WO2022128219A1

WO2022128219A1 - Method for protecting an electrical installation from a short circuit and measurement system for carrying out the method

Info

Publication number: WO2022128219A1
Application number: PCT/EP2021/079865
Authority: WO
Inventors: Tim Roesinger; Alexander SCHIES
Original assignee: Sma Solar Technology Ag
Priority date: 2020-12-18
Filing date: 2021-10-27
Publication date: 2022-06-23
Also published as: DE102020134291A1

Abstract

The application describes a method for protecting an installation from a short circuit. The installation comprises a DC circuit (200) comprising a DC source (201) and a DC load (210), which is connected to the DC source (201) via feed lines, wherein the connection between the DC source (201) and the DC load (210) has a connection-intrinsic impedance (202). The method comprises the following steps: - causing a short circuit of the DC source (201) by closing a switching unit (102), which is arranged in the DC circuit (200) between the DC source (201) and the DC load (210), such that the DC source (201) is short-circuited at least by means of a portion of the connection-intrinsic impedance (202) actually present in the DC circuit (200), - detecting a time profile of a short-circuit current ISC arising during the short circuit, - limiting and/or interrupting the short-circuit current ISC by opening the switching unit (102) when a termination criterion is satisfied so that damage to components of the electrical installation, in particular of the DC circuit (200), by the short-circuit current ISC is reliably prevented, and - analysing the detected short-circuit current ISC, in particular the time profile thereof, taking into account that portion of the connection-intrinsic impedance (202) between the DC source (201) and the DC load (210) by means of which the short circuit of the DC source (201) has been caused, and - checking a design of a fuse (220) present in the electrical installation, in particular in the DC circuit (200), by means of the analysis, or determining a design of a fuse (220) still to be installed in the electrical installation, in particular in the DC circuit (200), by means of the analysis. The application also describes a measurement system for carrying out the method.

Description

Method for protecting an electrical system against a short circuit and measuring system for carrying out the method

Technical field of the invention

The invention relates to a method for protecting an electrical installation against a short circuit. In particular, the method should allow an improved design of safety fuses within the system, which takes into account the individual conditions of the system and their influence on any short-circuit current that may occur to a greater extent than was previously the case. The invention also relates to a measuring system for carrying out the method.

State of the art

Large storage power plants based on rechargeable batteries and an inverter that can be operated in both directions are increasingly being used to support and stabilize an energy supply network (EVN). At a high frequency of the EVN, the batteries can take excess active power from the EVN, store it, and return it to the EVN in a low frequency state. Using the battery storage power plants, it is also possible to have a stabilizing effect on an amplitude of an AC voltage in the EVN by exchanging reactive power.

The storage power plants typically have an energy content of between a few MWh and 100 MWh and include a large number of battery modules as DC sources. These are electrically connected to each other using a combination of series and parallel circuits. Usually, several battery modules are electrically connected to one another in the form of a series circuit to form a so-called battery string. Several battery strings are in turn connected parallel to one another with a first busbar, a so-called string busbar, and thus form a group. Several groups of this type are in turn connected parallel to one another with their string busbars to a second busbar, a so-called group busbar, which in turn is electrically connected to an input of the inverter.

The immense energy content of such storage power plants can, in the event of a fault, for example in the event of a short-circuit fault in a component in the connected Inverters, quickly lead to a fault current of 500 kA to 1000 kA, which, if not interrupted in time, can cause serious damage to the inverter or other components of the storage power plant, and possibly also to people in the vicinity.

In order to minimize and/or prevent the damage that occurs in the event of a fault, the connection between the batteries and the inverter is protected by a large number of fuses. Specifically, each battery string is usually connected to its associated string busbar via a fuse—a so-called string fuse. Each string busbar is connected to the group busbar assigned to it via another fuse - a so-called group fuse. In addition to the safety fuses, electromechanical switches can also be provided.

The triggering behavior of a fuse is mainly characterized by its triggering current and its so-called limit load integral, also known as melting integral or Ft value. If the tripping current is exceeded, the safety fuse is heated by the current flow passing through it in such a way that it can no longer, at least not completely, dissipate the power loss transmitted by the current to the safety fuse to the environment in the form of waste heat. As a result, the fuse will heat up, melt and break the circuit. The tripping current of the safety fuse acts as a kind of trigger parameter for the tripping process, with the tripping process only starting/being triggered at a current above the tripping current. If the current is below the tripping current, the fuse will not trip. The fuse load limit integral is a measure of the energy required to blow the fuse. It therefore describes the dynamics of the triggering of the fuse. A fast-acting fuse has a comparatively small load limit integral relative to a slow-acting fuse.

In the battery storage power plants in question, the rise behavior of a short-circuit current is significantly influenced by the individual electrical connection between the battery modules as the DC source and the inverter as the DC load. In particular, an impedance inherent in the connection, also referred to as connection-immanent in the following, is relevant here. For example, long supply lines and their connection-immanent inductance can lead to a throttling of the rise in the short-circuit current. If, on the other hand, the connection has a relatively small inductance inherent in the connection, the rise in current is significantly steeper. If, for example, a fuse that reacts too sluggishly is used, the current, depending on the rate of increase, may already have risen to values that damage components of the inverter and/or other components of the storage power plant before the fuse melts. A fuse that reacts too quickly can lead to an undesired triggering of the fuse that is not associated with a real fault. On the other hand, quick-reacting fuses usually also have a higher power loss during normal operation of the storage power plant, which is also undesirable.

