Synchronous generators are the primary voltage sources in electrical power systems and play therefore a central rule to voltage regulation. As voltage is a local value subject to both slow variations (e.g., load changes, network topology modifications) and fast disturbances (e.g., short‑circuits), maintaining system voltage within TSO grid‑code limits is essential for equipments protection, losses minimization, and overall system stability.

Voltage regulation is achieved through generator excitation systems, which adjust the internal EMF to supply or absorb reactive power as required by the system. Rotor current is limited to prevent thermal overload of the field winding and to preserve generator stability. As a result, the P–Q capability diagram is bounded by overexcitation and underexcitation limits implemented within the AVR, while protection schemes act only after these limits, thereby ensuring maximum voltage support duration.

A detailed transmission and distribution model based on the CIGRE benchmark [CIG14] is implemented in PowerFactory to evaluate these phenomena. A 24‑hour simulation with a 10‑minute resolution captures a wide range of operating scenarios involving varying load levels and renewable generation. Overvoltage conditions primarily occur during low‑demand nighttime periods (00:00–06:00), driving synchronous generators toward regions of high reactive power leading in their P–Q diagrams. Results also demonstrate that a premature activation of generator protection prior to reaching underexcitation limits can lead to cascading overvoltage events and to the system blackout.

The study concludes with a description of EDF Producer’s expert framework and quality system entitled “Critical Parameters for Power System Stability”, established following different stability events.

The “Critical Parameters for Power System Stability” quality system includes dedicated training programs, reference frameworks for control settings, full traceability of operational feedback, as well as a support network for operators of power plants. The required controls are integrated into preventive maintenance practices. This organization therefore supervises AVR and Governors controls settings, as well as protection relay configurations, all of which are critical to system stability. How can we choose the different parameters supervised, how can we control them, what tolerances can be granted?

In addition, EDF‑developed Hardware‑in‑the‑Loop platforms and monitoring tools further reinforce the robustness of this framework, that will be explained.

Particular attention will be devoted to electrical systems exposed to elevated voltage levels, notably in configurations featuring a high density of decentralized generation or networks with extensive underground cabling.