Summary
The Technical Brochure of JWG B1/A1/A2/A3.44 investigates operational limitations of high-voltage equipment caused by frequent power-frequency temporary overvoltages (TOV). Firstly, the network situations leading to short- and long-duration TOVs are systematically classified. It is shown that grid codes specify TOVs in a non-uniform manner and that long-duration TOVs (typically up to 1.05–1.10 × Um over seconds to minutes) are already observed in present transmission and distribution networks.
Using a typical overvoltage level UTOV = 1.05 × Um, the impact of recurrent TOV on insulation lifetime of different equipment is assessed. A continuous increase of operating voltage above maximal voltage for equipment Um can significantly reduce insulation life. Therefore, the introduction of higher continuous voltage levels is not recommended. For recurrent TOV, lifetime reduction must be evaluated from an asset-management perspective.
Apart from insulation ageing, the equipment-specific limitations related to the performed functions are analyzed. For specific equipment its relevant functions, especially in near-limit applications, are analyzed and the possible mitigating options are procedures are discussed. Moreover, the potential risks are named. For example, circuit breakers are able to switch nominal short-circuit currents at TOV instance, if the transient recovery voltages are lower as tested according to the standard, i.e. first-pole-to-clear factor is lower as 1.3 in reality. Furter example for cables, it has been identified that TOV does not lead to function limitations of HV cable.
The brochure concludes that many types of equipment can handle recurrent TOV with acceptable risk of equipment failure, but this cannot be assumed in general. Users must compare their specific TOV conditions (magnitude, duration, frequency) with tested limits and involve manufacturers in the assessment. These actions refer to HV equipment and users should also keep in mind how far the TOV may lead to instability of the system in general.
For standardisation, an update of TOV definitions in insulation coordination standards is recommended, clearly distinguishing between stresses covered by existing type tests and permanent or recurrent operation at TOV levels. In view of increasing short-circuit power and complex interactions of power-electronic-based devices, the need for technical guidance on the application of high-voltage equipment will grow further.
Table of content
1. Introduction
1.1. Motivation
1.2. Scope
2. Voltage Stresses
2.1. Introduction
2.2. Typical Network Conditions Resulting in Temporary Overvoltages
2.3. Review of some of the International Network Codes on Temporary Overvoltages
2.4. Review of the User Experience with TOV
3. Lifetime of the Equipment
3.1. Failure Probability
3.2. Testing Philosophy
3.3. Modelling of Insulation Ageing
3.4. General Lifetime Characteristics covered by Standard Tests
3.5. Conclusion
4. Current and Voltage Instrument Transformers
4.1. Introduction
4.2. Insulation Ageing
4.3. Measurement Performance
4.4. Short Circuit Currents
4.5. Ferro-resonance
4.6. Discharging of Cables and MSCDN by Inductive VTs
4.7. Summary
5. Generators in Power Plants
5.1. Typical Network Conditions
5.2. Major Differences in Design of Power Generation Units
5.3. Impacts of TOV on Ageing of Generators
5.4. Standards for Generators with a Link to the TOV
5.5. Summary
6. Switchgear
6.1. Circuit Breakers
6.2. Gas Insulated Substations
6.3. Disconnectors
6.4. Earthing Switches
7. Cable
7.1. General Considerations
7.2. HV Extruded Cable Ageing
7.3. Impact of TOV on Current Rating
7.4. Absorption of Energy Stored in Cable
7.5. Summary
8. Transformers
8.1. Introduction
8.2. Bushing
8.3. Windings
8.4. Core
8.5. Tap Changer
8.6. Cooling
8.7. Transformer Noise Emissions
8.8. Summary
9. Surge Arrester
9.1. Introduction
9.2. Considerations when Selecting the Appropriate Surge Arrester for a Particular Application
9.3. IEC and IEEE Surge Arresters
9.4. Surge Arrester Dimensioning/Sizing
10. Conclusion
10.1. Major Findings
10.2. Recommendation for the Users
10.3. Recommendation for the Standardisation
Appendix A. Definitions
Appendix B. References
Appendix C. User Examples of TOV
C.1. Introduction
C.2. TSO 1 - 400 kV Network Dominated by Renewable Generation
C.3. TSO 2 - Example for 63, 90, 245 and 400 kV Network
C.4. TSO 3 - Example for 400 kV Network
C.5. DSO 4 - Urban 110 kV Network
C.6. DSO 5 - Urban and land 110 kV Network
Appendix D. CIGRE Survey
D.1. Survey Content and Invitation Letter
D.2. Survey Responses
Appendix E. Surge Arrester User Use Cases
Appendix F. Background Information for Switching Duties of Circuit Breaker
F.1. Transient Recovery Voltage (TRV)
F.2. Initial Magnitude of the TRV
F.3. Terminal Fault
F.4. Short-Line Fault
F.5. Maximum (peak) TRV Values
F.6. Switching of Capacitances
F.7. Switching of Inductances
Appendix G. Voltage Control in French Power Plants
Additional informations
| Publication type | Technical Brochures |
|---|---|
| Reference | 980 |
| Publication year | |
| Publisher | CIGRE |
| ISBN | 978-2-85873-685-0 |
| Study committees | |
| Working groups | JWG B1/A1/A2/A3.44 |
| File size | 6 MB |
| Pages number | 113 |
| Price for non member | 210 € |
| Price for member | Free |
Authors
B. RUSEK, Convenor (DE)
C. RÉMY (FR), W. GIL (PL), T. VINAS (FR), S. NGUEFEU (FR), C. PALLEM (US), J.-O. DEBUS (DE), S. DEMMIG (DE), J. AMON (BR), E. SPERLING (CH).
Corresponding members
T. WIJNHOVEN (BE), K. VELITSIKAKIS (NL), F. TRAUTMANN (DE), E. STELLA (IT), S. REINCKE (DE), R. PARKE (UK), A. KOTHARI (IN), TAO-WEN (CN), L.R. RODRIGUEZ (SP), R.-M. CERNAT (DE)
Keywords
Power-Frequency Temporary Overvoltages (TOV), Insulation Ageing, High-Voltage Equipment Limitations, Recurrent Temporary Overvoltage, Circuit breaker, Transformer, Cable, Instrument transducer, Surge arrester, Power plant, GIS
