System Awareness, Protection and Security for the Future – Experience and Solutions in Exploiting Low-Inertia Grids 

 

During their exploitation, classical power systems face different phenomena, such as sub-synchronous resonance (SSR) and Inter-area oscillations (IAO). SSR results from significant energy exchange of the electricity grid with a turbine generator at one or more of the natural frequencies of the combined system. As a result of imperfections when the torsional modes of turbine-generator shafts interact with series-compensated transmission lines, in some cases, severe mechanical shaft failures may occur. SSR occurs in steady-state and transient conditions; the first occurs as self-excitation, and the second occurs during faults and switching operations.

 In modern grids with high penetration of renewable resources and power electronics, SSRs have become more prominent, driven by interactions involving HVDC converters, wind turbine dynamics, and other power electronic interfaces. These phenomena can impose stress on rotating equipment and, in extreme cases, trigger large-scale instability if not detected, identified, and alleviated.

IAOs typically occur in the 0.1–1 Hz range and represent electromechanical swings between coherent groups of generators across weakly interconnected areas. Long-distance power transfers, fast-acting digital controllers of generating units, and insufficient damping and synchronizing torques are the main factors in these oscillations. Properly tuning and coordinating the power system stabilizers (PSSs) installed on generating units to provide sufficient damping torque is a practical action against these low-frequency oscillations. While grid-following (GFL) converters do not inherently supply this function and may even amplify oscillations if not properly coordinated. Poorly damped inter-area oscillatory modes limit transfer capacity and increase the risk of cascading outages or system separation.

Future power systems face low System Inertia (SI). It results from the stored kinetic energy in rotating equipment, such as traditional synchronous machines, that naturally resists frequency excursions immediately following a load-generation imbalance. As rotor-based synchronous generating units are increasingly replaced by non-synchronous inverter-based generations, SI in modern power systems sharply decreases, resulting in faster and deeper frequency nadirs and reduced frequency stability margins. This transition necessitates the deployment of fast frequency response technologies, such as energy storage systems, and advanced grid-forming (GFM) inverter controls, to provide synthetic inertia and mitigate fast Rate of Change of Frequency (RoCoF). Thus, unlike GFL inverters, GFM inverters can emulate the dynamic behaviour of synchronous machines by actively controlling system frequency and voltage, which is increasingly essential to maintain stability under diverse contingencies.

Traditionally, voltage stability has been supported by synchronous machines through their reactive power capability and excitation systems. They provide steady-state and dynamic voltage stability during contingencies. In inverter-dominated grids, however, the ability to provide reactive support is limited by converter design and control. The occurrence of voltage instability, especially under high levels of GFL inverter penetration, raises serious concerns.  

Furthermore, in traditional power systems, angular stability is maintained by large synchronous machines, keeping them able to remain in synchronism after disturbances. In modern power grids, this natural synchronizing mechanism is greatly reduced as GFL inverters have replaced synchronous machines. As such, the power system becomes more vulnerable to loss of synchronism, oscillatory instability, or even large-scale blackouts.

Traditional protection schemes are designed to detect short-circuit fault currents contributed by synchronous machines, and fault detection as well as relay coordination rely heavily on these current levels. With the widespread integration of inverter-based generation, fault current magnitudes are significantly lower, resulting in delayed or failed fault detection. Under such conditions, directional relays, distance protection, and overcurrent relays may malfunction. These issues are further exacerbated in wind and solar farms connected through long transmission lines, where reduced short-circuit strength and high network impedance complicate protection systems. Therefore, traditional protection schemes must be re-evaluated and adapted to address these emerging challenges.

This workshop will elaborate on recent experiences and solutions provided by transmission and distribution system operators and manufacturers such as General Electric and Siemens. The workshop will also refer to the existing grid codes of some system operators and challenges they face in future power systems, as well as lessons learned with the exploitation of large-scale renewables.

 

We are looking forward to meeting you in Delft,

Marjan Popov
Power System Protection Centre