Optimisation and Performance Evaluation of Non-Overlap Wound-Field Converter-Fed and Direct-Grid Wind Generators

Non-overlap winding technology continues to remain relevant in the design of wound-field machines as an alternative for high torque density permanent magnet machines. In this paper, the finite element analyses-optimisation of two variants of the non-overlap wound-field machines viz., wound-field flux switching machine (WF-FSM) and phase shifting wound rotor synchronous machine (WRSM), are compared, first in terms of their performance for large-scale converter-fed wind generator drives, then experimentally as sub-scaled converter-fed versus direct grid-connected wind generators, respectively. This study is unique because there are no prior attempts to design, optimize and analyse on these machines for medium-speed wind power generation, as well as experiment on sub-scale prototypes for direct-grid and converter-fed operation. All investigations are contemplated in the medium-speed wind generator drivetrain which provides a tradeoff for generator efficiency and size. From the global optimisation of both machines at large-scale power levels, the torque per mass of the WF-FSM is found to be 50 % lesser compared to the WRSM. This is due to approximate volume, with closely matched optimal split and aspect ratios. In terms of the sub-scaled experimentation, both generators can easily vary their generated output power to match with varying wind resource, but direct grid-connected WRSM generator yields better efficiency performance compared to the WF-FSM converter-fed operating mode, given that the generator terminal voltage of the former is highly regulated.

and sourcing constraints [9]. Non-permanent magnet machines, such as wound-field machines, have generally lower overall material costs. Classical wound-field machines, such as the wound rotor synchronous machine (WRSM), have typically high losses associated with the brushed rotor windings as well as low power density [8]. On the other hand, the wound-field flux-switching machine (WF-FSM) is an example of a non-classical wound-field machine and it is characterised by high torque densities due to the flux modulation nature [10]. The WF-FSM is designed with the windings on the stator, which make them brushless among other things. Also, due to their large pole numbers for designs exhibiting high winding factors, as well as the so-called DC winding induced voltage pulsation, it is difficult to fix their voltages and frequency to the conventional grid, thus they are usually converter-operated [11], [12].
Meanwhile, interest in non-overlap windings has reignited due to the associated lower manufacturing costs, lower cogging torque and improved torque density [13]. They have been common in permanent magnet (PM) machines, but recent developments have focused on non-PM electric machines [14]. Overlap windings are characterized by a sinusoidal magnetomotive force (MMF) when the coils/slot/phase is high. This means that some MMF harmonic content is reduced, which translates to improved machine performance, but the winding manufacturing becomes more complex. Moreover, end-winding lengths are also very large, and the slot filling ability is low. By the adoption of nonoverlap windings, many of the associated disadvantages of the distributed winding layout can be negated such as leading to a simple winding layout and reduced end-winding losses. The main disadvantage, however, is that the MMF spectrum of non-overlap windings consists of many sub-and higher-order harmonics. Harmonic reduction techniques based on phaseshift winding shows an improvement of the main harmonic, while the sub-and higher-order harmonics are largely diminished [15], [16].
The phase-shift winding WRSM, which exhibit fluxvariation characteristic and reactive power compensation, typical in wound-field excited generators, is a very attractive option for direct grid-connected wind power generation [17]- [18]. The flux-variation characteristics allow for regulation of the phase voltage when the wind speed varies.
To this end, this study is a follow-up on the study discussed in [19], wherein the non-classical non-overlap phase shifting winding WRSM is designed, optimised and compared alongside the WF-FSM for 3 MW medium-speed wind power generation, for the first time. At such power levels and generator drivetrain, the attempt is to produce alternative wind generator technologies, which could compete with wellknown PM variants in the niche utility scaling and drivetrain technology of future of wind power generation [2], [20], [21]. The intent of the current study is to experimentally evaluate the sub-scale performance of the phase-shift winding WRSM for direct-grid wind power generation as against the WF-FSM converter-fed wind generator drive, for the first time and with both machines designed in the medium-speed wind turbine operating regime. The study still highlights as already done in [19], the optimization and theoretic performance evaluation of both generator variants for medium-speed converter-fed systems at 3 MW power levels.
The design optimisation conducted is to be based on a multi-objective multi-variable optimization algorithm -NSGA-II -originally proposed by authors in [25]. The highlight of this algorithm is in its ability to evolve multiple solutions of the competing objective parameters from a pool of both parental and offspring populations into a converged true Pareto-optimal front. It is important to emphasise that the accuracy of the optimisation is further linked to the tuning of specific parameter settings in line with the population size and the maximum number of generations. Hence, similar parameter settings are implemented at fixed population size of 20 and maximum iterations of 100, for both machines. The optimisation problem is formulated as: where F( ̅ ) is a vector of the objective parameter functions, comprising the total active mass (MA) and per unit torque ripple (κδ) of the prescribed machines. These objectives have been nominated based on the following considerations: • A lower active mass is adjudicated to ensure a better power/mass ratio. The lower the mass, the less material is required and in turn, the cheaper the machine and the lower the top-tower mass. • Torque ripple is important for large electric machines, and especially for wind generator applications. Thus, very high torque ripple translates to vibrations in the machine, affecting its structural stability and causing excessive wear on the drivetrain components. • The constraints placed on the efficiency and power factor are also important for the optimum performance of the proposed wind generator drivetrains.
In [26], it is reported that a high efficiency/power factor unit reduces the kVA and costs of the drivetrain converters.
A number of design parameters are independently varied in both machines such as the phase current angle (α), armature current density (J), field current density (JF), stack length (lst), stator outer diameter (Dout), stator interior diameter (Din) and rotor shaft diameter (Dsh), to mention a few. Based on the number of dimensional and non-dimensional variables, a total of 14 and 21 design variables were realised in the WF-FSM and WRSM designs, respectively. A reflection on some of the key base machine parameters conceived before the optimisation process, and the common design constants in both machines are presented in Table I; the shaded columns report data captured for the optimal benchmark designs as later elucidated in Section III.
Dimensional and non-dimensional parametric optimisation is a common technique in modern electrical machine design and optimisation [27]. The choice of dimensional parameters such as stator outer and inner diameter because of the objective to reduce active mass, which we understand depends to a large extent on the conceived split ratios for wound-field machines [28]. In addition, the selection of nondimensional parameters such as current density and current angle is because of tracking of performance variables such as torque and power factor, for which their optimal values yield tradeoffs [23].
The analysis algorithm as shown in Figure 3 is based on an in-house 2-D finite element analysis (FEA) package (SEMFEM) [29], which is then coupled to the optimiser (VisualDOC ® ) for the optimisation process as shown in Fig Figure 4, where the converge criteria is calculated and validated in FEA ( Figure 3). The problem is looped continuously for convergence until the assigned generations are completed. The convergence to the true Pareto-optimal front in both machines is then adjudicated after the optimisation process, with the results and the benchmark performance comparison of both machines considered next.   Figures 5 and 6 show the optimum data of the Pareto-optimal front, as well as dispersion of the feasible solutions. Two optimal benchmark designs, one from each of the studied machines, earlier presented in Table I,  A tradeoff between the contrasted objectives (active mass and torque ripple) of both machines is portrayed in Figures 5  and 6, where increase in active mass results in torque ripple reduction, and vice versa. Also, the optimised torque ripple values are minimised to less than 5 %, especially for the WF-FSM which is notorious for high cogging torque due to its double salient structure. For the same torque ripple value, the active mass of the WF-FSM design is two times more than the WRSM design, although both seem to maintain the same torque per litre volume. The significantly higher mass of the WF-FSM is due to its inherent stator-mounted nature as both the phase and field windings housed on the stator, with the split and aspect ratio closely matched as shown in Table II.

