Current Sensing in Electric Drives. A Future and History Based on Multiple Innovations. Eric FAVRE, Wolfram TEPPAN. LEM Group, Switzerland. Abstract: This ...
Current Sensing in Electric Drives A Future and History Based on Multiple Innovations Eric FAVRE, Wolfram TEPPAN LEM Group, Switzerland Abstract: This paper presents the many various technologies which allow to measure the electric current, highlighting a very dynamic innovation process which in the past allows to measure many different current waveforms and, in the future, will be a key part of future technical evolutions.
1
HISTORY
From the beginnings of electrical engineering the quantification of the basic electrical units was a requirement for the work of physicists and engineers as well as the basis for the commerce with electrical energy. For the engineers, besides the electromagnetic field quantities, most notably the integral basic electric units, voltage and current, are of interest. At first, it was the physical effects directly associated to the moving charge, like the force action of the magnetic field generated by a conductor, Joule’s heat or the dissociating action of a current passing through a conductive liquid. Therefore, the first measuring instruments combined measurement transducer and display to an inseparable unit. At the end of the 19th century, the currents used for technical purposes had reached orders of magnitude impeding a direct measurement : Laying of massive bus bars to the instruments became inefficient and the danger of high potentials of components with respect to ground became evident. This development led to the separation of displaying and sensing units and the first “current transducers” originated, even though this term came into use much later. In 1837, the tangent galvanometer (Figure 1) was first described in a paper by Claude-Servais-Mathias Pouillet (1790-1868). It is a simple compass in the earth’s magnetic field influenced by the magnetic field of a current flowing around it. In 1884, Kohlrausch reported a measuring shunt made from a copper conductor allowing for the extension of the current range up to 200 A, which was considered sufficient at this time. Not much later, in 1893, current intensities of more than 10 kA were already measured. The instruments did not have closed magnetic circuits yet so that the results were impaired by magnetic stray fields as well as by the temperature dependence of the bus bars used as measuring shunts. An early article about current transformers was edited in 1901, where the primary current was only partially linked to the secondary winding in order to reduce the effect of external magnetic fields.
Figure 1: tangent galvanometer.
In 1912, Rogowski and Steinhaus published their article "Die Messung der magnetischen Spannung", Archiv für Elektrotechnik, 1912, 1, Pt.4, pp.141-150, although A.P. Chattock of Bristol University, ("On a magnetic potentiometer", Philosophical Magazine and Journal of Science, vol. XXIV, no. 5th Series, pp. 94– 96, Jul-Dec 1887) described a similar device earlier. In the following years, up to about 1920, together with improvements and extension of the measuring range, the theoretic foundations where refined. A graphical method to deal with the measuring errors of voltage transformers in real operating conditions was given by Möllinger and Gewecke (1911) and Schering dealt in a similar way with current transformers (1918). Also the first ideas to measure large DC currents came up in this period : as early as 1919 E. Besag described a circuit where a DC current to be measured influences an AC current in a saturable choke. In the twenties of the past century, current transformers for currents up to 50 kA and for rated voltages of up to 250 kV were available, many of the magnetic materials for instrument transformers still in use today were known, and also the isolation systems were similar to the ones used today – so the basic development for AC current transformers had been completed. The first construction achieving good accuracies for the measurement of large DC currents was given by Krämer in 1939 (Figure 2), it could measure 30 kA with an error of 0.5%.
Figure 4: High accuracy current measurement Figure 2: First accurate solution for DC current
Before 1930, the development of instruments to measure small magnetic fields, the so-called fluxgate magnetometers, had begun, although the Hall-effect was described much earlier, in 1879. But it was not until the 1950s when semiconductors with Hall constants large enough were available so that practical magnetometers could be built. Also current transducers for large currents came up using Hall cells. In the first transducers the output voltage of a Hall cell in an air-gap was taken directly as a measure for an electric current flowing through the aperture of the core, later a compensation winding was added to achieve smaller errors. The measuring devices built at the end of the 1950s for aluminum plants reached errors smaller than 0.1%, which was sufficient for industrial purposes.
