CWT Ultra Mini Technical Notes

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CWT Ultra Mini Technical Notes All measuring instruments are subject to

The purpose of these technical notes is to explain some of those limitations and

1 CWT Ultra Mini Technical Notes All measuring instruments are subject to limitations. The purpose of these technical notes is to explain some of those limitations and to help the engineer maximise the many advantages of PEM s Rogowski current transducers. These technical notes should be read in conjunction with the CWT Ultra Mini short-form datasheet. Power Electronic Measurements Ltd Nottingham UK Tel: +44 (0) Fax:+ 44 (0) info@pemuk.com

2 Contents 1. Basic operation What are the advantages of the CWT over other Current Transducers? Accuracy and Calibration Calibration Accuracy with current magnitude linearity and internally generated noise Low frequency noise DC offset Linearity Positional accuracy and rejecting external currents Interference due to external voltages Temperature Rogowski Coil Temperature Coefficient Integrator Temperature Coefficient Using the CWT UM outside the operating temperature range (-20 to +70 o C) Rated Current, overloads and saturation Peak di/dt Absolute maximum (peak) di/dt Absolute maximum (rms) di/dt Frequency response Low frequency Droop High frequency Rise-time and Delay Output cabling and loading Product safety and standards How does PEM rate the voltage insulation of its Rogowski coils? Product safety and EMC compliance Figures Figure 1. Basic operation... 3 Figure 2. Low Frequency Noise Measurements... 5 Figure 3. Positional accuracy measurements... 6 Figure 4. External voltage pick-up... 7 Figure 5. rms di/dt of a pulsed waveform... 9 Figure 6. Low frequency bode plots of the CWT ULTRA MINI Range Figure 7. Droop definition Figure 8. Droop measurements Figure 9. High frequency bode plots of the CWT ULTRA MINI Range Figure 10. Rise time and delay definitions Figure 11. Maximum rise time

3 1. Basic operation The CWT Ultra mini is an ac current sensor. It comprises a thin, flexible, clip-around sense coil (termed a Rogowski coil) connected to an electronic integrator. The voltage induced in a Rogowski coil is proportional to the rate of change of current enclosed by the coilloop. It is therefore necessary to integrate the coil voltage in order to produce a voltage proportional to the current. Figure 1. Basic operation The coil is uniformly wound with N turns/m on a non-magnetic former of constant cross section area A m 2. If formed into a closed loop then the voltage e induced in the coil is given by the equation e = o NA di/dt = H di/dt where H (Vs/A) is the coil sensitivity and I is the current to be measured passing through the loop. The loop does not need to be circular and e is independent of the current position in the loop. To reproduce the current waveform as a measurement signal which can be displayed on an oscilloscope or quantified using a DVM, all that is required is means for accurately integrating the coil voltage, such that V out = 1/T i e.dt = R SH.I where T i =R o C 1 and R sh =H/T i is the transducer sensitivity in (mv/a). 3

