transmitting power at high voltage and in dc form instead of ac is a new technology proven t

8/8/2019 Transmitting Power at High Voltage and in DC Form Instead of AC is a New Technology Proven t

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Transmitting power at high voltage and in DC form instead of AC is a new technology

proven to be economic and simple in operation which is HVDC transmission. The HVDC (High Voltage

Direct Current) technology is used to transmit electricity over long distances by overhead

transmission lines or submarine cables. It is also used to interconnect separate power systems. A

further development in this technology is HVDC light where HVDC light converters are used for

faster and efficient conversion of power.

HVDC Light® was introduced in 1997.HVDC Light is a fundamentally new power transmission

technology developed recently. It is particularly suitable for medium to small-scale power

transmission applications. This new transmission and distribution technology, HVDC Light provides

an important role to todays requirements on our network systems and opens up new opportunities

for both investors and environmentalist alike.

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HVDC light is a high voltage, direct current transmission technology I.E., Transmission up to 330MWand for dc voltage in the ± 150kv range.

It consists of two ac to dc converter stations and a pair of underground cables interconnecting each

converter station. The converter stations are designed to be unmanned andvirtually maintenance-

free.

This technology provides the hvdc light converter with a switching speed 27 times faster than a

traditional hvdc, thyristor controlled converter.HVDC light® is HVDC technology based on voltage

source converters (vscs). With extruded DC cables, power ratings from a few tens of megawatts up

to several hundreds of megawatts are available.

Slide-4

Layout of hvdc:

Here Is a layout 330 mw hvdc light converter station.in this there components are used .let see how

it works?.

Dc cable: dc underground cable which are entering from this section.

Dc filter:DC filter circuits, which are connected in parallel to the station poles, are an effective tool

for combating these effects.

The configuration of the DC filters very strongly resembles the filters on the AC side of the HVDCstation. There are several types of filter design. Single and multiple-tuned filters with or without the

high-pass feature are common. One or several types of DC filter can be utilized in a converter

station.



Therefore a switchyard will contain; current carrying conductors, grounding wires and switches,

transformers, disconnects, remotely controlled arc snuffing breakers, metering devices, etc.

Slide -5

Technical features:

Advantageous for long distance transmissions .

Power reversal without interruption .

Can start up dead ac network.

No increase of short circuit current.

Eqal and longer life then XLPE ac cable.

What is XLPE?Xlpe means (Cross-Linked Polyethelene)There are actually two semi-conductive layers

on high voltage cable. One is between the actual conductors and the XLPE. The other semi-con is on

outside of the XLPE insulation underneath the concentric neutral.

Slide -6

There are two types of transmission technique :

1. Classic HVDC

2. HVDC light

The classic HVDC technology is used to transmit electric power over long distances by overhead

transmission lines or submarine cables. It is also used to interconnect separate power systems,

where traditional alternating current (AC) connections can not be used.

HVDC Light® is an underground and submarine cable power transmission technology developed by

ABB, that offers additional benefits compared to HVDC Classic.

The Classic HVDC Transmission

A HVDC converter station uses thyristor valves to perform the conversion from AC to DC and vice

versa. The valves are normally arranged as a 12-pulse converter. The valves are connected to the AC

system by means of converter transformers. The valves are normally placed in a building and the

converter transformers are located just outside.

The 12-pulse HVDC converter produces current harmonics (11th, 13th, 23rd, 25th, 35th, 37th etc.)

on the AC side. These harmonics are prevented from entering into the connected AC network byAC

filters, i.e. resonant circuits comprising capacitors, inductances (reactors) and resistors. The filters

also produce a part of the reactive power consumed by the converter.

The HVDC converter also gives rise to voltage harmonics on the DC side (12th, 24th, 36th etc.). A



large inductance (smoothing reactor) is always installed on the DC side to reduce the ripple in the

direct current. In addition, a DC filter is also normally needed to reduce the level of harmonic

currents in the DC overhead line. The harmonics may otherwise cause interference to telephone

circuits in the vicinity of the DC line.

