Methods, electrical circuit arrangements and insulation monitoring devices for interference-resistant insulation monitoring of an ungrounded power supply system with a converter

Description

[0001] The invention relates to a method and an electrical circuit arrangement for interference-resistant insulation monitoring of an ungrounded power supply system with a converter (frequency converter).

[0002] Furthermore, the invention relates to an insulation monitoring device with the proposed electrical circuit arrangement.

[0003] For the supply of electrical equipment, an ungrounded (IT) power system (French: Isolé Terre) is used when increased operational, fire, and contact safety requirements are met. In this type of power system, all active parts of the supply network are isolated from earth potential.

[0004] Due to the inherent safety of the unearthed power system against dangerous touch voltages, a continuous power supply to the loads connected to the unearthed power system can be maintained even if an initial insulation fault occurs.

[0005] The electrical resistance of active conductors of the power supply system to earth (insulation resistance - in the event of a fault also insulation fault resistance or fault resistance) is therefore constantly monitored, since a possible further fault on another active conductor (second fault) would create a fault loop and the resulting fault current, in conjunction with an overcurrent protection device, would result in the system being switched off and brought to a standstill.

[0006] As a rule, the insulation resistance is monitored by an insulation monitoring device (IMD), which actively superimposes a measuring voltage on the power supply system and records and evaluates a measuring signal corresponding to the insulation resistance.

[0007] However, there are applications in ungrounded power systems, such as mining or power plants, where the power supply needs to be shut down as quickly as possible even when an insulation fault occurs. The insulation monitoring device should then detect the first fault as quickly as possible and trigger a switching signal.

[0008] Unlike in an earthed power supply system, the requirement for the shortest possible shutdown time is not determined with the aim of avoiding the risk of electric shock - this risk does not exist by definition in the unearthed power supply system installed in accordance with the standards in the first case of a fault - but by the requirements with regard to fire protection and system protection.

[0009] Particularly in the case of insulation monitoring in conjunction with high-quality (frequency) converters and controlled drives, short shutdown times in the range of a few tens of seconds would theoretically be possible with active insulation monitoring, since the system leakage capacitances in these drives often have sufficiently small values.

[0010] In practice, however, these shutdown times are not achievable in many converter applications, since the converter drive (converter-controlled electric motor) generates application-related low-frequency displacement voltages, i.e. voltages between an active conductor and earth, with an amplitude that is often several times the amplitude of the measuring voltage applied by the insulation monitoring device and can therefore seriously interfere with the detected measuring signal.

[0011] If, in addition, the frequency of the low-frequency residual voltage components is very close to the (fundamental) frequency (measurement frequency) of the applied and specially clocked measurement voltage, monitoring is not possible in this operating state.

[0012] Across manufacturers, the interference caused by low-frequency residual voltage components is taken into account in the technical documentation of insulation monitoring devices by specifying a (monitoring) gap in the specified frequency range. For example, the monitorable frequency range of a state-of-the-art insulation monitoring device is specified such that reliable insulation monitoring is only possible for mains voltages "DC and 15 to 460 Hz," meaning a monitoring gap exists between DC (direct current) and 15 Hz.

[0013] Due to the low-frequency interference components in the recorded measurement signal, the evaluation of the recorded measurement signal is complex and the shutdown times achievable using state-of-the-art technology are significantly longer than required in some critical applications (mining, power plants).

[0014] Extreme requirements for shutdown times arise, especially when, for example, an existing 3AC 50 / 60 Hz IT system with very high system leakage capacitances is expanded with a powerful frequency converter drive. For example, a ground fault on a motor phase of the controlled frequency converter drive can cause residual voltages that can destroy undesigned equipment in the existing 3AC 50 / 60 Hz IT system. Shutdown times of less than 100 ms are often required in these cases.

[0015] In addition to the low-frequency interference not further pursued in this invention, which requires a computationally intensive and time-consuming evaluation of the detected measurement signal and thus impairs rapid shutdown, interference components considered in this invention, generated by the converter, switching frequency, i.e. corresponding to the switching frequency of the converter, occur in the range of approximately 10 kHz or more, which, due to their detrimental effect on the equipment, require rapid shutdown of the power supply.

