Phase-selective VAr management

Abstract

Method for operating an arrangement with an inverter (9) which has - a control and regulation unit (3) - one electronic component (11a, 11b, 11c) for each of the three phases (L1, L2, L3) of a 3-phase three-phase power supply network, wherein the electronic components (11a, 11b, 11c) are configured on the AC side for connection to the respective phase (L1, L2, L3), - a DC busbar (7) to which the electronic components (11a, 11b, 11c) are connected, and - a control input via which an operating mode can be changed upon application of a control signal in such a way that an asymmetrical feed-in to the or an asymmetrical reference from the supply network takes place with respect to the three phases, wherein the AC voltage side of the inverter (9) is connected to the three phases (L1,L2,L3) of the 3-phase three-phase power supply network, wherein the voltage of all three phases is measured at any network connection point in the supply network, and that the control signal is generated using the three voltage measurements, characterized by that at least one of the three phases draws reactive power or power while simultaneously supplying reactive power or power to another phase.

Description

[0001] The invention relates to an inverter comprising a control and regulating unit, which can be connected to a 3-phase three-phase power supply network, in particular for feeding in regeneratively generated energy, and to a method for its operation.

[0002] Systems for generating renewable energy are widely known. Every photovoltaic generator, hereinafter also referred to as a PV generator, produces a direct current (DC) which is converted into an alternating current (AC) by means of an inverter. Both purely electronic devices and electromechanical converters can be used as inverters. For the purposes of this invention, an inverter is defined as any device that can electronically generate an alternating voltage from a DC voltage. For example, wind turbines directly generate an alternating current, which must be adapted to the conditions of the public power grid via a frequency converter. These frequency converters also include inverters with an internal DC link, to which the present invention is applicable.

[0003] To date, measures for maintaining the stability of a power grid are known that act uniformly on the grid. A disadvantage of these measures is that while a simultaneous voltage change across all phases may be necessary or at least beneficial for one phase to prevent a voltage threshold from being undershot or exceeded, it may be undesirable for another phase of the three-phase grid. For example, if a voltage increase is implemented to prevent an impending undervoltage at one point in the grid, this voltage increase may be undesirable at another point where no large loads are currently connected. The invention therefore proceeds from the premise that the ability of electronic inverters to act on each phase separately can be used to contribute to voltage stabilization. These devices, due to their inherent components, such as...IGBTs offer the ability to make a phase-selective VAr contribution. This utilizes an existing capability of the devices for an additional purpose.

[0004] From WO 2010 / 088 928 A1, a method is known for supplying a multi-phase electrical network by an electric generator of a renewable energy source, which is connected to the network via a converter. When an asymmetrical network fault occurs, the feed-in to the phases not affected by the fault is interrupted, and a current is fed into the affected phases to stabilize the network voltage.

[0005] WO 2006 / 032 685 A1 describes a renewable energy system. This system consists of a first energy generator and a second energy generator. The first energy generator has a generator that produces electrical energy, which is fed into an electrical grid to which several consumers are connected. The second energy generator, which has a generator and an associated combustion engine, has a fuel tank that can be supplied to the combustion engine as needed. A plant for producing the fuel from renewable raw materials is included, and the fuel production plant draws electrical energy for its operation from the first energy generator.

[0006] German patent DE 10 2007 016 140 A1 discloses a method and a device for controlling the output voltages of an inverter designed for a 4-wire network when voltage imbalances occur in the network. Two control branches are proposed for this control. In the first control branch, a symmetrical voltage system characterized by equal amplitudes is subjected to conventional vector control, while in the second control branch, differences resulting from voltage imbalances are taken into account. These differences are obtained by calculating the difference between the amplitudes of the symmetrical voltage system and the amplitudes of the output voltages. Both control branches are used to generate control signals, which are summed to form overall control signals and then used to control the inverter.

[0007] Accordingly, the present invention is based on the objective of increasing the stability in a supply network by equalizing the individual phase voltages.

[0008] An inverter has a control input through which its operating mode can be changed when a control signal is applied, resulting in an asymmetrical feed-in to the supply network with respect to the three phases.

[0009] Asymmetrical operation means that the generated power and / or reactive power of the photovoltaic generator is not distributed equally across the three phases by the inverter. This can also mean that one or two phases receive only a small share or no share of the generated power, or even consume power, while power is being fed into the grid via at least one other phase. The generation of reactive power is independent of the PV generator's operation. It can also occur at night when there is no sunlight and no power generation.

