California Rule 21 ride-through requirements

Low-voltage ride-through, High-voltage ride-through, Frequency ride-through for the United States. Every breakpoint below is read from the code itself, and carries the clause it came from — so you can check it, not just cite it.

3Envelopes
LVRT · HVRT · FRTEvents
Distribution Connection level
LVRT

What is the low-voltage ride-through envelope for California Rule 21?

The plant must not disconnect for any voltage that stays on or above this envelope. The envelope holds 0 pu from t = 0 to 1 s; then 0.5 pu from 1 s to 10 s; then 0.7 pu from 10 s to 20 s; then 0.88 pu from 20 s onward — the last band the envelope defines, with no stated end time.

The clause defines part of this envelope as a momentary-cessation region, where output ceases but the plant does not disconnect; elsewhere inside it the clause requires mandatory operation. Which region a point falls in is in the clause below, not in the curve. The last band has no stated end time. The clause below states a continuous region — highlighted.

Applies to Smart-inverter DER at the distribution PCC (PG&E tariff)

California Rule 21 — low-voltage ride-through envelopeCalifornia Rule 21 low-voltage ride-through envelope. The plant must not disconnect for any voltage that stays on or above the line; below it, the envelope no longer applies. Logarithmic time axis from 0.1 s to 100 s; linear voltage axis in per-unit. The first band applies from the instant of the event (t = 0). The envelope holds 0 pu from t = 0 to 1 s; then 0.5 pu from 1 s to 10 s; then 0.7 pu from 10 s to 20 s; then 0.88 pu from 20 s onward — the last band the envelope defines, with no stated end time (drawn as an open dashed tail).California Rule 21 — low-voltage ride-through envelopenominal 1.00 pu0.11101000.000.200.400.600.801.001.20Time from event start (s, log scale) · first band applies from t = 0Voltage (pu)0.88 puMUST NOT DISCONNECTmust not disconnect on or above the lineopen — no stated end time
Low-voltage ride-through envelope for California Rule 21. Time runs on a logarithmic axis. Scroll the chart sideways for the rest of the time axis →
Low-voltage ride-through breakpoints for California Rule 21
From To Voltage Between the points
0 1 s 0 pu Held flat
1 s 10 s 0.5 pu Held flat
10 s 20 s 0.7 pu Held flat
20 s no stated end 0.88 pu Held flat
The clause this came from

PG&E Electric Rule 21, Sheet 193, Table Hh-1b 'Smart Inverter Voltage Ride-through Settings' (V>1.20 cease to energize; 1.10<V<=1.20 momentary cessation 12 s; 0.88<=V<=1.10 continuous; 0.70<=V<0.88 mandatory 20 s; 0.50<=V<0.70 mandatory 10 s; V<0.50 momentary cessation 1 s) and Table Hh-1a shall-trip settings (UV1 0.88 / 21.0 s; UV2 0.50 / 2.0 s), Advice 7692-E effective August 29, 2025, Corroborated by the legacy SDG&E Rule 21 Sheet 122 Table Hh-1 (Advice 2860-E, effective July 5, 2016): LV3 V<50% ride-through until 1 sec; LV2 50-70% until 10 sec; LV1 70-88% until 20 sec; NN 88-110% indefinite.

HVRT

What is the high-voltage ride-through envelope for California Rule 21?

The plant must not disconnect for any voltage that stays on or below this envelope. The envelope holds 1.2 pu from t = 0 to 12 s; then 1.1 pu from 12 s onward — the last band the envelope defines, with no stated end time.

The clause defines part of this envelope as a momentary-cessation region, where output ceases but the plant does not disconnect. Which region a point falls in is in the clause below, not in the curve. Every breakpoint is stated in the clause cited below.

