20-04-23

Challenge the future

DelftUniversity ofTechnology

Fault Ride-Through Strategies for VSC-Connected Wind Parks

Ralph L. Hendriks, Ronald Völzke, Wil L. Kling

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Contents

• Introduction• Technical requirements for grid connection• VSC transmission system outline• Influence of converter (de-)rating• Energy dissipation• Fast power reduction

• Direct communication• Voltage reduction• Frequency droop

• Design optimization• Conclusions

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Introduction

Future wind parks•will be situated far from load centres, long transmission

distances•will have high power ratings (hundreds of megawatt)

Application of HVAC transmission is limited by charging current of cables

HVDC transmission can overcome these limitations. Two types:•Current-source converter•Voltage-source converter

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Grid connection of wind power

Transmission system operators require well defined technical behaviour from wind power plants

During faults in the power system, wind power plants are usually required to:

•remain connected during and after the fault (fault ride through)

•support system restoration by supplying reactive current

Wind turbine generators have been further developed to comply to these requirements

Technical requirements

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Grid connection of wind power

Technical capabilities are required at the point of connection

For VSC-connected wind power plants, the behaviour during faults is completely determined by the properties of the VSC transmission system

Different types of wind turbines!

Technical requirements

U

t0 t1 t2

U0

Umin

Un

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VSC transmission system overview

BR

BR

WPVSC GSVSC

WP

• Two-terminal link connecting wind park to active network• WPVSC functions as a slack node, absorbs all power• GSVSC controls direct voltage• Converter type does not impact general applicability of

presented strategies

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Converter (de-)rating

Power electronic switches have hardly over-load capability

Current limit must be maintained at all times

De-rating could improve FRT performance

Q

P

Un = 1.1 p.u.

Un = 1.0 p.u.

Un = 0.9 p.u.

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Energy dissipation

• Control of the direct voltage during faults using a dissipative device• Power electronic control is required (chopper)• Power electronic switches will constitute a high price for this solution• Thermal aspects need to be considered

BR

BR

WPVSC GSVSC

WP

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Fast power reduction

Wind-park side VSC signals power reduction order to turbines through a communication link

Only applicable for turbines with controllable converters

Typical time delay 10–100 ms

Reliability is an issue

Communication

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Fast power reduction

Wind-park side VSC sinks the AC voltage to reduce the incoming power

Inherent reaction from directly coupled induction generators

The success for wind turbines with power electronic converters depends on the ratings and controls of the converters

Standard FRT methods need to be disabled

Voltage reduction

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Fast power reduction

The frequency in the wind park network is increased to signal power reduction

Inherent response from directly-coupled induction generators

Additional droop characteristic in turbine control necessary

Speed of frequency measurement is an issue, PLLs tend to be slow

Frequency droop

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Design optimization

Converter de-rating and dissipation load to higher investment costs

Strategies can (parly) be combined to realize reliable FRT solutions

System can be optimized by formulating boundary conditions and optimization methods, such as linear programming

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Conclusions

The FRT behaviour of VSC-connected wind parks is greatly determined by the design and control of the VSC-system

Grid-code compliance with respect to FRT could be achieved by de-rating, dissipation of excess energy and fast reduction of incoming wind power

Fast power reduction methods yield lowest additional costs

Optimized design could combine several FRT strategies