EU’s InterOpera: GFM requirements for multi-terminal multi-vendor HVDC systems released

By 2050, the European Union (EU) aims to deploy 300 GW of offshore wind (OSW) capacity, necessitating advanced integration solutions like multi-terminal high-voltage direct current (HVDC) systems. These systems are required to not only transmit electricity directly to the shore but also to serve as interconnectors between countries and transmit electricity between different wind farms.

Supporting this, “Enabling interoperability of multi-vendor HVDC grids” (InterOPERA) project was launched in January 2023 by 21 European partners under the EU’s key funding programme for research and innovation, Horizon Europe Framework Programme. With EUR50 million in EU funding, the project’s (2023-2027)main objective is to make future HVDC systems mutually compatible and interoperable by design, to improve the grid forming (GFM) capabilities of offshore and onshore converters, and to pave the way for the first HVDC multi-terminal, multi-vendor, multi-purpose real-life projects in Europe. It focuses on ensuring that HVDC transmission systems and components procured from different suppliers work together – making them “interoperable” by defining technical frameworks and standards.

In January 2024, the second deliverable (D2.2) was published addressing GFM capabilities to enable robust system integration, as defined in Task 2.4 of InterOPERA project (T_2.4).

Task 2.4 of InterOPERA project

The main objectives of T_2.4 include defining GFM functional requirements for onshore HVDC converter stations to provide synthetic inertia support to the onshore AC grid through the multi-terminal, multi-vendor HVDC system. Additionally, it also aims to establish GFM requirements for DC-connected power park modules (PPMs) to ensure they can remain operational despite temporary interruptions at remote HVDC converter stations, thus improving the reliability of OSW power transmission.

D2.2 details the GFM functional requirements across three types of subsystems within the HVDC network:

• HVDC converter stations with an AC synchronous area connection point: These stations are crucial for providing synthetic inertia support to the onshore AC grid, enhancing stability and reliability.
• DC-connected PPMs: These modules are designed to maintain operation and ride through temporary blockages at remote HVDC converter stations, improving the reliability of OSW power transmission.
• Remote-end HVDC converter stations operating in isolated AC networks: These stations need specific GFM requirements to operate effectively in isolated networks.

Multi-vendor interoperability aspect of GFM control

GFM control involves a fast dynamic coupling between the AC connection point and DC circuit of the HVDC system, for energy exchange and transient in DC voltage and AC active power may occur across the HVDC system. For a single-vendor multi-terminal HVDC project, the boundary of responsibility lies at the connection point between the HVDC system owner and the relevant system operator of the onshore AC grid. In such case, the relevant transmission system operator (TSO) only specifies the GFM functional requirements at the connection point for the HVDC system, while the DC grid and interdependencies between the AC power and the DC side control could be considered an internal problem where everything is coordinated and solved by that single-vendor.

However, for a multi-vendor HVDC project, which consists of converter stations provided by different vendors, there will be responsibility boundaries not only at the connection point, but also at the DC side in the HVDC system (DC connection point). Thus, GFM control of multi-terminal multi-vendor HVDC systems needs an alignment of functional requirements, detailed specifications with parameter settings and coordination of AC and DC voltage control and protection schemes, careful control tuning across all HVDC converter stations and DC connected PPMs as a coordinated response is needed.

Consequently, the multi-vendor multi-terminal specific aspect of the GFM functional requirements presented in this document involves:

• Standardisation of definitions and nomenclature: To ensure effective implementation, all stakeholders in the multi-vendor HVDC system must agree on a unified set of definitions and terminology related to GFM control. This common understanding is essential for seamless integration and operation across different vendors and technologies.
• Dynamic coupling of AC and DC connection points: The behaviour of GFM control needs to be well-understood in terms of how it dynamically interacts with both AC and DC connection points within the multi-terminal HVDC system. This involves a comprehensive understanding of how changes or disturbances at one connection point can affect the other.
• Interdependencies between DC and AC interface functionalities: The GFM control system must account for the interdependencies between the functionalities of the DC and AC interfaces. This ensures that operations on the DC side are properly coordinated with those on the AC side, facilitating overall system stability and efficiency.
• Coordination of GFM control and DC voltage control: GFM control must be harmonised with DC voltage control mechanisms. Additionally, DC system security constraints must be integrated into the GFM control strategy to maintain system reliability and prevent potential issues.