Applies to Smart-inverter DER at the distribution PCC (PG&E tariff)

California Rule 21 — high-voltage ride-through envelopeCalifornia Rule 21 high-voltage ride-through envelope. The plant must not disconnect for any voltage that stays on or below the line; above it, the envelope no longer applies. Logarithmic time axis from 1 s to 50 s; linear voltage axis in per-unit. The first band applies from the instant of the event (t = 0). The envelope holds 1.2 pu from t = 0 to 12 s; then 1.1 pu from 12 s onward — the last band the envelope defines, with no stated end time (drawn as an open dashed tail).California Rule 21 — high-voltage ride-through envelopenominal 1.00 pu11025200.951.001.051.101.151.201.25Time from event start (s, log scale) · first band applies from t = 0Voltage (pu)1.1 puMUST NOT DISCONNECTmust not disconnect on or below the lineopen — no stated end time
High-voltage ride-through envelope for California Rule 21. Time runs on a logarithmic axis. Scroll the chart sideways for the rest of the time axis →
High-voltage ride-through breakpoints for California Rule 21
From To Voltage Between the points
0 12 s 1.2 pu Held flat
12 s no stated end 1.1 pu Held flat
The clause this came from

PG&E Electric Rule 21, Sheet 193, Table Hh-1b: '1.10 < V <= 1.20 | Momentary Cessation | 12 | 0.083' and 'V > 1.20 | Cease to Energize | NA | 0.16'; Table Hh-1a OV2 1.20 / 0.16 s, OV1 1.10 / 13.0 s. Advice 7692-E effective August 29, 2025, Corroborated by legacy SDG&E Rule 21 Sheet 122 Table Hh-1, HV1 110<V<120, 'Ride-Through Until 12 sec', Momentary Cessation, Maximum Trip Time 13 sec.

FRT

What is the frequency ride-through envelope for California Rule 21?

The plant must ride through between the upper and lower bands. The upper (over-frequency) band holds 61.8 Hz from t = 0 to 299 s; then 61.2 Hz from 299 s onward — the last band the envelope defines, with no stated end time. The lower (under-frequency) band holds 57 Hz from t = 0 to 299 s; then 58.8 Hz from 299 s 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 Smart-inverter DER at the distribution PCC (PG&E tariff)

California Rule 21 — frequency ride-through envelopeCalifornia Rule 21 frequency ride-through envelope. The plant must ride through for any frequency that stays between the lower and upper bands. Logarithmic time axis from 10 s to 1000 s; linear frequency axis in hertz. The first band applies from the instant of the event (t = 0). upper (over-frequency) band: holds 61.8 Hz from t = 0 to 299 s; then 61.2 Hz from 299 s onward — the last band the envelope defines, with no stated end time (drawn as an open dashed tail). lower (under-frequency) band: holds 57 Hz from t = 0 to 299 s; then 58.8 Hz from 299 s onward — the last band the envelope defines, with no stated end time (drawn as an open dashed tail).California Rule 21 — frequency ride-through envelopenominal 60 Hz10100100020502005005657585960616263Time from event start (s, log scale) · first band applies from t = 0Frequency (Hz)61.2 Hz58.8 HzMUST RIDE THROUGHUpper band (over-frequency)Lower band (under-frequency)must ride through between the bandsopen — no stated end time
Frequency ride-through envelope for California Rule 21. Time runs on a logarithmic axis. Scroll the chart sideways for the rest of the time axis →

Upper limit

Frequency ride-through breakpoints for California Rule 21 — Upper limit
From To Frequency Between the points
0 299 s 61.8 Hz Held flat
299 s no stated end 61.2 Hz Held flat

Lower limit

Frequency ride-through breakpoints for California Rule 21 — Lower limit
From To Frequency Between the points
0 299 s 57 Hz Held flat
299 s no stated end 58.8 Hz Held flat
The clause this came from

