Although there are many instruments that can accurately measure small DC currents (up to 3A), few instruments can accurately measure DC currents above 50A (less than 1%). Such large current ranges are typical values ​​for loads such as electric vehicles (EV), grid energy storage, and photovoltaic (photovoltaic) renewable energy devices. In addition, these systems require accurate prediction of the state of charge (SOC) of the associated energy storage battery. Estimation of the state of charge can be based on current and charge (Coulomb count) measurements, and accurate measurement data is necessary for accurate charge state estimation.
In general, any system used for current or charge measurement is designed to include built-in data acquisition components such as suitable amplifiers, filters, analog-to-digital converters (ADCs), and more. A current sensor is used to detect the current. The output of the current sensor needs to be converted into a usable form (ie voltage) by a circuit. The signal is then filtered to reduce electromagnetic and radio frequency interference. Then zoom in and digitize. Each current data sample is then multiplied by a suitable time interval, and the charge value is accumulated (by digitization calculation).
On the other hand, if digitization is performed at a constant frequency, the first accumulated current samples are then multiplied by the appropriate time interval when the accumulated charge values ​​are read or utilized in some way. Also consider choosing the appropriate minimum Nyquist sampling rate and using a sufficiently narrow anti-aliasing filter before the analog-to-digital converter.
Figure 1: Signal chain in a typical modern current measurement system.
Practical sensor technology for high current measurement
Of the techniques used to measure large currents, two are the most common. The first technique is to detect the magnetic field around the conductor carrying the current. The second technique is to measure the voltage drop across a resistor (often referred to as a shunt) that carries the current (and charge) to be measured. This pressure drop follows Ohm's law (V = I × R).
Devices used for large current measurements are commonly referred to as Hall effect current sensors. This sensor has a current-carrying element built in. When a current and an external magnetic field are applied to the element, a pressure difference perpendicular to the direction of the current and perpendicular to the direction of the external magnetic field appears on both sides of the element. The Hall effect pressure difference in ordinary metals is small. It is worth noting that not all DC current sensors that measure the magnetic field around the current carrying conductor are based on the Hall effect. The differences between them are briefly described below.
High current Hall effect sensor
In order to make a current sensor with a Hall effect device, it is necessary to use a magnetic core to concentrate the magnetic field around the conductor current, and a slot is formed in the core for accommodating the actual Hall element. A relatively small size slot (relative to the entire length of the magnetic circuit) creates a magnetic field that is nearly uniform and perpendicular to the plane of the Hall element. When the Hall element obtains current energy, it produces a voltage proportional to the field current and the magnetic field. This Hall voltage is amplified and output from the output of the current sensor.
Figure 2: Schematic diagram of the magnetic field around the conductor, the linear open-loop Hall effect sensor, and the closed-loop sensor.
Since there is no electrical connection between the current carrying conductor and the magnetic core (only the magnetic field is coupled), the sensor is actually isolated from the circuit under test. The current-carrying conductor may have a very high voltage, and the output of the Hall-effect current sensor can be safely connected to the ground circuit or to a circuit with an arbitrary potential relative to the current-carrying conductor, thus providing clearance and creepage that meet the most stringent safety standards. The value is also relatively easy.
However, such linear sensors also have some disadvantages. Perhaps the least important drawback is the fact that Hall effect sensors require a constant field current. In addition, amplification and regulation circuits that process signals from Hall effect sensors typically consume significant energy. Of course, this energy consumption may not be so significant, depending on the specific application. Nevertheless, Hall sensors used to continuously measure currents cannot consume as little as milliwatts.
Hall effect sensor: large drift, available operating temperature range is small
Because the typical linear sensor output is scaled (depending not only on the strength of the measured magnetic field, but also on the value of the field current), the stability of the field current will greatly affect the magnitude of the current to be measured and zero when no current flows. Offset. In general, the latter two depend on the stability of the supply voltage and the temperature change (because the Hall sensing element resistance that affects the excitation current and the Hall voltage itself depends on the operating temperature).
Sensor variants that measure the field current and take this factor into account in the output are possible. But it requires sophisticated external components and large processing circuitry. Moreover, the Hall voltage is a nonlinear function of the magnetic field to be measured, which further increases the error of the sensor.
