sponsoredPOWER-OVER-FIBER IN HIGH VOLTAGE ENGINEERING A fundamental shift for galvanic isolation and auxiliary power supplies in critical infrastructure

Power engineering is undergoing a fundamental transformation driven by the integration of renewable energies and the need for efficient long‑distance power transmission. This article shows how Power‑over‑Fiber (PoF) provides true galvanic isolation and reliable auxiliary power for SSTs and HVDC breakers.

Key technologies such as High Voltage Direct Current (HVDC) transmission and Solid State Transformers (SST) form the backbone of this. Controlling and protecting systems has become extremely difficult with the transition from line-commutated thyristor-based converters (LCC) to self-commutated Voltage Source Converters (VSC)—and specifically to Modular Multilevel Converters (MMC).

In parallel, Wide-Bandgap (WBG) semiconductors, Silicon Carbide (SiC) and Gallium Nitride (GaN), enable far higher switching frequencies than conventional silicon IGBTs, allowing blocking voltages in the range of 10 kV to 15 kV. Traditional isolation is pushed to its limits when faced with the extreme voltage slew rates associated with WBG semiconductors (>100 kV/µs).

In this environment, conventional inductive transformers or capacitive coupling face limitations regarding insulation, electromagnetic interference (EMI) immunity and size.

Power-over-Fiber (PoF) technology—transmitting electrical energy via laser light through optical fibers—offers a unique solution. By completely decoupling the energy path from electrical conductors, PoF enables galvanic isolation defined solely by the dielectric properties of glass, effectively eliminating parasitic couplings.

PoF can be used in several different high voltage systems: gate driver supplies in MV/HV SiC converters, auxiliary power in hybrid HVDC circuit breakers and active sensors at high voltages.

Here we look at two key applications: Solid State Transformers and HVDC Circuit Breakers.

Principle of power of fiber

Power-over-Fiber (PoF) systems operate on the core principle of converting electrical energy into infrared light, transmitting this light through an optical fiber, and then converting it back into

electrical energy at the destination. A standard PoF system comprises of three key elements: the laser, the fiber and the optical power converter.

Application 1: solid state transformers (SST)

Solid State Transformers (SSTs) incorporate power electronics to perform the functions of a traditional transformer, allowing for voltage conversion, regulation, and direct current (DC) connectivity. In Medium Voltage (MV) SSTs (e.g., 13.8kV to 35 kV), auxiliary power supplies present a significant challenge.

Power density

One of the key primary functions of SSTs is to increase power density and reduce volume compared to 50/60 Hz transformers by using Medium Frequency Transformers (MFT). Conventional 50 Hz isolation transformers face limitations in terms of volume and insulation when used to power high-voltage switches (e.g., 10 kV SiC MOSFETs).

PoF to bridge the isolation gap

PoF decouples the volume of the auxiliary supply from the isolation voltage. A fiber isolating 10 kV is physically almost identical to one isolating 100 kV—only the length changes.

Compact: Replacing bulky isolation transformers with thin optical fibers significantly increases the power density of Power Electronics Building Blocks (PEBB) in SSTs.

Input-Series Output-Parallel (ISOP) Architectures: Many MV SSTs use cascaded modules on the input side. The top module sits at full line potential against earth and daisy-chaining electrical auxiliary power is complex and risky. PoF systems, where a central laser unit supplies all modules individually, simplify the isolation topology immensely.

Immunity at high switching frequencies

SSTs often operate at high switching frequencies (>20 kHz) to minimize magnetic components. The resulting high-frequency common-mode noise can disrupt conventional inductive supplies or couple into control circuits. PoF, with its minimal coupling capacitance (<1-2 pF ensures signal integrity in high-frequency SST designs.

Application 2: HVDC circuit breakers (DC breaker)

Either full semiconductor based or Hybrid HV DC breakers will be needed to achieve meshed DC grids. Hybrid DC breakers combine mechanical switches with power circuits (IGBTs/BIGTs/SiC) to interrupt fault currents within milliseconds.

The "cold" breaker problem

An HVDC breaker is primarily in a closed, low-loss state. When a fault occurs, it must react instantly. The main branch uses hundreds of series-connected semiconductors to block high voltage (e.g., 320 kV) when "Off." The gate drivers for these devices must be powered at floating potential, which means galvanically isolated from ground level. Energy harvesting from the load

current fails when the current is low or zero and is anyhow difficult in DC applications. Harvesting from voltage across the breaker only works when the breaker is open.

PoF for guaranteed availability

PoF offers a predictable power supply independent of the state of the grid.

Gate drivers can be "Ready": using PoF, even on a disconnected line. This is critical for maintenance cycles and shutdown after faults.

Transient Immunity During Interruption: During faults, the voltage across the breaker rises from near zero to hundreds of kilovolts in microseconds (Transient Recovery Voltage). This massive voltage jump causes strong interference energy, potentially leading to driver failure or false triggering. PoF prevents this and ensures the breaker logic remains stable during any disruptions.

Broadcom is showcasing its Power over Fiber solutions at PCIM in Nuremberg from 9-11th June 2026 –Hall 9-526

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