PG&E Electric Rule 21, Sheet 196, Table Hh-2b 'Frequency Ride-Through Settings Table' (f>62.0 no RT; 61.2<f<=61.8 Mandatory Operation 299 s; 58.8<=f<=61.2 Continuous Infinite; 57.0<=f<58.8 Mandatory Operation 299 s; F<57.0 no RT), section Hh 'Smart Inverter Generating Facility Design and Operating Requirements for UL1741SB Inverters', Advice 7692-E effective August 29, 2025, Superseded values from SDG&E Rule 21 Sheet 124 Table Hh-2 (Advice 2860-E, effective July 5, 2016): f>62 no ride-through 0.16 s trip; 60.5<f<62 mandatory 299 s; 58.5<f<60.5 continuous; 57.0<f<58.5 mandatory 299 s; f<57.0 no ride-through 0.16 s trip. Cross-checked against IEEE 1547-2018 Table 18 shall-trip defaults (OF2 62.0/0.16 s, OF1 61.2/300 s, UF1 58.5/300 s, UF2 56.5/0.16 s) as reproduced in NERC Reliability Guideline 'BPS Reliability Perspectives on the Adoption of IEEE 1547-2018', March 2023, Table 1.3.

Beyond ride-through

Operating requirements beyond the envelopes.

The envelopes above are what California Rule 21 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. A row marked secondary source was read from an authoritative summary, not the standard itself, and is labelled so it cannot pass as clause-stated text.

Supplementary operating requirements for California Rule 21
Requirement What the code states Clause
Generating Facility power-factor range Reactive capability Power factor near unity at rated output, or a utility-specified power factor within 0.9 leading to 0.9 lagging (PF cone) Binds all Generating Facilities (storage included) interconnecting to the distribution system under Rule 21 Section H design and operating requirements; the specific setpoint inside the 0.9 lead/lag cone is set by the Distribution Provider per local system conditions. Section H.2.i
The clause this row cites

Section H.2.i — “ Producer shall provide adequate reactive power compensation on site to maintain the Generating Facility power factor near unity at rated output or a Distribution Provider specified power factor within a power factor range from 0.9 leading to 0.9 lagging, based on local system conditions. ” (Electric Rule No. 21 — Generating Facility Interconnections (PG&E tariff book), accessed 2026-07-28)

Smart inverter reactive capability category Reactive capability IEEE 1547-2018 Clause 5.2 Category B reactive capability (Rule 21 states the category, not the percentage figures) Binds UL1741SB Smart Inverter Generating Facilities; the companion default volt-var curve (Table Hh-3, Section Hh.2.j) is voltage-dependent: 30 % reactive injection at 92.0 % voltage to 30 % absorption at 107.0 % voltage, unity between 97-103 %. Section Hh.2.i
The clause this row cites

Section Hh.2.i — “ Smart Inverter Reactive Power capabilities shall comply with IEEE 1547-2018, Section 5.2 Category B requirement. ” (Electric Rule No. 21 — Generating Facility Interconnections (PG&E tariff book), accessed 2026-07-28)

Frequency droop (frequency-watt) defaults Frequency response Droop coefficient kOF = kUF = 0.05 (5 %); deadband 36 mHz (dbOF and dbUF); open-loop response time 5 s Binds UL1741SB-certified Smart Inverter Generating Facilities (including storage inverters) interconnecting under Rule 21, implementing IEEE 1547-2018 clause 6.5.2.7 frequency-droop with these California default settings. Section Hh.2.l
The clause this row cites

Section Hh.2.l — “ Smart Inverters shall change their real power production as function of system frequency in accordance with IEEE 1547-2018, 6.5.2.7 with the following default settings: Deadband 36 mHz, dbOF and dbUF. kOF and kUF would be 0.05, open loop response time of 5 seconds. ” (Electric Rule No. 21 — Generating Facility Interconnections (PG&E tariff book), accessed 2026-07-28)