Most linear Hall device manufacturers decompose the total error into many separate components because of the different errors that can occur under different conditions. Sometimes it is difficult to calculate the total synthesis error.
Closed loop current sensor
In order to solve the nonlinear problem of Hall sensing elements, another technology has been developed in the industry. This technique relies on detecting the presence or absence of a magnetic field in the sensing core rather than measuring the strength of such a magnetic field. In addition, it avoids measurement errors due to unstable excitation currents in the Hall element.
This technique adds a winding to the core to generate a magnetic field of opposite sign, but the intensity is exactly equal to the magnetic field produced by the current to be measured. Hall sensing elements are now only used to detect magnetic field symbols rather than magnetic field strength. This winding is connected in a circuit with an op amp. The circuit maintains this current in the compensation winding and causes the magnetic field sensed by the Hall sensor to be zero. The current in the compensation winding is many times smaller than the current in the conductor to be tested (perhaps more than 1000 times). This function can be achieved only by winding a few turns on the core when making the winding, and the number of turns can be precisely controlled. .
In view of the role of the compensation winding in the op amp feedback loop, such current sensors are often referred to as "closed loop" sensors. In contrast, the aforementioned simple linear Hall effect sensors are often referred to as "open loop" sensors to emphasize that there is no feedback mechanism during their operation.
In Hall effect devices, the (offset) error in detecting a zero magnetic field cannot be reduced to an arbitrarily small value due to various drifts, and most of them are due to temperature-dependent drift. This is why some of the higher performance current sensors use technology that does not rely on the Hall effect. However, these sensors are still commonly referred to as Hall effect sensors simply because they are very similar in appearance to Hall effect devices.
Other magnetic field detector
In non-Hall devices, some sensors based on various physical phenomena can be used to perform the functions of the magnetic field detector. One of these techniques is based on the magnetoresistance effect, which means that when a magnetic field is applied to the sensor, the resistance of the sensor changes.
Another technique for magnetic field detectors utilizes the nonlinear properties exhibited by ferrite between magnetic field strength (indicated by H), magnetic flux density (represented by B), and a special phenomenon known as saturation. As the H field increases, the magnetic flux density B will eventually reach a point that no longer increases significantly - this point is called the saturation point. Some specially formulated materials have very low saturation points and are widely used in devices called fluxgates.
In fact, a fluxgate-based sensor converts a constant magnetic field into a "selective" or "cut-off" magnetic field that alternates between full-scale and near-zero. This change in magnetic field can be easily picked up by a winding on the core and then amplified by an AC amplifier. Finally, the so-called synchronous detection (because the circuit itself controls the cutting action) recovers a value proportional to the constant magnetic field to be measured.
It is worth noting that the mechanical structure of the sensor and the associated circuitry are much more complex than closed-loop sensors. In addition, they are very difficult to work with - when the sensor does not gain energy, or because of the loose connection to the external sense resistor, the current measurement is performed under the condition that the compensation winding circuit is open - often resulting in unrecoverable offset and gain specifications. Since the compensation winding cannot cancel the magnetic field from the current to be measured, the magnetic components in this sensor will be permanently magnetized.
Need precision resistor
The output signal of the closed-loop sensor is the current in the compensation winding (its value is many times smaller than the current to be measured). This current is usually converted to a voltage value for further processing and digitization. Just use a normal resistor.
However, the accuracy and stability of this resistor will directly affect the accuracy and stability of the closed-loop current sensor. If a 1% accurate sense resistor is used, the closed loop sensor with a basic accuracy of 0.0.01% will soon be reduced to 1% accuracy.
However, it is difficult to purchase a certain number of commercially available resistors with an accuracy of more than 0.01%, even if they only work in a narrow temperature range.
High current shunt
As mentioned earlier, the second current measurement technique uses a voltage drop across the resistor. When determining the current according to Ohm's law, a unique set of factors needs to be considered, depending on the current magnitude. For relatively small currents, the voltage drop across the shunt resistor can be made quite large to overcome any errors due to heat dissipation from the sense connections and shunt resistors or from temperature differences created by the operating environment. However, when the current exceeds 50A, heat dissipation and thermoelectric error are the most important. Also, since the shunt resistor is always heated by the flowing current and may work in an unstable temperature environment, the stability of the shunt resistor relative to temperature is particularly important.