In order to correctly design the fuses to protect the storage power plant against a short circuit, it is therefore desirable to know as well as possible the transient behavior of a short circuit current that occurs in the event of a fault. Conventionally, this is currently estimated using calculations that include, for example, the actual cable lengths between the DC source and the DC load. However, the estimation is partly associated with a high degree of inaccuracy. Alternatively or in addition to this, laboratory tests on the short-circuit behavior of a planned system can be carried out. However, the laboratory setup does not correspond 100% to the real setup of the system, which is why the results obtained in the laboratory test cannot be directly transferred to the future real system.

An electrical switching device operating as an electronic fuse is known from the publication WO 2018185051 A1. The switching device comprises at least one semiconductor switch arranged in a current path, electrical measuring devices and a control device, which is designed to evaluate the measured values determined and to control the semiconductor switches. When specific measured values are recorded, the control device automatically triggers specific activations of the semiconductor switches. The conditions for driving the semiconductor switches can be transmitted to the control device by means of an initiation process via a data interface. In this way, fuses that are structurally identical per se can be programmed with a different tripping behavior. However, electronic fuses of this type are usually associated with higher costs, which are disadvantageous in particular when there are a large number of fuses to be used. In addition, rapid and reliable triggering of the electronic fuse depends on proper and error-free operation of the semiconductor switch. It has been shown that safety fuses, if they are selected correctly, generally operate more error-free and more reliably.

The publication DE 3446958 A1 discloses an arrangement for monitoring short circuits or overloads in electronic proximity switches. A current detection is provided in the output circuit, which influences the current flow by switching off or limiting. A query cycle for current detection has a delay time for the influence and a waiting time between two queries, the waiting time being significantly longer than the delay time, and only the length of the first delay time is selected with regard to a time constant of the line to be connected.

The publication DE 199 46 826 A1 describes a safety device for an electric circuit in vehicles with a safety fuse which permanently interrupts the electric circuit when the current load is above the rated current. The safety device has at least one thermal heating element, which is in thermal connection with the safety fuse and additionally supplies a limited amount of heat to the safety fuse. The limited amount of heat supplied by the heating element is less than the energy required to trigger the safety fuse.

Document DE 19741828 A1 discloses an electrical safety switch for motor vehicles, in which a positive pole of a vehicle battery is connected to the vehicle electrical system via a fuse link. A negative pole of the vehicle battery is connected via a short-circuit actuation device to a connection of the fuse link that is on a supply line to the vehicle electrical system. If the vehicle is involved in an accident, the short-circuit actuation device uses an information transmitter to short-circuit the battery connections.

The publication DE 10 2017 107517 A1 discloses an electronic circuit with an electronic switch and a control circuit for controlling the electronic switch. The control circuit is configured to operate in one of a first mode and a second mode based on a level of a load current of the electronic switch. In the first operating mode, the control circuit is designed to generate a first protection signal based on a current-time characteristic of the load current and to control the electronic switch based on the first protection signal.

object of the invention

The invention is based on the object of specifying an improved method for protecting a system with a DC source and a DC load against a short circuit. In concrete terms, the method should allow one or more safety fuses to be designed that take into account the individual conditions of the system, in particular the connection between the DC source and the DC load, the impedance inherent in the connection and its influence on the fault current that occurs in the event of a fault. The method should be able to be carried out as simply and inexpensively as possible. It is also the object of the invention to provide a measuring device suitable for carrying out the method.

solution

The object of demonstrating a method for protecting a system against a short circuit is achieved according to the invention with the features of independent patent claim 1 . The object of demonstrating a measuring system suitable for carrying out the method is achieved according to the invention with the features of independent claim 5 . Advantageous embodiments of the method are set out in claims 2 to 4. Claims 6 to 11 specify advantageous embodiments of the device.

Description of the invention

The invention aims at a method for protecting an electrical installation against a short circuit. The electrical installation comprises a DC circuit with a DC source and a DC load connected to the DC source via supply lines, the connection between the DC source and the DC load having an impedance inherent in the connection. The procedure includes the steps: Bringing about a short circuit of the DC source by closing a switching unit, which is arranged between the DC source and the DC load in the DC circuit, so that the DC source has at least a portion of the connection inherent in the DC circuit that is actually present impedance is short-circuited,

Detection of a short-circuit current Isc occurring during the short-circuit, in particular a time profile of the short-circuit current Isc,

Limiting and/or interrupting the short-circuit current Isc by opening the switching unit when a termination criterion is met, so that damage to components of the electrical system, in particular the DC circuit, is reliably prevented by the short-circuit current Isc, and

Analysis of the detected short-circuit current Isc, in particular its time profile, taking into account at least that part of the connection-immanent impedance between the DC source and the DC load, via which the short circuit of the DC source was brought about, and

Checking a design of a fuse present in the electrical system, in particular in the DC circuit, using the analysis, or determining a design of a fuse still to be installed in the electrical system, in particular in the DC circuit, using the analysis.

