ECE 8830 Electric Drives

1
ECE 8830 - Electric Drives
Topic 9 Current-Fed Inverters
Spring 2004
2
Introduction

• Current-fed inverters requires a stiff
  constant current source input - thus are
  sometimes referred to as CSI (current source
  inverters or current stiff inverters).
• A large inductance can be used to change a
  variable voltage input to a variable current
  input.
• VSI-inverters and CSI-inverters are dual to
  each other.

3
Introduction (contd)

• Power semiconductor devices used in CSI
  inverters must be able to withstand large reverse
  voltages. Therefore, power MOSFETs, BJTs, IGBTs,
  MCTs, IGCTs and GTOs.
• Symmetric blocking GTOs and thyristors can be
  used in CSI inverters.
• Generally CSI inverters are now used in very
  high power applications.

4
General Operation of a 6-Step Thyristor Inverter

• General Schematic of Thyristor Inverter

5
General Operation of a 6-Step Thyristor Inverter
(contd)

• Initially, ignore commutation considerations.
• Induction motor load is modeled by back emf
  generator and leakage inductance in each phase of
  the winding.
• The constant dc current Id is switched through
  the thyristors to create a 3? 6-step symmetrical
  line current waves as shown on the next slide.

6
General Operation of a 6-Step Thyristor Inverter
(contd)

7
General Operation of a 6-Step Thyristor Inverter
(contd)

• The load or line current may be expressed by a
  Fourier series as
• where the peak value of the fundamental
  component is given . Each thyristor
  conducts for radians. At any instant one
  upper thyristor and one lower thyristor conduct.

8
General Operation of a 6-Step Thyristor Inverter
(contd)

• The dc link is considered harmonic-free and
  the commutation effect between thyristors is
  ignored.
• At steady state the voltage output from the
  rectifier block input voltage of inverter.
• For a variable speed drive the inverter can be
  operated at variable frequency and variable dc
  current Id.

9
General Operation of a 6-Step Thyristor Inverter
(contd)

• If thyristor firing angle ? gt 0, inverter
  behavior.
• If thyristor firing angle ?0, rectifier
  behavior.
• Max. power transfer occurs when ??.

10
Inverter Operation Modes

• Two inverter operation modes are established
  depending on the thyristor firing angle
• 1) Load-commutated inverter
• Applies when ?/2lt?lt?.
• 2) Force-commutated inverter
• Applies when ?lt?lt3?/2.

11
Load-Commutated Inverter Mode

• Consider ?3?/4. In this case vca lt 0 gt
  thyristor Q5 is turned off by the load. This
  requires load to operate at leading power factor
  gt motoring mode of a synchronous machine
  operating in over-excitation.
• Vd-Vd0cos?

12
Force-Commutated Inverter Mode

• Consider ?5?/4. In this case vcagt 0 and so
  thyristor Q5 is not turned off by the load. Thus
  some type of forced commutation is required in
  this case. Lagging VAR is consumed by the load gt
  motoring mode of an induction motor. Vd-Vd0cos?

13
Force-Commutated Inverters

• For driving an induction machine, a
  force-commutated inverter is required because of
  the phase lag characteristic of the induction
  motor.
• The topology of a 3? bridge inverter with an
  auto-sequential method of forced commutation is
  shown on the next slide.

14
Force-Commutated Inverters (contd)

Ref D.W. Novotny and T.A. Lipo, Vector Control
and Dynamics of AC Drives
15
Force-Commutated Inverters (contd)

• The current is switched sequentially into one
  of the motor phases by the top half of the
  inverter and returns to the dc link from another
  of the phases via the bottom half of the
  inverter. By switching every 2?/3 radians, a
  6-step current waveform can be applied to the
  motor.
• The series diodes and delta-connected
  capacitors force the commutation of the
  thyristors. The capacitors store a charge with
  the correct polarity for commutation and the
  diodes isolate them from the load.

16
Force-Commutated Inverters (contd)

• Since current is constant, voltage drop across
  stator windings 0 and voltage drop across
  winding resistances constant.
• Thus the motor terminal voltage is set by the
  motor not by the inverter.
• Since the motor is wound with sinusoidally
  distributed windings, the voltages at the motor
  terminals are nearly sinusoidal.

