Electrical quantity measurement sensors in power equipment

Electricity is an energy source that uses electrical energy as a driving force. The power system refers to the power production, transmission and consumption system composed of power generation, transmission, transformation, distribution, and power consumption. Common generators, transformers, switch cabinets, motors, etc. in the power system are all power devices.

The electrical quantity measurement sensors in the power device mainly include sensors for measuring electrical quantities such as current, voltage, resistance, electric power, phase, and electric energy. The common detection methods of electrical quantity measurement sensors can be roughly divided into two categories: one is a direct-reading electrical quantity indicating instrument made using the principle of electromagnetic action, and the other is to compare the measured electrical quantity with the same electrical quantity as a standard to determine the size of the measured quantity.

Electrical quantity measurement sensors have a series of advantages such as simple structure, flexible installation, low cost, high stability and reliability, easy use and maintenance, and easy to achieve long-distance measurement, so they are widely used. There are many types and specifications of electrical quantity measurement sensors/meters. According to the name (or unit) of the measurement object, there are ammeters (ammeters, milliammeters, microammeters), voltmeters (voltmeters, kilovoltmeters, millivoltmeters, microvoltmeters), power meters (wattmeters, kilowattmeters), phase meters (power factor meters), energy meters (watt-hour meters, watt-hour meters), frequency meters, resistance meters (ohmmeters, megohmmeters) and multimeters with multiple measurement purposes. According to the type of working current of the sensor, there are DC meters, AC meters and AC/DC dual-purpose meters. The

working principles of electrical quantity measurement sensors mainly include magnetoelectric, electromagnetic, electric, ferromagnetic electric, induction, rectification, electrostatic and thermoelectric types. The working principles of various typical sensor types are described as follows. Magnetoelectric

sensor

Magnetoelectric sensors work by using the interaction between the magnetic field of a permanent magnet and the magnetic field generated by a current-carrying coil (see Figure 1), and are used to measure DC voltage, DC current and resistance. Therefore, this type of sensor can be used to make ammeters, voltmeters, resistance meters, galvanometers or clamp meters, etc. The advantages are uniform scale; high sensitivity and accuracy (up to 0.05 level); strong damping; small power consumption (less than 100mW); and little influence by external magnetic fields. The disadvantages are that it can only measure DC; the price is relatively high; and it cannot withstand large overloads. Its measurement range is: current 10-11~102A, voltage 10-3~103V.

Electromagnetic sensor

Electromagnetic sensors work by using the attraction of the magnetic field of the current-carrying coil to attract the moving iron sheet or the deflection torque generated by the repulsive force between the moving iron sheet and the fixed iron sheet in the current-carrying fixed coil. They are used to measure AC voltage and AC current, etc. Based on this principle, AC ammeters and AC voltmeters, phase meters, synchronization meters, clamp meters, etc. The advantages are simple structure; low price; can be used for AC and DC; can measure larger currents; and allow larger overloads. The disadvantage is that the scale is uneven; it is easily affected by the external magnetic field and the hysteresis and eddy current in the iron sheet (when measuring AC), so the accuracy is not high (the highest accuracy is 0.1 level). Its measurement range is: current 10-3~102A, voltage 1~103V. The frequency is industrial frequency, but it can be extended to 5kHz.

Electric sensor

Electric sensors work by using the deflection torque generated by the interaction of the magnetic field between two fixed coils and one movable coil, and thus voltmeters, ammeters, and power meters are made. The advantages are that they can measure the true effective value of AC with high accuracy (up to 0.05 level), and can be used for both AC and DC, and are not affected by the AC frequency (usually the application frequency does not exceed 2.5kHz). The disadvantages are that the manufacturing cost is high, the power loss is greater than that of the magnetoelectric type, the overload capacity is small, and the magnetic field itself is weak, and the measurement is easily disturbed by the external magnetic field. Its measurement range is: current 10-3~102A, voltage 1~103V. The frequency is industrial frequency, but it can be extended to 5kHz.

