This article provides a detailed analysis of the frequency and voltage regulation mechanism and output waveform characteristics of a variable frequency controller (VVVF), and proposes several issues that should be noted in the design of variable frequency cables, providing a certain basis for the design of variable frequency cables.

In the past two decades, the application of variable frequency speed control systems has become increasingly widespread. A research report on the Chinese variable frequency drive market shows that the market has maintained a high-speed growth rate of 15% -20% in the past few years. Due to the rapid development of the industrial and construction industries, as well as a large amount of investment in various industries, the year-on-year growth reached nearly 40%, and the market size exceeded 8.5 billion yuan With the widespread application of frequency converters, the demand for frequency conversion cables used in conjunction with them is increasing day by day, and the annual market demand is also increasing at a rate of 30% per year. As a specialized cable for power and signal transmission between frequency converters and loads, the design and use of frequency conversion cables must meet the special requirements under frequency conversion conditions.

Working principle of frequency converter

The working principle of a frequency converter is to convert the mains power (380V, 50Hz) into smooth DC through a rectifier, and then use a three-phase inverter composed of semiconductor devices (GTO, GTR, or IGBT) to convert the DC power into variable voltage and variable frequency AC power. Due to the use of a sine pulse width modulation method programmed by a microprocessor, the output waveform approximates a sine wave, which is used to drive asynchronous motors and achieve stepless speed regulation. The above two transformations can be simplified as AC-DC-AC (AC-DC-AC) frequency conversion method.

The current variable frequency power supply is regulated by power semiconductor devices, which greatly changes the waveform characteristics and brings new problems to motors and cables. In frequency converters, high-power self turn off switching devices (BJTs, IGBTs, etc.) are usually used for rectification, and then the DC voltage is PWM inverted. As a result, high-order harmonics of the voltage are generated in the input and output circuits, which interfere with the power supply system, loads, and other adjacent electrical equipment, especially the I/O signals of the control system. At the same time, due to the presence of high-order harmonics, frequency conversion cables should have higher insulation safety margins. In practical use, the interference problem of high-order harmonics in frequency converters is often encountered. Below is a brief introduction to the mechanism and propagation path of harmonic generation. The main circuit of a frequency converter is generally composed of AC-DC-AC. An external input of 380V/50Hz power supply is non controllably rectified into a DC voltage through a three-phase bridge circuit. It is then filtered by a filtering capacitor and inverted into a variable frequency AC voltage using high-power thyristor switching elements. In the rectification circuit, due to the presence of irregular rectangular waves, the waveform is decomposed into fundamental waves and various harmonics according to Fourier series, and the higher harmonics will interfere with the input power supply system. In the inverter circuit, the output current waveform is a pulse waveform modulated by PWM carrier signal. For GTR high-power inverter components, the PWM carrier frequency is 2-3kHz, while for IGBT high-power inverter components, the highest PWM carrier frequency can reach 15kHz. Similarly, the output circuit current can also be decomposed into the fundamental wave containing only sine waves and other harmonics. Higher harmonic currents radiate into space through cables, interfering with adjacent electrical equipment. Therefore, based on the working characteristics of the frequency converter, the frequency conversion cable should focus on solving the following problems: the cable body emits electromagnetic waves to the outside, suppressing the interference of high-order harmonics through the cable to the outside world; The impact of pulse voltage on insulation, to prevent the influence of pulse voltage on cables. It is particularly important to address the anti-interference capability and insulation safety and reliability of variable frequency cables from the perspective of cable structure design.

The working characteristics of variable frequency cables

Understanding the working characteristics of frequency converters, the design of frequency conversion cables should focus on controlling the following aspects:

The cable body emits electromagnetic waves externally. Generally, household appliances with variable frequency are powered by single-phase power, with a short length and low power. When designing, the variable frequency power supply, connecting cables, and variable frequency motor are all placed in a metal shell to suppress the emission of electromagnetic waves to the outside world. However, in the industrial field, the motor power is relatively high, and the length of the cable connecting the variable frequency motor and the variable frequency power supply is long. During operation, the cable is an effective carrier for high-frequency electromagnetic waves to be emitted outward. It will cause interference to communication tools or amplitude modulation receivers in the surrounding areas, and sometimes the situation is quite serious, which is called electromagnetic wave environmental pollution. Foreign countries have put forward requirements for this type of cable, and we have also proposed relevant EMC testing and control methods. Although there is currently no national standard for assessing the environmental pollution caused by the emission of electromagnetic waves from cables, it is necessary to suppress external high-frequency interference. To achieve effective suppression of high-frequency interference, the shielding structure of variable frequency cables is particularly important. Shielding structure is the best method to suppress external high-frequency interference, and shielding structures are divided into copper wire braided shielding and copper tape shielding. When using copper wire braided shielding for cables, the shielding suppression coefficient increases continuously with the increase of copper wire braiding density. The higher the braiding density, the better the shielding effect. When using copper tape braided shielding for cables, the shielding effect is equivalent to that of copper tape shielding only when the braiding density reaches 90% or more. Therefore, frequency conversion cables should be shielded with copper tape as much as possible to ensure shielding effectiveness. Manufacturers are accustomed to using copper wire braided shielding, but in reality, this is not the best method due to high material consumption, slow processing speed, and suboptimal shielding effect. The use of copper strips for covering, wrapping, and rolling is a relatively advanced structure and process, forming a fully enclosed metal layer that can achieve effective shielding function.

