Method for adjusting voltage target value for adjustment of adjustable transformer, device for adjusting voltage target value for adjustment of adjustable transformer

Abstract

The invention relates to a method for adjusting a voltage target value (Usoll) for the voltage regulation of a variable transformer (200) by means of an on-load tap changer (10), said method comprising the following steps: determining a reverse power flow on the low-voltage side of the variable transformer (200) by measuring a current (I20) and a voltage (U20); operating the on-load tap changer (10) from the current tap position n to a further tap position (n-1, n + 1) and measuring a voltage (U21, U19) and a current (I21, I19) in the further tap position (n-1, n + 1); determining a value m from the powers (L19, L20, L21) of the different tapping points (n, n-1, n + 1); the determined value m is used as the slope of the first section of the straight line for the voltage target value (Usoll).

Description

Technical Field

[0001] The invention relates to a method for setting a voltage target value for regulating a variable transformer by means of an on-load tap changer.

[0002] Furthermore, the invention relates to a device for adjusting a voltage target value for regulating a variable transformer. Background Art

[0003] Usually, active power is consumed in the grid by electrical consumers on the low-voltage side of a variable transformer. In order to keep the voltage constant when the consumption increases or decreases, voltage regulation is performed using on-load tap changers. However, since more and more generators are to be found on the low-voltage side, negative active power flows can occur. For this purpose, new regulation concepts are required. Summary of the invention

[0004] The object of the present invention is therefore to provide a method for adjusting a voltage target value for regulating a variable transformer by means of an on-load tap changer, by which the voltage at the end consumer does not deviate from a predetermined voltage band and additionally does not exceed the maximum load of the system, which corresponds to the current through the line.

[0005] This object is achieved by a method for setting a voltage target value for voltage regulation of a variable transformer by means of an on-load tap changer, wherein the method comprises the following steps:

[0006] determining reverse power flow on the low voltage side of the variable transformer by measuring current and voltage;

[0007] operating the on-load tap changer from a current tap position to a further tap position and measuring a voltage and a current in the further tap position;

[0008] The value m is determined by the power of different tap positions;

[0009] The determined value m is used as the slope of the first section of the straight line for the voltage target value.

[0010] The method can adjust the voltage target value in a simple manner, particularly efficiently and quickly, in the case of reverse power flow, thereby preventing overvoltage at the electrical appliance in the power grid, and the voltage target value is used for the regulation of the on-load tap changer in the variable transformer. In addition, the method guarantees the maximum feed-in power. The fixedly set voltage target value cannot react to changes in the power grid on the low-voltage side. What the method can achieve is that, for example, PV facilities can be simply connected in the power grid without having a negative impact on the power grid. The voltage target value under the new given conditions of the power grid is then adjusted accordingly by the method. For this purpose, the power (apparent power and / or active power) at different tap positions is determined after determining the reverse power flow. When the on-load tap changer moves to different tap positions, the power is based on the measured current and voltage on the low-voltage side of the variable transformer. The value m of the slope of the straight line representing the voltage target value is then depicted by the quotient of the power after it is determined.

[0011] The power can be determined in any desired manner, for example as apparent power and / or active power.

[0012] The reverse power flow can be determined in any manner, for example by measuring the current and voltage on the low-voltage side of the variable transformer. In particular, when a reverse power flow is detected, active current flows from the consumers and generators, ie from the low-voltage side, via the variable transformer to the high-voltage side.

[0013] The on-load tap changer can be designed in any desired manner and can be, for example, an on-load tap changer with a load switch and a selector or a load selector, and can also have mechanical switching elements, such as contacts and vacuum switching tubes, or semiconductor switching elements. The on-load tap changer can be operated by a motor drive or by electronic control of the semiconductor switching elements.

[0014] The power can be determined in any desired manner, for example as the product of the measured current and the measured voltage. In this case, the apparent power or the active power can be determined.