The short-circuit current Isc can be detected via one or more current sensors arranged in the DC circuit. A time profile of the short-circuit current Isc means any information that provides information about a change in the short-circuit current Isc over time. In particular, the time profile can be detected by detecting one or more corresponding value pairs of the time-dependent short-circuit current Isc(t) and the time t corresponding thereto. Within the scope of the invention, the detection of only one corresponding pair of values for short-circuit current and the corresponding time also falls under the term "detection of the time profile of the short-circuit current ISC", provided that another pair of values of short-circuit current and time that are adjacent in time can be assumed to be known. For example, the pair of values can be assumed to be known at the time when the short circuit is brought about by the closing of the switching unit. Specifically, it can correspond to the current flowing at the location of the current sensor during operation of the system. In particular, if no Power flow between DC source and DC load flows, the current when closing the switching unit is OA

A measuring system according to the invention enables protection of an electrical installation against a short circuit. In this case, the electrical system comprises a DC circuit with a DC source and a DC load connected to the DC source via supply lines, the connection between the DC source and the DC load having an impedance inherent in the connection. The measuring system comprises: a switching unit which is designed to bring about a short-circuit current Isc in the closed state and to interrupt a short-circuit current Isc in the open state in the DC circuit, at least one current sensor which is designed to detect a short-circuit current Isc occurring in the closed state of the switching unit , in particular its time profile, is an evaluation unit (115) connected to the at least one current sensor, which is set up to analyze the detected short-circuit current Isc, in particular a time profile of the short-circuit current Isc, and a control unit connected to the evaluation unit for controlling the measuring system, in particular the switching unit. When installed in the DC circuit, the measuring system is designed and set up for carrying out the method according to the invention.

The at least one current sensor for detecting the short-circuit current Isc can comprise only one current sensor, but also a plurality of current sensors. The evaluation unit and the control unit can each be present as separate and/or separate units. As an alternative to this, however, it is also possible for the control unit and the evaluation unit to be combined in the form of a common control and evaluation unit. The evaluation unit can also be set up to signal a correct or incorrect design of a fuse present in the electrical system, particularly in the DC circuit of the electrical system, depending on the analysis of the detected short-circuit current Isc, in particular its time profile. To this end, it can have appropriate signaling means. Alternatively or in addition to this, it can be set up to incorporate one or more design parameters into the electrical system, in particular into to determine and output the fuse still to be installed in the DC circuit of the electrical system. To output the design parameters, the evaluation unit can have a communication unit or be connected to one.

Within the scope of the invention, a short circuit is brought about in a targeted manner via the switching unit on the actual electrical installation, in particular at a suitable point between the DC source and the DC load. In this case, the switching unit can be installed where, for example after a risk analysis, a possible short-circuit fault can be assumed to be probable. This can also involve a number of points at which a short circuit is generated one after the other by means of the switching unit. In the forced short-circuit, the DC source, or at least a part of the DC source, drives an increasing short-circuit current Isc through the switching unit and that part of the connection-immanent impedance between the DC source and the DC load through which the switching unit with the short-circuit current Isc driving DC source is electrically connected. This results in an increase in the short-circuit current Isc over time, which is recorded by one or more current sensors.

The increase in the short-circuit current over time depends on the proportion of the impedance inherent in the connection between the DC source and the DC load that caused the short-circuit in the DC source. In concrete terms, this is that part of the connection-immanent impedance that is located between the DC source driving the short circuit and the switching unit. The connection-immanent impedance of the connection between DC source and DC load summarizes the impedances of all components that are inherent in the connection between DC source and DC load of the actual system.

As also explained in more detail in connection with FIGS. 1 and 2 , the impedance inherent in the connection can be represented as a series connection made up of a resistive and an inductive impedance component. When the short circuit is brought about, the short-circuit current Isc increases over time, usually exponentially, depending on the shape

In this case, the parameters Isc.o and T are characteristic of the DC circuit, in particular of the current short-circuit path of the DC circuit. Specifically, the parameter Isc.o is a theoretical maximum current that would flow in the short-circuit path in the extreme case of long times (t -> °°) if the components arranged in the short-circuit path would withstand this current load without being damaged or destroyed beforehand. The value of the parameter Isc.o mainly depends on the internal resistance Ri of the DC source and the resistive impedance component R _a of the connection-immanent impedance - also slightly on the ohmic resistance of the switching unit RSE causing the short-circuit. In many cases, however, the internal resistance Ri is high compared to the remaining resistive components Ra and RSE and is therefore the limiting factor in relation to the maximum short-circuit current Isc.o of the DC source. The DC time constant T describes the dynamics of the increase and depends according to

on the inductive impedance component L. The inductive impedance component is influenced on the one hand by the length of the supply lines between the DC source and the DC load, or more precisely in the short-circuit path. Specifically, the smaller the inductive impedance component in the short-circuit path, the steeper the rise in the short-circuit current over time. Conversely, the greater the inductive impedance component in the short circuit path, the slower the rate of rise. On the other hand, the inductive impedance component L is also influenced, for example, by the laying of positive and negative supply lines relative to one another. In concrete terms, for example, positive and negative supply lines that are close together have a lower inductive impedance component L than the supply lines that are otherwise of the same length but are laid further apart from one another. This is due to the fact that a magnetic field is largely compensated for in an outer area of the supply lines when the supply lines are close together and is only formed to a significant extent in a small spatial area between the supply lines. On the other hand, if the leads are far apart, the mutual compensation is low and the magnetic field forms around each of the positive and negative leads. In the short-circuited state of the DC source, compliance with a termination criterion is continuously and repeatedly checked over time. According to the invention, the short-circuit current Isc is interrupted when the termination criterion is met by opening the switching unit. The short-circuit current Isc is usually interrupted well before the theoretical maximum current Isc.o characterizing the equilibrium case is reached, ie at significantly lower values of the short-circuit current Isc. The repeated checking of the termination criterion ensures that no component of the electrical system is damaged when the short circuit is caused.

