Power capacitors. Low-voltage power factor correction banks
1Key Takeaways
This standard provides specifications for low-voltage power factor correction devices used in electrical systems. It outlines the requirements for design, performance, and testing of such equipment to ensure compatibility and efficiency in power distribution networks. The document covers essential parameters related to…
2Expert Interpretation
An in-depth look at BS EN IEC 61921:2025, the international standard for low-voltage power factor correction capacitor banks, covers technical changes, safety requirements, verification methods, and implementation recommendations. This standard replaces BS EN 61921:2003 and will become obsolete on August 31, 2028, providing the latest technical specifications for reactive power compensation in power systems.
Standard Overview and Technical Evolution
BS EN IEC 61921:2025 is an international standard for low-voltage AC shunt capacitor banks, published by the British Standards Institution (BSI). It applies to low-voltage AC shunt capacitor banks used for power factor correction. This standard, the UK implementation of IEC 61921:2017, completely replaces the 2003 edition of BS EN 61921:2003, which will officially become obsolete on August 31, 2028.
This standard, developed by IEC Technical Committee TC 33 (Power Capacitors and Their Applications), has undergone significant technical revisions. Compared to the 2003 edition, the new version features significant improvements in verification methods, marking requirements, and routine verification. In particular, it aligns with IEC 61439-1, the standard for low-voltage switchgear and controlgear assemblies.
Main technical changes
Compared with the 2003 version, the 2025 version of the standard has undergone a number of important technical updates, mainly including:
| Change areas | Requirements of the 2003 version | New requirements of the 2025 version | Technical impact |
|---|---|---|---|
| Verification method | Independent verification system | Consistent with IEC 61439-1 | Improve test consistency and comparability |
| Labeling requirements | Basic labeling information | Add key parameters such as rated short-time withstand current | Enhance equipment selection and safe use information |
| Routine verification | Rated output verification is not clear | Rated output verification requirement is added | Ensure the actual performance of the equipment meets the declared value |
| Temperature rise verification | The test method is unclear | New Appendix D provides guidance on temperature rise verification | Standardize the test method and improve the reliability of the results |
These changes reflect the latest developments in power capacitor technology and the increased requirements for safety performance, especially in terms of operating reliability and equipment protection in harmonic environments.
Key Terms and Technical Definitions
The standard clearly defines the core concepts of low-voltage AC capacitor banks (power factor correction banks):
Low-voltage AC capacitor bank: A bank consisting of one or more low-voltage capacitor units combined with associated switchgear and control, measurement, signaling, protection, regulation equipment, etc., fully assembled under the responsibility of the assembly manufacturer, including all internal electrical and mechanical interconnections and structural components.
Capacitor step: A combination of one or more capacitor units switched together by a single switch, which may include tuning reactors, connecting lines and associated switchgear and control equipment devices.
Automatic reactive power controller: A device designed to calculate the reactive power absorbed by the loads connected to the power line and control the switching of the steps of the automatic bank to compensate for the reactive power.
Equipment marking and nameplate requirements
The standard stipulates that manufacturers must provide the following minimum information on the nameplate of the capacitor bank:
| Information category | Specific requirements | Description |
|---|---|---|
| Basic marking | Manufacturer's name or trademark, identification number or model designation, manufacturing date | For equipment identification and traceability |
| Electrical parameters | Rated reactive power (kvar), rated voltage (V), rated frequency (Hz) | Core performance parameters |
| Standard reference | IEC Citation of 61921 standard and its publication year | Declaration of Conformity |
| Additional Information | Class rating, series reactor value, ambient temperature range | Installation and operating conditions |
| Safety parameters | Protection level, rated short-time withstand current, maximum allowable current | Key parameters for safe operation |
Design verification and routine testing requirements
Chapter 7 and Chapter 8 of the standard specify the requirements for design verification and routine verification respectively to ensure the design and manufacturing quality of capacitor banks.
