Transmission System Operators (TSOs) are in charge of delivering electricity in an efficient, resilient and safe manner. Renewable energies (wind and photovoltaic) have a rising role in the electrical systems and are going to become an important element to control. These new generation units are not based on conventional generation technologies and due to their uncertainty and its different technology, it is necessary to define an optimum performance. Transmission System Operators (TSOs) need innovative ideas to keep the power system safe, reliable and up to the required quality standards.

Scientific and technological progress has improved the functionalities that renewable generation units can provide to support the grid. TSOs must be aware and take advantage of the new capabilities that these new technologies can achieve and use them for the electric power system to operate optimally. We’ll develop a tool that will help REE define how these new agents such as solar generation, wind plants or mass storage elements will behave, to optimize the frequency response of the system. New energy resources based on power electronics offer the possibility of a wide range of behaviours.​

It is expected for ENIGMA to be a strategic tool for REE, , helping to envision and define the future grid in diverse business areas: Planning Department, Demand management, Control System, etc. ENIGMA addresses future challenges in several aspects:​

• Define the minimum technical requirements for PV and WTG.​
• Suggest control functions to be included within new versions of Grid Codes, or services.
• Simulate future scenarios and predict the impact on grid stability.​
• Define optimum parameters for control functions of elements connected to the grid.

After more than 3 years of research, the FST (Flexible Smart Transformer) project has been successfully completed. Funded by the Grid 2030 innovation programme, the project has consisted in the development of a wireless power transfer module with a high capacity of isolation between the primary and the secondary side, allowing the development of new applications.

The FST project consists of the development of a new multi-purpose device with additional advantages compared to very high voltage power converters. Firstly, it proposes a magnetic coupling through a dielectric medium instead of a closed ferrite core in the high-frequency transformer stage. This modification reduces the weight significantly, gives intra-modularity and scalability to the system, and achieves a simple design of the very high voltage isolation. Secondly, the use of SiC (silicon carbide) technology helps to reduce the volume of the entire system. All these features help to drastically reduce maintenance and transportation costs.

On the other hand, it is expected to contribute to the decisive transformation of the current transmission grid paradigm to meet the challenges of decarbonisation of society. In fact, this innovation contributes to remove traditional insulations such as mineral oil, which means a significant reduction of pollutants. This new insulation method also allows for a significant volume reduction, which will contribute to a better integration from an environmental impact point of view.

Some of the potential applications are: Solid State Transformers (SST); AC/AC Converters; HV Flexible AC Transmission Systems (FACT); High Voltage Energy Storage Systems (ESS); HVDC Links; HV DC/DC; Unified Power Flow Controllers (UPFC) and Offshore Wind Farms.

The particularity of this development lies in the fact that it is designed for use in high-voltage transmission grids (230kV and 400kV in Spain) as a unified power flow controller (UPFC). This uniqueness poses two fundamental challenges, which have been tackled throughout the execution of the project:

• - The device has had to be designed (based on a patent registered at the Spanish Patent and Trademark Office, reference number ES3462.6) in order to be connected to very high voltage networks. This challenge has been fixed using an inductive coupling in order to transfer power, solving the isolation problems presented by traditional dry transformers. On the other hand, the design has also considered achieving high voltage values by means of serialisation and parallelisation of several modules. This is possible because the system works with floating voltage references.
• - The usual design of these devices is unidirectional, due to the nature of inductive power transfer applications (always from the grid to the load). Nevertheless, the FST module allows a bidirectional power transfer. This key feature enables an effective control of power flows within the network.

Both challenges have been successfully overcome, resulting in a module capable of overcoming these two technological barriers. The technical specifications of this device are listed in the following table:

A test bench has also been successfully executed to check the operating characteristics of the device, where it has been possible to evaluate the correct performance of the module.

The manufacturing of an UPFC based on the technology developed in the FST project will provide an ideal power electronics solution for the decarbonisation of the energy sector demanded by the current energy transition, achieving a substantial improvement in several aspects:

• - Controllability of the electrical grid, as it will allow leading power flows, as well as controlling voltage levels, thus facilitating the penetration of renewable energies.
• - Stability of the power grid, allowing the integration of additional impedance to avoid inter-area oscillations.
• - Cost reduction, achieving a more efficient grid by means of power flow control, reducing transport costs as it is a totally modular system, reducing maintenance costs, etc.
• - Environmental, as it is a cleaner equipment, with no need for coolant, thus eliminating the risk of contamination by waste.

The GRID2030 RITSE project of two (2) years duration addressed a specific power system stability issue - the transient stability. To build the most valuable solution for TSOs like Red Eléctrica, it was essential to sort and focus the solution features in order to align with what the customers find most compelling, i.e. the implementation of the technical solution as well as the market needs. Through the “Reduced Inertia Transient Stability Enhancement” (RITSE) project, SuperGrid Institute and IMDEA created a partnership to improve the transient stability of the AC networks by coordinating the use of batteries (BATTERTIA concept) and HVDC links supplementary control (Dynamic Virtual Admittance Control -DVAC concept). in the context of inertia reduction for system stability and ancillary services, the project development lines were as follows:

• - To assess the proposed control solutions on realistic power network by considering existing measurements as well as industrial hardware implementation.
• - To provide guidelines on technical requirements usable for Red Eléctrica to elaborate technical specifications of solutions.
• - To describe the value chain of such solutions in both regulation and market design for ancillary services.
• - To elaborate the go-to-market strategy.

