Fast outer-loop voltage controllers are commonly implemented in energy parks to regulate voltage and reactive power at the point of connection (POC). Operating similarly to plant-level Power Plant Controllers (PPC), these are often utilised in synchronous condenser installations, hybrid generation plants, and renewable energy projects. This paper presents outcomes of a recent Australian project examining coordinated tuning of synchronous condenser excitation systems, fast outer-loop voltage regulators, and conventional Power System Stabilisers (PSS). Driven by grid codes that demand rapid voltage and reactive power response at the POC, the study evaluates the practical implications and stability risks introduced by aggressive control strategies. Specifically, fast outer-loop voltage controllers impose challenging bandwidth requirements on automatic voltage regulators (AVRs), which can compromise small-signal stability and reduce the damping provided by conventionally tuned PSS.

A particular focus is the adverse interaction arising when fast outer-loop voltage regulators introduce biases at the AVR summing junction, typically shared by PSS signals based on rotor speed and active power. This configuration inherently alters PSS behaviour and phase compensation requirements, reducing damping effectiveness or causing unintended oscillations. To quantify these interactions, RMS-based simulations employing Fourier-based sinusoidal injections at the POC were conducted. Frequency-domain assessments illustrated through Bode plots clearly reveal resonances and identify critical frequencies where outer-loop control negatively impacts damping performance. Eigenvalue analyses complement these results, validating the observations.

The study identifies that integrating dead bands within fast outer-loop controllers is essential to minimise unnecessary controller interventions and reduce adverse interactions with inner-loop AVRs and PSS. Additionally, conventional PSS must be carefully retuned or fully redesigned when operating alongside aggressive outer-loop controllers, particularly if assessed against rapid voltage response criteria measured at the POC. Instead, it is recommended that voltage response and settling-time criteria be evaluated directly at machine terminals. Time-domain simulations and eigenvalue analyses demonstrate superior stability and voltage regulation performance from deliberately slower outer-loop controllers.

Practical guidelines for coordinated tuning strategies, strategic use of dead bands, slower outer-loop controllers, and appropriately retuned or redesigned PSS are provided. These recommendations enhance overall system stability, improve damping, and optimise voltage performance in grids dominated by variable renewable energy sources.