DC-Connected Hybrid Offshore-wind and Tidal-Turbine (HOTT) Generation System
This article describes a ‘Hybrid Offshore-wind and Tidal-Turbine’ (HOTT) generating system, which uses an interconnecting method to link a DC-side cluster of wind and tidal turbine generators. An interconnected combination of four tidal and one wind turbine generators is proposed, and it is anticipated that this will give an electrical output of high reliability and quality. This method is able to send generated power through a long-distance DC transmission.
By Mohammad Lutfur Rahman and Yasuyuki Shirai, Graduate School of Energy Science, Kyoto University, Japan
Wind Turbine and Results
Figure 2 shows the offshore wind turbine individual simulation output using PSCAD/EMTDC. It shows the real power (PwindMW) and the mechanical torque (Tmwindpu) of the offshore wind turbine, and also the AC voltage line-to-line (Vwind-L-L (RMS) kV). The per unit machine speed is controlled to be 1.014 per unit constant throughout the simulation. As shown in Figure 2, the wind generator is in starting-up condition until 0.9s. Wind speed noise is given throughout the simulation period. The noise amplitude controlling parameter is 1rad/s, the number of noise components is 30, the surface dreg coefficient is 0.0192, the random wind speed is 8m/s, and the time interval of random generation is 0.35s. Additionally ramp wind starts at 6s, the number of ramps is 3, ramp period is 1s and ramp wind maximum velocity is 3m/s. Gust wind starts at 10s, the wind adds gust wind force to the blades to rotate the generator shaft, gust peak velocity is 3m/s, gust period is 1s and the number of gusts is 3. The actual power for this system fluctuates between 1.8 and 2.7MW, while the torque changes between 0.7 and 1.2 per unit. AC line to line voltage RMS is 5.8kV almost constant.
Tidal Turbine and Results
Figure 3 shows the tidal turbine individual simulation output using PSCAD/EMTDC. The tidal turbine models were modified from wind turbine IEEE models available in the PSCAD master library. The figures shows the real power (PtidalMW) and mechanical torque (Tmtidalpu) of the turbine, and also the tidal AC voltage line-to-line (Vtidal-L-L(RMS)kV). The tidal generator is in starting up condition until 0.8s. The generator condition is assumed to be in steady-state while the tidal speed is between 2 and 3m/s. Figure 3 shows that the tidal power delivers 4.2MW to the system at around 1.48 to 15s. The input torque also has an almost steady value of 1.0 per unit. Tidal AC line-to-line RMS voltage is 8kV almost constant.
HOTT System
Figure 4 shows that the AC power generated by the wind and tidal turbine generators are converted into DC power with the rectifier and transmitted onto land via an underwater power cable. It is converted again into AC power through the inverter. The hybrid DC link unit is used to convert the output AC–DC–AC through a 6-pulse GTO converter and inverter.
Figure 5 shows the hybrid turbine simulation output using PSCAD/EMTDC and illustrates from top to bottom the converter DC current (ICON-DCpu) and inverter DC current (IINV-DCpu), DC transmission power (PDCpu), the converter DC voltage set point (VCON-DCkV), the inverter DC voltage set point (VINV-DCkV), AC voltage line-to-line (RMS) (VINV-ACkV).
As shown in the figure, the generator starting situation (t=0.2s to t=0.5s) is caused by simulation difficulty, so it does not need explanation. The DC voltage (VCON-DCkV) is kept to the setting point 22kV from time t=0.51s until t=15.0s. AC voltage line-to-line (RMS) (VINV-L-LkV) inverter side is 77kV. The DC transmission line power (PCON-DC pu) shows that the total hybrid system line power is almost steady, even in the ramp and gust wind period. The DC transmission line power is 0.42 per unit (6.3MW). The DC current is 0.40 per unit.
HOTT Advantage
The proposed HOTT system is more flexible than a single system, because the stable generation ranges of the wind/tidal turbines can be extended using an adequate system control strategy. Since the rotational speed of the wind turbine is changed irregularly with fluctuations in natural wind, the output voltage and frequency of the wind turbine generator vary widely. However, the dynamic performances of the systems have rarely been reported . In order to maintain a high quality power load, it is desirable for a HOTT system to keep its output voltage and power constant, even when the wind (or tide) velocity changes. So, in order to design effective controls for the hybrid system, it is essential to develop the dynamic model of the system. This paper presents a dynamic model of the hybrid wind and tidal turbine generator system. The dynamic performances of the system in response to variations in wind velocity were then investigated in order to design a useful control system for the HOTT. Based on the discussion of the system performances, we propose a control system to keep the output voltage and power of the whole system constant. The hybrid system method is considered to be one of the best techniques available for converting tidal energy and wind energy into electricity.
