Isolation Design of a 14.4Kv, 100Khz Transformer with a High Isolation Voltage (115Kv)

Isolation design of a 14.4kV, 100kHz transformer with a high isolation voltage (115kV) Michael Jaritz, Jurgen¨ Biela Laboratory for High Power Electronic Systems, ETH Zurich, Physikstrasse 3, CH-8092 Zurich, Switzerland Email: [email protected] Abstract—In this paper, the isolation design procedure of a 14:4kV output voltage, 100kHz transformer with an isolation CP 1 : n voltage of 115kV using Litz wire is presented. For designing CS LS CP CP the isolation, a comprehensive design method based on an VG V01 400V V analytical maximum electrical field evaluation and an electrical Cf 15kV 01 + - field conform design is used. The resulting design is verified VIn by long and short term partial discharge measurements on a 800V SPRC basic module1 CP prototype transformer. V G SPRC bm2 400V 2 SPRC ISOP stack Keywords— High voltage, high frequency transformer; V G SPRC bm3 isolation design 400V Klystron/IOT V02 V G SPRC bm4 400V I. INTRODUCTION VOut 2 SPRC ISOP stack 115kV For the new linear collider at the European Spallation V Source (ESS) in Lund, 2:88MW pulse modulators with pulsed G SPRC bm15 400V output voltages of 115kV and pulse lengths in the range V08 V G SPRC bm16 of a few milliseconds are required (pulse specifications see 400V 2 SPRC ISOP stack Tab.I). For generating these pulses, a long pulse modulator SPRC modulator system based on a modular series parallel resonant converter (SPRC) topology has been developed [1]. This converter is operated at Fig. 1: SPRC modulator system with 2 SPRC basic modules high switching frequencies (100kHz-110kHz) to minimize the forming an ISOP stack [2] and 8 of them are dimensions of the reactive components and the transformer. To connected in series to achieve the required output achieve the required output voltage of 115kV, 8 SPRC basic voltage. modules each with an output of 14:4kV are connected in series [2], see Fig.1. Due to the series connection of the SPRC basic modules, the insulation of the last oil isolated transformer in signed with respect to an isolation voltage of 15kV, a nominal the row has to withstand the full pulse voltage. output voltage of 3:8kV and 3kHz operation frequency. In In the literature several approaches are presented for designing [7] the design is carried out for a nominal output voltage a high voltage, high frequency transformer [3, 4, 5] with of 3kV, a switching frequency of 10kHz and provides partial nominal output voltages between 50kV-60kV and a switching discharge measurements for short term tests(1 min, test voltage frequency of 20kHz. The transformer presented in [6] is de- 28kV). However, all of these transformers are either tested only under nominal field conditions [3]-[6] and/or no values for long term partial discharge measurements which are an TABLE I: Pulse specifications essential life time key parameter for high voltage components 115 Pulse voltage VK 115 kV are given [7]. In addition the isolation voltage of kV and the switching frequency range of 100kHz-110kHz exceed by Pulse current IK 25 A far the designs in [3]-[7]. Therefore, in this paper an isola- P 2:88 MW Pulse power K tion design procedure for a 14:4kV nominal output voltage, Pulse repetition rate PRR 14 Hz 100kHz transformer with an isolation voltage of 115kV is Pulse width TP 3:5 ms given and verified by long term (60 min) nominal test voltage Pulse duty cycle D 0:05 and short term (5 min) extended test voltage (up to 136%) Pulse rise time (0::99 % VK ) trise 150 µs partial discharge measurements. In section II, an isolation design procedure which is part of Pulse fall time (100::10 % V ) t 150 µs K fall a transformer optimization procedure is presented, which is used to design the transformer for ESS. Afterwards, in section Pulse specifications (V , I , P ,...) K K K Primary Secondary winding winding Electrical SPRC basic modul model Ni Tmax, Emax, VSec, IPrim , N, f, B,... Transformer optimizer (Geometric parameters) Vsec Vsec Core geometry Winding arrangement Emax design Analytical Core Winding Analytical N N calaculation of 1 i N1 losses losses maximum electrical parasitics Lσ, Cd field evaluation Core window Δx Δx No No (b) Thermal model T<T (a) max E<Emax Vsec =15kV, VN1 = 100kV, VNi = 115kV Yes Ni Optimal design Ni Ni Post isolation field conform design check Field V sec Vsec Vsec shape 2D FEM evaluation of detailed geometry including all permittivities rings Obtain most critical oil and creepage paths N1 N1 Calculate safety factor “q” Δx = 0 Δx = 0 N1 (c) Δx (d) (e) No q > 1 Fig. 3: Five basic winding configurations: (a) Standard Yes winding, (b) flyback winding, (c) s-winding, (d) s-winding with ∆x = 0, and (e) s-winding Checked optimal design arrangement with field shape ring and ∆x = 0. Fig. 2: Transformer optimization with integrated isolation design procedure (grayed shown areas). the flyback winding configuration (see Fig.3(a) and (b)) lead 0 to high Emax, WE and VWS values [8]. The s-winding