Low Voltage Ride Thru Technology

In renewable power generation the wind energy has been noted as the most rapidly growing technology; it attracts interest as one of the most cost-effective ways to generate electricity from renewable sources.

Because of some challenges such as conventional energy sources consumption,  pollution,  global climate change and security of energy supply, significant efforts have been made to develop renewable energy sources such as wind energy. Wind power growth with a 20% annual rate has experienced the fastest growth among  all  renewable energy sources since five years ago. It is predicted that by 2020 up to 12% of the world’s electricity will have been supplied by wind power.

What is Low Voltage Ride Thru Technology

In terms of wind power generation technology as  a result of numerous technical benefits (higher energy yield, reducing power fluctuations and improving var  supply) the modern MW-size wind turbines always use variable speed operation which is achieved by electrical converters.

These converters are typically associated with individual generators and they contribute significantly to the costs of  wind turbines. The variable speed  wind  turbine generators such as doubly fed induction generators (DFIGs) and  permanent magnet synchronous  generators (PMSGs) with primary converters are emerging as the  preferred technologies.

As a result of large-scale wind power generation, interconnecting large wind farms to power grids and the relevant influences  on the  host grids  need to be carefully investigated. Wind farms are now required to comply with stringent connection  requirements including reactive power support, transient  recovery, system stability and voltage/frequency regulation. Further to increase the maximum power extraction the  variable speed  generators are employed. These variable speed generators necessitate a AC-DC-AC conversion systems.

The  generator side converter controls the electromagnetic torque and therefore the extracted power while the grid side converter controls both the DC link  voltage and the power factor.  Moreover  when designing the control strategy it seems that the generator-side converter must control the extracted  power as it is located closer to the incoming power. Hence the grid-side converter would control the DC voltage.

Fulfilling the  new  grid codes constitutes one of the main challenges for the  wind power industry. There are low voltage ride thru technology requirements. Enhancing the operation of wind turbines in front of the grid faults is  mandatory requirement. The wind turbines must stay  connected to the grid during grid disturbances. They should continuously feed the reactive power in  addition to  limited active power. In modern wind turbines the increasing integration of power electronics enables to control the  behavior  of wind generation system under faulty scenarios.

The  function  of an electrical generator is providing a means or energy conversion between the  mechanical torque from the wind rotor turbine as the prime mover and the local load or the electric grid. Different types  of generators are being used with wind turbines. Small wind turbines are equipped with DC generators  of up to a  few kilowatts in capacity. Modern wind turbine systems use three  phase  AC generators.

The common types of  AC generator that are possible candidates in modern wind turbine systems are as follows:

• Squirrel-Cage (SC) rotor Induction Generator (IG);
• Wound-Rotor (WR) Induction Generator;
• Doubly-Fed Induction Generator (DFIG);
• Synchronous Generator (With external field excitation); and
• Permanent Magnet (PM) Synchronous Generator