Continuous voltage operating region 0.88–1.10 pu Continuous operating range 0.88 pu ≤ V ≤ 1.10 pu of nominal voltage: 'Continuous Operation' operating mode with minimum ride-through time 'Infinite' (Rule 21 Table Hh-1b); outside 0.88–1.10 pu the trip settings of Table Hh-1a apply (UV1 0.88 pu / 21.0 s, OV1 1.10 pu / 13.0 s defaults) Binds Smart Inverter Generating Facilities interconnecting under CPUC Electric Rule 21 (PG&E tariff), Section Hh (UL 1741 SB inverters); Voltage evaluated at the Reference Point of Applicability (RPA) per Hh.2.b.ii. Storage applicability: binds storage explicitly through the generation-scoped chain — Section Hh applies to all Smart Inverter Generating Facilities and contains storage-inverter-specific provisions (e.g., Hh.2.s 'Storage Inverter Normal Operation Ramp Control Requirements'), so BESS smart inverters are covered. The Table Hh-1b values coincide with the IEEE 1547-2018 abnormal-performance Category III continuous operation region (0.88–1.10 pu); Rule 21 expressly assigns Category III elsewhere in the same section (ROCOF 'as specified in IEEE 1547-2018, section 6.5.2.5 category III'). Scope caveats: Section B.5 limits the IEEE 1547-2018 regime to POI below 50 kV, and Hh.2.p exempts qualifying load-shedding facilities from Hh.2.b.ii and Hh.2.f.i by name. Storage is in scope directly via the Section C definition: 'Generating Facility: … for the purpose of producing electric power, including storage.' The IEEE 1547-2018 Category III attribution is evidenced by the Hh-1a defaults (OV2 1.20/0.16, OV1 1.10/13.0, UV1 0.88/21.0, UV2 0.50/2.0), which match Table 13 Cat III exactly; note the 0.88–1.10 pu continuous region is identical across Cat I/II/III and is therefore not Cat-III-distinguishing. PG&E Electric Rule 21, Section Hh.2.b.ii, Table Hh-1b 'Smart Inverter Voltage Ride-through Settings', Sheet 193 (Cal. P.U.C. Sheet No. 60223-E, Advice 7692-E, effective 29 Aug 2025) + NERC Reliability Guideline 'Bulk Power System Reliability Perspectives on the Adoption of IEEE Std 1547-2018' (March 2023), Chapter 2, discussion of IEEE 1547-2018 Clause 6.4.2 voltage disturbance ride-through
The clauses this row cites

PG&E Electric Rule 21, Section Hh.2.b.ii, Table Hh-1b 'Smart Inverter Voltage Ride-through Settings', Sheet 193 (Cal. P.U.C. Sheet No. 60223-E, Advice 7692-E, effective 29 Aug 2025) — “ Table Hh-1b – Smart Inverter Voltage Ride-through Settings — Voltage Range (p.u.): 0.88 ≤ V ≤ 1.10 | Operating mode/response: Continuous Operation | Minimum ride-through time (s): Infinite | Maximum response time (s): NA ” (PG&E Electric Rule 21 — Generating Facility Interconnections, accessed 2026-08-09)

NERC Reliability Guideline 'Bulk Power System Reliability Perspectives on the Adoption of IEEE Std 1547-2018' (March 2023), Chapter 2, discussion of IEEE 1547-2018 Clause 6.4.2 voltage disturbance ride-through — “ For voltage perturbations within the continuous operation region of the ride-through curves, DERs must remain in operation and continue delivering available active power of magnitude at least as great as its predisturbance level ” (NERC Reliability Guideline: Bulk Power System Reliability Perspectives on the Adoption of IEEE Std 1547-2018 (March 2023), accessed 2026-08-09)

