hvdc control

© ABB Group September 18, 2011 | Slide 1

Control System for HVDC Classic

Power Systems – HVDC/ Dipti Khare

Cross Border Electricity Transmission with High Voltage Direct Current (HVDC) Executive ExchangeDhaka, Bangaldesh


CONTENTS

§AC/DC Conversion Principle

§Basic functions

§Additional functions

§Operating modes


AC/DC Conversion Principle

The 6-pulse Bridge with Uncontrolled Valves

§R §S §T

§wt §wt

§T§S§R

§I§d

§R

§S

§T

§V1 §V3 §V5

§V4 §V6 §V2

§U§d

§X=0 §X=0

§d§U

§V2§V6§V4

§V5§V3§V1

§T

§S

§R§~

§d§I

§~

§~

§~

§~

§~

§Phase voltages §Phase voltages

§6-pulse Graetz rectifier bridge

§To load

§From load

§+

§-§0

Passive rectifier operation


§To load

§From load

§0

§+

§-



§To load

§From load

§0§+

§-



§To load

§From load

§0

§-

§+



§To load

§From load

§0§-

§+



§To load

§From load

§0

§+

§-


Wave Shapes of a Three Phase Diode Rectifier

§Suppose that diode #1 is on and #3 is fired and starts to conduct. Due to the transformer inductance the #1 diode continues to conduct until the stored magnetic energy in the

winding is gone. During the commutation, diode #1 and #3 share Id. id1 is reduced and id3

increased.

Commutation of the Current in a Three Phase Diode Rectifier

Current Pulses with Overlap

Commutation in a Controlled Bridge

u R

u S

u T

1 3 5

4 6 2

I d

U d

IR

IS

IT

uS

uR

uT

α u

§Average of the switched

phase


Basic functions


The Control system principal tasks:

q To determine the exact instant if and when to generatea firing pulse to the thyristor valve

q To control the ratio of the converter Transformer Tap Changer

q To determine the proper position of breakers,disconnectors and grounding switches

q To optimize the dynamic and static cooperation betweenthe two stations

HVDC Control SystemCore functions of the control system


U d

Id

I0rd conv 1I0rd conv2

Current control (conv 1)Current control (conv 2)

Normal operating point

§ The station with the highest current order (Iord) is operating as rectifier

§ The station with the highest available DC voltage (Ud) controls the direct current (Id)

§ The station with the lowest available DC voltage determines the DC voltage

HVDC Control System Relationships between the converter stations


U d

Id

I0rd conv 1I0rd conv2

Current control (conv 1)Current control (conv 2)

Normal operating point

HVDC Control SystemRelationship between the converter stations

Direct voltage in Rectifier mode

d cosU α•= UdI0 - ?Ud

Direct voltage in Inverter mode

d cosU ?•= -( UdI0 - ?U )d


HVDC control system

Powercontrol

Iorder Direct current- &

Firing Control systemId

Iresponse

Voltagemeasuring

system

Porder

Pmod

Ud response

To other station

UacUd

+

-

HVDC Control SystemCore functions of the control system


VDCOL CCAFiring

Control CPG

UAC

ORDER CP

BLOCK / DEBLOCK

IO

UD ID

CP (calc)6/12

IOLIM

q Provide a fast current control system with a very low steady state error

q Avoid power instability during and after disturbances in the AC network

q Minimize the risk of communication failure during AC-network disturbances

q Perform a fast and controlled restart after clearence of AC and DC faults

HVDC Control SystemCFC - Converter Firing Control

§VDCOL §CCA §Firing§Control §CPG

§UAC

§ORDER §CP

§BLOCK / DEBLOCK

§IO

§UD §ID

§CP (calc)§6/12

§IOLIM

HVDC Control System VDCOL - Voltage dependent current order limit

§ Avoids power instability during and after disturbances in the AC network

§ Defines a fast and controlled restart after clearence of AC and DC faults

§ Avoids stresses on the thyristors at continuous commutation failure

§ Suppresses the probability of consecutive commutation failures at recovery

§

HVDC Control System CCA - Current Control Amplifier

q Fast enough step response

q Zero current error at steady state

q Stable current control

q Fast reduction of over-current at faults

q Permits two current controllers (in rectifier vs. inverter) to operate together


§UAC

§ORDER§CP

§BLOCK / DEBLOCK

§IO

§UD §ID

§CP (calc)§6/12

§IOLIM


§UAC

§ORDER§CP

§BLOCK / DEBLOCK

§IO

§UD §ID

§CP (calc)§6/12

§IOLIM

HVDC Control System FC - Firing Control

§ In the Firing Control the time to fire a valve is calculated (CP calc) based on the a-order for each individual thyristor valve in a converter.

