Introduction to HVDC

UNIT-1
HVDC System Configuration and
Components

Transmission System
An efficient transmission system has to meet
the following requirements:
• Bulk power transmission over long distances,
• Low transmission losses.
• Less voltage fluctuations.
• Possibility of power transfer through
submarine cables.
• System of interconnection.

Transmission System
• Up to the 1980s, ultra high voltage ac (UHV-
AC) transmission lines above 765 kV were
used for bulk power transmission, and due to
the development of accurate control in
thyristor, the HVDC (high voltage direct
current) transmission lines are using which are
having a distinct superiority over UHV-AC
transmission lines.

What is High Voltage DC (HVDC)
Transmission System?
• The High Voltage Direct Current (HVDC)
transmission system uses direct current for the
transmission of power over long distances.
• The HVDC transmission system provides efficient
and economic transmission of power even to very
long distances that meet the requirements of
growing load demands.
• Due to its simple constructional feature and less
complexity, research and development authority
discovered its usage in modern power
transmission.

Principle of HVDC Transmission
• The HVDC transmission system mainly consists of
converter stations where conversions from ac to dc
(rectifier station) are performed at sending end and at
the receiving end the dc power is inverted into ac
power using an inverter station. Hence, the converter
stations are the major component of the HVDC
transmission system.
• Also, by changing the role of the rectifier to inverter
and inverter to rectifier the power transfer can be
reversed which can be achieved by suitable converter
control. The below shows the schematic diagram of the
HVDC transmission system.

Principle of HVDC Transmission
• The ac substations at both ends of the HVDC line
consist of ac switchgear, bus bars, current
transformers, voltage transformers, etc.
• The converter transformers are connected
between converter values and ac bus valves
which transfers power from ac to dc or vice-versa.
Smoothing reactors are necessary for converter
operation, and for smoothing the dc current by
reducing ripples obtained on the dc line.
• The electrode line connects the midpoint of
converters with a distant earth electrode.

Comparison Between HVDC and HVAC
System
HVDC Transmission System
• It is economical for
transmission of power
above break-even point i.e.,
for long distances.
• The number of conductors
required for transmitting
power is less.
• Does not require any
intermediate substations for
reactive power
compensation.
HVAC Transmission System
• It is economical for
transmission of power
below break-even point i.e.,
for small distances.
• The number of conductors
required for transmitting
power is more.
• Requires intermediate
substations for
compensation.

Comparison Between HVDC and
HVAC System
• Very fast and accurate power
flow control is possible.
• Skin effect is absent resulting
in uniform distribution of
current density across the
cross-section of the conductor.
• Corona loss and radio
interferences are absent
resulting in less insulation level
required for the transmission
line.
• Power flow control is slow and
I
• Skin effect is present due to
which current density is non-
uniformly distributed across
the cross-section f the
conductor. s very difficult.
• Corona loss and radio
interferences are more due to
which high insulation level is
required for the transmission
line.

HVAC System
• Voltage in the line does not
fluctuate with the load.
• Does not require a double
circuit, in this systems earth
return is used.
• Transmission through
underground or marine is
also possible.
• Transmission losses are less
due to the absence of flow
of reactive power.
• Voltage in the line
fluctuates with the load.
• Always requires a double
circuit.
• Limit is imposed on the
length of the cable.
• Transmission losses are
more due to the flow of
reactive power.

HVAC System
• The fault levels of the two
networks are unaffected and
remain unchanged when
interconnected.
• The cost of right of way is less
and the cost of supporting
towers is less, as this system
requires narrow towers.
• DC breakers used in this
system are of high cost, as it is
difficult to break dc currents.
• Fault levels of two networks
get added up and are
increased after the
interconnection.
• The cost of right of way is
more and the cost of
supporting tower is more as
this system requires lattice-
shaped towers.
• The circuit breakers used in
this system are of low cost
when compared to dc
breakers.

Components of an HVDC Transmission
System
• The essential components in a HVDC
transmission system are 6/12/24 pulse
converters, converter transformer with
suitable ratio and tap changing, filters at both
DC and AC side, smoothening reactor in DC
side, shunt capacitors and DC transmission
lines.

