DESIGN AND DEVELOPMENT OF 500 kV, 100 mA DC HIGH …...The HVHF step-up transformer is a ferrite core three winding transformer. The centre tap of the two series connected secondary

ASHOK D. MANKANI et.al

DESIGN AND DEVELOPMENT OF 500 kV, 100 mA DC HIGH VOLTAGE POWER SUPPLY FOR PARTICLE ACCELERATORS AT IPR ASHOK D. MANKANI Institute for Plasma Research (IPR), Bhat, Gandhinagar, India Email: [email protected] AMAL S, SAURABH KUMAR, ARITRA CHAKRABORTY, URMIL THAKER, PAUL CHRISTAN, U. K. BARUAH Institute for Plasma Research (IPR), Bhat, Gandhinagar, India

Abstract

At IPR Neutral Beam Injection (NBI) facility to heat the plasma and drive the plasma current in Tokamak has been built by accelerating the positive / negative ion beam of energy/power around 100 keV/ 5 MW. Under the current R&D plan the projection is to develop the technology for future Mega Volt range DC Power Source facility to accelerate ion beam of energy to the tune of 1 MeV and power of the order of few MW. To meet this objective a compact 500 kV, 100 mA DC upgradable to 1000 kV Power supply is being designed and developed as a first step. This power supply shall not only serve as test facility for future larger systems but also be used for several other applications within IPR related to particle accelerator.

The 500 kV, 100 mA, 50 kW DC Regulated High Voltage Power Supply (500 kV-RHVPS) for particle accelerator is being designed using a symmetrical Cockcroft-Walton Voltage Multiplier (CW-VM) topology owing to its design simplicity and economical construction. This paper will present the design and simulation results of 500 kV-RHVPS modelled in MATLAB Simscape toolbox. The paper will explain the optimization / sensitivity study performed in selecting and sizing of CW-VM, High Frequency Power Source (HFPS), and High Voltage High Freqency Step-up Transformer (HVHF-Tr); the primary components of 500 kV-RHVPS taking into account the possible operational difficulties and future expansion. Both steady state and transient study results will be explained. This paper will briefly cover the engineering assembly design aspects of voltage multiplier unit in general and of a 250 kV prototype voltage multiplier developed.

1. INTRODUCTION

High Voltage DC Powers Supplies are widely used in the field of R&D and in industry. They are used in scientific instruments, TV sets and CRTs, Oscilloscope, X – ray, particle accelerators, laser systems and many other applications. The method of stepping up the voltage is commonly done by a step-up transformer in an AC system. Voltage multipliers are preferred owing to its design simplicity and economical construction for stepping up the DC voltage. Cockcroft-Walton voltage multiplier developed by Greinacher and later enhanced by Cockcroft and Walton intending to produce high-energy positive ion beams [1]. The symmetrical Cockcroft-Walton Voltage Multiplier (CW-VM) is constructed by ladder network of capacitors and diodes for generation of high voltage DC from a low input voltage e.g. 400 V, 50 Hz AC supply. Other advantages of such cascade generators are: (a) low voltage rating of components, (b) balanced voltage w.r.t. ground, (c) gradual build-up of voltage, and (d) modular construction. The use of a high frequency power source instead of power frequency (50 Hz) gives an added advantage of low stored energy, less ripple, better regulation and faster response. This paper describes the design of 500 kV, 100 mA DC Power Supply which primarily composed of a regulated low frequency to high frequency converter (high frequency power source - HFPS), a step-up transformer, and a symmetrical CW-VM. The output high voltage of the power supply is controlled / regulated by controlling the output voltage of the front end HFPS operating in close loop feedback control.

2. DESIGN CONSIDERATIONS

The basic requirements / parameters for the design of 500 kV DC RHVPS considered are tabulated in Table 1

TABLE 1. RHVPS DESIGN PARMAETRS

Parameters Data Nominal output voltage 500 kV DC Nominal output current 100 mA Nominal output power 50 kW Output ripple ≤ 0.5 % at rated output power

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Range of output voltage regulation 5% - 100% Regulation ≤ 1 % (no load to full load) Polarity Positive Duty Continuous Overall efficiency ≥ 85% at rated output power Response time < 100 msec

Two governing equations for the design of CW-VM based 500 kV, 100 mA DC Regulated High Voltage Power Supply (RHVPS) given below have been considered. At no load the output voltage is twice the peak input voltage (V) multiplied by the number of stages (n). However on load due to drainage of charge the output voltage never reaches 2nVm. The ripple voltage (δV) can be obtained from Equation-1, and the maximum output voltage is given by Equation-2 [2].

