Independent power flow control of multiple energy sources using a single electric machine

In this thesis, an independent power flow control of different energy sources connected to a single electric machine with a multitude of three-phase winding sets has been investigated. These machines are highly suitable for high power and critical applications. Additionally, these machines utilise the well-established three-phase power electronics technologies. The interest towards electrification of the transportation systems makes having multiple energy source a viable solution in the near future. Independent power flow control will enable the integration of hybrid energy storage systems on electrical vehicles such that the regenerative power can be directed to a super-capacitor while the cruising power is consumed from a battery bank. Nevertheless, this technique can be envisaged for different applications, from wind turbines to microgrids. In order to make all of this possible, the current amplitude of each winding set needs to be controlled first. Therefore, the control of the individual winding set’s currents’ amplitude and direction for multiple three-phase machines is the main subject of this thesis. The developed control schemes are based on vector space decomposition (VSD) rather than multi-stator (MS) approach. The former approach has a unique harmonic mapping and a single flux and torque producing subspace. Primarily in the thesis, current sharing strategy has been developed for both symmetrical and asymmetrical multiple three-phase machines with a common mode of operation for all the winding sets (motoring or generation). The strategy is based on the correlation of the xi-yi currents of the VSD and the αi-βi currents of the MS approach. These links enable the control of the current amplitude of the winding sets separately while maintaining the same torque and speed. The correlations between these modelling approaches combine good features of both modelling methods, the ability of the MS approach to control each winding set individually, and the VSD feature to perform the control in a completely decoupled subspace. Afterwards, the same strategy is employed to change the power flow direction as well as the amplitude of the multiple three-phase winding sets currents such that concurrent motoring and generation mode of operation is established. Two novel power sharing schemes have been proposed and analysed in this thesis. Both are based on VSD. The first scheme is sharing the flux and torque producing currents equally, while the second one is controlling the power by the torque producing current while preserving the same flux producing current. The transferred power efficiency has been improved significantly using the second approach. The same power sharing technique has been applied to an unorthodox type of machine – a twelve-phase machine implemented as a six-phase machine with double winding (hence, consisting of two six-phase sub-machines). The proposed power sharing scheme here is using a hybrid control approach combining two vector control schemes, based on MS and VSD. The control based on MS is controlling the power transfer from one six-phase sub-machine to the other one, while the control based on VSD, and with auxiliary current control, is sharing and directing power to a specific three-phase winding set within each sub-machine. Last but not least, two novel regenerative test methods have been proposed for multiple three-phase machines. The first approach is based on utilising a modified power sharing control strategy to operate the machine with iv rated current while maintaining the speed and circulating the power among the winding sets. The approach can be implemented differently based on the number of winding sets. With an even number of neutral points, half of the winding sets will be in motoring while the other half are in generation mode. However, when there is an odd number of winding sets, one of the winding sets will be in no-load mode of operation. The second approach is implementing the motoring and generation of the winding sets using a unique y-current component of the VSD. This method is only applicable to multiple three-phase machines with an even number of neutral points. The regenerative test can be applied to induction and synchronous machines equally but with a completely different outcome. For synchronous machines, the test can be used for efficiency evaluation and temperature rise test while for the induction machines the test can provide a straightforward experimental approach to segregate constant losses (core and mechanical losses) from load dependent losses (copper losses). All the proposed control methods have been validated by simulation and experimentally, except for the double winding machine where experiments were not done.