[0015] The value m of the slope of the first section of the straight line for the voltage target value is determined as a quotient formed from the powers of the different tap positions of the on-load tap changer.

[0016] The method can be carried out in any desired manner, wherein the value m, after its determination, is used as the slope of the straight line for the voltage target value in the first section.

[0017] The method may be performed in any manner, wherein:

[0018] The power in the current tap position is determined as the product of the measured current and the measured voltage in the current tap position;

[0019] Operate the on-load tap-changer and move from the current tap position to a higher tap position;

[0020] determining the power of the higher tap position as the product of the measured current and the measured voltage of the higher tap position;

[0021] The value m is determined as the quotient formed from the power of the higher tap position and the power of the lower tap position.

[0022] After it has been determined, the value m is then used as the slope of the straight line for the voltage target value in the first section and thus replaces the previous first section of the straight line.

[0023] The method may be performed in any manner, wherein:

[0024] determining the power in the current tap position as the product of the measured current and the measured voltage in the current tap position;

[0025] Operate the on-load tapchanger and move it from the current tap position n to a lower tap position;

[0026] determining the power of the lower tap position as a product of the measured current and the measured voltage of the lower tap position;

[0027] The value m is determined as the quotient of the power of the higher tap position n and the power of the lower tap position n-1.

[0028] The method may be performed in any manner, wherein:

[0029] Operate the on-load tap-changer and move from the current tap position to a higher tap position;

[0030] determining the power of the higher tap position as the product of the measured current and the measured voltage of the higher tap position;

[0031] Secondary operation of the on-load tapchanger and moving from the current tap position to a lower tap position;

[0032] determining the power of the lower tap position as a product of the measured current and the measured voltage of the lower tap position;

[0033] The value m is determined as the quotient formed from the power of the higher tap position and the power of the lower tap position.

[0034] The method may be performed in any manner, wherein:

[0035] Operate the on-load tap-changer and move from the current tap position to a lower tap position;

[0036] determining the power of the lower tap position as a product of the measured current and the measured voltage of the lower tap position;

[0037] Secondary operation of the on-load tapchanger and moving from the current tap position to a higher tap position;

[0038] determining the power of the higher tap position as the product of the measured current and the measured voltage of the higher tap position;

[0039] The value m is determined as the quotient formed from the power of the higher tap position and the power of the lower tap position.

[0040] When determining the voltage target value, the on-load tap changer is always switched from the current tap position to the adjacent tap position. The voltage and current are measured in the respective tap positions. The quotient formed by the power of the higher tap position and the power of the lower tap position forms the slope of the straight line that plots the voltage target value. Higher and lower tap positions here mean that the value of the higher tap position of the on-load tap changer is higher than the value of the lower tap position.

[0041] In addition, it is possible to switch from the current tap position to a higher tap position. Then switch twice to a lower tap position. When moving to the tap position, the voltage and current are measured accordingly. The value m for the slope of the voltage target value thus becomes more accurate. The slope triangle becomes larger and the value m thus determined becomes more accurate.

[0042] Furthermore, the object of the present invention is to provide a device for adjusting a voltage target value for voltage regulation of a variable transformer, comprising:

[0043] at least one measuring device for measuring current and voltage on the low voltage side of the variable transformer;

[0044] A regulating device is connected to the at least one measuring device for receiving the measured current and voltage.

[0045] The measuring device can be constructed in any manner, for example, the measuring device can have a current sensor and a voltage sensor. These sensors are connected to the regulating device via a cable or without a cable. The device is set and constructed to perform the improved method described above for adjusting the voltage target value for voltage regulation of a variable transformer by means of an on-load tap changer, and in particular to detect the measured current and voltage, determine the reverse power flow, calculate the corresponding power, control the drive device of the on-load tap changer, in order to move the on-load tap changer to different tap positions, determine the value of the slope of the straight line for the voltage target value, and change the straight line of the voltage target value accordingly and store the straight line.

