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Understanding HVDC System for Offshore Wind Power

image credit: bauaelectric.com

sushank sachan's picture
owner bauaelectric

I write on Electric Vehicles and Grid Related Topics at www.bauaelectric.comI work on DC/DC Converters used in EV, HEV for power conversion application

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HVDC Power Flow Overview

  • Off shore wind farm generates 30kV AC power, which is than fed to HVDC system
  • HVDC system converts to AC power to DC power with a higher voltage of 350kV, why DC power because it helps in transferring more real power with minimal losses over a long distances
  • HVDC system helps in interconnecting two networks with different characteristics and frequencies.
  • Once converted to DC it is transmitted over long distances and fed to onshore converter station where the power is again converted to AC with country standard power specifications

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  • Once converted to AC it is fed to grid distribution network, from where it reaches to end consumers after going through again different step down processes 

Why HVDC System ?

Following is the reason why HVDC is preferred

  • HVDC helps in interconnecting two AC power with different characteristics such as frequency.
  • Helps in transferring more power due to less losses (no reactive power loss, least corona loss, no skin effect, no induction losses)
  • Decreased transmission cost above the breakeven distance as compared to HVAC transmission
  • Break-even distance vary’s for Overhead lines, submarine cables and underground cables.
  • The least breakeven distance is for submarine cables

Off Shore converter station

  • AC power generated from wind farm is fed to Transformer to boost the voltage level through switchgears for safety and protection purpose
  • Supply from the transformer is fed to Valves room where AC power is converted to HVDC, in between these two AC reactors are used for protection aginst such as surge current, etc
  • DC power generated is fed to onshore converter station where DC is converted back to AC

HVDC Generator or AC-DC converter

Line Commutated Converter (LCC)

  • LCC also known as current source converter works on thyristor based technology for power conversion
  • Has high current and voltage carrying capability in the order of kV and kA
  • Turns off only at the zero current crossing
  • Inductive energy storage, hence the name current source converter
  • Current polarity remains constant at the DC lines
  • Requires converter transformer
  • Poor in reactive power control
  • Requires AC and DC filter for harmonics and distortion removal
  • Used for bulk power transfer

Voltage Source Converter (VSC)

  • Voltage source converter works on IGBT technology for power conversion
  • Has lower current and voltage carrying capability
  • Provides ease of control as it can be turned off at any instance of AC voltage
  • Capacitive energy storage hence the name voltage source
  • Voltage polarity remains constant at the DC link
  • Requires conventional AC transformer 7.Good in reactive power control 8.Used for power transfer from remote areas with renewable energy

Main components of Voltage Source Converter

Transformer

Usually, the converters are connected to the AC system via transformers. The transformer has the main purpose of transforming the AC voltage in to a level suitable to the converter.

Converter Reactor

Converter reactor blocks the harmonic current and limit the short circuit current

DC capacitor

Stores the energy and smooth’s the DC voltage

Converter

Converts the power form AC to DC

AC filter

AC filter act as high pass filter which bypasses the high frequency components to ground and reduces the harmonics in the input AC voltage

Types of Voltage Source Converter (VSC) – Overview

Two level converter – 2 level converter means switches are controlled in such away that AC voltage has only +Vdc/2 and –Vdc/2 voltage levels. Shown picture is of single phase AC

Three level converter – in 3 level converter switches are controlled in such away that AC voltage at the output of converter has 3 voltage levels, +Vdc/2, 0, -Vdc/2. Shown picture is of single phase AC

Multi level converter – in multilevel converter the AC voltage output has many levels of voltage for positive and negative cycles this is done to improve the harmonics and get AC voltage more sinusoidal

Modular Multilevel converter (MMC)

Modular multilevel converter topology gives the flexibility of increasing voltage level for both positive and negative half cycles unlike two level or level converter where the voltage levels can be increased upto +vdc, 0, -Vdc only.

Shown in the picture, DC to AC conversion using MMC, each phase contains multiple string and each string contains multiple cell of half bridge.

The cells are controlled in such away that the output line to line fundamental AC voltage is sinusoidal, thus requires less compensation for reactive power and least AC filters.

Shown voltage output is of 5 level converter, +vdc/4, +vdc/2, 0, -vdc/4, -vdc/2

HVDC System Conclusion

in this article we studied about about the basics of HVDC System for offshore wind power and the Overview of Converter that is used to enable this technology viz LCC, VSC, MMC.

we have also seen the overview of the HVDC Substation and the power flow through it.

sushank sachan's picture
Thank sushank for the Post!
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Ron Rebenitsch's picture
Ron Rebenitsch on May 10, 2021

A couple questions...

What is a typical breakeven distance for DC vs AC for submarine cable?

Would ultracapacitor vaults, with IGBT located ahead of the grid AC transformers be of value; either in displacing one or more of the DC to AC converter components, smoothing surges/dips, or improving power quality?

sushank sachan's picture
sushank sachan on Jun 9, 2021

hello ron

if cables needs to run beyond 50km under water than HVDC is preferred to save the cost, because HVAC will cost more for transmission due to high capacitive losses.

 

ultracapacitors will again cost higher, which in turn make the electricity generation costlier also the maintenance would also require. the best solution is to use HVDC and convert to AC

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