EP3447602A1 - Method and device for controlling the voltage of a transformer system - Google Patents

Abstract

Provided is a method for controlling a value of a voltage on a conductor (3) to which at least one secondary winding (5A) of a first stepping transformer (7A) and a secondary winding (5B) of a second stepping transformer (7B) are connected Method comprises: if a voltage _deviation (D _V ) of the voltage on the conductor (3) from a voltage _setpoint within a first bandwidth ([-B _{CC_DV} , B _{CC_DV} ]) is around the voltage _setpoint , and if a total deviation (D _A , D _B ) a sum of the voltage deviation (D _V ) and a reactive current deviation (D _CCA , D _CCB ) from the voltage setpoint respectively for the first and the second transformer outside a second, greater than the first, bandwidth ([-B _C , B]) around the voltage setpoint: setting a delay time (T1) to the stages of the first transformer (7A) and / or the second transformer (7B) such that one of the voltage deviation counteracts Staging the first or the second transformer is prioritized.

Description

The present invention relates to a method and apparatus for controlling a value of a voltage on a conductor to which at least one secondary winding of a first step-up transformer and a secondary turn of a second step-down transformer are connected. Furthermore, the present invention relates to a transformer system comprising the device and two gradable transformers.

From the prior art it is known that two or more transformers are connected in parallel to a bus bar in order to divide the power to be supplied between the transformers so that the single transformer is not overloaded. However, if two or more transformers feed on the same busbar and regulate the voltage regulators independently of each other to the setpoint voltage value, balancing currents may occur due to the different open circuit voltages of the transformers. Since the impedance of the transformers is mainly inductive, essentially circular reactive currents can form.

Such circular reactive currents are undesirable because they can lead to increased power dissipation of the transformers. In addition to a voltage deviation from the voltage setpoint, the circulating reactive current has therefore traditionally been used as an additional control criterion of the voltage regulation in order to minimize this.

In a conventional voltage regulator, a voltage band or a bandwidth B is defined around the nominal voltage value. If the measured voltage is outside this bandwidth, a gradation command is given to a tap changer after an adjustable time delay, which is determined by the

Change in the open circuit voltage of the transformer, the voltage returns to the voltage band. The set value of the bandwidth is set at least so large that the bandwidth is not passed through in the case of a higher or lower step around the setpoint. Otherwise it would result in an endless back and forth step. With the voltage regulation for parallel transformers according to the circulating reactive current method, in addition to the voltage deviation at each parallel transformer, the circulating reactive current is determined and a control deviation based on the circulating reactive current is added to the control deviation by the voltage.

• k - einstellbarer Kreisblindstromregelfaktor
• ICC - Kreisblindstrom
• X - Transformatorlängsreaktanz berechnet aus Kurzschlussspannung
• UN - Transformatornennspannung

The control deviation DCC resulting from the circulating reactive current can generally be calculated according to the following formula: $$\mathrm{DCC}=\left(\mathrm{k}*\mathrm{ICC}*\mathrm{X}*\mathrm{<figure-callout\; id="3"\; label="root"\; filenames="imgf0001.png"\; state="\{\{state\}\}">root</figure-callout>}\left(\mathrm{3}\right)*\mathrm{100}\%\right)/\mathrm{U.N.}$$

• k - adjustable circulating reactive current control factor
• ICC - Circulating reactive current
• X - transformer longitudinal reactance calculated from short-circuit voltage
• UN transformer rated voltage

From the sum of the voltage deviation and the circulating reactive current deviation, the total deviation D is determined and compared with the bandwidth: $$\mathrm{D}=\mathrm{DV}+\mathrm{DCC}$$

In the case of two parallel transformers, the difference between the no-load voltages results in exactly the opposite circulating reactive current and thus exactly the opposite circulating reactive-field deviation D _CC . If the measured voltage at both transformers corresponds to the setpoint voltage, then exactly the opposite total deviation D results for both voltage regulators.

By way of example, a first transformer A has a higher stage and thus a higher open-circuit voltage than a second transformer B. Thus, the circulating reactive-current voltage deviation D _CC positive. In this example, according to a conventional control method, both regulators would step to control the transformers because in one of the transformers the reactive current deviation is greater than the threshold of the bandwidth and in the other transformer the reactive current deviation is less than the negative of the threshold of the bandwidth.

