On-load tap changer for an electrical network

Abstract

The invention relates to a load tap changer (1) with multiple switching positions for an electrical network, comprising switching elements that can be moved between the different switching positions by means of a drive (2), and an electronic control unit (SE) for controlling the drive (2), wherein the control unit (SE) is communicatively connected to a current sensing device (3) for a load current (116b) flowing through the load tap changer (1), and / or wherein the control unit (SE) is communicatively connected to a device (4) for recording a switching history (116d) of the load tap changer (1), and wherein the drive (2) can be actuated by means of the control unit (SE) depending on the load current (116b) detected by the current sensing device (3) and / or the switching history (116d) detected by the device (4).

Description

[0001] The present invention relates to a load tap changer with multiple switching positions for an electrical network, comprising switching elements that can be moved between the different switching positions by means of a drive, and an electronic control for controlling the drive. Load tap changers serve for the uninterrupted switching between multiple winding taps of a transformer, in particular a power transformer. Furthermore, the present invention relates to a method for actuating such a load tap changer.

[0002] On-load tap changers thus enable the alteration of the turns ratio of transformers under load and are well known in various designs from the prior art. According to one embodiment of on-load tap changers known to those skilled in the art, they comprise a load changeover switch and a selector. The load changeover switch is equipped for uninterrupted switching with switching elements, such as vacuum interrupters, and switching resistors, which are arranged in a cylindrical housing filled with oil or gas. The selector is usually located below the housing and consists, for example, of a plurality of rods arranged in a circle. Contacts are arranged on these rods at different levels, serving as connections for a control winding. Inside the selector, two selector arms are attached to a switching column. These arms make contact with the contacts on the rods.The load switch and selector can be connected via a gearbox. Furthermore, the motor drive can be mounted externally on the step-down transformer and connected to the load tap changer via a linkage.

[0003] According to another design of load tap changer, the so-called load selector, which differs structurally from the load tap changers described in the previous paragraph but is equally well-known, the functions of selecting and switching are combined in a single switch. This means that the selector and load switch are not spatially separated but are located in a common oil reservoir. Load tap changers can now also be implemented using semiconductor elements.

[0004] On-load tap changers are primarily used to maintain a constant voltage level in an electrical network. Therefore, in a network with highly fluctuating loads, it may be necessary to operate the on-load tap changer more frequently. This can lead to excessive heating of the on-load tap changer, its drive mechanism, or other components required for its operation, resulting in malfunctions or a significant reduction in the service life of the on-load tap changer and / or the aforementioned components.

[0005] It is therefore an object of the present invention to create a load tap changer that is protected against overload and premature aging.

[0006] This problem is solved by a load tap changer with multiple switching positions for an electrical network, comprising the features of claim 1, and by a method according to claim 9. Advantageous embodiments of the invention are the subject of the associated dependent claims. Further developments, advantages, and possible applications of the invention will also become apparent from the following description of exemplary embodiments and from the figures. All features described and / or illustrated are, individually or in any combination, fundamentally the subject of the invention, irrespective of their inclusion in the claims or their cross-reference. The content of the claims is also incorporated into the description.

[0007] According to a first aspect, the invention relates to a load tap changer with multiple switching positions for an electrical network, comprising switching elements that can be moved between the different switching positions by means of a drive. Furthermore, the load tap changer includes an electronic control unit for controlling the drive.

[0008] According to the invention, the control unit is connected to a current sensing device for the load current flowing through the tap changer. The load current is the electrical current flowing through the transformer and through the tap changer and its components. Alternatively or additionally, the control unit is connected to a device for recording the switching history of the tap changer. The switching history refers to the switching operations in the recent past of the tap changer, that is, over a period in which the switching operations still have an influence on the current temperature of the tap changer and / or its components. This is typically a period of several minutes to one hour. For example, if many switching operations have taken place in the last half hour, this leads to a corresponding heating of the switching elements and / or the switching resistances.Thus, the switching history, i.e., the number and timing of switching operations in the recent past, allows one to deduce the stress level and heat buildup on the load tap changer. Within the scope of the present invention, a switching operation is specifically defined as a changeover of the load tap changer between two switching positions.

