Lecture by

P.RANGA RAO
Chief Manager / Kolar

BASIC PRINCIPLES
OF
HVDC TRANSMISSION

TOPICS
AC Generation and Transmission
Advantages of HVDC
Principles of AC /DC Transmission
Thyristor Evolution
Conversion principles
Analysis of 12 pulse bridge
Operation of HVDC
Types of HVDC
Salient features of Talcher-Kolar HVDC

REASONS FOR AC GENERATION AND TRANSMISSION

Due to ease of transformation of voltage levels (simple

transformer action) and rugged squirrel cage motors,
ALTERNATING CURRENT is universally utilised.
Both for GENERATION and LOADS and hence for
TRANSMISSION.
Generators are at remote places, away from the
populated areas i.e. the load centers
They are either PIT HEAD THERMAL or HYDEL
Turbines drive synchronous generators giving an
output at 15-25 kV.
Voltage is boosted up to 220 or 400 KV by step-up
transformers for transmission to LOADS.
To reach the loads at homes/industry at required safe
levels, transformers step down voltage.

COMPARISION OF HVAC & HVDC SYSTEMS

CONVENTIONALLY POWER TRANSMISSION IS EFFECTED
THROUGH HVAC SYSTEMS ALL OVER THE WORLD.

HVAC TRANSMISSION IS HAVING SEVER LIMITATIONS

LIKE LINE LENGTH , UNCONTROLLED POWER FLOW,
OVER/LOW VOLTAGES DURING LIGHTLY / OVER
LOADED CONDITIONS,STABILITY PROBLEMS,FAULT
ISOLATION ETC

CONSIDERING THE DISADVANTAGES OF HVAC SYSTEM

AND THE ADVANTAGES OF HVDC TRANSMISSION ,
POWERGRID HAS CHOOSEN HVDC TRANSMISSION FOR
TRANSFERRING 2000 MW FROM ER TO SR

ADVANTAGES OF HVDC OVER HVAC TRANSMISSION

CHEAPER THAN HVAC SYSTEM DUE TO LESS
TRANSMISSION LINES & LESS RIGHT OF WAY FOR
THE SAME AMOUNT OF POWER TRANSMISSION

COST: AC vs DC Transmission
Line Cost AC

Line Cost DC

Terminal Cost DC
Terminal Cost AC
Break Even Distance

2000 MW HVDC VIS- A- VIS HVAC SYSTEMS

HVDC BIPOLAR TRANSMISSION SYSTEM

2 DOUBLE CIRCUIT HVAC TRANSMISSION SYSTEMS

AC

DC

DC

ADVANTAGES OF HVDC OVER HVAC TRANSMISSION

CONTROLLED POWER FLOW IS POSSIBLE

VERY PRECISELY
ASYNCHRONOUS OPERATION POSSIBLE
BETWEEN REGIONS HAVING DIFFERENT
ELECTRICAL PARAMETERS
NO RESTRICTION ON LINE LENGTH AS NO
REACTANCE IN DC LINES

ADVANTAGES OF HVDC OVER HVAC TRANSMISSION

STABILISING HVAC SYSTEMS -DAMPENING OF POWER

SWINGS AND SUB SYNCHRONOUS FREQUENCIES OF
GENERATOR.

FAULTS IN ONE AC SYSTEMS WILL NOT EFFECT THE

OTHER AC SYSTEM.

CABLE TRANSMISSION
.

Types of HVDC
HVDC is the unique solution
to interconnect asynchronous
systems or grids with
different frequencies.
Solution: HVDC Back-to-Back
Back-to-Back Station

Up to 600 MW

AC

AC

50 Hz

60 Hz

Types of HVDC

HVDC represents the most

economical solution to
transmit electrical energy
over distances greater than
approx. 600 km
Solution: HVDC Long
Distance

Long Distance Transmission

Up to 3000 MW

AC

AC

DC line

Types
of
HVDC
HVDC is an alternative for
submarine transmission.
Economical even for shorter
distances such as a few
10km/miles
Solution: HVDC Cable
Long Submarine Transmission

