3.hvdc controls(1)
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HVDC Control System -
Overview
What are the basic control principlesfor HVDC Systems?
HVDC Control & Protection
AC System A AC System B
U1 U2
Id
Simplified HVDC System diagram
What are the basic principles of HVDC Controls?
HVDC Control
AC System A AC System B
U1 U2
Id
What are the basic principles of HVDC Controls?
HVDC Control
= =U U1 2
Id in one direction only
Id
Change of Power Direction
Power Direction
U1 U2
Magnitude of Id or power iscontrolled depending on the difference in theterminal voltages (U1, U2)
U1 U2
Direction of power iscontrolled depending on thepolarity of the terminalvoltages (U1, U2)
What are the basic principles of HVDC Controls?
HVDC Control
AC System A AC System B
U2U1
Id
Rectifier
Control
Id-Control
Converter
Id: DC Current
Converter Control
Inverter
Control
Ud-Control
Converter
Ud: DC Voltage
Reactive Power Control
ReactivePower Control(AC VoltageLimitation Control)
capacitors capacitors
ReactivePower Control(AC VoltageLimitation Control)
Sending End Receiving End
Tap Changer Control
Tap ChangerControl
Tap ChangerControl
ACF
ACFACF
ACF
ACF: AC Filter
What are the basic principles of HVDC Controls?
HVDC Control & Protection
Principles of HVDC ControlsControl of DC Voltage
V 1 V 3 V 5
V 2V 6V 4
Phase A
Ud
Phase B
Phase C
Id
Power FlowAC System DC System
V 1 V 3 V 5
V 2V 6V 4
Phase A
Ud
Phase B
Phase C
Id
AC System DC SystemPower Flow
30 60 90 120 150 180
0
+Ud
-Ud
160
5
Rectifier
Operation
Inverter
Operation
α
Rectifier Operation Inverter Operation
Principles of HVDC ControlsControl of DC Voltage
• DC voltage is varied by means of a converter bridge
• In rectifier operation the power flow is from the AC system to the DC system
• The power flow is changed from the DC system to the AC system byreversing the DC voltage.
• The DC current does not change it’s direction.
• The operating range of the ideal converter - theory from 0° (+1.0 p.u. DC voltage) to 180° (-1.0 p.u. DC voltage)
• The operating range of a real converter is from approx. 5° to approx. 160°
• In 90° operation the DC voltage of the converter is 0 V.
Principles of HVDC ControlsRelationship of DC Voltage Ud and Firing Angle α
30 60 90 120 150 180
0α
+Ud
-Ud
160
LimitαInv
5
Limitα Rect.Rectifier
Operation
Inverter
Operation
tω
ο60=α
Ud
ο30=αο
0=α
ο90=α ο
120=α ο150=α
-Ud
tω
Ud
Ud
Principles of HVDC ControlsConverter Control Functions, Fixed Firing Angles α
AC SYSTEM
AAC SYSTEM
B
Inverter
Ud Rect
Id
Ud Inv
Fixed ααααoFixed ααααo
Triggerset
Rectifier
Triggerset
Converter Control Functions, Fixed Firing Angles α
• Firing angle is controlled by use of controllers and the trigger set.
• Task of the trigger set - convert the firing angle in appropriate firing pulses for each individual valve and the synchronisation of these pulses to the AC system
Principles of HVDC Controls
Converter Characteristics, Fixed Firing Angles α
Ud=Udio*cos(α) = k*UAC*cos(α) Station A (Rectifier)
Fixed αo (~15°)
Ud=Udio*cos(α) − dx*Id
∆U=dx*Id
Id (p.u.)
Ud (p.u.)
1.0
1.0
-1.0
Station A (Inverter)
Fixed αo (~140°)
Station B (Rectifier)
Fixed αo (~15°)
Station B (Inverter)
Fixed αo (~140°)
Operating
Point
Converter Characteristics with Fixed Firing Angles (no controller)
• Rectifier is operated with fixed firing angle of e.g. 15 degree.
• The slope of this characteristic depends on the commutation impedance (AC System and transformer) of the converter.
• The weaker the AC system, the steeper the slope.
• Inverter is operated with fixed firing angle of e.g. 140 degree
• Operating point of the HVDC is intersection of the two converter characteristics
• Operating point is strongly dependent on the AC voltage
Principles of HVDC ControlsConverter Characteristics, Fixed Firing Angles a
1.0
Ud (p.u.)
1.0Id (p.u.)