[0009] The actuator can be operated by the control unit depending on the load current detected by the current sensing device and / or the switching history recorded by the device. Before a new switching operation, the magnitude of the load current to be switched and the number of recent switching operations are taken into account to evaluate whether the new switching operation would place an excessive load on the tap changer. If the load current is deemed too high, the actuator's operation is blocked for the next future switching operation of the tap changer. The same applies if the switching history shows that too many switching operations have occurred recently, for example, in the last half hour. In this case, another switching operation could damage the tap changer or at least significantly reduce its service life.In both cases, a switching operation can alternatively be enabled. Furthermore, it is possible to enable a switching operation that was initially blocked, for example, for a defined period, only after that defined period has elapsed. In this way, the invention allows for an evaluation of the current operating state of the load tap changer and its components, and can thus effectively counteract overloading of the load tap changer, in particular overheating.

[0010] In an advantageous embodiment of the invention, the load tap changer further comprises at least one temperature sensor, which communicates with the control system, for detecting the temperature of the load tap changer. The drive can then be controlled by the control system based on the temperature of the load tap changer detected by the at least one temperature sensor. The temperature of the load tap changer can be detected at any suitable location that allows conclusions to be drawn about the temperature of the switching elements and / or the switching resistors or other switching-relevant components of the load tap changer, for example, in the insulating medium of an oil- or gas-cooled load tap changer or at the switching resistors of the load tap changer. Advantageously, several temperature sensors can also be provided.In particular, the temperature sensor can include at least one surface acoustic wave (SAW) sensor or similar sensor technology, enabling direct temperature measurement at the switching resistances of the load tap changer. If the load tap changer does not have its own switch housing, the transformer's temperature sensor can also be used for temperature measurement, as the load tap changer and the transformer share a common oil system in this configuration. Temperature measurement allows for direct inference of the load on the switching elements and / or the switching resistances of the load tap changer, enabling or disabling a switching operation accordingly.The verification of the release before an actual actuation of the load tap changer thus takes into account another important parameter by recording the temperature, which provides information about the current load of the load tap changer and its components.

[0011] Preferably, for example, the load tap changer can comprise a selector unit and a load switching unit, wherein at least the load switching unit has a housing with a closing cover. In this case, the temperature sensor is arranged on the closing cover, which is advantageous for detecting a maximum temperature, since the temperature is usually highest under the top cover of the housing. A temperature sensor arranged there is therefore best able to detect a critical operating temperature of the load tap changer.

[0012] It should be noted that the load transfer unit comprises the housing, usually a GRP cylinder, with its cover. Inside the housing is a load transfer insert, which may include switching elements designed, for example, as vacuum tubes. The load transfer insert is an integral part of the load transfer unit. The selector unit is typically attached to the underside of the load transfer unit, but is located outside the housing. The load transfer unit thus consists of the load transfer insert with its switching elements (e.g., vacuum tubes) and the housing with its cover.In the case of a gas-insulated or oil-insulated load tap changer, the housing contains the gas or oil for insulating the load switching unit, and the arrangement of the temperature sensor on the cover of the housing provides very reliable results about the temperature and thus about the load state of the switching elements and / or the switching resistances of the load tap changer.

[0013] In a further advantageous embodiment of the invention, the temperature sensor is configured to detect the temperature of a gas-insulated or oil-insulated tap changer, particularly at at least one switching resistor of the tap changer and / or at a motor winding of the tap changer's drive. For example, the temperature sensor can be a resistance thermometer. In particular, the resistance thermometer can be a Pt100 sensor. Resistance thermometers are ideal for measuring the temperature at critical points such as the switching resistors and motor windings of the tap changer because they are precise and reliable. Optimal placement of the temperature sensors at the switching resistors and / or the motor winding significantly improves the operational reliability, service life, and efficiency of the tap changer.The use of various sensor technologies, depending on the specific requirements (contact measurement, non-contact, wireless), ensures reliable and continuous monitoring of the thermal load of the load tap changer.

