There are many methods of the current measurement. Each method is suitable for different occasions, and each method has its own characteristics. This article introduces several common current measurement methods and compares their respective characteristics.

An electromagnetic current transformer is the most used measurement equipment in the power system. It has mature technology, low cost, and very high precision. It is the most used measurement equipment at present. But electromagnetic current transformers have many limitations:

1. Insulation is difficult, especially above 500kV, the volume, quality, and price of the transformer are increased due to insulation;

2. When the dynamic range is small and the current is large, the CT will be saturated, and the saturation will make the secondary protection unable to correctly identify the fault phenomenon;

3. The output signal of the transformer needs to be laid with cables to the secondary equipment, and it must be converted into a digital quantity twice;

4. CT open circuit will generate high voltage, endangering personal and equipment safety;

5. It is easy to produce ferromagnetic resonance;

6. The current transformer has high accuracy at the specified operating frequency, but the frequency range it can adapt to is very narrow, especially it cannot transmit DC. In addition, there is an excitation current when the current transformer is working, so it is an inductive element. , has the same disadvantages as the shunt.

Hall current sensor, both DC and AC current can be measured, the ordinary current transformer can only measure AC current, ordinary current transformer, when using, the secondary side can not be open circuit, the Hall current sensor can be open circuit. The output voltage of the Hall current sensor is proportional to the magnitude of the current flowing through the primary side, and the output polarity also changes when the direction of the primary side current changes, so it can measure alternating current and direct current, and there is no special requirement for the waveform; the applicable frequency range is also wide. Generally used in electronic circuits, such as frequency converters. The AC transformer can only measure AC, and the frequency must be the rated frequency. For example, the 50Hz transformer has a large error in measuring 60Hz, and the output signal cannot directly enter the electronic detection circuit.

Other advantages of Hall sensors are:

1. Good linearity: better than 0.1%;

2. Good dynamic performance: the response time is less than 1μs and the tracking speed di/dt is higher than 50A/μs. The excellent dynamic performance of the Hall sensor module provides a key basis for improving the performance of modern control systems. Compared with this, the response time of the common transformer is 10-12ms, which can not meet the needs of the development of the work control system;

3. Operating frequency bandwidth: The accuracy is 1% in the frequency range of 0-100kHz. 0.5% accuracy over the 0-5kHz frequency range.

4. Large measurement range: The Hall sensor module is a system product, the current measurement can reach 50KA, and the voltage measurement can reach 6400V.

5. Strong overload capacity: when the primary side current is overloaded and the module reaches saturation, it can be automatically protected, even if the overload current is 20 times the rated value, the module will not be damaged;

6. Wide measurement range: it can measure the current and voltage of arbitrary waveforms, such as DC, AC, pulse, triangular waveform, etc., and even the transient peak current and voltage signals can be faithfully reflected;

7. Fast response speed: the fastest response time is only 1us;

8. Good dynamic performance: the response time is fast, which can be less than 1us; the response time of ordinary transformers is 10~20ms.

A shunt is a resistor that is connected in parallel with the current circuit of a measuring instrument to expand its measuring range. The shunt is made according to the principle that a voltage is generated across the resistor when the DC current passes through the resistor. The shunt is actually a resistor with a small resistance value. The measurement is simple, and the DC measurement accuracy can reach a relatively high level. The biggest problem with shunts is that there is no electrical isolation between the input and output. In addition, when a shunt is used to detect high frequency or large current, it is inevitably inductive, so the connection of the shunt will not only affect the measured current waveform but also cannot truly transmit a non-sinusoidal waveform.