Reactors

MRI reactors are used to limit the inrush peak current to less than 100 times the rated current of the bank of capacitors. The inrush current peak is due to the transitory phenomena of high amplitude and high frequency that are generated when the bank of capacitors cuts in on the supply network; this peak is considerably accentuated when a bank of capacitors is inserted in parallel with other banks that are already energized.

The reactors are single phase with an air core and copper winding and they are set in series with the bank of capacitors; they can be made for either indoor installation or outdoor installation. The reactors must be installed on post insulators with an insulation class equal to or greater than that of the bank of capacitors, considering the minimum distances for the installation indicated in the catalog.

The reactors must be chosen considering the following main factors:

• the sizing voltage must be equal to or greater than the network voltage;
• the sizing current must take account of the rated current of the bank and the current peaks;
• the reactor must be sized appropriately to withstand the dynamic and thermal stresses caused by the current peaks at the time of inrush;
• the influence of other banks of capacitors on the same network;
• the type of network in which to operate;
• the environmental conditions in which to operate.

MRA reactors are filters for harmonics, used to prevent resonances occurring, which are extremely dangerous situations for the electric system because of the ensuing overcurrents and overvoltages.

The “block” reactors are used with the sole purpose of correcting the power factor of the loads generating harmonics; the filter is therefore tuned only to a single frequency below the lowest one present. The most commonly used tuning frequencies are:

• 210 Hz corresponding XL = 5,7% XC
• 189Hz corresponding XL = 7% XC
• 134 Hz corresponding XL = 14% XC

Normally, reactors to block harmonics are singlephase with an air core, but three-phase reactors can be used with an iron core for limited powers; the main difference is that reactors with an air core do not saturate but they are bulky, while reactors with an iron core saturate but they are smaller. In both cases the reactors are set in series with the bank of three-phase capacitors and types can be made for either indoor or outdoor installation. The “filter” reactors are used with the aim of reducing the factor of distortion; the filter is therefore tuned to a frequency close to that of the harmonics present. If there are several harmonics to filter, then different filters will need to be made associated with separate banks of capacitors. The reactors are typically single-phase with an air core. The filter reactors are set in series with the bank of three-phase capacitors and can be made for either indoor or outdoor installation.

When used for indoor installations, it is fundamental to verify whether the power dissipated by the reactors affects the temperature of the installation environment; in such cases it is necessary to use air exchangers and/or air conditioning systems. If reactors with an air core are used, they can be arranged side by side in a line, triangle or overlapping (to reduce the overall dimensions), but in all cases the minimum distances between them must be respected, as towards metal bodies, magnetic bodies, current and/or voltage transformers and components made by winding closed coils; the catalog shows the minimum distances to respect in the installation.