Electrical Diagnostics For Pulsed Power
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ELECTRICAL DIAGNOSTICS FOR PULSED POWER
Rishi Verma, R. S. Rawat, P. Lee, S. V. Springham, T. L. Tan NSSE, NIE, Nanyang Technological University 1 Nanyang Walk, 637616, Singapore
M. Krishnan Alameda Applied Sciences Corporation, San Leandro, CA 94577, USA
Abstract Pulsed power systems are integral part of any pulsed plasma radiation device and hence the associated electrical diagnostics plays vital role in investigating the overall device performance and its characteristics. The typical diagnostic parameters of interest in any pulsed power system are linked with the measurement of high frequency, high voltages and currents. There is wide range of available diagnostics being used by practicing researchers for the measurement of mentioned parameters but even though they operate on simple laws of electromagnetics and the conceptual understanding is clear; the bandwidth response of such diagnostics is often limited by various parasitic effects that impairs the factual measurement of parameters. The scope of the paper is to introduce various invasive and noninvasive electrical diagnostics used in pulsed power systems and highlight the concealed causes that affect their behavioral response.
Purpose This talk is meant to provide an overview of standard electrical diagnostic techniques used in pulsed power systems driving pulsed plasma devices. …….. Impulse
Measurements!
The main focus will be on pulsed electric and magnetic field (Voltage & Current) measurement techniques having bandwidth response in ns to ms regimes.
Parasitic effects that impairs the factual measurement of parameters will be discussed.
Overview of design methodology. Noise and Shielding. International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Categorization Pulsed Power Electrical Diagnostic Tools
Current measuring devices
Non-intrusive
Voltage measuring devices
Intrusive
Intrusive
Non-intrusive
Rogowski Coils Current Transformers Current Shunt Simple resistive dividers Compensated dividers Capacitive Voltage dividers International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Rogowski Coils “most effective, economic and extensively used diagnostic” r r i = ∫ H .d l Amperes Law It is an air-cored toroidal coil that surrounds the conductor carrying the current to be measured.
Faraday’s Law
Vcoil
dφ = n× dt
# Gennadiy Frolov et al., Microbridge Technologies; EE Times-India, December 2007
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Sensitivity of Rogowski Coil The current to be measured is related to the induced voltage by a proportionality constant i.e. the mutual inductance of the coil.
Vcoil
di = − M 21 × dt
M = µ 0 nA
M = Coil Sensitivity (Vs/A) (depends on the coil winding design) di/dt = rate of change of current (A/s) n & A = design and geometry parameters International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Sensitivities for different cross-sections Rectangular Cross-section
Circular Cross-section
Oval Cross-section
# Jan Hlavacek et al., 16th IMEKO TC4 Symposium, Exploring New Frontiers of Instrumentation and Methods for Electrical and Electronic Measurements, Sept. 22-24, 2008, Florence, Italy
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Time response consideration Differentiating / Integrating ! - depends on circuit parameters dI c I c dφ =L + dt dt R 1 dφ L dI c = + Ic R dt R dt
Self-Integrating
Differentiating L dI c << I c R dt
R >> ωL
dφ Ic α dt
L dI c >> I c R dt
ωL >> R
International Workshop on Plasma Diagnostics and Applications, Singapore
Ic α φ July 2 – 3, 2009
Realistic lumped circuit model
I(t)
- solution is complex ! # M. Argueso et al., www.aedie.org/9CHLIE-paper-send/252-argueso.pdf
High frequency response (bandwidth) is determined by : Coil inductance (Lc) Stray capacitance of winding (Cc) Coil resistance (Rc) Termination impedance (Z) International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
High bandwidth issues 1. The rise time (tr) of the measuring pulse is limited by the wave transit time (T) in the coil winding. tr >T always
2. Role of termination impedance (Z) is very important. R >> ωL ωL >> R
20 ns/div 5 ns/div
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
High bandwidth issues 3. Highest frequency 4. Non-uniform excitation measurement limited by due to dislocation of resonant frequency (LC) current centroid may lead of the coil. to strong oscillations in “distributed capacitance the sensor signal due large no. of turns”