After an opposite step of both controllers, the identical case would occur with reversed roles. The prior art regulators would subsequently infinitely scale. The control thus conventionally does not work stable and thus may be connected to unnecessary wear of the tap changer. The controls would subsequently increment endlessly. Here additional measures would be necessary to avoid unnecessary steps.

Traditionally, voltage regulators with two different timing characteristics have also been used. Regarding the linear time characteristic, a constant control deviation is controlled independently of the control deviation. In the case of the inverse time characteristic, the delay time is inversely proportional to the control deviation D. The setting of the delay time for voltage deviations is predetermined by the network operator, so that higher-level coordination is possible with radial networks. Thus a fixed different parameterization is not desired. In addition, in the event of a voltage drop due to load connection, both controllers should switch to the same delay time.

In the prior art, a voltage regulator has been proposed, in which one adjusts a controller for the circulating reactive current sensitive than the other. However, this has the disadvantage that in the case of a voltage deviation within the voltage band, no control deviation results in the sensitive controller, since D _V and D _CC cancel each other out and the bandwidth B in the insensitive controller D does not exceed. It can thus result in a poor quality control and the circulating reactive current is not regulated satisfactorily.

Thus, it is an object of the present invention to provide a method and apparatus for controlling a value of a voltage on a conductor to which at least one secondary winding of a first stepping transformer and a secondary winding of a second stepping transformer are connected, with reliable control being achieved , whereby wear of components, in particular wear of step switches of transformers is reduced.

The object is solved by the subject matters of the independent claims. The dependent claims specify particular embodiments of the present invention.

Embodiments of the present invention employ temporal stage prioritization to stabilize voltage regulation. The stabilization of the regulator is used in terms of the temporal stage prioritization, if the voltage deviation is within a certain range of the bandwidth. This particular range of bandwidth is also referred to below as a prioritization bandwidth, the prioritization bandwidth being smaller than the bandwidth traditionally used.

According to one embodiment of the present invention, there is provided a method of controlling a value of a voltage on a conductor to which at least one secondary winding of a first stepping transformer and a secondary winding of a second stepping transformer are connected, the method comprising: if a voltage deviation of the voltage the conductor is of a voltage setpoint within a first bandwidth around the voltage setpoint, and if a total deviation of a sum of the voltage deviation and a reactive current deviation from the voltage setpoint is respectively for the first and the second Transformer outside a second, greater than the first, bandwidth around the voltage setpoint: Setting a delay time to the stages of the first transformer and / or the second transformer such that a voltage deviation counteracting grading of the first or the second transformer is prioritized.

The method may be partially implemented in hardware and / or software, in particular implemented by a computer method. The conductor to which the secondary windings of the first and second transformers are connected is also referred to as a busbar.

The transformers may include tap changers of any known type. By actuating the tap changer, an open circuit voltage of the respective transformer can be set. In this case, a grading on a primary winding or the secondary winding can take place. A grading made at the primary stage may be advantageous because, due to the higher voltage at the primary winding, the current (especially in load operation) may be less than a current flowing through the secondary coil.

For controlling the target voltage value on the busbar (which is electrically connected to corresponding secondary windings of the transformers), step-type transformers are used, in which the primary winding and / or the secondary winding can be tapped at different points. For stages of transformers so-called tap changer can be used, which serve to adjust the transmission ratio. For this purpose, the winding or the coil of the transformer on the primary side and / or the secondary side have a main winding and a control or step winding with a plurality of taps, which are guided to the tap changer. Among the tap changers are so-called on-load tap changer ( OLTC) and diverter (NLTC no load tape changer, DETC deenergized tap changer or OCTC, off circuit tap changer ). known. On-load tap-changers can be used for no-load switching and can be subdivided into load selectors and load switches. In this case, a tap changer column either one or more phases, for example, switch three phases.

