6pulse fully controlled bridge basic operation

1
6-pulse fully controlled bridge - basic operation

• See notes for circuit diagram
• Each thyristor has a gate firing circuit
  associated with it (not shown) which provides
  gate current when required to do so to fire the
  device
• A control circuit, synchronized to the supply
  voltages, controls the firing of the thyristors
  via the gate firing circuits
• Each thyristor is fired with respect to the point
  on the supply voltage waveform where its
  corresponding diode in a 6-pulse diode rectifier
  would have started conducting
• Each thyristor firing is delayed with respect to
  the corresponding diode turn-on point. The delay
  (?) is normally measured in degrees and is called
  the delay angle or firing delay angle
• For example, T3 is fired ? degrees after the
  negative zero crossing of VAB, since D3 in a
  diode bridge starts conducting at the negative
  zero crossing of VAB
• Provided ? lt 180O (see later) firing T3 will
  automatically reverse bias T1and turn it off
  (natural commutation - same for other thyristors)
• Controlling the delay angle allows the average
  output voltage to be controlled (see notes)

2
Notching - effect of overlap on supply voltage
(1)

• Affects diode and thyristor bridges - but is more
  pronounced with thyristor circuits
• Consider the T1/T3 overlap

• VAB is a sinewave
• During overlap, Vab 0 (supply voltage is
  dropped across the supply inductance)
• Vab has notches in it (where it falls to
  zero) each time there is a T1/T3 overlap (or a
  T4/T6 overlap)
• The other 2 line voltages are also distorted at
  these points and have either a notch or a pulse
  1/2 the size of the Vab notch (see example)
• Consequently each line voltage at the converter
  terminals has 6 notches (2 large ones dropping to
  zero and 4 smaller ones) corresponding to the 6
  overlaps per cycle (see example)

3
Notching - effect of overlap on supply voltage
(2)

• Distortion due to notching is most severe at the
  converter terminals but distortion is seen at
  other points in the network since the supply
  inductance is distributed

• PCC - Point of Common Coupling. Place where other
  users are connected to the network and normally
  the point where power quality regulations are
  applied
• Distortion due to notching at PCC is attenuated
  from that at the converter terminals by the ratio
  L1/(L1L2)
• L2 often added inside the equipment to reduce
  distortion seen by the network
• Note - large notches occur ? degrees after each
  zero crossing of the line voltage. Notches are
  very large for values of ? around 90O - hence
  generally worse with thyristor circuits rather
  than diode circuits

4
Notching - example to do

• Doing this example will test your ability to draw
  rectifier waveforms and will clearly illustrate
  the distortion caused by notching
• On a 3-phase template, draw VXN, VYN and VXY for
  a firing delay angle of 60O and an overlap angle
  of 15O
• Label next to each overlap, the thyristors
  involved in each overlap
• Draw up a table listing the overlaps in order
  and, corresponding to each overlap, the voltages
  VaN and VbN. Hence determine the voltage Vab
  at each overlap and add this to the table
• Hence draw the waveform of Vab and see the
  distortion due to notching. Note that Vab
  VAB except during overlaps

5
Fully controlled Bridge - Inverting Operation

• If ? gt 90O then the mean converter output voltage
  (VXY) will be negative and power can flow from
  the DC side to the AC side - this is called
  INVERSION

• For this to be sustainable there must be a source
  of energy on the DC side - eg battery, or motor
  acting as a generator
• Very useful when the converter feeds a motor
  since it allows mechanical energy to be returned
  to the supply (useful to avoid energy wastage
  when slowing things down - see H5CEDR)
• Theoretically ? can be taken to 180O during the
  inverting mode, but in practice it is limited by
  the need to provide sufficient reverse bias time
  for the thyristors to turn off properly

6
Fully controlled Bridge - Maximum delay angle (1)

• Consider commutation from T1 to T3

• T3 is fired ? degrees after the negative zero
  crossing of VAB. There is then an overlap with T1
  and T1 turns off (??) degrees after the negative
  zero crossing of VAB. After T1 has turned off,
  the voltage across it is VAB.

• T1 will be reverse biassed after turn-off for a
  time equivalent to 180 - (??) degrees.

7
Fully controlled Bridge - Maximum delay angle (2)

• In seconds, the reverse bias time is given by
• trv 180 - (??)?/180?
• For T1 to turn off properly, trv must be greater
  than the turn off time (normally given the symbol
  tq) for the particular thyristor used.
• It it isnt, T1 will start conducting again when
  VAB goes positive (even though it isnt fired)
  and a catastrophic short circuit of the DC side
  will occur when T4 is fired
• This is called a COMMUTATION FAILURE and can
  normally be recovered from by blowing fuses
• To avoid commutation failure (??) is limited to
  typically 150O
• Even so commutation failure can occur in practice
  due to abnormal conditions such as
• Excessive DC side current, resulting in an
  increase in ?
• Reduction in supply voltage, resulting in an
  increase in ?
• ? too large due to control circuit error (loss of
  synchronisation for example)

8
Dual Controlled Bridge - 4 Quadrant Arrangement(1)

• Very important for DC machine drives
• One fully controlled bridge can provide 1
  direction of current and both polarities of
  voltage
• For a DC machine Voltage ? Speed and Current ?
  Torque
• Hence a single bridge can drive a machine in 2
  quadrants of the torque-speed plane

• Many applications require operation in all 4
  quadrants (accelerating and braking in both
  directions)
• Need to use 2 bridges in a dual arrangement
• Very common arrangement for DC motor drives

9
Dual Controlled Bridge - 4 Quadrant Arrangement(2)

• At any time, only one of the bridges is enabled
  (otherwise the supply would be short circuited)
• The controller must sense the desired current
  direction and enable the appropriate bridge.

10
Inverting operation - example to do

• Doing this example will (definitely!) test your
  ability to draw rectifier waveforms and will
  clearly illustrate the reverse bias period
  following thyristor turn off
• On a 3-phase template, draw VXN, VYN and VXY for
  a firing delay angle of 120O and an overlap angle
  of 15O
• Label next to each overlap, the thyristors
  involved in each overlap. Label the conducting
  thyristors during the non-overlap periods
• Draw up a table listing the thyristor conduction
  patterns in sequence (including overlaps) for a
  whole cycle.
• Next to each conduction pattern, tabulate the
  voltages VXN and VaN and hence tabulate the
  voltage VT1 (voltage across thyristor 1)
• On another template re-draw VXN, VYN on the upper
  part and using the table, draw VT1 on the lower
  part
• Make sure you can identify the reverse bias time
  on the VT1 waveform and make sure it is equal to
  180 - (??) degrees
• Note this exercise will take some time and
  methodical working to do - but it will be
  invaluable practice