29-05-2012, 02:53 PM
Harmonic problem in a three-phase converter system.
[attachment=23357]
INTRODUCTION:
About HVDC transmission
The history of electric power transmission reveals that transmission was originally developed with DC. However, DC power at low voltage could not be transmitted over long distances, thus it led to the development of alternating current (AC) electrical systems. Also the availability of transformers and improvement in ac machines led to the greater usage of ac transmission. The advent of the mercury arc valve for high power and voltage proved to be a vital breakthrough for High Voltage Direct Current (HVDC) transmission. These mercury valves were the key elements in the converter stations and the filtering was done using oil immersed components. The control was analog and most of the operations were left to the operator. After enough experiments conducted on mercury valves, the first HVDC line was built in 1954 with a 100 km submarine cable with ground return between the island of Gotland and the Swedish mainland. The development of thyristors is another milestone in the development of HVDC technology. The first solid-state semiconductor valves were commissioned in 1970. The mercury arc valves in the primitive projects were replaced by thyristor valves. The semiconductor devices like thyristors, IGBTs and GTOs, in conjunction with microcomputers and digital signal processors have proved to be very effective compared to older mercury valves. The wider usage of semiconductor technology in present day HVDC systems has initiated great leaps in the research of power electronics. With increased demand for high quality power, application of power electronics in the field of power distribution and transmission systems is attracting wide attention throughout the world.
Why HVDC?
There are many different reasons as to why HVDC is to chosen instead of ac transmission. A few of them are listed below. Cost effective HVDC transmission requires only two conductors compared to the three wire ac transmission system. One-third less wire is used, thus readily reducing the cost of the conductors. This corresponds to reduced tower and insulation cost, thereby resulting in cheaper construction. However, the ac converters stations involve high cost for installation; thus the earlier advantage is offset by the increase in cost. If the transmission distance is long, a break-even distance is reached above which total cost of HVDC transmission is less than the ac. Asynchronous tie HVDC transmission has the ability to connect ac systems of different frequencies. Thus it can be used for intercontinental asynchronous ties. For example, in Japan HVDC could be used to connect an ac system operating at 60 Hz with one operating at 50 Hz. Lower line losses similar to ac transmission, HVDC transmission has I2R losses too. However, for the same amount of power transfer, DC losses are less due to the lower resistance of the conductors because of only two-thirds of the conductor length. The main losses are converter losses that offer better stability and control ensures low environmental impact and reduces construction time.
HVDC CONSTRAINTS:
Even though HVDC has many advantages, the whole power system cannot be made DC, because of the fact that generation and distribution of power is ac. So HVDC technology is restricted to transmission. As no system is perfect, even HVDC transmission has some disadvantages and drawbacks. A few of them are listed below, Converter station costs the power electronic converters involve high installation and maintenance costs. This expenditure offsets the cost savings mentioned as one of the advantages; for this reason, short overhead HVDC lines are more expensive compared to ac. Reactive power requirement both the rectifier and inverter in converter stations consume large amounts of reactive power (VARs). Even though the capacitors used in the converters supply reactive power to some extent, the rest should be supplied by additional capacitors or taken from the ac system. Harmonic converters at both ends of an HVDC system inject a certain amount of harmonics into the ac system.
COMPONENTS OF HVDC TRANSMISSION SYSTEM:
The converter station:
The converter stations at each end are identical and can be operated either as an inverter or rectifier based on the control. Hence, each converter is equipped to convert ac to DC and vice versa. One of the main components of a converter substation is the thyristor converter is usually housed in a valve hall. As shown in Fig. 1.2, the substation also essentially consists of converter transformers. These transformers transform the ac system voltage based on the DC voltage required by the converter. The secondary or DC side of the converter transformers is connected to the converter bridges. The transformer is placed outside the thyristor valve hall and the connection has to be made through the hall wall.
This is accomplished in two ways:
1) With phase isolated bus bars where the bus conductors are housed within insulated bus ducts with oil or SF6 as the insulating medium,
2) With wall bushings and these require care to avoid external or internal breakdown [1].
Filters are required on both ac and DC sides since the converters generate harmonics. The filters are tuned based on the converter operation (6 or 12 pulse). DC reactors are included in each pole of the converter station. These reactors assist the DC filters in filtering harmonics and mainly smooth the DC side current ensuring continuous mode of operation. Surge arrestors are provided across each valve in the converter bridge, across each converter bridge and in the DC and ac switches to protect the equipment from overvoltages.