Up to 600 MW

AC

AC

DC cable

HVDC BIPOLAR LINKS IN INDIA

NR
NR

NER
NER

ER
ER

RIHAND-DELHI

-- 2*750 MW

CHANDRAPUR-PADGE 2* 750 MW
TALCHER-KOLAR
2*1000 MW
ER TO SR
SILERU-BARASORE - 100 MW
EXPERIMENTAL PROJECT
ER SR

SR
SR

HVDC IN INDIA
Bipolar
HVDC LINK

CONNECTING CAPACITY
LINE
REGION
(MW)
LENGTH

Rihand
Dadri

North-North

1500

815

Chandrapur Padghe

West - West

1500

752

Talcher
Kolar

East South

2000

1367

ASYNCHRONOUS LINKS IN INDIA

NR
NR

NER
NER

ER
ER

VINDYACHAL (N-W) 2*250 MW

CHANDRAPUR (W-S) 2*500 MW
VIZAG (E-S)

- 2*500 MW

SASARAM (E-N)

- 1*500 MW

SR
SR

HVDC IN INDIA
Back-to-Back

HVDC LINK CONNECTING CAPACITY

REGION
(MW)
Vindyachal

North West

2 x 250

Chandrapur

West South

2 x 500

Vizag I

East South

500

Sasaram

East North

500

Vizag II

East South

500

AC Transmission Principle

HVDC Transmission Principle

USE OF DC
Direct current is put to use in common life for driving our
portable devices, UPSs, battery systems and vastly in
railway locomotives.

DC AS A MEANS OF TRANSMISSION
This has been possible with advent of

High power/ high current capability thyristors

&
Fast acting computerised controls

The Evolution of Thyristor Valves in HVDC

High Voltage Thyristor Valve History Highlights
1967

First Test Valve: 2 parallel 35 mm Thyristors @ 1650 V

1969

World's First Contract for an HVDC System with Thyristor Valves

2 parallel 35 mm thyristors @ 1650 V for 2000 A

1975

World's First Contract for Watercooled HVDC Thyristor Valves

2 parallel 52 mm thyristors @ 3500 V for 2000 A

1980

World's First Contract for HVDC System with 100 mm Thyristors

no parallel thyristors @ 4200 V for 3600 A

1994

First HVDC Contract Using 8kV Thyristors

100 mm thyristors @ 8000 V

1997

First Thyristor Valve with Direct-Light-Triggering

100 mm thyristors with breakover protection @ 8000 V for 2000 A

2001
First complete HVDC System using Direct-Light-Triggered
Thyristors with integrated breakover protection @ 8000 V

If DC is required to be used for transmission

&
since our primary source of power is A.C,
the following are the basic steps:
1. CONVERT AC into DC (rectifier)
2. TRANSMIT DC
3. CONVERT DC into AC ( inverter)

Purpose & function of Thyristor Valve

Connects AC phases to DC system

Conduct High Current currents upto 3000A without the requirement

of paralleling of thyristors

Block High Voltage Blocks high voltage in forward and reverse

direction up to 8KV

Controllable thyristor triggering /conduction possible with the gate

firing circuits

Fault tolerant and robust

SINGLE PHASE HALF WAVE RECTIFIER

SINGLE PHASE
FULL WAVE
RECTIFIER

SINGLE PHASE FULL WAVE BRIDGE RECTIFIER

6-Pulse Convertor Bridge

Ld

E1

Ls

iA

Ls

iB

Ls

iC

Vd

V'd

Id

2
Id

Voltage and Current of an Ideal

Diode 6 Pulse Converter
Alpha = 0
Overlap =
0

Operation of Converter

Each thyristor conducts for 120

Every 60 one Thyristor from +ve limb and one Thyristor from ve limb is triggered

Each thyristor will be triggered when voltage across it becomes positive

Thyristor commutates the current automatically when the voltage across it becomes ve. Hence, this
process is called natural commutation and the converters are called Line Commutated converters

Triggering can be delayed from this point and this is called firing angle

Output voltage of the converter is controlled by controlling the Rectifier action

If > 90 negative voltage is available across the bridge Inverter action

Due to finite transformer inductance, current transfer from one thyristor valve to the other cannot
take place instantly

This delay is called over lap angle and the reactance called commutating reactance. This also
causes additional drop in the voltage

