Protection Strategies for IGBT Current Source Inverters
12
M. Haberberger1, F. W. Fuchs2
Power Electronics and Electrical Drives
Christian-Albrechts-University
Kiel, Germany
E-Mail: 1mkh@tf.uni-kiel.de, 2fwf@tf.uni-kiel.de
Phone (49) 431 880-6100 Fax (49) 431 880-6103 WWW: http://www.tf.uni-kiel.de/etech/LEA
Abstract— An overview of overcurrent and overvoltage CSI
faults is presented and strategies, how to protect a CSI
equipped with IGBT power switches against these failures are
developed and explained. The shown strategies and the
related circuits are applicable at low expense and require only
minor changes in the CSI and its control.
I.
INTRODUCTION
Current source inverters (CSI) with slow switching
symmetrically blocking GTOs are commercially used for
many years, especially in high voltage and high power
applications [1].
With the availability of fast switching IGBTs and the
possibility to increase the switching frequency, one of the
major drawbacks of CSIs, the large and expensive dc link
inductor and bulky line and motor side filters [2], have
been overcome and CSIs became also attractive in medium
power applications due to their sinusoidal input and output
currents and their inherent ability for regenerative
operation. But with the reduction of the inductive and
capacitive components as well as with the increased
switching speeds of IGBTs, protection becomes more
difficult because of the shorter time available to react on
failures. The missing freewheeling diodes as they can be
found e.g. in VSIs (voltage source inverters) require extra
passive or active protection mechanisms in a CSI to
provide continuous conduction paths for the inductor
currents.
In literature related to CSIs only sparse information can
be found about protection against overcurrents, current
interruptions and overvoltages [3] [4]. The aim of this
paper is to show cause and effect of possible fault
situations in a CSI and to describe strategies how to
prevent especially the CSI using IGBTs as power switches
from beeing damaged.
In this publication a general overview on CSI operation
(II) is given and the failures are divided into the three
major groups - converter short circuits, overcurrents and
current interruptions (III). These faults are explained in
detail and possible protection measures are shown (IV, V
and VI). The analysis is mainly done for the line side
converter of an example 22 kW CSI drive. Its electrical
parameters are published in the appendix. The
investigation applies in adequate manner to the machine
side as well as to other CSI configurations.
It should be noted, that the investigation given in this
paper does not cover motor winding short circuits and
earth faults.
OPERATION OF A CSI
II.
The basic circuit of an IGBT current source inverter
consisting of two three-phase bridges with a total of 12
reverse-blocking switches (IGBT with series diode) is
shown in figure 1. Both bridges usually operate in PWM
mode. Various modulation and control techniques can be
applied depending on the allowed switching frequency of
the IGBTs and the required dynamic response time of the
drive [5].
S6
S1
S3
S4
S2
S5
L1
L2
L3
3~
motor
S5
S2
S4
S3
S1
S6
Line side converter
Motor side inverter
Fig. 1. Basic circuit of a CSI drive
It is common for all modulation schemes, that always
four switches must conduct to provide a continuous path
for the dc link current. This may flow in the dc link only
(shorted bridge legs) or through the line and/or motor
phases. Capacitors across the input and output terminals
ensure, that the currents in the mains and motor
inductances can decay, if the related bridge leg was
actively turned off and the current has commutated to the
next phase.
III.
OVERVIEW OF CSI FAULTS
The cause of CSI faults, that may damage the
converter/inverter are devided into three groups:
· Short circuits
· Overcurrents
· Current interruptions
There are various conditions, that could lead to these
faults. An overview is shown in figure 2.
The effect of all failure conditions in a CSI is an
overvoltage which will be applied to one or more power
devices (diodes, IGBT, capacitor). As the fault energy,
mainly the magnetic energy stored in the dc link, mains or
motor inductors, is high enough to destroy even more than
one device, a chain reaction may result.
driver, switch
failure or
pulse off
control failure
mains unsymmetry
modulation fault
mains blackout
series diode failure
short circuit
current interruption
overcurrent
overvoltage
Fig. 2. Overview of cause and effect of CSI faults
In the following the faults and effects of these unnormal
converter conditions will be explained and it is described,
how they can be handled by protective measures to prevent
the CSI from being damaged or to limit the extent of
damage.