[0014] In an advantageous embodiment of the invention, the control unit has a unit for recording each switching operation of the load tap changer, and the device for recording the switching history is communicatively connected to this unit. The unit for recording each switching operation can be a circuit or simply software within the control unit. As already explained above, the switching history refers to the distribution of switching operations in the recent past, for example, in the last half hour or quarter hour. The control unit has a first memory for storing the switching history and a first evaluation unit that determines a minimum time interval for the next switching operation of the load tap changer between two switching operations and / or derives a blocking or enabling signal for actuating the drive from the switching history.In the first case, a minimum time interval for future switching operations is derived from the recent switching history. This ensures that the interval between permissible switching operations is increased if several switching operations have occurred recently, for example, in the last 10 minutes, and it is therefore expected that, for example, the switching elements and / or the switching resistances of the load tap changer are heated. In addition to increasing the minimum time interval between two switching operations, a blocking or enabling signal for the pending switching operation—i.e., for actuating the drive during the pending switching operation—can also be derived.

[0015] Preferably, the controller has a second memory for recording the load current of the tap changer as detected by the current sensing device, and the controller includes a second evaluation unit that determines the minimum time interval for the next switching operation of the tap changer between two switching operations from the load current and / or derives the blocking / enabling signal for actuating the drive. As already described, within the scope of the present invention, a switching operation is understood to be a switching of the tap changer between two switching positions. As already explained above with regard to the switching history, the current load current to be switched is also considered here, and a signal for the minimum time interval between two future switching operations is determined from it.Here, too, an alternative or additional blocking or enabling signal for actuating the drive can be derived, corresponding to the approval or rejection of an upcoming switching operation. If, for example, a higher load current is switched, the switching elements and / or the switching resistors are subjected to greater thermal stress. Therefore, it is advantageous to increase the minimum time interval between two subsequent or future switching operations when dealing with a higher load current, so that the switching elements and / or the switching resistors of the load tap changer do not overheat.

[0016] In an advantageous embodiment of the invention, the control unit comprises a third memory for load tap changer-specific parameters and a third evaluation unit, which evaluates these parameters to determine the minimum time interval for the next switching operation of the load tap changer between two switching operations and / or to enable or disable the actuation of the drive. This feature thus allows not only the load current, temperature, or switching history to be considered for determining the minimum interval between the next switching operations or for enabling or disabling a switching operation, but also other load tap changer-specific parameters, such as gas formation in the insulating oil of the load tap changer or contact abrasion on the switching elements of the load tap changer, or the like.The locking or unlocking of a switching operation, or the setting of a minimum time interval between two future switching operations, can thus be made dependent on a multitude of different parameters of the load tap changer. This makes it possible to always operate the load tap changer, especially considering all its components, in a tolerable load state. The control system can also have a shared memory for the first, second, and third memory locations.

[0017] Preferably, the load tap changer is designed for seamless switching between winding taps of a tap-changer. Accordingly, the load tap changer has at least one selector unit for power-free preselection of a selected winding tap, as well as at least one load switching unit with several switching elements that can be switched or moved by means of the drive for the actual load switching from the previous winding tap to the preselected winding tap. The switching between winding taps can therefore be carried out without interruption, which counteracts the formation of arcs during the switching process. Since the current flows through both winding taps for a brief moment during switching, so-called switching resistors are provided so that the current does not short-circuit during this short time, but rather flows through the switching resistors connected to the switching elements.

[0018] In an advantageous embodiment of the invention, the drive of the load tap changer is formed by a motor-preloaded spring energy storage device, or it incorporates a spring energy storage device. Such a spring energy storage device has the advantage that a very high energy can be released in a very short time with minimal equipment, thus enabling the switching process between two winding taps to take place extremely quickly and reliably, which in turn counteracts overheating of the switching resistors.

[0019] According to a further aspect, the invention also relates to a method for actuating a load tap changer of the type described above for an electrical network. According to the method according to the invention, a load current flowing through the load tap changer is detected by means of a current sensing device. The load current does not have to be detected precisely at the moment the switching elements of the load tap changer change; rather, the load current can also be detected shortly beforehand or continuously. Alternatively or additionally, the switching history of the load tap changer can be recorded. According to the invention, a drive for actuating the switching elements of the load tap changer is then enabled or disabled by the control system depending on the load current detected by the current sensing device and / or the switching history recorded by the device.Regarding the features and advantages of this method, reference is made to the above description of the load tap changer.

