A number of emerging trends indicate that the interaction between Transmission System Operators (TSOs) and Distribution System Operators (DSOs) will evolve in the coming years.
Among these is the increasing volume of distributed generation being connected to the distribution grid. These partly fluctuating generation units change the behaviour of the entire system, making it more challenging, for example, to balance generation and demand at every single point in time. Since balancing the grid is a responsibility of the TSO, it is clear that greater cooperation between TSOs and DSOs is to be expected.
Future grid operation challenges
In a new discussion paper from the International Smart Grid Action Network (ISGAN), six specific grid operation challenges for which collaboration between TSOs and DSOs is helpful or necessary are investigated:
1. Congestion of transmission-distribution interface
2. Congestion of transmission lines
3. Balancing challenge
4. Voltage support
5. (Anti-)islanding, re-synchronization and black-start
6. Coordinated protection.
Based on input from experts in Austria, Belgium, Canada, China, France, Ireland, South Africa, Sweden and USA, the technical solutions required for a closer interaction between TSOs and DSOs are found to be very similar for almost all the cases (with the exception of islanding and black-start).
From a high level viewpoint, grid monitoring has to be implemented, communication between TSO and DSO has to be established and means of communication between the DSO and its flexible customers have to be available.
The roles and tasks of the DSO are expected to evolve more than the role of the TSO, based on the possible future interaction proposals. New tasks include intensified data management and flexibility management. New technical requirements for the DSO include a two-way communication to its flexible customers and to the TSO, and the ability to perform (quasi) real-time network simulations with input from measurements on the grid.
According to the paper with the current status of technology, the technical requirements for an evolved interaction between TSOs and DSOs can be met. However, the paper cautions that such technical requirements should not be underestimated regarding implementation and operational cost, complexity and skills required. These could be a challenge, especially for smaller distribution network operators.
Non-technical grid operation issues
Several non-technical issues were also identified, which are closely related to the regulated environment grid operators are working in.
• Maintaining a balance between infrastructure investments and use of flexibility: A minimum use of flexibility will be necessary, but the impact on the processes and business cases of flexible customers will have to be limited. The use of flexibility of renewable energy sources needs to be limited to avoid a high loss of renewable energy.
• The role of markets: In this work, it is proposed to use market mechanisms only for the balancing challenge. Coping with local grid operation challenges such as critical transformer loading, line loading and voltages is proposed to be managed by the network operators, interacting optimally with each other. To handle such challenges, markets would not work efficiently without intervention. Instead, a regulatory framework is required to address bilateral contracts between flexible customers and network operators.
• Setting a level playing field for flexibility: When the combined flexibility of customers on the distribution and transmission grid is used, favouring one set of customers at the cost of the other should be avoided. Some mechanism, probably in discussion with the regulator, should be implemented to cope with this.
• The role of regulation: Closely related to the previous statement, is the discussion point on how grid operation should evolve, either more regulated, with clearer and stricter roles, or more open, with guaranteed interaction between grid operators and new market players. In both cases, a clear definition of the roles and responsibilities of all actors in future grid operation will be necessary.
A clear policy framework will, in every case, push forward investments in smart grid solutions to deal with the challenges that grid operators are facing.