Patent Description:
In the present application, the expression "high voltage" means a voltage greater than <NUM> volts, while "low voltage" means a voltage less than <NUM> volts.

Maintaining the output voltage of high-voltage electrical equipment, such as transformers, within the allowed or desired margins depending on the load circumstances has traditionally been carried out by changing the transformation ratio of said equipment, so that the relationship between the primary and secondary winding voltages of said electrical equipment changes accordingly.

To do this, high-voltage electrical equipment is provided with a device called a tap changer, which can consist of an off-load or on-load tap changer, that is, the tap changes can be carried out with the electrical equipment de-energized (voltage regulation interrupting the load current) or energized (voltage regulation without interrupting the load current). The tap changer increases or reduces the number of turns of the primary winding, thus changing the transformation ratio, or what is the same, varying the voltage in the secondary winding.

With the advent of smart power grids (Smart Grids), voltage fluctuations have increased in the power grid, especially in distribution networks with a high integration of distributed generation sources (small photovoltaic, wind, cogeneration, etc.) and a large deployment of electric vehicles, which makes electricity grids more unstable. On-load tap changers respond effectively and efficiently to voltage fluctuations in load levels. The use of the on-load tap changer (named with the acronym OLTC from the term "On-Load Tap changer") is common in electrical equipment, such as high/low voltage distribution transformers, whose service cannot be interrupted without seriously harming the operation of the distribution system and with the consequent damage to the users of the distribution network.

The transformer with on-load tap changer allows transforming the voltage level from high voltage to low voltage while automatically regulating the latter within a range compatible with the load to be powered. The on-load tap changer therefore operates automatically, where an automatic voltage regulator (named with the acronym AVR from the term "Automatic Voltage Regulator"), also known as automatic voltage controller (named with the acronym AVC from the term "Automatic Voltage Controller"), it is in charge of giving the pertinent orders to the tap changer in charge of increasing or reducing the number of turns of the primary winding of the transformer to obtain the desired voltage in the secondary winding of the transformer.

The automatic voltage regulator or controller makes the decision to increase or reduce the number of turns based on a control policy or control strategy that it has programmed; these decisions being later communicated to the on-load tap changer. The objective of the control policy is to maintain the voltage of the secondary winding of the transformer as close as possible to an optimal voltage setpoint determined from the setpoint optimization policies or criteria established by the Distribution System Operator, known by the acronym DSO (from the term "Distribution System Operator") or the policies set by the regulatory limits, making the fewest number of tap changes possible in order to maximize the useful life of tap changers, avoiding unnecessary tap changes.

To determine a control policy, it is necessary to measure the voltage deviation from the optimal voltage setpoint and calculate the waiting time before each tap change, relating the deviation from the voltage setpoint and time in a functional way. The voltage deviation from the optimal voltage setpoint is measured from a voltage reference value that is currently continuously measured on a single phase (R, S or T) selected or in the three phases (R, S and T), calculating in the latter case the average voltage measured in the three phases (R, S and T) and taking said average as the value of reference. Within the electrical network, specifically in the energy generation and transmission steps, it is currently continuously measured in a single phase (R, S or T) because the voltage imbalance between phases is minimal and, therefore, measuring one phase is enough since they all have practically the same voltage. However, in the power distribution path or in the distribution networks the imbalance of the voltage between phases is greater and, therefore, the needs vary, the fact of measuring only in one phase (R, S or T) being insufficient or inconvenient.

Selecting a single phase to continuously measure the voltage reference value in that phase can mean that if the least critical phase is selected, the critical phase being understood as the phase that has a voltage value higher or lower than the rest of the phases, the consumers of the other phases can incur overvoltage and/or undervoltage states, that is, the following can occur:.

On the other hand, using the average voltage measured in the three phases as the voltage reference value may have the disadvantage that, if one phase is more critical, the other two phases may affect the average and correct or necessary decisions cannot be taken.

