1. HIGH VOLTAGE DIRECT CURRENT
(HVDC) TRANSMISSION
Dr.J K Garg
M.Tech, PGDOM, ACDOM, CIC, PhD.
DGM (Retd.) BSES Rajdhani Power Ltd.
04|12|2020
2. The massive transmission of electricity in the
form of DC over long distances by means of
submarine cables or overhead transmission
line is the high voltage direct current
transmission. This type of transmission is
preferred over HVAC transmission for very
long distance when considering the cost,
losses and many other factors. The names
Electrical superhighway or Power
superhighway are often used for HVDC
3. HVDC TRANSMISSION SYSTEM
We know that AC power is generated in the
generating station. This should first be
converted into DC. The conversion is done
with the help of rectifier. The DC power will
flow through the overhead lines. At the user
end, this DC has to be converted into AC.
For that purpose, an inverter is placed at the
receiving end.
4. Thus, there will be a rectifier terminal
in one end of HVDC substation and an
inverter terminal in the other end. The
power of the sending end and user
end will be always equal (Input Power
= Output Power).
5.
6. When there are two converter stations at both ends and a
single transmission line is termed as two terminal DC systems.
When there are two or more converter stations and DC
transmission lines is termed as multi-terminal DC substation.
7. THE COMPONENTS OF THE HVDC
TRANSMISSION SYSTEM AND ITS FUNCTION
Converters: The AC to DC and DC to AC conversion
are done by the converters. It includes transformers
and valve bridges.
Smoothing Reactors: Each pole consist of smoothing
reactors which are of inductors connected in series
with the pole. It is used to avoid commutation failures
occurring in inverters, reduces harmonics and avoids
discontinuation of current for loads.
Electrodes: They are actually conductors which are
used to connect the system to the earth.
Harmonic Filters: It is used to minimize the
harmonics in voltage and current of the converters
used.
DC Lines: It can be cables or overhead lines.
8. Reactive Power Supplies: The reactive
power used by the converters could be
more than 50% of the total transferred
active power. So the shunt capacitors
provide this reactive power.
AC Circuit Breakers: The fault in the
transformer is cleared by the circuit
breakers. It also used to disconnect the DC
link
9. HVDC SYSTEM CONFIGURATIONS
The classification of HVDC links are as follows:
Mono Polar Links
Single conductor is required and water or ground act as
the return path. If the earth resistivity is high, metallic
return is used.
10. BIPOLAR LINKS
Double converters of same voltage rating are
used in each terminal. The converter junctions
are grounded.
11. HOMOPOLAR LINKS
It consists of more than two conductors which is
having equal polarity generally negative. Ground
is the return path.
12. COMPARISON OF BOTH HVAC AND
HVDC TRANSMISSION SYSTEM
HVDC Transmission System HVAC Transmission System
Low losses.
Losses are high due to the skin
effect and corona discharge
Better Voltage regulation and
Control ability.
Voltage regulation and Control
ability is low.
Transmit more power over a longer
distance.
Transmit less power compared to a
HVDC system.
Less insulation is needed. More insulation is required.
Reliability is high. Low Reliability.
Asynchronous interconnection is
possible.
Asynchronous interconnection is
not possible.
14. The transmission distance at which the overall
investment cost for HVAC start increasing than HVDC is
called Break-even Distance. This distance depends on
the type of transmission. The break-even distance
for overhead transmission is estimated at around 400 –
500 miles (600- 800 in kilometers) while the underwater
transmission is 20-50 Km & underground is 50-100
km. Therefore, the HVDC is a far more efficient &
economically cheaper choice for power transmission
over the break-even distance.
15. Advantages of the HVDC transmission system
1) Economical transmission of the bulk power
In a conventional transmission line, the distance cannot be more than the
breakeven distance. But in the HVDC transmission line, the distance can
be more than the breakeven distance.
Although, this system is more economical when the distance is more than
the breakeven distance. Because at this distance, the cost of conductors
and poles balanced.
2) Decrease in the number of conductors
In the HVAC system, the power transmitted in the form of three-phase AC
power. Therefore, three or four conductors need as per the type of
transmission line.
But in the case of HVDC transmission lines, only two conductors
required. Hence, the cost of the conductor decreased.
3) Corona
Corona effect appears in both HVAC and HVDC systems. But, in an
HVDC system, the effect of the corona is very less compared to the
HVAC system. And there is no disturbance to the nearby communication
line.
16. 4) Size of tower
In the HVDC transmission line, phase-phase and phase-
ground clearance required is less compared to the HVAC
line. Therefore, the height and width of the tower required is
less.
