HVDC & FACTS

Question Number 1 → Comparision

🔄 Comparison of AC and DC Transmission – Economics

Aspect DC Transmission AC Transmission

Effect of Line Inductance & capacitance don’t Power transfer is affected

Parameters affect power transfer; no by inductance &
charging/leakage current capacitance

Conductor Needs 2 conductors Needs 3 conductors ( for

Requirement 3-phase system)

Line Losses (steady No charging current, so better for Charging current present,
state) cables especially over long
distances

Terminal Higher due to HVDC converters Lower

Equipment Cost

Line/Cable Cost Lower for long distance Higher, especially over

500 km

Economical More economical above 500 km Better for shorter

Distance distances
(Break-even)

Installation Cost Depends on DC voltage level, line Less sensitive to voltage

Factors type, materials, labor choice

Loss Evaluation Uses life cycle cost analysis Not always required

Technology Options Thyristor valves or VSC (IGBT) Traditional EHVAC

based systems systems
⚙️ Comparison of AC and DC Transmission – Technical Performance

Aspect DC Transmission AC Transmission

Power Control Full control over bidirectional Not easily achievable

Direction power flow

System Stability Improves stability when DC link May face transient

(Transient & Dynamic) is embedded instability

Fault Management Fast fault current control via Fault currents are harder
converters to control

System DC link can act as an Requires frequency

Synchronization asynchronous tie to stabilize synchronization
frequency differences

Large System Helps dampen oscillations in May lead to instability

Interconnection interconnected AC systems without DC support

Tower & Conductor Requires 2 conductors (bipolar), Requires 3 conductors

Requirements lighter and smaller towers, hence (3-phase), heavier and
more cost-effective costlier towers
Question Number 2 → Different types of dc links Comparison: 3-Phase AC vs Bipolar DC
vs Homopolar DC
✅ 1. Types of DC Links
DC links are used in HVDC transmission systems to connect two AC systems or for
long-distance power transmission. Based on their configuration and usage, there are three
main types:

🧩 a) Monopolar Link

●​ Uses: One conductor (either positive or negative).​

●​ Return Path: Ground or sea return.​

●​ Economical for initial stages (low power).​

●​ Used in: Early/low-cost HVDC projects.​

🧠 Easy to Remember:​
"Mono = One conductor, return through Earth."

🧩 b) Bipolar Link

●​ Uses: Two conductors – one positive and one negative.​

●​ Return Path: Ideally no return, but in case one pole fails, current can return
through ground (temporary).​

●​ Advantage: Can transmit power even if one pole fails (half capacity).​

●​ Used in: Most modern HVDC systems.​

🧠 Easy to Remember:​
"Bi = Two poles, +ve and -ve conductors. Reliable even if one fails."
🧩 c) Homopolar Link

●​ Uses: Two (or more) conductors at same polarity, usually negative.​

●​ Return Path: Through the ground.​

●​ Advantage: Simpler insulation, less voltage stress.​

●​ Not common due to environmental concerns with ground return.​

🧠 Easy to Remember:​
"Homo = Same polarity, all negative conductors use Earth as return."

✅ 2. Comparison: 3-Phase AC vs Bipolar DC vs

Homopolar DC

Feature 3-Phase AC System Bipolar DC Link Homopolar DC

Link

Number of 3 conductors 2 conductors (+ve 1 or 2 conductors

Conductors and -ve) (same polarity)

Return Path No return, closed loop Normally none, Ground return

ground used in
fault

Power Good, but affected by line High, stable, Moderate

Handling parameters reliable

Power
Equation
Power Losses Higher due to skin effect, Lower (no reactive Lower
reactive power power, no skin
effect)

Fault Handling Slower isolation and One pole can Less reliable due to
recovery continue during ground return
fault

System Size Large towers, heavy lines Smaller towers, Smaller towers
lighter conductors

Environment Neutral Slight (during Significant

Impact monopolar (ground current
operation) issues)

Application Traditional power grid Long-distance Rare, earlier

HVDC (reliable) systems

Question Number 4 → layout

Question Number 5 → Limitations of HVDC Transmission Lines

🔴 Limitations of HVDC Transmission Lines

Sl. Limitation Explanation
No.

