EP2361435B2 - Transformer for transformation between medium and low voltage with a step switch and its operating method - Google Patents

Description

The invention relates to a transformer for the transformation between medium and low voltage with tap-change.

DE 10 102 310 C1 discloses a transformer according to the preamble of claim 1.

Energy distribution networks are subject in particular by the impedances of the network components and by changing loads voltage fluctuations. It is desirable to keep the fluctuations as low as possible. For this purpose, transformers use high-voltage and medium-voltage tap changers. The tap changer compensates for the voltage fluctuations that occur during load changes by changing the transmission ratio. For this purpose, at least one of the windings of the transformer is provided with a series of taps, which can be electrically connected by a selector mechanism. Furthermore, a diverter switch is provided which makes the switching between two selector positions without interruption even under load. Winding short circuit is avoided by briefly forcing current flow through resistors.

It is possible that in the future the energy supply will be more decentralized compared to the current situation. This means that power generation takes place closer to the consumer than today in a larger number of smaller systems. Such systems are for example photovoltaic systems, wind power plants and biomass power plants or smaller cogeneration units. Smaller power plants are at least in principle very advantageous because of the more feasible combined heat and power. If the electricity generated can not be taken directly, it is necessary to feed it from the low-voltage grid into the medium-voltage grid in order to enable low-loss transmission over long distances. For this purpose, it is necessary to provide a variable transmission ratio in the distribution transformer.

Object of the present invention is to provide a transformer for the transformation between medium and low voltage with tap changer, which is particularly simple. Another object of the invention is to provide an operating method for such a transformer.

This object is achieved by a transformer having the features of claim 1. Advantageous embodiments and further developments are specified in the dependent claims. The object is also achieved by a method having the features of claim 7.

The transformer according to the invention has a tap changer. In this case, one of the windings of the transformer, preferably the low-voltage side secondary winding, two end taps and at least two center taps. Furthermore, at least one switching device is provided for the switchable electrical connection of at least one of the taps with an output line of the transformer. Finally, at least one semiconductor switching device is provided, which is electrically connected to the output line and directly to one of the end taps.

Preferably, a first of the end taps is directly connected to a first output line of the transformer and is not further changed or used in a special way for the tap changer. The second end tapping at the other end of the winding, however, is used together with the one or more taps for the tap changer and the taps are for this purpose connected in a more complex manner with the second output line of the transformer.

The switching device according to the invention comprises mechanical switches, which advantageously have a particularly low on-resistance, and particularly preferably allows independent connection and disconnection of individual taps.

In this case, the switching device expediently alternately connects one of the taps to the second output line of the transformer.

The transformer according to the invention advantageously has a simple and low maintenance by the semiconductor switch construction and allows a tap changer without load interruption in the middle to low voltage range.

Suitably, but not necessarily, the tap changer contains a control device that automatically performs a control of the load switching. For this purpose, the control device expedient means, which allow detection when a switch should occur. For example, these may be means for determining voltage and / or current on the input or output side. This determines whether a changeover is necessary, for example by detecting the corresponding slight decrease in the output voltage when the load on the output side is higher. Alternatively, the control of the load switching can be made from outside the tap changer. In this case, the tap changer expedient means on which allow external control. This can be an indirect, for example, digital remote control, which is implemented in the tap changer by a control device in the actual control of the switch. It may also be a direct, analog control from the outside, which if necessary. Even without an internal control device can be done, for example, by directly applying an actuator of the switching element from the outside with electricity.

It is particularly advantageous if the switching device is connected only to the further taps, that is to say it is not connected in negative form to the end taps. It is useful if there are several more taps, so at least two. It is also useful if the semiconductor switching device connected to one of the end taps is. This structure allows a particularly advantageous operation.

In this case, as soon as a switch appears necessary, the semiconductor switching device is turned on. This switching is preferably carried out at the zero crossing of the AC voltage, wherein expediently a turn-on delay of the semiconductor switch or the semiconductor switching device is taken into account, that is, for example, an ignition delay of thyristors. As a result, a voltage jump is avoided.

If the semiconductor switching device is turned on, a short circuit occurs, since there is a direct connection between one of the further taps via the switching element, the semiconductor switching device for final tapping. As a result, a current will build up in this circuit. In order to decelerate the structure of this current, an inductance is preferably provided in series with the semiconductor switching device. Alternatively or additionally, a resistance element may also be provided to limit the current.