LINE / MOTOR SIDE SHORT CIRCUIT
IV.
Short circuit conditions with very low dynamic
impedance, e.g. as they can occur in VSIs when shorting
the dc link capacitor, should at first sight not be a problem
in a CSI, because there is no direct short circuit path across
the only capacitive element - the line and motor side filter
capacitors. Even if all bridge IGBTs are simultaneously
turned on, at least one of the series diodes is located in
reverse direction and blocks the current.
This is no longer true, if one of the diodes is defective
and remains in a short circuit state. This is typically the
case after a diode breaks through and was thermally
overloaded.
Because this type of fault was experienced in an
experimental CSI setup in the laboratory, the effect of a
shorted series diode was analyzed in detail and is described
now.
A. Current commutation from phase L1 to L2
D1
vD1
S1
~
i1
vl2
Ll1
Ll2
S3
B. Short circuit behaviour with defective series diode
A short circuit will happen, if D3 is defective. The
commutation path with the filter capacitor C12 as voltage
source is now statically shorted as long as V1 is on. The
short circuit current starts to rise with a slope only limited
by parasitic inductances in the commutation path and the
semiconductor conductance. This is outlined by iS3,b and
iS1,b in figure 4.
v/i
Id
iS3,b
iS3
vD3,b
0
t
vD3
-vC12
v/i
vC12
Id
0
vS1
iS1,b
iS1
t
Fig. 4. Commutation switch voltages and currents
vC23
vC12
vC31
Ls3
Ls1
is1
C12
vD3
V3
V1
vl1
D3
with antiparallel diode. The latter is required to conduct the
reverse current of the related series diode during
commutation.
It is supposed, that at the beginning of the investigation
S3 conducts the dc link current and is now turned off
(figure 4). The commutation voltage vC12 is assumed
positive in this moment.
Immediately after turning V1 on the current commutates
to V1/D1. When the current through D3 becomes zero, the
diode gets the commutation voltage as reverse voltage and
must block. Otherwise the inverse IGBT diode would start
conducting.
is3
vC12
~
i2
iS1
Fig. 3. Commutation path from phase L1 to L2 (top half bridge)
Figure 3 shows a part of the top half bridge of the line
side converter with the switches S1 and S3 to represent the
commutation path from phase L1 to L2. Every switch
consists of a series diode (D1, D3) and an IGBT (V1, V3)
t
Fig. 5. Capacitor voltages and switch current at short circuit in the
commutation circuit (simulation)
Figure 5 shows a SPICE simulation result for the
described fault case using typical values for the elements in
the commutation path of a 22 kW drive built up for CSI
measurements on a 400 V grid with a nominal phase
current of 35 A (Ls1 = Ls3 = 150 nH, C12 = 48 µF).
The current peak in figure 5 results of the short circuit of
the filter capacitor caused by the defective series diode.
Although this current reaches a very high level, this may
not necessarily destroy the IGBT, inverse diode or the
remaining series diode, because it flows only for a short,
limited time. Todays IGBTs are capable to withstand a
capacitive short of the dc link of a VSI for some
microseconds without damage. Compared to the dc link in
VSIs, the stored energy in the line or motor side filter
capacitors in a CSI is rather low.
With the mean pulse power and the transient thermal
response of IGBT and diode (datasheet parameter) the
junction temperature rise can be calculated using equation
(1).
(
)
∆T j = Z th t pulse ⋅ P d
(1)
C. Protection measures against short circuit conditions
If the calculated junction temperature exceeds the
maximum rating of the devices, a short circuit detection
and protection circuit for every IGBT is proposed the same
way as they are used in VSIs, e.g. by supervising the
IGBT's collector-emitter voltage and actively turning the
switch off.