1️⃣ Harmonics Generation Converter switching → non-sinusoidal currents →

in converters harmonics → interference in telephone lines; requires
filters.

2️⃣ No Reactive Power HVDC converters can’t generate VARs → need SVCs or
Generation STATCOMs from AC side.

3️⃣ Lack of Multi-Terminal Due to the absence of fast-acting DC circuit breakers;

DC Systems only recently being addressed.

4️⃣ Complex Control Requires sophisticated electronics and coordination

between ends.

5️⃣ High Conversion Cost Costly converters → rectifier/inverter stations more

expensive than AC substations.
 6️⃣ No Transformers for Unlike AC, voltage in DC can’t be changed directly with
Voltage Change transformers → needs complex electronics.

✅ Modern Advances That Overcome Limitations

Advance Benefit

1. Development of DC Breakers Enables protection and realization of multi-terminal

DC networks.

2. Modular Thyristor Valves Simplifies construction and increases system

flexibility.

3. Higher Thyristor Ratings Allows for higher voltages and currents, improving
efficiency.

4. 12-Pulse Converters Reduces harmonics, improving power quality.

5. Metal Oxide Gapless Provides better over-voltage protection.

Arresters

6. Fiber Optics & Digital Enhances control, monitoring, and speed of

Electronics operation.

🧠 Memory Tip:

Limitations → “H R C C H T”​
(Harmonics, Reactive, Circuit breaker, Control, High cost, Transformers)

Solutions → “B T V C A F”​
(Breakers, Thyristors, Valve rating, Converter pulses, Arresters, Fiber optics)
Question Number 6→ Reliability of HVDC Systems

✅ Reliability of HVDC Systems

🔹 What is Reliability in HVDC?

Reliability refers to the ability of a system to perform its function without failure. In
HVDC systems, this means:

Transmission Reliability = (Number of times HVDC operates as designed) /

(Number of recordable AC system faults)

🔹 Key Points on Reliability

Aspect Details

Current HVDC Very reliable and often more reliable than AC transmission
Systems systems.

Technology Used Thyristor valves with Light Triggered Thyristors (LTT) enhance
performance and reduce auxiliary power use.

Fault Tolerance HVDC links can withstand certain AC faults (e.g. short circuits) as
long as the voltage does not fall below converter limits.

Advantages of Even if one pole fails, the other can carry up to 50% of load with
Bipolar Links ground return.
📊 MTTF & MTTR Data Table

(MTTF = Mean Time To Failure, MTTR = Mean Time To Repair)

S. No. Component MTTF (years) MTTR (hours)

1️⃣ Thyristor Valve 16.3 1.1

2️⃣ Smoothing Reactor 157.1 16.4

3️⃣ AC Filter 10.2 1.0

4️⃣ DC Filter 12.0 1.0

5️⃣ AC Breaker 13.3 7.2

6️⃣ DC Breaker 10.0 5.3

7️⃣ Converter 30.0 12.1

Transformer

🧠 Summary / Easy Recall Points

●​ HVDC systems are very reliable due to modern components (LTT, advanced
protection).​

●​ Bipolar HVDC links can still operate at 50% power even if one pole fails.​

●​ Components like Smoothing Reactors & Converter Transformers have very high
MTTF, meaning rare failures.​
⚡ HVDC-VSC (Voltage Source Converter) Transmission
Systems

🔹 What is HVDC-VSC?

●​ HVDC-VSC uses IGBT-based voltage source converters introduced in the

mid-1990s.​

●​ These converters can control both active and reactive power independently.​

●​ Suitable for underground, underwater, and renewable energy integration

applications.​

●​ Offers higher controllability, compact design, and better grid stability.​

✅ Important Features & Advantages

No. Feature Explanation

(i) Cable-based Uses solid insulation DC cables → ideal for

transmission underground & underwater → less risk of damage
than oil-filled cables.

(ii) No external magnetic Due to two-core/single-core bundled cables with

field opposite current flow → magnetic field cancels out.

(iii) Independent active & Unlike LCC (line commutated converters), VSC offers
reactive power control full control.

(iv) Feeds power into passive No need for local generation to support grid voltage.
networks
(v) Compact and modular Easy to fabricate and install → faster deployment.

