METHOD AND DEVICE FOR IMPEDANCE MONITORING FOR PROTECTION AGAINST ELECTRIC SHOCK

Description

[0001] The invention relates to a method and a device for impedance monitoring for a single- or multi-phase ungrounded power supply system.

[0002] Within the framework of the basic safety standard VDE 0100-410 and the internationally corresponding standard IEC 60364-4-41, protective measures (protection against electric shock) are defined in an ungrounded power supply system.

[0003] The ungrounded power supply system is also known as an isolated network (French: isolée terre - IT) or an IT system. In this type of power supply system, the active components are separated from earth potential.The advantage of these networks lies in the fact that in the event of an insulation fault (first (insulation) fault), such as an earth fault (conductive connection between a live conductor and earth) or a ground fault (conductive connection between a live conductor and the body or housing of a consumer or equipment), the function of the connected electrical consumers is not impaired, since their bodies are grounded separately from the live conductors of the IT system, either individually or jointly, via a protective conductor, and thus, due to the ideally infinite impedance value – neglecting network leakage capacitances – between a live conductor of the network and earth, no closed circuit can form.

[0004] This inherent safety of the ungrounded power supply system ensures a continuous power supply to the consumers powered by the ungrounded power supply system even if an initial insulation fault occurs.

[0005] The resistance of the ungrounded power supply system to earth (insulation resistance - in case of a fault also insulation fault resistance or fault resistance) is therefore constantly monitored, because a possible further fault, for example another earth fault, short circuit to body or human contact, on another active conductor (second (insulation) fault) would create a fault loop via the protective conductor and the fault current flowing in conjunction with an overcurrent protection device would result in a shutdown of the system with operational standstill.

[0006] Protection against electric shock is intended to prevent a dangerous electric current from flowing through the human body (personal protection) or the body of an animal. The standard cited above does not exclude the case of contact with a live conductor (direct contact) in the ungrounded power supply system after a failure of the basic insulation, although the ungrounded power supply system is only considered in conjunction with the protective measure "automatic disconnection of the power supply assuming two zero-resistance insulation faults on different live conductors".

[0007] The prerequisite for the protective effects prescribed by the aforementioned standards is that the bodies of the connected consumers with conductive housings (protection class 1 equipment) are connected to the protective conductor with low resistance and that protection in the event of contact with, for example, a device housing (indirect contact) after a fault to a body or earth (first fault) and after a further resistance-free insulation fault on another active conductor (second fault) is ensured for automatic disconnection by a designated overcurrent protection device.

[0008] It is known from the prior art to carry out continuous monitoring of the insulation resistance using an insulation monitoring device in accordance with the product standard IEC 61557-8. However, as described above, the insulation monitoring is only performed with regard to indirect contact of the equipment in the ungrounded power supply system with an intact protective conductor. The other known methods for determining the insulation resistance also assume a functioning protective conductor connection of the equipment.

[0009] For example, the patent application DE 10 2014 204 038 A1 discloses a method for the insulation monitoring of an ungrounded power supply system, in which the insulation resistance is determined separately for each phase conductor.

[0010] The patent application EP 3 832 324 A1 also shows a circuit arrangement for determining an insulation resistance in an ungrounded power supply system, whereby a conductor-selective determination from a complex-valued insulation impedance is possible, but only for switched-off parts of the ungrounded power supply system.

[0011] Furthermore, the patent application EP 3 598 151 A1 shows the phase-selective distribution of a total insulation resistance and a total network leakage capacitance in an ungrounded power supply system.

[0012] Patent EP 0 061 704 B1 discloses a method for determining the total leakage impedance and the maximum conductor touch current that can be derived therefrom in an ungrounded AC power supply system.

[0013] US patent 7,538,454 B2 describes the derivation of fault currents in semiconductor circuits, preferably used in circuit breakers. An electrical RC circuit model of the human body is used as a simulation model for verification.

[0014] In the aforementioned state of the art, it proves to be a disadvantage that potentially hazardous body currents and potential interruptions of the protective conductor are not considered, or only considered as a simulation model for verification.

[0015] Furthermore, there are no normative requirements for the case of contact with an active unearthed conductor after failure of the basic insulation and simultaneous loss of the protective conductor, for example due to an interruption or breakage, or in the general case of high resistance.

