HVDC transmission was first used commercially in 1954 between Sweden and an island. It allows for bulk power transmission over long distances and undersea cables. HVDC systems use converters to change AC to DC and back. Components include smoothing reactors, filters, and electrodes. Configurations include monopolar, bipolar, and back-to-back links. Control is done by firing angle adjustment and tap changing. Advantages are long distance transmission and connecting unsynchronized grids, while disadvantages are costly converters and circuit breakers.
2. History
First commercial application of HVDC between Swedish
mainland and the island of Gotland in 1954.
Underwater link of 90 km and 20 MW.
With reduced size, cost and improved reliability of power
electronic converters, has made HVDC transmission more
widespread.
3. Need of HVDC systems
For bulk transmission of electrical power.
In AC systems there arises some technical
problems in long distances.
For high power transmission through
underground cables.(AC-charging current
problems)
In the view of the drawbacks HVDC
transmission came into picture.
5. HVDC System Configurations and Components
HVDC links can be broadly classified into:
Monopolar links
Bipolar links
Homopolar links
Multiterminal links
Back-to-back links
6. Monopolar Links
It uses one conductor
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
7. Bipolar Links
It uses two conductors, one positive and the other negative.
Each terminal has two converters of equal rated voltage,
connected in series on 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 ground and carry half the rated load.
8. 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
9. Multiterminal links
Transmits power between three or more AC
substation.
Frequency conversion is possible.
In Europe where there is international grid,
power can be exchanged between nations.
10. Back-to-back links
For frequency conversion.
For asynchronous interconnection.
Both Rectifier & Inverter are at same place,
connected in DC loop.
There is no DC transmission line.
11. Components of HVDC Transmission Systems
1. Converter
transformer
2. Converters
3. Smoothing
reactors
4. Harmonic filters
5. Reactive power
supplies
6. Electrodes
7. DC lines
8. AC circuit
breakers
12. Converter transformer
Connected between converter and the AC bus.
Specially deigned as they have dc component coming
from converter side.
They may be three single units or a 3 ph unit
13. Converters
They perform AC/DC and DC/AC conversion.
They consist of thyristor bridges and transformers.
Thyristor bridge consists of high voltage thyristor connected in a 6-pulse or
12-pulse arrangement.
The transformers are ungrounded such that the DC system will be able to
establish its own reference to ground.
14. Smoothing reactors
They are high reactors with
inductance as high as 1 H in
series with each pole
They function to smooth out
the DC output from the rectifier
circuit.
15. Harmonic filters
Converters generate harmonics in voltages
and currents.
These harmonics may cause overheating of
capacitors and interference with
telecommunication systems
Harmonic filters are used to mitigate these
harmonics
twelve-pulse bridge converter is used
because it creates harmonics of order 12n+1
on AC side and 12n on DC side
16. Reactive power supplies
Under steady state condition conditions, the
reactive power consumed by the converter is
about 50% of the active power transferred
Under transient conditions it could be much
higher
Reactive power is, therefore, provided near
the converters
For a strong AC power system, this reactive
power is provided by a shunt capacitor
17. Electrodes
Electrodes are conductors that provide
connection to the earth for neutral. They have
large surface to minimize current densities and
surface voltage gradients.
DC lines
They may be overhead lines or cables
DC lines are very similar to AC lines
18. AC circuit breakers
They used to clear faults in the transformer
and for taking the DC link out of service
They are not used for clearing DC faults
DC faults are cleared by converter control
more rapidly
20. Multiple Bridge Converters
Two or more bridges are connected in series
to obtain as a high a direct voltage as
required
These bridges are series on the DC side,
parallel on the AC side
A bank of transformers is connected
between the AC source and the bridges
The ratio of the transformers are adjustable
under load
Multiple bridge converters are used in even
numbers and arranged in pairs for 12-pulse
arrangement
21. Multiple Bridge Converters
Two banks of transformers, one connected in Y-Y and the other Y- are used to supply
each pair of bridges
The three-phase voltage supplied at one bridge is displaced from the other by 30 degrees
These AC wave shapes for the two bridges add up to produce a wave shape that is more
sinusoidal than the current waves of each of the 6-pulse bridges
This 12-pulse arrangement effectively eliminates 5th and 7th harmonics on the AC side.
This reduces the cost of harmonic filters
This arrangement also reduces ripple in the DC voltage
23. Basic means of control
This can be accomplished by:
Controlling firing angles of the rectifier and inverter (for fast
action)
Changing taps on the transformers on the AC side (slow
response)
Power reversal is obtained by reversal of polarity of
direct voltages at both ends
24. Control implementation
Tap changer control
It is used to keep the converter firing angles ( and )
within the desired range
They are sized to allow for minimum and maximum steady
state voltage variation
Current limits:
Maximum short circuit current is limited to 1.2 to 1.3
times normal full load current to avoid thermal damage
to equipment
Minimum current limit is set to avoid ripple in the current
that may cause it to be discontinuous or intermittent
Minimum firing angle limit:
In case of a DC fault, the inverter station may switch to
rectification mode. This would result in reversal of power flow
To prevent this, the a minimum value for firing angle is set
25. Advantages
In a number of applications HVDC is more effective than AC
transmission. Examples include:
Undersea cables, where high capacitance causes additional AC
losses. (e.g. 250 km Baltic Cable between Sweden and Germany)
Long power transmission without intermediate taps, for example,
in remote areas
Power transmission and stabilization between unsynchronized AC
distribution systems
Connecting a remote generating plant to the distribution grid
Reducing line cost: 1) fewer conductors 2) thinner conductors
since HVDC does not suffer from the skin effect
Facilitate power transmission between different countries that use
AC at differing voltages and/or frequencies
Synchronize AC produced by renewable energy sources
26. Disadvantages
The disadvantages of HVDC are in conversion,
switching and control.
Expensive inverters with limited overload capacity
Higher losses in static inverters at smaller transmission
distances
The cost of the inverters may not be offset by reductions
in line construction cost and lower line loss.
High voltage DC circuit breakers are difficult to build
because some mechanism must be included in the circuit
breaker to force current to zero, otherwise arcing and
contact wear would be too great to allow reliable
switching.