UK G99 (Type D) ride-through requirements
Low-voltage ride-through, Frequency ride-through for the United Kingdom. Every breakpoint below is read from the code itself, and where a point was taken off a published figure rather than stated in the clause, the section under that chart says so — an envelope that stops where its published chart stops.
What is the low-voltage ride-through envelope for UK G99 (Type D)?
The plant must ride through for any voltage that stays on or above this envelope. The envelope holds 0 pu from t = 0 to 140 ms; then ramps linearly from 0 pu at 140 ms to 0.85 pu at 2.2 s; then 0.85 pu from 2.2 s, plotted to 3 min.
The clause states this boundary as a line; intermediate points are interpolated between its stated vertices. The envelope stops where the published chart stops; the code states no end time.
Applies to Type D power park modules connected at or above 110 kV (Type D = ≥50 MW and/or ≥110 kV)
| From | To | Voltage | Between the points |
|---|---|---|---|
| 0 | 140 ms | 0 pu | Held flat |
| 140 ms | 2.2 s | 0 → 0.85 pu | Ramps linearly |
| 2.2 s | plotted to 3 min | 0.85 pu | Held flat |
The clause this came from
ENA EREC G99 Issue 2, 10 March 2025, Table 13.6 and Figure 13.11 'Voltage against time curve applicable to Type D Power Park Modules connected at or above 110 kV': Uret 0, Uclear 0, Urec1 0, Urec2 0.85; tclear 0.14, trec1 0.14, trec2 0.14, trec3 2.2, figure axis to 180 s. Clause 13.3.1.1 makes the heavy black line of Figures 13.8-13.11 the ride-through boundary. The code defines a linear recovery ramp; the vertices are read from the clause and the chart draws the segment between them.
What is the frequency ride-through envelope for UK G99 (Type D)?
The plant must ride through between the upper and lower bands. The upper (over-frequency) band holds 52 Hz from t = 0 to 15 min; then 51.5 Hz from 15 min to 90 min; then 51 Hz from 90 min onward — the last band the envelope defines, with no stated end time. The lower (under-frequency) band holds 47 Hz from t = 0 to 20 s; then 47.5 Hz from 20 s to 90 min; then 49 Hz from 90 min onward — the last band the envelope defines, with no stated end time.
The last band has no stated end time. The clause below states a continuous region — highlighted; check which band it covers, because it is not always the last one.
Applies to Type C-D power park modules (clause 13.2.1); Type D = ≥50 MW and/or ≥110 kV connection
Upper limit
| From | To | Frequency | Between the points |
|---|---|---|---|
| 0 | 15 min | 52 Hz | Held flat |
| 15 min | 90 min | 51.5 Hz | Held flat |
| 90 min | no stated end | 51 Hz | Held flat |
Lower limit
| From | To | Frequency | Between the points |
|---|---|---|---|
| 0 | 20 s | 47 Hz | Held flat |
| 20 s | 90 min | 47.5 Hz | Held flat |
| 90 min | no stated end | 49 Hz | Held flat |
The clause this came from
ENA EREC G99 Issue 2 (2025), clause 13.2.1 (a)-(e) for Type C and Type D Power Generating Modules: 20 s at 47-47.5 Hz; 90 minutes at 47.5-49.0 Hz; continuous 49.0-51.0 Hz; 90 minutes at 51.0-51.5 Hz; 15 minutes at 51.5-52 Hz.
Operating requirements beyond the envelopes.
The envelopes above are what UK G99 (Type D) demands during a disturbance. The rows below are the same regime’s operating and fault-response requirements — reactive capability, frequency response, continuous operating range, ramp rate, RoCoF withstand and fault-current injection — each researched from the document its own row cites, separately from the plotted corpus. Every row carries its clause and a verbatim quote, so you can check it the same way.