[0020] Preferably, the temperature of the load tap changer is also detected, particularly at at least one switching resistor of the load tap changer and / or at a motor winding of a drive of the load tap changer. The drive for actuating the switching elements of the load tap changer is then enabled or disabled by the control system depending on the detected temperature data. Thus, for the authorization of a switching operation, not only the load current and / or the switching history, but also the temperature of the load tap changer and its relevant components are taken into account. In this way, the essential parameters that reflect the current load on the load tap changer can be used for its operation, ensuring that it is always operated within a permissible load range.

[0021] In an advantageous embodiment of the invention, as the load current through the tap changer increases, the minimum time interval between switching operations of the tap changer is extended. A higher load current results in greater heat generation during the switching operation of the tap changer, so that increasing the minimum time interval between future switching operations effectively prevents overheating of the tap changer.

[0022] Preferably, the minimum time interval for the next switching operation of the load tap changer between two switching operations is set or adjusted depending on the detected temperature of the load tap changer, in particular the temperature of the at least one switching resistor of the load tap changer and / or the temperature of the motor winding of the load tap changer drive. Thus, the higher the temperature of the load tap changer, the greater the minimum time intervals between future switching operations.

[0023] According to an advantageous embodiment of the invention, the load current is determined via a current transformer of a step-down transformer connected to the load tap changer. This is technically simple to implement and does not require any new hardware components on the load tap changer.

[0024] In the event that the drive is blocked, the time until the next possible switching operation of the load tap changer is calculated and displayed and / or applied to the blocked switching operation, meaning that the scheduled switching operation is delayed by the corresponding time. In this way, the intended grid adjustment can be carried out by the load tap changer, albeit with a delay, taking into account the current load state of the load tap changer.

[0025] In an advantageous embodiment of the invention, the temperature of the switching elements and / or a switching resistor is derived from the switching history and / or the load current. It is easy to establish a correlation between the switching history and the temperature of the switching elements or the switching resistor, or a correlation between the load current and the temperature of the switching elements or the switching resistor, in just a few tests. From such data, which can be presented, for example, in tabular form, the temperature of the switching elements or the switching resistor can then be directly derived from the switching history or the load current.

[0026] It is obvious to the person skilled in the art that the embodiments of the invention described above, both of a device-related and a process-related nature, can be combined with each other in any way.

[0027] The invention and its advantages are described in more detail below with reference to the accompanying drawings. These show: Figure 1 shows a highly schematic view of an embodiment of a load tap changer according to the invention with several switching positions for an electrical network; and Figure 2 shows a flowchart of a method for operating such a load tap changer.

[0028] For identical or similarly functioning elements of the invention, identical reference numerals are used in the figures. Furthermore, for the sake of clarity, only those reference numerals necessary for describing the respective figure are shown in the individual figures. The invention is also presented in the figures only as a schematic view to illustrate its operation. In particular, the illustrations in the figures serve only to explain the fundamental principle of the invention. For the sake of clarity, not all components of the device are shown.

[0029] Fig. 1Figure 1 shows an exemplary embodiment of a load tap changer 1 according to the invention with multiple switching positions for an electrical network. The load tap changer 1 comprises switching elements that can be moved between the different switching positions by means of a drive 2. In particular, the load tap changer 1 is installed in a transformer tank of a tap transformer 14 and is designed for uninterrupted switching between winding taps of the tap transformer 14. In the illustrated embodiment, the load tap changer 1 has at least one selector unit 7 for power-free preselection of a selected winding tap and at least one load switching unit 8 with several of the switching elements that can be switched / moved by means of a drive 2 for the actual load switching from the previous winding tap to the preselected winding tap.

[0030] Furthermore, the load tap changer 1 comprises a spring energy storage device 15 and a drive 2, in particular a motor drive, arranged, for example, laterally on the transformer tank. The drive 2, in turn, has a housing 2.2 in which a motor 2.1 with at least one motor winding 12 is arranged. The drive 2 is connected to the load tap changer 1 via a drive shaft 20. To perform a switching operation, the motor 2.1 drives the load tap changer 1 via the drive shaft 20. During the switching operation, the selector unit 7 is first actuated, whereby a winding tap to be connected is preselected by a corresponding selector arm. Preferably in parallel, the spring energy storage device 15 is wound up. At a defined point in time, the spring energy storage device 15 is released and abruptly releases the energy previously stored during winding.This energy is used to actuate the load switching unit 8 and, in particular, the switching elements of the load switching unit 8.

