The multiDC project started in 2017 and is active until 28 February, 2022. As in other complex project, the Work Break Down structure (WBS) is adopted. The WBS breaks down project into smaller, easier to manage portions called work packages (WP’s), and highlights information exchange between them.
The project environment is organized into 6 WP’s. The first work package, WP1, lays the foundation by specifying tasks for each WP, final results at the end of the project, and work package leader. Based on the finding of WP1 – “Specification, description and organization of Work Packages”, WP2-4 direction is set with specific technical topic and tasks. The results from WP2-4 are used to implement and test the findings in real-life environment, and give final technical recommendations (WP5). Throughout the project runs WP7 – ” Communication and dissemination” supported by all project partners, and WP6 – “Administration and management” from Technical University of Denmark. The detailed work package description with task are presented below.
The work package 1 objective was to gather existing research results and current trends in power systems or energy infrastructure projects. From such a standpoint, detailed project scope, direction, and result requirements were identified, from which each work package was formed with specific tasks and responsibilities.
The project partners have a different technical background related to electrical systems, which needs to be coordinated. Therefore, the WP1 discussion outcome defines the clear function of each partner and its role in the project.
Through enabling these discussions WP 1 laid important groundwork for the multiDC project as a whole and particularly the outcomes that will emerge towards the end of the project.
Deliverable 1.1: Gather existing research and current trends to define the scope of the project
Deliverable 1.2: Form technical work packages and define package task, deliverables
Deliverable 1.3: Create project partners function, roles, and responsibilities in the project context
Large-scale integration of renewable generation, usually interfaced with the network through power electronics, has led to an overall decrease in power system inertia. Due to the increased integration of RES, power systems are gradually shifting from synchronous-based to converter-interfaced power generation. Conventional power plants are also responsible for providing various system services, such as Short Circuit Capacity (SCC), frequency control, and rotational inertia. If conventional power plants can no longer provide these services other solutions need to be developed. To ensure the feasibility of such a transition, stability analysis needs to be conducted and advanced control design should be employed. The scope of WP2 is on the fundamental stability properties of such systems, as well as identifying the limit for high penetration of voltage source converters. Moreover, the scope of WP2 seeks to find high fidelity model order reduction techniques for evaluating existing simulation tools and their appropriateness of modeling low- and zero-inertia systems.
Task 2.1: Stability enhancement and control of low- and zero-inertia systems
This task first investigates the impact of time-varying inertia and damping on the frequency dynamics of the system. An analysis is conducted using tools from the control system theory on how the uncertainty of those parameters affects frequency dynamics. The focus of this task is on proposing a structured robust control design suitable for low-inertia systems, in order to increase the security of the system operation under uncertainty. Moreover, with Denmark dedicated to maintaining its leading position in the integration of massive shares of wind energy, the construction of new offshore energy islands has been recently approved by the Danish government. These new islands will be zero-inertia systems, meaning that no synchronous generation will be installed on the island and those power imbalances will be shared only among converters. To this end, this task also proposes a telecommunication-free frequency droop controller to maintain the active power balance in the offshore system and guarantee N-1 security. Although offshore systems are the main focus of this task, the presented methodology could be applied to any other zero- or low-inertia system. The frequency droop gains are calculated by solving an optimization problem that takes into consideration the small-signal and transient stability of the system. As a consequence, the proposed controller allows for greater loadability of the offshore converters at the pre-fault state and guarantees their safe operation in the event of any power imbalance.
Task 2.2: Appropriateness of RMS modeling in AC systems with penetration of converter-based resources
Time-domain simulations are a critical tool for power system operators. Depending on the instability mechanism under consideration and the system characteristics, such as the time constants of controllers, either phasor or Electro-Magnetic Transient (EMT) models should be employed. On the one hand, EMT models provide detailed modeling of the system dynamics, thus increase the reliability of stability analysis; on the other hand, using these models increase the computing time of simulations, slowing down the security assessment process. To decrease computational time, system operators could resort to phasor-mode simulations for a subset of disturbances. This task of WP2 investigates the appropriateness of phasor-approximation models on simulating events related to the power supply and balance stability. Moreover, it provides sufficient conditions and bounds for the control parameters of the converters under which the phase-approximation modeling of the component is still valid. Time-domain simulation is performed to validate the analysis using the North Sea Wind Power Hub (NSWPH) test case, which is developed in the project.
WP3 focuses on employing the advanced functionalities of HVDC interconnections through supplementary power control, in order to enable the provision of cheaper ancillary services from neighboring areas. This task demonstrates that such capabilities could be beneficial for the dynamic stability of areas that experience a disturbance without jeopardizing the stability of neighboring ones. Moreover, the study aims to provide robust solutions that would improve the overall system controllability, without implying high costs of implementation. The proposed approaches will be demonstrated and tested on a detailed realistic Nordic system model that is developed in Task 5.4.
Task 3.1: Mitigation of emergencies through preventive coordinated control
The first task is the support of Frequency Containment Reserves (FCR) actions by utilizing HVDC interconnections in the form of Emergency Power Control (EPC). This study mainly proposes a novel droop frequency-based control approach for EPC provision, which is expected to improve the frequency response and reduce the currently required EPC reserves (in the Nordic Power System). Moreover, the properties of FCR are carefully analyzed to define the minimum requirements for EPC to maintain the system N-1 secure in case of various (low) inertia scenarios. Finally, the study tackles the challenge of long-term adaptive operation to reduce the potential cost of such services with and without the usage of remote measurements.