[0010] When measuring the voltages of all three phases, L1, L2, and L3, values ​​of 230 volts, 225 volts, and 228 volts, for example, are obtained. This can occur for a variety of reasons, including the current behavior of consumers or a large number of rooftop photovoltaic systems feeding into the grid independently. In such cases, it serves as a precautionary measure to stabilize the grid by boosting the weakest phase, L2, which has a voltage of only 225 volts. This is achieved by feeding the generated power into this phase via an inverter equipped with the appropriate control input. In cases of significant deviations, the inverter can also utilize spare capacity to feed reactive power into phase L2 to increase the voltage. Spare capacity of the inverter exists when the current power output of the photovoltaic generator is below the inverter's rated power.If, for example, the inverter is designed for 12 kVA, but only 7 kVA of solar energy is generated and converted into alternating current due to the prevailing solar irradiance, the inverter has a free capacity of 5 kVA available for reactive power injection (VAr). This 5 kVA corresponds to the difference between the currently supplied power and the inverter's rated power. In this way, the reactive power is limited to the power remaining to achieve the inverter's rated power. This does not restrict or reduce the energy generated by the power producer itself. The inverter's free power capacity is simply used to fulfill an additional function, namely that of a phase shifter or reactive power supplier. Typically, the VAr value is less than the VA value of 5 kVA calculated above as the difference.

[0011] The request is then implemented via a corresponding control signal applied to the control input. It is irrelevant whether the control input only receives the measured phase voltages, which are then converted into a feed-in mode by a processing unit in the inverter, or whether the appropriate feed-in mode is directly entered at the control input, which the inverter then simply implements.

[0012] Four basic operating modes can be distinguished: i) the standard operating mode towards a point of maximum power generation is maintained and the photovoltaically generated power is distributed asymmetrically across the three phases; ii) an operating mode is set towards a point of maximum reactive power generation, which is distributed asymmetrically across the three phases; iii) the standard operating mode towards a point of maximum power generation is maintained as a priority, and additionally, free capacity of the inverter is used to feed reactive power unevenly into the three phases or to draw unevenly from the three phases; and iv) the standard operating mode towards the maximum power point is suspended and replaced by an operating mode in which, depending on the control signal, a predeterminable amount of reactive power is fed into or drawn from at least one of the three phases.

[0013] The standard operating mode leading to the point of maximum power is well known and will not be discussed further here. What is important for the purpose of this inverter is that it can convert a control signal into an asymmetrical distribution of its power and / or that its electronic components can be influenced by the control signal in such a way that additional reactive power can be generated.

[0014] The problem is solved by a method for operating the inverter by measuring the voltage of all three phases at any grid connection point in the supply network, in particular at the grid connection point that connects the inverter to the supply network, and by generating the control signal using the three voltage measurements.

[0015] The term "network connection point" is usually understood to mean the position of the metering point between the consumer and the grid, as well as between the feed-in point and the grid. For the purposes of the present invention, it also encompasses any position within the public supply network, as well as the networks of consumers and feed-in points, where voltage measurement is carried out.

[0016] Choosing a measuring point in the immediate vicinity of the inverter allows for immediate correction of the three phase voltages at the point of intervention. Besides this location, all grid connection points with high voltage sensitivity are particularly suitable. This means that a particularly suitable location for placing the voltage measuring device is at the end of a branch line, or, in relation to a network transformer feeding a ring line, in the middle of the ring line, or, in the case of multiple feed-in transformers, in their vicinity. These locations are where the consumers are located that, in the normal reference direction of the grid, have the lowest grid voltage with the greatest difference in their phase voltages, especially if one of the transformers fails, e.g., due to maintenance work.Due to the other consumers upstream, all of which cause a marginal voltage drop, the available voltage is usually lowest there, unless a power supplier is nearby. The end of a branch line and the connection point relative to a network transformer in the middle of a ring line that feeds the line are relative to each other. For example, if a branch line has 200 connection points, the end is considered one of the last 10%, i.e., one of the last 20 connection points. Similarly, in a ring line with 200 connection points, the connection points to the left and right of the transformer connection points would be considered.

[0017] Generally, the most voltage-sensitive grid connection point is chosen, particularly under the assumption of a transformer failure. This could also be a different location, for example, if it houses a consumer operating heavy machinery with high starting currents that are switched on and off frequently throughout the day. Alternatively, the consumer with the highest fluctuation in reactive power consumption is identified, and the voltage value is measured at the grid connection point of this consumer. In general, the most voltage-sensitive point is characterized by the highest voltage variation relative to the power being fed into or drawn from the grid (P). Furthermore, the voltage variation can be defined by a percentage change rather than by the absolute voltage values ​​under consideration.In the case of a ring main, this is usually the case in the middle in relation to the transformer to the next higher network, that is, at the point where the distance to the transformer is the same in both directions.

[0018] It follows that measuring the voltage of at least two phases at the connection point of a transformer is particularly suitable for generating the control signal. Additionally, the current through at least one of the transformer outputs should be measured and used in the calculation of the control signal.

[0019] As already mentioned, it is beneficial if the operation of the inverter is maintained by its inherent control device at the maximum power point (MPP) of the photovoltaic generator or wind turbine, and if it additionally feeds in or draws from the supply level a reactive power quantity that corresponds at most to the difference between the currently supplied power and the nominal power of the inverter.

