lecture power electronics interactions between electrical machine and power electronics technische...
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LecturePower Electronics
InteractionsBetween Electrical Machine and Power
Electronics
Technische Universität MünchenLehrstuhl für Elektrische Antriebssysteme
und LeistungselektronikProf. Dr.-Ing. Ralph Kennel
ralph.kennel@tum.de
Additional Losses
Additional LossesCurrent Harmonics
Quelle : Prof. A. Binder, Technische Universität Darmstadt
with increasingswitching frequency
the current harmonicscaused by the inverter
decrease
Iron Losses under Inverter Supply
Quelle : PTB
f / Hz (fundamental oscillation)
Additional LossesInfluence of Switching Frequency
Quelle : Prof. A. Binder, Technische Universität Darmstadt
mains supply inverter supply
2 pole squirrel cage induction machine 3 kW, 380 V, Y connectionrated frequency 50 Hz, slip 4.5 %, torque 10 Nmvoltage source inverter 8.3 kVA, 400 V
at frequency 9.6 kHz motor efficiency is high ( less temperature rise)the overall efficiency, however, is the same as at Frequency 4.8 kHzAt frequency 19.2 kHz motor current harmonics are low, but switching losses increase
EMC
Electro Magnetic Compatibility
E
M
C
even
more
confusion
Cabinet Designwith Modern Servo Drives
signalelectronics
powerelectronics
Shielding and Grounding
following these requirements and advicethere is „conductive EMI“ only
when using power electronics inverters(… usually no „radiation EMI“)
Shielding and Grounding
Reactive Power
Reactive Power
+
-
+
-
+
-
Motor(e. g. induction machine)
U0
induction machines needreactive power
for magnetization
… in this cablereactive power
can be measured !
this is a DC link there is
no reactive powerby definition
… in this cable reactive power
cannot be measured !
Reactive Power
+
-
+
-
+
-
Motor(e. g. induction machine)
U0
induction machines needreactive power
for magnetizationthis is a DC link there is
no reactive powerby definition
… in this cable reactive power
cannot be measured !
… where doesreactive powercome from ???
… as the sumof reactive power
in all 3 phases is zero („0“) !
Reactive Power
+
-
+
-
+
-
Motor(e. g. induction machine)
U0
induction machines needreactive power
for magnetization… where does
reactive powercome from ???
… it is no problem for the inverter to provide it
… as the sumof reactive power
in all 3 phases is zero („0“) !
Reactive Power
+
-
+
-
+
-
Motor(e. g. induction machine)
U0
… it is no problem for the inverter to provide it
… as the sumof reactive power
in all 3 phases is zero („0“) !
… with regard to reactive power the inverter is like a marshalling yard (switching station) for trains !
Reactive Power
+
-
+
-
+
-
Motor(e. g. induction machine)
U0
… with regard to reactive power the inverter is like a marshalling yard (switching station) for trains !
… therefore inverters can be used easily for compensating reactive power in grids !
… especially in regenerative energy applicationslike wind power farms or solar power arrays !
Reactive Power
voltage
current
reactivepower
t
t
t
Reactive Power
voltage
current
reactivepower
t
t
t
Active Power
activepower
voltage
current
t
t
t
Active Power
activepower
…, of course, this can be split mathematicallyinto a constant term (which is active power)and a fluctuating term(which can be considered reactive power)
t
Active Power
activepower
…, of course, this can be split mathematicallyinto a constant term (which is active power)and a fluctuating term(which can be considered reactive power)
… this is, however,a mathematical operation only
… because there is no momentwith power flowing in backward direction
… in fact this is pulsating active power only !!!
… from physical perspectivethere is not really reactive power
t
Active Power
activepower
… in fact this is pulsating active power only !!!
Pulsating Active Power ≠ Reactive Power
PFC
=PowerFactorControl
Power Factor=
PW (active power)
PS (apparent power)
PFC
… when using line commutated (Thyristor-)convertersthis was an issue indeed
voltaget
currentt
cos = 10,60,2- 0,3
PFC
… when using line commutated (Thyristor-)convertersthis was an issue indeed
voltaget
currentt
cos =
firing angle control takes carefor a phase shift between current wave and voltage wave !
10,6
… for that reason the „inductive“ reactive power had to be compensated by „kapacitive“reactive power
PFC
… in case of diode rectifiers
the power factor is usually cos = 1
voltaget
currentt
cos = 1
and in case of fully controlled rectifier bridges
… PFC is meant to include harmonics as well !
