Light High Voltage Direct Current Systems - Coursework Example

Light High Voltage Direct Current (HVDC) systems. A High Voltage Direct Current (HVDC) is an electric power transmission system that uses direct current from the bulk transmission of electric power, in contrast with the more common alternating current - AC. High Voltage Direct Current (HVDC) systems is less expensive when transmitting electric power over long distances. It suffers lower electrical losses. The HVDC system also reduces the heavy currents required to charge and discharge the cable capacitance each cycle in under water power cables (Kim, Pg. 125). The Light High Voltage Direct Current (HVDC) system allows power transmission between unsynchronized AC transmission systems. The power flow through the HVDC link can be controlled independently of the phase angle between the source and load hence can stabilize a network against disturbances due to rapid changes in power. The Light High Voltage Direct Current (HVDC) system also allows power transfer between grid systems running at varied frequencies, such as 50 Hz and 60 Hz. This improves the stability and economy of each grid, allowing exchange of power between incompatible networks. The history of the Light High Voltage Direct Current (HVDC) systems dates back to 1930s. This technology bases its originality in Sweden and Germany. Early commercial installation included the one for Soviet Union in 1951 between Moscow and Kashira, and the 100 kV, 20 MW System between Gotland and Sweden ( Tang, Pg. 35). With modern technology, smaller HVDC systems have been made economical. This has been managed through the development of higher rated Insulated -gate bipolar transistors (IGTBs), gate turn - off thyristors (GTO) and integrated gate commutated thyristors. The manufacture of this is called ABB Group. The company terms this concept as HVDC light ( Bollen, Pg. 85). The application and use of the HVDC systems have been extended to blocks as small as few tens of Megawatts and the lines are short as few kilometers of overhead line. A current installation includes HVDC PLUS and HVDC MaxSine. These two applications are based on variants of converter called Modular Multi - level converter (MMC). The Modular Multi - level converter (MMC) have the advantage that they allow harmonic filtering equipment’s to be reduced and eliminated altogether. Comparing the HVDC Harmonic filters and the harmonic filters of the typical power lines commutated converter stations; the harmonic filters of the ordinary lines cover nearly half of the station area. With time, voltage - source converter systems will probably replace all installed simple systems using thyristor, including the highest DC power transmission applications. Application of HVDC transmissions employing voltage source converters (VSC) is rapidly growing worldwide due to several distinctive advantages versus the conventional HVDC employing current source converters (CSC). For instance, the voltage source converter ensures Independent control of active and reactive power. They also have the Capability to perform the AC voltage or reactive power flow control at points of interconnection to the power system (Laughton, Pg. 96). The HVDC transmission can be based on either line commutated converters (LCCs) or voltage-source converters. However, the most applied and recommended is the voltage source converters (VSC) .When making the use of voltage source converters (VSC) in an HVDC transmission system, there is no need for heavy reactive power compensation. They HVDC transmission system with the voltage source converters (VSC) have significantly smaller footprint and it’s also reliable to operation with a weak or even passive system, including black start. Similar to a conventional HVDC, the application for voltage source converters (VSC) in HVDC systems includes interconnection transmissions designated for delivering power from one part of a power system to another. This technology is also ideal for delivery of power from off-shore wind turbines, which cannot be done with the conventional HVDC technology without significant additional investments. Several simulations can also be undertaken in the HVDC transmission system. Such simulations are like simulation of transmission line, simulation of clearing fault, simulation of detecting harmonics waves and control, simulation of VSC, and the simulation of light line fault located. DC grid can be defined as a VSC based Multi-terminal HVDC grid, where several converter terminals are connected in parallel with the DC buses ( Lagoni, Pg. 89). The advantages of such a DC grid include reduction in the number of converters compared to several point-to-point HVDC connections and improved power flow control and energy trading. The DC grid also facilitates