The power transmitted over the HVDC transmission is controlled by means of a control system. Itadjusts the triggering instants of the thyristor valves to obtain the desired combination of voltage

and current in the DC system.

Several other apparatus are needed in a converter station, such as circuit breakers,current and

voltage transducers, surge arresters, etc.

Smoothing reactor:HVDC smoothing reactors can be of air-core design as well as and oil-insulated

units.A DC reactor is normally connected in series with the converter. The main objectives of the

reactor are:

y To reduce the harmonic currents on the DC side of the converter.

y To reduce the risk of commutation failures by limiting the rate of rise of the DC line current

at transient disturbances in the AC or DC systems.

HVDC light

The key part of the HVDC Light converter consists of an IGBT valve bridge. No special converter

transformers are necessary between the valve bridge and the AC-grid. A converter reactor can

separate the fundamental frequency from the raw PWM waveform. If the desired DC voltage does

not match the AC system voltage, a normal AC transformer may be used in addition to the reactor. A

small shunt AC-filter is placed on the AC-side of the reactor. On the DC-side there is a DC capacitor

that serves as a DC filter too.

Active and reactive power control

The fundamental frequency voltage across the converter reactor defines the power flow between

the AC and DC sides. Changing the phase angle between the fundamental frequency voltage

generated by the converter (Ug) and the voltage on the AC bus controls the active power flow

between the converter and the network. The reactive power flow is determined by the amplitude of

Ug, which is controlled by the width of the pulses from the converter bridge.

The control is performed by the MACH2 system developed by ABB. All functions for control,

supervision and protection of the stations are implemented in software running in a family of

microprocessor circuit boards.

HVDC Light® is a state-of-the-art power system designed to transmit power underground and under

water, also over long distances. It offers numerous environmental benefits, including invisible

power lines, neutral electromagnetic fields, oil-free cables and compact converter stations.



HVDC Light® was introduced in 1997. A number of underground transmissions up to 350 MW are in

commercial operation and more are being built. The first commercial project delivery - Gotland

HVDC Light - turned 10 years in November 2009

Slide-9

Differences between HVDC Light and classic HVDC:

Power range

Classical HVDC is most cost effective in the high power range, above approximately some 250 MW.

HVDC Light, on the other hand, comes in unit sizes ranging from a few tens of MW. In the upper

range, the technology now reaches 1,200 MW and ±320 kV.

DC transmission circuit

The DC transmission circuit for classical HVDC can be an overhead line or a DC cable. The cables are

normally mass-impregnated (MIND) cables suitable for HVDC with a copper conductor and insulationmade of oil impregnated paper. The vast majority of classical DC cables are submarine cables.

The DC transmission circuit for HVDC Light is made by extruded polymer cables both for land

transmissions (underground) and across water (submarine). HVDC Light is by nature bipolar. The DC

circuit is not connected to ground. Therefore two cables are needed. HVDC Light can also be build

with over head lines - exemple Caprivi Link Interconnector

Converter station circuit

HVDC Light converter stations are Voltage Source Converters (VSCs) employing state of the art turn

on/turn off IGBT power semiconductors. (IGBT = Integrated Gate Bipolar Transistors.)

Active and reactive power control

Classic HVDC terminals can provide limited control of reactive power by means of switching of filters

and shunt banks and to some level by firing angle control. But this control requires additional

equipment and therefore extra cost.

The HVDC Light control makes it possible to create any phase angle or amplitude, which can be done

almost instantly. This offers the possibility to control both active and reactive power independently.

In fact the same converter can be used as a SVC and it is then called SVC Light. It is extremely

effective to eliminate flicker e.g. from arc furnaces.

The power is which is actually utilised in the circuit is

known as active power and Reactive power is the loss

occures in the system due to its inertia.



Slide -10

HVDC light cables:

There are two types of cable: The cable system is complete with cables, accessories and installation

services. The cables are operated in bipolar mode, one cable with positive polarity and one cable

with negative polarity. The cables have polymeric insulating material, which is very strong androbust.

