Common current sensor classification
Published time: 2020-07-25 10:54:06
A current sensor is a detection device that can feel the information of the measured current, and can transform the detected and felt information into electrical signals that meet certain standards or other required forms of information output according to certain rules to meet Information transmission, processing, storage, display, recording and control requirements.
With the development of technology, current sensors are constantly innovating and developing. Let's compare the current common current sensors.
Current sensors, also known as magnetic sensors, can be used in household appliances, smart grids, electric vehicles, wind power generation, etc. Many magnetic sensors are used in our lives, such as computer hard drives, compasses, household appliances and so on. It is a detection device that can feel the information of the measured current, and can transform the sensed information into an electrical signal that meets certain standards or other required forms of information output according to certain rules to meet the transmission of information , Processing, storage, display, recording and control requirements. Mainly can be divided into: shunt, electromagnetic current transformer, electronic current transformer, etc.
1. The resistance shunt is used to measure direct current.
It is made according to the principle that a voltage is generated across the resistor when the direct current passes through the resistor. The advantage of resistive shunt is high precision, corresponding speed and low cost; but the disadvantage is that the measuring circuit is not electrically isolated from the measured current. Resistive shunts are suitable for low frequency and small amplitude current measurement.
2. The Hall current sensor
It is made according to the principle of Hall effect, applying Ampere's law, that is, a magnetic field proportional to the current is generated around the current-carrying conductor, and the Hall device is used to measure this magnetic field. Therefore, non-contact measurement of current is possible. The Hall current sensor can measure both DC and AC, with a frequency of up to 100KHz, high accuracy and good isolation; its disadvantage is that the impact speed is slow, and the small current test accuracy is low. It can be used in AC and DC tests with DC-100KHz.
3. The fluxgate current sensor
It uses the non-linear relationship between the magnetic induction intensity and the magnetic field intensity of the high permeability magnetic core in the measured magnetic field under the saturation excitation of the alternating magnetic field to measure the weak magnetic field. This physical phenomenon seems to be a "gate" to the measured environmental magnetic field. Through this "gate", the corresponding magnetic flux is modulated and induced electromotive force is generated. Use this phenomenon to measure the magnetic field generated by the current, so as to achieve the purpose of measuring current indirectly.
Electronic current transformers include Hall current sensors, Rogowski current sensors, and AnyWay variable frequency power sensors (which can be used for voltage, current and power measurement) dedicated to variable frequency power measurement. 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 are 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 optical fiber as the medium.
When linearly polarized light propagates in the medium, if a strong magnetic field is applied parallel to the propagation direction of the light, the direction of light vibration will be deflected. The deflection angle ψ is proportional to the product of the magnetic induction intensity B and the length l of the light passing through the medium. That is, ψ=V*B*l, and the proportional coefficient V is called the Feld constant, which is related to the properties of the medium and the frequency of light waves. 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.
The shunt is used to measure the direct current, and is made according to the principle that a voltage is generated across the resistance when the direct current passes through the resistance. The shunt is actually a resistor with a small resistance value. When a direct current passes through, a voltage drop is generated for the direct current meter to display. The so-called shunt refers to dividing a small current to drive the meter indication. The smaller the ratio of the small current (mA) to the current in the large loop (1A-tens of A), the better the linearity and accuracy of the ammeter reading.
Figure 1. Detection of high-current shunt is composed of one or more conductors
Current Transformer (CT)
A current transformer is an instrument that converts a large current on the primary side into a small current on the secondary side based on the principle of electromagnetic induction (only for AC testing). The current transformer is composed of a closed core and windings. Its primary winding has a few turns and is stringed in the line of the current to be measured. Under normal working conditions, the voltage drop on the primary and secondary windings is very small, which is equivalent to a transformer in a short-circuit state, so the magnetic flux in the iron core is also very small. At this time, the magnetic potential of the primary and secondary windings F (F= IN) are equal in size and opposite in direction. That is, the current ratio between the primary and secondary of the current transformer is inversely proportional to the number of turns of the primary and secondary windings, that is, I1/I2=N2/N1.
Figure 2. Schematic diagram and physical diagram of current transformer
When the current transformer is running, no open circuit on the secondary side is allowed. Because once the circuit is opened, the primary side current becomes the excitation current, so that the magnetic flux and secondary side voltage greatly exceed the normal value and endanger the safety of people and equipment. Therefore, it is not allowed to connect a fuse in the secondary circuit of the current transformer, and it is also not allowed to remove the ammeter, relay and other equipment without bypass during operation.
