# Grid-Forming & Weak-Grid Customization grid-forming VSG control and weak-grid adaptive microgrid engineering for weak grids at long feeder ends, mining areas and regions with high renewable penetration. **Fig. 1｜**A VSG grid-forming architecture: storage establishes bus voltage and frequency, while diesel, hydro and PV feed in as followers or supplements, holding up supply even where the upstream grid is very weak. Grid-forming customization · becoming the voltage source yourself Grid-forming customization: building frequency and voltage at the end of a weak feeder Long radial overhead feeders, remote mountainous grids and islanded sites share the same signature: a **low short-circuit ratio, large voltage swings and trips as soon as renewable penetration rises**. Conventional grid-following converters rely on a phase-locked loop and can actually aggravate oscillation there. A grid-forming converter behaves as a voltage source, using VSG virtual-synchronous-machine control to create frequency and voltage and inject virtual inertia — so the battery becomes the machine that holds the grid up. **SCR ≥ 1.5**Operating point our grid-forming designs remain stable at **below 20 ms**Transfer and fast island detection **Inertial response**VSG virtual inertia damps frequency steps **Ride-through**Customizable low-voltage ride-through curves 1. First question: how weak is your grid? Three indicators decide whether grid-forming is mandatory: short-circuit ratio, voltage deviation and any history of oscillation. Two of three means rethink. Stiff grid Fault level far exceeds the plant rating. SCR above 10; voltage barely responds to output Standard grid-following converters behave well Weak grid Long, thin conductors; low terminal voltage. SCR around 2–3; output changes visibly move voltage Parallel converters can fall into control-coupled oscillation Very weak or islanded Storage must create the reference itself. SCR below 2, or no upstream support at all Grid-forming control with its own primary frequency response 2. Following versus forming In one line: a grid-following unit is a current source that needs somebody else's voltage; a grid-forming unit is a voltage source that provides it. Control philosophy comparison AspectGrid-followingGrid-forming Role on the gridTracks grid phase through a PLLBecomes the reference by creating voltage and frequency Weak-grid behaviourLoses stability as SCR fallsRemains stable at very low SCR Inertia and droopNeeds added frequency supportInherent virtual inertia and droop Black startNot supportedCan energise load and build the system ParallelingSynchronisation depends on the gridDroop shares the load between units 3. Six engineering essentials 01 Assess SCR and strength first Divide fault level at the point of connection by total converter rating. Everything downstream — necessity, overload requirement, control tuning — depends on this number. 02 Tune inertia and droop together The inertia constant sets the rate of change of frequency, the droop coefficient the steady-state offset. Both must agree with the genset governor, or the two plants will fight over load. 03 Reactive support and impedance shaping Weak lines often have a low X/R ratio, coupling active power into the voltage. Virtual impedance and reactive control decouple them and suppress voltage rise at the far end. 04 Genset coordination Storage takes the instant-to-instant swings while the genset carries average power in its efficient band, avoiding low-load carbon build-up. 05 Paralleling and circulating current Multiple forming units need harmonised droop and line-impedance matching to suppress zero-sequence circulating current and power oscillation. 06 Islanding and seamless transfer An STS moves between connected and islanded states while grid-forming storage instantly takes over the voltage, so critical load never sees the disturbance. 4. What we calculate SCR = Sfault level at POI ÷ Ptotal converter rating; _SCR below 3 is weak, below 2 calls for grid-forming_ ΔU% ≈ (P·R + Q·X) ÷ Urated2 × 100%; _with a low X/R ratio, active power alone lifts the terminal voltage_ Virtual inertia target: deliver power support within tens of milliseconds to limit RoCoF 5. Applications Standard approaches by scenario ScenarioMain symptomRecommended approach Rural feeder endLong line, low voltage, unbalanceGrid-forming storage with reactive support, isolation transformer where needed Island or remote clusterNo or negligible main gridStorage forms the bus; genset supplies energy Mine captive networkLarge induction-motor starting surgesGrid-forming with overload margin plus soft starting High renewable shareVoltage excursions from variable outputGrid-forming regulation and active-power smoothing **Fig. 2｜**Terminal voltage regulation: a storage cabinet paralleled on the transformer low-voltage side uses grid-forming control and reactive output to pull voltage back into compliance, while providing short-term backup. 6. Recommended platforms [SP250HAG2T Hybrid inverter with isolation transformer Isolation Built-in, improves earthing and immunity Transfer Integrated switching unit Paralleling Multiple units supported Enclosure IP54 View details →](/en/product/sp250hag2t) [SP125HCPS Microgrid storage converter Rated power 125 kVA Control Grid-forming / grid-following modes Transfer below 10 ms Peak efficiency 98.5% View details →](/en/product/sp125hcps) [SP60 / 50 / 40 / 30HCPS Compact microgrid converters Transfer Built-in fast switching Paralleling Multiple units supported Genset hybrid Supported Use case Feeder ends, islands, farms, villas View details →](/en/product/sp60-50-40-30hcps) [STS200 / 400 / 600 Static transfer switch Rated power 200 / 400 / 600 kW Transfer time below 20 ms Frequency 50/60 Hz, ±5 Hz Purpose Island transfer and load grading View details →](/en/product/sts200-400-600) 7. Delivery sequence Grid strength measurement Fault level, line parameters and historical voltage curves; compute SCR. Stability study Model lines, transformers, loads and machines; simulate connected, islanded and faulted cases. Control tuning Set inertia constant, droop, virtual impedance and protection limits. Bench or site testing Full load, step response and island ride-through with recorded waveforms. Start-up accompaniment On site through the first week; refine against real grid behaviour. 8. Frequently asked How do I know if I need grid-forming? Compute SCR. Below 3, conventional grid-following units carry real risk; below 2, or with several converters in parallel, go straight to grid-forming. Can grid-forming cause problems of its own? Only if parameters are mismatched to the network. We run time-domain studies first, deliver a tuning sheet, and verify with measured waveforms at commissioning to avoid multi-unit oscillation. How does it live with an existing genset? Storage handles instantaneous swings and frequency; the genset covers energy and holds its loading band. Coordinated droop means neither fights the other. Do we need an isolation transformer? It depends on earthing practice and DC-content requirements. Models with a built-in isolation transformer avoid a separate unit and improve immunity and fault segregation. Give us your point of connection and we will assess stability With fault level, line length and load composition we return the SCR calculation, time-domain conclusions and a platform recommendation. [Request a weak-grid proposal](/en/contact)