[0016] For insulation monitoring, it is known from the state of the art to select the measuring voltage component from the noisy, recorded measuring signal by analog or digital filtering measures.

[0017] Published patent application EP 2 230 522 A1 discloses a method and device for insulation monitoring of a direct current IT network that is coupled to a grounded alternating current network via a galvanically connected inverter. To determine the insulation resistance, a resistance measuring device is provided that alternately connects a reference resistor to the conductors of the direct current IT network via a changeover switch. As a special feature, an additional current measuring device on the direct current side can detect a leakage current flowing through the operating inverter.

[0018] EP 2 490 032 A2 discloses a method for detecting ground faults in a motor drive system with a converter. The drive system is subjected to a common-mode voltage (CMV) to generate motor control signals that enable clear identification of fault currents. High-quality insulation monitoring devices typically employ complex digital signal processing methods to suppress interference components whose frequency is very close to the measurement frequency. The closer the interference signal frequency is to the measurement frequency, the more computational effort and processing time are required for adequate interference signal suppression.

[0019] One approach used in earthed power systems - as opposed to the unearthed ones considered here - to achieve rapid shutdown of system components in the event of a critical fault is to use residual current-based protective devices (RCDs - Residual Current Protective Devices).

[0020] However, in applications with high-performance frequency converter drives, large DC components must be expected in the fault current in the particularly critical fault scenario of a motor phase earth fault. The use of a Type A residual current device (for detecting AC fault currents and pulsating DC fault currents) is therefore usually not possible. Suitable Type B residual current devices (for detecting AC fault currents, pulsating DC fault currents, and pure DC fault currents) for high-performance frequency converter drives are often unavailable.

[0021] However, the use of residual current-based protective devices in unearthed power systems is fundamentally problematic, since this type of protective device only functions reliably if it is ensured that the residual current transformer of the residual current device is actually located in the fault loop.

[0022] The requirement for short measuring and shutdown times, which is necessary in special applications, cannot be fully met according to the state of the art.

[0023] The present invention is therefore based on the object of specifying a method and a device for interference-resistant insulation monitoring in an ungrounded power supply system with a converter drive.

[0024] This object is achieved with reference to a method in conjunction with the preamble of claim 1 by the characterizing features of claim 1.

[0025] In contrast to the methods known from the prior art, which are based on a selection of the measuring voltage component from the noisy, detected measuring signal, the solution according to the invention is based on detecting and evaluating the switching frequency signal components generated by the converter independently of the detection of the measuring signal and, if necessary, deriving a switching (off) signal from it.

[0026] Using a coupling device, a measuring voltage is applied between one—in multi-phase ungrounded power systems, preferably between all—active conductors of the power system and earth. The measuring voltage superimposed on the power system generates a measuring signal, which is recorded in the coupling device and used to determine the insulation resistance of the power system and, if necessary, to shut down faulty system components. An unadulterated measuring signal is therefore essential.

[0027] In the event of an asymmetrical, very low-resistance earth fault on a motor phase (active conductor) of the inverter drive, the inverter causes a switching-frequency common-mode voltage, which is noticeable on the input side of the inverter (mains side) as a displacement voltage between the affected active conductor of the power supply system and earth and has an amplitude that corresponds approximately to the intermediate circuit voltage of the frequency inverter.

[0028] If very large system leakage capacitances are present in the unearthed power supply system on the mains side of the frequency converter - the capacitive conductance is proportional to the capacitance and the frequency - then these switching frequency components of the residual voltage in particular cause a high, potentially dangerous current via an earth connection, e.g. a protective conductor of the connected loads.

[0029] To detect the switching-frequency signal component, the residual voltage is measured between the affected active conductor of the power system and ground on the input side of the converter. In multi-phase ungrounded power systems, this residual voltage measurement is preferably performed between all active conductors and ground.