Conclusion
The PSCAD simulation results with a HOTT 6.3MW+ test system demonstrate a satisfactory performance for a range of wind and tidal speeds using a 6-pulse GTO rectifier and inverter. The key challenges for offshore wind and tidal power are design, electric transmission and connection, system and stability operation, system investigation, reactive power and voltage control strategy, and finally the interaction between offshore wind and tidal turbines. The advance simulation study has to be carried out in order to ensure stability and also to give a better understanding of the control aspects which will be required to make it more efficient.
An output voltage control system to keep the voltage, as well as output power, constant for the cases when the wind velocity is changing has been proposed. It has been demonstrated that the output voltage can be kept almost constant with the proposed control system.
Biography of the Authors
Mohammad Lutfur Rahman was born and grew up in Bangladesh. He gained his bachelor and master degrees in the Philippines in 2000 and 2003 (BSc Computer Engineering and a Masters in Information Technology). He has worked in Thailand as a lecturer at Eastern Asia University and Rajamangala University of Technology, Thanyaburi. Between 2007 and the present, he has been a PhD student at the Graduate School of Energy Science, Kyoto University, Japan.
Yasuyuki Shirai was born in Kyoto Prefecture, Japan. He received his BE, ME and DE degrees in Electrical Engineering from Kyoto University in 1980, 1982 and 1988 respectively. He was an assistant professor at Kyoto University from 1985, an associate professor from 1996 and he is now a professor of the Graduate School of Energy Science, Kyoto University. His areas of interest are applied superconductivity to power system apparatus, next generation power systems (including renewable energy sources), and energy infrastructure.{/access}
This article describes a ‘Hybrid Offshore-wind and Tidal-Turbine’ (HOTT) generating system, which uses an interconnecting method to link a DC-side cluster of wind and tidal turbine generators. An interconnected combination of four tidal and one wind turbine generators is proposed, and it is anticipated that this will give an electrical output of high reliability and quality. This method is able to send generated power through a long-distance DC transmission.
By Mohammad Lutfur Rahman and Yasuyuki Shirai, Graduate School of Energy Science, Kyoto University, Japan
{access view=!registered}Only logged in users can view the full text of the article.{/access}{access view=registered}The utilisation of natural energy sources such as offshore wind and tidal power is one of the most effective answers to some important global environmental problems. An offshore wind turbine generator system supplies electrical power to the utility, but a stable power supply to the grid is not feasible because the output power of the offshore wind generator system fluctuates at all times with wind conditions. Hence, we are proposing a hybrid generator system using offshore wind turbine generators in combination with a tidal generator system; the steady-state characteristics of this hybrid system are discussed in this paper.
The proposed HOTT generator system is illustrated in Figure 1. As this figure shows, the system consists of an offshore wind and tidal turbine with three possible modes of operation: first situation, swing situation and maintenance situation. The hybrid model is designed to simulate a realistic situation that would lead to a power fluctuation during the continuous operation of the tidal and offshore wind turbine. The model has been built using a combination of three phases. The first phase is the offshore wind model, which simulates the wind power. The second phase is the tidal turbine model, which simulates the tidal power. The third phase investigates the workings of the hybrid system, where the tidal turbine and offshore wind turbine are linked using 6-pulse GTO (Gate Turn-Off thyristor) rectifier DC side connections and inverter. HOTT energy will have both environmental and socioeconomic benefits; for example, an unenclosed HOTT system can avoid many of the detrimental effects of CO2 emission, which has become a key environmental issue, while providing significant amounts of distributed renewable energy in areas that need it.Wind Turbine and Results
Figure 2 shows the offshore wind turbine individual simulation output using PSCAD/EMTDC. It shows the real power (PwindMW) and the mechanical torque (Tmwindpu) of the offshore wind turbine, and also the AC voltage line-to-line (Vwind-L-L (RMS) kV). The per unit machine speed is controlled to be 1.014 per unit constant throughout the simulation. As shown in Figure 2, the wind generator is in starting-up condition until 0.9s. Wind speed noise is given throughout the simulation period. The noise amplitude controlling parameter is 1rad/s, the number of noise components is 30, the surface dreg coefficient is 0.0192, the random wind speed is 8m/s, and the time interval of random generation is 0.35s. Additionally ramp wind starts at 6s, the number of ramps is 3, ramp period is 1s and ramp wind maximum velocity is 3m/s. Gust wind starts at 10s, the wind adds gust wind force to the blades to rotate the generator shaft, gust peak velocity is 3m/s, gust period is 1s and the number of gusts is 3. The actual power for this system fluctuates between 1.8 and 2.7MW, while the torque changes between 0.7 and 1.2 per unit. AC line to line voltage RMS is 5.8kV almost constant.