The impact of pulse voltage on insulation. The frequency adjustment range of variable frequency power supply is wide, regardless of the frequency, it has a waveform profile of the main frequency, which contains many high-order harmonics. As a traveling wave, it can be reflected multiple times and the amplitude superposition can reach several times the working voltage. The longer the cable, the higher the amplitude. If the cable insulation safety factor is not high, it may be broken down. Therefore, to ensure cable safety, we will start from the following three aspects:

Increase insulation thickness, improve insulation voltage resistance, and select materials with better insulation performance. The insulation thickness of the cable can be specified according to the corresponding voltage level. If it is appropriately thickened, it will certainly be more reliable, which is more advantageous for variable frequency cables. In general, using polyvinyl chloride insulation for land use is not ideal because its dielectric constant is relatively high, and under the action of alternating electric fields, its dielectric loss is also significant. The use of cross-linked polyethylene insulation is more suitable, as cross-linked polyethylene material has a low dielectric constant, low dielectric loss, and better temperature resistance and mechanical properties than polyvinyl chloride. It also has excellent organic, electrical, and thermal properties. Using cross-linked polyethylene as insulation material is a more suitable choice.

Add a semiconducting layer outside the conductor to homogenize the electric field and reduce tip discharge. During the processing of conductors, defects (such as burrs) may occur on the surface. If there is no non-conductive layer outside the conductor, electric field distortion may occur at the defect site, which can easily lead to breakdown and insulation damage. After applying a semiconducting layer, the presence of the semiconducting layer homogenizes the surface electric field of the conductor, effectively avoiding insulation breakdown.

The cable adopts a symmetrical structure to achieve uniform electric field and phase balance. For four core low-voltage cables, the first step is to improve the arrangement of insulated wire cores. If the four cores of the cable are directly cabled, it is an asymmetric structure. If the fourth core is decomposed into three insulated cores with smaller cross-sections, the three large and three small wire cores can be symmetrically cabled.

Grounding measures for shielding layer. Good grounding of the shielding layer is a necessary condition for suppressing the external emission of electromagnetic waves. The grounding method of copper wire braided shielding is relatively easy to solve, while the longitudinal copper strip corrugated shielding requires special fixtures for grounding. The contact surface between the fixture and the corrugated copper tube should match, and the grounding wire should be led out from the tail end of the fixture.

Outer sheath. This type of cable is mostly laid indoors and generally does not require armor. Although it is not completely ruled out to use PVC sheath, high-density polyethylene is more suitable.

Additional tests on cables. Generally, low-voltage cables do not require pulse voltage testing. For example, the IEC 60502 standard only specifies pulse voltage testing for cables of 3.6/6kV and above. The connection cable of the variable frequency motor is slightly different and needs to withstand high-frequency pulse voltage. The amplitude of high-frequency waves can reach 1200-1900V, and the ringing frequency is about 100-2000 kHz. Conducting pulse voltage tests (type tests) on cables is to verify their insulation level. The test can refer to the IEC 60502 standard, which applies positive and negative pulse voltages ten times each. The test voltage can be considered as 40kV, but further verification is required. The factory can also decide whether it is necessary or not.

The development of 3.6/6-6/10 kV medium voltage variable frequency cables requires the expansion of motor capacity due to the large-scale mechanical equipment, and the corresponding output current of the variable frequency power supply also needs to be increased. However, due to the limitations of high current variable frequency components, the further development of current capacity technology is restricted. On the other hand, it is relatively easy to increase the output voltage of the variable frequency power supply. After increasing the voltage, the power of the medium voltage variable frequency motor can be significantly increased, and the voltage level of the cable must also keep up. At present, 3.6/6-6/10kV medium voltage variable frequency cables have been put into use. In terms of insulation structure, electrical, mechanical, and physical properties, they can be equivalent to power cables. Cross linked polyethylene is obviously the preferred insulation material. If flexibility is required during installation, using ethylene propylene rubber insulation also has certain advantages. Due to the increase in working voltage, the emission capability of high-frequency electromagnetic waves is significantly enhanced, so the shielding structure requires more complete requirements. Under the working conditions of variable frequency cables, coaxial cables are a suitable structure, so the three main cores of the variable frequency cable adopt a coaxial structure, and the overall shielding structure is the same as that of low-voltage variable frequency cables.