Current / Voltage Imbalance

  • Fig.: Balance
  • In a three-phase system, balance is achieved when the three phase voltages and currents are equal in size and phase-shifted by 120° relative to each other.

    Imbalance occurs when one or both of these conditions are not met, often caused by inconsistent loads.

  • In high- and medium-voltage power grids, loads are generally three-phase and symmetrical, but large one- or two-phase loads may exist (e.g., mains frequency induction furnaces, resistance furnaces, etc.). In the low-voltage network, electrical loads are frequently single-phase (e.g., PCs, consumer electronics, lighting systems, etc.), and their associated load current circuits should be distributed as evenly as possible within the electrical wiring on the three-phase conductors. Depending on the symmetry of the single-phase loads, the network operates on a balanced or unbalanced basis.

    The acceptable degree of voltage imbalance in stationary operation induced by all mains loads is defined as ≤ 2%. For individual load systems, the resultant degree of imbalance is limited to = 0.7%, when an average over 10 minutes must be measured.

  • Fig.: Unbalance
  • Voltage imbalance can lead to:

    • Increased current loading and network losses.
    • With equal load power, the phase currents can be 2-3 times greater, and losses can multiply by 2 to 6 times, limiting the load of lines and transformers to half or a third of their rated power.
    • Increased losses and vibration in electrical machinery.
    • The negative sequence component of the currents creates a field that runs against the phase sequence of the rotor, inducing currents that result in increased thermal load.
    • Uncharacteristic harmonic currents in rectifiers and inverters that react to power supply imbalance.
    • In star connection three-phase systems, current flows through the neutral conductor.
  • Fig.: Illustration of unbalance in the Vector diagram