| Budget Amount *help |
¥2,200,000 (Direct Cost: ¥2,200,000)
Fiscal Year 1997: ¥500,000 (Direct Cost: ¥500,000)
Fiscal Year 1996: ¥1,700,000 (Direct Cost: ¥1,700,000)
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| Research Abstract |
In order to simplify the configuration of mutli-parallel connected current source converters used for superconducting magnetic energy storage (SMES), nuclear fusion and etc., two methods such as 1) Reduction of harmonics by multi-parallel configuration using only DELTA-DELTA and DELTA-Y connection ans without using zigzab connection of transformer windings, 2) Series connected primary windings of transformers to balance the current without DC reactors, were examined. As for the converter circuit, snubber energy recoverable current source converter was chosen, and characteristics of the circuit were also carefully examined. Obtained results are summarized as below.
1. Current balancing method by series connected primary side windings was examined by digital simulation. Current balancing was performed in the operation without short-through mode in which series connected positive and negative side devices are simultaneously turned on. Operation with such short-through mode prevents current
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of bridges from balancing, in which current does not flow through transformer windings.
2. N harmonics can be canceled by the N-th power of 2 sets of converters without using transformers. For example, either 11th or 13th harmonic current can be canceled by two sets of converters in which current phases are shifted each other to cancel the harmonics, both 11th and 13th harmonic current can be canceled by four sets of converters. By using transformers with DELTA-DELTA and DELTA-Y connection, 5th and 7th harmonics are also canceled.
3. Snubber capacitors are charged, even if the switching devices beside the capacitor is keeping off-state. This phenomenon is due to the effect of switching of other devices and often occurs when the capacitance value is larger. If such phenomenon occurs frequently, applied voltage to switching devices can not be decreased corresponding to increase of the capacitance value of snubber capacitors, and harmonics of ac line are increased due to the charging current.
4. As for application of sinusoidal Pulse Width Modulation (PWM) control to the snubber energy recoverable current source conveter, snubber capacitor should be designed as small as possible to suppress the rate of rise of voltage across switching devices within the allowed value. Additional capacitors to ac line are required to suppress the voltage across switching devices and absorb high frequency ripple current produced by PWM. Less
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