UK G99 (Type D) — ride-through requirements

The LVRT, FRT envelopes a grid-scale battery must meet under UK G99 (Type D), digitized breakpoint by breakpoint and checked against the clause it came from.

By Sergey Syrvachev — Director of Product Engineering at HyperStrong

Applies to: transmission connections — Type D power park modules connected at or above 110 kV (Type D = >=50 MW and/or >=110 kV).

Part of: the grid-code ride-through chart — compare this against 65 other standards in 44 countries.

What this is, and is not. A reading aid compiled from the clause cited below — not a compliance document. The primary text governs. Every value here was read from a current primary source.

Breakpoints

Times are seconds from the start of the disturbance. Voltage is per-unit of nominal; frequency in hertz. An LVRT curve is a lower boundary (stay connected while voltage is on or above it), an HVRT curve an upper boundary, and FRT gives the band between its High and Low boundaries. These are the boundaries, not the behaviour: what holding one demands of the converter — reactive current injection, thermal stress, protection timing — is stepped through in the LVRT / HVRT ride-through visual.

EventDutyBreakpointsStatus
low-voltage ride-through (LVRT)ride-through0.001s → 0 pu; 0.14s → 0 pu; ↗ 2.2s → 0.85 pu; 180s → 0.85 puverified
frequency ride-through (FRT)ride-throughHigh: 0.1s → 52 Hz; 900s → 52 Hz; 900.001s → 51.5 Hz; 5400s → 51.5 Hz; 5400.001s → 51 Hz | Low: 0.1s → 47 Hz; 20s → 47 Hz; 20.001s → 47.5 Hz; 5400s → 47.5 Hz; 5400.001s → 49 Hzverified

Where these values come from

  • LVRTENA 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.
  • FRTENA 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.

Any event not in the table above is not plotted for this code. Where an absence is itself the finding — a code that defines no high-voltage envelope, or one whose curves circulate under another country's name — it is recorded in the grid codes with no ride-through curve.

Last checked against the source document on .

How UK compares

Each event this code defines, put side by side with every other code that defines it:

Other grid codes in Europe