Introduction
Transmission losses play a decisive role in power system economics and energy efficiency. While generating power at high efficiency is crucial, delivering it across long distances without excessive energy loss is equally important.
In most countries, the grid backbone relies on overhead transmission lines, operating in either:
- Common HVAC extra-high voltages used in bulk transmission include 220 kV, 380 kV, 500 kV and 765 kV
- HVDC (High Voltage Direct Current), typically at ±320 kV, ±450 kV, ±500 kV, ±533 kV, ±800 kV and even ±1100 kV in recent UHV projects — all are implemented in actual projects worldwide
Both HVAC and HVDC are subject to power losses, but their mechanisms, magnitude, and mitigation strategies differ significantly.
This article provides a detailed comparison of losses in 380 kV HVAC and ±500 kV HVDC transmission lines, referencing industry standards (IEEE 738, IEC 61803), case studies, and practical engineering insights.
International Standards for Transmission Loss Calculations
- IEEE 738-2023:
Defines current–temperature relationships of overhead conductors, addressing resistance increase with temperature and its effect on I²R losses. - IEC 61803-2020:
Provides standardized methods to determine losses in HVDC converter stations, including valves, transformers, filters, and auxiliary equipment. - CIGRÉ Technical Brochures:
Widely used for corona loss evaluation and field-effect studies. - IET Research Papers:
Provide measured values for corona losses in ultra-high-voltage AC and DC systems.
Types of Losses in Overhead Transmission
| Loss Type | HVAC (380 kV AC) | HVDC (±500 kV DC) |
|---|---|---|
| Resistive (Joule) Losses | Calculated as I² × R. Increased by skin effect and proximity effect. | Same formula, but no skin/proximity effect → slightly lower. |
| Reactive & Charging Losses | Major factor in long AC lines. Shunt compensation required. | None along line (only converter-side reactive power). |
| Corona Losses | Higher in AC, weather dependent. | Present but typically lower in HVDC. |
| Converter/Transformer Losses | Transformer losses in substations. | Converter station losses: 0.5–1% each end. |
Key Formulas (WordPress-friendly)
1. Resistive (Joule) Losses
P_loss = I^2 × R
Where:
- I = RMS current (A)
- R = Conductor resistance (Ω)
2. Corona Power Loss (Peek’s Formula for AC)
P_c = 241 × (f + 25) × (r + 25)^2 × sqrt(δ / r) × (V_ph - V_d)^2 × 10^-5
Where:
- f = frequency (Hz)
- r = conductor radius (cm)
- δ = air density factor
- V_ph = phase-to-neutral voltage (kV)
- V_d = disruptive critical voltage (kV)
3. DC Corona Loss (empirical)
P_c (DC) ≈ k × (V - V_d)^2
Where k depends on conductor condition and weather.
Quantitative Comparison (380 kV HVAC vs ±500 kV HVDC)
| Parameter | 380 kV HVAC | ±500 kV HVDC |
|---|---|---|
| Resistive Losses | 2–4% over 500–800 km | 1–2% |
| Corona Losses | ~1–2 kW/km (wet weather) | ~0.1–0.3% of transmitted power |
| Reactive Losses | 0.5–1% | Negligible |
| Station Losses | ~0.5–1% (transformers, shunt reactors) | ~1–2% (converters, filters) |
| Total Losses | ~5–8% | ~3–5% |
Real-World Case Study: Rihand–Delhi HVDC Line
- Type: Bipolar overhead HVDC line
- Voltage: ±500 kV
- Length: 814 km
- Capacity: 1,500 MW
Findings:
- HVDC line maintained high efficiency, with measured total losses around 3.5%, confirming long-distance superiority.
- The equivalent HVAC line would have experienced 6–7% losses over the same distance.
Factors Affecting Transmission Losses
- Conductor type – ACSR, ACCR, HTLS conductors reduce resistive losses.
- Conductor bundling – lowers corona onset and surface gradients.
- Weather conditions – fog, rain, and ice significantly increase corona losses.
- Altitude – higher elevations lower air density, increasing corona losses.
- Frequency (AC only) – amplifies skin and proximity effects.
- Converter technology (DC only) – newer voltage source converters (VSC) have lower losses compared to older line commutated converters (LCC).
Break-Even Distance: HVAC vs HVDC
- Overhead transmission: HVDC more efficient beyond 500–800 km.
- Underground/submarine cables: HVDC preferable after 100 km, since HVAC suffers heavy capacitive charging current.
