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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Innovative concepts for drive train components' taking place on Thursday, 13 March 2014 at 11:15-12:45. The meet-the-authors will take place in the poster area.

Eduardo Prieto CITCEA-UPC, Spain
Co-authors:
Eduardo Prieto-Araujo (1) F P Oriol Gomis (1) David Lavèrnia (2) Adria Junyent-Ferre (3) Oriol Gomis-Bellmunt (1)
(1) CITCEA-UPC, Barcelona, Spain (2) IREC, Barcelona, Spain (3) Imperial college, London, United Kingdom

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Abstract

Distributed control of a nine phase permanent magnet generator and scaled platform validation

Introduction

Nowadays, there is an increasing number of offshore wind farms. In these locations, larger wind turbines can be installed and usually the average wind speeds are higher. However, offshore installations imply a limited accessibility to the wind power plants, thus reliable machines with low maintenance needs should be installed. The Direct drive Permanent Magnet Synchronous Generator (PMSG) with more than three phases is one of the topologies that accomplishes these requirements, increasing the redundancy and the fault tolerance capability of the system.


Approach

Recently, with the growing interest on the offshore wind power plants, factors as reliability and efficiency gained importance. Three-phase PMSGs are one of the topologies which could be installed in the offshore plants. However, back-to-back converters offer other possibilities which could be profitable for offshore operation. Since the generator is isolated from the grid, the number of the generator phases is not limited to three, so redundant structures based on multi-phase machines can be implemented [1].

One of the proposed technologies is the direct driven multipolar PMSG with multiple stators, which may include two or more three-phase winding sets with isolated neutral points [2]. Each stator is connected to a three-phase back-to-back converter in order to control the power flowing through each of them. This structure increases the reliability by means of redundancy, allowing to perform different fault tolerance strategies. Moreover, these redundant structures reduce the amount of power flowing through each converter, compared to a single three-phase one, decreasing the stress suffered by the power electronics.

These new multi-phase topologies show several advantages in comparison with other classical structures. However, the system control complexity is higher. Different machine stators are now included in the same machine slots, interacting between them. This interaction not only must be known, but also must be taken into account during the design stage of the machine torque controllers. In order to increase the concept viability, this controllers should be designed to extract the maximum profit of the machine topology minimizing the effects of having more that one stator inside a machine.


Main body of abstract

This work proposes a distributed control of a direct driven triple three-phase stator machine connected to the grid by means of three independent back-to-back converters (Figure 1). Due to the fact that the windings of the different stators are wound within the same stator slots, the magnetic interaction between them must be analized in detail, because changes in one of the stator phases can interact with the others. For these reason a detailed model of the machine must be obtained, assuming non-zero mutual couplings between the three stators. This model is developed combining classical methodologies of electrical machine modelling and finite element modelling software to confirm the theoretical calculations.

Once the machine model is defined, the torque controller of the machine is designed based on the obtained system equations, considering that no communications are available between the different stators. For this reason, the torque control is divided in three different controllers, each performed by the machine side converter of each stator. Each converter carries out its own controller allowing the system to work with three different modes of operation, considering one, two or three active stators generating power at the same time. Also, the transitions between the different operating states are analized, assuming that any of the three stators could be connected or disconnected depending on different factors. The controller is also designed to reduce the dynamic interaction between stators during current transients due to the coupling effect, avoiding large currents in the stators caused by a current change in another stator. Also, the controller is able to regulate active and reactive power independently on each stator. For example, flux weakening strategies injecting reactive power while maintaining the power production can be achieved, among others. Besides, the controller increases the failure tolerance of the system, because each stator is operated totally independent of the others. Then, if one of the three-phase stators is disconnected for any reason, the system remains operative, if the failure is not affecting the healthy stators and converters. Regarding the grid side converters, they perform the DC bus voltage control of the three different back-to-back converters respectively, injecting all the power that is being generated by the PMSG to the grid. These converters are also able to regulate active and reactive power independently. Simulations have been carried out to validate the system control behavior, showing a proper independent operation of the generator control system.

Once the controllers are tested in simulation, the control algorithms have been validated in a scaled wind turbine test rig. This rig is composed by two permanent magnet machines, one acting as a motor and the other as a generator joined mechanically by their axis. The motor is connected to a frequency converter and the generator is a nine-phase generator with three independent stators, connected to the grid by means of three back-to-back converters. Figure 2 shows the machines joined by the mechanical coupling and Figure 3 illustrates the cabinet where the frequency converter and the back-to-back converter for the multi-phase machine are enclosed. The motor acts as a wind emulator, using the frequency converter to regulate the speed of the axis. The generator regulates the machine torque by means of the designed distributed controller. The grid side converters are also including the designed DC voltage controllers to inject the active power to the grid, and its independent reactive power control for grid support. Figure 4 shows the currents flowing through the phase 'a' of each stator while each converter is regulating the same amount of machine torque. As it is expected, their values are identical and it can be seen that the current flowing through stator two and three are shifted 40º and 80º, from the current flowing through stator one, respectively (the stators are shifted 40º).This and other experiments developed with the test rig show a proper behavior of the distributed control of the generator, validating the developed control design.

Figure 1 - Nine-phase generator connected to the grid by means of three back-to-back converters


Figure 2 - Test rig generator and motor test bench


Figure 3 - Converters cabinet


Figure 4 - Currents flowing through the phase 'a' of each stator


Conclusion

The design of a distributed control of a nine phase wind turbine generator is presented. The control is performed by three back-to-back converters connected to the generator to inject the produced power to the grid. Each converter carries out its own torque control without communications between the other stators. The controllers are designed to achieve the torque demand of the wind turbine controller while reducing the interaction between the different stators. This design is based on the system equations obtained through theoretical development and finite element modelling of the generator. Also, the designed regulators can control the reactive power flowing each stator, if it is desired for flux weakening strategies for example. The grid side converter controllers are also designed to inject the produced active power into the AC grid and to support the grid with reactive power if it is needed. These controllers are tested in simulation and validated in a scaled wind turbine generator test rig. Different experiments are performed to test the behavior of the machine torque control, considering the operation with one, two or three active stators at the same time, also producing the same or different amount of power. All these tests have shown satisfactory results confirming the concept viability and the controller desing. These experiments also show that this generator configuration along with the designed torque control is an interesting proposal for the offshore wind farms. It shows additional capabilities in comparison with the classical three-phase structures, in terms of redundancy, fault operation and control possibilities, which must be considered during the generator topology selection of the future wind turbines.



Learning objectives
This abstract show an interesting generator topology for the offshore wind farms. The control scheme is addressed to maximize the possibilities of the concept. The control proposal is validated with a scaled wind turbine test rig, showing a good performance in different scenarios.



References
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[2] M.J. Duran, S. Kouro, Bin Wu, E. Levi, F. Barrero, and S. Alepuz. Six-phase pmsg
wind energy conversion system based on medium-voltage multilevel converter. In Power
Electronics and Applications (EPE 2011), Proceedings of the 2011-14th European Con-
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