Shielding measures for variable frequency cables

The frequency conversion cable is mainly used to connect the power supply with the inverter, inverter, and electrical equipment. Its laying space is relatively small, and the voltage level is relatively high (up to 8.7/15kv). During its operation, a large amount of electromagnetic waves will be generated, which will generate strong interference with the surrounding power supply and power system. This requires better shielding measures for the inverter cable. Therefore, phase-frequency shielding and turn-key shielding are required for frequency conversion cables with voltage ratings of 3.6/6 kV and above. Using multi-layer shielding can achieve very good results.

The frequency conversion cable, as its name implies, is a dedicated cable for the inverter. It is used to transmit electrical energy and has a higher voltage rating. This requires us to not only consider the influence of the external environment on the inverter cable when designing the structure of the inverter cable. Since most of them are laid in the room, we must also consider the influence of the inverter cable on the external environment. So there are special requirements for the structure of the frequency conversion cable. Although the current domestic major companies have different opinions on the structure of the variable frequency cable, they have formulated their own corporate standards accordingly, but they all prefer the symmetrical 3+3 structure. I believe it will be unified in the near future. Here, the author collected and summarized some of the information on the symmetric 3+3 structure of the frequency conversion cable, hoping to make a contribution to the development of the variable frequency cable.

Inverter cables currently use cross-linked polyethylene as the insulating material, and the frequency variation range that can withstand the actual work is 30~300HZ. The frequency conversion cable has the advantages of resisting high-order harmonics, reducing mutual interference with the external environment, etc., and the main laying site is Indoors, this makes the operation of the inverter cable have a very close relationship with the surrounding power supply or electrical equipment, so there needs to be a special structure to solve this complex interrelationship. As a result, a symmetrical 3+3 structure results.

However, if the circuit in the shield is eccentric, the effect of electromagnetic shielding must be reduced, and the eddy current loss generated in the shield will increase. For the eccentric cable, set the attenuation value of the shield to Ap, then there is Ap=As+1/Sp. As is the attenuation value when the core is located at the shield center. Sp is the eccentricity factor analysis: In the eccentric cable, Sp is much larger than 1 , then 1/Sp becomes a negative value. In this way, we get a conclusion: ApAs: The effect of metal shielding is reduced when the cable is eccentric. Eccentricity is absolute, which means that Ap is always less than As. The problem is that we have to try to minimize the value of ApAs in order to enhance the effectiveness of the metal shielding, thereby reducing the interference of the inverter cable to the outside world.

Other reasons for designing a symmetric 3+3 inverter cable:

a) The symmetry 3+3 structure of the frequency conversion cable core is interchangeable, so that it will have better electromagnetic compatibility, play a certain role in suppressing interference, and can be odd harmonics in inefficient higher harmonics Waves improve the cable's immunity to interference.

b) Using a symmetric 3+3 variable frequency cable can effectively prevent the generation of high frequency axis current. So how can we minimize the eccentricity? Only symmetry. The 3+3 inverter cable is symmetrical. This symmetric structure plus the corresponding metal shielding can reduce the shielding factor of the cable to 0.7 or even less. This effectively shields the leakage of electromagnetic waves and makes the metal shielding work better. This is the second reason why the symmetric 3+3 structure is selected for the frequency conversion cable.

Earlier we discussed the importance of symmetrical structures for variable-frequency cables. This problem is very serious. How to solve it? According to the actual cable engineering data, the voltage rating of the variable frequency cable laid in this way is almost below 1.8/3kv, and this voltage level of the frequency conversion cable does not require phase-shielding.