In parallel with the evolution of the inductive current transducers, measuring shunts were improved. Constructions (co-axial shunts, disk shunts) were found which suppress any inductive voltage and can be used up to the GHz range where eddy currents effects can influence the measurement. Other physical effects used for the indirect) measurement of electrical currents are the Faraday effect (rotation of the polarization plane of a light beam by a magnetic field), nuclear magnetic resonance (NMR), the quantum Hall effect, magneto-resistance (anisotropic - AMR, giant - GMR, tunneling - TMR) and magneto-impedance. The current comparator has been refined by superconducting magnetic shields and NMR zero detectors, its uncertainties are well below one ppm now. 2
MAIN INDUSTRIAL TECHNOLOGIES
In term of products today available on the market, the measurement of the electric current can be made with more than 15 different designs or technologies, depending on the specific application requirements, such as peak or RMS current, accuracy, bandwidth, environmental robustness or simply cost [1]. Those technologies can be divided into six main categories:
Figure 3: Basic isolated AC/DC current measurement
Federal institutes responsible for the reproduction of basic units had much higher requirements for the measuring devices for calibration purposes : The institutes that contributed significantly to the development of current metrology are the Physikalisch -Technische Bundesanstalt (PTB) in Brunswick, Germany, the National Bureau of Standards (NBS) in the USA and the National Research Council (NRC) in Canada. It was here were in the 1960s the current comparator was developed, a sophisticated transducer designed to compare AC and DC currents with uncertainties below one ppm up to the audio frequency range (Figure 4).
• AC transducers, limited to AC measurements, including the classical current transformers (CTs), Rogowski coil based transducers or a recently developed printed-circuit-board (PCB) technology, • Hall effect transducers, which can be derived into ‘Closed-loop’, ‘Open-loop’ and ‘ETA’ technologies, • Fluxgate transducers, with more than six main designs, each showing specific performance, • Use of other field sensing technologies: alternatives to the Hall cells have been deployed into current measurement (e.g. magneto resistance), • Shunt based technologies, and • MEMS current transducers: micro-electromechanical systems (MEMS) have been used for current measurement even if none of them is yet at a real industrial stage.
Among these, the isolated technologies often measure the current by sensing the magnetic field it creates, which different performances depending on the considered field sensing technologies. The latter are shortly traded-off in the next chapter, focusing on the technologies potentially usable for current measurement. The resulting main current transducer designs are then presented, showing their respective performance, focusing on the less conventional designs. 3
INDUCTION SENSORS
The isolated measurement of an electric current is in many cases made with an induction sensor, used to detect the field created by the current to be measured. The following induction sensing principles are considered in general: • Hall cell: The classical solution, considering different types of cells (GaAs, InSb or silicon), • Magneto impedance, • Magneto resistance (MR): anisotropic (AMR), giant (GMR), tunnel (TMR), colossal (CMR), • Fluxgates, using saturation in a soft magnetic material, • Magneto-optic, • Nuclear magnetic resonance (NMR): magnetic "induction to frequency“ conversion, • Magnetostriction & piezo effect: the measured current creates a field modifying the size of a magnetostrictive element. It applies a force on a piezo element and produces a voltage signal; and • Others: paramagnetic Meissner effect, quantum Hall effect (superconductive, needs cooling), Zeemann effect, SQUIDs (superconductive, needs cooling), moving coils to convert a DC flux into an AC signal. The development of these field sensing technologies is often focused on the performance expected in the highest sales potential markets, namely position measurement, reading heads for hard disks data collection and, more recently, static magnetic memories. Due to its marginal size, the current sensing market presents limited interest for designing a specific field sensing device and thus the performance of existing solution are often not the most adequate. Of all the presented induction sensing principles, only the Hall effect, Fluxgate, AMR, and GMR are today used for current transducers made in large quantities. Some lead users have nevertheless designed other specific solutions. Indeed, TMR, CMR, NMR or the other mentioned effects are either in an early state of their development (universities), or intended to provide “binary information” or only used in small quantities, in laboratory equipments because of their high price (NMR, quantum Hall effect, SQUIDs). Current transducers based on magneto-optic effects (e.g. Faraday effect) have been developed for high currents applications (e.g. 100 kA) and seem to demonstrate good performance for AC measurement.
