Download Caution - leakage currents!

Caution - leakage currents!
Leakage currents in fault-current
protected environments
Herbert Blum
Product Manager EMC
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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
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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.
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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
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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.
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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.
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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.
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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.
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Leakage currents from frequency inverters
Schematic frequency inverter system
Leakage currents
parasitic capacity
Leakage currents
through capacitors
> Leakage currents in frequency inverters arise through internal
interference-suppression measures and all parasitic capacitances
in the inverter and motor cables.
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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
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inverter output
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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.
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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.
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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
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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
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RCD
> Characteristics of the RCDs
RCD
Symbol
Fault current
Characteristics
sensitive to AC
sinusoidal AC
A
sensitive to pulsed
current
sinusoidal AC
pulsed DC
B
sensitive to all
currents
all currents up to 2 kHz
B+
sensitive to all
currents
all currents up to 20
kHz
AC
Only 50Hz currents
all currents up to 20kHz = good for inverters
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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
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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.
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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 !
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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
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Measurement
> Select RCD type
> RCD limites are shown
Selection of RCD
Frequency inverter with filter
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Measurement
> Mesurement over frequency range
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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)!
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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.
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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.
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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)
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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.
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Example
> 2 different filters for a motor drive
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Example
> 2 different filters for a motor drive
FMAC ECO (single stage)
3 x 0.57 mH
FMBC ECO (double stage)
3.3 F = max. 240mA
6 x 0.9 mH
47 nF = max. 4mA
Quasi Peak
Average
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Example
> Big Y-capacitors to ground
= cheap, compact, good
attenuation but high leakage
currents!
FMAC ECO (single stage)
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> Replacment of the Ycapacitors with an additional
choke
= expensive, bigger, good
attenuation and small
leakage currents
FMBC ECO (double stage)
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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.
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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
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 small leakage current
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Conclusion
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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 3conductor filters
An output filter (sinewave filter)
Low leakage-currents filters
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