. The network whose granularity matches the per-GSP measurement layer one-to-one - the default estimation target. See Fold map.

GDA [GS]. Grid Data Archive - the read-only lake of GB grid data (DNO LTDS, Elexon settlement, NESO ETYS/TEC/inertia/frequency) GridSim's GB networks, measurements and estimated history are built from. Kept out of this repository. See 09-gda-integration.

Generator [PS]. The Generator record (Core/Model/Model.cs): Pg/Qg, setpoint Vg, limits Qmin/Qmax/Pmax, machine base MBase, inertia constant H (dynamics only - never enters the load flow).

gsp_ic_flows [GS]. Derived/gsp_ic_flows - per-period net interconnector import at each landing GSP, from B1610 settlement volumes (ic_mw = 2 x sum signed MWh of the I_* units). Settlement-grade and era-deep (2019+); the estimator's preferred interconnector source, with the operator-frames columns as fallback. See 09-gda-integration.

GSP [GB]. Grid Supply Point - a transmission/distribution interface substation. The NESO anchor scheme's GSP names key the measurement layer, the crosswalk and the spine nodes.

GVA.s [PS]. Gigavolt-ampere-seconds - the unit of system inertia (total stored energy / 1000). The GB inertia risk curve sweeps 260/155/120/102/50 GVA.s.

H

H - inertia constant [PS]. Stored kinetic energy per MVA of machine rating, in seconds (Generator.H). WSCC: 23.64 / 6.40 / 3.01 s.

Horizon [GS]. The 169-hour (7 days + 1 h) minimum lag every dated datum must satisfy. Anything newer - recent or future - is rejected at ingestion with a thrown ForwardInferenceException. The GDA side enforces the same cutoff (Scripts/horizon.py; the bridge server masks every emitted tick and rejects future starts/queries). The bridge client applies the exact horizon to streamed ticks, with no clock-skew allowance (removed in 1.95.030). See Forward-inference guard.

HVDC [PS]. High-voltage direct current - GB interconnectors and offshore links. Converter-connected (zero intrinsic inertia); corridor-correlated trips drive the credible loss.

I

ILU0 [PS]. Incomplete LU keeping the exact sparsity pattern (Core/Numerics/Sparse.cs), the BiCGSTAB preconditioner.

Inertia (intrinsic vs market/contracted) [GS]. GridSim splits system inertia into intrinsic (real synchronous rotating mass, GVA.s(i)) and market/contracted (converter-provided response; may include real synchronous condensers). Only intrinsic inertia enters the purchased-time denominator. The bridge operator block streams the stratification (synchronousInertiaGvaS, outturnInertiaGvaS, marketInertiaGvaS).

Interconnector [GB]. A cross-border HVDC link (IFA, IFA2, BritNed, Moyle, Nemo, NSL, ElecLink, Viking, Greenlink, EastWest). Its flow enters the estimator as a landing-GSP injection (see gsp_ic_flows) and its correlated-trip status the RP1 credible loss.

IWlsStep [GS]. The estimation-step seam (Core/Estimation/WlsStep.cs:IWlsStep) - the estimator-side sibling of ILinearSolver/IPowerFlowSolver. The estimator owns the loop (start point, damping, convergence, chi-square, bad-data removal); the step backend owns representation: how H is held, how the gain matrix is formed and solved, how observability is gated, how residual covariance is computed. Implementations: DenseWlsStep (up to EstimationOptions.SparseBusThreshold, default 200 buses), SparseWlsStep (CSR gain, Gilbert-Peierls LU, structure built once per pass), ConstrainedDenseWlsStep (Hachtel-style KKT, --method constrained), FeasibleWlsStep (--method feasible; both KKT methods are dense only and refuse models above 3,000 buses) and QrWlsStep (EstimationMethod.Qr, library API only - the CLI --method does not offer it; orthogonal, no normal equations); robust reweighting drops in behind the same seam. See 06-state-estimation.

J

Jacobian [PS]. The matrix of partial derivatives of calculated injections with respect to the voltage unknowns, in four polar blocks. Each Newton step solves J*deltax = f. GridSim uses the full exact Jacobian. See 03-power-flow.

L

LFDD [GB]. Low-Frequency Demand Disconnection - automatic demand shedding to arrest a falling frequency; GB stage 1 arms at 48.8 Hz (FrequencyLimits.LfddFirstStageHz). An LFDD shed is a RecordedAction (positive deltaP) and the substrate for avoided cost. Distinct from the 49.2 Hz working set-point.

LNR - largest normalised residual [PS]. The bad-data identification statistic: r/sqrtOmega with Omega the residual covariance diagonal. See Bad data.