[0030] Using a filter circuit, the switching-frequency signal component is first filtered out from the detected zero-sequence voltage(s) and then evaluated. If the evaluation shows that the switching-frequency signal component exceeds a critical amplitude value, a switching signal is triggered.

[0031] According to the invention, the filtering and evaluation of the switching frequency signal component from the detected displacement voltage is designed in such a way that mains frequency components and / or DC components in the displacement voltage do not lead to the triggering of a switching signal or to the shutdown of the faulty converter drive.

[0032] In contrast to the described fault scenario involving a motor phase of the converter drive ("downstream" of the converter), a first fault on the line-frequency side of the ungrounded power system ("upstream" of the converter) or in a DC voltage circuit of the converter results in significantly smaller currents, even with very large system leakage capacitances. If the specifications of standards for the installation of electrical systems are observed, these currents do not cause damage to system components. In this case, a rapid shutdown is not required, as is usually expected with an ungrounded power system.

[0033] Preferably, the converter is disconnected from the power supply system if the switching signal is triggered.

[0034] According to the invention, interference resistance in insulation monitoring is achieved by detecting and evaluating switching-frequency signal components that could corrupt the measurement signal detected for monitoring the insulation resistance, independently of the detected measurement signal.

[0035] The disturbing, switching-frequency signal components in the zero-sequence voltage generated by the converter are filtered out, evaluated, and a switching signal is triggered when a critical amplitude value of these switching-frequency signal components is exceeded.

[0036] The switching signal can be used for rapid, selective shutdown of the frequency converter drive. This enables selective, rapid shutdown of a faulty converter drive in the event of an asymmetrical ground fault.

[0037] Disconnecting other parts of the power system "upstream" of the converter (on the grid side) is not necessary. Therefore, all other connected loads—except for the faulty converter drive—continue to benefit from the advantages of the ungrounded power system; they can continue to operate without interruption.

[0038] With regard to a device, the object underlying the invention in conjunction with the preamble of claim 3 is achieved by the characterizing features of claim 3.

[0039] To carry out the method according to the invention according to claim 1, the electrical circuit arrangement according to the invention has as essential functional blocks a coupling device for applying the measuring voltage, a detection device for detecting the displacement voltage and for filtering out the switching frequency signal component generated by the converter and an evaluation circuit for evaluating the filtered out, switching frequency signal component and for triggering a switching signal.

[0040] The detection device is designed as an analog RC high-pass filter circuit or has a digital signal processing circuit for filtering out the switching frequency signal component.

[0041] This digital filter circuit can advantageously be integrated together with the evaluation circuit in an insulation monitoring device.

[0042] In In a further embodiment, the electrical circuit arrangement has a separating device which is connected to the evaluation circuit via a signal line for transmitting the switching signal.

[0043] If a switching signal is triggered by the evaluation circuit and forwarded to the disconnecting device via the signal line, the converter is disconnected from the power supply network.

[0044] The insulation monitoring device for monitoring insulation resistance in an ungrounded power system with a converter advantageously comprises an inventive electrical circuit arrangement for interference-resistant insulation monitoring, which extracts and evaluates the switching-frequency signal component generated by the converter. The insulation monitoring device, which is prescribed for insulation monitoring and designed according to standards, can be made less sensitive to switching-frequency interference components and provided with a shutdown function by expanding it with the inventive electrical circuit arrangement.

[0045] Further advantageous design features will become apparent from the following description and the drawings, which illustrate preferred embodiments of the invention using examples. It shows: Fig. 1 :an embodiment of a circuit arrangement according to the invention with evaluation of a switching frequency signal component.

[0046] Fig. 1 shows, in a functional block diagram, a first circuit arrangement according to the invention for interference-resistant insulation monitoring of an ungrounded power supply system 2 with a converter 4. The converter 4 controls an electric motor 5 and, together with the latter, forms a converter drive 6.