While medium term drift issues was still seen as critical for DC current measurements, some products are now available on the market claiming for good performances. The main advantage of NMR is a temperature independent “conversion constant”. The drawbacks are a zero frequency at zero field, no detection of the field polarity and the need of using Magnetic Resonance Imaging (MRI) for integration over a volume. Table 1 shows the main performance expected for the most common magnetic field sensors. For the sensitivity, the unit µV / (A/m) = µΩm was chosen, corresponding to the ratio "sensing element output signal (µV)“ / "measured magnetic field (A/m)". Indeed, a common unit was needed to be in position to compare technologies defined in many different ways in the abundant literature (e.g. suppliers data-sheets), the magnetic field being defined in A/m, A/cm, kA/m, Oersted, Gauss, mT, or T and output quantity commonly referred as ∆R/R, ∆Z/Z, V/mV, V/mA ! Table 1 sensitivity values are given under the assumption that “bridge type” sensors are fed by a voltage of 1 V, giving comparable µV output values. For sensors made of a non magnetic material, induction is more common as input quantity. For others that use some kind of flux concentration, it is the magnetic field strength. Indeed, the fact that A/m is chosen as input quantity does not mean that the physical effect is dependent on this quantity, but rather that the induction level inside the sensor is produced by a given field strength in air, as it can be produced for example by a pair of Helmholtz coils. The same applies for the offset, defined in A/m. Hall
Hall
Hall
Si
GaAs
InSb
Sensitivity (µΩm)
0.06
0.29
Offset (A/m)
800 typ.
Offset drift (A/m/K) Sensitivity drift (%/K)
AMR
GMI
Fluxgate
4
16
1250
25000
13600 max
1760 max
94
?
0.08
0.6
19
32
typ
max
max
0.375
?
0.035
-0.05
-1.8
-0.4
?
0.8 × 10
–6
0.0002
Table 1: Comparison of field sensing principles
For all Table 1 technologies and if no correction / compensation is used, non-linearity is quite high, close to 1 % within the sensor nominal working range. The values for Hall and AMR sensors are given without any additional electronics, GMI and fluxgate need some electronics to produce an output signal. The cause for the low drift values for Fluxgate is the fact that it is a compensating principle (type of local closed-loop): the even harmonics are controlled to be nearly zero by applying the field of a compensation coil.
A disadvantages of AMR is the risk of destruction by a strong external field. It can be mitigated by the use of a coil that produces a field perpendicular to the direction of the field to be measured. By applying an alternating current to this coil the AMR sensor is reset in each switching cycle. Regarding complexity, the difference between a GMI device and a miniature fluxgate is not very big. At least one coil around a small piece of ferromagnetic material is needed in both cases, as GMI is not sensitive for the sign of a magnetic field. Table 1 technologies are all used in current measurements, taking profit of their respective performance in specific current measurement segments, considering for example the need in term of sensitivity, offset behavior, saturation field, sensitivity to external perturbations, design complexity, and implementation cost.
The three Hall effect based technologies used for AC & DC current measurement are discussed hereafter.
applications. Compared to more sophisticated technologies, the main drawbacks of the open-loop transducers are both a moderate bandwidth / response time and a larger temperature dependence of the measurement accuracy. In specific applications, high frequency eddy-current losses may be a limiting factor. Nominal currents go from several Ampere to 10 kA, exceptionally up to >30 kA. The technology withstands short time current pulses going significantly beyond the maximum measurable value (e.g. 5-10 times). It may nevertheless create a large magnetic offset, resulting in an additional permanent measurement error, requiring then a dedicated demagnetization procedure. Overall accuracy is in general in the range of a few percent, due to the combination of the following: (1) DC offset at zero current - e.g. offsets of the Hall cell and processing electronic, (2) DC magnetic offset, (3) gain and linearity error, (4) amplitude attenuation and phase shift when the bandwidth limit is reached, and (5) noise. Additional errors are due to temperature changes, namely gain variations and offset drift.
