Abstract: This research paper presents the comparison of commutation technology in HVDC transmission system. In the 19th century HVAC system was used for the long distance transmission. Mid of 19th century first HVDC system was adopted. The mercury valve was used in the commutation process. After development in the commutation process thyristor valves were used for the better commutation process. Now a day to reduce the losses VSC based technology uses IGBT for the commutation technology. This use of VSC based technology in HVDC transmission is under process.
BASIC IDEAIn the 21st century requirement of power is increasing now a days. Rising energy demand because of the developing and fast growing megacities. The requirement of power increases. In India transmission of power is done by the HVAC transmission line for the long distance. Only the one HVDC transmission line developed in the Delhi and the other HVDC line is under development at ADANI, Mundra. This is the longest transmission line in HVDC. In both the HVDC transmission system for the commutation process thyristor valve used. This commutation is classic or light HVDC system. This commutation process reduces the losses than HVAC system. But for the more reduction in losses we can use the IGBT for the commutation process. That will reduce the cost also. Use of UHVDC (Ultra HVDC) system will give the better environment and reduce the environment problems. In India the natural resources are available. Using these sources we can develop the HVDC transmission and reduce the cost structure and losses. The advantages of using this new technology for the commutation process are given in the paper. The first project is under development using UHVDC at CHINA 800kV transmission line (2014). The other advantages and detail study of technology is described in my IDP.
Advantages of using HVDC…· Greater power per conductor.
· Simpler line construction.
· Ground return can be used.
· Hence each conductor can be operated as an independent circuit.
· No charging current.
· No Skin effect.
· Cables can be worked at a higher voltage gradient.
· Line power factor is always unity: line does not require reactive compensation.
· Less corona loss and radio interference, especially in foul weather, for a certain conductor diameter and rms voltage.
· Synchronous operation is not required.
· Hence distance is not limited by stability.
· May interconnect A.C systems of different frequencies.
· Low short-circuit current on D.C line.
Technical, environmental & economical advantages of using UHVDC…· The most economical solution for long-distance bulk power transmission, due to lower losses, is transmission with High Voltage Direct Current (HVDC).
· A basic rule of thumb: for every 1,000 kilometres the DC line losses are less than 3% (e.g. for 5,000 MW at a voltage of 800 kV).
· Typically, DC line losses are 30–40% less than with AC lines, at the same voltage levels, and for long-distance cable transmission DC is the only solution, technically and economically.
· Siemens UHV DC is a newly developed system that provides the key to increased performance and robustness of the transmission grid, to keeping pace with the steadily growing energy demand, and to a highly economical way of CO2 emissions reduction.
· 60 % reduction in transmission losses and CO2 emissions with UHV DC compared with standard ± 500 kV HVDC
· Significantly smaller footprint and lower OHL costs compared with 800 kV AC solutions daily suited for bulk power transmission over very long distances of 2,000 km and more for infrastructure up rating escapable of interconnecting large grids and of stabilizing parallel AC systems advanced high-speed system control with Win TDC.
· UHV DC – More than 50 % Reduction in Right-of-Way Requirements.
· Siemens UHV DC will be the bulk power energy highway and security backbone of the future power grids.
· The next level of HVDC technology, Siemens UHV DC, is characterized by its innovative 800 kV voltage level, its transmission capacity of up to 7,200 Megawatts, and a substantial loss reduction.
· Thanks to thorough R&D efforts, Siemens is able to produce the entire range of components required for 800 kV DC power transmission itself and supply complete UHV DC systems from a single source.
· An example of the station Layout and the converter arrangement with two 400 kV systems in series in each pole for n-1 redundancy is given in the following figures.
COMPARISON OF CLASSIC HVDC & VSC HVDC
| Attributes | Classical HVDC | VSC-HVDC |
| Converter technology | Thyristor valve, grid commutation | Transistor valve (IGBT), self commutation |
| Max converter rating at present | 6400 MW, ±800 kV (overhead line) | 1200 MW, ±320 kV (cable) |
| Relative size | 4 | 1 |
| Typical delivery time | 36 months | 24 months |
| Active power flow control | Continuous ±0.1Pr to ±Pr (Due to the change of polarity, normally changing the power direction takes some time, which is not the case for VSC-HVDC) | Continuous 0 to ±Pr |
| Reactive power demand | Reactive power demand = 50% power transfer | No reactive power demand |
| Reactive power compensation & control | Discontinuous control (Switched shunt banks) | Continuous control (PWM built-in in converter control) |
| Independent control of active & reactive power | No | Yes |
| Scheduled maintenance | Typically < 1% | Typically < 0,5% |
| Typical system losses | 2.5 - 4.5 % | 4 - 6 % |
| Multiterminal configuration | Complex, limited to 3 terminals | Simple, no limitations |
