Summary of Dynamic Model for Full-Converter Wind Turbines employing Permanent Magnet Alternators

There are a number of different wind turbine technologies competing for market

share in this growing industry. One of these technologies is the full converter wind

turbine (FCWT) employing a permanent magnet alternator (PMA). This

technology has a number of significant advantages. It effectively decouples the

generator from the grid, improving fault response. It allows the turbine to operate

over a wide speed range, leading to improved power extraction from the wind and

other advantages. So the combined advantages of the FCWT and PMA lead to

search for reliable models to evaluate the impacts of integrating these FCWTs into

the existing grid.

This paper describes the development and testing of a dynamic model for full

converter wind turbines employing permanent magnet alternators. The model

described here is not proprietary and is a generic, manufacturer-independent model

with no restrictions on its use.

In order to tackle complexity, wind turbines can be thought of as a collection of

subsystems which can be modeled individually. The individual subsystem models

can then be assembled into a complete wind turbine model. From a modeling

standpoint, a full converter PMA wind turbine consists of the following

mechanical and electrical subsystems:

• Aerodynamic model for rotor

• Mechanical two-mass model for drive train

• Reference power calculation block

• Pitch controller

• Permanent magnet Alternator (PMA) model

• Rectifier and buck/boost converter models (for DC link voltage control)

• Inverter model (current-controlled)

• Unit transformer and grid representation

The interaction between each of the components listed above determines the wind

turbine model’s steady-state and dynamic response.

After modeling each components complete model was assembled and subjected to

testing. The results show that the desired power curve is found and sharp edges are

smoothened using smoothening technique. In order to test if independent real and

reactive power control has been achieved, two tests were carried out: real power

drop and reactive power rise. The results conclusively show that a change in either

real power or reactive power demand does not affect the other quantity except

momentarily. The pitch control is tested by subjecting it to speed greater than the

rated speed. The result show that the pitch control responding actively and indeed

works in a stable fashion. The turbine dynamics is tested by increasing the wind

speed greater than the rated speed the pitch controller was active. Voltage sag on

the grid was simulated, and the real and reactive power response of the wind

turbine was observed. The intent of the test is to show that the model does indeed

respond to events occurring in the dynamic timescale and that the response of the

machine to this event is realistic. The results of the test shows that the model does

indeed respond to the grid event as expected.

The converter topology of the model described in this paper is a popular one; the

PMA is interfaced to the grid through an AC-DC-AC conversion system. The

converter interfacing the turbine to the grid has to handle the entire output of the

generator (unlike in a DFIG turbine where the converter handles only 30% to 40%

of the generator output) and hence is more cost and lossy, but also provides more

headroom to supply reactive power to the grid. In the past many various Converter

topologies was implemented. The model described in this paper differs from those

models on various points. The model presented here employs a buck-boost

converter which is intended for DC link voltage control.

In conclusion, the development and testing of a full converter wind turbine

employing a permanent magnet alternator has been presented here. This model is

unique in that it employs a buck-boost converter to control DC link voltage. All the

desired outputs are achieved from the tests. An example of the model’s dynamic

response has also been provided. In the future, the model will be used as a platform

to model various controls such as those needed to provide low voltage ride through

(LVRT) and inertial support.