4.1 Hall Effect Open-Loop Current Transducers
4.2 Hall Effect Closed-Loop Current Transducers
4.1.1 Working Principle Open-loop Hall effect transducers use a Hall sensing element placed into the airgap of a magnetic circuit (Figure 5). The design is such that the magnetic induction detected by the Hall cell is theoretically proportional to the primary current to be measured. Measurement inaccuracies are mainly due to magnetic and electronic non-linearity, as well as offsets created by the Hall cell, the processing electronics and the magnetic circuit hysteresis. The use of a magnetic circuit offers several advantages, namely to focus the field on the Hall cell, to amplify the field magnitude and to shield against external magnetic perturbations.
4.2.1 Working Principle Compared to the open-loop transducers of 4.1, the closed-loop Hall effect transducers have a built-in compensation circuit which improves performance. The Hall cell of closed-loop transducers (Figure 6) is used as a counter-reaction signal, driving a “secondary coil” current IS in a way that the magnetic field in the air-gap equals zero. The secondary winding is made of more turns (NS) than the primary (NP). At zero flux, the current linkage of the two coils is identical and thus the secondary current IS proportional to the current IP to be measured (IS = IP • NP / NS).
4
HALL EFFECT TECHNOLOGIES
Figure 5: Open-loop transducer. Figure 6: Closed-loop transducer.
4.1.2 Performance Open-loop transducers are capable of measuring DC, AC and complex current waveforms while ensuring galvanic isolation. They stand out by their low power consumption, reduced weight and sizes, particularly interesting when higher currents are considered. While not involving insertion losses in the circuit to be measured, open-loop transducers are particularly resistant to current overloads. They are relatively low priced and, in general, well suited to industrial
The maximum working frequency of this closed-loop principle using the Hall cell is given by the bandwidth of the processing electronics and by the capability to feed the secondary coil with the Is current (typically max. 2-10 kHz). To bypass this limitation, the secondary coil is used as a regular current transformer, converting the primary AC current into a secondary current, extending significantly the transducer bandwidth.
4.2.2 Performance Hall effect closed-loop transducers are capable of measuring DC, AC and complex current waveforms while ensuring galvanic isolation. They stand out by their: (1) excellent accuracy & linearity, (2) low temperature drift, (3) fast response time and high bandwidth, (4) no insertion loss in the primary circuit, and (5) current output robust to EMI. The main drawbacks of the closed-loop technology are the higher power consumption on the secondary supply, the larger dimensions (more noticeable for high current transducers) and higher costs compared to simpler open-loop designs. Nominal currents go from several Ampere to more than 20 kA. Specific designs reach 500 kA. With the closed-loop technology, the maximum measurable current is limited, specially at low frequencies. Closed-loop current transducers have an excellent linearity over a wide measuring range, with a total accuracy remaining below typically 1 %. Factors limiting the accuracy are the one mentioned for the open-loop transducers (see §4.1.2) but have a significantly smaller impact due to the considered zero flux principle (suppression of magnetic non-linearity and offsets, more stable gain versus temperature). At high frequencies, the measuring performance is set by the current transformer performance. Closed-loop transducers can still be affected by magnetic offset if once used in abnormal conditions (e.g. low frequency primary current strongly exceeding the nominal value). Finally, they have an excellent frequency response, surpassing 100 kHz in most cases. It occasionally goes up to 300 kHz. It is possible to measure di/dt’s reaching several hundreds of A/µs. 4.3 Hall Effect ETA Transducers 4.3.1 Working Principle In terms of construction, Hall effect ETA transducers (Figure 7) are similar to closed-loop transducers, with the same layout of the magnetic circuit, Hall cell and secondary winding. The differences are the magnetic core design and the electronic processing, leading to the ETA specific features. Indeed, Hall effect ETA transducers employ a mix of open-loop and closedloop technologies: (1) at low frequencies (up to typ. 210 kHz), they work as open-loop transducers, the Hall cell providing a signal proportional to the primary current to be measured – (§4.1). (2) At high frequencies, they work as a current transformers (§4.2). Both Hall effect and transformer signals are electronically added to form a common output signal (Figure 7). 4.3.2 Performance ETA transducers are capable of measuring DC, AC and complex current waveforms while ensuring galvanic isolation. ETA is better suited when the followings performance is expected: (1) high bandwidth and fast response time, (2) low power consumption due to the fact that the secondary coil is never actively energized, and (3) use of a low
secondary voltage supply (e.g. +5 V). For frequencies leading to the current transformer mode (> 2-10 kHz), the accuracy and temperature drifts are good (similar to closed-loop performance). ETA transducers do not involve insertion losses in the circuit to be measured and are particularly resistant to current overloads.
