Advancements in HVDC converter and inverter technology for power transmission

HVDC

There have been a number of recent and important developments in the High Voltage Direct Current (HVDC) method of electrical power transmission. Many of these developments are directly related to the inverter and converter aspects of transmission. These developments become clear as to their importance with the usage trend towards HVDC and away from Alternate Current (AC) transmissions. Since most experts agree that HVDC is a much more efficient method of transmission than AC, it is likely that the global trend of using and implementing HVDC over AC will continue; therefore any improvements in the manner in which it HVDC is converted (especially if such conversions improve the transmission method) become important as well.

One of the primary advances that have come about concerning power transmission is the manner in which engineers can now interact with simulations of various changes, and the effects those changes will have on the overall HVDC process. Simulations such as these were not as readily available in previous years as they are today. A number of design tools and design software ensures that engineers can now "rapidly and easily build models that simulate power systems" (Alasooly, Redha, 2010, p. 120). It is certain that technology has had a strong influence on the manner in which improvements are now made to transmission systems; these technologies have opened doors through which engineers can simulate different methods and styles of inverters and converters to determine almost immediately how those changes will affect the overall transmission process. As Alasooly and Redha explain "requirements for drastically increased efficiency have forced power system designers to use power electronic devices and sophisticated control system concepts that tax traditional analysis tools and techniques" (p. 120).

The Alasooly and Redha study determined that Metlab SimPowerSystems provided the capabilities to test various inverters and converters in an efficient and effective manner. Having the technology available to test various methodologies should translate into continual improvements in electrical transmission processes. One type of inverter improvement that is now being studied is called the three-level space phasor generation method. This is accomplished through the use of hybrid pulse width modulation (PWM) for dual two-level inverters. A recent study found that "it is determined through stimulation studies that three-level space phasor generation is possible using the proposed hybrid PWM switching strategy for the dual two-level inverter feeding induction motor with open-end windings" (Srinivas, Ramachandrasekhar, 2010, p. 146). The style of induction motor Srinivas and Ramachandrasekhar refer to is a three-phase open end winding inductions motor drive. Transmission can be obtained and regulated through this type of motor drive "by opening the neutral point of the star connected stator windings of the conventional three-phase induction motor and feeding the motor from both ends with two three-phase two-level inverters" (p. 141). This study would not have likely taken place without the assistance of the simulation tool(s) that are now available for engineer's use.

Additionally, technology has also allowed studies in the efficiency of conductors and how effective they are in transmissions. Technology can also determine the effects of changes in conductor methodology takes place (negatively or positively). One study determined that something as simple as an air-gap slit "eliminates flux contribution from two adjacent line sectors and allows only one dominant flux to exist -- that which is generated by a line conductor in its line sector" (Meah, Ula, 2008, p. 214).

A line conductor that allows this type of generation is a good thing because there are limitations to how line-commutated converters can be used in HVDC transmissions. A line commutated converter is what is commonly used in today HVDC transmission process but that is changing. One of the earlier changes made is the implementation of a capacitor-commutated converter (CCC). The CCC is a conventional converter used in HVDC systems by inserting capacitors within the line connections. However, the CCC is quickly being done away with due to another more practical converter called the voltage-sourced converter (VSC). The VSC completely does away with the need for 'extinction time' and as one recent study determined "voltage source multi-level power converter structures are being considered for high power high voltage applications where they have well-known advantages (such as no extinction time)" (Chaves, Margato, Silva, Pinto, Santana, 2011, p. 1436). Their study also showed that for both converter sides "the control strategy considers active and reactive power to establish ac grid currents on sending and receiving ends while guaranteeing the balancing of both NPC dc bus capacitor voltages" (p. 1436). Another study determined that "this technology had captured a significant proportion of the HVDC market" and that VSC technology "will probably eventually replace all simple thyristor-based systems now in use" (Arrillage, Liu, Watson, Murray, 2009).

One of the future trends seems to be towards the self-commutated converters (SCC). The problem with self-commutated converters is that it loses a lot of energy during the shut-down process. A recent study however, determined that self-commutated power converters that use zero-current soft-switching techniques are much more likely to have reduced switching losses.

The study found that "switching losses of HVDC are reduced by adding a commutation circuit" (Senjyu, Kurohane, Miyagi, Urasaki, 2009, p. 316). This reduction is accomplished by introducing the self-commutated converter. Evidence of this accomplishment was verified by the Matlab/Simulink as discussed earlier in this paper.