[0046] The device can be designed in any desired manner, wherein the variable transformer is a linear regulator with variable impedance, in particular a high-voltage transformer. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] In the following, the invention will be explained in detail according to exemplary embodiments with reference to the accompanying drawings. Elements that are identical or functionally identical or have the same effect may be provided with the same reference numerals. Identical elements or elements with the same function may only be annotated in the figure in which they first appear. The annotations are not necessarily repeated in subsequent figures.

[0048] Figure 1 The power supply system is shown;

[0049] Figure 2 A circuit diagram showing an idealized electrical network;

[0050] Figure 3 is a diagram used to visualize voltage regulation. DETAILED DESCRIPTION

[0051] Figure 1 The power supply system 100 including a variable transformer 200 is shown, the variable transformer having a plurality of primary windings 300 and a plurality of secondary windings 400, the plurality of primary windings and the plurality of secondary windings being inductively coupled. An on-load tap changer 10 is coupled to each primary winding. The primary winding 300 has a plurality of taps. The on-load tap changer 10 is connected to the primary winding 300 via the taps. The on-load tap changer 10 is designed to connect the taps and thus to adjust the variable transformer 200. The motor drive 11 operates the on-load tap changer 10, whereby the taps are connected for adjusting the variable transformer 200. In addition, a device 20 for voltage regulation is provided. The device 20 has a regulating device 21, which is connected to the motor drive 11 and the measuring device 15. The regulating device 21 is constructed and arranged to control the motor drive 11 and thus control the operation of the on-load tap changer 10, thereby adjusting the variable transformer 200.

[0052] The variable transformer 200 is connected to a high voltage grid on its first side 30, i.e., the high voltage side. In addition, the variable transformer 200 is connected to a low voltage grid on its second side 40, i.e., the low voltage side. For example, 110 kV is applied to the high voltage side and 20 kV is applied to the low voltage side. The grid is preferably a three-phase grid. Usually, the voltage of the first grid is converted into a lower voltage of the second grid by means of the variable transformer 200. The variable transformer 200 is preferably configured as a linear regulator or a high voltage transformer with variable impedance.

[0053] The device 20 is provided for voltage regulation with a regulating device 21 on the variable transformer 200. The regulating device 21 can be arranged directly on the transformer housing or separately in a control room.

[0054] On the second side 40, i.e., on the low-voltage side, in addition to the electrical consumers, generators are also connected, so the voltage on this side or in the power grid may fluctuate. The voltage fluctuations on the second side 40 can be equalized by operating the on-load tap changer 10 on the first side 30 of the first power grid (high-voltage power grid). For this purpose, at least one measuring device 15 is arranged on the low-voltage side, which measures voltage and current changes. The measuring device specifically refers to at least one current sensor and at least one voltage sensor, which are arranged on at least one conductor 16 of the second power grid (low-voltage power grid). The measuring device 15 transmits the measured voltage and the measured current and transmits them to the regulating device 21. In addition, the measuring device 15 can also be arranged on the high-voltage side, i.e., on the first side 30.

[0055] Based on the transmitted current and voltage, a voltage regulation takes place and the on-load tap changer 10 is therefore actuated in particular according to the method for voltage regulation by the motor drive 11 .

[0056] The device 20 having the regulating device 21 has means or is designed and provided for additionally carrying out the method according to the invention.

[0057] Figure 2 The circuit diagram of an idealized power grid 41 with a power supply system 100 is shown. A variable transformer 200 is integrated into the power grid. In addition, in the circuit diagram of the power grid 41, conductors with conductor impedance 45, electrical consumers with electrical consumer impedance 46, a high-voltage power grid with a grid impedance 43, and a generator 47 are marked. The generator 47 represents all elements (generators) that feed power into the power grid without extracting power. This can be, for example, a photovoltaic system, a wind power system, and therefore generally a renewable energy generator.

[0058] The conductor impedance 45 represents the impedance of the entire conductor (or high voltage conductor or cable).

