Compact high voltage, high peak power, high frequency transformer for converter type modulator applications

Abstract

The design and development of a compact high voltage, high peak power, high frequency transformer for a converter type modulator of klystron amplifiers is presented. The transformer has been designed to operate at a frequency of 20 kHz and at a flux swing of ±0.6 T. Iron (Fe) based nanocrystalline material has been selected as a core for the construction of the transformer. The transformer employs a specially designed solid Teflon bobbin having 120 kV insulation for winding the high voltage secondary windings. The flux swing of the core has been experimentally found by plotting the hysteresis loop at actual operating conditions. Based on the design, a prototype transformer has been built which is per se a unique combination of high voltage, high frequency, and peak power specifications. The transformer was able to provide 58 kV (pk-pk) at the secondary with a peak power handling capability of 700 kVA. The transformation ratio was 1:17. The performance of the transformer is also presented and discussed.

Hybrid Foil-Litz Windings for Highly Efficient and Compact Medium-Frequency Transformers

“…With V hv = 12 kV, the thickness of resin needed to limit E max < 3.5 kV/mm is D hv,c = 45 mm, as shown in Fig. 11 These analyses confirm that the litz rings preserve the E shielding effect of EQP rings [18]- [20], [25]; if necessary, the litz bundle can be covered with semiconductive tape or coatings to smooth possible irregularities on its surface, as often done in MV rotating machinery, pulse transformers and MFTs [26]- [29]. Since these concepts are already established, the next sections focus on the advantages offered by the new topology in terms of reduced ohmic loss, conductor usage and winding size.…”

“…Foil windings exhibit a uniform voltage distribution in the case of steepfronted PWM voltages [16], which is advantageous for MV SST applications; however, the electric field hotspots at the foil winding corners limit the insulation rating [17]. Equipotential rings are crucial for mitigating the electric field hotspots in MV applications [18]- [20], with the disadvantage of increasing the winding footprint and possibly introducing additional losses. Similarly, the state-of-the-art solutions based on interleaving [21], [22], magnetic field shielding [23] or conductor shaping [24] for reducing ohmic losses in foil conductor are hardly applicable in an MV winding due to the persistence of electric field hotspots and the manufacturing complexity introduced by the insulation requirements.…”

“…1, mitigating both the current crowding and the electric field hotspots: the foil winding package is extended and connected in series to two turns of litz wire, forming the first and last turn, one above, and one below the foil winding, such that they act as equipotential shielding rings; besides, their current deflects the magnetic field curvature away from the foil edges, decreasing the ohmic losses due to the current crowding. Since electric field shielding by equipotential rings is a concept already established in the literature on both LFTs and MFTs [18]- [20], [25]- [29], this work focuses on the advantages offered by the new topology in terms of ohmic loss, conductor usage and winding size.…”

Simulation of electro-thermal ageing and breakdown in polymeric insulation under high frequency trapezoidal-wave pulses

“…Thus the ability of neighboring bonds to resist degradation is ineffective at this field if their ability to concentrate field energy is at or greater than the characteristic value. At E1, the rate constants kf + kb that determine the progress of the reaction equation (11) are about ten times their value when the simulation starts in the uniform field E=40 MV/m, and kf ∼ 10 -50 kb. At E2=128 MV/m, Aeq=0.99 even when the neighboring bonds have the smallest values of C allowed in the simulation.…”

“…More recently quantitative theoretical expressions have been derived for insulation lifetimes under AC and DC electrothermal ageing lifetimes with some success [2][3][4]. However the emergence of power electronic technologies [5][6][7][8][9][10] has posed several new problems because of their highly compact configurations [5,11], dramatically larger thermal dissipation [5,[12][13][14], very rapid polarity changes (high dv/dt) and high repetition frequencies of applied field pulses [5][6][7] far in excess of the typical power system value of 50/60 Hz. To limit high dv/dt stress within the insulation, modular multilevel converter topologies with trapezoidal modulation have been proposed for medium and high voltage DC-DC transformers [15].…”