Page type: primary-anchored (mirrors Grid Code Connection Conditions, CC)
Grid Code -- Connection Conditions (CC)
The Connection Conditions are the section of the Grid Code that specifies the minimum technical, design and operational criteria that must be met by every User connected to or seeking connection with the National Electricity Transmission System. CC.6 is the heart of the section: it sets the performance standards that generators must meet and the system characteristics that NESO must maintain.
Source file: grid-code.md Parent page: Grid Code
Scope (CC.3)
The CC applies to NESO and to: - Generators (except those with only Embedded Small Power Stations) - Network Operators - Non-Embedded Customers - DC Converter Station owners - BM Participants and Externally Interconnected System Operators (for CC.6.5 Operational Metering only)
CC.6.1 -- System performance characteristics NESO must maintain
These are the parameters NESO guarantees (subject to plant availability and User compliance):
Frequency (CC.6.1.2--CC.6.1.3)
| Parameter | Value | Clause |
|---|---|---|
| Nominal frequency | 50Hz | CC.6.1.2 |
| Normal operating range | 49.5--50.5Hz | CC.6.1.2 |
| Extreme range (exceptional) | 47--52Hz | CC.6.1.3 |
User withstand obligations (CC.6.1.3): all Users' plant must be designed to operate across the full 47--52Hz range with the following time limits:
| Frequency band | Minimum operation time |
|---|---|
| 51.5--52Hz | 15 minutes per occurrence |
| 51--51.5Hz | 90 minutes per occurrence |
| 49--51Hz | Continuous |
| 47.5--49Hz | 90 minutes per occurrence |
| 47--47.5Hz | 20 seconds per occurrence |
Frequency/speed-based relay disconnection is not permitted in the range 47.5--51.5Hz unless agreed with NESO under CC.6.3.12.
Voltage (CC.6.1.4)
| Voltage level | Normal operating range | Notes |
|---|---|---|
| 400kV | +/-5% of nominal | Max +10%, min -10%; +5% to +10% limited to 15 minutes |
| 275kV | +/-10% of nominal | |
| 132kV | +/-10% of nominal | |
| Below 132kV | +/-6% of nominal |
Under fault conditions, voltage may collapse transiently to zero at the point of fault.
Power quality (CC.6.1.5--CC.6.1.8)
- Harmonics: must comply with Engineering Recommendation G5/4 (CC.6.1.5(a))
- Phase unbalance (weekly 95th percentile): below 1.5% for E&W above 150kV; below 2% for Scotland above 150kV; below 2% for all GB at 150kV and below (CC.6.1.5(b))
- Rapid voltage changes: limited by Table CC.6.1.7 (3 categories based on magnitude and frequency of occurrence)
- Sub-synchronous oscillation: NESO must ensure Users' plant is not subjected to unacceptable conditions (CC.6.1.9)
CC.6.2 -- Plant and apparatus at the connection site
Protection requirements for generators (CC.6.2.2)
| Requirement | Specification | Clause |
|---|---|---|
| Fault clearance time at 400kV | >= 80ms (minimum) | CC.6.2.2.2(a)(i) |
| Fault clearance time at 275kV | >= 100ms | CC.6.2.2.2(a)(ii) |
| Fault clearance time at 132kV and below | >= 120ms | CC.6.2.2.2(a)(iii) |
| Back-up protection clearance | 300ms (new plant, 400/275kV, 2 main protections) | CC.6.2.2.2(b) |
| System Fault Dependability Index | >= 99% | CC.6.2.2.2(d) |
| Loss of excitation detection | Must detect and initiate unit trip | CC.6.2.2.3.3 |
| Work on protection equipment | NESO representative required | CC.6.2.2.4 |
| Quality assurance | BS EN ISO 9000 series | CC.6.2.1.2(d) |
Protection requirements for network operators / demand (CC.6.2.3)
Parallel structure to CC.6.2.2 with identical fault clearance time thresholds. Back-up protection obligations also apply.