δV = nI/ (2fC) (1)

Vmax = 2nVm – (n2/2 + 1) (nI/3fC) (2)

The ripple and voltage regulation of the CW-VM largely depends on the supply frequency and multiplier component sizing. By through choice of higher supply frequency to obtain the required ripple and regulation; the size and cost of the CW-VM can be sufficiently reduced.

The frequency of HFPS is fixed to 20 kHz after the optimization study performed on various factors such as switching losses in HFPS solid state switches (e.g. IGBT), CW-VM output DC voltage ripple and regulation requirement and CW-VM primary component viz. diode and capacitors sizing and availability, and step-up transformer design. The Fig. 1 shows the main components of RHVPS system, they are:

- High Frequency Power Source (HFPS); - High Voltage High Frequency Step-up Transformer; and - Symmetrical Cockcroft-Walton Voltage Multiplier (CW-VM).

FIG. 1. Genral Block Diagram of 500 kV, 100 mA DC Regulated High Voltage Power Supply (RHVPS) System.

2.1 High Frequency Power Source (HFPS)

A typical HFPS configuration/topology consist of a Rectifier, DC link Capacitor and full H-Bridge Inverter [3]. In view of the voltage control required at the RHVPS system output the HFPS output voltage also requires to be regulated/control and tuned to the requirement of RHVPS system output. Several control techniques have been studied viz. Phase Shift Control, PWM control, firing angle control of Rectifier, and a Buck-voltage-fed control. Considering the power and the switching frequency, a buck-voltage-fed full-wave bridge topology seems to be the potential control technique for the development [4].

The Fig. 2 as shows the topology selected for the HFPS. The buck converter converts the DC link voltage to variable DC voltage and supplies power to the full H-Bridge Inverter. The buck converter comprising of a IGBT switch operates at a switching frequency of 20 kHz same as of H-Bridge switching frequency, a free-wheeling diode to prevent the DC voltage from going negative, and a LC filter to produce ripple free DC voltage equal to the average of the duty-cycle-modulated raw DC input. By varying the duty cycle of the IGBT, the output DC voltage could be controlled as per the requirement.

SourceInput: 415 V, 50Hz, 3-ph

AC

HFPSOutput:

100 kVA, 400V (RMS),

1-Phase, 20kHz

HVHF Step-up

TransformerOutput: 80 kVA

25kV-0-25kV(RMS)

1-Phase, 20kHz

Symmetrical CW Voltage multiplier(CW-VM)

Output: 500kV,

100mA DC

LOADDC Particle Accelerator

OR Resistive Load

Bank

ASHOK D. MANKANI et.al

FIG. 2. Schematic of High Frequency Power Source (HFPS).

2.2 High Voltage High Frequency Step-up Transformer (HVHF-Tr)

The HVHF step-up transformer is a ferrite core three winding transformer. The centre tap of the two series connected secondary windings is grounded. The insulation between the windings and core is designed for 60 kV. As this transformer will operate at high frequency its parasitic capacitance component cannot be ignored which shall lead to high current spikes in the primary winding of transformer. The transformer leakage inductance and parasitic capacitance has a inverse relationship, hence the design of HVHF-Tr particularly for this application is designed for minimum parasitic capacitance.

2.3 Symmetrical Cockcroft-Walton Voltage Multiplier (CW-VM)

The symmetrical CW-VM is being designed considering 10 stages. Each stage components viz. capacitor, diodes, resistor, and protection devices are mounted on a circular FR-4 acrylic sheet of diameter 750 mm forming one deck. The spacing between the two stages is 150 mm height. Each deck assembly corresponds to a single stage of CW-VM. The 10 deck assembly then is tanked into fibre reinforced plastic (FRP) / carbon steel tank filled with pressurised dry nitrogen gas or mixture of nitrogen and SF6 gas. As shown in Fig. 3 the CW-VM is formed using a symmetrical (full wave) circuit as its load characteristics are superior to those of a conventional (half wave) circuit. The capacitance (C1, C1’, C1”; ….; C10, C10’, C10”) value in both AC (oscillating) and DC (smoothing) column chosen at this moment is same i.e. 20 nF considering the margin of 30% and operating de-rating factors. The fast recovery diode of current rating 1.5 - 2 A considered, to limit the capacitor charging current and diode surge current a resistor (R2) in series with each diode provided. To limit the output current in case of a fault a current limiting resistor (R3) after the last i.e. 10th stage is considered. Both capacitor and diode stacks are formed using series parallel combination of standard value capacitor and diode available in market. The voltage divider (R4) made of metal oxide resistor network for measuring the terminal voltage is designed for 10000:1 ratio and draw maximum 1-2 mA of leakage current. The load current is measured at the transformer secondary centre tap grounding point.

