IET：Prototype design for a high-voltage high-frequency rectifier transformer for high power use

Published in IET Power Electronics Received on15th October2009 Revised on17th February2010 doi:

10.1049/iet-pel.2009.0284

ISSN1755-4535 Prototype design for a high-voltage high-frequency recti?er transformer for high power use

Y.A.Wang D.M.Xiao

School of Electronic,Information and Electrical Engineering,Shanghai Jiaotong University,Minhang District,

Shanghai200240,People’s Republic of China

E-mail:wangyananxj@https://www.360docs.net/doc/ce13210997.html,

Abstract:The traditional high-voltage high-frequency transformer has a drawback of low power density owing to the rigorous requirements of high-voltage insulation.This study proposes a new con?guration for the magnetic core based on planar EE cores. The parallel connection of planar cores was adopted as a unit,and several units were cascaded to form the high-voltage transformer.The electrical potential distribution of the proposed transformer is more uniform than a traditional transformer, and enables a decrease in the insulation distances.The mechanical con?guration of a laboratory prototype is discussed,as well as the electrical,parasitic and thermal behaviours.A prototype transformer has been designed and built with the following characteristics:30kV output voltage,60kW output power and20kHz inverting frequency.The transformer was tested and found to have an ef?ciency of better than96%.Compared with traditional high-voltage transformers,this transformer has good thermal behaviour,good line insulation properties and a high power density.

1Introduction

In this paper,the discussion is focused on the design of a high-voltage high-frequency power transformer in switched supply for an electrostatic precipitator(ESP)application. There are several improvements in the switched supply compared with the conventional high-voltage DC power supply(50or60Hz transformer is adopted).For example, high-frequency switching operation will allow:(i)much more precise control over the operating parameters(such as output voltage level,current level,voltage rise times and response to variations in load demand),(ii)a reduction in the size and weight of the high-voltage transformer and enhancing the power density of the transformer[1,2]. However,the reduction in the size of the transformer is limited in high-voltage step-up transformers(.30kV).In order to obtain the required output high-voltage,it is necessary to employ a transformer with a large turns ratio and high insulation distances between primary and secondary windings, secondary windings and cores,and in the secondary itself[3]. The requirements for high-voltage insulation distances will become more and more rigorous with rated voltage of transformer https://www.360docs.net/doc/ce13210997.html,monly,the main insulation distance is proportional to the1.5th power of rated withstand voltage of the transformer.Consequently,the method of reduction transformer’s size only by increasing the transformer’s rated frequency is limited in high-voltage transformer.

The other methods that may reduce in the size and weight of the high-voltage high-frequency transformer are as follows: 1.Adoption of high-performance magnetic materials such as nanocrystalline core[4,5];2.Adoption of superconducting material[6];

3.Improvement of the transformer’s insulation structure[7,8]. The method(1)is able to increase the?ux density of core, and methods(2)will increase the current density of the windings of transformer and(3)enhances insulation structure of the high-voltage transformer mainly.But those methods do not overcome the contradiction between transformer’s power density decreasing at high voltage and its increasing at high frequency.In order to deal with the contradiction mentioned above,a new con?guration for the transformer magnetic core was proposed based on planar EE cores.The parallel connection of planar cores magnetic circuit was adopted as a unit,and several units were cascaded to form the high-voltage transformer.Cascade transformers have already been used for high-voltage test systems[9,10]and multilevel pulse width modulation (PWM)inverter applications[11,12].In[9],the frequency effect of cascade transformer was studied but it was used for 60Hz high-voltage test systems.In[11,12],high-quality output voltage waves were synthesised by a large number of output voltage levels using transformers,which have a series-connected secondary.In this paper,the cascade transformer has been used for high-frequency high-voltage applications.It has the merits of cascade transformer and planar cores[13].First,the output high-voltage is shared in the multistage voltage of transformer units,and the main insulation distance may decrease with voltage decreasing. Second,the planar cores provide a relatively large surface area for the transfer of dissipated heat to the environment. Thus,the contradiction of high-frequency and high-voltage impact on power density of transformer is eased largely.