The value pairs of short-circuit current Isc(t) and the corresponding time t detected during the short-circuit that was caused are analyzed. For example, the time constant T, which characterizes the increase dynamics of the short-circuit current, can be determined via the time profile of the short-circuit current. According to the present invention, it is not necessary to detect the lapse of time in the entire current range. Rather, it is sufficient to detect only a relatively short period of time after the short circuit was brought about. The remaining time profile can be extrapolated with knowledge of the initial time profile and in particular with knowledge of the basic exponential time behavior according to Equation 1. Knowing the initial course of time, it is possible to check whether a fuse installed in the DC circuit, in particular a fuse installed in the short-circuit path, is designed correctly. Likewise, a design parameter for a fuse that is to be installed in the DC circuit, in particular in the short-circuit path, can be determined from the time profile of the short-circuit current Isc. If the check reveals that a fuse installed in the electrical system is not designed correctly, the fuse installed in the system can be exchanged for a fuse with a different tripping behavior. Alternatively or in addition to this, the connection of the DC source to the DC load can also be changed, and via this the inductive impedance component L of the impedance inherent in the connection can be changed. Specifically, for example, the routing of the supply lines relative to one another and/or their length can be changed, which causes the inductive impedance component L to change. A similar procedure can be followed if the determined design parameters of the fuse still to be installed indicate a fuse that is not available in this form. In this case, the inductive impedance component L of the real electrical equipment can be modified as follows that an available backup can be used. The fuse to be checked and/or the fuse still to be installed can be designed in particular as a fuse.

Before the short circuit is brought about via the switching unit, individual components of the electrical system can be separated from the DC circuit. The components can remain separated from the DC circuit until the short circuit is ended again by opening the switching unit. This is possible in particular when the corresponding components do not affect the short-circuit current Isc in the DC circuit at all or only to an insignificant extent. Specifically, for example, before and during the short circuit that is brought about, the DC load can be separated from the DC circuit if the switching unit—as shown in FIGS. 1 and 5—is arranged in parallel with the DC load.

According to the invention, the connection-immanent impedances present in the real electrical installation are taken into account when designing the planned fuse and/or checking the existing fuse. A possibly deviating replica of an electrical system and subsequent tests under laboratory conditions are deliberately avoided. This takes account of the rise behavior of the short-circuit current Isc, which varies greatly in some cases, in different electrical systems despite at least largely the same DC sources and/or DC loads. Since the connection-immanent impedances that are actually present are included in the design/checking of the fuses, they are significantly more precise than can be achieved with a laboratory replica or a simulation calculation. In concrete terms, the fuse can be optimized both with regard to the lowest possible on-state resistance during normal operation of the electrical system and still react sufficiently quickly in the event of a short-circuit current that may occur. In particular, the compromise between the two fundamentally opposing parameters "low on-state resistance" and "highly dynamic triggering behavior" of a fuse is much better possible by taking into account the actual connection-immanent impedances of the electrical system in question than using conventional laboratory tests or simulation calculations.

The measuring system can be produced comparatively inexpensively. It can be designed to be mobile, with a specific measurement system divided into a variety of electrical Systems installed, and can be removed again after the process has been carried out. The method can be automated and carried out in a simple manner. Damage to components of the electrical system is largely ruled out, since the short-circuit caused is only brief, includes constant monitoring of the short-circuit current Isc during the short-circuit caused, and is limited in its maximum occurring short-circuit current by a timely current interruption. In summary, the method according to the invention can be used to protect the electrical system against short circuits in a cost-effective and yet comparatively precise manner, which takes into account the individually present connection-immanent impedances of the individually different electrical systems.

Advantageous refinements of the invention are specified in the following description and the dependent claims, the features of which can be used individually and in any combination with one another.

In an advantageous variant of the method, the DC time constant T assigned to the DC circuit can be determined during the analysis of the detected short-circuit current Isc, in particular its time profile. This can be done, for example, by determining a slope of a tangent to the time profile that describes the point in time at which the short circuit was brought about. Alternatively, it is possible to adapt a theoretical curve of the exponential time behavior according to Equation 1 by varying the parameters that occur (here: the DC time constant T to be determined and the theoretical maximum current Isc.o) to the detected measured values in such a way that the sum of the squared errors is between the measured values and the theoretical curve becomes minimal. As an alternative or in addition to determining the time constant, an optimal value for a parameter that describes a triggering threshold or a triggering behavior of a fuse can also be determined during the analysis. Such a parameter can be, for example, a tripping current and/or a limit load integral of the fuse.

In a variant of the method, the termination criterion can include one or more of the following events: reaching or exceeding a threshold value ITH for the short-circuit current Isc, reaching or exceeding a threshold value Atm for a period of time At that has elapsed since the switching unit was closed, reaching or exceeding a threshold value for a limit load integral i ² tm assigned to the short-circuit current Isc.

The method can preferably be used in a battery storage power plant, in a photovoltaic power plant (PV power plant), or in a combined power plant (i.e. a combination of battery storage power plant and PV power plant). Specifically, the process can use the DC -Source include a variety of batteries and / or a variety of PV modules, which are arranged in series and / or parallel to each other.In addition, the combined cycle power plant can also include fuel cells as DC sources.Furthermore, the DC load can have a DC/AC converter and/or a DC/DC converter that is unidirectional in relation to a power flow passing through it—in the case of a PV power plant—but alternatively also bidirectional—in the case of a battery storage power plant or combined cycle power plant - are operable.