RITSE Inter-area and intra-area transient stability concept

The DVAC concept proposed and developed by SuperGrid Institute consists of an innovative method to control embedded into the AC network VSC-HVDC links in order to contribute to the transient and small signal stability improvement of electrical systems. The control scheme computes the necessary active power references for each station of a VSC-HVDC link allowing it to emulate the behaviour of a self-tuning loss-less AC transmission line connected in parallel with this VSC-HVDC link. The associated control law is derived in order to determine the instantaneous value of the virtual admittance based on the state variables and the parameters of the system, brought by performant measurement systems. The main objective is to improve the rotor angle stability of the system and implicitly ensure the synchronization of two AC interconnected networks.

The motivation behind the deployment of the DVAC control is the participation in the system stability after being exposed to instabilities essentially due to the reduction of its inertia. The ambitious target set by the EU concerning the integration of renewable energies will increase the sharing of power electronics and as a consequence reduce the inertia. As a result, supplementary control to deal with instability issues is needed. The DVAC control concerns essentially the embedded into the AC network VSC-HVDC links. The growth of VSC-HVDC links market and the numerous installed and planned HVDC projects gives the DVAC a promising future from the market point of view.

DVAC Proposed control structure for providing support to the surrounding AC grid

IMDEA Energy Institute introduced of a novel control concept for battery-supported power electronics interfaces called BATTERTIA. Unlike the existing power battery converters offering only active and reactive power controls, BATTERTIA has the same inertial and voltage support characteristics as a synchronous generator offering enhanced voltage support and inertial services to electricity network operators. In this way BATTERTIA helps the network operators retain and improve transient and voltage stability properties during the on-going process of power system decarbonization. BATTERTIA control system is based on a grid power converter coupled with the battery system. A custom made Virtual Synchronous Machine (VSM) concept was used for the control of the power converter and a battery State of Charge (SOC) controller was tuned to provide only the inertial response. Inertial properties can be configured according to the user specifications.

It was demonstrated that BATTERTIA represents the enabling technology for the batteries to enter ancillary service market and compete with existing inertia reserves provided by rotating machines. As such, it has a direct impact on DSO and TSO capacity to operate MV and HV networks, secure the power system stability and allow a massive and safe connection of Renewable Energy Sources.

BATTERTIA overview – power stage and the measured quantities

The ENIGMA project, developed under the Grid2030 program, looks for solutions based on AI to optimize the stability of the Spanish power system. Its main objective is to develop new control techniques which would allow frequency stabilization in the event of possible future contingencies, based on smarter management of renewable capacity. The increasing incorporation of renewable plants into the grid, connected through power electronics, poses a new scenario where traditional control systems may be overwhelmed. A new system capable of adapting to such changes and controlling the grid in an optimized way is needed.

Reinforcement Learning (RL) is one of the Machine Learning paradigms with the greatest potential for the development of new control systems to solve these new scenarios. In this paradigm, an RL agent interacts with an environment, generally simulated, and receives a reward depending on the quality of its actions. The objective of this agent is to maximize this reward signal by exploring an immense space of possible control policies. Once trained, this agent could be deployed in the real environment to achieve the desired objectives.

Within the ENIGMA project, a training platform for RL agents has been developed. One of the pillars of this platform is a power grid simulator in which the island of Gran Canaria has been modeled. Thanks to its fast execution times, the agents have been able to be trained in hundreds of thousands of possible scenarios, thus learning to control the future renewable plants of the grid and to respond appropriately to possible contingencies. These agents have been developed under different communication configurations, emulating possible scenarios that may occur in the future. These agents manage to improve the existing response and to propose the most convenient new scenarios for the future.

In future scenarios where the energy mix is mainly composed of controllable renewable generation and little conventional generation, the developments of the Enigma project facilitate the control of the frequency response of the former systems and lead the way to the incorporation of AI in control centers. Results of the project showed a reduction in the contribution of conventional energies by up to 42% in favor of renewable generation.

The following figure shows an example of an episode response in which a significant loss of demand is simulated at t=0, resulting in an over-frequency episode. On the left is shown the frequency response of the RL agent compared to the actual system response (PO12.2). On the right is shown the active power of the controlled renewable energy inverter generators along with the setpoint (dotted line) sent by the RL agents. The response of the agents not only allows, as can be seen in the image on the left, to reduce the frequency deviation, but also produces a response that would not be possible to obtain using the current proportional controller.

With the Enigma project it has been demonstrated that, faced with the future concern of system inertia loss and its implication on frequency stability, AI-based technologies and simulation environments are a valid alternative to guarantee system stability.