Device for correcting alternating currents

Abstract

Device (10) for correcting alternating currents with - a supply connection (5) for connecting the device (10) to an electrical power supply network, - a consumer connection (6) for connecting the device (10) to an electrical consumer, - a power line (9) connecting the supply connection (5) with the consumer connection (6) for conducting a primary alternating current between the energy supply network and the consumer, - at least one current transformer (2a, 2b) connected to the power line (9) for converting the primary alternating current into a reduced secondary alternating current (7) and - at least one active power factor correction filter (1) connected to the current transformer (2a, 2b) and the current conductor (9) for filtering the primary alternating current depending on the secondary alternating current (7), - a measuring converter (3) connecting the current transformer (2a, 2b) with the correction filter (1) for converting the secondary alternating current (7) into a measuring signal, wherein the correction filter (1) is configured to filter the primary alternating current depending on the measuring signal, - a signal bus (8) arranged between the measuring converter (3) and the correction filter (1) for transmitting the measurement signal from the measuring converter (3) to the power factor correction filter (1), an output current meter connected to the correction filter (1) for measuring an output current flowing between the correction filter (1) and the current conductor (9), wherein the correction filter (1) is configured to filter the primary alternating current by superimposing it with the output current, - the output current meter is integrated into the correction filter (1), characterized in that - a computer connected to the output current meter and the correction filter (1) for calculating the measurement signal and the output current to form a control variable, wherein the correction filter (1) is configured to superimpose the primary alternating current with the output current depending on the control variable.

Description

[0001] The present invention relates to a device for correcting alternating currents according to the preamble of claim 1.

[0002] A device of the above-mentioned type is known from the document DE 697 04 602 T2 and the document DE 43 27 894 A1.

[0003] The branch of electrical engineering that deals with the conversion of electrical energy using electronic components is commonly referred to as power electronics. Typical power electronics applications include converters or frequency converters, such as those used in electrical drive technology.

[0004] Of particular importance here is the quality of the supplied energy, which is usually measured by the proportion of electrical power available for conversion into mechanical, thermal, chemical, or other non-electrical forms of energy – taking into account transmission losses, for example. The quality benchmark used is therefore usually an electrical quantity known to experts as the power factor, which DIN 40110-1 defines as the ratio of the active power (P) to the apparent power (S). Consequently, in power supply systems, a power factor is aimed for that is as close as possible to the ideal value of 1 or, with a view to good grid stability, maintains a defined, small distance from this ideal value.

[0005] To achieve this goal, so-called power factor correction filters (PFC filters) are known from the state of the art. These filters are used, for example, in the operation of electronic devices equipped with switching power supplies. Active filters, which generally have their own switching power supply for this purpose, meet particularly high quality requirements. Under optimal conditions, such an active filter ensures the provision of the harmonic components of the current consumed by the electrical devices, so that only the sinusoidal component of the fundamental oscillation is provided by the supply device. For this purpose, it is generally essential that the filter has access to various basic measured values, which particularly relate to the AC mains voltage provided by the power grid.

[0006] The availability of relevant sensor data proves particularly problematic where multiple filters of the same type are used simultaneously. State-of-the-art parallel arrangements of correction filters typically pass the relevant signals from one filter element to the next, requiring all filters to actively participate in the data transmission. In addition to the immense wiring complexity required due to the numerous signal lines required, such conventional devices suffer from a high risk of failure and complicate the replacement of individual filter elements and the integration of additional filter elements and other system components.

[0007] An object of the invention is therefore to provide a correction device which is characterized by high reliability and ease of maintenance while having low structural complexity.

[0008] This object is achieved by means of an alternating current correction device having the features of claim 1.

[0009] A fundamental idea of ​​the invention is therefore to connect the current transformer, which serves as a measured value sensor for the purpose of AC current correction, not directly to the individual filters, but via a measuring converter configured according to the invention. This converter converts the secondary AC current generated by the current transformer into a suitable measurement signal, which is then passed on to all filters via a dedicated signal connection. In this way, the secondary current of the current transformer, which serves as the input variable of the measuring converter, can be converted into an output variable with a completely independent signal structure, which can be adapted to the requirements of the individual filters within the framework of the development of an inventive device for AC current correction—detached from the current signal supplied by the current transformer.Depending on the application, the measurement signal generated by the measuring converter can have an analog or digital signal structure without departing from the scope of the invention.