Due to the relative arrangement of the semiconductor switching device at the end tapping and the switching element at one of the further taps, the current in the winding short-circuit is opposite to the load current. As a result, therefore, a time is reached at which the current in the winding short-circuit assumes the same amount as the load current and thus as good as no current flows through the switching element. In other words, the load current at this time is completely commutated to the semiconductor switching device.

This time is used to turn off the connection through the switching element. Since in the time in which no or very little current flows through the switching element, and accordingly less voltage drops over it, the shutdown without an arc is possible and therefore particularly gentle for the switching element. Alternatively, the opening of the Switching element can be made before the zero crossing, in particular at a time at which the semiconductor switching device safely already passes. In this case, when opening the switching element, an arc will occur, but in the course of Stromkommutierung very fast, typically in the range of microseconds, extinguished because the current disappears via the switching element yes. Once the arc extinguished, it does not emerge when the voltage across the switching element increases.

It is advantageous if the transformer comprises means for determining a value representing the voltage across the switching element and / or the current through the switching element, since then the time for opening the switching element can be determined directly. This time in the operating method described above, for example, given when the current is just zero. Another possibility is to effect the opening of the switching element when the current or the voltage, in particular the maximum amounts thereof within each period, falls below a certain threshold, which is slightly greater than zero. Another alternative is to set the timing for the opening of the switching element based on a timing in response to the turn-on of the semiconductor switching device, for example, 2 ms after switching on the semiconductor switching device.

After opening by the switching element, the semiconductor switching device carries the load current and the short-circuiting of the winding is canceled.

In the following, the switching element is closed again to make a connection with another of the other taps of the winding. Again, it is advantageous if a suitable time for closing the connection is selected. For this purpose, for example, a time can be selected at which the voltage across the switching element corresponds exactly to the voltage across the semiconductor switching device. It is assumed that by the semiconductor switch due to the semiconductor switching device always a low voltage drops. When closing the switching element, it is advantageous to take into account the closing time, which requires the switching element for producing the electrical contact. Closing at said time makes it possible to avoid voltage jumps. An alternative approach is to cause the switching element to close when the voltage is just passing through a zero crossing due to the line frequency.

After the switching element has again made an electrical connection, the semiconductor switching device can be turned off or depending on the semiconductor switch used the ignition can be canceled.

In one embodiment of the invention, a thyristor circuit is provided as a semiconductor switch. It is advantageous that this is selbstabschaltend and thus allows easy control. The thyristor circuit preferably consists of two anti-parallel connected thyristor elements, wherein each of the thyristor elements consists of a thyristor or a parallel and / or series connection of thyristors. Other electrical components can be used together with the thyristors.

As an alternative to the thyristors, turn-off semiconductor switches can also be used as semiconductor switches, in particular transistors, GTOs (Gate Turn-off Thyristor) or IGCTs (Integrated Gate Commutated Transistor). As a result, an active shutdown of the line is made possible by the semiconductor switch, which in turn shortens the time of the short-circuiting of the winding by the closed switching element and the conductive semiconductor switch.

In a preferred embodiment of the invention, means for determining the current in the region of the switching element or semiconductor switch are provided.

Figur 1

einen ersten Transformator mit durchgehender Sekundär-Wicklung mit Stufenschalter,

Figur 2

ein Ablaufdiagramm für die Stufenschaltung mit dem ersten Transformator,

Preferred, but by no means limiting embodiments of the invention will now be explained in more detail with reference to the drawing. The features are shown schematically and corresponding features are marked with the same reference numerals. The figures show in detail

FIG. 1

a first transformer with continuous secondary winding with tap changer,

FIG. 2

a flow chart for the tap-changer with the first transformer,

The figures refer to embodiments for transformers. These will be carried out expediently in a real implementation three-phase. For a better clarity, the figures but only a single-phase design. For the same reason, the tap changer in the embodiments, only three setting options for the gear ratio, while actually tap changer can often set more than three ratios. The invention is equally applicable with more than three gear ratios. The voltage on the side of the primary windings should be exemplified 10 kV, while on the side of the secondary winding, a voltage 400 V is output.