[0059] The grid impedance 43 represents the impedance of the higher-level grid (high-voltage grid) at the connection point of the variable transformer.

[0060] The variable transformer 200 represents an adjustable linear impedance.

[0061] Since both consumers and generators are connected to the grid 41, it is possible that energy is not only extracted from the high-voltage grid by the consumers but also fed in by the generators. Extraction here means that power (active power) is consumed primarily by the consumers on the second side 40, i.e. on the low-voltage side. This is, for example, a so-called forward power flow (FPF).

[0062] During the feed-in, power (active power) is supplied from source 47 to grid 41 via a PV system or the like on second side 40. In other words, active power flows from the consumer side, i.e. second side 40, to the superordinate grid or power is transferred from the low-voltage side to the high-voltage side. This is the so-called reverse power flow (RPF).

[0063] When determining whether there is RPF or FPF, the voltage U20 between points A and B of the power grid is measured. Point A is located between the adjustable transformer 200 and the conductor impedance 45, the electrical impedance 46 and the generator 47. Point B is after the conductor impedance 45, the electrical impedance 46 and the generator 47. In addition, the current I20 is measured directly at point A. All measurements and especially the measurement at point A are performed with the aid of the measuring device 15. In FPF, the current I20 (active current) flows from the high-voltage power grid to the electrical consumer via the adjustable transformer 200 and the conductor. In RPF, the current (active current) I20 flows from the combination formed by the electrical consumer and the generator via the conductor through the adjustable transformer 200 into the high-voltage power grid. When measuring the RPF at points A and B, the sign of the measured power (active power) is negative. When measuring the FPF at points A and B, the sign of the measured power (active power) is positive. Therefore, it can be determined whether there is a reverse power flow or a forward power flow based on the sign.

[0064] Figure 3 A diagram is shown for visualizing the voltage regulation on the variable transformer 200. The power (active power) P is shown on the X-axis, which is drawn from the variable transformer 200 or fed into or supplied to the variable transformer. Drawing here means that power (active power) is consumed mainly by electrical consumers on the second side 40, i.e. on the low-voltage side. This is the so-called forward power flow (FPF).

[0065] During the feed-in, power (active power) is supplied to the power supply system 100 via a PV system or the like on the second side 40. In other words, power (active power) is transmitted from the consumer side, i.e. the second side 40, to the grid, or from the low-voltage side to the high-voltage side. This involves so-called reverse power flow (RPF).

[0066] The zero point of the X-axis is shown in the middle of the diagram in order to be able to depict the corresponding power state of the power grid, i.e. FPF or RPF. For this purpose, the diagram can be divided into a first region present in FPF and a second region present in RPF. In FPF, the power measured on the low voltage side is shown as positive. In RPF, the measured power is provided with a negative sign.

[0067] The reference voltage Uref is shown on the Y axis. For example, a reference voltage of 100 V is predetermined, wherein the reference voltage can deviate by 10%, that is, between 90 V and 110 V. Alternatively, any voltage value can be shown here. In principle, the reference voltage Uref directly or indirectly depicts the measured voltage on the second side 40 in the low-voltage network.

[0068] In the diagram, a straight line 50 is recorded, which is used as a target value for the device 20 for voltage regulation; the straight line 50 depicts the voltage target value Usoll. During operation, the voltage Uist on the second side 40 of the variable transformer 200 is permanently monitored by means of at least one measuring device 15. The measured value of the voltage Uist is recorded in the diagram. Depending on the position of the measured value in the diagram, the on-load tap changer 10 is operated by the motor drive 11 until the measured actual value of the voltage Uist is at the voltage target value Usoll represented by the straight line 50 or is near the voltage target value Usoll. Here, a voltage band or tolerance range is predetermined around the voltage target value Usoll, that is, around the straight line 50, in which the actual value of the voltage Uist should be located without operation.

[0069] The straight line 50 of the voltage target value Utarget is divided into a first section and a second section and may have a different slope in each section.