[0016] Regulations are also lacking for ungrounded power supply systems, where both the power supply system and the bodies of the connected devices are ungrounded. Such an ungrounded power supply system is to be incorporated into national and international standards in the future as a Type II system (French: isolée-isolée - II).

[0017] The considerations relating to the invention therefore apply both to an ungrounded power supply system in the form of the IT system and to a future ungrounded power supply system in the form of the II system.

[0018] The present invention is therefore based on the objective of proposing a method and a device for protection against electric shock (personal protection) for an ungrounded power supply system (IT and II systems) that are effective even when the protective conductor is interrupted or missing.

[0019] This problem is solved by a method for impedance monitoring for a single- or multi-phase, ungrounded power supply system with a power source, active conductors, and a load, comprising the following method steps according to claim 1: measuring a complex impedance to earth simultaneously for each active conductor by means of a measuring device, calculating a complex touch current for each active conductor, which would flow after the occurrence of a first insulation fault on one of the active conductors (L1, L2, L3) upon contact with another active conductor (L1, L2, L3), as the quotient of a conductor-to-earth voltage of the respective conductor and a complex total impedance (Ztotal) resulting from a combination of parallel and series connections of the complex impedances, wherein a total body impedance is connected in parallel to the complex impedance of the active conductor that is touched by the person.The processing unit checks whether the respective complex touch current exceeds an adjustable body current threshold, generates a switching signal in the processing unit if the body current threshold is exceeded, and controls a switching element with the switching signal to switch off or disconnect the power source.

[0020] The basic idea of ​​the method according to the invention is to first determine the respective complex impedance to earth in the ungrounded power supply system to be monitored, selectively for each conductor. From this, a complex touch current is derived using a total body impedance of an adjustable body current model. This complex touch current would flow if a person were to touch another conductor (second insulation fault). If this complex touch current, which would flow in the event of contact, exceeds an adjustable, hazardous body current threshold, the power source driving the supply current is switched off or disconnected.

[0021] First, a measuring device with a coupling branch for each active conductor is used to simultaneously measure the complex impedance to ground for each active conductor. The complex impedance corresponds to the complex-valued insulation resistance of the respective active conductor in the ungrounded power supply system and consists of the real part, formed by the (real) insulation resistance, and the capacitive imaginary part, determined primarily by the leakage capacitances.

[0022] The complex touch current is calculated as the quotient of the conductor-to-ground voltage of the respective conductor and a combination of parallel and series connections of the complex impedances. Using a body current model stored in the device with adjustable body impedance values, the total body impedance (of the human body) is taken into account when calculating a complex touch current for each active conductor by reducing the complex impedance of the conductor touched by the total body impedance.

[0023] The processing unit then checks the complex touch current associated with the respective active conductor to determine whether, in the event of contact, it exceeds an adjustable body current threshold, given the previously set total body impedance. Body current thresholds mark the boundary between physiological effects that pose varying degrees of danger to humans when an electric current passes through the body.

[0024] If an exceedance of the body current threshold is detected, the processing unit generates a switching signal.

[0025] The switching signal controls a switching element to shut down or disconnect the power source.

[0026] In contrast to methods known from the prior art for insulation monitoring, the value of a complex touch current is used as a criterion for detecting a risk to persons, rather than the determined insulation resistance.

[0027] The method according to the invention can therefore be used in the following cases as a monitoring and protection device in conjunction with a switching element without or with separating properties: a) in the case of indirect contact with a live body of equipment of protection class 1 (protective measure with protective conductor) with an interrupted protective conductor in an IT system, b) in the case of direct contact with a live conductor with an interrupted protective conductor in an IT system, c) in the case of indirect contact with a live body in a Class II system and d) in the case of direct contact with a live conductor in a Class II system.

[0028] Impedance measurement technology based on this method significantly increases personal safety in ungrounded power supply systems.

[0029] In an advantageous embodiment, the body current threshold is taken from an implemented current-time characteristic curve, which represents the duration of the body current versus the magnitude of the body current.

