Adjustable local network transformer

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A controllable local network transformer (ront) is a special transformer housed in a transformer station , the electric voltage from the medium voltage grid to the low voltage network (generally 230/400 volts in Europe) transformed used (local area network) lower voltage. Compared to conventional, non- controllable transformers in transformer stations, it can change the transformation ratio during operation and thus enable, for example, an increased feed-in of solar power without the grid voltage rising or falling in an unacceptable manner.

Basics

Adjustable local network transformer
Switch group of an adjustable local network transformer

Network operation with uneven demand and generation and various load flows in the transport network cause voltage fluctuations. In order to compensate for fluctuations, larger power transformers, which feed the medium voltage level operated with 10–20 kV from the 110 kV distribution network level in subordinate substations , are usually equipped with on- load tap-changers . In this way, even when the voltage in the high-voltage network fluctuates, an approximately constant voltage can be supplied to the end-user connections.

Permissible voltage fluctuations

When the existing energy distribution networks were built, it was assumed that energy would be generated almost exclusively in central power stations. The decentralized feeding of electricity into the low-voltage network played practically no role. With the increasing expansion of renewable energies as part of the energy transition , more and more electrical power is fed directly into the low-voltage network, for example from photovoltaic or biogas systems . If no countermeasures are taken, this can lead to inadmissibly high line voltages at the consumer, since the instantaneous values ​​of the voltage must be higher than those obtained from the power grid for feeding in.

The DIN EN 50160 allows a load-side voltage fluctuation of ± 10% of nominal voltage. With single-phase connections in the household, the permissible voltage range is 207-253 V, with three-phase alternating current 360-440 V.

However, since common local transformer stations have a fixed transformation ratio from the medium-voltage network to the low-voltage network, a portion of the maximum permissible fluctuation range must be "reserved" within the tolerance range, e.g. B. for feeding electrical energy into the medium-voltage network. According to the current FNN application rule "Generation systems on the low-voltage network" (VDE-AR-N 4105), the voltage may only increase by a maximum of 3% in practice due to the low-voltage feed. Conversely, the voltage drop that occurs in the low-voltage network may amount to a maximum of 5%, even if there is a strong demand for electricity, since a portion of the permissible 10% must also be kept for the medium-voltage network, for example. The worst-case scenario must always be assumed - it is assumed that the voltage drop is not compensated for by the medium- or low-voltage feed-in and vice versa.

If the fluctuation ranges mentioned were exceeded, for example if a large amount of electrical power was fed in from photovoltaic systems, the operators of conventional distribution networks were previously forced to undertake expensive network expansion measures.

Mode of action

A controllable local network transformer decouples the voltage in the low-voltage network from that in the medium-voltage network by means of a transformation ratio that can be changed during operation. On the low-voltage side, it always provides a constant voltage, regardless of the fluctuations in the medium-voltage network. With an assumed hysteresis of 4% and a maximum permissible voltage drop in the low-voltage network of unchanged 5%, the power fed in on the low-voltage side may cause a voltage increase of up to 11% (based on the nominal voltage of 230/400 V), since now the entire 20% Fluctuation range (± 10%) can be exploited. In many cases, this means that expensive new laying of low-voltage lines can be dispensed with. The higher costs for the controllable local network transformer itself are disadvantageous compared to conventional local network transformers with a fixed gear ratio.

technical structure

A controllable local network transformer consists of the actual transformer , a device for measuring the voltage in the medium-voltage and low-voltage network and an on- load tap-changer . In its basic functionality, it is thus similar to a conventional power transformer, as can be found in every substation . Due to the significantly lower voltages, however, the dimensions are significantly smaller, so that a RONT can usually be built into the existing transformer stations. The on-load tap-changers are installed on the medium-voltage side and are usually designed as vacuum switches . Compared to oil switches, vacuum switches have the advantage of less maintenance. The measuring points for voltage measurement can be installed not only directly on the RONT, but also at particularly critical points within the supplied low-voltage network.

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