10 stage Voltage Multiplier Circuit inside HV TankC1

C1

C1’’

n1 n10

A

V

V

V

R1R1R1R1R2

R1R1R1R1R2

R1R1R1R1

R2R1R1R1R1

R2

R5R4

R3

R6

D

’

400V/25kV-0-25kV(RMS),20kHz, 80kVA

Transformer

FIG. 3. Schematic of Symmetrical Cockcroft-Walton Voltage Multiplier (CW-VM)

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3. PROTECTION AND CONTROL

All the components of 500 kV-RHVPS system have been designed considering various internal fault cases such as shoot-through fault in HFPS, arching in CW-VM etc... and external fault cases viz. CW-VM terminal fault. In the design of each component appropriately the protection devices have been considered. A simple shunt resistor or a hall-effect sensor or a over voltage detector on the DC Link can detect the peak of the current/voltage and triggers the mains AC circuit breaker and shut-off all the four gates of inverter. Fuses provided on DC link or at the output of HFPS acts as a back-up protection, in case the electronic protection fails. In any case the fault (internal or external) shall be clear in maximum 100 µsec.

The current and the voltage signal from CW-VM output is used as a feedback signal to control the duty cycle of the buck converter through a PID controller. By controlling the duty cycle the CW-VM output voltage varied from 10% to 100 % of nominal voltage. Also the overall regulation of 500kV-RHVPS considering the load and line voltage variation can be brought to less than 1% upon duty cycle control on HFPS.

4. SIMULATION AND RESULTS

The simulation study of the 500 kV- RHVPS system is performed in MATLAB and its toolboxes Simulink and Simscape. Discrete solver with time step of 1 µsec is used for performing study. The study is performed under both steady state and transient state condition. Following are the important results obtained from the study.

4.1 Steady State Analysis

Fig. 4 shows the CW-VM output voltage and current profile at 100 % and at 5% duty cycle set for the buck converter of HFPS. Hence the requirement of output voltage control range is achievable. The CW-VM voltage regulation i.e. from 0 % to 100 % loading considering the isolated source obtained is 20%; this due to the voltage drop in capacitor, diodes, protective resistors and leakage inductance of the step-up transformer [5]. This could will be further improved upon the experimental results obtain on 250 kV prototype testing.

FIG. 4. CW-VM Voltage and Current Profile @ 100 % and 5& duty

Fig. 5 shows the CW-VM output voltage and current ripple profile at 100 % load. As can been seen the ripple voltage is less than 0.1% as against the required 0.5% i.e. 2500 Vpp.

FIG. 5. CW-VM Output Voltage and Current Ripple

ASHOK D. MANKANI et.al

Fig. 6 shows the HFPS output voltage and current profile at 100 % load. The LC filter at the output of HFPS further requires further tuning to achieve sine wave or THD less than 5%. With the connected LC filter the THD obtained is 15%.

FIG. 6. HFPS Output Voltage and Current Profile

The HVHF step-up transformer primary and secondary voltage profile is shown in Fig 7.

FIG. 7. HVHF Step-up Transformer Primary and Secondary Voltage Profile

4.2 Transient State Analysis

In case of terminal short at the CW-VM, as shown in Fig. 8 the voltage drops instantly at time 55 millisecond but the current shoots to about 5 A which gradually reduces to around 400 mA and remains until the IGBT pulses of the HFPS are blocked at time 60 millisecond. In the same time interval of time i.e. between 55 millisecond to 60 millisecond as shown in Fig 9, the HFPS current rises to around 3 times nominal current and goes to zero when the HFPS inverter pulses are blocked. Effect of CW-VM terminal short is also seen at the DC link capacitor as shown in Fig. 10, about 10% voltage rises across the capacitor during this transient state.