But even if the devices could withstand this first short
circuit current pulse and short circuit protection measures
are eliminated due to equation (1) and economical reason,
a second effect has to be considered.
vC23
vC12
vC31
iL1
iL3
iL2
t
Fig. 6. Behaviour of the line currents at repetitive short circuit in the
commutation circuits (simulation)
It can be derived from the schematic in figure 3 that the
short circuit condition does not end after dissipating the
filter capacitor energy. Besides the short of the capacitor,
there is also a unidirectional short between two mains
phases, which may lead to an almost unlimited power flow
and an excessive rise in phase currents together with heavy
unsymmetries in input voltages and currents. Figure 6
shows the capacitor voltages and phase currents, if the CSI
continues pulsing and the capacitor C12 is repetitively
shorted. The voltage drop causes the currents, especially
the current through phase L1 in the shown example, to rise
beyond their nominal values.
To prevent the CSI from further damage, two strategies
are proposed depending on the cycle time of the digital
control, which characterizes the time delay between
occurance of the fault and possible detection. It should be
noted, that the cycle time must not necessarily be equal to
the switching frequency of the drive. Especially in low
switching CSIs, the control cycle time is often much lower
to provide a good accuracy for model calculations.
a) Cycle time < 250 µs
The slope of the phase current is limited by the
rather large grid or motor inductances, so in CSIs
using high sampling rates, additional algorithms in the
control scheme, that could detect such an unnormal
current rise as it appears in the shown fault within two
to three cycles, should be sufficient for protection. No
extra hardware is required.
The reaction on the detected fault should be an
immediate pulse off of the complete CSI. Even if the
defective series diode could by detected by special
algorithms within the control scheme, it seems not
realistic to power the CSI safely down using only the
remaining operative devices.
b) Cycle time > 250 µs
CSIs with low update rates especially in
combination with a small mains impedance (ohmic
and inductive) as it can be found in high power drives
may cause the overcurrents to reach already values
too high to pulse off the CSI when the fault was
detected. Later in the section about overvoltage
protection it is shown that a pulse off is a critical state
itself and requires protective measures. Their size and
extent mainly depends on the initial current at pulse
off, so it is not advisable to wait until the control
detects the fault and the overcurrent has reached a
high level.
An additional protection circuit in the analog signal
path is suggested instead. It compares the measured
line currents with maximum limits, that could be
determined by the capability of the pulse off
protection shown in section VI.D. The comparator
should bypass the control and trigger the pulse off
directly. This allows the CSI to react much faster on
overcurrents and it provides a deterministic maximum
current for the dimensioning of other protection
circuits.
Figure 7 shows an outline where to place the
overcurrent detection comparators.
current
measurement
3
3
A/D
conversion
D/A
conversion
current limits
overcurrent
detection
digital
control
6
4
pulse-off
pulse
generator
L1
L2
L3
Fig. 7. Detection of overcurrents
V.
OVERCURRENTS
Dynamic overcurrents are usually not a critical problem
in CSIs, because the rather large inductors on the line side
(mains inductance), in the dc link and on the motor side
(machine inductance) lead to a limited current slope, which
in most cases allows overcurrents to be easily handled by
the control scheme without extra protection hardware. But
some aspect should still be kept in mind.
A. Filter capacitor voltage ripple and commutation
overvoltages
The filter capacitors across the input and output
terminals are designed to ensure a limited voltage ripple on
the input and output terminals when the CSI works within
its operating range (nominal dc link current). The ripple
voltage across the capacitors is directly related to dc link
current. As the dc link and the line/motor currents can be
supposed as constant during one PWM period, the ripple
has a triangular shape with its amplitude proportional to the
dc link current level. Figure 8 shows an exemplary
simulation time plot for two different dc link current levels.
vC
a)
b)
t
Fig. 8. Voltage across the input/output terminals of the CSI at two
different dc link current levels (a: Id = 40 A, b: Id = 20 A)
An increased dc link current has two major effects:
- higher capacitor voltage ripple
The slope of the triangular voltage ripple is linearly
dependent on the dc link current. As the pwm cycle
time (= ripple period) is constant, the ripple
amplitude rises with a higher current level, shown in
plot a) in figure 8. Because the fundamental rms
value of the line/motor voltage remains constant at a
constant power flow, the increase in ripple voltage
also causes an increase in the maximum voltage level
across the capacitors at the CSI terminals.