Design Verification Items
Design verification aims to verify whether a given type of capacitor bank meets the requirements of this standard, including:
- Material and component strength verification: Refer to relevant clauses of IEC 61439-1 and IEC 61439-2
- Enclosure protection level verification: Ensure appropriate IP protection level
- Electrical clearance and creepage distance verification: Prevent electrical breakdown and surface discharge
- Dielectric performance verification: Withstand voltage test after all electrical equipment is connected
- Temperature rise limit verification: Test current is at least 1.2 times the rated current
Routine Verification Items
Routine verification aims to detect material and workmanship defects. Each newly manufactured capacitor bank must undergo:
- Enclosure protection level inspection
- Electrical clearance and creepage distance verification
- Verification of protection against electric shock and integrity of protective circuits
- Verification of incorporation of built-in components
- Verification of rated output: by capacitance impedance measurement or current measurement
Safety Requirements and Installation Guidelines
Chapter 6 provides detailed design, installation, operation, and safety guidelines, with particular attention to the following key safety aspects:
Discharge Device Requirements
Each capacitor bank or stage must provide a means of discharging the capacitors after disconnection from the network. The specified discharge time can be met by applying an internal (built-in) discharge resistor to each capacitor or by rating an external discharge device for the entire capacitor installation.
Important Safety Warning: Before touching any live parts, allow at least 5 minutes for the bank to self-discharge, then short-circuit and ground each capacitor terminal.
Fire Hazard Protection
Capacitors contain flammable materials, i.e., dielectric film and/or paper, oil, etc. The arrangement of banks should take into account the potential fire hazard in the event of component failure, with particular attention to:
- Areas adjacent to capacitors: Capacitors are typically installed in metal cans or in isolated metal parts
- Areas around reactors: Reactors (chokes and filters) should be installed with minimal power and control cabling
Special considerations for harmonic environments
The standard specifically highlights the risks of connecting capacitor banks in systems containing harmonics and provides mitigation measures:
The presence of harmonics can shorten the service life of capacitor banks. The damaging effects of harmonics can be mitigated by connecting suitable detuning reactors in series with each capacitor stage. If iron-core reactors are used, attention should be paid to the potential for core saturation and overheating caused by harmonics.
For more detailed information, refer to IEC 61642, "Industrial AC networks affected by harmonics – Application of filters and shunt capacitors."
Implementation recommendations and transitional arrangements
Based on the requirements of BS EN IEC 61921:2025, the following implementation recommendations are provided to manufacturers and users:
Recommendations for manufacturers
- Update product design to comply with the new verification method requirements, especially consistency with IEC 61439-1
- Improve nameplate marking information and add all required electrical and safety parameters
- Establish a routine test procedure for rated output verification to ensure that each product leaving the factory meets the declared performance
- Revise technical documentation and installation guides to reflect the safety requirements of the new standard
Recommendations for users
- Specify products that comply with BS EN IEC 61921:2025 when purchasing new equipment
- Revise existing BS EN 61921:2003 equipment should develop an update and replacement plan and complete the transition before August 31, 2028.
- Pay attention to the enhanced safety requirements in the new standard, especially discharge time and fire protection measures.
- When used in a harmonic environment, ensure that appropriate demodulation reactors are installed.
Transition period arrangements
The standard stipulates a clear transition timetable:
- August 31, 2026: The latest date by which the document must be implemented at the national level by issuing the same national standard or recognizing it.
- August 31, 2028: The latest date by which conflicting national standards must be withdrawn.
Relevant parties are advised to start preparations as early as possible to ensure the smooth completion of technology updates and standard conversions during the transition period.
Technical Annex Application Guide
The standard contains four important technical annexes, providing guidance on specific technical issues:
Appendix A: Minimum and Maximum Cross-Sections of Copper Conductors for Connection
Referring to the relevant annexes of IEC 61439-1, the maximum allowable current and its harmonic spectrum should be considered when selecting the size of copper conductors.
Appendix B: Formulas for Capacitors and Installation
Provides a series of practical calculation formulas, including:
- Calculation of the output of a three-phase capacitor from three single-phase capacitance measurements
- Resonant frequency calculation
- Voltage rise calculation
- Inrush transient current calculation
- Discharge resistance calculation
Appendix C: Definition of Similar Design for Capacitor Banks
Defines the standard for similar design of capacitor banks. Capacitor banks with similar mechanical structure and electrical design can be considered as similar designs.
Appendix D: Method of Connecting Additional Capacitors for Performing Temperature Rise Tests
Provides a method for increasing the capacitance value to ensure that all additional losses affect the internal temperature rise of the capacitor bank while keeping the layout unchanged.
These appendices provide important technical references for the practical application of the standard, especially in design verification and failure analysis.