| Requirement | What the code states | Clause |
|---|---|---|
| Reactive capability at Registered Capacity (Connection Point >33 kV) Reactive capability | PF 0.95 leading (consumption) to 0.95 lagging (production) at Registered Capacity, delivered inside a voltage-dependent envelope spanning 0.875-1.10 pu Connection Point voltage (Figure 13.13); shape = voltage-dependent envelope, not a simple rectangle Binds all Power Park Modules (the class covering inverter-based storage) under G99 Section 13 (Type C & D; Type D = Connection Point >=110 kV and/or Registered Capacity >=50 MW), measured at the Connection Point, for Connection Point voltage above 33 kV. The ENA copy of G99 is access-controlled, so the linked PDF is the publicly hosted dcode.org.uk mirror of the identical Issue 2 (10 Mar 2025) document; issue currency was confirmed against the issuer’s published issue register. | 13.5.4 (Figure 13.13) |
The clause this row cites13.5.4 (Figure 13.13) — “ All Power Park Modules with a Connection Point voltage above 33 kV, shall be capable of satisfying the Reactive Power capability requirements at the Connection Point as defined in Figure 13.13 when operating at Registered Capacity. ” (Engineering Recommendation G99 Issue 2 — Requirements for the connection of generation equipment in parallel with public distribution networks on or after 27 April 2019, accessed 2026-07-28) | ||
| Reactive capability at Registered Capacity (Connection Point <=33 kV) Reactive capability | PF 0.95 lead (Qmin) to 0.95 lag (Qmax) at Registered Capacity within a sloped, voltage-dependent envelope over 0.95-1.05 pu Connection Point voltage (Figure 13.14): full lagging capability only at <=1.00 pu, full leading capability only at >=1.00 pu Binds all Power Park Modules under G99 Section 13 (Type C & D) with Connection Point voltage at or below 33 kV — the common case for ~50 MW GB BESS connected at 33 kV that are Type D by capacity; requirement is at the Connection Point. The ENA copy of G99 is access-controlled, so the linked PDF is the publicly hosted dcode.org.uk mirror of the identical Issue 2 (10 Mar 2025) document; issue currency was confirmed against the issuer’s published issue register. | 13.5.5 (Figure 13.14) |
The clause this row cites13.5.5 (Figure 13.14) — “ All Power Park Modules with a Connection Point voltage at or below 33 kV shall be capable of satisfying the Reactive Power capability requirements at the Connection Point as defined in Figure 13.14 when operating at Registered Capacity. ” (Engineering Recommendation G99 Issue 2 — Requirements for the connection of generation equipment in parallel with public distribution networks on or after 27 April 2019, accessed 2026-07-28) | ||
| Reactive capability below Registered Capacity (P-Q envelope) Reactive capability | Q/Pmax = +/-0.33 maintained from 100% down to 50% Active Power output; below 50% the limits reduce linearly (Figure 13.15 shows the leading limit tapering from -0.33 at 50% P to -0.12 at 20% P), unless the DNO specifies retention of full absorbing capability down to 20% P; limits reduce pro rata to plant in service Binds all Power Park Modules under G99 Section 13 (Type C & D) at the Connection Point when operating below Registered Capacity; defines the part-load P-Q envelope shape. The ENA copy of G99 is access-controlled, so the linked PDF is the publicly hosted dcode.org.uk mirror of the identical Issue 2 (10 Mar 2025) document; issue currency was confirmed against the issuer’s published issue register. | 13.5.6 (Figure 13.15) |
The clause this row cites13.5.6 (Figure 13.15) — “ All Power Park Modules, shall be capable of satisfying the Reactive Power capability requirements at the Connection Point as defined in Figure 13.15 when operating below Registered Capacity. With all plant in service, the Reactive Power limits will reduce linearly below 50% Active Power output as shown in Figure 13.15 unless the requirement to maintain the Reactive Power limits defined at Registered Capacity under absorbing Reactive Power conditions down to 20% Active Power output has been specified by the DNO. These Reactive Power limits will be reduced pro rata to the amount of plant in service. ” (Engineering Recommendation G99 Issue 2 — Requirements for the connection of generation equipment in parallel with public distribution networks on or after 27 April 2019, accessed 2026-07-28) | ||
