The type of possible grid bottlenecks makes a difference for potential solutions. It depends, for example, on which grid level problems exist and whether bottlenecks occur due to load or supply. Effective solutions determine which flexibility mechanisms are suitable for which grid areas or grid levels and to what extent they are compatible with each other. They also help identify, which possibilities exist to avoid undesired interactions between local and zonal electricity markets.
Voltage-related grid bottlenecks are already occurring today in distribution grids with high feed-in of renewable energy (RE), low load and long line lengths - i.e. especially in rural areas. In the medium to long term, a large part of the congestion situations can be efficiently eliminated by expanding grid capacity. In the short term, however, the use of grid-serving flexibility can also be helpful and enable the integration of further RE capacities into the grids. For this purpose, reactive power is provided from decentralised generators such as wind farms, thus supporting voltage maintenance. In addition, innovative voltage regulating equipment can be used.
In order to address these challenges as comprehensively as possible in Germany, various blueprints have been developed as part of the SINTEG synthesis field Grid-Serving Flexibility Mechanisms. They are intended to support users in practice, managers in the economy and in politics in scaling up and replicating solutions. One blueprint deals with cross-grid level reactive power management.
SINTEG has investigated a wide range of innovative solutions for the future energy system in Germany in practice. Until SINTEG, the provision of reactive power from distribution grid to transmission grid in particular was still largely untested. In the future, however, the importance of active, cross-grid level reactive power management in the distribution grid will increase. This can contribute to compensating for the lack of reactive power provided by conventional power plants in the future. Reactive power management makes it possible to reduce the demand for reactive power at superordinate grid levels, so that the transmission capacities of the superordinate grid and transformers are available for active power transport. In addition, reactive power can be provided to the superimposed grid level. Coordination between transmission grid operators and subordinate distribution grid operators is therefore essential. The systemic approach of SINTEG made it possible to develop cross-grid level solutions.
The SINTEG showcases successfully demonstrated that higher voltage levels can be supported by targeted control of customer plants at low voltage levels. The showcases C/sells and enera, for example, demonstrated that reactive power can be transported from the medium-voltage level to the high-voltage level. Even transportation from the high-voltage level to the extra-high voltage level was successfully demonstrated by the showcases C/sells, NEW 4.0 and WindNODE. However, the transport of reactive power across two grid levels and transformers is generally not target-oriented. It was shown that cross-grid level coordination in accordance with FNN application rules (VDE-AR-N-4140/4141) can also be implemented for reactive power management. The losses in the transformer during cross-grid level reactive power provision are relatively low. This does not result in any economically relevant losses for grid operators.
A prerequisite for cross-grid level reactive power management is therefore to first optimise local reactive power management. SINTEG has made important contributions here. Among other things, the showcases successfully demonstrated reactive power management by means of an automated grid controller and its integration into existing systems (grid control room, grid management). In addition, in the DESIGNETZ showcase, innovative grid operating equipment such as rONT or longitudinal controllers were used and integrated into the reactive power management. In the C/sells showcase, different control strategies (Cosphi control, VDE FNN 4120, Statcom) were tested.
"The SINTEG results have shown that reactive power management coordinated across grid levels is an effective element in secure and efficient system operation with high shares of decentralised generation and storage.
The increasing equipment of distribution grids with sensor technology and communication technology creates ideal conditions for the potential at suitable grid nodes to be tapped in a meaningful way.
The necessary close coordination of the actors is seamlessly embedded in processes that are actively designed for the development of other flexibility potentials anyway."
Dr.-Ing. Karsten Burges, SINTEG subject Mmtter expert
After the successful testing of grid-serving flexibility mechanisms within the framework of SINTEG, there are still many challenges to be solved before these can be used in practice. These relate in particular to the broad implementation of the technical prerequisites, standardisation of the technology and economic evaluation as well as suitable regulatory framework conditions.
In order to enable reactive power management in the distribution grid, a higher penetration of sensors and actuators will be necessary in the future. There is a need to catch up in many distribution grids, especially at the medium-voltage level. For the transferability of the SINTEG approach, it must be possible to integrate the reactive power management and the coordination cascade into the existing IT systems of the grid operators and grid control room. SINTEG has shown that this is usually possible in the short term. However, it only makes sense to apply it if there is a need for dynamic reactive power management, for example due to regularly occurring voltage-related bottlenecks at medium-voltage or high-voltage level.
In order to also enable the provision of reactive power from the distribution grid to the transmission grid independently of the current supply, secured reactive power potentials are necessary in the distribution grid. This can be provided, among other things, by upgrading supply-dependent DCs to STATCOM capability.
Cross-industry agreement on general and industry-wide applicable interfaces and protocols facilitates the future broad application of the demonstrated concepts. For example, interfaces are needed in the grid control rooms for the integration of a reactive power management system. Protocols are used to transmit reactive power control commands to the generation plants. The broad introduction of the approaches into grid planning and grid operation will also support a legally binding methodology for the evaluation of costs and benefits.
Scaling the approach also requires the appropriate regulatory framework. In particular, the procurement of reactive power must be coordinated between plant and grid operators - possibly by means of market mechanisms. The legal basis for this has already been created with the new §12 h EnWG (2020). The concrete design of the regulatory framework is the task of the Federal Network Agency. The technical concepts for providing reactive power from customer plants are largely mature and universally applicable.