The switching unit typically has a first connection, a second connection, a connecting line arranged between the first and the second connection and at least one switch arranged in the connecting line, for example a semiconductor switch or an electromechanical switch. In one embodiment of the measuring system, the switching unit can include a series connection of an actively controllable semiconductor switch and an electromechanical switch arranged in the connecting line between the first and second connection or a series connection of an actively controllable semiconductor switch and a safety fuse. In a further embodiment, the switching unit can have a series connection of an electromechanical switch and a safety fuse between the first and the second connection. Due to the combination of a semiconductor switch on the one hand and an electromechanical switch or fuse on the other hand, the switching unit is able to react in a fast-switching manner (through the semiconductor switch) as well as to ensure a galvanically isolating current interruption (through the electromechanical switch or the fuse). In one embodiment of the measuring system, the at least one current sensor can include a large number of current sensors which are connected to the evaluation unit and which are each designed to detect a short-circuit current Isc flowing in the DC circuit, in particular a time profile of the short-circuit current Isc. Alternatively or cumulatively, the measurement system can include one or more voltage sensors connected to the evaluation unit, which are each designed to detect a voltage Use prevailing in the DC circuit, in particular a time profile of the voltage Use. Since the measuring system has multiple current sensors and/or multiple voltage sensors, multiple time curves of short-circuit current Isc and/or short-circuit voltage Use can be detected simultaneously when a short-circuit occurs, for example at different points in the DC circuit. In this way, the connection-immanent impedances present in the DC circuit can be analyzed more efficiently.

In an advantageous variant of the measuring system, the current sensor or one of the current sensors can be arranged in a connecting line between the first connection and the second connection of the switching unit. Specifically, the current sensor can be set up to detect the short-circuit current Isc flowing between the terminals of the switching unit, in particular its time profile. Furthermore, the voltage sensor or one of the voltage sensors of the measuring system can be connected to the first connection of the switching unit on the one hand and the second connection of the switching unit on the other hand and be set up in this way to detect a voltage Use prevailing between the connections of the switching unit, in particular its time profile.

Brief description of the characters

The invention is illustrated below with the aid of figures. From these show

1 shows an embodiment of a measuring system installed in a DC circuit of an electrical installation;

FIG. 2a shows a first embodiment of a switching unit of the measuring system from FIG. 1; FIG.

Fig. 2b shows a second embodiment of a switching unit of the measuring system from Fig.

1 ; FIG. 2c shows a third embodiment of a switching unit of the measuring system from FIG. 1 ;

3 shows a flow chart of an embodiment of the method according to the invention;

4 shows an analysis of the measured values detected using the example of a time profile of the short-circuit current Isc(t);

5 battery storage power plant as an electrical system with a measuring system installed therein;

character description

1 shows an embodiment of a measuring system 100 installed in a DC circuit 200 of an electrical installation. The DC circuit 200 includes a DC source 201 (illustrated in FIG. 1 by way of example as a battery with an effective internal resistance Ri) and a DC load 210 which is connected to the DC source 201 via supply lines 203 . The connection of the DC load 210 and the DC source 201 has a fuse 220 . In addition to the fuse 220 and the supply lines 203, the connection between the DC load 210 and the DC source 201 can contain other components, for example isolating switches, which are not explicitly shown in FIG. 1 . The connection between the DC source 201 and the DC load 210 has _a connection-immanent impedance 202, which is illustrated in FIG.

A correct design of the safety fuse 220 is now to be checked with the measuring system 100, taking into account the impedance 202 inherent in the connection. For this purpose, the measuring system 100 includes a switching unit 102 with a series connection of an actively controllable semiconductor switch 103 and a further safety fuse 104, as well as a control device 101 that activates the switching unit 102. By way of example, the switching unit 102 is connected in the DC circuit 200 in such a way that the DC Source 201 is short-circuited in the closed state of the switching unit 102 via the connection-immanent impedance 202 and the safety fuse 220 to be checked. The measurement system 100 also includes an evaluation unit connected to the control unit 101 115, a current sensor 110 for detecting a short-circuit current Isc flowing in the DC circuit, here in particular in the short-circuit path. The measurement system 100 also includes a first voltage sensor 111a and a second voltage sensor 111b, each for detecting a voltage Use prevailing in the DC circuit 200 . By way of example, the first voltage sensor 111a is set up to detect a voltage Uma prevailing between connection terminals of the DC source 201, in particular its time profile. The second voltage sensor 111b, on the other hand, is set up to detect a voltage Un prevailing between a first connection 106 and a second connection 107 of the switching unit 102, in particular its time profile. An operation of the measurement system 100 is explained in more detail in connection with FIGS.

Different embodiments of the switching unit 102a - 102c, which can preferably be used in the measuring system 100, are illustrated in FIGS. 2a - 2c. A first embodiment of the switching unit 102a is shown in FIG. 2a. The switching unit 102a comprises a first terminal 106 and a second terminal 107. In a connecting line between the terminals 106, 107 is an actively controllable semiconductor switch 103, for example an IGBT (insulated gate Bipolar transistor) or a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) arranged. A further safety fuse 104 is arranged in series with the semiconductor switch 103 and also between the terminals 106, 107. To bring about the short circuit, the actively controllable semiconductor switch 103 is closed, while it is opened to interrupt the short-circuit current Isc. The additional safety fuse 104 is dimensioned here so that it triggers when a relatively low current threshold value is reached, which is below a value for a short-circuit strength of that component that would most likely be damaged by an overcurrent in the short-circuit path. This ensures that even if the semiconductor switch 103 is operating incorrectly, none of the components in the short-circuit path will be damaged by the short-circuit that has been brought about.