17
Force-Commutated Inverters (contd)

• The current ideally follows a six-step
  waveform. However, current cannot change
  instantaneously through the winding inductances
  and so the current transitions have a finite
  slope.
• During these transitions the current transfers
  from one thyristor to the next via one of the six
  commutating capacitors.

18
Force-Commutated Inverters (contd)

• Example Commutation from Q2 to Q4

19
Force-Commutated Inverters (contd)

• When Q4 is fired, Q2 is impressed with a
  reverse voltage across the capacitor bank. gt Q2
  turns off almost instantaneously. Id flows
  through Q3 and D3, phases b and c, D2, the
  capacitor bank and Q4. The capacitor bank charges
  linearly with Id. During this time D4 is
  reverse-biased. When the capacitor bank voltage
  equals the line voltage, diode D4 turns on and
  the current Id flows through D4 and terminates
  the commutation process.

20
Force-Commutated Inverters (contd)

Ref D.W. Novotny and T.A. Lipo, Vector Control
and Dynamics of AC Drives
21
Force-Commutated Inverters (contd)

• Note the large voltage spikes (Ldi/dt). These
  can be suppressed either by designing the motor
  with small leakage inductance or by using a diode
  bridge at the motor terminal with a zener diode
  load.

22
Force-Commutated Inverters (contd)

• Two positive features of CSI inverters compared
  to VSI inverters
• 1) CSI inverters are able to ride through a
  commutation failure and return naturally to
  normal operation costly preventive measures used
  for VSI inverters.
• 2) CSI inverters can be switched to regenerative
  mode simply by reversing the polarity of the dc
  rectifier output voltage. This is automatically
  accomplished when an induction motor operates in
  a negative slip mode. In the VSI inverter, the
  current flow must be reversed - much harder.

23
Force-Commutated Inverters (contd)

• On the other hand, CSI drives cannot be
  operated in open loop operation as can VSI
  drives. The torque-speed characteristics of an
  induction motor driven by a voltage source and
  a current source are shown below

Ref D.W. Novotny and T.A. Lipo, Vector Control
and Dynamics of AC Drives
24
Force-Commutated Inverters (contd)

• A distinct peaking occurs in the current
  source case.
• Two possible operating points
• 1) One on the stable, negatively sloped region,
  and
• 2) one above breakdown torque on the positively
  sloped region where operation is generally
  unstable (depending on load torque vs. speed
  characteristics).

25
Force-Commutated Inverters (contd)

• On the stable side, the working flux in the
  machine is is high gt saturated operation and
  excessive magnetizing current and iron losses.
  Thus, continuous operation is not feasible on
  this side.
• On the unstable side, the flux in the machine
  is near its rated value and losses are
  reasonable. However, being on the unstable side,
  feedback control must be used to maintain the
  operating point.

26
Force-Commutated Inverters (contd)

• One system uses a motor voltage control loop
  (see next slide) which regulates the motor
  voltage by controlling the input phase controlled
  rectifier. Also, an internal current control loop
  is used with the voltage error serving as a
  reference signal for the current regulator. Some
  IR drop compensation is often added as are
  additional compensating circuits to improve
  system dynamics.

27
Force-Commutated Inverters (contd)

Ref D.W. Novotny and T.A. Lipo, Vector Control
and Dynamics of AC Drives
28
Force-Commutated Inverters (contd)

• ASCI inverter-fed induction motor drives for
  medium to high power applications were popular.
• However, the size and cost of the commutating
  capacitors and the dc link inductor are the major
  disadvantages of this type of inverter.
• ASCI inverters are being replaced with
  inverters using self-controlled devices (e.g.
  GTOs).

29
Six-Step CSI with Self-Commutated Devices

• Self-controlled symmetric blocking devices,
  e.g. GTOs can be turned on and off by gate
  current pulses. This allows the 6-step waveform
  to be directly controlled.