§ Assures that firing of a valve is accurately synchronized with the AC-voltage

§ Avoid firing a valve outside certain time limitations

HVDC Control System CPG - Control Pulse Generator

§ Turns the calculated times to fire a valve into individual Control Pulses (CP) which are distributed to the corresponding thyristor valves.

§ Can be ordered to Deblock or Block the Control Pulses and also select Bypass Pairs (Block with By-pass Pairs)


§UAC

§ORDER§CP

§BLOCK / DEBLOCK

§IO

§UD §ID

§CP (calc)§6/12

§IOLIM


Determine the exact instant if and when to generate a firing pulse to the thyristor valve

Powercontrol

Iorder Direct current &Firing Control

Id

Iresponse

Voltagemeasuring

system

Porder

Pmod

Ud response

To other station

HVDC control system

Uac Ud

-

+

VDCOL CCA FiringControl CPG

UAC

ORDER CP

BLOCK / DEBLOCK

IO

UD ID

CP (calc)6/12

IOLIM

HVDC Control System Summary


Additional functions


Control system descriptionAdditional tasks for the control system

§ Create and distribute reference values to the control system such as:

§ Determining target value for the firing delay angle ain rectifier operation

§ Determining target value for the extinction angle ? in inverter operation

§ Determining target value for the direct voltage level

This is achieved partly by using the ratio of the Converter Transformer Tap Changer


Udref

VARC

CFC TCC

VARC

TCC CFC

TCOM TCOM

Rd

Alpha_ref/Udref Gamma_ref/Udref

Alpha GammaStep orders Step orders

The angles are sent to the TCC and can, for example, be used to keep the reactive power balance Iord

Ud

Ud Ud

UdR

+

-UdI

+

-

P=UdR(UdR-UdI)R

The objective of the VARC function is to calculate reference valuesfor the extinction angle gamma, DC voltage and firing angle alpha.These reference values are then distributed, normally to the TCC

IdId

IoR IoI

Control system description VARC - Voltage and Angle Reference Calculation


q In rectifier operation the TCC primarily maintains the ordered firing angle a by altering the value of Udi0

q In inverter operation TCC primarily maintains the DC voltage

q In inverter operation TCC is also able to maintain the extinction angle ?

The reference values are distributed by the VARC

UdrefAlpha ref Gamma ref

Control system description TCC - Transformer Tap Changer


ALPHACONTROL

NO LOADCONTROL

VOLTAGECONTROL

UDI0LIMIT

GAMMACONTROL

MANUALCONTROL

ALPHAALPHA Reference

Ud

Ud Reference

GAMMA

GAMMA Reference

RECTIFIER

DECREASE UDI0 AT LIMIT

1>&

STEPTAPCHANGERS

Udi0 ReferencePERMIT INCREASE OF UDI0

1>

Udi0

AUTO

RESYNCHRONISATION

1>Udi0

No load control

Control system description TCC - Tap Changer Control


Rectifier Operation

Typical purpose: To maintain the ordered firing angle ato the reference value set by the VARC

When AC voltage level differs, the CFC alters the firing delay angle a inorder to keep Id = Io

If a becomes higher/lower than the reference value set by the VARC,TCC alters Udi0 in order to bring a back to the reference value

REMEMBER!Ud ˜ Udi0 ? cos a

Remains constant!



Inverter Operation

Typical purpose: To maintain Ud in the rectifier toreference value set by the VARC

When AC voltage level differs little, the CFC maintains ? and so Ud alterscorrespondingly

If Ud becomes higher/lower than the reference value set by the VARC,TCC alters Udi0 in order to bring Ud back to the reference value

REMEMBER!- Ud ˜ Udi0 ? cos ?