| FSM droop, deadband and insensitivity (Table 13.1) Frequency response | Droop settable 3-5%; Frequency Response Deadband 0 mHz; Frequency Response Insensitivity <= +/-15 mHz (+/-0.03% of 50 Hz); minimum Active Power frequency-response range |dP1|/Pmax = 10% of Registered Capacity Binds Type C & D Power Generating Modules under G99 Section 13.2.6; by default modules operate in LFSM — FSM operation requires a specific contract with the ISOP (13.2.6.1); Deadband and Droop must be resettable at any time as required by the DNO. The ENA copy of G99 is access-controlled, so the linked PDF is the publicly hosted dcode.org.uk mirror of the identical Issue 2 (10 Mar 2025) document; issue currency was confirmed against the issuer’s published issue register. The 10%-of-Registered-Capacity response range is cited from Table 13.1 but is not carried in the quoted excerpt. | 13.2.6.4(a), Table 13.1 |
The clause this row cites13.2.6.4(a), Table 13.1 — “ Frequency Response Insensitivity in mHz (|Δfi|) ±15mHz … Frequency Response Deadband in mHz 0 (mHz) … Droop (%) 3 – 5% ” (Engineering Recommendation G99 Issue 2 — Requirements for the connection of generation equipment in parallel with public distribution networks on or after 27 April 2019, accessed 2026-07-28) | ||
| FSM activation timing (Table 13.2) Frequency response | Initial delay t1 <= 2 s for modules with inertia, <= 1 s for modules without inertia (i.e. inverter-based storage), unless technically justified; full activation time t2 = 10 s for the 10%-of-Registered-Capacity response range Binds Type C & D Power Generating Modules in FSM under G99; full and stable Active Power frequency response on a frequency step per Figure 13.7/Table 13.2; justification for longer delays goes to the DNO, who passes it to the ISOP. The ENA copy of G99 is access-controlled, so the linked PDF is the publicly hosted dcode.org.uk mirror of the identical Issue 2 (10 Mar 2025) document; issue currency was confirmed against the issuer’s published issue register. The full activation time t2 = 10 s is cited from Table 13.2 but is not carried in the quoted text. | 13.2.6.4(c)-(d), Table 13.2 |
The clause this row cites13.2.6.4(c)-(d), Table 13.2 — “ For Power Generating Modules with inertia the delay in initial Active Power Frequency Response shall not be greater than 2 s. For Power Generating Modules without inertia the delay in initial Active Power Frequency Response shall not be greater than 1 s. ” (Engineering Recommendation G99 Issue 2 — Requirements for the connection of generation equipment in parallel with public distribution networks on or after 27 April 2019, accessed 2026-07-28) | ||
| LFSM-O response gradient and threshold Frequency response | Activates above 50.4 Hz (50.5 Hz if contracted in FSM); minimum response 2% of output per 0.1 Hz deviation (= 10% droop); droop settable but must be >= 2%; proportional reduction achieved within 10 s; initial delay > 2 s must be justified with technical evidence Binds every Power Generating Module (default mandatory mode, not an ancillary service) under G99 Section 13 for Type C & D; response continues until frequency returns to or below 50.4 Hz (13.2.4.2). The ENA copy of G99 is access-controlled, so the linked PDF is the publicly hosted dcode.org.uk mirror of the identical Issue 2 (10 Mar 2025) document; issue currency was confirmed against the issuer’s published issue register. The 50.5 Hz activation variant when contracted in FSM, the 10 s proportional-reduction time and the delay-justification rule are cited from clause 13.2.4.1 but are not carried in the quoted text. | 13.2.4.1(a) (Figure 13.2) |
The clause this row cites13.2.4.1(a) (Figure 13.2) — “ The rate of change of Active Power output shall be at a minimum a rate of 2% of output per 0.1 Hz deviation of system frequency above 50.4 Hz (ie a Droop of 10%) as shown in Figure 13.2. For the avoidance of doubt, this would not preclude a Generator from designing the Power Generating Module with a Droop of less than 10%, (for example between 3 – 5%), but in all cases the Droop should be 2% or greater. ” (Engineering Recommendation G99 Issue 2 — Requirements for the connection of generation equipment in parallel with public distribution networks on or after 27 April 2019, accessed 2026-07-28) | ||
| Storage-specific LFSM droop (importing mode) Frequency response | Droop between 0.6% and 1.2% (typical value 0.6%), agreed with the DNO, governing the import-to-export transition below 49.5 Hz per Figure 13.4; as much as possible of the response within 10 s; import must reach zero by 48.9 Hz or the module is instantaneously tripped Storage-specific clause binding Electricity Storage Power Generating Modules (Type C & D, G99 Section 13) when importing (charging); also applies when exporting if frequency falls below 49.5 Hz (13.2.5.2.3). The ENA copy of G99 is access-controlled, so the linked PDF is the publicly hosted dcode.org.uk mirror of the identical Issue 2 (10 Mar 2025) document; issue currency was confirmed against the issuer’s published issue register. The 48.9 Hz zero-import point, the instantaneous-trip rule and the 10 s response expectation are cited from clause 13.2.5.2.1 but are not carried in the quoted text. | 13.2.5.2.1(a) (Figure 13.4) |
