07915a0204-hvdc

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Department of EEE

Friday, July 13, 2012 V.J.I.T - Dept of EEE

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Electrical engg technicalseminar


Krishna . K EEE B07915A0204

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HVDC LIGHT, A TOOLFOR TRANSMISSION OFELECTRIC POWER


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Need for HVDC


Because of large charging

currents

use of hvac is prohibited.

Inter connection of two large

AC systems by a DC

transmission tie line.

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CONTENTS INTRODUCTION

HVDC LIGHT CONVERTER STATION

HVDC CABLES CONTROL OF ACTIVE AND REACTIVE POWER

FEATURES OF HVDC LIGHT

APPLICATIONS

CONVERTER CONTROL AND PROTECTION CONCLUSION

REFERENCES


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INTRODUCTION HVDC light is high voltage direct current

transmission system based on solid statevoltage source technology.

Utilizes most advanced power electronics andsemiconductors.

Power is transmitted via

underground/undersea cables. More efficient way of transmitting power over

distances above ~50km.


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HVDC light installations


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HVDC light converter station


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Power transformer The transformer is an ordinary single phase or three

phase with a tap changer on the secondary side

The filter bus voltage will be controlled with the tap

changer to achieve the maximum active and reactivepower from the converter

The current in the transformer winding containshardly any harmonics and is not exposed to any dc

voltage The transformer may be provided with a tertiary

winding to feed the station auxiliary power system


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Converter reactors


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The main purposes of theconverter reactors are

To provide low-pass filtering of thePWM pattern to give the desiredfundamental frequency voltage

To provide active and reactive power

control

To limit the short circuit currents


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DC capacitors The primary objective of the valve dc side

capacitor is to provide a low inductance path

for the turned off current and also to serve asan energy store

It reduces the harmonic ripple on the directvoltage

The ability to limit the dc voltage variationscaused by the disturbances in the systemdepends on the size of the dc side capacitor


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The DC capacitor is a DryQ

capacitor

The DryQ design has:1)Twice the capacity in half

the volume

2)Corrosion-free plastichousing

3)Low inductance

4)Shortened production time

and simplified installation


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AC filtersExample of an HVDC light ac filter

The sinusoidalvoltage to the

VSC is made

possible by

means of converter reactors

and ac filters


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Ac filters A shunt filter is designed as low

impedance to all the ac harmonic

currents and capacitive impedance tofundamental frequency.

As aresult the voltage across the filter is

constrained to fundamental frequencysine wave


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Valves-The IGBT position IGBT is the semiconductor used in

HVDC light

To increase the power handling,sixIGBT chips and three diode chips areconnected in parallel in a sub-module

The IGBT has two,four or six sub-modules, which determine the currentrating of the IGBT


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A complete IGBT position consists of anIGBT,a gate unit,a voltage divider and awater cooled heat sink

The gate driving electronics control the gatevoltage and current at turn on and turn off

The voltage across the IGBT is measured andthe information is sent to the valve control

unit through an optical fibre The voltage divider connected across the

IGBT provides the current needed to drivethe gate


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IGBT with four and six sub-

modules


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Valve function To switch voltages higher than the rated

voltage of one IGBT,several positions are

connected in series in each valve Each IGBT position can be individually

regulated in the valve to the correct voltagelevel

The flexibility of the IGBT makes it possible toblock the current immediately if a short circuitis detected


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Mechanical design of valves


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Valve cooling system


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It consists of water cooled heat sinks whichprovides high efficiency cooling

The water is circulating through the heat sink in close contact with each IGBT,whichefficiently transports the heat away from thesemiconductor

The water passes continuously through a de-ionizing system to keep the conductivity of the water low


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HVDC

Cables


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Advantages of HVDC cables HVDC light cables are installed close in

bipolar pairs with anti parallel currents

and thus eliminating the magnetic fields They have no technical limitation for

distance as in ac cables

Its strength and flexibility makes it wellsuited for several installation conditions


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Control of active and reactive

power


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The fundamental base apparent power at thefilter bus between the Converter reactor andthe ac filter is ,

Sb= P+jQ= 3.Uf .IR

The active and reactive components are

P=UF.UC. sin L

Q=UF.(UF –UC). cos

LFriday, July 13, 2012 V.J.I.T - Dept of EEE

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Active power flowIf UC is in phase lag,the

active power flows from

ac to dc side(rectifier)

If UC is in phase lead,the

active power flows from

dc to ac side


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Reactive power flow

If UF > UC, there is

reactive power

consumption.

If UC > UF, there is

reactive power

generation.


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Features Independent control of active and reactive

power

Independent power transfer and powerquality control

Power reversal

Reduced power losses in connected ac

systems Increased transfer capacity in the existing

system


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Fast restoration after blackouts

Flexibility in design

No relevant magnetic fields Low environmental impact

Indoor design

Short time schedule


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Kinds of dc links Monopolar lines

Bipolar lines

Homopolar lines Line has one conductor and earth is used

as return conductor(mono)_ve

Line has two conductors one +ve and

one_ve(bipolar) Line have two or more conductor having

same polarity usually _ve


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Simplified HVDC circuit


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Dc links


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Ground return It uses t/m lines use ground or sea

water as the return conductor

For the same length of the t/m line theresistance offered by the gnd in case of dc is much less as compared to ac

Earth resistance in case of dc isindependent of line length


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Applications Wind power generation

Multi terminal dc grid

Interconnecting networks

Islanded operation

City infeed


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Reliability and Quality To assure high reliability and availability,the

HVDC light principles include

1.Simple station design 2.Use of components with proven high

reliability

3.Automatic supervision

4.Use of back up control systems andequipment such as measurements,pumps etc

Maintainability

Quality assurance/standardsFriday, July 13, 2012 V.J.I.T - Dept of EEE

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Difference betn hvac and hvdcHigh voltage ac t/m High voltage dc t/m

Ac power is given byp= (E1E2)sinΦ

line construction is simplerIt has same reliability of dc linefor 3phase line having sixlineconductorspower per conductor is lesscompared to dc

stability problemsynchronous systemInterconnections of two acsystem increases fault levelexisting switch gear has to bereplaced

Dc power is given byP =(Ed1-Ed2)Ed2/R

line circuit is simpler bipolar dc line has same reliabilityPower per conductor in case of dc ismoreno stability problemasynchronous tie line system(dc link)

Fault dc link do not draw the largecurrents from ac system


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Conclusions Opportunity to transmit small scale power

long distances via cable

Opportunity to connect to passive load Separate control of active and reactive power

No contribution to short circuit currents

No need of fast communication Opportunity to operate without transformers

Small and compact


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References1.DC Transmission based on voltage

source converters,Gunnar Asplund,Kjell

Eriksson and Kjell Svesson,19972.The ABCs of HVDC transmission

technologies,IEEE Power and Energy

Magazine,2006


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Thank you

Krishna. K

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