Historical developments:
Evolution of power systems
late
1870s
|
Commercial
use of electricity
|
1882
|
First
electric power system (generators, under ground cables, fuse, and load mostly
of lamp loads (incandescent lamps) were developed by Thomas alwa Edison at
pearl street station in new York. It was a dc system of 110 v load, 59
customers, supplying an area of 1.5 km radius.
|
1884
|
Motors
were developed by frank Sprague ( father of electric traction)
|
1886
|
Limitation
of dc become apparent
As it has
high losses and more voltage drop
Transformation
of voltage is also required
Transformers
and ac distribution (150 lamps) was developed by William Stanley of westing
house
|
1888
|
Nikola
testla developed poly phase systems and had patents of generators, motors,
transformers and transmission lines. Westing house bought it.
|
1889
|
First ac
transmission system in USA between Willamette falls and Portland oregaon.
Single phase 4000 v and over 21 km in length.
|
1890s
|
Controversy
on whether industry should standardize ac or dc, Edison advocated dc and
westing house ac, the main advantage of using ac is in Voltage increase,
simpler and cheaper generators and motors.
|
1893
|
First 3
phase line 2300 v, 12 km in California. Ac was chosen at Niagara falls (30
km)
|
1922
1923
1935
1953
1965
1966
1969
1990's
|
Early
voltage (highest)
165kv
220kv
287kv
330kv
500kv
735kv
765kv
110kv
Standards
are
115,138,161,230
kv- high voltage
345,500,765kv-
Extra high voltage
Earlier
frequencies were
25,50,60,125,133
Hz; USA-60Hz and some countries-50 Hz
|
HVDC Transmission system
A system
of HVDC transmission was designed by a French engineer, Rene Thury when the
ac system was in their infancy.
|
|
1880-1911
|
At least
11 Thury systems were installed in Europe. The prominent was mouteirs to Lyons (France) in 1906. 180 km of 4.5km
underground cable of 4.3MW, 57.6 kv, 75 A.
DC series
generators were used
Constant
current control mode of operation
|
1920
|
Transverter
were developed. It is poly phase transformer commutated by synchronously
rotating bus gear but not used commercially.
|
1938
|
All the
thury systems were dismantled
|
1950s
|
Mercury
arc valve
|
1954
|
First
HVDC transmission between Sweden and Gotland island by cable.
|
Historical
background of HVDC
|
The most significant contribution to HVDC came when the Gotland Scheme
in Sweden was commissioned in 1954 to be the World's first commercial HVDC
transmission system. This was capable of transmitting 20 MW of power at a
voltage of -100 kV and consisted of a single 96 km cable with sea return.
With the fast development of converters (rectifiers and inverters) at
higher voltages and larger currents, d.c. transmission has become a major
factor in the planning of the power transmission.
In the beginning all HVDC schemes used mercury arc valves, invariably
single phase in construction, in contrast to the low voltage poly phase units
used for industrial application. About 1960 control electrodes were added to
silicon diodes, giving silicon-controlled-rectifiers (SCRs or Thyristors).
In 1961 the cross channel link between England and France was put into
operation. The a.c. systems were connected by two single conductor submarine
cables (64km) at ± 100kV with two bridges each rated at 80MW. The mid-point
of the converters was grounded at one terminal only so as not to permit
ground currents to flow. Sea return was not used because of its effect on the
navigation of ships using compasses. The link is an asynchronous link between
the two systems with the same nominal frequency (60Hz).
The Sakuma Frequency Changer which was put into operation in 1965
interconnects the 50Hz and the 60Hz systems of Japan. It is the first d.c.
link of zero length, and is confined to a single station. It is capable of
transmitting 300 MW in either direction at a voltage of 250 kV.
In 1968 the Vancouver Island scheme was operated at +250 kV to supply
300 MW and is the first d.c. link operating in parallel with an a.c. link.
In 1970 a solid state addition (Thyristors) was made to the Gotland
scheme with a rating of 30MW at - 150kV.
Also in 1970 the Kings north scheme in England was operated on an
experimental basis. In this scheme transmission of power by underground d.c.
cable at ± 200 kV, 640 MW is used to reinforce the a.c. system without
increasing the interrupting duty of a.c. circuit breakers.
The first converter station using exclusively Thyristors was the Eel
River scheme in Canada. Commissioned in 1972, it supplies 320 MW at 80 kV
d.c. The link is of zero length and connects two a.c. systems of the same
Nominal frequency (60Hz).
|
HVDC Transmission system in India
One of the most exciting new technical
developments in electric power system in the last three decades has been
“High Voltage Direct Current transmission”. From the first of HVDC links to
the recent, the voltage has increased from 100 KV to 800 KV, the rated power
from 20 MW to 6300 MW and the distance from 96 km to 1370 km.
Preceding and accompanying this rapid growth of
Direct Current Transmission were developments in High Voltage, High power
valves, in control and protection system, in DC cables and in insulation for
overhead DC lines.
|
Limitations of HVAC Transmission
|
Advantages of HVDC transmission
|
Reactive power loss
|
No reactive power loss
|
stability
|
No stability problem
|
Current carrying capacity
|
No charging current
|
Skin and Ferranti effect
|
No skin and Ferranti effect
|
Power flow control is not possible
|
Power control is possible
|
|
Requires less space as compared to ac for same voltage rating and size
|
Disadvantages of HVDC
Transmission
|
Ground can be used as return conductor
|
Cost of terminal equipments is high
|
Less corona loss and radio interference
|
Introduction of harmonics
|
Cheaper for long distance transmission
|
Blocking of reactive power
|
Asynchronous operation possible
|
Point to point transmission
|
No switching transient
|
Limited overload capacity
|
No transmission of short circuit power
|
Huge reactive power requirements at the converter terminals
|
No compensation problem
|
|
Low short circuit current
|
|
Fast fault clearing time
|
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