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2Transformer principle proposed Fig.1illustrates the schematic circuit of the proposed transformer and power conversion circuit.It consists of a high-frequency transformer,a full-bridge inverter,a voltage multiplier and the ESP.The full-wave series multiplier is connected to the secondary side of the transformer.The full-wave series multiplier is AC–DC power conversion devices composed of the diodes (D 5,D 6,D 7and D 8)and the capacitors (C 5,C 6and C 7)that produce high DC voltage from a low-voltage AC source.The output voltage of the voltage multiplier is 22E pk ,where E pk is the peak value of the transformer output voltage.The term C E and the non-linear resistor R E represent the equivalent load of the ESP.

In this circuit topology,there is no dc current that could create a transformer core saturation problem,and a higher transformer utilisation factor than half-wave recti?er and full-wave recti?er versions is achieved [14].Additionally,compared with half-wave series multiplier,the full-wave series multiplier has lower voltage drops D V and ripple

voltage components d V

D V ?

I d 4fC 6(1)d V ?

I d 2fC 6

(2)

where I d is the average value of the output DC current,f is the frequency of the inverter and C 6is the voltage multiplier capacitance,which also has a role as a ?lter.

Fig.2shows the schematic structure of the proposed transformer.It consists of two same ‘transformer branches’(branch 1and branch 2).Each branch consists of three ‘transformer units’(unit 1,unit 2and unit 3).Three transformer units are cascaded to form one transformer branch.Each transformer unit has an ef?ciency of 98%.Unit 1,unit 2and unit 3have 30.6,20.2and 10kW output power,respectively.Thus,one transformer branch’s output power and ef?ciency can be calculated;they are 30kW and 96%,respectively.So,the overall cascade transformer has 60kW output power and an ef?ciency of 96%.

The electric potential to earth of the three transformer unit cores are 5,15,and 25kV.The turn ratios of the three transformer units are equal (?20)with the same input voltage (510V,20kHz).Thus,n 1?n 3,where n 1is the primary turns and n 3is the cascaded winding turns that supplied energising voltage for the next transformer unit.The current ratings of all units and all windings in one transformer branch are shown in Table 1.

By observing the potential difference of the secondary winding with respect to its core in Fig.2,it is clear that the maximum potential difference is 5kV.However,the corresponding potential difference of the conventional transformer (as shown in Fig.3)with the same output voltage is 30kV.Thus,the insulation distances of a cascaded transformer are much shorter than in a conventional transformer,and the cascaded transformer has a much higher power density.

3

Transformer design

In this section a high-voltage high-frequency cascade transformer for an ESP power supply was designed with

the

Fig.1Schematic circuit of the proposed transformer and power conversion

circuit

Fig.2Schematic structure of the proposed transformer Table 1

Current rating of all units and all windings

Transformer units Current ratings of n 1,A

Current ratings of n 2,A

Current ratings of n 3,A

transformer unit 166.44 1.0844.92transformer unit 243.84 1.0822.78transformer unit 3

21.70

1.08

–

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following electrical speci?cation:510V input voltage (quasi-square wave),30kV output voltage,60kW output power and 20kHz inverting frequency.Because the transformer branch 1is same as branch 2completely,the structure of transformer unit 1is similar to unit 2and 3(note:besides unit 3has a coupling windings,unit 1and unit 2have three windings,as shown in Fig.2).Thus,in the following subsections,the design of unit 1of the transformer branch 1is an example to the design of the overall cascade transformer mainly.The cascade transformer design required consideration of the insulation,the magnetic material and the management of electrical,magnetic and thermal stresses.3.1

Magnetic design

The area product AP is the product of the winding window area and the cross-sectional area of the core,and it is a useful design parameter in selecting the core and the number of turns per volt [15].

AP =

P t ×104

4B m f K u J

(3)

where AP is the area product (cm 4

),P t is the apparent power handling capability (VA),B m is the maximum core ?ux density (T),f is the operating frequency (Hz),K u is the window utilisation factor and J is the wire current density (A /cm 2).

