Shunt reactors and series reactors

Shunt reactors and series reactors

Maximum flexibility and cost effectiveness

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 in increasing grid efficiency.   Range: series reactors, variable and fixed shunt reactors Rated power: from ≤10MVAr to 300MVAr Rated voltage: from 33kV to 800kV Features: compact, robust design; stable performance over service life; 3ph testing up to highest voltage and power levels; state-of-the-art technology and design tools Optional features: low-noise/low-loss version, extended 80%-regulation range, green design options


Pushing the boundaries of reactor technology

Reactors are real all-rounders and grid trouble-shooters. In conventional electricity transmission grids shunt reactors control voltage and compensate reactive power, while series reactors change load flow and limit short-circuit currents. 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. Siemens' reactors come with benefits at various levels to help grid operators achieve and maintain a reliable and secure power supply system:

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.


MTBF1) of Siemens shunt reactors

Number of units:                        820

Total service years:                     4,361

Number of failures:                    5

External failure rate:                  0.115 %

Meantime between failure:      872 years

1) 2004-2013 evaluated

Case study: How to measure transformer reliability

Our philosophy is quality

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

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.

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.

  • During their long service life, Siemens reactors reward customers with an outstandingly low failure rate and stable product performance over their entire service life.
  • To help customers find the most cost-efficient solution – a solution that not only matches specifications but fulfils their needs – Siemens Transformers offers in-house grid consulting.
  • Projects are executed smoothly and on-time. Compliance with specified values in test field is a matter of course.
  • Compact reactor designs keep the cost of substation construction low.
  • State-of-the-art technology and highly qualified personnel, also designing and manufacturing most complex products like HVDC transformers or phase shifters.

Get the most out of our reactor designers

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)

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

Innovative technology drives performance

Cores: Precision powered by experience

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.

Springs and clamping: Lowest noise and vibration values

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:

Design features for a smaller footprint and more efficiency


Enhancing grid stability and economic efficiency

Siemens manufactures the full range of reactors for all customers’ needs in a variety of application areas. Series reactors are deployed for short-circuit current limitation and load flow changes. Shunt reactors provide voltage control and reactive power compensation. They can be designed as variable shunt reactors with tap changers for customers requiring a more flexible solution.

Technical detail

Increasing efficiency and flexibility

Shunt reactors (SHR) 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. While overhead power lines are of comparably low capacitance and therefore only require compensation at high voltages and/or along very long power lines, cables have a significantly higher conductor-to-ground capacitance (by a factor of 20). Therefore, compensation should be provided already for lower voltages and shorter cable lengths.


Typical application areas

  • Grid access of large wind farms and solar power arrays
  • Line compensation for long HV overhead lines or AC cable connections
  • Distributed generation leading to load fluctuations
  • Prosumers leading to load fluctuations and power flow reversion in MV&HV grid
  • Large Energy storage systems leading to line loading when charging and no load conditions during storage time

Fixed shunt reactor

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.


Typical application areas

  • 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

Variable shunt reactor

A profitable investment thanks to full flexibility

Variable shunt reactors are used for voltage control and reactive power compensation. They continuously adjust reactor power rating to actual needs. Use of variable reactors is particularly recommended if fluctuations occur. This is the case for a growing number of grids, due to the increased use of renewable energy sources. Depending on actual demand, the reactive power can be adjusted to the actual grid.

Variable shunt reactors by Siemens are not only designed as compact units for on-going adjustment to changes in grid conditions, but are also low-maintenance units with minimal service demands. Siemens uses the reliable VACUTAP VRG tap changer series by Maschinenfabrik Reinhausen.

These on-load tap changers execute up to 300,000 switching operations maintenance-free.  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. Other interesting advantages of this flexibility: Switching in a variable shunt reactor with a low reactive power rating results in a smaller switching impulse. Also, operator profit from lower losses and noise emissions when the variable shunt reactor operates at a low power rating.


Typical application areas

  • 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

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.


Typical application areas

Series reactors are used to control current and increase impedance. As their name suggests, these reactors are arranged in series with the existing line. The series reactor constitutes an impedance in the grid and thereby increases the resistance of the network segment to limit short circuit currents or shift load flows. Arc-furnace series reactors, for example, provide additional reactance to stabilize the arc and increase efficiency. In motor start-up reactors, the starting current is limited and - where appropriate - combined with a speed controller.

Enjoy total customer focus

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 highly customized units.

Shunt reactors: fixed or variable?

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.

Advantages of variable shunt reactors over fixed shunt reactors

Would a VSR be cost effective for your grid?

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.
Talk to our experts and find out what Siemens reactors can do for you.

Contact the Energy Support Center


Driving innovation for future success

In 2008, Siemens emerged as the world’s first supplier of 800-kV HVDC transformers and, since then, has completed several 800kV HVDC projects every year. We followed up with the successful tests of the world’s first 1100kV HVDC mock-up, and, in 2013, of the world’s first 400kV transformer with alternative liquids. Our customers benefit from our dedication to their future success.
Get insider insights

Variable Shunt Reactors - Technical design, benefits and economic payback

This Whitepaper looks into the applications, components and uses of shunt reactors. It delves into topics such as the advantages and disadvantages of using shunt reactors versus using variable shunt reactors (VSRs) and transition resistance, demonstrating benefits by utilising business cases and use cases.


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Do you need help on a concrete project? Do you need a quote for any of our products or solutions? Do you need support for your installed equipment? Do you have a technical question? Do you need something totally different? You can contact us 24/7. We are looking forward to your call or online request. Phone +49 180 524 70 00