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

Abstract

Method for interference-resistant insulation monitoring of an ungrounded power supply system with a converter, comprising the following method steps: Applying a measuring voltage between an active conductor of the power supply system and earth and detecting a measuring signal driven by the measuring voltage, characterized by Simulating a pulse width modulation signal from the converter, Evaluating the simulated pulse width modulation signal with filter characteristics that correspond to a filtering of the acquired measurement signal in order to obtain a low-frequency interference signal, Subtracting the low-frequency interference signal from the acquired measurement signal.

Description

The invention relates to a method and an electrical circuit arrangement for fault-resistant insulation monitoring of an ungrounded power supply system having a converter (frequency converter).The invention further relates to an insulation monitoring device having the proposed electrical circuit arrangement.For the supply of electrical operating means, the network form of an ungrounded (IT) power supply system (frz) comes under increased requirements for operational, fire and contact safety. Isole Terre). In this type of power supply system, all active parts of the supplying network are disconnected from ground potential.Due to the inherent safety of the ungrounded power supply system with respect to hazardous touch voltages, a continuous power supply of the loads connected to the ungrounded power supply system can thus be maintained even if a first insulation fault occurs.The electrical resistance of active conductors of the power supply system to ground (insulation resistance-in the event of a fault also insulation fault resistance or fault resistance) is therefore constantly monitored since a fault loop would result from a possible further fault on another active conductor (second fault) and the fault current flowing in this case would result in the system being shut down when the operation is stopped in conjunction with an overcurrent protection device.As a rule, the insulation resistance is monitored by an insulation monitoring device (IMD), which actively superimposes a measurement voltage on the power supply system and detects and evaluates a measurement signal corresponding to the insulation resistance.However, there are applications in the ungrounded power supply system, for example in mining or in power plants, in which the power supply is to be switched off as quickly as possible even in the event of a first insulation fault. The insulation monitoring device should then recognize a first fault as quickly as possible and trigger a switching signal.The requirement for a shutdown time which is as short as possible is not determined, unlike in a grounded power supply system, with the aim of avoiding the risk due to electrical shock-this risk does not exist in the standard installed ungrounded power supply system by definition in the first fault case-but rather by the requirements with regard to fire protection and plant protection.In particular 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 10 s would theoretically be possible with active insulation monitoring, since the grid discharge capacities in these drives frequently have sufficiently small values.In practice, however, these switch-off times cannot be reached in many converter applications, since low-frequency displacement voltages, i.e. voltages between an active conductor and ground, are generated by the converter drive (converter-controlled electric motor) in particular as a result of the application, with an amplitude which is frequently a multiple of the amplitude of the measurement voltage applied by the insulation monitoring device and can therefore disturb the detected measurement signal in a sensitive manner.If, in addition, the frequency of the low-frequency displacement voltage components is very close to the (base) frequency (measurement frequency) of the applied and specifically clocked measurement voltage, it is not possible to monitor it in this operating state.Across the manufacturer, the interference due to low-frequency displacement voltage components in technical documentation of the insulation monitoring devices is taken into account with the specification of a (monitoring) gap in the specified frequency range. For example, the monitored frequency range in an insulation monitoring device according to the prior art is specified to the effect that reliable insulation monitoring is possible only for mains voltages "DC and 15 to 460 Hz", that is to say a monitoring gap exists between DC (direct current) and 15 Hz.Due to the low-frequency interference components in the detected measurement signal, the evaluation of the detected measurement signal is complicated and the switch-off times that can be achieved according to the prior art are significantly higher than is required in some critical applications (mining, power plants).Extreme requirements for the switch-off times occur in particular when, for example, an existing 3AC 50 / 60 Hz IT system with very large grid discharge capacities is extended with a high-performance frequency converter drive. Thus, in the event of a ground fault at a motor phase of the regulated converter drive, displacement voltages can arise which lead to the destruction of operating means not designed for this purpose in the existing 3AC 50 / 60 Hz IT system. Turn-off times of less than 100 ms are often required here.In addition to low-frequency interference, which requires a computationally intensive and time-consuming evaluation of the detected measurement signal and thus impairs a rapid shutdown, interference components generated by the converter at switching frequencies, i.e. corresponding to the switching frequency of the converter, also occur in the range of approximately 10 kHz or above, which, because of their impairing effect on the operating means, make a rapid shutdown of the power supply necessary.For insulation monitoring, it is known from the prior art to select the measurement voltage component from the detected measurement signal affected by faults by analog or digital filter measures.As a rule, complex methods of digital signal processing are used in high-quality insulation monitoring devices in order 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 computation effort and processing time are required for sufficient interference signal suppression.Patent document DE 10 2015 208 725 B3 discloses a method and a device for switching off a plant part of an ungrounded power supply system provided with a connected operating means in the event of an insulation fault in a direct current circuit connected to the plant part to be switched off. The method is based on first determining