III. OPTIMISATION RESULTS AND COMPARISON
In Table I, a further comparative data of the optimal benchmark designs, a higher phase current density is realised in the WRSM compared to the WF-FSM, due to larger slot filling areas of the phase windings. A linear interpolation of the optimisation dataset is undertaken of which the relationship between the stator outer diameter and the phase current density, as evinced in Figure 7, clearly shows an inverse relationship. The behaviour of Dout reflects the slot area allocation in such a way that the stator slots house only the phase windings in the WRSM, whereas for the WF-FSM, both the phase and field windings are collocated on the stator. Hence, one can see that as Dout decreases in the WRSM, J increases, whereas, for the WF-FSM, Dout increases with increase in J.
The per unit reactance of the optimal benchmark machines are compared as shown in Table III. Note that, Xd is reportedly greater than Xq because they are both evaluated at α = 70º. Although with 12 % deviation, note that Xs can be approximately prescribed at 0.5 pu for both machines. This is due to similarities in constraints imposed on the airgap length, power factor, power and supply current of both machines. It is reported in [30] that such small values (Xs << 1 pu) would be profitable to the converters, in terms of reduced loss, KVA rating and cost. However, it can be argued that the fixed airgap length ( = 3 mm), assumed for both machines, influenced the low Xs value obtained in the WRSM more than the WF-FSM considering the smaller stator outer diameter of the former [31]. Figure 8 maps the 2-D FEA flux line distributions and flux densities for the optimal machine benchmarks at rated load conditions. The peak airgap flux density is observed at 2.84 T and 2.52 T in the WF-FSM and WRSM, respectively. The higher peak airgap flux density recorded in the WF-FSM is due to cross-coupling effects [23].
The higher-level saturation in the rotor core of the WRSM compared to the WF-FSM is due to very high ampere-turns of its rotor field windings, as elucidated by the optimum current density values in Table I. Such oversaturation of the rotor core has been attributed to high torque ripple effects in WRSMs [32]. The issue could be addressed by decreasing the slot fill factor of the field windings, but at the expense of declining the optimal torque density of the WRSM. In Figure  9, the 2-D FEA predicted torque waveforms of the optimal benchmarks under rated load conditions are shown.
An effective comparison on the studied machines has been undertaken as shown in Table IV. A substantial reduction in size and mass of the proposed WRSM wind generator for the same power level and performance constraints as that of the WF-FSM has been observed. However, other factors need to be properly weighed such as fabrication and maintenance costs, which should be lower for the WF-FSM. But more importantly, the non-utilisation of PMs has been fully demonstrated in the studied wound-field machine topologies.

