hvdc presentation eee

8/8/2019 Hvdc Presentation Eee

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Presented by,PREETHA .S



INTRODUCTION



o Electric power transmission was originally developed with directcurrent.

o However, DC power at low voltage could not be transmitted overlong distances, thus giving rise to high voltage alternating current (AC) electrical systems.

o With the development of high voltage valves, it was possible toonce again transmit DC power at high voltages and over long distances, giving rise to HVDC(High Voltage DC) transmissionsystems.

HVDC TECHNOLOGY



o The first commercial HVDC line built in 1954 was a 98 kmsubmarine cable with ground return between the island of

Gotland and the Swedish mainland.o Thyristors were applied to d.c. transmission in the late 1960·s

and solid state valves became a reality.o Today, the highest functional d.c. voltage for d.c. transmission

is +/- 600 kV for the 785 km transmission line of the Itaipuscheme in Brazil.



W HY USE DC TRANSMISSION?

o Large AC cable capacitance will limit the maximum possibleunderground and underwater cable transmission distance .

o DC allows the use of no DC allows the use of no

environmental impacts.o In almost all instances DC Transmission has lower losses than

AC Transmission.o For the DC aerial construction the tower cost and land cost is l

ower.o The D.C. transmission line can have a lower visual profile than

an equivalent A.C. line and so contributes to a lowerenvironmental impact.



W HY USE DC TRANSMISSION?(CONTINUES«««)

o Some A.C electric power systems are not synchronized toneighboring networks even though their physical distancesbetween them is quite small. It is physically impossible toconnect the two together by direct A.C methods in order toexchange electric power between them.

o If transmission is by submarine or underground cable, thebreakeven distance is much less than overhead transmission.It is not practical to consider A.C cable systems exceeding 50

km but D.C cable transmission systems are in service whoselength is in distances of 600 km or greater have beenconsidered feasible.



W HY HVDC SYSTEMS?.........

o HVDC is the unique solution to interconnect asynchronoussystems.e.g. different grid frequencies.

o HVDC represents the most economical solution for distancesgreater than 600 km.

o HVDC is the solution for long submarine transmission.



HVDC BASICS

o A typical HVDC transmission system consists of two converter stations connected by a transmission path (aerial/underground cable).

o Each converter station acts as either a rectifier (AC DC) or an inverte

r (DC AC)

o The converter uses high speed switching to create a DC voltage fromthe AC voltage or an AC voltage from a DC voltage

o Depending upon the design of the HVDC system, filtering of the AC signal may be signal may be required in order to smooth the resultant waveform



CONFIGURATIONS

o The integral part of an HVDC power converter is the valve orvalve arm. It may be controllable if constructed from one ormore thyristors in series.

o Electric power flowing between the HVDC valve group and thea.c. system is ¶three phase·.

o When electric power flows into the d.c. valve group from the a.c.

system then it is considered a rectifier. If power flows from thed.c. valve group into the a.c. system, it is an inverter. Each valveconsists of many series connected thyristors in thyristormodules.



HVDC SUBSTATION -REPRESENTATION



PRINCIPLES OF HVDC



HVDC TERMINAL



ADVANTAGES OF HVDCTECHNOLOGYo No limits in transmitted distance. This is valid for both OH lines

and sea or underground cables.o Very fast control of power flow, which implies stability

improvements, not only for the HVDC link but also for thesurrounding AC system.o Direction of power flow can be changed very quickly (bi-

directionality).o An HVDC link don´t increase the short-circuit power in the

connecting point. This means that it will not be necessary tochange the circuit breakers in the existing network.



ADVANTAGES OF HVDCTECHNOLOGY (CONTINUES«..)

o HVDC can carry more power for a given size of conductor.o The need for ROW (Right Of Way) is much smaller for HVDC

than for HVAC, for the same transmitted power. The

environmental impact is smaller with HVDC.o VSC technology allows controlling active and reactive power

independently without any needs for extra compensating equipment.

o VSC technology gives a good opportunity to alternative energysources to be economically and technically efficient.

o HVDC transmissions have a high availability and reliability rate,shown by more than 30 years of operation.



DISADVANTAGES OF HVDCTECHNOLOGYo Converters are expensive.

o Converters require much reactive power.

o Converters generate harmonics, requiring filters.

o Converters have little overload capability.

o Lack of HV dc circuit breakers hampers multiterminal ornetwork operation.



APPLICATIONS OF HVDC

o For cables crossing bodies of water wider than 32km.

o For interconnecting ac systems having different frequencies orwhere asynchronous operation is desired.

o For transmitting large amounts of power over long distances byoverhead lines.

o In congested urban areas or elsewhere where it is difficult toacquire right of way for overhead lines and where the lengthsinvolved make ac cables impractical.



OVERVIE W OF HVDC APPLICATIONS



SHORT CIRCUIT CONTRIBUTIONOF HVDC LIGHT



HVDC LIGHT

o HVDC Light is the newly developed HVDC transmissiontechnology, which is based on extruded DC cables and voltagesource converters consisting of Insulated Gate BipolarTransistors (IGBT·s) with high switching frequency.

o Under more strict environmental and economical constraintsdue to the deregulation, the HVDC Light provides the mostpromising solution to power transmission and distribution.

o One of the most concerned issues from customers is thecontribution of HVDC Light to short circuit currents.



FEATURES OF HVDC LIGHT

o Active and reactive power exchange can be controlled flexiblyand independently.

o The power quality and system stability can be improved viacontinuously adjustable reactive power support with AC voltagefeedback control.

o Feed AC systems with low short circuit power or even passivenetworks with no local power generation.



SHORT CIRCUIT RATIO OF HVDCLIGHTo SHORT CIRCUIT RATIO

The strength of the a.c. network at the bus of theHVDC substation can be expressed by the short circuit ratio (SCR),defined as: µ the relation between the short circuit level in MVA at the

HVDC substation bus at 1.0 per-unit a.c. voltage and the d.c. power inMW.µThe possible maximum relative short circuit current

Increment (¨Imax) is determined by the short circuit ratio(SCR) as,

ISC :the short-circuit current of the original AC system alone at a 3-ph fault.ISC_HVDC_L : the short-circuit current of the AC system with converter stationconnected and in operation at the same fault.



SHORT CIRCUIT RATIO OF HVDC

LIGHT (CONTINUE««)

The maximum possible short circuit increment (¨Imax) due toHVDC Light is determined by the SCR. It is inversely inproportional to the SCR and it occurs when the transmissionsystem is operating at zero active power.

The contribution to the short circuit current is irrelevant tothe fault location if the fault current is evaluated in per unit

with the base value equal to the 3-ph fault current at thecorresponding fault location and without HVDC Lightconnected.



CONCLUSION

HVDC systems remain the best economical and environmentallyfriendly option for the above conventional applications. However,around the world, a quantum leap in efforts to conserve theenvironment - are demanding a change in thinking that could make

HVDC systems the preferred alternative to high voltage AC systems inmany situations.

The HVDC Light, in contrast to the conventional HVDC which does notcontribute any short circuit current, may contribute some short circuitcurrent. With HVDC Light, the voltage dip due to distant fault ispossibly reduced and thereby the connected electricity consumersmay suffer less from disturbances.



THANKS«.

hvdc presentation eee

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