[0030] In the current-time characteristic curve, maximum permissible current flow durations (duration of the current through the body) are assigned to possible body current thresholds. The current-time characteristic curve thus defines a boundary line between physiologically different levels of danger when a current flows through the body. The maximum time interval within which a shutdown or disconnection occurs after the body current threshold has been exceeded (trigger time) results from the current flow duration specified by the current-time characteristic curve, ensuring that no harmful effect on the person occurs if the current source is shut down / disconnected within this time interval. Since the shutdown or disconnection occurs preventively in the inventive method, i.e., before the occurrence of the dangerous complex touch current, the required trigger times are implicitly met.

[0031] Preferably, one of the current-time characteristic curves shown in Fig. 20 of the IEC 60479-1 standard is used.

[0032] Based on the current-time characteristics described in the international standard IEC-60479-1:2018, the personal-endangering, adjustable body current threshold, at which a shutdown or disconnection of the power source should take place, can be determined.

[0033] For preventive shutdown to increase the level of protection, a shutdown device can be used as a switching element. This shutdown device does not need to meet the normative requirements for disconnection.

[0034] Alternatively, in the sense of a protective device, a safer separation of the power source is achieved by a switching device suitable according to the standard VDE 0100-530 or IEC 60364-5-53 in the form of a disconnect switch.

[0035] Protection against electric shock is ensured by the safe isolation of the power source in conjunction with a tripping time sufficient according to the selected current-time characteristic curve.

[0036] Advantageously, the total body impedance is formed as a vector sum of an adjustable internal body impedance and an adjustable skin impedance.

[0037] The electrical impedances of the human body consist essentially of the internal body impedance and skin impedances, which together constitute the total body impedance. For calculating the complex touch current, the magnitude of the total body impedance can be implemented as a value specified in the IEC 60479-1 standard, preferably a variable value between 0.5 kΩ and 2 kΩ, and the lowest value can be used in a worst-case scenario.

[0038] The corresponding process steps of the method according to the invention are implemented by the structural features of the impedance monitoring device according to claim 5. Thus, the requirements of the

[0039] The technical effects and resulting benefits of the processes also applied equally to the impedance monitoring device.

[0040] For this purpose, the impedance monitoring device is equipped with a measuring device for simultaneously measuring a complex impedance to earth for each active conductor, with a computing unit designed to calculate a complex touch current for each active conductor that would flow after the occurrence of a first insulation fault on one of the active conductors (L1, L2, L3) upon contact with another active conductor (L1, L2, L3), as the quotient of a conductor-to-earth voltage of the respective conductor and a complex total impedance (Ztotal) resulting from a combination of parallel and series connections of the complex impedances, whereby a total body impedance is connected in parallel to the complex impedance of the active conductor that is touched by a person, for testing purposes.whether the respective complex touch current exceeds an adjustable body current threshold and to generate a switching signal when the body current threshold is exceeded, and with a switching element which is controlled with the switching signal to switch off or disconnect the power source.

[0041] The measuring device is connected via a separate coupling branch between each of the active conductors and ground. The simultaneous measurement of the respective complex impedances is performed using an active measurement method in which a measuring voltage is superimposed on the network. The respective conductor-related measuring circuits run via the respective active conductors of the ungrounded power supply system, across their complex impedances to ground – consisting of the real, resistive component and the imaginary, capacitive component – ​​and via the grounding conductor of the connected loads back to the respective coupling branch of the measuring device. The resulting measuring current for each active conductor is detected via a voltage drop across a measuring resistor and evaluated to determine the complex impedances.

[0042] The impedance monitoring device also includes a processing unit that determines a complex touch current for each active conductor from the conductor-to-ground voltage of the respective conductor and the parallel combination of the complex impedances according to Ohm's law. The total body impedance is added to the complex impedance of the active conductor with which the human body is in contact.

[0043] The complex touch current determined for each active conductor is then checked in the processing unit to determine whether it exceeds a user-hazardous, adjustable body current threshold. If such an exceedance is detected, the processing unit generates a switching signal to activate a switching element for disconnecting or isolating the power source.

[0044] Preferably, a current-time characteristic curve is stored in the processing unit, which represents the duration of the body current versus the magnitude of the body current and from which the body current threshold is taken.

[0045] The computing unit incorporates a current-time characteristic curve that can be adjusted to the electrical properties of the ungrounded power supply system to be monitored and depending on the selected body current model, from which a body current threshold corresponding to the protection goal is extracted.

