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Posted on 14 March 2019

Methods of Current Measurement in Mid-Frequency Power Circuits

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Fast-changing power current precise measurements are very important in modern measuring equipment for power semiconductor devices. It is quite important for converter equipment as well, since improved control algorithms are used there, which require knowledge of instantaneous current value in a certain period of time.

By Alexey Poleshchuk, R&D Center, Proton-Electrotex JSC

There are many methods to measure current, which differ depending on the used physical effect. Electronic components and the chosen technique play a critical part because they set complexity of circuit solution.

The situation is complicated by the fact that current may vary from ten amps up to hundreds kiloamps, and frequency may vary from industrial 50/60 Hz up to hundreds kHz. If it is necessary to run current measurements in a high-voltage pulse cascade which requires increased insulation, additional limitations occur. There is no single way to solve the problem that is why it is necessary to know all pros and cons of certain methods to manage the issue.

Further there is a comparative analysis of the accepted methods to measure current, the areas of their usage and differential peculiarities. At first, the pros and cons of various sensors are described depending on the operational approach. Secondly, analysis of the issues connected with a insertion point of the sensor into scheme and following processing of received data received is described.

Current measurement principles and features

Measurement Transformer

Power current transformer is a device which is widely used in industrial electrical engineering. It is fully described in various literature. There is no its detailed description in this article since it is used only in industrial frequency networks.

Disadvantages of this device:

  • Slow response, determined by core magnetism, which does not let using it at a frequency higher than assigned (60 or 400Hz). At higher or lower frequency its transmission ratio is distorted due to nonlinear characteristics of core magnification;
  • Distortion of transmission ratio due to heating-up and core saturation and necessity to apply turns correction;
  • High mass-dimensional characteristics and overheat danger of the secondary coil in the case of measuring device disconnection.

General advantages:

  • Simple design and low cost comparing to the other solutions;
  • Availability of regulatory standards and normative documents for accuracy class index;
  • Possibility of high voltage galvanic insulation, no additional power supply requirements.

There are broadband power current transformers, which can operate in the frequency range different from the industrial network range, but such solutions are more expensive and less popular. For example, transformers by Pearson Electronics can register current in the 10Hz- 20MHz range with 100A current amplitude. Other models produced by Pearson Electronics allow to measure current flowing in coaxial cable in the 300Hz-200MHz range.

Rogowski Coil

The Rogowski Coil is an alternating-current component sensor in conductor. It is a coreless coil around the current-carrying conductor. Coil design provides with high protection from the external electromagnetic interference and high output linearity to the measured current.

Coil output voltage follows the formula:

V=\frac{-AN\mu_0}{l}\cdot \frac{dI}{dt}

where A = πa2 – small area (one coil around the central wire)
l – coil length, N – number of coils around the central wire.

Since the coil registers alternating-current component only, to achieve absolute current value it is necessary to integrate the coil input signal. This process is done by the sensor internal circuit, which is supplied together with the coil (figure 1).

The Rogowski Coil is an alternating-current component sensor in conductor

Since the coil is not closed and sensor output does not depend on the coil path, the sensor can be quickly installed in any conductor line without structural changes.

Due to fast response and high linearity the sensor registers alternating and pulse current with high accuracy, but the integration scheme generates distortion on direct current linked with the accumulative error during registration of high frequency transient phenomenon and insensitivity to the low frequency current changes.

Since transmission characteristics of each coil vary during production process, typically the sensor, amplifier and integrator are considered as a whole thing calibrated by the manufacturer.

Advantages of the method:

  • High linearity and fast response of the sensor which enable it to register high frequency current pulses;
  • Lightness and installation simplicity at any circuit location;
  • Galvanic isolation of the sensor. Disadvantages:
  • Accumulative error on direct current connected with integration errors and influence of the noises;
  • External power supply requirements.

