Voltage unbalance and current unbalance
Symmetry in a three-phase system is said to exist when the three phase-to-phase voltages and currents are equal and 120° out of phase with each other. Unbalance occurs when one or both conditions are not met. In most cases, the cause for unbalance lies in the loads.
In high voltage and MV grids, the loads are usually three-phase and symmetrical, although large single-phase or two-phase loads may also be present here (e.g. grid frequency induction furnaces, resistance furnaces, etc.). In the low-voltage network, the electrical loads are often also single-phase (e.g. PCs, consumer electronics, lighting systems, etc.) and the associated load circuits should be distributed as evenly as possible over the three phase conductors within the electrical cabling. Depending on the balancing of the single-phase loads, the grid is operated more or less symmetrically or asymmetrically, and the compatibility level for the degree of unbalance in steady-state operation of the voltage caused by all grid consumers is set at ≤2%. In relation to individual consumer installations, the resulting degree of unbalance is limited to = 0.7%, with averaging over 10 minutes.
Unbalance in the voltage results in the following effects:
- Increased current load and losses in the network.
- For the same load power, phase currents can reach 2 to 3 times the value, and losses can reach 2 to 6 times the value. Lines and transformers can then only be loaded to half or one third of their rated power.
- Increased losses and vibration torques in electrical machines.
- The field built up by the negative sequence component of the currents runs against the direction of rotation of the rotor and induces currents in it that lead to increased thermal stress.
- Rectifiers and inverters react to an unbalanced supply voltage with uncharacteristic harmonic currents.
- In three-phase systems with a Y connection, a current flows through the neutral conductor.
The detailed formulas can be found in the formulary.