[0046] Preferably, one of the current-time characteristic curves shown in Fig. 20 of the IEC-60479-1 standard is used.

[0047] The physiologically hazardous effect on persons of a calculated complex touch current and the body current threshold to be set are evident from the area limits and boundary lines shown in Fig. 20 of standard IEC 60479-1 and are adopted as a criterion for evaluation when testing the calculated complex touch currents.

[0048] Furthermore, the impedance monitoring device has a switching device as a switching element for the preventive shutdown of the power source if an exceedance of the body current threshold is detected.

[0049] If the power source is to be disconnected preventively after an initial insulation fault, a disconnecting device is used as the switching element. Such a disconnecting device as a preventive measure does not have to meet the separation requirements specified in the standards VDE 0100-530 or IEC 60364-5-53.

[0050] Alternatively, the impedance monitoring device includes a disconnect switch as a switching element for disconnecting the power source in accordance with standards. In this configuration, for safety reasons, the power supply to all sections or individual sections of the electrical installation is interrupted by disconnecting the electrical power source.

[0051] Preferably, the computing unit is designed to implement a total body impedance, which is formed as a vector sum of the internal body impedance and the skin impedances.

[0052] Different body current models with different resistance values ​​for the human body can be stored in the processing unit.

[0053] Further advantageous design features will become apparent from the following description and the drawing, which illustrates a preferred embodiment of the invention by way of an example.

[0054] TheFigure shows an impedance monitoring device 10 according to the invention for an ungrounded power supply system 2.

[0055] A functional block diagram illustrates an impedance monitoring device 10 according to the invention for a three-phase, ungrounded power supply system 2 (IT system), in which a power source 4 supplies a load 6 via active conductors L1, L2, L3 protected by overcurrent fuses F1, F2, F3. The power supply system 2 is characterized for each active conductor L1, L2, L3 by the ohmic insulation resistances R1, R2, R3 and the leakage capacitances C1, C2, C3, which form the real and imaginary parts of the respective conductor-specific complex impedance Z1, Z2, Z3.

[0056] The impedance monitoring device 10 includes a measuring device 12 which comprises a coupling branch (shown here only in simplified form) for each active conductor L1, L2, L3 between the respective active conductor L1, L2, L3 and earth PE. The coupling branch in turn includes a coupling resistor Ri, a measuring voltage generator G – which can also be implemented jointly for all three coupling branches – and a measuring resistor Rm.

[0057] The voltage measured at the measuring resistor Rm is used in a computing unit 14 to first calculate the respective complex impedance Z1, Z2, Z3 for each active conductor L1, L2, L3.

[0058] To determine a complex touch current I1, I2, I3, a body current model 16 with an adjustable total body impedance Zk is stored in the processing unit 14. The calculated complex impedance Z1, Z2, or Z3 of the conductor with which the human body comes into contact is reduced by the value of the total body impedance Zk (parallel connection of the impedances). Using the conductor-to-ground voltage UL1_E, UL2_E, UL3_E of the respective conductor L1, L2, L3 and a complex total impedance Zges, the respective complex touch current I1, I2, I3 can be calculated according to Ohm's law. The complex total impedance Zges results from a combination of parallel and series connections of the complex impedances Z1, Z2, Z3, with the total body impedance Zk being connected in parallel to the respective complex impedance Z1, Z2, or Z3.

[0059] In the case of contact with the active conductor L3, the following applies to the complex touch current I3. I 3 = UL 3 _ E / Zges with Zges = Z 1 ∥ Z 2 + Z 3 ∥ Zk .

[0060] Subsequently, the respective complex touch current I1, I2, I3 is evaluated based on a current-time characteristic curve 18 stored in the processing unit. For each complex touch current I1, I2, I3, a check is carried out to see if it exceeds a body current threshold.

[0061] If a personal-endangering, adjustable body current threshold value taken from the current-time characteristic curve 18 is exceeded, a switching signal 20 is generated as soon as an exceedance is detected.

[0062] The switching signal 20 controls a switching element 22 designed as a shutdown device or as a disconnect switch, which causes the shutdown of the power source 4 or, in accordance with the standard VDE 0100-530 or IEC 60364-5-53, the safe separation of the power source 4.