In order to measure the voltage deviation and the reactive current, a measuring device can be provided which measures both the current at the secondary windings and in each case the current flowing from the secondary windings to the conductor. Both the voltage deviation and the reactive current deviation may be e.g. is specified or defined as a fraction or percentage of the difference to the voltage setpoint or a reactive current setpoint of zero.

$$\mathrm{TermA}=\mathrm{X}/\left(\mathrm{X}*\mathrm{BP}-\mathrm{1}\right)$$

• k - einstellbare Kreisblindstromregelfaktor
• ICC - Kreisblindstrom
• X - Transformatorlängsreaktanz berechnet aus Kurzschlussspannung
• BP - Gesamtblindleitwert bzw. Gesamtsuszeptanz aller parallelen Transformatoren
• UN - Transformatornennspannung

An embodiment of the present invention uses an optimized calculation of the control deviation as a result of the circulating reactive current: $$\mathrm{DCC}=\left(\mathrm{k}*\mathrm{ICC}*\left(\mathrm{X}+\mathrm{TermA}\right)*\mathrm{<figure-callout\; id="3"\; label="root"\; filenames="imgf0001.png"\; state="\{\{state\}\}">root</figure-callout>}\left(\mathrm{3}\right)*\mathrm{100}\%\right)/\mathrm{U.N.}$$

$$\mathrm{TermA}=\mathrm{X}/\left(\mathrm{X}*\mathrm{BP}-\mathrm{1}\right)$$

• k - adjustable circulating reactive current control factor
• ICC - Circulating reactive current
• X - transformer longitudinal reactance calculated from short-circuit voltage
• BP - total reactive conductance or total acceptance of all parallel transformers
• UN transformer rated voltage

With the addition of the term X / (X * BP - 1) to the transformer reactance, the control sensitivity is amplified to the point where, with a minimum step difference, the control deviation caused by the circulating reactive current DCC is exceeded. Through this optimization, the setting value of the control sensitivity can be maintained in most cases to the default value of 1, and thus is without time-consuming commissioning given a very good control sensitivity and control stability. With conventional controllers, the optimum factor must be determined during commissioning.

Embodiments of the present invention use two bandwidths, in particular a first bandwidth and a second bandwidth. Conventional methods use only a single bandwidth, especially the second bandwidth. Embodiments of the present invention address control instability in the case where the voltage deviation is relatively low and the reactive current deviations are opposite and of equal magnitude. Conventional methods and devices have for this case constantly up and stepped, which has led to damage and wear on components.

According to embodiments of the invention, however, a delay time is set in the event that the voltage deviation lies within a first bandwidth around the voltage setpoint. The delay time (one of the first or the second transformer) may define how long a certain deviation must be at least in order to perform a grading of the corresponding transformer gate. To prioritize the grading of a particular transformer, the delay time of the other transformer can be increased. If the delay time of a particular transformer is increased, this corresponds to a stricter criterion for triggering a step of the respective transformer. If e.g. the voltage deviation is negative, a higher level of that transformer can be prioritized (by increasing the corresponding delay time of the other transformer), which shows a negative total deviation. This provides a simple method of stabilizing the control.

According to one embodiment of the present invention, a delay time of that transformer for which the total deviation has a different sign than the voltage deviation, set larger than another delay time for the other transformer.

Thus the transformer is prioritized in the staging, which has a total deviation that has the same sign as the voltage deviation. In this case, the gradation or the voltage step in the case of higher stages or low stages and the second bandwidth as a function of one another can be selected such that the upgrading or, in the case of low grading, the open-circuit voltage at the output of the secondary winding does not lead out of the second band range. Thus, a constant back and forth can be prevented.

The method may be configured such that if the total deviation and / or reactive current deviation is outside the second bandwidth for at least the greater set delay time, the corresponding transformer is stepped up if the overall deviation is negative and is stepped lower if the overall deviation is negative wherein the grading is left if the total deviation is shorter than the larger set delay time outside the second bandwidth.

A given transformer will only be stepped up or down if the corresponding or respective total deviation is outside the second band range for a period of time that is at least as long for the currently set delay time. By changing the delay time, a criterion can thus be changed according to which a grading takes place.

