Where virtual inertia and damping come from: understanding VSG through the swing equation
Summary: VSG is not a deliberately sluggish ramp added to an inverter; it is a synchronous generator simulated inside a digital controller. Virtual inertia corresponds to rotor inertia, virtual damping to damper windings, active power-frequency droop to the governor and reactive power-voltage droop to the excitation regulator. Once that mapping is clear, frequency rate-of-change and recovery time under a load step become readable.
VSG is not a deliberately sluggish ramp added to an inverter; it is a synchronous generator simulated inside a digital controller. Virtual inertia corresponds to rotor inertia, virtual damping to damper windings, active power-frequency droop to the governor and reactive power-voltage droop to the excitation regulator. Once that mapping is clear, frequency rate-of-change and recovery time under a load step become readable.
What a synchronous machine does
A conventional generator stores kinetic energy in its rotor. When load rises suddenly the rotor releases energy and slows the frequency drop - that is inertia. The governor and damper windings then adjust output to restore frequency - that is damping. A grid-forming inverter reproduces the rotor swing equation in software and produces an inertial response to load steps or grid frequency changes, instead of being indifferent like a conventional grid-following inverter or even amplifying the disturbance through PLL dynamics.
From FOC to VSG
The technical library uses field-oriented control of permanent-magnet machines as an analogy: d-q transformation, motion equation, torque equation and flux equation describe the same machine running backwards, since dragging it with external torque turns it into a generator. Put that model into an inverter and virtual inertia maps to moment of inertia J, virtual damping to damper windings, P-f droop to the governor and Q-V droop to the excitation regulator. VSG parameters are therefore not arbitrary tuning constants but quantities with explicit physical counterparts.
How to verify it on site
Apply step loads and record the frequency nadir, rate of change of frequency and recovery time, then compare responses at different virtual inertia settings. Confirm that the supported PQ, VF, droop and VSG modes align with the EMS strategy. Note the trade-off: higher inertia slows the frequency excursion but also slows the power response and demands more DC-side energy buffer, so tuning must be coordinated with the battery SOC window and the PCS overload curve of 1.1x continuous, 1.25x for 60 s and 1.5x for 100 ms.
Frequently asked questions
Does virtual inertia create energy from nothing?
No. The inertial response discharges energy stored in the DC-side battery; the control law only shapes the power response to look inertial. Battery power and the SOC window must sustain that interval.
If droop already shares power, why use VSG?
Droop mainly solves steady-state sharing. VSG adds controllable transient inertia and damping, giving a more predictable frequency trajectory under load steps, genset paralleling and weak-grid disturbances.
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SP1350H54SW · SP250HAG2T · STS200 · SP125HCPS · All technical guides · Product catalogue。
FAQ
Does virtual inertia create energy from nothing?
No. The inertial response discharges energy stored in the DC-side battery; the control law only shapes the power response to look inertial. Battery power and the SOC window must sustain that interval.
If droop already shares power, why use VSG?
Droop mainly solves steady-state sharing. VSG adds controllable transient inertia and damping, giving a more predictable frequency trajectory under load steps, genset paralleling and weak-grid disturbances.
