current sensor

The current sensor is a detecting device that can sense the information of the measured current, and can transform the detected information into a certain signal to meet the requirements of a certain standard or other required form of information output to meet the requirements. Requirements for the transmission, processing, storage, display, recording and control of information.

Current sensors, also called magnetic sensors, can be used in home appliances, smart grids, electric vehicles, wind power, etc., and we use many magnetic sensors in our lives, such as computer hard drives, compasses, household appliances, and so on.

The current sensor is mainly divided into: a shunt, an electromagnetic current transformer, an electronic current transformer, etc. according to different measurement principles.

Electronic current transformers include Hall current sensors, Rogowski current sensors, and AnyWay variable frequency power sensors (for voltage, current, and power measurements) dedicated to variable frequency fuel measurements.

Compared with electromagnetic current sensors, electronic current transformers have no ferromagnetic saturation, transmission frequency bandwidth, small secondary load capacity, small size and light weight, which is the development direction of current sensors in the future.

The fiber-optic current sensor is a new type of current sensor based on the Faraday magneto-optical effect and using fiber as the medium.

When linearly polarized light propagates in the medium, if a strong magnetic field is applied in parallel with the direction of propagation of the light, the direction of the light vibration will be deflected, and the deflection angle ψ is proportional to the product of the magnetic induction intensity B and the length l of the light traversing medium. That is, ψ=V*B*l, the proportional coefficient V is called the Feld constant, and is related to the properties of the medium and the frequency of the light wave. The direction of deflection depends on the nature of the medium and the direction of the magnetic field. The above phenomenon is called the Faraday effect. Discovered by M. Faraday in 1845. [1]

AIC is the abbreviation of "specialized integrated circuit", which is a high-tech product that developed rapidly in the late 1980s. From the design idea, the development method, to the test method, it is qualitatively different from the traditional general-purpose integrated circuit. It is the process technology, computer-aided design (CAD) and automatic test technology (ATE) of VLSI. The combination of fruitful results. Applied to the transmitter, it is a special thick film circuit for the transmitter. The transmitter of the ASIC circuit integrates the converter's conversion circuit and output circuit (that is, most of the electronic circuit) into a customized chip, which greatly reduces the number of components. The entire transmitter has only CT, PT, A few devices, such as power supplies, large capacitors, and ASIC chips, can greatly improve the reliability and long-term stability of the entire transmitter.

The Hall principle current sensor is based on the two basic principles of Hall magnetic balance principle (closed loop) and Hall direct measurement (open loop). [2]

The principle of the open-loop current sensor: the magnetic flux generated by the primary current IP is concentrated in the magnetic circuit by the high-quality magnetic core, the Hall element is fixed in a small air gap, and the magnetic flux is linearly detected, and the output of the Hall device is After the Hall voltage is processed by a special circuit, the secondary side outputs a follow-up output voltage consistent with the primary side waveform, and this voltage can accurately reflect the change of the primary current.

Hall current sensors can measure various types of current, from direct current to tens of kilohertz of alternating current, which is based on the Hall effect.

When the primary side wire passes through the current sensor, the primary current IP will generate magnetic lines of force, and the 2 primary magnetic lines are concentrated around the core. 3 The Hall electrode built into the core air gap can produce a size proportional to the primary magnetic line. With only a few millivolts, the 4 electronic circuit can convert this tiny signal into the secondary current IS,5 with the following relationship:

(1)

Where IS-secondary current;

IP—primary current;

NP—the number of turns of the primary side coil;

NS—the number of turns of the secondary side coil;

NP/NS—the turns ratio, generally takes NP=1.

The output signal of the current sensor is the secondary current IS, which is proportional to the input signal (primary current IP). The IS is usually small, only 100~400mA. in case

The output current passes through the measuring resistor RM, and an output voltage signal of a few volts proportional to the primary current is obtained.

Standard rating IPN and rated output current ISN

IPN refers to the standard rating that the current sensor can test. It is expressed in terms of rms (Arms). The size of the IPN depends on the model of the sensor product.

ISN refers to the rated output current of the current sensor, which is generally 100~400mA, and some models may be different.