[0020] The inverter's operation is such that a measured phase voltage below a threshold value leads to the injection of reactive power into that phase by the inverter, and an impending overvoltage of a phase, e.g. upon reaching an upper threshold value, leads to the targeted extraction of reactive power from that phase by at least one inverter.

[0021] Some energy suppliers may require that a fixed ratio of active to reactive power, i.e., a fixed cos phi value, be maintained. In such cases, to be able to draw or supply more reactive power to a selected phase, it makes sense to increase the power output P on that phase. This will then also change the reactive power on that selected phase accordingly, based on the fixed cos phi value.

[0022] The asymmetrical distribution can be so pronounced that the direction of reactive power generation can also vary from phase to phase. Thus, it can be advantageous to draw reactive power from at least one of the three phases while simultaneously supplying reactive power to another. This can be modified as desired, for example, by supplying reactive power to two phases and drawing reactive power from the third phase, or by drawing reactive power from two phases and supplying it to the third phase, and so on.This means creating the possibility: i) that at least one of the three phases receives reactive power or power while simultaneously supplying reactive power or power to another phase, and ii) that the power direction of at least one phase differs from the power direction of at least another phase, while simultaneously reactive power is received on at least one phase and reactive power is supplied to at least another phase.

[0023] Further advantages and embodiments of the invention will become apparent from the description of an exemplary embodiment with reference to the figure, which shows a schematic overview of a photovoltaic system with an inverter according to the invention.

[0024] The figure shows a three-wire power supply system with conductors L1, L2, and L3, all connected to a measuring point 1. This measuring point allows at least the voltage across each phase L1, L2, and L3 to be measured. In certain cases, it is also conceivable to additionally determine the current through the conductors to detect potential overloads and, for example, direct the system's power supply to conductors that are not fully utilized. For instance, when tapping into transformers or cables, it may be advantageous to measure only the current and not the voltage. The measured values ​​are transmitted to a control unit 3, which also includes the control unit for setting the maximum power point (MPP).

[0025] A photovoltaic generator 5 is used as a renewable energy source. The photovoltaic generator 5 produces a direct current, which is fed to a DC busbar 7 via electrical supply lines (not shown). The DC busbar 7 is connected to an inverter 9, whose AC side is connected to the three phases L, L2, and L3. The actual conversion of the generated direct current is carried out separately for each phase using electronic components, currently preferably IGBTs. This is indicated by three inverter symbols 11a, 11b, and 11c within the inverter 9. Three separate signal lines S1, S2, and S3 lead from the control unit 3 to the three IGBT blocks, which are designated with the inverter symbols 11a, 11b, and 11c. The IGBTs are controlled via these signal lines S1, S2 and S3 so that they make the settings calculated by the control unit 3 depending on the voltage measurements.For clarity, the inverter 9 and the control unit 3 are shown separately. In reality, the control unit 3 is integrated into the inverter 9 along with the MPP element. Therefore, we have an inverter 9 whose control unit 3 additionally performs the described phase-selective reactive power management.

[0026] In operation, the inverter 9 is used as follows. To provide a concise numerical example illustrating the invention, the three phase voltages L1, L2, and L3 are assumed to be 230 volts, 235 volts, and 227 volts, respectively. This can occur for various reasons, including the current behavior of consumers or a multitude of rooftop photovoltaic systems feeding independently into the grid. This information is determined by the measuring point 3 and transmitted to the control unit 3. This unit is programmed to accept, for example, a maximum voltage difference of 4 volts between phases L1, L2, and L3, whereby the phase voltage should lie within a predefined range, here 228 volts to 232 volts. The criterion, in this case 4 volts, can be set according to the needs of the energy supplier and grid operator.Other criteria, such as the absolute voltage of the three phases L1, L2 and L3, can also be used, without taking into account the difference in voltage between the phases.

[0027] The requirements for the grid can also change, which is why a second embodiment of the invention provides for equipping the control unit 3 with a setting input E. The currently required criterion is transmitted to the inverter 9 from a control room (not shown) via the setting input E; this could be, for example, the currently required setpoint values ​​for the three phase voltages L1, L2, and L3.

[0028] To continue with the previously mentioned numerical example, the control unit, based on the measured voltage values, determines that phase L1 should remain unchanged, that phase L2 should have its voltage reduced by at least 3 volts by drawing reactive power, and that phase L3 should be boosted by one volt by supplying reactive power. After the relevant IGBTs have been adjusted, the resulting voltage profile for the three phases is L1 = 230 volts, L2 = 232 volts, and L3 = 228 volts. In the case of significant deviations, the inverter can utilize not only the unbalanced distribution of power generated by PV generator 5 but also spare capacity to feed reactive power into phase L3 to boost the voltage and to draw reactive power from phase L2 to lower the voltage. Reference symbol list 1 Messstelle 3 Regel- und Steuereinheit 5 PV-Generator 7 Gleichstromschiene 9 inverters 11a-11c Inverter symbol E setting input L1, L2, L3 Phase S2, S2, S3 Signal input

Images