[attachment=23357]
INTRODUCTION:
About HVDC transmission
The history of electric power transmission reveals that transmission was originally developed with DC. However, DC power at low voltage could not be transmitted over long distances, thus it led to the development of alternating current (AC) electrical systems. Also the availability of transformers and improvement in ac machines led to the greater usage of ac transmission. The advent of the mercury arc valve for high power and voltage proved to be a vital breakthrough for High Voltage Direct Current (HVDC) transmission. These mercury valves were the key elements in the converter stations and the filtering was done using oil immersed components. The control was analog and most of the operations were left to the operator. After enough experiments conducted on mercury valves, the first HVDC line was built in 1954 with a 100 km submarine cable with ground return between the island of Gotland and the Swedish mainland. The development of thyristors is another milestone in the development of HVDC technology. The first solid-state semiconductor valves were commissioned in 1970. The mercury arc valves in the primitive projects were replaced by thyristor valves. The semiconductor devices like thyristors, IGBTs and GTOs, in conjunction with microcomputers and digital signal processors have proved to be very effective compared to older mercury valves. The wider usage of semiconductor technology in present day HVDC systems has initiated great leaps in the research of power electronics. With increased demand for high quality power, application of power electronics in the field of power distribution and transmission systems is attracting wide attention throughout the world.
Why HVDC?
There are many different reasons as to why HVDC is to chosen instead of ac transmission. A few of them are listed below. Cost effective HVDC transmission requires only two conductors compared to the three wire ac transmission system. One-third less wire is used, thus readily reducing the cost of the conductors. This corresponds to reduced tower and insulation cost, thereby resulting in cheaper construction. However, the ac converters stations involve high cost for installation; thus the earlier advantage is offset by the increase in cost. If the transmission distance is long, a break-even distance is reached above which total cost of HVDC transmission is less than the ac. Asynchronous tie HVDC transmission has the ability to connect ac systems of different frequencies. Thus it can be used for intercontinental asynchronous ties. For example, in Japan HVDC could be used to connect an ac system operating at 60 Hz with one operating at 50 Hz. Lower line losses similar to ac transmission, HVDC transmission has I2R losses too. However, for the same amount of power transfer, DC losses are less due to the lower resistance of the conductors because of only two-thirds of the conductor length. The main losses are converter losses that offer better stability and control ensures low environmental impact and reduces construction time.
HVDC CONSTRAINTS:
Even though HVDC has many advantages, the whole power system cannot be made DC, because of the fact that generation and distribution of power is ac. So HVDC technology is restricted to transmission. As no system is perfect, even HVDC transmission has some disadvantages and drawbacks. A few of them are listed below, Converter station costs the power electronic converters involve high installation and maintenance costs. This expenditure offsets the cost savings mentioned as one of the advantages; for this reason, short overhead HVDC lines are more expensive compared to ac. Reactive power requirement both the rectifier and inverter in converter stations consume large amounts of reactive power (VARs). Even though the capacitors used in the converters supply reactive power to some extent, the rest should be supplied by additional capacitors or taken from the ac system. Harmonic converters at both ends of an HVDC system inject a certain amount of harmonics into the ac system.
COMPONENTS OF HVDC TRANSMISSION SYSTEM:
The converter station:
The converter stations at each end are identical and can be operated either as an inverter or rectifier based on the control. Hence, each converter is equipped to convert ac to DC and vice versa. One of the main components of a converter substation is the thyristor converter is usually housed in a valve hall. As shown in Fig. 1.2, the substation also essentially consists of converter transformers. These transformers transform the ac system voltage based on the DC voltage required by the converter. The secondary or DC side of the converter transformers is connected to the converter bridges. The transformer is placed outside the thyristor valve hall and the connection has to be made through the hall wall.
This is accomplished in two ways:
1) With phase isolated bus bars where the bus conductors are housed within insulated bus ducts with oil or SF6 as the insulating medium,
2) With wall bushings and these require care to avoid external or internal breakdown [1].
Filters are required on both ac and DC sides since the converters generate harmonics. The filters are tuned based on the converter operation (6 or 12 pulse). DC reactors are included in each pole of the converter station. These reactors assist the DC filters in filtering harmonics and mainly smooth the DC side current ensuring continuous mode of operation. Surge arrestors are provided across each valve in the converter bridge, across each converter bridge and in the DC and ac switches to protect the equipment from overvoltages.