Advantages of Electronic Current Transformer_ Application of Electronic Current Transformer

Advantages of electronic current transformer1) Complete isolation of high and low voltage, high safety, excellent insulation performance, no iron core, eliminating magnetic saturation and ferromagnetic resonance. The measured signal of the electromagnetic transformer is coupled with the secondary coil through the iron core, the insulation structure is complex, and its cost increases exponentially with the voltage level. The unconventional transformer transmits the high-voltage side signal to the secondary equipment through the optical fiber with good insulation performance, which greatly simplifies its insulation structure. The higher the voltage level, the more obvious the cost performance advantage.2) There are no potential flammable, explosive and other hazards due to oil filling. The insulation structure of unconventional transformer is relatively simple. Generally, oil is not used as the insulation medium, which will not cause fire, explosion and other hazards.

3) Large dynamic range, high measurement accuracy and wide frequency response range. During the normal operation of the power grid, the current flowing through the current transformer is small, but the short-circuit current is generally large, and the short-circuit current increases with the increase of the power grid capacity. Unconventional transformer has a wide dynamic range, which can meet the needs of measurement and relay protection at the same time.4) The anti electromagnetic interference performance is good, and there is no risk of open circuit and high voltage at the low-voltage side. There is only optical fiber connection between high-voltage side and low-voltage side of unconventional transformer. The signal is transmitted through optical fiber. The high-voltage circuit and secondary circuit are completely isolated electrically. The transformer has good anti electromagnetic interference ability, and there is no high-voltage risk caused by open circuit at low-voltage side.5) Strong anti-interference ability of data transmission. Electromagnetic transformer transmits analog signals. Traditionally, measurement, control and relay protection in power station transmit the electrical signals measured by electrical sensors to the control room through coaxial cable.

6) Small size and light weight. Unconventional transformer has no iron core, and its weight is much smaller than that of electromagnetic transformer with the same voltage level.Application of electronic current transformerElectronic current transformers usually have higher bandwidth and are suitable for fundamental and harmonic measurement of currents with large harmonic content. In order to accurately measure power, voltage and current combined electronic transformer can also be used, because the combined electronic transformer can better control the phase difference of voltage and current signal and improve the accuracy of power measurement.

1 Output current waveform of two-level frequency converterThe voltage waveform and current waveform are close to sine wave, the voltage waveform is PWM wave, and the higher burr in the figure is overshoot voltage. Shortening the cable and adding DV / dt filter can reduce or eliminate overshoot voltage.PWM wave has less low-order harmonics, and the harmonics are mainly concentrated near the integer multiple of carrier frequency.

The fundamental frequency of the frequency converter is recorded as FS; The carrier frequency of the frequency converter is recorded as FC;Then, n = FC / Fs is the carrier frequency ratio.For SPWM modulated three-phase frequency converter, when n is an integer, it does not contain harmonics below n-2.

Assuming FS = 50z and FC = 2KHz, n = 40, and the output of the frequency converter does not contain harmonics below 38 times.And the harmonic frequency is KFC MFs, where KS = 1, 2, 3, 4, 5, 6, 7 k1=12457. Generally speaking, the load of the frequency converter is the motor, and the motor is the inductive load, which can better suppress high-order harmonics. Therefore, theoretically, the higher the carrier frequency ratio of the frequency converter, the less the harmonic content of the output current of the frequency converter.

2 Input current and voltage waveform of six pulse rectifier frequency converterThe input current waveform of six pulse rectifier frequency converter contains large harmonics, which are mainly 6K 1 times, K is an integer, and K 1. Similarly, the harmonic of the input current of 12 pulse rectifier is mainly 12K 1, K is an integer, and K 1.Assuming that the frequency converter is not p-pulse rectified, the input current of the frequency converter does not contain harmonics below P-1. The harmonics are mainly PK 1, K is an integer, and K 1.

3 Transformer no-load input waveformThe above figure shows the no-load input waveform of single-phase transformer. The current waveform in the figure has large distortion. According to its shape characteristics, it is generally called "spike wave". It can be seen from the spectrum on the right that the harmonics of spike wave are mainly odd harmonics. The higher the harmonic order, the smaller the harmonic content.4 Superimposed frequency wave of stator thermal test by superimposed frequency method

The waveform above contains two frequency components, 50Hz and 40Hz respectively. 50Hz is called the main power supply and 40Hz is called the auxiliary power supply. In the voltage waveform, the amplitude of the auxiliary power supply is about 25% of that of the main power supply.Due to the low impedance of 40Hz auxiliary power supply, the relative content in the current waveform increases and the waveform fluctuation increases.