The planar ferrite EE-type core ‘R 49938EE’from MAGNETICS Inc is selected for the cascade transformer.The material of ‘R 49938EE’is magnetic R-type material,which has low AC core losses and decreasing losses to temperature of 1008C.The dimensions of the core are shown in Fig.4and Table 2.The area product AP EE of a pair of EE is calculated from Fig.4and Table 2,

AP EE ?50.27cm 4.In each transformer branch design,the parameters of (3)were calculated or selected as follows:P t ?61.2×103W,B m ?0.3T (ferrite),f ?20×103Hz,K u ?0.3and J ?300A /cm 2.Hence,we obtained AP ?283.5cm 4from (3).Thus,transformer unit 1needs six pairs of ‘R 49938EE’(and same as the transformer unit 2,and transformer unit 3need four and two pairs of ‘R 49938EE’,respectively).

Once a core is chosen,the calculation of primary and secondary turns and wire size is readily accomplished.The selection of wire size also considers the existing skin and proximity effects in the current distribution in the copper wire [16].The important parameters of transformer unit 1are listed in Table 3.The conductor of winding n 1is three layer copper foils in parallel and each foil with 20mm width,0.4mm thick.The conductor of winding n 2is the ‘three-layer insulated round wire’with 0.75mm diameter and it is rated withstand voltage is 10kV.Two-layered copper foils in parallel are composed of the conductor of winding 3and the size of each foil is same as winding n 1.Other speci?cations and design results of the transformer unit 1windings is shown in Table 3.3.2

Insulation design

The design of the insulation is one of the important issues in this research.In order to convenience industry process,oil-paper insulation co-ordination is adopted as non-uniform insulation of the transformer.Oil is prior to use to prevent insulation breakdown when the electric ?eld intensity is too high.Since compared with insulating paper,the electric strength of oil is high (160kV /cm)and its dielectric constant is small (2.0–2.25).So the parasitic capacitance of the transformer may decrease when oil is adopted.Insulating paper is used to increase creepage distance and prevent creeping discharge.The insulation data of one window of transformer unit 1are listed in Table 4and the structure of windings of transformer unit 1is given in Fig.5.In Fig.5,the insulation of the x -axis direction is already shown in Table 4.In the y -axis direction,the 3mm oil clearance between bobbin and core yoke is reserved and the thickness of bobbin is 1mm.As shown in Figs.2and 5and Table 4,turn 1and turn 69of the n 2secondary windings are earthed and connected to the core,respectively.The primary winding is ‘sandwiched’between two groups of secondary windings.This type of winding arrangement has the lowest product of the parasitic inductance L s and capacitance C s ,and the product of L s

and

Fig.3Schematic circuit of the conventional

transformer

Fig.4Structure of the planar cores Table 2

Mechanical dimension of R 49938EE (mm)

A

B

C

D

102+1.5220.3+0.2537.5+0.4

13.3+0.25

E

F L M 86+1

14+0.25

8

36

Table 3

Parameters and copper losses of transformer unit 1windings

Winding Turn Wire

Current density,A /mill 2

Length of winding,mm

Copper losses,W

n 17three-layer copper foil 2.768425423n 2131three-layer insulated wire 2456937209n 3

7

two-layer copper foil

2.808

4710

15

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C s determines the overall high-frequency properties of the transformer[8,13].

3.3Thermal analysis

The power handling ability of a ferrite transformer is limited by either the saturation of the core material or,more commonly,the temperature rise.Temperature rise is important for overall circuit reliability,and staying below a given temperature insures that wire insulation is valid.On the other hand,as core temperature rises,core losses can rise and the maximum saturation?ux density decreases commonly.R-type material is adopted in our design transformer,which attempts to mitigate this problem by being tailored to have decreasing losses to temperature of 1008C.One of the two major factors effecting temperature rise is core loss,which is a function of the operating?ux density[15]

P core=af c B d m×10?3(4) where P core is the loss density(W/cm3),a,c and d is the factors(a?0.074,c?1.43,d?2.85if R-type material is adopted,and f,100kHz),f is the operating frequency (Hz),B m is the maximum core?ux density(kG, 10kG?1T).B m is calculated by

B m=

E

4A′c N1f×10?8

(5)

where E is the applied voltage of primary windings(V),A′c is the core area(cm2),N1is the number of turns of primary windings and f is the operating frequency(Hz).

The B m and P core can be calculated according to the datum of Tables2and3,namely,B m?2.89kG, P core?110.58mW/cm3.With reference to Fig.4,the total volume of the six pairs of‘R49938EE’is;?478.8cm3. Thus,the total core losses are53W.Copper loss is the second major contributor to temperature rise.According to Table3,the total copper loss of the windings is47W. Furthermore,the overall total losses are100W.For this situation,the power dissipation of transformer unit1is ,1%of the total output power.