Converter Unit
• HVDC transmission requires a converter at each end of the
line. The sending end converter acts as a rectifier which
converts AC power to DC power and the receiving end
converter acts as an inverter which converts DC power to
AC power.
• This unit usually consists of two three phase converter
which are connected in series to form a 12 pulse converter.
The converter consists of 12 thyristor valves and these
valves can be packaged as single valve or double valve or
quadri valve arrangements.
• Due to the evaluation of power electronic devices, the
thyristor valves have been replaced by high power handling
devices such as gate turn-off thyristors (GTOs), IGBTs and
light triggered thyristors.

Converter Transformers
• The transformers used before the rectification of AC in HVDC
system are called as converter transformers. The different
configurations of the converter transformer include three phase-
two winding, single phase- three winding and single phase- two
winding transformers.
• The valve side windings of transformers are connected in star and
delta with ungrounded neutral and the AC supply side windings are
connected in parallel with grounded neutral.
• The design of the control transformer is somewhat different from
the one used in AC systems . These are designed to withstand DC
voltage stresses and increased eddy current losses due to harmonic
currents.
• The content of harmonics in a converter transformer is much higher
compared to conventional transformer which causes additional
leakage flux and it results to the formation of local hotspots in
windings. To avoid these hotspots, suitable magnetic shunts and
effective cooling arrangements are required.

Filters
• Due to the repetitive firing of thyristors,
harmonics are generated in the HVDC system.
These harmonics are transmitted to the AC
network and led to the overheating of the
equipment and also interference with the
communication system.
• In order to reduce the harmonics, filters and
filtering techniques are used. Types of filters
include: AC filter, DC filter and High frequency
filter

AC filters
• These are made with passive components and they provide low
impedance and shunt paths for AC harmonic currents. Tuned as
well as damped filter arrangements are generally used in HVDC
system.
DC filters
• Similar to AC filters, these are also used for filtering the harmonics.
Filters used at DC end, usually smaller and less expensive than
filters used in AC side. The modern DC filters are of active type in
which passive part is reduced to a minimum.
• Specially designed DC filters are used in HVDC transmission lines in
order to reduce the disturbances caused in telecommunication
systems due to harmonics.
High frequency filters
• These are provided to suppress the high frequency currents and are
connected between converter transformer and the station AC bus.
Sometimes these are connected between DC filter and DC line and
also on the neutral side.

Reactive Power Supplies (Shunt
capacitors)
• Due to the delay in the firing angle of the
converter station, reactive volt-amperes are
generated in the process of conversion. Since
the DC system does not require or generate
any reactive power, this must be suitably
compensated by using shunt capacitors
connecting at both ends of the system.

Smoothening reactor
These are large reactors having high inductance as high
as 1 H connected in series with each pole of converter
station. It can be connected on the line side, neutral
side or at an intermediate location They serve the
following purpose:
• Decrease harmonic voltages and currents in DC line.
• Prevent commutation failure in inverters.
• Prevent current from being discontinuous at light load.
• Limit the crest current in the rectifier during the short
circuit in DC line.

Transmission medium or lines or
cables
• Overhead lines act as a most frequent transmission
medium for bulk power transmission over land. Two
conductors with different polarity are used in HVDC
systems to transfer the power from sending end to
receiving end.
• The size of the conductors required in DC transmission is
small for the same power handling capacity to that of AC
transmission. Due to the absence of frequency, there is no
skin effect in the conductors.
• High voltage DC cables are used in case of submarine
transmission. Most of such cables are of an oil filled type.
Its insulation consists of paper tapes impregnated with high
viscosity oil.

DC and AC switchgear
• The switchgear equipment provides the
protection to the entire HVDC system from
various electrical faults and also gives the
metering indication. The switchgear
equipments include isolator switches,
lightening arrestors, DC breakers, AC breakers,
etc.

Types of HVDC Transmission Systems
The HVDC transmission systems are mainly classified into the
following types on the basis of arrangement of the pole (line) and
earth return. They are:
• Mono-polar HVDC System - An HVDC system having only one pole
and earth return.
• Bipolar HVDC System - An HVDC system with two poles of opposite
polarity.
• Homo-polar HVDC System - It has two poles of the same polarity
and earth return.
• Back to Back HVDC Coupling System - It has no dc transmission line.
The rectification and inversion are taken place at the same
substation by a back-to-back converter.
• Multi-Terminal HVDC Systems - It has three or more terminal
substations.

