equalizer

In electrical power systems, an equalizer is a device or circuit that redistributes current or voltage across parallel paths to prevent one path from carrying excessive load. Where the audio definition describes frequency adjustment in sound, the power systems equalizer solves a different problem: ensuring that multiple conductors, transformer windings, or bus sections operating in parallel share the electrical burden fairly. Without equalization, manufacturing tolerances, temperature differences, or aging cause some paths to hog current while others sit idle, leading to overheating and premature failure in the overloaded path.

The most common application is in parallel conductor bundles used in high-voltage transmission and distribution lines. When two or three conductors run alongside each other to increase current capacity, they must share load evenly. An equalizer reactor (a small inductor) can be inserted between the conductors at their midpoint or at intervals. This reactor forces current to redistribute if one conductor starts carrying more than its share, maintaining roughly equal current distribution across the bundle. The equalizing effect improves with conductor length; longer parallel runs benefit more from mid-span equalizers.

Another critical use is in transformer banks. Where two or more transformer units are connected in parallel to serve a load, equalizing reactors or resistors may be added in series with each primary winding. These devices match the impedances of the transformers, preventing one unit from supplying most of the power while others coast. Without equalization, the transformer with lowest impedance claims the lion's share of current and may overheat while its parallel partner remains cool and underutilized. The choice between series reactors and resistors depends on whether the transformers' impedances are fixed or variable with load.

Equalizers in battery and generator systems

Battery banks and parallel-connected diesel generators rely on equalizers to prevent unbalanced discharge or load division. In battery systems, equalizers are resistive or reactive networks that encourage uniform voltage drop across cells or modules in series. In generator paralleling, an equalizer network across the bus ensures each generator contributes proportional current, especially critical during synchronization and load transients. Without proper equalization, one generator can suddenly become overloaded, causing it to slip out of synchronism and trip offline.

The term 'equalizer' reflects its function: to equalize (make equal) the voltage drop, current share, or impedance across parallel elements. It appears throughout utility substations, large industrial power distribution, and backup power systems. Engineers size equalizers based on the impedance mismatch between parallel paths and the allowable current imbalance, typically targeting no more than 5 to 10 percent variation in steady state.