Tidal Turbine and Results
Figure 3 shows the tidal turbine individual simulation output using PSCAD/EMTDC. The tidal turbine models were modified from wind turbine IEEE models available in the PSCAD master library. The figures shows the real power (PtidalMW) and mechanical torque (Tmtidalpu) of the turbine, and also the tidal AC voltage line-to-line (Vtidal-L-L(RMS)kV). The tidal generator is in starting up condition until 0.8s. The generator condition is assumed to be in steady-state while the tidal speed is between 2 and 3m/s. Figure 3 shows that the tidal power delivers 4.2MW to the system at around 1.48 to 15s. The input torque also has an almost steady value of 1.0 per unit. Tidal AC line-to-line RMS voltage is 8kV almost constant.
HOTT System
Figure 4 shows that the AC power generated by the wind and tidal turbine generators are converted into DC power with the rectifier and transmitted onto land via an underwater power cable. It is converted again into AC power through the inverter. The hybrid DC link unit is used to convert the output AC–DC–AC through a 6-pulse GTO converter and inverter.
Figure 5 shows the hybrid turbine simulation output using PSCAD/EMTDC and illustrates from top to bottom the converter DC current (ICON-DCpu) and inverter DC current (IINV-DCpu), DC transmission power (PDCpu), the converter DC voltage set point (VCON-DCkV), the inverter DC voltage set point (VINV-DCkV), AC voltage line-to-line (RMS) (VINV-ACkV).
As shown in the figure, the generator starting situation (t=0.2s to t=0.5s) is caused by simulation difficulty, so it does not need explanation. The DC voltage (VCON-DCkV) is kept to the setting point 22kV from time t=0.51s until t=15.0s. AC voltage line-to-line (RMS) (VINV-L-LkV) inverter side is 77kV. The DC transmission line power (PCON-DC pu) shows that the total hybrid system line power is almost steady, even in the ramp and gust wind period. The DC transmission line power is 0.42 per unit (6.3MW). The DC current is 0.40 per unit.
HOTT Advantage
The proposed HOTT system is more flexible than a single system, because the stable generation ranges of the wind/tidal turbines can be extended using an adequate system control strategy. Since the rotational speed of the wind turbine is changed irregularly with fluctuations in natural wind, the output voltage and frequency of the wind turbine generator vary widely. However, the dynamic performances of the systems have rarely been reported . In order to maintain a high quality power load, it is desirable for a HOTT system to keep its output voltage and power constant, even when the wind (or tide) velocity changes. So, in order to design effective controls for the hybrid system, it is essential to develop the dynamic model of the system. This paper presents a dynamic model of the hybrid wind and tidal turbine generator system. The dynamic performances of the system in response to variations in wind velocity were then investigated in order to design a useful control system for the HOTT. Based on the discussion of the system performances, we propose a control system to keep the output voltage and power of the whole system constant. The hybrid system method is considered to be one of the best techniques available for converting tidal energy and wind energy into electricity.
Conclusion
The PSCAD simulation results with a HOTT 6.3MW+ test system demonstrate a satisfactory performance for a range of wind and tidal speeds using a 6-pulse GTO rectifier and inverter. The key challenges for offshore wind and tidal power are design, electric transmission and connection, system and stability operation, system investigation, reactive power and voltage control strategy, and finally the interaction between offshore wind and tidal turbines. The advance simulation study has to be carried out in order to ensure stability and also to give a better understanding of the control aspects which will be required to make it more efficient.
An output voltage control system to keep the voltage, as well as output power, constant for the cases when the wind velocity is changing has been proposed. It has been demonstrated that the output voltage can be kept almost constant with the proposed control system.
Biography of the Authors
Mohammad Lutfur Rahman was born and grew up in Bangladesh. He gained his bachelor and master degrees in the Philippines in 2000 and 2003 (BSc Computer Engineering and a Masters in Information Technology). He has worked in Thailand as a lecturer at Eastern Asia University and Rajamangala University of Technology, Thanyaburi. Between 2007 and the present, he has been a PhD student at the Graduate School of Energy Science, Kyoto University, Japan.
Yasuyuki Shirai was born in Kyoto Prefecture, Japan. He received his BE, ME and DE degrees in Electrical Engineering from Kyoto University in 1980, 1982 and 1988 respectively. He was an assistant professor at Kyoto University from 1985, an associate professor from 1996 and he is now a professor of the Graduate School of Energy Science, Kyoto University. His areas of interest are applied superconductivity to power system apparatus, next generation power systems (including renewable energy sources), and energy infrastructure.{/access}