Outcomes and conclusion of D2.2 study

By addressing these aspects, D2.2 aims to foster interoperability and ensure that GFM control systems can effectively manage the complex interactions within multi-vendor, multi-terminal HVDC networks.

The main outcomes and conclusions of D2.2 document are as follows:

• GFM functional requirements: The GFM functional requirements proposed in D2.2 are based on the ‘voltage source behind an impedance’ model, which is the most prevalent and accepted definition in the industry. The document outlines five core mandatory functionalities for GFM, along with optional functionalities and withstand capabilities (more details in Table 1).
• Operational conditions: The proposed GFM converter requirements specify that converters must meet these requirements during normal operation, adhering to current, voltage, and energy limits. In cases where these limits are reached and the converter operates in a withstand mode, it must maintain its GFM capabilities as much as possible while ensuring stable operation and continued grid connection.

Table 1. Mandatory and optional GFM control functions and withstand capabilities as proposed in InterOPERA D2.2

Note: SCR – short circuit ratio; POC – point of connection; RoCoF – rate-of-change-of-frequency; PPM – power park module
Source: InterOpera

• Functional requirements: The functional requirements for GFM control are consistent for both HVDC converter stations and DC-connected PPMs, unless otherwise specified. It is recommended to define dynamic performance requirements individually for each type of GFM converter.
• Transient active power management: In multi-terminal, multi-vendor HVDC systems, it is crucial to recognise that HVDC converters mainly function as energy conversion devices. Any transient active power from GFM control needs to be managed through energy transfer in the DC system and proper coordination with the DC voltage control scheme.
• GFM and DC voltage droop (V_dc) control: All HVDC converter stations should be equipped with both GFM control and V_dc droop control functionalities which should be operable simultaneously. V_dc droop control refers to a purely proportional and fast change of DC voltage based on the changes in the active power exchange. The gain or droop of this controller can vary based on the DC voltage range in the multi-terminal system.
• Active power contribution limitations: Under certain conditions, such as when DC voltage levels are outside acceptable limits, the GFM controller’s active power contributions (including phase jumps, inertial response, and positive damping) may be limited. However, self-synchronisation and inherent reactive power contributions may still be available. A set of minimum performance capabilities for GFM control in HVDC systems with DC voltage control responsibilities is proposed.
• DC voltage range for GFM control: GFM control functionality should be fully available within the normal DC voltage range. Its functionality should be gradually reduced when the DC voltage moves into the alert range, as specified by the connection point requirements.
• Compliance with D2.1: The V_dc droop control function and the associated DC voltage ranges should adhere to the specifications outlined in D2.1 of InterOPERA, which covers the functional framework and basic functional requirements for HVDC grid systems and subsystems.
• Remote-end converter specifications: Remote-end HVDC converters can be either classical frequency (V/f) control converters or GFM converters. When a remote-end converter is specified as a GFM converter, it can be tuned for low inertial active power, showing dynamic characteristics similar to those of a V/f converter.
• Parallel operation of remote-end converters: The capability for remote-end HVDC converters and DC-connected PPMs to operate in parallel with other similar components is outlined. It is recommended to specify remote-end HVDC converters as GFM converters if they need to operate in parallel with other HVDC voltage source converter (VSC) converters.
• Islanded operation of DC connected PPMs: The functionality of DC-connected PPMs to withstand temporary islanding by self-synchronising during a blockage of the remote-end HVDC converter station is specified, which enhances the robustness and availability of power generation in the system.
• GFM use cases: Four detailed use cases of GFM control in multi-vendor, multi-terminal HVDC systems are discussed, with specific control operations assigned to various components. Use cases 3 and 4 involve GFM control from DC-connected PPMs.

Table 2. Use cases of GFM control assignment in a multi-terminal multi-vendor HVDC system

Source: InterOpera

In summary, by developing detailed functional requirements for GFM control in a multi-terminal, multi-vendor HVDC system, the work in T_2.4 and D2.2 report advance the practical implementation of GFM control in such HVDC systems. This progress supports the stability of onshore AC systems and facilitates the further OSW integration.