Finding the Best Solution for a 5MW Wind Turbine
The power system of the offshore wind turbine is critical to the cost of energy of the installed park. The generator and converter type and configuration are important issues to be addressed during the concept design phase. However, the effects of these choices on the system design and park operation and income are too often overlooked. Annual energy output and system reliability are primary considerations.
By Rain Byars, CEO, Nextwind, Inc., USA
{access view=!registered}Only logged in users can view the full text of the article.{/access}{access view=registered}In this article we present an overview of a study for a 5MW offshore wind turbine designed for manufacture and installation in China. At the time of the study, the wind turbine manufacturer had already specified rotor size, drive-train configuration and gearbox ratio. With these constants, the following question was put forward: What is the best choice for power system architecture in terms of annual energy output, weight, cost of service, fulfilment of grid requirements and up-front cost – all of which affect the cost of energy?
First we examine the choice of generator type with focus on the two most popular generator types for wind turbines: doubly fed induction (DFIG) and permanent magnet (PMG) generators. Second, the system voltage is assessed, to identify the optimum configuration for a balance between all design requirements.
Nominal Turbine and Site
All calculations are based on a 5MW nominal wind turbine. The configuration is an upwind, three-bladed turbine with active pitch and variable speed operation. The turbine has a 148-metre-diameter rotor with standard blades, and a three-stage gearbox with a 97:1 gear ratio. The hub height is 100 metres. The site conditions used for the comparison are based on an IEC class II site. The site is nearshore, with 8.5mps annual average wind speed at hub height. Sea level density and 18% turbulence are assumed. Figures for up-front costs, service costs and reliability have been estimated based on past and ongoing experience of reviewing operational data of wind turbines with different power system configurations.
Generator Type
The DFIG stator is directly connected to the grid, while the rotor current, comprising approximately 30% of the total output, passes through slip rings and is controlled by a four quadrant converter (Figure 1).
The PMG has a rotor equipped with permanent magnets. The stator is connected to a full four quadrant converter, which is used to control torque on the generator (Figure 2).
Voltage Level
In the low voltage configuration, the generator windings and converter are at 690V. The medium voltage transformer is from 690V to transmission voltage (typically 36kV for offshore installations). In the medium voltage configuration, the generator windings and converter are at a medium voltage (e.g. 2 to 12kV). In the case of DFIG, the stator may be at an even higher voltage (i.e. the transmission voltage). Figure 3 compares the low and medium voltage systems. A medium voltage level 3.3kV was selected as a reference point for this study, because the converter can be manufactured with standard components, without stacking.
Annual Energy Production (AEP) Calculation
AEP is not directly a result of peak efficiency or peak power. In order to approach an accurate calculation of AEP, it is important to realise that a wind turbine spends the majority of its operating hours at partial power. Each component of the power train has an efficiency profile, with actual efficiency depending on wind speed, revolutions per minute (rpm), or percentage of nominal power. The blade, the gearbox, the generator and the converter have different efficiencies based on these factors. Figure 4 compares DFIG and PMG aggregate efficiency curves.
The PMG has the highest annual power output, with a 2.2% increase in AEP over the DFIG for an IEC class II site. The PMG has a higher efficiency curve, with much higher efficiency at partial power, where the highest number of operating hours are spent.
The DFIG has a higher efficiency at rated speed. Additionally, the converter losses are only applied to 30% of the power output of the generator. This results in lower overall losses at full power.
A significant difference in power output becomes apparent when the operating speed range is taken in to account. The PMG can begin producing power at very low rpms, but the DFIG is limited to the synchronous speed less 30% – in the case of the six-pole DFIG on a 50Hz grid this is 700 rpm. This affects the cut in of the wind turbine and contributes to a higher overall AEP from the PMG.
Effect of Wind Class on Generator Type
As shown in Table 1, the difference in AEP becomes larger at lower average wind speeds and becomes less significant at higher average wind speeds. This is due to better performance of the PMG at partial power.
Table 1. Comparison of AEP differences for various classes/wind speeds
AEP Comparison of System Voltage
The medium voltage system has a higher annual power output, with an ~1% increase in AEP over the low voltage system for an IEC class II site. This is due to the lower losses in the converter and cables.
Up-front Costs
The low voltage DFIG has the lowest up-front cost. Because of the higher cost of magnets, a DFIG generator is usually less expensive. Additionally, when a partial converter is used, the converter can use smaller (or fewer) modules and is therefore less expensive. It can be estimated that the DFIG (generator and converter) will represent a 30% savings in up-front costs compared to the PMG (generator and converter).
The medium voltage converter is more expensive than a low voltage converter (25%). On the other hand, over four times the number of cables are required between the generator and converter with the low voltage configuration compared to the 3.3kV design. In addition the components are more compact, reducing tower top weight. The converter and transformer can also be placed down tower, further reducing tower top weight.
