Renewables at >50% in European grids

To comply with the European goal of reducing greenhouse gas emissions, power systems have been integrating ever higher levels of renewable energy sources (RES) – and similar upwards trends are expected for the next few years. To accommodate this high level of renewable generation – mainly from non-synchronous sources – power systems must undergo structural changes while maintaining a reliable and cost-efficient service.

One of the main evidences of these structural changes is the high percentage of this non-synchronous RES that is expected to be connected at the distribution level, forcing distribution and transmission system operators (DSO and TSO) to change operational and planning procedures and increase their coordination. Historically, for the power system planning and operation, the TSO would model distribution networks as a passive constant power model without any dynamic characterization, whilst the DSO would model only their voltages levels without considering any important interaction with the transmission network. This coordination between DSO and TSO is one of the most important challenges to be tackled.

To this mean EDP and INESC TEC have jointly designed the Flexibility Hub (FlexHub), a concept that is going to be implemented and demonstrated within the EU-SysFlex project, which is a TSO-DSO coordination platform trying to address several of these challenges.

Flexibility Hub (FlexHub)

The FlexHub tools and demonstration aim to address the challenges coming from scenarios of high RES power levels (above 50%) where new strategies are needed to allow maximum flexibility at minimum cost, while maintaining a secure and stable system operation. In such scenarios, and due to the progressive decommissioning of conventional gas and coal power plants (to comply with the EU decarbonisation objectives) there will be:

  1. An increasing TSO need for frequency regulation and balancing reserves provision due to the variability of RES
  2. A need for additional reactive power regulation

In addition, the increase of distributed resources will transform the distribution grids (traditionally more resistive) into grids with more complex dynamic behaviours, and therefore, with larger impacts on the whole power system dynamics.

The Flexibility Hub (or “FlexHub” for short) is a new platform concept to promote the interaction and coordination between TSO and DSO for an enhanced system operation.

Service provided:

To illustrate the capabilities of the FlexHub in addressing the aforementioned challenges, the following solutions and services are being developed and will be demonstrated in the project:

1. Reactive Power Provision – a new local market

This service proposes an innovative local market design to provide reactive power – according to the needs of both the DSO and the TSO – from resources connected to the distribution grid.

This local market is managed and cleared by the DSO through an optimal power flow that ensures that none of the accepted market bids compromise the secure operation of the grid.

Comparing to the classic market structure, this proposed market arrangement increases the temporal granularity, decreasing the product time duration and allowing a closer to real time optimization of the network assets. To ensure that a broad range of assets can participate in it, the market combines an extended delivery time with complex bids allowing the DSO to facilitate the adaptation of the cleared schedules to the real operating constraints of the new assets providing the service.

2. Active Power Provision – mFRR/RR-type reserves

This service proposes a new market design to integrate active power bids of assets in the distribution grid for the manual balancing reserve of the TSO.

This market is a redesign of the current restoration reserve (RR) market, with increased temporal granularity (as for the previous case), reducing the time-duration of the products. It also increases the delivery horizon, so that, in combination with complex bids (designed according to the resources that could provide the service), it helps market agents to adapt the clearing schedules to the real operating constraints of their assets.

To ensure the coordination between the TSO (requesting the activation) and the DSO (responsible for the network where the asset is connected) a Traffic Light Quantification (TLQ) algorithm was developed. When the TSO selects a bid with resources located at the DSO grid, the DSO triggers the TLQ of the bid to inform the TSO of the amount of active power flexibility that can be activated without causing congestion or voltage issues in the distribution grid. In addition, a colour code indicates if, due to the aforementioned constraints, the resources offered can be totally (green), partially (yellow) or cannot (red) be activated.

3. Equivalent dynamic model of the distribution grid provision

This service provides an equivalent dynamic model of the distribution grid to replace the actual (more static) model use by the TSO. This dynamic model intends to represent the dynamic behaviour of the distribution grid under small voltage and frequency disturbances for TSO stability and dynamic grid analysis.

This model allows the TSO to incorporate more realistic representations of the dynamic behaviour of the distribution grids at the TSO/DSO connection points. This is of key relevance as the increasing penetration of distributed resources is transforming the distribution grid into more complex and dynamic structures with a much more significant impact on the transmission grid dynamics. The proposed model allows to include a larger diversity of distributed generation technologies allowing a more realistic representation of all the RES connected to the distribution grid.

Demonstration

EDP and INESCTEC will carry out demonstrations of the services in Portugal, according to the diagram below.

For the Reactive Power Provision, the tests will be made in the 60kV distribution grid with two wind farms, totalling 35MW installed capacity and 2 capacitor banks. For the Active power provision, the setting will take place in the 15kV distribution grid and will include a 2.5MW PV farm and a 480kW/360kWh Storage unit.

These tests will be performed in close collaboration between INESCTEC (technology provider and developer of FlexHub), EDP NEW (EDP’s R&D Centre and WP/ leader), EDP Distribuição (as the Portuguese DSO) and EDP Renewables (wind and PV farm operator) and aim to validate in a real scenario the functionalities developed for the FlexHub.

 

Conclusions

The FlexHub is an innovative platform for the TSO-DSO coordination that addresses three basic coordination issues, namely, the provision (from distributed resources) of reactive power to TSO and DSO and of active power to the TSO, and the improved and simplified modelling of distribution networks for TSO stability and dynamic grid analysis. This coordination issues are becoming essential and a matter of continuous research in the current decarbonization context with the decommissioning of conventional power plants and the increase of non-dispatchable distributed renewable generation.

Although the FlexHub already provides promising solutions for the integration of high voltage level distributed resources, there is still an important way to go. Other important issues relate, for example, to the standardization of local flexibility market solutions, the integration of lower voltage level resources and coordination of different voltages levels in the provision of grid services, the improved digitization for the provision of local markets and energy services or the engagement of final customers in the energy and flexibility markets among others, which are already been tackled in other projects where EDP and INESC TEC continue to research together.

 

Authors: Miguel Jorge Marques (EDP), José Villar (INESC TEC) and Nuno Marinho (EDP)

 

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