{"id":1131,"date":"2026-08-26T13:05:52","date_gmt":"2026-08-26T13:05:52","guid":{"rendered":"https:\/\/www.cemengineering.it\/shunt-reactors-reactive-power-compensation-design-and-maintenance-in-power-systems\/"},"modified":"2026-08-27T13:48:35","modified_gmt":"2026-08-27T13:48:35","slug":"shunt-reactors-reactive-power-compensation-design-and-maintenance-in-power-systems","status":"publish","type":"post","link":"https:\/\/www.cemengineering.it\/it\/shunt-reactors-reactive-power-compensation-design-and-maintenance-in-power-systems\/","title":{"rendered":"Shunt Reactors: reactive power compensation, design and maintenance in power systems"},"content":{"rendered":"\n<p>Shunt reactors are among the least visible yet most operationally critical pieces of equipment in high-voltage power systems. While transformers and circuit breakers dominate the attention of plant engineers and grid operators, the shunt reactor sits quietly in parallel with the network absorbing excess reactive power, suppressing dangerous overvoltages, and protecting equipment from the consequences of lightly loaded transmission lines and cable circuits. <\/p>\n\n<p>This guide explains what shunt reactors are, the physical phenomenon they address, the design variants used across different applications, and how they are maintained and supported over their service life. It also clarifies the important overlap between shunt reactor service and the industrial transformer expertise that CEM Engineering brings to this field. <\/p>\n\n<div class=\"wp-block-rank-math-toc-block\" id=\"rank-math-toc\"><h2> <\/h2><nav><ul><li class=\"\"><a href=\"#what-is-a-shunt-reactor\">What is a Shunt Reactor?<\/a><\/li><li class=\"\"><a href=\"#the-ferranti-effect-why-shunt-reactors-are-necessary\">The Ferranti effect: Why shunt reactors are necessary<\/a><\/li><li class=\"\"><a href=\"#other-functions-of-shunt-reactors-in-power-systems\">Other functions of Shunt Reactors in power systems<\/a><\/li><li class=\"\"><a href=\"#design-of-shunt-reactors-gapped-core-construction\">Design of Shunt Reactors: gapped core construction<\/a><\/li><li class=\"\"><a href=\"#types-of-shunt-reactors\">Types of Shunt Reactors<\/a><\/li><li class=\"\"><a href=\"#shunt-reactors-in-industrial-power-systems\">Shunt Reactors in industrial power systems<\/a><\/li><li class=\"\"><a href=\"#maintenance-and-service-of-shunt-reactors\">Maintenance and service of Shunt Reactors<\/a><\/li><li class=\"\"><a href=\"#why-cem-engineering-for-shunt-reactor-support\">Why CEM engineering for Shunt Reactor support<\/a><\/li><li class=\"\"><a href=\"#faq-shunt-reactors\">FAQ &#8211; Shunt Reactors<\/a><ul><\/ul><\/li><\/ul><\/nav><\/div>\n\n<h2 class=\"wp-block-heading\" id=\"what-is-a-shunt-reactor\"><strong>What is a Shunt Reactor?<\/strong><\/h2>\n\n<p>A shunt reactor is a high-capacity inductive device connected in parallel (shunt) with the power system, typically at the terminals of a transmission line, at a substation busbar, or across the tertiary winding of a power transformer. Its function is to absorb reactive power from the network, counteracting the excess capacitive reactive power generated by lightly loaded or unloaded long transmission lines and high-voltage cable circuits. As documented bye <a href=\"https:\/\/eroots.tech\/glossary\/reactor-shunt-reactor\" target=\"_blank\" rel=\"noopener\">eRoots Power Systems Glossary<\/a>, shunt reactors are an important counterpart to capacitor banks: where capacitor banks generate reactive power to support voltage under heavy load, shunt reactors absorb reactive power to suppress overvoltage under light load.  <\/p>\n\n<p>Structurally, a shunt reactor resembles a power transformer: it has a core of grain-oriented electrical steel, copper or aluminium windings, an oil-filled steel tank, and bushings for connection to the network. The critical design difference is the introduction of non-magnetic gaps in the core, a feature that fundamentally changes the magnetic behaviour of the device and is the key to its function as a controlled inductive impedance. <\/p>\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p><em>A shunt reactor is not a transformer. It converts no voltage, transfers no power between circuits, and has only one winding. Its sole function is to consume reactive power, absorbing the excess charge of the line and holding system voltage within safe limits.  <\/em><\/p>\n<\/blockquote>\n\n<h2 class=\"wp-block-heading\" id=\"the-ferranti-effect-why-shunt-reactors-are-necessary\"><strong>The Ferranti effect: Why shunt reactors are necessary<\/strong><\/h2>\n\n<p>The physical phenomenon that makes shunt reactors necessary is the Ferranti effect, a voltage rise that occurs at the receiving end of a lightly loaded or open-ended AC transmission line. <a href=\"https:\/\/resources.system-analysis.cadence.com\/blog\/msa2021-how-to-reduce-the-ferranti-effect-in-ac-transmission-lines\" target=\"_blank\" rel=\"noopener\">Cadence System Analysis explains<\/a> that this effect arises from the distributed capacitance to ground that exists along every transmission line. When the line is unloaded or lightly loaded, these capacitances generate reactive power, leading reactive power that flows back toward the sending end, raising the voltage progressively along the line. <\/p>\n\n<p>In extreme cases, long lines at high voltage, or extensive underground cable circuits which have far higher capacitance per kilometre than overhead lines, the Ferranti effect can raise the receiving-end voltage by 10\u201320% or more above nominal. This excess voltage overstresses transformer insulation, threatens equipment at the receiving end, and can interfere with protection system settings designed around nominal voltage. <\/p>\n\n<p>Shunt reactors suppress this effect by providing an inductive current that counteracts the capacitive current of the line. <a href=\"https:\/\/strongpowerelectric.com\/what-is-a-shunt-reactor-its-definition-working-principle-in-power-systems\/\" target=\"_blank\" rel=\"noopener\">Strong Power Electric&#8217;s technical analysis<\/a> summarises it clearly: by installing a parallel reactor, increasing the inductive reactive power absorbed (QL), the net reactive power (QC-QL) is reduced, bringing the voltage back within the acceptable range.<\/p>\n\n<h2 class=\"wp-block-heading\" id=\"other-functions-of-shunt-reactors-in-power-systems\"><strong>Other functions of Shunt Reactors in power systems<\/strong><\/h2>\n\n<p>Beyond Ferranti effect suppression, shunt reactors serve several other important functions in transmission and distribution systems:<\/p>\n\n<ul class=\"wp-block-list\">\n<li><strong>Prevention of generator self-excitation: <\/strong>generators connected to lightly loaded or long transmission lines can enter a condition of leading power factor self-excitation, where the capacitive charging current of the line provides sufficient reactive power to sustain the generator&#8217;s field without external excitation. Shunt reactors prevent this condition by absorbing the excess capacitive reactive power. <\/li>\n\n\n\n<li><strong>Reduction of overvoltages following single line-to-ground faults:<\/strong> during a phase-to-ground fault, the healthy phases can experience overvoltage due to the redistribution of capacitive charging currents. Shunt reactors reduce this transient overvoltage on the sound phases, protecting equipment from insulation stress. <\/li>\n\n\n\n<li><strong>Reactive power management in cable-intensive networks:<\/strong> underground cable circuits generate reactive power at rates several times higher per kilometre than equivalent overhead lines. In networks with significant cable penetration, urban transmission grids, offshore wind farm connections, shunt reactors are essential for managing the reactive power balance throughout the day and night load cycle. <\/li>\n\n\n\n<li><strong>Support for long-distance HVDC interconnections:<\/strong> the AC networks at either end of an HVDC link often require shunt reactors at the converter station to manage reactive power balance during the varying loading of the DC link.<\/li>\n<\/ul>\n\n<h2 class=\"wp-block-heading\" id=\"design-of-shunt-reactors-gapped-core-construction\"><strong>Design of Shunt Reactors: gapped core construction<\/strong><\/h2>\n\n<p>The defining design feature of an oil-immersed shunt reactor is the gapped core. <a href=\"https:\/\/studyelectrical.com\/2024\/09\/shunt-reactors-types-working-and-design.html\" target=\"_blank\" rel=\"noopener\">Study Electrical&#8217;s technical overview<\/a> explains that shunt reactors are constructed in the same way as power transformers, with one critical difference: non-magnetic gaps are inserted between packets of reactor core steel. These gaps are precisely dimensioned to control the effective inductance of the reactor and to prevent saturation of the