Zai-Qiang Jiang | Jiang-Ping Hu

Abstract—The electromagnetic characteristics and iron loss of a high-temperature superconductor wind generator(HWG) equipped with an overlapped field coil arrangement (OFCA) are studied by comparing with the one equipped with the conventional field coil arrangement (CFCA).Through a quantitative analysis, it was found that HWG with OFCA exhibits better electromagnetic characteristics than HWG with CFCA and can reduce the iron loss by eliminating the magnetic flux sag caused by the adjacent field coil sides with the same current flow direction.In addition, the OFCA topology can further reduce the volume of the wind generator.

1.lntroduction

In the past decade, the threat imposed by the climate change has driven the rapid development of the wind energy technology.The Global Wind Energy Council predicted an increase in the global wind capacity from 237.7 GW at the end of 2011 to 493.3 GW at the end of 2016[1].The Integrated Wind Turbine Design Upwind project in Europe found that 20-MW wind turbines are feasible and is planning a 20-MW offshore wind-turbine design[2].In 2011, China reached the highest installed wind power capacity in the world (62.4 GW)[1]and is currently undertaking a project for 10-MW-class offshore wind turbines[3],[4].

Modern research has focused on lightweight, small-volume, and high-efficiency superconducting wind generators that efficiently utilize wind energy[4],[5].A 10-MW superconducting direct-drive generator weighing approximately 160 tons has been designed by American Superconductor (AMSC).The AMSC design is approximately 50% lighter than the 10-MW permanent direct-drive wind (PMDD) generator[6], whose weight is more than 300 tons and diameter is 10 m.The large weight and diameter of the PMDD generator increase the cost and limit the practical application of the generator from laboratories to factories.The advantage of a high-temperature superconductor (HTS) wind generator (HWG) becomes obvious at the capacities above 8 MW[7].

To realize the practical application of HWGs, many companies, including Siemens[8],[9], General Electric Company[10], Converteam[11], American Superconductor Corporation[6], and Dongfang Electric Corporation[12], have developed superconducting prototype generators to prove the technical viability of applying HTS to large electric machines.However, the high cost of superconducting wires hinders the production of large-capacity HWGs[4],[12].Reducing the cost of HTS wires would render HWGs a very competitive option for large-capacity wind-turbine applications.

The communication loss remains a crucial issue in superconducting armatures.Accordingly, most HWGs are configured with a rotary superconducting field coil and a static copper armature winding[12].The HTS field coil is the key component of a superconducting wind generator because the structure and arrangement of the field coil affect the electromagnetic characteristics of the generator.In general, the HTS field coils in prototype superconducting generators are sequentially arranged outside the support structure, with intervals between adjacent field coil sides with the same current direction[6],[8]-[12].However, in this configuration, magnetic flux sag is generated above each interval, introducing high-order harmonics and iron losses[13].

This study proposes an overlapped field coil arrangement (OFCA) that optimizes the electromagnetic characteristics.The design process of OFCA is described in detail, and the characteristics of HWG with OFCA are compared with those of HWG with the conventional field coil arrangement (CFCA).

2.Configuration and Design Process

Fig.1.Schematic of 10-MW HWG: 1) coil former, 2) HTS field coil, 3) torque tube, 4) refrigerator, 5) coil support, 6)coolant pipe, 7) junction, 8) rotor support, 9) thermal shield, 10) first cold head, 11) second cold head, 12)compressor, 13) aftercooler, 14) oil separator, 15) buffer tank, 16) shaft, 17) rotating joint, and 18) vacuum shield.

2.1.Main Topology of HWG

The 10-MW wind generator is schematized in Fig.1.The system is fixed on a shaft 16) and rotates synchronously with the HTS rotor.The cryogenic system includes an oil scroll compressor 12) with a coolant tank, aftercoolers 13), an oil separator 14), a buffer tank 15) for oil adsorption, and cold heads 10)and 11).The oil scroll compressor reduces the friction between the two scroll sheets, thereby increasing the service life.The compressor is cooled by the oil and is power-supplied by a slip ring that includes a fixed end,an electric brush, and a sliding end.The electric brush and sliding end are fixed on the shaft and rotate synchronously with the rotor.An aftercooler installed adjacently to the compressor precools the coolant vented from the compressor and reduces the refrigeration load of the cold head.The aftercooler can be cooled using the air-cooling method.Because the compressor is oiled, an oil separator is required to separate the oil admixture from the coolant.For easy separation of the oil admixture from the coolant at lower temperatures, the oil separator is installed adjacently to the aftercooler.The separated oil is recycled to the compressor through oilreturn passages in the oil separator.The oil scroll compressor operates in the continuous mode, with the periodic intake and exhaust of the cold head.The high pressure at the exhaust end of the scroll compressor (approximately 1.6 MPa) can harm the cold head if entering directly.To prevent such damage, a buffer tank is required.The buffer tank contains activated carbon that further absorbs the oil admixture.This design does not require cryogenic rotary joints[14].