Sensor supply voltage VA

VA refers to the supply voltage of the current sensor, which must be within the range specified by the sensor. Above this range, the sensor does not work properly or the reliability is reduced. In addition, the supply voltage VA of the sensor is further divided into a positive supply voltage VA+ and a negative supply voltage VA-.

Measuring range Ipmax

The measurement range refers to the maximum current value that the current sensor can measure. The measurement range is generally higher than the standard rating IPN. The measurement range can be calculated by:

(2) It is necessary to pay attention to the single-phase power supply sensor. The power supply voltage VAmin is twice the two-phase power supply voltage VAmin, so the measurement range is higher than that of the dual-phase power supply sensor.

overload

See Figure 2 for the overload capability of the current sensor. When a current overload occurs, the primary current will increase outside the measurement range, and the duration of the overload current may be short, and the overload value may exceed the allowable value of the sensor. The overload current value sensor is generally not measured, but not Damage to the sensor.

Precision

The accuracy of the Hall effect sensor depends on the standard rated current IPN. At +25 ° C, the curve of the sensor measurement accuracy affected by the primary current is shown in Figure 3. The accuracy can be calculated using the following formula:

(3)

Among them, K = NS / NP.

The effects of offset current, linearity, and temperature drift must be considered when calculating accuracy.

Offset current ISO

The offset current is also called residual current or residual current, which is mainly caused by the unstable operation state of the operational amplifier in the Hall element or electronic circuit. When the current sensor is produced, at 25 ° C, IP = 0, the offset current has been minimized, but the sensor will generate a certain amount of offset current when leaving the production line. The accuracy mentioned in the product technical documentation has taken into account the effects of increased offset current.

Linearity

The linearity determines the degree to which the sensor output signal (secondary current IS) is proportional to the input signal (primary current IP) within the measurement range. ABB's current sensor linearity is better than 0.1%.

Temperature drift

The offset current ISO is calculated at 25 ° C. When the ambient temperature around the Hall electrode changes, the ISO changes. Therefore, it is important to consider the maximum variation of the offset current ISO, which can be calculated by:

Among them, CV (Catalogue value) refers to the temperature drift value in the current sensor performance table. For example, for the CS2000BR type, the CV is 0.5×10-4/°C, the maximum temperature Tmax is -40°C, and the rated output current is 400mA. , the maximum change in offset current is: Ma

The Hall current sensor product description is generally composed of two parts: “sensor product model” and “production date” [5]. The “Sensor Product Model” is used to indicate the sensor model, rated measurement, standard or non-standard type. The “sensor production date” is composed of 8 digits, indicating the year and date of production of the sensor (the first few days of the year) and the sensor serial number.

There are many Hall current sensor products, each of which has different shape, size and size. Here are some typical outline structures and installation and wiring methods.

MP25P1 type

The MP25P1 current sensor is a small measuring sensor in ABB. The rated current can be 5, 6, 8, 12, 25A. The different connection methods of the primary pins can determine the rated current. See the figure. 5.

ES300C type

Like the MP25P1, the general sensor has three pins: positive (+), negative (-), and measuring (M). However, ES300C does not have these three pins, but has three red, black, and green leads. Corresponding to the positive electrode, the negative electrode and the measuring end. At the same time, there is an inner hole in the ES300C type sensor. When measuring the primary current, the wire should be passed through the inner hole.

Regardless of the current sensor such as MP25P1 or ES300C, the wiring of the pins should be connected according to the measurement conditions.

(1) When measuring AC power, the bipolar power supply must be forced. That is, the positive (+) of the sensor is connected to the "+VA" terminal of the power supply, and the negative terminal is connected to the "-VA" terminal of the power supply. This connection is called a bipolar power supply. At the same time, the measuring terminal (M) is connected to the "0V" terminal of the power supply through a resistor.

(2) When measuring DC current, a unipolar or single-phase power supply can be used, that is, the positive or negative electrode is short-circuited with the "0V" terminal, so that only one electrode is connected, and there are four connections.

In the sensor product, the "-N" mark indicates that the sensor has no power supply accidental inversion protection; the "-P" mark indicates that the sensor has protective measures. Figure 6 shows the unipolar power supply installation wiring method when there is no protection diode, and Figure 7 shows the connection of the sensor with protection.