Mono-polar HVDC System
• An HVDC link that uses only a single conductor is
known as a mono-polar link.
• Usually, in this type of link, only a single conductor
with negative polarity is used, in order to reduce
corona and interference.
• Earth or water is used as the return path. However, a
metallic conductor is used as a return path when earth
resistivity is very high.
• The power and current flows only in one direction.
• For mono-polar transmission systems, the rated
current is from 200A to 1000A. The below figure
represents the mono-polar HVDC link.

Advantages and Disadvantages of
Mono-polar Link
Advantages of Mono-polar Link :
• It uses only a single conductor. Hence, the design is very simple.
• It requires less maintenance.
• Because of high charging currents, these links are technically
feasible than HVAC systems.
• It is economical.
Disadvantages of Mono-polar Link :
• When a fault occurs on the conductor the entire transmission
system is shut down.
• These are used only for low-power rating links, like cable
transmission.
• It affects the magnetic compasses of ships when it passes over
underwater cables.

Bipolar HVDC Transmission System
• An HVDC link that uses two conductors for
transmitting the power and current is known
as bipolar links. Generally, these type of
systems uses two conductors. One with
positive polarity and the other with negative
polarity.

Bipolar HVDC Transmission System
• Under normal conditions, the current in the two poles
is the same. Hence, the ground current is absent.
Whenever a fault occurs on these systems then they
automatically switch to the mono-polar system by
using earth as a return path conductor i.e., when one
pole undergoes fault condition, the other will continue
to supply the load.
• A single bipolar high voltage direct current line is equal
to two ac transmission lines.
• When compared to the mono-polar link the voltage is
twice between the poles in this system. The mid-point
of the converters are grounded.

Advantages of Bipolar HVDC Link
• The transmission of power between two stations
or on the mainline is continuous.
• The fault on one link does not affect the
operation of another link.
• During fault conditions, this link can also be used
as the monopolar link.
• The direction of power flow can be changed by
changing the polarities of the two poles.
• The voltage in the bipolar link is twice between
the poles when compared to the voltage between
the pole and the earth of a monopolar link.

Disadvantages of Bipolar HVDC Link
• Corona and radio interference is more when
compared with a homo-polar link.
• The connection of a converter to a pole is
complicated.
• It is quite costly when compared to mono-
polar links.

Homo-polar HVDC Transmission
System
• These links also use two conductors but of the
same polarity, usually of negative polarity.
• When a fault occurs on the conductor the
converters of the healthy pole are quite enough
to feed the remaining conductors, Which are able
to supply more than 50% of the power.
• In this type of link, the earth is used as a return
conductor.
• It also acts as a mono-polar link during faulty
conditions.

Advantages of Homo-polar HVDC Link
• It is comparatively cheaper than a three-phase
ac line of the same ratings.
• Corona and radio interference are greatly
reduced with the use of negative polarity
conductors.
• These links can be operated independently
under faulty conditions.
• The connection of the converter to the pole is
not so complicated as the bipolar link.

Disadvantages of Homo-polar HVDC
Link
• The presence of ground current may have an
adverse effect on the pipelines passing
through the nearby areas.
• It has limited applications due to the presence
of ground currents.
• The cost of the line increases for higher
voltages.

Back to Back HVDC Coupling System
• It has no dc transmission line. Rectification and
Inversion are done in the same substation by a
back-to-back converter. The figure below shows
the back-to-back HVDC coupling.
For example, the Vindhyanchal back-to-back system in India, which has a capacity
of 250MW is capable of transmitting and receiving power in between Uttar
Pradesh and Madhya Pradesh power grids i.e., from the northern region to the
western region.

• The back-to-back HVDC coupling is mainly used to
interconnect two ac networks operating at different
frequencies. It also provides features like improving system
stability, rapid variations in the power exchange, and
control over the magnitude of voltage and frequency
independently in two networks.

