With the rising penetration of wind and solar new energy and continuous decommissioning of traditional synchronous units, power systems are entering a low-inertia era. Power conversion systems (PCS) for photovoltaic and energy storage, as inverter-based power electronic devices, have no rotating rotors and possess no natural rotational inertia. Once power grid disturbances occur, the Rate of Change of Frequency (RoCoF) will surge sharply, easily triggering cascading faults and large-scale off-grid events.
One of the core technologies of Grid-Forming (GFM) PCS is the Synthetic Synchronous Generator (VSG) algorithm. It solves the rotor motion equation of synchronous generators in software and constructs Synthetic rotational inertia via code, enabling static power electronic equipment to deliver inertia support similar to conventional steam or hydroelectric generating units. This is also the key for stable operation of energy storage in weak grid scenarios.
I. Fundamental Principle of VSG Synthetic inertia
Conventional synchronous generators rely on the kinetic energy stored in massive rotors. When grid frequency drops, rotors release kinetic energy to hinder rapid frequency variation; when frequency rises, they absorb excess power to curb frequency spikes, essentially forming a physical flywheel effect.
VSG does not depend on mechanical components. Instead, it solves the rotor motion equation inside the PCS controller:
J: Synthetic moment of inertia (configured by software parameters)
Pₘ: Synthetic mechanical power reference
Pₑ: Actual electromagnetic power output of the PCS
D: Synthetic damping coefficient
ω: Internal Synthetic angular velocity of the PCS
ωg: Measured grid angular velocity
When power shortage occurs in the power grid and grid angular velocity ωg decreases, an unbalanced power term appears in the equation. The controller immediately draws electric energy from the energy storage battery and outputs extra inertia power to limit the Rate of Change of Frequency (RoCoF) and slow down frequency drop. Conversely, in case of power surplus and frequency rise, the PCS absorbs excess power to suppress rapid frequency escalation.
II. Analysis of Typical VSG Synthetic inertia Code (MATLAB Example)
Below is a typical implementation code for Synthetic inertia control:
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matlab |
Define the Synthetic inertia time constant Tᵥᵢᵣₜ , typically ranging from 2~8 s, representing the strength of inertia support;
Calculate the base angular frequency ωbₐₛₑ from the rated frequency, then derive the Synthetic moment of inertia Jᵥᵢᵣₜ ;
Sample grid frequency at two consecutive timestamps and compute the angular frequency change rate dω/dt (corresponding to RoCoF);
Calculate the additional inertia power Pᵢₙₑᵣₜᵢₐ using Jᵥᵢᵣₜ and angular frequency variation rate;
Superpose the inertia power on the original power reference Pᵣₑf to obtain the final PCS output power command Pᵣₑfₜₒₜₐₗ .
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Key point: The response speed of inertia power is determined by the switching frequency of PCS hardware. Traditional silicon IGBT-based PCS only achieves a switching frequency of 3~5 kHz, with control latency up to hundreds of microseconds and obvious phase lag in Synthetic inertia. In contrast, GFM PCS adopting SiC modules boosts the switching frequency to the 100 kHz level, compressing control latency to around a dozen microseconds. Its inertia response better matches real synchronous generators, and stability in weak grids is greatly improved. |
III. Essential Differences Between VSG Synthetic inertia and Grid-Following (GFL)
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Type |
Core Characteristics |
Inertia Capability |
Applicable Scenarios |
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Grid-Following (GFL) PCS |
Controlled current source, relies on PLL to track grid voltage phase |
No native inertia; only limited auxiliary Synthetic inertia, prone to loss of phase lock under faults |
Strong power grid, high Short-Circuit Ratio (SCR) >3 |
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Grid-Forming VSG PCS |
Controlled voltage source, autonomously establishes voltage/frequency reference |
Native Synthetic inertia + damping, independently supports frequency, capable of black start |
Weak grid SCR<3, high-renewable-energy bases, islanded microgrids |
The premise for grid-following converters is that the grid itself acts as a rigid voltage source. In weak grids with large line impedance, voltage at the Point of Common Coupling (PCC) drifts easily, the phase-locked loop tends to lose stability, and the effect of additional Synthetic inertia is greatly weakened.
GFM VSG PCS no longer depends on an external rigid grid and serves as a voltage anchor itself. Even when the grid Short-Circuit Ratio (SCR) drops to 1.5 or lower, it can stably output Synthetic inertia and suppress system oscillation.
IV. Design Considerations for Engineering Implementation
Tuning of Synthetic inertia Parameters
Tᵥᵢᵣₜ is not better when larger. Excessively large Jᵥᵢᵣₜ introduces low-frequency oscillation; an overly small value provides insufficient inertia support and fails to restrain RoCoF. In engineering practice, adaptive variable inertia is commonly adopted: increase J during rapid frequency fluctuations and reduce J under steady-state conditions to balance stability and dynamic response.
Energy Storage Capacity Constraints
All energy for Synthetic inertia comes from the energy storage battery. Short-term inertia support triggers charging and discharging. During design, it is necessary to verify battery power and State of Charge (SOC) range to prevent out-of-limit protection actions caused by frequent inertia response.
Hardware Bandwidth Bottleneck
No matter how well the VSG Synthetic inertia algorithm is designed, it is limited by the switching speed of power devices. Control delay introduced by low switching frequency weakens inertia performance and may even induce subsynchronous oscillation. High-bandwidth SiC power modules form the hardware foundation for high-performance GFM PCS.
V. Summary of Industrial Value
Grid codes worldwide (ENTSO-E NC RfG 2.0, UK GC0137, China's Code for Design of Electrochemical Energy Storage Station) have gradually upgraded grid-forming capability from an optional function to a mandatory grid interconnection threshold.
VSG Synthetic inertia essentially replicates the rotor inertia of synchronous generators on energy storage PCS via software algorithms and high-speed power electronic hardware. In the new-type power system with large-scale wind and solar grid integration, Synthetic inertia of grid-forming PCS is becoming the core technology to maintain grid frequency stability and guarantee grid-connection safety of new energy bases.