30
Six-Step CSI with Self-Commutated Devices (contd)

• In this circuit, the capacitors are freed from
  their commutating requirement and are simply
  placed across the terminals of the induction
  motor. These capacitors are much smaller and
  serve two roles
• 1) primarily, to allow commutation from the
  outgoing GTO to the incoming GTO,
• 2) secondarily, to load filter higher harmonics

31
Six-Step CSI with Self-Commutated Devices (contd)

• Example Commutation from Q1 to Q3.

32
Six-Step CSI with Self-Commutated Devices (contd)

• Initially current flows through Q1, phase a,
  phase c, and Q2. The equivalent capacitance Ceq
  and polarity of vba are as shown.
• Next, Q3 is turned on at time A. But because
  of voltage across Ceq, Q1 does not automatically
  turn off.
• Next, Q1 is turned off.The current Id
  transfers to Q3 and through Ceq.
• Ceq charges up overcoming the motor back emf
  b/w phases a and b. Gradually the current
  transfers to phase b. Commutation is completed
  when ibId.

33
Six-Step CSI with Self-Commutated Devices (contd)

• Total commutation time is tc.
• Once commutation is complete, current can be
  commutated back to Q1. This back and forth
  current commutation can be used to create a PWM
  current wave and with suitable selection of notch
  angles, can be used to suppress higher harmonics
  (just as in the VSI inverter).

34
Six-Step CSI with Self-Commutated Devices (contd)

• A major disadvantage of this scheme is the
  potential for resonance between the capacitors
  and the motor inductance. Care must be taken to
  avoid impressing current harmonics into the
  motor/capacitor network which will excite one of
  the system resonance frequencies. This can be
  avoided by careful use of PWM. However, since the
  motor parameters must be known to implement such
  an approach, this drive is not popular for
  general-purpose applications.

35
PWM Inverters

• The six-step CSI inverter has several
  disadvantages primarily associated with harmonics
  in the current waves. Pulse width modulation can
  be used to reduce the harmonic content of the
  current waves. The PWM methods are somewhat
  different from those for the voltage-fed
  inverters.

36
Trapezoidal PWM

• Similar to the sinusoidal PWM method for
  voltage-fed converter. This method is shown
  below

37
Trapezoidal PWM

• Trapezoidal wave has max. amplitude of B and
  is compared to a triangle wave of amplitude A.
• For the first ?/3 radians both waves are
  compared. For the next ?/3 radians no triangular
  wave is applied. For the final ?/3 radians both
  waves are compared again.
• Two variables 1) modulation index mB/A
• 2) pulse number M in
  half-
• cycle of inverter
  operation.

38
Trapezoidal PWM (contd)

• For M21, harmonics vs. m is as shown below
• At m0.82,
• 5th harmonic 0, 7th harmonic4,
  11th harmonic1 and 13th harmonic2.

39
Trapezoidal PWM (contd)

• The output current waves for these conditions
  is shown below

40
Trapezoidal PWM (contd)

• To limit switching losses it is necessary to
  control the device switching frequency,
  irrespective of the fundamental frequency of the
  current waveform. This can be achieved by making
  the parameter M constant in many segments of the
  fundamental frequency (see next slide for
  switching frequency ?1kHz).
• Note In multi-MW GTO inverters the switching
  frequency generally does not exceed a few hundred
  Hz.

41
Trapezoidal PWM (contd)

• Trapezoidal PWM can reduce harmonic components
  up to order n1.5(M1) for M gt 9 but does produce
  a pair of harmonics of order 3(M-1)?1.

42
Selected Harmonic Elimination PWM

• SHE-PWM can both lower the harmonic content of
  the output current and, more importantly, remove
  the resonant harmonic. Unlike SHE-PWM for
  voltage-fed inverters, several restrictions apply
  for application of SHE-PWM to current-fed
  inverters.
• Consider the 3? current waveforms for M5
  shown in the next slide.

43
Selected Harmonic Elimination PWM (contd)

44
Selected Harmonic Elimination PWM (contd)

• Angles ?1 and ?2 are the variables and all the
  other switching angles are in terms of these two
  variables. With two variables, two switching
  harmonics (e.g. 5th and 7th) can be eliminated.
  The fundamental is controlled by the dc link
  current frequency. The general relation between
  of harmonics removed (K) and of pulses per half
  cycle (M) is given by
• K(M-1)/2
• Both K and M are odd numbers.

