This paper discusses newly developed control strategies of a self‐commutated converter applied to the HVDC system. An upper and lower power‐limited dc voltage control establishes high performance in a two‐terminal HVDC system. A voltage margin method to reverse dc transmission power also is introduced. Even when the communication system is shut down, if required, dc transmission power can be changed with the authors' proposed controls applied to the terminal control. Furthermore, the dc voltage control scheme with two‐stage voltage control characteristic is presented for multiterminal HVDC system use. When one terminal is collapsed, the multiterminal HVDC system can be operated because of the proposed control characteristics.
Simulator studies were carried out to verify the proposed control performance in the two‐terminal HVDC system and successful test results such as power reversal and a terminal start‐up during other terminal operations are shown.
A 50‐MVA self‐commutated Static Var Compernsator (SVC) using the worlds's largest GTO thyristors rated 6 kV‐2500 A has been developed for voltage control of electric power systems. The rated dc voltage is 16.8 kV which is four times higher than the highest one manufactured so far. To achieve the high dc voltage, eight GTO thyristors are connected in series. Voltage unbalance between GTOs during switching transient was investigated. A new overcurrent limiting method is proposed, since conventional fuse protection cannot be applied due to such high voltage. Newly developed technologies for this large and high‐voltage self‐commutated SVC are described.
Although simple star‐delta connection transformers are used, harmonics generated from SVC are almost equivalent to the 24‐pulse converter by shifting the pulse timing appropriately. The PWM control based on 150 Hz is applied and the SVC is designed to continue operation during system faults. The performance of the controller is tested successfully by a simulator, and typical rest results are introduced.
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