Inverter Fixed αo (~140°)
Rectifier Fixed αo (~15°)
Operating Point
Rectifier Fixed αo (~15°)
AC Bus Voltage reduction
at rectifier
DC Current reduction
(depending on AC Voltage)
Operating Point with reduced rectifier
AC Voltage
DC Voltage reduction DC Current Control at Rectifier
Principles of HVDC ControlsConverter Control Functions, Id control at Rectifer
ααααo
Triggerset
Fixed ααααo
AC SYSTEMA
Rectifier Inverter
Ud Rect
Id
AC SYSTEMB
Ud Inv
Triggerset
PI Controller
I DC Ref
I DC Control
-+I DC Act
DC Current Control at rectifier station
• The DC current control - to keep the DC current constant as
long as possible
• The DC current is controlled according to the reference value
• When the actual DC current becomes lower than the ordered
current, the current control increases the PI controller output
resulting in an increase of the rectifier DC voltage (UdRect) in
order to maintain the DC current.
• In case the actual current is too high the PI controller reacts in
opposite direction.
Principles of HVDC ControlsConverter Control Functions, Id control at Rectifer
1.0
1.0
Ud (p.u.)
Id (p.u.)
Rectifier Fixed αmin (5°) Inverter Fixed α (~140°)
Operating Point
(e.g. alpha 15°)
Rectifier Id Control
Operating range with
reduced AC Bus
Voltage at rectifier
Operating range with
increased AC Bus
Voltage at rectifier
AC Bus Voltage reduction
at inverter
new Operating Point with reduced
inverter AC Voltage and fixed
Inverter α (~140°)
No change in DC Current,but change in DC Voltage
DC Voltage Control at Inverter
No Change in Operating Point but
increased alpha (e.g. 30°)No Change in
Operating Point
• Angle of the rectifier is not fixed to 15° anymore but controlled to lets
say 15°
• In case of AC voltage reductions at recitifer station, the current
controller reduced the Angle in order to keep the required DC Voltage
• Angle can be decreased down to 5°
i.e. no change in rectifier DC voltage as long as the DC voltage referring
to 5°
• Reaction of AC voltage Decrease at inverter station
• To avoid DC Voltage changes due to inverter AC bus voltage changes, a
DC voltage control is applied at the inverter station
Principles of HVDC ControlsConverter Control Functions, DC Voltage Control
AC SYSTEMA
Rectifier Inverter
Ud Rect
Id
AC SYSTEMB
Ud Inv
Triggerset
ααααo
PI Controller
Triggerset
PI Controller
ααααo
I DC Ref
I DC Control
-+I DC Act
U DC Ref
+
U DC ActU DC Control -
Principles of HVDC ControlsConverter Control Functions, DC Voltage Control
1.0
Ud (p.u.)
1.0 Id (p.u.)
Inverter Ud Control
No change in DC Voltage,but change in DC Current
Operating Point
(e.g. αRect =15°, αInv =140°)Inv Fixed αmax (~160°)
Operating Range for
AC Bus Voltage
reduction at inverterOperating Range for
AC Bus Voltage
increase at inverter
Rectifier Fixed αmin (5°)
Constant Operating Point for a wide range of inverter and rectifier side AC
Voltage variations
DC Current Margin Control at inverter
AC Bus Voltage reduction
at rectifier
new Operating Point with reduced rectifier
AC Voltage and constant DC Voltage
control
DC Current reduction
(depending on AC Voltage)
Rectifier Id Control
Rectifier Fixed αmin (5°)
Principles of HVDC ControlsConverter Control Functions, Id Margin Control
I DC Ref
MAX
I DC Ref
I DC Control
-+I DC Act
AC SYSTEMA
Rectifier Inverter
Ud Rect
Id
AC SYSTEMB
Ud Inv
Triggerset
ααααo
PI Controller
Triggerset
PI Controller
ααααo
U DC Ref
+
U DC ActU DC Control -
-
I DC Ref
I DC ActI DC Control +
I marg
-+
Rectifier Fixed αmin (5°)
Principles of HVDC Controls
Converter Control Functions, Id Margin Control
1.0
Ud (p.u.)
1.0 Id (p.u.)
Operating Point
(e.g. αRect =15°, αInv =140°)
AC Bus Voltage reduction
at rectifier
new Operating Point with inverter
Id Margin Control
Inverter Ud Control
Rectifier Fixed αmin (5°)
DC Current margin
Rectifier DC Current Control
Inverter DC Current Control
Operating Point without
inverter Id Margin
Control
Inv Fixed αmax (~160°)
Principles of HVDC ControlsConverter Control Functions, Extinction Angle Control
Inverter Ud Control
1.0
Ud (p.u.)