Ideal No-Load Condition

1

Vd

B
2

Effect of Control Angle

1

Vd

B
2

RECTIFIER VOLTAGE

INVERTER VOLTAGE

DC Terminal Voltage
RECTIFICATION

120 180 240 300

60 120 180
0.866E . 2
LL

E . 2
LL

DC Terminal Voltage
INVERSION

0.866E . 2
LL

120 180 240 300

60 120 180

E . 2
LL

DC Voltage Verses Firing Angle

1 Vd
0.8
0.6
0.4
0.2

alpha

0
-0.2 0

30

60

90

120

150

180

-0.4
-0.6
-0.8
-1

Vd=Vac*1.35 *(cos alpha-uk/2)

Valve Voltage and Valve Current

RECTIFICATION
=15
+
u

A
S
R
120 180 240 300
0
u
P
u
u
B

EG J L N
D
H
M
K
C F
60

60

60

60

A
120 180

E . 2
LL

0.866E . 2
LL

Valve Voltage and Valve Current

60 60

INVERSION
=15

u
Q

60

J
F H K

Q
0.866E . 2
LL

C
AS
R
120 180 240
B

R
60 120 180

E . 2
LL

12-Pulse Convertor Bridge

Commonly Used in HVDC systems

12-Pulse Convertor Bridge

Commonly

adopted in all HVDC applications

Two 6 pulse bridges connected in series
30 phase shift between Star and Delta
windings of the converter transformer
Due to this phase shift, 5th and 7th harmonics
are reduced and filtering higher order
harmonics is easier
Higher pulse number than 12 is not
economical

DC VOLTAGE AT = 15

DC VOLTAGE AT = 90

DC VOLTAGE AT = 165

HVDC Link Voltage Profile

RECTIFIER
Vdio

INVERTER

cos

Vdio I

Id Xc
2
Id Er

cos

Id RL

Id Xc
2

Id Er
DC CABLE or O/H LINE

VdR=VdioR cos-Id Xc+Er

2

VdI=VdioI(cos-Id Xc+Er
2

Control of DC Voltage
Rectifier Operation
A C S y s te m

P o w e r F lo w

Inverter Operation
D C S y s te m

A C S y s te m

D C S y s te m

P o w e r F lo w

Id
V 1

V 3

Id

V 5

V 1

P hase A

V 3

V 5

P hase A

P hase B

P hase B

P hase C

P hase C
V 4

V 6

V 2

V 4

V 6

V 2

+Ud
R e c tifie r
O p e r a tio n

160

0
5

-U d

30

60

90

120

150

In v e r te r
O p e r a tio n

180

Relationship of DC Voltage Ud and Firing

Angle

R e c t.

L im it

+Ud

R e c tifie r
O p e r a tio n
160

0
5

30

60

90

120

150

180

In v e r te r
O p e r a tio n

-U d
Ud

0o

30o

60o

In v

L im it

Ud

Ud

90o

120o

150o

t
-U d

How does HVDC

Operate?

NORMAL POWER DIRECTION

REVERSE POWER OPERATION

Schematic of HVDC

Modes of Operation
Bipolar
Smoothing Reactor
Thyristor
Valves

DC OH Line

Smoothing Reactor
Thyristor
Valves

Current

Converter
Transformer

Converter
Transformer

Current
400 kV
AC Bus
AC Filters,
Reactors

400 kV
AC Bus
AC Filters, shunt
capacitors

Modes of Operation
Monopolar Ground Return
Smoothing Reactor

DC OH Line

Thyristor
Valves

Thyristor
Valves

Converter
Transformer

400 kV
AC Bus
AC Filters,
Reactors

Smoothing Reactor

Current

Converter
Transformer

400 kV
AC Bus
AC Filters

Modes of Operation
Monopolar Metallic Return
Smoothing Reactor

DC OH Line

Thyristor
Valves

Thyristor
Valves

Converter
Transformer

400 kV
AC Bus
AC Filters,
Reactors

Smoothing Reactor

Current

Converter
Transformer

400 kV
AC Bus
AC Filters

TALCHER KOLAR
SCHEMATIC

TALCHER

Electrode
Station
Electrode
Station

KOLAR

+/- 500 KV DC line

1370 KM

400kv System
Blore

Hoody
Cudappah

220kv system

Hosur
Salem
Udumalpet

Kolar

Madras

Chintamani

Sharing of Talcher Power

Tamil

Nadu - 636 MW

A.P.