- higher commutation overvoltages
These are overvoltages caused by the parasitic
inductances in the commutation path. As the
switching speed of an IGBT, resp. the current rise
and fall time is almost independent from the collector
current, an increase in dc link current directly causes
an increase in the current slope and with it in the
voltage drop across the stray inductances in the
commutation path. These overvoltages are mainly
arising on the switch being turned off and are also
distributed to other non conducting devices.
The sum of the maximum capacitor voltage and the
maximum commutation overvoltage is the maximum
forward and/or reverse voltage imposed to the IGBTs and
series diodes in normal as well as in critical or fault
conditions. This voltage has always to be below the
absolute maximum ratings of the devices, e.g. typically
1200 V for CSIs working on a 400 V grid. Considering
this, a maximum allowed dc link current can be calculated
and the control has to ensure the current below this level,
e.g. by reducing the active or reactive power.
B. Line side overcurrents, motor side overcurrents or
unsymmetric current distribution
A faulty control or modulation scheme, special dynamic
conditions or unsymmetries in the mains voltages may lead
to overcurrents or unbalanced input or output currents. All
this could cause an unsymmetric charge of the capacitors,
which may result in input/output phase-to-phase voltages,
that are higher than the nominal value. This behaviour can
be considered by using appropriate factors of safety for the
maximum allowed current or as unsymmetries usually have
a history, the control scheme could early detect these
situations and reduce the power or power the CSI totally
down.
In the overcurrent case B. as well as in A. the
comparator shown in figure 7 could be helpful to hold the
currents below absolute maximum limits under all
circumstances. This ensures, that as a last measure a total
pulse off is still possible without damaging the CSI.
VI.
CSI CURRENT INTERRUPTIONS
A. Dc link current interruptions
A total pulse off or any other intentional or unintentional
turn off of one of the four conducting switches interrupts
the dc link current path.
A simplified equivalent circuit diagram with the relevant
components for the analysis of current interruptions in a
CSI is shown in figure 9.
Sline 1,3,5
vline
vd
Sline 2,4,6
Ld
id
Smotor 2,4,6
vmotor
CLd+Coes
Smotor 1,3,5
Fig. 9. Equivalent circuit diagram of the CSI simplified
for analyzing current interruptions
The voltage sources vline and vmotor represent the
momentary value of the line and motor side voltage applied
to the dc link. Sline/motor 1,3,5 is an equivalent for the switch
currently conducting in the top half bridge, Sline/motor 2,4,6 for
the switch of the bottom half bridge of the line and motor
side bridge. CLd stands for the parasitic capacitance of the
dc link inductor referenced to the line side dc link
terminals. Coes is the sum of all applicable output
capacitances of the line side bridge (IGBTs and diodes).
If one of the line side switches fails or is turned off, the
voltage across the inductor will start rising with a slope
only limited by the parasitic capacitances. For the example
of the 22 kW CSI drive (CLd = 5 nF, Ld = 30 mH,
Coes = 400 pF) a slope of 8.3 kV/µs will result at the rated
dc link current of 45 A, which is shown by the simulation
time plot in figure 10. The time available from detecting
such an overvoltage (e.g. at 850 V) until the protection
must become active (below the IGBT breakdown voltage
of 1200 V) is in this example about 40 ns.
-850V
-1200V
40ns
t
Fig. 10. Dc link voltage slope after interrupting the inductor current
(400 V initial dc link voltage)
B. Protection measures against dc link overvoltages
To provide this short response time, a very fast
protection circuit is required.
Well known from VSI protection is active clamping [4]
of the turned off switch, what has almost zero delay and
could on principle be also applicable in a CSI. For this, all
12 switches must be equipped with clamping circuits.
One major drawback of active clamping is the high
momentary power dissipation. In an economically
dimensioned CSI, an IGBT is able to clamp at his
breakdown voltage for a maximum of only some ten
microseconds, because the initial junction temperature is
already very high. In the shown 22 kW drive, clamping at
1200 V is required for more than 1 ms. It seems unrealistic
to dimension the CSI in a way to be capable of allowing
such a long clamping time.
Another disadvantage of the clamping strategy is the
fact, that the turned off IGBT still conducts and current
flows towards the mains or motor capacitors. This could
cause unsymmetric voltages and may result in line or motor
side overvoltages.