Continuous frequency operating region 58.8–61.2 Hz Continuous operating range 58.8 Hz ≤ f ≤ 61.2 Hz (60 Hz nominal): 'Continuous Operation' with minimum time 'Infinite' (Rule 21 Table Hh-2b); 61.2<f≤61.8 Hz and 57.0≤f<58.8 Hz are Mandatory Operation for 299 s; no ride-through requirements above 62.0 Hz or below 57.0 Hz Binds Smart Inverter Generating Facilities under Rule 21 Section Hh.2.f.i; grid frequency at the PCC/RPA. Storage applicability: binds storage explicitly through the generation-scoped chain — Section Hh applies to all Smart Inverter Generating Facilities including storage inverters (Hh.2.s names storage inverters). The Rule 21 continuous band matches the IEEE 1547-2018 continuous operation region reproduced by NERC (58.8–61.2 Hz, subject to V/f ≤ 1.1). Companion trip table Hh-2a (same sheet) sets OF2 62.0 Hz/0.16 s, OF1 61.2 Hz/300 s, UF1 58.5 Hz/300 s, UF2 56.5 Hz/0.16 s defaults. Scope caveats: Section B.5 limits the IEEE 1547-2018 regime to POI below 50 kV, and Hh.2.p exempts qualifying load-shedding facilities from Hh.2.b.ii and Hh.2.f.i by name. Storage is in scope directly via the Section C definition: 'Generating Facility: … for the purpose of producing electric power, including storage.' The IEEE 1547-2018 Category III attribution is evidenced by the Hh-1a defaults (OV2 1.20/0.16, OV1 1.10/13.0, UV1 0.88/21.0, UV2 0.50/2.0), which match Table 13 Cat III exactly; note the 0.88–1.10 pu continuous region is identical across Cat I/II/III and is therefore not Cat-III-distinguishing. PG&E Electric Rule 21, Section Hh.2.f.i 'Frequency Ride-Through Requirements', Table Hh-2b 'Frequency Ride-Through Settings Table', Sheet 196 (Cal. P.U.C. Sheet No. 60226-E, Advice 7692-E, effective 29 Aug 2025) + NERC Reliability Guideline (March 2023), Chapter 2, 'Clause 6.5.2: Frequency Disturbance Ride-Through Requirements' (reprinting the IEEE 1547-2018 Table 19 continuous-operation requirement)
The clauses this row cites

PG&E Electric Rule 21, Section Hh.2.f.i 'Frequency Ride-Through Requirements', Table Hh-2b 'Frequency Ride-Through Settings Table', Sheet 196 (Cal. P.U.C. Sheet No. 60226-E, Advice 7692-E, effective 29 Aug 2025) — “ Table Hh-2b: Frequency Ride-Through Settings Table — 58.8≤f≤61.2 | Continuous Operation | Infinite ” (PG&E Electric Rule 21 — Generating Facility Interconnections, accessed 2026-08-09)

NERC Reliability Guideline (March 2023), Chapter 2, 'Clause 6.5.2: Frequency Disturbance Ride-Through Requirements' (reprinting the IEEE 1547-2018 Table 19 continuous-operation requirement) — “ Frequency disturbances of any duration for which system frequency remains between 58.8 Hz and 61.2 Hz and the per-unit ratio of voltage to frequency is less than or equal to 1.1 V/Hz shall not cause the DER to trip. The DER shall remain in operation during any such disturbance and shall be able to continue to exchange active power at least as great as its predisturbance level of power. ” (NERC Reliability Guideline: Bulk Power System Reliability Perspectives on the Adoption of IEEE Std 1547-2018 (March 2023), accessed 2026-08-09)

Storage inverter normal-operation ramp control Ramp rate Optional ramp-up rate control; default 100 % of maximum current output per second (or slower if the Applicant requires); other settings by mutual agreement Storage-specific clause for UL1741SB smart inverters; per Table Hh-4 (Section Hh.2.o) this 'Storage Inverter Normal Operation Ramp Control' function is Deactivated by default and may be activated by mutual agreement. Section Hh.2.s
The clause this row cites

Section Hh.2.s — “ Smart Inverters used for energy storage applications may optionally include ramp-up rate control. The default value shall be 100% of maximum current output per second or slower if required by Applicant. Other ramp-up control settings can be used, when required, as mutually agreed by the Distribution Provider and the Applicant. ” (Electric Rule No. 21 — Generating Facility Interconnections (PG&E tariff book), accessed 2026-07-28)