Hall Effect current sensor

Unlike the above described sensors, the ones based on Hall effect measure real current in the circuit. This allows to precisely register processes in the circuit in the range from direct current to high-speed transients.

There are many types of this sensor in different design: for semiconductor mounting and for mounting around the conductor line. According to the principle of operation there are 2 types of sensors: open and closed-loop. Open type sensors use Hall effect to register the flowing current and distribute converted Hall voltage. Unlike the open closed-loop sensors generate current, which compensate magnetic field produced by primary current, and, as a result, provides with zero magnetic flux linkage.

Open type sensors use Hall effect to register the fl owing current and distribute converted Hall voltage

The following formula describes connection between coils volume and current: primary coil and control coil.

I_PN_P = I_SN_S

The main advantage of closed-loop sensors is increased accuracy and wide frequency range. The sensors have current output, which provides increased resistivity to electromagnetic interference.

For the low current range (up to 30A) integrated design sensors are widely used to be mounted on semiconductors produced by Allegro Microsystems, for example. One of the main disadvantages of such microcircuits is output noises related to internal compensation scheme. LF filter used to eliminate the noises decreases system response rate down to 30-40kHz.

Advantages of the method:

  • Wide range current measurement including direct current;
  • High measurement accuracy, good linearity especially in closed loop sensors;
  • Overcurrent withstand without damage;
  • Complete galvanic separation with measured circuit.

Disadvantages:

  • Output noise in some kind of sensor;
  • Limited frequency range (below 500kHz);
  • Saturation mode when measured current exceeds rated range;
  • High cost and large dimensions especially for closed loop sensors designed for high current (from 1kA);
  • Additional power supply requirements.

Resistive Sensor

Resistive shunt is one of the most common but in many cases the most complicated device due to the absence of galvanic separation with the operating circuit.

In the low current range film technology SMD-resistor shunts are used, which provide good heat sink and low inductivity. The next power class is represented by the resistors produced with the thick film technology in TO-220 housing for through-hole mounting; these resistors have very good thermal characteristics through the external heatsink and low inductivity.

There are two major problems associated with the usage of resistive shunts with high current - shunt power losses, which require its cooling and internal shunt inductance which distorts voltage with increasing frequency transients. Galvanic insulation between high voltage circuit and control circuit, especially at high nominal and pulse voltage, is another important question.

It is possible to include a compensating element with the transfer function for approximate compensation of reactance into the measurement scheme:

W(s) = \frac{R_\omega^{-1}}{\frac{L_\omega}{R_\omega}\cdot s + 1}

Lω and Rω - shunt characteristics

However, frequency band of the whole device is limited by the band of first order filter, and accuracy of measured values doesn’t meet the metrological requirements.

To have very accurate measurement of high frequency processes special design coaxial shunts are used, where influence of the internal inductivity is compensated what allows to measure current in the range up to several MHz. Coaxial shunts produced by LEMSYS allow to register peak current up to 100kA in the 200MHz frequency range (figure 3).

Coaxial shunts produced by LEMSYS allow to register peak current up to 100kA in the 200MHz frequency range

As a result, there are the following major advantages and disadvantages of the resistive shunts.

Advantages:

  • Simple design;
  • High metrological accuracy and very wide frequency range (for coaxial shunts);
  • High overload capability and possibility to measure current peak values;
  • Need no power supply.

Disadvantages:

  • Need to be directly connected to power bus, power losses during heating-up of the shunt;
  • External galvanic isolation is required.

Magneto-optic Sensor

Current sensors based on magneto-optical effect are used for galvanically isolated measurement of high currents (up to 500kA) in high voltage buses.

Advantages:

  • Possibility to measure high current values;
  • Stability against the influence of the cross-magnetic field;
  • Complete galvanic isolation;
  • No power losses in the sensor.

Disadvantages:

  • High cost and large dimensions.

