Hvdc 1

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HVDC, FACTS AND HV TECHNOLOGY PRESENTED BY:T.RADHA PRASANTHI

J.NALINI SREE

Reg.No:-05761A0249

Reg.No:-05761A0240

MAIL ID: [email protected]

MAIL ID: [email protected]

LAKIREDDY BALIREDDY COLLEGE OF ENGINEERING, MYLAVARAM.

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ABSTRACT:HVDC transmission has a number of technical advantages compared with HVAC transmission. During the latest 20 years, HVDC has become the dominating technology for long distance transmission of bulk power.Overcoming the problem of AC transmission of growing trends towards the rapid increase of load has lead to development of DC transmission. In a combined AC and DC system generated AC voltage is converted into DC voltage at sending end and DC voltage is inverted into AC voltage at the receiving end for distribution purpose. DC transmission and FACTS (Flexible AC Transmission Systems) has developed to a viable technique with high power ratings since the 60s. From the first small DC and AC "mini networks", there are now systems transmitting 3 - 4 GW over large distances with only one bipolar DC transmission: 1.000 - 2.000 km or more are feasible with overhead lines. As a multi terminal system, HVDC can also be connected at several points with the surrounding three-phase network. FACTS is applicable in parallel connection or in series or in a combination of both. The rating of shunt connected FACTS controllers is up to 800 Mvar, series FACTS devices are implemented on 550 and 735 kV level to increase the line transmission capacity up to several GW.

DocumentToPDF trial version, to remove this mark, please register this software. INDEX:-

HVDC is often the preferred as the

• Introduction

transmission line costs less and has

• Characteristics of HVDC

lower losses than an AC line for the

transmission

same transmission capacity. It is very

• HV technology

easy to control the active power in

1) Capacitor Commutated

the link and the control is based on a constant

Converter 2) Voltage Source Converter

power

transfer.

HVDC

transmission does not contribute to the

• Advantages

short

circuit

• Conclusion

interconnected

current

of

AC system.

the

With an

HVDC system, the power flow can be controlled rapidly and accurately as

INTRODUCTION:HVDC stands for High

to both the power level and the direction.

Voltage Direct Current and is today a Flexible

well proved technology employed for power transmission all over the world. HVDC power

is

to interconnect separate

systems,where

alternating current(AC)

traditional connections

cannot be used.The HVDC technology is used to transmit electricity over long distances

by

overhead

lines or submarine cables.

transmission

AC

Transmission

Systems (FACTS)and HVDC controllers based on power electronics have been developed to improve the performance of long distance AC transmission. Later their use has been extended to load-flow control in meshed and interconnected systems.

Excellent

on-site operating

experience is being reported, and the FACTS and HVDC technology became mature and reliable. Characteristics

of

HVDC

transmission: A

brief

over

view

of

the

main

characteristics of the technology and its present implementation will be provided here.

HVDC

transmission

is

now

DocumentToPDF trial version, to remove this mark, please register this software. available in two different technologies.

the magnitude, phase angle or frequency

i.e.,

of the ac voltage at either end of the dc

1)Line

commutated

current

sourced

converter (LCC HVDC) 2)Self

commutated

scheme. The ability to rapidly control the active power can be very beneficial,

voltage

sourced

e.g. for the damping of power swings in

converter (VSC).

one

Both technologies convert ac into dc and

commutated converter depends on the ac

vice versa. This can do the transmission

system

between the asynchronous networks.

operation. Thyristor switch the converter

1)

ac

Capacitor

commutated

converter

(CCC):

of

the

voltage

network.

for

its

The

line

satisfactory

terminals between the two

dc

terminals. The thyristor can be turned on by a gate signal when the voltage across it is forward biased. The thyristor can conduct in one direction only. When the current is reversing it will turn off. LCC operates at lagging power factor, partly because of inductance of the converter

Above fig . shows the diagram of mono polar LCC hvdc scheme, which has one converter at each end and provides a single transmission block. It is generally considered equivalent to a single circuit ac transmission link. The rectifier takes power from its ac network and the inverter

injects

power

into

its ac

network. Control systems control the two converters such that the desired active power is transmitted between the two. One terminal controls the dc voltage, and the direct current. The active power between the converters is fully controlled and does not depend on

transformer. But primarily because the firing of the thyristors in the rectifier has to be delayed relative to the voltage crossing to control the dc voltage. Similarly,

the

commutation

at

the

inverter has to be completed atleast 10 degrees electrical before the voltage crossing, to enable the thyristor to build up it’s withstand voltage before the voltage becomes forward biased. The commutation process also results in the generation of substantial amounts of harmonics current. AC harmonic filters are

used

to

reduce

the harmonic

distortion at the ac terminals. The filters

DocumentToPDF trial version, to remove this mark, please register this software. are capacitive at fundamental frequency,

disturbances,

and are subdivided into banks, which are

risk of commutation failures. Ø

switched in and out as required to limit the harmonic distortion and to provide

with

reduced

Lower load rejection over voltages

Ø

reactive power compensation ac voltage

No needs to switch AC filters

control. The ac harmonic filters and

or shunt capacitor banks to

reactive power banks together with ac

compensate

switchgear occupy very large land area.

