POWER UPGRADING OF TRANSMISSION LINE BY COMBINING AC–DC TRANSMISSION
INTRODUCTION • Long extra high voltage (EHV) ac lines cannot be loaded to their thermal limits in order to keep sufficient margin against transient instability. • The scheme proposed in this project, it is possible to load these lines very close to their thermal limits. • The conductors are allowed to carry usual ac along with dc superimposed on it.
Electric power transmission Electric power transmission, a process in the delivery of electricity to consumers, is the bulk transfer of electrical power. A power transmission network typically connects power plants to multiple substations near a populated area.
At the generating plants the energy is produced at a relatively low voltage between about 2300 volts and 33,000 volts, depending on the size of the unit. The generator terminal voltage is then stepped up by the power station transformer to a higher voltage (66 kV to 440 kV AC, varying by country) for transmission over long distances.
Usually transmission lines use three phase AC current. Single phase AC current is sometimes used in a railway electrification system. High-voltage direct current systems are used for long distance transmission, or some undersea cables, or for connecting two different ac networks.
HVDC High voltage direct current (HVDC) is used to transmit large amounts of power over long distances or for interconnections between asynchronous grids. When electrical energy is required to be transmitted over very long distances, it is more economical to transmit using direct current instead of alternating current. For a long transmission line, the lower losses and reduced construction cost of a DC line can offset the additional cost of converter stations at each end.
COMPARISION OF AC AND DC TRANSMISSION We can transmit the electric power either in dc transmission and in ac transmission. DC Transmission The Transmission of Electric power by DC has been receiving the Active Consideration of engineers due to its numerous Advantages
Advantages • It requires only two conductors as compared to three for AC Transmission. • There is no inductance , capacitance, phase displacement and surge problems in Dc Transmission. • Due to the Absence of inductance , the voltage drop in DC transmission line is less than the AC Transmission line for the same sending end Voltage . For the reason DC transmission is better for voltage Regulation. • There is no Skin effect in DC System . Therefore , entire cross-section of the line conductor is utilized
For the Same working Voltage , the potential stress on the insulation is less in case of DC System than that in AC System . There fore , DC line requires less insulation . • A DC line has less corona loss and reduced interference with Communication circuits. • The high voltage DC Transmission is free from dielectric losses , Particularly in Case of Cables . • In DC Transmission , there is no Stability Problem and Synchronizing Problems.
Disadvantages • Electric Power Cannot be generated at high DC voltage due to Commutation Problem. • The DC Voltage Cannot be stepped up for transmission of power at High voltage. • The DC Switches and Circuit Breakers have their own Limitations.
AC Transmission • Electrical Energy is almost exclusively generated , Transmitted and Distributed in the Form of AC.
Advantages • The power can be generated at High Voltage. • The maintenance of AC Sub-station is easy and Cheaper. • The AC Voltage can be Steeped up or steeped down by transformer with ease and Efficiency. This permits to Transmit power at High voltages and Distribute it at Safe Potentials.
Disadvantages • An AC line requires more Copper than a DC lines. • The Construction of AC System , the effective resistance of the line is Increased. • An AC line has Capacitance . Therefore , there is a Continuous loss of Power due to Charging Current even When the line is Open.
Advantages of High Transmission voltage • Reduces Volume of Conductor Material • Increases Transmission Efficiency. • Decreases Percentage Line Drop.
Limitations 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 increased cost of insulating the Conductors. • The increased cost of Transformers , Switch gear and Other terminal apparatus.
For a short line, the heating of conductors due to line losses sets a "thermal" limit. If too much current is drawn, conductors may sag too close to the ground, or conductors and equipment may be damaged by overheating. • For intermediate-length lines on the order of 100 km (60 miles), the limit is set by the voltage drop in the line. • For longer AC lines, system stability sets the limit to the power that can be transferred.
FACTS – Flexible Alternating Current Transmission Systems For Cost Effective and Reliable Transmission of Electrical Energy • Statcom- Static Compensator • SSSC-Static Synchronous Series Capacitor • UPFC-Unified Power Flow Controller • TCSC-Thyristor Controlled Switched Capacitor • SVC-Static VAR Compensator • UPQC-Unified Power Quality Conditioner • IPFC-Interline Power Flow Controller • FFFTs-Fractional Frequency Transmission System
Skin Effect Skin effect is a tendency for alternating current to flow mostly near the outer surface of a solid electrical conductor, such as metal wire, at frequencies above the audio range. The effect becomes more and more apparent as the frequency increases.
Thermal Stability Thermal Limits 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. Thermal limits usually determine the maximum power flow for lines
Parallel AC-DC Transmission line the simplified single line diagram of the parallel ac-dc system
By using a low power low voltage DC link in parallel with the ac system we can improve the small-signal stability and Consequently increase the power transfer in a parallel high voltage-high power AC line is the use of a small-parallel dc link transferring about 2% of the normal ac power substantially increases the power transfer capability of the ac line.
The added dc power flow does not cause any transient instability. • The flexible ac transmission system (FACTS) concepts, based on applying state-of-the-art power electronic technology to existing ac transmission system, improve stability to achieve power transmission close to its thermal limit. • Simultaneous ac–dc power transmission was first proposed through a single circuit ac transmission line. • In these proposals Mono-polar dc transmission with ground as return path was used. • There were certain limitations due to use of ground as return path.
This project gives the feasibility of converting a double circuit ac line into composite ac–dc power transmission line to get the advantages of parallel ac–dc transmission to improve stability and damping out oscillations. • The feasibility study of conversion of a double circuit ac line to composite ac–dc line without altering the original line conductors, tower structures, and insulator strings has been presented.
The dc power is obtained through line commutated 12-pulse rectifier bridge used in conventional HVDC. • This is injected to the neutral point of the zigzag connected secondary of sending end transformer and is reconverted to ac again by the conventional line commutated 12-pulse bridge inverter at the receiving end. • The inverter bridge is again connected to the neutral of zig-zag connected winding of the receiving end transformer.
The double circuit ac transmission line carriers both three-phase ac and dc power. • Each conductor of each line carries one third of the total dc current along with ac current. • Zig-zag connected winding is used at both ends to avoid saturation of transformer due to dc current. • Two fluxes produced by the dc current flowing through each of a winding in each limb of the core of a zig-zag transformer are equal in magnitude and opposite in direction.
The dotted lines in the figure show the path of ac return current only. • The second transmission line carries the return dc current, and each conductor of the line carries along with the ac current per phase. • And are the maximum values of rectifier and inverter side dc voltages and are equal to times converter ac input line-to-line voltage.
Neglecting the resistive drops in the line conductors and transformer windings due to dc current. • Expressions for ac voltage and current, and for active and reactive powers, parameters of each line may be written as • Transmission loss for each line is
Ia being the rms ac current per conductor at any point of the line, the total rms current per conductor becomes • Power loss of each line is • The net current through the conductor equal to its thermal limit
Let be per-phase rms voltage of original ac line and also be the per-phase voltage of ac component of composite ac–dc line with dc voltage superimposed on it. • As insulators remain unchanged, the peak voltage in both cases should be equal. • The total power transfer through the composite line
Approximate value of ac current per phase per circuit of the double circuit line may be computed as • The rectifier dc current order is adjusted online as