Presented by: Aazim Rasool
1134200011
Presented to: Dr. Chongru Liu
North China Electric Power University,
Beijing , China 1
AC
DC
North China Electric Power University,
Beijing 2
North China Electric Power University,
Beijing 3
Cost of HVDC is less
One cable required instead of three
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Same poles can be use. Moreover, slim and smart
poles are used for DC transmission
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ACNorth
Transmission Line Corridor
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DCNorth
Transmission Line Corridor
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DCNorth
Transmission Line Corridor
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DC transmission system
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In 6-phase;each transistor
operate for 120o .
Ebc -- T3&T2 Ebc -- T3&T4 Eba - T5&T4
Eac -- T1&T2
Eca - T5&T6 Ecb - T1&T6
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Graph representation of operation.
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Figure representing, when firing
delay angle ‘α’ changes
To make eac(α=0) ; switch ON
transistors 1 & 2
at ‘-60o ‘
for ‘60o ‘.
To make eac(α≠0) ; switch ON
transistors 1 & 2
at ‘-60o + α’
For ‘60o + α‘.
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Rectifier Operation Inverter Operation
AC System Power Flow DC System AC System Power Flow DC System
Id Id
V1 V3 V5 V1 V3 V5
Phase A Phase A
Phase B Phase B Ud
Ud
Phase C Phase C
V4 V6 V2 V4 V6 V2
+Ud
Rectifier
Operation
160
0
a
5 30 60 90 120 150 180
Inverter
Operation
-Ud
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a Rect. Limit
Rectifier
+Ud Operation
160 a
0
5 30 60 90 120 150 180
Inverter
Operation
-Ud
a Inv Limit
a = 0o a = 30o a = 60o
Ud
Ud
wt
a = 90o a = 120o a = 150o
Ud
wt
-Ud
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15
Direct current from the rectifier to the inverter
Vdor cos a Vdoi cos
Id =
Rcr Rl Rci
Power at the rectifier terminal
Pdr = Vdr I d
Power at the inverter terminal
Pdi = Vdi I d = Pdr RL I d
2
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α:
Ignition delay angle for rectifier
α min = 5 o (Required to charge thyristor)
α op. = 15-20 o (Room for VR )
α ≤ 900
γ:
Extinction advance angle
γmin = 15o (50Hz)/ 18o (60Hz) – avoid comm. failure
** 1800 ≥ α ≥ 900 (For inverter mode)
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* µ= overlap angle
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1 α= firing Angle
3
μ= Commutation
u u u
a a a Interval
C
A Vd
B
2
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Internal voltages, Vdorcosa and Vdoicos are used to control
the voltages at any point on the line and the current flow
(power)
This can be accomplished by:
Controlling firing angles of the rectifier and inverter (for fast action)
Changing taps on the transformers on the AC side (slow response)
Power reversal is obtained by reversal of polarity of direct
voltages at both ends
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Ideal Characteristic:
Under normal Condition;
Rectifier maintains CC (Constant Current)- α
Inverter maintains CEA (Constant Extinction Angle) γ min
Vd = Vdoi cos ( RL Rci ) Id
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Actual Characteristic
Rectifier
Abnormal Condition
FA represents min. ignition angle (CIA mode)
AB represents Constant Current (CC mode)
*CIA shows maximum
rectifier voltage
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Actual Characteristic
Inverter
Abnormal Condition
GD represents min. extinction angle (CEA mode)
GH represents Constant Current (CC mode)
Operating Point
Operating Point
at abnormal *CEA shows maximum
inverter voltage
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Each converter can work as a rectifier as well as
Operating Point 1
inverter.
O.P 1
C1=rectifier(CC)
Current is same
C2=inverter(CEA)
O.P 2
C2=rectifier(CC)
C1=inverter(CEA)
Operating Point 2
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Decrease voltage at station B or increase voltage at station A. power flows from A B Normal
direction
Decrease voltage at station B or increase voltage at station A. power flows from A B Normal
direction North China Electric Power University,
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Power reversal is obtained by reversal ofElectric
North China polarity
Powerof direct voltages at both ends.
University,
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CONSTANT VOLTAGE MODE CONSTANT B MODE
V-I characteristic is flat Back-up type
Higher value of γ γ is comparatively less
γ is set at higher; maintain low constant voltage
γ is se at medium; make greater voltage then CVM
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Small change in AC-Voltage cause large change in
DC-Current.
There is a Mode Ambiguity.
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Fig a, represents constant β mode.
Fig b , represents constant Voltage mode.
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Voltage-Dependent Current-Order Limit.
Under low voltage(drop >30%);current also decreases to
low level
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Graph shows the function of VDCOL in control graph
of rectifier and inverter characteristic
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“Power system stability and control”, parabha
qundar
Course Lectures “HVDC” , A.M Gole.
“Presentation of HVDC Transmission”,Zunaib Ali
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North China Electric Power University,
Beijing 33