Remains constant!


Reactive Power Requirement

•HVDC converters absorb reactive power, approximately 50% to 60% of their active power.

•Harmonic filters are installed on the AC side for filtering the AC current and for generation of reactive power.

•The reactive power absorption of a converter increases with the transmitted active power. Also the need for filtering of harmonics is increased.

•The need for reactive power grows slowly at low power, and more pronounced at high power, whereas the filter needs behave in the opposite fashion.

•The reactive power compensation scheme has to take care of the unbalances for the AC system requirement, by switching of filters

0,13filter

converterunbalance

1,0

0,5

Id

Q

Classic

HVDC Control Reactive Power Control

•The reactive power balance of each side of the HVDC transmission will normally be performed by reactive power controller (RPC).

•Each RPC is located in the pole control level and operates independently from the RPC in the other end of the HVDC transmission.

•Switching of filter banks or sub-banks is ordered by the RPC or by protections.

•Switching priority restrictions are determined by limits in the reactive power compensation study for the different control modes.

HVDC Control Reactive power for typical AC filter switching sequence

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

0.00 0.20 0.40 0.60 0.80 1.00 1.20

p (pu)

q (

=Q/P

dN

)

1: qexchng

2: qdc

3: qf

4: qac(limit)

1

2

3

4


HVDC ControlExtinction angle ? control

§ Manually or externally triggered short time increase of ? (gamma)

§ Reduces the risk of commutation failure when distortion of the AC voltage is caused by switching of components like AC filters and capacitor banks

§ Affects reactive power


HVDC ControlSupervision and switch over logic

§ The supervision function supervises the control system itself and reports any faults that occur

§ The Switchover function manages the transition of the control system computers between Test, Off, Standby and active states. It ensures that the healthiest control system is active and that there is an active computer


Operating modes


Operating modesPC – Power Control

§ The orders between the stations are automatically coordinated

§ To keep the power constant variations in DC voltage are compensated by adjusting the DC current

§ The ramp rate is set [MW/min]

§ The power order is set [MW]

§ Orders can be given both localy and remotely

Powercontrol

Iorder

Porder

Pmod

Porder

UD= Iorder

42(17)

Operating modesPole Synchronous Current Control

§ The current order in both stations are synchronized. (the inverter current order follows the rectifier current order)

§ The current order is given in [A]

§CFC §CPG §CPG §CFC

§CCA §CCA

§Converter 1 §Converter 2

§U§ac1 §U§ac2

§12 §12

§I§d2

§Iresp. §Iresp.

§I§o2§I§o1

§U§d1 §U§d2

§I§d1

43(17)


§CCA §CCA


§U§ac1 §U§ac2

§12 §12

§I§d2

§Iresp. §Iresp.

§I§o2§I§o1

§U§d1 §U§d2

§I§d1

§ The purpose in BSC mode is to maintain the Current Control in the rectifier during telecom outages.

§ The rate of change limit is decreased compared with operating in Synchronous Control.

§ The inverter uses the measured DC current as its current order.

Operating modesBSC - Backup Synchronous Control


Master ControlPower Modulations

Frequency Control§ Keeps the steady-state frequency of the AC grids within its design

limits

§ Measures the frequency deviation in either AC network

§ Can only be active in one station at a time

Damping Control§ Damps the AC networks power oscillations (0.1-2 Hz) in order to obtain

network stability

Emergency Power Control§ Enables fast power change or even reversal of the transmission in order to

support either of the AC networks

§ Supplies the Power Control with a power order reference and a predefined ramp speed reference

Powercontrol

Iorder

Porder

Pmod

§Ud

45(17)

SummaryOne of the Control system principal tasksTo optimize the dynamic and static cooperation between the two stations.


§CCA §CCA


§U§ac1 §U§ac2

§12 §12

§I§d2

§Iresp. §Iresp.

§I§o2§I

§o1

§U§d1 §U§d2

§I§d1

hvdc control

Documents

passive rectifier operation

graetz rectifier bridge

reactive power balance

firing delay angle

phase diode rectifier

ordered firing angle

tcc alters udi0

extinction angle