Physical composition of the shunt
At first glance, the shunt device is a simple resistor. Some conductive materials having suitable properties in terms of volume resistivity, (temperature and time) stability, and suitable mechanical profile can be used as shunt resistors. The low precision shunt resistor can be entirely a length of wire or a rectangular shape constructed of a suitable alloy and simply soldered in series (or with some electrical connection) to the current carrying conductor. However, it is almost impossible to insert such a shunt element into the measuring circuit without affecting its resistance (due to the change in the number of solder joints or the change in mechanical details).
In addition, for stability reasons, it is advantageous to arrange the shunt resistors in a manner that the current density in any given cross-section of the shunt resistor is mostly uniform. This prevents the formation of so-called hot spots - defined as internal regions of the shunt resistor that have a higher temperature than the rest of the material. In addition to simple resistance changes, elevated temperatures at the hot spot may bring the resistive material to the annealing point temperature at which the material resistance (by careful control of chemical composition and processing) may begin to change permanently.
Even if the actual presence of the hot spot does not affect the accuracy, it is not possible to ensure that they are formed in exactly the same place when calibrating the shunt resistor. The design of the shunt resistor therefore includes a method of distributing the current evenly across the cross-section of the resistive material, or between a single parallel resistive portion and the interior of each portion.
This is why most of the higher precision shunt resistors are made up of three different parts: the two areas are terminals for accessing the circuit (almost always made of thick, highly conductive material, such as copper), One or more parallel regions make up the majority of the shunt resistor. The two terminal areas are connected by a resistive section or a section using a welding or metallurgical process with a very uniform seam.
The resistive portion (also referred to as the active portion) of the precision shunt resistor must have a low temperature dependence impedance property. One of the most common alloys for precision shunt resistors is Manganese Copper, developed in 1892 by Edward Weston (known for developing electrochemical cells - Weston batteries) due to its suitable resistance and low temperature resistivity (TCR).
Heat dissipation in shunt resistor
The heat dissipated by the resistor is proportional to the square of the current and the resistance (W = I2 × R). For example, a 1mΩ shunt resistor consumes 2.5W when flowing through a 50A current. This power dissipation is a controllable value with moderate heat sink and still air conditions. Conversely, when the current is 1kA, the same shunt resistor will dissipate 1 kW of heat, which requires a large physical size and may force air-cooled (or liquid-cooled) devices.
Figure 3: The relationship between the heat dissipated in the shunt resistor and the resistance and current.
Figure 4: The relationship between the heat dissipated in the shunt resistor and the full-scale output voltage and current.
It should be clear from the above diagram that the only way to reduce the amount of heat dissipated in the shunt resistor under given current conditions is to reduce its resistance. However, this also reduces the voltage value measured across the shunt resistor, which becomes more sensitive to errors caused by the shunt resistor and the detection circuit, resulting in degradation of accuracy in the case of small currents.
Error source in shunt measurement method
The high operating temperature and temperature difference in the shunt resistor will have a negative impact on gain and offset errors. For a shunt-based measurement system, not only does the ambient temperature act, but the measured current itself also works because a large current heats the shunt resistor.
Although the resistive (active) portion of the shunt element is made of a low TCR material, a high operating temperature will inevitably promote the resistance deviation from the calibration value, no matter how small the change. This will produce a sensitivity (gain) error.
Since different materials are used in the shunt resistor structure (that is, the material of the connection terminal and the detection lead is generally different from the resistance part of the shunt resistor), there is a so-called thermoelectric error (such as the Seebeck effect), which affects the bias. Shift error (current reading is reported when the actual current is zero). Since the heat dissipation effect of the shunt resistor can be measured and can be expressed in a predictable manner, some shunt resistor based systems can compensate for the shunt resistance thermal effects that cause offset and gain errors. In any case, when designing a shunt resistor-based current measurement system as shown in Figure 1 (the signal chain of a typical modern current measurement system), careful selection of components that provide the least error and drift is required.
Choose the right measurement method
The most basic problem for measuring large DC currents is measurement accuracy and cost. Other important considerations include the working environment (especially the temperature range), power consumption, size and durability (taking into account possible overloads, transients and non-energizing work). In order to determine the measurement accuracy of any given method, it is important to consider all possible sources of error under all relevant extreme operating conditions.
Table 1: Comparison of current dividers.
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