Selection Logic and Technical Drivers for EHV Transmission Lines: A Deep Dive

I. Clarify the Concepts First: EHV and UHV

In one sentence: EHV/UHV is the main corridor for "cross-province, cross-region, and cross-large-base" power transmission.

II. Why Use EHV/UHV Transmission?

1. The Physics: Higher Voltage, Lower Line Loss

Transmitted power P = √3 × U × I, while line loss:

Ploss = 3 × I² × R = P² × R / U²

Every doubling of voltage cuts line loss to 1/4 for the same power.

2. Spatial Mismatch Between Resources and Loads

China's energy layout is a classic "west-to-east power, north-to-south power" pattern:

• West/North: abundant hydro, wind, solar, and coal resources, but limited local consumption
• East/Coast: developed economies with dense electricity demand but scarce energy

Without UHV, western power cannot reach the east — this is why UHV is vividly called the "power highway."

3. Economics: For Long Distances, Higher Voltage Pays Off

Total cost of long-distance transmission = line construction cost + line loss + station investment. As distance grows, the loss advantage of high voltage becomes increasingly significant — within the economic transmission distance, the higher the voltage level, the more cost-effective it is.

III. AC or DC? — Choosing Between Two Technical Routes

AC Transmission

DC Transmission (HVDC/UHVDC)

Selection Rules

Scenario	Recommendation
Point-to-point over very long distance (>1000km)	UHVDC (±800kV/±1100kV)
Regional grid interconnection, intermediate tapping	EHV AC (500kV/1000kV)
Offshore wind transmission	HVDC (±320kV etc.)
Large base to grid transmission	UHVDC as the main choice

China's practice: "DC for transmission, AC for networking" — cross-region point-to-point bulk transfer uses DC; in-region grids use AC.

IV. Conductor Selection: Why Do UHV Lines Use "Bundle Conductors"?

1. Corona Loss and Radio Interference

When the electric field on the conductor surface is too high, air is ionized producing corona discharge:

• Wastes energy (corona loss)
• Produces audible noise ("buzzing")
• Interferes with radio communications

2. The Principle of Bundle Conductors

Split one large conductor into 2–8 sub-conductors, fixed at a certain spacing by spacers, forming a phase conductor:

      ○          ○      ○      ○
     ○ ○   or   ○ ○   ○ ○   ○ ○   (2-bundle, 4-bundle, 6-bundle, 8-bundle)
      ○          ○      ○      ○

Effect: The "equivalent radius" of the conductor increases, lowering surface electric field strength, thereby:

Typical bundle numbers:

• 500kV: 4-bundle
• 1000kV: 8-bundle
• ±800kV: 6-bundle (e.g., 6×625mm²)

3. Conductor Material and Cross-Section

• Material: mainly Steel-Cored Aluminum Conductor (ACSR); higher aluminum/steel ratio = better conductivity
• Large cross-section: UHV commonly uses 500–1250mm² large cross-section conductors to reduce resistance loss
• Trend: energy-efficient conductors (e.g., mid-strength aluminum-alloy core) to reduce line loss

V. Tower and Insulator Selection

1. Tower Types

2. Insulator Selection

EHV/UHV line insulators face the triple challenge of high voltage + long distance + pollution:

Key parameters:

• Creepage ratio: the more polluted the area, the longer the creepage distance required (Class IV pollution ≥ 31mm/kV)
• Lightning withstand: jumper and shield-wire configuration
• V-string / tension strings: V-shaped suspensions are common in UHV to prevent conductor galloping collisions

VI. Key Engineering Challenges for EHV/UHV Lines

VII. Core Logic Summary of EHV/UHV "Selection"

Step 1: Determine transfer capacity and distance
    ↓
Step 2: Choose DC or AC (distance >1000km and point-to-point → DC)
    ↓
Step 3: Determine voltage level (best economics for capacity & distance)
    ↓
Step 4: Conductor design (bundle number, cross-section, material → control corona & loss)
    ↓
Step 5: Insulation design (insulator type, disc count, creepage ratio → prevent pollution flashover)
    ↓
Step 6: Mechanical structure (towers, anti-galloping, anti-icing)
    ↓
Step 7: Supporting equipment (circuit breakers, arresters, CTs/VTs, substations)

VIII. ROCKWILL and EHV/UHV Supporting Equipment

Although line construction is a large-scale project led by grid companies, ROCKWILL provides a key link in the supporting equipment for EHV/UHV systems:

ROCKWILL Related Products

Why Choose ROCKWILL for Supporting Equipment?

• Full-range surge arresters: 220kV-class arresters designed to IEC 60099-4, high nonlinearity ZnO varistors, low residual voltage, effective protection of line insulation
• KEMA certification: core products pass international authoritative type tests
• Outdoor weather-resistant design: validated in Class IV pollution, coastal salt spray, and -45°C extreme cold
• Global supporting experience: from Southeast Asian distribution networks to Russian grid projects

The reliable operation of EHV/UHV lines depends on the reliability of every piece of supporting equipment. ROCKWILL provides a full range of high-voltage supporting products from arresters to switchgear, safeguarding the "power highway."

Contact us:

• Web: www.rockwill.com 

Edited From：Dyson