According to one embodiment of the present invention, the method is configured such that when upgrading one of the transformers, a higher, with lowering of one of the transformers a lower voltage at the output of the secondary winding of the respective transformer is applied, the upgrading and / or subscribers by appropriate tap on a respective primary winding (and / or secondary winding) takes place, whose input is connected to a further conductor to which in particular a high voltage between 70 kV and 400 kV is applied, the voltage setpoint is in particular between 5 kV and 20 kV.

Lowering or upgrading can be achieved by changing the tap on the primary turn and / or on the secondary turn.

According to an embodiment of the present invention, the larger set delay time is between 1.5 and 2.5 times, more preferably 2 times the other delay time. Thus, an effective stabilization of the control can be achieved.

According to an embodiment of the present invention, the total deviation for the other transformer (whose delay time is not increased) has the same sign as the voltage deviation. This other transformer is prioritized for grading to counteract the voltage drift.

According to one embodiment of the present invention, the method is configured such that when the voltage deviation is negative, the delay time for the voltage regulator or voltage regulators which exceed the bandwidth, that is to say lower stages, is set greater.

$$-\mathrm{BCC\_DV}<\mathrm{DV}<\mathrm{BCC\_DV}$$

$$\mathrm{DCC},\mathrm{D}>\mathrm{B}$$

This applies to: $$\mathrm{DV}<=0$$

$$-\mathrm{BCC\_DV}<\mathrm{DV}<\mathrm{BCC\_DV}$$

$$\mathrm{DCC}.\mathrm{D}>\mathrm{B}$$

in which
the first bandwidth is given by the range [-BCC_DV, BCC_DV],
the second bandwidth is given by the range [-B, B], DV is the voltage deviation,
DCC is the reactive current deviation,
D is the total deviation,
For example, the larger set delay time can be between 1.5 and 2.5 times, especially 2 times, the other delay time.

According to an embodiment of the present invention, the method is such that when the voltage deviation is positive, the delay time for the voltage regulators that are below the bandwidth, that is, higher, is set larger.

$$-\mathrm{BCC\_DV}<\mathrm{Dv}<\mathrm{BCC\_DV}$$

$$\mathrm{DCC},\mathrm{D}<-\mathrm{B}$$

This applies to: $$\mathrm{dv}>0$$

$$-\mathrm{BCC\_DV}<\mathrm{dv}<\mathrm{BCC\_DV}$$

$$\mathrm{DCC}.\mathrm{D}<-\mathrm{B}$$

According to one embodiment of the present invention, the first bandwidth is between 0.3 and 0.7 times, in particular 0.5 times the second bandwidth. Other values are possible.

It should be understood that features that may be used individually or in any combination in the context of a method for controlling a value of a voltage on a

Conductors are described, illustrated or provided, as well, individually or in any combination, can be used for a device for controlling a value of a voltage to a conductor, according to an embodiment of the present invention, or vice versa.

According to one embodiment of the present invention, there is provided a device for controlling a value of a voltage on a conductor to which at least a secondary winding of a first stepping transformer and a secondary winding of a second stepping transformer are connected, the device comprising: a logic unit formed if a voltage deviation of the voltage is at the conductor of a voltage setpoint within a first bandwidth around the voltage setpoint and if a total deviation of a sum of the voltage deviation and a reactive current deviation from the voltage setpoint for each of the first and second transformers is out of a second, greater than the first, bandwidth around Voltage setpoint is to set a delay time to stages of the first transformer and / or the second transformer such that a voltage deviation counteracting grading of the first or the second transformer is prioritized.

The apparatus may further include a measuring device for measuring the value of the voltage and values of a reactive current of the first transformer and the second transformer.

According to one embodiment of the present invention, there is provided a transformer system comprising: a first step-up transformer and a second step-up transformer connected in parallel therewith; and an apparatus for controlling a value of a voltage according to any of the above-described embodiments.

In the transformer system, at least one of the first and second transformers may include an on-load tap changer.

• Fig. 1 illustriert schematisch eine Transformatorsystem mit einer Vorrichtung zum Steuern eines Wertes einer Spannung an einem Leiter gemäß einer Ausführungsform der vorliegenden Erfindung; und
• Fig. 2 und 3 illustrieren Balkendiagramme, mit Abweichungen, die in Ausführungsformen der vorliegenden Erfindung betrachtet werden.

