A high-voltage, direct current (HVDC) electric power transmission

system uses direct current for the bulk transmission of electrical power,

in contrast with the more common alternating current systems

WHY HVDC?Reduces energy losses due to the

resistance of transmission lines

• Cheaper over long distances (sustainable energy generation sites are usually far away from consumption)

• Electricity can be imported (Pakistan’s salvation)!!!!

•

•Doesn’t generate reactive power

•No synchrony demands (the stations can even be at different frequencies , so buffer is created between the two systems)

• Only two conductors are needed instead of three as used in AC so cost of transmission reduced.

• Increasing the capacity of an existing power gridin situations where additional wires are difficult or expensive to install

• More power transmitted since HVDC does not suffer from the skin effect. (No need for STRANDED conductors!!!)

• Undersea cables, where high capacitance causes additional AC losses. (e.g., 250 km Baltic Cable between Sweden and Germany

Two types of HVDC

transmission

•Conventional HVDC

•VSC-HVDC

•No risk of commutation failures in VSC

•VSC can connect to weak or dead AC networks

•Faster response of VSC due to high switching frequency

•Minimal environmental impact

ELECTRICAL FAULT!!!

OR

PERFORMANCE ANALYSIS OF A VSC

TRANSMISSION SYSTEM UNDER FAULTED

CONDITIONS

Fundamentals of VSC

Schematic of the

interconnected VSC by a

DC link

,

Power flows

Phase locked loop, outer

active and reactive power

and voltage loop

Inner Current Loop

Converting into the rotating frame gives:

and

Rearrange the equations…

SIMULATIONS

Integration gives…

SIMULATED USING

SIMULINK…

DATA ANALYSIS AND RESULTS

RESULTS:Step responses:

RESULTS CONTINUED…

Station 1 Station 2

AC Perturbations:

- The proposed strategy has been shown to provide fast and satisfactory dynamic responses

- The simulation showed it can be used for fast and bi-directional power transfers

- During a single phase fault, transmitted power can be kept constant except a small oscillation

- During a three phase fault however, the voltage strongly reduces the power flow

- When the fault is cleared, normal operation is recovered fast

REFERENCES

• HVDC Light: A new technology for a better environment, IEEE Review 1998

• Performance analysis of VSC based HVDC system under fault conditions, Khatir Mohammed, Amiri Rabie 2000

• Topologies for VSC transmission, Anderson 1999

• Power system stability benefits(sweden 2004)

FUTURE??????