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

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Comparison HVAC & HVDC

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Comparison HVAC & HVDC

Cost of HVDC is lessOne cable required instead of three

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Comparison HVAC & HVDC

Same poles can be use. Moreover, slim and smart poles are used for DC transmission

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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

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Control

30 60 90 120 150 180

0a

+Ud

-Ud

160

L im ita In v

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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

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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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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)

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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 =

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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

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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

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Thanks