Embodiments of the present invention will now be explained with reference to the two drawings. The present invention is not limited to the described or illustrated embodiments.

• Fig. 1 schematically illustrates a transformer system having a device for controlling a value of a voltage on a conductor according to an embodiment of the present invention; and
• FIGS. 2 and 3 illustrate bar graphs with variations considered in embodiments of the present invention.

Fig. 1 schematically illustrates a transformer system 1 for controlling a value of a voltage on a conductor 3 to which at least one secondary winding 5A of a first step-up transformer 7A and a secondary winding 5B of a second step-up transformer 7B are connected, according to an embodiment of the present invention. This in Fig. 1 schematically illustrated transformer system 1 comprises a first step-up transformer 7A and a second step-wave transformer 7B connected in parallel therewith, and a device 13 for controlling a value of a voltage at a conductor 3.

The device 13 has a logic unit 15, which is formed in the illustrated embodiment by a first voltage regulator 17A and a second voltage regulator 17B, which are interconnected by a communication line 21. The logic unit 15 is _configured if a voltage _deviation D _{V of} the voltage on the conductor from a voltage _setpoint within a first bandwidth [-B _{CC_DV} , B _{CC_DV} ] is around the voltage _setpoint , and if a total _deviation D is a sum of the voltage _deviation D _V and a Reactive current deviation D _CC of the voltage setpoint for each of the first and the second transformer outside a second, larger than the first, bandwidth [-B, B] by the voltage setpoint, a delay time T1 to the stages of the first transformer 7A and / or the second transformer 7B set such that a voltage deviation counteracting grading of the first or the second transformer is prioritized.

In particular, the logic unit 15 is formed, a method of controlling a value of a voltage in a conductor 3, on which at least one secondary winding 5A of a first step-up transformer 7A and a secondary winding 5B of a second step-type transformer 7B are connected, according to an embodiment of the present invention.

The first transformer 7A has a first primary winding 9A and the second transformer 7B has a second primary winding 9B. Further, switches 37 are provided between the high voltage rail 27 and the bus bar 3 to selectively disconnect the first and / or second transformers 7A, 7B from the high voltage rail 27 and / or from the bus bar 43.

In the illustrated embodiment, the device 13 further comprises a measuring device 23, which is formed by partial measuring devices 23A and 23B, wherein the measuring device 23A is communicatively present with the first voltage regulator 17A to generate a first (blind) current I _A and a first open-circuit voltage or Load voltage U _A at the outputs of the secondary winding 5 of the first transformer 7A to measure. The partial measuring device 23B is communicatively connected to the second voltage regulator 17B and configured to measure the reactive current I _B or generally the current I _B and the output voltage U _B at the output terminal of the second secondary winding 5B of the second transformer 7B and supply it to the second voltage regulator 17B.

To the stages of the first transformer 7A, a tap changer 25A is provided, which receives control signals 26A from the first voltage regulator 17A, whereupon a corresponding step (tap on the primary winding 9A or on the secondary winding 5A of the first transformer 7A) is performed. To the stages of the second transformer 7B, a further tap changer 25B is provided, which receives control signals 26B from the second voltage regulator 17B, whereupon it performs a desired grading on the second transformer 7B.

The two transformers 7A and 7B are electrically connected in parallel between a high voltage rail 27 and the conductor 3 (also referred to as a busbar). Point the two transformers 7A and 7B have different no-load voltages (or also voltages under load), this can lead to a circulating reactive current 29 (I _KBS ), which is undesirable and is regulated in accordance with the embodiment of the present invention.

In particular, embodiments of the present invention stabilize the control to a target voltage value on the busbar 3 in the case where the output voltages of the two transformers 7A and 7B are slightly different but close to the target voltage value.

