Functions and Classifications of Reactors

Reactor is also called an inductor. When a conductor is energized, it generates a magnetic field within a certain space it occupies. Therefore, all current-carrying electrical conductors exhibit general inductive properties. However, a long straight energized conductor has low inductance and produces a weak magnetic field. In practice, reactors are made by winding wires into a solenoid, known as an Air-core Reactor; sometimes an iron core is inserted into the solenoid to increase inductance, forming an Iron-core Reactor.

 

Functions of Reactors

1.With the expansion of power grid capacity, the rated short-circuit capacity of the system increases rapidly. For example, at the 35kV low-voltage side of a 500kV substation, the maximum effective value of three-phase symmetrical short-circuit current approaches 50kA. To limit short-circuit current in transmission lines and protect power equipment, reactors must be installed. Reactors reduce short-circuit current and keep system voltage stable during short circuits.

2.Installing a Damping Reactor (Series Reactor) in a capacitor circuit suppresses inrush current when the capacitor circuit is energized. It also forms a harmonic circuit with the capacitor bank to filter out various harmonics.

⑴.For instance, in the capacitor circuit of a 35kV reactive power compensation device at a 500kV substation, damping reactors are required to limit capacitor switching inrush current and suppress system harmonics. To suppress 3rd harmonics, a 35kV rated voltage, 26.2mH rated inductance, 350A rated current dry-type air-core single-phase outdoor damping reactor is used, forming a 3rd harmonic resonant (filter) circuit with a 2.52Mvar capacitor.

⑵.Similarly, to suppress 5th and higher harmonics, a 35kV, 9.2mH, 382A single-phase outdoor damping reactor forms a resonant circuit for 5th and higher harmonics with a 2.52Mvar capacitor. Note that the use and technical specifications of damping reactors are specified in the national standard GB 10229-88 Reactors and international standard IEC 289-88.

 

Role of Reactors in Reactive Power Compensation Devices

The development of 500kV power systems, electrified railways, and large iron and steel bases requires the installation of Static Var Compensators (SVC) in major hub substations.

SVCs respond quickly to load changes (typical response time 0.02–0.04s) and provide smooth reactive power and voltage regulation. They stabilize grid voltage, effectively compensate the system reactive power factor, suppress voltage fluctuations, maintain three-phase balance, and damp sub-synchronous oscillations.

SVCs installed at grid hubs also reduce transient overvoltages. Major power grids therefore require large and medium-sized substations to install reactors for local capacitive reactive power compensation and balancing to ensure safe operation.

Reactors are key components of reactive power compensation equipment. Shunt Reactors provide inductive reactance to absorb excess capacitive reactive power, which is essential during low early‑stage transmission and late‑night light loads.

In these cases, transmission line reactive loss is low; due to the capacitance effect, generated reactive power exceeds consumed reactive power, leaving surplus capacitive reactive power. Shunt reactors must absorb this surplus to maintain reactive balance and voltage levels; otherwise, overvoltage endangers system safety.

To reduce thyristor count and save SVC investment, there is a trend to maximize Thyristor Switched Capacitor (TSC) and Thyristor Controlled Reactor (TCR) capacity.

Some SVCs eliminate the TSC branch and use Fixed Capacitor (FC) banks instead.

To retain smooth, continuous reactive power and voltage regulation, total shunt reactor capacity must be increased.

Thus, reactor usage continues to grow. Damping reactors in series with capacitor circuits also provide reactive power compensation in addition to limiting inrush current and harmonics.

 

Application of Reactors in Frequency Converters

Function of Input Reactors

Input reactors limit current surges from grid voltage fluctuations and switching overvoltages, smooth voltage spikes in the supply, and correct commutation‑induced voltage defects in bridge rectifiers. They protect frequency converters, improve power factor, block grid interference, and reduce harmonic pollution from rectifier units.

Function of Output Reactors

Output reactors mainly compensate for distributed capacitance in long (50–200m) cables, suppress output harmonic current, raise output high‑frequency impedance, effectively limit dv/dt, reduce high‑frequency leakage current, protect converters, and lower equipment noise. Capacitors in power compensation are vulnerable to harmonic voltage and current, which cause failure and degraded power factor, so harmonic treatment is required.

Function of DC Reactors

DC reactors are connected between the DC rectifier and inverter sections of a variable‑frequency drive. Their main purpose is to limit AC ripple superimposed on DC current, maintain continuous rectifier current, reduce current pulsation, stabilize inverter operation, and improve converter power factor.

 

Types of Reactors

Shunt Reactor

Reactors used for generator full‑load testing are prototypes of shunt reactors. Due to attractive forces from alternating magnetic fields between segmented cores, core‑type reactors are typically about 10dB noisier than equal‑capacity transformers.

