Elsevier

Renewable Energy

Volume 48, December 2012, Pages 473-481
Renewable Energy

Technical note
Comparison of fault-ride-through capability of dual and single-rotor wind turbines

https://doi.org/10.1016/j.renene.2012.06.010Get rights and content

Abstract

The majority of wind turbines currently in operation have the conventional concept design. That is a single-rotor wind turbine (SRWT) which is connected through spur gearbox to a generator. Recently, dual-rotor wind turbine (DRWT) has been introduced to the market. It has been proven that the steady state performance of the DRWT system for extracting energy is better than the SRWT. But, a comparison of fault-ride-through capability of these two types of turbines requires further research.

In this paper, the fault-ride-through capability of DRWT and SRWT are evaluated and compared when generating units are operating at constant pitch angle and constant speed modes. Constant pitch angle mode is simulated to investigate the natural damping of DRWT and SRWT. To verify the time domain simulation results, damping characteristics of DRWT and SRWT are also compared through eigenvalue analysis and speed droop characteristics of the control system. The accuracy of the aerodynamic model of the DRWT is enhanced by including the stream tube effect in the simulation. It was uncovered that DRWT introduces higher damping torque to the network in both constant speed and constant pitch angle modes. This advantage improves the transient performance of DRWT-based wind farms.

Highlights

► Dynamics of dual-rotor wind turbine (DRWT) and single-rotor wind turbine (SRWT) are studied. ► Fault-ride-through capability of DRWT and SRWT are evaluated and compared. ► Damping characteristics of DRWT and SRWT are compared through eigenvalue analysis. ► Accuracy of the aerodynamic model of DRWT is enhanced by including the stream tube effect. ► DRWT introduces higher damping torque in both constant speed and constant pitch angle modes.

Introduction

Wind energy is one of the fastest growing energy resources and it is going to have remarkable share in the energy market. Thus, the consequences of the connection of wind turbine, specifically in the form of a wind farm, to the electrical grid must be investigated from steady state, dynamic and transient point of view. Different approaches have been introduced to improve the static and dynamic responses of the wind turbines [1], [2].

The electrical, mechanical and aerodynamic performance quality of the wind turbine is very important to absorb energy as much as possible from wind. In this direction, a new wind turbine generator system (WTGS) has been recently introduced as shown in Fig. 1. This new WTGS, which is called as dual-rotor wind turbine (DRWT), has two sets of rotor systems and is more efficient than the conventional single-rotor wind turbine (SRWT) from the energy extraction point of view [3]. Because most of the aerodynamic torque is generated from the tip portion of the blade, a relatively small auxiliary rotor which is positioned at the upwind location, would compensate for the less effective portion of the main rotor located downwind.

At the time of writing this paper, the authors could trace [4] as the only reference about the dynamic performance of the dual-rotor system. Multi-body dynamics is the employed approach. Although in this paper a model is provided to present the detailed procedures used to show the system dynamic and aerodynamic, however the authors did not compare the dynamic response of the dual-rotor wind turbine with a single-rotor wind turbine. According to [4], the commercial types of dual-rotor wind turbines are able to generate power up to 1 MW.

Even though at the same wind speed and environmental conditions the efficiency of the dual-rotor is higher, nevertheless it does not signify that the transient performance of DRWT is better than SRWT. Obviously, the transient behaviours of the dual-rotor and single-rotor wind turbines are different, because in the dual-rotor system the number, type and arrangement of the components are different.

The objective of this investigation is comparing synchronizing and damping torque introduced by DRWT and SRWT to the network. For getting to this stage both type of wind turbines have been set up in PSCAD software. Drive train method has been employed for modelling the mechanical system of DRWT and SRWT. The electrical characteristics of generator, transformer, transmission line and power system used for DRWT and SRWT are identical to have a fair comparison.

Synchronizing torque is mostly dominated by electromagnetic torque imposed by electrical side. Damping factor of generating units is mostly influenced by their control system mode and natural damping characteristic, which is imposed by mechanical drive. To assess the transient response of DRWT and SRWT, when they are operating in constant speed mode, a temporary three phase short circuit is applied to the power system and post-fault fluctuations of the variable of interest are recorded and compared. To verify the validity of the time domain simulation, the control system is approximated by its speed droop characteristic and damping factors which are introduced by the control system are evaluated analytically.

