EP3365906B1 - Controllable local network transformer - Google Patents

Description

The invention relates to a controllable local network transformer for transforming the electrical voltage from the medium-voltage network to the lower voltage used in the low-voltage network (local network).

Controllable local network transformers can change the transformation ratio between their high-voltage side, i.e. the medium voltage of 10-20 kV, for example, and their low-voltage side, i.e. the local network voltage of, for example, 400 V in Europe. In this way, changes in voltage can be compensated that arise, for example, from feed-in from decentralized photovoltaic systems.

Known controllable local network transformers have an on-load tap-changer on the high-voltage side, which includes, for example, one medium-voltage vacuum tube per phase and stage of the tap-changer, i.e. 15 medium-voltage vacuum switches for three phases and 5 stages. Medium-voltage vacuum switches are disadvantageously complex and therefore expensive and therefore make the construction of the on-load tap-changer unfavorable. For this, a preferred value for the voltage adjustment steps of 2.5% can be set.

Another solution places the on-load tap-changer on the low-voltage side of the controllable local network transformer. Here, cheaper and simpler low-voltage vacuums are used. The minimum setting step depends on the number of turns of the undervoltage coil and is, for example, 5% with 20 turns. This setting step is disadvantageously larger than the preferred setting step of 2.5%.

From the DE 10 2013 109 289 A1 a step transformer with an on-load tap changer according to the preamble of claim 1 is known.

The object of the present invention is to specify a controllable local network transformer which reduces or avoids the disadvantages of known controllable local network transformers mentioned at the beginning.

This object is achieved by a controllable local network transformer with the features of claim 1. The subclaims relate to advantageous configurations of the local network transformer.

The controllable local network transformer according to the invention comprises at least one high-voltage coil on the high-voltage side and at least one low-voltage coil on the low-voltage side. Furthermore, the controllable local network transformer comprises a first on-load tap-changer for the low-voltage side, the first on-load tap-changer being designed to switch between at least two different numbers of turns of the undervoltage coil and comprising at least one second switch for this purpose. Furthermore, the controllable local network transformer comprises a second on-load tap-changer for the high-voltage side, the second on-load tap-changer being designed to switch between at least two different numbers of turns of the high-voltage coil and comprising at least one second switch for this purpose.

The invention thus creates a controllable local network transformer which has an on-load tap-changer on both the high-voltage side and on the low-voltage side. The on-load tap-changer present on the high-voltage side advantageously means that the voltage level can be switched in relatively fine steps that could not be achieved from the low-voltage side alone. At the same time, however, as much of the structural effort as possible remains with the on-load tap-changer on the low-voltage side, where the lower voltage also places lower demands on the structure of the on-load tap-changer.

Advantageous refinements of the invention emerge from the claims dependent on claim 1. The embodiment according to claim 1 can be combined with the features of one of the subclaims or preferably also with those from several subclaims. Accordingly, the following features can also be provided for the current transformer:
The ideal compromise between the switching accuracy and the structural effort has proven if exactly two levels of the number of turns can be achieved on the high-voltage side. These are then preferably such that during the switchover a change in the number of turns of the high-voltage coil between 1% and 4%, in particular between 2% and 3%, preferably 2.5%, is brought about. These ranges are smaller than those that can usually be achieved on the low-voltage side. By taking coarse setting steps with the first on-load tap-changer and finer setting steps with the second on-load tap-changer, a broad setting range with fine setting steps is achieved overall. The first on-load tap-changer can be designed to switch between exactly three different numbers of turns of the undervoltage coil. This configuration results in six different gear ratios. However, only six switches are advantageously required on the high-voltage side for the three phases, while a further 12 switches are required on the low-voltage side, one switch being used for bridging the switching process for each phase.

The first and second switches include or are vacuum switches. A switch is expediently used per phase and per switchable step of the on-load tap-changer.

The controllable local network transformer can have a bridging circuit for power consumption during the switchover, the bridging circuit having a further switching device includes and expediently a current limiting resistor.

The controllable local network transformer can include a controller which is designed to determine a point in time of the current zero crossing for a phase to be switched, to open a switch for this phase at a time interval of less than 2 ms before the current zero crossing and a further switch for this phase to close at a time interval of less than 2 ms after the current zero crossing. Such a structure makes the bypass circuit unnecessary. A switching arc in the disconnecting switch is extinguished when the current passes through zero. Since the switch to be closed has no contact at this point in time, a circulating current is avoided. For this purpose, it is expedient if switches are used which have a short switching delay, preferably less than 5 ms, the switching delay being essentially independent of influences such as temperature.

