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Download Point out the Limitations and difficulties of HVDC Transmission System
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Q1: Discribe The principle of HVDC system opertaion . PRINCIPLE OF HVDC SYSTEM OPERATION A typical HVDC transmission system is shown fig. At sending end there is one rectifier unit whereas one inverter unit at the receiving end. The two ends ar interconnected by a DC transmission line. The A.C produced by genrating station after stepping up is converted to D.C by rectifier whereas th inverter conversts D.C to A.C. The converter makes use of thyristor for controlled operation. Thus by varying the finger angle of thyristor, the dc output voltage magnitude is controlled. The firing angle is 0 degree and 90 degree in rectifier while inverter the firing angle is 90 deg and 180 degree. The converter and inverter station in HVDC use three phase controlled bridge converter From the figure Id is given by VR= D.C output voltage at rectifier side VI= D.C input voltage at inverter side The power transfer I given by, Q:2 discuss technical and economical advantage of D.C system over A.C system. Economical advantages: (i) The HVDC transmission lines require only one conductor when compare to Ac transmission system which requires several conductors so the cost of HVDC is less when compare to the AC transmission system. (ii) The supporting structure required for an HVDC transmission system is narrow, whereas an AC transmission requires a lattice structure. So, the cost of supporting tower is less when compare to that of A.C system. (iii) Line losses in HVDC transmission are less when compare to the AC transmission for the same power transfer capacity so the energy cost in HVDC transmission is less when compare to A.C transmission line. (iv) The HVDC lines can be built in two stages. The second stage can be built whenever the extra power transfer capability is needed. The first stage can be operated as a monopolar line with two conductors without ground return. The investment on the second stage can, thus, be postponed. Technical advantages: (i) Reactive power requirement: When the load impedance is equal to surge impedance of line, the reactive power generated by the line capacitance equals the reactive power absorbed by the line inductance. The line cannot always be operated at its natural load since the loads varies with time. In D.C transmission, no reactive power is transmitted over the line the reactive power is independent of line length but varies with transmitted power. (ii) System stability: In order to maintain stability, the length of an uncompensated AC’ line must be less than 500 km. whereas, when series compensation is used, the length may be longer than 500 km. A DC transmission system has no such stability problems. (iii) Short-circuit current: If two AC systems are interconnected by an AC line. Then the short circuit current in the system increases. Therefore. The existing circuit breakers (CBS) have to be replaced with new CBs of high ratings. However, in DC lines, the contribution of short-circuit current is the same as the rated current-carrying capacity of a dc line. (iv) Independent control of AC systems: The AU systems which arc interconnected by a DC line can be controlled independently. They are independent with reference to frequency, system control. Short- circuit rating, future extension etc. (v) Fast change of energy flow: The transmitted power is proportional to the difference in terminal voltage. So, the direction of energy flow can he changed by changing the values of DC voltages. (vi) Less corona loss and radio interference: The corona loss of a transmission system is proportional to (f+ 25), the frequency of a DC system is zero. So. Corona losses in a DC system are less when compared to an AC system of the same conductor diameter and voltage. (vii) Greater reliability: If a fault occurs in one conductor of a bipolar system, the other conductor continues to operate s with ground return. So,. a two-conductor bipolar DC line is more reliable than a threeconductor three-phase AC line. Q3: Point out the Limitations and difficulties of HVDC Transmission System The lack of HVDC circuit breakers is regarded as a limitation to HVDC transmission. In ac circuits, circuits breakers take advantage of the current zeros occurring twice per cycle. The arc does not restrike between contacts because the design is such that the breakdown strength of the arc path between contacts is increased rapidly as to enable extinction. Grid control is converter valves on radial lines are used to block the dc temporarily. The realization of multi-terminal systems requires the use of HVDC circuit breakers. A number of breaker concepts have been described, and several laboratory prototypes have been developed. With the availability of these commercially, utility planners can proceed with serious consideration of multi-terminal HVDC systems. The reliability and maintenance of converters have been a major problem for dc systems with mercury-arc converters. This difficulty has been resolved in projects using thyristor valves. These valves have a little overload capacity, which can present a problem when one bipolar line is involved. In this case it is not possible to meet the requirement of 100 percent half-hour overload capability to take care of a pole outage. The production of harmonics due to converter operation leads to audio frequency telephone line interference. Filter on both sides of the dc system are required to suppress these harmonics Q:4 what is DC link. Classify the type of HVDC links. Discuss the application of each of these links. A DC link is the DC power transmission network which consists of transformers, converter units, and conductors. The classification of HVDC systems depends upon the arrangement of the pole and the earth return. They are classifying as follows: (1) Monopolar link (ii) Bipolar link (iii) Homopolar link 1. Monopolar link Monopolar HVDC transmission system is represented in the Fig. This system has only one pole and the return path is provided by permanent earth or sea. The pole generally has negative polarity with respect to earth. Full power and current is transmitted through a line conductor with earth or sea as a return conductor. The earth electrodes are designed for continuous full current operation. The sea or ground return is permanent and of continuous rating. 2. Bipolar link This system has two poles, one positive and one negative pole with respect to earth. During fault on one pole the bipolar system is changed to monopolar mode. The system is represented in the Fig. This system is more commonly used for transmission of power over long distance. The mid points of convertors at each terminal are earthed through electrode line arid earth electrode. Power rating of one pole is about half of bipolar power rating. The earth carries only small out of balance current during normal operation. The normal bipolar HVDC system consists of two separate monopolar systems with a common earth. The two poles can operate independently. Normally they are operated with equal currents and hence ground carries no current. 3. Homopolar link This system consists of two poles of same polarity and the return is through permanent earth. It is shown in the Fig. Q:5 Describe the application of HVDC transmission system HVDC transmission is advantageous in the following areas of application: (i) For long underwater cable crossings (wider than 32 km). In six of the first seven commercial schemes. submarine cables are the medium of power transfer. The success of the Gotland scheme justified later underwater connections as mentioned earlier. A 25-kin submarine cable between New Brunswick and Prince Edward Island was completed in 1977. The initial operation was at 138 kV ac. The design is such that the forecast (ii) (iii) increase in transmission capacity will be met by HVDC operation at 1200 kV. For long-distance, bulk-power transmission by overhead lines, when the savings in cost of a dc line would more than compensate for the cost of converter stations. For the same power capability, the cost per unit length of a dc line is lower than that of an ac line. In Figure 7-1., we show the comparative costs of ac and dc overhead lines versus distance of transmission. The break-even distance is the abscissa of the intersection of the dc transmission cost with the ac transmission cost. If the transmission distance is longer than the break-even distance, then dc is cheaper than ac. The breakeven distance vanes with the power transmitted, the transmission voltage, the type of terrain, the cost of equipment, and other factors. This particular aspect will be treated Later on in the chapter. Thus dc transmission plays a significant role in situations where it is more economical to generate power at the mine mouth, hydrosite, or gas well and to transmit it electrically to the load center. The dc systems have an inherent short-time overload capacity that can be used for damping system oscillations. Two systems when interconnected by ac lines sustain instability. A dc link interconnecting the two would overcome this difficulty. The Eel River tie, Canada, has operated in this mode for the past several years. The Stegall project in Nebraska was constructed to connect east-west systems in the United Stat at a point along what might be termed the “electric continental divide.” (iv) (v) (vi) A requirement to provide an intertie between two systems without raising the short-circuit level appreciably can be met by using a HVDC link. Two systems having different frequencies may be tied together through a dc interconnection. For transmission in underground metropolitan cable systems where long distance are involved.