[0063] Further simplified representations include case A of direct contact, where there is direct contact between the human body and one of the live conductors L1, L2, L3, and case B of indirect contact, where a hazard occurs when an electrical voltage is present on parts of the electrical system, such as a motor housing, which are normally de-energized (short circuit to body), due to an insulation fault.

Claims

1. A method for impedance monitoring for a single-phase or multiphase, ungrounded power supply system (2) having a power source (4), active conductors (L1, L2, L3) and a consumer (6), the method comprising the following steps: measuring a complex impedance (Z1, Z2, Z3) against ground (PE) for each active conductor (L1, L2, L3) simultaneously using a measuring device (12), computing a complex touch current (I1, I2, I3), which would flow upon a touching of another active conductor (L1, L2, L3) after a first insulation fault has arisen at one of the active conductors (L1, L2, L3), for each active conductor (L1, L2, L3) using a computing unit (14) as a quotient from a conductor-to-ground voltage (UL1_E, UL2_E, UL3_E) of the corresponding conductor (L1, L2, L3) and a complex total impedance (Zges), which results from a combination of parallel connection and series connection of the complex impedances (Z1, Z2, Z3), a total-body impedance (Zk) being switched parallel to the complex impedance (Z1, Z2, Z3) of that active conductor (L1, L2, L3) which is touched by the person, testing in the computing unit (14) whether the corresponding complex touch current (I1, I2, I3) exceeds a settable body-current threshold value, generating a switch signal (20) in the computing unit (14) upon the body-current threshold value being exceeded, controlling a switch element (22) using the switch signal (20) for shutting off or isolating the power source (4).

2. The method according to claim 1, characterized in that the body-current threshold value is derived from an implemented current-time characteristic curve (18) which represents a duration of the touch current over the level of the body current.

3. The method according to claim 2, characterized in that one of the shapes shown in Fig. 20 of standard IEC 60479-1 is used as the current-time characteristic curve (18).

4. The method according to any one of the claims 1 to 3, characterized in that the total-body impedance (Zk) is constituted as the vectorial sum of a settable body inner impedance and a settable skin impedance.

5. An impedance monitoring device for a single-phase or multiphase, ungrounded power supply system (2) having a power source (4), active conductors (L1, L2, L3) and a consumer (6), the impedance monitoring device having a measuring device (12) for simultaneously measuring a complex impedance (Z1, Z2, Z3) against ground (PE) for each active conductor (L1, L2, L3), the impedance monitoring device having a computing unit (14) which is configured for computing a complex touch current (I1, I2, I3), which would flow upon a touching of another active conductor (L1, L2, L3) after a first insulation fault has arisen at one of the active conductors (L1, L2, L3), for each active conductor (L1, L2, L3) as a quotient from a conductor-to-ground voltage (UL1_E, UL2_E, UL3_E) of the corresponding conductor (L1, L2, L3) and a complex total impedance (Zges), which results from a combination of parallel connection and series connection of the complex impedances (Z1, Z2, Z3), a total-body impedance (Zk) being switched parallel to the complex impedance (Z1, Z2, Z3) of that active conductor which is touched by the person, for testing whether the corresponding complex touch current (I1, I2, 13) exceeds a settable body-current threshold value and for generating a switch signal (20) upon the body-current threshold value being exceeded, the impedance monitoring device having a switch element (22) which is controlled for shutting off or isolating the power source using the switch signal (20).

6. The impedance monitoring device according to claim 5, characterized in that the computing unit (14) is configured for deriving the body-current threshold value of an implemented current-time characteristic curve (18) which represents a duration of the body current over the level of the body current.

7. The impedance monitoring device according to claim 6, characterized in that the computing unit is configured to use one of the shapes represented in Fig. 20 of standard IEC 60479-1 as the current-time characteristic curve (18).

8. The impedance monitoring device according to any one of the claims 5 to 7, characterized by a shut-off device as a switch element (22) for preventively shutting off the power source (4).

9. The impedance monitoring device according to any one of the claims 5 to 7, characterized by an isolator as a switch element (22) for standardized isolation of the power source.

10. The impedance monitoring device according to any one of the claims 5 to 9, characterized in that the computing unit (14) is configured for implementing the total-body impedance (Zk) which is formed as the vectorial sum of the body inner impedance and the skin impedance.