The FIG. 1 shows a transformer 1 with a tap changer. The transformer 1 has, in addition to a primary winding which is not significant in this exemplary embodiment, a continuous secondary winding. The continuous secondary winding consists of a first to fourth part 17a ... d. The first part 17a comprises approximately 70% of the winding length of the secondary winding, while the second, third and fourth parts 17b ... d each comprise approximately 10% of the winding length. The representation in Fig.1 is not exactly to scale. From the relative proportions of the secondary winding arise the adjustable ratios and it is clear that even very different divisions of the secondary winding are possible. The parts 17a ... d are defined by first, second and third taps 2, 3, 4, wherein the first tap 2 is at 70% of the winding length of the secondary winding, the second tap 3 is at 80% of the winding length of the secondary winding and the third tap 4 is at 90% of the secondary winding , With the beginning of the secondary winding, a first output line 11 of the transformer 1 is connected. A second output line 12 of the transformer 1 is connected in a more complex manner with the taps 2, 3, 4 in order to realize the tap changer.

For the tap changer, a mechanical switch 20 is provided, the middle connection according to FIG. 1 is connected to the second output line 12. The switch 20 can establish a connection between its center connection and a first, second or third connection 13, 14, 15. The first connection 13 connects the tap 2 and one of the connections of the mechanical switch 20. The second connection 14 connects the second tap 3 to a further connection of the switch and the third connection 15 connects the third tap 4 to a last connection of the mechanical switch 20 In this case, the switch 20 is expediently designed so that the separation and production of the connection between the terminals can be done independently of one another, that is to say a plurality of mechanical switching elements together form the switch 20.

Furthermore, there is a fourth connection 18 between the end tap 52 of the secondary winding and the second output line 12, which leads via a thyristor 5, which consists of two antiparallel connected thyristors. The structure of two thyristors is exemplary in this case. Depending on the expected load, one of the thyristors can represent one series connection and / or parallel connection of a plurality of actual thyristor elements. Also, other elements such as IGBTs, GTOs or similar can be used here instead of the thyristors. be used. In series with the thyristor circuit 5, an inductance 53 is provided, which is the delay of the current in the case of a winding short circuit.

At each of the connections 13, 14, 15, 18 and in the region of the central connection of the switch 20, a measuring point 7 ... 10 is provided. Furthermore, a controller 6 is present. The controller 6 can determine the voltage at the measuring points 7... 10 and control the thyristor circuit 5 and the switch 20 on the basis of the determined values.

The sequence of an exemplary load switching with the structure according to FIG. 1 will now be based on FIG. 2 explained. It is assumed in a first step 21 that the mechanical switch 20 establishes an electrical connection between the second output line 12 and the first connection 13. A first current path 26 thus leads from the first output line 11 via the first part 17a of the secondary winding and the first connection 13 to the second output line 12. Thus, about 70% of the secondary winding is used. The thyristors are not ignited.

In a second step 22, a changeover is performed. In this case, the mechanical switch 20 switches between its terminals so that instead of the first tap 2, the second tap 3 is connected to the second output line 12. During the switching, the thyristor circuit 5 takes over the power. This happens without interruption, the exact circuit is shown below, for example. The second current path 27 thus leads during the switching from the first output line 11 over all parts 17a ... d of the secondary winding. Next it leads via the fourth connection 18 and thus the thyristor 5 to the second output line 12. It is thus used the entire secondary winding. Once the mechanical switch 20 has switched, the ignition of the thyristor circuit 5 is terminated.

After switching, the state used in the third step 23 results. This is about 80% of the secondary winding and the third current path 28 leads from the first output line 11 via the first and second part 17a, b of the secondary winding and the second connection 14 to the second output line 12.

In a fourth step 24, a changeover is performed again. In this case, the mechanical switch 20 switches between its terminals so that instead of the second tap 3, the third tap 4 is connected to the second output line 12. During the switching, in turn, the thyristor circuit 5 takes over the power. The fourth current path 29 thus leads during the switching from the first output line 11 over the entire secondary winding. Further, it leads via the fourth connection 18 and thus the thyristor circuit 5 to the second output line 12. As soon as the mechanical switch 20 has switched, the ignition of the thyristor circuit 5 is terminated.

After switching, the state used in the fifth step 25 is obtained. Here, 90% of the secondary winding is used and the current path leads from the first output line 11 via the first, second and third part 17a... C of the secondary winding and the third connection 15 to the second output line 12.