45
Selected Harmonic Elimination PWM (contd)

• For M3, only one harmonic (e.g. 5th) can be
  eliminated and for M7, three harmonics (e.g.
  5th, 7th and 11th) can be eliminated.

46
Selected Harmonic Elimination PWM (contd)

47
Double-Sided CSI Converter

• As mentioned earlier, the CSI converter can
  easily be used to send power back into the
  rectifier when the machine acts as a generator.
  In this case the load-side converter acts as a
  rectifier and the line-side converter acts as an
  inverter.

48
Duality of Current-Fed and Voltage-Fed Inverters

Ref D.W. Novotny and T.A. Lipo, Vector Control
and Dynamics of AC Drives
49
Current-Fed vs. Voltage-Fed Inverters

• Current-Fed Inverters Voltage-Fed Inverters
• 1. More interactive with Not so interactive
  with
• the load and hence machine and can
  thus
• require a close match be designed to be
  more
• to the machine. general
  purpose.
• 2. Inherent 4-quadrant Requires additional
  
• operation. circuitry to
  operate in
• all 4
  quadrants.
• 3. Robust through load Shoot-through
  faults
• short circuits/inverter need to be
  avoided (use
• misfirings. freewheel
  diodes).

50
Current-Fed vs. Voltage-Fed Inverters

• Current-Fed Inverters Voltage-Fed Inverters
• 4. Devices must be Devices must be
• symmetric blocking. assymmetric
  blocking.
• 5. Multi-machine or Normally used
  for
• multi-inverter system multi-machine or
  multi-
• inverter system very inverter system
• difficult to implement. applications.
• 6. Relatively sluggish PWM inverters
  can
• response. demonstrate
  relatively
• fast
  dynamic response.

51
Current-Fed vs. Voltage-Fed Inverters

• Current-Fed Inverters Voltage-Fed Inverters
• 7. Cannot be operated Can be operated
  open-
• open-loop. loop.
• 8. Minimum load required. Can operate at
  no-load.
• Based on these differences, PWM voltage-fed
  inverters are most widely used for motor drives.
  However, current-fed inverters are used for
  high-power applications, particularly
  load-commutated synchronous motor drives.

52
d,q Model for CSI Inverter

• The duality of VSI and CSI systems implies that
  the switching function models for CSI systems
  should be the duals of those for the VSI systems.
  
• The exact dual of a VSI feeding a Y connected
  load is a CSI feeding a ? connected load.
  However, since we generally want to consider Y
  connected loads, the model for the CSI inverter
  will not be the exact dual of the VSI inverter
  (but it will be close).

53
d,q Model for CSI Inverter (contd)

• The d,q equations for each switching mode for
  the CSI inverter are obtained in the same way as
  for the VSI inverter. The six switching modes for
  the CSI inverter are shown on the next slide.

54
d,q Model for CSI Inverter (contd)

Ref D.W. Novotny and T.A. Lipo, Vector Control
and Dynamics of AC Drives
55
d,q Model for CSI Inverter (contd)

• The d,q equations, in the stationary stator
  reference frame, can be written in terms of CSI
  inverter switching functions, h as
• where the switching functions (shown on the
  next slide) can be expressed as Fourier series
  by

56
d,q Model for CSI Inverter (contd)

Ref D.W. Novotny and T.A. Lipo, Vector Control
and Dynamics of AC Drives
57
d,q Model for CSI Inverter (contd)

• These equations can be written in complex form
  as
• where
• Note that these are very similar to the VSI
  equations. In particular, only the signs in hqdss
  are altered.

58
d,q Model for CSI Inverter (contd)

• As before we observe that the complex vector
  current is constant in each mode and simply
  shifts by 60? at each mode transition. We can
  write
• for k1, 2, 3, 4, 5, and 6. The six vectors
  corresponding to the switching of a CSI inverter
  are shown in the next slide.

59
d,q Model for CSI Inverter (contd)

Ref D.W. Novotny and T.A. Lipo, Vector Control
and Dynamics of AC Drives
60
d,q Model for CSI Inverter (contd)

• See handout for d,q model for CSI inverter in
  stationary reference frame.