reduction in the effect of intermittency for HVDC connections. For a successful VSC-HVDC link operation, the system highly depends the control of grid connected VSC. Vector-current control is most commonly used by the industries due to its current limiting capability and efficient. Due to the lower losses in DC cables, HVDC transmission has become more popular than HVAC for long distance offshore wind integration. In the HVDC system and simulation, a three-terminal high-voltage direct current (HVDC) grid connected to a doubly-fed induction generator (DFIG) based wind farm. Invented control strategy, known as power synchronization control, is applied for the integration of the wind farm to the HVDC grid. The HVDC transmission can be based on either line-commutated converters (LCCs) or voltage-source converters (VSCs).Among them, the VSCs being the most appropriate because it does not need any additional reactive-power support and does not depend on external voltage sources for commutation.. ABB has developed a state-of-the-art real-time hardware-in-loop simulation Centre. The prime objective of the DC grid simulation center is to develop a real-time simulation setup for a multi-terminal HVDC system connecting several unsynchronized HVAC grids and offshore wind farms. A three-terminal HVDC grid is modeled. One of the terminals of the modeled DC grid is connected to a doubly-fed induction generator (DFIG) based wind farm and the other two stations are connected to three-phase ideal voltage sources ( Sood, Pg. 120). The AC sides of the converters are connected to the point of common coupling (PCC) through a 425kV/400kV transformer with tap changer. The DC side pole-to-ground voltage of the system is kept at 320 kV. At this stage the HVDC converter stations consist of two-level VSC. The DFIG converters are also two-level converters connected to a common DC link. The active and reactive power capabilities of all the converter stations are +/-600 MW and +/-250 MVAR respectively ( Bollen, Pg. 56). One phase of a two-level Voltage Source Converter There are altogether five converters in the system, out of which two are rotor side and grid side converters for the DFIG based wind farm, and the remaining three are HVDC converters.In HVDC installations, AC harmonic shunt filters are used. They are installed in the system in order to reduce harmonic voltages and currents in the power system, and to supply the reactive power consumed by the converter. An illustration of this concept can be made by use of a 1000-MW (500 kV, 2kA) HVDC rectifier is which is simulated. The HVDC rectifier is built up from two 6-pulse thyristor bridges connected in series. The converter is connected to the system with a 1200-MVA Three-Phase transformer (three windings). A 1000-MW resistive load is connected to the DC side through a 0.5 H smoothing reactor. The filters set is made of four components of the power ;one capacitor banks (C1) of 150 Mvar modeled by a "Three-Phase Series RLC Load", three filters modeled using the "Three-Phase Harmonic Filter" ,One C-type high-pass filter tuned to the 3rd (F1) of 150 Mvar and one double-tuned filter 11/13 the (F2) of 150 Mvar . One high-pass filter tuned to the 24th (F3) of 150 Mvar is also used. The total Mvar rating of the filters set is then 600 Mvar. A three-phase circuit breaker (Brk1) is used to connect the filters set on the AC bus. Simulation of line fault locators is also inclusive in the HVDC simulation model. Cable fault locators are studied for use on the overhead electrode lines in the HVDC Light project.. The cable fault locator operates with the principle of traveling waves, where a pulse is sent in the tested conductor. The time difference is measured from the injection moment to the moment when the reflection is received. Knowing the propagation speed of the pulse is known enables calculation of the distance to the fault (Holt, Dabkowski & Hauth, pg. 37). This type of unit is typically referred to as a TDR (Time Domain Reflectometer). This simulation process is computerized, where a simplified model of a TDR unit is created and applied to an electrode line model by using PSCAD/EMTDC. Staged faults of open circuit and ground fault types are placed at three distances on the electrode line model, different parameters of the TDR units such as pulse width and pulse amplitude along with its connection to the electrode line are then studied and evaluated. The results of the simulations show that it is possible to detect faults of both open circuit and ground fault types. However, the ground faults with high resistance occurring at long distances can be hard to detect. This is because of the low reflection amplitudes from the injections. Nevertheless, this problem can somewhat be resolved with a function that lets the user compare