[0010] In a preferred embodiment, a transformer is arranged between a supply connection of the device and the said current transformer, usually a medium-voltage transformer with, for example, 10 kV / 400 V, which transforms the primary alternating current supplied via the supply connection from the - typically public - electrical power supply network or the corresponding alternating voltage applied to the supply connection in a predetermined ratio.

[0011] If transformer-based isolation of the device from the power grid is omitted, or as an additional measure for supplying particularly sensitive system components, or in the event of an increased risk of injury to the operator, the device according to the invention has its own device protection at the corresponding input of the measuring converter to prevent damage caused by overvoltages occurring during operation, such as those caused by capacitive or inductive coupling from other systems or by lightning strikes. While the use of a conventional protective diode should prove sufficient for fine voltages, higher voltages of up to 450 volts—commonly used in three-phase systems, for example—generally require a surge arrester, preferably a gas-filled one.If the latter has a low capacitance in the range of up to approximately 2 picofarads, the power factor correcting effect of the device according to the invention is usually only slightly impaired by the additional overvoltage protection of the measuring converter.

[0012] With regard to the arrangement of the current transformer relative to the filter output, there are essentially two basic alternatives. On the one hand, the alternating current drawn directly from the power grid via the supply connection can be used as the basis for the current conversion in the conventional manner. On the other hand, the current transformer can be connected upstream of the load connection, so that the detection and consideration of the output current generated by the filters—in addition to the measurement signal of the measuring converter based on the secondary current of the current transformer—may be unnecessary. In any case, the options outlined offer the skilled person a multitude of possible circuit variants from a manufacturing perspective, without having to depart from the scope of the invention.

[0013] For the purpose of transmitting the measurement signal generated by the measuring converter to the filters, any digital or analog connections are conceivable in principle. According to the invention, the device is supplemented by a signal bus arranged between the measuring converter and the correction filter for transmitting the measurement signal from the measuring converter to the power factor correction filter. Such a suitable bus system, in accordance with the operating principle of a field bus, can establish a communication connection between the filters, which in the present scenario function as actuators, on the one hand, and the measuring converter, which acts in the role of a control unit, on the other. The cabling effort required during production is thus considerably reduced compared to conventional parallel wiring of all filters, while the reliability and availability of the device according to the invention are even potentially increased. In addition,The use of an open bus system between the measuring converter and filters ensures uniform measurement signal transmission and physical connection across manufacturers, which also allows additional filters or other field and control devices to be integrated with less effort.

[0014] Regardless of the topologies traditionally used to construct fieldbus systems, such a bus can be implemented, in particular, based on an Ethernet variant, which—given the advantageous possibility of using standardized cable types and connector shapes, as well as the communication protocols also encompassed by the Ethernet standard—enables average transmission rates above 10 Mbps. With regard to the network architecture, this approach enables what is commonly sought in automation technology as the so-called vertical integration of the computer network, from the control level in the office to field devices such as the device according to the invention, which can be used, for example, in the environment of an industrial production facility.

[0015] To overcome the problem of the comparatively high bus cycle times of 1 millisecond or more that occur with standard Ethernet, the use of a protocol variant that is capable of real-time in the broadest sense is particularly suitable in this context to ensure isochronous transmission of the measurement signal from the measuring converter to the filters. One example is a protocol family integrated into the international standards IEC 61158 and IEC 61784-2, known in automation technology as EtherCAT, which enables precise synchronization of a device according to the invention with short cycle times of less than 100 microseconds, low jitter, and moderate unit costs.

[0016] The device is preferably designed for operating a system with multi-phase alternating current, such as three-phase alternating current, commonly referred to as power current, construction current, heavy current, or three-phase current, which also includes systems with a neutral conductor. With appropriate configuration of the power line used in the device according to the invention, a significantly higher electrical power is transmitted in this way with the same material expenditure. Furthermore, a three-phase power transformer optionally connected downstream of the supply connection can be realized with superior efficiency and a significantly smaller core cross-section compared to similarly powerful single-phase transformers.

