An integrated control approach for standalone operation of a hybridised wind turbine generating system with maximum power extraction capability

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Abstract

This paper presents a novel configuration of a hybrid wind generating system which can be used as a remote area power supply (RAPS) system. The proposed wind energy conversion system consists of a doubly-fed induction generator (DFIG) and a permanent magnet synchronous generator (PMSG) where the latter is connected to the DC bus of the DFIG generator. In addition, a battery storage system is also incorporated into the DC bus to address the demand-generation mismatch. Control strategies for individual system components of the RAPS system are designed with a view to achieve an acceptable level of voltage and frequency regulation while extracting the maximum power from wind. The performance of the proposed RAPS system is investigated in terms of voltage and frequency regulation capability under changing wind and variable load conditions.

Highlights

► novel remote area power supply (RAPS) configuration. ► power flow control and control strategies. ► maximum power extraction.

Introduction

Renewable energy schemes are becoming popular among regional and remote communities as a viable method of supplying power [1], [2]. Selection of suitable energy sources to form a hybrid RAPS system depends entirely on the availability of resources within the locality. Among all renewable energy options, wind power has gained the momentum as one of the most widespread and commercially viable renewable energy generation technologies [3]. However, one of the major challenges associated with wind based power generating schemes is their intermittency [4], [5]. To overcome this issue, hybrid remote area power supply (RAPS) schemes can be formed to supply reliable power to the customers [6]. Although hybrid RAPS systems offer attractive advantages and solutions for rural electrification, the design, control and operational aspects involved with such power systems are still challenging [7].

Doubly-fed induction and permanent magnet synchronous machines are more popular and widely used and dominant variable speed wind turbine generator technologies [8]. They offer many advantages over other types of wind turbine systems1 including variable speed operation, maximum power extraction capability, active and reactive power control capabilities and ability to suppress the mechanical stresses [9]. However, the involvement of power electronic devices such as inverters in variable speed wind generators represents significant cost factor in their over-all arrangement and complexity in operation.

Due to the variable nature (i.e. intermittency) of wind, a wind turbine generator alone cannot supply power to meet the load demand continuously. To make it dispatchable, similar to other conventional generation units such as a diesel generator, the generated power has to be regulated at a desired level [10]. With rapid development currently taking place on energy storage devices, their application in wind energy systems is seen to provide a promising opportunity to mitigate the issues associated with wind power fluctuations. Typically, battery storage systems are widely advocated for such remote application owing to high energy density levels [11], [12].

In recent studies, the hybrid operation of standalone variable speed wind turbine generators has been explained predominantly with other types of RAPS components such as energy storage system and diesel generators. The hybrid operation of a standalone power supply system consisting of a battery storage integrated with a DFIG and a PMSG based wind turbine generator is explained in [13], [14] respectively. Furthermore, the modelling aspects associated with the standalone operation of DFIG and PMSG are demonstrated in [15], [16] respectively. However, the hybridised wind energy system consisting of a DFIG and a PMSG together with a battery storage system has not received any research attention and forms the basis of this paper.

The proposed novel wind energy conversion system shown in Fig. 1, consists of a DFIG and a PMSG where the latter is connected to the DC bus of the DFIG. Such an arrangement of two wind turbine generators essentially avoids the necessity of having an inverter for the PMSG compared to the situations where PMSG alone serves AC loads. In addition, the battery storage is used to meet the demand-generation mismatch during over-generation and under-generation situations. The main objectives of this research work are to address the followings: (a) development of a power management methodology, (b) development of individual controllers for each system component, and (c) extraction of the maximum power from the two types of wind turbine generators (i.e. DFIG and PMSG).

Section snippets

Design considerations of the hybridised wind energy system

If the DFIG and PMSG based wind turbines are operated independently, PMSG requires an additional inverter to satisfy the customer demand (i.e. AC loads). In addition, PMSG based wind turbine generating system needs an additional battery storage system and DC link capacitor. With the proposed arrangement shown in Fig. 1, the PMSG can be operated by sharing the components with DFIG such as an inverter (i.e. LSC), a battery storage system and a DC capacitor. However, there are some limitations

DFIG and associated control

The DFIG performs as the main source of energy in the proposed RAPS system. Therefore, the main contribution towards the load side voltage and frequency regulation has to be realised using the converter control associated with the DFIG. In this regard, RSC is used to achieve the voltage and frequency regulation, whereas the DC bus voltage regulation is achieved via the LSC. To achieve these objectives, vector control scheme has been employed for the RSC and LSC. Stator indirect flux orientation

Maximum power extraction from wind

The optimal RAPS operation can be achieved by operating both DFIG and PMSG at their maximum power extraction modes. In the existing literature, several methods have been discussed to extract maximum power from wind. In general, the maximum power extraction from wind can be obtained using (19), (20), (21), (22) [18], [19] .Pa=12Cp(λ,β)Aρvw3λopt=(ωr)optRvw(Pm)opt=kopt[(ωr)opt]3kopt=12(Cp)optρARλopt3where Pa is aero dynamic power, Pm is power output of the turbine, Cp is power coefficient of

PMSG and associated control

When a PMSG alone supplies power to the remote loads, usually it requires a back-to-back converter. However, in the proposed topology, the PMSG is connected to an uncontrolled three-phase diode bridge rectifier11 and it is interfaced to the DC bus of the DFIG via a DC/DC converter as shown in Fig. 4. Considering the voltage constant

Battery storage and associated control

Nickel–Cadmium battery model given in [20] is employed in this paper. Due to limited availability of RAPS system components, it is assumed that battery storage system is able to supply power without having any difficulties. Therefore, as in real life applications, the state of charge detection of the battery storage system has not been implemented within the converter control.

A bi-directional buck-boost converter is used to interface the battery storage with the DC bus of the back-to-back

Simulation results

The suitability of the proposed RAPS system has been investigated in relation to the ability of regulating the voltage and frequency on the load side and also the maximum power extraction capability of the two types of wind turbine generators (i.e. DFIG and PMSG). The RAPS system parameters are listed in Appendix B.

The entire RAPS system has been simulated under fluctuating wind and load conditions. Fig. 8 shows the system response and Fig. 9 illustrates the power sharing among different system

Conclusions

This paper has proposed a novel hybrid wind generating system which can be used to supply power to remote area customers. The overall operation of the RAPS system is enhanced by considering its power-electronic arrangements and by extracting maximum power from wind. These objectives have been realised by avoiding, the deployment of an inverter for the PMSG and developing the control strategies for DFIG, PMSG and battery storage system. In addition, the primary voltage and frequency control have

Acknowledgements

This work is supported by the Australian Research Council (ARC) and Hydro Tasmania Linkage Grant, LP0669245. The authors gratefully acknowledge the support and their cooperation.

References (20)

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