LTDS [GB]. Long Term Development Statement - the DNO-published network dataset (nodes, circuits, transformers, demand, generation) that ingestion normalises into the per-licence tables the assembler consumes.

M

MATPOWER [PS]. The open power-system package whose .m case format and published cases GridSim parses and validates against; case9's published solution is the ground truth.

MBase [PS]. A machine's own MVA rating (Generator.MBase), the base its H is quoted on.

Measurement set [GS]. A gridsim-measurements/1 document (Core/Estimation/MeasurementSet.cs): case telemetry with per-measurement kind, value, sigma and source, addressed by bus id / branch endpoints. Horizon-checked on load when takenUtc is present. See 10-file-and-wire-formats.

MIF - Minimum Intrinsic Floor [GS/GB]. The intrinsic stored energy needed to hold a RoCoF limit for a given loss (720 GVA.s for 3,600 MW at 0.125 Hz/s). Computed live as the Dynamic MIF.

Mismatch [PS]. Per-bus difference between specified and calculated injection, per-unit on BaseMva; convergence is max|f| < tolerance.

N

Nadir [PS]. The lowest frequency a fall reaches before response arrests it (FrequencyAssessment.NadirHz).

NESO [GB]. National Energy System Operator - the GB system operator. Source of the limit ladder, ETYS, TEC, inertia outturn and frequency traces.

Newton-Raphson [PS]. The iterative power-flow method: linearise via the Jacobian, solve J*deltax = f, step, repeat. GridSim's dense (NewtonRaphsonPowerFlow) and sparse (SparseNewtonRaphsonPowerFlow) engines are bit-identical on the CPU since 1.95.001: each assembles its own Y-bus, injections and Jacobian exactly, and both take the one shared step in NewtonShell (CPU direct sparse LU, damping above a 0.4 step, 0.2 pu Vm floor). DenseSparseParityTests pins it. See 03-power-flow.

N-1 [PS]. The security standard requiring survival of any single-element loss; nminus1/nminus1-sweep screen it.

O

Observability [PS]. Whether the measurement set determines the whole state. The dense backend proves gain-matrix rank and names free directions; the sparse backend does the same up to 2,500 state variables and above that runs the topology (island) sweep only, reporting rank = -1 (SparseWlsStep.ObservabilityDiagnosisMaxState). Unobservable runs exit 3. See 06-state-estimation.

OLTC [PS]. On-load tap changer (Oltc record): regulates a controlled bus voltage within a deadband; the SteadyStateSolver outer loop moves taps on model clones and exposes the result via RegulatedModel. It never steps a tap on a non-converged or NaN solve, and TapsSettled is false when it stopped on one or hit its pass cap (default 25) with taps still moving.

P

Per-unit (pu) [PS]. Quantities as fractions of a base: power / BaseMva, voltage / BaseKv, impedance x S_base/V_base^2. Actual kV = Vm x BaseKv.

PN fallback [GS]. The B-3 source ladder rung: when a settlement day has no (or a partial, < 40-period) B1610 file, the per-GSP generation measurements build from Physical Notification levels instead - submitted intent, not metering - with looser sigmas (10 MW / 10 % vs B1610's 2 MW / 2 %), provenance = "pn" end to end, and no group-demand sibling. Extends estimable history to 2016-03 (GDA NetworkModel/build_gsp_measurements.py). See 09-gda-integration.

Prior-backfill [GS]. Observability restoration on bad-data removal (--backfill-removals; EstimationOptions.BackfillRemovedFromPrior): a removed real-power measurement is replaced by a pseudo evaluated at the start state (the warm start / solved prior) with a deliberately looser sigma (3x, floored at 5 % of value), tagged source = "removal-backfill-prior" (WlsStateEstimator.BackfillSource) and itself never removable. Stops a removal in a weakly-redundant region letting the state run away from the prior. Reactive kinds are deliberately not backfilled. Off by default - it changes what "removed" means (excluded -> replaced-by-prior).

Provenance tag [GS]. The seven-value trust enum every evidence series carries (Core/Diagnostics/Provenance.cs): Recorded, DerivedFromRecorded, CuratedPlaceholder, Synthetic, Counterfactual, Interpolated, UnvalidatedAtScale. Only Recorded is admissible as a real event input. See 08-provenance-and-invariants.

Pseudo-measurement [PS]. A prior dressed as a measurement with a deliberately loose sigma - the folded-demand injections (pseudo-demand, 25 %; group- anchored 15 %), the reactive pseudos (pseudo-reactive), the voltage schedule (pseudo-voltage-schedule, 1.01 +/- 0.03 pu). Tagged by source, never presented as telemetry. See Virtual measurement.

