Hvdc dad

UNIT 2: HVDC TRANSMISSION
M&V Patel Department of Electrical Engineering
Faculty of Technology and Engineering
Charotar University of Science and Technology – Changa
Prepared by: Dharmesh A Dabhi
Assistant Professor

HVDC Transmission
Introduction, Comparison of AC and DC Transmission, Limitations of HVDC transmission
lines, Comparison of HVDC link with EHVAC link ,Equipment required for HVDC systems,
reliability of HVDC systems, , HVDC system configuration and components, fundamental
equations in HVDC system, HVDC links, converter theory and performance equation, valve
characteristic, converter circuits, converter transformer testing, multi bridge converters,
abnormal operation of HVDC system, control of HVDC system, harmonics and filters.
Influence of AC system strength on AC/DC system interaction, response to AC and DC system
faults, Concept of reactive power compensation- reactive Power balance in HVDC
substations-Effect of angle of advance and extinction angle on reactive power requirement of
converters.

Evolution of power system
Late 1870 Commercial use of electricity start.
1882 First electric power system by Thomas Edison at pearl street
station(Generator, Cable,Fuse, Load(LAMP)).
DC system,59 customers,1.5 km in radius.110v load, underground cable.
1886 Limitation of dc arise. Two aspect (i)High losses and (ii) Voltage drop
To minimize two aspect transformers and ac distribution(150 lamps) developed
by William Stanky of Westinghouse Electric Corporation.
1888 N.Tesla developed poly-phase systems and had patents of Generator, Motors,
Transformers, Transmission lines.(Westinghouse Electric Corporation brought
it)
1889 First ac transmission system in USA between Willamette falls and Portland
1-phase, 4000V over 21 km.
1890 Controversy on whether industry should standardize ac or dc.
Finally ac use because of some advantages.
Voltage increase, Simpler & Cheaper generators and motors.
1893 First 3-phase line, 2300V, 12km in California

EARLY VOLTAGE
1922-165kv 1965-500kv
1923-220kv 1966-735kv
1935-287kv 1969-765kv
1953-330kv 1990-1100kv
It is due to various advancement in insulating material.
EARLYER FREQUENCY
25,50,125,60,133 Hz USA 60 HZ,OTHER COUNTRY 50 HZ

HVDC trasmission
A system of HVDC transmission was designed by French engineer Rene Thury.
1880-1911 At least 11 Thury system were installed in Europe.
-DC series generators used and its operating in constant current control mode
1938 All Thury system were dismantled due to safety and maintenance problem.
1950 Mercury arc valve developed so it was possible to convert ac to dc.
1954 First HVDC transmission between Sweden and Gotland island developed.
It was 70 km long.ac was not suitable therefore go to HVDC.

• LIMITATION OF HVAC TRANSMISSION
1. Reactive Power loss.
2. Stability.
3. Less Current carrying capacity.
4. Skin effect and Ferranti effect.
5. Power flow control is not possible.
• ADVANTAGES OF HVDC TRANSMISSION
 No reactive power loss.
 No stability problem.
 No charging current.
 No Skin and Ferranti effect.
 Power control is possible.
 Cheaper for long distance.
 Asynchronous operation is possible.
 No transmission of short circuit power.
 Require less space compared to ac for same voltage rating and size.
 Ground can be used as return conductor.
 Less corona loss & radio interference.
 No compensation is required.
 Fast fault clearing time.

Why HVDC?
 DC is more efficient than AC for
transmitting large amounts of power
over long distances
 3,500 MW transmission line, 600 mi.
long
 Losses on AC line ~ 15%
 Losses on DC line ~ 4.54%
 Only 2 conductors vs. 9 (3 per phase)
 Smaller towers require less right of way
 Break even distances
 600 km (373 mi.) for above ground lines
 50 km (31 mi.) for submarine lines
 Allows two unsynchronized AC grids to
be connected
http://www.electricaleasy.com/2016/02/hvdc-vs-hvac.html

• DISADVANTAGES OF HVDC TRANSMISSION
 Cost of the terminal equipment is high.
 Introduction of Harmonics.
 Blocking of reactive power.
 Point-to-Point transmission.
 Limited overload capacity.
 Huge reactive power at the converter terminals.
• Objectives of HVDC transmission;
1. Bulk Power transmission.(Large Power over long distance)
2. Back to Back HVDC.(Control of power and voltage)
3. Modulation of AC.(Stability)

Monopolar link:-
• Having One conductor(-ve polarity) and ground is used as
return path.
• 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 low.
• This configuration type is the first step towards a bipolar link.

Bipolar link
• There are two conductors.
• One operates at +ve polarity and other is –ve polarity.
• During fault in one pole it works as Monopolar.
• Each terminal has two converters of equal rated voltage, connected
in series on the DC side.
• The junctions between the converters is grounded.
• If one pole is isolated due to fault, the other pole can operate with
ground and carry half the rated load (or more using overload
capabilities of its converter line).