Figure 7: Hall effect ETA principle.
The major drawback is the size / mass of the magnetic circuit, which combines both the need for an often cumbersome secondary coil (as for closed-loop transducers) and a relatively thick magnetic core (as for open-loop transducers). At low frequencies, ETA technology’s measurement accuracy has a larger sensitivity to temperature changes (as for open-loop transducers). ETA product cost is higher than simpler open-loop designs. Nominal currents range typically between 25 A and 150 A. This rather narrow working range is not limited by technical issues, but rather by market and cost considerations: for currents below 25 A, it is possible to work in similar conditions with the more efficient closed-loop technology, while for currents larger than 150 A, higher secondary voltage supplies (e.g. +/-15 V) are generally available anyway, leading to a preference for the more efficient closed-loop technology. Indeed, the reduced secondary power consumption of the ETA technology is often not a sufficient asset to promote it beyond the discussed nominal current range. ETA transducers have an accuracy which is frequency dependant: (1) for low frequencies (< 2-10 kHz), overall accuracy is a few percent as for Hall cell openloop designs; (2) for higher frequencies, the overall accuracy is typically below one percent. Risk of having a residual magnetic offset after a nonexpected current overload is similar to the one of open-loop transducers at low frequencies. The bandwidth, response time and di/dt behavior of ETA transducers is very close to the one of the Hall effect closed-loop technology, with slight performance reductions. The higher frequency limit is typically 100 kHz. Reaction time is fast, below 1 µs. 5
FLUXGATE TECHNOLOGIES
Fluxgate technologies used for current measurement [2] are discussed in this chapter. It covers several different designs, all based on the same basic
measurement principle, having specific performance and varying complexities. In this chapter, the basic Fluxgate principle is described, considering first a design somehow similar to the ones used when the field sensing element is made with an Hall cell. Then, the Fluxgate overview is broadened to the most common designs, highlighting their specific performance. 5.1 “Standard” Fluxgate Working Principle An isolated Fluxgate transducer (Figure 8) can be designed like a Hall cell closed-loop transducer (Figure 6), with the same layout for the magnetic circuit, including an airgap and a secondary winding. The main difference is the way the airgap field is sensed, using a “saturable inductor” (Figure 9) instead of a Hall cell. Of course, this influences the transducer performance and implies a major redesign of the transducer electronics used both to supply the sensing element and to process its output signal. The “saturable inductor” (Figure 9) can be made of a small and thin magnetic core with a coil wound around it. It is generally made of discrete pieces of material (core & copper wire) but various designs can be considered, including advanced concepts based on MEMS technologies. The inductance value of the “saturable inductor” depends on the magnetic properties of the core (permeability): When the flux density is high, the core is saturated, its permeability low, and the inductance value small. In contrast, for a low flux density the inductance value is high. The design of the current transducer is indeed made in a way that the inductance of the saturable inductor is affected by both the primary current I1 and the current Isi fed into the saturable inductor: • The fact that the inductance is affected by the primary current is used as a feedback signal for the closed-loop principle (Figure 8); • The waveform of the saturable inductor’s current is used to detect the inductance change. As for closed-loop Hall effect transducers (§4.2.1), the secondary coil in Figure 8 is often used as a normal current transformer, measuring current components at high frequencies. Depending on the selected fluxgate design, this is not always possible and implies bandwidth / response time limitations.