To evaluate and compare the natural damping characteristic of DRWT and SRWT, the maximum short circuit period for which both generating units are able to keep their stability are checked while the controller are deactivated and both DRWT and SRWT are rotating at constant pitch angle. To verify the simulation results regarding natural damping response, eigenvalue analysis is employed using MATLAB software. The real portion of eigenvalues is a good criterion for assessing the system natural damping.

Additionally, in calculating the aerodynamic torque, the stream tube effect behind the auxiliary rotor disk is neglected in Ref. [4]. This simplification can affect the accuracy of the simulations negatively. In this paper, we have included the stream tube effect into the dual-rotor aerodynamic model which improves the exactness of the aerodynamic model to be more realistic.

This paper is organized as follows: In Section 2 mechanical models of different components of the DRWT and SRWT are presented; In section 3 state space equations of turbine generator set has been derived for eigenvalue analysis; Stream tube effect has been discussed in section 4; The effect of pitch angle control on damping torque is obtained analytically in section; 5 Computer simulation results are conducted in section 6. Although other configurations of DRWT are introduced to enhance the performance of this technology, however, the focus of this paper is on the T gearbox type of DRWT. The authors intend to extend the studies for other types of dual-rotor wind turbines. For example, one promising configuration is created if two rotors are directly coupled to an asynchronous electrical machine: one rotor to the induction windings and the other rotor to the induced ones.

Section snippets

Mechanical dynamic model

In this section, the dynamic models of different components of single and dual-rotor wind turbines are discussed. Fig. 2.a and Fig. 2.b shows the elements of the single and dual-rotor wind turbines, respectively.

SRWT and DRWT state space model

To prove the validity of our studies regarding transient response of the dual and single-rotor wind turbines, the damping characteristic of DRWT and SRWT can be evaluated through eigenvalue analysis. The location of the eigenvalues in each system is a powerful aid to predict the damping factor of the system. For getting to this stage, the state space of induction generator, single-rotor and dual-rotor wind turbines must be calculated and combined appropriately. Both dual-rotor and single-rotor

Aerodynamic model for DRWT

Aerodynamic model of DRWT is different from SRWT to some extent. Since the wind which is flowing through the main turbine in DRWT is disturbed by the auxiliary turbine, then stream tube effect must be included in the aerodynamic torque calculations for DRWT. Through (15) aerodynamic torque introduced by the blades is as follows:TM=0.5ρ·π·R5·CP·ωM2/λ3

With R blade radius, λ the tip speed ratio, ρ the air density and ωM the mechanical speed of the rotor. Cp can be calculated as follows [16]:CP(λ,β)

Damping effect of pitch angle control

The conventional blade pitch angle control strategies are categorised mainly as; a) Generator power control and b) Generator rotor speed control. During the fault, electrical power drops down to a very low value and generators accelerate. Throughout the fault, constant speed wind turbines increase the pitch angle to reduce the captured aerodynamic power to keep the speed constant. On the other hand, constant power wind turbines decreases the pitch angle to restore the electric power and this

Simulation results

The objective of this study is to investigate and compare the dynamic behaviour of the dual and single-rotor wind turbines from different aspects. Both dual and single-rotor wind turbines are set up in PSCAD software. To facilitate, a simple power system has been chosen which is shown in Fig. 12. The dynamic model of the wind turbines are established based on the component models presented in previous sections. The generators are connected to the power system through a step up transformer and a

Conclusions

In this paper, by using PSCAD/EMTDC software, transient response of dual and single-rotor wind turbines have been evaluated and compared. The stream tube effect is included in aerodynamic torque calculation which was ignored in previous work in literature. So, the dual-rotor wind turbine (DRWT) aerodynamic model in this paper is more accurate compared to the previous investigations. The results of the methods uncovered that the DRWT presented higher damping torque to the network compared to

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