|Posted by Prasuna Pandalaneni on July 26, 2010 at 4:37 AM|
AC system is used in the transmission of bulk power, instead of DC (Direct Current), because of its ability to transform voltage to various levels using a transformer. The voltage transformation follows the faradays Law which states; the emf induced in a circuit is directly proportional to the time rate of change of magnetic flux through the circuit.
Ability to transform voltage and to flow power in two opposite directions (bidirectional) are the only major advantages of AC system over DC system. DC transmission system on the other hand has more advantages over AC transmission system.
The industrial growth of a nation requires increased consumption energy, particularly electrical energy. This has lead to increase in the generation and transmission facilities to meet the increasing demand. The generation can be increased to the required level but the problem is in transmission due to the thermal limit, because the transmission line loadability is fixed up to 60% of the power to be transmitted.
1.2.1 DISADVANTAGES OF HVAC TRANSMISSION
The following are the disadvantages in HVAC transmission lines:
• Thermal limit
• Corona loss
• Skin effect
• Ferranti effect
• Economics of transmission
• Technical performance
Thermal limits usually determine the maximum power flow for lines. Thermal power flow limits on overhead lines are intended to limit the temperature attained by the energized conductors and the resulting sag and loss of tensile strength.
The amount of power that can be sent over a transmission line is limited. The origins of the limits vary depending on the length of the line. The increase in thermal limit of the transmission system increases the cost of insulation and also it increases the cost of Transformers, Switch gear and other terminal apparatus.
126.96.36.199 CORONA LOSS
A corona is a process by which a current, perhaps sustained, develops from an electrode with a high potential in a neutral fluid, usually air, by ionizing that fluid so as to create a plasma around the electrode. The ions generated eventually pass charge to nearby areas of lower potential, or recombine to form neutral gas molecules.
188.8.131.52 SKIN EFFECT
The skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. That is, the electric current tends to flow at the "skin" of the conductor. The skin effect causes the effective resistance of the conductor to increase with the frequency of the current. Skin effect is due to eddy currents set up by the AC current.
The skin effect has practical consequences in the design of radio-frequency and microwave circuits and to some extent in AC electrical power transmission and distribution systems. Also, it is of considerable importance when designing discharge tube circuits.
This effect can be minimized by using ACSR (Aluminum conductor steel reinforce) conductors which has the property to minimize the skin effect. But it increase the cost compare to normal conductors.
184.108.40.206 FERRANTI EFFECT
The Ferranti Effect is a rise in voltage occurring at the receiving end of a long transmission line, relative to the voltage at the sending end, which occurs when the line is charged but there is a very light load or the load is disconnected.
This effect is due to the voltage drop across the line inductance (due to charging current) being in phase with the sending end voltages. Therefore both capacitance and inductance are responsible for producing this phenomenon.
The Ferranti Effect will be more pronounced the longer the line and the higher the voltage applied. The relative voltage rise is proportional to the square of the line length.
Due to high capacitance, the Ferranti Effect is much more pronounced in underground cables, even in short lengths
1.3 HVDC TRANSMISSION
Modern DC power transmission is relatively a new technology which made a modest beginning in the year 1954. The advent of thyristor valve and relater technological improvements over the last 18 years has been responsible for the acceleration of the growth of HVDC technology is still undergoing many changes due to continuing innovations directed at improving reliability and reducing cost of converter stations. The latest development of multi-terminal system operation has increased the scope of application of HVDC systems. However, the growth in the knowledge on HVDC technology remains limited.
When the number and size of dc system are small, it was common to consider HVDC power transmission as too specialized and fit only to be taken up by the manufacturers and consultants. With the growth of HVDC systems there is now a greater awareness among engineers from utilities, regarding the potential of dc transmission from the point of view of interaction with ac systems. Some of these interactions are beneficial, while others may pose problems unless investigated thoroughly during the design stage and solutions incorporated to overcome the adverse effects. While it is true that the HVDC systems are quite reliable and converter control allows flexibility in the system operation.
1.3.1 COMPARISION OF AC & DC TRANSMISSION
The relative merits of two modes of transmission (ac & dc) which need to be considered by a system planner are based on the following factors:
• Economics of transmission
• Technical performance
220.127.116.11 ECONOMICS OF POWER TRANSMISSION
The cost of transmission line includes the investment and operational costs. The investment includes costs of Right of Way (ROW), transmission towers, conductors, insulators and terminal equipment. The operational costs include mainly the cost of losses.
The characteristics of the insulators vary the type of voltage applied. For simplicity, if it is assumed that the insulator characteristics are similar for ac & dc and depend on the peak level of the voltage applied with the respect to the ground. Then it can be shown that for lines designed with the same insulation level, a dc line carry as much power with two conductors (with positive and negative polarities with respect to ground) as an ac line with three conductors for the same size. This implies that for a given power level dc line requires less ROW, simpler and cheaper towers and reduced conductor and insulation costs. The power losses are also reduced with dc as there are only two conductors. The absence of skin effect with dc is also beneficial in reducing power losses marginally. The dielectric losses in case of power cables is also very less for dc transmission.