core steel at the rated operating flux level. <\/p>\n\n<p>Without gaps, the core would saturate at high flux density, causing the inductance to drop sharply and the reactor to draw far more current than designed, potentially causing overloading of the connected network. The gaps linearise the flux-current relationship across the operating range, giving the reactor a stable, predictable inductive impedance. <\/p>\n\n<p>The gaps also create a source of mechanical noise and vibration. Magnetic forces across the air gaps cause the core laminations to vibrate at twice the power system frequency, a characteristic hum that is more pronounced in gapped-core reactors than in conventional power transformers. Modern shunt reactor design uses FEM (Finite Element Method) analysis of the tank structure to minimise vibration transmission to the tank surface, reducing audible noise at the installation site.  <\/p>\n\n<h2 class=\"wp-block-heading\" id=\"types-of-shunt-reactors\"><strong>Types of Shunt Reactors<\/strong><\/h2>\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><tbody><tr><td><strong>Type<\/strong><\/td><td><strong>Characteristics and Applications<\/strong><\/td><\/tr><tr><td>Oil-immersed, gapped core<\/td><td>Standard design for transmission and sub-transmission applications. Ratings from a few MVAr up to several hundred MVAr. Connected directly to HV busbar or line terminal. Maintainable with standard oil-immersed transformer service tools.   <\/td><\/tr><tr><td>Air-core (dry type)<\/td><td>No iron core the winding is suspended in air, supported by a fibreglass frame. Used at lower voltages, typically below 35 kV, and for harmonic filter reactors in industrial power quality applications. Very linear inductance, no saturation risk.  <\/td><\/tr><tr><td>Variable shunt reactor (VSR)<\/td><td>Oil-immersed reactor with OLTC for continuous reactance variation under load. Allows smooth adjustment of reactive power absorption without switching the reactor in or out. Increasingly deployed where renewable energy variability requires dynamic reactive power management.  <\/td><\/tr><tr><td>Transformer tertiary-connected reactor<\/td><td>Smaller reactors connected to the delta tertiary winding of a power transformer. Provides reactive compensation at the transformer location without a direct HV connection. Simplifies switchgear arrangement.  <\/td><\/tr><\/tbody><\/table><\/figure>\n\n<h2 class=\"wp-block-heading\" id=\"shunt-reactors-in-industrial-power-systems\"><strong>Shunt Reactors in industrial power systems<\/strong><\/h2>\n\n<p>While shunt reactors are most commonly associated with high-voltage transmission systems, they also serve important functions in industrial power systems, particularly in facilities with extensive cable infrastructure or significant non-linear loads. Industrial networks with long cable runs, extensive motor fleets, or renewable energy tie-ins often experience voltage instability and reactive power imbalance that shunt and damping reactors are specifically designed to address. <\/p>\n\n<p>In facilities with large arc furnace installations, such as the electric arc furnace steelmaking plants served by CEM Engineering, the flicker and reactive power fluctuations generated by the arc furnace process can create significant power quality challenges on the supply network. Shunt reactors and thyristor-controlled reactor (TCR) systems are deployed at the point of common coupling to manage these fluctuations and maintain compliance with utility power quality requirements. <\/p>\n\n<p>Our engineering experience with <a href=\"https:\/\/www.cemengineering.it\/it\/eaf-transformer-everything-you-need-to-know-about-electric-arc-furnace-transformers\/\">EAF transformers<\/a> and <a href=\"https:\/\/www.cemengineering.it\/rectifier-transformer\">rectifier transformers<\/a> gives us a thorough understanding of the reactive power dynamics in heavy industrial environments, an understanding that is directly relevant to the specification and maintenance of shunt reactors in these settings.