HVDC Lecture 2

7/28/2019 HVDC Lecture 2

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HVDC

Lecture 2

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HVDC System Configurations and

Components


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HVDC links can be broadly classified into:

Monopolar links

Bipolar links Homopolar links

Back-to-back links

Multiterminal links


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Monopolar Links

It uses one conductor, usually of negative polarity to reduce corona effect inDC lines

The return path is provided by ground or water

Use of this system is mainly due to cost considerations

A metallic return may be used where earth resistivity is too high

This configuration type is the first step towards a bipolar link


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Bipolar Links

This is the most commonly used conguration of HVDC power transmission

Systems.It uses two conductors, one positive and the other negative

Each terminal has two converters of equal rated voltage, connected in serieson the DC side

The junctions between the converters is grounded

Currents in the two poles are equal and there is no ground current

If one pole is isolated due to fault, the other pole can operate with groundand carry half the rated load (or more using overload capabilities of itsconverter line)


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Homopolar Links

It has two or more conductors all having the same polarity, usually negative.

Since the corona effect in DC transmission lines is less for negative polarity,

homopolar link is usually operated with negative polarity.

The return path for such a system is through ground.

Homopolar link have the disadvantage of earth return which is undesirable.


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This is the common configuration for connecting two adjacentasynchronous AC systems.

Two converter stations are located at the same site and transmission line

or cable is not needed.

The two AC systems interconnected may have the same or different

nominal frequencies, i.e. 50Hz and 60Hz. Examples of such system

configurationcan be found in Japan and South America .

Back-to-back links


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When three or more HVDC substations are geographically separated with

interconnecting transmission lines or cables, the HVDC transmission

system is multi-terminal.

If all substations are connected to the same voltage then the system is

parallel multi-terminal dc.

Multi-terminal links


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If one or more converter bridges are added in series in one or both poles, then the

system is series multi-terminal dc .

A combination of parallel and series connections of converter bridges is a hybrid

multi-terminal system.

Multi-terminal dc systems are more difficult to justify economically because of

the cost of the additional substations.

Examples of multi-terminal HVDC were implemented in the connection

Sardinia-Corsica-Italy (SACOI), the Pacific Intertie in the US and the connection

Hydro Quebec-New England Hydro from Canada to the US.


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Configuration of classic HVDC systems

A classic HVDC system operating in bipolar mode, consists of ac filters, shunt

capacitor banks or other reactive-compensation equipment, convertertransformers, converters, dc reactors, dc filters, and dc lines or cables


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Converters

The HVDC converters are the most important part of an HVDC system. They perform the conversion from AC to DC (rectifier) at the sending end

and from DC to AC (inverter) at the receiving end.

HVDC converters are connected to the AC system through transformers.

The classic HVDC converters are current source converters (CSCs) with line-

commutated thyristor switches.A six pulse valve bridge is the basic converter unit of classic HVDC for both

conversions, i.e. rectification and inversion.

A twelve pulse converter bridge can be built by connecting two six pulse

bridges.

The bridges are then connected separately to the AC system throughtransformers, one with Y-Y winding structure and the other with Y- winding

structure. In this way the 5th and 7th harmonic currents through the two

transformers are in opposite phase. This significantly reduces the distortion in

the AC system caused by the HVDC converters.


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Transformers

The transformers connect the AC network to the valve bridges and adjust theAC voltage level to a suitable level for the converters.

The transformers can be of different design depending on the power to be

transmitted and possible transport requirements .

AC-side harmonic filters

The HVDC converters produce harmonic currents on the AC side and the

harmonic currents entering into the connected AC network are limited by AC

filters.

For instance, AC filters installed on the AC side of a 12-pulse HVDC converterare tuned such that the 11th, 13th, 23th and 25th harmonic currents are limited.

In the conversion process the converters consume reactive power which is

partly compensated in the filter banks and the rest is provided by capacitor banks

or any compensation device.


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DC filters

The HVDC converters produce ripple on the DC voltage.

Voltage ripple causes the interference to telephone circuits near the DC line.

DC filters can be used to reduce the ripple.

Usually no DC filters are needed for pure cable transmission nor for back-to-

back HVDC stations.

However, it is necessary to install DC filters if overhead lines are used in the

transmission system.

The filter types used on the DC side are tuned filters and active DC filters.


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HVDC cables or overhead lines

HVDC cables are normally used for submarine transmission. No seriouslength limitation exists for HVDC cables.

For a back-to-back HVDC system no DC cable or overhead line is needed.

For connections over land overhead lines are typically used. However, dueto environmental concerns the tendency is to also use cables for connections

over land.


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Configuration of VSC-HVDC

A typical VSC-HVDC system, consists of AC filters, transformers,converters, phase reactors, DC capacitors and DC cables.

ConvertersThe converters are VSCs employing IGBT or GTO power semiconductors,

one operating as a rectifier and the other as an inverter.

The two converters are connected either back-to-back or through a DC

cable, depending on the application.


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Transformers

Normally, the converters are connected to the AC system via transformers.

The most important function of the transformers is to transform the voltage of

the AC system to a level suitable for the converter.

Phase reactorsThe phase reactors are used for controlling both the active and the reactive

power flow by regulating currents through them.

The reactors also function as AC filters to reduce the high frequency harmoniccontents of the AC currents which are caused by the switching operation of the

VSCs.


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AC filters

The AC voltage output contains harmonic components, caused by theswitching of the IGBTs or GTOs.

The harmonics emitted into the AC system have to be limited to prevent them

from causing malfunction of AC system equipment or radio and

telecommunication disturbances.

High-pass filter branches are installed to mitigate these high order harmonics.

With VSC converters there is no need to compensate any reactive power and

the current harmonics on the AC side are related directly to the PWM frequency.

Therefore, the amount of filters in this type of converters is reduced as

compared with line commutated converters.


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DC capacitors

On the DC side there are two capacitor stacks with the same size.

The size of these capacitors depends on the required DC voltage.

The objective of the DC capacitors is to provide an energy buffer to keep

the power balance during transients and reduce the voltage ripple on the DC

side.

DC cables

The cable used in the VSC-HVDC applications is a new developed type where

the insulation is made of an extruded polymer that is particularly resistant to

DC voltage.

Polymeric cables are the preferred choice for HVDC mainly because of their

mechanical strength, flexibility and low weight .

Documents

HVDC Lecture 2