Shunt reactors and series reactors

Welcome to a world of real allrounders and grid trouble-shooters

Shunt reactors and series reactors are used widely in AC networks to limit overvoltage or shortcut current in power transmission. With a growing number of high-voltage overhead lines in a fast-changing energy environment, both shunt and series reactors play a key role in stabilizing network systems and increasing grid efficiency. They are available as series reactors, variable and fixed shunt reactors with a rated power from ≤10 MVAr to 300 MVAr (33 kV to 800 kV). 

Reactors of Siemens Energy are highly economically efficient and enhance grid stability

Reactors from Siemens Energy come with benefits at various levels to help grid operators achieve and maintain a reliable and secure power supply system. For over 100 years, we have closely cooperated with energy providers and grid operators. Drawing on these decades of experience, we have tailored our processes − from consulting to design, from manufacturing to testing and after-sales services − to meet the needs of our customers and their individually customized units. Siemens Energy manufactures the full range of reactors for all customer needs in a variety of application areas. With the increase of renewable and distributed power generation reactors – and variable shunt reactors in particular – play a significant role in today’s energy landscape with its higher fluctuation in power feed-in. Reactors of Siemens Energy are compact and robust with stable performance over the complete service life. Customers can optionally choose features like low-noise/low-loss version, extended 80%-regulation range and green design options.

Measuring reliability

The high number of units in service – more than 800 units manufactured over the last decade alone – serves as proof of the validity of our design laws. In our effort to ensure long and reliable service life, we strive for minimum tolerances in manufacturing and insist on high-quality materials. Our suppliers worldwide are required to comply with the high standards, to which we hold ourselves. The result is a failure rate that is nothing short of impressive – we’ll gladly provide you with our latest mean time between failures (MTBF) figures.

Siemens' reactors work reliably for decades in high-voltage grids worldwide.

Our philosophy is quality

Our systematic approach to quality supports our relentless pursuit of precision and customer satisfaction. It rests on three pillars:

The highest quality standards govern all processes – from reactor design to manufacturing and testing – at all our sites. Each of our manufacturing sites has been certified to ISO 9001 and holds additional local certifications. The reactors manufactured meet all required standards, from IEC to ANSI and IEEE.

Siemens uses tried-and-tested technologies for windings, insulation, tank, monitoring devices, connection equipment, and the tap changer, sourced from rigorously screened suppliers. The sub-supplier qualification system – independent of region or site – sets the same high quality standards, to which we hold ourselves.

Every manufacturing step is subject to quality control, rounded off by a final inspection and an acceptance test in the high-voltage test bay of the respective manufacturing site. In addition, we can conduct special tests upon request. We invite customers at regular intervals to attend tests and inspections to give them the opportunity to personally ascertain the quality of the manufactured reactor.

Utmost precision at every stage

We build on decades of experience to achieve the high signature precision of our reactors. Low levels of vibration, noise, and losses, which remain stable over the entire service life, require utmost precision at every stage of the manufacturing process, from manufacturing to testing. To ensure this precision we employ highly qualified personnel and our very own design analysis tools. Every factory which is manufacturing reactors is testing units under real operating conditions, right up to the highest voltage levels and power ratings. 

  • Air cushion throughout manufacturing
  • Hydraulic lifting arrangement for core assembly
  • Two fully-equipped test fields to prevent bottlenecks
  • Large capacitor bank to test shunt reactors up to 250 MVAr

  • Constant pressure of the winding during drying process increases short-circuit capability.
  • Oil impregnation after drying reduces humidity in the windings.
  • “Desert Climate Hall” for final assembly after drying ensures long insulation life.

Dedicated to your commercial success

Siemens shunt and series reactors are the most cost-efficient solutions for reactive power compensation and short-circuit current limitation in the market. They come with strong commercial customer benefits, such as lower reactive power demand, lower losses and higher grid capacity. Their balanced load flows enable customers to avoid expensive grid extension.

  • Outstandingly low failure rate and stable product performance over entire service life
  • In-house grid consulting to help customers find the most cost-efficient solution
  • Projects are executed smoothly and on-time in compliance with specified values in test field
  • Compact reactor designs keep cost of substation construction low

Get the most out of our reactor design

The design process for reactors differs radically from that of transformers: Owing to their special core, the mechanical design of reactors tends to be more complex and demands particular attention to physical characteristics. In their communication with customers, Siemens experts share comprehensive advice on desired and necessary design features and will support customers with recommendations.