Further switches are carried out analogously. In this case, the mechanical switch 20 does not have to switch between adjoining taps 2, 3, 4, but the switching can take place between any of the taps, that is, for example, directly from the first tapping 2 to the third tapping 4 or vice versa.

The method with which a switchover is made will now be explained in more detail by way of example. It is assumed that the controller 6 determines that switching between two of the taps appears necessary. Thereafter, the controller provides for the ignition of the thyristors in the thyristor circuit 5. The timing of ignition is chosen so that no voltage jump occurs. Ideally, a point in time is used which lies before a zero crossing of the mains voltage for a period of time, the time span corresponding to the ignition delay of the thyristors. This ensures that the thyristors can take over the load current in principle at the zero crossing of the voltage.

When the thyristors are conductive, the connection across the switch 20 and the thyristors produces a short-circuited winding in which part of the secondary winding is contained. A very high current can flow in this short circuit. The speed of use is limited in this embodiment by the inductor 53.

Due to the relative arrangement of the thyristors at the end tap 52 and the switch 20 at one of the other taps 2, 3, 4, the current in the winding short-circuit is opposite to the load current. As a result, therefore, a time is reached at which the current in the winding short-circuit assumes the same amount as the load current and thus no current flows through the switch 20.

This time is used to turn off the connection through the switch. The opening of the switch is preferably already made shortly before the expected zero crossing, in particular at a time at which the thyristors already conduct safely. In this case, when opening the switching element, an arc will occur, but in the course of Stromkommutierung very fast, typically in the range of microseconds, extinguished because the current disappears via the switching element yes. Once the arc extinguished, it no longer arises when the voltage across the switching element increases. For a smooth switchover, it is expedient if the switch 20 for opening the connection is activated such that it only opens when the thyristors are already conducting.

As a result, the thyristors transport the load current and by opening the switch 20, the short-circuiting of the winding is canceled. Closing of the new connection of the switch 20 preferably takes place in a natural zero crossing of the mains voltage, in turn to achieve a smooth transition of the line. Since after the closing of the switch 20 again a indentation short-circuit exists, it is expedient to stop the ignition of the thyristors in time before the zero crossing in order to prevent simultaneous conduction of the thyristors with the switch 20.

Claims (10)

1. Transformer (1) for transformation between medium-voltage and low-voltage having a tapping circuit, wherein:

   - one of the windings (17a, ..., 17d) on the transformer has two end taps (51, 52) and at least two centre taps (2, 3, 4),

   - at least one switching device (20) is provided for switchable electrical connection of one of the centre taps to an output line (12) of the transformer,

   - the switching device (20) has mechanical switches (20), and

   - at least one semiconductor switching apparatus (5) is provided, and is electrically connected to the output line, characterized in that the semiconductor switching apparatus is directly connected to one of the end taps.
2. Transformer (1) according to Claim 1, in which means are provided for determination of a value which represents the voltage across the switching element and/or the current through the switching element.
3. Transformer (1) according to one of the preceding claims, in which means are provided for determination of a value which represents the voltage across the semiconductor switching apparatus.
4. Transformer (1) according to one of the preceding claims, in which the semiconductor switching apparatus (5) has two semiconductor switching elements, in particular two thyristors, which are connected back-to-back in parallel.
5. Transformer (1) according to one of the preceding claims, wherein the semiconductor switching apparatus (5) has semiconductor switching elements which can be turned off, in particular transistors, GTOs or IGCTs.
6. Transformer (1) according to one of the preceding claims, in which an inductance (53) or resistance is provided in series with the semiconductor switching apparatus (5).
7. Method for operation of a transformer (1) for transformation between medium-voltage and low-voltage having a tapping circuit according to one of the preceding claims, which has a semiconductor switching apparatus for temporarily accepting the current during the switching process of a switching device, in which case a first time is defined during a switching process of the switching device, at which the current flow through the switching device actually becomes zero, and the switching device is opened at this time.
8. Method according to Claim 7, in which the semiconductor switching apparatus is switched on shortly before a zero crossing.
9. Method according to Claim 7 or 8, wherein, after the switching device has been opened, a second time is defined, at which the voltage between the intended tap on the switching device and the output line of the transformer corresponds to the voltage across the semiconductor switching apparatus, and the switching device is closed at this second time.
10. Method according to Claim 9, in which a time period required for the switching element to close is taken into account when defining the second time.

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