EP2831894B1 - Secondary distribution network transformer - Google Patents

Description

The invention relates to a local network transformer for transforming a medium voltage in the kV range into a voltage for the local power supply.

Conventional fossil fuel generation, for example, was virtually 100% centralized and current power grids are designed for this situation. It is always to be expected with a voltage drop from the grid transformer to the terminals such as private households. Currently, however, the generation of electricity from renewable sources is growing rapidly. In contrast to the traditional generation of electricity, a not inconsiderable part of the generation of electricity from renewable sources takes place locally, for example in private households or small plants.

As a result, it can increasingly come to the previously impossible situation that reverses the power flow along spurs in the local network, for which, however, the local network is not designed. The result is voltage changes in the local network, which can exceed the limit of +/- 10% of the nominal voltage. Even in the medium-voltage network voltage overshoots are possible, for example, by connected wind turbines. These also entail an increased voltage in the low-voltage network (local area network).

One known way to deal with these problems is the significant expansion of local networks, ie an increase in the cable network laid in the local network. This leads to a reduction of the voltage drop across the cables and thus also to a reduction of voltage overshoots in the local network. However, this is associated with a considerable effort.

The DE 20 2010 012 811 U1 and the GB 1007496A show solutions for transformers with tap changers. In the DE 20 2010 012 811 U1 the equipment of the tap-changer are arranged in the oil-filled boiler. In the GB 1007496A Between each tapping point of the secondary winding and the output of the transformer, a permanent connection is made by means of a controlled anti-parallel connected thyristor pair. It is an object of the present invention to provide an improved local power transformer, with which the above-mentioned problem is reduced.
This object is achieved by a local power transformer with the features of claim 1.
The local network transformer according to the invention for the local network power supply has an active part with primary and secondary windings for converting a primary medium voltage into a secondary local network voltage, wherein the active part is arranged in a transformer tank with cooling and insulating liquid. Furthermore, the local network transformer comprises a plurality of switching means for adjusting the ratio between the primary voltage and the secondary voltage, wherein the switching means are connected on the secondary side between taps of the secondary side windings and the neutral point of the secondary side windings. Finally, the switching devices are arranged in the transformer boiler in the area of cooling and insulating liquid.
The primary voltage is a medium voltage in the range of a few kV, for example 10 kV, 20 kV or 30 kV. The secondary voltage is a voltage for the local supply of a local network or urban network, for example, 230V single-phase at 400V line-to-line voltage.
The switching devices are designed, for example, such that the secondary voltage can be varied in steps by means of a suitable control.
Due to the advantageous arrangement of the switching devices in the transformer boiler is achieved that bushings of Anzapfun conditions of the secondary windings out of the transformer box are unnecessary. Furthermore, it is achieved that the copper lines are short from the taps to the switching devices. Finally, it is additionally achieved that cooling of the switching devices automatically takes place, since the switching devices are arranged in the cooling and insulating liquid.

Furthermore, it is advantageous that the stated construction is modular, i. In addition to a structure with 3 or 5 taps and constructions can be realized with more taps without much additional effort. There are no substantial changes necessary, but it must only be adapted to the number of switching devices and appropriate taps are provided.

It is also particularly advantageous that a separate voltage control for the three phases is possible. In other words, the gear ratio of the grid transformer for the phases can be individually controlled. As a result, an improvement in the voltage value can be achieved even if a very uneven loading of the phases occurs in the local network.

The switching devices preferably each comprise a parallel circuit comprising a controllable mechanical switch and at least one controllable semiconductor switch. This is at the same time given a lossless power line in the normal state, i. when not currently switching, while at the same time a fast switchable bypass is possible when switching the voltage level. For an interruption-free switching is advantageously possible.

According to the invention, the switching devices each comprise a bistable mechanical switch. In this case, the controllable mechanical switch is expediently a bistable switch. This ensures that in the switching devices no performance is required, as long as no switching takes place, because on the one hand no holding power for the mechanical switch is necessary, the current flow through the mechanical switch and not about semiconductor and on the other hand, the transformer has a defined switching group even in the de-energized state.

It is particularly advantageous if the switching devices are arranged in the transformer box below the windings. "Below" refers to the area between the lower edge of the windings and the bottom of the tank. In this arrangement, the switching means are in a range of cooling and insulating liquid, which is still relatively cool due to the heat convection. This ensures good cooling of the switching devices.

The local network transformer expediently comprises a control device for detecting voltage and / or current of at least one of the phases and for controlling the switching devices on the basis of the detected values. The control is expediently such that a deviation of voltage values occurring in the local network from at least one predefinable setpoint voltage value for the local network is reduced.

It is advantageous if voltage and / or current are determined separately in all phases. This then makes it possible to compensate for voltage differences between the phases in the local network.

In a particularly advantageous embodiment of the invention, the control device and the transformer are constructed in the immediate vicinity of each other. You can, for example, form a structural unit, so be designed as a total device. Alternatively, for example, the control device may be an external device separate from the transformer, but which is arranged in the immediate vicinity of the transformer.

As a result, it is advantageous to control the work of the transformer exclusively from information available locally at the transformer by detecting and processing the voltage in the region of the transformer. A remote control is so advantageous not required. Likewise, no Remote measurement of, for example, voltage data required.

