isolated current voltage transducers

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Air-Core Technologies 5 Air-core technologies The performance of current and voltage transducers are often limited by the characteristics of the magnetic core material itself (e.g. remanence, hysteresis, non-linearity, losses, saturation) so the design of an air-core, or coreless, transducer is often considered. In this case the following issues have to be taken into account: • the measurement of DC current requires the use of a field sensing element; due to the absence of a field focused area (e.g. gap), a highly sensitive field sensing device must be considered (e.g. GMR, Hall cell) ideally in an array around the conductor. • when available, a magnetic circuit can be used as a shield to external magnetic field disturbances (e.g. earth’s field, external conductors); with air-core technologies the sensitivity to external disturbances must be managed in a different way, for example an array of field sensors instead of a single sensor or, when coils are considered, special design execution such as the Rogowski method of routing the return wire; the ability to accurately measure the desired current while also rejecting external fields is a significant challenge for air-core technologies. the waveform of the measured current requires the integration of the induced voltage. Therefore, the current transducer includes an integration function in the processing electronics. In the LEM~flex and PRiME transducer datasheets the value of sensitivity (S 12 ) is provided, linking the amplitude of a sinusoidal current to the amplitude of the transducer output voltage at a specific frequency. The same sensitivity parameter can also be used to link the RMS values of a sine wave primary current and the corresponding sine wave output voltage, namely: E PEAK = S 12 • f • I PEAK and E RMS = S 12 • f • I RMS To give an order of magnitude, the typical sensitivity is: • LEM~flex probe S 12 = 2.0 [µVs/A] • PRiME S 12 = 1.0 [µVs/A] 5.2 LEM~flex - the flexible AC current transducer LEM~flex is a transducer based on the Rogowski coil principle (Fig. 38), providing a flexible air-core sensor that can be opened, allowing it to be installed on a primary conductor without interrupting the circuit. Two efficient air-core transducer technologies are presented in this chapter, both related to AC measurements: the LEM~flex and PRiME(1) technologies. (1) patented and licensed to LEM by Suparule Ltd. 5.1 Basic working principle and sensitivity LEM~flex and PRiME technologies both work on the same basic principle; a pick-up coil is magnetically coupled with the flux created by the current to be measured. A voltage is induced on the pick-up coil proportional to the derivative of flux and thus proportional to the derivative of the current to be measured. Because the derivative of DC is zero these technologies are only useful for the measurement of AC or pulsed currents. I P Figure 38: Rogowski AC current measurement The instantaneous voltage induced in the pick-up coil is typically: E OUT (t) = L 12 • di(t)/dt where i(t) is the primary current [A] and L 12 is the mutual inductance [H] between the primary and pick-up coil. For a sinusoidal current, we have: i(t) = I PEAK • sin(2 • π • f • t) Hence: E OUT (t) = L 12 • I PEAK • 2 • π • f • cos(2 • π • f • t) = E PEAK • cos (2 • π • f • t) [V] As shown in this example, where a sinusoidal current i(t) creates a phase delayed (cosine) voltage E OUT (t), reproducing 34 [V] [A] 5.2.1 Construction and principle of operation The key element of the LEM~flex transducer is the flexible measuring head (Figure 39), which is a coil uniformly wound around a flexible former of insulating material. The end of the coil wire is returned to the same end as the start by returning it co-axially through the coil former (Fig. 38). This construction technique minimizes the sensitivity to external field disturbances. To make a measurement, it is not necessary to have the LEM~flex transducer formed as a circle or to have the primary conductor(s) centered within the perimeter. In practice, the flexible measuring head is wrapped around the conductor(s) carrying the current to be measured and the two ends are brought together and mechanically connected by a coupling latch (Fig. 40) to form a closed path.
Air-Core Technologies sectional area. Note that changing the length of a Rogowski coil does not affect the sensitivity. 5.2.2 Characteristics and features The LEM~flex family of transducers has been designed to conveniently measure single and 3-phase AC currents, as well as pulsed DC currents. Standard ranges include 30/300/ 3 kA RMS and 60/600/6 kA RMS , but scaling is easily designed for other currents. Theoretically the size of the measuring head and/or measurement range is unlimited. The standard LEM~flex current transducer provides a sensitivity of 50 or 100 mV/A at 50 Hz and at the output of the processing electronic. The analog output voltage, galvanically isolated from the measured current, is 0 to 3 V RMS , or to be precise 4.25 V PEAK . Fig. 39 LEM-flex transducers Due to the flexibility of the measuring heads, it is possible to position them around one or more irregularly shaped or difficult to access conductors or busbars. The transducer can be quickly and easily installed as well as removed. Installation and measurement is performed without mechanical or electrical interruption of the current carrying conductor, while also ensuring galvanic isolation. LEM~flex transducers are also very lightweight. In terms of bandwidth, the performance of the LEM~flex transducer is similar in concept to a band-pass circuit, exhibiting both high and low cut-off frequencies. As the integrator gain can be very high, shielding and appropriate filtering prior to the integrator must be used to minimize the influences from low frequency influences. For high frequency performance, the upper cut-off frequency is determined by the coil inductance and capacitance. It should also be noted that the integrator includes compensation circuits that limit the thermal drift. The typical bandwidth targeted by the LEM~flex portfolio is 10 to 100 kHz, with possible extension to 1 MHz. As the LEM~flex measuring head is fundamentally an air-core coil, there is no magnetic hysteresis, no saturation phenomena or non-linearity, as is present with magnetic cores. The main factors that affect the accuracy of this technology are: Fig. 40 Typical LEM-flex use, surrounding conductor(s) and latched. The sensitivity (§ 5.1) of a LEM~flex transducer can be detailed as follows: S 12 = 8 • 10 -7 • π 2 • N • A where N is the density of the coil turns made around the flexible insulating cylinder [turns/m] and A is the crosssectional area of the coil [m 2 ]. To design a Rogowski coil with a high sensitivity requires either a large number of turns per meter or a large cross- Relative Amplitude 1.30 1.25 1.20 1.15 1.10 1.05 1.00 0.95 4 Phase Degree 2 0 -2 -4 -6 -8 Output 8 10 40 100 400 1k 4k 10k 40k 100k Frequency (Hz) Figure 41: Integrator circuit & bandwidth -10 -12 -14 Phase Degree ( ) ° 35
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