Filter Reactors

Filter reactor 27,5 kV
Photo of filter reactors RFOS-35, RFTS-35. Filter reactors 35 kV
Photo of the RFTS-35 filter reactor. Filter reactor 35 kV
Three phase filter reactor
Photo of a 27.5 kV filter reactor
Photo of filter reactors 27.5
Photo of a filter reactor 6500 A

Dry filter reactors are manufactured by KPM, LLC based on the technology that has proven itself in manufacture of current-limiting reactors.

Intended Purpose

The necessity in balancing the reactive power and voltage across the consumer power substation busbars lead to wide use of capacitors. Resistance of such batteries is inversely proportional to the voltage frequency .

During any transient processes in an electrical network, transient components appear. These components are the power system response to changes. Unlike the forced components, the transient ones may have any oscillation frequency or may be aperiodic. Forced oscillations are oscillations with the mains frequency — in Russia it is 50 Hz.

During a transient process, the frequency content in circuits with capacitors changes — energy is re-distributed towards the high-frequency oscillations range, where resistance of capacitors is less than that for commercial-frequency oscillations. This poses a threat to the electrical network, since supercurrents may occur. Such supercurrents might lead to overheating and failure of capacitors and other circuit elements.

The usual method of the actual shape of current and voltage waveforms analysis is Fourier transform of complex waves. The Fourier transform represents the distortions as sine waveforms with frequencies being multiples of the commercial frequency (50 Hz, 100 Hz, 150 Hz, etc.) — the so-called high-order harmonics. That is why, expression "high-order harmonics" is frequently used, although, in general, distortion of current and voltage waveform in transient states is referred to.

Traditionally, other sources of high-order harmonics are feeders of railway, subway and municipal transport overhead contact systems. This is due to the operating regime which, as a rule, implies regeneration of current during braking of electrical transport.

The source of high-order harmonics may also be represented by most common devices in electrical networks — transformers and reactors with ferromagnetic cores, electric motors and generators. If they are overloaded (overexcited), their current-voltage diagram becomes explicitly nonlinear, which leads to appearance of the 5th, 7th and higher harmonics.

Current electrical systems are characterized by high percentage of nonlinear loads — electrical apparatuses and machinery with nonlinear current-voltage diagrams. For example, devices based on power electronics (thyristors, IGBTs, etc.) belong to this group of loads — rectifiers, softstarters, variable-frequency drives, static power supply units, etc. Their application is determined by the requirements to modern electrical devices — their high controllability, lower power consumption. Use of power electronics causes disturbances in electrical networks. An AC network starts having dead times, sharp edges ("spikes") of electrical quantities. All of it might require use of filter reactors.

Filter reactors are also required when devices rarely occurring in the Russian Federation are connected to the network: HVDC inserts and lines and SVC/STATCOM devices — static thyristor compensators of reactive power.

In many cases, appearance of high-order harmonics in the network is inevitable. But their presence is considered destructive and undesirable:

  • All electric machinery and apparatuses are designed to be fed with alternating electric current with commercial frequency. As a minimum, high-order sinusoidal components cannot be effectively used by such devices, and as a maximum, they might cause equipment failure.
  • Flow of high-frequency currents causes waste of electrical power. Leak currents and losses rise.
  • High-frequency currents may lead to local overheating of equipment and insulation, to their accelerated ageing.

Power-line filters are used to protect capacitors and other elements of the network against high-order harmonics. A simplest power-line filter consists of a single element — filter reactor. If it is necessary to ensure additional limiting of currents, such filter shall include resistors. If it is necessary to limit surge overvoltages, there shall be insulation coordination means — nonlinear overvoltage suppressors (OVS).

The operating principle of a power-line filter is based on the fact that the reactor is essentially an inductance coil. An inductance coil is characterized by direct correlation between its resistance and frequency of current flowing through it . Thus, the higher the frequency of the harmonics, the more the reactor impedes and attenuates them.

Power-line filters and filter reactors are connected to an electrical network in series and/or in parallel with sources of high-order harmonics. Selection of the power-line filter connection configuration and its parameters depends on the network characteristics and peculiarities of the high-order harmonic sources. The basis for selection shall be the design calculation for the respective part of the network.


Design of filter reactors is similar to that of current-limiting reactors manufactured by KPM, LLC. Filter reactors have the characteristics adapted to their application conditions — reinforced insulation.

The most important design features of a KPM, LLC reactor are:

  • The reactor is a solid construction, its base and main load-bearing element are represented by the reactor winding itself. The winding needs no support framework or other elements to ensure extra strength.
  • All metal parts of the reactor are under the same voltage as its winding. Absence of significant potential drops inside the reactor minimizes the probability of its internal damage. E. g., breakdowns between the layers, breakdowns between the cross-piece and winding, etc.
  • Secondary elements of the reactor (rods, bindings) are made of fully nonmagnetic materials that have no electrical conductivity. This fully prevents their interaction with the magnetic field of the reactor. Since such elements are secondary, their strength is many times greater than the loads applied to them in the process of operation.
  • The reactor has absolutely no dismountable mechanical connections (such as screw-and-nut connections, etc.). This ensures highest strength, durability and reliability of the whole structure; prevents the necessity in maintenance of mechanical connections in the process of operations.
  • All electrical connections are made by soldering (welding), which prevents their heating, deterioration of contact joints, minimizes the losses.
  • The reactor does not contain any liquids and highly flammable materials, it cannot be a source of fire and is explosion-proof. The reactor is designed for long-term maintenance-free service.
  • Presence of vertical and horizontal through channels between the windings ensures reliable natural cooling of the reactor.