MV STATCOM sizing starts with the reactive-power and voltage-performance requirement at a defined point of connection, not with the present power-factor penalty or the largest kvar value in one meter screenshot. A medium-voltage STATCOM must meet steady-state Mvar range, dynamic voltage recovery, low-voltage current, overload, redundancy and environmental requirements across credible operating cases. The coupling transformer, cables, filters and plant controller are part of that rating.
Quick answer: build a voltage-versus-Mvar envelope from load-flow and dynamic studies, then convert the point-of-connection duty to converter-terminal current across minimum and maximum voltage. Add transformer and cable reactive losses, define short-time overload and N-1 capacity, and verify the design in RMS and electromagnetic-transient models. Nameplate Mvar alone cannot prove that a STATCOM will meet the grid requirement.
| Input | Why it matters | Common mistake |
|---|---|---|
| Point of connection and voltage | Defines the measured target and current conversion | Sizing at the converter bus instead of the PCC |
| Required capacitive/inductive Mvar | Sets continuous operating envelope | Assuming a symmetrical range |
| Minimum and maximum grid strength | Affects voltage sensitivity and control stability | Using only the normal fault level |
| Voltage and active-power cases | Changes current, losses and other inverter capability | Checking only nominal voltage |
| Fault ride-through duty | Defines dynamic current priority and overload | Quoting response time without a disturbance |
| Ambient and altitude | Determines semiconductor and cooling derating | Applying indoor sea-level ratings outdoors |
| N-1 requirement | Sets installed modules and usable capacity | Confusing installed Mvar with available Mvar |
The project specification should identify the bus where voltage, power factor or reactive power is controlled. A solar plant may have a 35 kV collector bus and a 220 kV grid connection. The STATCOM injects current at medium voltage, but the grid code is evaluated at the high-voltage PCC. Transformer magnetizing current, leakage reactance, cable charging and renewable-inverter kvar all affect the relationship.
State whether the target is fixed Mvar, fixed power factor, fixed voltage or voltage droop. These modes produce different duties. A voltage controller may use little current during normal operation and reach its limit after a contingency. A fixed power-factor target follows MW dispatch. If several devices regulate the same bus, their droops and deadbands must be coordinated to avoid hunting.
Run load-flow cases for maximum and minimum active power, high and low grid voltage, normal and outage topology, daytime and nighttime cable charging, transformer tap positions and other reactive assets available or unavailable. For renewable plants, include the inverter P-Q capability at each active-power level and terminal voltage. For industrial sites, include production, idle and starting combinations.
The result should be a required envelope rather than a single ±Mvar number. If the plant needs +18 Mvar capacitive at 0.9 pu and −10 Mvar inductive at 1.1 pu, a nominal ±15 Mvar statement is already ambiguous. Convert each point to current and compare it with the converter and transformer capability at that voltage.
For a balanced three-phase system, reactive current is approximately:
I = |Q| / (√3 × VLL)
A 10 Mvar duty at 10 kV requires about 577 A. The same 10 Mvar at 9 kV requires about 642 A. If the converter is current-limited, available Mvar falls with voltage. This is why a STATCOM may be described as maintaining current during a voltage depression rather than maintaining full nameplate Mvar. Check the exact current limit, modulation range and duration.
At the coupling transformer, include leakage voltage drop and the reactive power consumed by leakage reactance. At high current, those losses can materially increase converter-terminal duty. Cable and reactor losses also change the PCC value. The supplier and system consultant should agree on whether guaranteed Mvar is gross converter output or net output at the contractual boundary.
Dynamic sizing follows the required event. Examples include recovery after a transmission fault, motor starting, electric-arc-furnace flicker, wind-farm voltage steps or loss of a parallel line. Specify pre-event voltage and output, disturbance depth and duration, required reactive-current rise, maximum overshoot, voltage-recovery curve and post-event settling.