(3) Connection method of sensor having shielding effect

Some of ABB's current sensors have electromagnetic shielding. There is an “E” mark on the product housing. There are two ways to connect them: connect the shield to the negative (-VA) or neutral (0V).

In addition, the use, model, range, and installation environment of the product must be fully considered during installation. For example, the sensor should be installed as much as possible for heat dissipation; if the environment is only suitable for vertical installation, the sensor with the “V” mark (such as CS300 BRV) must be selected.

In addition to the installation wiring, instant calibration calibration, and attention to the working environment of the sensor, the measurement accuracy can be improved by the following methods:

1. The primary side conductor should be placed in the center of the inner hole of the sensor, and should not be biased as much as possible;

2. The primary side wire fills the sensor inner hole as completely as possible, and no gap is left.

3. The current to be measured should be close to the standard IPN of the sensor. Do not differ too much. If the conditions are limited, there is only one sensor with a high rated value, and the current value to be measured is much lower than the rated value. In order to improve the measurement accuracy, the primary side wire can be wound a few times to make it close to the rated value. For example, when measuring a current of 10A with a sensor with a rating of 100A, the primary side conductor can be wound around the center of the inner hole of the sensor for nine times (in general, NP=1; one turn in the inner hole, NP=) 2;......; Around nine turns, NP=10, then NP×10A=100A is equal to the rated value of the sensor, which can improve the accuracy);

4. When the current value to be measured is IPN/5, it can still have higher accuracy at 25 °C.

1. Electromagnetic field

The closed-loop Hall-effect current sensor utilizes the electromagnetic field principle of the primary side conductor. Therefore, the following factors directly affect whether the sensor is interfered by external electromagnetic fields.

A1 DC Current Transmitter

(1) Whether the external current and the current frequency near the sensor change;

(2) the distance between the external wire and the sensor, the shape and position of the external wire, and the position of the Hall electrode in the sensor;

(3) Whether the material used to install the sensor is magnetic;

A3 single phase AC current transmitter

(4) Whether the current sensor used is shielded;

In order to minimize the interference of external electromagnetic fields, it is best to install the sensor according to the installation guide.

2, electromagnetic compatibility

Electromagnetic compatibility EMC (Electro-Magnetic Compatibility) is a coexistence state in which electrical and electronic equipment can perform their respective functions in a common electromagnetic environment, that is, all of the above-mentioned various devices in the same electromagnetic environment can work normally and interact with each other. A discipline that does not interfere with the "compatible" state [8]. The deterioration of the space electromagnetic environment is more and more likely to cause malfunction of the system due to mutual incompatibility between electronic components. Therefore, it is extremely necessary to detect electromagnetic compatibility of electricians and electronic equipment. Due to the urgent need for actual production, scientific research and marketing, the use of current and voltage sensors that have passed electromagnetic compatibility testing has formed a consensus and has become a mandatory standard. All ABB current sensors have passed EMC testing since January 1, 1996.

1, offset current ISO

The offset current must be calibrated at IP=0, ambient temperature T≈25°C, wired according to the method of Figure 9 (bipolar supply), and the measured voltage VM must satisfy:

VM≦RM×ISO (5)

2, accuracy

The measurement is performed under the conditions of IP=IPN (AC or DC), ambient temperature T≈25°C, sensor bipolar power supply, and RM is the actual measurement resistance. The wiring is as shown in Fig. 10, and the accuracy is calculated by formula (3).

3. Protective test

The Hall current sensor can be protected under the conditions of short circuit of measuring circuit, open circuit of measuring circuit, open power supply, current overload of primary side and accidental inversion of power supply. Examples of the above tests are as follows:

(1) Measuring circuit short circuit

This test must be carried out under the conditions of IP=IPN, ambient temperature T≈25°C, sensor bidirectional power supply, and RM for practical applications. The connection diagram is shown in Figure 11, and the switch S should be closed within one minute. turn on.