1.0 Id (p.u.)
Rectifier Fixed αmin (5°)
Operating Point
(e.g. αRect =15°, αInv =140°)
Rectifier DC Current Control
Inverter DC Current Control
Inverter Gamma (γ) min
Control (17°)
Principles of HVDC ControlsConverter Control Functions, Extinction Angle Control
MAX
AC SYSTEMA
Rectifier Inverter
Ud Rect
Id
AC SYSTEMB
Ud Inv
Triggerset
ααααo
PI Controller
Triggerset
PI Controller
ααααo
U DC Ref
+
U DC ActU DC Control -
I DC Ref
I DC ActI DC Control +
I marg
-+
-
Gamma Ref
-
Gamma ActGamma Control+
I DC Ref
I DC Ref
I DC Control
-+I DC Act
Extinction Angle Control (Gamma Control)
• The extinction angle control at inverter is provided in order to maintain stability margins.
• The extinction angle reference value is the minimum allowed value (e.g.17°)
• The extinction angle control becomes active when the minimum limit is reached.
Principles of HVDC ControlsConverter Control Functions, DC Voltage Limit Control
Inverter Ud Control
1.0
Ud (p.u.)
1.0 Id (p.u.)
Rectifier Fixed αmin (5°)
Operating Point
(e.g. αRect =15°, αInv =140°)
Rectifier DC Current Control
Inverter DC Current Control
Inverter Gamma (γ) min
Control (17°)
Rectifier DC Voltage
Limit Control
DC Voltage Margin
Principles of HVDC ControlsConverter Control Functions, DC Voltage Limit Control
MAX
AC SYSTEMA
Rectifier Inverter
Ud Rect
Id
AC SYSTEMB
Ud Inv
Triggerset
ααααo
PI Controller
Triggerset
PI Controller
ααααo
U DC Ref
+
U DC ActU DC Control -
Gamma Ref
-
Gamma ActGamma Control+
I DC Ref
I DC ActI DC Control +
I marg
-+
MIN
I DC Ref
I DC Control
-+I DC Act
I DC Ref
U DC Ref
-U DC Act U DC Control+
U marg
+
DC Voltage Limitation Controller at Rectifier
• A DC voltage limitation control at the rectifier is provided in order to limit the DC voltage
• A margin of typical 0.03. p.u. is add to the inverter DC voltage order.
• This prevents the DC voltage limitation controller to become active during normal operation
• Backup control which becomes active to prevent from excessive overvoltage in special fault situations
• It reduces the DC voltage to save valves by shifting the firing angle in inverter direction of in order to maintain the requested DC current
0.1
0.2
0.3
0.1 0.2 0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.4 0.5 0.6 0.7 0.8 0.9 1.0
Ud
(pu)
ld (pu)
DC Line Drop
Inverter VDCOL
Rectifier VDCOL
Inverter Ud Cont.
CEC
Rectifier Ud Cont.
Rectifier Id Cont.
d Contr.
Inverter Id Cont.
Abbreviations:
Ud DC Voltage
Id DC Current
VDCOL
CEC
Voltage Dependent Current Limit
Current Error Characteristic
Extinction Angle
Minimum DC Current
Operation Point
Principles of HVDC ControlsConverter Control Functions, Converter Control Characteristic
Principles of HVDC ControlsConverter Control Functions
AC SYSTEM
AAC SYSTEM
B
Rectifier Inverter
Ud Rect
Id
Ud Inv
Current Order
Calulator
P Ref
U DC Act
PI Controller
+-
+ -
+ -17°
I DC Act
γγγγ Act
I DC Control
γγγγ Control
I marg
I DC Ref
MAXMin
PI Controller
I DC Act
I DC Ref
I DC Control
- +
U DC Ref
U DC ActU DC Control
UDC Control
-+
UDC Ref
UDC Act
-+
ααααoααααo
Trigger
set
Trigger
set
+U marg
Principles of HVDC ControlsConverter Control Functions
� DC Power Control
• The steady state Power Order is normally determined by the Operator from the Operator Control System
• Current order is calculated by the Current Order Calculator • Current order calculated by dividing the DC power order by the measured DC
voltage.
� Summary Control Functions
� Control functions at the rectifier» DC Current Control (main control mode)» DC voltage limitation control (backup control mode)
� Control functions at the Inverter» DC Voltage Control (main control mode)» DC Current Control (backup control mode)» Extinction Angel Control (backup control mode)