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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Electrical aspects and grid integration' taking place on Thursday, 13 March 2014 at 09:00-10:30. The meet-the-authors will take place in the poster area.

mònica aragüés peñalba CITCEA-UPC, Spain
Co-authors:
Mònica Aragüés Peñalba (1) F P Oriol Gomis Bellmunt (1)
(1) CITCEA-UPC, Barcelona, Spain

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Abstract

Optimal power flow tool for mixed hvac and hvdc systems for grid integration of large wind power plants

Introduction

The progress in power electronics, as well as the improvement of cables performance have favored and still stimulate the development of HVDC (High Voltage Direct Current) technology, in parallel with HVAC (High Voltage Alternating Current). The choice of a technology or the other one will depend, to begin with, on the technical feasibility of each one for the specific link to be constructed.

Approach

Two examples of this large dependence on the technical requirements can be advanced. If the electrical connection to be constructed links two systems working at different frequency (asynchronous systems), HVDC must be used. With HVDC the power transmitted is practically independent of the distance, whereas, in HVAC the power capability transmission decreases with distance. However, the larger expenses of HVDC installations for certain distances compared to the HVAC option (around 50-80 km for submarine or underground transmissions and around 600-800 km for overhead transmission) can favor the HVAC construction.

Taking into consideration the before mentioned, a scenario for transmission networks based on interconnections between High Voltage Direct Current and High Voltage Alternating Current grids is feasible and seems probable. Specially, if HVDC technology is used for delivering the power produced in the wind power plants to the terrestrial grid. Thinking in remote wind power plants to be connected to different AC grids, then the HVDC system could be multiterminal. A multiterminal HVDC grid linked to the HVAC grid could look as sketched in , where the HVDC grid is constituted by 4 DC buses and the AC grid has 5 AC buses. The power is generated in two wind power plants. Each wind power plant is connected to a Voltage Source Converter (VSC), linked between each other and, at the same time, to two other VSC through high voltage submarine cables. The VSCs connected to the wind power plants will operate as rectifiers and the VSCs connected to the AC grid will operate as inverters.




Main body of abstract

In the system study, the VSC connected to the wind farms (wind farm rectifiers) inject the wind power to the HVDC grid. In normal operational mode, the wind farm rectifiers absorb all the power produced and inject it into the DC grid while supplying the reactive power needed to maintain the AC wind farm voltage. The power in the DC network is injected to the AC grid through grid connected inverters, responsible for the HVDC grid voltage control and which provide reactive power support to the AC grid when needed. Each converter station containing a grid connected inverter also includes a transformer for adapting the voltage level to the required for the HVAC transmission. The AC grid is constituted by three AC links (overhead lines or underground cables), forming a mesh.

The main objective of this paper is to present a tool for solving Optimal Power Flows (OPF) in mixed HVDC and HVAC systems for grid integration of large wind power plants, located either offshore or onshore. The OPF departs from the assumption that the power being produced from the wind power plants is known, as well as the demand from the AC grid. To model the interaction between the DC and AC grids, the active power conservation is expressed between the AC side and DC side of each converter, taking into consideration converter losses (modelled as a second order polynomial). The tool developed will determine the voltages (and angles in case of the AC grid), and the active and reactive power in each bus and branch that allow to ensure several objective functions. All the electrical variables are limited. Also, the currents flowing in each DC and AC branch. The maximum AC voltage that can be applied by the converters is also limited. To develop the tool, both HVDC and HVAC grids need to be represented appropriately through its impedances and admittances. The process for building the tool includes mainly four steps. To begin with, the specification of the tool requirements is done. Secondly, the system elements are modelled and its parameters are defined. Then, the tool is developed. Finally, the case studies are defined and the tool is applied to solve them. The voltage levels that will be used for study cases are +/- 80 kV (bipolar HVDC transmission) for the HVDC grid and 90 kV and 220 kV for the AC grid.

Several objective functions can be defined for the optimization tool. An interesting one that will be shown through a case study is the overall Joule losses in the HVDC-HVAC network. The OPF results will allow a comparison of the losses in the different system components: DC and AC cables/lines, converters and transformers, for different wind power plant injections. This will be applied to a study case in order to show how the results of the OPF can help in an operational perspective. By changing the objective function to minimum reactive power compensation in the AC grid, the OPF results will lead to interesting results in terms of planning.


Conclusion

To sum up, the authors have developed a tool that permits to solve Optimal Power Flows in mixed HVDC and HVAC systems for grid integration of large wind power plants. The electrical data of the HVDC and HVAC systems (that is to say, voltage levels for the HVDC grid and HVAC grid, transformers resistances and inductances, rated power of the lines, cables, transformers and converters, resistances, inductances, capacitances and lengths of all the cables and lines) is required information as input for the optimization tool. The power entering the HVDC grid is also needed, as well as the AC active and reactive power demand in the different AC buses. One of the AC buses is needed to be set as a slack (in Figure 1, bus number 3 has been selected). All the electrical variables are limited. Also, the currents flowing in each DC and AC branch. The maximum AC voltage that can be applied by the converters is also restricted. Any other restrictions to the optimization problem can be added. The objective function will be set as grid losses per default, but can be changed by the user of the tool (minimum reactive power compensation, minimum deviation from a desired operating point, etc). The tool also enables the evaluation of how any change in the parameters of the system affects the objective function. The output of the tool are the voltages (and angles in case of the AC grid), and the active and reactive power in each bus and branch that allow to ensure the specified objective function. In conclusion, the tool can be used by the industry for operation and planning purposes for the construction or replacement of existing transmission links.


Learning objectives
There are a large number of applications of OPF. The tool presented is specific for mixed HVAC and HVDC systems. The companies involved in the construction of future transmission systems can be interested in the results obtained with this tool.


References
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