(2) Measuring circuit open circuit

The test conditions are IP=IPN, ambient temperature T≈25°C, sensor bidirectional power supply, and RM is the resistor in practical applications. The test chart is shown in Figure 12. The switch S should complete the closing/opening switching action within one minute.

(3) Power supply accidental inversion test

In order to prevent the sensor from being damaged by accidental inversion, a protection diode is specially installed in the circuit. This test can test both ends of the diode with a multimeter. The test should be at IP=0, ambient temperature T≈25°C, the sensor is not powered, no Perform the connection with the measuring resistor. It can be tested in two ways:

The first type: the multimeter red pen end is connected to the sensor "M" end, and the multimeter black pen end is connected to the sensor "+" end;

The second type: the multimeter red pen is connected to the sensor negative pole, and the multimeter black pen is connected to the sensor M end;

In the test, such as the multimeter whistling, the diode is damaged.

Eight, sensor application calculation [5]

According to Figure 13, the main calculation formula of the current sensor is as follows:

NPIP=NSIS; Calculate primary or secondary current

VM=RMI; Calculate the measured voltage

VS=RSIS; Calculate the secondary voltage

VA=e+VS+VM; Calculate the supply voltage

Among them, e is the voltage drop inside the diode and the output of the transistor. Different types of sensors have different e values. Here we only take the ES300C as an example. The sensor has a turns ratio of NP/NS=1/2000, a standard rated current value of IPN=300A ​​rms, and a supply voltage VA of ±12V~±20V (±5%). Side resistance RS=30Ω, in the case of bipolar (±VA) power supply, with sensor measurement range >100A and no protection diode to prevent the power supply from being accidentally inverted, e=1V. Under the above conditions:

(1) Given the supply voltage VA, calculate the measured voltage VM and the measuring resistor RM:

Assumption: supply voltage VA = ± 15V

According to the above formula:

Measuring voltage VM=9.5V;

Measuring resistance RM = VM / IS = 63.33 Ω;

Secondary current IS = 0.15A.

Therefore, when we select the 63.33Ω measuring resistor, the output current signal is 0.15A and the measured voltage is 9.5V when the sensor is fully measured.

(2) Calculate the peak current to be measured given the supply voltage and the measured resistance;

Assumption: supply voltage VA = ± 15V, measuring resistance RM = 12Ω,

Then: VM+VS=(RM+RS)×IS=VA-e=14V

And: RM+RS=12W+30W=42W,

Then the maximum output secondary current: A

Primary peak current: IPmax=ISmax(NS/NP)=666A

This shows that under the above conditions, the maximum current that the sensor can measure is the primary peak current of 666A. If the primary current is greater than this value, the sensor will not be measured, but the sensor will not be damaged.

(3) The measurement resistance (load resistance) can affect the measurement range of the sensor.

Measuring resistance also has an effect on the sensor's measurement range, so we need to carefully select the measured resistance. The measured resistance can be calculated by the following formula:

Where VAmin - the minimum supply voltage after the error is subtracted;

E—the voltage drop of the internal transistor of the sensor;

RS—the resistance of the secondary winding of the sensor;

ISmax—The value of the secondary current when the primary current IP is the maximum value.

In addition, we can confirm the stability of the selected sensor by the following formula.

If VAmin does not meet the above formula, it will cause instability of the sensor. Once this happens, we can overcome the following three methods:

1) Replace the power supply with a larger voltage;

2) reduce the value of the measured resistance;

3) Replace the sensor with a sensor with a small RS.

For example, a certain type of current sensor has a standard rated current of IPN=1000A, a turns ratio NP/NS=1/2000, an e value of 1.5V, a secondary side resistance RS=30Ω, a measuring resistance of RM=15W, and a 15V power supply. Unipolar power supply. Then VA = 30V (unipolar power supply is 2 times that of bipolar power supply), and:

IS=IP×NP/NS =0.5A

VS=RS×IS=15V

VM=RM×IS=7.5V

by

The above test shows that the measurement of this sensor under these conditions can ensure stability. The maximum value of the primary current (ie, the measurement range) that it can measure is a general term for a device or device that can be measured and converted into a usable output signal according to a certain rule, usually composed of a sensitive component and a conversion component. When the output of the sensor is a specified standard signal, it is called a transmitter.

