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Power Systems ­ HVDC/ Dipti Khare Cross Border Electricity Transmission with High Voltage Direct Current (HVDC) Executive Exchange Dhaka, Bangaldesh

Control System for HVDC Classic

© ABB Group September 18, 2011 | Slide 1

CONTENTS

§ AC/DC §Basic

Conversion Principle functions modes

functions

§Additional §Operating

© ABB Group September 18, 2011 | Slide 2

AC/DC Conversion Principle

© ABB Group September 18, 2011 | Slide 3

The 6-pulse Bridge with Uncontrolled Valves

§§d I §~ §~ §~ §V4 §V6 §X=0 §V1 §R §S §T §V2 §Ud § §V3 §V5 §~ §~ §~ §V4 §V6 §X=0 §V1 §R §S §T §V2 §U §d §V3 §V5 §Id §

§Phase

§R

voltages

§S §T

§Phase

§R

voltages

§S §T

§wt

§wt

Passive rectifier operation

§+

§To

load

§0

§-

§From

§6-pulse

Graetz rectifier bridge

load

Passive rectifier operation

§+

§To

load

§0

§§6-pulse

§From

Graetz rectifier bridge

load

Passive rectifier operation

§To

load

§0

§+

§§6-pulse

§From

Graetz rectifier bridge

load

Passive rectifier operation

§-

§To

load

§0

§+

§From

§6-pulse

Graetz rectifier bridge

load

Passive rectifier operation

§-

§To

load

§0

§+

§6-pulse

§From

Graetz rectifier bridge

load

Passive rectifier operation

§To

load

§0

§-

§+

§6-pulse

§From

Graetz rectifier bridge

load

Passive rectifier operation

§+

§To

load

§0

§-

§From

§6-pulse

Graetz rectifier bridge

load

Passive rectifier operation

§+

§To

load

§0

§§6-pulse

§From

Graetz rectifier bridge

load

Wave Shapes of a Three Phase Diode Rectifier

Commutation of the Current in 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. i d1 is reduced and id3 increased.

Current Pulses with Overlap

Commutation in a Controlled Bridge

Id

uR uS uT

IR IS IT

1

3

5

Ud

4

6

2

uT

u

uR uS

§Average

of

Basic functions

© ABB Group September 18, 2011 | Slide 17

HVDC Control System Core functions of the control system

The Control system principal tasks: q To determine the exact instant if and when to generate a 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 between the two stations

© ABB Group September 18, 2011 | Slide 18

HVDC Control System Relationships between the converter stations

Ud Normal operating point

Current control (conv 2)

Current control (conv 1)

I 0rd conv2

I 0rd conv 1

Id

§

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

§

§

© ABB Group September 18, 2011 | Slide 19

HVDC Control System Relationship between the converter stations

Ud Normal operating point

Current control (conv 2)

Current control (conv 1)

I 0rd conv2

I 0rd conv 1

Id

Direct voltage in Rectifier mode

Ud = UdI0 · cos - ? U d

Direct voltage in Inverter mode

Ud = -( UdI0 · cos ? - ? Ud )

© ABB Group September 18, 2011 | Slide 20

HVDC Control System Core functions of the control system

HVDC control system

+ Uac

To other station

Ud -

P

order

I Power control

order

Direct current- & Firing Control system

I

d

P

mod

I

response

U

d response

Voltage measuring system

© ABB Group September 18, 2011 | Slide 21

HVDC Control System CFC - Converter Firing Control

IO

IOLIM

ORDER

VDCOL

CCA

Firing Control

CP (calc)

CPG

CP

6/12

UD

ID

UAC

BLOCK / DEBLOCK

q q q q

Provide a fast current control system with a very low steady state error Avoid power instability during and after disturbances in the AC network Minimize the risk of communication failure during AC-network disturbances Perform a fast and controlled restart after clearence of AC and DC faults

© ABB Group September 18, 2011 | Slide 22

HVDC Control System VDCOL - Voltage dependent current order limit

§IO §IOLIM §ORDER

§VDCOL

§CCA

§Firing §Control

§CP

(calc)

§CPG

§CP

§ 6/12

§UD

§ID

§UAC

§BLOCK

/ DEBLOCK

§ § § §

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

§IO

§IOLIM

§ORDER

§VDCOL

§CCA

§Firing §Control

§CP

(calc)

§CPG

§CP

§ 6/12

§UD

§ID

§UAC

§BLOCK

/ DEBLOCK

q q q q q

Fast enough step response Zero current error at steady state Stable current control Fast reduction of over-current at faults Permits two current controllers (in rectifier vs. inverter) to operate together

HVDC Control System FC - Firing Control

§IO

§IOLIM

§ORDER

§VDCOL

§CCA

§Firing §Control

§CP

(calc)

§CPG

§CP

§ 6/12

§UD

§ID

§UAC

§BLOCK

/ DEBLOCK

§

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

§IO

§IOLIM

§ORDER

§VDCOL

§CCA

§Firing §Control

§CP

(calc)

§CPG

§CP

§ 6/12

§UD

§ID

§UAC

§BLOCK

/ DEBLOCK

§

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)