The internal structure of the cryogenic system is shown in Fig.2.The copper thermal shield is surrounded by a vacuum shield and coil former to reduce the thermal radiation from the stator and torque tube.As shown in Fig.2, one side of the thermal shield is connected to the first stage of the cryocooler, which has an expected temperature of approximate 50 K.The torque tube (red part in Fig.2) transmits the torque from the coil former to the shaft.One end of the torque tube is fixed on the coil former, and the other end is fixed on the rotor support[14]-[18].

Fig.2.Configuration of the cryogenic system of the 10-MW HTS wind generator.

2.2.Arrangement of Field Coils

Fig.3 shows CFCA.Two HTS field coils are arranged outside the rotor support and separated by an interval.As the current flows along the same direction in the adjacent sides of the field coils, the norm value of the magnetic field is much smaller along the symmetry axes than in other space[19]-[23].Therefore, the magnetic flux sag is produced at each wave crest.Fig.4 shows the configuration of OFCA.The two field coil sides with the same current direction are arranged in a stepwise configuration to form a north or south pole.

Fig.3.Schematic of the magnetic flux sag produced between the adjacent field coil sides with the same current direction.

Fig.4.Configuration of OFCA.

2.3.Design Process

Fig.5 shows the design process of the field coil.Fig.5 (a) is an expanded view of CFCA.Here, the pole number is 2p, the inner diameter of the stator isD, the air-gap length isg, the width of the coil support iswb,the height ish, the width of the tape turns iswc, and the interval between the field coils isd.The equivalent length of the expanded field coils can be approximately expressed as

Fig.5 (b) is an expanded view of OFCA.The basic parameters of the field coil are the same as those of CFCA.The expanded length of OFCA can be approximately expressed as

Fig.5.Design process for the field coil: (a) CFCA and (b)OFCA.

When the inner stator diameterDis constant, the lengths of CFCA and OFCA approximately differ by

Fig.6.10-MW wind generator with (a) CFCA and (b) OFCA; the geometry of (c) conventional HTS field coil and (d)proposed HTS field coil.

Figs.6 (a) and (b) show 10-MW HWGs with CFCA and OFCA, respectively.The primary parameters of both topologies are listed in Table 1.Fig.6 (c) shows the geometry of the HTS field coil, and Table 2 compares the HTS tape consumption of the two configurations.

Table 1:Tape consumption of two field coils

3.Characteristic Analysis

3.1.Electromagnetic-Characteristic Analysis

The simulated magnetic field distributions of HWGs with CFCA and OFCA are compared in Fig.7.In HWG with CFCA, the magnetic field was minimal at the inner side of the adjacent field coil sides with the same current flow current direction, in which the flux sag appeared.In contrast, the magnetic field in HWG with OFCA was uniformly distributed in both sides of the field coil.

Fig.8 shows the norm values of the magnetic field in the air gap (BN) of both topologies.Clearly, the magnetic flux sag appeared in HWG with CFCA but was eliminated in HWG with OFCA.Moreover, the magnetic field fluctuation was larger in HWG with CFCA than in HWG with OFCA.The difference (ΔB) between the maximum and minimum fluxes (BmaxandBmin, respectively)was approximately 1.6 T for HWG with CFCA and was 0.45 T in HWG with OFCA.

Table 2:Primary parameters of HWGs with CFCA and OFCA

Fig.7.Magnetic field distributions in two HWGs: (a) HWG with CFCA and (b) HWG with OFCA.

The magnetic field (BN) in the air gap of both topologies is decomposed into a radial component (By)and a circumferential component (Bx).TheBydistributions of the two topologies are compared in Fig.9.TheByspectra are analyzed via the fast Fourier transform, and the results are compared in Fig.10.

The waveform (Fig.9) is smoother in HWG with OFCA than in HWG with CFCA.Fig.10 reveals more high-order harmonics in HWG with CFCA than in the proposed configuration.In the conventional configuration, the amplitudes of the fourth, seventh, and ninth harmonic components ofBywere approximately 1.20 T, 0.45 T, and 0.38 T, respectively.In HWG with OFCA, the main harmonic component ofBywas the third harmonic with the approximate amplitude of 1.5 T.

