High power operation of the polyphase resonant converter modulator system for the spallation neutron source linear accelerator

The Spallation Neutron Source (SNS) is a new 1.4 MW average power beam, 1 GeV accelerator being built at Oak Ridge National Laboratory. The accelerator requires 15 "long-pulse" converter-modulator stations each providing a maximum of I I MW pulses with a 1.1 MW average power. Two variants of the converter-modulator are utilized, an 80 kV and a 140 kV design, the voltage dependant on the type of klystron load. The converter-modulator can be described as a resonant zero-voltage-switching polyphase boost inverter. As noted in Figure 1, each convener modulator derives its buss voltage from a standard 13.8 kV to 2100 Y (1.5 MVA) substation cast-core transfonner. The substation also contains harmonic traps and filters to accommodate IEEE 519 and 141 regulations. Each substation is followed by an SCR pre-regulator to accommodate system voltage changes from no load to full load, in addition to providing a soft-start function. Energy storage and filtering is provided by special low inductance self-clearing metallized hazy polypropylene traction capacitors. Three "H-Bridge" Insulated Gate Bipolar Transistor (IGBT) switching networks are used to generate the polyphase 20 kHz transformer primary drive waveforms. The 20 kHz drive waveforrns are chirped the appropriate duration to generate the desired klystron pulse width. PWM (pulse width modulation) of the individual 20 kHz pulses is utilized to provide regulated output waveforms with DSP (digital signal processor) based adaptive feedforward and feedback techniques. The boost transformer design utilizes amorphous nanocrystalline material that provides the required low core loss at design flux levels and switching frequencies. By resonating the transformer secondary leakage inductance, voltage multiplication and IGBT zero-voltage-switching can be attained. The transformers are wound for leakage inductance, not turns ratio, and a 1:18 turns ratio results in a 1:60 output. The resonant topology has the added benefit of being deQed in a klystron fault (shorted output) condition, with little energy transfer during an arc-down situation. This obviates the need of crowbars and other related protective networks. A review of these design parameters, operational performance at design power levels, and production status will be presented.