FIG. 8. CW-VM Voltage and Current Profile under CW_VM terminal fault condition

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FIG. 9. HFPS Voltage and Current Profile under CW_VM terminal fault condition

FIG. 10. DCLink Voltage and Current Profile under CW_VM terminal fault condition

The Fig. 11 shows the current flowing through the 1st , 5th and 10th stage smoothing column capacitor and as seen the under steady state this current is 100 mA which shoots to around 5 A and dies down to zero when HFPS is switched off.

FIG. 11. Current Profile of 1st, 5th and 10th CWCapacitor under CW_VM terminal fault condition

ASHOK D. MANKANI et.al

Similarly the current through the diode rises from 300 mA to about 1.5 A goes to zero upon clearing of fault as shown in Fig. 12. The maximum voltage built-up across each capacitor and diode as observed to go upto 68 kV under steady state operating condition.

FIG. 12. Current and Voltage Profile CWDiode under CW_VM terminal fault condition

5. CAD DESIGN MODEL OF CW-VM

The CAD model of CW-VM is shown in Fig. 13. As said above the CW-VM assembly is made in 10 stages stacked one over the other using delrin insulators/spacers. The entire assembly is supported at the bottom plate of the tank. The components are mounted on FR-4 acrylic sheet. The top plate of the tank is made of polypropylene insulating sheet which facilitate high voltage terminal to be brought out through specially designed high voltage connectors. The overall dimensions of the CW-VM have been estimated to be 3900 mm height and 1500 diameter. The 500 kV silicon rubber insulated cable used for connection between 500 kV connector and load. The total losses estimated to be within 3 kW which is important to limit the temperature and pressure of gas inside the tank.

HV CAP80kV,20nF

HV CAP80kV,20nF

HV CAP80kV,20nF

FR-4 Acrylic Disc

25kV(RMS)

25kV(RMS)

GND

Delrin Spacer

Delrin Spacer

HV cable

FIG. 13. CW_VM CAD Model

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6. CONCLUSION

The ambitious 500 kV, 100 mA DC Regulated High Voltage Power Supply system is in design state. Various topologies / configurations to build this rating of RHVPS have been explored and studied; the Cockcroft-Walton based topology seems to be very promising owing to its simple design. The high voltage assembly, in particular the output high voltage termination remains a big challenge which is being worked out using a special designed high voltage receptacle and connector that is brought through the tank top insulating cover. The simulation study for some topologies of HFPS also has been analysed, the buck converter regulated inverter design is finalised to bring down the overall switching losses. The HFHF-Tr design is finalised and is under manufacturing. Owing to close inductive and / or capacitive coupling between the HFPS and HVHF-Tr, the design of output filter of HFPS is yet to be finalised. The 500 kV-RHVPS regulation using the duty cycle control of HFPS could be brought down to less than 1% when operating in close loop feedback control. The simulation results though seems to be satisfying the requirement, the real experimental results on prototype 250 kV CW-VM will assure/validate the design and performance.

7. FUTURE PLANS

Finite Element Analysis (FEA) to study the electrostatic field distribution and Computational Fluid Dynamics (CFD) analysis to check the overall thermal stability of CW-VM is being planned. The prototype testing of 250 kV CW-VM with the support of industry is also in progress.

ACKNOWLEDGEMENT

The authors want to thank all the colleagues of the division for their co-operation and support in design and development of ambitious 500 kV, 100 mA DC Power Supply systems. Special thanks to the Director, IPR, for allowing us to develop HV power supply.

REFERENCE

[1] Wagner L. Araujo, Tarcisio P.R. Campos, “Design of High DC Voltage generator and DT-Fusion based on Particle accelerators” 2011 International Nuclear Atlantic Conference.

[2] (Late) M.S. Naidu and V. Kamaraju, High voltage engineering, McGraw-Hill Education (India) Private Limited, 2013, pp. 152-155.

[3] Abraham I. Pressman, Switching power supply design, The McGraw-Hill Companies, Inc., 1999, pp. 13-24. [4] A Chakraborty, “Design concept of a high power high frequency power supply for feeding 500 kV, 100 mA

Cockcroft-Walton generator”, National Power Electronic Conference 2017. [5] Tong-Ling Su, “A 600 kV 15 mA Cockcroft-Walton High voltage power supply with high stability and low ripple

voltage”, Nuclear Instruments and Methods in Physics Research A 560 (2006) 613-616.