Residual current monitoring on production plants with Danisense residual current monitor SRCMH070IB+
Today, the speed-controlled, three-phase motor is a standard element in all automated process plants and commercial buildings. Highly efficient asynchronous motors, but especially motor technologies such as permanent magnet motors, EC motors and synchronous reluctance motors, require control via frequency converters; for many motor types, direct operation via a standard 3-phase power supply is even no longer possible at all.
This development is contrasted by decades of safety directives, which are intended to guarantee the protection of persons, fire and plant. For example, periodic inspection of low-voltage installations must be carried out in accordance with IEC 60364-6 (Edition 2.0 2016-04). Point 126.96.36.199 requires, among other things, a check of the insulation resistance, in which a test voltage is applied between the respective conductor and the PE protection potential. Many manufacturers of frequency converters expressly prohibit this test on their devices. Therefore, the frequency converter must be disconnected for this measurement to prevent possible damage. IEC 60364-6 also offers us a way out under point 188.8.131.52. Here the standard explains:
“Where a circuit is permanently monitored by an RCM in accordance with IEC 62020 … it is not necessary to measure the insulation resistance if the function of the … RCM is correct.”
IEC 62020, mentioned in connection with the RCM (Residual Current Monitoring device), describes the technical boundary conditions that a residual current monitor must fulfill to be recognized as a complete substitute for the conventional measurement of insulation resistance. An increase in the measured levels with the residual current monitor may indicate a fault in the insulation of the installation. A subsequent check of the plant can then be timed to avoid an uncontrolled shutdown of the plant and an unwanted interruption of production processes. In contrast to conventional insulation measurement, the system is monitored without interruption by residual current monitoring and faults in the insulation can be detected immediately.
It is therefore a procedure that can be classified as a predictive maintenance solution. When commissioning a residual current monitor, several boundary conditions often must be observed to ensure correct functioning.
Due to the use of frequency converters in production machines, in most cases a system-related leakage current is present, which can cause problems for the traditional Residual Current Protective Devices (RCDs). While fault currents mostly consist of a high resistive component, system-related leakage currents are predominantly capacitive. However, an RCD cannot distinguish between the different leakage currents. Therefore, it can already trip if the sum of all leakage currents is above the tripping threshold. This is also possible during normal operation.
As shown in the figure, different frequency components can occur in the residual current from DC up to several kHz. When analyzing the measured residual current, the system-related residual current must always be taken into account, because this is present despite perfect insulation and cannot be technically separated. Also, due to inductances (e.g., motor), high current peaks can be generated during the switch-on processes, which can lead to relay tripping at the RCDs and RCMs.
In general, the frequency components can be interpreted as follows.
When installing a residual current monitor, it is important to know the actual system-related leakage current. Only then an appropriate warning threshold and relay trip threshold can be set.
The residual current monitor from Danisense (SRCMH070IB+) can be read out via a USB socket using specially developed software for Windows systems. With this setup, we now move on to a production machine with a wide variety of robot systems and speed-controlled electric motors. The rated current of this big production plant is nearly 300 A. Due to the installed frequency converters, different frequency components of the system-related leakage current should be detectable.
The user interface of the software provides the following overview.
A true RMS value of 290.1 mA is detected over the integration interval of 1000 ms. We start with the maximum trigger threshold of the integrated relay of 1000 mA and look at the signal of the differential current via the FFT tab.
The signal is plotted over the time interval of 0.1 seconds. Over an interval of 20 ms (one sine wave @ 50 Hz) we detect 3 oscillations. A fundamental oscillation of 150 Hz thus forms the largest amplitude in our signal. The FFT analysis confirms our assumption.
It should be noted that the relay does not weight all frequency components of the residual current equally and therefore calculates a smaller true RMS value (210.6 mA) for
the relay function in the user interface. This is due to the normative regulation of RCDs, which also applies to RCMs according to IEC 62020.
The figure above shows an RCD type B+ which can detect a residual current between DC and 20 kHz. As shown in the figure above, only the frequency components between
50 and 100 Hz are included 1:1 in the current value relevant for the relay. Lower and higher frequency components are weighted weaker. The tripping value of 30 mA is
given in the range of the mains frequency of 50 Hz, since the possibility of a fault current is greatest there. The permissible tripping value increases with increasing frequency. This means that the high-frequency leakage currents of the frequency inverter are already partially taken into account. This weighting is also applied in the relay output of residual current monitors. For this reason, higher-frequency current components are significantly attenuated in the relevant waveform for the relay output and the true RMS value is smaller than the conventionally determined true RMS value.
The figure above shows the clear attenuation of the higher frequency components in the signal relevant for the relay output.
In order to generate stable monitoring and at the same time to be protected against false alarms, we now look at the different values of the residual current generated by the machine during different operating modes.
The values were generated from the Danisense software as a .csv file. At the same time the values are also provided for the 4-20 mA DC output. The machine has been previously subjected to an insulation measurement. Defects could not be found. Due to the integration interval over 1000 ms, the current peaks during switch-on and switch-off processes are smoothed, so that no clearly increased values are recognizable via the TRMS calculation. The differential current oscillates between 236.5 and 333.7 mA. Via the 4-20 mA interface, two alarm thresholds can now be defined in the PLC or in the universal measuring device at 450 or 550 mA. The relay output can be set to 1000 mA. According to the relevant standards, a trip between 50 and 100 percent (500 to 1000 mA) is defined here. Accordingly, the system should be reasonably monitored with these parameters.
Over a period of two months, no false alarms could be detected.
A reduction of the integration interval to 400 ms also provided usable values to have a reliable monitoring of the plant.
For quick commissioning of the RCM, automatic analysis of the differential current can also be performed by an integrated algorithm. This is performed by a specific combination of keys in the operating terminal.
In many critical properties, such as data centers or cost-intensive production facilities, residual current monitors are already used to prevent uncontrolled shutdown or to save the time-consuming insulation measurement. Likewise, residual current monitors can be used in parallel to the RCD (300 mA) in fire-hazardous operating sites in order to provide early information about an increase in residual current values.
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