INTRODUCTION TO HVDC V G RAO HVDC / KOLAR

They are either PIT HEAD THERMAL or HYDEL 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

HVDC: USE less current Direct current : Roll along the line ; opposing force friction (electrical resistance ) AC current will struggle against inertia in the line (100times/sec)-cuurent inertia –inductance-reactive power

Better Voltage utilisation rating

DC has Greater Reach Distance as well as amount of POWER determine the choice of DC over AC

DC The alternating current in a cable ”leaks” current (charging movements) in the same manner as a pulsating pressure would be evened out in an elastic tube.

DIRECT CURRENT CONSERVES FOREST AND SAVES LAND Fewer support TOWER, less losses

CONTROLLING or BEING CONTROLLED By raising the level in tank ;controlled water flow

CONTROLLING or BEING CONTROLLED ZERO IF Vr=VI=10V

HVDC leads to Better Use of AC TRANS SYS. FORCE HAS TO BE APPLIED IN RIGHT POSITION

HVDC provides increase power but does not increase the short circuit POWER

HVDC LEADS TO BETTER USE OF AC HVDC and HVAC SHOULD CO-OPERATE FOR OPTIMUM EFFICIENCY

HVDC LEADS TO BETTER USE OF AC If two networks are connected by an AC link, it can be in-efficient

CONTROLLED POWER FLOW IS POSSIBLE VERY PRECISELY 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 .

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

Types of HVDC HVDC is the unique solution to interconnect asynchronous systems or grids with different frequencies. Solution: HVDC Back-to-Back Up to 600 MW Back-to-Back Station AC 50 Hz 60 Hz

Long Distance Transmission Types of HVDC HVDC represents the most economical solution to transmit electrical energy over distances greater than approx. 600 km Solution: HVDC Long Distance Up to 3000 MW Long Distance Transmission 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 Up to 600 MW Long Submarine Transmission AC DC cable

HVDC BIPOLAR LINKS IN INDIA NER NER NR NR 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 Rihand – Dadri North-North 1500 815 HVDC LINK CONNECTING REGION CAPACITY (MW) LINE LENGTH Rihand – Dadri North-North 1500 815 Chandrapur - Padghe West - West 752 Talcher – Kolar East – South 2500 1367

ASYNCHRONOUS LINKS IN INDIA NER NER NR NR 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 REGION CAPACITY (MW) Vindyachal North – West 2 x 250 Chandrapur West – South 2 x 500 Vizag – I East – South 500 Sasaram East – North Vizag – II

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BASIC PRINCIPLES OF HVDC TRANSMISSION

AC Transmission Principle

HVDC Transmission Principle

DC AS A MEANS OF TRANSMISSION 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

Important Milestones in the Development of HVDC technology · Hewitt´s mercury-vapour rectifier, which appeared in 1901. · Experiments with thyratrons in America and mercury arc valves in Europe before 1940. · First commercial HVDC transmission, Gotland 1 in Sweden in 1954. · First solid state semiconductor valves in 1970. · First microcomputer based control equipment for HVDC in 1979. · Highest DC transmission voltage (+/- 600 kV) in Itaipú, Brazil, 1984. · First active DC filters for outstanding filtering performance in 1994. · First Capacitor Commutated Converter (CCC) in Argentina-Brazil interconnection, 1998 · First Voltage Source Converter for transmission in Gotland, Sweden ,1999

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: CONVERT AC into DC (rectifier) TRANSMIT DC 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 CONVERSION SINGLE PHASE HALF WAVE RECTIFIER

SINGLE PHASE FULL WAVE RECTIFIER CONVERSION SINGLE PHASE FULL WAVE RECTIFIER

SINGLE PHASE FULL WAVE BRIDGE RECTIFIER CONVERSION SINGLE PHASE FULL WAVE BRIDGE RECTIFIER

6-Pulse Convertor Bridge 1 3 5 L i s A E 1 L i s B V' d V d L i s C 4 6 2 I d 2

Voltage and Current of an Ideal Diode 6 Pulse Converter Alpha = 0 Overlap = 0

Operation of Converter Each thyristor conducts for 120º Every 60º 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

Operation of Converter Triggering can be delayed from this point and this is called firing angle α Output voltage of the converter is controlled by controlling the α – Rectifier action If α > 90º 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 μ and the reactance called commutating reactance. This also causes additional drop in the voltage

Ideal No-Load Condition 1 3 C A V d B 2 2

Effect of Control Angle 1 3 u u u C A V d B 2 2

RECTIFIER VOLTAGE

INVERTER VOLTAGE

DC Terminal Voltage 120 º RECTIFICATION 240 º 180 º 300 º 60 º 0.866 240 º 180 º 300 º 60 º 0.866 E . 2 LL 2