I2
I1 Φ1
L ≠ L (I1 = 0) ? Yes
No
Fluxgate Sensing head Adjust I2
I2 = I1 / N 2
Figure 8: Construction of the “Standard“ Fluxgate
Isi
Magnetic core
Bext
coil
Bsi
L = f(Bext;Isi) 0.5 mm
Figure 9: Fluxgate magnetic sensing head.
5.2 Detection of the Inductance Change To understand the Fluxgate principle, a good knowledge of the way Figure 9’s saturable inductor behaves is required. It can be apprehended by looking at Figure 10, describing the simple case of the inductor’s current waveform after a sudden voltage step: (1) The “constant inductance” case shows the expected exponential current response (2) The “real inductance” case with the “I1 = 0” reference gives the current response obtained with a well designed saturable inductor. The latter case is related to a zero primary current, where no external field is applied on the inductor. The current waveform can be divided into three main sections: (a) for small current values, the current variation is slow due to the fact that the inductor has been designed with a high inductance; (b) when the current exceeds a given “turning point”, the current rise becomes very fast due to a sudden drop of the inductance value, obtained by design; (c) the current reaches the asymptotic level fixed by the available supply voltage (U/R). (3) when the primary current is not equal to zero, an external field is applied to the saturable inductor and the current response is modified as shown in Figure 10.
5.3 Processing of the Fluxgate Signals Back to the general working principle of the Fluxgate current transducer, the saturable inductor is supplied by a square wave voltage u(t), leading to the sensing head’s alternative current i(t) shown in Figure 11 and Figure 12, for a zero or non-zero primary current respectively. The sharp current peaks are due to the inductors “saturated” behavior described above. It must be noted that Figure 10 ref. I1 ≠ 0 and Figure 12 assume a non-active transducer closed-loop, where the secondary coil I2 of Figure 8 is not energized. The presence of a primary current is detected by the changes of the inductor’s current, namely the differences between the current waveforms in Figure 11 and Figure 12. The most common ways to detect these differences are: (1) to measure the DC component of the inductor current; (2) to make a spectral analysis of the inductor current and sense the amplitude of a specific current harmonic, e.g. the dominant second harmonic, or (3) to measure the duty-cycle of Figure 12 u(t) voltage (duration of the positive / negative half-cycle – asymmetry). The detected parameter is then used as the feedback signal for the closed-loop (Figure 8).
current
ance duct in t n sta Con
(2)
I1 = 0 Bext = I1 = 0
5.5 Performance of Fluxgate Technologies It is difficult to compare in general the numerous possible Fluxgate designs, but the following general trends can be highlighted. Advantages: (1) low offset & offset drift, (2) very accurate, (3) high resolution, (4) large temperature working range, (5) large maximum / minimum measurable current ratio, (6) large bandwidth / fast rise time – up to 500 kHz. Drawbacks: (1) limited bandwidth for the simpler designs, (2) risk of current / voltage noise injection into the primary conductor, (3) relatively high secondary power consumption - similar to Hall based closed-loop transducers.
(3)
(1)
time
Figure 10: Current answer to a voltage step. U(t)
i(t) time
6
Figure 11: pulsed voltage and current answer (I1 = 0)
U(t)
i(t)
time
Figure 12: pulsed voltage and current answer (I1 ≠ 0)
5.4 Main Types of Fluxgate Transducers The main types of fluxgate transducers are shown on Figure 13 and briefly described hereafter: (1) “Standard” Fluxgate, as discussed before; (2) “Two magnetic cores” Fluxgates, where the performance is significantly improved by: (a) making the field sensing head with one (dark color) of the two main magnetic cores, no airgap being machined into it, and (b) ensuring the high frequency performance by using a separate core for the transformer effect, again without any airgap machined into it. (3) “Three magnetic cores” Fluxgates, where the performance is improved a step further by (a) duplicating the field sensing head, using two magnetic cores (dark color) with an excitation coil wound around each of them; (b) improving the way the high frequency current transformer is made and processed electronically. (4) “Low frequency” fluxgates, using only the low frequency part of the “Two magnetic cores” Fluxgate transducer, not including the current transformer.
(1)
(2)
(3)
Figure 13: Main types of fluxgate transducers.