[0070] In the area of ​​FPF, i.e. when power is extracted by the load, the straight line 50 has a defined predetermined slope in the second section 50.2 of the set voltage target value Uset. This slope is dependent on certain grid parameters. These parameters are predetermined by the transmission line (Rline, Lline) and the load (Rload, Lload). These parameters can be easily determined and therefore stored in the device 20 before being put into operation.

[0071] This ensures that the voltage on the consumer side remains within a predetermined range despite the increased load. Typically, the voltage is between 360V and 440V.

[0072] In the area of ​​RPF, i.e. when feeding in power on the low voltage side, the straight line 50 has a different slope in the first section 50.1 of the voltage target value Uset in this section of regulation, which slope is different from the slope in the area of ​​FPF. This slope can also be determined before commissioning based on the grid parameters. However, for this purpose, the parameters for the transmission line and the generator must be known. However, these parameters usually do not exist or may change over time.

[0073] However, the method according to the invention allows the slope of the straight line for the voltage target value Uset to be adjusted dynamically. This is symbolized by the double arrow 50.3. The grid parameters do not need to be predetermined in this case.

[0074] An exemplary method sequence is described below for setting a voltage target value Uset for voltage control of a tapped transformer 200 by means of the on-load tap changer 10 .

[0075] In the first step, the power flow is determined. Specifically, it is determined whether there is a reverse power flow or a forward power flow. For this purpose, the direction of the current I20 or the sign of the active current and the voltage U20 are determined and it is concluded whether there is a reverse power flow or a forward power flow. The current I20 and the voltage U20 measured at point A of the power grid are detected by the measuring device 15 and determined in the device 20. In addition, the power L20 is determined as the product formed by the measured voltage U20 and the current I20. The power can be related to the apparent power or the active power.

[0076] If the active power resulting from the voltage U20 and the current I20 is negative (has a negative sign), ie the active current at point A flows from the generator via the transformer 200 into the grid, there is a reverse power flow.

[0077] If the active power resulting from the voltage and the current is positive, ie the active current at point A flows from the grid via the transformer 200 to the load, there is a positive power flow.

[0078] In a further step, the on-load tap changer 10 in the variable transformer 200 is operated so that the on-load tap changer moves from the current initial tap position n to the next higher tap position n+1 or alternatively to the next lower tap position n-1. If the connection is made to the higher tap position, a first current I21 and a first voltage U21 and a power L21 resulting therefrom are determined. However, if the connection is made to the next lower tap position n-1, a second current I19 and a second voltage U19 and a power L19 resulting therefrom are determined.

[0079] In the next step, the on-load tap changer 10 is connected to the original tap position n.

[0080] In the next step, the value m is determined from the measured powers in the tap position moved to and in the current tap position. The value m is always formed as a quotient of the power of the higher tap position and the power of the lower tap position. Higher and lower tap positions here mean that the value of the higher tap position of the on-load tap changer is higher than the value of the lower tap position. In the previous case of switching to higher, the value m is the quotient of the power L21 of the higher tap position n+1 and the power L20 of the current tap position, or in the case of switching to lower or moving to a lower tap position or tap changer position, the value m is the quotient of the power L20 of the current tap position n and the power L19 of the lower tap position n-1.

[0081] In the next step, the determined value m is used as the slope of the straight line 50 for the voltage target value Utarget in the first section 50 . 1 of the RPF. The new straight line section forms the voltage target value Utarget and is used for regulating the on-load tap changer 10 .

[0082] The equation of the straight line for the voltage target value Usoll is then y=m*x. The voltage target value Usoll is determined by means of the device 20 for voltage regulation and in particular the regulating device 21. The device 20 is thus used not only for voltage regulation of the variable transformer 200 but also for adjusting the voltage target value Usoll. The adjustment can be performed as often as desired, in particular when weather conditions change. For example, in the case of clouds over a large PV installation; in the case of a changing feed-in power, that is, a reverse power flow with a high gradient, or when a grid configuration is started or shut down.