The specific reasons are as follows: the primary measured current and magnetic potential of the current transformer F1=I1N1 generates a magnetic flux Φ1 in the iron core, the secondary measuring instrument current magnetic potential F2=I2N2 generates a magnetic flux Φ2 in the iron core, and the current transformer core is magnetically combined It is generally Φ = Φ1 + Φ2. Since Φ1.Φ2 have opposite directions, are equal in size, and cancel each other out, Φ=0. If the secondary open circuit, that is, I2 = 0, then: Φ = Φ1, the current transformer iron core magnetic flux is very strong, saturated, iron core heating, burned out the insulation, leakage, and very high in the current transformer secondary coil N2 The high induced potential E forms a high voltage at both ends of the secondary coil of the current transformer, endangering the life and safety of operators. Therefore, one end of the secondary coil of the current transformer is grounded, which is a protective measure to prevent the danger of high voltage.
The principle of a voltage transformer is similar to that of a transformer. The primary winding (high-voltage winding) and the secondary winding (low-voltage winding) are wound on the same iron core, and the magnetic flux in the iron core is. According to the law of electromagnetic induction, the relationship between the voltage U (or electromotive force E) of the winding, the number of turns N of the winding, and the magnetic flux is: U1=-N1dφ/dt, U2=-N2dφ/dt. And then get: U1/U2=N1/N2. In the case of negligible no-load current, I1/I2=-N2/N1, so the power of the primary and secondary windings of an ideal transformer is equal to P1=P2. It shows that the ideal transformer itself has no power loss.
Figure 3. Schematic diagram and physical diagram of voltage transformer
According to different uses, current transformers can be roughly divided into two categories:
1. Measuring current transformer (or measuring winding of current transformer)
Provide current information of the power grid to measuring and metering devices within the normal working current range.
When measuring the large current of the alternating current, it needs to be converted into a relatively uniform current for the convenience of the secondary meter measurement (China stipulates that the secondary rating of the current transformer is 5A or 1A). Usually there are only 1 to a few turns on the primary side, and the wire has a large cross-sectional area, which is connected to the circuit under test in series. The secondary side has a large number of turns and a thin wire, which forms a closed circuit with a meter with a small impedance (current coil of the ammeter/power meter). During normal operation, the secondary side of the transformer is in an approximate short-circuit state and the output voltage is very low. During operation, if the secondary winding is open or the primary winding flows through abnormal currents (such as lightning current, resonance overcurrent, capacitor charging current, inductor starting current, etc.), overvoltages of thousands or even tens of thousands of volts will be generated on the secondary side .
2. Current transformer for protection (or protection winding of current transformer)
provides grid fault current information to relay protection and other devices under grid fault conditions.
Protection current transformers are divided into: 1) overload protection current transformer; 2) differential protection current transformer; 3) grounding protection current transformer (zero sequence current transformer). The working conditions of the current transformer for protection are completely different from the current transformer for measurement, and the protection transformer only starts to work effectively when the current is several tens of times larger than the normal current.
The essence of the Hall effect is that when the carriers in the solid material move in an external magnetic field, they are affected by the Lorentz force and the trajectory is shifted, and charges are accumulated on both sides of the material, forming a vertical direction to the current. The electric field finally balances the Lorentz force received by the carriers with the repulsion of the electric field, thereby establishing a stable potential difference, or Hall voltage, on both sides.
What people say is that for a given Hall device, when the bias current I is fixed, UH will completely depend on the measured magnetic field strength B. The Hall voltage changes with the change of the magnetic field strength. The stronger the magnetic field, the higher the voltage, the weaker the magnetic field, and the lower the voltage. The Hall voltage value is small, usually only a few millivolts, but it is amplified by the amplifier in the integrated circuit. The voltage can be amplified enough to output a stronger signal.
From an application point of view, the current transformer and the Hall sensor are the same in that they both need a coil to generate a magnetic field. One of the differences is that the transformer requires a changing magnetic field, while the Hall sensor can be a constant magnetic field. Therefore, the former can only be used for AC testing, while the latter can be used for AC and DC testing. The second difference is that the transformer has an iron core, while the Hall sensor does not have an iron core. The former is non-linear in terms of frequency, and the latter is linear. Therefore, the former applies to a narrow frequency band and is generally used for fixed frequency bands (such as 45~66Hz), the latter has a wider frequency band. The third difference is that more transformers are used for electric energy measurement, and the phase index is an important index for measuring transformers. However, Hall sensors are mostly used for control or simple voltage and current independent testing, and generally do not control the phase index or provide the phase index (such as the phase error index of 50 Hz).Tag: sensor