Grid Requirements
Stringent interconnection standards are a factor with regard to wind park compliance and costs. The PMG is a good choice for grid code compliance. Because of the full converter, all requirements for harmonics, power factor control and grid fault ride-through can be met easily. There is extra cost related to meeting new grid codes with the DFIG.
Reliability
It is estimated that the service costs per year for the DFIG and converter with the additional service costs for the gearbox will be 20 to 30% higher than the service costs for the PMG.
The PMG has high reliability and low maintenance cost due to better heat performance, no slip rings and no encoder. The PMG also solves gearbox issues seen with the DFIG due to increased cyclic tooth loads from grid transients. Slip rings in the DFIG design require inspection at six- month intervals and frequent replacement.
DFIG generators tend to have high induced shaft currents. This can be mitigated with insulated bearings and grounding brushes; however, these systems are expensive and the extra components bring the mean time between failures (MTBF) down.
The low voltage converter hardware is based on a standard and mature IGBT (insulated gate bipolar transistor) design. Converter modules are mass-produced for industrial application. The medium voltage converter modules may be considered a less mature technology in wind applications compared to the low voltage modules. Therefore, careful consideration needs to be given to system reliability and cost of service.
Conclusion
For a multi-megawatt offshore wind turbine, a permanent magnet generator is a clear choice for optimising all factors affecting the cost of energy of the installed turbine. The advantages include increased power capture and high system reliability. It is clear that the improvements in AEP and cost of service for the PMG outweigh the higher up-front cost, and that a wind turbine with a PMG will achieve a lower cost of energy compared to a wind turbine equipped with a DFIG.
The medium voltage power system configuration presents many technical advantages such as lower weight and increased efficiency. While it is true that the increased AEP of the medium voltage system will offset the increased up-front cost, the total impact on the service costs (increase or decrease) is not yet fully clear. The medium voltage solution is certainly worth careful consideration and assessment for any new development of a multi-megawatt turbine concept.
Biography of the Author
Rain Byars has worked in wind technology since 1998. With an engineering degree from Carnegie Mellon University, and a design focus on reliability and safety, Rain is currently employed in the role of CEO at Nextwind, leading consulting activities and development of wind turbines for license.{/access}
The power system of the offshore wind turbine is critical to the cost of energy of the installed park. The generator and converter type and configuration are important issues to be addressed during the concept design phase. However, the effects of these choices on the system design and park operation and income are too often overlooked. Annual energy output and system reliability are primary considerations.By Rain Byars, CEO, Nextwind, Inc., USA
{access view=!registered}Only logged in users can view the full text of the article.{/access}{access view=registered}In this article we present an overview of a study for a 5MW offshore wind turbine designed for manufacture and installation in China. At the time of the study, the wind turbine manufacturer had already specified rotor size, drive-train configuration and gearbox ratio. With these constants, the following question was put forward: What is the best choice for power system architecture in terms of annual energy output, weight, cost of service, fulfilment of grid requirements and up-front cost – all of which affect the cost of energy?
First we examine the choice of generator type with focus on the two most popular generator types for wind turbines: doubly fed induction (DFIG) and permanent magnet (PMG) generators. Second, the system voltage is assessed, to identify the optimum configuration for a balance between all design requirements.
Nominal Turbine and Site
All calculations are based on a 5MW nominal wind turbine. The configuration is an upwind, three-bladed turbine with active pitch and variable speed operation. The turbine has a 148-metre-diameter rotor with standard blades, and a three-stage gearbox with a 97:1 gear ratio. The hub height is 100 metres. The site conditions used for the comparison are based on an IEC class II site. The site is nearshore, with 8.5mps annual average wind speed at hub height. Sea level density and 18% turbulence are assumed. Figures for up-front costs, service costs and reliability have been estimated based on past and ongoing experience of reviewing operational data of wind turbines with different power system configurations.
Generator Type
The DFIG stator is directly connected to the grid, while the rotor current, comprising approximately 30% of the total output, passes through slip rings and is controlled by a four quadrant converter (Figure 1).
The PMG has a rotor equipped with permanent magnets. The stator is connected to a full four quadrant converter, which is used to control torque on the generator (Figure 2).
Voltage Level
In the low voltage configuration, the generator windings and converter are at 690V. The medium voltage transformer is from 690V to transmission voltage (typically 36kV for offshore installations). In the medium voltage configuration, the generator windings and converter are at a medium voltage (e.g. 2 to 12kV). In the case of DFIG, the stator may be at an even higher voltage (i.e. the transmission voltage). Figure 3 compares the low and medium voltage systems. A medium voltage level 3.3kV was selected as a reference point for this study, because the converter can be manufactured with standard components, without stacking.
