electrical diagnostics for pulsed power

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 non-

invasive electrical diagnostics used in pulsed power systems and highlight the

concealed causes that affect their behavioral response.

International Workshop on Plasma Diagnostics and Applications, Singapore July 2 – 3, 2009

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.


Categorization

Pulsed Power Electrical Diagnostic Tools

Current measuring devices Voltage measuring devices

Non-intrusive Intrusive Non-intrusive Intrusive

Rogowski Coils

Current Transformers

Current Shunt

Simple resistive dividers

Compensated dividers

Capacitive Voltage dividers


Rogowski Coils

“most effective, economic and extensively

used diagnostic”

∫= ldHirr

. Amperes Law

Faraday’s Law

It is an air-cored toroidal coil

that surrounds the conductor

carrying the current to be

measured.

dt

dnVcoil

φ×=

# Gennadiy Frolov et al., Microbridge Technologies; EE Times-India, December 2007


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.

dt

diMVcoil ×−= 21

nAM 0µ=

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


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


Time response consideration

Differentiating / Integrating !

Differentiating

- depends on circuit parameters

R

I

dt

dIL

dt

d cc +=φ

cc I

dt

dI

R

L

dt

d

R+=

φ1

cc I

dt

dI

R

L<< LR ω>>

dt

dIc

φα c

c Idt

dI

R

L>> RL >>ω φαcI

Self-Integrating


Realistic lumped circuit model

High frequency response (bandwidth) is determined by :

Coil inductance (Lc)

Coil resistance (Rc)

Stray capacitance of winding (Cc)

Termination impedance (Z)

I(t)

- solution is complex !# M. Argueso et al., www.aedie.org/9CHLIE-paper-send/252-argueso.pdf


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.

LR ω>> RL >>ω

20 ns/div

5 ns/div



3. Highest frequency

measurement limited by

resonant frequency (LC)

of the coil.

“distributed capacitance

due large no. of turns”

4. Non-uniform excitation

due to dislocation of

current centroid may lead

to strong oscillations in

the sensor signal

# http://www.pemuk.com



5. High voltage

consideration

6. Shielding - is placing the

Rogowski coil inside the

slotted metallic housing.

“Some times coupling capacitance b/w the winding

and shielding may affect the signal response”

# http://www.pearsonelectronics.com


Design methodology for differentiating Rogowski

Step 1: Estimate the di/dt in the circuit.

pkITdt

di×=

π2

Step 2: Fix the max. limit for the induced voltage (Vcoil).

Step 3: Use the basic equation:

# John Anderson, RSI 42,7,1971

Step 4: Choose optimum values for –

a,b, R and N.

dt

di

RNAVcoil ××=

π

µ

2

0

baA ×=

L

CVI chpk =LCT π2=


Current Monitors

- 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.

# http://www.pearsonelectronics.com# Chris Waters, PCIM Article 86; http://www.pearsonelectronics.com


Major limitation with CT’s

“Core Saturation i.e. ”

dt

dnVcoil

φ×= φ∫ ∆= ndttV )(

- is the change in flux in the coreφ∆

Since the max. flux is limited by core saturation there is

a corresponding limit on : TI ×

In terms of design parameters :R

ABndttI max

2

)( ≤∫

“Core may also get saturated by the DC component of the

current being measured, Biasing overcomes this problem”

TI ×

# Chris Waters, PCIM Article 86; http://www.pearsonelectronics.com


Important time domain parameters

- current – time product rating must not exceed ( )maxTI ×

- highest measurable current (related with )maxI ( )maxTI ×

%)9010( −rt - 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


Current Shunts/ CVR’s

“Use of shunt is based on measurement of the

voltage drop across the resistance of known value”

Current shunts/ CVR’s have:

High peak power

High frequency response

Large pulse energy handling capacity

“Ideally – Ohmic (Non-Inductive)”

dtIRE CVRCVR ∫= 2

max

# Mark E. Savage, Pulsed Power Electrical Diagnostics, IEEE Pulsed Power-Plasma Science Mini-course, June 23 2007

# Hansjoachim Bluhm, Pulsed Power Systems; ISBN-10 3-540-26137-0, ISBN-13 978-3-540-26137-7 Springer Verlag Berlin Heidelberg 2006


CVR’s from T & M Research, USA

Hi-Wattage (225W - R Series) CVR’s

Sub-milliohms

Up to 100’s of MHz

Up to sub-nanosecond

Up to 10’s of kJ’s# http://www.tandmresearch.com


High voltage impulse measurements

1. Simple resistive dividers

Limitation:

(R1)

Copper casing(grounded to chamber frame)

51 ΩBNCConnector

Insulator

Resistor chain (10×510 Ω )

To positive flange

HV Arm

(R2)

LV Arm

(HV)

GND

+=

21

2

RR

RVV inout

“Measurement of fast signals with

large division ratio”

inV

# E. Kuffel et al., High Voltage Engineering Fundamentals, ISBN 0 7506 3634 3



2. RC compensated dividers

L = 30nH , C = 534 fF

It is necessary to balance the time

constants of both the arms

Shunt capacitance and inductance of Resistor

For R = 1 kΩ

RC ≈ 500ps

L/R ≈ 30ps

Equivalent circuit of Resistor

2211 CRCR =# Mark E. Savage, Pulsed Power Electrical Diagnostics, IEEE Pulsed Power-Plasma Science Mini-course, June 23 2007



3. Capacitive voltage dividers

Equivalent circuit

( )( )

dt

dV

C

CC

CRR

V

dt

dV 2

1

21

121

11 ++

+=

Differentiating Integrating

( ) ( ) rtCCRR <<++ 2121

( )dt

dVCRRV 1

1212 +=

( ) ( ) rtCCRR >>++ 2121

+

+=

2

21

1

21

1

2.R

RR

C

CC

V

VRatioAttn

2

21

12 V

CC

CV

+=

# Hansjoachim Bluhm, Pulsed Power Systems; ISBN-10 3-540-26137-0, ISBN-13 978-3-540-26137-7 Springer Verlag Berlin Heidelberg 2006


Safe practices for maintaining wave shape fidelity

When long length cables are used it is advisable to use 50Ωtermination 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.

Noise reduction by ferrite cores. Enhances Shield Inductance.

Spurious ringing is produced

due to high frequency currents

flowing out side the cable shield

Using differential probes giving (Ch1 – Ch2):

– Ch1: +Vreal+Vparasite

– Ch2: -Vreal+Vparasite

# http://www.pearsonelectronics.com


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. –

r

signalt

BW4.0

=signalscope BWnBW ×=

tr – rise time

BW - bandwidth

For n = 3, 5GHz/80ps 1.66GHz/240ps 1/50ps ~20GS/s

(# 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)


Thanks

electrical diagnostics for pulsed power

Documents