[0017] According to the invention, it is further provided that the current intensity at the output of the filter itself is also detected. Specifically, the device comprises an output current measuring device connected to the correction filter for measuring an output current flowing between the correction filter and the current conductor, which is integrated into the correction filter. It is essential that the device further comprises a computer connected to the output current measuring device and the correction filter for calculating the measurement signal and the output current to produce a control variable. In this way, the magnitude of the said output current can be incorporated into a complex control variable determined by a calculating unit, which forms the basis for the generation of additional current components by the filter.

[0018] Further important features and advantages of the invention emerge from the subclaims, from the drawing and from the associated description of the figures based on the drawing.

[0019] It is understood that the features mentioned above and those to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

[0020] Preferred embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description, wherein the same reference numerals refer to the same or similar or functionally identical components.

[0021] The only Fig. 1 shows a device 10 according to the invention for alternating current correction, which is based on a number of three active correction filters 1, selected as an example. Each of these correction filters 1 can in turn comprise a series of different circuit elements such as a switching power supply, a rectifier, a boost converter connected directly or indirectly downstream of the boost converter, an electrical capacitor charged by the boost converter for the purpose of capacitive reactive current or reactive power compensation, as well as an upstream or downstream passive line filter, which Fig. 1 not reflected graphically for reasons of clarity.

[0022] The representation according to Fig. 1 is therefore to be understood functionally as an overview or block diagram, in which three parallel lines with the same component sequence are graphically grouped together, with the number of parallel conductors in a line symbolized by three diagonal lines. Electronic circuits such as the correction filters 1 are grouped analogously to functional blocks, which do not reflect the actual shape and arrangement of the components, but merely abstract them.

[0023] The meaning and purpose of Fig. The purpose of the device 10 shown in Figure 1 is to optimize the power flow or the ripple of the alternating current from a public electrical power supply network (not shown) to a plurality of electrical consumers which, for proper operation, depend on a form of multi-phase alternating current consisting of three individual alternating currents of essentially the same frequency, which have a fixed phase shift of approximately 120° with respect to one another. For this purpose, the device 10 comprises, on the one hand, a supply connection 5 in the form of a plug and, on the other hand, a consumer connection 6, which is designed as a 5-pin earthed socket. A segmented power line 9 in the form of a three-pole three-phase line, through which the three-phase alternating current generated by the power supply network flows, serves to connect these and other circuit elements of the device 1.An optional transformer 4, also known to experts as a converter or transformer, increases or decreases the alternating voltage applied to the supply connection 5 to a desired level.

[0024] The aforementioned three-phase alternating current forms the primary current of a first current transformer 2a downstream of the transformer 4, which, under normal operating conditions, outputs a secondary alternating current 7 that is approximately proportional to the primary alternating current and deviates by the smallest possible phase shift angle. For this purpose, the first current transformer 2a can have a few primary windings through which the primary alternating current flows, as well as a larger number of secondary windings inductively coupled to the primary windings. In particular, the primary winding can consist of a busbar guided through a toroidal core of the first current transformer 2a, which thus functions as the only primary winding. Thus, a secondary alternating current 7 is induced in the secondary windings of the first current transformer 2a, the current intensity of which is reduced compared to that of the primary alternating current inversely to the numerical ratio of primary to secondary windings.

[0025] The resulting secondary alternating current 7 is then fed as an input variable to a measuring transducer 3 (essential to the invention), which evaluates the current of the secondary alternating current 7 and converts it into a measurement signal of a defined structure according to a fixed relationship. As such, the measuring transducer 3 corresponds to one of the three basic types of measuring instruments standardized under the term "measuring transducer" according to DIN 1319, which in automation, control, and regulation technology is also known as a transmitter for specific applications.

[0026] The measuring converter 3, in turn, is connected at its output as a participant to a signal bus 8 serving as a fieldbus. In this context, a binary unit system (bus) is understood to be any system for data transmission between multiple participants via a common transmission path, in which not all participants necessarily participate in the data transmission between other participants. In the given embodiment, the signal bus 8 is specifically based on a local data network (LAN) implemented as Ethernet, which processes the measurement signal in the form of a protocol data unit known as a data frame and transmits it in this coded form from the measuring converter 3 to each of the power factor correction filters 1. To take into account predetermined quantitative real-time requirements for the signal bus 8, the signal bus uses an EtherCAT communication protocol disclosed in IEC 61158.