DC Terminal Voltage 120 º INVERSION 240 º 180 º 300 º 60 º 0.866 E . 2 240 º 180 º 300 º 60 º 0.866 E . 2 LL 2

DC Voltage Verses Firing Angle Vd alpha Vd=Vac*1.35 *(cos alpha-uk/2)

Valve Voltage and Valve Current 120 180 A u 0.866 240 60 F C D B E K G J L H N M 300 P S E . 2 LL R Q RECTIFICATION =15º +u 2

Valve Voltage and Valve Current M Q 120 º 180 º R N P u 240 º 120º B F S A C E D H 60 º J K G L INVERSION =15º 60º 0.866 E . 2 LL 2

12-Pulse Convertor Bridge Y  Commonly Used in HVDC systems 2

12-Pulse Convertor Bridge Commonly adopted in all HVDC applications Two 6 pulse bridges connected in series 30º 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 α = 15º

DC VOLTAGE AT α = 90º

DC VOLTAGE AT α = 165º

HVDC Link Voltage Profile RECTIFIER INVERTER V dio R cos V dio I cos I X d c 2 I E d r I R d L I X d c 2 I E d r DC CABLE or O/H LINE VdR=VdioR cos-Id Xc+Er VdI=VdioI(cos-Id Xc+Er 2 2 2

Control of DC Voltage Rectifier Operation Inverter Operation In the real HVDC system the DC voltage is varied by means of a converter bridge In rectifier operation the power flow is from the AC system to the DC system The power flow is changed from the DC system to the AC system by reversing the DC voltage. The DC current does not change it’s direction. The operating range of the ideal converter is in theory from 0° (+1.0 p.u. DC voltage) to 180° (-1.0 p.u. DC voltage). The operating range of a real converter is from approx. 5° to approx. 160°. In 90° operation the DC voltage of the converter is 0.

Relationship of DC Voltage Ud and Firing Angle α

How does HVDC Operate?

Decrease voltage at station B or increase voltage at station A Decrease voltage at station B or increase voltage at station A. power flows from A B Normal direction

NORMAL POWER DIRECTION

REVERSE POWER OPERATION

Schematic of HVDC

Modes of Operation Bipolar Smoothing Reactor DC OH Line Thyristor Valves Thyristor Valves Current Converter Transformer Converter Transformer Current 400 kV AC Bus 400 kV AC Bus AC Filters, Reactors AC Filters, shunt capacitors

Modes of Operation Monopolar Ground Return Smoothing Reactor DC OH Line Smoothing Reactor Thyristor Valves Thyristor Valves Current Converter Transformer Converter Transformer 400 kV AC Bus 400 kV AC Bus AC Filters, Reactors AC Filters

Modes of Operation Monopolar Metallic Return Smoothing Reactor DC OH Line Smoothing Reactor Thyristor Valves Thyristor Valves Current Converter Transformer Converter Transformer 400 kV AC Bus 400 kV AC Bus AC Filters, Reactors AC Filters

TALCHER KOLAR SCHEMATIC Electrode Station Electrode Station KOLAR +/- 500 KV DC line 1370 KM Kolar 400kv System 220kv system Hosur Hoody B’lore Chintamani Salem Madras Cudappah Udumalpet

Sharing of Talcher Power Tamil Nadu - 636 MW A.P. - 499 MW Karnataka - 466 MW Kerala - 330 MW Pondicherry - 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

AWARD OF HVDC TERMINAL STATION PKG - 14TH MAR 2000 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 Long time, 40C 1.25 pu per pole Half an hour 1.3 pu per pole Five Seconds 1.47 pu per pole

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 LONG TIME OVER LOAD POWER – 8/10 HOURS -- 2500 MW SHORT TIME OVER LOAD – 5 SEC- 3210 MW

HARMONIC FILTERS AT TALCHER TOTAL FILTERS – 14 DT 12/24 FILTERS EACH 120 MVAR - 7 NOS DT 3/36 FILTERS EACH 97 MVAR - 4 NOS SHUNT REACTORS 138 MVAR- 2 NOS SHUNT CAPCITORS 138 MVAR- 1 NOS DC FILTERS DT 12/24 & DT 12/36 – 1 No per pole. AT KOLAR TOTAL FILTERS – 17 DT 12/24 FILTERS EACH 120 MVAR - 8 NOS SHUNT CAPCITORS 138 MVAR- 5 NOS DC FILTERS DT 12/24 & DT 12/36 – 1 each pole

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

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