§

HVDC Control System Summary

HVDC control system

Uac

To other station

+ Ud -

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

P order

Power control

Iorder

Direct current & Firing Control

P mod Iresponse U response d

Voltage measuring system

Id

IO

VDCOL

UD

IOLIM

CCA

ORDER

Firing Control

CP (calc)

CPG

CP

6/12

ID

UAC

BLOCK / DEBLOCK

© ABB Group September 18, 2011 | Slide 27

Additional functions

© ABB Group September 18, 2011 | Slide 28

Control system description Additional tasks for the control system

§

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

§

Determining target value for the firing delay angle a in 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

© ABB Group September 18, 2011 | Slide 29

Control system description VARC - Voltage and Angle Reference Calculation

Rd

P=UdR(UdR-UdI) R

+

Step orders Alpha

+

UdI

Gamma Step orders

UdR

Id

Id

TCC

CFC

Ud

Ud

CFC

TCC

IoR

IoI

Gamma_ref/Udref VARC

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

Alpha_ref/Udref VARC

TCOM

Iord Udref

Ud

TCOM

© ABB Group September 18, 2011 | Slide 30

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

Control system description TCC - Transformer Tap Changer

q

In rectifier operation the TCC primarily maintains the ordered firing angle a by altering the value of Udi0 In inverter operation TCC primarily maintains the DC voltage In inverter operation TCC is also able to maintain the extinction angle ? The reference values are distributed by the VARC

q q

Alpha ref

Udref

Gamma ref

© ABB Group September 18, 2011 | Slide 31

Control system description TCC - Tap Changer Control

Udi0 Reference Udi0

DECREASE UDI0 AT LIMIT

UDI0 LIMIT

PERMIT INCREASE OF UDI0

No load control

AUTO

NO LOAD CONTROL

Udi0

>1

&

>1

STEP TAPCHANGERS

RECTIFIER ALPHA

ALPHA Reference CONTROL

MANUAL CONTROL ALPHA

Ud

Ud Reference

VOLTAGE CONTROL >1

RESYNCHRONISATION

GAMMA

© ABB Group September 18, 2011 | Slide 32

GAMMA

GAMMA Reference CONTROL

Control system description TCC - Tap Changer Control

Rectifier Operation Typical purpose:

To maintain the ordered firing angle a to the reference value set by the VARC

When AC voltage level differs, the CFC alters the firing delay angle a in order 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!

© ABB Group September 18, 2011 | Slide 33

Control system description TCC - Tap Changer Control

Inverter Operation Typical purpose:

To maintain Ud in the rectifier to reference value set by the VARC

When AC voltage level differs little, the CFC maintains ? and so Ud alters correspondingly If U d 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!

© ABB Group September 18, 2011 | Slide 34

Reactive Power Requirement

·HVDC

converters absorb reactive power, approximately 50% to 60% of their active power. filters are installed on the AC side for filtering the AC current and for generation of reactive power.

·Harmonic

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

Q 0,5

·The

Classic

0,13

filter

converter

1,0

Id

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

3 2 1: qexchng 2: qdc 3: qf 4: qac(limit) 1 4

q (=Q/PdN)

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)

HVDC Control Extinction 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

§

© ABB Group September 18, 2011 | Slide 38

HVDC Control Supervision 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

§

© ABB Group September 18, 2011 | Slide 39

Operating modes

© ABB Group September 18, 2011 | Slide 40

Operating modes PC ­ Power Control

P

order

I Power control

order

Porder = Iorder UD

P

mod

§ § § § §

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

© ABB Group September 18, 2011 | Slide 41

Operating modes Pole Synchronous Current Control

§ Converter

1

§ Converter

2

§U §ac1

§U §ac2

§U §d1

§U §d2

§ CCA §§I o1

§ 12 § CFC § CPG

§ 12

§ CCA §I §o2

§Id1 §

§I § d2

§ CPG

§ CFC

§ Iresp.

§ Iresp.

§ §

The current order in both stations are synchronized. (the inverter current order follows the rectifier current order) The current order is given in [A]

42(17)

Operating modes BSC - Backup Synchronous Control

§ Converter

1

§ Converter

2

§U §ac1

§U §ac2

§U §d1

§U §d2

§ CCA §§I o1

§ 12 § CFC § CPG

§ 12

§ CCA §§I o2

§Id1 §

§I § d2

§ CPG

§ CFC

§ Iresp.

§ Iresp.

§

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.

§

§

43(17)

Master Control Power Modulations

Frequency Control

§

P

order

I Power control

order

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

P

mod

§ §

§U

d

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

§

© ABB Group September 18, 2011 | Slide 44

Summary One of the Control system principal tasks

To optimize the dynamic and static cooperation between the two stations.

§ Converter

1

§ Converter

2

§U §ac1

§U §ac2

§U §d1

§U §d2

§ CCA §§I o1

§ 12 § CFC § CPG

§ 12

§ CCA §I §o2

§I d1 §

§Id2 §

§ CPG

§ CFC

§ Iresp.

§ Iresp.

45(17)

© ABB Group September 18, 2011 | Slide 46

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