The operating principle of the ferromagnetic electric sensor is similar to that of the electric type. The difference is that ferromagnetic materials are added to the fixed coil of the electric instrument to form a magnetic circuit to increase the magnetic field. The characteristics are low power consumption and the highest accuracy can reach 0.2 level. The disadvantage is that the overload capacity is small. The types of instruments made are voltmeters, ammeters, power meters, frequency meters, and phase meters. Its measurement range is: current 10-7～102A, voltage 10-1～103V, industrial frequency.

Inductive sensor

Inductive sensors are mainly used to measure electric energy, so they are usually called AC energy meters. The rotational torque in the meter is generated by the interaction between the current coil with AC current and the eddy current induced in the movable aluminum disk. The power consumption is small and medium, the highest accuracy can reach 0.5 level, and the overload capacity is large. Common inductive electrical measuring instruments include single-phase active energy meter, single-phase reactive energy meter, three-phase two-element active energy meter, three-phase two-element reactive energy meter, three-phase four-wire three-element active energy meter and three-phase four-wire three-element reactive energy meter.

Rectification sensor

Rectification sensors mainly use rectifiers to convert the measured AC into DC, and then use magnetoelectric measuring mechanisms for measurement. The characteristics are low power consumption, the highest accuracy is 1.0 level, and the overload capacity is small. The types of instruments include ammeter, voltmeter, ohmmeter, frequency meter, phase meter, multimeter, etc. Its measurement range is: current 10-5~10A, voltage 1~103V, frequency industrial frequency, but can be expanded to 5kHz.

Electrostatic sensor

The electrostatic sensor works by generating deflection torque based on the principle that like charges repel each other and opposite charges attract each other. It can measure true effective value and has a wide range of applications (can be used in the range of 10Hz to several MHz for DC and AC). The loss is very small, the highest accuracy can reach 0.1 level, and the overload capacity is large. It is often used to make voltmeters, power meters, quadrant meters, etc. Its measurement range is: voltage 10~5×105V, frequency can reach 108Hz.

Thermoelectric sensor

Thermoelectric sensor works by using magnetoelectric measuring mechanism to measure the thermoelectric potential generated by the measured current passing through the thermocouple. The instrument types formed by this include voltmeter, ammeter and power meter, etc. It can be applied to AC and DC occasions, with low power consumption and the highest accuracy can reach 0.1 level, but the overload capacity is small. Its measurement range is: current 10-3～10A, voltage 10～103V, frequency < 108Hz.

Digital sensor/instrument

With the development of microprocessor technology, a new generation of digital sensors/instruments has been born on the basis of traditional analog sensors. These digital instruments developed by integrating the latest achievements of electronic technology and computer technology, although of various varieties and models, are mainly composed of analog conditioning, analog-to-digital conversion, digital display and other special chips (such as latch, drive, calculation, etc.). In general, it is an instrument that realizes automatic detection through logic control, converts digital/analog, and directly displays the measurement results in digital form. Because of their advantages such as fast measurement speed, high accuracy (generally up to 0.5 level, or even 0.2 level), digital display, high resolution, good repeatability, strong anti-interference ability (series mode rejection ratio SMR can reach 100dB, common mode rejection ratio CMR can reach 120dB, if digital filtering is used, CMR can even reach 180dB), and suitable for multiple parameter measurement and centralized control, this type of sensor/instrument is widely used.

With the development of production and the advancement of science and technology, the technology of electrical detection sensors has leapt forward, and its measured objects have also expanded to a considerable detection range, such as currents as small as 10-17A and as large as hundreds of thousands of amperes; voltages from 10-9V to tens of millions of volts; resistances from a few hundredths of micro-ohms to 1018Ω; frequencies from 10-6Hz to 1012Hz, etc. As time goes by, the detectable range of the measured objects will be further expanded, and the performance will be further improved.