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CISPR 18-2: Interference Characteristics of Overhead Power Lines and High-Voltage Equipment – Limits
Limits for radio interference from overhead power lines above 1 kV, covering corona and gap discharge in the 0.15-30 MHz range
Introduction to CISPR 18-2
CISPR 18-2 specifies the limits for radio interference characteristics produced by overhead power lines and high-voltage equipment operating at voltages above 1 kV. This standard is a critical reference for utility companies, transmission system operators, and HV equipment manufacturers who must ensure their installations do not cause unacceptable degradation to radio reception in the frequency range 0.15 MHz to 30 MHz. The standard covers both corona-generated interference from conductors and hardware, as well as gap-type discharges from loose connections or defective hardware. Compliance with CISPR 18-2 limits is increasingly important as wireless communication systems become more sensitive and ubiquitous.
Corona discharge is the dominant source of radio interference from overhead power lines under fair weather conditions. The interference level is proportional to the square of the conductor surface voltage gradient — a 10% reduction in gradient can reduce interference by approximately 3-5 dB.
Interference Limits and Measurement Procedures
CISPR 18-2 establishes two categories of limits: general limits for areas with normal radio reception requirements, and relaxed limits for remote areas where radio services are sparse. The limits are specified as quasi-peak values measured with a CISPR 16-1-1 compliant receiver. The standard also provides guidance on statistical evaluation methods, recognizing that interference from power lines is inherently variable with weather conditions. Measurements are typically performed at a distance of 20 m from the outermost conductor projection, using a rod antenna positioned 3 m above ground.
| Frequency Range | General Limit (dBµV/m) | Relaxed Limit (dBµV/m) | Measurement Distance |
|---|---|---|---|
| 0.15 – 0.30 MHz | 34 – 28 (linear decrease) | 40 – 34 (linear decrease) | 20 m |
| 0.30 – 1.0 MHz | 28 – 24 (linear decrease) | 34 – 30 (linear decrease) | 20 m |
| 1.0 – 3.0 MHz | 24 (flat) | 30 (flat) | 20 m |
| 3.0 – 30 MHz | 24 – 18 (linear decrease) | 30 – 24 (linear decrease) | 20 m |
Rain and fog can increase corona-generated interference by 10-20 dB compared to fair weather conditions. CISPR 18-2 requires long-term statistical measurements (L_50 and L_95 levels) to properly characterize the interference environment.
Engineering Design for Interference Mitigation
Controlling the surface voltage gradient on conductors is the most effective method for reducing radio interference from overhead lines. Bundle conductor configurations — using two, four, or more sub-conductors per phase — reduce the surface gradient by distributing the electric field across multiple conductor surfaces. For existing lines, corona rings and grading shields at insulator assemblies and hardware connections can reduce local gradient enhancement. Selection of conductor diameter and sub-conductor spacing is a key design optimization; larger diameters reduce gradient but increase wind loading and cost.
Hardware design is equally important. Loose connections, corroded clamps, and damaged conductors create gap-type discharges that generate broadband interference extending well into the VHF band. Regular inspection using corona cameras (ultraviolet imaging) and acoustic detection methods can identify problem areas before they cause complaint-worthy interference. Application of anti-corona coatings and use of non-ferrous hardware materials reduce the propensity for gap discharges.
A well-designed 400 kV double-circuit line with 4-bundle conductors (each 31.5 mm diameter) typically produces radio interference levels 6-10 dB below the CISPR 18-2 general limit under fair weather conditions, providing adequate margin for adverse weather degradation.
Statistical Characterization and Compliance
CISPR 18-2 employs a statistical approach to interference characterization, recognizing the time-varying nature of power line noise. The L_50 (median) and L_95 (exceeded 5% of time) statistical levels are used to characterize interference. Compliance is typically assessed using the L_95 value — if the interference exceeds the limit for no more than 5% of observations, the line is considered compliant. This statistical method allows utilities to design for economic efficiency while maintaining acceptable radio reception quality.
Conductor surface gradients exceeding 22 kV/cm on AC lines or 15 kV/cm on DC lines should be avoided, as these levels correspond to the onset of intense corona discharge that causes both audible noise and radio interference far exceeding CISPR 18-2 limits.
Frequently Asked Questions
Q: Does CISPR 18-2 apply to DC transmission lines?
A: Yes, the standard covers both AC and DC overhead lines. DC lines typically produce lower radio interference than AC lines of equivalent voltage due to the absence of cyclical voltage polarity reversal.
A: Yes, the standard covers both AC and DC overhead lines. DC lines typically produce lower radio interference than AC lines of equivalent voltage due to the absence of cyclical voltage polarity reversal.
Q: How does altitude affect corona interference?
A: Corona onset voltage decreases with altitude due to lower air density. At 3000 m, interference levels can be 8-12 dB higher than at sea level for the same conductor configuration.
A: Corona onset voltage decreases with altitude due to lower air density. At 3000 m, interference levels can be 8-12 dB higher than at sea level for the same conductor configuration.
Q: What is the relationship between audible noise and radio interference?
A: While both are caused by corona discharge, they have different frequency characteristics. Radio interference extends to 30 MHz+ while audible noise is below 20 kHz. Mitigation measures for one generally benefit the other.
A: While both are caused by corona discharge, they have different frequency characteristics. Radio interference extends to 30 MHz+ while audible noise is below 20 kHz. Mitigation measures for one generally benefit the other.