[0031] According to the invention, the load tap changer 1 has an electronic control unit SE for controlling the drive 2, which is communicatively connected to a current sensing device 3 for a load current 116b flowing through the load tap changer 1. According to the embodiment shown, the control unit SE is also equipped with a device 4 for recording a switching history 116d (shown in Figure 2The current sensing device 3 and the device 4 for recording a switching history 116d of the load tap changer 1 are, according to the embodiment, provided as part or component of the control unit SE. The drive 2 can be actuated by the control unit SE depending on the load current 116b detected by the current sensing device 3 and / or the switching history 116d detected by the device 4.

[0032] Furthermore, the load tap changer 1, according to the Figure 1 The illustrated embodiment uses several, namely three, temperature sensors 6, 6', 6" connected to the control unit SE for detecting a temperature 116d of the load tap changer 1. For this purpose, the drive 2 can also be controlled by the control unit SE depending on the temperature 116d of the load tap changer 1 detected by the temperature sensors 6, 6', 6".

[0033] The first temperature sensor 6 is preferably arranged near the load tap changer 1, preferably near the load switching unit 8, in particular on the underside of a cover 10 of the housing 9 in which the load switching unit 8 is received. In other words, the load switching unit 8 comprises the housing 9 with a closing cover 10, and the temperature sensor 6 is arranged on the closing cover 10. A further temperature sensor 6' is arranged near or on at least one switching resistor 11 of the load tap changer 1, and a further temperature sensor 6" is arranged near or on the motor winding 12 of the drive 2 of the load tap changer 1.

[0034] Furthermore, the inventive method 100 is carried out by means of the control unit SE, or the load tap changer 1 executes the method 100. One possible actuation of the load tap changer 1 is described in the Figure 2shown. For the first process step 110 of the in Fig. 2 The method shown in 100 is achieved in particular when a command for the next pending switching operation is given with a smaller time interval than is specified for the corresponding type of load tap changer, for example 2 minutes for a gas-insulated load changer.

[0035] If a switching command for a switching operation does not maintain the minimum time interval between two switching operations for the next switching operation of the load tap changer 1, as specified for the corresponding type of load tap changer 1, then the next test step 112 checks whether the switching operation can be carried out or whether it must be blocked. For this purpose, in step 114, the temperature of the load tap changer 1 is first taken into account, which in the simplest case can be detected by at least one temperature sensor 6, 6', 6".

[0036] In the subsequent step 116, various influencing factors or parameters 116a to 116e are considered, which provide information about the current load state of the load tap changer 1. Parameter 116a includes, in particular, a permissible limit temperature of at least one switching resistor 11 of the load tap changer 1, gas formation in the oil, or the temperature of a motor winding 12 of the drive 2 of the load tap changer 1 or other limiting components. Parameter 116b considers the load current or, optionally, the circulating current through the switching resistors 11 at the time of switching. The third parameter, 116c, considers the load duration of the switching resistor 11 and, if applicable, other tap changer-specific parameters. Parameter 116d considers the switching history, e.g.,The last 5 to 30 minutes, or a temperature calculated therefrom, of the switching elements or the switching resistor 11 is taken into account. The final parameter 116e considers the cooling curve 121 of the switching resistor or the motor winding 12, including the properties of the cooling media used.

[0037] In calculation step 118, the temperature rise or temperature swing at the switching resistor 11 or at the motor winding 12 of the drive 2 is determined from the parameters 116b and 116c, in particular calculated.

[0038] In the following calculation step 120, the maximum temperature, hereinafter also referred to as maximum temperature, at the switching resistor 11 or in the motor winding 12 of the drive 2 is calculated from the calculation step 118 as well as the temperature and the switching history according to parameter 116d using the following formula: Temp max = Temp aktuell + Δ T where Current temperature = Maximum temperature ( last circuit ) x cooling factor (from cooling curve f(t)), and Δ T = temperature increase calculated in step 118.

