Common wisdom is that in Europe’s evolving power system much closer cooperation will be required between the transmission and distribution system operators (TSOs, DSOs). Increasing levels of renewables as well as increased interconnection of European grids and for example the development of local energy initiatives are bringing new challenges to both the TSOs and DSOs. Both are becoming reliant on a common set of supply and demand side resources and their respective networks can no longer be decoupled. [Engerati-In Focus Power In Europe]
Roles are also changing. Possible future roles of DSOs in Europe have come under the spotlight in the evolvDSO project – some roles extended, others evolving and still others new. [Engerati-Methodologies and tools for evolving DSO roles for renewable energy integration in distribution networks]
Alongside these the project also has developed a set of innovative tools to support these roles in four key areas, of which TSO-DSO cooperation is one.
“Cooperation is needed as presently the distribution system is a grey box to the TSO and vice-versa,” says Ricardo Bessa, senior researcher at the Portuguese research institute INESC TEC (Institute for Systems and Computer Engineering, Technology and Science). “In this context cooperation means bidirectional exchange of information regarding the operating conditions of the transmission and distribution systems. It also includes DSO support for TSO operational and planning tasks, such as controlling the active and reactive power ratio in the primary substation or elaborating a joint expansion plan of both systems.”
TSO-DSO cooperation tools
The two TSO-DSO cooperation tools, which were contributed by INESC TEC, are:
• Interval Constrained Power Flow tool, which estimates the flexibility range in primary substations by aggregating the distribution network flexibility
• Sequential Optimal Power Flow tool, which derives a set of control actions that keep the active and reactive power flow within pre-agreed limits at the primary substations.
Bessa explains that these tools derive from the evolvDSO business use case: “Manage TSO’s requests at different timeframes (network planning, operational planning, real-time operations, & ex-post)”. In turn this gives rise to the respective system use cases (i.e. functions required to execute/enable the business use case) – to estimate the flexibility range of the primary substations; and to optimise the network by providing active and reactive power profiles to the TSO.
“Components of the business use case include data exchange, coordination of maintenance actions and control execution of resources connected to the HV and/or MV networks that allow the DSO to respond to operational requests from the TSO. These two tools fulfill the processes and steps described in the system use cases, as well as the requirements.”
In practical terms an example of the use of two tools is with high renewables penetration in the distribution network, such as occurs in Portugal, Germany and Italy, which frequently results in reverse power flows and voltage limit violations in the transmission network. In order to solve these problems, the TSO can request control actions at the level of distribution network using the available levers (e.g. reactive power control, control set points for distributed generation, etc.).
Furthermore, under the new ENTSO-E ‘Operational Planning and Scheduling’ Network Code, TSOs should monitor the ratios of active and reactive power at the interface between the transmission and distribution systems, i.e. the primary substation.
Power flow tools
Luis Seca, coordinator for the Centre of Power and Energy at INESC TEC, explains the Interval Constrained Power Flow tool is aimed to estimate the flexibility range of active and reactive power in each primary substation, taking into account the available flexible resources and costs in each distribution network.
“The core function is an original optimal power flow (OPF) algorithm,” he says. “The approach is innovative as it or similar ones have not been proposed in previous literature. The tool also enables an evaluation of the distributed resource aggregators’ impact on the transmission network and provides a means for a cost benefit evaluation of the available levers.”
The output is a flexibility map between active and reactive power (see below).
Regarding the Sequential Optimal Power Flow tool, Seca says it includes network reconfiguration, active power control with evolutionary algorithms (Evolutionary Particle Swarm Optimization, EPSO) and voltage-Var control based on fuzzy logic controllers. The tool also integrates different types of flexibility (e.g. demand response, flexible generation, capacitors banks, network reconfiguration, etc.).
“This approach has had only limited attention in the literature and the tool fully tackles the problem of managing the active and reactive power flows at the TSO-DSO interface.”
Use of TSO-DSO cooperation tools
Seca says that both tools have been tested in the field by the Portuguese DSO, EDP Distribuição and its operators.
In the case of the Interval Constrained Power Flow tool this was integrated with the existing systems at EDP Distribuição and the French DSO Enedis and tested in real-world conditions between January and July of this year. The operators had access to the user interface and to the flexibility maps generated by the tool, which increased their awareness regarding the overall flexibility of the distribution network and future operating points.
The Sequential Optimal Power Flow tool was integrated with the existing systems at EDP Distribuição and the output (i.e. capacitor banks optimal operating period and network reconfiguration) was implemented in the field, with significant reductions in both active power losses and reactive power payments (i.e. the penalties imposed to the DSO for not respecting the pre-defined reactive power limits in the primary substations).
Availability of TSO-DSO cooperation tools
Bessa explains that as software application the two tools can be integrated in a Distribution Management System (DMS) installed at the DSO control centre level. They require a close interaction with the DSO and TSO SCADA systems, as most of the input information (e.g. current and future network topology, active and reactive power measurements, capacitor banks, OLTC operating state, etc.) comes from these two systems.
Indeed, a load and generation forecasting system is a key requirement as the paradigm behind the tools is to generate predictive information and control strategies.
He adds that as the tools are under development, some further work is required on both to bring them to commercial readiness. “The plan is to solve some limitations, such as the computational time, which were identified during the test-field phase, and the adoption of the Common Information Model (CIM - IEC 61970 and IEC 61968) for the network data representation. The Interval Constrained Power Flow tool also requires an improved graphical user interface in order to include the flexibility representation in multiple primary substations.”
In conclusion Bessa says that an important part of the work to promote the deployment of tools is the construction of attractive business cases and the evolution of DSO roles. “The project identified scalability and replicability issues that will be the basis to establish a road map for the tools deployment.”