control of hvdc system
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North China Electric Power University, Beijing , China
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CONTROL OF HVDC SYSTEM
Presented by: Aazim Rasool 1134200011
Presented to: Dr. Chongru Liu
North China Electric Power University, Beijing
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Comparison HVAC & HVDC
AC
DC
North China Electric Power University, Beijing
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Comparison HVAC & HVDC
North China Electric Power University, Beijing
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Comparison HVAC & HVDC
Cost of HVDC is lessOne cable required instead of three
North China Electric Power University, Beijing
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Comparison HVAC & HVDC
Same poles can be use. Moreover, slim and smart poles are used for DC transmission
North China Electric Power University, Beijing
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Comparison HVAC & HVDC
AC Transmission Line Corridor
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Comparison HVAC & HVDC
DC Transmission Line Corridor
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Comparison HVAC & HVDC
DC Transmission Line Corridor
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Operation
DC transmission system
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Operation In 6-phase;each transistor
operate for 120o .
Eac -- T1&T2
Ebc -- T3&T2 Ebc -- T3&T4 Eba - T5&T4
Eca - T5&T6 Ecb - T1&T6
Each small block representing 600
operation
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Operation
Graph representation of operation.
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Operation & Control
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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Control
V 1 V 3 V 5
V 2V 6V 4
P hase A
UdP hase B
P hase C
Id
Pow er FlowAC System DC System
V 1 V 3 V 5
V 2V 6V 4
P hase A
UdP hase B
P hase C
Id
AC System DC SystemPow er Flow
30 60 90 120 150 1800
+Ud
-Ud
160
5
RectifierOperation
InverterOperation
a
Rectifier Operation Inverter Operation
North China Electric Power University, Beijing
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Control
30 60 90 120 150 180
0a
+Ud
-Ud
160
L im ita In v
5
L im ita R e c t. RectifierOperation
InverterOperation
tw
o60=a
Ud
o30=ao0=a
o90=a o120=a o150=a
-Ud
tw
Ud
Ud
North China Electric Power University, Beijing
Basic principles of control Direct current from the rectifier to the inverter
Power at the rectifier terminal
Power at the inverter terminal
cilcr
doidord RRR
VVI
=a coscos
ddrdr IVP =
2dLdrddidi IRPIVP ==
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North China Electric Power University, Beijing
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Basic principles of control
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)
North China Electric Power University, Beijing
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Basic principles of control
* µ= overlap angle
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Effect of Control Angel
B
A
2
C
1
a u u
Vd
u
3
a a
α= firing Angleμ= Commutation
Interval
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Basic means of control Internal voltages, Vdorcos 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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Control CharacteristicIdeal Characteristic: Under normal Condition;
Rectifier maintains CC (Constant Current)- α Inverter maintains CEA (Constant Extinction Angle) γ min
dciLdoid IRRVV )(cos =
North China Electric Power University, Beijing
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Control CharacteristicActual Characteristic
Abnormal Condition FA represents min. ignition angle (CIA mode) AB represents Constant Current (CC mode)
Rectifier
*CIA shows maximum rectifier voltage
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Control CharacteristicActual Characteristic
Abnormal Condition GD represents min. extinction angle (CEA mode) GH represents Constant Current (CC mode)
Inverter
*CEA shows maximum inverter voltage
Operating Point
Operating Point at abnormal
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Rectifier & Inverter
Each converter can work as a rectifier as well as inverter. O.P 1
C1=rectifier(CC) C2=inverter(CEA)
O.P 2 C2=rectifier(CC) C1=inverter(CEA)
Operating Point 2
Operating Point 1
Current is same
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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, Beijing
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North China Electric Power University, Beijing
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Power reversal is obtained by reversal of polarity of direct voltages at both ends.
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Inverter - DC Voltage Control
CONSTANT VOLTAGE MODE CONSTANT B MODE
V-I characteristic is flat Higher value of γ
Back-up type γ is comparatively less
γ is set at higher; maintain low constant voltageγ is se at medium; make greater voltage then CVM
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Mode Stabilization
Small change in AC-Voltage cause large change in DC-Current.
There is a Mode Ambiguity.
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Mode Stabilization
Fig a, represents constant β mode. Fig b , represents constant Voltage mode.
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VDCOL
Voltage-Dependent Current-Order Limit. Under low voltage(drop >30%);current also decreases to
low level
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VDCOL
Graph shows the function of VDCOL in control graph of rectifier and inverter characteristic
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References
“Power system stability and control”, parabha qundar
Course Lectures “HVDC” , A.M Gole. “Presentation of HVDC Transmission”,Zunaib Ali
North China Electric Power University, Beijing
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Thanks
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