4 2. What are the advantages of the CWT over other Current Transducers? are intrinsically safe devices. There is no danger of an open circuit secondary. can take large current overloads without damage (provided the di/dt ratings are not exceeded see Sections 4.2 to 4.5) and do not suffer from magnetic saturation so are very linear (see Section 3.2) the size of the Rogowski coil can be chosen independently of the current magnitude. This is unlike other current transducers which become bulkier as the current magnitude increases. For currents of several ka's or more there is really no better alternative than the Rogowski transducer! are very easy to use - the coil is thin and flexible and easy to insert around a current carrying device. Ideal for retrofit applications. are non-intrusive. They draw no power from the main circuit carrying the current to be measured. The impedance injected into the main circuit due to the presence of the transducer is only a few pico- Henries! have a wide bandwidth, from 3.2Hz with typical high frequency (-3dB) 20MHz. provide an isolated measurement at ground potential similar to other current transducers (except coaxial shunts) i.e. there is no direct electrical connection to the main circuit. can measure AC signals superimposed on large DC. The transducer does not measure direct currents - as a result it can measure small AC currents in the presence of a large DC component 3. Accuracy and Calibration 3.1 Calibration Every CWT UM is supplied with a calibration certificate. The calibration certificate contains details of all comparative measuring devices and recording equipment used in the calibration with reference to their traceable United Kingdom Accreditation Service (UKAS) calibration certificates. A copy of our traceability chart is available on request. 3.2 Accuracy with current magnitude linearity and internally generated noise Low frequency noise For the CWT ULTRA Mini Rogowski transducers the predominant sources of low frequency noise are Low frequency noise This is generated within the integrator op-amp. This random noise is distributed around the low frequency cut-off f L where the integrator gain is at a maximum. 50Hz pick-up noise. A Rogowski coil can never be perfectly wound and necessarily has a discontinuity where the coil clips together. It is therefore susceptible to interference from electromagnetic fields external to the Rogowski coil (albeit this pick up is very small see Positional accuracy and external currents ). Typically predominant fields are those at power frequency 50/60Hz. When the integrator gain is very high these fields create measurement disturbance. Hence it is necessary to limit the low frequency cut-off of the CWT015 and CWT03 probes to greater than 60Hz to ensure the pickup noise is not significant. 4

CWT015 Noise Noise max: 20mVp-p Measured value approx 15.0mVp-p Volts per div 5.0mV / Timebase 40ms per div CWT1 Noise Noise max: 10mVp-p Measured value approx 8.0mVp-p Volts per div 5.0mV / Timebase 40ms per div Figure 2.

transformers. 3.2.2 DC offset For the CWT UM the maximum DC offset is ±3.0mV at 25 o C. 3.2.3 Linearity Linearity error is the difference ΔI between the true current value, I,and the measured value V out / R SH (where R SH is the sensitivity in V/A).

5 CWT015 Noise Noise max: 20mVp-p Measured value approx 15.0mVp-p Volts per div 5.0mV / Timebase 40ms per div CWT1 Noise Noise max: 10mVp-p Measured value approx 8.0mVp-p Volts per div 5.0mV / Timebase 40ms per div Figure 2. Low Frequency Noise Measurements Care should also be taken to keep the coil and particularly the coil clip-together mechanism as far as possible from large 50Hz sources such a power supply transformers DC offset For the CWT UM the maximum DC offset is ±3.0mV at 25 o C Linearity Linearity error is the difference ΔI between the true current value, I,and the measured value V out / R SH (where R SH is the sensitivity in V/A). For a fixed frequency and fixed current position the linearity error will vary with the current magnitude over the rated range of the CWT ULTRA MINI. Rogowski current transducers contain no magnetic materials thus there are no saturation or non-linear effects associated with the magnitude of the current. Additionally the integrator gain is set by highly stable low drift passive components which ensure excellent linearity. In tests the linearity of the CWT has been found to be better than 0.05% of full scale or 0.1% of actual reading. The linearity may in fact be better than this since the accuracy of measuring the current was of the same order as the differences. 3.3 Positional accuracy and rejecting external currents Due to small variations in the winding density and coil cross sectional area the transducer output varies slightly depending on the position of the current in the coil and also the size of the current conductor relative to the coil. To quantify this positional variation a conductor of area (1mm 2 ) is moved around the Rogowski loop as shown overpage. The Rogowski coil is calibrated with the conductor central in the Rogowski loop,. The deviation in reading relative to the calibrated value, as the conductor is moved around the loop, is shown in the table. The positional variation is at its worst where the coil clips-together,, every effort must be made to keep the conductor away from this area. 5