[0047] This circuit arrangement according to the invention is based on the evaluation of a switching frequency signal component in order to separate the converter drive 6 from the power supply system 2 in the event of an insulation fault Rfu occurring, for example in the event of an earth fault on the electric motor 5 (drive unit) connected downstream of the converter 4.

[0048] The unearthed power supply system 2 is in this case 3-phase and is characterized by the leakage resistance (insulation resistance) Rf and the leakage capacitance Ce.

[0049] The circuit arrangement according to the invention comprises a coupling device 8, which is preferably arranged in an insulation monitoring device 10. The coupling device 8 applies a measuring voltage between at least one active conductor 12 of the power supply system 2 and ground 14 and detects a measuring signal driven by the measuring voltage.

[0050] Furthermore, the circuit arrangement according to the invention comprises a detection device 16 for detecting a displacement voltage Uv present between the active conductor 12 of the power supply system 2 and earth 14. In multi-phase ungrounded power supply systems 2, the displacement voltages Uv are detected between all active conductors 12 and earth 14. Switching-frequency signal components generated by the converter as a result of the earth fault Rfu are filtered out of the detected displacement voltage(s) Uv in the detection device 16.

[0051] The detection device 16 is shown in an embodiment as an analog RC high-pass circuit; alternatively, an implementation as a digital filter structure is also possible.

[0052] The high-frequency or switching-frequency signal component tapped across resistor R of the detection device 16 is fed to an evaluation circuit 18, which triggers a switching signal if the evaluation of the switching-frequency signal component reveals that this signal component exceeds a critical amplitude value. The evaluation circuit 18 can be implemented, for example, as an overvoltage relay.

[0053] The switching signal is transmitted via a signal line 20 to a disconnecting device 22, which disconnects the faulty converter drive 6 from the power supply system 2.

Claims

1. A method for an interference-resistant insulation monitoring of an ungrounded power supply system (2) having a converter (4), comprising the method steps: applying and actively superimposing a measuring voltage between an active conductor (12) of the power supply system (2) and ground (14) by means of a coupling device (8) and detecting a measuring signal driven by the measuring voltage to determine an insulation resistance of the power supply system (2), characterized by detecting a displacement voltage between the active conductor (12) or a different active conductor of the power supply system (2) and ground (14) on an input side of the converter (4) by means of a detection device (16), filtering a switching-frequent signal portion, which is generated by the converter (4), from the detected displacement voltage by means of an analogous RC high-pass filter circuit or a digital signal processing circuit, evaluating the filtered, switching-frequent signal portion, triggering a switching signal should the evaluation of the switching-frequent signal portion yield that the switching-frequent signal portion surmounts a critical amplitude value.

2. The method according to claim 1, characterized by separating the converter (4) from the power supply system (2) should the switching signal be triggered.

3. An electrical circuit arrangement for an interference-resistant insulation monitoring of an ungrounded power supply system (2) having a converter (4), having a coupling device (8) for applying a measuring voltage between an active conductor (12) of the power supply system (2) and ground (14) and for detecting a measuring signal driven by the measuring voltage to determine an insulation resistance of the power supply system (2), characterized by a detection device (16) for detecting a displacement voltage between the active conductor (12) or a different active conductor of the power supply system (2) and ground (14) on an input side of the converter (4) and for filtering a switching-frequent signal portion, which is generated by the converter (4), from the detected displacement voltage; and by an evaluation circuit (18) for evaluating the filtered switching-frequent signal portion and for triggering a switching signal should the evaluation of the switching-frequent signal portion yield that the switching-frequent signal portion surmounts a critical amplitude value, the detection device (16) being realized as an analogous RC high-pass filter circuit or having a digital signal processing circuit for filtering the switching-frequent signal portion.

4. The electrical circuit arrangement according to claim 3, characterized by a separating device (22) connected to the evaluation circuit (18) via a signal line (20) for forwarding the switching signal.

5. An insulation monitoring device for monitoring an insulation resistance in an ungrounded power supply system (2) having a converter (4), characterized by an electrical circuit arrangement for an interference-resistant insulation monitoring according to claim 3 or 4.