(vi) Remote control Ideal for unmanned operation and remote locations.

(vii) High reliability Rugged against grid disturbances and environmental

effects.

(viii) Low noise, eco-friendly Enclosed container → minimal acoustic noise and
less environmental impact.

🌐 Additional Advantages Over AC Systems

1.​ Precise Power Control​

AC systems depend on converter firing angle → VSC allows independent
active/reactive control.​

2.​ Higher Transfer Capacity​

Operates at higher voltages, enabling more power transfer with fewer losses.​

3.​ Fast Fault Recovery​

Quick response post-blockage → faster restoration compared to traditional
systems.​

🧠 Easy Memory Aids

🧩 "CANCER-FIR" (mnemonic for the features)

●​ C: Cable-based transmission​

●​ A: Active/Reactive power control​

●​ N: No magnetic field​
 ●​ C: Compact design​

●​ E: Environmental friendly​

●​ R: Remote/unmanned operation​

●​ F: Feeds passive grid​

●​ I: Insulated (solid) cables​

●​ R: Rugged & reliable​

Here's a detailed, structured, and easy-to-remember answer for the topic:

⚡ Modern Trends in HVDC Transmission

With technological advancement and growing energy demands, especially from
renewables and long-distance power transfer, HVDC systems have evolved significantly.
Here are the modern trends shaping the future of HVDC:

🔹 1. Voltage Source Converter (VSC)-Based HVDC

●​ Use of IGBTs and modular multilevel converters (MMC).​

●​ Key benefits:​

○​ Independent control of active and reactive power​

○​ Black start capability​

○​ Compact, modular, and scalable​

○​ Suitable for offshore wind farms and urban areas​

🧠 Remember: VSC = Versatile, Scalable, Controllable

🔹 2. Multi-Terminal HVDC (MTDC) Systems

●​ Unlike point-to-point systems, MTDC allows multiple sending and receiving

stations.​

●​ Useful for:​

○​ Renewable integration (e.g., wind farms)​

○​ Interconnecting countries or regional grids​

🔧 Still evolving due to complexity in control and protection systems.

🔹 3. Integration with Renewable Energy Sources

●​ HVDC enables grid integration of:​

○​ Offshore wind farms​

○​ Solar power plants in remote areas​

●​ Provides stable, long-distance transmission with lower losses.​

🌍 Supports the global energy transition to green sources.

🔹 4. Underground and Underwater HVDC Cables

●​ Use of solid-insulated DC cables for:​

○​ Urban power delivery​

○​ Submarine interconnections (e.g., between countries or islands)​

●​ Less electromagnetic interference and higher safety.​

🔹 5. Hybrid AC-DC Grids

●​ Emerging concept to combine the strengths of AC and DC.​

●​ Enables better load sharing, redundancy, and grid flexibility.​

🔹 6. Advanced Control & Protection Systems

●​ Use of:​

○​ Digital control systems​

○​ Wide Area Monitoring (WAM)​

○​ Artificial intelligence (AI) and machine learning for fault detection and
control​

🔹 7. Compact and Modular Substations

●​ Smaller footprint → better suited for urban substations​

●​ Prefabricated modules allow quick deployment​

🔹 8. Higher Voltage Levels and Power Ratings

●​ From ±500 kV → now reaching up to ±1100 kV UHVDC​

●​ Power transfer capacity up to 10,000 MW​

●​ Reduces current → minimizes losses​

🔹 9. Development of HVDC Breakers

 ●​ Especially for VSC-based MTDC systems​

●​ Enables fast fault isolation and protection​

🔹 10. Environmental Considerations

●​ Low visual and acoustic impact​

●​ Less land use → ideal for dense or sensitive areas​

🧠 Easy Mnemonic: "VIP HER CUTER"

●​ V: VSC-based HVDC​

●​ I: Integration of Renewables​

●​ P: Protection & control systems (advanced)​

●​ H: Hybrid AC-DC grids​

●​ E: Environmental benefits​

●​ R: Renewable-ready (offshore/remote)​

●​ C: Compact substations​

●​ U: UHVDC (Ultra High Voltage)​

●​ T: Trend of multi-terminal systems​

●​ E: Emerging cable tech (underwater/underground)​

●​ R: Reliable breakers for VSC​