HVDC Control System - Overview

Transcription

1 HVDC Control System - Overview

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

3 HVDC Control What are the basic principles of HVDC Controls? I d U 1 U 2 AC System A AC System B Simplified HVDC System diagram

4 HVDC Control What are the basic principles of HVDC Controls? I d U 1 U 2 AC System A AC System B

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

6 HVDC Control & Protection What are the basic principles of HVDC Controls? Tap Changer Control Sending End Receiving End Tap Changer Control U1 U 2 I d ACF capacitors ACF Converter Control Rectifier Id-Control Converter Control Inverter Ud-Control ACF capacitors ACF AC System A Converter Control Id: DC Current Ud: DC Voltage Reactive Power Control (AC Voltage Limitation Control) Reactive Power Control ACF: AC Filter Reactive Power Control (AC Voltage Limitation Control) AC System B Tap Changer Control

7 Control of DC Voltage Rectifier Operation Inverter Operation AC System Power Flow DC System AC System Power Flow DC System I d I d V 1 V 3 V 5 V 1 V 3 V 5 Phase A Phase A Phase B U d Phase B U d Phase C Phase C V 4 V 6 V 2 V 4 V 6 V 2 +Ud 0 -Ud 5 Rectifier Operation Inverter 180 Operation 160 α

8 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 Relationship of DC Voltage Ud and Firing Angle α +Ud Limit α Rect. Rectifier Operation 0 -Ud Inverter Operation 160 α Ud ο α = 0 ο α = 30 ο α = 60 α Inv Limit Ud ωt Ud ο α = 90 ο α = 120 ο α = 150 ωt -Ud

10 Converter Control Functions, Fixed Firing Angles α Fixed α o Fixed α o Trigger set Trigger set Id AC SYSTEM A Ud Rect Ud Inv AC SYSTEM B Rectifier Inverter

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 Converter Characteristics, Fixed Firing Angles α Ud (p.u.) Ud=Udio*cos(α) = k*u AC*cos(α) Station A (Rectifier) Station Fixed A (Inverter) α o (~15 ) Fixed α o (~140 ) Ud=Udio*cos(α) dx*id U=dx*Id Station B (Inverter) Fixed α o (~140 ) 1.0 Operating Point 1.0 Id (p.u.) Station B (Rectifier) Fixed α o (~15 ) -1.0

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 Converter Characteristics, Fixed Firing Angles a Rectifier Fixed Fixed α o (~15 ) α o (~15 ) Inverter Fixed α o (~140 ) Ud (p.u.) 1.0 AC Bus Voltage reduction at rectifier Operating Point Operating Point with reduced rectifier AC Voltage DC Voltage reduction DC Current Control at Rectifier DC Current reduction (depending on AC Voltage) 1.0 Id (p.u.)

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

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 (Ud Rect ) in order to maintain the DC current. In case the actual current is too high the PI controller reacts in opposite direction.

17 Converter Control Functions, Id control at Rectifer Rectifier Fixed α min (5 ) Inverter Fixed α (~140 ) Ud (p.u.) 1.0 Rectifier Id Control AC Bus Voltage reduction at inverter Operating range with increased AC Bus Operating Point Operating Voltage range rectifier No Change in with Operating (e.g. alpha Point 15 ) but reduced AC Bus increased alpha Voltage at rectifier No (e.g. Change 30 ) in Operating Point 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 1.0 Id (p.u.)

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 Converter Control Functions, DC Voltage Control I DC Act - + I DC Ref I DC Control PI Controller α o α o PI Controller U DC Control - U DC Ref + U DC Act Trigger set Trigger set Id AC SYSTEM A Ud Rect Ud Inv AC SYSTEM B Rectifier Inverter

20 Converter Control Functions, DC Voltage Control Rectifier Fixed α min (5 ) Ud (p.u.) 1.0 Rectifier Fixed α min (5 ) Inverter Ud Control Operating Range for AC Bus Voltage reduction Operating Range inverterfor AC Bus Voltage increase at inverter AC Bus Voltage reduction at rectifier Inv Fixed α max (~160 ) Operating Point (e.g. α Rect =15, α Inv =140 ) Rectifier Id Control Constant Operating Point for a wide new range Operating of inverter Point and with rectifier reduced side rectifier AC AC Voltage Voltage and constant variations DC Voltage control DC Current reduction (depending on AC Voltage) No change in DC Voltage, but change in DC Current DC Current Margin Control at inverter 1.0 Id (p.u.)

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

22 Converter Control Functions, Id Margin Control Rectifier Fixed α min (5 ) Ud (p.u.) Rectifier Fixed α min (5 ) Inverter Ud Control 1.0 Inv Fixed α max (~160 ) Operating Point without inverter Id Margin Control new Operating Point with inverter Id Margin Control Operating Point (e.g. α Rect =15, α Inv =140 ) Rectifier DC Current Control Inverter DC Current Control DC Current margin AC Bus Voltage reduction at rectifier 1.0 Id (p.u.)

23 Converter Control Functions, Extinction Angle Control Rectifier Fixed α min (5 ) Ud (p.u.) Inverter Ud Control 1.0 Inverter Gamma (γ) min Control (17 ) Operating Point (e.g. α Rect =15, α Inv =140 ) Rectifier DC Current Control Inverter DC Current Control 1.0 Id (p.u.)

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

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 Converter Control Functions, DC Voltage Limit Control Rectifier Fixed α min (5 ) Ud (p.u.) Inverter Ud Control 1.0 Inverter Gamma (γ) min Control (17 ) Rectifier DC Voltage Limit Control DC Voltage Margin Operating Point (e.g. α Rect =15, α Inv =140 ) Rectifier DC Current Control Inverter DC Current Control 1.0 Id (p.u.)

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

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 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 Converter Control Functions, Converter Control Characteristic Ud (pu) Rectifier U d Cont. Inverter U d Cont. 1.0 d Contr. 0.9 DC Line Drop Operation Point 0.8 Inverter I d Cont. CEC 0.7 Minimum DC Current 0.6 Rectifier I d Cont. 0.5 Inverter VDCOL Rectifier VDCOL Abbreviations: VDCOLVoltage Dependent Current Limit CEC Current Error Characteristic Ud DC Voltage Id DC Current Extinction Angle ld (pu)

30 Converter Control Functions P Ref U DC Act Current Order Calulator I DC Ref + - I marg I DC Act DC Ref U DC Act U I DC Ref I DC Control UDC Control + U marg Min PI Controller Trigger set α o Trigger set α o PI Controller MAX I DC Control U DC Control γ Control I DC Act U DC Ref - + U DC Act γ Act Id AC SYSTEM A Ud Rect Ud Inv AC SYSTEM B Rectifier Inverter

31 DC Power Control Principles of HVDC Controls Converter Control Functions 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)

32