Protection Filters

//Protection Filters保护滤波器

Protection filters are used, in supply networks having a high level of harmonic distortion, when the final objective is reactive power compensation at the fundamental frequency.

Their purpose is to avoid that harmonic currents overload the capacitors by diverting them to the mains. Protection filters are made by connecting reactors in series with capacitors, in such a way that the tuning frequency of the whole unit is set at a value between the fundamental frequency and the frequency of the lowest present harmonic, which is usually the 5th order harmonic.
它们的目的是通过将电容器转移到电源来避免谐波电流使电容器过载。 保护滤波器是通过将电抗器与电容器串联连接而成的,使得整个单元的调谐频率设定在基频和最低谐波频率之间的值,该谐波通常是5次谐波。

Connection of a reactor in series with a power capacitor, makes the capacitor to work at a voltage higher than the supply voltage. Because of this, capacitors to be connected to protection reactors, need to be designed to work at higher voltages than standard capacitors. The choice of the tuning point of the filter is a balance between the quantity of harmonics rejected by the filter and the voltage increase produced in the capacitor at the fundamental frequency.
将电抗器与功率电容器串联连接,使电容器工作在高于电源电压的电压。因此,要连接到保护电抗器的电容器需要设计为在比标准电容器更高的电压下工作。 滤波器调谐点的选择是滤波器抑制的谐波量与基频电容器产生的电压增量之间的平衡。

It has to be also kept in mind that reactive power supplied by the filter at rated frequency (50 or 60 Hz), is different to the one that the capacitor would supply without the reactor.
还必须记住,滤波器在额定频率(50或60 Hz)下提供的无功功率与电容器在没有电抗器时提供的无功功率不同。

Taking all the above into account, the reactor is normally chosen in such a way that its impedance is about 7% the impedance of the capacitor that protects. This will give a tuning frequency, for example at 50 Hz, of 189 Hz. Other tuning frequencies are also available.
考虑到上述所有因素,通常选择电抗器,使其阻抗约为保护电容器阻抗的7%。 这将给出调谐频率,例如50Hz,189Hz。 其他调谐频率也可用。

Example 例如

Protection filterThe need for using protection filters for capacitors can be seen in the here below case (Fig. 4). Let us suppose that the converter current of the figure is 550 A, with the following harmonic distribution:
在下面的例子中可以看到对电容器使用保护滤波器的需求(图4)。 让我们假设图的转换器电流为550 A,具有以下谐波分布:

The 150 kvar 400 V capacitor bank QC has a rated current of 217 A. The harmonic currents that will circulate through the capacitor can be calculated from the formula (3):
150 kvar 400 V电容器组QC的额定电流为217 A.通过电容器循环的谐波电流可通过公式(3)计算得出:

Then, the rms current the capacitor bank will absorb will be:

IC which is far above the 217 A marked on the rating plate of the capacitor bank. This overcurrent exceeds the safety limit set by IEC standards and makes impossible the connection of the bank without protection filters.
远高于217A的IC标记在电容器组的铭牌上。 这种过电流超过了IEC标准规定的安全限制,并且无法在没有保护滤波器的情况下连接电容器柜。