And what also has to be considered is that clamping
could only work, if the turned off IGBT is still operative. It
will fail, if a defective IGBT is the cause of the current
interruption.
Besides active clamping there are several other
protection strategies using passive devices like varistors or
zener diodes. These are applicable, but is has to be
considered, that they all have a finite v/i characteristic [6],
which is not very steep. Devices, that block below the
maximum voltage during normal CSI operation (e.g.
850 V) and at the same time are able to limit the voltage at
full dc link current below the IGBT/diode breakdown
voltage (e.g. 1200 V) are rather large.
Because of all these drawbacks of existing protection
measures, a novel two step strategy as presented in [7] is
proposed against CSI dc link current interruptions. This
could provide both - a fast and "hard" overvoltage
limitation and a freewheeling path to safely dissipate the
inductor energy. The protection works as follows:
1) The dc link is equipped with an extra freewheeling
path (IGBT + series diode) across the line and motor
side dc link terminals. This path is triggered by an
overvoltage detection circuit or if the CSI should
intentionally be pulsed off, by the control circuitry.
The freewheeling path is held on, until the inductor
energy becomes zero. Compared to active clamping
this reduces the momentary power dissipation at the
expense of a longer time to get the current to zero,
which allows a rather small IGBT being used.
Depending on the size of the inductor, freewheeling is
required for a maximum of only some hundred
milliseconds.
2) Because the freewheeling switch is off during normal
converter operation and there are no devices available
that could be turned on (delay time + rise time) within
the required response time, a fast and "hard" limiting
voltage clamp is installed to span the time from
overvoltage occurance to getting the freewheeling
path active. The limitation circuit has to withstand the
high power dissipation during clamping only for the
short time of some hundred nanoseconds. Devices
working in the avalanche breakdown (e.g.
MOSFETs) are the best choice for this purpose. They
have a much steeper v/i characteristic compared to
varistors or zener diode [4], which allows very small
device being used.
The CSI with the detection/protection circuits and the
freewheeling path for the line side of the dc link is shown
in figure 11. It applies to the motor side as well.
overvoltage
limitation
L1
overvoltage
limitation
L2
L3
≥1
overvoltage
limitation
line-side converter
from control
for pulse off
Fig. 11. CSI with overvoltage protection circuit against dc link
current interruptions (shown is only the line side converter)
For experimental verification the three oscilloscope plots
shown in figure 12 were taken on current interruption
testcases applied to the experimental 22 kW CSI drive. The
initial current was always the nominal dc link current of
45 A.
total bridge pulse off
top half bridge pulse off
C. Line or motor side current interruptions
Similar to the dc link, the line and motor side of the CSI
is highly inductive. But even if all switches are
simultaneously turned off during a total pulse off, the line
and motor current could not be interrupted. The filter
capacitors always provide a current path.
What has to be considered is the fact, that the capacitors
are only dimensioned to limit the voltage ripple during
normal converter operation. After a pulse off, the
capacitors alone cannot provide a full overvoltage
protection, because the energy situation in the line/motor
inductors leads to heavy voltage overshoots as will be
shown now.
Fig. 13 shows a SPICE simulation of a randomly pulsed
off 22 kW CSI, that was running with nominal line current
of 35 A at a grid voltage of 400 V.
vC12
bottom half bridge pulse off
vd,lim
vC23
vtop,lim
vbottom,lim
Ch3
Ch2
500 V Ch4
500 V M
500 V
t
1 us
t
t
Fig. 12. Measurement results for failure cases caused by dc link current
interruptions (v: 500V/div, t: 1µs/div)
The figure on the left shows a total converter pulse off,
where all switches were turned off simultaneously, e.g.
after detecting a fault condition or if the power supply fails.
The dc link voltage rise is here distributed to both, the top
and the bottom half bridge, which causes none of their
limitation circuits to become active. In this fault case, the
limitation circuit across the dc link terminals limits the
voltage (vd,lim).
In the mid figure only the conducting switch of the top
half bridge was turned off to represent an IGBT, diode or
driver failure. As the IGBT in the bottom half bridge still
conducts, the bottom dc link terminal is connected to one
of the mains phases. The protection chain of the top half
bridge therefore sees the rising dc link inductor voltage
superimposed to the rectified phase-to-phase mains
voltage. This causes the voltage across the top protection
circuit to be higher than the voltage across the dc link
protection. The top limiting circuit is therefore activated
(vtop,lim).