• The dc voltage between two converters can be freely
selected because of the short length of the conductor.
• A back-to-back system gives or provides more stability
for the system.
• The power can be transmitted very fast and accurately.
• The power flow can be controlled in a system by
controlling the magnitude and direction of power in a
network.
• By using these types of systems the power can be
transmitted from one station to another station or it
can be received from the other terminals i.e., these
systems possess the ability to receive or transmit
power from the same station itself.

Advantages of Back to Back HVDC
System
• The voltage and frequency can be controlled
independently in two networks.
• The power flow is fast, accurate, and fully
controllable.
• We can determine the power flow in the link.
• Short circuit levels can be limited.
• Coupling of two networks at different
frequencies.
• Daily and seasonal costs can be determined.

Disadvantages of Back to Back HVDC
System
• Harmonics are generated.
• These systems are very expensive because of
complicated converters and dc switchgear.
• When the system is nearer to the sea coast,
water gets contaminated with insulators.

Multi-Terminal HVDC System
• A multi-terminal HVDC system consists of three or more
converter substations in which some of the converter
stations act as the rectifiers and some of them as the
inverters. The substations are either connected in series or
parallel according to the requirements. The below shows
the bipolar multi-terminal HVDC system.

Multi-Terminal HVDC System
• The multi-terminal HVDC system configuration consists
transmission line and more than two converters
connected in parallel or in sequential. In this multi-
terminal HVDC configuration, the power is transmitting
between two or more AC substations and the
frequency conversion is possible in this configuration.

Advantages of HVDC Transmission
System
• The HVDC transmission requires narrow towers, whereas ac systems require lattice
shape towers, this makes the construction simple and reduces cost.
• The ground can be used as the return conductor.
• No charging current, since dc operates at unity power factor.
• Due to less corona and radio interference, it results in an economic choice of the
conductor.
• Since there is no skin effect in dc transmission the power losses are reduced
considerably.
• Large or bulk power can be transmitted over long distances.
• Synchronous operation is not required.
• Low short-circuit current on dc line.
• Tie-line power can be easily controlled.
• Power transmission can be also possible between unsynchronised ac distribution
systems (interconnection of ac systems of different frequencies).
• Cables can be worked at a high voltage gradient, which makes them more suitable
for undersea cables.
• Power flow through the HVDC line can be quickly controlled.

Disadvantages of HVDC Transmission
System
• It is very difficult to break the dc currents hence it requires a
high cost of dc circuit breakers.
• It is not possible to use transformers to change the voltage
levels.
• Due to the generation of harmonics in converters, it requires ac
& dc filters, hence the cost of converting station is increased.
• It requires continuous firing or triggering thyristor valves hence
is it is complex.
• Converters have little overload capability.
• HVDC substations have an additional loss at converter
transformers and valves. These losses are continuous.

Applications of HVDC Transmission
System
• Long-distance bulk power HVDC transmission by
overhead lines.
• Underground or underwater cables.
• Interconnection of ac systems operating at
different frequencies.Back-to-back HVDC coupling
stations.
• MTDC asynchronous interconnection between 3
or more ac networks.
• Control and stabilization of power flow in ac
interconnection of large interconnected systems.

Application1:
Interconnection of two AC systems
• DC link is an economical option than the AC link to
interconnect two AC systems. This system is more
effective, efficient and technically superior compared
to the AC link.
• The biggest advantage is the there is no effect of
frequency in the DC link. And the frequency
disturbance of one system does not transfer to other
systems.
• It does not affect the transient stability and there is no
change in the short circuit levels of both the systems.
• The direction of power flow maintains properly
through the DC link.

Application2:
Long-distance power transmission line
• This is the main purpose to use the HVDC system. Because in the HVAC
system, the length of a line is the biggest constraint. The length of the line
cannot more than a certain length to keep control of the thermal effect of
the conductor. And it needs an intermediate substation every 300 km of
line.
• But this problem solved by the use of the HVDC line. In the HVDC line, the
generated AC power is stepped up by the transformer. The high voltage AC
converted in High voltage DC with the help of a rectifier at sending end of
the line.
• Power transmitted to long-distance with the help of the HVDC line. To
transmit more power bipolar HVDC system used.
• At the receiving end, high voltage DC power converted into high voltage
AC with the help of an inverter.
• The HVDC line is economical only for long-distance. The breakeven point is
at 800 km. hence, at 800 km of line, the HVDC line is more economical
than the HVAC line.
• And there is no need to build an intermediate substation in between of
lines irrespective the length of a line.
• The cost of the tower and the conductor is less in the HVDC line.

