High Voltage DC Transmission: Comparison and Principles, Essays (university) of Computer-Aided Power System Analysis

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High Voltage DC Transmission

Presented By

FA13-R09-005 Muqadsa Iftikhar

FA13-R09-013 Zunaib Ali

FA13-R09-024 Madiha Naeem

Principle of AC Transmission

Schematic of AC system

COMPARISION OF HVAC & HVDC SYSTEMS

• HVAC transmission is having several limitations like line length , uncontrolled power flow, over/low voltages during lightly / over loaded conditions, stability problems, fault isolation etc
• The advantage of HVDC is the ability to transmit large amounts of power over long distances with lower capital costs and with lower losses than AC.
• In a number of applications HVDC is more effective than AC transmission.

Examples include:

• Undersea cables, where high capacitance causes additional AC losses. (e.g. 250 km Baltic Cable between Sweden and Germany).
• 600 km NorNed cable between Norway and the Netherlands
• HVDC transmission allows efficient use of energy sources remote from load centers. Depending on voltage level and construction details, losses are quoted as about 3% per 1,000 km.

COMPARISION OF HVAC & HVDC SYSTEMS

• In HVDC Long power transmission without intermediate taps, for example, in

remote areas.

• Increasing the capacity of an existing power grid in situations where additional wires are difficult or expensive to install
• Power transmission and stabilization between unsynchronized AC distribution systems
• Reducing line cost:

 fewer conductors

 thinner conductors since HVDC does not suffer from the skin effect

• Connecting a remote generating plant to the distribution grid
• Asynchronous operation possible between regions having different electrical parameters.
• Facilitate power transmission between different countries that use AC at differing

voltages and/or frequencies

COMPARISION OF HVAC & HVDC SYSTEMS

• No restriction on line length as no reactance in dc lines
• HVDC can carry more power per conductor because, for a given power rating, the constant voltage in a DC line is lower than the peak voltage in an AC line.

COMPARISION OF HVAC & HVDC SYSTEMS

• AC current will struggle against inertia in the line (100times/sec)-electrical resistance – inductance- reactive power
• Direct current : Roll along the line ; opposing force friction (electrical resistance )
• HVDC uses less current i.e. low losses.

COMPARISION OF HVAC & HVDC SYSTEMS

• Direct current conserves forest and saves land
• The towers of the dc lines are narrower, simpler and cheaper compared to the

towers of the ac lines.

COMPARISION OF HVAC & HVDC SYSTEMS

AC Transmission Line Corridor

COMPARISION OF HVAC & HVDC SYSTEMS

DC Transmission Line Corridor

COMPARISION OF HVAC & HVDC SYSTEMS

• HVDC has greater reliability. i.e.

bipolar dc is more reliable than 3 phase HVAC

• DC requires less insulation.
• An optimized DC link has smaller towers than an optimized AC link of equal capacity.
• Lesser Corona Loss than HVAC at same voltage and conductor diameter and less Radio interference.
• Direction of power flow can be changed very quickly
• DC line in Parallel with AC link.

Corona → (f+25)

HVDC technology is used to transmit electricity over

long distances by overhead transmission lines or

submarine cables.

HVDC Introduction

HVDC Principle

Components of HVDC Transmission Systems

1. Converters
2. Smoothing reactors
3. Harmonic filters
4. Reactive power supplies
5. Electrodes
6. DC lines
7. AC circuit breakers

Components of HVDC

Reactive power supplies

• Under steady state condition, the reactive power consumed by the converter is about 50% of the active power transferred
• Under transient conditions it could be much higher
• Reactive power is, therefore, provided near the converters
• For a strong AC power system, this reactive power is provided by a shunt capacitor

Electrodes

• Electrodes are conductors that provide connection to the earth for neutral. They have large surface to minimize current densities and surface voltage gradients

DC lines

• They may be overhead lines or cables
• DC lines are very similar to AC lines

AC circuit breakers

• They used to clear faults in the transformer and for taking the DC link out of service
• They are not used for clearing DC faults
• DC faults are cleared by converter control more rapidly

Components of HVDC Transmission Systems….

Application based HVDC Transmission Types

Upto 600MW

50 Hz 60 Hz

HVDC is the unique solution to interconnect Asynchronous

systems or grids with different frequencies.