Hereinafter, a prioritization _bandwidth B _{CC_DV is} introduced as B _{CC_DV} = factor * B, where the factor may be, for example, 0.5 and B is a conventionally used bandwidth. In the case of the two transformers 7A and 7B with approximately the same transformer reactance and D _V = 0, the opposite circulating reactive-current voltage deviation D _CC and thus a total deviation D result in a step difference. The voltage deviation D _V is then close to 0 and thus within the range Prioritization bandwidth (also referred to as first bandwidth). If D _V ≤ 0, the higher level is prioritized in time. That is, with the regulator in transformer A (transformer 7A), where a positive circulating reactive current is measured, twice the delay time is applied (criterion D> B). Accordingly, at D _V > 0, the lower level is prioritized.

Fig. 2 12 schematically illustrates a bar graph wherein the voltage deviation D _V , the total deviation D _A for the first transformer 7 A, the reactive current deviation D _CCA for the first transformer 7 A, the total deviation D _B for the second transformer 7 _B and the reactive current deviation D _CCB for the second transformer 7 _B. are applied. The deviations are applied relative to the setpoint voltage 31 plotted as a proportion. How out Fig. 2 is apparent, the voltage deviation D _{V is} within the first bandwidth 33 and is negative. Furthermore, D _A and also D _{CCA are} outside the second bandwidth 35 and in particular greater than B. Furthermore, D _B and D _{CCB are} outside the second bandwidth 35 and are in particular> -B.

In this case, the first delay time (the delay time of the control for the first transformer 7A) is increased, in particular, set to double the value of the delay time used for the second transformer 7B. In order for a higher stages of the second transformer 7B is prioritized. VACT illustrates the actually measured at the busbar voltage, to the voltage command value is the difference D _V.

Fig. 3 illustrated in a similar bar chart as in Fig. 2 a situation during a regulatory procedure, with different values of the different deviations. In particular, the voltage deviation D _{V is} within the first bandwidth 33 and at the same time greater than 0 (> 0). Furthermore, the total deviation and the reactive current deviation of the first transformer and the second transformer are outside the second bandwidth 35. In this case, the delay time for the control of the second transformer 7B is increased, in particular set to twice the value compared to the value T1 of the delay time, which is used for the first transformer 7A. This prioritizes a lowering of the first transformer 7A.

Embodiments of the present invention can ensure the user a control stability, with safe design of the circulating reactive current or regulating the circular reactive current. The set value introduced by the prior art is not sufficient for this and is difficult to master by the user. Due to the low conventional control quality and the associated circulating reactive current, the power loss of the transformers is increased and thus the efficiency is lowered.

The stability achieved by embodiments of the invention simultaneously prevents unnecessary steps and thus increases the life of the on-load tap-changer (OLTC). This can reduce costs for the network operator.

The delay time T1 can be predetermined by the network operator and relate to the compensation of voltage fluctuations in the network. The doubling of this delay time relates to the compensation of the circulating reactive current and thus has no effect on the coordination of voltage regulation in radial networks.

Claims (13)

A method of controlling a value of a voltage on a conductor (3) to which at least one secondary winding (5A) of a first step-up transformer (7A) and a secondary winding (5B) of a second step-down transformer (7B) are connected, the method comprising: if a voltage _deviation (D _V ) of the voltage across the conductor (3) from a voltage _setpoint within a first bandwidth ([-B _{CC_DV} , B _{CC_DV} ]) is around the voltage _setpoint , and if a total deviation (D) of a sum of the voltage deviation (D _V ) and a reactive current deviation (D _CCA , D _CCB ) from the voltage setpoint for each of the first and second transformers is out of a second, greater than the first, bandwidth ([-B , B]) lies around the voltage setpoint: Setting a delay time (T1) to the stages of the first transformer (7A) and / or the second transformer (7B) such that a voltage deviation counteracting grading of the first or the second transformer is prioritized. The method of claim 1, wherein a delay time (T1) of the transformer for which the total deviation has a sign other than the voltage deviation is set greater than another delay time for the other transformer. Method according to claim 1 or 2, wherein if the total deviation D and / or reactive current deviation (D _CC for at least the greater set delay time is outside the second bandwidth ([-B, B]), the transformer is stepped up if the total deviation is negative is and is stepped lower if the total deviation is negative,
wherein the staging is left if the total deviation is shorter than the larger set delay time outside the second bandwidth. Method according to one of the preceding claims, wherein when upgrading one of the transformers, a higher, with lowering of one of the transformers, a lower voltage at the output of the secondary winding (5A, 5B) of the respective transformer is applied, the upgrading and / or subscribing by appropriate tap on a respective primary winding takes place whose input is connected to a further conductor (27) to which in particular a high voltage is applied between 70 kV and 300 kV,
wherein the voltage setpoint is in particular between 5 kV and 20 kV. Method according to one of the preceding claims, wherein the larger set delay time (2 * T1) is between 1.5 and 2.5 times, in particular 2 times, the other delay time (T1). Method according to one of the preceding claims, wherein the total deviation for the other transformer has the same sign as the voltage deviation. Method according to one of the preceding claims, wherein, if the voltage deviation is negative, the delay time for the voltage regulator (s) is exceeded which exceed the bandwidth, ie lower, if: $$\mathrm{DV}<=0$$

