M V1 Frequency
factor F
15
10
Fig. 34 : fault voltage.
Voltages V1 and V2 are responsible for
5
circulating any fault current that may occur, as shown in figure 35 .
f
(Hz)
1
50/60
100
300
1,000
V1
V2
Fig. 33 : variation of the cardiac fibrillation threshold as a function of the frequency (according to IEC 60479-2).
Shape of the fault current If there is a dead short to earth at the drive output, with a TN system, the overcurrent trips the internal drive protection or the overcurrent Fig. 35 : fault current.
protection devices placed upstream.
Cahier Technique Schneider Electric no. 204 / p.23
The fundamental frequency of voltage V1, Its shape is shown in figure 38 . This fault between the neutral of the 3-phase supply and the current also contains the HF currents described central point of the rectifier, is 150 Hz (see fig. 36 ).
in the preceding sections, but not included here in order to simplify the illustrations.
V
A
100
80
0.5
60
0.4
40
0.3
20
0.2
t
0
0.1
-20
(s)
t
0
-40
(s)
-0.1
-60
-0.2
-80
-100
-0.3
0
0.01
0.02
0.03
0.04
-0.4
Fig. 36 : voltage of the rectifier neutral point with a
-0.5
3-phase supply.
0
0.01
0.02
0.03
0.04
Fig. 38 : fault current with a 3-phase supply.
Voltage V2 (see fig. 37 ), between the central point of the rectifier and one output phase is the As shown in figure 39 , the amplitude of the result of PWM. It therefore contains a low-various components changes as a function of frequency component equal to the drive output the motor operating frequency:
frequency (40 Hz in this example) and a component at the PWM frequency (1 kHz in this example).
c The total rms value of the current remains constant, as does the 150 Hz component.
c Components at the motor supply frequency and the PWM frequency vary in opposite ways.
V
400
mA
300
200
300
100
250
t
0
200
(s)
-100
150
-200
100
-300
-400
50
0
0.01
0.02
0.03
0.04
Hz
0
Fig. 37 : output voltage of the inverter stage.
0
10
20
30
40
50
60
Total rms value
This results in a fault current containing all of the Motor frequency component
following components:
150 Hz component
c 150 Hz,
1 kHz component
c drive output frequency,
c modulation frequency,
Fig. 39 : evolution of the fault current components.
and their harmonics.
Cahier Technique Schneider Electric no. 204 / p.24
The shape of the fault current for a single-phase Solution
power supply is shown in figure 40 . Note that The complex shape of the fault current requires there is a 50 Hz component and not a 150 Hz the use of a type A RCD (see fig. 41 ).
component as with a 3-phase supply.
A
0.3
0.2
0.1
t
0
(s)
-0.1
-0.2
-0.3
Fig. 41 : example of a type A RCD suitable for use 0
0.01
0.02
with an LV circuit-breaker (C60-300 mA Vigi unit –
Fig. 40 : fault current with single-phase power supply.
Merlin Gerin).
5.4 Fault at the drive output with an IT system Rapid fluctuation of the line voltage with respect to earth
In an IT system, an earth fault at the drive output does not necessitate tripping, but will cause a V2
rapid fluctuation in the line voltage relative to earth.
M Unlike the TN system, the line voltage relative to V1
earth is not actually fixed, and will follow the fluctuations set by the PWM. This is shown in the diagram in figure 42 .
Z
Vz
Any load connected to the line supply is therefore subject to the same fluctuations, Fig. 42 : earth fault in IT system.
including significant voltage gradients (see fig. 43 ). These gradients may result in damage to the capacitive filters connected between the V
line supply and earth.
400
Solutions 300 The use of EMC filters to improve
200
Electromagnetic Compatibility is not
100
recommended on supplies with IT systems t
(see standard IEC 61800-3).
0
(s)
When it is essential that HF emissions are
-100
reduced, a suitable solution is to place an EMC
-200
filter with no earth connection at the drive input.
-300
To eliminate the phenomenon of rapid voltage
-400
fluctuation, it is advisable to install a “sinus” filter 0
0.01
0.02
0.03
0.04
at the drive output. This eliminates any high voltage gradient applied to the motor and the Fig. 43 : fluctuation of the line supply voltage.
power supply cable.
Cahier Technique Schneider Electric no. 204 / p.25
5.5 Fault current with DC component Description
Conventional protection devices are suitable for measuring AC fault currents. However, insulation A
faults on the DC bus of the drive or on the 0.5
braking energy dissipation circuit (function performed by a resistor which is usually external 0.4
to the drives) cause the circulation (see fig. 44 ) of a current with a DC component (see fig. 45
0.3
with a 3-phase supply, and see fig. 46 with a 0.2
single-phase supply).
0.1
t
0
(s)
-0.1
M 0 0.02
0.04
Fig. 46 : current in the event of a fault on the braking resistor, for a single-phase supply and a fault Rb
resistance of 1 kΩ.
Solution The protection devices must remain operational Fig. 44 : fault between the braking resistor and earth.
despite this DC component.
If an insulation fault is possible on the DC bus, or on the braking resistor circuit, a type B RCD must A
be used when the drive has a 3-phase supply.
0.5
When the drive has a single-phase supply, a type A RCD should be used.
0.4
Practical rule for using RCDs 0.3
c In the first situation, with an IT system, illustrated by figure 47 , the fault current has a 0.2
DC component. The RCDa used to provide
0.1
protection against direct contact must therefore be sensitive to this type of current.
t
0
c In the second situation (see fig. 48 ), two (s)
-0.1
RCDs are connected in cascade.
0
0.01
0.02
0.03
0.04
If there is a fault on the DC bus, the fault current may not be sufficiently high to trip RCD2.
Fig. 45 : current in the event of a fault on the braking Conversely, this current, which has a DC
resistor, for a 3-phase supply and a fault resistance of component, may be sufficiently high to saturate 1 kΩ.
the measurement toroid of RCD1, preventing it from tripping if there is a fault on another feeder.
Cahier Technique Schneider Electric no. 204 / p.26
RCDb
RCD1
RCD2
RCDa
Fig. 47 : risk of “blinding” the RCDa.
Fig. 48 : risk of “blinding” the RCD1.
c The following rule is therefore used: is essential in particular in the following If the fault current may have a DC component, a situations:
type A or B RCD is necessary, depending on the v when these RCDs are installed in series, type of power supply. Thus all the RCDs in v in IT systems, since the RCDs may be affected which this current may circulate should be by double faults occurring on different feeders.
identical type A or type B RCDs. This condition Cahier Technique Schneider Electric no. 204 / p.27