Purchased time [GS]. Seconds from a credible loss to the 49.2 Hz working set-point on the initial-RoCoF approximation, given intrinsic stored energy (StabilityMetrics.PurchasedTimeSeconds). A physical duration: for a 3,600 MW loss, 720 GVA.s -> 6.4 s, 155 -> 1.4 s, 102 -> 0.9 s.

Q

Q-limit switching (PV->PQ) [PS]. The outer-loop model of a generator's excitation ceiling: a PV bus whose aggregate reactive output violates its limits is released to PQ with Q pinned at the binding limit, and the flow re-solves, up to PowerFlowOptions.MaxQLimitPasses (default 12; below 1 is refused). If the cap is reached right after a switch, the engines solve once more and report the result not converged (MaxIterations).

R

Recorded [GS]. The highest-trust provenance tag - a datum directly from an archived record; the only class admissible as a real event input.

Reproduced-baseline gate [GS]. No counterfactual until the recorded event's RoCoF and nadir are reproduced with no device (RoCoF +/-15 %, nadir +/-0.15 Hz -> Reproduced; within twice both -> Marginal; else Failed; a per-sample RMSE above 0.05 Hz caps the verdict at Marginal; ReplayValidator). Anything short of Reproduced exits 3 and unlocks no counterfactual. See 07-evidence-engine.

Residual check [GS]. The independent NumPy instrument (tools/crossval/residual_check.py) that rebuilds Y and evaluates S = V .* conj(Y*V)*baseMVA at every bus. Its estimation sibling is se_residual_check.py (honesty / objective / optimality / leverage). See 11-validation.

RoCoF [PS]. Rate of change of frequency, df/dt = f0*deltaP/(2E) at the instant of loss - a clean window onto inertia.

RoCoF (dual threshold) [GB]. GB carries two loss-of-mains relay settings: legacy 0.125 Hz/s and modern (ALoMCP-retuned) 1.0 Hz/s (FrequencyLimits). Every assessment reports breaches against both.

S

Settle [PS]. The frequency the system stabilises at after primary response deploys (FrequencyAssessment.SettleHz).

Sigma (sigma) [PS]. A measurement's declared standard deviation, in its own units; the WLS weight is 1/sigma^2. The GDA sigma models are documented per product in 09-gda-integration; their calibration in ch. 22 (Calibrate the sigmas).

Slack bus [PS]. The reference bus: voltage magnitude and angle fixed, absorbing the system balance including losses. One per electrical island.

SolverSelector [GS]. The single source of truth for engine choice (Core/PowerFlow/SolverSelector.cs): radial -> BFS; otherwise sparse Newton, because DefaultSparseThreshold is 0 and every model has more buses than that (dense Newton only by explicit choice or a caller's own threshold; the two Newton engines are bit-identical on the CPU). DenseSparseCrossoverBuses (200) is kept for the estimator's observability report, the GPU routing and the solve export. ResolveNewton / CreateNewton apply the same rule without the radial check. The CLI (SolverBackend.Solver / NewtonSolver / Pick), the GUI, the protection cascade, contingency, AC-OPF, CPF, the tap optimiser, restoration, the parameterless SteadyStateSolver() (REST /solve, viz export) and the estimator's backend threshold all use it. The two Newton engines are bit-identical, so the choice changes speed and memory, never a number.

Sparse LU [PS]. Gilbert-Peierls left-looking sparse LU with partial pivoting over a reverse Cuthill-McKee column ordering (Core/Numerics/SparseLu.cs, SparseLuFactorization.cs; CpuSparseLuSolver in the power flow). The canonical linear solve of every CPU Newton step, dense engine included. Returns null (singular) when a pivot magnitude is at or below the shared relative floor, 1e-12 x max|A| (NumericTolerances.RelativePivotFloor; an absolute 1e-300 before October 2026).

Spine [GS]. The GSP-level GB transmission backbone built by GDA NetworkModel/spine/build_spine.py: one bus per deduplicated NESO GSP site, synthetic Delaunay->Gabriel->prune+MST topology, curated corridor overlay, boundary limits from NESO constraint files. gb-full's DNO fragments attach to it; gb-spine is its estimation-grade reduction.

Spine overlay (SPINE_ETYS=1) [GS]. The opt-in replacement of the spine's synthetic per-km impedances with the real ETYS Appendix B circuit list (parallel-combined per site pair, provenance etys-appendix-b). Off by default because of the era-mismatch caveat; enable it for investigations of the present-day network only. See Era-mismatch.

Synthetic GB [GS]. The licence-free GB case (Core/Synthetic/SyntheticGb) that lets the GB workflow run with no GDA lake present. Distinct from the materialised gb-full.

System inertia [PS]. Total stored kinetic energy, sum H*MBase / 1000 GVA.s (GridModel.SystemInertiaGvaS).