Shunt reactors carry AC current and compensate system capacitive reactance. They are usually series‑connected with thyristors for continuous reactance current regulation. They mitigate power‑frequency overvoltage from long‑line capacitance effects under no‑load or light‑load conditions, improve voltage and reactive power distribution, reduce line loss, decrease secondary arc current, accelerate secondary arc extinction, improve automatic reclosing success rate, and are widely used in long‑distance power transmission and distribution.

Series Reactor

Series reactors carry AC current and are connected in series with compensation capacitors to create series resonance for steady‑state harmonics (5th, 7th, 11th, 13th). They are typically 5–6% reactors with high inductance.

Series reactors are essential supporting equipment for power system reactive power compensation. When combined with power capacitors, they effectively suppress grid harmonics, limit switching inrush current and operating overvoltages, improve voltage waveform, raise power factor, and greatly enhance the safe operation of capacitors and other power equipment.

Tuned Reactor

Tuned reactors carry AC current and are series‑connected with capacitors to create series resonance for a specified nth harmonic (usually n=5,7,11,13,19) to absorb that harmonic.

Output Reactor

Output reactors limit capacitive charging current in motor cables and restrict the motor winding voltage rise rate to within 540V/μs. They are recommended when cable length between a 4–90kW converter and motor exceeds 50m. They also soften the steepness of converter output voltage and reduce disturbance to inverter components such as IGBTs.

Output Reactor Instructions: To increase converter‑to‑motor distance, use appropriately thicker, higher‑insulation, preferably unshielded cables.

Output Reactor Features:

1.Suitable for reactive power compensation and harmonic control.

2.Mainly compensates long‑line distributed capacitance and suppresses output harmonic current.

3.Effectively protects frequency converters, improves power factor, blocks grid interference, and reduces harmonic pollution from rectifier units.

Input Reactor

Input reactors limit grid‑side voltage drop during converter commutation, suppress harmonics, decouple parallel converter groups, and restrict current surges from voltage steps or system operations. When the ratio of grid short‑circuit capacity to converter capacity exceeds 33:1, the relative voltage drop of input reactors is 2% for single‑quadrant and 4% for four‑quadrant operation.

Input reactors are permitted when grid short‑circuit voltage exceeds 6%. 12‑pulse rectifier units need at least one grid‑side input reactor with 2% relative voltage drop. Input reactors are widely used in industrial/ factory automation systems, installed between converters/speed controllers and the power supply to suppress surge voltages and currents and attenuate high and distorted harmonics.

Input Reactor Features:

1.Suitable for reactive power compensation and harmonic control.

2.Limits current surges from grid voltage fluctuations and switching overvoltages; filters harmonics to suppress waveform distortion.

3.Smooths voltage spikes in the power supply and corrects commutation‑induced voltage defects in bridge rectifiers.

Current-limiting Reactor

Current-limiting reactors are generally used in distribution lines. They are often installed in series on branch feeders from the same busbar to limit feeder short-circuit current and maintain busbar voltage during feeder faults.

Arc-suppression Coil

Arc-suppression coils are widely used in 10kV–63kV resonant grounding systems. Due to the oil‑free trend in substations, most arc-suppression coils below 35kV are dry‑cast type.

Damping Reactor

(Commonly called Series Reactor) Connected in series with capacitor banks or dense capacitors to limit capacitor switching inrush current. Similar to current‑limiting reactors. Filter reactors form resonant filters with filter capacitors, typically for 3rd to 17th harmonic filtering or higher‑order high‑pass filtering. Harmonic sources include DC transmission converter stations, phase‑controlled SVCs, medium/large rectifiers, electrified railways, and all high‑power thyristor‑controlled power electronic circuits; these must be filtered to prevent grid pollution. Power authorities specify harmonic limits.

Smoothing Reactor

Used in DC circuits after rectification. Rectifier circuits have finite pulse numbers, so output DC voltage contains ripple that is often harmful and must be suppressed by smoothing reactors. All DC transmission converter stations are equipped with smoothing reactors to approximate ideal DC. They are also essential in thyristor‑based DC electric drives. As key components in rectifier circuits, smoothing reactors in medium‑frequency power supplies mainly:

1.Limit short‑circuit current (simultaneous conduction during inverter thyristor commutation is equivalent to direct short‑circuit; no reactor causes direct short).

2.Suppress medium‑frequency components affecting the mains power grid.

3.Filter (rectifier current contains AC; high‑frequency AC has difficulty passing large inductance) to keep rectifier output continuous. Discontinuous current causes zero‑current periods, stopping the inverter bridge and opening the rectifier bridge.

4.Absorb reactive power in parallel inverter circuits; energy‑storage reactors are required in inverter input circuits.