The clause this row cites13.2.5.2.1(a) (Figure 13.4) — “ A typical value of the Droop would be 0.6% where this does not result in control system instability or plant difficulties. In all cases the Droop shall be between 0.6% and 1.2% and shall be agreed with the DNO. ” (Engineering Recommendation G99 Issue 2 — Requirements for the connection of generation equipment in parallel with public distribution networks on or after 27 April 2019, accessed 2026-07-28) | ||
| GB Grid Code FSM droop and deadband (transmission, ECC) Frequency response | Droop 3-5%; Frequency Response Deadband 0 mHz; Frequency Response Insensitivity +/-15 mHz (+/-0.03% of 50 Hz); Active Power response range 10% of Maximum Capacity; initial delay t1 <= 2 s (with inertia) / <= 1 s (without inertia), activation time t2 = 10 s per Table 6.3.7.3.3(b) Binds Type C and Type D Power Generating Modules and DC Connected Power Park Modules under the GB Grid Code European Connection Conditions (transmission-connected / EU-code users); source is the NESO-hosted GC0101 ECC legal-text extract dated 08/01/2018, which pre-dates the current consolidated Grid Code issue. | ECC.6.3.7.3.3(i), Table 6.3.7.3.3(a) |
The clause this row citesECC.6.3.7.3.3(i), Table 6.3.7.3.3(a) — “ Frequency Response Insensitivity in mHz (|Δfi|) ±15mHz … Frequency Response Deadband in mHz 0 (mHz) … Droop (%) 3 – 5% ” (GC0101 Extract of European Connection Conditions Legal Text (GB Grid Code ECC.6), accessed 2026-07-28) | ||
| GB frequency bands: 49.0-51.0 Hz continuous, timed operation 47-52 Hz Continuous operating range | Continuous operation required for 49.0-51.0 Hz. Time-limited bands: 47-47.5 Hz at least 20 s (each time in range); 47.5-49.0 Hz at least 90 minutes; 51.0-51.5 Hz at least 90 minutes; 51.5-52 Hz at least 15 minutes. Clause 13.2.1 sits in the Type C & D section and binds 'all Power Generating Modules' in that scope; sections 11.2.1 and 12.2.1 record identical bands for the other module types. Storage binds explicitly: G99 §2.2 (quote 2) requires Power Generating Modules comprising Electricity Storage devices commissioned on or after 01-Sep-2022 to comply with EREC G99 in full, and §13.2.5.2 (p.165-166) contains storage-specific frequency-response provisions. Requirement applies in parallel operation with the DNO's Distribution Network. Source copy: the ENA's own document-catalogue copy of EREC G99 is access-controlled, so the linked PDF is the publicly hosted dcode.org.uk mirror of the identical Issue 2 (10 Mar 2025) document; issue currency was confirmed against the issuer's own published issue register. | EREC G99 Issue 2 (2025), clause 13.2.1 (a)-(e), printed page 161 + EREC G99 Issue 2 (2025), §2.2 (Scope), printed page 20 |
The clauses this row citesEREC G99 Issue 2 (2025), clause 13.2.1 (a)-(e), printed page 161 — “ Under abnormal conditions automatic low-frequency load-shedding provides for load reduction down to 47 Hz. In exceptional circumstances, the frequency of the DNO’s Distribution Network could rise above 50.5 Hz. Therefore all Power Generating Modules shall be capable of continuing to operate in parallel with the Distribution Network in accordance with the following: a) 47 Hz – 47.5 Hz Operation for a period of at least 20 s is required each time the frequency is within this range. b) 47.5 Hz – 49.0 Hz Operation for a period of at least 90 minutes is required each time the frequency is within this range. c) 49.0 Hz – 51.0 Hz Continuous operation of the Power Generating Module is required. d) 51.0 Hz –51.5 Hz Operation for a period of at least 90 minutes is required each time the frequency is within this range. e) 51.5 Hz – 52 Hz Operation for a period of at least 15 minutes is required each time the frequency is within this range. ” (ENA Engineering Recommendation G99 Issue 2 (2025), accessed 2026-08-08) EREC G99 Issue 2 (2025), §2.2 (Scope), printed page 20 — “ Power Generating Modules comprising Electricity Storage devices commissioned on or after 01 September 2022 shall comply with EREC G99 in full. ” (ENA Engineering Recommendation G99 Issue 2 (2025), accessed 2026-08-08) | ||
| Voltage immunity ±10% of declared voltage - mandatory for Type D Continuous operating range | Immunity of the Power Generating Module to voltage changes of ±10% of the declared voltage at the Connection Point is recommended for Power Generating Modules generally, and is mandatory for Type D Power Generating Modules, subject to design appraisal of individual installations. The declared voltage and voltage range are agreed with the DNO where the module is remote from a network voltage control point. Binds Type D Power Generating Modules (mandatory) under the Type C & D section; for other modules the same immunity is only 'recommended' - modal preserved from source. Storage binds via the Power Generating Module scope (G99 §2.2: storage devices commissioned on/after 01-Sep-2022 comply in full). Note this is an immunity/withstand band around the declared voltage, not a statutory steady-state voltage limit; G99 does not restate GB statutory voltage limits in 13.4.1. Source copy: the ENA's own document-catalogue copy of EREC G99 is access-controlled, so the linked PDF is the publicly hosted dcode.org.uk mirror of the identical Issue 2 (10 Mar 2025) document; issue currency was confirmed against the issuer's own published issue register. | EREC G99 Issue 2 (2025), clause 13.4.1 (Voltage Limits and Control), printed page 177 |