Reactor design criteria

  • Reactive power and reactance (reactor-specific)
  • Linearity range and knee-point (reactor-specific)
  • Losses and loss capitalization (only total losses for reactors)
  • Sound level limits
  • Vibrations (reactor-specific)

The low levels of vibration, noise, and loss, which have become Siemens' trademark in the industry, require utmost precision at every stage of the manufacturing process and exacting standards in supplier management. To prevent torsion flexion, Siemens Nuremberg uses core stacking tables designed for cores of 300 hundred tons, which shift core from their horizontal layer position into the vertical assembly position. Our oil-filled reactors are manufactured in two design types − with air core or with iron core.

Air core and magnetic flux current

Series reactors up to 800 kV and up to 1,500 MVAr are generally designed and manufactured without an iron core (air core), and with only a magnetic-flux return circuit. Precise manufacturing and robust designs ensure that these reactors operate reliably in all types of climates and in polluted and corrosive environments.

Iron core devided by air gaps

Siemens uses an iron core divided by air gaps in shunt reactors up to 800 kV and up to 300 MVAr. The result is a compact reactor with low levels of noise, vibration, and losses. The core is made from radially laminated iron packages, while ceramic spacers ensure precise compliance with the specific air-gap requirements.

Held in place in a clamping frame, the iron core in a reactor is clamped together by tie rods made of steel and/or limbs. Siemens’ unique spring technology between the tie rod and crossbeam ensures constant core pressing. This way, Siemens' spring and clamping design constantly maintains lowest noise and vibration values over the entire service life of these units. Siemens offers two types of clamping:

Inner clamping

Inner clamping technology offers particular advantages for high-voltage reactors: The tie rods are inserted into the grounded core. The resulting optimized force transmission allows for compact design and minimum noise.

Outer clamping

With outer clamping, the tie rods are located outside the core and winding which, at low voltages or in single-phase reactors, reduces the unit weight (in particular of core and winding).

Full range of insulations liquids

Siemens Transformers has been one of the first manufacturers building transformers filled with ester instead of mineral oil.  The capabilities of esters are nearly unlimited, as are the reasons why transformer operators decide to choose these fluids. As the aging performance of esters outweighs that of mineral oil, transformers can operate at higher temperatures than conventionally filled units –  an approach in compliance with IEC60076-14.

Lowest noise emissions in the world

When designing low-noise reactors we focus on avoiding core resonances and winding resonances. We use internal and external damping measures to reduce excess vibrations and noise. In most cases, Siemens succeeds in meeting even ambitious noise-level requirements without having to rely on an expensive sound house. However, Siemens does offer sound-house solutions, if required. Subject-matter experience and a specialized software –  developed by our R&D experts to determine a reactor’s total noise emission – enables us to build reactors with the lowest noise emissions in the world.

Reactor design on the cutting edge

In addition to developing its own reactor design software, Siemens R&D collaborates with major universities and partners worldwide to improve the quality of its products and services. Siemens' skilled experts use state-of-the-art design software and simulation tools, such as Siemens software based on the Finite Elements Method, i.e. 3DFEM for core and yoke losses of shunt reactors, and 3rd-party tools, such as ANSYS© and Open Foam®.


  • Compliance with guarantee values
  • High first-pass yield
  • Fast response to change requests during design stage of project
  • Smooth and timely project execution
  • Verified test results across four sites

Flux distribution in core - 3DFEM Calculation

Flux in core, air-gaps and winding - 2DFEM Calculation

Software tools

Increasing efficiency and flexibility

In conventional electricity transmission grids, shunt reactors control voltage and compensate reactive power. Shunt reactors provide voltage control and reactive power compensation. They are arranged between line voltage and ground. Their place of installation is usually located at the start or end of a long overhead power line or cable connection, or in central substations. They can also be assambled as variable shunt reactors with an individual design of tap changers for customers requiring a more flexible solution. Series reactors change load flow and limit short-circuit currents. They are deployed for short-circuit current limitation and load flow changes.

Reactive power compensation and voltage control

Fixed shunt reactors are designed for defined system condition and used for voltage control & reactive power compensation. They are budget and easy ON/OFF device. However, when multiple units are placed in parallel, fixed shunt reactors can be more expensive than variable shunt reactors. Their compact design and low maintenance needs make them a perfect solution for increasing efficiency.

