Direct Answer
A reactor in a power system is a high-power inductor intentionally added to the network to introduce inductive reactance. A shunt reactor is connected in parallel and primarily absorbs reactive power to control high voltage, while a series reactor is placed in the current path and primarily adds impedance to limit fault current, switching current, or other current flow.
Reactor application is study-driven. Engineers must evaluate voltage profile, MVAR demand, available short-circuit current, protection sensitivity, switching transients, harmonic behavior, insulation duty, thermal limits, magnetic clearances, noise, grounding, and physical installation—not just the nominal inductance.
What Does an Electrical Reactor Look Like?
Power-system reactors are physically much larger than the inductors used on circuit boards. Depending on voltage class and construction, a reactor may look like a large open coil on insulators, a dry-type magnetic assembly, or tanked equipment that resembles a transformer.
| Part / feature | Function | Engineering concern |
|---|---|---|
| Winding | Creates inductance and carries reactor current | Conductor heating, short-circuit forces, insulation, losses |
| Support insulators | Provide electrical insulation and mechanical support | Voltage class, contamination, creepage, seismic/mechanical duty |
| Air space / magnetic path | Controls magnetic coupling and inductance | Stray magnetic field, required clearances, nearby induced heating |
| Terminals / bus connections | Connect reactor to bus, feeder, capacitor bank, or neutral | Current rating, corona, thermal joints, mechanical stress |
| Support structure | Holds the reactor in its designed geometry | Magnetic/nonmagnetic materials, fault forces, grounding, access |
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A reactor is an inductor designed specifically for power-system duty. The underlying physics are the same, but a reactor must withstand utility/industrial voltage, current, fault forces, thermal duty, switching transients, insulation stress, and installation conditions.
Shunt Reactor vs. Series Reactor
The most important reactor distinction is where it is connected. A shunt reactor is connected across the system and primarily changes reactive-power balance. A series reactor is inserted in the current path and primarily changes circuit impedance.
| Characteristic | Shunt reactor | Series reactor |
|---|---|---|
| Connection | Parallel with bus, line, cable, or tertiary | In series with feeder, bus tie, source, or capacitor branch |
| Primary effect | Absorbs inductive reactive power | Adds inductive impedance |
| Typical problem solved | Light-load overvoltage / excess capacitive VARs | Excess fault current / inrush / current sharing |
| Common rating emphasis | MVAR, voltage, losses, insulation | Current, reactance, short-time current, voltage class |
| Main tradeoff | Can depress voltage when VAR support is needed | Creates normal-load voltage drop and losses |
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How Shunt Reactors Control Voltage
Shunt reactors absorb reactive power generated by the capacitance of long transmission lines, underground cables, and lightly loaded high-voltage networks.
Long Lines, Cables, and the Ferranti Effect
Long high-voltage lines and cables have distributed capacitance. Under light load, charging current can cause the receiving-end voltage to rise above the sending-end voltage—a behavior associated with the Ferranti effect. Cable systems can be especially capacitive because conductors are closely coupled to grounded shields.
Fixed, Switched, and Controlled Shunt Reactors
A fixed reactor remains energized under its normal configuration. A switched reactor can be connected or disconnected as system conditions change. Controlled shunt reactors—including thyristor-controlled implementations—can vary effective reactive absorption more continuously. The preferred approach depends on the voltage-control problem, switching frequency, equipment cost, dynamic performance, losses, and reliability requirements.
Shunt reactors help during high-voltage/light-load conditions but absorb VARs whenever connected. A reactor that is useful overnight or during an open-ended line condition may be undesirable during heavy transfer or depressed-voltage operation.
Series and Current-Limiting Reactors
A series reactor reduces current by adding inductive reactance to the path between the source and the downstream system.
Fault-Current Limiting
When a bus or feeder fault level exceeds existing equipment capability, a series reactor can reduce prospective short-circuit current. Applications include bus ties, feeder circuits, generator auxiliaries, and systems where replacing switchgear or restructuring the network would otherwise be required.
Series-Reactor Tradeoffs
The same reactance that reduces fault current causes voltage drop and absorbs reactive power during normal operation. It may also influence motor starting, load flow, stability, relay sensitivity, distance-relay reach, and coordination margins.
For system-level fault calculations, see Short Circuit Analysis.
