3.hvdc controls

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


U1 U2

Id


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)


HVDC Control


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


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


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


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


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

+


-

I DC Ref

I DC ActI DC Control +

I marg

-+


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°)


at rectifier

new Operating Point with inverter

Id Margin Control

Inverter Ud Control


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.)


Operating Point

(e.g. αRect =15°, αInv =140°)



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

+


I DC Ref


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.)


Operating Point

(e.g. αRect =15°, αInv =140°)



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

+


Gamma Ref

-

Gamma ActGamma Control+

I DC Ref


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)

3.hvdc controls

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