Multipulse Converters 1

1
Multipulse Converters (1)

• Defined as diode/thyristor converters where the
  pulse number is gt 6 (remember, the ones we have
  looked at so far have been 6-pulse)
• Generally the pulse number is a multiple of 6
  (assuming 3-phase system), so 12, 18, 24-pulse
  circuits etc are possible
• A 12-pulse converter, for example, consists of
  two 6-pulse converters fed from a 6-phase supply
  and connected in series or parallel on the DC
  side (18-pulse has three 6-pulse circuits and so
  on)
• Since a 6-phase supply is not normally available,
  it is generated from the 3-phase supply using a
  phase shifting transformer (6-phase source is
  equivalent to two 3-phase sources with 30O phase
  shift between them)
• The advantage of using the two 6-pulse circuits
  like this, rather than just connecting them in
  parallel (or series) is that harmonic
  cancellation takes place and the power quality at
  the input and output is improved

2
Multipulse Converters (2)

• Higher pulse number means fewer input harmonics
  (better power factor), and smoother output
  (smaller smoothing components)
• Used at high power levels (gt500kW) for this
  reason - sometimes also at lower power levels
  where power quality is critical
• Cross channel (UK-France), electricity link
  (2000MW, 270kV) uses 48-pulse converters at each
  end, for example

3
12-Pulse Converter (1)

• Transformer with two secondaries (Star and
  Delta), provides two 3-phase outputs with 30O
  phase shift between them
• RMS line voltages are the same so delta winding
  has more (?3) turns per phase
• Output voltages look like (assuming diode
  circuit)

4
12-Pulse Converter (2)

• Input currents iABCDEF all have the same shape as
  in the normal 6-pulse circuit
• iDEF are mutually displaced from each other by
  120O
• iABC are mutually displaced from each other by
  120O
• iD lags IA by 30O (?/6) and so on
• It can be shown (not done here) that the effect
  of the currents combine in the transformer so
  that

• Similar result for 7th, 17th, 19th etc harmonics
• Only 12K?1 harmonics are left

5
12-Pulse Converter (3)
Line Current waveform

• Parallel connection is also possible (same
  advantages) - but needs inter-phase reactor
  (IPR) for correct operation