T

Tap ratio [PS]. A transformer's off-nominal turns ratio (Branch.TapRatio, 1.0 nominal). A MATPOWER ratio of 0 parses as 1.0, and the admittance stamp treats any ratio <= 0 as 1.0.

TEC [GB]. Transmission Entry Capacity - the NESO register of contracted transmission-connected capacity; a crosswalk rung and a fleet-assembly source.

Tolerance [PS]. The convergence threshold on the mismatch (PowerFlowOptions.Tolerance, default 1e-8 pu). The estimator's is EstimationOptions.Tolerance (default 1e-8) on max |deltax|.

TSGB [GS]. Twin Scroll Grid Balancer - the device the frequency/evidence apparatus models; TsgbSizer inverts the swing equation to size inertia and FFR for target RoCoF and nadir.

V

[VERIFY] convention [GS]. The in-source marker that always accompanies a CuratedPlaceholder figure. It describes the trust level of the software's data and is preserved deliberately - stripping a [VERIFY] without sourcing the figure defeats the integrity apparatus. See 08-provenance-and-invariants.

Virtual measurement [PS]. A structural zero-injection constraint at a pure switching node, tagged virtual-zero-injection - soft (tight-ish sigma) under WLS, promoted to a hard KKT equality under --method constrained (ConstrainedDenseWlsStep.ConstraintSource).

VoLL [GB]. Value of Lost Load, GBP/MWh of unserved demand. Default GBP 6,000/MWh, a curated placeholder (GB estimates span GBP 6k-17k/MWh).

W

Warm start [PS]. Seeding a solve or estimate from a previous solution. The assembler bakes a converged operating point into GDA cases; the estimator accepts --warm-start (iterate from the case's solved point - standard EMS practice, needed for real data) and --prior solve (solve the measured-injection power flow first; its solution replaces the flat voltage schedule as source = "prior-solved" and becomes the warm start - Core/Estimation/MeasuredInjectionPrior.cs).

Working set-point (49.2 Hz) [GS/GB]. The operational floor purchased time is measured to (StabilityMetrics.LfddWorkingSetpointHz) - deliberately distinct from the 48.8 Hz LFDD relay arming point.

WSCC [PS]. The Western System Coordinating Council 9-bus system (IeeeCases.Case9()), GridSim's physics ground truth.

Y

Y-bus [PS]. The nxn complex bus admittance matrix, Y*V = I. Extremely sparse for real grids - the fact behind the CPU-first numerics (04-sparse-and-gpu-numerics).

Reference tables

The GB frequency-limit ladder (Core/Dynamics/FrequencyLimits.cs; StabilityMetrics.cs):

Rung Constant Value
Nominal NominalHz 50.0 Hz
Operational OperationalLow/HighHz 49.8 / 50.2 Hz
Statutory StatutoryLow/HighHz 49.5 / 50.5 Hz
Working set-point LfddWorkingSetpointHz 49.2 Hz
LFDD stage 1 LfddFirstStageHz 48.8 Hz
RoCoF legacy RocofLimitLegacyHzPerS 0.125 Hz/s
RoCoF modern RocofLimitModernHzPerS 1.0 Hz/s

The seven provenance tags (Core/Diagnostics/Provenance.cs):

Tag Rule
Recorded Only class admissible as a real event input
DerivedFromRecorded Computed from records
CuratedPlaceholder Unsourced stand-in; always carries [VERIFY]
Synthetic Model-generated; never fed to the integrator as a record
Counterfactual Device what-if; never an input to a replay
Interpolated Gap-filled; usable for fidelity, never as an event source
UnvalidatedAtScale Distributed-inertia result with no GB-scale reference; never presented as validated

Protections & asset dynamics

Term Meaning
Asset dynamics Authored per-asset behaviour (grid-following/forming inverters, passive plant) contributing a pAssetFleet swing term; ch. 5, 13
Protection stage Monitored quantity -> threshold + dwell -> action (TripInfeed/ShedDemand/InjectStep/OpenBranch/RestoreBranch); ch. 15
LoM / RoCoF trip Loss-of-Mains protection tripping embedded generation on rate-of-change-of-frequency (legacy 0.125, modern 1.0 Hz/s)
LFDD Low-Frequency Demand Disconnection - a demand-shed protection arming at 48.8 Hz (modelled dynamically)
Recloser A protection that opens a branch (topology) and auto-recloses after a delay
Island partition The pre-pass that finds connected components over in-service branches and elects a per-island slack or blacks out a dead island (IslandPartition)
Reduction gate The invariant that an absent/never-operating fleet leaves the frequency AND power-flow solve byte-identical

See also