The core shape also affects temperature and those that dissipate heat well are desirable.The planar cascade transformer is different from the conventional transformer in switching-mode power converters.The?uid

Table4Data of insulation section for transformer unit1

Core/insulation/ windings(x-axis direction)Electric potential

to earth,V

Potential difference

between

conductors,V

Insulation

distance,mm

Insulation

material

x-axis value,

mm

magnetic core25000––isolation with tank–bobbin25000––– 1.00 n269–51250001377 1.25thickness of bobbin 2.00 50–322362327540.50three-layer insulated wire 3.00 31–132224627540.50three-layer insulated wire 4.00 12–128702246.0.50three-layer insulated wire 5.00 n1+2552546(1170) 1.33(0.33)two-layers insulating paper

(0.08mm)

15.00

spacer and oil-duct from255to

25000

(absolute)

–––20.00

n270–88263776677 5.33two-layers insulating paper

(0.08mm)+pressboard+

three-layer

insulated wire

21.00

89–1072775427540.50three-layer insulated wire22.00 108–1262913027540.50three-layer insulated wire23.00 127–131294931739.0.50three-layer insulated wire24.00 n3132295661812(362) 1.33(0.33)two-layers insulating paper

(0.08mm)

25.04

––––––138210000730.16two-layers insulating paper

(0.08mm)

31.28 oil from210000

to25000

(absolute)

–––36.00

magnetic core2500050007.44isolation with tank

–

Fig.5Structure of windings of transformer unit1 https://www.360docs.net/doc/ce13210997.html,

encapsulation such as transformer oil was considered.Transformer oil is a good medium for heat transport.Under ideal natural convection conditions it has heat transfer coef?cient of 95W /m 2K 1;this is equivalent to forced air-cooling with a ?ow velocity of 25m /s [2].On the other hand,Oil cooling has many merits as follows:?It is favourable for improving the cooling,especially when multiple planar transformers are paralleled to form a higher-power transformer (this type of transformer is easy to integrate,which will be shown later).

?It is favourable for enhancing the insulation of the planar transformer (the insulation strength of oil is 16kV /mm).?Compared with sulfur hexa?uoride,transformer oil has little or no greenhouse effect.

Besides the planar transformer’s cores offer a large ?at surface,the ?exible oil circulation cooling system of windings is adopted as shown in Fig.6,where Fig.6a is the platform of the transformer unit 1,and Fig.6b is the photograph of it.Note that there are two sets of unit 1in Fig.6b ,one being the transformer branch 1and another being the transformer branch 2.The 6mm spacing is reserved between each pair of planar cores.The 2mm diameter of oil hole is adopted in the middle of spacing,which is convenient for oil circulation.So the conventional method of calculating temperature rise with natural cooling is not applicable simply.Consequently,a method described by Fothergill et al.[2]was used for the temperature rise calculations.A 258C temperature rise was determined by these calculations.3.4

Electrostatic analysis

The logical and physical structure of the branch 1of the overall planar cascade transformer is shown in Fig.7,where the planar cores compressed by two pairs of 3mm-thick epoxy compressing plate are composed of a power unit.The 3mm pure oil clearance is reserved between the power units in the

transformer active part assembly.According to electrostatic analysis,the electrical potential distribution in the direction of x 1,x 2,x 3plane (corresponding U 1,U 2and U 3),and x 4,x 5,x 6plane (corresponding U 4,U 5and U 6)can be drawn,as shown in Fig.8.The curves in Fig.8describe the maximum absolute values of the voltage of the windings in a work period,and the x -axial data are the size of the practical prototype transformer.