The general application areas of the various current sensors

General Application Areas of Various Sensors

During development of the measuring equipment there are some key questions: the precise measurement of current peak value in the wide frequency range due to high rate transient processes in power circuit and tested elements. Hall effect sensors and resistive shunts correspond to the current value registration accuracy criterion.

In most cases the Hall sensor frequency band is not enough for precise registration of the transient processes, moreover, closed-loop sensors for high current values are very expensive. Coaxial design resistive shunts have very wide passband and high measurement accuracy combined with ability to register very high peak current values limited by shunt heat capacity only (fig. 5).

The general application areas of the various current sensors

These advantages determine their usage for measurement despite the disadvantages linked to requirement of additional galvanic isolation.

Current measurement in bridge and half-bridge schemes

Shunt connection point

Since the shunt is installed in measured circuit, it is under power circuit operating voltage, where its connection point is very important (figure 6).

There are two ways of measurement shunts connection

Supposing that ground measurement and control design is connected to the common line of the main circuit, there are two ways of measurement shunts connection in any power circuit - between power supply and load (high-side current sensor) and between load and common line (low-side current sensor).

Connecting shunt to the negative load pole we have the following:

  • Measurement is done with respect to the control scheme ground;
  • Low common mode voltage on the input of the buffering operational amplifier.

Connecting to the positive pole:

  • Measurement is done using differential method (since floating shunt connection point potential);
  • Adapter scheme is required to transmit and convert input differential signal;
  • High common mode voltage on the input of the buffering circuit.

To simplify the implementation of measurement circuits, connection of the shunt between load and common lines has some advantages, but sometimes it is not possible due to different reasons. To implement the scheme a lot of instrumental amplifiers can be used, for example, AD627 produced by Texas Instruments. There are no issues to implement this scheme.

Connection of the shunt to the positive load pole has the one major problem - fast-changing common-mode input voltage. There is a set of instrumental amplifiers to measure differential-mode signal in shunt at common-mode input voltage out of the scheme supply range. LT1990 and AD629 belong to the set of such amplifiers. AD629 has ±270V common-mode input voltage with bipolar supply, and its maximum is ±500V. Rejection level of common-mode voltage is defined by CMRR characteristic (common-mode rejection ratio), which reaches 90dB in the frequency range up to 1kHz.

In the scheme key switching time is 200ns and supply voltage is 0V and +300V. Then:

\frac{dV}{dT}=\frac{\Delta V}{\Delta T}=\frac{300V}{200ns}=1500V/\mu s

Thus, current shunt installed in the scheme changes its potential from 0 up to supply voltage with defined rate (figure 7).

Current shunt installed in the scheme changes its potential from 0 up to supply voltage

Unfortunately, CMRR characteristic drops with common-mode voltage frequency growth, and usage of this type of amplifiers is impossible at the frequency about several tens of kilohertz, and it also cannot block the noise formed by dV/dt.

To implement the scheme considering all issues described above it is necessary to use the methods which guarantee blocking of the common-mode noises through the galvanic separation. Such methods consist of the following: tied part, which makes preliminary signal adjustment, galvanic isolation, untied part, which normalizes output signal. All these parts can be combined in one unit or implemented as separate components and units.

Primary areas of this method implementation:

  • High-linearity analog optocoupling devices;
  • Insulated operational amplifiers;
  • Insulated analog-digital converters with digital separation.

Analog optocoupling devices are used for signal separation through highly linear couple LED - 2 photo diodes, where one of the photo diodes is used to make feedback and ensure signal accuracy. An example of such components type is Avago HCNR200. Principle application diagram is shown in the following figure (figure 8):

Principle application diagram

Optocoupling devices have very high transmission ratio accuracy and wide dynamic range (over 5MHz). If there are good characteristics to block common-mode interference with 50Hz frequency, its inner structure doesn’t provide with high CMTI (common-mode transient immunity), and high frequency common-mode interference goes into the secondary circuit insufficiently filtered.