the insulation resistance of the power supply system and performing a measurement of the DC displacement voltage present in the power supply system and subsequently determining in a combined evaluation of the measurement results whether there is a risk for the operating behavior of the connected operating medium.The reference DE 10 2010 030 129 A1 describes a circuit arrangement for determining a voltage fluctuation of conductor potentials in an ungrounded electrical grid. At the output of an inverter, a measurement voltage with respect to a reference potential is measured, which directly represents the voltage fluctuation of the supply potentials of a DC voltage intermediate circuit. The measurement voltage is subjected to a Fourier transformation and evaluated at multiples of an electrical angular speed of an electrical machine connected to the grid via the inverter.U.S. Pat. No. 6,856,137 B2 shows a system and a method for detecting ground faults. A direct current system supplied by a voltage source with the active conductors is described, to which a motor is driven via a switching mechanism. In a circuit for detecting ground faults, a (displacement) voltage is measured, which has an oscillating profile in the event of a fault. This (switching-frequency) oscillation is detected in the circuit for detecting ground faults and leads to a shutdown of the motor.An approach, followed in grounded-in comparison to the ungrounded-power supply systems considered here, for realizing a quick shutdown of plant parts in the case of a critical fault, is to use differential current-based protection devices (residual current protection devices RCD).In applications with high-power frequency converter drives, however, large DC components in the fault current must be expected in the particularly critical fault case, the ground fault of a motor phase. The use of a fault current protection device type A (for detecting fault alternating currents and pulsating fault direct currents) is therefore usually not possible. Suitable fault current protection devices of type B (for detecting fault alternating currents, pulsating fault direct currents and pure fault direct currents) for high-performance frequency converter drives are frequently not available.However, the use of differential current-based protection devices in ungrounded power supply systems is fundamentally problematic, since this type of protection device functions reliably only if it is ensured that the differential current converter of the differential current protection device is actually also located in the fault loop.The requirement for short measurement and switch-off times, which is offered in special application cases, cannot be fulfilled fully according to the prior art.The present invention is therefore based on the object of specifying a method and a device for insulation monitoring which is resistant to interference in an ungrounded power supply system having a converter drive.The object on which the invention is based is achieved by applying a measurement voltage between an active conductor of the power supply system and ground and detecting a measurement signal driven by the measurement voltage, simulating a pulse width modulation signal of the converter, evaluating the simulated pulse width modulation signal with filter characteristics which correspond to filtering of the detected measurement signal in order to obtain a low-frequency interference signal and subtracting the low-frequency interference signal from the detected measurement signal.This procedure achieves interference resistance during insulation monitoring with respect to low-frequency interference components generated by the converter by suppressing these low-frequency interference components to such an extent that (frequency) continuous monitoring of the insulation resistance is possible. Thus, even symmetrical errors in the case of low-frequency disturbances can be detected quickly and accurately.In contrast to switching-frequency interference components, interfering low-frequency components arise in the displacement voltage of an ungrounded power supply system having a converter drive by the pulse width modulation (PWM) used in the converter for generating drive signals. These interference components can have amplitudes at the level of the intermediate circuit voltage of the frequency converter.Instead of the slowly settling analog and / or digital filters known from the prior art for eliminating the switching-frequency interference components, system knowledge about the frequency converter causing the interfering low-frequency components is used for simulating the pulse width modulation (PWM) signal. Accessible, internal system variables of the converter that can be used are, for example, the frequency and the duty cycle of the pulse width modulation signal or the amplitude of the DC link voltage.The pulse width modulation signal of the frequency converter is reconstructed from the existing system knowledge and evaluated with the same filter characteristics, which correspond to a filtering of the detected measurement signal. In order to obtain a low-frequency interference signal, the simulated pulse width modulation signal is thus subjected to exactly the filter algorithms which also pass through the measurement signal detected for insulation monitoring in an insulation monitoring device.The low-frequency interference signal obtained in this way is then subtracted from the detected measurement signal, which contains as mixed signal both a useful component-on account of the applied measurement voltage-and the interference component caused by the converter drive, in order to obtain a measurement signal freed of the low-frequency interference for ascertaining the insulation resistance.If the modulation type (e.g. block commutation or sinusoidal commutation) of the frequency converter is also known, the PWM signal of the frequency converter can be simulated without knowledge of further signal components such as grid frequency or measurement signal.The evaluation of the simulated pulse width modulation signal using the same filter algorithms, which are applied to the detected measurement signal in an insulation monitoring device-preferably a filtering with a low-pass characteristic-extracts the desired low-frequency interference components, which the frequency converter causes.If this low-frequency interference signal is subtracted from the detected measurement signal in the insulation monitoring device, a clear suppression of interference can be achieved. In this way, in an ungrounded power supply system with a converter, insulation monitoring which is compliant with standards and operates quickly and is particularly resistant to interference in the low-frequency range is possible in all operating states of the converter without a monitoring