HVDC Valvesppt ABB

7/30/2019 HVDC Valvesppt ABB

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HVDC Valves---ABB

Basic Requirement:

1.shall be able to carry current only in

forward direction.

2. Non conducting valve shall be able to

sustain a forward and reverse blocking

voltage. The valve is designed for a max

blocking voltage about three times the rated

direct voltage of the 6-pulse converter bridge.


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HVDC Valves

3. transition from the blocking condition to the conductingcondition takes place when the valve is given a controlpulse(firing pulse)It remains in the conducting state untilthe current through the valve is reduced to zero.

4. the valve is also designed for over current condition .themaximum over current occurs for a direct short circuitacross a valve an the amplitude is mainly determined bythe reactance of the converter transformer.

In addition it should have;

High reliability Low losses

Minimum installation and maintenance costs.


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HVDC Converter

HVDC converter normally built with two 6-pulseconverter bridges in cascade.

Transformers feeding 6-p bridges are star-star andstar-delta (YY and YD) respectively ,introducing a

phase shift of 30o between two bridges. a12-pulse converter with thyristor valves is normally

built with three physical units , quadruple valves,

Quadruple valve is air insulated and water cooled

Each single valve is protected with a valve arresterwhich is mounted on one side of the quadruplevalve.


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The Thyristor

cvvcv

Cathode

Anode

Gate

v

v

v

v


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Hvdc valves

Ref power semiconductor devices

and circuits edited by Andre A Jacklin


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The AC/Dc converter is subjected to very high voltagesand each valve in the converter bridge must be able towithstand several hundred kV. The line current isnormally in the range of 1000 to 4000 A.

The modern HVDC valves are based on thyristors.

The voltage capability of the single thyristors is limitedto less than 10 kV and a large number of thyristorsmust therefore be connected in series to support this

voltage.

The thyristor string is often organized in thyristormodules with current limiting inductors in between.


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Most thyristors are located at a high electric

potential, making certain demands on the valve

designs.

The necessary means for firing and protection of

thyristors must be energized by valve current at

each particular thyristor level.

Triggering signals between the firing andprotection means and controls system at ground

potential must be transmitted optically.


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The LTT should be self protected against voltage

triggering i.e.

1. the thyristor must not be damaged by spurious

triggering due to an excessively high off statevoltage (leakage current),

2. excessively fast dVo/dt ( displacement current)

or3. an unintended forward blocking voltage during

the reverse recovery period (stored charge)


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HVDC valve requirement

HVDC valve design is installation specific .themain aspects are ;

i) converter must be able to operate at steady

state under specified condition within ratherwide current and voltage ranges.

ii) uninterrupted operation must continue

during transitory AC fault and disturbances .iii)Lightning strikes and earth faults must notdamage the converter valves.


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Features of Full size self protected

LTTs

The main data for a 45 cm2 LTTs

Off state voltage 7000 V

Line current 1600 A

On state voltage 1.7 V at 1600 A

Minimum optical triggering power 10 mw at=940 nm for LED and 5mw at 840 nm for LD.

dVo/dt capability 4000 V/ s to 7000 V/ s

dI/dt , single pulse capability 3000 A/ s dI/dt , 50 Hz capability 800 A/ s

Max operating temp 80o


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Valve firing

current

voltage

Indicating

pulse

Control

pulse

Firing pulse


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Optical indicating pulse is generated at each

thyristor level as soon as the anode cathode

voltage across the thyristor has reached an

appropriate level for safe turn on.

When the converter control system generates

the control (CP) the valve control system will

send optical firing pulses to all thyristor levelsof the string.


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Triggering of ETT

In ETT(Electricallytriggeredthyristor) case

FP triggers theTCU (Thyristorcontrol unit) torelease gate

pulse from aprechargedcapacitor.

TCU

FP

IP

ETT

R R

C


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ETT Triggering

TCU

FP

IP

ETT

R R

C

LTT

FP triggers the LTT

(light Triggered Thyristor)

And supplies gate pulse

to ETT


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LTT Triggering

TCU

FP

IP

LTT

R R

C

A new 45 cm 2 7 kV self

protected LTT was

developed in 1987. For

short turnon delay a

high optical power ( > 30

mw, =940 nm) results in

td ,< 4 s.

A


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Optical firing systems

One LED per LTT carrying 3.5 A and generate

45 mw for 10 sOR

One LD (Laser diode) per two LTT carrying 1.5A supplying 70 mw for 100 s


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Optical firing systems

When using direct light triggering a new principle forgenerating firing pulses from the valve control electronics.

A high frequency indicating pulses are generated by TMU (Thyristor monitoring unit) as long as LTT voltage exceeds110 volts . The optical firing signal (FP) starts at thebeginning of the control pulse command and will last aslong as IP is generated.

The FP signal is then cut off as soon as all the thyristors inthe valve have turned on , thus reducing the stress on thelight sources. The firing pulse length will normally be about20 sec but operating at low delay angle or low ac voltagewill require longer pulses. The maximum pulse length islimited to 100 sec.


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Improved LTT firing system

Improved LTT firing system based oncommercially available 50 mw continuouslaser diode (originally intended for office

automation products) are used as light source.No light reserve redundancy is needed as thethyristor will operate by self triggering in caseof missing regular pulse. The light pulses are

cut off by TMU (thyristor Monitoring Unit)signals when thyristor have turned on in orderto save LD life time.


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Voltageacross

thyristor

110V

Safer firing voltage

IP

CP

FP

t

TMU

LTT

LD

FP

50mw

IP

Improved LTT Triggering System

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HVDC Valvesppt ABB