Grid-Forming STATCOM Sizing (Supercapacitors)

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A STATCOM on its own has no energy store, so it can only exchange reactive power. To act as a grid-forming unit (voltage source, synthetic inertia, damping of power oscillations, short frequency support in an island) it has to deliver and absorb active power for seconds. That energy comes from supercapacitors on its DC side. This study answers two questions: how much active power and energy the grid-forming duties need, and which converter rating and supercapacitor bank deliver them at end of life.

Where to find it

Simulate → Grid-Forming STATCOM Sizing (Supercapacitors). Requires a subscription. The diagram needs a STATCOM (SSC) and a Load Flow that converges.

When to use

  • Wind or PV plants whose connection conditions ask for grid-forming behaviour from the STATCOM
  • Checking whether a STATCOM rating leaves room for active power on top of its reactive power
  • A first size of the supercapacitor bank (cells in series, strings in parallel) before talking to suppliers

How to run

  1. Model the plant with its STATCOM, and confirm that Load Flow converges with the controllers you use.
  2. Click Simulate → Grid-Forming STATCOM Sizing (Supercapacitors).
  3. Pick the STATCOM to size and the connection point bus. Check the STATCOM rating and the plant rated power.
  4. Choose a requirement set, or Custom, and check every duty value against your connection conditions.
  5. Enter the data of the supercapacitor cell you plan to use, and click Calculate.

Check the duty values. Connection conditions usually leave the numbers (inertia constant, rate of change of frequency, phase jump, damping power) to be agreed with the grid operator. The values a requirement set fills in are starting assumptions, not requirements. Replace them with the agreed values.

Requirement sets

A requirement set switches the duties on or off. Only duties that need active power call for supercapacitors.

Set What it switches on
PSE (Poland): synthetic inertia, oscillation damping, grid-forming STATCOM The grid-forming duties of the newer connection conditions (items 15.3 and 15.4).
PSE (Poland): island operation with FSM, LFSM-O, LFSM-U No duty. The older connection conditions ask for no grid forming, so the result is that no supercapacitors are needed. The frequency modes are a matter of plant control.
Custom Keeps the values in the dialog.

Duties

The frequency event (rate of change of frequency, lowest and highest frequency) is the disturbance the STATCOM has to ride through, once falling (the bank discharges) and once rising (the bank charges). Each duty can be switched off.

Duty Inputs Active power it needs
1. Synthetic inertia Inertia constant H [s], given on the plant rated power or on the STATCOM rating P = 2·H·S·(f/f0)·(df/dt)/f0, against the change of frequency
2. Voltage source behaviour (phase jump) Phase jump [degrees], decay time constant [s] P = V²·sin(Δθ) / (Xcoupling + Xgrid), decaying. The voltage and both reactances come from the model.
3. Power oscillation damping Damping power amplitude [% of STATCOM rating], oscillation frequency [Hz] A sine wave of that amplitude and frequency
4. Frequency support in island operation Droop [%], deadband [mHz], largest power [% of plant rated power], bridging time [s], and when island mode starts The droop response, held for the bridging time until the turbines have taken over

Converter and supercapacitor bank

  • Reactive power kept free: the converter must carry the active power of the duties on top of reactive power for voltage control. Choose what the STATCOM carries in the load flow of the model, a share of its rating, or its full rating.
  • Short-time current capability of the converter [p.u.]: how far above its rating the converter may go during the event.
  • Supercapacitor cell: capacitance, rated voltage, series resistance and peak current. The defaults are a common 3000 F cell; use the data sheet of your cell.
  • String: highest voltage of a string, and the lowest working voltage in percent of the highest.
  • End of life: capacitance and resistance in percent of new. The bank is sized on these values.
  • Converter efficiency (one way) and a design margin on the bank.

Results

  • Summary tiles: STATCOM rating, converter rating needed, peak active power, energy to deliver, inertia provided, and grid short-circuit power.
  • Duties: peak power and energy of each duty, and of all duties together.
  • Event charts: the bank while the frequency falls (discharge) and rises (charge).
  • Supercapacitor bank: cells in series per string, strings in parallel, capacitance and resistance new and at end of life, usable energy, resting voltage, lowest and highest voltage in the events, peak current per cell and of the bank, and peak loss in the cells.
  • Read from the model: reactive power in the load flow, coupling reactance, grid short-circuit power at the STATCOM bus and at the connection point, and the short-circuit ratios of the STATCOM and of the plant.
  • Checklist: each item of the chosen requirement set, with what the study covers and what is a matter of plant control.
How it works
  • The STATCOM rating, its coupling reactance and the plant rated power are read from the diagram.
  • The reactive power of the STATCOM comes from a load flow with the Park Controller, Plant Supervisory Controller, reactor steps and tap changers active.
  • The grid short-circuit power at the STATCOM bus and at the connection point is calculated with the turbines out of service, so it describes the grid alone.
  • The bank is sized by simulating the falling-frequency and the rising-frequency event with end-of-life capacitance and resistance. The result is the smallest number of parallel strings that keeps the bank voltage and the cell current inside their limits.

Limits of the study

  • It is a sizing study with prescribed power profiles, not a dynamic simulation of the converter control. It does not prove stability or tune the control.
  • Thermal design of the bank, cell balancing and the DC/DC stage are outside its scope.
  • The result depends on the duty values. Agree them with the grid operator before using the sizes.

Last updated: November 2026

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