CC.6.3 -- Generator performance requirements
This is the section most relevant to anyone developing, connecting, or operating a generator in GB. It applies to all Generating Units, DC Converters, and Power Park Modules (but not Small Power Stations or individual Power Park Units).
Reactive power capability (CC.6.3.2)
| Plant type | Requirement | Reference point | Clause |
|---|---|---|---|
| Synchronous generators (onshore) | 0.85 lagging to 0.95 leading at Rated MW; enhanced 0.9 lag where CEC increased above Rated MW post 1 May 2009 | Unit terminals | CC.6.3.2(a) |
| Non-synchronous / PPMs (onshore) | Zero reactive transfer at all output levels; tolerance +/-5% Rated MW in MVAr | Grid Entry Point | CC.6.3.2(b) |
| Non-synchronous / PPMs (onshore, post 1 Jan 2006) | 0.95 lagging to 0.95 leading at Rated MW; P-Q envelope per Figure 1 | Grid Entry Point | CC.6.3.2(c) |
Short circuit ratio (CC.6.3.2(a)): - Synchronous generators <1600MVA: >= 0.5 - Synchronous generators >=1600MVA: >= 0.4
Other generator obligations (CC.6.3.3--CC.6.3.16)
| Clause | Subject | Key requirement |
|---|---|---|
| CC.6.3.3 | Voltage control | AVR or equivalent required |
| CC.6.3.4 | Frequency response | Primary, secondary, high frequency response per Appendix 3 |
| CC.6.3.5 | Black Start | Where contracted via Bilateral Agreement |
| CC.6.3.6 | Operating range | Minimum stable generation, start/shutdown times |
| CC.6.3.7 | Fault ride through | Withstand voltage dips per Appendix 4 envelope |
| CC.6.3.8 | Power oscillation damping | Power system stabiliser requirements |
| CC.6.3.9 | Generator transformers | Specification requirements |
| CC.6.3.10 | Negative phase sequence | Withstand capability |
| CC.6.3.11 | Short circuit ratio | See above |
| CC.6.3.12 | RoCoF withstand | Rate of change of frequency ride-through |
| CC.6.3.13 | Control telephony | Communication requirements |
| CC.6.3.14 | Information requirements | Data provision to NESO |
| CC.6.3.15 | Frequency sensitive mode | FSM/LFSM operating modes |
| CC.6.3.16 | Active power during frequency events | Output constraints |
Appendices
- Appendix 3: Minimum Frequency Response Requirement Profile and Operating Range
- Appendix 4: Fault Ride Through Requirements (4A: general; 4B: specific envelopes)
- Appendix 5: Low Frequency Relay Technical Requirements
- Appendix 6: Excitation Control (Synchronous Generating Units)
- Appendix 7: Voltage Control (Non-Synchronous / PPMs / DC Converters)
CC.7 -- Site related conditions
Covers Operation Diagrams, Site Responsibility Schedules (Appendix 1), site naming conventions, and site common drawings.
CC.8 -- Ancillary services
Covers Mandatory Services Agreements and System Ancillary Services. The commercial terms for ancillary services are in the CUSC and individual contracts; CC.8 sets the technical framework.
European Connection Conditions (ECC)
The ECC mirrors the CC structure (ECC.1--ECC.8) but applies to EU Code Users -- generators connecting under the retained EU Regulation 2016/631 (Requirements for Generators). Key differences: - Generators classified as Type A/B/C/D by MW threshold - Additional Appendix 4EC: Fast Fault Current Injection Requirements - Additional Appendix E8: Voltage Control for Configuration 2 AC/DC connected offshore PPMs - References EU Regulations 2016/1388 (Demand Connection Code) and 2016/1447 (HVDC Code)
The ECC is 137 pages in Issue 5 R25. It is structurally identical to CC but with EU-specific additions.
Character positions
(None yet captured.)
Last updated: 2026-04-05