Multi-level conversion involves considerably more hardware than the two- and three-leveltopologies and increases the station size. On the other hand, the valve design is simpler and the stresses lower, because for the same switching frequency each valve (in the nth-level circuit) is only switched 1/n 1 times and at 1/n 1 times the voltage; therefore theswitching losses are lower than for a three-level converter. However, the valve stresses are also affected by inductance in the commutation loop, which increases with the level number; thus for high-level numbers it may be necessary to increase the value of the snubber capacitances, which will in turn increase the switching losses.
At the time of writing, only the three-level diode-clamped VSC topology, complementedby PWM, has been used in HVDC transmission. The main factors affectingthe potential application of the multi-level configurations to HVDC transmission are asfollows.
The main advantage of multi-level clamping is a considerable reduction in switching losses and its ability to control the harmonic content. The converter can act as a sink or generator of reactive power, but in the case of a DC link this cannot be achieved at both ends with complete independence from each other. Other negative issues are the large number of clamping diodes required (almost proportional to the square of the level number), the difficulty of maintaining the balance of DC capacitors when the power angle is not exactly equal to 90 , the unequal power rating of the power switches and the complexity involved in providing redundant switches to increase reliability.
It is, thus, unlikely that this configuration will play an important part in future HVDCtransmission. Capacitor clamping shares the main advantages of the diode clamping alternative. It has some flexibility in the choice of switching combinations and, therefore, more control of the clamping capacitor current, thus keeping the clamping capacitor voltage at the required level.
Multi-level conversion involves considerably more hardware than the two- and three-leveltopologies and increases the station size. On the other hand, the valve design is simpler and the stresses lower, because for the same switching frequency each valve (in the nth-level circuit) is only switched 1/n 1 times and at 1/n 1 times the voltage; therefore theswitching losses are lower than for a three-level converter. However, the valve stresses are also affected by inductance in the commutation loop, which increases with the level number; thus for high-level numbers it may be necessary to increase the value of the snubber capacitances, which will in turn increase the switching losses.
At the time of writing, only the three-level diode-clamped VSC topology, complementedby PWM, has been used in HVDC transmission. The main factors affectingthe potential application of the multi-level configurations to HVDC transmission are asfollows.
The main advantage of multi-level clamping is a considerable reduction in switching losses and its ability to control the harmonic content. The converter can act as a sink or generator of reactive power, but in the case of a DC link this cannot be achieved at both ends with complete independence from each other. Other negative issues are the large number of clamping diodes required (almost proportional to the square of the level number), the difficulty of maintaining the balance of DC capacitors when the power angle is not exactly equal to 90 , the unequal power rating of the power switches and the complexity involved in providing redundant switches to increase reliability.
It is, thus, unlikely that this configuration will play an important part in future HVDCtransmission. Capacitor clamping shares the main advantages of the diode clamping alternative. It has some flexibility in the choice of switching combinations and, therefore, more control of the clamping capacitor current, thus keeping the clamping capacitor voltage at the required level.
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