HVDC

TRANSMISSION

H-HIGH

V-VOLTAGE

D-DIRECT

C-CURRENT

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WHY HVDC ?

BULK POWER TRANSMISSION WITHLOWER LINE LOSSES AS NO REACTIVE POWER TRANSFER TAKES PLACE

CONNECT TWO ASYNCHRONOUS SYSTEM. PROVIDES SYSTEM STABILITY.

CONTROLABILITY OF POWER FLOW AT HIGH SPEED.

LESS RIGHT OF WAY FOR TRANSMISSION LINES.

NO CONTRIBUTION TO SHORT CIRCUIT LEVEL OF THE AC BUS CONNECTED.

NO CHARGING CURRENT TO DIMINISH THE USEFUL CAPACITY OF THE LINE.

DISTANCE IS NOT LIMITED BY STABILITY POINT OF VIEW.

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BREAKEVEN DISTANCE FOR HVDC

DC LINE COST

DC TERMINAL COST

AC LINE COST

AC TERMINAL COST

EXPENDITURE /

COST

DISTANCE

BREAKEVEN

POINT

BREAKEVEN POINT FOR HVDC TRANSMISSION IS

ABOVE 700KMS

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CONFIGURATION

NON CONTROLLABLE VALVE

CONTROLLABLE VALVE

NON CONTROLLABLE VALVE GROUP

CONTROLLABLE VALVE GROUP

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TYPES OF CONVERTER BRIDGES

6-PULSE CONVERTER

BRIDGE

12- PULSE CONVERTER

BRIDGE / TWO 6PULSE BRIDGES

CONNECTED IN SERIES

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BACK -TO-BACK This is application where two AC systems to be connected are physically in same substation and no transmission line or cable is required.This is used for interconnection between asynchronous Power System networks.

TRANSMISSION BETWEEN TWO SUBSTATION.

This is for bulk power transfer through dc line or cable from one Substation to another which are geographically far away from each other.

MULTITERMINAL SYSTEM

More than two HVDC Substation, geographically separated are connected to each other by same transmission line then it is multiterminal system.

PARALLEL MULTITERMINAL SYSTEM

If one or more converters are connected at same voltage level then system is parallel multiterminal.Here voltage remains same but the current capacity increases.

SERIES MULTITERMINAL SYSTEM

If one or more converters are connected in series then system is series multiterminal.Here voltage increases.

UNIT CONNECTION

This is scheme when generator is directly connected to the converter transformer of the rectifier so that generated power is directly fed into dc line

Different types HVDC Converter Bridge Arrangements

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MONOPOLAR GROUND CONNECTION

BIPOLAR CONNECTION

TYPES OF CONNECTIONS OF HVDC CONVERTER BRIDGE

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MONOPOLAR METALLIC RETURN

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RECTIFIER INVERTER

RECTIFIER INVERTER

POLE-1

POLE-2

MRTB

RECTIFIER INVERTER

RECTIFIER INVERTER

POLE-1

POLE-2

MRTB

GRTS

GRTS

c:\ V.B.Bhandarkar

9

1 3 5

4 6 2

1 3 5

4 6 2 15 3

2 6 4

2 6 4

5 3 1

PRINCIPLE OF WORKING

CHANDRAPUR PADHE HVDC

PROJECT

FEATURES:

A. BIPOLE PROJECT

B. TRANSMISSION VOLTAGE 500KV

C. POWER CARRYING CAPACITY 1500 MW OR 750MW /POLE.

D. OVERLOAD CAPACITY : 1650 MW FOR 2HRS AMBIENT ABOVE 33C . 2200 MW FOR 5 SECS AMBIENT BELOW 33C .

E. TYPE OF THRISTOR VALVE QUARAPULE VALVE.

F. TYPE OF CONDUCTOR -QUARAPULE CONDUCTOR/POLE.

G. NO.OF FILTERS/STATION-4NOS.

H. FILTER MVAR CAPACITY /STATION-800MVAR.

I. LINE LENGTH 753 KM.

J. INTERSTATION COMMUNICATION AND COMMUNICATION BETWEEN LD,KALWA THROUGH OPTICAL FIBRE

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Static Ud/Id Characterstics

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Io(rectifier)Io(inv erter)

Current Margin

DC Voltage

DC Current

c:\ V.B.Bhandarkar

12 pulse bridge with three phase transformer connected

12

DCCT

DCCT

To control

system

To

control

system

DC voltage

divider

DC voltage

divider

Smoothing

reactor

c:\ V.B.Bhandarkar

PRINCIPLE OF POWER TRANSFER BETWEEN TWO CONVERTER STATIONS

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I=(V1-V2)/R

V1V2

RECTIFIER INVERTER

TRANSMISSION LINE

RESISTANCE (R)

CURRENT

(I)

P=V2*I

10V9V

RECTIFIER INVERTER

TRANSMISSION LINE

RESISTANCE

(R=1OHMS)

