The document discusses the basic control principles for HVDC systems. It describes how the magnitude and direction of DC power flow is controlled by regulating the DC current and voltage. The key control functions at the rectifier include DC current control and DC voltage limitation control. At the inverter, the main control functions are DC voltage control, with DC current control and extinction angle control as backup modes. Diagrams are provided to illustrate how different controllers interact to maintain stable operation over a range of conditions.

1. HVDC Control System -
Overview

2. What are the basic control principles
for HVDC Systems?
HVDC Control & Protection

3. AC System A AC System B
U1 U2
Id
Simplified HVDCSystem diagram
What are the basic principles of HVDC Controls?
HVDC Control

4. AC System A AC System B
U1 U2
Id
What are the basic principles of HVDC Controls?
HVDC Control

5. = =
U U
1 2
Id in one direction only
Id
Change of Power Direction
Power Direction
U1 U2
Magnitude of Id or power is
controlled depending on
the difference in the
terminal voltages (U1, U2)
U1 U2
Direction of power is
controlled depending on the
polarity of the terminal
voltages (U1, U2)
What are the basic principles of HVDC Controls?
HVDC Control

6. AC System A AC System B
U2
U1
I
d
Rectifier
Control
Id-Control
Converter
Id: DC Current
Converter Control
Inverter
Control
Ud-Control
Converter
Ud: DC Voltage
Reactive Power Control
Reactive
Power Control
(AC Voltage
Limitation
Control)
capacitors capacitors
Reactive
Power Control
(AC Voltage
Limitation
Control)
Sending End Receiving End
Tap Changer Control
Tap Changer
Control
Tap Changer
Control
ACF
ACF
ACF
ACF
ACF: AC Filter
What are the basic principles of HVDC Controls?
HVDC Control & Protection

7. Principles of HVDC Controls
Control of DC Voltage
V 1 V 3 V 5
V 2
V 6
V 4
Phase A
Ud
Phase B
Phase C
Id
Power Flow
AC System DC System
V 1 V 3 V 5
V 2
V 6
V 4
Phase A
Ud
Phase B
Phase C
Id
AC System DC System
Power Flow
30 60 90 120 150 180
0
+Ud
-Ud
160
5
Rectifier
Operation
Inverter
Operation
α
Rectifier Operation Inverter Operation

8. Principles of HVDC Controls
Control 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 by
reversing 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.

9. Principles of HVDC Controls
Relationship of DC Voltage Ud and Firing Angle α
30 60 90 120 150 180
0
α
+Ud
-Ud
160
Limit
α Inv
5
Limit
α Rect.
Rectifier
Operation
Inverter
Operation
t
ω
ο
60
=
α
Ud
ο
30
=
α
ο
0
=
α
ο
90
=
α ο
120
=
α ο
150
=
α
-Ud
t
ω
Ud
Ud

10. Principles of HVDC Controls
Converter Control Functions, Fixed Firing Angles α
AC SYSTEM
A
AC SYSTEM
B
Inverter
Ud Rect
Id
Ud Inv
Fixed α
α
α
αo
Fixed α
α
α
αo
Trigger
set
Rectifier
Trigger
set

11. 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

12. 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

13. 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

14. Principles of HVDC Controls
Converter Characteristics, Fixed Firing Angles a
1.0
Ud (p.u.)
1.0
Id (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

15. Principles of HVDC Controls
Converter Control Functions, Id control at Rectifer
α
α
α
αo
Trigger
set
Fixed α
α
α
αo
AC SYSTEM
A
Rectifier Inverter
Ud Rect
Id
AC SYSTEM
B
Ud Inv
Trigger
set
PI Controller
I DC Ref
I DC Control
-
+
I DC Act

16. 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.

17. Principles of HVDC Controls
Converter 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

18. • 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

19. Principles of HVDC Controls
Converter Control Functions, DC Voltage Control
AC SYSTEM
A
Rectifier Inverter
Ud Rect
Id
AC SYSTEM
B
Ud Inv
Trigger
set
α
α
α
αo
PI Controller
Trigger
set
PI Controller
α
α
α
αo
I DC Ref
I DC Control
-
+
I DC Act
U DC Ref
+
U DC Act
U DC Control -

20. Principles of HVDC Controls
Converter 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 inverter
Operating 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°)

21. Principles of HVDC Controls
Converter Control Functions, Id Margin Control
I DC Ref
M
A
X
I DC Ref
I DC Control
-
+
I DC Act
AC SYSTEM
A
Rectifier Inverter
Ud Rect
Id
AC SYSTEM
B
Ud Inv
Trigger
set
α
α
α
αo
PI Controller
Trigger
set
PI Controller
α
α
α
αo
U DC Ref
+
U DC Act
U DC Control -
-
I DC Ref
I DC Act
I DC Control +
I marg
-
+

22. 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°)

23. Principles of HVDC Controls
Converter 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°)

24. Principles of HVDC Controls
Converter Control Functions, Extinction Angle Control
M
A
X
AC SYSTEM
A
Rectifier Inverter
Ud Rect
Id
AC SYSTEM
B
Ud Inv
Trigger
set
α
α
α
αo
PI Controller
Trigger
set
PI Controller
α
α
α
αo
U DC Ref
+
U DC Act
U DC Control -
I DC Ref
I DC Act
I DC Control +
I marg
-
+
-
Gamma Ref
-
Gamma Act
Gamma Control+
I DC Ref
I DC Ref
I DC Control
-
+
I DC Act

25. 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.

26. Principles of HVDC Controls
Converter 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

27. Principles of HVDC Controls
Converter Control Functions, DC Voltage Limit Control
M
A
X
AC SYSTEM
A
Rectifier Inverter
Ud Rect
Id
AC SYSTEM
B
Ud Inv
Trigger
set
α
α
α
αo
PI Controller
Trigger
set
PI Controller
α
α
α
αo
U DC Ref
+
U DC Act
U DC Control -
Gamma Ref
-
Gamma Act
Gamma Control+
I DC Ref
I DC Act
I DC Control +
I marg
-
+
M
I
N
I DC Ref
I DC Control
-
+
I DC Act
I DC Ref
U DC Ref
-
U DC Act U DC Control
+
U marg
+

28. 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

29. 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 Controls
Converter Control Functions, Converter Control Characteristic

30. Principles of HVDC Controls
Converter Control Functions
AC SYSTEM
A
AC SYSTEM
B
Rectifier Inverter
Ud Rect
Id
Ud Inv
Current Order
Calulator
P Ref
U DC Act
PI Controller
+
-
+ -
+ -
17°
I DC Act
γ
γ
γ
γ Act
I DC Control
γ
γ
γ
γ Control
I marg
I DC Ref
MAX
Min
PI Controller
I DC Act
I DC Ref
I DC Control
- +
U DC Ref
U DC Act
U DC Control
UDC Control
-
+
U
DC Ref
U
DC Act
-
+
α
α
α
αo α
α
α
αo
Trigger
set
Trigger
set
+
U marg

31. Principles of HVDC Controls
Converter 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)