It can be shown that the electrostatic ?eld of the transformer branch is approximately uniformly distributed across the three transformer units.On the other hand,it can be seen that the electric ?eld intensity is high between the magnetic core yoke and the windings,and between the different windings in the same transformer unit.And the electric ?eld intensity between the different transformer units is also high.Consequently,the 1mm epoxy resin bobbin and 3mm oil clearance were placed between the yoke and windings,and the 3mm epoxy compressing plate and 3mm pure oil clearance is reserved between the different transformer units.Other insulation data are shown in Fig.5and Table 4.3.5

Component

One of the objectives in choosing the planar cascade con?guration was the characteristic of component of

the

Fig.6Transformer unit 1

a Platform

b Photograph (there are two unit 1in the

photograph)Fig.7Logical and physical structure of the transformer branch

a Logical structure

b Physical structure

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transformer unit as shown in https://www.360docs.net/doc/ce13210997.html,ly the transformer unit may act as an independent power transform voltage unit.The transformer units are cascaded to form a higher-voltage transformer,and a higher-power transformer can be derived from the transformer units in parallel.However,there is a lower-power transformer utilisation factor when over three stages of transformer units were cascaded.Hence,it should be a trade-off among insulation distance,operating frequency and stages of cascaded in practical transformer design.

4Leakage inductance and parasitic capacitance analysis

Fig.10a illustrates the equivalent circuit of the planar cascade transformer in Fig.2.In order to simplify the analysis of the planar cascade transformer,the mutual capacitance and resistances of the n 1,n 2and n 3windings (shown in Fig.2)were neglected.Because the transformer branch 1is same as branch 2completely,the transformer branch 1is analysed only here.Fig.10b illustrates an equivalent electric circuit of the transformer branch 1with all the n 1and n 3winding parameters referred to the n 2high-voltage

winding.These parameters are indicated by:L ′p1,L s1,L ′

k1,C ′p1,C s1and C ′

k1that are the leakage inductances and self-capacitances of transformer unit 1’s n 1,n 2and n 3

windings;L ′p2,L s2,L ′k2,C ′p2,C s2and C ′

k2which are the leakage inductances and self-capacitances of transformer

unit 2’s n 1,n 2and n 3windings;and L ′p3,L s3,C ′

p3and C s3which are the leakage inductances and self-capacitances of transformer unit 3’s n 1and n 2windings.Fig.11shows the simpli?ed equivalent circuit of Fig.10b ,where L e and C e are the lumped equivalent leakage inductance and parasitic capacitance [3,17].

According to Figs.10and 11,the following expression can be obtained

12L e i 2=12(L s1+L s2+L s3+L ′k2+L ′p3)i 2+1

2

L ′p1(3i )2+1

2(L ′k1+L ′p2)×(2i )2(6)We can show from formula (6)

L e =L s1+L s2+L s3+L ′k2+L ′p3+9L ′p1+4(L ′k1+L ′p2)

(7)

If the in?uence of leakage inductances on C with voltage is neglected,the following expression can be obtained 12C e u 22?12(C s1+C s2+C s3)×u 23 2+12C ′p1u 2

3 2

+12

(C ′k1+C ′

p2)×u 2

3 2+12(C ′k2+C ′

p3)×

u 23 2(8)

We can show from formula (8)

C e ?

C s1+C s2+C s3+C ′p1+C ′k1+C ′p2+C ′k2+C ′

p3

9

(9)So the total leakage inductances and the total parasitic capacitances of the overall cascade transformer are 0.5L e and 2C e ,respectively.From expression (7)and (9)we can see that the total leakage inductances are increasing,and the total parasitic capacitances are decreasing in the cascade transformer.According to the design in above sections,0.5L e is equal to 0.13mH and 2C e is equal to 0.6m F.

5Prototyping and testing results

Following the design and detailed analysis above,a prototype of the planar cascade transformer with output voltage 30kV,output power 60kW and work frequency 20kHz,was developed and tested.The whole structure of the transformer consists of two same ‘transformer branches’(branch 1and branch 2).Each branch consists of three ‘transformer units’(unit 1,unit 2and unit 3).According to the method of experimental

determination

Fig.8Potential distribution of the transformer branch

a x 1,x 2and x 3

b x 4,x 5and x

6

Fig.9Illustration of the component of the transformer unit

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parasitic parameters of the transformer described by Lu et al.[17],the lumped equivalent leakage inductance and parasitic capacitance of the overall cascade transformer are tested,and equal to 0.11mH and 0.62m F,respectively.5.1

Power density calculation

In Section 3.1and Fig.6,we can see that the lengths of transformer unit 1,unit 2and unit 3are 32.7,24and 15.3cm,respectively.All units have same highness and width.From Figs.7and8,we can see that the width and highness of transformer branch 1(includes unit 1,unit 2and unit 3)are 10.2and 13.98cm.So the volume of the transformer branch 1is 3422cm 3,but consider the margins,the volume may be 4000cm 3,then the total transformer’s volume is 8000cm 3.Thus,the power density of the overall planar cascade transformer is 7.5W /cm 3.