CURRENT

(I)

I=(10-9)/1

=1AMP

P=9*1=9W

129V

RECTIFIER INVERTER

TRANSMISSION LINE

RESISTANCE

(R=1OHMS)

CURRENT

(I)

I=(12-9)/1

=3AMP

P=9*3=27W

c:\ V.B.Bhandarkar

AC SIDE VALVE HALL DC SIDE

A) AC FILTERS A)THYRISTOR VALVES A)DC FILTERS

B) CONVERTER BREAKERS

B)DC BUS ARRESTORS B)DC BREAKER (METALLIC RETURN TRANSFER BREAKER, GROUND RETURN TRANSFER SWITCH,NEUTRAL BUS SWITCH)

C) MEASURING EQUIPMENTS

C)VALVE COOLING SYSTEM

C) SMOOTHING REACTOR

D) CONVERTER TRANSFORMERS

D)DC CTs, DC VOLTAGE DIVIDERS.

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CONVERTER

TRANSFORMER

AC SIDE

DC SIDE

VALVE

HALL

CONVERTER

BREAKERAC FILTER

DC BUS

ARRESTOR

DC

FIL

TE

R

DC VOLTAGE

DIVIDER

DC CT

DC LINE

ISOLATOR

NBS

c:\ V.B.Bhandarkar

AC SIDE EQUIPMENTS AC Side mainly Constitutes of

1)Ac filters: They serve two purpose

A) They offer low impedance path to the harmonics which travel on AC side generated during the conversion process.

B)Filters constitutes of R,L&C .These filter caters part of capacitive MVAR to the AC Bus which helps in boosting the system voltage.

Harmonics Generated are:

i)Harmonics of the order of h=2 to 5 due to voltage imbalance caused by firing angle and transformer commutation inductance.

ii)h=Kn1 order Harmonics are generated on AC side of Converter where K is a value dependent on 6 or 12 pulse Bridge and n=1,2,

Switching of the Ac filters through Circuits Breakers are done by

Reactive Voltage Controller depending on the DC power flow through

the link.

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U

(a) (b) (c)

Single Tuned Filter Double Tuned Filter

HP-12 HP-3 HP-24/36

AC SIDE EQUIPMENTS 2) Converter breakers: Breakers generally SF6 are used for catering AC

supply to the Converter Transformer. In Bipolar Projects sometimes it is required that only one Converter Breaker caters load to both poles depending on the type of Bus bar arrangements .In such cases Converter Breakers of high current carrying capacity are employed s. Also Circuit Breakers are employed for Switching in and out of the AC Filters.

3) Measuring Equipments: Equipments like CVTs are installed for measurement of bus voltages which useful in for control of power on HVDC link. CTs installed in AC yard as well as Filter area are employed for protection of AC Bus and Filters respectively.

4) Converter Transformers: This is one of the key equipments in HVDC link.AC supply is fed to the Converter through the transformer .As the name suggests the transformer is connected to the converter. The secondary or the valve winding of the Transformer is directly connected to valve through either wall mounted bushing or the bushings of the transformer are protruding inside the valve hall and are connected to the valve. The secondary of the converter transformer is carrying AC as well as DC valve current through it. The transformers may be of following type in case of 12- pulse converters:-

a) 3phase

i) 3phase 3 winding transformer 1 unit/pole.

ii) 3phase 2 winding transformer 2 units/pole.

b) 1 phase

i) 1phase 3 winding transformer 3units/pole.

ii)1phase 2winding transformer 6units/pole.

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CONVERTER TRANSFORMER

17

.

3A

3B

A

B1 B2

2A

2B

1PHASE THREE WINDING CONVERTER TRANSFORMER

PRIMARY

WINDING

SECONDARY

WINDING 1

SECONDARY

WINDING 2

.

.

.

....

c:\ V.B.Bhandarkar

CONVERTER TRANSFORMER

12 -PULSE CONVERTER FED BY THREE PHASE TRANSFORMER

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VALVE HALL EQUIPMENTS 1) Thyristor Valves: Thyristor valves are heart of HVDC .These valves

are air insulated , water cooled and suspended in controlled enviroment. Previously the thyristor valves where erected on ground but nowadays they are suspended to meet the seismic requirements. The 12-pulse bridge consist of 3 nos. quadruple valve. Each quadruple valve is connected to one phase. One valve consists of no. of thyristors connected in series depending on the rating of the thyristor. Each thyristor is equipped with damping circuit for dv/dt protection, series connected reactor for di/dt protection for group of thyristor and thyristor control unit for the firing as well as for status monitoring of thyristor .

Configuration for Chandrapur Padghe project:

3 nos. Quadruple valve

1 valve consist of 16 modules

1 module consist of 6 thyristors

1 valve consist of 96 thyristors

Each phase quadruple valve consist of 384 thyristor

One pole converter consist of 1152 thyristor

Series connected thyristor

Fine Water cooled valves.