Annual Energy Production (AEP) Calculation
AEP is not directly a result of peak efficiency or peak power. In order to approach an accurate calculation of AEP, it is important to realise that a wind turbine spends the majority of its operating hours at partial power. Each component of the power train has an efficiency profile, with actual efficiency depending on wind speed, revolutions per minute (rpm), or percentage of nominal power. The blade, the gearbox, the generator and the converter have different efficiencies based on these factors. Figure 4 compares DFIG and PMG aggregate efficiency curves.
The PMG has the highest annual power output, with a 2.2% increase in AEP over the DFIG for an IEC class II site. The PMG has a higher efficiency curve, with much higher efficiency at partial power, where the highest number of operating hours are spent.
The DFIG has a higher efficiency at rated speed. Additionally, the converter losses are only applied to 30% of the power output of the generator. This results in lower overall losses at full power.
A significant difference in power output becomes apparent when the operating speed range is taken in to account. The PMG can begin producing power at very low rpms, but the DFIG is limited to the synchronous speed less 30% – in the case of the six-pole DFIG on a 50Hz grid this is 700 rpm. This affects the cut in of the wind turbine and contributes to a higher overall AEP from the PMG.
Effect of Wind Class on Generator Type
As shown in Table 1, the difference in AEP becomes larger at lower average wind speeds and becomes less significant at higher average wind speeds. This is due to better performance of the PMG at partial power.
| Constants clean blade, steady power curve 148 meter rotor 97:1 gearbox ratio |
||||
| EIC Class Average windspeed DFIG PMG increase in AEP |
Class I 10 mps baseline 1.39% |
Class II 8.5 mps baseline 2.20% |
Class III 7.5 mps baseline 3.24% |
Class IV 6 mps baseline 6.91% |
Table 1. Comparison of AEP differences for various classes/wind speeds
AEP Comparison of System Voltage
The medium voltage system has a higher annual power output, with an ~1% increase in AEP over the low voltage system for an IEC class II site. This is due to the lower losses in the converter and cables.
Up-front Costs
The low voltage DFIG has the lowest up-front cost. Because of the higher cost of magnets, a DFIG generator is usually less expensive. Additionally, when a partial converter is used, the converter can use smaller (or fewer) modules and is therefore less expensive. It can be estimated that the DFIG (generator and converter) will represent a 30% savings in up-front costs compared to the PMG (generator and converter).
The medium voltage converter is more expensive than a low voltage converter (25%). On the other hand, over four times the number of cables are required between the generator and converter with the low voltage configuration compared to the 3.3kV design. In addition the components are more compact, reducing tower top weight. The converter and transformer can also be placed down tower, further reducing tower top weight.
Grid Requirements
Stringent interconnection standards are a factor with regard to wind park compliance and costs. The PMG is a good choice for grid code compliance. Because of the full converter, all requirements for harmonics, power factor control and grid fault ride-through can be met easily. There is extra cost related to meeting new grid codes with the DFIG.
Reliability
It is estimated that the service costs per year for the DFIG and converter with the additional service costs for the gearbox will be 20 to 30% higher than the service costs for the PMG.
The PMG has high reliability and low maintenance cost due to better heat performance, no slip rings and no encoder. The PMG also solves gearbox issues seen with the DFIG due to increased cyclic tooth loads from grid transients. Slip rings in the DFIG design require inspection at six- month intervals and frequent replacement.
DFIG generators tend to have high induced shaft currents. This can be mitigated with insulated bearings and grounding brushes; however, these systems are expensive and the extra components bring the mean time between failures (MTBF) down.
The low voltage converter hardware is based on a standard and mature IGBT (insulated gate bipolar transistor) design. Converter modules are mass-produced for industrial application. The medium voltage converter modules may be considered a less mature technology in wind applications compared to the low voltage modules. Therefore, careful consideration needs to be given to system reliability and cost of service.
Conclusion
For a multi-megawatt offshore wind turbine, a permanent magnet generator is a clear choice for optimising all factors affecting the cost of energy of the installed turbine. The advantages include increased power capture and high system reliability. It is clear that the improvements in AEP and cost of service for the PMG outweigh the higher up-front cost, and that a wind turbine with a PMG will achieve a lower cost of energy compared to a wind turbine equipped with a DFIG.
The medium voltage power system configuration presents many technical advantages such as lower weight and increased efficiency. While it is true that the increased AEP of the medium voltage system will offset the increased up-front cost, the total impact on the service costs (increase or decrease) is not yet fully clear. The medium voltage solution is certainly worth careful consideration and assessment for any new development of a multi-megawatt turbine concept.
Biography of the Author
Rain Byars has worked in wind technology since 1998. With an engineering degree from Carnegie Mellon University, and a design focus on reliability and safety, Rain is currently employed in the role of CEO at Nextwind, leading consulting activities and development of wind turbines for license.{/access}
