AC AND DC GENERAL OVERVIEW
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. The total power loss due to corona is less
than 5 MW for a 450 kV and 895 kilometers HVDC transmission line . The long HVAC overhead
lines produce and consume the reactive power, which is a serious problem. If the transmission
line has a series inductance L and shunt capacitance C per unit of length and operating voltage
V and current I, the reactive power produced by the line is and is called natural load. So the
power carried by the line depends on the operating voltage and the surge impedance of the
line. Pmax is the steady-state stability limit. For a long distance transmission system the line has
the most of the reactance and very small part is in the two terminal systems, consisting of
machines, transformers, and local lines. The inductive reactance of a single-circuit 60 Hz
overhead line with single conductor is about 0.8 /mi (0.5/km); with double conductor is
about 3/4 as greater. The reactance of the line is proportional to the length of the line, and thus
power per circuit of an operating voltage is limited by steady-state stability, which is inversely
proportional to length of line.
HIGH VOLTAGE DIRECT CURRENT TRANSMISSION PRINCIPLE
An HVDC converter converts electric power from high voltage alternating current (AC) to high-
voltage direct current (HVDC), or vice-versa. HVDC is used as an alternative to AC for
transmitting electrical energy over long distances or between AC power systems of different
frequencies. HVDC converters capable of converting up to 2000 megawatts (MW) and with
voltage ratings of up to 900 kilovolts (kV) have been built, and even higher ratings are
technically feasible. A complete converter station may contain several such converters in series
and/or parallel. Almost all HVDC converters are inherently bi-directional; they can convert
either from AC to DC (rectification) or from DC to AC (inversion). A complete HVDC system
always includes at least one converter operating as a rectifier (converting AC to DC) and at least
one operating as an inverter (converting DC to AC). Some HVDC systems take full advantage of
this bi-directional property (for example, those designed for cross-border power trading, such
as the Cross-Channel link between England and France). Others, for example those designed to
export power from a remote power station such as theItaipu scheme in Brazil, may be
optimised for power flow in only one preferred direction.
Technological University of the Philippines
College of Engineering
Department of Mechanical Engineering
ASSIGNMENT NO. 1
IN
AC & DC MACHINERY (LEC)
Submitted By: Submitted To:
Jerikc Rafael D. Jazmin Engr. Frederick F. Ole
AC AND DC GENERAL OVERVIEW
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. The total power loss due to corona is less
than 5 MW for a 450 kV and 895 kilometers HVDC transmission line . The long HVAC overhead
lines produce and consume the reactive power, which is a serious problem. If the transmission
line has a series inductance L and shunt capacitance C per unit of length and operating voltage
V and current I, the reactive power produced by the line is and is called natural load. So the
power carried by the line depends on the operating voltage and the surge impedance of the
line. Pmax is the steady-state stability limit. For a long distance transmission system the line has
the most of the reactance and very small part is in the two terminal systems, consisting of
machines, transformers, and local lines. The inductive reactance of a single-circuit 60 Hz
overhead line with single conductor is about 0.8 /mi (0.5/km); with double conductor is
about 3/4 as greater. The reactance of the line is proportional to the length of the line, and thus
power per circuit of an operating voltage is limited by steady-state stability, which is inversely
proportional to length of line.
HIGH VOLTAGE DIRECT CURRENT TRANSMISSION PRINCIPLE
An HVDC converter converts electric power from high voltage alternating current (AC) to high-
voltage direct current (HVDC), or vice-versa. HVDC is used as an alternative to AC for
transmitting electrical energy over long distances or between AC power systems of different
frequencies. HVDC converters capable of converting up to 2000 megawatts (MW) and with
voltage ratings of up to 900 kilovolts (kV) have been built, and even higher ratings are
technically feasible. A complete converter station may contain several such converters in series
and/or parallel. Almost all HVDC converters are inherently bi-directional; they can convert
either from AC to DC (rectification) or from DC to AC (inversion). A complete HVDC system
always includes at least one converter operating as a rectifier (converting AC to DC) and at least
one operating as an inverter (converting DC to AC). Some HVDC systems take full advantage of
this bi-directional property (for example, those designed for cross-border power trading, such
as the Cross-Channel link between England and France). Others, for example those designed to
export power from a remote power station such as theItaipu scheme in Brazil, may be
optimised for power flow in only one preferred direction.
Technological University of the Philippines
College of Engineering
Department of Mechanical Engineering
ASSIGNMENT NO. 1
IN
AC & DC MACHINERY (LEC)
Submitted By: Submitted To:
Ellijah Phillip T. Chin Engr. Frederick F. Ole