Homopolar link:
• Two or more conductors having the same polarity.
• Normally negative polarities are used.
• Ground is always used as returns path.
• During fault in one pole it works as Monopolar.

Principle parts of HVDC transmission:

CONVERTERS
• Converters are the main part of HVDC system.
• Each HVDC lines has atleast two converters, one at each end.
• Sending end converter works as Rectifier (It converts AC power to
DC power). However converter as receiving end works as Inverter (
it converts DC power to AC power).
• In case for reversal of operation, Rectifier can be used as inverter or
vice versa. So generally it is call it as CONVERTERS.
• Several thyristors are connector in series and/or in parallel to form a
valve to achieve higher voltage / current ratings.
Note*- Valves (Combinations of several thyristors) .

Thyristor Valves
 Module
 Contains 6-10 high
power thyristors,
 High power thyristors
 1-10 kV
 1-5 kA
 Cooling
 Constant circulation
of deionized water
 Isolation
 Thyristors operating a
high potential relative
to ground the gate
signal needs to be
isolated
http://www.industrial-electronics.com/elec_pwr_3e_22.html

Continues…
• How to achieve required voltage and current ratings?
The current rating of converter stations can be increased by
putting
 Valves in parallel
 Thyristors in parallel
 Bridges in parallel
 Some combinations of above.
The voltage ratings of converter stations can be increased by
putting
 Valves in series
 Bridges in series
 Combination of above.
Bridge converters are normally used in HVDC systems.

Main requirement of the Valves are:
• To allow current flow with low voltage drop across it during
the conduction phase and to offer high resistance for non
conducting phase.
• To withstand high peak inverse voltage during non conducting
phase.
• To allow reasonably short commutation angle during inverter
operation.
• Smooth control of conducting and non conducting phases.

Continues…
• Two versions of switching converters are feasible depending
on whether DC storage device utilized is.
• An inductor-Current source converter
• A Capacitor-Voltage source converter.
• CSC is preferable for HVDC system
• VSC is preferable for FACTS like STATCOM,SVC,etc

Comparison of CSC and VSC:
CSC VSC
Inductor is used in DC side Capacitor is used in DC side
Constant current Constant voltage
Higher losses More efficient
Fast accurate control Slow control
Larger and more expensive Smaller and less expensive
More fault tolerant and more reliable Less fault tolerant and less reliable
Simpler control Complex control

CONVERTER TRANSFORMERS:
• For six pulse converter, a conventional three phase or three
single phase transformer is used.
• However for 12 pulse configuration, following transformer are
used.
 Six single -phase two windings
 Three single- phase three windings
 Two three- phase two windings

Continues…
• As leakage flux of a converter transformer contains very high
harmonic contents, it produces greater eddy current loss and hot spot
in the transformer tank.
• In case of 12-Pulse configuration, if two three phase transformers
are used, one will have star-star connection, and another will have
star delta connection to give phase shift of 30°.
• Since fault current due to fault across valve is predominantly
controlled by transformer impedance, the leakage impedance of
converter transformer is higher than the conventional transformer.
• On-line tap changing is used to control the voltage and reactive
power demand.

SMOOTHING REACTORS:
• As its name, these reactors are used for smoothing the direct
current output in the DC line.
• It also limits the rate of rise of the fault current in the case of
DC line short circuit.
• Normally Partial or total air cored magnetically shielded
reactor are used.
• Disc coil type windings are used and braced to withstand the
short circuit current.
• The saturation inductance should not be too low.

Harmonic filters
• Harmonics generated by converters are of the order of np±1in AC side and
np is the DC side. Where p is number of pulses and n is integer.
• Filter are used to provide low impedance path to the ground for the
harmonics current.
• They are connected to the converter terminals so that harmonics should not
enter to AC system.
• However, it is not possible to protect all harmonics from entering into AC
system.
• Magnitudes of some harmonics are high and filters are used for them only.
• These filters are used to provide some reactive power compensation at the
terminals.

Overhead lines:
• As monopolar transmission scheme is most economical and the first
consideration is to use ground as return path for DC current.
• But use of ground as conductor is not permitted for longer use and a bipolar
arrangement is used with equal and opposite current in both poles.
• In case of failure in any poles, ground is used as a return path temporarily.
• The basic principle of design of DC overhead lines is almost same as AC
lines design such as configurations,towers,insulators etc.
• The number of insulators and clearances are determined based on DC
voltage.
• The choice of conductors depends mainly on corona and field effect
considerations.