Currently, the selection of the phase or phases (if the average voltages of the three phases is taken as a reference value) is carried out only once, normally when the electrical equipment is installed, and the regulation of the on-load tap changer device is always based on the reference value measured on the selected phase or calculated if all three phases are selected for voltage measurement (the average of the three voltages is calculated and said average voltage is taken as a reference value). This selection of phase/s is unique in time, that is, the single selection of the phase/s and the continuous measurement/calculation of the reference value in said phase/s can cause the aforementioned drawbacks.

The present invention relates to a determination method of the reference value (Vref), such as a voltage reference value, of an automatic voltage regulator or controller for regulating an on-load tap changer device based on to a pre-defined and/or calculated optimal voltage setpoint (Vset) and based on voltage measurements of the three phases (R, S and T), which aims to solve each and every one of the problems mentioned above.

The method of the invention allows dynamically determining at least one reference value (Vref), whereby said reference value (Vref) may vary at different times or may be constantly changing. A moment is understood as each instant of time in which an automatic regulation algorithm of the on-load tap changer device evaluates the difference between the reference value (Vref) and the optimal voltage setpoint (Vset) to determine if a tap change is carried out or not.

In this sense, the method includes a dynamically selection step of at least one phase (R, S, T) for measurement and/or calculation of at least one reference value (Vref), selecting the phase (R), the phase (S), the phase (T) or the three phases (R, S, T) at each moment to measure and/or calculate the reference value (Vref), in such a way that the problems that can arise from selecting a phase (R, S or T) and continuously use as reference value (Vref) the measurement of the voltage in the selected phase or by selecting the three phases (R, S and T) and continuously use the average voltage measured in the three phases (R, S and T) as reference value (Vref). In short, the selection step of at least one phase (R, S, T) for measurement and/or calculation of at least one reference value (Vref) is executed at each instant of application of the automatic regulation algorithm associated with the automatic voltage regulator or controller, which means that the selection of the reference value (Vref) is dynamic over time.

The determination method of the voltage reference value (Vref) generally comprises the following steps:.

An example of a preferred embodiment is described below, making reference to the aforementioned figures, without this limiting or reducing the scope of protection of the present invention.

<FIG> shows a determination method of the reference value (Vref) of an automatic voltage regulator or controller for regulating an on-load tap changer device based on an optimal voltage setpoint (Vset) pre-defined and/or calculated and based on voltage measurements of the three phases (R, S and T), for application in transformers with on-load tap changer device.

The method of the invention comprises a series of steps that, when executed, allow to dynamically select at least one phase (R, S, T) to measure and/or calculate at least one reference value (Vref), so that the reference value (Vref) can vary at different moments, understanding the term moment as each instant of time in which an automatic regulation algorithm of the on-load tap changer device evaluates the difference between the reference value (Vref) and the optimal voltage setpoint (Vset) to determine whether a tap change is performed or not.

The objective of dynamically measuring and/or calculating a reference value (Vref) is to be able to perform a more precise measurement of the voltage deviation at any time with respect to the optimal voltage setpoint (Vset) and to calculate the time waiting time before each tap change, with the ultimate goal of maintaining the voltage of the secondary winding of the transformer as close as possible to said optimal voltage setpoint (Vset) and making the fewest possible tap changes, as well as the one of reduce over- and under-voltage events.

To do this, as shown in <FIG>, the method comprises the following steps:.

In the determination step (<NUM>) of the maximum voltage (Vsup) and minimum voltage (Vinf) levels, the regulatory limits between which the voltage must be maintained (normally, in Spain, 230V ±<NUM>%) and a value (C) made up of the measurement error (E), the imbalance between phases (D) and/or the voltage tolerance (T).

Therefore, once the calculating and/or loading step (<NUM>) of at least one parameter (E, D, T) has been executed, the calculation step (<NUM>) of the value (C) is executed, which determines the potential deviation of the reference value (Vref): <MAT>.

Once the value (C) has been calculated in the calculation step (<NUM>), the determination step (<NUM>) of the maximum voltage (Vsup) and minimum voltage (Vinf) levels is executed. As an example, the regulatory limits established by the regulation in Spain will be taken into account, namely: <MAT> <MAT>.

So, the maximum voltage (Vsup) and minimum voltage (Vinf) levels will be: <MAT> <MAT>.

In the case of another country, the regulatory limits could be different and should be changed to adjust the method.