Advantageously, one of the voltage sensors of the measuring system 100 - here the second voltage sensor 111b - can be connected to the connections 106, 107 of the switching unit 102 be connected and set up to detect a voltage Un prevailing between the terminals 106, 107 when the short circuit is caused. Furthermore, one of the current sensors - in this case the second current sensor 110b - can be arranged in the connecting line between the first connection 106 and the second connection 107 and set up to detect the short-circuit current Isc flowing in the connecting line, in particular its time profile Isc, during the short circuit that has been brought about. The connection shown of the current sensor 110b and the voltage sensor 111b is optionally also possible in the second and the third embodiment of the switching unit 102 .

The second embodiment of the switching unit 102b according to FIG. 2b is similar to the first embodiment according to FIG. To bring about the short circuit, the electromechanical switch 105 is closed if it is not already closed. After that, the semiconductor switch

103 closed. The short-circuit current Isc is interrupted by the actively controllable semiconductor switch 103 being opened. Thereafter, the electromechanical switch 105 can, but does not have to be opened. The fast-response function of the switching unit 102b is provided by the semiconductor switch 103 as in the first embodiment. The electromechanical switch 105, like the further fuse 104 in the first embodiment of the switching unit 102a, provides a galvanic isolation function for the switching unit 102b.

The third embodiment of the switching unit 102c comprises a series connection of a further fuse 104 and an electromechanical switch 105. The short circuit is brought about here by closing the electromechanical switch 105, while the short-circuit current flows through the triggering further fuse

104 is interrupted. Thus, in the third embodiment, the further fuse 104 provides the fast switching function of the switching unit 102c. The electromechanical switch 105 is used here mainly for a redundant separation function for the short-circuit current Isc, should the additional fuse 104 not be able to withstand the forced short-circuit current Isc—for example due to an arc burning across the tripped further fuse 104 - to separate. For this purpose, the electromechanical switch 105 can have a comparatively high current-carrying capacity and suitable means for extinguishing the arc (not explicitly shown in FIGS. 2b and 2c).

FIG. 3 shows a variant for operating the method according to the invention in the form of a flow chart, as can be carried out, for example, with the measuring system 100 from FIG. 1 installed in the DC circuit 200 of an electrical installation. The method starts with a step A1. In step A2, a short circuit in DC circuit 200 is deliberately brought about by closing switching unit 102 . As a result, a short-circuit path is formed in the DC circuit 200 in which the DC source 201 is short-circuited via at least part of the impedance 202 inherent in the connection and the switching unit 102 . Due to the short circuit, an increasing short circuit current Isc is generated in the short circuit path. A time profile of the short-circuit current Isc(t) is now detected via the at least one current sensor 110 in step A3. In addition, a time course of the voltage Uma (t) which is present between the connection terminals of the DC source 201 is detected via the first voltage sensor 111a. Furthermore, a time course of a voltage Unib(t) prevailing between the terminals 106, 107 of the switching unit 102 is detected via the second voltage sensor 111b. In step A4, the fulfillment of an abort criterion is checked continuously by the evaluation unit 115 and in each case with the last detected values of the short-circuit current Isc(t). The termination criterion can be reaching or exceeding a current threshold value ITH due to the increasing short-circuit current lsc(t), reaching or exceeding a time threshold value Atm, the time Atm that has elapsed since the short circuit was caused, and/or reaching or exceeding a predefined threshold value l ² tm of a limiting load integral lsc ² t (t) formed by means of the short-circuit current lsc(t).

As long as the termination criterion is not met, the method jumps back to step A3. If, on the other hand, the termination criterion is met, the switching unit 102 is opened in step A5, as a result of which the short-circuit current Isc(t) is interrupted. In step A6, the detected time curve for the short-circuit current Isc(t) is analyzed, possibly also taking into account the detected time curves for the voltages Uma, Un. The aim of the analysis is to set a trigger threshold to determine the parameters characterizing the fuse 220, in which the connection-immanent impedances 202 are taken into account, as they actually exist between the DC source 201 and the DC load 210 of the electrical installation. The triggering threshold characterized by the specific parameter can be compared with a triggering threshold of the fuse 220 already installed in the DC circuit 200 . In this way, a correct design of the fuse 220 can be checked and, if necessary, adjusted. Alternatively or in addition to this, it is possible to design a fuse 220 to be installed in the DC circuit 200 appropriately and as optimally as possible using the specific parameters. The method ends in step A7.

FIG. 4 shows a variant of an analysis using the example of a time curve for the short-circuit current Isc(t). The analysis pursues the goal of determining a DC time constant T ZU characterizing the DC circuit 200—or more precisely: a DC time constant TZU that characterizes the respective short-circuit path. The course illustrated in FIG. 4 reflects, by way of example, a situation in which reaching or exceeding a predefined current threshold value ITH is set as a termination criterion for measuring system 100 . At time t=0, the short circuit is brought about by a closing of the switching unit 102, causing an increasing short circuit current Isc(t) in the short circuit path. At the time ti, the short-circuit current Isc reaches the predefined current threshold value ITH, which is registered by the evaluation unit 115 and the current sensor 110 connected to it. In response to this, the switching unit 102 is immediately opened, thereby interrupting the short-circuit current Isc. In FIG. 4 there are thus actually detected values for the short-circuit current Isc(t) in the region below the current threshold value ITH, which is symbolized by the solid curve section 401a over time of the short-circuit current Isc(t). In contrast, the curve portion 401b above the current threshold value ITH only describes a theoretical time profile of the short-circuit current Isc, which is, however, extrapolated taking into account the measured values detected below ITH. This is symbolized in FIG. 4 by the curve section 401b drawn in broken lines. Due to the series connection of the resistive and the inductive impedance components of the connection-immanent impedance 202, a curve 401 for the time profile of the short-circuit current Isc(t) results that tends exponentially towards the limit value Isc.o. To determine the DC time constant T, a tangent 402 is now nestled against the curve section 401a at t=0—ie at the point in time at which the short circuit is brought about—and its slope m is determined. It can be shown that the gradient m of the tangent 402 according to m = - = Y (3) corresponds to the quotient of open-circuit voltage Uo of the DC source 201 and the inductive impedance component L of the connection-immanent impedance 202 and also a measure of the DC time constant T of the short-circuit path. Furthermore, it is easy to see from Fig. 1 that in the event of a short circuit, the maximum possible short circuit current Isc.o in the short circuit path shown consists of the no-load voltage Uo of the DC source 201, the internal resistance Ri of the DC source 201, the resistor R102 of the switching unit 102, and the resistive impedance component R _a of the connection-immanent impedance 202 according to

