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HYBRID CONVERTER SYSTEM FOR ENERGY- AND COST-EFFICIENT OFFSHORE WIND TRANSMISSION
Increasing the integration of wind energy to the electricity grid requires the development of new powerconversion technologies that reduce cost and line losses to improve profitability and energy efficiency.
A Higher Colleges of Technology (HCT) led research project has proposed a new high voltage direct current (HVDC) converter system to efficiently transmit power from offshore wind farms (OWF) to the onshore power grid.
OWFs are wind farms located on the ocean or seabed. As the speed of the wind over water tends to be higher overall and more stable than wind over land, the average OWF can produce more than twice as much electricity as a medium-sized onshore farm. The International Renewable Energy Agency (IRENA) has reported that the global installed wind generation capacity has increased by a factor of almost 75 in the past two decades, jumping from 7.5 gigawatts (GW) in 1997 to some 564 GW by 2018. The UAE has recently turned its attention to wind power, with His Highness Sheikh Mohammed bin Rashid Al Maktoum, Vice President and Prime Minister of the UAE, Ruler of Dubai, announcing plans to build the country’s first wind farm in Hatta.
Electricity can flow one of two ways. It is either one directional, which is called direct current (DC), or it can flow both backwards and forwards, which is called alternating current (AC). Electric grids supply electricity as AC, which is a legacy of the “war of the currents” waged by Thomas Edison and Nikola Tesla in the 20th century, while renewable energy (RE) systems that harness wind and solar energy produce electricity as DC power. For RE supplies to be integrated on an electric grid, they must first be transformed from DC to AC, which is done through power converters, which convert the voltage from AC to DC, and inverters, devices that invert the voltage from DC to AC.
“For OWF developers, HVDC transmission systems are an attractive way to get the energy from the windfarm to the grid, as they cost less and lose less of the electricity for transmissions over 50 kilometers. If you are using an HVDC transmission system, you need an HVDC converter to control and stabilize the operation and delivery of power between the offshore windfarm to the grid on the mainland. There are many HVDC topologies available, but each of them has its own inherent drawback. The topology we have proposed has a lower cost, loss, and volume compared to other HVDC converter systems, while still proving the fundamental operation of the HVDC system,” explained HCT Assistant Professor of Electrical Engineering Dr. Thanh Hai Nguyen.
An HVDC converter system topology refers to the structure of electric circuits consisting of power electronic devices, inductors, capacitors, and transformers, which convert the voltage forms between AC and DC types.
Dr. Nguyen and his co-author, Can Tho University Electrical Engineering Department Deputy Head Dr. Ngoc Thinh Quach, recently published a paper on their hybrid HVDC converter topology in the International Journal of Electrical Power & Energy Systems.
In an OWF, a series of massive wind turbines are installed on the seabed or on floating platforms. When the wind blows, it pushes the blades of the turbine, causing the blades to spin. The blades are connected to a drive shaft, so when the blades turn, the drive shaft powers an electric generator to produce electricity. That electricity is then transmitted via a series of current conversion devices and cables to the onshore electric grid, where it can contribute to the available electricity supply.
To transmit the electricity produced by the OWFs to the electrical grid, an electricity transmission system is required. Transmission systems based on HVDC have been attracting attention from the electric industry due to the reduced line losses and lower costs they offer compared to the AC-based alternative. The two main ways HVDC topologies integrate OWFs to the main grid are line- communicated converter (LCC) based and voltage-source converter (VSC) based. Both have documented limitations.
OFFSHORE WIND FARMS (OWF) ARE WIND FARMS LOCATED ON THE OCEAN OR SEABED, AS THE SPEED OF WIND OVER WATER TENDS TO BE HIGHER OVERALL AND MORE STABLE THAN WIND OVER LAND. THIS ENABLES THE AVERAGE OWF TO PRODUCE OVER DOUBLE THE ELECTRICITY OF A MEDIUMSIZED ONSHORE FARM
The system proposed by Dr. Nguyen and his team consists of a full-scale modular multilevel converter (M2C) at the grid site, and a hybrid scheme of a reduced rating M2C and 12-pulse diode rectifier (12PD) without DC capacitors in the offshore station. The HVDC transmission system would deliver the power from the OWF to the grid, where the M2C located in the onshore grid is used to maintain the HVDC link voltage without change, enabling the power delivered from the offshore converter to be fully transmitted to the main grid.
Explaining the benefits of their proposed system, Dr. Nguyen said, “The most surprising finding from this work is about the converter itself, which combines the diode rectifiers and the voltage-source converter. The diode rectifiers also don’t need DC capacitors, which are known to be costly and prone to breakdown. Without the DC capacitors, our converter system has a lower cost, loss, and volume compared to other existing topologies.”
The team has tested the feasibility of their proposed topology through simulations. They also assembled a smaller prototype, which has demonstrated the basic operations of the system.
Explaining the value of the system within the UAE, Dr. Nguyen presented examples of where it can be used, as well as how it can be adapted to other power systems.
“THE TOPOLOGY WE HAVE PROPOSED HAS A LOWER COST, LOSS, AND VOLUME COMPARED TO OTHER HVDC CONVERTER SYSTEMS WHILE STILL PROVING THE FUNDAMENTAL OPERATION OF THE HVDC SYSTEM.”
-Dr. Thanh Hai Nguyen, Assistant Professor of Electrical Engineering Higher Colleges of Technology
“I believe that this topology can be applied to some systems in the UAE. As the HVDC system is used to deliver power over a long distance, this topology is suggested for power transmission from offshore wind farms. In the UAE, there are many offshore oil platforms that require power from the mainland. Our topology can be modified to transmit the power from the mainland to the oil platforms on the offshore side. In addition, the converter can be modified to apply in other applications such as photovoltaic (PV) power plants, machine drives, or battery charging systems,” Dr. Nguyen revealed.
Clean energy is a major focus of efforts to meet the UAE’s Energy Strategy 2050 goals, which include increasing the contribution of clean energy in the total energy mix from 25% to 50% by 2050 and reducing the carbon footprint of power generation by 70%.
To fully capitalize on the potential of the project’s findings, Dr. Nguyen will be exploring modifications and improvements to the novel topology for both high voltage and low voltage applications.