The first and / or the second on-load tap changer can include exactly one changeover switch per phase and switching stage. In other words, a changeover switch is used instead of two switches that work as contacts. It is thus possible to advantageously design the first and / or second on-load tap-changer with exactly three changeover switches. Thus, in one embodiment, the first and second on-load tap-changer can be constructed together with exactly six changeover switches and thus achieve four switching stages.

FIG 1

den Aufbau eines regelbaren Ortsnetztransformators,

FIG 2

einen regelbaren Ortsnetztransformator mit sechs Schaltstufen,

FIG 3

einen regelbaren Ortsnetztransformator mit vier Schaltstufen und Wechselschaltern.

The invention is explained in more detail below on the basis of preferred exemplary embodiments with reference to the drawing. They each show in schematic form

FIG 1

the construction of a controllable local network transformer,

FIG 2

a controllable local network transformer with six switching stages,

FIG 3

a controllable local network transformer with four switching stages and changeover switches.

FIG 1 shows a rough scheme for the structure of a controllable local network transformer 10. The controllable local network transformer 10 comprises a high-voltage side 11 and a low-voltage side 12. Typically in Germany the high-voltage side 11 is designed for voltages in the range of 20 kV, while the low-voltage side 12 for voltages of 400 V is designed.

On the high-voltage side 11, connecting lines 13 for connection to the medium-voltage network lead into the controllable local network transformer 10. On the low-voltage side, connecting lines 14 lead to the connection of the local local network from the controllable local network transformer 10. The connecting lines 14 on the low-voltage side 12 are connected to a controller 15, which also includes voltage monitoring.

The controller 15 is connected to a first on-load tap-changer 16 on the high-voltage side 11 and controls the first on-load tap-changer 16. The controller is also connected to a second on-load tap-changer 17, which is arranged on the low-voltage side 12. The controller 15 monitors the voltage levels on the connection lines 14 to the local network and controls the on-load tap-changers 16, 17 according to their position.

FIG 2 shows schematically the structure of the on-load tap changer 16, 17 for an exemplary controllable local network transformer 20, this controllable local network transformer 20 allowing six different switching stages.

FIG 2 shows a main winding 23 and for one phase of the high-voltage side 21 of the controllable local network transformer 20 a control winding 24. One end of the main winding 23 is connected to two taps of the control winding 24 via vacuum switches 27, 28. In the case of one of the taps, the vacuum switch 27 can be bridged by a bridging switch 32 arranged in parallel, a current limiting resistor 35 being arranged in series with the bridging switch 32. The taps are arranged in such a way that switching results in a change in the number of turns of 2.5% of the total number of turns. When switching over, the bridging can be used in a known manner to switch under load and thereby reduce circulating currents. For a switchover, the bypass switch 32 is activated and takes over the current when the vacuum switch 27, 28, which was closed at the time before the switchover, opens. Only after opening this vacuum switch 27, 28 does the other of the vacuum switches 27, 28 close and then the semiconductor switch 32 is switched off again.

On the low voltage side 22 of the controllable local network transformer 20 there is an analog structure with vacuum switches 29, 30, 31 for low voltage and a bypass switch 33 and a current limiting resistor 36. On the low voltage side 22, however, there are three taps and therefore also three vacuum switches 29, 30, 31 for each phase. The taps on the low voltage side 22 are arranged in such a way that switching over the vacuum switches 29, 30, 31 leads to relative changes in the voltage of approx. + 5% of the total number of turns, i.e. just one turn for a total of 20 turns. A smaller step is therefore not easy to carry out on the low voltage side 22.

The two on-load tap-changers can therefore control six different voltage levels together, which are between -7.5% and + 5% of the standard voltage in 2.5% steps. Six medium-voltage vacuum switches and twelve low-voltage vacuum switches are used for this purpose, while in a known controllable local network transformer, In which five switching stages can only be switched on the high-voltage side, 15 medium-voltage vacuum switches are required.