an old trace of a “healthy” line with the new trace. This indicates that most of the faults can be detected and a distance to the fault can be calculated within an accuracy of ± 250 m. The pulse width of the TDR needs to be at least 10 μs, preferable 20 μs to deliver high enough energy to the fault to create a detectable reflection. In this simulation, the pulse amplitude is of less significance although higher pulse amplitude is likely to be more suitable in a real measurement due to the higher energy delivered to the fault. The basic VSC HVDC topology is simply a converter at each end of a pair of extruded DC-transmission cables. The station feeding energy into the dc-circuit is called the rectifier, the converter taking energy from the dc-circuit and feeding it into the receiving AC-network is called the inverter. In VSC, the roles of inverter and rectifier can be interchanged between the stations at any time without delay, without breaker switching and without polarity reversal. Both stations can independently generate or consume reactive power as suitable at the connection point Rectifier VSC Inverter VSC Basic topology of a VSC HVDC transmission with cables HVDC light has a multiple of advantages. It has facilitated the transfer of power long distances. It is also quick to install and offers a alternative to conventional AC transmission systems and local generators. This is achieved in several ways such as converting wind farms to power grids, underground power links and providing shore power supplies to islands and offshore oil and gas platforms ( Tang, Pg. 78). HVDC light also enables transmission of power underground or under water. This type of power is environmental friendly in that it reduces environmental pollution. It makes the use of invisible power lines; hence that space which could be used for installation of electric poles or power cables can be used for other activities such as agriculture or construction of infrastructure. The HVDC light system also ensures neutral electromagnetic fields around the power transmission line. This reduces destructive and dangerous effects of power transmission such as electro-magnetization, heating due to resistance within the power cables and shock effects. HVDC light increases the reliability of power grids, and the technology extends the Economic power range of HVDC transmission down to several Megawatts (MW). However this Light High Voltage Direct Current (HVDC) systems looks advantageous, it also has a number of disadvantages. The main disadvantage is the cost factor. The converters required at both ends of the transmission line are much more expensive to purchase. The cost of the HVDC light circuit breakers is relatively high. The converters also have very little over load capacity and absorb relatively little amounts of reactive power. The unabsorbed reactive power may be a hazard to the environment; more so on animal and plant life. The HVDC light converters also produce a lot of harmonics, both on AC and DC sides. This will cause interference with the audio frequency communication line ( Kim, Pg. 77). In conclusion, HVDC system is very useful in the latest electrical world because HVDC have a great reliability and lower losses. We can control the HVDC active and reactive power Controlling the harmonics and give protection of HVDC system through simulation process. Any fault within the transmission line can easily be located. The simulation will also enhance controlling frequency and other dynamics characteristics within the HVDC system. An application of the Light High Voltage Direct Current (HVDC) systems in the modern line power transfer will be of great importance, putting into consideration its diverse advantages, efficiency and importance. Work cited Holt, R.J, J Dabkowski, and R.L Hauth. Hvdc Power Transmission Electrode Siting and Design. Washington, DC: United States. Dept. of Energy. Office of Energy Efficiency and Renewable Energy, 1997. Internet resource. Sood, Vijay K. Hvdc and Facts Controllers: Applications of Static Converters in Power Systems. Boston, MA: Springer US, 2004. Internet resource. Lagoni, Rainer. Legal Aspects of Submarine High Voltage Direct Current (hvdc) Cables. Hamburg: LIT, 1998. Print. Bollen, Math H. J. Smart Grid: Adapting the Power System to New Challenges. San Rafael, Calif: Morgan & Claypool, 2011. Internet resource. Laughton, M A, and D F. Warne. Electrical Engineers Reference Book. Oxford [England: Newnes, 2003. Internet resource. Kim, Chan-Ki. Hvdc Transmission: Power Conversion Applications in Power Systems. Singapore: IEEE Press, 2009. Internet resource. Tang, Doan . Hvdc-user Defined Model for Dynamic Simulation: Making User Define Hvdc Model Based on Rms Value in Commercial Pss/e Programme. Saarbrücken: lap Lambert, 2011. Print. Read More