gap occurring in low-frequency ranges.The omission according to the invention of a selection of the low-frequency interference from the detected measurement signal by complicated and / or slow filter algorithms enables the interference suppression to operate significantly more quickly-and that also in cases where the frequencies of the interference signal and the measurement signal are close to one another.In a further refinement, a frequency and a duty cycle of the pulse width modulation signal are provided by the converter via a digital interface or can alternatively be calculated using methods of signal processing.The frequency and the duty cycle of the pulse width modulation signal can be made available directly by the frequency converter, for example via a digital interface, or can be calculated from communication technology using very fast control techniques (e.g. PLL circuit).Furthermore, an amplitude of the pulse width modulation signal is directly predefined or determined by measurement as an amplitude of an intermediate circuit DC voltage of the converter.Preferably, the measurement of the amplitude of the pulse width modulation signal is carried out by a DC voltage measurement at the intermediate circuit of the converter or by a conductor voltage measurement with a correction factor.The measurement can be carried out as a DC voltage measurement at the intermediate circuit of the converter or can be determined by measuring the phase-phase voltage using a known correction factor. The relationship Uzwk=(1.32... 1.4) Un, with the intermediate circuit DC voltage Uzwk, the mains rated voltage Un and the correction factor between 1.32 and 1.4.The object on which the invention is based is furthermore achieved by an electrical circuit arrangement having a coupling device for applying a measurement voltage between an active conductor of the power supply system and ground and for detecting a measurement signal driven by the measurement voltage, having a reconstruction unit for simulating a pulse width modulation signal of the converter, having an evaluation circuit for evaluating the simulated pulse width modulation signal, wherein the evaluation circuit has filter characteristics which correspond to filtering of the detected measurement signal in order to obtain a low-frequency interference signal, and having a subtraction circuit for subtracting the low-frequency interference signal from the detected measurement signal.To carry out the method according to the invention as claimed in claim 1, the electrical circuit arrangement according to the invention has, as essential functional blocks, a coupling device for applying a measurement voltage, a reconstruction unit for simulating a pulse width modulation signal, an evaluation circuit for evaluating the simulated pulse width modulation signal and a subtraction circuit for subtracting the low-frequency interference signal from the detected measurement signal.With this circuit arrangement, the pulse width modulation signal of the converter is simulated in the reconstruction unit and low-pass filtered in the evaluation circuit. The signal modeled in this way corresponds to the low-frequency interference signal generated by the converter and is subtracted from the detected measurement signal in the subtraction circuit in order to obtain a measurement signal freed of low-frequency interference components.In a further embodiment, the electrical circuit arrangement comprises a digital interface via which a frequency and a duty cycle of the pulse width modulation signal are provided by the converter.Alternatively, the electrical circuit arrangement can comprise a computing unit for calculating a frequency and a duty cycle of the pulse width modulation signal using methods of signal processing.Advantageously, the insulation monitoring device for monitoring an insulation resistance in an ungrounded power supply system having a converter comprises an electrical circuit arrangement according to the invention for interference-resistant insulation monitoring, which simulates the pulse width modulation signal of the converter and eliminates the low-frequency interference signal obtained therefrom from the detected measurement signal.By this extension of the insulation monitoring device prescribed according to the standard, the insulation monitoring can be carried out reliably even when low-frequency interference signals occur.Further advantageous design features are evident from the following description and the drawings, which explain preferred embodiments of the invention on the basis of examples. It shows the following:FIG. : shows an embodiment of the circuit arrangement according to the invention with simulating a pulse width modulation signal.In the FIG., a functional block diagram shows a second circuit arrangement according to the invention for interference-resistant insulation monitoring of an ungrounded power supply system 2 having a converter 4.This circuit arrangement simulates a pulse width modulation signal of the converter 4 in order to obtain a fault-free measurement signal in the low-frequency range, preferably between DC and approximately 15 Hz. For older insulation monitors, this low-frequency range can be DC to 40 Hz because of the larger monitoring gap, current, high-quality insulation monitors reduce the monitoring gap from DC to below 1 Hz.In this embodiment, the circuit arrangement comprises, in addition to the coupling device 8 arranged in the insulation monitoring device 10, a reconstruction unit 30, which simulates the pulse width modulation signal of the converter 4. For this purpose, reconstruction unit 30 is connected via a digital interface 32 to converter 4 in order to use internal system variables of the converter, such as frequency and duty cycle of the pulse width modulation signal, directly for simulating the pulse width modulation signal.The circuit arrangement furthermore has an evaluation circuit 34 which subjects the simulated pulse width modulation signal to filtering in order to obtain a low-frequency interference signal which reflects the interference components generated by the converter 4 in the event of a fault. The filter characteristic used in this case corresponds to that which is also used in the insulation monitoring device 10 for filtering the detected measurement signal.In a subtraction circuit 36 downstream of the evaluation circuit 34, the low-frequency interference signal obtained by filtering is subtracted from the detected measurement signal, so that the low-frequency interference component contained in the detected measurement signal and the low-frequency interference component obtained from the reconstructed pulse width modulation signal compensate one another and a measurement signal largely free of interference is available in the low-frequency range.

