This document discusses firing angle control and constant current control techniques for HVDC systems. It describes two main firing angle control schemes: Individual Phase Control (IPC) and Equidistant Pulse Control (EPC). IPC determines firing pulses individually for each valve but causes harmonic instability. EPC produces pulses at equal intervals and has three methods - pulse frequency control, pulse period control, and pulse phase control. It also discusses constant current control and provides references for further reading.
3. 3
Firing Angle Control
Individual Phase Control (IPC) Equidistant Pulse Control (EPC)
Constant α control
Two Schemes
Inverse cosine control
Pulse Frequency control Pulse period control Pulse Phase control
4. Individual Phase Control(IPC)
– These where used in early days when Mercury arc valves where used.
– Firing instances is determined individually for each valve i.e., phase position of
each control pulses is determined separately for each valve and related to
commutation voltage.
– Six parallel delay circuit is required.
Two ways in which this can be achieved:
1. Constant α control:
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5. Individual Phase Control(IPC)
2. Inverse cosine control:
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The delay angle α is proportional
to the Inverse cosine of the control
voltage.
6. Individual Phase Control(IPC)
Drawbacks of IPC scheme:
• The major drawback of IPC is the harmonic instability.
• This is mainly due to the fact that any distortion in the system voltage leads to
perturbations in the zero crossing. This effects the instants of firing pulses.
• Thus non characteristic harmonics are generated in AC current which amplifies
the harmonic content in AC voltage at the converter end.
This drawback can be overcome by:
• Use of filters for filtering these non characteristic harmonics. This may be
problematic due to variation in the supply frequency and will add to control
delays.
• Use of firing angle control independent of zero crossings of the AC voltages.
This solution leads to equidistant pulse firing scheme.
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7. Equidistant Pulse Control (EPC)
• Equidistant pulse control (EPC)
– No synchronization of control pulses with applied ac voltage.
– Used in modern HVDC.
– It produces pulses at equal intervals of 1/( f*p )
– This method gives low dc output voltage but successful in weak ac system.
Three methods in EPC scheme:
1. Pulse frequency control:
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8. Equidistant Pulse Control (EPC)
• The frequency of voltage control oscillator (VCO) is determined by the control
voltage Vc related to the error in current, gamma or voltage.
• The VCO consists of an integrator, comparator and a pulse generator.
• The output pulses of the generator drive the ring counter and also resets the
integrator.
• The instant (tn ) of the firing pulse is determined from following equations
Where V1 is bias voltage and V3 is proportional to the system period.
At steady state Vc=0,
The gain K1 of the integrator is chosen as,
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9. Equidistant Pulse Control (EPC)
2. Pulse period control:
• This is similar to PFC except that Vc is summed with V3 instead of V1.
• Thus the instant (tn ) of the pulse generation is given by
Thus we get,
3. Pulse phase control:
• A train of pulses are generated proportional to the control voltage Vc.
• The analog circuit is configured to generate firing pulses according to the
following equation:
• Response of this system is fast as it doesn’t have integrator characteristics.
• Capacitor is used here for charging and discharging. It is maintained between
± ∆V. 9
Vc
V3
11. References
• “HVDC Power Transmission Systems : Technology and
system Interactions”, K.R.Padiyar, new age
international,1990.
• High voltage DC transmission : NPTEL lecture videos.
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