The number of conductors required in this system is less.
So, the size of the tower is less which results in less cost of
the tower.
5) Earth return
For the monopolar HVDC transmission system, earth return
can be used. That means, only one conductor required to
transmit the power. This is not possible in the HVAC
transmission line.
6) Charging current
In the DC transmission line, the capacitance is not produced
between two phases or between the phase and ground.
Therefore, the charging current is absent in the HVDC
system.
17. 7) Skin effect
The current density is uniform throughout the line. Hence,
there is no skin effect in the HVDC system.
And it utilizes an entire cross-section area of the
conductor. So, the resistance of the line is not increasing
and the power loss is less.
8) Reduction in line loss
The line loss reduced due to the absence of the reactive
power in the HVDC transmission line. This increases the
efficiency of the system.
9) Reduction in size of the conductor
When equal power transmitted for the same distance, less
volume of conductor required for the HVDC two-wire
system compared to the HVAC three-phase three-wire
system.
18. 10) Underground cable
The underground system can be established for the HVDC
system because of the absence of the charging current.
In the HVAC system, the distance of underground cables is
a constraint. For example, 145 kV line the distance is 60
km, for 245 kV it is 40 km and for 400 kV it is 25 km.
This constraint is not affected in the HVDC underground
system. So, it is possible to establish more distance
underground and marine lines for the HVDC system.
11) Reduction in the number of intermediate
substations
For compensation of reactive power, intermediate
substations required to be installed at 300 km in HVAC
lines. Because of the absence of reactive power, this is not
required in the HVDC line. Hence, the cost is reduced.
19. 12) Power factor
Generally, we are not considered a power factor in the case
of DC. Similarly, for the HVDC system also, the power factor
is not considering.
13) Stability and line loading
The line can be loaded up to its thermal limit or the thermal
limit of thyristors because of the absence of the transient.
While in the case of the AC transmission line, a transient is
present. Therefore, the line can be loaded up to one-third of
the thermal rating of the conductor.
14) Flexibility in operation
When a fault occurs in a bipolar HVDC system, the earth can
be used as a return path. So, the system will continue in
operation in case of a fault in one conductor.
But this is not possible in case of a three-phase AC
transmission line. Once fault occurred, the line will be going
in maintenance.
20. 15) Quick power transfer and control
The magnitude and direction of power flow in the
transmission line controlled by the converters. Due to this
the limit of transient stability can be increased.
16) Short circuit level
Parallel lines used to transmit the bulk power in an HVAC
system. When this system interconnected, there is an
increase in short circuit kVA of both the systems.
But if two systems interconnected with HVDC lines, the fault
level of each system remains the same.
17) Voltage regulation
Due to the change in load, the voltage of the AC
transmission line is changing. And for the long-distance line,
the voltage is change with the distance. This difficulty does
not arise in the HVDC transmission system with the control
of the rectifier and inverter.
21. 18) Asynchronous tie
The frequency is the most important quantity in the case of
the AC fundamental. For the interconnection of the tie line, it
is necessary to match the frequency of both tie lines.
If the frequency is not matched, this is known as an
asynchronous tie. This cannot interconnect directly.
But this is possible in the case of the HVDC transmission
line. And the disturbance of one system is not transferred to
another system. Hence, the total shutdown and blackout can
be prevented.
22. DISADVANTAGES/LIMITATIONS
1) Cost of terminal equipment
In the HVDC transmission line, the rectifier used at the
sending end and the inverter used at the receiving end. The
smoothing filters need at receiving end. The cost of this
equipment is very high.
2) DC circuit breaker
The DC circuit breaker is still under development and the cost
is high compared to the AC circuit breaker.
3) Additional equipment
This system needs some additional equipment like converter
transformer, electrical and mechanical auxiliaries, pole control,
valve control, and many more. All this equipment is of high
technology and the cost of this equipment is high.
23. 4) Complicated control
The converter used to control the transmission line. But it is
difficult to control the converter under certain abnormal
conditions.
5) Change the voltage level
In the AC system, with the help of a transformer, the voltage
can be easily stepped up and stepped down. Therefore, this
system cannot use for low voltage transmission.
6) System failure
There is some abnormal operating condition in which the
system may fail to operate.
7) Harmonic filter
In the input side, the AC supply is given to the rectifiers. To
mitigate these harmonics, a large amount of filter required.
And the cost of this equipment is high.
24. 8) Complicated cooling
The converter used power electronics switches. When this is
in operation, a very high amount of heat produced in the
thyristor.
9) Overload capacity
The converters cannot operate on overload conditions.
Therefore, it is not permissible.