Land cable and submarine cable

DC underground cables provided significant advantages ,compared with over head power lines.

The cables are operated in bipolar mode. One cable with positive polarity and one cable with

negative polarity

The strength andT he submarine cables can be laid in deeper waters and on rough bottoms.

The rough cables can be install less costly with ploughing technique . flexibility make the HVDC lightcables perfect for severe installation . With HVDC Light cables there is no need to get permits for an

overhead line. The cables are installed close in bipolar pairs with anti-parallel currents and thus

eliminating the magnetic fields.

Slide-11

Environmentally friendly:

y Magnetic fields are eliminated since HVDC light cables are laid in pairs with anti-parallel dc

currents.

y risk of oil spill, as in paper-oil-insulated cables, is eliminated.

y The cable insulation is PE(Polyethylene) and not dangerous.

y The cable metals can be reused

Power transmission via cables gives:

y No visual impact.

y No ground current

y No electromagnetically fields

Advantage:

y Reduced environmental impact.an underground cable has no visual impact on the

landscape.

y Faster and easier issue of permits using dc underground cable. underground cables rarely

meet public opposition and often receive political support.

y The installation of DC cable has no environmental impact ,the land can continue to be used

and there is virtually no magnetic radiation associated with bi-polar DC cable.



Slide -12

y The dc cable require only two cables between each converter station .

y Unlike AC cable, ehichgenrellay have technical limit of around 100 km due to the reactive

power and losses, DC cable have no technical limit at distance.

y DC cables can carry up to 50% more power than the equivalent AC cable.

y There is no need to install group of cables to achieved the required power rating.

y DC cables have longer life expectancy thanthan AC cable due to its lower operational stress

level.

In summary, when considering the cost of installing an HVDC Light underground transmission it is

important to consider the total life cost benefits and not just the initial up front capital costs.

Slide-14

Applications of HVDC Light

HVDC Light® has important advantages, such as undergound cables instead of overhead lines, short

delivery times, compact stations, controllability of power and voltages, possibility for multiteminal

operation, etc.

The fact that it is possible to build a long electric power transmission underground and avoid public

opposition and long uncertain approval processes, makes the HVDC Light system very attractive.

Possible application:

Connecting wind farms

Underground power links

Powering islands

Oil & gas offshore platforms; power from shore

Asynchronous grid connection

City centre in-feed

Slide-15

SOME HVDC light application

Underground power links

Connecting wind farms

Powering islands and remote loads



Interconnecting networks

City center infeed

Oil and gas offshore platforms

Multiterminal HVDC Light grid

Power system benefits:

An HVDC Light transmission can do much more than just transmit power between two points - it can

also improve conditions in the connected AC networks.

"The Pulse Width Modulated Voltage Source Converter is a close to ideal component in the

transmission network. From a system point of view it acts as a motor or generator without mass that

can control active and reactive power almost instantaneously."

In the congested power networks of today, where permission for new transmission lines are not

easy to get, building a HVDC Light® cable transmission may give more benefits than just the added

Megawatts that come over the DC cable. ABB would therefore want to present some of the power

system related advantages HVDC Light can offer.

Conclusion :

HVDC light is a new technology that has been specifically developed to match the requirements of

the new competitive electricity markets.

It provides the ability to connect renewable generation to the ac grid.

It provides accurate control of the transmitted active power and independent control of the reactive

power in the connected ac networks.

A pair of lightweight dc cables can be laid direct in the ground in a cost-effective way which is

comparable to or less than a corresponding total life cycle cost of ac overhead line.

Hällsjön, Sweden,1997 Hallsjon

-Granges berg(1st testing project)

Gotland HVDC Light,

Sweden,1999

-Nas

-backs (1st HVDC commercial

project)

Estlink, Estonia - Finland -Harku, Estonia

Tjaereborg HVDC Light, DenmarkTjaereborg substation

-TjaereborgEnge-Tjaereborg substation

transmitting power at high voltage and in dc form instead of ac is a new technology proven t

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