Unbalanced load

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Under unbalanced load refers to the uneven loading of the outer conductor of a three-phase alternating current network ( three-phase system ). Larger unbalanced loads can cause damage due to overheating in without compensation in extreme cases, power plant generators and power transformers lead.

The reason for damage in generators is that in particular the rotor of the synchronous generators used in power plants is designed as a solid and forged full-drum rotor without lamination. The rotor of a synchronous generator is only penetrated by a constant magnetic field with uniform load and in synchronous operation, and no eddy current losses occur. An unbalanced load results in an inverse rotating field in the rotor, which can lead to impermissible heating and in extreme cases to the destruction of the generator. The inverse rotating field causes a current in the damper winding , the frequency of which is twice as high as the mains frequency. The damper winding is designed with low resistance in order to keep the losses low.

The measure of the unbalanced load is the negative system in the system of symmetrical components .

Examples

Unbalanced load occurs, for example, when a railway line is operated with mains frequency and the single-phase traction current is taken from the three-phase public power grid by a transformer . This was the case in Germany with the Rübelandbahn .

Another example of the occurrence of unbalanced loads are single-phase three-wire networks , such as those found mainly in North America. In this system, due to the structure, single-phase loads at a transformer station can only be connected to an external conductor of the medium-voltage network.

Countermeasures

An even distribution of the load on the three outer conductors can avoid unbalanced loads. This is ensured, for example, in the area of ​​the sub-distribution , in that the individual single-phase circuits of a house or apartment are evenly distributed over the individual external conductors. Even if complete balancing is not ensured for each house or apartment - this depends, among other things, on the current consumption in the individual single-phase circuits - the balancing on average over a region and larger supply area is in many cases well met.

In the case of three-phase transformers , the addition of compensating windings or the design of the three-phase transformer as a five-leg design can help. In any case, this involves more effort and higher costs.

For balancing of smaller unbalanced loads in low voltage networks that can zigzag circuit are used on the low-voltage winding in three-phase transformers. This is used for unbalanced loads in local transformer stations , among others .

Since the 2000s, cost-intensive power electronics technologies such as the Unified Power Flow Controller (UPFC) have been available to compensate for unbalanced loads in electrical high-voltage networks.

literature

  • Rene Flosdorff, Günther Hilgarth: Electrical energy distribution . 9th edition. Teubner + Vieweg, 2005, ISBN 978-3-519-36424-5 .