Future electricity systems will have large shares of inverter-based resources, i.e. wind and solar generation and batteries. Inverter-based resources are often connected to the distribution grid rather than directly to the transmission grid, which leads to reduced dynamic reactive power response in the case of a voltage disturbance. Additionally, they are not always operated in voltage control mode, further reducing available dynamic reactive power. To address the reduced dynamic reactive power response, new grid codes have been suggested, which include requirements for inverter-based resources such as changes in control mode, control point position, and STATCOM operation. Here, STATCOM operation implies delivery of reactive power from inverter-based resources even at zero active power generation.

To evaluate the impact of new grid codes on voltage stability, we analyze the N-1 post contingency static voltage security for a 400 bus Nordic synchronous transmission system (i.e. Norway, Sweden, Finland and Eastern Denmark). Both a historic (2022) and a future (2050) case are analyzed. The future case was created by a capacity expansion energy system optimization model with the same topology used in this N-1 analysis. We apply various types of contingencies and evaluate whether post contingency states are statically voltage secure.

The results show that the historic case has less dependence on grid code changes than the future case and is generally more stable, with a maximum of 1.6 % of voltage insecure contingencies across all considered grid codes. In the future case, 17 % of contingencies are not voltage secure when inverter-based resources are operating in reactive power control mode. A large 80 % reduction in voltage insecure contingencies is achieved when instead operating inverter-based resources in voltage control mode. Additionally, a further reduction in voltage insecure contingencies by 27 % is found when implementing a change in control point position and requiring STATCOM operation. The change in control point position and STATCOM operation each show similar individual impacts, but the former is available at a lower cost to generation owners. Thus, moving the voltage control points of inverter-based resources closer to the transmission grid seems to be an impactful and cost-effective grid code requirement. Overall, our findings underscore the importance of future-appropriate reactive power grid code requirements for inverter-based resources, especially for units that will soon connect to the grid and will remain connected to the transmission system for many years to come.