reactive power consumption.

for

converter

Therefore, an hvdc converter station is

The CCC is now an interesting solution

many

in conjunction with the development of

times

(>10)

larger

than

an

equivalently rated substation.

automatically

Because of their capacitance the ac

(ConTune). These filters can be built to

harmonic filters/ reactive power banks

generate small quantities of Reactive

can result in large ac over voltages

power but still provides good filtering.

during the load rejection and dynamic

These

conditions.

fault

characteristics of the CCC, which has a

HVDC

much-reduced need for reactive power

e.g.,

recovery.Typically

during an

the

LCC

scheme needs to be connected to a point

tuned

properties

AC

match

filters

the

due to the commutation.

in the ac network where the short circuit power is atleast 2.5 times the rating of

VSC transmission:

the HVDC scheme, in order to achieve

The figure shows the simplified diagram

stable and satisfactory operation.

of a vsc transmission scheme. The scheme has one converter at each end

CCC results in: Ø

Ø

and is a single transmission block, and it

Significantly better stability,

is considered as a single circuit ac

in particular when connected

transmission link. VSC creates an ac

to AC networks with low

voltage by switching the ac terminals

short circuit capacity and in

between the dc terminals. The switches

transmissions with long DC

use IGBTs which can be switched on

cables.

and off by a gate signal. Even if there a

Dependable performance in

current flowing through the switch at the

the

time it is instructed to switch. The IGBT

event

of AC

system

DocumentToPDF trial version, to remove this mark, please register this software. can withstand voltage and conduct current in one direction only, to enable the converter

to

conduct

in both

directions a diode is connected in anti parallel in some converters like 3level neutral point clamped converter. One or more intermediate dc voltage levels can be created, such that the wave shape more closely resembles a sinusoid. The

Fig: VSC transmission scheme operating capability.

converter switches can be switched on and off several times during each power frequency cycle, if required .Typically the converter switches are operated at a mean frequency of about 1khz.and is switched in such way that certain lower order harmonics are eliminated. As a result, filters are required only for higher frequency harmonics, and can be much lower rating than those used for LCC HVDC schemes

The above fig shows the operating capability of a vsc terminal, with the three circles indicating the capability for different ac voltages. The active power and reactive power can be controlled by vsc. The vsc converters ability to change the stator frequency of the induction generator with in 30 to 65 Hz gives the possibility to optimize the power output from the wind turbine by adjusting the frequency in relation to the wind. The

space

occupied

by

a

vsc

transmission substation is less than 35% of that of the LCC hvdc terminal. Another significant difference is that the Fig: simplified vsc transmission scheme

vsc transmission scheme generates its own ac voltage from the dc capacitor,

Seen from the ac network, the vsc transmission terminal is equivalent to a voltage source with an amplitude and phase angle determined by the control system.

which means that it can operate as a power supply to a passive ac network. The scheme in this Operating mode is same as motor drive.

DocumentToPDF trial version, to remove this mark, please register this software. HVDC LIGHT:

commutate against. By the use of high

HVDC light is an electric power transmission

technology

based

switching frequency components e.g.

on

IGBT’s it is possible to use PWM. Thus

voltage source converters with pulse

it is possible to create any phase angle or

width modulation and modern HVDC

amplitude by changing PWM pattern,

cables. The advantage of hvdc light

and it offers the possibility to control

includes that the flow of energy over the

both

link can be

independently.

precisely defined and

active

and

reactive

power

controlled, so that the capacity rights required for fully commercial network

In

service are readily defined. Its suitability

schemes described below, the switching

is based on accurate control of the

of the IGBT valves follows a pulse

transmitted

and

width modulation (PWM) pattern. This

independent control of the reactive

switching control allows simultaneous

power in the connected AC networks.

adjustment of the amplitude and phase

HVDC light is designed at standard units

angle of the converter AC output voltage

between 10 and 300MW.Through out

with constant dc.

active

power

the

HVDC

Light

transmission

the world this technology is used in four countries(Sweden, Denmark, Australia and US).Key to success for this light projects is that the speed and accuracy of both active and reactive power control. Voltage source converters with PWM: In industrial drives the PCC (phase commutated

converter)

technology, which is used in Hvdc is now almost totally replaced by VSC (voltage source converter) technology. The advantage in vsc is that the current can

be

switched

off

and

on

by

controlling semi -conductor valves. Thus there is no need for a network to

Fig : PWM pattern and the fundamental frequency.