Economics of power transmission:
• The cost of transmission line includes the investment and
operational costs.
Investment cost includes,
 Right of way
 Transmission towers
 Conductors
 Insulators
 Terminal equipment
Operational costs includes
 It mainly due to cost of losses

Right of Way(RoW):
• An electric transmission line right-of-way (ROW) is a strip of
land used by Electrical utilities to construct, operate, maintain
and repair the transmission line facilities.
• Rights of way may also include the purchase of rights to
remove danger trees. A danger tree is a tree outside the right of
way but with the potential to do damage to equipment within
the right of way. If the danger tree falls or is cut down, it could
strike poles, towers, wires, lines, appliances or other
equipment and disrupt the flow of electricity to our customers.

Continues…
• This Implies that for a given power level, DC lines requires
less RoW, Simpler , and cheaper towers and reduced
conductors and insulator costs.
• The power losses are also reduced with DC as there are only
two conductors are used.
• No skin effect with DC is also beneficial in reducing power
loss marginally.
• The dielectric losses in case of power cables is also very less
for DC transmission.
• The corona effects tends to less significant on DC conductors
than for AC and this leads to choice of economic size of
conductors with DC transmission.

Continues…
• The other factors that influence the line cost are the cost of
compensation and terminal equipment.
• In dc lines do not require compensation but the terminal
equipment costs are increased due to the presence of
converters and filters.

Reactive power source.
• As such converter does not consume reactive power but due
to phase displacement of current drawn by converter and the
voltage in AC system, reactive power is drawn. Reactive
power requirement at a converter station is about 50-60% of
real power transfer, which is supplied by filters, capacitors
and synchronous condensers.
• Synchronous condensers are not only supplying the reactive
power but also provide AC voltage for natural commutation
of the inverter.

Earth electrodes:
• The earth resistivity of at upper layer is higher (~4000 ohm-m) and
electrodes cannot be kept directly on the earth surface.
• The electrode are buried into the earth where the resistivity is around (3-10
ohm-m) to reduce transient over voltages during line faults and gives low
DC electric potential and potential gradient at the surface of the earth.
• The location of earth electrode is also important due to
 Possible interference of DC current ripple to power lines, communication
systems of telephone and railway signals,etc.
 Metallic corrosion of pipes, cable sheaths ,etc.
 Public safety.
 The electrode must have low resistance (Less than 0.1 ohm) and buried
upto 500 meters into the earth.

HVDC BIPOLAR LINKS IN INDIA
NER
ER
SR
NR
NER
ER
SR
NR
RIHAND-DELHI -- 2*750 MW
CHANDRAPUR-PADGE – 2* 750 MW
TALCHER-KOLAR – 2*1000
MW
ER TO SR
SILERU-BARASORE - 100 MW
EXPERIMENTAL PROJECT
ER –SR

HVDC IN INDIA
Bipolar
HVDC LINK CONNECTING
REGION
CAPACITY
(MW)
LINE
LENGTH
Rihand –
Dadri
North-North 1500 815
Chandrapur -
Padghe
West - West 1500 752
Talcher –
Kolar
East – South 2500 1367

ASYNCHRONOUS LINKS IN INDIA
NER
ER
SR
NR
NER
ER
SR
NR
VINDYACHAL (N-W) – 2*250 MW
CHANDRAPUR (W-S)– 2*500 MW
VIZAG (E-S) - 2*500 MW
SASARAM (E-N) - 1*500 MW

HVDC IN INDIA
Back-to-Back
HVDC LINK CONNECTING
REGION
CAPACITY
(MW)
Vindyachal North – West 2 x 250
Chandrapur West – South 2 x 500
Vizag – I East – South 500
Sasaram East – North 500
Vizag – II East – South 500

Three Phase Full wave bridge converter

Analysis including commutation overlap
• Due to the inductance Lc of ac source, the phase
currents cannot change instantly. Therefore, the
transfer of current from one phase to another
requires finite time, called the commutation or
overlap time.
• It is denoted by µ. Typically the full load values of
µ are between 15˚ to 25˚.
• During commutation overlap, three valves
conduct at the same time
• Other wise only two valves conduct for period
(60˚-μ) with no firing angle delay.