# http://www.pemuk.com
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
High bandwidth issues 5. High voltage consideration
6. Shielding - is placing the Rogowski coil inside the slotted metallic housing.
# http://www.pearsonelectronics.com
“Some times coupling capacitance b/w the winding and shielding may affect the signal response” International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Design methodology for differentiating Rogowski Step 1: Estimate the di/dt in the circuit. di 2π = × I pk dt T
T = 2π LC
I pk = Vch
C L
Step 2: Fix the max. limit for the induced voltage (Vcoil). Step 3: Use the basic equation: Vcoil = NA ×
µ 0 di × 2πR dt
A = a×b
Step 4: Choose optimum values for – a,b, R and N. International Workshop on Plasma Diagnostics and Applications, Singapore
# John Anderson, RSI 42,7,1971
July 2 – 3, 2009
Current Monitors
# http://www.pearsonelectronics.com
# Chris Waters, PCIM Article 86; http://www.pearsonelectronics.com
- are similar to self integrating Rogowski Coils in response but utilize high permeability magnetic core for coil winding. - the presence of high permeability core is important for the extension of flat response to low frequency. - Usage: CT’s – Universal / Rogowski Coil - Customized. International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Major limitation with CT’s “Core Saturation i.e. I × T ” Vcoil
dφ = n× dt
∫ V (t )dt = n ∆φ
∆φ - is the change in flux in the core
Since the max. flux is limited by core saturation there is a corresponding limit on : I × T n 2 B max A In terms of design parameters : ∫ I (t ) dt ≤ R
“Core may also get saturated by the DC component of the current being measured, Biasing overcomes this problem” # Chris Waters, PCIM Article 86; http://www.pearsonelectronics.com
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Important time domain parameters
(I × T )max - current – time product rating must not exceed I max - highest measurable current (related with (I × T )max ) t r (10 − 90%) - Useable rise time (<10% overshoot)
Sensitivity(Volt/Ampere) - typical range 0.001 - 0.01V/A Droop – it’s the distortion in the pulse shape of longer duration current pulses (ms to 100’s of ms) # http://www.pemuk.com
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Current Shunts/ CVR’s “Use of shunt is based on measurement of the voltage drop across the resistance of known value”
2 ECVR = RCVR ∫ I max dt # Mark E. Savage, Pulsed Power Electrical Diagnostics, IEEE Pulsed Power-Plasma Science Mini-course, June 23 2007
Current shunts/ CVR’s have: High peak power
“Ideally – Ohmic (Non-Inductive)” # Hansjoachim Bluhm, Pulsed Power Systems; ISBN10 3-540-26137-0, ISBN-13 978-3-540-26137-7 Springer Verlag Berlin Heidelberg 2006
High frequency response Large pulse energy handling capacity International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
CVR’s from T & M Research, USA Hi-Wattage (225W - R Series) CVR’s
Sub-milliohms
Up to sub-nanosecond
Up to 100’s of MHz
Up to 10’s of kJ’s
# http://www.tandmresearch.com
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
High voltage impulse measurements 1. Simple resistive dividers C o p pe r casin g (grou n de d to ch a m be r fram e )
GND Vin (HV)
Vout In su lato r R esisto r ch ain (1 0×5 10 Ω ) To p o sitive flan ge
(R1) HV Arm
51 Ω
R2 = Vin R1 + R2
BNC C o n n e cto r
(R2) LV Arm
Limitation:
“Measurement of fast signals with large division ratio” # E. Kuffel et al., High Voltage Engineering Fundamentals, ISBN 0 7506 3634 3
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
High voltage impulse measurements 2. RC compensated dividers Shunt capacitance and inductance of Resistor L = 30nH , C = 534 fF For R = 1 kΩ RC ≈ 500ps L/R ≈ 30ps Equivalent circuit of Resistor
It is necessary to balance the time constants of both the arms # Mark E. Savage, Pulsed Power Electrical Diagnostics, IEEE Pulsed Power-Plasma Science Mini-course, June 23 2007
R1C1 = R2C 2
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
High voltage impulse measurements 3. Capacitive voltage dividers
Equivalent circuit
V C + C2 R1 + R2 Attn. Ratio 2 = 1 V C R 1 2 1