10) Multi-terminal network
HVDC transmission line is not suitable for a multi-terminal
network.
11) Power loss
The losses occur in the converters and other auxiliaries, which
nullify the reduced loss in the line.
•Application of HVDC Transmission
•Undersea and underground cables
•AC network interconnections
•Interconnecting Asynchronous system
25. STATIC VAR COMPENSATOR
(SVC OPERATION)
A static VAR compensator (SVC) is a set of
electrical devices for providing fast-acting reactive
power on high-voltage electricity
transmission networks. SVCs are part of the Flexible
AC transmission system device family, regulating
voltage, power factor, harmonics and stabilizing the
system. A static VAR compensator has no significant
moving parts (other than internal switchgear). Prior
to the invention of the SVC, power factor
compensation was the preserve of large rotating
machines such as synchronous condensers or
switched capacitor banks.
26. •The SVC is an automated impedance matching device,
designed to bring the system closer to unity power factor.
SVCs are used in two main situations:
•Connected to the power system, to regulate the
transmission voltage ("Transmission SVC")
•Connected near large industrial loads, to improve power
quality ("Industrial SVC")
•In transmission applications, the SVC is used to regulate
the grid voltage. If the power system's reactive load
is capacitive (leading), the SVC will use thyristor controlled
reactors to consume VARs from the system, lowering the
system voltage. Under inductive (lagging) conditions, the
capacitor banks are automatically switched in, thus
providing a higher system voltage. By connecting the
thyristor-controlled reactor, which is continuously variable,
along with a capacitor bank step, the net result is
continuously variable leading or lagging power.
27. PRINCIPLE
Typically, an SVC comprises one or more banks of
fixed or switched shunt capacitors or reactors, of
which at least one bank is switched by thyristors.
Elements which may be used to make an SVC
typically include:
Thyristor controlled reactor (TCR), where the
reactor may be air- or iron-cored
Thyristor switched capacitor (TSC)
Harmonic filter(s)
Mechanically switched capacitors or reactors
(switched by a circuit breaker)
29. By means of phase angle modulation switched by the thyristors, the
reactor may be variably switched into the circuit and so provide a
continuously variable VAR injection (or absorption) to the electrical
network.[2] In this configuration, coarse voltage control is provided by
the capacitors; the thyristor-controlled reactor is to provide smooth
control. Smoother control and more flexibility can be provided with
thyristor-controlled capacitor switching.[7]
Thyristor Switched Capacitor (TSC), shown with Delta connection
The thyristors are electronically controlled. Thyristors, like all
semiconductors, generate heat and deionized water is commonly
used to cool them.[5] Chopping reactive load into the circuit in this
manner injects undesirable odd-order harmonics and so banks of
high-power filters are usually provided to smooth the waveform.
Since the filters themselves are capacitive, they also export MVARs
to the power system.
CONTINUED
30. More complex arrangements are practical
where precise voltage regulation is
required. Voltage regulation is provided by
means of a closed-
loop controller.[7] Remote supervisory
control and manual adjustment of the
voltage set-point are also common.
32. Connection
Generally, static VAR compensation is not done at line
voltage; a bank of transformers steps the transmission
voltage (for example, 230 kV) down to a much lower
level (for example, 9.0 kV). This reduces the size and
number of components needed in the SVC, although
the conductors must be very large to handle the high
currents associated with the lower voltage. In some
static VAR compensators for industrial applications such
as electric arc furnaces, where there may be an existing
medium-voltage busbar present (for example at 33 kV
or 34.5 kV), the static VAR compensator may be directly
connected in order to save the cost of the transformer.
33. Another common connection point for SVC is
on the delta tertiary winding of Y-connected
auto-transformers used to connect one
transmission voltage to another voltage.
The dynamic nature of the SVC lies in the use
of thyristors connected in series and inverse-
parallel, forming "thyristor valves"). The disc-
shaped semiconductors, usually several
inches in diameter, are usually located indoors
in a "valve house".
34. ADVANTAGES
•The main advantage of SVCs over simple mechanically
switched compensation schemes is their near-
instantaneous response to changes in the system
voltage.For this reason they are often operated at close to
their zero-point in order to maximize the reactive power
correction they can rapidly provide when required.
•They are, in general, cheaper, higher-capacity, faster and
more reliable than dynamic compensation schemes such
as synchronous condensers. However, static VAR
compensators are more expensive than mechanically
switched capacitors, so many system operators use a
combination of the two technologies (sometimes in the
same installation), using the static VAR compensator to
provide support for fast changes and the mechanically
switched capacitors to provide steady-state VARs.
