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 (ORNL). The accelerator requires 15 converter-modulator stations each typically providing 10 MW pulses with a 900 kW average power. Five different klystron load configurations are required in this installation. For the first RF station, 3 parallel 2.5 MW (RF peak output), 402 MHz klystrons are operated with reduced power at ∼113 kV. For the remaining 402 MHz stations, a pair of 2.5 MW klystrons is operated at fall power with ∼125 kV. For the high power 805 MHz stations, a single large 5 MW (RF peak output) klystron is operated at 140 kV. In the low Beta and high Beta superconducting portions of the accelerator, 550 kW, 805 MHz klystrons are grouped into modules of either 11 or 12 parallel-connected tubes. These tubes operate at 75 kV. Two variants of the converter-modulator are utilized, an 80 kV and a 140 kV design, which share a common topology with many interchangeable parts. The converter-modulator can be described as a zero-voltage-switching 20 kHz polyphase bridge, boost inverter for pulse application. Each converter modulator derives its buss voltage from a standard 13.8 kV to 2100 Y (1.5 MVA) substation cast-core transformer. 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. These capacitors do not fail short, but clear any internal anomaly, providing a lifetime of over 300000 hours. As in traction application, these capacitors are hard-bussed parallel. 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 waveforms 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) bas- ed 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. Capacitive peaking is used on the transformer secondary to boost output voltage and resonate transformer leakage inductance. The resonant condition also provides for polyphase resonant voltage multiplication, the transformers are wound with a 1:19 turns ratio, but the output voltage ratio is over 1:60. With the appropriate transformer leakage inductance and peaking capacitance, zero-voltage-switching of the IGBT\´s is attained, minimizing switching losses. 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 for crowbars and other related protective networks. A review of these design parameters, operational performance, and production status will be presented.