# Hansjoachim Bluhm, Pulsed Power Systems; ISBN-10 3-540-26137-0, ISBN-13 978-3-540-26137-7 Springer Verlag Berlin Heidelberg 2006
Differentiating
(R1 + R2 ) (C1 + C2 ) << tr V2 = (R1 + R2 )C1
dV1 dt
dV1 V1 (C +C ) dV2 = + 1 2 (R1 + R2 )C1 dt C1 dt Integrating
(R1 + R2 ) (C1 + C2 ) >> tr C1 V 2 V2 = C1 + C2
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Safe practices for maintaining wave shape fidelity
When long length cables are used it is advisable to use 50Ω termination at scope end. Noise travels faster in air than cables. The instrument grounds must be isolated from the equipment ground for avoiding ground loop noise. Using differential probes giving (Ch1 – Ch2): – Ch1: +Vreal+Vparasite – Ch2: -Vreal+Vparasite
Noise reduction by ferrite cores. Enhances Shield Inductance. Spurious ringing is produced due to high frequency currents flowing out side the cable shield # http://www.pearsonelectronics.com
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Conclusion Awareness of data quality is an important issue and it can only be improved by proper understanding of response of the diagnostic tools and the factors limiting their bandwidths. Noise problems are often challenging and shield currents are the main cause. Good cabling and grounding practices solve most noise problems (e.g. use of double shield cables) High bandwidth response of data acquisition system i.e. oscilloscopes/ fast digitizers is equally important for good data quality for e.g. – tr – rise time BW - bandwidth
BWsignal =
0 .4 tr
BWscope = n × BWsignal
(# http://www2.tek.com/cmsreplive/pirep/3802/55W_18024_2_2009.04.07.10.03.56_3802_EN.pdf) (# http://cp.literature.agilent.com/litweb/pdf/5989-5733EN.pdf)
For n = 3, 5GHz/80ps
1.66GHz/240ps
1/50ps ~20GS/s
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Thanks International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Rishi Verma, R. S. Rawat, P. Lee, S. V. Springham, T. L. Tan NSSE, NIE, Nanyang Technological University 1 Nanyang Walk, 637616, Singapore
M. Krishnan Alameda Applied Sciences Corporation, San Leandro, CA 94577, USA
Abstract Pulsed power systems are integral part of any pulsed plasma radiation device and hence the associated electrical diagnostics plays vital role in investigating the overall device performance and its characteristics. The typical diagnostic parameters of interest in any pulsed power system are linked with the measurement of high frequency, high voltages and currents. There is wide range of available diagnostics being used by practicing researchers for the measurement of mentioned parameters but even though they operate on simple laws of electromagnetics and the conceptual understanding is clear; the bandwidth response of such diagnostics is often limited by various parasitic effects that impairs the factual measurement of parameters. The scope of the paper is to introduce various invasive and noninvasive electrical diagnostics used in pulsed power systems and highlight the concealed causes that affect their behavioral response.
Purpose This talk is meant to provide an overview of standard electrical diagnostic techniques used in pulsed power systems driving pulsed plasma devices. …….. Impulse
Measurements!
The main focus will be on pulsed electric and magnetic field (Voltage & Current) measurement techniques having bandwidth response in ns to ms regimes.
Parasitic effects that impairs the factual measurement of parameters will be discussed.
Overview of design methodology. Noise and Shielding. International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Categorization Pulsed Power Electrical Diagnostic Tools
Current measuring devices
Non-intrusive
Voltage measuring devices
Intrusive
Intrusive
Non-intrusive
Rogowski Coils Current Transformers Current Shunt Simple resistive dividers Compensated dividers Capacitive Voltage dividers International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Rogowski Coils “most effective, economic and extensively used diagnostic” r r i = ∫ H .d l Amperes Law It is an air-cored toroidal coil that surrounds the conductor carrying the current to be measured.