Common Types of Power-System Reactors
Reactor names usually describe the connection point or the system problem being controlled.
| Reactor type | Connection | Primary purpose | Key review item |
|---|---|---|---|
| Shunt reactor | Parallel | Absorb VARs / control overvoltage | Light-load vs. heavy-load voltage cases |
| Current-limiting reactor | Series | Reduce available fault current | Normal voltage drop and protection sensitivity |
| Bus-tie reactor | Series between buses | Limit fault contribution between bus sections | Load transfer and contingency behavior |
| Line reactor | Series with feeder/equipment | Add impedance, smooth current, reduce input stress | Voltage drop, harmonics, thermal duty |
| Shunt-capacitor reactor | Series with capacitor bank | Detuning / harmonic or inrush control | Tuning frequency and capacitor voltage stress |
| Filter reactor | Part of LC branch | Create intentional frequency response | Harmonic current, tuning, losses |
| Neutral grounding reactor | Neutral-to-ground | Limit ground-fault current | Fault detection and insulation stress |
| Converter reactor | AC/DC converter interface | Current smoothing / harmonic / commutation duty | Waveform, harmonics, converter control |
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Shunt Reactor vs. Capacitor Bank
A capacitor bank supplies leading reactive power; a shunt reactor absorbs inductive reactive power. They are complementary voltage-control tools, not interchangeable devices.
See Capacitors for the opposite VAR behavior and harmonic/resonance interaction.
Reactor Ratings and Equations
Reactor ratings should be interpreted on the correct voltage, frequency, phase, and connection basis. MVAR is common for shunt reactors, while ohms, percent reactance, and continuous/short-time current are especially important for series reactors.
Inductive Reactance
Three-Phase Shunt-Reactor Reactive Power
Using per-phase voltage and reactance:
For an equivalent wye-connected representation where \(V_\phi=V_{LL}/\sqrt{3}\), this becomes:
Shunt-Reactor Current
Why Shunt-Reactor MVAR Changes With Voltage
At constant frequency and reactance, the reactive-power magnitude changes approximately with voltage squared:
Series-Reactor Fault-Current Approximation
If source resistance is neglected for a conceptual calculation:
Real short-circuit studies include source resistance/reactance, transformer and conductor impedance, motor/generator contribution, X/R ratio, topology, grounding, and applicable calculation standards. The simplified equation only shows why added series reactance reduces current.
Worked Reactor Examples
Example 1: 50 MVAR Shunt Reactor at 230 kV
The reactor line current at rated voltage is:
If the bus drops to 220 kV and the reactor remains connected, approximate reactive absorption becomes:
Example 2: Series Reactor Reducing a 13.8 kV Fault
Assume a simplified purely reactive source produces 25 kA symmetrical fault current at a 13.8 kV bus. The source reactance is approximately:
Add a 0.10 Ω series reactor:
Reactor Switching, Harmonics, and Protection
Reactor application is not only a steady-state impedance problem. Switching can produce severe transient recovery voltage and overvoltage behavior, while filter/capacitor reactors intentionally interact with frequency-dependent network impedance.
Shunt-Reactor Switching
High-voltage shunt-reactor switching can involve current chopping, reignition/restrike, transient overvoltage, and challenging transient recovery voltage across the breaker. IEEE C37.015-2017 is the active IEEE guide for applying AC high-voltage breakers to shunt-reactor switching, while IEEE has an active revision project underway.
Reactors in Harmonic Filters and Detuned Banks
A reactor connected in series with a capacitor changes the branch resonant frequency. This can be used to create a tuned harmonic filter or to detune a capacitor bank away from a troublesome harmonic. Reactor inductance, capacitor tolerance, system impedance, harmonic spectrum, and thermal current all matter.
Reactor Protection
| Concern | Typical protection / monitoring | Why it matters |
|---|---|---|
| Internal phase fault | Differential / overcurrent depending design | Rapid isolation of reactor winding fault |
| Ground fault | Ground / differential elements | Depends strongly on winding and grounding configuration |
| Overtemperature | Temperature sensors / alarms / trips | Protects insulation and prevents thermal damage |
| Unbalance | Neutral/residual/current comparison where applicable | Can reveal abnormal winding or bank condition |
| Switching stress | Breaker/surge arrester application and transient studies | Controls overvoltage and TRV duty |
| Harmonic overload | Current/temperature monitoring and harmonic study | Non-fundamental current can raise losses and heating |
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Air-Core vs. Iron-Core Reactors
Construction changes saturation behavior, losses, magnetic-field exposure, footprint, noise, cooling, and installation requirements.
| Construction | Advantages | Limitations / special checks |
|---|---|---|
| Dry-type air-core | No ferromagnetic-core saturation; predictable reactance at high current | Large magnetic field, footprint, clearance, induced heating in nearby metal, audible noise |
| Iron-core / gapped core | More compact magnetic path for some duties | Saturation, nonlinear inductance, core losses, heating |
| Liquid-immersed shunt reactor | High-voltage insulation and cooling similar to transformer technology | Oil/fluid systems, bushings, fire/environmental controls, condition monitoring |
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Strong stray magnetic fields from air-core reactors can induce current and heating in nearby closed conductive loops, reinforcing steel, fences, cable trays, and structures. Manufacturer magnetic-clearance requirements are part of the electrical design.
