Lecture 11 - HVDC

Question 1

What is HVDC and why is it used instead of AC?

Answer 1

Transmission system that uses (high voltage) DC system with large amount of power.
AC-cables have capacitors, which creates reactive power, and therefore are AC-cables only used for a few km. HVDC is therefore better for long distances.

Question 2

Describe the cost relationship between AC and DC lines

Answer 2

DC lines have a higher investment cost, but since AC lines needs reactive power compensation the total cost for DC lines will be lower if the transmission distance is big.

Question 3

Name the basic functions for power flow control

Answer 3

Active power control
DC-voltage control
AC-voltage or reactive-power control

Question 4

Name the additional functions for power flow control

Answer 4

• Frequency control
• Flicker control
• Harmonics mitigation

Question 5

Name the different HVDC configurations

Answer 5

Back to back
Multi-terminal systems
Point to point transmissions (Mono + Bipolar)
(Lättast att kolla på bilderna på s. 12 på L11)

Question 6

Name 3 different HVDC technologies

Answer 6

• Line-commutated Converters (LCC) (Classic)
• Capacitor-commutated Convertes (CCC)
• VSC-HVDC

Question 7

What is EU:s vision overall with a super grid?

Answer 7

Use different energy sources in EU and a super grid (=interregional HVDC grid) could work as a “super highway” for electricity in EU
For example:
Hydro power & pump storage - Scandinavia
Solar power - S.Europe, N.Africa & Middle East

Question 8

Name some development focus for a supergrid / interregional HVDC grid

Answer 8

• Needs several protection zones for DC earth faults
• HVDC breakers and fast protections
• Grid power flow control
• Master control: start/stop, re-dispatching (big aspect!)