Finally the figure on the right shows the turn off of the
conducting switch in the bottom half bridge, which causes
the bottom protection circuit to be issued.
vC31
t
Fig. 13. Voltage across at the input/output terminals of the CSI
(capacitor voltage) in the case of a line or motor side
current interruption (e.g. pulse off)
If the pwm immediately stops, an uncontrolled
oscillation process in the second-order input and output
filters will start, activated by the remaining energy in the
inductors. Depending on the initial conditions at pulse off,
these oscillations may reach levels higher than the
maximum ratings of the semiconductors and capacitors.
D. Protection against line and motor side overvoltages
To protect the CSI against overvoltages at the line and
motor side caused by current interruptions, the use of
varistors across the input and output terminals as shown in
figure 14 is well suited to handle the voltage overshoots in
a CSI and absorb the remaining inductive energy. This
strategy has already been presented for matrix converter
protection [8], which uses similar line and motor side filter
circuitry.
It was already mentioned in section IV.C.b, that a pulse
off is a very critical state itself and the dimensioning of the
protection varistors depends mainly on the initial current at
pulse off. In the case of a pulse off initiated by a fault
condition, this current could be much higher than the CSI's
nominal current, so the varistors have to be chosen
according to the absolute maximum appearing current
level. If using the comparator circuit from figure 7, this
current can be well defined. Otherwise an adequate factor
of safety has to be considered.
circuits to fully protect the CSI against these failures were
explained.
The presented circuits and strategies are applicable at
rather low expense and need only minor changes in the
standard IGBT CSI structure. The dc link current
interruption protection can even be used completely
independent from the CSI and if its power supply is
buffered by rechargable batteries or chemical capacitors, it
can even protect the IGBT CSI in the case of a total grid
and power supply fault.
APPENDIX
TABLE 1
PARAMETERS OF THE 22kW CSI DRIVE
Nominal dc link current
Nominal line current
Line-to-line voltage
Total line inductance (mains
+ additional filter inductor)
Filter capacitor capacitance
(delta connection)
Dc link inductor inductance
Parasitic inductor capacitance
Max. IGBT blocking voltage
Total bridge capacitance
referenced to dc link
Parasitic inductance of one
switch path
L1
CSI
L2
3~
motor
L3
Fig. 14. Input / output overvoltage protection with varistors
E. Special note on a total inverter pulse off
In section VI.B and VI.D protection strategies are shown
to prevent overvoltages in the dc link as well as on the line
and motor side of the CSI.
One may argue, that the extra freewheeling paths in the
dc link proposed in figure 11 are unnecessary, because the
legs of the line and motor side bridge could also be used to
short the dc link terminals and provide a path for the
inductor current.
It must be noted, that this strategy is not advisable,
because in the case of an overcurrent pulse off initiated by
a defective series diode, such a bridge leg short circuit
could be equal to a static phase-to-phase short circuit of
two mains or motor phases, depending on what bridge leg
is activated and which diode is defective. The effect of
such a short circuit was described in chapter section IV.C
(figures 5 and 6).
It is therefore proposed to never turn on a leg of one of
the CSI bridges in the case of a detected fault condition.
Both line and motor side bridge must be completely turned
off to avoid such side effects.
VII.
45 A
35 Arms
400 Vrms
4 mH
C12, C23,
C31
Ld
CLd
VIGBT,max
30 mH
5 nF
1200 V
Coes
400 pF
LS
150 nH
48 µF
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
CONCLUSION
In a CSI, as long as current flows, all switches must be
operative. Different from IGBT VSIs, where the best
protection strategy in the case of a fault condition is
usually to simply turn the converter off, an IGBT CSI has
to be still actively controlled. Especially if one or more of
the main power switches in the converter bridges are the
cause of the fault, it is not possible to safely power down
the converter without additional protection hardware.
In this paper an overview of common IGBT CSI faults
caused by short-circuit conditions, overcurrents and current
interruptions was given and strategies including protection
Id,nom
IL,nom
VLL
LL
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