Subsequently, the verification steps (<NUM>, <NUM>, <NUM>) are executed:.

The following table summarizes the verification steps (<NUM>, <NUM>, <NUM>):.

<FIG>, <FIG> and <FIG> show the voltage curve obtained for phase (S) according to the reference value (Vref) used by controller of the automatic voltage regulator or controller of the on-load tap changer, in particular, when the reference value (Vref) is a phase (R, S or T) (the (Vref) is static), using as a reference value (Vref) average voltage (Vavg) of the three phases (R, S, T) and when the reference value (Vref) dynamically selects between phases (R, S, or T) and average voltage (Vavg) of the three phases (R, S, T).

The abscissa axis represents the time course in hours, on the other hand, the ordinate axis determines the voltage level (V). Each measurement is given by a gray point on the graph, the dashed lines that join them are to give a sense of time curve to the graphic representation.

The dashed gray horizontal line represents the upper voltage limit established by the regulatory framework in Spain, that is, <NUM>. 1V (<NUM>+<NUM>%). Any voltage measurements above it are highlighted with a black outline that contrasts with the gray color of normal measurements. These measurements above the regulatory limit pose a risk for customers connected to phase (S), since, for example, if the voltage drop is not sufficient, these users would potentially suffer overvoltages.

<FIG> represents the voltage curve of phase (S) when the control has been carried out using a single phase (R, S or T) as reference, specifically phase (R). Given the existing imbalance in low voltage networks, on certain occasions, as in this example, one phase (R) is at lower voltage levels than another phase (S). When regulating using said phase (R), and raising the tap using the on-load tap changer to bring it closer to the optimum voltage setpoint (Vset), there is a risk of raising the voltage of the other phase too high (S). In this way, as can be seen in <FIG>, the phase (S) repeatedly exceeds the limit of <NUM>. 1V, even reaching <NUM>.

<FIG> represents the voltage curve of the phase (S) when the control uses the average voltage (Vavg) as reference value of the three phases (R, S, T). As can be seen, on this occasion, the number of moments in which the voltage limit of <NUM>. 1V is exceeded is less than the previous case (<FIG>). However, not only is it still exceeded, but also, in many moments, it is very close to the regulatory limit.

Finally, <FIG> shows the voltage curve of the phase (S) when the control has been carried out using a reference value. (Vref) variable according to the present invention, i.e. the reference value is selected dynamically (Vref), switching between a phase (R, S or T) (the optimal) or the average voltage (Vavg) of the three phases (R, S, T), as appropriate. As can be seen, on this occasion the limit of <NUM>. 1V has not been exceeded on any occasion. Also, the curve is much further from the upper limit in general terms.

These numbers of times when the regulatory limit is exceeded for each method used to measure and/or calculate the reference value (Vref) are represented deterministically in <FIG>. Thus, the comparison can be taken to more precise numerical terms. In particular, for the case of selection of the reference value (Vref) equal to one static phase (R, S or T), the regulatory limit for <NUM> measurements has been exceeded, for the case of using the average voltage (Vavg) of the three phases (R, S, T), <NUM> times, and finally for the reference value prompt (Vref), the regulatory limit has not been exceeded on any occasion. In <FIG>, the abscissa axis represents the method used to measure and/or calculate the reference value (Vref) and the ordinate axis represents the number of events or tap changes.

Claim 1:
Dynamic determination method of the reference value (Vref) of an automatic voltage regulator and/or controller for regulating an on-load tap changer device based on a pre-defined and/or calculated optimal voltage setpoint (Vset) and voltage measurements of the three phases (R, S and T), comprising at least one selection step (<NUM>, <NUM>', <NUM>") of at least one phase (R, S, T) for measurement and/or calculation of at least one reference value (Vref), characterized in that the selection step (<NUM>, <NUM>', <NUM>") of at least one phase (R, S, T) is executed at each instant of application of an automatic regulation algorithm associated with the automatic voltage regulator and/or controller, selecting at each moment the phase (R), the phase (S), the phase (T) or the three phases (R, S, T) to measure and/or calculate the reference value (Vref), so that the reference value (Vref) can vary at different times.