6 The Rogowski loop circumference is 80mm Conductor Position Typical Error (%) 0.2% ± 1.0% ± 2.0% - 5.0% Note that with a larger conductor the variation of error with conductor position will decrease and approach the calibrated value. Figure 3. Positional accuracy measurements Error can arise due to the presence of current close to but outside the Rogowski coil which ideally should not provide any reading. However an external current of magnitude 100A adjacent to the coil will give a reading of up to ±2A. The error will significantly decrease as the external current becomes more distant from the coil. Care must be taken to keep any source of external current away from the shaded area position as the error will be worse in this region. If the external current (outside the Rogowski loop) is much greater than the current being measured (inside the current loop) then the error maybe significant. This is particularly relevant if the external current is flowing in a nearby multi-turn coil such as a transformer. 3.4 Interference due to external voltages If there is a surface with a very high voltage very close to the coil and the voltage is subject to high rates of change (>1kV/µs) or has high frequency oscillations in the MHz range then interference can occur due to capacitive coupling on to the coil. The CWT ULTRA Mini coil does not have an electrostatic screen. It would be virtually impossible to fit a screen and maintain the 1.7mm coil cross section and such a screen would dramatically reduce the high frequency bandwidth. Recreating the complex interaction of the various electrostatic fields in a power electronics circuit is very difficult. To provide the user with an example of the magnitude of the error due to a very fast slewing dv/dt adjacent to the coil, the following experimental set up is arranged. This is considered a harsh test for the CWT ultra mini as the aluminium plate presents a very large area with which to couple to the coil. 6

7 Please note this is an untypically harsh test and the interference should be significantly less for a typical application. ch1 ch1 ch2 ch2 ch1: 50V/div dv/dt = 27kV/µs adjacent to the coil ch2: 20mV/div CWT1 ULTRA Mini output Timebase is 40ns / div Error current is approx 4.0 A peak ch1: 50V/div dv/dt = 27kV/µs 1cm from the coil ch2: 20mV/div CWT1 ULTRA Mini output Timebase is 40ns / div Error current is approx 1.5A peak Figure 4. External voltage pick-up As a check for the effect of external voltages or currents the user should place the Rogowski coil in approximately the same position as used for measuring the desired current, but not looped around the desired current. Ideally there should be no measured signal. If there is interference then the same interference will be superimposed on the current waveform when it is measured and this can be taken into account when interpreting the measurement. 3.5 Temperature The variation in accuracy of the CWT ULTRA MINI with temperature results from 1. Expansion of the plastic former length onto which the Rogowski coil is wound. This reduces the sensitivity of the Rogowski coil. 2. Drift with temperature of the passive components that set the integrator time constant To overcome these problems the Rogowski coil is wound onto a plastic former with a very low co-efficient of expansion. High stability resistors and capacitors set the integrator time constant. 7

8 3.5.1 Rogowski Coil Temperature Coefficient A sample batch of Rogowski coils have been tested over the rated temperature range of the CWT ultra mini coil, -20 to +70 C, inside a temperature controlled environment (Vötsch VT4002 chamber). The coils inside the chamber measure a known current and the sensitivities (Vs/A) of the coils are recorded at each temperature interval. The typical temperature coefficient of the Rogowski coil varies slightly with model type. The temperature coefficients which best describe the measured results shown in the table below: Coil Type Operating Temp range Temperature coefficient C ppm/ C CWT UM 1,3,6-10 to CWT UM 015, 03, to Temperature coefficient of CWT ULTRA MINI coil (1m cable) Integrator Temperature Coefficient The integrator sensitivity is set using a number of passive components which are selected for their low temperature drift. The temperature coefficient for the CWT ULTRA MINI integrator is given in the table below: Temperature Coefficient (ppm/ C) 0 C to +40 C Integrator time constant, Ti ±150 The overall measurement uncertainty can thus be calculated from the sum of the Rogowski coil and integrator time constant temperature coefficients Using the CWT UM outside the operating temperature range (-20 to +70 o C) CWT ultra mini coils have a very small cross section and, unlike our standard coils, it is not possible to fit an temperature compensation mechanism within the coil. As a consequence, these types of coils more prone to break at elevated temperatures. There were no failures in the small batch of coils used for temperature testing (-40 to 150 degrees) but the effect of temperature cycling on lifetime has not yet been predicted. It is worth noting for ALL coils that the inner conductor may fail prematurely if the coil is used outside the recommended operating temperature range and operation outside the rated conditions is not covered by warranty. 8