Mitigation Strategies
- Raise voltage level → reduces current, cutting I²R losses.
- Bundled conductors → mitigate corona losses and improve efficiency.
- Reactive compensation (for AC) → shunt reactors and series capacitors.
- Efficient converters (for DC) → modern IGBT-based VSC reduces converter losses.
- Surface maintenance → polishing and cleaning reduces corona inception voltage.
Conclusion
- 380 kV HVAC: Losses accumulate from resistive heating, reactive power, and corona discharge, typically totaling 5–8% over long distances.
- ±500 kV HVDC: Despite converter station losses, HVDC achieves 3–5% total losses, making it more efficient for bulk long-distance power transfer.
- Standards: IEEE 738 and IEC 61803 provide frameworks for consistent and reliable loss evaluation.
- Verdict: For distances >500 km overhead or >100 km submarine, HVDC is the superior transmission technology.
Frequently Asked Questions (FAQs)
Q1. What are the main losses in a 380 kV HVAC transmission line?
The major losses are resistive (I²R), corona discharge, and reactive power losses due to line capacitance and inductance.
Q2. Why does HVDC have lower losses than HVAC?
HVDC eliminates reactive and skin-effect losses, which dominate long HVAC lines. Only resistive and corona losses remain, along with converter station losses.
Q3. At what distance does HVDC become more efficient than HVAC?
Typically beyond 500–800 km for overhead lines and 100 km for submarine cables.
Q4. Which standards are used for calculating line losses?
- IEEE 738 → conductor resistive losses
- IEC 61803 → HVDC converter losses
Q5. What role does weather play in transmission line losses?
Rain, fog, and ice increase corona discharge losses, especially in HVAC systems.
Q6. Can transmission line losses be reduced?
Yes, by increasing voltage, using bundled conductors, compensating reactive power, and applying advanced HVDC converter designs.
ADDITIONAL INFO:
| Example (route / name) | Type | Nominal DC/AC Voltage | Country / Notes | Length / Capacity | Sources |
|---|
| Xiangjiaba – Shanghai | HVDC (UHVDC) | ±800 kV DC | China — large UHVDC bulk link | ~2,000 km, 6,400 MW. | Wikipedia+1 |
| Xingu – Estreito (and Xingu–Rio series) | HVDC (UHVDC) | 800 kV DC | Brazil — UHVDC north→southeast hydropower links | ~2,000–2,500 km (projects vary), multi-GW capacity. | Wikipedia+1 |
| Zhundong–Wannan (record UHVDC) | HVDC | ±1100 kV DC (record capacity networks) | China — ultra-UHV developments; highest capacity examples referenced in literature. | Highest-capacity UHVDC systems referenced (12 GW level). | Wikipedia |
| Rihand – Dadri (Rihand–Delhi) | HVDC (bipole) | ±500 kV DC | India — classic long overhead HVDC line | 814 km, 1,500 MW. | Wikipedia+1 |
| Inga – Kolwezi (Inga–Shaba) | HVDC | ≈500 kV DC | DRC — historically long continental HVDC line | ~1,700 km, ~560 MW (original rating). | Wikipedia+1 |
| Cahora Bassa (Mozambique – South Africa) | HVDC (bipole) | ±533 kV DC | Mozambique → South Africa (large interconnector) | ~1,420 km, 1,920 MW. | Wikipedia+1 |
| NorNed (subsea) | HVDC (submarine) | ±450 kV DC | Norway ↔ Netherlands — long submarine interconnector | 580 km, 700 MW. | Wikipedia+1 |
| Agra – Greater Noida | HVAC (AC) | 765 kV AC | India — example of 765 kV AC overhead project (UHV AC in India) | ~200 km project example (part of national 765 kV rollout). | Power Technology+1 |
| Berlin Nordring (example 380 kV) | HVAC (AC) | 380 kV AC | Germany / European networks routinely use 380 kV as core transmission level | Local/municipal 380 kV line example; 380 kV is common in Europe. | Wikipedia+1 |
| Typical national AC lists / Wikipedia summary | HVAC | 110 kV, 220 kV, 380/400 kV, 500 kV, 765 kV (varies by country) | General reference for typical AC transmission voltage classes | Summary of typical system voltage classes and records. | Wikipedia |
| Historic/long AC lines (examples) | HVAC | various (e.g. 500 kV) | e.g., many countries use 500 kV AC (US, parts of Europe/Asia) | See national grid operators / country project lists. |