The corona effects tend to be less significant on dc conductors than for ac and this also leads to the choice of economic size of the conductors with dc transmission. The other factors that influence the line cost are the cost of compensation and terminal equipment. Dc lines do not require compensation but the terminal equipment costs are increased due to the presence of the converter and filters.
Graph 1: variation of costs with distance for ac and dc transmission
Ac tends to be more economical than dc for distance less than break even distance and costlier for longer distances. The break even distance can vary from 500 to 800 km in overhead lines depending on the per unit line costs.
18.104.22.168 TECHNICAL PERFORMANCE
The DC transmission has some positive feature which are lacking in AC transmission. These are mainly due to the fast controllability of power in DC lines through converter control.
The following are the advantages:
• Full control over power transmitted.
• The ability to enhance transient and dynamic stability in association AC networks.
• Fast control to limit fault currents in DC lines. These make it feasible to avoid DC breakers in two terminal DC links.
In addition, the DC transmission overcomes some of the problems of AC transmission. These are described further:
The power transfer in AC lines is dependent on the angle difference between the voltage phasors at the two ends. For a given power level, these angle increases with distance. The maximum power transfer is limited by the considerations of steady state and transient stability.
Graph 2: Power transfer capability vs. distance
The power carrying capability of AC lines as a function of distance is shown in the figure. The same figure also shows the power capability of the DC lines which is unaffected by the distance of transmission.
The voltage control in AC lines is complicated by the line charging and inductive voltage drops. The voltage profile in an AC line is relatively flat only for the fixed level of power transfer corresponding to surge impedance loading (SIL). The voltage profile varies with the line loading. For the constant voltage at the line terminals, the mid point voltages reduced for the line loading higher then SIL and increase for loading less than SIL. This is shown in figure followed:
Figure 1: Variation of voltage along the line
The maintenance of constant voltages at the two ends requires reactive power control from inductive to capacitive as the line loading is increased. The reactive power requirements increase with the increase in the line lengths.
Although dc converter stations require reactive power related to the line loadings, the line itself does not require reactive power. The steady state charging currents in ac lines pose serious problems in cables this puts the break even distance for the cable transmission around 40 km.
The reliability of dc transmission systems is quite good and comparable to that of Ac systems. An exhaustive record of existing HVDC links in the world is available from which the reliability statistics can be computed. It must be remembered that the performance of the thyristor valves is much more reliable than mercury arc valves and further development in devices control and protection is likely to improve the reliability level for example the development of direct light triggered (LTT) is expected to improve reliability because of the elimination of the high voltage pulse transformers and auxiliary supplies for turning on the device.
Both energy availability and transient reliability of existing dc systems with thyristor valves is 95% or more.
1.3.2 APPLICATIONS OF DC TRANSMISSION
The detailed comparison of ac & dc transmission in terms of economics and technical performance leads to the following areas of application for dc transmission.
• Long distance bulk power transmission.
• Underground or underwater cables.
• Asynchronous interconnections of ac systems operating at different frequencies or where independent control of systems is desired.
• Control and stabilization of power flows in ac ties in an integrated power system.
1.3.3 DISADVANTAGES OF DC TRANSMISSION
The scope of application of DC transmission is limited by the following factors:
• The difficulty of breaking dc currents which results in high cost of dc breakers.
• Inability to use transformers to change the voltage levels.
• High cost of conversion equipment.
• Generation of harmonics which require ac & dc filters, adding to the cost of converter stations.
• Complexity of control.
There are different types of HVDC systems which are as follows:
5.1.1 MONO-POLAR HVDC SYSTEM:
In the mono-polar configuration, two converters are connected by a single pole line and a positive or a negative DC voltage is used. In Fig. There is only one Insulated transmission conductor installed and the ground or sea provides the path for the return current.
Figure 5: Mono polar HVDC system
5.1.2 BIPOLAR HVDC SYSTEM:
This is the most commonly used configuration of HVDC transmission systems. The bipolar configuration, shown in Fig. below uses two insulated conductors as Positive and negative poles. The two poles can be operated independently if both Neutrals are grounded. The bipolar configuration increases the power transfer capacity.
Under normal operation, the currents flowing in both poles are identical and there is no ground current. In case of failure of one pole power transmission can continue in the other pole which increases the reliability. Most overhead line HVDC transmission systems use the bipolar configuration.
Figure 6: Bipolar HVDC system
5.1.3 HOMO-POLAR HVDC SYSTEM:
In homo polar configuration, shown in Fig. Two or more conductors have the negative polarity and can be operated with ground or a metallic return. With two Poles operated in parallel, the homopolar configuration reduces the insulation costs. However, the large earth return current is the major disadvantage.
Figure 7: Homopolar HVDC system