Analysis including commutation overlap
Effect of commutation over lap on periods of conduction of valve

Effect of commutation overlap with ignition
delay
The commutation begins at ωt = α and ends at ωt = α+μ=δ,
where δ = extinction angle

Equivalent circuit diagram during commutation

Voltage waveforms showing the effect of
overlap from valve 1 to valve 3

Power flow in HVDC links
• The HVDC system has two converters, either of them can be
made to work as rectifier/inverter. Thus the power flow can
be bidirectional.
• The power flow can be controlled by controlling the DC
voltage on rectifier side or inverter side.
• Thus the current flowing through the DC link can be given by
• Where Vd1 = DC voltage on rectifier end
• Vd2= DC voltage on inverter end
• R = resistance of the DC line per pole
• The DC voltage in the middle of the line is given by
• Thus the DC power through the line is given by
RVVI ddd /21 
RVVV ddd /213 
dddc IVP 

Control of Voltages Vd1 and Vd2
• There are two ways to control the value of DC Vd1 and Vd2
 With the use of tap changer control which is slow and may
take 8 to 10 seconds.
 Fast variation in the DC voltages can be achieved through gate
control , by varying the firing angle for a few milliseconds.
 The value of Pdc can be controlled quickly because of small
value of R.
 The DC side values of converter and inverter depend on the
position of the tap changer connected to their respective
transformers.
 The general practice is to hold inverter side voltage Vd2
constant at certain value and control the current by rectifier
terminal.
 The current through the rectifier and inverter is maintained
same.

Power at rectifier and inverter end
• Power at the rectifier end (Pd1) is given by,
=
• The power thus can be controlled by controlling the difference
of the DC voltage at rectifier and inverter end.
• Power at inverter end is given by,
=
=
ddd IVP 11 
ddd IVP 22 
The negative sign of the first term indicates that the
power is received by inverter

Power loss in DC system
• The power loss in DC system is
• Line loss increases with increase in power flow.
• Power loss in middle of line Pdm = (Pd1+Pd2)/2
• =
• =
RIP dL *2)^(

Abnormal operation:
Arc-back• Arc-back: This fault refers to the reverse conduction of the
valves. This can happen only when inverse voltage appears
across the valves. In rectifier inverse voltage appears in 2/3rd
cycle. So arc back are more common phenomenon in rectifier.
• The effect of Arc back is to place a short circuit between two
phases, due to the reverse current flow from one valve.
• To avoid this a bypass valve is placed in parallel with rectifier
to provide a diversion for the fault current.

Abnormal operation: Commutation
failure
• This abnormal condition is more common in inverter.
Commutation failure means, failure to turn-off a valve, before
the commutation voltage across it reverses.
• If valve 1 has to turn OFF and valve three is to be turnd ON,
and the valve 1 fails to turn off then after 60 degree valve 4
will be triggered, thus it will place a short circuit across the
phase.

Thus from the equations above it can be said that the DC
voltage and current at any point in the line can be controlled
by controlling the α for rectifier and γ for inverter.
Initially for rapid action firing angle control is used and than
followed by to tap changing transformer to restore the
converter quantities α & γ to their normal range.

Constant voltage mode compensates for the RI drop and thus its
characteristic shows constant V. This mode maintains γ to be
around18 degree. Thus this mode is less prone to commutation
failure.
The β control maintains constant β, so the voltage increases
slightly with increase in current. In this mode at lower load value
of μ is less, but at higher load it increases thus decreasing the
value of γ, so it can lead to commutation failure.

Maximum current limit:
The maximum current limit is limited to 1.2 to 1.3 times the normal full load
current to avoid damage to the valve.
Minimum current limit:
If the current is discontinuous, then in case of a 12 pulse converter , it will
stop and start 12 times. This will lead to high Ldi/dt voltages in transformer
windings and due to lower overlap there will be high stress on the switch

Steady state VI characteristic for inverter and rectifier

Control implementation
• The control scheme is divided into four levels:
• Bridge converter control unit: Defines the αmin and γmin, Fastest
response
• Pole control: Controls the bridges in a pole; current order to
firing angle order; tap changer control and protection
sequences such as starting up, deblocking and balancing
bridge controls
• Master control: current order to all the poles with co-
ordination; interface between the pole control and overall
control
• Overall control: Power flow scheduling; Ac system stabilization
and communication between both the terminals

Hierarchy of different levels of HVDC
system

Converter firing control systems
• Converter firing system decide the firing insants for different control
modes: CC,CIA,CEA
• Two types of control are used:
 Individual pulse control: Firing pulses are decided individually for each
valve depending on the zero crossing of their commutation voltage
• Advantage: achieve highest possible direct voltage under unsymmetrical
or distorted supply waveforms since firing angle is determined
independently
• Disadvantage: Any deviation of the AC system voltage from its ideal
waveform will lead to asymmetry in current waveforms; introduces non-
characteristic harmonics
 Equidistant pulse control: Valves are ignited at equal time intervals, the
ignition angles are retarded or advanced equally
• Advantage: Results in lower level of uncharacteristic harmonics and stable
control with week AC systems
• Disadvantage: If AC network asymmetry is large it results in lower power
and DC voltage

Starting, Stopping and power flow reversal
Normal stopping (blocking) sequence:
The following steps are taken:

• Normal Starting sequence:

• Power flow reversal:
• Power reversal can be done in around 20 to 30 ms.

Controls of enhancement of AC system performance
The following are the major reasons for using supplementary control:

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