High Power Inverters 1

1
High Power Inverters 1

• Application power range of inverter circuits
  using the basic "inverter leg" building block is
  now vast (lt1kW to 10MW)
• Very large application area is in industrial (PWM
  controlled induction motor) drives (see H5CEDR)
  in the 3kW to 100kW power range. IGBT devices are
  used almost exclusively in this power range.
• Recently the application area for these circuits
  has extended to power levels (gt1MW), previously
  serviced only by naturally commutated thyristor
  circuits (for example naturally commutated
  thyristor synchronous motor drives).
• Inverter drive provides more functionality and
  flexibility in control and allows induction
  motors to be used (see H5CEDR).
• Typical high power applications
• Railway locomotives (1-5MW)
• Ship propulsion (eg Frigate 20MW)
• Power systems applications, for example FACTS
  (Flexible AC Transmission Systems) - see H5CPNW -
  up to 100MW
• Devices used IGBT (Insulated Gate Bipolar
  Transistor), GTO (Gate Turn-off Thyristor), IGCT
  (Insulated Gate Commutated Thyristor).

2
High Power Inverters 2

• Design of high power converters (MW range)
  presents problems
• Single devices cant handle the V and I.
• For example a 1MW drive would be typically
  supplied at 3.3kV (UK) or 4.16kV (US) giving a DC
  link of 5 to 7kV. The voltage supplied to the
  motor is also 3.3kV (or 4.16kV).
• Device voltage rating required 8-10kV - not
  available.
• Handling high currents by putting devices (or
  converters) in parallel is fairly well
  established. Getting the voltage handling
  capability remains the problem.
• Possible solutions are
• Use standard converter topologies with devices in
  series.
• Use alternative topology converters which a
  number of low voltage devices and that have some
  means for distributing the voltage stress amongst
  those devices (Multi-level converters).

3
Series Devices 1
One switch - all devices switched together
(N/2)(E/2)
DC supply
A
O
VAO is 2-level
(N/2)(E/2)
Total number of devices N per leg
One inverter leg

• Assuming that each device is capable of operating
  at voltage E, then theoretically by using N
  devices in series we can operate with a DC
  voltage of (N/2)E

4
Series Devices 2

• Problems
• The entire DC voltage appears across each switch
  when it is off. This will be greater than the
  voltage rating of the individual devices.
• The devices will not automatically share the
  voltage in the off state because of differences
  in leakage current - high value parallel
  resistors can be used to overcome this (static
  sharing).
• More seriously, the devices will not share the
  voltage during switching due to variations in
  switching speed. Special gate drive techniques
  and/or special snubbers are required (dynamic
  sharing). Not well established yet.
• Two level output causes very large voltage steps
  on the load - can be a problem for motor
  insulation (for example)
• Harmonic content (distortion) is larger for a
  given switching frequency than with multi-level
  techniques (see later)

5
Series Devices 3

• Advantages
• Standard PWM techniques can be used.
• Number of power circuit components is less than
  with other (multi-level) circuits.
• Redundancy can be incorporated (to improve
  reliability) by using more series devices than
  actually required - the circuit can then still
  work if one fails (provided it fails short
  circuit).

6
Multi-level Converters 1

• As the name suggests, these circuits produce a
  waveform with more levels than a standard
  (2-level) inverter leg

Example of 3-level line to line waveform
produced by an inverter with standard 2-level
inverter legs
Example of 5-level line to line waveform
produced by an inverter with individual legs that
can produce 3-levels
7
Multi-level Converters 2

• Multi-level converters produce a better output
  spectrum than 2-level converters employing the
  same device switching frequency. PWM generation
  is more complex

Comparison of spectra for conventional (3-level)
line to line waveform and 5-level line to line
waveform

• The are 3 basic types of multi-level converter
• Isolated H-bridge
• Diode clamped converter
• Flying capacitor converter

8
Isolated H-bridge 1
O
1a
2a
3a
1b
2b
3b
C
A
B
Output

• Each H-bridge must have an isolated DC supply -
  usually derived from an isolated AC supply via a
  diode bridge
• Each bridge can produce E, 0, -E independently
• With K bridges per phase, VAO etc has 2K1 levels
  and VAB has 4K1 levels
• Circuit above has 5-levels in VAO and 9-levels in
  VAB

9
Isolated H-bridge 2

• Problems
• Each H-bridge needs an isolated DC supply
  compared to the other solutions which need only
  one supply. Normally this requires some sort of
  complicated transformer arrangement. Also the
  capacitors associated with each supply can be
  large.
• Advantages
• Device voltage sharing is automatic because of
  the independent DC supplies. There is no
  restriction on switching pattern.
• With N devices (each capable of operating at
  voltage E) per-phase, the circuit can produce an
  output varying between (N/2)(E/2). This is the
  same as the series device solution. The advantage
  is that the output has more levels, giving
  smaller voltage steps to the load and less
  harmonic distortion (for a given switching
  frequency).
• Some redundancy is possible by using more
  H-bridges per phase than is actually required.
• By using a lot of H-bridges, very high voltage
  converters can be made this way.
• The circuit is modular the is an advantage for
  manufacture and maintenance.

