Copper Conductor Size vs Temperature Rise in 145kV Disconnectors

The relationship between the temperature-rise current of a 145 kV disconnector and copper conductor size lies in balancing current-carrying capacity and heat dissipation efficiency. The temperature-rise current refers to the maximum continuous current a conductor can carry without exceeding its specified temperature rise limit, and the copper conductor size directly influences this parameter.

Understanding this relationship begins with the physical properties of the conductor material. Copper’s conductivity, resistivity, and thermal expansion coefficient determine both heat generation under load and heat dissipation rate. Larger cross-sectional areas reduce resistance per unit length, thereby generating less heat at the same current. For example, a 2.5 mm² copper wire exhibits lower temperature rise than a 1.5 mm² wire when carrying 20 A.

When selecting conductor size, three key factors must be evaluated holistically:

• Load characteristics, including current fluctuation magnitude and duration. Equipment with frequent starts/stops or short-term overloads requires consideration of transient temperature rise effects on insulation.

• Ambient temperature: Higher ambient temperatures necessitate larger conductors to offset additional thermal stress.

• Installation method: Enclosed conduits offer poor heat dissipation; conductor size should be increased by at least 20% compared to open installations.

Critical thresholds can be estimated using the formula:
ΔT = (I² · R · t) / (m · c)
where I is current, R is resistance per unit length, t is time, m is conductor mass, and c is specific heat capacity. In practice, quick-reference tables are commonly used—for instance, at 40°C ambient temperature, standard BV wires have the following ampacities: 1.5 mm² → 16 A, 2.5 mm² → 25 A, 4 mm² → 32 A.

Common misconceptions must be avoided. Some assume that simply increasing conductor size solves overheating—but poor terminal contact, oxidation at joints, or loose connections can cause localized hotspots. In one case, a poorly crimped 4 mm² copper connection reached 120°C at just 15 A, far exceeding the conductor’s bulk temperature rise of 65°C.

Copper purity significantly affects temperature rise. Oxygen-free copper (99.9% Cu) has 8–12% lower resistivity than recycled copper, enabling ~10% higher current capacity at the same size. It is recommended to use copper wire complying with GB/T 395 standards for electrical applications.

Practical application strategies can be structured into three tiers:

• Tier 1 (Basic Matching): Select conductor size based on 1.2× the rated current.

• Tier 2 (Dynamic Compensation): Adjust for power factor—inductive loads require 5–8% larger conductors.

• Tier 3 (Redundancy Design): Reserve 20% current margin on critical circuits for unexpected surges.

Heat dissipation can be enhanced through structural and material improvements:

• Stranded conductors offer >30% more surface area than solid-core wires.

• Tin-plating reduces contact resistance by 15–20%.

• In enclosed switchgear, replacing bundled cables with copper busbars improves heat dissipation by 40% while reducing connection points.

Maintenance intervals impact long-term stability. Inspect connection tightness every 500 operating hours, use thermal imaging to monitor temperature distribution, and replace oxidized terminals promptly. In humid environments, apply anti-corrosion coatings to prevent electrochemical degradation that increases resistance.

Special scenarios demand tailored approaches:

• High-frequency equipment (>1 kHz): Skin effect becomes significant; use multiple parallel fine strands instead of a single thick conductor.

• Unbalanced three-phase systems: Size conductors based on the highest phase current; neutral conductors should be no smaller than phase conductors.

Experimental validation is essential. Build a test rig and run at 1.5× rated current for 2 hours, recording temperature-rise curves at critical points. Acceptance criteria: Ambient temp + Conductor temp rise ≤ Insulation thermal rating (e.g., ≤70°C for PVC).

Cable layout geometry affects cooling:

• Maintain spacing ≥2× cable diameter for parallel runs.

• Vertical installation dissipates heat 15–20% better than horizontal routing—prefer for high-current lines.

• Minimum bend radius should be ≥6× conductor diameter to avoid localized heat trapping.

Monitor conductor aging dynamically: under normal use, copper resistance increases ~0.5% annually. After five years, re-evaluate ampacity. Install temperature sensors at critical nodes and implement real-time warning thresholds.

Copper-aluminum transition joints require special attention. Galvanic corrosion occurs at dissimilar metal interfaces—always use certified bi-metallic connectors and apply antioxidant grease. One substation failure analysis showed unprotected Cu-Al joints in humid conditions tripled contact resistance within three months, leading to meltdown.

Voltage drop must also be considered, especially in long-distance runs. Ensure terminal voltage remains ≥95% of nominal value. When both temperature rise and voltage drop constraints apply, select the conductor size dictated by the stricter requirement.

Insulation thermal resistance matters significantly. Thermal conductivity varies widely—e.g., silicone rubber is twice that of PVC, allowing 8–12% higher current at the same size. For high-temperature applications, use XLPE (cross-linked polyethylene) insulation, rated for continuous operation up to 90°C.

Finally, electromagnetic effects—skin effect and proximity effect—reduce effective conductor area in AC systems. For large single-core conductors, using multiple smaller parallel conductors is more effective for temperature control than a single oversized one.
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