What Are Smoothing Reactors?

Smoothing reactors are large inductors connected in series with high-voltage direct current (HVDC) transmission lines to reduce ripple in the transmitted current and suppress transient overvoltages. They are passive components, but they perform a structural role in HVDC converter stations: without adequate series inductance, the rectified DC output of a line-commutated converter carries significant harmonic content that would degrade power quality, interfere with communication cables in the same right-of-way, and stress converter valves. Smoothing reactors sit at the junction between the converter valve hall and the DC line, acting as a first line of defense against both steady-state harmonic currents and fault-driven current surges.

The electrical function of a smoothing reactor is rooted in Faraday's law: inductance resists rapid changes in current, so a reactor with sufficient henries of inductance limits the rate of rise of fault current during line-to-ground faults and commutation failures. This limiting action gives protection relays and converter controls time to respond before current reaches destructive levels. At the same time, the reactor forms a low-pass filter in combination with the DC-side capacitors and filter banks, attenuating higher-order harmonics that the converter's switching action would otherwise inject into the DC link.

Construction and Types

Smoothing reactors are manufactured in two principal configurations: oil-immersed core reactors and dry-type air-core reactors. Oil-immersed designs use a laminated iron core submerged in insulating oil, which provides a compact unit with high inductance per unit volume; they are typically deployed on the pole line side of the converter station. Air-core reactors eliminate magnetic saturation concerns by removing the iron core entirely, relying on the air-filled winding geometry to produce inductance; this makes them better suited to environments where the reactor must remain linear under the large DC bias current flowing through an HVDC link. The IEEE Standard 1277-2020, which covers both dry-type and oil-immersed smoothing reactors for DC power transmission, specifies electrical, mechanical, and thermal requirements and defines the test code that manufacturers and utilities use to verify performance before installation.

Role in HVDC Systems

In a conventional line-commutated converter (LCC) HVDC scheme, the smoothing reactor is sized to meet two criteria simultaneously: keeping steady-state DC ripple below a defined percentage of rated current, and limiting peak fault current to a value that circuit breakers or thyristor valves can interrupt safely. For back-to-back HVDC stations, where rectifier and inverter share the same site with no transmission line between them, the reactor inductance is the dominant impedance in the DC circuit, and its value is a key variable in system stability studies. Research published in IEEE Transactions on Power Delivery has examined how varying smoothing reactor inductance in back-to-back systems affects commutation margin, DC current stability, and the overall short-circuit performance of the converter.

In modular multilevel converter (MMC) based HVDC systems, smoothing reactors continue to appear, though their sizing differs because MMC topologies generate far lower harmonic currents than LCC converters. In multi-terminal HVDC networks, the reactor also limits the propagation of fault currents from one cable or overhead line segment to the rest of the network, a function that research on LCC-MMC hybrid networks has shown is sensitive to the reactor's physical location within the converter station layout.

Applications

Smoothing reactors have applications across a range of HVDC and power conversion contexts, including:

• Long-distance overhead HVDC transmission links, where line inductance alone is insufficient to suppress ripple
• Submarine HVDC cable interconnections between national grids
• Back-to-back HVDC stations linking asynchronous AC networks
• DC microgrids and solid-state circuit breaker installations requiring transient current limitation
• Industrial rectifier systems supplying high-current DC loads such as electrolysis plants