A Voltage Source Converter (VSC) is a solid-state power electronics device that converts AC to DC, or DC to AC, using a stiff DC-link capacitor and high-frequency IGBT or SiC MOSFET switching. But if you are searching what is voltage source converter to actually size one for a grid-tied, microgrid, or motor-drive application, you are really looking for a unit conversion: translating your Real Power (kW) into the Apparent Power (kVA) and Ampere ratings the hardware must handle. For a benchmark 100 kW load at 480V 3-phase with a 0.9 power factor, the direct conversion yields a 111.1 kVA VSC requirement, drawing 133.6 Amps per phase. You need a 150A-rated VSC drive, such as the Siemens SINAMICS G120 or ABB ACS580.
What Is a Voltage Source Converter (VSC)?
At the bench level, a VSC is defined by its DC-link topology. Unlike a Current Source Converter (CSC) which relies on a massive series inductor to maintain stiff DC current, a VSC uses a parallel capacitor bank to maintain a stiff DC voltage. This allows the VSC to independently control active power (kW) and reactive power (kVAR), making it the default choice for modern HVDC transmission (like Hitachi Energy's HVDC Light), solar farm inverters, and variable frequency drives (VFDs). When you buy a 'VSC', you are buying a bidirectional or unidirectional bridge of semiconductor switches that chop DC into a simulated AC sine wave via Pulse Width Modulation (PWM).
The Power Conversion Formula: kW to VSC Sizing
To select the right VSC hardware, you must convert the mechanical or thermal load (kW) into the electrical stress the converter's IGBTs and busbars will endure (kVA and Amps). This conversion is fixed by three assumptions: nominal line-to-line voltage, power factor (PF), and phase topology.
1. Apparent Power:
kVA = kW / PF2. Line Current (3-Phase):
I = (kVA × 1000) / (√3 × V_LL)
Substituting our benchmark values (100 kW, 480V, 0.9 PF):
- kVA = 100 / 0.9 = 111.11 kVA
- Current = 111,111 / (1.732 × 480) = 111,111 / 831.36 = 133.65 Amps
Because VSCs must handle transient overloads and thermal derating in enclosed panels, you never size to the exact decimal. You round up to the next standard hardware frame size, which is 150 Amps.
Neighboring Load Values (±20% Sizing Table)
Loads rarely sit perfectly at 100 kW. Here is the conversion table for a ±20% range around our benchmark, assuming a constant 480V 3-phase supply and 0.9 PF. This table dictates the physical frame size of the VSC you must order.
| Real Power (kW) | Apparent Power (kVA) | Line Current (A) @ 480V | Required VSC Frame Rating |
|---|---|---|---|
| 80 kW | 88.9 kVA | 106.9 A | 110A - 125A Frame |
| 90 kW | 100.0 kVA | 120.3 A | 125A - 150A Frame |
| 100 kW | 111.1 kVA | 133.6 A | 150A Frame |
| 110 kW | 122.2 kVA | 147.0 A | 150A Frame (Maxed) |
| 120 kW | 133.3 kVA | 160.3 A | 175A - 200A Frame |
Voltage and Phase Shifts: 120V, 230V, and 3-Phase
The 133.6A answer above is strictly for 480V 3-phase. If your site voltage changes, the current shifts inversely. Presenting a single-voltage answer as universal is a critical engineering error.
- 208V/230V 3-Phase: At 208V, that same 100 kW load (111.1 kVA) pulls 308 Amps. You must jump to a much larger, physically bulkier VSC frame (e.g., 350A) and use parallel 500 kcmil THHN conductors.
- 230V Single-Phase: Single-phase VSCs are rare above 5 kW due to DC-link ripple. If forced to convert 10 kW at 230V 1-phase (assuming 0.9 PF = 11.1 kVA), the current is 48.3 Amps. The VSC must be rated for heavy 120Hz ripple current on the DC capacitors.
- 120V Single-Phase: Reserved for fractional horsepower or lab equipment. A 1 kW load at 120V (0.9 PF) draws 9.2 Amps. Standard plug-in VFDs or micro-inverters handle this.
VSC Selection Decision Tree
Use this decision matrix to terminate your sizing process with a concrete hardware pick. This aligns with interconnection standards like IEEE 1547 for distributed energy resources.
| Application Condition | Required VSC Topology | Concrete Hardware Pick |
|---|---|---|
| Motor Drive (Uni-directional, 480V) | Standard 6-pulse Diode Rectifier + IGBT Inverter | Siemens SINAMICS G120 (150A frame) |
| Grid-Tied Solar / BESS (Bi-directional) | Active Front End (AFE) IGBT Rectifier + Inverter | ABB ACS880 Regenerative Drive |
| High-Frequency / High-Temp (e.g., EV Charger) | SiC MOSFET 3-Level NPC Topology | Wolfspeed / Infineon SiC Modules (Custom build) |
FAQ: When VSC Conversion Fails
When is the kW-to-kVA conversion meaningless?
The conversion becomes meaningless if the Power Factor (PF) is unknown or if the load is highly non-linear with massive Total Harmonic Distortion (THD) exceeding 40%. In environments with heavy arc furnaces or unmitigated 6-pulse rectifiers, the apparent power (kVA) can be double the real power (kW). If you size a VSC using a assumed 0.9 PF on a 0.5 PF distorted load, the IGBTs will overcurrent and trip instantly. You must measure the load with a Fluke 435 Power Quality Analyzer first.
Does the VSC rating change if I use SiC MOSFETs instead of IGBTs?
The electrical conversion (111.1 kVA) remains identical because the physics of the load haven't changed. However, the thermal sizing changes. Silicon Carbide (SiC) MOSFETs switch at 20kHz+ with drastically lower switching losses compared to IGBTs (typically 4kHz). A SiC-based VSC can often be physically one frame size smaller due to reduced heatsink requirements, though the semiconductor cost is roughly 30-40% higher.
What happens if my grid voltage sags by 10%?
If your 480V grid sags to 432V, the VSC must draw more current to maintain the same 100 kW output (Current = 111,111 / (1.732 × 432) = 148.4A). If your VSC is sized exactly to 133A, a voltage sag will trigger an overcurrent fault. Always apply a 1.15x service factor to your final Ampere calculation if the local grid is prone to brownouts.