Enter-service ramp defaults Ramp rate Enter-service delay 15 s; default enter-service (ramp) duration 50 s, per IEEE 1547-2018 clause 4.10.3 Binds UL1741SB Smart Inverter Generating Facilities when entering or returning to service; this is the enter-service ramp, distinct from the normal-operation ramp control in Hh.2.s. Section Hh.2.k
The clause this row cites

Section Hh.2.k — “ Enter Service ramp control requirements as outlined in IEEE 1547-2018 section 4.10.3 with following default settings: o Delay enter service shall be 15 seconds per Hh.1.a.ii o Default Enter Service Duration shall be 50 seconds ” (Electric Rule No. 21 — Generating Facility Interconnections (PG&E tariff book), accessed 2026-07-28)

ROCOF ride-through RoCoF withstand No trip for |ROCOF| <= 3.0 Hz/s (IEEE 1547-2018 clause 6.5.2.5 Category III); ride-through preferred above 3 Hz/s while frequency stays inside the ride-through regions; no averaging window stated in the tariff text Binds UL1741SB Smart Inverter Generating Facilities under Rule 21 (Frequency, sub-item ii on Sheet 196); Rule 21 adopts the IEEE 1547-2018 Category III ROCOF withstand. Section Hh.2.f.ii
The clause this row cites

Section Hh.2.f.ii — “ Smart Inverter shall not trip for frequency excursion having magnitude rates of change of frequency (ROCOF) that is less than or equal to 3.0Hz per second as specified in IEEE 1547-2018, section 6.5.2.5 category III. For ROCOF greater than 3Hz per second, it is preferred for Smart Inverter to ride-through as long as frequency remains in the continuous operating region, low frequency ride-though region(and corresponding duration times), or high frequency region (and corresponding duration times). ” (Electric Rule No. 21 — Generating Facility Interconnections (PG&E tariff book), accessed 2026-07-28)

Dynamic voltage support: not mandated (optional under IEEE 1547-2018) Fault-current injection No mandatory dynamic reactive-current injection during faults as of Advice 7692-E (29 Aug 2025). Rule 21 Hh.2.n makes the Dynamic Reactive Power Support Function mandatory only for Interconnection Requests submitted twelve months after a nationally recognized standard adopts the function, and Table Hh-4 lists Dynamic Reactive Power Support Mode as Optional / activated under mutual agreement; the only dynamic-voltage-support text, Hh.2.u, invokes IEEE 1547-2018 6.4.2.7.1 (restore output without dynamic voltage support), and IEEE 1547-2018 6.4.2.6 leaves dynamic voltage support to mutual agreement. What Rule 21 does require during voltage dips is Mandatory Operation at maximum available current for 0.50 ≤ V < 0.88 pu (20 s / 10 s) and Momentary Cessation to below 10 % current within 0.083 s below 0.50 pu (Table Hh-1b); reactive-power capability is IEEE 1547-2018 Section 5.2 Category B (Hh.2.i). No k-factor, reactive-current gain, response time or negative-sequence injection requirement appears anywhere in Rule 21 ('reactive current' has zero occurrences). Applies to Smart Inverter Generating Facilities under Rule 21 Section Hh, including storage inverters (storage applicability: explicit via Section Hh scope and Hh.2.s storage-inverter provision). The modal is the claim: dynamic voltage support under the incorporated IEEE 1547-2018 framework is 'may / by mutual agreement' between the area EPS operator and the DER operator, not 'shall'. Searched the full Rule 21 tariff text: 'dynamic voltage support' appears only in the Hh table of contents and Hh.2.u; 'reactive current' has zero occurrences — no fault reactive-current gain (k-factor), response time, or negative-sequence requirement exists anywhere in Rule 21. Scope of the absence: established by full-text search of the live 290-page tariff for 'dynamic voltage support' (3 hits, all Hh.2.u/toc), 'dynamic reactive' (5 hits: the Hh contents, the Smart Inverter definition, Hh.2.j Dynamic Volt/Var Operations — a steady-state Volt/Var duty, not fault injection — Hh.2.n and Table Hh-4), 'reactive current' and 'reactive-current' (0), 'current injection' (harmonic and DC only), 'negative sequence' (an X2 data field and two test steps), plus a clause-by-clause read of Hh.2 (a)–(w). Rule 21's only fault-current provisions are limits, not duties (Screen F, short-circuit contribution ≤ 1.2 pu). Residual scope caveat: Rule 21 delegates settings outward in Hh.2.o (Distribution Provider's Interconnection Handbook) and Section B.5 (POI ≥ 50 kV), so this is 'Rule 21 does not mandate…', not 'California does not'. Marked secondary because the load-bearing IEEE 1547-2018 clause 6.4.2.6 content is quoted via the NERC reliability guideline — an officially published secondary source (the IEEE text itself is paywalled); the Rule 21 quote is primary. PG&E Electric Rule 21, Section Hh.2.u 'Restore output without dynamic voltage support', Sheet 204 (Cal. P.U.C. Sheet No. 60234-E, Advice 7692-E, effective 29 Aug 2025) + NERC Reliability Guideline (March 2023), Chapter 2, discussion of IEEE 1547-2018 Clause 6.4.2.6 (dynamic voltage support) secondary source
The clauses this row cites

