High voltage series reactors for load flow control

摘要

Liberalization efforts are continuing in electricity markets all over the world. The change from monopolistic to free market structures leads to a significant increase in the trading of electrical energy. Since current power plant operation is driven by price aspects, the load flows in the networks are mainly determined by the economic situation and are no longer driven by the technical design of the networks. However, as the networks were built according to the former monopolistic market conditions and requirements, the desired load flows resulting from energy trading in the free market might cause problems when forced into the physical limitations of the actual networks, such as overloading, loop flows or stability/security limit violations. Also within the Austrian transmission grid, the effects of liberalization have led to bottlenecks caused by changes in the load flow due to the new power plant regime. To ensure a safe grid operation Verbund-Austrian Power Grid (APG), the Austrian transmission system operator, must apply cost-intensive measures for congestion management (CM). APG plans to build new 380 kV overhead lines to overcome the bottleneck situations, but the erection of new lines is delayed by extensive authorisation procedures and strong opposition by the public. In general, there are a number of approaches to overcome or mitigate load flow problems, including the installation of high voltage (HV) dry-type series reactors. The paper describes in some detail the design and construction of HV series reactors. It focuses on dry-type air core reactors which have become the technology of choice for many applications because of their design features and their cost effectiveness. Based on the actual situation in the Austrian transmission grid, the relevance for load flow control in the APG network is analyzed. The result shows that the present high loading of the 220 kV north to south lines within Austria can be reduced by installation of HV series reactors on condition that operational limits (e.g. node voltages and voltage angles) are kept within a certain range. Especially in combination with other measures, like the usage of phase shifting transformers and special switching conditions within the grid, the installation of (switchable) series reactors becomes an attractive option for power flow control. The comparison of cost for power plant re-dispatch versus the investment cost for series reactors provides a good reason to consider the installation of series reactors if the intended extension of the 380 kV grid cannot be realized in the short term.