Transmission System Operators (TSO)
The ambitious European 20-20-20 targets will naturally increase the demand for renewable electricity and push for high-efficiency means of generation, like Combined Heat & Power (CHP). The large share will be wind on-shore and off-shore, however a significant part of renewable and efficient means of generation is small, and distributed in medium and low voltage networks. Combined with demand flexibility, these trends will introduce breakthrough changes in the way that electricity is generated, transported and used. Design and operation of existing transmission and distribution networks, together with the associated regulatory and market rules, were decided on the basis of a centralised system, thus leading to unidirectional power flows in the distribution network. Given the rapid development of Distributed Energy Resources (DER), the assumptions made when designing the system do not hold true any more.
The position of DER in the electricity supply chain and the associated techno-economic consequences impose new challenges on grid operators, electricity suppliers, market operators and regulators. In order to capitalise on the potential benefits offered by the distributed resources these challenges need to be adequately met. Being responsible for system operational security, TSOs are on the front line of the DER integration process.
- How will TSO activity be affected by Distributed Generation (DG)?
- How to maintain security as the increasing penetration of DG units replaces large centralized power plants?
- How can “DER Aggregators” help TSOs in the task of balancing the system?
- How will TSO-DSO relations be affected by the increasing penetration of DER?
EU-DEEP demonstrated that the basic principles of power systems control remain applicable in the presence of a large amount of DER
DER will impact on the system as a whole, as well as transmission and distribution networks. However, the fundamental principles behind power systems operation remain valid. Preventive security margins, voltage and frequency controls are the cornerstones of design and operation of electrical power systems. This ensures robustness against a predefined list of contingencies.
Created initially for rotating generators, these practices remain compatible with both generation using power electronic interfaces and demand response. The structure of control such as it has been used since the initial developments of power systems, can be prolonged.
The increasing penetration of DER into the power system requires their “integration”, especially for the management of system security
Maintaining a continuous balance between generation and consumption is one of the tasks of a TSO. However, it requires sufficient control capacities that, with increasing penetration of DER, need to be adapted from conventional generators to DER. If they reach significant penetration levels covering a substantial amount of the demand, they need to be fully integrated into system security management. They have to participate in system control and in provision of reserves, similarly to large conventional power stations.
TSOs serve as the entity responsible for system security, and need to consider the impact of DER on the system, especially during critical moments. It is therefore necessary that the TSO, in addition to information about demand and its evolution, also collects information about the amount of DER on-line, by estimating the power injections at the boundaries between the TSO and DSO grids. Indeed, given the possible changing behaviour of the DSO grid from passive (i.e. only consuming) to active (i.e. injecting into the DSO grid), monitoring of the net injection will not be enough, as the TSO will need to get information about generation per se.
Aggregation of multiple small customers can be the provisional interface needed in the future between the DER market and the TSO
Due to their typically small size, individual DER cannot directly interact with the market and the TSO. DER aggregation becomes the provisional interface that is lacking between the dispersed resources and the TSO prefiguring SmartGrid solutions. Aggregators can provide balancing power and power-frequency control as an alternative to large centralised power plants, increasing the competitiveness of reserve markets.
Good integration of DER in the system can only be ensured when information necessary for guaranteeing system security is available. Aggregation will make DER more visible to TSOs at acceptable costs, aiding TSOs in their task of managing the system balancing and helping them to improve the coordination of defence schemes in the DSO and TSO grids.
For the stability of power system operation, coordination of frequency ranges between the TSO and the DSO is mandatory, and sufficient “low voltage ride-through capability” for DER must be ensured
For normal operation, system frequency remains near to its nominal setting, within a range of about ± 200 mHz around 50 Hz for the UCTE system. At a “secured event” (standard assumed loss of 3000 MW) the frequency can transiently decrease down to 49.25 Hz, and even to 47.5 Hz, within a sub-network, in case of contingencies beyond the “secured event”. It is then mandatory that generation of whatever type remains connected to the system to avoid further deterioration of the system’s state. The September 2003 and November 2006 incidents demonstrated this necessity empirically.
A high proportion of DER means a reduced number of large generators on-line, which reduces short-circuit power leading to higher system voltage sensitivity during transient conditions. Faults in the transmission network will lead to deeper and geographically more wide-spread dips in voltage. DER must therefore present sufficient robustness and low voltage ride-through capability at least facing normally cleared faults in the transmission network.
In systems with a large proportion of DER, mature control concepts must be extended. In particular, load shedding schemes must be adjusted and in the long-term replaced by load shedding implemented locally in LV
Load shedding schemes are generally implemented at HV – MV substations by tripping MV feeders. With high penetration of DER units, this practice is no longer optimal as feeder tripping also means disconnection of generation. In the medium-term, this could be tackled by adequate selection of feeders for implementing the load shedding schemes. In the long-term however, the best solution is to include emergency demand response and load shedding within the smart system–customer interface (i.e. upgraded smart meter).
Challenges not covered by EU-DEEP
- Do short-term alternatives to aggregation exist? EU-DEEP assumes this is the first step to DER integration and to SmartGrids.
- How will services via aggregation interact with increasing shared resources between control zones? Cross-border balancing is only beginining to be developed today.
- How to reconcile commercial and security demand response? Aggregation brings new interesting elements, but decision making requires a careful examination by the main stakeholders under the lead of regulatory bodies.