If the vacuum switch is equipped with a fast drive, the bridging with the bridging switch 32, 33 can be omitted. To this end, the controller 15 is designed to detect the current zero crossing and to align the switching process for the vacuum switches 27... 31 with it. The respective opening vacuum switch 27 ... 31 then opens shortly before a current zero crossing, for example within 2 ms before that. The arc that forms is extinguished when the current passes zero. The closing vacuum switch 27 ... 31 does not close until shortly after the current has passed zero, for example within 2 ms thereafter. Since both switches are never closed, a circulating current is avoided and switching under load is achieved at the same time. This saves one bypass switch per phase and page 21, 22.

For this purpose, it is expedient to use switches that either have a very short switching delay, ideally in the range of less than 10 ms, even better in the range of less than 2 ms. Alternatively or additionally, it is advantageous if the switching delay of the switches depends only slightly on ambient conditions such as the temperature. With such switches it is possible to sufficiently synchronize the switching time with the current zero crossing.

FIG 3 shows a further embodiment for the structure of the on-load tap changer 16, 17 for an exemplary controllable local network transformer 40, this controllable local network transformer 40 allowing four different switching stages. The controllable local network transformer 40 similarly has main windings 43, 45 and control windings 44, 46 for the high and low voltage sides 41, 42.

Instead of the vacuum switch in the previous example, fast changeover switches are used here. In this case, only one changeover switch 47, 48 is used per phase on both the high voltage and the low voltage side 41, 42. The changeover switch 47, 48 ensures that a current path is always present and bridging is also unnecessary. In total, only six of the changeover switches 47, 48, namely one per phase and side 41, 42 are used, which represents a very low structural effort for the implementation of an on-load tap-changer with four stages.

Claims (10)

1. Regulatable local grid transformer (10, 20, 40) having at least one high-voltage coil (23, 24, 43, 44) on the high-voltage side (21, 41) and at least one low-voltage coil (25, 26, 45, 46) on the low-voltage side (22, 42), comprising a first on-load tap changer (17) for the low-voltage side, wherein the first on-load tap changer (17) is configured to switch between at least two different numbers of turns of the low-voltage coil (25, 26, 45, 46) and comprises at least one first switch (29, 30, 31) for this purpose, characterized by a second on-load tap changer (16) for the high-voltage side (21, 41), wherein the second on-load tap changer (16) is configured to switch between at least two different numbers of turns of the high-voltage coil (23, 24, 43, 44) and comprises at least one second switch (27, 28) for this purpose.
2. Regulatable local grid transformer (10, 20, 40) according to Claim 1, wherein the second on-load tap changer (16) is configured to switch between exactly two different numbers of turns of the high-voltage coil (23, 24, 43, 44).
3. Regulatable local grid transformer (10, 20, 40) according to Claim 1, wherein the first and second switches (27...31) comprise vacuum switches or are vacuum switches.
4. Regulatable local grid transformer (10, 20, 40) according to Claim 1 or 2, wherein the first on-load tap changer (17) is configured to switch between exactly two or exactly three different numbers of turns of the low-voltage coil (25, 26, 45, 46) .
5. Regulatable local grid transformer (10, 20, 40) according to one of the preceding claims, wherein the second on-load tap changer (16) is configured to effect a change in the number of turns of the high-voltage coil (23, 24, 43, 44) of between 1% and 4%, in particular between 2% and 3%, at switching.
6. Regulatable local grid transformer (10, 20, 40) according to one of the preceding claims, having a bypass circuit for drawing current at switching, wherein the bypass circuit comprises a further switching device (32, 33) and a current limiting resistor (35, 36).
7. Regulatable local grid transformer (10, 20, 40) according to one of the preceding claims, having a control means (15) that is configured to determine a time of the current zero crossing for a phase to be switched, to open a switch (27...31) for this phase within a time interval of less than 2 ms before the current zero crossing and to close a further switch (27...31) for this phase within a time interval of less than 2 ms after the current zero crossing.
8. Regulatable local grid transformer (10, 20, 40) according to one of the preceding claims, in which the first on-load tap changer (17) comprises exactly two switches per phase.
9. Regulatable local grid transformer (10, 20, 40) according to one of the preceding claims, in which the first on-load tap changer (17) comprises exactly one changeover switch (48) per phase.
10. Regulatable local grid transformer (10, 20, 40) according to one of the preceding claims, in which the second on-load tap changer (16) comprises exactly one changeover switch (47) per phase.

Images