Each thyristor of blocking voltage 7kv ,1700amp DC.

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VALVE HALL EQUIPMENTS

2) Fine Water Cooling system For Converter Valves:

Dissipation of losses in the form of heat from Thyristor Valves.

Fine water / Demineralised water used for heat dissipation. Few percent of water routed through water treatment circuit.

Temperature, Conductivity and leakage is monitored.

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DC SIDE EQUIPMENTS DC Side mainly constitutes: 1)DC filters:-The harmonics generated on DC side are of the order of h=kn where k=6 or 12 depending on the type of converter 6 or 12 pulse. These harmonics result in current harmonic in DC line. As no. of harmonics increases the amplitude of the harmonics decreases. 2)DC Switches/Breakers :- DC side Switches are basically High speed isolator which connect the neutral bus to earth electrode line or metallic return path or station ground .They divert/isolate the dc current in event of any fault on DC side. They do not perform the function of current breaking. There are 4 types of DC Switches: A) Neutral Bus Switch (NBS): This DC Switch performs the function of connecting the the converter to the neutral bus. In event of any fault on the neutral bus the NBS isolates the neutral bus and the converter after the operation of respective protection. B) Neutral Bus Grounding Switch (NBGS): This Switch provides path for local / station grounding in event of breakdown on electrode line or any maintenance on electrode line or on Metallic return transfer breaker during bipolar balanced condition . C)Metallic return transfer Breaker (MRTB):-This Switch is used to commutate the current from ground return path to metallic return path. In this type of transfer .The current that is commutated from Ground path to metallic path is near about 70 t0 80 percent of pole current due to difference in the resistance offered by these two paths. Hence Special Arrangement is adopted for current quenching from Ground to metallic path which shown in figure on page 21. D) Ground return transfer Switch (GRTS) :The GRTS is used to commutate current from metallic return to ground return The maximum DC current that is to be commutated in this case is only 20 to 30 percent.

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DC SIDE EQUIPMENTS

3) DC Smoothing Reactor: Smoothing Reactor is the first equipment connected in series on DC side i.e. HV side .As the name suggest it is for Smooth ripple free DC and also limits the sudden change in DC current during fault on DC line or DC bus. The reactor are generally air insulated or oil filled smoothing reactor. At low DC voltage air insulated smoothing reactor are used but for high voltage oil insulated smoothing reactor is used. Specification of Smoothing reactors in Chapad Project: Inductance (L): 365mH Rated Voltage : 500KV DC Rated current 1700 amp DC

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Arrangement for MRTB

BREAKER

CONTACTS

SPARK GAP

CAPACITOR

NON-LINEAR

RESISTORc:\ V.B.Bhandarkar

DC SIDE EQUIPMENTS 4) DC MEASURING DEVICES: Unlike AC side , measurement are also to be carried out on DC side of voltage and current. Following are used to carry out the measurement : A)DC Voltage Divider:-This is used to carry out the measurement of DC voltage on Line side and neutral side. As shown in figure voltage Across H-L is transmitted upto control room via screened cable where another small Voltage divider is installed .The final voltage received from line side Divider is for 500kv is 5v DC and for neutral side the ratio is for 50kv it is 2v DC. Accordingly from the measured dc voltage the actual voltage on line is fed to control system .

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.

.

.

HV

R1

R2F

H

L

E

c:\ V.B.Bhandarkar

DC SIDE EQUIPMENTS B) DC CT(Zero Flux CT): On Ac side the Current measured is carried out by AC Current Transformer, but the measurement of DC current is not possible with normal CT.Hence DC CT is used for the measurement of current on DC side . DC CT is a DC Current Transducer or it is also called Zero Flux CT. This Transducer just like normal CT only difference is that an electronic circuit is connected in the secondary circuit which detects the static field / flux developed by the DC current flowing in the primary. This circuit gives input to the Power Amplifier which injects current in the secondary .This secondary current nullifies the field / flux produced by the primary and the output is in the form of voltage drop measured across a resistor in secondary ckt. Measurement Ratio of DC CT is 1500A / 2V.

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Magneticfeild

detection

circuit

PowerAmplifier

OutputAmplifier

I1

I2

K U2=k*I2

C:\ V.B.Bhandarkar

Quadruple Converter Valves

Quadruple

valve

Double valve

Single valve

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ELECTRODE

LINE

ELECTRODE STATION

EARTH ELECTRODE STATION

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Electrode Station:-

In HVDC system the terminal which is formed to be a neutral is not grounded locally. The reason behind this is that the DC corrodes and destroys the metal structures , earthmats,etc. very fast. Hence the earthing in done in earth electrode station geographically away from the converter station in a remote place in the periphery of 20 -30 kms .The current flowing through earth electrode line reaches to the electrode station from where it takes the path to the earth through various electrodes which are buried inside the earth about 3 to 3 mtrs deep.

ELECTRODE BURIED INSIDE EARTH

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THANK YOU