The 33rd China International Exhibition on Electric Power Equipment and Technology
Shanghai International Energy Storage Technology Application Expo / Hydrogen Energy Expo
Power system stability is the property of a power system that enables it to remain in a state of operating equilibrium under normal operating conditions and to regain an acceptable state of equilibrium after being subjected to a disturbance. Stability is classified into three main categories: rotor angle stability (the ability of synchronous generators to maintain synchronism after a disturbance — subdivided into transient stability for large disturbances and small-signal stability for small perturbations); voltage stability (the ability to maintain acceptable voltages throughout the system after a disturbance — voltage collapse is a major cause of large-scale blackouts); and frequency stability (the ability to maintain system frequency within acceptable limits following a large imbalance between generation and load). The increasing penetration of inverter-based renewable energy resources, which lack the inherent inertia of synchronous generators, is creating new stability challenges that require advanced control strategies including synthetic inertia, grid-forming inverters, and fast-acting energy storage.
5 Key Questions About Power System Stability
Transient instability occurs when a large disturbance — typically a fault on a high-voltage transmission line — causes one or more generators to lose synchronism with the rest of the system. During a fault, the electrical power output of nearby generators is reduced while their mechanical input power remains approximately constant, causing the generators to accelerate. If the fault is not cleared quickly enough (within the critical clearing time, typically 80–150 ms), the generator rotors advance too far and cannot recover synchronism after fault clearing. Prevention measures include fast fault clearing by high-speed protection systems, power system stabilisers (PSS) on generators, and controlled islanding to prevent cascading instability.
Voltage collapse is a form of voltage instability in which a power system experiences a progressive, uncontrollable decrease in voltage, typically caused by excessive reactive power demand that exceeds the reactive power supply capability of the system. Voltage collapse can be triggered by heavy loading, loss of reactive power sources (capacitor banks, generators), or contingency events (line or transformer outages). Prevention measures include: maintaining adequate reactive power reserves through capacitor banks, STATCOMs, and generator reactive capability; undervoltage load shedding schemes that automatically disconnect load before voltage collapse; and online voltage stability monitoring using PMU data and real-time security assessment tools.
System inertia — the kinetic energy stored in the rotating masses of synchronous generators and motors — provides the first line of defence against frequency deviations by absorbing or releasing energy instantaneously when generation-load balance is disturbed. As synchronous generators are displaced by inverter-based renewable energy (which has no inherent inertia), system inertia decreases, causing faster and larger frequency deviations following generation-load imbalances. This requires faster-responding frequency regulation services — including battery storage providing synthetic inertia and fast frequency response — and more sensitive under-frequency load shedding schemes to prevent frequency collapse.
Phasor measurement units (PMUs) measure voltage and current phasors (magnitude and angle) at multiple points in the transmission network with GPS-synchronised timestamps, providing real-time wide-area situational awareness of system stability margins. PMU data is used for: real-time monitoring of inter-area oscillations that can lead to instability; post-disturbance analysis to understand the sequence of events during stability incidents; model validation to verify that simulation models accurately represent system behaviour; and advanced control applications including wide-area damping control and adaptive protection systems that adjust settings based on real-time stability margins.
Grid-forming inverters are a new class of power electronic converter that can establish voltage and frequency references — behaving like a voltage source rather than a current source — enabling them to provide grid-forming functions previously only available from synchronous generators. Grid-forming inverters can provide synthetic inertia by modulating their power output in response to frequency deviations, support black start and islanded operation, and damp inter-area oscillations. The deployment of grid-forming inverters in large-scale battery storage systems and renewable generators is a key strategy for maintaining power system stability as synchronous generation is displaced by inverter-based resources.
Key Takeaways
Power system stability is a fundamental requirement for reliable electricity supply, encompassing rotor angle, voltage, and frequency stability. The energy transition — with its displacement of synchronous generators by inverter-based renewables — is creating new stability challenges that require advanced monitoring, control, and storage technologies. EP Shanghai showcases the latest stability analysis tools, PMU systems, STATCOM equipment, and grid-forming inverter technologies from leading suppliers.
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