Integration of renewable energy sources: Strong challenge for a weak grid
Integration of renewable energy sources whether into transmission or distribution level focuses on incorporation of such sources in a reliable and economic fashion in the bulk electric power system while maintaining the grid integrity, reliability and load deliverability. Integration of electronic based renewable energy sources such as wind, solar and storage units equipped with complex fast-acting control systems to the weaker portion of the grid has been a challenge for equipment manufacturers, grid planners and operators. This challenge stems from the fact that connecting these renewable energy sources to a weak grid may result in transient over voltages, voltage oscillations and system instability. If this issue is overlooked and/or not addressed during the planning and design stage of the integration project, postponing the generation interconnection or curtailing the generation output will be inevitable imposing significant financial risk on generation developers. Furthermore, recognition of this issue after commercial operation of the generation unit will make the grid vulnerable from reliability standpoint. This article briefly discusses the key challenges associated with integration of renewable energy sources into a weak grid.
Strength of the electric power system is defined as the ability of the system to maintain its voltage during the injection of reactive power. In comparison with weaker systems, stronger systems will experience less voltage change following an injection of reactive power. Short Circuit Ratio (SCR), defined as ratio of the interconnected grid’s short circuit MVA (before connecting the generator) to the MW size of the interconnecting generator, has been utilized to quantify the strength of the electric power system with respect to the interconnecting generator. The lower the SCR, the weaker the electric power grid will be. Weak systems become more troublesome when renewable sources with fast controllers connect to them. This is due to the fact that the voltage/reactive power control loops within these electronic based generation units are capable of almost instantaneous reactive power injection in response to any voltage change at the point of interconnection. Fast reactive power injection/absorption to a weak grid, characterized by high Volt/VAR sensitivity, may translate to un-damped voltage oscillations. Therefore, apart from the system strength, the speed of the voltage controllers associated with the renewable sources has a substantial impact on the dynamic response of the renewable source and the stability of the interconnected grid. While reducing the voltage controller gain will slow down the voltage controller response associated with the renewable sources and could mitigate the voltage oscillations, it will also slow down the post-contingency voltage recovery. The balance between the post-fault transient voltage recovery and a stable response is critical to grid integration studies for renewable sources connecting to weaker portions of the grid.
Having defined the system strength and its impact on integrating fast acting renewable sources, it is worthwhile to discuss why the integration of renewable energy sources to weaker portions of the grid is still a challenge and why equipment manufacturers, grid planners and operators are still struggling with this issue despite notable progresses and developments in this area.
The first reason is related to the fact that in most of situations equipment manufacturers may not have access to the precise SCR level/system strength of the system to which the renewable source is planned to connect. Therefore, they normally tune the control systems for a typical SCR level which is very likely different than the actual interconnected grid’s SCR. Additionally, the grid’s strength level may drastically vary during contingencies which will be another roadblock for equipment manufacturers to precisely tune the control systems to ensure stability and acceptable post-contingency voltage recovery.
The second reason stems from the fact that traditional system strength index may not be effective for all grid topologies. Traditional SCR is utilized to evaluate the strength of a single bus within a power grid to which a single generation unit is planned to connect. However, there may be a situation in which a cluster of fast-acting renewable energy sources with different control philosophies and parameters connect to a multiple neighboring buses within a system forming a super node. In this scenario, the strength of the individual bus holds less relevance. Indeed, in this scenario, the system strength of super node relative to the cluster of renewable energy sources needs to be evaluated in order to find the balance and tune the control systems.
In a nutshell, integration of fast acting renewable energy sources into a weak grid requires comprehensive planning studies in order to evaluate the possible instability and reliability risks. DNV GL Power System Planning team can help bridge the gap between equipment manufacturers, generation entity and interconnected grid to evaluate the project feasibility from reliability standpoint, determine any potentials for reliability risks under normal and contingency conditions and to propose mitigation plans to be performed either by equipment manufacturers or by utility entity depending on grid topology and situation in which instability has been observed.
In the view of aforementioned challenges, following scope of practice could be performed in order to effectively plan the reliable integration of the renewable energy sources into power grids from transient stability standpoint:
- Developing equipment specific user-defined dynamic models or customizing publicly available generic dynamic models for the renewable energy source under study and benchmarking the developed dynamic models with real time measurements and responses.
- Utilizing the load flow and dynamic parameters along with the dynamic models to represent the renewable energy campus in simulation software.
- Incrementally adding the renewable energy campus model to the load flow and dynamic dataset of the interconnected grid and conducting transient stability analysis to evaluate the stability of the proposed renewable energy source and interconnected grid under normal and contingency conditions and various control strategies.
- If any instability is observed following the integration of the renewable energy source into the power grid, performing detailed analysis to identify the underlying reasons for such instabilities.
- If it is observed that the instability issues are caused by the renewable energy source and its controllers, investigation of generation-level mitigation options such as re-tuning controllers associated with the renewable source and/or static/dynamic shunt devices.
- If it is observed that the reliability concerns are caused by the interconnected grid or by neighboring facilities, investigation of system-level mitigation options such as transmission/distribution system expansion, static/dynamic shunt devices, re-tuning plant level controllers to improve coordinated control strategies, HVDC and synchronous condensers.
In conclusion, investigation of stability impact is a crucial step in integration of renewable energy sources into the weak portions of the power grid. DNV GL’s Power System Planning team has proven expertise, experience and comprehensive scope for modeling of renewable energy sources, conducting detailed transient stability studies and developing generation-level as well as system-level mitigation options for integration of renewable energy sources into the power grid. Contact us at email@example.com to learn about how we can help you.