9 4. Rated Current, overloads and saturation The CWT ULTRA MINI has an output of 6V peak corresponding to the peak current. If the peak current exceeds the rating then the integrator will saturate and the measured waveform will be completely corrupted (unlike an amplifier for which the output waveform is merely clipped). Exceeding the peak current rating will not damage the CWT ULTRA MINI provided the di/dt ratings are not exceeded. It will return to normal operation after the current surge has passed. 4.1 Peak di/dt This is the maximum di/dt above which the transducer will fail to correctly measure the current. Values are given on the specification sheet. 4.2 Absolute maximum (peak) di/dt The transducer can be damaged by excessive di/dt due to the voltage generated in the coil. The specification sheet gives an absolute maximum rating for di/dt for each transducer that must not be exceeded. 4.3 Absolute maximum (rms) di/dt The transducer can also be damaged by sufficiently high repetitive di/dt even though the peak di/dt rating is not exceeded. A damping resistor is used to provide correct termination of the Rogowski coil and cable to prevent reflections (seen as high frequency damped oscillations) appearing on the measured waveform. A high repetitive di/dt will cause excessive power to be dissipated in this resistor. For sinusoidal waveforms the calculation of rms di/dt is straight-forward, di/dt rms = 2 fi rms (where f is the measured frequency and I rms the rms value of the measured current) For pulsed waveforms an example of how to calculate the di/dt rms is shown below, 5000A I t(us) (a) 5000A/us 0 di/dt t -5000A/us (b) Figure 5. rms di/dt of a pulsed waveform Consider the current waveform shown in Figure (a) with a repetition frequency of 20kHz. Figure (b) shows the corresponding di/dt waveform. The rms di/dt is given by 5000 A/µs x (1µs/25µs) 0.5 = 1 ka/µs rms. 9

10 5. Frequency response The CWT has a wide-bandwidth and is optimised to give a flat sensitivity (V/A) and small phase error over a wide range of frequencies. 5.1 Low frequency The low frequency bode plot of the CWT ULTRA MINI for the various current ratings is included below: Figure 6. Low frequency bode plots of the CWT ULTRA MINI Range 10

5.2 Droop For non-sinusoidal current waveforms (such as chopped or rectified current) the effect of the phase displacement at low frequencies can cause some distortion of the measured waveform.

Droop definition The droop rate for a rectangular pulse is worst case and in general % offset = x (mean value / peak value) x (droop in % / ms) The calculation of droop assumes that the measured

11 5.2 Droop For non-sinusoidal current waveforms (such as chopped or rectified current) the effect of the phase displacement at low frequencies can cause some distortion of the measured waveform. This also applies for pulses of relatively long duration. This distortion is termed droop, the droop for a rectangular pulse for a particular CWT Ultra mini model is quoted on the datasheet. Figure 7. Droop definition The droop rate for a rectangular pulse is worst case and in general % offset = x (mean value / peak value) x (droop in % / ms) The calculation of droop assumes that the measured pulse duration <<T, where T is related to the low frequency bandwidth by approximately T 1/2πf L. As an example, the response of both a CWT015 and CWT1 ULTRA Mini to a 100µs pulse cf. a dc shunt is shown below CWT015 Ultra Mini Offset = Droop x 100 = 10% (approx as measured) Timebase 20µs per div CWT1 Ultra Mini Negligible droop distortion Timebase 20µs per div Figure 8. Droop measurements 11

12 5.3 High frequency The transducer behaviour at frequencies approaching and exceeding its specified (-3dB) bandwidth is very complicated. It is related to the distributed inductance and capacitance of both the coil and the co-axial cable (which have different characteristic impedances) and their terminations, and to the gain-frequency characteristic for the op-amp IC used for the integrator. It also varies depending on the position of the current within the loop although up to the (-3dB) bandwidth the variation is small. PEM has produced several publications regarding the high frequency behaviour of Rogowski transducers and these can be downloaded in The typical (-3dB) high frequency bandwidth of the CWT Ultra mini range is shown below. Figure 9. High frequency bode plots of the CWT ULTRA MINI Range The high frequency phase response includes the inherent transport lag in the 1m cable connecting the Rogowski coil to the electronic integrator. The low frequency limit for -1% gain is quoted on the datasheet. 12