Figur 1 und 2

Schnittansichten eines Ortsnetz-Transformators,

Figur 3

ein elektrisches Schaltbild für die Sekundärseite des Ortsnetz-Transformators mit Schalteinrichtungen und

Figur 4

eine Schalteinrichtung.

Further expedient embodiments and advantages of the invention are the subject of the following description of an embodiment of the invention with reference to the figures of the drawing, wherein like reference numerals refer to the same components. Show

FIGS. 1 and 2

Sectional views of a grid transformer,

FIG. 3

an electrical circuit diagram for the secondary side of the local network transformer with switching devices and

FIG. 4

a switching device.

The FIGS. 1 and 2 show sectional views of a local network transformer 10, in the case of FIG. 1 a side view and FIG. 2 a front view. In this case, a transformer tank 11 makes the main part of the local network transformer 10. In the transformer tank 11, the active part 12 of the local network transformer 10 with windings 311, 312, 313 is housed in about halfway up. Towards the upper end of the transformer tank 11, copper lines lead to passages through the wall of the transformer tank 11. These passages lead into connecting elements for the phases of the local network and the medium voltage.

Towards the lower part of the transformer tank 11 out of the active part 12 terminals 14 of taps of the windings 311, 312, 313 led out. The connections lead in each case to an input connection of switching devices 13 arranged in the lower part of the transformer tank 11. The output connections of the switching devices 13 are in turn connected to one another and to a neutral conductor connection 16 via a bus line 21 arranged horizontally at the bottom in the transformer tank 11.

By the arrangement shown, the switching devices 13 are in that part of the transformer tank 11, which will be the coolest under normal conditions, since the heated in the region of the active part 12 cooling and insulating liquid will strive upward. It is also advantageous that the additional arrangement of the switching devices 13 in the transformer tank 11 additional complex and expensive feedthroughs from the transformer tank 11 are unnecessary.

The electrical wiring is in the FIGS. 3 and 4 clarified. FIG. 3 schematically shows the secondary coils 311, 312, 313. At these taps 32 are arranged to one end of the coil, in the present example exactly five taps 32 per secondary coil 311, 312, 313. The last of the taps 32 is the respective coil end itself. Each of the taps 32 is connected to the input contact 135 of each switching device 13. The output contacts 134 of all switching devices 13 are in turn combined and connected to the neutral terminal 16.

The structure of the switching devices 13 in this example is in FIG. 4 clarified. Each of the switching devices 13 comprises a parallel connection of a bistable mechanical switch 131 with a series of a resistor 133 and an anti-parallel pair of thyristors 132. The parallel circuit is connected on both sides with the input contact 135 and the output contact 134. Out FIG. 4 is also apparent that the switching devices 13 have no preferred operating direction and thus can be used with reversed input and output contact 134, 135. The respective control contacts of the individual elements are guided together or separately from one another to a control device 33.

FIG. 4 shows the switching device 13 with closed bistable switch 131. This will be the case in normal operation only in three of the switching devices 13, one per phase. In the other switching devices 13 is the bistable Switch open and the thyristor pair 132 inactive and thus no current flow possible. It is particularly advantageous in the present switching device 13 that the bistable switch 131 as well as the other elements in normal operation requires power. Likewise, there is a very low volume resistance, since the current is not passed through semiconductor. Overall, this is - as well as a change in voltage - as little power needed as in a transformer of the prior art without voltage regulation.

The control device 33 is configured in this example to receive and evaluate measured values for the voltage. For this purpose, for example, the measured voltages are compared with a nominal voltage value of 230 V or 400 V. If a clear deviation is detected, then a change in the voltage level on one of the phases in the local network is necessary. Then the gear ratio is switched. It is advantageous that this can take place specifically for the phase or phases in which the change is necessary, independently of the other phases. The switchover takes place without interruption, by first making a power line via thyristor pairs 132. The concept of switching is described in the DE 10 2008 064 487 A1 ,

Claims (5)

1. Local grid transformer (10) for the local grid voltage supply, having:

   - an active part (12) having primary and secondary windings (311, 312, 313) for converting a primary medium voltage to a secondary local grid voltage, wherein the active part (12) is arranged in a transformer tank (11) having cooling and insulating liquid,

   - a plurality of switching devices (13) for adjusting the ratio between the primary voltage and the secondary voltage, wherein the switching devices (13) are connected on the secondary side between taps (32) of the secondary windings (311, 312, 313) and the star point of the secondary windings (311, 312, 313) and wherein the switching devices (13) are arranged in the transformer tank (11) in the region of the cooling and insulating liquid and wherein the switching devices (13) each comprise a bistable mechanical switch (131).
2. Local grid transformer (10) according to Claim 1, in which the switching devices (13) each have a parallel circuit composed of the controllable bistable mechanical switch (131) and at least one controllable semiconductor switch (132).
3. Local grid transformer (10) according to either of the preceding claims, in which the switching devices (13) are arranged in the transformer tank (11) below the windings of the active part (12).
4. Local grid transformer (10) according to one of the preceding claims, in which a control device (33) for detecting the voltage of at least one of the phases and for controlling the switching devices (13) based on the detected voltage is comprised, wherein the control is effected in such a way that a deviation of voltage values arising in the local grid from at least one prescribable setpoint voltage value for the local grid is reduced.
5. Local grid transformer (10) according to Claim 4, in which the control device (33) is designed to detect the current in at least one phase, in particular in all of the phases.

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