Current / voltage unbalance

Current / voltage unbalance

Fig.: Balance
Fig.: Balance

One speaks of balance in a three-phase system if the three phase voltages and currents are of an equal size and are phase-shifted at 120° to each other.

Unbalance arises if one or both conditions are not fulfilled. In the majority of cases the cause of unbalance lies in the loads.


Fig.: Unbalance
Fig.: Unbalance

In high and medium voltage power grids the loads are usually three-phase and symmetrical, although large one- or two-phase loads may also be present here (e.g. mains frequency induction furnaces, resistance furnaces, etc.). In the low voltage network electrical loads are frequently also single-phase (e.g. PCs, consumer electronics, lighting systems, etc.), and the 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 is operated on a more balanced or unbalanced basis.

The compatibility level for the degree of unbalance of the voltage in stationary operation caused by all mains loads is defined as ≤ 2 %. Related to individual load systems the resultant degree of unbalance is limited to = 0.7 %, whereby an average over 10 minutes must be obtained.

Fig.: Illustration of unbalance in the Vector diagram
Fig.: Illustration of unbalance in the Vector diagram

The following effects arise due to unbalance in the voltage:

  • Increased current loading and losses in the network.
  • With equal load power the phase currents can attain 2 to 3 times the value, the losses 2 to 6 times the value. It is then only possible to load lines and transformers with half or one third of their rated power.
  • Increased losses and vibration moments in electrical machinery.
  • The field built up by the negative sequence component of the currents runs against the phase sequence of the rotor and therefore induces currents in it, which lead to increased thermal loading.
  • Rectifiers and inverters react to unbalance in the power supply with uncharacteristic harmonic currents.
  • In three-phase systems with star connection, current flows through the neutral conductor.

 

You can find the related detailed formulas in the collection of formulas on page 318.