8 Power Electronics for Wind and Photovoltaic Power Plants

• Market liberalization

  allowed decentralized power generation, creating prosumers, typically small scale power

  generation (CHP) and REN sources (mostly volatile sources, such as PV and wind)

• 𝐂𝐎𝟐 certificates, Emission Regulations

• Unbundling of power generation and grid operation

• Unbundling of TSO and DSO

275 trillion$

or

9 trillion$/year

= 10 % World economic product

Fixed speed generators with direct grid connection

▪ Induction generator with direct grid connection („Danish concept“)

Variable speed generator systems with frequency converter

▪ Synchronous generator (permanent magnet excited) with full converter

▪ Doubly-fed induction generator (active synchrounous generator)

je vor/nachteile siehe vl

• Advantages

  ▪ Variable speed

  ▪ Low dynamic stress

  ▪ Better utilization of aerodynamic efficiency of the rotor

  ▪ Higher efficiency due to permanent magnets

  ▪ Decoupled control of active and reactive power

• Disadvantages

  ▪ Converter and filter must be designed for full rated

  power of the plant

  ▪ Complex and expensive construction for high power

  ratings (MW)

  ▪ Overall efficiency is affected by the efficiency of the

  converter

• Advantages

  ▪ Variable speed (high efficiency and low dynamic stress)

  ▪ Decoupled control of active and reactive power

  ▪ Design of converter just for 25% of rated power of the

  plant

• Disadvantages

  ▪ Slip rings cause

  ▪ Higher wear and tear

  ▪ Resistance

  ▪ sensitive to network disturbances (no converter between

  stator and grid)

• The frequency 𝑓2 of the rotor currents has to be adjusted by the inverter when the speed of the rotor changes

wind power plant with direct grid connection

• Advantages

  ▪ Simple construction

  ▪ Cheap and low maintenance (especially for squirrel-

  cage rotors)

  ▪ Blade angle adjustment not required

• Disadvantages

  ▪ High dynamic stress of wind power plant

  ▪ Fluctuations in wind power are fed unfiltered to the

  grid

  ▪ Stiff coupling to the grid

  ▪ Reactive power compensation for magnetizing current

  even for open loop operation required

central requires string diodes,If one string fails (e.g. due to shading), it must not take up the current

of the other strings

string invertor

hybrid structures

micro inverter

DC-DC converter

DC-AC inverter

dependent on solar radiation

dependent on temperature