For comparison purpose,a conventional prototype transformer has been designed and built with the following characteristics:30kV output voltage,60kW output power,20kHz inverting frequency and rectangular magnetic core-coaxial cylindraceous winding structure.The volume of the transformer based on a conventional method [2]is 61000cm 3.So the power density of the conventional high-frequency transformer is 0.98W /cm 3.Clearly,the planar cascade transformer is considerably smaller than the conventional high-frequency transformer.5.2

Experiment

In this paper,the insulation,the current and voltage waveform performance of the transformer is focused particularly.5.2.1Dielectric test:Short-duration power-frequency tests which supplied to high-voltage winding –low-voltage winding of each transformer unit is passed,where the voltage is 20kV,the duration is 60s.So the 30kV-rated voltage requirement of the overall cascade transformer is satis?ed.As the sum voltage of three transformer units is 48kV by adding appropriate margins that may be non-uniform electrical potential distribution between the transformer units.

(20×3)×0.8=48

kv

Fig.10Equivalent circuit

a Overall cascade transformer

b Transformer branch

1

Fig.11Simpli?ed equivalent circuit of the transformer branch 1

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5.2.2Commissioning test:The experimental circuit is shown as Fig.1.In the experimental circuit,the SKM 300GB125D switching devices from Semikron are chosen as the switches S 1,S 2,S 3and S 4.It is the maximum collector current I CP and the rate current I F of the anti-parallel diode is 210A and 180A,respectively.The pairs S 1(S 3)and S 2(S 4)operate at a symmetrical duty ratio.This operation requires a short and well-de?ned dead time (3.41m s)between conduction intervals.The terms D 1,D 2,D 3and D 4are the anti-paralleled diodes for the insulated gate bipolar transistor (IGBTs).The terms C 1,C 2,C 3and C 4are the paralleled non-inductive capacitances for the IGBTs,which are used to achieve zero-voltage switching.The simulated load C E and R E on the experimental circuit is 2.7nF and 50k V ,respectively.

Test waveforms are shown in Fig.12.The term v AB and i P are the transformer input voltage and current,respectively,and v O and i O are the full-wave series multiplier output voltage and current,respectively.The positive direction and symbols for the above variables are shown in Fig.1.

Experiment 1:The waveform at maximum power is shown in Figs.12a and b .From the primary input voltage current waveform and secondary output voltage current waveform,the ef?ciency of 96.7%of the planar cascade transformer can be derived,and the output power and voltage of the power conversion circuit (through transformer and voltage multiplier)are 60kW and 50kV,respectively.Considering multiplier and load voltage drops,we believe that the output power and voltage values of the transformer have met the design demands (60kW and 30kV).

Experiment 2:The waveforms from the initiation of our power source are shown in Figs.12c and d .These plots show that the output voltage can rapidly rise up in a transient state.

Additionally,the temperature rise of the actual transformer is 24+18C in previous experiments,and it is better than the design value.

6Conclusion

This paper describes a novel transformer with magnetic cores based on planar EE cores and cascade transformer technology.The main features of the proposed transformer can be summarised as follows:

?The electrical potential distribution is more uniform than the conventional high-frequency transformer,and enables a considerably decrease in the insulation distances.

?The power density of the planar cascade transformer is high enough to 7.5W /cm 3.

?The transformer unit of the planar cascade transformer has the characteristic of component.It is ?exible to output voltage extending and output power extending.

?Cooling is good and the ef?ciency is .96%.

A prototype transformer with an output voltage of 30kV,output power of 60kW and inverting frequency of 20kHz was built,and the design and analysis were veri?ed by the test

results.

Fig.12Experimental waveforms

a ,

b Waveforms at the maximum power output

c ,

d Waveforms at th

e beginning

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7Acknowledgments

The authors are grateful to manager Shenghe Tian from Shanghai Yongjiu Power Transformer Company,senior engineer Ming Hu from Shanghai Power Company.

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