III the resulting design is evaluated by long term nominal configuration (see Fig.3(c) and (d)) has the advantage of a test voltage and short term over-voltage partial discharge minimum withstand voltage, but still high Emax values occur. measurements. Adding a field shape ring to Fig.3(d) results in the winding arrangement given in Fig.3(e). The first and the last turn of II. ISOLATION DESIGN PROCEDURE the winding are mounted inside field shape rings leading to a reduced Emax. Fig.4(a) and (b) show the electrical field Due to the high number of degrees of freedom during distribution of the s-winding configuration with and without the transformer design process as for example the geometric field shape rings. Comparing case (d) and (e) in Tab.II, the parameters of the core or the windings, an optimization occuring peak field is reduced by 43:3%. The field shape procedure has been developed for optimally designing the rings are on the same potential as the respective turn and one transformer (see Fig. 2) [1]. In the first step, an electrical model of the SPRC basic module determines the input parameters and constraints for the transformer optimization, TABLE II: Emax evaluation results for the given pulse specifications. Before the transformer 0 design procedure is started, first a specific core and winding ∆x Emax WE VWS geometry has to be chosen. For the core geometry an E-type Config. [mm] [kV/mm] [mJ/m] [kV] core is used. For the winding geometry, there are five basic (a) 6.1 16:53 435:22 Vsec winding configurations possible (Fig.3) which are investigated (b) 6.1 15:8 433:66 Vsec=2 with respect to the maximum electrical field, lowest electrical (c) 2.1 15:96 401:28 4Vsec=Ni 0 (d) 0 16:11 385:86 4Vsec=Ni energy per length WE and maximum wire to wire withstand (e) 0 11:24 461:64 4Vsec=Ni voltage VWS by varying the distance ∆x. The standard and losses do not increase much because most of the load current Core 17 is still conducted by the Litz wire and not by the field shaping 16 ring. With the defined core and winding geometry (Fig.3(e)) all losses and parasitics are calculated and also the maximum Emax=16.11 kV/mm 14 electrical peak field is estimated. Afterwards, in a FEM based post design check a detailed model of the transformer is 12 evaluated regarding oil gap widths and creepage paths. Primary winding In the following the isolation design procedure (areas high- Secondary 10 winding lighted in gray in Fig.2) which can be divided into an analytical a) maximum electrical field evaluation and a FEM supported post Core 8 (kV/mm) max isolation field conform design check are described more in E detail. 6 E =11.24 kV/mm Primary max A. Evaluation of the maximum electrical field winding Due to the complexity of the transformer isolation structure, 4 Field it is not computationally efficient to use a comprehensive shape analytical model of the transformer including all details as e.g. ring 2 bobbins, winding fastenings and oil gap barriers (see Fig.5) in the optimization procedure. Instead an analytical maximum Secondary winding 0 electrical field calculation is used, which is based on the image b) charge method [1] and allows a quick basic isolation design Fig. 4: (a) Maximum E-field of s-winding and (b) maximum check considering the maximal electrical field which has to E-field of s-winding with a field shape ring (10mm be below a certain constraint value (see Fig. 2). This method diameter) inside the core window. considers a single insolation material permittivity and is ≥ 7 times faster than FEM, because only a few points along the surface of the turn with the highest potential are evaluated to end of the turn is soldered to the corresponding field shape estimate the highest Emax value. ring, see Fig.5(d). Due to this arrangement the high frequency B. FEM supported field conform design In the following, first, the material characteristic of the components are presented and afterwards the FEM supported Primary-secondary bobbins fastening plate Secondary bobbin field conform post design procedure is discussed. For the built Primary POM POM Primary-secondary prototype emphasis was put on the choice of proper insulation oil gap barrier PC Equipotential lines 22cm Fixing plate Oil gap barrier Secondary Primary bobbin Oil gap barriers Sintered PA2200 Core Core Oil gap barrier P clamping rods 4 P Electrical field lines EPR S1 Primary 3 Equipotential lines 22cm winding P1 P2 27.3cm Primary P5 P6 Primary 100% (a) (b) bobbin 100% bobbin Core No triple points Field shape ring Secondary between bobbin fastenings winding 80% and ring inside the 80% core window Field shape rings Last turn Field shape rings 40% mounted inside field shape ring 60% 60% Secondary-core Primary oil gap barrier winding Secondary Soldered potential 40% PC winding connection between 20% P last secondary winding Secondary 7 20% and field shape ring Secondary Electrical field lines bobbin (c) (d) bobbin Primary outlets Fig.

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