[0083] A further exemplary method sequence is described below for setting a voltage target value Uset for voltage control of a tapped transformer 200 by means of the on-load tap changer 10 .

[0084] In the first step, the power flow is also determined. Specifically, it is determined whether there is a reverse power flow or a forward power flow. For this purpose, the direction of the current (active current) I20 and the voltage U20 are determined and it is concluded whether there is a reverse power flow or a forward power flow. The current I20 and the voltage U20 measured at the point A of the power grid are detected by the measuring device 15 and determined in the device 20.

[0085] If the active power resulting from voltage U20 and current I20 is negative (or has a negative sign), ie the current at point A flows from the generator via transformer 200 into the grid, there is a reverse power flow.

[0086] If the power resulting from the voltage and the current is positive, ie the current at point A flows from the grid via the transformer 200 to the load, there is a positive power flow.

[0087] In a further step, the on-load tap changer 10 in the variable transformer 200 is operated so that the on-load tap changer moves from the current initial tap position n to the next higher tap position n+1. In this process, the first current I21 and the first voltage U21 and the power L21 resulting therefrom are determined. The power can be the apparent power or the active power.

[0088] In a further step, the on-load tap changer 10 in the variable transformer 200 is operated so that the on-load tap changer moves downward twice from the current initial tap position n+1 to the lower tap position n-1. The second current I19 and the second voltage U19 and the power L19 resulting therefrom are determined. The power can be the apparent power or the active power.

[0089] In the next step, the on-load tap changer 10 is connected to the original tap position n.

[0090] In the next step, the value m is determined from the measured power L21 in the tap position n+1 moved to and the measured power L19 in the second tap position n-1 moved to. The value m is always formed as a quotient of the power of the higher tap position and the power of the lower tap position. Higher and lower tap positions here mean that the value of the higher tap position of the on-load tap changer is higher than the value of the lower tap position. In this case, the value m is the quotient of the power L21 of the higher tap position n+1 and the power L19 of the tap position n-1.

[0091] In the next step, the determined value m is used as the slope of the straight line 50 for the voltage target value Uset in the first section 50.1 of the RPF. This new straight line forms the voltage target value Uset. The equation for the straight line for the voltage target is then y=m*x. The voltage target value Uset is determined by means of the device 20 for voltage regulation and in particular the regulating device 21. Thus, the device 20 is not only used for voltage regulation of the variable transformer 200 but can also be used for adjusting, changing and storing the voltage target value Uset. This adjustment can be performed as often as desired.

[0092] Alternatively, first a connection can be made once downwards to tap position n-1 and twice upwards to tap position n+1. Compared to the previously described method, the quotient formed from the powers of tap positions that are farther apart from one another is more accurate than the quotient formed from the powers of tap positions that are closer together.

[0093] The device 20 having the regulating device 21 has means or is designed and arranged to carry out the method described above.

[0094] List of reference numerals:

[0095] 10On-load tap changer

[0096] 11Motor drive device

[0097] 15. Measuring device

[0098] 16 wires

[0099] 20 devices

[0100] 21 Adjustment device

[0101] First side of 30200

[0102] Second side of 40200

[0103] 41 Power Grid

[0104] 43 Grid impedance of high voltage grid

[0105] 45 Wire impedance

[0106] 46 Electrical Impedance

[0107] 47 Power generation device

[0108] 50 Voltage target value Usoll

[0109] The first section of 50.150

[0110] The second section of 50.250

[0111] 50.3 Double Arrow

[0112] 100 power supply system

[0113] 200 adjustable transformer

[0114] 300 primary winding

[0115] 400 secondary winding

Claims

1. A method for setting a voltage target value (Usetll) for voltage regulation of a variable transformer (200) by means of an on-load tap changer (10), wherein: The method comprises the following steps: determining a reverse power flow on the low voltage side of the variable transformer (200) by measuring a current (I20) and a voltage (U20); Operating the on-load tap changer (10) from a current tap position n to another tap position (n-1, n+1), and measuring the voltage (U21, U19) and the current (I21, I19) in the other tap position (n-1, n+1); The value m is determined by the powers (L19, L20, L21) of the different tap positions (n, n-1, n+1); The determined value m is used as the slope of a first section of a straight line ( 50 ) for a voltage target value (Uset).