13 5.4 Rise-time and Delay Figure 10. Rise time and delay definitions The CWT ULTRA mini has an inherent measurement delay. Provided the actual current rise time is within the limits of the high frequency bandwidth of the transducer the delay is predictable. The delay is a combination of T a the transit delay for the cable connecting the coil to the integrator (4.2ns/m) T b the delay of the electronic integrator. This is a function of the GBW product of the integrating opamp and the various parasitic impedances that determine the hf performance of the integrator. T c the delay for the Rogowski coil. This is dependent on the distributed inductance and capacitance of the Rogowski coil. T b and T c cause an attenuation of the measurement, T a does not. The fastest 10 to 90% rise-time for which PEM would recommend the CWT ULTRA MINI is used is 40ns. This is a conservative value. For faster rise-times the transducer may exhibit distortion and the settling behaviour will become increasingly oscillatory. As an example, the pulse response below is that of a CWT015 to a rise-time of 50ns and also 20ns. The comparative device is an 800MHz bandwidth co-axial shunt. From the traces Recommended rise time is conservatively 40ns for the CWT ULTRA Mini range. Delay is typically 28ns (this includes 2.5ns from the 0.5m output BNC-BNC cable) Rise time 10 to 90% 50ns Current peak 2A Timebase 40ns per div Figure 11. Maximum rise time Rise time 10 to 90% 20ns Current peak 2A Timebase 40ns per div 13

14 6. Output cabling and loading The minimum input impedance for any measuring device (oscilloscope, DVM, power recorder etc.) connected to the CWT ULTRA MINI must be 100kΩ or greater for rated accuracy. The output impedance is approximately 50Ω. Any cables used to connect the output of the transducer to the data acquisition device, longer than the 0.5m coaxial cable supplied with the unit, should be 50Ω singly screened coaxial cable. Although at present cables longer than 0.5m have not been included in the immunity tests and may decrease RF noise immunity, PEM does not consider the use of extension cables to be problematic from the noise viewpoint. PEM has conducted tests using a 25m extension and no discernible attenuation of measured current signal has occurred although, as is to be expected, there is an increased measurement delay of 5ns/m. The CWT ULTRA MINI can be terminated into a 50Ω impedance for driving long output cables (>10m). A load of 50Ω will reduce the sensitivity to half its nominal value. It will also reduce the peak output to ±2V. 7. Product safety and standards 7.1 How does PEM rate the voltage insulation of its Rogowski coils? The CWT ranges of Rogowski current transducers are intended for instrumentation use and not for permanent installation on equipment. The peak voltage insulation ratings for these transducers reflect the fact the transducers are not to be used continuously at high voltages. Every Rogowski coil supplied by PEM is given a peak voltage insulation rating. The rating is derived from the following test: The coil is exposed to an ac test voltage (kv) = (2 x Peak voltage rating + 1) / 2 (kv), for 60 seconds at 50Hz. The CWT ULTRA MINI is rated at 1.2kV and will be flash tested at 3kVrms (4.3kV peak), 50Hz, for 1 minute. The user should visually inspect the Rogowski coil and cable for insulation damage each time the transducer is used. Every Rogowski coil has at least two layers of insulation covering the winding. These are always different colours making visual inspection of the integrity of the insulation easier. It is imperative that the user grounds the BNC connector from a safety viewpoint so that in the event of an insulation breakdown at the coil (due to exceeding the voltage rating or due to mechanical damage), a fault current path exists via the co-axial cables to the grounded BNC connector. The practice of floating the oscilloscope which results in the BNC connections being isolated from ground is strongly deprecated. 14

15 7.2 Product safety and EMC compliance The CWT range of current transducers has been designed and assessed to ensure they comply with relevant EU standards and all products carry the CE mark of conformity. The CE Declaration of Conformity can be found on our website. All CWT products comply with: Safety: IEC :2001; Pollution Degree 2 Refer to the Instructions for use document before use. 15

LFR: flexible, clip-around current probe for use in power measurements

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