– in spite of harmonics having nothing to do with „PFC“ „PFC“ is often meant
to compensate harmonics
Rectifiers
not allowed in the public grid(with respect to impact
to the grid)
RectifiersB4 bridge with capacitive load
current shape
… today‘s discussion is dealing with harmoncs !
– in spite of harmonics having nothing to do with „PFC“ „PFC“ is often meant
to compensate harmonics
… filtering effort would be significant!
0
20
40
60
80
100
120
H1 H3 H5 H7 H9 H11 H13 H15 H17 H19 H21
without filtering
harmonics spectrum
RectifiersB4 bridge with capacitive load
not allowed in the public grid(with respect to impact
to the grid)
… different (better) solution :fully controlled front end rectifier
current shape harmonics spectrum
RectifiersB4 bridge with capacitive load
… filtering effort would be significant!
Quelle : Prof. A. Binder, Technische Universität Darmstadt
Additional LossesCurrent Harmonics
with increasingswitching frequency
the current harmonicscaused by the inverter
decrease
line voltage
line current
motoring
regeneration
… different (better) solution :fully controlled front end rectifier
+
-
+
-
+
-
Netz
U0 ≈
line voltage
line current
motoring
regeneration
current shape
Rectifiers
+
-
+
-
Netz
U0 ≈
harmonics spectrum
0
20
40
60
80
100
120
H1 H3 H5 H7 H9 H11 H13 H15 H17 H19 H21
with optimized filtering
Rectifiers
current shape harmonics spectrum
fully controlled front end rectifier
• … nevertheless !!! … even with a good 1phase „PFC“ …• either the load has to be charged by pulses (law of energy conservation !)• or an energy storage device must be implemented
… no tricky control schemecan change that !!!
… or – after all – can it?
Rectifiers
PFC
… how must the current shape look liketo provide a constant power flow ?
voltaget
currentt
power
t… of course, at u = 0 and/or i = 0 no power can be transmitted (law of energy conservation !)The time to be bypassed by the energy storage device (e. g. capacitance),
(= energy), however, is significantly smaller !
PFC
… is this current shape allowed ???
voltaget
currentt
power
t… please calculate the harmonics spectrum …it is surprising,
how close one can get to this current shapewithout exceeding the standard limits of grid harmonics
PFC
… is this current shape allowed ???
voltaget
currentt
power
t… please calculate the harmonics spectrum …some companies make use of this effect,
to reduce the size of the DC link capacitance, … but, of course, they do not tell that in public.
Travelling Waves
-800
-600
-400
-200
0
200
400
600
800
0 0.002 0.004 0.006 0.008 0.01
vo
lta
ge
in
V
time in s
typical „voltage pattern“at the output of a PWM voltage source inverter
Travelling Waves
-100
0
100
200
300
400
500
600
700
0 4e-06 8e-06 1.2e-05 1.6e-05 2e-05
volta
ge
in
V
time in s
Travelling Waves
typical „voltage step“at the output of a PWM voltage source inverter
M
time
term
inal
vol
tage
Travelling Waves
M
… what now ?
… which case is it ?
„fix“ end
„loose“ end
adaptation… „it depends“ … on what ?
… whether we consider currents or voltages !!!
… in our case : voltages
… for voltages the motor is a „loose“ end
Travelling Waves
time
term
inal
vol
tage
M
… what now ?
… which case is it ?
„fix“ end
„loose“ end
adaptation
… for voltages the inverter is a „fix“ end
… if the inverter output voltagedid not change meanwhile,
the wave is inverted and travels back again
Travelling Waves
time
term
inal
vol
tage
M
„loose“ end
… on the inverter side the voltage remains constant(fix end !)
… on the motor side voltage oscillations occurrup to the double value of DC link voltage
(loose end !)
Travelling Waves
time
term
inal
vol
tage
0
200
400
600
800
1000
1200
0 4e-06 8e-06 1.2e-05 1.6e-05 2e-05vo
ltage
in
V
time in s
-100
0
100
200
300
400
500
600
700
0 4e-06 8e-06 1.2e-05 1.6e-05 2e-05
volta
ge
in
V
time in s
… so far … so good… the matter, however, is getting much worse,
as soon as the inverterswitches simultaneously „into“ the back travelling voltage wave
Travelling Waves
… on the inverter side the voltage remains constant(fix end !)
