09-08-2012, 12:37 PM
Comparative Evaluation of HVDC and HVAC Transmission Systems
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INTRODUCTION
THE first electric generator was the direct current (DC)
generator and hence, the first electric power transmission
line was constructed with DC. The basic discoveries of
Galvani, Volta, Oersted, Ohm, and Ampere were in the DC
field. Thomas A. Edison built the first electric central station
in the world in 1882, on the Pearl Street, in the New York,
which was the DC current. Despite the initial supremacy of
the DC, the alternating current (AC) supplanted the DC for
greater uses. This is because of the availability of the
transformer, the induction motor, and polyphase circuits in
the 1880s and 1890s [1]. The transformer is very simple and
easy to change the voltage level for the transmission,
distribution and use. The induction motors are the workhorse
in the industries and work only with AC. That is why AC has
become very useful for the commercial and domestic uses.
But for the long transmission, DC is still more favorable than
AC because of its economical, technical, and environmental
advantages.
High voltage DC (HVDC) Transmission system consists of
three basic parts: 1) converter station to convert AC to DC 2)
transmission line 3) second converter station to convert back
to AC. HVDC transmission systems can be configured in
many ways on the basis of cost, flexibility, and operational
requirements. The simplest one is the back-to-back
interconnection, and it has two converters on the same site
and there is no transmission line.
HVDC VERSUS HVAC TRANSMISSION
Alternating current (AC) became very familiar for the
industrial and domestic uses, but still for the long
transmission lines, AC has some limitations which has led to
the use of DC transmission in some projects. The technical
detail of HVDC transmission compare to high voltage AC
(HVAC) transmission is discussed to verify HVDC
transmission for long distances.
Current and voltage limits are the two important factors of
the high voltage transmission line. The AC resistance of a
conductor is higher than its DC resistance because of skin
effect, and eventually loss is higher for AC transmission. The
switching surges are the serious transient over voltages for the
high voltage transmission line, in the case of AC transmission
the peak values are two or three times normal crest voltage
but for DC transmission it is 1.7 times normal voltage. HVDC
transmission has less corona and radio interference than that
of HVAC transmission line [2]. The total power loss due to
corona is less than 5 MW for a ± 450 kV and 895 kilometers
HVDC transmission line [3-4].
ECONOMICAL ASPECT
Bulk power could be transferred using HVDC or HVAC
transmission system from a remote generating station to the
load center. Direct cost comparisons between AC and DC
alternatives should be conducted before make a decision. In
order to compare the cost, all main system elements must be
taken into consideration. For the DC alternative, capital cost
for the converter terminals, AC input/output equipment,
filters, the interconnecting transmission line must be
accounted. For the AC alternative, capital cost for the stepup/
step-down transformer, the overhead line, light load
compensation if required, reactive power compensation,
circuit breaker, building should be evaluated. Control system
cost need to be considered for the both case. Table II shows
generic cost comparison elements [6-7]. For the preliminary
planning stage, the capital cost for the terminals and
transmission line are the main concern. For example, Nelson
River HVDC Transmission line Bipole 1 is considered here
for economic analysis.
DC STATION COST AND LINE COST
The main equipment of the D.C station is converters and
more than 50% costs of HVDC transmission system are
related to the converters. The converter stations are the key
component to make an economical comparison between DC
and AC transmission system. For an AC system the line costs
predominate and station costs are small and for the DC
system stations costs predominate and line costs are small.
Table IV shows the percentage of each main component cost
relative to the total station cost for DC system [6-7].
CONCLUSION
Long distances are technically unreachable by HVAC line
without intermediate reactive compensations. The frequency
and the intermediate reactive components cause stability
problems in AC line. On the other hand HVDC transmission
does not have the stability problem because of absence of the
frequency, and thus, no distance limitation. The cost per unit
length of a HVDC line lower than that of HVAC line of the
same power capability and comparable reliability, but the cost
of the terminal equipment of a HVDC line is much higher
than that of the HVAC line. The breakeven distance of
overhead lines between AC and DC line is range from 500 km
(310 miles) to 800 km (497 miles). The HVDC has less effect
on the human and the natural environment in general, which
makes the HVDC friendlier to environment.