Shunt reactors

Shunt reactors in power systems

The presence, in an electrical system, of capacitive loads such as LED and neon lighting may translate in a non-optimal power absorption from the grid. The main power transformer of a plant, such as all appliances that are connected in series along the network between source and loads, find themselves under continuous overload because of the excess reactive power required by the capacitive impedance. The situation that arises may impact rather heavily on the service life of the plant and of its electrical insulation, since a permanent overheating has been proved to dramatically reduce the duration the devices involved. Moreover, the presence of a leading (capacitive) current on the line makes its control way more difficult.

These issues can be easily checked by installing a shunt reactor operating as load between phase and neutral or ground (wye connection) or between phases (delta connection) so that the overall power factor in absorption may tend to unity, which represent the best setup by efficiency. This is equivalent to saying that the capacitive current gets trapped by an inductive impedance placed in proximity of the load requiring that current.

FDUEG produces special shunt reactors for voltages up to the insulation class 36 kV, in any of the main configurations that the market requires.

Special Shunt reactors in power systems: voltage up to 36 kV and current up to 3 kA

Since they are not directly placed in series with the power flow, shunt reactors do not need, provided no overvoltages are present, to sustain currents of relatively large magnitude. However, since they are constantly connected to the power line and exposed to its voltage (or phase voltage for wye connected devices), constructive precautions are needed in order to ensure an adequate dielectric strength for the whole service life of the reactor.

FDUEG carefully analyzes this aspect in particular. Apart from respecting, with due tolerances, the creepage and clearance distances requested by the standards, if a shunt reactor bears some tens of kV (MV), the device’s electrodes are shaped so that no electric field accumulations can locally arise, thus reducing the overall degradation of the equipment.

Another important characteristic involves the limitation of the losses generated. Being it always under stress, this kind of reactor is designed to show low resistance and adequate magnetic field. The purpose of this is not to provoke issues from a thermal point of view, nor to reduce the overall power system efficiency too consistently.

The technology FDUEG exploits for its shunt reactors is dry type VPI (Vacuum Pressure Impregnated), with iron core construction. Thanks to this, the voltage containment is optimized through the usage of a fluid dielectric, which is constantly replaced, while the role of the polyester resin is limited, by its thin deposition, to mechanical containment.

Oil immersed or cast resin equivalent models are anyway available through custom designs. Moreover, the company offers air core reactors of identical performance, and single phase devices.

Whatever the final utilization of the reactor, FDUEG’s technical department is ready to assist the customer in its design and realization.

Shunt Reactors – Types and applications

Shunt reactors are used anywhere an alternating capacitive current has to be compensated. This opens up to numerous application and large rated power, voltage and current ratings.

• Industrial plants: a typical situation comprehends a grid whose power rating is relevant, which includes motors working as intermittent loads already compensated by a capacitive bank. If, given the static nature of this solution, this compensation is excessive when the system is lightly loaded, FDUEG proposes to install a tapped shunt reactor or a confined bank whose inductive value can be made up through composition and automatic insertion. This way, the power factor is progressively adjusted depending on the load variations.
• Transmission lines: the standard mathematical model of an overhead line or underground cable involves a capacitance per unit length, representing the electric fields generated with respect to earth. If this component prevails on the distributed inductance for long transmission lines, the insertion (maybe actively commanded) of inductive loads allows to balance the reactive power. These can also be made as variable shunt reactors (VSR)
• Harmonic filters: since they include a capacitor bank, they can generate an excess in VAr towards the grid. This may not be desirable and could interfere with the power electronics governing the filters. A shunt reactor can be the ideal solution.