DocumentToPDF trial version, to remove this mark, please register this software. Voltage in a Voltage Source

frequency controller.The active power

Converter:

exchange

The combined active and reactive power capabilities are most easily seen

with

the ac network

is

controlled primarily by the phase angle of the voltage created by vsc.The active power balance on the dc side needs to be preserved, and therefore one terminal is allocated to control the direct voltage. While the other controls the active power exchanged at its ac network

The stations are compact and need little

connection.On theGotland network the

space; a 65MVA station occupies an

introduction of an hvdc light links

area of approx.800sq.m. Here power is

permits the active power flow in the

transmitted via a pair of underground

network to be controlled. Continuous

cables there is no visual impact along the

calculation is considering both reactive

transmission.

compensation and active power flow and the optimization are made for the losses

HVDC light in the network:

in the entire Gotland system. This makes

The hvdc light links in operation have a

the hvdc light a very powerful tool for

common basic control design and then

power flow control for the Gotland

each one of them has its specific control

system.

features to act in its specific network

Important advantage of hvdc light:-

environment. There by each one of them

That the flow of energy over the link can

can give sample support to the connected

be precisely defined and controlled, so

ac networks.

that the capacity rights required for fully

(1)Active power:-

commercial network service are readily

An hvdc light transmission can control

defined.

the active power transmission in an exact

(2)Reactive power:

way, so that the contracted can be

The hvdc light converters can

delivered when required. The power

provide reactive power and combined

transmission can be combined with a

with a master controller, provide ac

frequency controller that varies the

voltage

power in order to support the network

connected to the converter stations. An

control

to

the

networks

DocumentToPDF trial version, to remove this mark, please register this software. ac voltage controller can also be used for

parameters and control the power flow

improving

by

through the interconnection. Fig. 5

inclusion of flicker control. The reactive

shows the principal configurations of

power exchange with the ac network is

FACTS devices. Main shunt connected

controlled primarily by the magnitude of

FACTS application is the Static Var

the voltage created by vsc. Because of

Compensator

the controllability of the reactive power,

commutated thyristor technology. A

the ac harmonic filters are not used. In

further development is STATCOM using

the Gotland HVDC light transmission

voltage source converters. Both devices

the reactive capabilities are used to

provide fast voltage control, reactive

control the ac voltages of the networks

power control and power oscillation

connected to the converter stations.

damping features. As an option, SVC

the

power

quality

(SVC)

with

line-

can control unbalanced system voltages HVDC Light is proved to be the most economical and efficient among the power transmission

technology

systems.Development environmentally

of

friendly generation

sources is made easier. In this manner Power

Transmissions

are

made

convenient and useful.

The main idea of FACTS and HVDC can be explained by the basic equation

for

transmission

.

Power

transmitted between two nodes in the systems depends on voltages at both ends

of

the

interconnection,

the

impedance of the line and the angle

Fig.5: Basic configurations of FACTS

difference

devices

between

both

systems.

Different FACTS devices can actively influence

one

ore

more

of

these

DocumentToPDF trial version, to remove this mark, please register this software. Ø HVDC can carry more power for a given

Advantages of HVDC Systems:

size of conductor. Modern HVDC system combines the

Ø The need for ROW (Right Of Way) is

good experience of the old installations

much smaller for HVDC than for

with recently developed technologies

HVAC, for the same

and materials. The result is a very

power. The environmental impact is

competitive, flexible and efficient way

smaller with HVDC.

of transmitting electrical energy with a

Ø VSC

technology

allows

controlling

very low environmental impact. It is

active

important to remark that an HVDC

independently without any needs for

system not

extra compensating equipment.

only transmit

electrical

power from one point to another, but it

Ø VSC

and

transmitted

reactive

technology

gives

also has a lot of value added which

opportunity to

should have been necessary to solve by

sources to

another means in the case of using a

technically efficient. Ø HVDC

conventional AC transmission.

power

a

good

alternative energy

be economically and

transmissions

have

a

high

availability and reliability rate, shown Some of these aspects are: Ø

by more than 30 years of operation.

No limits in transmitted distance. This is valid for both OH lines and sea or underground cables.?c Very fast control of power flow, which implies stability improvements,

not

only for

the

HVDC

but

also

the

link

for

surrounding AC system. Ø Direction of power flow can be changed very quickly (bi-directionality). Ø An HVDC link don’t increase the shortcircuit power in the connecting point. This means that it will not be necessary

to

change

the

circuit

breakers in the existing network.

DocumentToPDF trial version, to remove this mark, please register this software. Conclusion:-

interconnections have to be improved by new investments, including the use of

Power systems develop on line with the

Power Electronics like HVDC, FACTS

increasing demand on energy. With

and

time, large interconnected systems came

Further developments in the future will

into existence. System interconnections

be also influenced by the liberalization

offer

of power industry.

technical

and

economical

other

advanced

technologies.

advantages. These advantages are high when

medium

interconnected.

sized

systems

However,when

are

References:-

using

synchronous AC interconnection, the

• Electrical India Magazine,

advantages diminish with an increasing

• IEEMA Magazine,

size of the systems to be interconnected

• EHVAC, HVDC Transmission

and on the other hand, the costs to adjust

and distribution systems by

the

Gupta, Electrical India

AC

systems

for

synchronous

operation increase.In addition, to avoid large

cascading

transmission

system

systems

and

outages, system

Magazine,

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