Application3:
Multi-terminal HVDC interconnection
• The frequency does not affect in DC system.
Therefore, if the frequency is not the same,
then also these systems can connect with the
HVDC link.
• Three or more AC systems can be
interconnected asynchronous using a multi-
terminal HVDC system. Due to this, bulk
power can be transferred.

Application4:
Parallel AC and DC link
• DC link operates with the parallel to the existing
AC line. In this way, more amount of power can
transmit.
• Due to this, there is a decrease in the fault level
and an increase in the stability of the system.

Application5:
Underground or submarine cable transmission
• In the AC system, it is difficult to transmit power
through underground cable or submarine cable
because of the temperature rise due to the
charging current.
• This will limit the length of the line. But this
problem solved in the HVDC line as an absence of
the charging current.
• Therefore, it is easy to implement the
underground and submarine cable with the HVDC
transmission line.

Application6:
Back to back asynchronous tie station
• If two tie lines have different frequencies than it tends to not
possible to interconnect. Therefore, back to back asynchronous tie
station becomes very useful for the interconnection of two AC
systems which has different frequencies.
• For example. One tie line has 50 Hz and the other has 60 Hz
frequency. (Generally, is much of frequency difference will not
create in the same country. But tie lines which connect the different
countries which use different frequencies.) The interconnection of
these tie lines can be done by the HVDC system. This cannot be
done by the AC system.
• The converter substation used to connect two asynchronous AC
systems. There is no DC transmission line used.
• The two AC lines connected through back to back converters. Power
flow can easily control from one system to other systems.
• Smoothing reactor, filters and converter transformer used in this
station.

Analysis of HVDC Converters
• Introduction:
• HVDC converters converts AC to DC and transfer the DC
power, then DC is again converted to AC by using inverter
station.
• HVDC system mainly consists of two stations, one in
rectifier station which transfers from AC to DC network and
other is inverter station which transfers from DC to AC
network.
• For all HVDC converters twelve pulse bridge converters are
used. Same converter can act as both rectifier as well as an
inverter depending on the firing angle ‘α’.
• If firing angle α is less than 90 degrees the converter acts in
rectifier mode and if the firing angle α is greater than 90
degrees the converter acts in inverter mode.

Choice of Converter configuration

• For a given pulse number select the configuration such a
way that both the valve and transformer utilization are
minimized.
• In general converter configuration can be selected by the
basic commutation group and the no. of such groups
connected in series and parallel.
• Commutation group means set of valves in which only one
valve conducts at a time.
Let ‘q’ be the no of valves in a commutation group,
‘r’ be the no of parallel connections,
‘s’ be the no of series connections, then
the total no of valves will be = qrs

• Valve Voltage Rating:
• Valve voltage rating is specified in terms of peak inverse voltage
(PIV) it can withstand.
• The valve utilization is the ratio of PIV to average dc voltage.
• Converter average DC voltage is

i) Peak inverse voltage(PIV):
• If q is even:
then the maximum inverse voltage occurs when the
valve with a phase displacement of π radian in
conducting and this is given by PIV = 2Vm
• If q is odd:
the maximum inverse voltage occurs when the
valve with a phase shift of π+π/q in conducting
and this is given by PIV = 2Vm Cosπ/2q

Analysis of Graetz circuit (6-pulse
converter bridge)

• This Graetz circuit utilizes the transformer and the
converter unit to at most level and it maintains low voltage
across the valve when not in conduction.
• Due to this quality present in Graetz circuit, it dominates all
other alternative circuits from being implemented in HVDC
converter.
• The low voltage across the valves is nothing but the peak
inverse voltage which the valve should withstand.
• The six-pulse Graetz circuit consists of 6 valves arranged in
bridge type and the converter transformer having tapings
on the AC side for voltage control.
• AC supply is given for the three winding of the converter
transformer connected in star with grounded neutral.
• The windings on the valve side are either connected in star
or delta with ungrounded neutral.
• The six valves of the circuit are fired in a definite and fixed
order and the DC output obtained contains six DC pulses
per one cycle of AC voltage wave.