Faraday’s Law
Vcoil
dφ = n× dt
# Gennadiy Frolov et al., Microbridge Technologies; EE Times-India, December 2007
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Sensitivity of Rogowski Coil The current to be measured is related to the induced voltage by a proportionality constant i.e. the mutual inductance of the coil.
Vcoil
di = − M 21 × dt
M = µ 0 nA
M = Coil Sensitivity (Vs/A) (depends on the coil winding design) di/dt = rate of change of current (A/s) n & A = design and geometry parameters International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Sensitivities for different cross-sections Rectangular Cross-section
Circular Cross-section
Oval Cross-section
# Jan Hlavacek et al., 16th IMEKO TC4 Symposium, Exploring New Frontiers of Instrumentation and Methods for Electrical and Electronic Measurements, Sept. 22-24, 2008, Florence, Italy
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Time response consideration Differentiating / Integrating ! - depends on circuit parameters dI c I c dφ =L + dt dt R 1 dφ L dI c = + Ic R dt R dt
Self-Integrating
Differentiating L dI c << I c R dt
R >> ωL
dφ Ic α dt
L dI c >> I c R dt
ωL >> R
International Workshop on Plasma Diagnostics and Applications, Singapore
Ic α φ July 2 – 3, 2009
Realistic lumped circuit model
I(t)
- solution is complex ! # M. Argueso et al., www.aedie.org/9CHLIE-paper-send/252-argueso.pdf
High frequency response (bandwidth) is determined by : Coil inductance (Lc) Stray capacitance of winding (Cc) Coil resistance (Rc) Termination impedance (Z) International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
High bandwidth issues 1. The rise time (tr) of the measuring pulse is limited by the wave transit time (T) in the coil winding. tr >T always
2. Role of termination impedance (Z) is very important. R >> ωL ωL >> R
20 ns/div 5 ns/div
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
High bandwidth issues 3. Highest frequency 4. Non-uniform excitation measurement limited by due to dislocation of resonant frequency (LC) current centroid may lead of the coil. to strong oscillations in “distributed capacitance the sensor signal due large no. of turns”
# http://www.pemuk.com
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
High bandwidth issues 5. High voltage consideration
6. Shielding - is placing the Rogowski coil inside the slotted metallic housing.
# http://www.pearsonelectronics.com
“Some times coupling capacitance b/w the winding and shielding may affect the signal response” International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Design methodology for differentiating Rogowski Step 1: Estimate the di/dt in the circuit. di 2π = × I pk dt T
T = 2π LC
I pk = Vch
C L
Step 2: Fix the max. limit for the induced voltage (Vcoil). Step 3: Use the basic equation: Vcoil = NA ×
µ 0 di × 2πR dt
A = a×b
Step 4: Choose optimum values for – a,b, R and N. International Workshop on Plasma Diagnostics and Applications, Singapore
# John Anderson, RSI 42,7,1971
July 2 – 3, 2009
Current Monitors
# http://www.pearsonelectronics.com
# Chris Waters, PCIM Article 86; http://www.pearsonelectronics.com
- are similar to self integrating Rogowski Coils in response but utilize high permeability magnetic core for coil winding. - the presence of high permeability core is important for the extension of flat response to low frequency. - Usage: CT’s – Universal / Rogowski Coil - Customized. International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Major limitation with CT’s “Core Saturation i.e. I × T ” Vcoil
dφ = n× dt
∫ V (t )dt = n ∆φ
∆φ - is the change in flux in the core
Since the max. flux is limited by core saturation there is a corresponding limit on : I × T n 2 B max A In terms of design parameters : ∫ I (t ) dt ≤ R
“Core may also get saturated by the DC component of the current being measured, Biasing overcomes this problem” # Chris Waters, PCIM Article 86; http://www.pearsonelectronics.com
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Important time domain parameters
(I × T )max - current – time product rating must not exceed I max - highest measurable current (related with (I × T )max ) t r (10 − 90%) - Useable rise time (<10% overshoot)
