Caution - leakage currents! Leakage currents in fault-current protected environments Herbert Blum Product Manager EMC
> General situation > Leakage current vs. fault current > Leakage currents from frequency inverters > Transient leakage currents > Properties of RCDs > Measuring leakage currents > Leakage currents in filters > Reducing leakage currents in filters > Conclusion 18.01.2013/BLU Product Marketing 2
General situation To provide for improved protection of operating personnel, residual current operated circuit breakers are seeing increased use in electrical installations. These, however, often trip unnecessarily due to leakage currents caused by electrical systems. The results are machine downtime and costs that can otherwise be prevented with design consideration to high leakage currents and targeted countermeasures. Because frequency inverters and power-line filters are significant causes of ground currents, they deserve special attention. 18.01.2013/BLU Product Marketing 3
General situation > Fuses or circuit breakers protect electrical systems from short circuit currents and fire > Personal protection : use of RCD 30mA > RCDs (residual current devices) register fault currents flowing to ground, and cut them off before anyone can be harmed. no dangerous situation for human beings 18.01.2013/BLU Product Marketing 4
General situation > If a person directly touches a live conductor, the fault current flows through the body to ground. > When the ground conductor is disconnected and bad isolation dangerous voltage can occure on chassis. 18.01.2013/BLU Product Marketing 5
Leakage current vs. fault current > Beside fault curents there are also leakage currents flow to ground. > Leakage currents, are currents that under normal operation does not return through the neutral conductor. > Fault current: high resistive component > Leakage current: capacitive reactance > The RCD cannot distinguish between the different types of ground currents. 18.01.2013/BLU Product Marketing 6
Leakage current > Leakage currents are mainly given through capacitive coupling from line to ground. > Direct coupling through capacitors e.g. Y-capacitors in mains filter wired from line to ground > Capacitive coupling from line to ground e.g. cable shield or long cables to chassis > Long cable = high capacity > The amount of leakage current depends on the design of a drive system, on the grid voltage, the inverter's pulse-width modulation frequency, the length of cables and the interference filters being used. 18.01.2013/BLU Product Marketing 7
Leakage currents from frequency inverters > Frequency inverters are used for energy efficient operation of motors. > In both 1-phase and 3-phase inverters, the grid voltage is first rectified through a bridge circuit and smoothed. From this, the inverter generates an output voltage that can vary in amplitude and frequency corresponding to the desired motor speed. 18.01.2013/BLU Product Marketing 8
Leakage currents from frequency inverters Schematic frequency inverter system Leakage currents through capacitors Leakage currents parasitic capacity > Leakage currents in frequency inverters arise through internal interference-suppression measures and all parasitic capacitances in the inverter and motor cables. 18.01.2013/BLU Product Marketing 9
Leakage currents from frequency inverters > The largest leakage currents, though, are caused by the method of operation of the inverter. It controls motor speed continuously using pulse-width modulation (PWM), which generates leakage currents far above the grid frequency of 50 Hz. > The inverter generates an output voltage that can vary in amplitude and frequency corresponding to the desired motor speed. mains input inverter output 18.01.2013/BLU Product Marketing 10
Leakage currents from frequency inverters > For instance, the switching frequency of an inverter might be 4 khz, and the associated harmonics can have very large amplitudes at higher frequencies. > These frequencies then travel over the motor cables to the motor, and so the motor cables with their grounded shields act like a capacitor to ground. > Current is then diverted to earth through this capacitance. > The same currents are flowing to ground inside the inverter and motor. 18.01.2013/BLU Product Marketing 11
Transient leakage currents > Transient leakage currents can arise when the system is turned on or off. > Depending on the phase angle, turning the system on, can result in steeply rising voltage spikes as a result of the fast voltage increase. > The same kind of spikes occure when the unit is turned off due to inductivity in the circuit. 18.01.2013/BLU Product Marketing 12
Transient leakage currents > These fast voltage spikes generate a transient leakage current to ground through the filter capacitors. > It can arise that the RCD shuts down operation when the system is first turned on. > One way to prevent this is to use a RCD with delayed response characteristics. > Also start machine step by step 18.01.2013/BLU Product Marketing 13
RCD > The residual current device must unterruppt the current immedely in case of an fault. > For 1 phase house installation required. > For 3 phase required for plugable connection up to 32 A. (DIN VDE 0100-410) > Limit of 300mA for fire protectiom > Limit of 30mA for human protection 18.01.2013/BLU Product Marketing 14
RCD > Characteristics of the RCDs RCD Symbol Fault current Characteristics AC sensitive to AC sinusoidal AC A sensitive to pulsed current sinusoidal AC pulsed DC B B+ sensitive to all currents sensitive to all currents all currents up to 2 khz all currents up to 20 khz Only 50Hz currents all currents up to 20kHz = good for inverters 18.01.2013/BLU Product Marketing 15