The concept of a transmitter is an instrument that converts a non-standard electrical signal into a standard electrical signal. A sensor is a device that converts a physical signal into an electrical signal. In the past, physical signals were often used, and other signals would also appear. The primary instrument refers to the on-site measuring instrument or the base control table. The secondary instrument refers to the instrument that uses the primary signal to complete other functions: functions such as control and display.

Sensors and transmitters are the concept of thermal instrumentation. The sensor converts non-electrical physical quantities such as temperature, pressure, liquid level, material, gas characteristics, etc. into electrical signals or sends physical quantities such as pressure, liquid level, etc. directly to the transmitter. The transmitter amplifies the weak electrical signal collected by the sensor to transfer or activate the control element. Or a signal source that converts the non-electricity of the sensor input into an electrical signal while amplifying it for remote measurement and control. The analog quantity can also be converted to a digital quantity as needed. The sensor and transmitter together form an automatically controlled monitoring signal source. Different physical quantities require different sensors and corresponding transmitters. There is also a transmitter that does not convert physical quantities into electrical signals, such as a "differential pressure transmitter" for a boiler water level gauge. It sends the lower water in the level sensor and the condensate from the upper steam through the meter tube. On both sides of the bellows of the transmitter, a differential meter on both sides of the bellows drives a remote meter that the mechanical amplifying device uses a pointer to indicate the water level. Of course, it is also possible to convert the electrical analog quantity into a digital quantity. The above just conceptually illustrates the difference between a sensor and a transmitter.

* Executive standard: IEC688:1992,

B1 single phase AC current transmitter

* Accuracy level: ≤1.0%.FS

* Linearity: better than 0.2%

* Response time: ≤10Us

* Frequency characteristics: 0~10KHz

* Offset voltage: ≤20mV

* Temperature characteristics: ≤150PPM/°C (0~50°C)

* Machine power consumption: ≤30 mA

B2 DC leakage current sensor

* Isolation withstand voltage: AC2.0KV/min*1mA between input/output/outer casing

* Overload capability: 2 times continuous current, 30 times 1 second

* Flame retardant properties: UL94-V0

* Working environment: -10 ° C ~ 50 ° C, 20% ~ 90% without condensation

* Pay attention to the auxiliary power information on the product label. The auxiliary power level and polarity of the transmitter cannot be connected incorrectly, otherwise the transmitter will be damaged.

* The positive output can only be obtained when the current direction is in the same direction as the arrow marked on the product casing;

* The temperature of the original side busbar should not exceed 60 °C, and the best measurement accuracy is obtained when the current busbar fills the primary edge threading hole;

* There is no lightning protection circuit inside the series of transmitters. When the transmitter input and output feeders are exposed to outdoor harsh weather conditions, lightning protection measures should be taken;

* The transmitter is an integrated structure, non-removable, and should avoid collision and drop;

* Do not damage or modify the product's label, logo, do not disassemble or modify the transmitter, otherwise the company will no longer provide "three guarantees" (including replacement, return, repair) service.

Hall voltage and current sensors are mainly used for industrial control and independent voltage and current measurement. Therefore, the angular difference index which is closely related to the power measurement accuracy is generally not specified, so it is not suitable for high-precision power measurement.

With the development of frequency conversion technology and energy-saving technology, it is necessary to accurately evaluate the energy efficiency of various types of variable frequency speed control devices, and electromagnetic voltage and current transformers can only accurately measure the power of the power frequency sinusoidal circuit. The new type of variable frequency power sensor is a combined voltage and current sensor. This type of sensor directly outputs digital quantity and transmits it by optical fiber, which can effectively avoid loss and interference in the transmission link. And with a small ratio of difference and angular difference in a wide frequency range, it can accurately measure various types of inverter power (voltage, current, power and harmonics, etc.). Widely used in product testing and energy efficiency evaluation of hybrid electric vehicles, electric vehicles, solar power, wind power, inverters, inverter motors and fuel cells .