Isc.o = _R ^K i. ₊ ⁺ _R ^K i0 ⁰ 2 ₊ ⁺ _ß ^K a (4) results. The no-load voltage Uo and the internal resistance Ri of the DC source are usually known with sufficient accuracy from technical data sheets for the DC source 201 used. The variable R102 describes the ohmic resistance of the switching unit 102 used, which is also known with sufficient accuracy from technical data sheets for the components used for the switching unit 102. A fairly good approximation value can be calculated for the resistive impedance component R _a taking into account the material and the geometry of the leads in the short-circuit path. Thus, all variables are known in order to calculate according to Eq. 4 to calculate the maximum possible short-circuit current Isc.o in the short-circuit path. With the determined slope m of the tangent, according to Eq. 3 calculate the DC time constant T of the short-circuit path.

In some cases, the internal resistance of the battery Ri is the current limiting factor and is large compared to the resistive impedance component R _a and also the resistance of the switching unit 102, ie Ri >> R _a and Ri >> R102. In this case Equation 3 simplifies to

In these cases, the maximum short-circuit current of the DC source can also be used as a good approximation for the maximum short-circuit current in the short-circuit path, which is also known with sufficient accuracy from data sheets for the DC source. This can also be used to determine the DC time constant T of the short-circuit path from the gradient m determined for the tangent and taking into account equation (3).

FIG. 5 shows a battery storage power plant 300 as an electrical system, such as is used to support an energy supply network 330, for example. The storage power plant is secured against a short circuit with a large number of fuses, in particular fuses. A correct design of the fuses can be checked in the battery storage power plant 300 with the measuring system 100 connected therein.

The battery storage power plant 300 comprises, as a DC source, a multiplicity of battery modules which are connected to one another by a combination of series and parallel circuits. A series connection of a plurality of battery modules forms a so-called battery string 302a.1-302a.n, 302b.1-302b. n. For reasons of clarity, in FIG. 5 in each of the battery strings 302a.1-302a. n, 302b.1 - 302b. n the series connection of the battery modules is only shown in the form of a battery module. Several battery strings 302a.1-302a are now in the battery storage power station. n connected parallel to one another via an isolating circuit 310 consisting of an isolating switch 312 and a string fuse 311 to a first busbar 315a—a so-called string busbar. The parallel connected battery strings 302a.1 - 302a. n form a first group 303a in this way. A comparably constructed second group 303b with a plurality of battery strings 302b.1-302b. m is connected to another first busbar 315b. Both groups 303a, 303b are connected parallel to one another to a second busbar 316—a so-called group busbar. The second bus bar 316 is connected to an input of a DC/AC converter 320 as a DC load 210 . The DC/AC converter 320 is connected to the power grid 330 for the exchange of electrical power and for the purpose of grid support. In the connecting lines between the first busbars 315a, 315b and the second busbar 316 are each overcurrent fuses 220a, 220b arranged. The function of the overcurrent fuses 220a, 220b is to be checked with the measuring system 100 and, if necessary, adjusted.

For this purpose, a temporary short circuit is generated by closing the switching unit 102, which is connected here in parallel to a DC input of the DC/AC converter 320. The short-circuit currents Isc flowing via the connecting lines between the first busbar 315a, 315b and the second busbar 316 are recorded with their time profiles for each group 303a, 303b via one of the current sensors 110a, 110b. In addition, time profiles of voltages on the first busbars 315a, 315b by corresponding

Voltage sensors 111 a, 111 b detected. A current sensor and a voltage sensor are also arranged in switching unit 102 (not explicitly shown in FIG. 5 ), which detects the short-circuit current Isc flowing through switching unit 102 and the voltage U102 dropping across switching unit 102 during the short circuit. After a predefined termination criterion has been reached or exceeded - for example a current threshold value Ith for the short-circuit current Isc flowing through the switching unit 102 has been reached or exceeded, the switching unit 102 is opened again, as a result of which the short-circuit current Isc flows through the switching unit 102 and in each of the connecting lines between the first busbars 315a , 315b and the second bus bar 316 is interrupted. The evaluation unit 115 analyzes the detected time curves of the detected short-circuit currents Isc and voltages and can use this to determine an optimum value for a design parameter of the fuses 220a, 220b, taking into account the impedances 202 inherent in the connection that are actually present. The triggering parameters of the existing fuses 220a, 220b can be compared with the optimum value determined via the measuring system 100, whereupon the fuses 220a, 220b can be exchanged for fuses with the determined optimum value, if necessary. Alternatively or in addition to this, it is possible to adjust the connection-immanent impedance of the connection between the DC source and the DC load of the storage power plant 300 by changing the length of the supply lines and/or their position relative to one another. Reference character list