Task 3.2: Control of System Response and Stability Enhancement
The second task assesses the influence of such HVDC frequency support services developed in Task 3.1 to rotor angle stability properties. The main concern is the local interaction between defined HVDC frequency support and (electrically) nearby generators. The study mainly focuses on finding a proper range of operation for which rotor angle stability properties such as damping and synchronizing components of the influenced generators are improved. Since there are high uncertainties in generators (control) parameters and system operation, an analytical approach is used to drawn general conclusions.
From a market point of view, HVDC interconnectors facilitate the exchange of energy and ancillary services between countries. Thus, if HVDC and AC grids are operated in an optimal way, significant cost and energy savings can be achieved. The scope of WP4 is to develop methods for the integration of the advanced functionalities of HVDC operation in a market setting, and the fair allocation of possible costs.
Task 4.1: Internalize HVDC losses in the market clearing process
Contrary to AC interconnections, which usually span only a few hundred meters, HVDC interconnectors are often hundreds of kilometers long. Thus, when considering the operation of such long HVDC lines, the cost of thermal losses becomes non-negligible. Focusing on HVDC interconnections, which, by definition, connect two different control areas, the question that arises is: who should bear these costs? This first task of WP4 will focus on developing a rigorous framework for internalizing HVDC losses in the market clearing algorithm, and then on different options for a fair allocation of such costs.
Task 4.2: Optimal coordination between asynchronous areas for the exchange of frequency products through HVDC lines
The ongoing decrease of system inertia is classified as one of the major future challenges for the Nordic Power System. To make sure that the system can withstand the loss of the most critical component, Transmission System Operators ensure that enough frequency balancing resources are procured and that the balancing responsible parties are able to supply the system with the primary and secondary reserves in case of a component failure. However, the advanced functionalities of HVDC lines can assist in delivering fast active power from neighboring regions, and, thus, ensure the system security. This service can help control areas avoid committing expensive generating units (which usually offer such reserves). The second task of WP4 will push the state-of-the-art by formulating, for the first time, an optimization problem which considers the unit commitment, the system inertia, the frequency constraints, and the HVDC converter frequency response among several neighboring regions. The results will be validated using a dynamic model representing the Nordic system.
The findings and control methods of WP2-WP4 focuses on concepts of zero and low inertia systems stability and operation, integration of HVDC lines in the market-clearing algorithm, and reserves sharing using emergency power control from HVDC. The first WP5 objective is to verify the results by creating a testbed for online and physical simulations. The developed online simulation tools are exchanged back to WP2-WP5 to strengthen the impact, and to implement the most promising methods in PowerLabDK and TSOs SCADA system.
Task 5.1: Flow control of combined grid for offshore wind power integration
This task implements and tests in real-time simulations the Kriegers Flak flow control scheme. The coordination scheme of combined VSC HVDC and AC grids will be tested to realize the planned power flow to Denmark and Germany according to the market-clearing. The VSC HVDC control will be developed and tested in real-time digital simulations.
Task 5.2: Coordinated voltage control for offshore wind power with combined grid
The task develops and tests coordinated automatic voltage control and reactive power control schemes of the combined grid and offshore wind power plants (WPPs) in order to maintain a good voltage profile within the offshore grid of Krieger’s Flak and WPPs, and maximize dynamic var reserve. The control scheme will be tested in real-time digital simulations.
Task 5.3: Implementation of the basic functionalities of the Kriegers Flak Master Controller in the SCADA system of EnerginetDK
Based also on the insights gained through Task 5.1 and 5.2, this task will focus on the implementation of the basic master controller functionalities in the SCADA system of Energinet.DK. This includes the reactive power control scheme for offshore wind integration and the coordinated voltage control scheme.
Task 5.4: Development of a realistic model of the Nordic system, including the North Sea Wind Power Hub
The task has two goals. First, using real data, this task aims to develop a state-of-the-art realistic model of the Nordic system for:
- Dynamic Simulations, focusing on frequency stability
- Market simulations
Second, develop a realistic model of the North Sea Energy Hub, for different possible AC topologies.
The ultimate goal is to merge these two models in an integrated one, which will be used for the development and testing of our proposed approaches in WP2, WP3, WP4.
Task 5.5: Demonstration at PowerlabDK
The last task will focus on the implementation of the North Sea Energy Hub model along with the Nordic System in PowerlabDK. The goal is to employ the RTDS infrastructure of PowerlabDK and Power Hardware-in-the-Loop, in order to test and demonstrate the zero-inertia and low-inertia topologies of the North Sea Energy Hub. The real hardware will be the synchronous condenser controller, which is available at the lab, for the low-inertia NSEH topology.
The work package is expected to deliver the following:
- Project financial administration and reporting to project funding members
- Coordinate project decision and activities through technical and steering committee meetings
- Prepare status reports and project progress
- Ensure proper cooperation and synergy between the WP’s
The work package is expected to deliver the following:
- Dissemination of project findings to all target groups and the wider public via website, workshops, academic publications, and social media channels
- Present the work outside the project to other institutions via invited talks
- Present the work to a wider audience through journal articles
- Organize multiDC Demonstration Event by the end of the project