Isolated operating amplifiers have a wider application area. These are Texas Instruments AMC1200 and Avago ACPL-790x. Avago ACPL- 790x provides a wider band pass up to 200kHz. It has the following advantages: integral design, which requires minimum additional elements and high withstand to transient processes up to 15kV/μs.

Major disadvantages: limited band pass, which is not enough for many applications in measurement equipment and high level of interference with frequency over 50kHz (figure 9) related with operation of sigma-delta converter used in this microcircuits. However, these microcircuits are very convenient to use for current measurement in control systems such as drive controllers and power converter controllers, where there is no need in precise current feedback.

Interference of frequency over 50kHz related by operation of sigma-delta converter

In the case when high resistivity to switching noises is required, wide frequency range and high measurement accuracy with low noise level, it is justified to use analog-digital converter located in unisolated part and further isolation of digital signal through digital isolators. Depending on the application this can be done by analog-digital converter with serial or parallel interface; after digital isolation it is possible to convert signal back to analog from using digital-analog converter, or connect it to the controller input (figure 10).

Digital isolation it is possible to convert signal back to analog from using digital-analog converter

Such scheme provides maximum quality of signal measurement, but it is more complicated, expensive and bulky. Simultaneously, it gives an opportunity to measure current in high voltage circuit, because digital signal insulation does not have any major issues and can be done using high voltage optocoupling devices and POF type optical fiber transmission system.

Influence of Power Circuit Parasitic Parameters

Usage of shunt connection scheme in the negative load pole in some cases is complicated by influence of high current flow and fast switching processes of power circuit stray characteristics such as power bus line inductivity. Such influence leads to the fact that current shunt is getting under floating potential defined by EMF power bus line self-inductance in different moments (figure 11).

Shunt connection in the negative load pole suffers from stray inductivity

Switching off 1000A current for 500ns the voltage value on short copper bus reaches 100V, and rate of voltage rise equals 200V/ μs. This allows treating this situation equal to the first, which occurs during shunt connection to the positive load pole, because the load is distributed.

Current Measurement in Bridge Circuit

During development of the bridge circuits switching low currents (up to 100A) with necessity to precisely control these currents value, it has sense to use current shunts connection into the lower arms of half-bridges instead of using shunt in the load area (figure 12).

In bridge circuits the current shunts are in the lower arms of half-bridges

Current in the shunt Rh is the real current flowing in the load, but its measurement causes additional problems, especially during high frequency key elements switching (80kHz). Current in the Rw shunt shows general power consumption of the power circuit, but it doesn’t reflect the processes, which occur during load short circuiting by keys Q2 and Q4. Current measurement in this resistor is used to define power line short-circuit to an external devices (housing, for example), what cannot be traced by the data from the other circuit points.

Thus, measuring voltage drop on resistors R1 and R2 it is possible to completely restore current flowing in the load in the almost all switching modes.

Summary

Measuring equipment for power semiconductors has very high requirements to the quality of current measurement circuit and its characteristics. These are wide range of the measured current in consideration of peak values measurement necessity, and wide frequency range up to several MHz. Considering the price factor, the coaxial resistive shunts correspond to the above mentioned characteristics. Its usage in fast main circuits is linked to the inevitability to solve some tasks like signal galvanic isolation and switching noises suppression. These tasks can be solved in different ways depending on the required accuracy and passband using integrated solutions like ACPL-790x by Agilent, or using special circuits on discrete elements, which allow to achieve dynamic and precise characteristics.

List of references
• Kawamura T., Haginomori E., Goda Y., Nakamoto T. Recent Developments on High Current Measurement Using Current Shunt/Transactions on electrical and electronic engineering. – 2007. – V. 2. – № 5. – P. 516–522.
• Ming-Hian Chew. Measuring motor drive and inverter currents. PCIM July/August 2003.
• Isolated current and voltage transducers. Characteristics-Application- Calculation. 3-rd Edition. LEM Components, CH24101 E/US, 05.04, www.lem.com.

 

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