(4)
CURRENT TRANSFORMERS
When only AC or pulsed primary currents have to be measured, the use of the classical transformer principle is generally considered, coupling a secondary coil to the variable flux created by the primary currents. Current transformers are robust and used for isolating and down-scaling of a larger primary alternating current to a secondary current that can easily be measured with a shunt. A vast variety of CTs is available for primary currents in the range of mA to kA and isolation voltages going up to the largest ones used in electrical engineering (MV). Apart from the types for technical frequencies (16.66 Hz to 400 Hz) they can be built even for frequencies in the MHz range if the turns count is low enough to avoid capacitive errors. 7
AIR-CORED TECHNOLOGIES
Performance of current and voltage transducers are often limited by the imperfections introduced by the core magnetic material (e.g. remanence, hysteresis, non-linearity, losses) and thus the design of air-cored or coreless transducers is often envisioned. The challenge in this case is to get enough measurement sensitivity and to be insensitive to external magnetic perturbations (e.g. terrestrial field, external conductors), functions generally realized by the magnetic core. The following main designs to mention are: • Use of very sensitive field sensing elements (e.g. AMR, GMR, MI [2]). The use of very sensitive field sensing devices provides the current transducer sensitivity but the robustness to external fields becomes an issue: Mounting four sensing elements in a bridge configuration mitigates this influence. • Rogowski coil: For AC or pulsed current measurements, an air-cored pick-up coil is made with a coil uniformly wound around a flexible cylinder of insulating material. To be insensitive to external field perturbations, the connections of the coil’s terminals are made in a special way. The design of a Rogowski coil with a high sensitivity requires either to have a large turns density or a large coil cross section. • Planar Rogowski coil: Different designs of Rogowski coil made of printed-circuit-board (PCB) have been considered, to reduce manufacturing costs, size and TM mass. A mature design (PRiME technology - Figure
14) masters the insensitivity to external perturbations by using an innovative coils design / construction [1] [3]. A very impressive performance has been reached with this technology.
Base PCB Conductor to be measured
10 REFERENCES [1]
R. Bürkel et al, “LEM technology and application brochure”, edition 3, May 2004.
[2]
Pavel Ripka (ed.), “Magnetic Sensors and Magnetometers”, 2001 Artech House Inc., Norwood, MA
[3]
D. Porto et al, "Design of a new air-cored transformer – th Analytical modeling and experimental validation", 39 IEEE-IAS annual conference record, Seattle, USA, 2004
[4] PRIME Faraday Partnership, “An Introduction to MEMS (Micro-electromechanical Systems)”, Dec. 2001, http://www.primetechnologywatch.org.uk/documents/Mems.pdf
Sensor PCB
Figure 14: PRiME
8
TM
current transducer.
SHUNTS
In general, a classical shunt measurement is adequate when the shunt drawbacks are acceptable for the considered application, namely: power losses, bandwidth, noise, non isolated measurement. Indeed, for real-life applications, the voltage drop at the shunt has to be kept constant whatever the nominal value of the current to be measured. Therefore losses increase in proportion to the current which leads to cooling problems at higher current levels. Also magnetic effects are getting stronger with higher currents, so the shunt upper cut-off frequency decreases and the signal-to-noise ratio is getting worse. Finally, there is a need for additional hardware for signal isolation, transmission and amplification. An advantage of the shunt compared to the CT is the suitability for currents containing DC components. The problems due to inductive effects can be mitigated by special constructions (coaxial shunt, planar shunt). 9
MEMS
Micro-Electro-Mechanical-Systems (MEMS, [4]) are basically not a new current sensing technology but more a new design / production opportunity, opening new ways of designing current transducers. There are two main types of MEMS developments which may impact current measurement in the future: (1) development of MEMS magnetic field sensing devices (e.g. Fluxgates), which could then be deployed into current measurement, (2) real new concepts of measuring the current, not possible or not effective with classical technologies. In both cases, industry and universities are still facing technical challenges to get efficient results. A key question is also still linked to the sustainability of the MEMS production sources and related investments, since quantities required for current measurement are often perceived as marginal.