Sensitivity(Volt/Ampere) - typical range 0.001 - 0.01V/A Droop – it’s the distortion in the pulse shape of longer duration current pulses (ms to 100’s of ms) # http://www.pemuk.com
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Current Shunts/ CVR’s “Use of shunt is based on measurement of the voltage drop across the resistance of known value”
2 ECVR = RCVR ∫ I max dt # Mark E. Savage, Pulsed Power Electrical Diagnostics, IEEE Pulsed Power-Plasma Science Mini-course, June 23 2007
Current shunts/ CVR’s have: High peak power
“Ideally – Ohmic (Non-Inductive)” # Hansjoachim Bluhm, Pulsed Power Systems; ISBN10 3-540-26137-0, ISBN-13 978-3-540-26137-7 Springer Verlag Berlin Heidelberg 2006
High frequency response Large pulse energy handling capacity International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
CVR’s from T & M Research, USA Hi-Wattage (225W - R Series) CVR’s
Sub-milliohms
Up to sub-nanosecond
Up to 100’s of MHz
Up to 10’s of kJ’s
# http://www.tandmresearch.com
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
High voltage impulse measurements 1. Simple resistive dividers C o p pe r casin g (grou n de d to ch a m be r fram e )
GND Vin (HV)
Vout In su lato r R esisto r ch ain (1 0×5 10 Ω ) To p o sitive flan ge
(R1) HV Arm
51 Ω
R2 = Vin R1 + R2
BNC C o n n e cto r
(R2) LV Arm
Limitation:
“Measurement of fast signals with large division ratio” # E. Kuffel et al., High Voltage Engineering Fundamentals, ISBN 0 7506 3634 3
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
High voltage impulse measurements 2. RC compensated dividers Shunt capacitance and inductance of Resistor L = 30nH , C = 534 fF For R = 1 kΩ RC ≈ 500ps L/R ≈ 30ps Equivalent circuit of Resistor
It is necessary to balance the time constants of both the arms # Mark E. Savage, Pulsed Power Electrical Diagnostics, IEEE Pulsed Power-Plasma Science Mini-course, June 23 2007
R1C1 = R2C 2
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
High voltage impulse measurements 3. Capacitive voltage dividers
Equivalent circuit
V C + C2 R1 + R2 Attn. Ratio 2 = 1 V C R 1 2 1
# Hansjoachim Bluhm, Pulsed Power Systems; ISBN-10 3-540-26137-0, ISBN-13 978-3-540-26137-7 Springer Verlag Berlin Heidelberg 2006
Differentiating
(R1 + R2 ) (C1 + C2 ) << tr V2 = (R1 + R2 )C1
dV1 dt
dV1 V1 (C +C ) dV2 = + 1 2 (R1 + R2 )C1 dt C1 dt Integrating
(R1 + R2 ) (C1 + C2 ) >> tr C1 V 2 V2 = C1 + C2
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Safe practices for maintaining wave shape fidelity
When long length cables are used it is advisable to use 50Ω termination at scope end. Noise travels faster in air than cables. The instrument grounds must be isolated from the equipment ground for avoiding ground loop noise. Using differential probes giving (Ch1 – Ch2): – Ch1: +Vreal+Vparasite – Ch2: -Vreal+Vparasite
Noise reduction by ferrite cores. Enhances Shield Inductance. Spurious ringing is produced due to high frequency currents flowing out side the cable shield # http://www.pearsonelectronics.com
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Conclusion Awareness of data quality is an important issue and it can only be improved by proper understanding of response of the diagnostic tools and the factors limiting their bandwidths. Noise problems are often challenging and shield currents are the main cause. Good cabling and grounding practices solve most noise problems (e.g. use of double shield cables) High bandwidth response of data acquisition system i.e. oscilloscopes/ fast digitizers is equally important for good data quality for e.g. – tr – rise time BW - bandwidth
BWsignal =
0 .4 tr
BWscope = n × BWsignal
(# http://www2.tek.com/cmsreplive/pirep/3802/55W_18024_2_2009.04.07.10.03.56_3802_EN.pdf) (# http://cp.literature.agilent.com/litweb/pdf/5989-5733EN.pdf)
For n = 3, 5GHz/80ps
1.66GHz/240ps
1/50ps ~20GS/s
International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
Thanks International Workshop on Plasma Diagnostics and Applications, Singapore
July 2 – 3, 2009
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