2They are either PIT HEAD THERMAL or HYDEL REASONS FOR AC GENERATION AND TRANSMISSIONDue 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 centersThey are either PIT HEAD THERMAL or HYDELTurbines drive synchronous generators giving an output at 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.
3COMPARISION 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 ETCCONSIDERING THE DISADVANTAGES OF HVAC SYSTEM AND THE ADVANTAGES OF HVDC TRANSMISSION , POWERGRID HAS CHOOSEN HVDC TRANSMISSION FOR TRANSFERRING 2000 MW FROM ER TO SR
4HVDC: USE less currentDirect 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
6DC has Greater ReachDistance as well as amount of POWER determine the choice of DC over AC
7DC The alternating current in a cable ”leaks” current (charging movements) in the same manner as a pulsating pressurewould be evened out in an elastic tube.
8DIRECT CURRENT CONSERVES FOREST AND SAVES LAND Fewer support TOWER, less losses
9CONTROLLING or BEING CONTROLLED By raising the level in tank ;controlled water flow
10CONTROLLING or BEING CONTROLLED ZERO IF Vr=VI=10V
11HVDC leads to Better Use of AC TRANS SYS. FORCE HAS TO BE APPLIED IN RIGHT POSITION
12HVDC provides increase power but does not increase the short circuit POWER
13HVDC LEADS TO BETTER USE OF AC HVDC and HVAC SHOULD CO-OPERATE FOR OPTIMUM EFFICIENCY
14HVDC LEADS TO BETTER USE OF AC If two networks are connected by an AC link, it can be in-efficient
15CONTROLLED POWER FLOW IS POSSIBLE VERY PRECISELY ADVANTAGES OF HVDC OVER HVAC TRANSMISSIONCONTROLLED POWER FLOW IS POSSIBLE VERY PRECISELYASYNCHRONOUS OPERATION POSSIBLE BETWEEN REGIONS HAVING DIFFERENT ELECTRICAL PARAMETERSNO RESTRICTION ON LINE LENGTH AS NO REACTANCE IN DC LINES
16ADVANTAGES 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.
17ADVANTAGES 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
18COST: AC vs DC Transmission Line Cost ACLine Cost DCTerminal Cost DCTerminal Cost ACBreak Even Distance
192000 MW HVDC VIS- A- VIS – HVAC SYSTEMS HVDC BIPOLAR TRANSMISSION SYSTEM2 DOUBLE CIRCUIT HVAC TRANSMISSION SYSTEMS
23Types of HVDCHVDC is the unique solution to interconnect asynchronous systems or grids with different frequencies.Solution: HVDC Back-to-BackUp to 600 MWBack-to-Back StationAC50 Hz60 Hz
24Long Distance Transmission Types of HVDCHVDC represents the most economical solution to transmit electrical energy over distances greater than approx. 600 kmSolution: HVDC Long DistanceUp to 3000 MWLong Distance TransmissionACDC line
25Types of HVDC HVDC is an alternative for submarine transmission. Economical even for shorter distances such as a few 10km/milesSolution: HVDC CableUp to 600 MWLong Submarine TransmissionACDC cable
26HVDC BIPOLAR LINKS IN INDIA NERNERNRNRERERRIHAND-DELHI *750 MWCHANDRAPUR-PADGE – 2* 750 MWTALCHER-KOLAR – 2*1000 MWER TO SRSILERU-BARASORE MWEXPERIMENTAL PROJECTER –SRSRSR
34DC AS A MEANS OF TRANSMISSION USE OF DCDirect 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 TRANSMISSIONThis has been possible with advent ofHigh power/ high current capability thyristors&Fast acting computerised controls
35Important 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
36The Evolution of Thyristor Valves in HVDC High Voltage Thyristor Valve History Highlights1967 First Test Valve: 2 parallel 35 mm 1650 V1969 World's First Contract for an HVDC System with Thyristor Valves2 parallel 35 mm 1650 V for 2000 A1975 World's First Contract for Watercooled HVDC Thyristor Valves2 parallel 52 mm 3500 V for 2000 A1980 World's First Contract for HVDC System with 100 mm Thyristorsno parallel 4200 V for 3600 A1994 First HVDC Contract Using 8kV Thyristors100 mm 8000 V1997 First Thyristor Valve with Direct-Light-Triggering100 mm thyristors with breakover 8000 V for 2000 A2001 First complete HVDC System using Direct-Light-Triggered Thyristors with integrated breakover 8000 V
37If 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 DCCONVERT DC into AC ( inverter)
