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Unbalanced load

Lexicon> letter S> unbalanced load

Definition: an asymmetrical load on the phases of a three-phase network

English: unbalanced load

Category: electrical energy

Author: Dr. RĂ¼diger Paschotta

How to quote; suggest additional literature

Original creation: 07/22/2020; last change: 02/21/2021

URL: https://www.energie-lexikon.info/schieflast.html

An unbalanced load in a three-phase network means an asymmetrical (unequal) current load on the various phases (outer conductor). In the ideal case without any unbalanced load, the currents and electrical power transported are identical in all phases. This is approximately the case, for example, if only one three-phase motor is connected to the mains, or if each phase supplies the same type and number of consumers (e.g. electric heating elements).

As long as there is no unbalanced load and no harmonics occur in the currents, the neutral conductor is no longer loaded with current, since the currents in the three phases add up to zero at any moment. Of course, this changes with unbalanced loads. In the simplest case, current only flows through one of the phases, and this current naturally has to flow back in full through the neutral conductor.

The sockets in the household as evenly as possible for the various phases. Above all, there are additional strong averaging effects due to the large number of households in a power grid.

In practice, certain unbalanced loads often occur. For example, single-phase loads are mainly used in households. (This includes everything that is supplied via a normal household socket.) For example, the sockets in a house are roughly evenly distributed over the various phases, but it is always possible that a powerful device loads a phase heavily during the others receive significantly lower benefits. This is why the unbalanced load in relation to an individual household is often quite high in relative terms. However, such effects are largely averaged out through the large number of households in a distribution network; it is unlikely that a large number of households will always burden the same phase more than the others.

The averaging mentioned works less well in countries in which households are only supplied with one phase at a time. It is also unfavorable if a network only feeds relatively few, but strong, single-phase loads.

Single-phase feeds can also result in unbalanced loads. For this reason, for example, for photovoltaic systems (except for those with very low power), the inverter must feed in three-phase, i.e. the power is evenly distributed over all three phases.

With some larger consumers, which can only be operated in single phase, there are significant unbalanced loads.

Larger unbalanced loads are possible when operating powerful consumers that can only use one phase. For example, practically all electric locomotives are operated with single-phase alternating current, as otherwise the system of overhead lines and pantographs would be too complicated. If a railway line is simply connected to one of the phases of a three-phase network, this leads to a considerable unbalanced load during operation. Even if the other phases were used for other railway lines, their loads would not be the same at all times, since trains need a lot of electricity when starting or at high speed, while braking usually even feeds in energy (recuperation), and different trains do this in general of course not do it at the same time.

Extreme unbalanced loads in the high-voltage network can arise for a short time due to short circuits - if, for example, power pylons fall over during storms - or if conductors break in an accident. In such cases, a line is usually switched off quickly.

Problems due to unbalanced loads

One of the problems with unbalanced loads is that the existing line capacities are not used well: For a given total output, they lead to increased energy losses.

The occurrence of a neutral point shift, where this is not prevented by grounding, is unfavorable, potentially even dangerous. This means the creation of a voltage to earth for the star point of the alternating current system. One consequence of this is the occurrence of an overvoltage between at least one of the phases and earth. Under certain circumstances, this can lead to the destruction of equipment.

In addition, a significant unbalanced load in generators of the power plants can lead to massively increased energy losses, which are associated with unusual heating of the rotor and could lead to destruction if such a generator is not relieved in time. Unbalanced loads are also unfavorable for transformers.

Countermeasures

It is usually best to minimize unbalanced loads from the outset through a suitable design of the systems. Here, of course, the supply of larger consumers is of correspondingly greater importance. For example, it is desirable that the batteries of electric cars are not charged via simple household sockets (which can also provide little power), but via charging devices supplied with three-phase current. Photovoltaic systems should also feed symmetrically into all phases. On the other hand, no additional effort is worthwhile for small devices.

There are also technical measures to compensate for unbalanced loads. For example, there are three-phase transformers with additional compensating windings or other extensions. Modern power electronics offer various other options. Basically, however, such facilities increase the construction costs.

In the case of low-voltage transformers in transformer stations, a so-called zigzag circuit with two secondary windings per low-voltage phase is sometimes used with moderate additional effort, whereby the power is distributed fairly evenly to the three primary phases even with very asymmetrical loads on the secondary side. An unbalanced load in the low-voltage network does not affect the medium or high-voltage side. However, this approach not only leads to slightly higher construction costs, but also to higher energy losses in the transformer. This is why low-voltage transformers with a zigzag connection are only used in special cases, for example to supply larger end consumers (e.g. farms).

Questions and comments from readers

21.02.2021

I have an emergency power generator for a family house with 7.5 kW, 400 V / 230 V, AVR-regulated. The connection (three-phase current) to the distribution box is professionally laid, with a manual switch. I am unsure about the operation of electronic devices such as PCs, televisions, etc., since the unbalanced load of the individual phases can cause malfunctions or damage to such devices. Does it make a difference how much the power generator is loaded and would surge protection after a socket make sense?

Answer from the author:

I share your concern, as devices could be seriously damaged, especially by the overvoltages that occur with unbalanced loads. The central question is whether you can and want to rely on the quality of the voltage regulation (for each phase separately). In the event of damage, you would probably have quite a problem getting this replaced by the manufacturer. Therefore, it would be safer to only operate very robust and / or cheap devices with something like this.

I also very much doubt that surge protection would provide sufficient safety. Something like this is intended for short-term voltage peaks such as lightning strikes, not z. B. for permanent exposure to unbalanced loads.

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See also: three-phase current, transformer, phase, neutral conductor
as well as other items in the electrical energy category