How does a shunt reactor work

Differences between shunt reactor and power transformer

Main differences

Shunt Reactor and Transformer both appear similar in structure. Reactors are often also equipped with fans for cooling, similar to transformers.

However, there are big differences between the two. While a power transformer is designed for efficient power transfer from one voltage system to another, a shunt reactor is provided just to consume reactive VArs (or in other words it can be said to generate trailing VArs)

Differences between shunt reactor and power transformer

So there is more than one winding on a power transformer with magnetic core that carry the mutual flux between the two in the reactor there is only one winding. The core is therefore not intended to just provide a low reluctance path for this winding to flow to increase inductance.

In the case of a power transformer, primary Ampere-Turns (AT) is the sum of the exciting AT and secondary AT. The AT loss (in terms of winding resistance, eddy loss and hysterical loss) is kept as low as possible. The exciting AT is small compared to the secondary AT. The rated current is based on the load transfer requirement.

Detailed view of an iron core divided by air gaps

The magnetizing current is small and negligible compared to the secondary rated current. Since the mutual flux is the main flux that leads to the conversion, the leakage flux is kept small and is based on fault current limitation.

In the case of a shunt reactor due to the lack of other windings, the total primary AT is equal to that exciting AT. Similar to a power transformer, AT losses (winding resistance, eddy current and hysteresis) are kept to a minimum due to the design. Magnetize AT is the main component of a shunt reactor. The choke magnetizing current is the rated current.

Since a shunt reactor magnetizing current is large, if only iron is provided as the power transformer, there is a large hysteresis loss. In a shunt reactor, air gaps are provided in the iron core to reduce this loss and minimize the remaining flux in the core.

This means that a shunt reactor can also be set up without iron (Air core).

A shunt reactor can be constructive Dipped in oil or dry type both with and without an iron core.

Dry reactors are constructed in a single phase and are arranged in such a way that the stray magnetic field in the environment (without metallic shielding) is minimized. When such an arrangement is difficult, some form of magnetic shielding carefully designed to minimize eddy current losses and arcing at any juncture in the metal loops is required. One of the advantages of a dry type reactor is the lack of inrush current.

Oil bath reactors can be seedless or with Iron core with gaps. This is either single phase or three phase with or without fan cooling. These are installed in tanks that contain oil and serve as metallic magnetic shields.


In some cases there may be a shunt reactor with an additional small capacitance winding that can provide power for small station power loads. Since the shunt reactor rating is usually based on the MVAr rating, this additional station load VA must be taken into account when designing the reactor for such applications.

Types of shunt reactors


Shunt chokes are used in high-voltage networks to compensate for the capacitive generation of long overhead lines or extended cable networks.


The reasons for using shunt reactors are mainly two

The first reason is to limit overvoltages and the second reason is to limit the transfer of reactive power in the network. If reactive power transfer is minimized, i. e. If the reactive power is balanced in the different parts of the network, a higher active power can be transmitted in the network.

Reactors to limit overvoltages are most urgently needed weak power systemsi.e. when the network short-circuit power is relatively low.

The voltage increase in a system due to the capacitive generation is:

ΔU (%) = QC. x 100 / ssh.c

where from:

Qc is the capacitive feed of reactive power into the network
S.sh.c is the short circuit power of the network

As the short-circuit power of the network increases, the increase in voltage decreases and the need for compensation to limit overvoltages is less accentuated.

Reactors to achieve a reactive power balance in heavily loaded networks in which no new lines can be built for environmental reasons, different parts of the network are most urgently needed. Reactors for this purpose are mostly Thyristor controlled in order to adapt quickly to the required reactive power.

Especially in industrial areas with Arc furnaces The reactive power requirement fluctuates between each half cycle.

In such applications there are usually combinations of:
  1. Thyristor-controlled chokes (TCR) and
  2. Switched capacitor banks (TSC).
Both together make it possible absorb, and Generate reactive power according to the current demand.

Four-leg reactors can also be used to extinguish secondary ones during single-phase reconnections in long transmission lines. Since there is always a capacitive coupling between the phases, this capacitance results in a current that holds the burning, secondary arc.

By adding a single phase reactor in the neutral conductor, the secondary arc can be extinguished and the single phase reclosing is successful.

Resource: Shunt reactors and shunt reactor protection - S.R. Javed Ahmed

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