[0039] The calculated maximum temperature therefore refers to the maximum temperature at the last switching operation, taking into account the calculated temperature increase, whereby the cooling since the last switching operation is taken into account according to cooling curve 121 in accordance with parameter 116e.

[0040] In the subsequent decision step 122, it is evaluated whether the maximum temperature calculated in step 120 is lower than the permissible limit temperature of the corresponding type of tap changer 1. If this is the case, the activation of the switching operation, i.e., the actuation of the actuator 2, is enabled in the final release step 124. If this is not the case, i.e., if the expected temperature rise from the current temperature for the next switching operation would exceed the limit temperature of the tap changer 1, e.g., one of the switching elements, the switching resistor 11, or the motor winding 12 of the actuator 2 of the tap changer 1, the execution of the switching operation is blocked in the blocking step 126.

[0041] From blocking step 126, procedure 100 proceeds to calculation step 128, in which either a defined number of seconds is waited before the next planned switching operation is carried out, or a necessary switching interval is calculated. Procedure 100 then branches back to verification step 112, which repeats the verification of the relevant parameters for the execution of the switching operation as described above, in order to determine whether the temperature increase expected from the actual temperature of the load tap changer 1 due to the planned switching operation exceeds the permissible limit temperature.

[0042] This method 100 ensures safe operation of the load tap changer 1 within a permissible load range, taking into account multiple parameters of the load tap changer 1.

[0043] The invention is not limited to the illustrated embodiment, but can instead be varied within the scope of protection of the following patent claims. Reference symbol list:

[0044] 1 Load tap changer 2 Drive 2.1 Motor 2.2 Housing 3 Current sensing device 4 Switching history recording device 6, 6', 6'' Temperature sensors 7 Selector unit 8 Load tap changer unit 9 Housing 10 Cover 11 Overload resistor 12 Motor winding 14 Step transformer 15 Spring energy storage 20 Drive train SE control 100 Implementation form of a method for taking into account the temperature of a load tap changer for carrying out a switching operation 110 First method step when a minimum time interval between two switching operations is not reached 112 Test step 114 Method step for taking the temperature into account 116 Method step for taking into account further load tap changer-specific parameters, corresponding to parameters 16a to 16e 116a First further parameter: Limit temperature at the switching resistor, gas formation in the oil,Motor winding 116b Second additional parameter: Load current and / or circuit current 116c Third additional parameter: Load time of the switching resistor 116d Fourth additional parameter: Temperature and switching history 116e Fifth additional parameter: Cooling curve of switching resistor and motor winding 118 Calculation step to determine the expected temperature rise from parameters 16b and 16c 120 Calculation step to determine a maximum temperature from calculation step 18 and parameter 16d 122 Decision step for enabling or disabling a pending switching operation 124 Execution of the switching operation 126 Disabling of the switching operation 128 Delaying the switching operation or returning to step 16 after a defined time,

Claims

1. Load tap changer (1) with multiple switching positions for an electrical network, comprising switching elements that can be moved between the different switching positions by means of a drive (2), and an electronic control (SE) for controlling the drive (2), - wherein the control (SE) is communicatively connected to a current sensing device (3) for a load current (116b) flowing through the load tap changer (1), and / or - wherein the control (SE) is communicatively connected to a device (4) for recording a switching history (116d) of the load tap changer (1), and wherein the drive (2) can be actuated by means of the control (SE) depending on the load current (116b) detected by the current sensing device (3) and / or the switching history (116d) detected by the device (4).

2. Load tap changer (1) according to claim 1, characterized by the fact thatthe load tap changer (1) further comprises at least one temperature sensor (6, 6', 6'') connected to the control unit (SE) for detecting a temperature (116d) of the load tap changer (1) and the drive (2) can be controlled by the control unit (SE) depending on the temperature (116d) of the load tap changer (1) detected by the at least one temperature sensor (6, 6', 6'').

3. Load tap changer (1) according to claim 2, characterized by the fact that the load tap changer (1) comprises a selector unit (7) and a load switching unit (8), wherein at least the load switching unit (8) comprises a housing (9) with a closing cover (10), and the temperature sensor (6) is arranged on the closing cover (10).

