HVDC

Why High Voltage Direct Current ?

High Voltage Direct Current (HVDC) History

The transmission and distribution of

electrical energy started with direct

current. In 1882, a 50-km-long 2-kV DC

transmission line was built between

Miesbach and Munich in Germany.

At that time, conversion between

reasonable consumer voltages and

higher DC transmission voltages could

only be realized by means of rotating

DC machines.

In an AC system, voltage conversion is

simple. An AC transformer allows high

power levels and high insulation levels

within one unit, and has low losses. It is

a relatively simple device, which requires

little maintenance. Further, a three-phase

synchronous generator is superior to a

DC generator in every respect. For these

reasons, AC technology was introduced

at a very early stage in the development

of electrical power systems. It was soon

accepted as the only feasible technology

for generation, transmission and distribution

of electrical energy.

However, high-voltage AC transmission

links have disadvantages, which may

compel a change to DC technology:

• Inductive and capacitive elements of

overhead lines and cables put limits

to the transmission capacity and the

transmission distance of AC transmission

links.

• This limitation is of particular significance

for cables. Depending on the

required transmission capacity, the

system frequency and the loss evaluation,

the achievable transmission

distance for an AC cable will be in the

range of 40 to 100 km. It will mainly

be limited by the charging current.

• Direct connection between two AC

systems with different frequencies is

not possible.

• Direct connection between two AC

systems with the same frequency or

a new connection within a meshed

grid may be impossible because of

system instability, too high short-circuit

levels or undesirable power flow

scenarios.

Engineers were therefore engaged over

generations in the development of a

technology for DC transmissions as a

supplement to the AC transmissions.

1 Technical Merits of HVDC

The advantages of a DC link over an AC

link are:

• A DC link allows power transmission

between AC networks with different

frequencies or networks, which can

not be synchronized, for other reasons.

• Inductive and capacitive parameters

do not limit the transmission capacity

or the maximum length of a DC

overhead line or cable. The conductor

cross section is fully utilized because

there is no skin effect.

For a long cable connection, e.g. beyond

40 km, HVDC will in most cases offer

the only technical solution because of

the high charging current of an AC cable.

This is of particular interest for transmission

across open sea or into large

cities where a DC cable may provide the

only possible solution.

• A digital control system provides

accurate and fast control of the active

power flow.

• Fast modulation of DC transmission

power can be used to damp power

oscillations in an AC grid and thus

improve the system stability.

1.3

Economic Considerations

For a given transmission task, feasibility

studies are carried out before the final

decision on implementation of an HVAC

or HVDC system can be taken

considering:

• AC vs. DC station terminal costs

• AC vs. DC line costs

• AC vs. DC capitalised value of losses

The DC curve is not as steep as the AC

curve because of considerably lower line

costs per kilometre. For long AC lines

the cost of intermediate reactive power

compensation has to be taken into

account.

The break-even distance is in the range

of 500 to 800 km depending on a number

of other factors, like country-specific cost

elements, interest rates for project

financing, loss evaluation, cost of right

of way etc.

1.4

Environmental Issues

An HVDC transmission system is basically

environment-friendly because

improved energy transmission possibilities

contribute to a more efficient

utilization of existing power plants.

The land coverage and the associated

right-of-way cost for an HVDC overhead

transmission line is not as high as that

of an AC line. This reduces the visual

impact and saves land compensation for

new projects. It is also possible to increase

the power transmission capacity

for existing rights of way. A comparison

between a DC and an AC overhead line

is shown in Fig. 1-2.

There are, however, some environmental

issues which must be considered for the

converter stations. The most important

ones are:

• Audible noise

• Visual impact

• Electromagnetic compatibility

• Use of ground or sea return path

in monopolar operation

In general, it can be said that an HVDC

system is highly compatible with any

environment and can be integrated into

it without the need to compromise on

any environmentally important issues of

today