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System Stability Impacts: Grid Code Trade-off Analysis

Public summary

The large scale grid connection of FLOW power plants triggers technical concerns about the frequency and voltage stability of the interconnected Dutch power system. These issues are not normally observed in current traditional power system where conventional generation units dominate the generation fleet. However, future power system is going to integrate large amounts of variable renewable energy resources of which FLOW plants would be a dominant contributor.

Transmission system operators (TSO) in order to ensure a certain level of the delivered power quality and security of supply, introduce interconnection criteria or grid codes as commonly termed for variable renewable power generation units such as FLOW plants. These set of grid codes include normal operation (steady state) and faulted condition requirements. The fault response of the FLOW plants influence significantly the power system stability performance and the risk of blackouts. For FLOW power plants in order to be grid code compliant, without negatively affecting the security of the inter-connected power system, a set of an intensive certification processes need to be fulfilled.

The certification process requires initially practical field tests, where critical parameters for the wind turbines models can be defined. Next, offline simulation-based interconnection studies are required by system operators in order to prove that the FLOW plants are grid code compliant and do not jeopardize the stability of the power system. In that prospect, steady-state and dynamic models for FLOW plants are needed by the project developers and by the power system operators. Building such models is a time consuming process which involves additional investments costs for the project developers. Obtaining black box dynamic models from wind turbine and high voltage direct current transmission (HVDC) manufacturers is not efficient as there are confidentiality and intellectual property conflicts/concerns. Let alone that black box models do not always provide a lot of insights due to their restricted in use nature. Finally, system operators and project developers need to have deep understanding of the plant dynamic behaviour in order to reduce risks or optimize their plant.

The main objective of this FLOW project is to provide the project partners with offline desktop simulation tools for FLOW wind power plants in high voltage direct current transmission technology. Deliverables 2,3 and 6 provide a description of the models. Secondary, the simulation tools are used within the project in order to perform control parameters trade-off analysis. These are documented in deliverable 3 and 6. In that prospect the system stability impact of FLOW on the interconnected power system is demonstrated for a variety of grid case studies. Recommendations on the grid code parameters are given in Deliverables 3 and 6. In addition, an optimal grid code compliance methodology is proposed. It reduces the cost of compliance, as it eliminates the use of hardware, and optimizes the dynamic behaviour of the FLOW plants from technical perspective.

The developed in this project simulation tools and methodologies for optimal grid code compliance documented in the deliverables, cover all the spectrum of the needed grid connection simulation studies including namely: power flows, detailed transient and dynamic stability models for FLOW plants with high voltage direct current (HVDC) transmission. The results and the tools can be directly applied by the FLOW project developers in future projects in the Netherlands (i.e RWE) and by transmission system operators (i.e TenneT TSO B.V.) in order to assess the system stability impact from the large scale grid connection of FLOW power plants. It can be used to define transient stability metrics and design grid codes for the Dutch power system and evaluate its stability for various scenarios of wind power penetration levels. In addition, project developers and system operators can use the tools in order to prove that the future FLOW plants are optimally grid code compliant reducing risks both for the power system and for the FLOW plants.

In this work, it is shown that by using the grid code compliance optimal trade-off tool, along with adequate analysis (deliverable 6A, 6B, 6C) it is possible to achieve a cost reduction of 0.5% according to the FLOW model, mainly due to the contributions in CAPEX, DEVEX and OPEX. The substitution of hardware equipment (as the HVDC DC-chopper) by coordinated software based control schemes for fault ride through compliance, would decrease the CAPEX for the HVDC link to at least by 5% (at the worst scenario), as it reduces significantly the size of the onshore HVDC converter station. Furthermore, clearly defined offshore grid codes at national level, harmonized to the European grid codes released by ENTSO-E, and confidence that the offshore HVDC networks can be safely operated will encourage the development of offshore wind resources reducing DEVEX. The developed in this project software tools can be used as platform for the preliminary design of HVDC grids and wind plants where the anticipated total system response can be studied beforehand by TSOs and FLOW project developers. Finally, an anticipated 10% OPEX reduction of the unscheduled maintenance cost can be achieved since the imposed electrical stresses due to the AC faults are minimized using the proposed optimization scheme.

Finally, the project has provided insights into technical risks associated with the grid code compliance of HVDC connected FLOW power plants during AC grid faults. Risks arise from the operation of the HVDC link and the wind plants and from badly tuned control loops installed in order to support the system during fault conditions. In addition, the use of such tools reduces the delays in the preliminary design phase making it possible to reduce the initial costs for preliminary studies.