38Purpose & function of Thyristor Valve Connects AC phases to DC systemConduct High Current – currents upto 3000A without the requirement of paralleling of thyristorsBlock High Voltage – Blocks high voltage in forward and reverse direction up to 8KVControllable – thyristor triggering /conduction possible with the gate firing circuitsFault tolerant and robust
43Voltage and Current of an Ideal Diode 6 Pulse Converter Alpha = 0Overlap = 0
44Operation of Converter Each thyristor conducts for 120ºEvery 60º one Thyristor from +ve limb and one Thyristor from –ve limb is triggeredEach thyristor will be triggered when voltage across it becomes positiveThyristor 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
45Operation of Converter Triggering can be delayed from this point and this is called firing angle αOutput voltage of the converter is controlled by controlling the α – Rectifier actionIf α > 90º negative voltage is available across the bridge – Inverter actionDue to finite transformer inductance, current transfer from one thyristor valve to the other cannot take place instantlyThis delay is called over lap angle μ and the reactance called commutating reactance. This also causes additional drop in the voltage
52DC Voltage Verses Firing Angle VdalphaVd=Vac*1.35 *(cos alpha-uk/2)
53Valve Voltage and Valve Current 120180Au0.86624060FCDBEKGJLHNM300PSELLRQRECTIFICATION=15º+u2
54Valve Voltage and Valve Current MQ120 º180 ºRNPu240 º120ºBFSACEDH60 ºJKGLINVERSION=15º60º0.866ELL2
5512-Pulse Convertor Bridge YCommonly Used in HVDC systems2
5612-Pulse Convertor Bridge Commonly adopted in all HVDC applicationsTwo 6 pulse bridges connected in series30º phase shift between Star and Delta windings of the converter transformerDue to this phase shift, 5th and 7th harmonics are reduced and filtering higher order harmonics is easierHigher pulse number than 12 is not economical
60HVDC Link Voltage Profile RECTIFIERINVERTERVdio RcosVdio IcosI Xdc2I EdrI RdLI Xdc2I EdrDC CABLEorO/H LINEVdR=VdioR cos-Id Xc+Er VdI=VdioI(cos-Id Xc+Er2
61Control of DC Voltage Rectifier Operation Inverter Operation In the real HVDC system the DC voltage is varied by means of a converter bridgeIn rectifier operation the power flow is from the AC system to the DC systemThe 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.
62Relationship of DC Voltage Ud and Firing Angle α
74TACLHER-KOLAR ± 500 kV HVDC TRANSMISSION SYTEM Project HighlightsFOR TRANSMITTING 2000 MW OF POWER FROM NTPC TALCHER STPS -II AND FOR SHARING AMOGEST SOUTHERN STATES THE MW HVDC BIPOLAR TRANSMISSION SYSTEM IS ENVISAGED ASEAST SOUTH INTERCONNECTOR II (ESICON –II).THIS IS THE LARGEST TRANSMISSION SYSTEM TAKEN UP IN THE COUNTRY SO FARTHE PROJECT SCHEDULE IS QUITE CHALLENGINGAGAINST THE 50 MONTHS FOR SUCH PROJECTS, THE PROJECT SCHEDULE IS ONLY 39 MONTHSSCHEDULED COMPLETION BY JUNE 2003
75AWARD OF HVDC TERMINAL STATION PKG - 14TH MAR 2000 Project HighlightsKEY DATESAWARD OF HVDC TERMINAL STATION PKG - 14TH MAR 2000AWARD OF HVAC PACKAGE TH APR 2000APPROVED PROJECT COST - RS CRTHIS IS THE FIRST OF SUCH SYSTEM WHERE THE ENTIRE GENERATION IN ONE REGION IS EARMARKED TO ANOTHER REGION.
76Salient Features Rectifier Talcher, Orissa Inverter Kolar, Karnataka Distance 1370 kmRated Power MWOperating Voltage 500 kV DCReduced Voltage 400 kV DCOverloadLong time, 40C pu per poleHalf an hour pu per poleFive Seconds pu per pole
77SYSTEM CAPACITIES BIPOLAR MODE OF OPERATION -- 2000 MW MONO POLAR WITH GROUND RETURN MWMONO POLAR WITH METALLIC RETURN MODE MWDEBLOCKS EACH POLE AT P min 100 MWPOWER DEMAND AT DESIRED LEVELPOWER RAMP RATE – 300 MW /MINPOWER REVERSAL IN OFF MODE
78SYSTEM CAPACITIES OVER LOAD CAPACBILITIES RATED POWER -- 2000 MW LONG TIME OVER LOAD POWER – 8/10 HOURS MWSHORT TIME OVER LOAD – 5 SEC MW
79HARMONIC FILTERSAT TALCHERTOTAL FILTERS – 14DT 12/24 FILTERS EACH 120 MVAR - 7 NOSDT 3/36 FILTERS EACH MVAR NOSSHUNT REACTORS MVAR- 2 NOSSHUNT CAPCITORS MVAR- 1 NOSDC FILTERS DT 12/24 & DT 12/36 – 1 No per pole.AT KOLARTOTAL FILTERS – 17DT 12/24 FILTERS EACH 120 MVAR - 8 NOSSHUNT CAPCITORS MVAR- 5 NOSDC FILTERS DT 12/24 & DT 12/36 – 1 each pole
80SYSTEM CAPACITIESMONOPOLAR GROUND RETURN 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 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 .