$$-\mathrm{BCC\_DV}<\mathrm{DV}<\mathrm{BCC\_DV}$$

$$\mathrm{DCC}.\mathrm{D}>\mathrm{B}$$

in which
the first bandwidth is given by the range [-BCC_DV, BCC_DV],
the second bandwidth is given by the range [-B, B], DV is the voltage deviation,
DCC is the reactive current deviation,
D is the total deviation,
wherein the greater set delay time is in particular between 1.5 and 2.5 times, more particularly 2 times, the other delay time. Method according to one of the preceding claims,
if the voltage deviation is positive, the delay time for the voltage regulator (s) is set larger than the bandwidth, ie higher, if: $$\mathrm{dv}>0$$

$$-\mathrm{BCC\_DV}<\mathrm{dv}<\mathrm{BCC\_DV}$$

$${\mathrm{D}}_{\mathrm{CC}}.\mathrm{D}<-\mathrm{B}\mathrm{,}$$

Method according to one of the preceding claims, wherein the first bandwidth ([-B _{CC_DV} , B _{CC_DV} ]) is between 0.3 and 0.7 times, in particular 0.5 times, of the second bandwidth -BC, B. The method of any one of the preceding claims, further comprising: Optimizing the calculation of the control deviation D _{CC as a} result of the circulating reactive current according to: $$\mathrm{DCC}=\left(\mathrm{k}*\mathrm{ICC}*\left(\mathrm{X}+\mathrm{TermA}\right)*\mathrm{root}\left(\mathrm{3}\right)*\mathrm{100}\%\right)/\mathrm{U.N.}.$$

in which $$\mathrm{TermA}=\mathrm{X}/\left(\mathrm{X}*\mathrm{BP}-\mathrm{1}\right).$$

k an adjustable circulating reactive current control factor, ICC the circulating reactive current, X is the transformer longitudinal reactance calculated from short-circuit voltage, BP of the total reactive conductance or total acceptance of all parallel transformers, UN the rated transformer voltage are. Device (13) for controlling a value of a voltage on a conductor (3), on which at least one secondary winding (5A) of a first step-up transformer (7A) and a secondary winding (5B) of a second step-up transformer (7B), the device comprising: a logic unit (15, 17A, 17B) which is formed if a voltage _deviation (D _V ) of the voltage on the conductor from a voltage _setpoint within a first bandwidth ([-B _{CC_DV} , B _{CC_DV} ]) is around the voltage _setpoint , and if a total deviation (D _A , D _B ) of a sum of the voltage deviation (D _V ) and a reactive current deviation (D _CCA , D _CCB ) from the voltage setpoint respectively for the first and second transformers is out of a second, greater than the first, bandwidth ([-B _C , B]) is around the voltage setpoint, a delay time (T1) to the stages of the first transformer (7A) and / or the second transformer (7B) to set such that a voltage deviation counteracting grading of the first or the second transformer is prioritized. Apparatus according to the preceding claim, further comprising: a measuring device (23A, 23B) for measuring the value of the voltage and values of a reactive current of the first transformer and the second transformer. Transformer system (1), comprising: a first step-up transformer (7A) and a second step-up transformer (7B) connected in parallel thereto; and a device (13) for controlling a value of a voltage according to claim 11 or 12.

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