Sub-Synchronous Control Interaction in a Utility-Scale Solar PV Plant: First evidence and countermeasures in Vietnam’s Power Grid

In parallel, parts of the transmission system employ series capacitor compensation, creating conditions where converter-driven subsynchronous phenomena can emerge.

This paper documents and analyses Vietnam’s first confirmed Sub-Synchronous Control

Interaction (SSCI) event in a utility-scale full-converter solar PV plant. The event occurred at a 600 MW plant connected to the 500 kV system only through a series-compensated corridor during a planned N-1 condition. During low-output condition, energization of the remaining line directly from the plant (with a shunt reactor at the end of line) initiates a high-magnitude resonance oscillation around 33 Hz. The oscillation was sustained for approximately 10 seconds and was finally cleared by the simultaneous, undesired tripping of a GSU transformer

(differential protection – F87T operating due to heavily distorted currents) and the tripping of the energized line (line protection operating for a line-end fault - surge arrester failure because of high-magnitude distorted voltage). The phenomenon was not visible in the reporting rate of 50 Hz (Wide Area Monitoring System – WAMs) or 0.5 Hz (SCADA system), but was captured in detail by 16 kHz fault recorder waveforms. The paper presents guidance on a comprehensive workflow—incorporating both frequency-domain analysis (simplified and impedance-based methods) and EMT time-domain validation—consistently identifies a poorly damped mode at 33–34 Hz within the series-compensated configuration. This indicates a significant risk of SubSynchronous Control Interaction (SSCI) during switching operations involving adjacent elements, particularly large shunt reactors which could potentially introduce ferroresonance.

Drawing on research findings and historical weak-grid events, the Power System Operator of

Vietnam has established operating procedures for renewable energy (RE) plants interfaced with 500 kV series-compensated grids. These measures address the current lack of effective SSCIsuppression solutions and the practical limitations of reconfiguring control settings within existing RE units. To mitigate oscillation risks, the operational framework prioritizes avoiding switching maneuvers under weak-grid conditions and requires that line energization be initiated from the system side rather than the plant side. In cases where switching is unavoidable, the RE plant is temporarily shut down to ensure stability. While these protocols have successfully prevented SSCI recurrence in the Vietnamese power system to date, the ongoing transition toward Net Zero by 2050 introduces heightened risks of SSCI and ferroresonance. This evolving landscape necessitates the advancement of simulation, monitoring, control and protection systems to ensure the early detection and prevention of SSCI or other instability phenomena relating to renewable energy.