[0027] Each of the three power factor correction filters 1 generates a specifically shaped output current on this basis, whereby the circuit of the Fig. 1 causes all output currents to essentially combine to form a common total output current. This, in turn, is fed into the power line 9 via an electrical connection 11 of the outputs of the correction filters 1, so that it harmoniously superimposes itself with the primary alternating current to form a sinusoidal current waveform. A second current transformer 2b, connected downstream of the connection 11 in this sense and structurally corresponding to the first current transformer 2a, may serve to check the quality of the filtering result made available to the electrical consumers via the consumer connection 6. In this case, the result of the check is to be fed to the measuring converter 3 as an additional input variable.

[0028] In an alternative, in Fig.In the embodiment not shown in Figure 1, the second current transformer 2b can completely replace the first current transformer 2a without departing from the scope of the invention.

Claims

[1] Device (10) for correcting alternating currents with - a supply connection (5) for connecting the device (10) to an electrical power supply network, - a consumer connection (6) for connecting the device (10) to an electrical consumer, - a power line (9) connecting the supply connection (5) to the consumer connection (6) for conducting a primary alternating current between the power supply network and the consumer, - at least one current transformer (2a, 2b) connected to the power line (9) for converting the primary alternating current into a reduced secondary alternating current (7) and - at least one active power factor correction filter (1) connected to the current transformer (2a, 2b) and the current conductor (9) for filtering the primary alternating current as a function of the secondary alternating current (7), - a measuring converter (3) connecting the current transformer (2a, 2b) to the correction filter (1) for converting the secondary alternating current (7) into a measuring signal, wherein the correction filter (1) is configured to filter the primary alternating current as a function of the measuring signal, - a signal bus (8) arranged between the measuring converter (3) and the correction filter (1) for transmitting the measuring signal from the measuring converter (3) to the power factor correction filter (1), an output current measuring device connected to the correction filter (1) for measuring an output current flowing between the correction filter (1) and the current conductor (9), wherein the correction filter (1) is configured to filter the primary alternating current by superimposing it with the output current, - the output current measuring device is integrated into the correction filter (1), characterized by , that - a computer connected to the output current measuring device and the correction filter (1) for calculating the measurement signal and the output current to form a control variable, wherein the correction filter (1) is configured to superimpose the primary alternating current on the output current as a function of the control variable. [2] Device (10) according to claim 1, characterized by - a transformer (4) arranged between the supply connection (5) and the current transformer (2a, 2b) for transforming the primary alternating current. [3] Device (10) according to claim 1 or 2, characterized by - an overvoltage protection device, in particular a protective diode, arranged between the current transformer (2a, 2b) and the measuring converter (3) for protecting the measuring converter (3) against an overvoltage. [4] Device according to one of claims 1 to 3, characterized by - an electrical connection (11) arranged between the supply connection (5) and the current transformer (2b) for connecting the correction filter (1) to the current conductor (9). [5] Device according to one of claims 1 to 3, characterized by - an electrical connection (11) arranged between the current transformer (2a, 2b) and the consumer connection (6) for connecting the correction filter (1) to the current conductor (9). [6] Device (10) according to one of the preceding claims, characterized by , that - the signal bus (8) comprises an Ethernet. [7] Device (10) according to claim 6, characterized by , that - the Ethernet is essentially real-time capable, in particular an EtherCAT. [8] Device according to one of claims 1 to 7, characterized by , that - the measuring converter (3) is integrated into the correction filter (1). [9] Device according to one of claims 1 to 8, characterized by , that - the power line (9) is a multi-pole, in particular three-pole, three-phase line, wherein the primary alternating current is a multi-phase, in particular three-phase alternating current. [10] Device (10) according to claim 9, characterized by - at least two correction filters (1) which are connected in parallel in such a way that the output currents of the correction filters (1) complement each other to form a total output current which is superimposed on the primary alternating current.

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