… on the motor side voltage oscillations occurrup to the double value of DC link voltage
(loose end !)
M
Travelling Waves
time
term
inal
vol
tage
M
„loose“ end
… until here everything is like before …
Travelling Waves
time
term
inal
vol
tage
M
… what now ?
„fix“ end
… in case the inverter has switchedthe voltage at its output meanwhile,
the wave travels back with amplification
Travelling Waves
time
term
inal
vol
tage
M
„loose“ end
Travelling Waves
time
term
inal
vol
tage
… on the inverter sidethe voltage is „impressed“
(fix end !)
… on the motor sidevoltage oscillations occurr
up to 2,7 timesthe DC link voltage
(loose end !)
… what is so critical ?
Travelling Waves
Voltage Flashover within Winding
… what is so critical ?
„Horror“ Picture
… the danger is real …- behind such pictures, however,
there is acommercial interest !!
… what is so critical ?
Extract from IEC paper IEC 2 (CD) 5661991 (in Germany) : appendix to IEC 34
as long as you supply standard induction motorsby inverters with• voltage peaks below 1000 V• voltage rise times below 500 V/µsyou should not expect any danger for the motor
Compatibility between Inverter and Motor
… these are realistic valuesfor modern inverters !!!
Insulation of WireReasoning
• the critical voltage resulting in a flashoverdoes not depend at all on the diameter of the wire
• doubling the thickness of wire insulationincreases the critical voltage by 15 %
(the must significant effect results fromcovering faults of the first layer by the second layer)
that is „state of the art“ today !!!
• increasing the operation temperature to 155 °Clowers the critical voltage by 15 %
Voltage Stress on Partial Coils
Voltage Stress on (Partial) Coils
0
200
400
600
800
1000
1200
0 4e-06 8e-06 1.2e-05 1.6e-05 2e-05
volta
ge
in
V
time in s
0
200
400
600
800
1000
1200
0 4e-06 8e-06 1.2e-05 1.6e-05 2e-05
volta
ge
in
V
time in s
-100
0
100
200
300
400
500
600
700
800
0 4e-06 8e-06 1.2e-05 1.6e-05 2e-05
volta
ge
in
V
time in s
Voltage Stress on (Partial) Coils
0
200
400
600
800
1000
0 2e-06 4e-06 6e-06 8e-06 1e-05
volta
ge
in
V
time in s
Voltage Stress on (Partial) Coils
… to explain this effect,the representationas a simple equivalent circuit
containing a serial connectionof concentrated inductances
is not sufficient!!!
… in this casethe motor winding has to be represented
– like an electric cable – by a serial network
of two-ports
Voltage Stress on (Partial) Coils
… voltage “waves“ spread out within ther motor windings– as in an electrical cable –
according to the laws of cable equation
Voltage Stress on (Partial) Coils
… the motor windings only has inductive behaviour,if the rising time of the voltage edge
is significantly larger than the group delay of the complete motor winding
if the rising time of the voltage edge is smallerthan the group delay of the complete motor winding,
the capacitive behaviour is predominant !!!
Voltage Stress on (Partial) Coils
0
200
400
600
800
1000
0 2e-06 4e-06 6e-06 8e-06 1e-05
volta
ge
in
V
time in s
Voltage Stress on (Partial) Coils
0
100
200
300
400
500
600
700
0 2e-06 4e-06 6e-06 8e-06 1e-05
volta
ge
in
V
time in s
Voltage Stress on (Partial) Coils
-200
0
200
400
600
800
1000
0 4e-06 8e-06 1.2e-05 1.6e-05 2e-05
volta
ge
in
V
time in s
… that is alarming !!!the first voltage pulse
appears nearly completely at the entrance coil
Voltage Stress on (Partial) Coils
… using cost effective winding processesthe single wires are distributed randomly in the slot !!
… therefore the insulation of the single wiremust be designed with respect to
the full voltage stress !!!
Voltage Stress on (Partial) Coils
… remember:
… on the motor side entstehenvoltage oscillations occurr
up to 2,7 timesthe DC link voltage
Voltage Stress on (Partial) Coils… using cost effective winding processes
the single wires are distributed randomly in the slot !!
… therefore the insulation of the single wiremust be designed with respect to
the full voltage stress !!!
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