A STATCOM may have short-time overload above its continuous current, but semiconductor junction temperature and cooling history limit the duration and repetition. Ten seconds at 1.2 pu once per hour is different from one second at 1.5 pu every minute. Provide the event frequency and worst initial temperature. The thermal model should show that the complete duty cycle remains within limits.
Short-circuit ratio is useful for screening, but it does not describe frequency-dependent impedance, nearby inverter controls or every contingency. Use RMS dynamic studies for plant-level voltage control and electromechanical behavior. Use electromagnetic-transient studies for low SCR, converter interaction, unbalanced faults, current limiting and grid-forming functions.
The 2026 ENTSO-E stability recommendations emphasize the need to address stability as grids become more converter dominated. In April 2026, India’s government also highlighted STATCOM deployment for grid stability. These system-level trends make validated converter models and clear current-limit behavior as important as the nameplate rating.
Assume a project requires 20 Mvar after any one power module is unavailable. If the design uses five 5 Mvar modules, installed capacity is 25 Mvar and usable N-1 capacity is 20 Mvar. However, a common transformer, cooling loop or controller can remove all modules. Redundancy must cover the failure modes named in the availability requirement.
The coupling transformer must carry continuous and overload current across the required taps and ambient conditions. Its impedance influences converter voltage demand, fault current and harmonic attenuation. Winding arrangement and grounding affect zero-sequence behavior. Specify insulation, cooling, sound level and losses on the same duty cycle as the STATCOM.
Multilevel converters can achieve low harmonic emission, but the project still needs guaranteed individual harmonics and THDv at the defined point. Include background distortion and resonance with cables, capacitor banks and filters. Protection must account for converter-limited fault current, DC-link faults, transformer differential zones, phase imbalance and safe isolation. Relay studies should use the delivered converter behavior rather than a synchronous-machine approximation.
A 100 MW solar plant connects through a 35/220 kV transformer. Load-flow cases show that the PCC needs +22 Mvar at 0.92 pu voltage during maximum export and −14 Mvar at 1.08 pu during light export. Reactive losses from the 35 kV STATCOM bus to the PCC reach 2.5 Mvar in the capacitive case and 1.2 Mvar in the inductive case. The converter-terminal steady-state requirement is therefore at least +24.5/−15.2 Mvar before margin.
Dynamic studies require 1.25 pu capacitive current for two seconds after a fault, and the owner requires 22 Mvar capacitive capability with one module unavailable. A five-module arrangement of 6 Mvar each provides 30 Mvar installed and 24 Mvar after one module loss at nominal voltage. The final design must verify that the N-1 capacitive current still meets the low-voltage PCC target after transformer drop and that the two-second overload is thermally available from the worst pre-fault output.
This example does not prove that 5 × 6 Mvar is sufficient. EMT studies may require a different current ceiling, and altitude or hot ambient conditions may derate each module. It illustrates the correct sequence: PCC requirement, losses, current conversion, dynamic duty, redundancy and environmental verification.
Footprint drawings should show cable bending, module removal path, cooling clearances, fire separation and maintenance access. A compact converter that cannot be serviced without removing adjacent equipment does not provide practical modularity.
Often, but not always. Transformer, converter voltage and project topology can produce different limits. Require P-Q or V-Q capability curves for both directions across the operating voltage range.
It can often maintain high reactive current until it reaches its current and modulation limits. Because Q is proportional to voltage times current, available Mvar normally decreases as voltage falls. Specify current and recovery performance.
There is no universal percentage. Margin should cover model uncertainty, expansion, voltage and environmental derating, measurement error and the required degraded state. Excessive margin increases losses and cost, so document each allowance.
Elumotive’s 3–35 kV MV SVG/STATCOM platform supports modular industrial and renewable applications. For an MV STATCOM sizing review, send the load-flow envelope, grid equivalents, dynamic requirements and environmental data. The result should be a traceable rating tied to the point-of-connection performance that the project must deliver.