<\/p>\n\n<h2 class=\"wp-block-heading\" id=\"maintenance-and-service-of-shunt-reactors\"><strong>Maintenance and service of Shunt Reactors<\/strong><\/h2>\n\n<p>Oil-immersed shunt reactors are maintained on essentially the same programme as oil-immersed power transformers of equivalent voltage class and rating. The shared construction, oil tank, bushings, conservator, protection devices means that transformer service expertise translates directly to shunt reactor maintenance. <\/p>\n\n<p>The core of the maintenance programme is <a href=\"https:\/\/www.cemengineering.it\/it\/attivita-di-servizio\/\">regular Dissolved Gas Analysis (DGA)<\/a>, conducted at intervals appropriate to the criticality and loading of the unit. Shunt reactors generate specific gas patterns in DGA that differ somewhat from power transformers, the gapped core introduces localised magnetic field concentrations that can cause partial discharge if gap spacing is not maintained correctly. Experienced DGA interpretation must account for the specific operating characteristics of gapped-core reactors.  <\/p>\n\n<p>Additional maintenance activities include:<\/p>\n\n<ul class=\"wp-block-list\">\n<li><strong>Oil quality testing:<\/strong> dielectric strength, moisture content, acidity and interfacial tension at annual intervals, with correction treatment (filtration, degassing) as required.<\/li>\n\n\n\n<li><strong>Bushing inspection: <\/strong>infrared thermography under load, visual inspection for leaks or surface contamination, capacitance and power factor measurement where instrumented bushings are fitted.<\/li>\n\n\n\n<li><strong>Vibration monitoring:<\/strong> gapped-core reactors are inherently noisier than equivalent transformers. Changes in vibration signature can indicate gap degradation or mechanical loosening of core components. <\/li>\n\n\n\n<li><strong>OLTC maintenance (for variable shunt reactors): <\/strong>calibrated by operation count, with oil sampling from the OLTC compartment and contact wear measurement at the manufacturer-specified intervals.<\/li>\n\n\n\n<li><strong>Tank and protection device inspection:<\/strong> Buchholz relay test, pressure relief device check, winding temperature indicator calibration.<\/li>\n<\/ul>\n\n<h2 class=\"wp-block-heading\" id=\"why-cem-engineering-for-shunt-reactor-support\"><strong>Why CEM engineering for Shunt Reactor support<\/strong><\/h2>\n\n<p>Our deep expertise in oil-immersed transformer engineering and service, built across more than 20 years of work with EAF, LF, rectifier, and large power transformers, translates directly to shunt reactor maintenance and diagnostic support. We understand the oil-immersed technology, the DGA interpretation methodology, the bushing inspection protocols, and the OLTC maintenance requirements that apply equally to shunt reactors and power transformers. <\/p>\n\n<p>We provide shunt reactor diagnostic services, DGA analysis, oil quality testing, thermographic inspection coordination, and technical consulting, for units in transmission and industrial service, with <a href=\"https:\/\/www.cemengineering.it\/it\/attivita-di-servizio\/\">24\/7\/365 technical support availability<\/a> for critical assets. <\/p>\n\n<p>Contact our <a href=\"https:\/\/www.cemengineering.it\/contact\">engineering team<\/a> to discuss a shunt reactor service programme.<\/p>\n\n<h2 class=\"wp-block-heading\" id=\"faq-shunt-reactors\"><strong>FAQ &#8211; Shunt Reactors<\/strong><\/h2>\n<div id=\"rank-math-faq\" class=\"rank-math-block\">\n<div class=\"rank-math-list \">\n<div id=\"faq-question-1787749218632\" class=\"rank-math-list-item\">\n<h3 class=\"rank-math-question \"><strong>What is the difference between a shunt reactor and a capacitor bank?<\/strong><\/h3>\n<div class=\"rank-math-answer \">\n\n<p>A capacitor bank generates capacitive reactive power, supporting voltage under heavy load conditions. A shunt reactor absorbs inductive reactive power, suppressing voltage under light load conditions, the two devices are complementary. Capacitor banks are switched in during peak demand; shunt reactors are typically switched in at night or during low-demand periods when the Ferranti effect is most pronounced.  <\/p>\n\n<\/div>\n<\/div>\n<div id=\"faq-question-1787749233372\" class=\"rank-math-list-item\">\n<h3 class=\"rank-math-question \"><strong>What is the Ferranti effect and why is it dangerous?<\/strong><\/h3>\n<div class=\"rank-math-answer \">\n\n<p>The Ferranti effect is a voltage rise at the receiving end of a lightly loaded or open-ended AC transmission line, caused by the line&#8217;s distributed capacitance generating leading reactive power. In severe cases, the voltage at the receiving end can exceed nominal by 10\u201320% or more, overstressing transformer and equipment insulation and triggering protection system operations. Shunt reactors suppress the Ferranti effect by absorbing the excess capacitive reactive power.  <\/p>\n\n<\/div>\n<\/div>\n<div id=\"faq-question-1787749246961\" class=\"rank-math-list-item\">\n<h3 class=\"rank-math-question \"><strong>Why do shunt reactors have gaps in their cores?<\/strong><\/h3>\n<div class=\"rank-math-answer \">\n\n<p>Non-magnetic gaps are inserted between packets of core steel to prevent saturation and to control the effective inductance of the reactor. Without gaps, the core would saturate at rated flux, causing the inductance to collapse and the reactor to draw uncontrolled current. The gaps linearise the flux-current relationship, giving the reactor a stable, predictable inductive impedance across its operating range.  <\/p>\n\n<\/div>\n<\/div>\n<div id=\"faq-question-1787749254182\" class=\"rank-math-list-item\">\n<h3 class=\"rank-math-question \"><strong>Can shunt reactor maintenance be performed by transformer service engineers?<\/strong><\/h3>\n<div class=\"rank-math-answer \">\n\n<p>Yes, oil-immersed shunt reactors use the same fundamental technology as oil-immersed power transformers: oil tank, bushings, conservator, winding temperature monitoring, and Buchholz protection. Transformer service expertise transfers directly. The key additional considerations are the interpretation of DGA results from gapped-core reactors (which have different gas generation patterns than standard transformers) and vibration monitoring of the core. Our <a href=\"https:\/\/www.cemengineering.it\/it\/attivita-di-servizio\/\">service team<\/a> has the technical background to address both.   <\/p>\n\n<\/div>\n<\/div>\n<div id=\"faq-question-1787749262887\" class=\"rank-math-list-item\">\n<h3 class=\"rank-math-question \"><strong>What is a variable shunt reactor (VSR) and where is it used?<\/strong><\/h3>\n<div class=\"rank-math-answer \">\n\n<p>A variable shunt reactor incorporates an on-load tap changer to allow continuous adjustment of its reactive power absorption under load, without switching the reactor in or out. This allows smooth, dynamic reactive power compensation increasingly important in networks with significant renewable energy penetration, where generation variability creates rapidly changing reactive power requirements. VSRs provide finer-grained reactive power control than conventional switched reactors, reducing circuit breaker wear and enabling more precise voltage management.  <\/p>\n\n<\/div>\n<\/div>\n<\/div>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>Shunt reactors are among the least visible yet most operationally critical pieces of equipment in high-voltage power systems. While transformers and circuit breakers dominate the attention of plant engineers and grid operators, the shunt reactor sits quietly in parallel with the network absorbing excess reactive power, suppressing dangerous overvoltages, and protecting equipment from the consequences [&hellip;]<\/p>\n","protected":false},"author":3,"featured_media":1130,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_et_pb_use_builder":"off","_et_pb_old_content":"","_et_gb_content_width":"","footnotes":""},"categories":[16],"tags":[],"class_list":["post-1131","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-notizie"],"_links":{"self":[{"href":"https:\/\/www.cemengineering.it\/it\/wp-json\/wp\/v2\/posts\/1131","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.cemengineering.it\/it\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.cemengineering.it\/it\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.cemengineering.it\/it\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/www.cemengineering.it\/it\/wp-json\/wp\/v2\/comments?post=1131"}],"version-history":[{"count":3,"href":"https:\/\/www.cemengineering.it\/it\/wp-json\/wp\/v2\/posts\/1131\/revisions"}],"predecessor-version":[{"id":1151,"href":"https:\/\/www.cemengineering.it\/it\/wp-json\/wp\/v2\/posts\/1131\/revisions\/1151"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.cemengineering.it\/it\/wp-json\/wp\/v2\/media\/1130"}],"wp:attachment":[{"href":"https:\/\/www.cemengineering.it\/it\/wp-json\/wp\/v2\/media?parent=1131"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.cemengineering.it\/it\/wp-json\/wp\/v2\/categories?post=1131"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.cemengineering.it\/it\/wp-json\/wp\/v2\/tags?post=1131"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}