PG&E Electric Rule 21, Section Hh.2.u 'Restore output without dynamic voltage support', Sheet 204 (Cal. P.U.C. Sheet No. 60234-E, Advice 7692-E, effective 29 Aug 2025) — “ Restore output without dynamic voltage support shall be in accordance with IEEE 1547-2018, 6.4.2.7.1. ” (PG&E Electric Rule 21 — Generating Facility Interconnections, accessed 2026-08-09)

NERC Reliability Guideline (March 2023), Chapter 2, discussion of IEEE 1547-2018 Clause 6.4.2.6 (dynamic voltage support) — “ Clause 6.4.2.6 allows for dynamic voltage support from DERs by mutual agreement coordinated between the area EPS operator and the DER owner. Original equipment manufacturers are expected to implement any dynamic voltage support setting differently (e.g., reactive-current injection, P or Q priority) and IEEE 1547-2018 does not require any particular capability. ” (NERC Reliability Guideline: Bulk Power System Reliability Perspectives on the Adoption of IEEE Std 1547-2018 (March 2023), accessed 2026-08-09)

Scope

Who does California Rule 21 bind, and since when?

Smart-inverter DER at the distribution PCC (PG&E tariff). The LVRT clause quoted above carries the date of effect — "effective August 29, 2025".

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

Questions this page answers about California Rule 21

Does California Rule 21 require ride-through, or only that the plant does not trip?
Ride-through, with one qualification. On its LVRT and HVRT envelopes the clause defines part of the region as momentary cessation, where output ceases but the plant does not disconnect; on its LVRT envelope the clause requires mandatory operation elsewhere inside the region. Which region a point falls in is in the clause quoted under those charts, not in the curve.
Are the numbers on this page taken from California Rule 21 itself?
Yes. Every breakpoint is read from the code, and each envelope carries the clause it came from, quoted in full under its chart — so you can check the figure against the standard rather than cite it from this page.
Which events are charted for California Rule 21?
California Rule 21 is charted here with 3 envelopes — low-voltage ride-through, high-voltage ride-through, frequency ride-through (LVRT, HVRT, FRT).
Related

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 States: ERCOT (NOG 2.6.2.1), ERCOT (NOG 2.9.1.2 legacy), ERCOT (NOG 2.9.1), IEEE 1547-2018 Cat III, IEEE 2800-2022, NERC PRC-024-4 (ERCOT), and 5 more

All 66 grid codes Open the chart