IV. EXPERIMENTAL DEMONSTRATION
In this section, sub-scaled prototypes of the WF-FSM [28] and the phase-shifted winding WRSM are demonstrated [24]. The machine design and rated parameters of both prototypes are presented in Table V. As earlier indicated, the WF-FSM is tested as a converter-fed generator while the WRSM is tested as a direct grid-connected generator. Figure  10 shows the test rig for the 10 kW WF-FSM generator prototype, while Figure 11 shows that of the 3 kW WRSM. Due to voltage limitation of the power converters, measurements conducted for the WF-FSM were done at half the prescribed rated conditions. The comparison of the no-load and rated torque profiles, as well as voltage regulation, between FEA and measurement for both prototype machines, is shown in Table VI. The disagreement between simulated and measured results is due to manufacturing defects on the prototypes as well as unsolicited fringing effects in the FEA models, especially for the WF-FSM. The finite element analysis process cannot fully account for the end leakages, especially with errors during the manufacturing process. The improper estimation of the end leakages means that the actual phase winding inductances are not in line with the theoretical values. This is also compounded by constraints on the fill factors while mounting the windings. In Figures 12 and 13, the generator efficiency is evaluated for the WF-FSM and WRSM, respectively, under rated phase current and varying load conditions of the field current. It is clear from both figures that more power can be generated as field current is increased, which can be easily aligned to the varying wind resource for optimum power capability. Additionally, it can be noted that, unlike the WF-FSM converter-fed generator, the WRSM direct grid-connected generator offers higher efficiency performance across a diverse operating load regime. No doubt, the wider improved efficiency range of the WRSM is due to a better voltage regulation recorded in Table VI, since the generator terminal voltage is tied to the grid voltage.
Also, the experimented steady-state heat maps for the WF-FSM at different temperature hotspots, as well as the thermal   Figures 14 and 15. These hotspots are prioritized by careful inspection of the broad surrounding of the machine's stator under prolonged short-circuit tests as captured in Figure 16 using a digital thermographic camera. The different hotspots show the rapid thermal build up in parts of the field coils compared to the armature coils. This is due to the intrinsic positioning and higher current density of the former, although the performance of the machine is not threatened as the risk of demagnetization is zero. Regarding the WRSM, only the temperature rise over time of the prototype casing under rated field and phase current is shown in Figure 17. This is because, unlike the WF-FSM prototype, the WRSM prototype is built within an enclosed frame. Number of rotor poles 10 16 Number of stator slots 12 18 Rated speed (r/min) Rated terminal power (kW) 10 3 Lastly, the electromagnetic torque versus load current profiles of the WF-FSM and WRSM are provided in Figures 18  and 19, respectively. The comparison between FEA and experiments show good match, although due to its stator-mounted nature, some notable discrepancy is observed for the WF-FSM due to increased saturation with increasing load current, as well as manufacturing defects.

V. CONCLUSION
In this study, finite element analysis-based optimisation on large-scale and experimentation on sub-scaled power levels of two distinct non-conventional, non-overlap wound-field machines have been investigated for medium-speed wind generator drivetrains in converter-fed versus direct grid-connected modes, for the first time. The comparison is based on 10/12 pole/slots wound-field flux switching machine (WF-FSM) and 16/18 wound-rotor synchronous machine (WRSM). Based on benchmark design candidates from the global optimisation of both machines at large-scale power levels, the torque per mass of the WF-FSM is observed to be half of that of the WRSM subjected under the same torque per litre. The optimal mass discrepancy in both machines could be attributed to the evolution of closely matched optimal split and aspect ratios. In terms of sub-scaled experimentation, the WF-FSM was tested in converter-output mode while the WRSM was tested in direct grid-connected mode.
The experimental results clearly show that both generators easily vary their generated output power to match with varying wind resource, but the latter can offer an improved efficiency range since the generator terminal voltage is better regulated.