- 499 MW

Karnataka
Kerala
Pondicherry

- 466 MW
- 330 MW
-

69 MW

KOLAR SINGLE LINE DIAGRAM

TACLHER-KOLAR 500 kV HVDC TRANSMISSION SYTEM

Project Highlights
FOR TRANSMITTING 2000 MW OF POWER FROM NTPC
TALCHER STPS -II AND FOR SHARING AMOGEST
SOUTHERN STATES THE

2000 MW HVDC BIPOLAR

TRANSMISSION SYSTEM IS ENVISAGED AS

EAST SOUTH INTERCONNECTOR II (ESICON II).
THIS IS THE LARGEST TRANSMISSION SYSTEM TAKEN
UP IN THE COUNTRY SO FAR
THE PROJECT SCHEDULE IS QUITE CHALLENGING

AGAINST THE 50 MONTHS FOR SUCH PROJECTS, THE

PROJECT SCHEDULE IS ONLY 39 MONTHS

SCHEDULED COMPLETION BY JUNE 2003

Project Highlights
KEY DATES
AWARD

OF HVDC TERMINAL STATION PKG -

14TH MAR 2000

AWARD

OF HVAC PACKAGE

27TH APR 2000

APPROVED PROJECT COST - RS. 3865.61 CR
THIS IS THE FIRST OF SUCH SYSTEM WHERE THE
ENTIRE GENERATION IN ONE REGION IS
EARMARKED TO ANOTHER REGION.

Salient Features
Rectifier

Talcher, Orissa

Inverter

Kolar, Karnataka

Distance

1370 km

Rated Power

2000 MW

Operating Voltage

500 kV DC

Reduced Voltage

400 kV DC

Overload

Two Hour, 50C

Two Hour, 33C
Half an hour, 50/33C
Five Seconds
To be upgraded to

1.1 pu per pole

1.2 pu per pole
1.2/1.3 pu per pole
1.47 pu per pole
2500MW

SYSTEM CAPACITIES
BIPOLAR MODE OF OPERATION

-- 2000 MW

MONO POLAR WITH GROUND RETURN --- 1000 MW

MONO POLAR WITH METALLIC RETURN MODE --- 1000 MW
DEBLOCKS EACH POLE AT P min 100 MW
POWER DEMAND AT DESIRED LEVEL
POWER RAMP RATE --

1 300 MW /MIN

POWER REVERSAL IN OFF MODE

SYSTEM CAPACITIES
OVER LOAD CAPACBILITIES
RATED POWER

-- 2000 MW

OVER LOAD POWER 2 HOURS -- 2500 MW

SHORT TIME OVER LOAD 5 SEC- 3210 MW

HARMONIC FILTERS
AT TALCHER
TOTAL FILTERS 12
DT 12/24 FILTERS EACH 120 MVAR - 6 NOS
DT 3/36 FILTERS EACH 97 MVAR - 3 NOS
SHUNT REACTORS
138 MVAR- 2 NOS
SHUNT CAPCITORS
138 MVAR- 1 NOS
DC FILTERS
AT KOLAR
TOTAL FILTERS 14
DT 12/24 FILTERS EACH 120 MVAR - 6 NOS
DT 3/36 FILTERS EACH 97 MVAR - 3 NOS
SHUNT CAPCITORS
138 MVAR- 5 NOS
DC FILTERS

SYSTEM CAPACITIES
MONOPOLAR GROUND RETURN - 1000 MW POWER
CAN BE TRANSMITTED THROUGH THIS MODE WHERE
THE RETURN PATH IS THROUGH THE GROUND
WHICH IS FACILITATED THROUGH A EARTH
ELECTRODE STATION SITUATED AT ABOUT 35 KMS
FROM THE TERMINALS AND CONNECTED BY A
DOUBLE CIRCUIT TRANSMISSION LINE.
MONOPOLAR METALLIC RETURN - 1000 MW POWER
CAN BE TRANSMITTED THROUGH THIS MODE WHERE
THE RETURN PATH IS THE TRANSMISSION LINES OF
OTHER POLE.
BALANCED BIPOLAR MODE 2000 MW CAN BE
TRANSMITTED THROUGH THIS MODE WHERE WITH
ONE +VE AND OTHER VE .

TALCHER-KOLAR HVDC & EHVAC SYSTEM