30 Table 2. Overview of selected VSC projects showing some examples of the trade-off between the key development factors for VSC HVDC. Estlink is the latest and represents the state of the art of VSC HVDC. lnstallation in service year Converler Switshing pattern Switching Contrllability Circuit complexity Losses Topology frequency Hz] Gotland HVDC Light ® 1999 2-level Sinus PWM 1950 Excellent Low High Tjaereborg 2000 Low Directlink 2000 Low · Hagfors SVC Light 1999 3-leve SFOPWM 1260 Excellent Medium Medium (and ali subsequent NPC, 1650 installations) ungrounded Cross Sound Cable 2002 3-level ali 3PWM Sinus PWM 1260 Excellent Medium Low Murraylink IGBT. 1350 Medium Grounded Estlink 2006 2-level Optimum PWM 1150 Very Good Low Low studied, manufactured and delivered. in the optimization process, the considered key factors were production cost, reliability, losses, delivery time and controllability. Production cost, reliability and delivery time are closely related to the circuit complexity. The range of possible applications for VSC HVDC has expanded due to the increase of voltage and power rating. The ± 300 kV de enables very large transmission distances. The realistically reachable power rating of 1000 MW makes VSC an alternative for large scale transmission in a way that was almost unthinkable ten years ago. 5. Control system and VSC application benefits During the lası ten years, a lot of effort has been put into development and refinement of the control system. The main focus has been robustness, high reliability and maximum availability. The control system is designed to ensure desired performance for a range of normal operation conditions and to ensure proper behavior under different AC disturbances. With the voltage source converter there is no commutation failure problem when an AC faul! occurs. However, effective suppressing of transient over- voltage and over-current is a challenge to the control design. it has been widely recognized that VSC based HVDC has many advantages over the classic HVDC. AC faults, or other disturbances on one AC network do not propagate to the other if two terminals Table 3. Expanded matrix with present attainable ratings [Dec 2005) of the converters and cable system. Maximum ratings under typical conditions. Distances are for 4% cable losses. DCVoltage\ 580Arms 1 140Arms 1740Arms -- ±80 kV 95 MW / 75 km 185 MW / 150 km 285 MW / 230 km ± 150 kV 175 MW f 140 km 350 MW / 280 km 525 MW / 420 km ± 300 kV 350 MW / 280 km 700 MW / 560 km 350 MW / 1050 km ENERJi DÜNYASI EYLÜL 2006 of HVDC are connected to two different AC networks. The transmission line can keep transferring the active power duririg one-phase faults and distant three-phase faults if transient o.vervoltage and overcurrent is avoided by effective control action. This makes the VSC HVDC superior to AC transmission and classic HVDC in transmitting wind power, and feeding power to sensitive loads. Another attribute that may be interesting to network planners is that the VSC HVDC can enhance the strength of the AC network via fast dynamic AC voltage control, without increasing the short-circuit current for the existing AC network .This is achieved by flexible AC current control. During a large size AC fault, the AC current from the converter is limited to zero by suitable control action. This attribute will save the cost for renewing AC breakers that would be needed when introducing new AC lines. The development of a black start function makes it possible for the VSC HVDC to act as a virtual generator for one of the networks in case there is a complete loss of generation. in this case, grid restoration may start from the VSC, or use the VSC as a balancing device when adding more and more generators during the reactivation of the network. it should also be noted that the ability to control active and reactive power at both AC terminals makes the VSC HVDC a very, powerful tool for power flow control. 6. Conclusions The development work for VSC HVDC systems has been discussed. The progress in four key technical aspects of VSC HVDC system indicates that the attainable power rating is increased to 1 000 MW. As a result, the range of possible applications for VSC HVDC can be expanded to large scale ônn Tho -..-:ı-ı..,;ı:_•• -' + �nn kV nl"' m,,,k,u::: it possible to transmit large amounts of power long distance. Under restrictions in right-of-way, the VSC HVDC system provides a solution for adding new transmission lines of distance longer than 1 00 km. Compared to AC, VSC HVDC has additional advantages such as full power flow control in both directions, voltage stabilization via continuous reactive power control and minimized disturbances by preventing fault propagation. t
RkJQdWJsaXNoZXIy MTcyMTY=