In the UK, a current sensor suitable for installation on the main line of a 240-V-600-amp substation has been developed. This sensor monitors the substation's power output and reduces the power outage caused by local grid faults. The current sensor can monitor the current of the power supply cable. If the cable outlet is overloaded, these current sensors can transfer part of the load to other phases, or the newly laid cable can protect the cable for safe use and operation.

With the continuous development and upgrading of smart grids, current sensors are constantly improving and perfecting in terms of technology, design and utility, and have a significant effect on current measurement in metallurgical and chemical industries.

Smart grid based fiber optic current sensor

The new fiber optic current sensor is the technological product of the rapid development of the smart grid. China has introduced the XDGDL-1 fiber-optic current sensing system, which realizes the full digital closed-loop control of the pipeline current sensing system. It has the characteristics of stability, linearity and sensitivity, and meets the high-precision measurement requirements of a large range of ranges.

At the same time, the system has developed a telescopic structure that can be wound on site. It is easy to install and avoids the interference of stray magnetic field. The measurement error of busbar eccentricity is less than plus or minus 0.1%, realizing a high-precision signal conversion scheme and is a rectifier. The control unit provides high precision analog signals and a standard digital communication interface.

Current sensor based on TMR (tunnel magnetoresistance) effect:

TMR magnetic induction technology was first applied to the computer hard disk field in 2004, which made the storage density of the hard disk a qualitative leap. The single-disc TB-class storage hard disk entered the civilian market. After nearly 10 years of development, TMR technology is still alive and well. The TMR magnetic induction effect is similar to the Hall technology and is considered to be the fourth generation of magnetic induction technology. Sensitivity, resolution, power consumption, and temperature characteristics all increase by more than 10 times. Full chip-level process control provides reliable quality and reasonable price. Now some domestic manufacturers have begun to introduce current sensors for TMR technology. Current sensing based on TMR chips can perform well in high sensitivity, temperature stability, noise immunity, miniaturization, integration, intelligence and low power consumption.

[3]

Industrial upgrade development promotes current sensor improvement

Driven by the industrial development and upgrading of China, the safety of power equipment is receiving more and more attention. As a tool with both protection and monitoring, the current sensor will play a more important role in the future power grid. Compared with foreign similar products, there is still a big gap in domestic current sensor technology that needs to be compensated and improved.

There are many new industries emerging in China, all of which require sensor support. Whether for safety reasons or market efficiency considerations, current sensors will tend to be more efficient and reliable. Under the requirements of low carbon and environmental protection, miniaturization is also the future. A major trend, this will also promote domestic sensor manufacturers to invest more experience in developing new technologies and products. In the near future, current sensors will be widely used in more industries, and will lay the foundation for the emerging Internet of Things.

Current sensors are used in wind power generation: As a clean and renewable energy source, wind energy is receiving more and more attention from all over the world. Its huge amount of energy, the global wind energy is about 2.74 × 109GW, of which the available wind energy is 2 × 107GW, 10 times larger than the total amount of water energy that can be developed and utilized on the earth. The wind has been used very early—mostly through windmills to pump water, grind surfaces, etc., and in the new century, people are interested in how to use wind to generate electricity and how to maximize power generation. Current sensors are the primary detection component and play a vital role.

The future development trend of current sensors has the following characteristics:

1, high sensitivity. The intensity of the detected signal is getting weaker and weaker, which requires the sensitivity of the magnetic sensor to be greatly improved. Applications include current sensors, angle sensors, gear sensors, and space environment measurements.

2. Temperature stability. More application areas require more and more harsh working environments of sensors, which requires magnetic sensors to have good temperature stability, and industrial applications include the automotive electronics industry.

3. Anti-interference. In many fields, there is no comparison of the sensor environment, and the sensor itself is required to have good anti-interference. Including automotive electronics, water meters and more.

4. Miniaturization, integration, and intelligence. To achieve the above requirements, this requires chip-level integration, module-level integration, and product-level integration. 5. High frequency characteristics. With the promotion of the application field, the working frequency of the sensor is required to be higher and higher, and the application fields include the water meter, the automobile electronics industry, and the information recording industry.

6, low power consumption. In many areas, the sensor itself requires very low power consumption to extend the life of the sensor. Applied to magnetic biochips, compasses, etc. implanted in the body.