100 measurement system

101 control unit

102 switching unit

103 semiconductor switches

104 backup

105 electromechanical switch

106 connection

107 connection

110, 110a, 110b current sensor

111, 111a, 111b tension sensor

115 evaluation unit

200 DC circuit

201 DC source

202 connection inherent impedance

203 lead

210 DC load

220, 220a, 220b fuse

300 Battery Storage Power Plant

302.a.1 - 3O2.a.n battery string

302. b.1 - 3O2.b.n battery string

303a, 303b group

315a, 315b string busbar

316 group busbar

320 DC/AC converter

330 power grid

Isc short-circuit current

401a, 401b curve section

402 tangent

A1 - A7 process step

Claims

- 24 -

Method for protecting a system against a short circuit, the system comprising a DC circuit (200) with a DC source (201) and a DC load (210) connected to the DC source (201) via supply lines, and wherein the connection between the DC source (201) and the DC load (210) has a connection-inherent impedance (202), comprising the steps of:

- Causing a short circuit of the DC source (201) by closing a switching unit (102) which is arranged between the DC source (201) and the DC load (210) in the DC circuit (200), so that the DC source (201) is short-circuited at least over a portion of the connection-immanent impedance (202) actually present in the DC circuit (200),

- Detection of a time profile of a short-circuit current Isc occurring during the short-circuit,

- Limiting and/or interrupting the short-circuit current Isc by opening the switching unit (102) when a termination criterion is met, so that damage to components of the electrical system, in particular the DC circuit (200), is reliably prevented by the short-circuit current Isc, and

- Analysis of the detected short-circuit current Isc, in particular its time profile, taking into account that portion of the connection-immanent impedance (202) between the DC source (201) and the DC load (210) through which the short circuit of the DC source (201) was brought about , and

- Checking a design of a fuse (220) present in the electrical system, in particular in the DC circuit (200) by means of the analysis, or determining a design of a fuse still to be installed in the electrical system, in particular in the DC circuit (200). (220) using the analysis. Method according to claim 1, wherein during the analysis of the detected short-circuit current Isc a DC time constant T assigned to the DC circuit (200) and/or a parameter characterizing a tripping threshold of a fuse (220), for example a tripping current and/or a limit load integral, is determined becomes. Method according to one of the preceding claims, wherein the termination criterion comprises at least one of the following events:

- reaching or exceeding a threshold value ITH for the short-circuit current Isc,

- a threshold value At™ being reached or exceeded for a period of time At which has elapsed since the switching unit (102) was closed,

- reaching or exceeding a threshold value for a limit load integral i ² t™ assigned to the short-circuit current Isc. Method according to one of the preceding claims, wherein the DC source (201) comprises a plurality of batteries (5) and/or a plurality of PV modules which are arranged in series and/or parallel connection with one another, and wherein the DC Last (210) comprises a DC / AC converter (501) and / or a DC / DC converter. Measuring system (100) for protecting a system against a short circuit, the system comprising a DC circuit (200) with a DC source (201) and a DC load (210) connected to the DC source (201) via supply lines , and wherein the connection between the DC source (201) and the DC load (210) has a connection-immanent impedance (202), comprising a switching unit (102) which in the closed state for bringing about and in the open state for interrupting a short-circuit current Isc is designed in the DC circuit (200), at least one current sensor (110) which is designed to detect a short-circuit current Isc occurring in the closed state of the switching unit (102), in particular its time profile, with the at least one current sensor (110) connected evaluation unit (115), which is set up to analyze the detected short-circuit current Isc, in particular its time profile, and a control unit (101) connected to the evaluation unit (115). Control of the measuring system (100), in particular the switching unit (102), characterized in that the measuring system (100) is designed and set up when installed in the DC circuit (200) for carrying out the method according to one of the preceding claims. Measuring system (100) according to Claim 5, characterized in that the switching unit (102) comprises a series connection of an actively controllable semiconductor switch (103) and an electromechanical switch (105), or a series connection of an actively controllable semiconductor switch (103) and a safety fuse (104 ) includes. Measuring system (100) according to Claim 5, characterized in that the switching unit (102) is a series circuit of an electromechanical switch

(105) and a safety fuse (104). Measuring system (100) according to one of Claims 5 to 7, characterized in that the at least one current sensor (110) comprises a multiplicity of current sensors (110a, 110b). Measuring system (100) according to one of Claims 5 to 8, additionally comprising one or more voltage sensors (111, 111a, 11b) connected to the evaluation unit (115), each of which is/are used to detect a voltage in the DC circuit (200). Voltage Use, in particular a time profile of the voltage Use prevailing in the DC circuit (200). Measuring system (100) according to Claim 9, characterized in that the voltage sensor (111) or one of the voltage sensors (111, 111a, 111b) is connected to a first connection (106) and a second connection (107) of the switching unit (102). and is set up to detect a voltage Use prevailing between the terminals (106, 107), in particular its time profile. Measuring system (100) according to one of claims 5 to 10, characterized in that the current sensor (110) or one of the current sensors (110, 110a, 110b) in a connecting line between a first connection

(106) and a second terminal (107) of the switching unit (102) and is set up to detect the short-circuit current Isc flowing between the terminals (106, 107), in particular its time profile.