Claims

Method for interference-resistant insulation monitoring of an ungrounded power supply system with a converter, comprising the method steps: applying a measurement voltage between an active conductor of the power supply system and ground and detecting a measurement signal driven by the measurement voltage, characterized simulating a pulse width modulation signal of the converter, evaluating the simulated pulse width modulation signal with filter characteristics which correspond to filtering of the detected measurement signal in order to obtain a low-frequency interference signal, subtracting the low-frequency interference signal from the detected measurement signal.Method according to Claim 1, characterized in that a frequency and a duty cycle of the pulse width modulation signal are provided by the converter via a digital interface.Method according to Claim 1, characterized in that a frequency and a duty cycle of the pulse width modulation signal are calculated using methods of signal processing.Method according to one of Claims 1 to 3, characterized in that an amplitude of the pulse width modulation signal is directly predefined or determined by measurement as an amplitude of an intermediate circuit DC voltage of the converter.Method according to Claim 4, characterized in that the measurement of the amplitude of the pulse width modulation signal is carried out by a DC voltage measurement at the intermediate circuit of the converter or by a conductor voltage measurement with a correction factor.Electrical circuit arrangement for interference-resistant insulation monitoring of an ungrounded power supply system, having a converter, having a coupling device for applying a measurement voltage between an active conductor of the power supply system and ground and for detecting a measurement signal driven by the measurement voltage, characterized a reconstruction unit for simulating a pulse width modulation signal of the converter and having an evaluation circuit for evaluating the simulated pulse width modulation signal, wherein the evaluation circuit has filter characteristics which correspond to filtering of the detected measurement signal in order to obtain a low-frequency interference signal and having a subtraction circuit for subtracting the low-frequency interference signal from the detected measurement signal.Electrical circuit arrangement according to Claim 6, characterized bya digital interface via which a frequency and a duty cycle of the pulse width modulation signal are provided by the converter.Electrical circuit arrangement according to Claim 6, characterized bya computing unit for calculating a frequency and a duty cycle of the pulse width modulation signal using methods of signal processing.Insulation monitoring device for monitoring an insulation resistance in an ungrounded power supply system having a converter, characterized byan electrical circuit arrangement for insulation monitoring which is resistant to interference according to one of Claims 6 to 8.

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