2. The method according to claim 1, wherein: After it has been determined, the value m is defined as the slope of the straight line (50) in a first section (50.1) of the voltage target value (Uset) and replaces the existing straight line (50) in the first section (50.1).

3. The method according to any one of claims 1 to 2, wherein: determining a power (L20) in a current tap position (n) as a product of a measured current (I20) and a measured voltage (I20) in the current tap position (n); The on-load tap changer (10) is operated and moved from the current tap position (n) to a higher tap position (n+1); determining the power (L21) of the higher tap position (n+1) as the product of the measured current (I21) and the measured voltage (U21) of the higher tap position (n+1); The value m is determined as the quotient formed from the power ( L21 ) of the higher tap position (n+1) and the power ( L20 ) of the lower tap position (n).

4. The method according to any one of claims 1 to 2, wherein: determining a power (L20) in a current tap position (n) as a product of a measured current (I20) and a measured voltage (I20) in the current tap position (n); The on-load tap changer (10) is operated and moved from a current tap position (n) to a lower tap position (n-1); determining the power (L19) of the lower tap position (n-1) as the product of the measured current (I19) and the measured voltage (U19) of the lower tap position (n-1); The value m is determined as the quotient formed from the power (L20) of the higher tap position (n) and the power (L19) of the lower tap position (n-1).

5. The method according to any one of claims 1 to 2, wherein: The on-load tap changer (10) is operated and moved from the current tap position (n) to a higher tap position (n+1); determining the power (L21) of the higher tap position (n+1) as the product of the measured current (I21) and the measured voltage (U21) of the higher tap position (n+1); A second operation of the on-load tapchanger (10) is performed and it moves from the current tap position (n+1) to a lower tap position (n-1); determining the power (L19) of the lower tap position (n-1) as the product of the measured current (I19) and the measured voltage (U19) of the lower tap position (n-1); The value m is determined as the quotient formed from the power ( L21 ) of the higher tap position (n+1) and the power ( L19 ) of the lower tap position (n−1).

6. The method according to any one of claims 1 to 2, wherein: The on-load tap changer (10) is operated and moved from a current tap position (n) to a lower tap position (n-1); determining the power (L19) of the lower tap position (n-1) as the product of the measured current (I19) and the measured voltage (U19) of the lower tap position (n-1); A second operation of the on-load tapchanger (10) is performed and it moves from the current tap position (n-1) to a higher tap position (n+1); determining the power (L21) of the higher tap position (n+1) as the product of the measured current (I21) and the measured voltage (U21) of the higher tap position (n+1); The value m is determined as the quotient formed from the power ( L21 ) of the higher tap position (n+1) and the power ( L19 ) of the lower tap position (n−1).

7. The method according to any one of claims 1 to 6, wherein: The power is apparent power and / or real power.

8. A device (20) for adjusting a voltage target value (Usoll) for voltage regulation of an adjustable transformer (200), comprising: at least one measuring device (15) for measuring current (I19, I20, I21) and voltage (U19, U20, U21) on the low-voltage side of the variable transformer (200); a regulating device (21), which is connected to the at least one measuring device (15) for receiving the measured currents (I19, I20, I21) and voltages (U19, U20, U21); The regulating device (21) is designed to carry out the method according to claims 1 to 6.

9. The device (20) for adjusting a voltage target value (Uset) for voltage regulation of a variable transformer (200) according to claim 8, wherein: The adjustable transformer (200) is an automatic transformer.

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