Reactor Selection and Engineering Review Checklist
The reactor rating should come from the system study—not the other way around. Start by defining the problem and then verify that the selected reactor does not create an unacceptable secondary problem.
| Review item | Verify | Risk if missed |
|---|---|---|
| Design objective | Overvoltage, fault current, harmonic tuning, grounding, converter duty | Wrong reactor type or connection |
| Load flow | Normal/light/peak/contingency voltage and MVAR | Voltage too high or too low in another operating case |
| Short circuit | Fault duty before/after reactor | Equipment still underrated or protection too insensitive |
| Continuous current / MVAR | Normal duty and overload range | Thermal overload or wrong reactive absorption |
| Short-time current | Thermal/mechanical withstand | Damage during fault before breaker clears |
| Switching study | TRV, overvoltage, reignition, inrush/outrush | Breaker/insulation/surge-arrester overstress |
| Harmonics | Frequency spectrum and resonances | Unexpected current and heating |
| Protection | Relay pickup, reach, differential zone, backup coordination | Failure to trip or unnecessary trip |
| Physical layout | Magnetic clearance, structure, grounding, ventilation, noise | Induced heating or maintainability problems |
| Condition monitoring | Temperature, insulation, bushings, vibration/noise, terminations | Undetected deterioration |
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Field Inspection Indicators
- Hot or discolored terminals and connections
- Cracked, contaminated, or tracking insulators
- Abnormal vibration or audible noise
- Loose supports or signs of mechanical movement
- Corrosion or damaged weather protection
- Unexpected current or temperature imbalance between phases
- For liquid-immersed units: leaks, bushing condition, oil/fluid condition, alarms
Power-System Reactor Engineering References
- IEEE C57.21-2021 — Shunt Reactors Rated Over 500 kVA Active IEEE standard covering liquid-immersed and dry-type shunt reactors over 500 kVA, including ratings, losses, impedance, temperature rise, dielectric tests, insulation levels, construction, and testing.
- IEEE C57.16-2025 — Dry-Type Air-Core Series-Connected Reactors Active IEEE standard for series-connected dry-type air-core reactors used for power-flow control, fault-current limitation, filter duty, shunt-capacitor applications, converter duty, and related reactor applications.
- IEEE C37.015-2017 — Application of Shunt Reactor Switching Active IEEE guide covering high-voltage breaker application and overvoltage behavior for grounded, ungrounded, and neutral-reactor-grounded shunt reactor switching; IEEE has an active revision project underway.
- GE Vernova — Series Reactors in Power Systems Industry technical guidance on air-core series reactors for fault-current reduction and the associated voltage-drop and network tradeoffs.
Frequently Asked Questions
What is a reactor in a power system?
A power-system reactor is a high-power inductor intentionally added to the network to provide inductive reactance for voltage control, current limiting, harmonic filtering, grounding, or converter applications.
What is the difference between a shunt reactor and a series reactor?
A shunt reactor is connected in parallel and primarily absorbs reactive power to control voltage. A series reactor is inserted into the current path and primarily adds impedance to limit current or modify circuit behavior.
Why are shunt reactors used on long transmission lines and cables?
Long lines and especially cables can generate substantial capacitive reactive power under light load. A shunt reactor absorbs part of those VARs to reduce receiving-end voltage rise.
How does a current-limiting reactor reduce fault current?
It adds inductive reactance to the source-to-fault path. The higher Thevenin impedance lowers the prospective short-circuit current, although normal voltage drop and protection sensitivity must also be reviewed.
What is the difference between a reactor and an inductor?
Reactor is the power-system term for an inductor designed for much higher electrical, thermal, mechanical, insulation, and switching duty than a typical electronics inductor.
What is an air-core reactor?
An air-core reactor creates inductance without a ferromagnetic core. This avoids magnetic-core saturation, which is valuable for predictable current-limiting behavior, but it creates large stray magnetic fields and requires careful physical clearances.
Does a shunt reactor absorb or supply reactive power?
A shunt reactor absorbs inductive reactive power. A shunt capacitor bank does the opposite: it supplies capacitive reactive power.
Why is shunt-reactor switching difficult?
Shunt-reactor current is highly inductive and relatively low compared with fault current. Interruption can produce current chopping, transient overvoltages, reignition/restrike, and demanding transient-recovery-voltage conditions for the switching device.
Can reactors cause harmonic resonance?
Yes. Reactors interact with system capacitance and capacitor banks. That interaction can be intentionally designed for filtering or detuning, but an incorrect combination can create unwanted resonance and excessive harmonic current or voltage.
Summary and Next Step
Power-system reactors are large-duty inductors used to deliberately change system reactance. Shunt reactors primarily absorb reactive power and control high voltage, while series and current-limiting reactors primarily add impedance and reduce current.
The correct reactor application depends on more than MVAR or ohms. Voltage profile, fault level, current duty, harmonics, switching transients, relay settings, insulation, thermal performance, magnetic fields, physical clearances, and maintenance all influence whether the reactor actually improves the system.