  • HV lines and cables with stable load and voltage and a fixed line length
  • Grid access of large wind farms and solar power arrays, when variable reactive power is supplied from other sources


  • Flexibility on network changes (VSR)
  • Independence of other grid operators connected to own network


  • Cost efficient solution for reactive power supply
  • Less purchase of reactive power
  • Reduced losses (line & connected equipment)
  • Increased active power capacity of line
  • Minimal space requirements vs. other solutions


  • Better network voltage control
  • Reduced reactive power loading of the grid
  • Compliance with contractual reactive power limits
  • Optimized reactive power compensation (VSR)

A profitable investment thanks to full flexibility

Variable shunt reactors (VSR) are used for voltage control and reactive power compensation for continuously adjusting reactor power rating to actual needs. Due to the increased use of renewable energy sources, operators deal with fluctuating power flows.

With the help of VSR, the reactive power can be adjusted to the actual grid.  Operators profit from lower losses and noise emissions when the variable shunt reactor operates at a low power rating. Switching in a low reactive power rating results in a smaller switching impulse. With a large control range of max. 20 to 100%, variable reactors offer a grid flexibility that enables operators to achieve the highest grid efficiency. VSR are not only designed as compact units, but are also low-maintenance units with minimal service demands. On-load tap changers execute up to 300,000 switching operations, maintenance-free. 

  • Network changes, e.g. planned extensions, shut-down of power plants without replacement
  • Continuous fluctuations of line loading due to fluctuating load or generation, e.g. distributed generation, renewables, large energy storage
  • ON/OFF of different cable sections
  • Flexible replacement unit
  • Downsizing of SVC


  • Flexibility on network changes (VSR)
  • Independence of other grid operators connected to own network


  • Cost efficient solution for reactive power supply
  • Less purchase of reactive power
  • Reduced losses (line & connected equipment)
  • Increased active power capacity of line
  • Minimal space requirements vs. other solutions


  • Better network voltage control
  • Reduced reactive power loading of the grid
  • Compliance with contractual reactive power limits
  • Optimized reactive power compensation (VSR)

Choose the design which fits best

Shunt reactors provide voltage control and reactive power compensation, but can also be designed as variable shunt reactors with tap changers. VSR are used to compensate for capacitive reactive power of transmission lines – particularly in grids operating at low-load or no-load condition. They reduce network-frequency overvoltage in case of load variation, shedding, or network operating at no load. Moreover, VSR improve the stability and economic efficiency of power transmission. The decision to opt for a fixed SHR often has technical reasons, but the - admittedly more substantial - investment in a variable shunt reactor does pay off: With increasing load fixed shunt reactors will overcompensate the voltage increase, resulting in an overall voltage drops, while variable shunt reactors always compensate accurately and adjust to the current load situation.

If your answer to one or more of the following questions is “yes”, you should consider opting for a VSR:

  • Parts of the grid / lines are facing a change of idle / low load and load operation.
  • Parts of the grid / lines are regularly overloaded with reactive power of secondary network operators.
  • There are tight borders for keeping the voltage and acquiring reactive power agreed upon with a preceeding network operator.
  • In my grid, there is a large share of distributed generaion and / or wind-/solar power generation.
  • My grid or the conditions of my grid will face considerable changes in future, but I cannot define those changes exactly at the moment.
  • I already use fix shunt reactors in different voltage levels and need a flexible spare unit.

  • Lower noise emissions, when operating at reduced power rating
  • Lower losses, when operating at reduced power rating
  • Flexibility on network changes
  • Cost-efficient solution for flexible reactive power
  • Profitable due to reduced reactive power purchase
  • Increased grid efficiency
  • Reduced reactive power loading
  • Better network voltage control
  • Independence of adjacent grid operators
  • Cost-efficient spare concept when combining variable shunt reactor with the Pretact® concept

Increased reliability

Siemens Transformers manufactures series reactors up to 800 kV and 1,500 MVAr, usually as oil-filled units with an air core. Depending on customer needs, however, they can also be designed with an iron core, especially for neutral-point grounding reactors.

In 2014, Siemens delivered the largest series reactor in the world for use at the 400-kV level at 408 MVAr rated power and 2,770 MVA throughput power.

Short-circuit current limitation reactors

  • Limit short-circuit current in the event of a fault
  • Reactance is designed to achieve effective short-circuit current limitation, while voltage drop is still acceptable during normal operation

Grounding reactors

  • Increase the impedance at the neutral point of transformers
  • Limits the fault current at the neutral point, precisely compensating the capacitive grounding current and supporting restoration of the line

Phase shift oscillator for one-time impedance adjustment

  • In order to change load flows and flexible load flow control
  • Most economical solution

Making a difference

Siemens Energy reactors are built for operations and environments all over the world. They not only meet highest customer requirements, but comply with both national and international standards.

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Alternative insulation fluids
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