RCD > Tripping characteristic curve of a RCD sensitive to all currents > Limit 30mA in the range of mains frequency 50Hz > accepts higher currents 1 20kHz 18.01.2013/BLU Product Marketing 16
RCM > If it is not possible to bring the leakage currents in a system below the RCD's response threshold, the option exists to replace that device with a differential RCM (residual current measuring > device). > Here the system's highest constant leakage current and the fault interrupter trigger value are added together and used as the setpoint. Example: leakage current 60mA trigger value 30mA Limite RCM 90mA > The RCM allows normal leakage current in the system but immediately interrupts any excess level above the limit of the sum. 18.01.2013/BLU Product Marketing 17
Measurement > It is recommended to measure the leakage current for every newly installed machine. > The simplest method for doing so is to measure the current on the ground conductor with a clip-on ammeter > Most clip-on ammeters display only 50-Hz current! 18.01.2013/BLU Product Marketing 18
Measurement > Most clip-on ammeters display only 50-Hz current, and thus a better way to measure the value is with a leakage-current analysis system. 50Hz leakage current 18.01.2013/BLU Product Marketing 19
Measurement > Select RCD type > RCD limites are shown Selection of RCD Frequency inverter with filter 18.01.2013/BLU Product Marketing 20
Measurement > Mesurement over frequency range 18.01.2013/BLU Product Marketing 21
Measurement > Mesurement with leakage current analysis system 50Hz 6kHz 10kHz > Leakage current in higher frequency ranges (14 ma @ 6 khz) can be larger than at 50 Hz (6 ma @ 50 Hz)! 18.01.2013/BLU Product Marketing 22
Measurement > The leakage current analysis system gives you a full overview of the leakage current situation. > When measuring leakage current, it is important to measure the current during various operating conditions. > A change in motor speed can have a major influence on the resulting leakage current. 18.01.2013/BLU Product Marketing 23
Leakage currents in filters > In EMC filters, capacitors from all conductors are wired to ground. > Current is continually flowing through each of these Y-capacitors, and the amount depends on the size of the capacitor, grid voltage and the frequency. Example leakage current > Most filter manufacturers specify the maximum expected leakage current so that it is easier to select the most suitable filter. 18.01.2013/BLU Product Marketing 24
Leakage currents in filters > Leakage currents on filters data sheets Leakage current under normal condition acc. IEC60950-5.2.5 > Worst case leakage current acc. to IEC60950 - Annex G4 (situation with two interrupted lines) 18.01.2013/BLU Product Marketing 25
Leakage currents in filters > In an ideal 3-phase power network with sinusoidal voltages, the sum of all these currents is zero. > In practice, however, there is a continuous leakage current to ground due to strong distortion in the grid voltage. This is also present even if the machine is not running, in other words even if voltage is applied only to the filter. > Many frequency inverters are delivered with integrated filters or what are known as footprint filters. > These are generally simple, inexpensive filters with small chokes and large capacitors between the phase conductors and ground that cause large leakage currents. 18.01.2013/BLU Product Marketing 26
Example > 2 different filters for a motor drive 18.01.2013/BLU Product Marketing 27
Example > 2 different filters for a motor drive FMAC ECO (single stage) FMBC ECO (double stage) 3 x 0.57 mh 3.3 F = max. 240mA 6 x 0.9 mh 47 nf = max. 4mA Quasi Peak Average 18.01.2013/BLU Product Marketing 28
Example > Big Y-capacitors to ground = cheap, compact, good attenuation but high leakage currents! > Replacment of the Y- capacitors with an additional choke = expensive, bigger, good attenuation and small leakage currents FMAC ECO (single stage) FMBC ECO (double stage) 18.01.2013/BLU Product Marketing 29
Reducing leakage currents in filters > Placing the frequency inverter close to the motorplace the inverter close to motor = short cables long cables = high capacity = high leakage current > Output filter (sinewave filter) on the drive output It effectively attenuates leakage currents above 1 khz by reducing the slew rate of the motor voltage. > Central filter at the grid input instead of a filter for each individual inverter. saves money and space but also reduces the leakage current > Power-line choke reduces the current's ripple factor along with harmonics and thus provides for smaller leakage currents. 18.01.2013/BLU Product Marketing 30
Reducing leakage currents in filters > 4-conductor filter with a neutral instead of a 3-conductor filter Y-capacitors are connected between the phase conductors and the neutral conductor 3 conductor filter FMAC 4 conductor filter FMBD 8 Y-capacitors to PE 1 Y-capacitors to PE big leakage current small leakage current 18.01.2013/BLU Product Marketing 31
Conclusion > Separate circuits in RCD protected/nonprotected areas > Separate filtered and unfiltered cables > Starting up the frequency inverter in steps > Placing the frequency inverter close to the motor (short motor cables) > Overvoltage protection to protect against voltage spikes > A RCD with delayed response characteristics > A differential RCM (residual current measuring device) > Power grid chokes > A central filter at the grid input instead of multiple individual filters > Use 4-conductor filters with a neutral conductor instead of 3- conductor filters > An output filter (sinewave filter) > Low leakage-currents filters 18.01.2013/BLU Product Marketing 32
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