4. Load tap changer (1) according to claim 2 or 3, characterized by the fact thatthe temperature sensor (6', 6'') is designed to detect the temperature (116d) of the load tap changer (1) in the case of a gas-insulated load tap changer (1) or an oil-insulated load tap changer (1), in particular at at least one switching resistor (11) of the load tap changer (1) and / or at a motor winding (12) of the drive (2) of the load tap changer (1).

5. Load tap changer (1) according to one of the preceding claims, characterized by the fact thatthe control (SE) has a unit for recording each switching operation of the load tap changer (1), and the device (4) for recording the switching history (116d) is communicatively connected to the unit, and wherein the control (SE) has a first memory for storing the switching history (116d) and a first evaluation unit which determines from the switching history (116d) a minimum time interval for the next switching operation of the load tap changer (1) between two switching operations and / or derives a block / release signal for actuating the drive (2).

6. Load tap changer (1) according to one of the preceding claims, characterized by the fact thatthe control unit (SE) has a second memory for recording the load current (116b) of the load tap changer (1) detected by the current sensing device (3), and that the control unit (SE) has a second evaluation unit which determines from the load current (116b) the minimum time interval for the next switching of the load tap changer (1) to be carried out between two switching operations and / or derives the blocking / enabling signal for actuating the drive (2).

7. Load tap changer (1) according to one of the preceding claims, characterized by the fact that The control unit (SE) has a third memory for load tap changer-specific parameters and a third evaluation unit, which evaluates these parameters to determine the minimum time interval for the next switching of the load tap changer (1) to be carried out between two switching operations and / or to enable / disable the actuation of the drive (2).

8. Load tap changer (1) according to one of the preceding claims, characterized by the fact that The load tap changer (1) is designed for uninterrupted switching between winding taps of a step-down transformer (14) such that the load tap changer (1) has at least one selector unit (7) for powerless preselection to a selected winding tap and at least one load switching unit (8) with several of the switching elements which can be switched / moved by means of the drive (2) for the actual load switching from the previous winding tap to the preselected winding tap.

9. Method for actuating a load tap changer (1) according to any one of claims 1 to 8 for an electrical network, characterized by the fact thata load current (116b) flowing through the load tap changer (1) is detected by means of a current detection device (3) and / or a switching history (116d) of the load tap changer (1) is / are detected by means of a device (4), and that a drive (2) for actuating the switching elements of the load tap changer (1) is enabled or disabled by means of the control (SE) depending on the load current (116b) detected by the current detection device (3) and / or the switching history (116d) detected by the device (4).

10. Method according to claim 9, characterized by the fact thatthe temperature (116d) of the load tap changer (1), in particular at at least one switching resistor (11) of the load tap changer (1) and / or at a motor winding (12) of a drive (2) of the load tap changer (1), is detected and the drive (2) is enabled or disabled for actuating the switching elements of the load tap changer (1) by means of the control (SE) depending on the detected temperature data.

11. Method according to claim 9 or 10, characterized by the fact that With increasing load current (116b) via the load tap changer (1) a minimum time interval for the next switching operation of the load tap changer (1) between two switching operations is increased.

12. Method according to any one of claims 9 to 11, characterized by the fact thatthe minimum time interval for the next switching operation of the load tap changer (1) between two switching operations is adjusted depending on the detected temperature (116d) of the load tap changer (1), in particular the temperature of the at least one switching resistor (11) of the load tap changer (1) and / or the temperature of the motor winding (12) of the drive (2) of the load tap changer (1).

13. Method according to any one of claims 9 to 12, characterized by the fact that the load current (116b) is determined via a current transformer of a step transformer (14) connected to the load tap changer (1).

14. Method according to any one of claims 9 to 13, characterized by the fact that In the event that the drive (2) is locked, the time until the next possible switching of the load tap changer (1) is calculated and displayed or applied to the locked switching operation.

15. Method according to any one of claims 9 to 14, characterized by the fact that The temperature (116d) of the switching elements and / or a switching resistor (11) is derived from the switching history (116d) and / or the load current (116b).

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