The clause this row citesEREC G99 Issue 2 (2025), clause 13.4.1 (Voltage Limits and Control), printed page 177 — “ Where a Power Generating Module is remote from a Network voltage control point it may be required to withstand voltages outside the normal statutory limits. In these circumstances, the DNO should agree with the Generator the declared voltage and voltage range at the Connection Point. Immunity of the Power Generating Module to voltage changes of ± 10% of the declared voltage is recommended, but is mandatory for Type D Power Generating Modules, subject to design appraisal of individual installations. ” (ENA Engineering Recommendation G99 Issue 2 (2025), accessed 2026-08-08) | ||
| Active-power ramp rate on (re)connection Ramp rate | No general maximum admissible ramp rate; the assumed maximum ramp rate must be declared in the connection application, and a network-specific maximum may be imposed via the Connection Agreement G99 Section 10 (Protection) requirement on connection/reconnection conditions, applying to G99 Power Generating Modules including Type D; G99 sets no numeric ramp limit anywhere — limits are project-specific via the Connection Agreement. The ENA copy of G99 is access-controlled, so the linked PDF is the publicly hosted dcode.org.uk mirror of the identical Issue 2 (10 Mar 2025) document; issue currency was confirmed against the issuer’s published issue register. | 10.3.4 |
The clause this row cites10.3.4 — “ There is in general no maximum admissible ramp rate for Active Power output on connecting or reconnecting, although it is a requirement to state the assumed maximum ramp rate for the Power Generating Module as part of the application for connection. If a network specific issue requires a maximum admissible ramp rate of Active Power output on connection it will be specified by in the Connection Agreement. ” (Engineering Recommendation G99 Issue 2 — Requirements for the connection of generation equipment in parallel with public distribution networks on or after 27 April 2019, accessed 2026-07-28) | ||
| RoCoF withstand capability RoCoF withstand | Stay connected and operate at rates of change of frequency up to 1 Hz/s, measured over a 500 ms period Binds Type C & D Power Generating Modules (G99 Section 13) staying connected to the Distribution Network; exceptions only for RoCoF-type loss-of-mains protection operation or the module's own protection acting on a co-incident internal fault. The ENA copy of G99 is access-controlled, so the linked PDF is the publicly hosted dcode.org.uk mirror of the identical Issue 2 (10 Mar 2025) document; issue currency was confirmed against the issuer’s published issue register. | 13.2.2 |
The clause this row cites13.2.2 — “ With regard to the rate of change of frequency withstand capability, a Power Generating Module shall be capable of staying connected to the Distribution Network and operate at rates of change of frequency up to 1 Hzs-1 as measured over a period of 500 ms unless disconnection was triggered by a rate of change of frequency type loss of mains protection or by the Power Generating Module’s own protection system for a co-incident internal fault as detailed in paragraph 10.6.18. ” (Engineering Recommendation G99 Issue 2 — Requirements for the connection of generation equipment in parallel with public distribution networks on or after 27 April 2019, accessed 2026-07-28) | ||
| Fast Fault Current: reactive current rising with retained-voltage fall below 0.9 pu, reactive priority Fault-current injection | For Transmission System faults (clearing within 140 ms, seen as a voltage depression), each Power Park Module shall inject reactive current IR not less than its pre-fault reactive current, increasing as a minimum with the fall in retained voltage each time Connection Point retained voltage falls below 0.9 pu, within the module's overall rating (Figure 13.16 locus). Injection over time must lie above the Figure 13.17 envelope, with priority given to reactive current injection and any residual capability supplied as active current. The module is not required to exceed its transient or steady-state rating. Binds Power Park Modules (inverter-connected plant, hence BESS) in the Type C & D section; storage binds via G99 §2.2 (storage commissioned on/after 01-Sep-2022 complies in full). §13.6.1 (printed p.182) frames scope: designed for Transmission System faults clearing within 140 ms, not Distribution Network faults, and voltage/current quantities are positive-phase-sequence values - i.e. no negative-sequence injection requirement is stated. Per Figure 13.16 (printed p.183, figure reading): IR reaches 1.0 pu at 0.5 pu retained voltage and 'Operation is not required beyond 1.0pu'. Per Figure 13.17(a) axis annotations (figure reading): the envelope steps from IPrefault, reaching 0.65 x delta-IR + IPrefault at 60 ms from fault inception, for voltage depressions of ≤140 ms duration. Source copy: the ENA's own document-catalogue copy of EREC G99 is access-controlled, so the linked PDF is the publicly hosted dcode.org.uk mirror of the identical Issue 2 (10 Mar 2025) document; issue currency was confirmed against the issuer's own published issue register. | EREC G99 Issue 2 (2025), clause 13.6.2(b) and Figure 13.16, printed page 183 + EREC G99 Issue 2 (2025), clause 13.6.2(c), printed pages 183-184 |
The clauses this row citesEREC G99 Issue 2 (2025), clause 13.6.2(b) and Figure 13.16, printed page 183 — “ Figure 13.16 defines the reactive current IR that is to be supplied during a fault on the Transmission System and which is dependent on the pre-fault operating conditions, and the voltage retained at the Connection Point. Each Power Park Module shall inject a reactive current IR which shall not be less than its pre-fault reactive current and which shall as a minimum increase with the fall in retained voltage each time the retained voltage at the Connection Point falls below 0.9 pu, whilst ensuring that the overall rating of the Power Park Module is not exceeded ” (ENA Engineering Recommendation G99 Issue 2 (2025), accessed 2026-08-08) EREC G99 Issue 2 (2025), clause 13.6.2(c), printed pages 183-184 — “ In addition each Power Park Module shall be required to satisfy the reactive current requirements shown in Figures 13.17 which shows how the reactive current should be injected over time from the fault inception. The injected current shall be above the shaded area shown in Figure 13.17 (a) or Figure 13.17 (b) with priority being given to reactive current injection with any residual capability being supplied as active current. Under any fault condition, where the voltage falls below 0.9 pu, there would be no requirement for any Power Park Module or constituent Generating Unit to exceed its transient or steady state rating. ” (ENA Engineering Recommendation G99 Issue 2 (2025), accessed 2026-08-08) | ||
Who does UK G99 (Type D) bind, and since when?
Type D power park modules connected at or above 110 kV (Type D = ≥50 MW and/or ≥110 kV). No date of effect is published here — grid codes are reissued, so check the current issue before relying on it.
These figures are a reading aid, not legal advice. Grid codes are reissued: verify against the current edition before you design to them.
Curve data last checked against the code on — 66 standards, 130 envelopes, published by bess.engineer under CC BY 4.0.
Questions this page answers about UK G99 (Type D)
- Does UK G99 (Type D) require ride-through, or only that the plant does not trip?
- Ride-through. Every envelope on this page is a performance duty the plant must meet, not a protection-setting no-trip boundary — they are different obligations. That is what these curves are; they are not everything UK G99 (Type D) contains. The requirement sentence under each chart says which side of the curve is the compliant one.
- Are the numbers on this page taken from UK G99 (Type D) itself?
- Every breakpoint is read from the code, and where a point was taken off a published figure rather than stated in the clause, the section under that chart says so — on this page that includes an envelope that stops where its published chart stops. The clause each curve came from is quoted under it.
- Which events are charted for UK G99 (Type D)?
- UK G99 (Type D) is charted here with 2 envelopes — low-voltage ride-through, frequency ride-through (LVRT, FRT). No high-voltage envelope is charted here — check UK G99 (Type D) itself before concluding it sets none.
Ride-through, in context.
An envelope is a compliance boundary; understanding why it exists is a different question. The ride-through and grid-forming entries cover the engineering, and the Engineering Foundations course builds it from the physics up.
Also for the United Kingdom: UK G99 (Type A-C)
Elsewhere in Europe: Switzerland (Swissgrid CESS minimum), Switzerland (Swissgrid TC-CH 2019), Turkey (TEİAŞ Şebeke Yönetmeliği Ek-18), Austria (E-Control TOR Typ D), Belgium (Federal Grid Code), and 13 more