Operation without overlap
• The six pulse converter without over lapping
valve construction sequence are 1-2, 2-3, 3-4, 4-
5, 5-6, 6-1.
• At any instant two valves are conducting in the
bridge. One from the upper group and other from
the lower group.
• Each valve arm conducts for a period of one third
of half cycle i.e., 60 degrees.
• In one full cycle of AC supply there are six pulses
in the DC waveform. Hence the bridge is called as
six pulse converter.

Operation without overlap
For simple analysis following assumptions are much:
i) AC voltage at the converter input is sinusoidal and constant
ii) DC current is constant
iii) Valves are assumed as ideal switches with zero impedance
when on(conducting) and with infinite impedance when
off(not conducting)
In one full cycle of AC supply we will get 6-pulses in the
output. Each pair of the devices will conduct 60 degrees.
The dc output voltage waveform repeats every 60 degrees
interval. Therefore for calculation of average output
voltages only consider one pulse and multiply with six for
one full cycle. In this case each device will fire for 120 deg.

Firing angle delay
• Delay angle is the time required for firing the pulses in a converter for its
conduction.
• It is generally expressed in electrical degrees.
• In other words, it is the time between zero crossing of commutation
voltage and starting point of forward current conduction.
• The mean value of DC voltage can be reduced by decreasing the
conduction duration, which can be achieved by delaying the pulses ie., by
increasing the delay angle we can reduce the DC voltage and also the
power transmission form one valve to another valve can also be reduced.
• when α = 0, the commutation takes place naturally and the converter acts
as a rectifier.
• when α > 60 deg, the voltage with negative spikes appears and the
direction of power flow is from AC to DC system without change in
magnitude of current.
• when α = 90 deg, the negative and positive portions of the voltage are
equal and because of above fact, the DC voltage per cycle is zero. Hence
the energy transferred is zero.
• when α > 90 deg, the converter acts as an inverter and the flow of power
is from DC system to AC system.

• Let valve 3 is fired at an angle of α. The DC output
voltage is given by
From above equation we can say that if firing angle varies, the DC output
voltage varies

DC Voltage waveform
• The dc voltage waveform contains a ripple whose frequency is six
times the supply frequency.
• This can be analyzed in Fourier series and contains harmonics of the
order h=np , Where p is the pulse number and n is an integer.
• The r.m.s value of the hth order harmonic in dc voltage is given by

• Although α can vary from 0 to 180 degrees, the full
range cannot be utilized. In order to ensure the firing
of all the series connected thyristors, it is necessary to
provide a minimum limit of α greater than zero, say 5
deg.
• Also in order to allow for the turn off time of a valve, it
is necessary to provide an upper limit less than 180
deg.
• The delay angle α is not allowed to go beyond 180-γ
where γ is called the extinction angle (sometimes also
called the marginal angle).
• The minimum value of the extinction angle is typically
10 deg, although in normal operation as an inverter, it
is not allowed to go below 15deg or 18deg.

• AC current waveform:
• It is assumed that the direct current has no ripple (or harmonics)
because of the smoothing reactor provided in series with the bridge
circuit.
• The AC currents flowing through the valve (secondary) and primary
windings of the converter transformer contain harmonics. The
waveform of the current in a valve winding is shown in fig.

• By Fourier analysis, the peak value of a line
current of fundamental frequency component is
given by,

Now the RMS value of line current of fundamental
frequency component is given by
where I = Fundamental current
n = nth order harmonic.
The harmonics contained in the current waveform are of the order given by
h = np + 1
where n is an integer, p is the pulse number.
For a 6 pulse bridge converter, the order of AC harmonics are 5, 7, 11, 13 and higher
order.
They are filtered out by using tuned filters for each one of the first four harmonics and a
high pass filter for the rest.

Introduction to HVDC

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