Fig.8.Magnetic field distributions (norm values) in the air gap of OFCA and CFCA topologies.

Fig.9.Distributions of By in (a) HWG with CFCA and (b) HWG with OFCA.

3.2.Size Reduction

The diameter (volume) of a large-capacity wind generator can be decreased using the OFCA topology.

A flat race-track structure of the field coil is favored for practical applications of a large-capacity superconducting motor[6].The length difference (Ld)between the two topologies can be calculated using (3).The diameter difference (ΔD) can then be expressed as

As shown in Fig.11, ΔDis a linear function of the tape-turn width (wc).

Fig.10.Comparison of harmonic orders in OFCA and CFCA topologies.

4.lron Loss Comparison

4.1.lron Loss Calculation Model

Theoretically, the iron loss is separated by the quasistatic hysteresis loss (Ph) and the dynamic modified eddy-current loss

Considering harmonic effects, the modified eddycurrent loss (in watts per kilogram) is calculated as follows[4]:

Fig.11.Relationship between ΔD and wc.

Table 3:Parameters of the stator iron core

whereKc=π2d2/6mρis the modified eddy-current loss coefficient,iis the order of the time harmonics included in the flux density waveform,fis the fundamental frequency, andBiis the amplitude of theith harmonic flux density.The parameters of the stator iron core are listed in Table 3[16].

The hysteresis loss mainly depends on the fundamental frequency, the peak value (Bm) of the flux density waveform, and the number of minor hysteresis loops (N).The hysteresis loss (Ph) (in watts per kilogram) is expressed as follows[15]:

whereKhis the hysteresis loss coefficient (approximately equal to 1.79×10?2), ΔBiis the flux density change during the excursion around a minor loop, and the coefficientkis set to 0.6 (a suitable value forBmvalues between 1.0 T and 2.0 T).

Considering the radial component (By) and circumferential component (Bx) of the flux variation, the iron loss (in watts per kilogram) is ultimately determined as follows[15]:

whereBy,iandBx,i(By,mandBx,m) are the amplitudes (peak values) of theith harmonic flux density of the radial and circumferential components, respectively.

4.2.lron Loss at Different Field Currents

As shown in Fig.12, the iron losses in both topologies increase with increasing the field current.At field currents of 95 A and 130 A, the iron losses were approximately 2.5% and 1.8% lower per kilogram,respectively, in HWG with OFCA than in HWG with CFCA.When the current exceeded 105 A, the magnetic saturation became severe and the iron loss gradually increased with increasing the field current.

4.3.lron Loss at Different Air-Gap Lengths

As shown in Fig.13, the iron loss is reduced by lengthening the air gap.As the air gap increased, its reluctance increased and the flux density decreased;however, the output power also decreased.For any air gap, the iron loss was approximately 2% per kilogram lower in HWG with OFCA than in HWG with CFCA.The rapid iron loss with the increase in the air gap can be explained by the desaturation in the stator iron core.

Fig.12.Comparison of iron loss versus field current in the two topologies.

Fig.13.Comparison of iron loss versus air gap length in the two topologies.

4.4.lron Loss at Different Revolution Speeds

As shown in Fig.14, the iron losses in both topologies increase with increasing the revolution speed.At any revolution speed, the iron loss was approximately 1.8% lower per kilogram in HWG with OFCA than in HWG with CFCA.

4.5.Efficiency Comparison

The efficiency (η) under a rated condition can be simply expressed as

Fig.14.Comparison of iron loss versus revolution speed in the two topologies.

At a revolution speed of 9 rpm, the iron losses in HWG with CFCA and that with OFCA were approximately 24353 W and 22722 W, respectively.The proposed configuration reduced the iron loss by 1631 W, thus improving the efficiency by 0.016%.

5.Conclusions

The electromagnetic characteristics of 10-MW HWG with OFCA were compared with those of HWG with CFCA.The main conclusions of the comparative analysis are summarized here:

1) The overlapped topology improved the electromagnetic characteristics over that of the sequential topology.

2) Under various conditions, the iron loss was lower in HWG with OFCA than in HWG with CFCA.

3) The efficiency can be increased by reducing the iron loss.

4) OFCA allows a smaller diameter than CFCA, enabling easier transportation and installation at lower manufacturing cost.

Disclosures

The authors declare no conflicts of interest.