10
Diode Clamped Circuit 1
3-level diode clamped circuit (neutral point
clamped circuit) One inverter leg shown
DC Supply
A1, B1 ? VXO E A0, B1 ? VXO 0 A0, B0?
VXO -E
DC Supply
Example current paths for VXO 0 state
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Diode Clamped Circuit 2
Complete 3-phase inverter with 3-level diode
clamped legs Popular solution for 500kW few MW
motor drives
E
DC Supply
O
O
O
E
B
C
A
VAO etc are 3-level (E,0,-E) VAB etc are
5-level (2E, E, 0, -E, -2E)
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Diode Clamped Circuit 3
ABCD1111? VXO 2E ABCD0111? VXO
E ABCD0011? VXO 0 ABCD0001 ? VXO
-E ABCD0000 ? VXO -2E
DC Supply
D1
O
X
D2
Note D1 has to block 3E when ABCD1111 D2 has to
block 3E when ABCD0000
5-level diode clamped circuit One inverter leg
shown
13
Diode Clamped Circuit 4

• Problems
• Except in the 3-level circuit, the capacitor
  voltages do not share automatically. Either some
  form of extra balancing circuit is needed or
  multiple (non-isolated) DC supplies are required
  (one for each capacitor).
• As the number of levels increases, some diodes
  have to block large voltages (see D1 and D2 in
  previous slide) makes the circuit unattractive
  for more than 5-levels since many diodes in
  series are required (currently).
• Difficult to build in redundancy.

14
Diode Clamped Circuit 5

• Advantages
• Only one isolated DC supply is required.
• By operating the circuit such that the output
  from each leg changes one level at a time, the
  problem of device voltage sharing during
  transients (dynamic sharing) is avoided. This
  however makes the switching pattern more
  restricted than the isolated H-bridge solution.
  Note static voltage sharing is not guaranteed
  without parallel resistors (see earlier notes).
• With the same number of devices (ignoring clamp
  diodes) of equal voltage rating, the circuit can
  produce the same output voltage levels as the
  isolated H-bridge solution and operate with the
  same DC link voltage as the series switch
  solution.

15
Flying Capacitor Circuit 1
3-level flying capacitor circuit One inverter leg
A1, B1 ? VXO E A0, B1 ? VXO 0 A1, B0?
VXO 0 A0, B0? VXO -E
Flying capacitor

• Flying capacitor must be pre-charged to E.
• Note the difference between the A0, B1 and A1,
  B0 states. The voltage produced is the same in
  both cases, but the current path through the
  flying capacitor is reversed this provides the
  means to keep the capacitor charged to the
  correct voltage during circuit operation.

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Flying Capacitor Circuit 2
Complete 3-phase inverter with 3-level flying
capacitor legs
VAO etc are 3-level (E,0,-E) VAB etc are
5-level (2E, E, 0, -E, -2E)
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Flying Capacitor Circuit 3
ABCD VXO 1111 2E 1110 E 1101 1100 0 1011 E 1010
0 1001 1000 0111 E 0110 0101 0 0100 0011 0 0010
-E 0001 -E 0000 -2E
A
2E
B
C
DC Supply
D
E
2E
3E
X
O
D
C
B
2E
A
5-level flying capacitor circuit One inverter leg
shown

• Duplicate states (4 ways to get E, 4 ways to get
  -E, 6 ways to get 0) provide the means for
  keeping the capacitor voltages correct.

18
Flying Capacitor Circuit 4

• Problems
• "Flying" capacitors need to be pre-charged.
• Switching strategy must be used to maintain
  flying capacitors at the correct voltage. This
  must be done via some form of feedback mechanism.
• As the number of levels increases, the number of
  capacitors required increases rapidly (note that
  the flying capacitors supporting more than E will
  generally be made from a number of series
  connected capacitors).
• Difficult to build in redundancy.

19
Flying Capacitor Circuit 5

• Advantages
• Only one isolated DC supply is required.
• Device voltage sharing is guaranteed (both static
  and dynamic) if the flying capacitors are kept
  charged the correct voltage.
• No high voltage silicon devices are required
  (unlike the diodes in the diode clamped circuit)
• With the same number of devices of equal voltage
  rating, the circuit can produce the same output
  voltage levels as the isolated H-bridge solution
  and the diode clamped solution and operate with
  the same DC link voltage as the series switch
  solution.

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Summary

• Power range for inverter circuits is extending
  upwards all the time.
• Improvements in device voltage and current
  ratings can't keep up.
• Series devices or multilevel converter structures
  are required to get voltage handling capability
  required.
• 3 basic types of multilevel converter
• Isolated H-Bridge converter
• Diode Clamped converter
• Flying Capacitor converter
• This is a big growth area for Power Electronics
  at the moment.