This article provides a detailed analysis of the frequency and voltage regulation mechanism and output waveform characteristics of variable frequency controllers (VVVF), and proposes several issues that should be paid attention to in designing variable frequency cables, providing a certain basis for designing variable frequency cables.
In the past two decades, the application of variable frequency speed control systems has become increasingly widespread. According to research reports on the Chinese frequency converter market, the market has maintained a rapid 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 of the same year reached nearly 40%, and the market size exceeded 8.5 billion yuan. With the widespread application of frequency conversion devices, The demand for frequency conversion cables used in conjunction with them is also increasing day by day, and the annual market demand is also increasing at an annual rate of 30%. 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 a smooth DC through a rectifier, and then use a three-phase inverter composed of semiconductor devices (GTO, GTR, or IGBT) to convert the DC into variable voltage and variable frequency AC. Due to the use of a microprocessor programmed sine pulse width modulation method, the output waveform is approximately 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 through power semiconductor devices, which greatly changes the waveform characteristics and brings new problems to motors and cables. In frequency converters, high-power self closing switching devices (BJT, IGBT, etc.) are usually used for rectification, followed by PWM inversion of the DC voltage. As a result, high order harmonics of the voltage are generated in the input and output circuits, interfering with the power supply system, load, and other adjacent electrical equipment, especially the I/O signals of the control system. Meanwhile, due to the presence of higher-order harmonics, frequency conversion cables should have higher insulation safety margins. In the actual use process, we often encounter the interference problem of high-frequency harmonics in frequency converters. Below is a brief introduction to the mechanism of harmonic generation, propagation paths, and other issues. The main circuit of the frequency converter is generally composed of AC-DC-AC, and the external input 380V/50Hz power frequency power supply is uncontrollably rectified into a DC voltage through a three-phase bridge circuit. It is filtered by a filtering capacitor and inverted into a variable frequency AC voltage through high-power thyristor switching elements. In the rectifier circuit, due to the presence of irregular rectangular waves, the waveform is decomposed into fundamental waves and various harmonics in the Fourier series, and the higher-order harmonics will interfere with the input power supply system. In the inverter circuit, the output current waveform is a pulse waveform modulated by a PWM carrier signal. For GTR high-power inverter components, the PWM carrier frequency is 2-3kHz, while the PWM maximum carrier frequency of IGBT high-power inverter components can reach 15kHz. Similarly, the output circuit current can also be decomposed into fundamental waves containing only sine waves and other harmonics. Higher harmonic currents radiate into space through cables, interfering with adjacent electrical equipment. Therefore, in response to the working characteristics of frequency converters, frequency conversion cables should focus on solving the following problems: the cable body emits electromagnetic waves to the outside world, suppressing the interference of higher-order harmonics through the cable to the outside world; The impact of pulse voltage on insulation to prevent the impact of pulse voltage on cables. Frequency conversion cables are particularly important in terms of cable structure design to address anti-interference capabilities and insulation safety and reliability.
Working characteristics of variable frequency cables
Understand the working characteristics of frequency converters, and the design of frequency conversion cables should focus on controlling the following aspects:
The cable body emits electromagnetic waves externally. Generally, variable frequency household appliances are powered by single-phase power, with a short length and low power. During the design, the variable frequency power supply, connecting cables, and variable frequency motor have been installed in a metal shell to suppress electromagnetic wave emissions. 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, which will cause interference to communication tools or amplitude modulation receivers in nearby areas. Sometimes, the situation is also serious, known as electromagnetic wave environmental pollution. Foreign countries have already put forward requirements for this type of cable, We have also proposed relevant EMC testing and control methods. Although there is currently no national standard for assessing environmental pollution caused by electromagnetic waves emitted by 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. The shielding structure is the best method to suppress external high-frequency interference, and the shielding structure is divided into copper wire braided shielding and copper tape shielding. When cables are shielded with copper wire braiding, the shielding suppression coefficient increases with the increase of copper wire braiding density. The higher the braiding density, the better the shielding effect. When using copper tape braiding 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 the shielding effect. Manufacturers are accustomed to using copper wire weaving for shielding, but in reality, this is not the best method. Material consumption is high, processing speed is slow, and the shielding effect is not ideal. The use of copper tape covering, wrapping, and embossing is a relatively advanced structure and process, forming a fully enclosed metal layer that can achieve effective shielding function.
The effect of pulse voltage on insulation. The frequency adjustment range of variable frequency power supply is relatively wide, regardless of the frequency. It has a main frequency waveform contour, which contains many high-order harmonics. As a traveling wave, after multiple reflections, 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 the insulation thickness, improve the 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 is of course more reliable, which is more beneficial for variable frequency cables. In general, using PVC insulation for land use is not ideal because its dielectric coefficient is too high and its dielectric loss is also significant under the action of alternating electric fields. However, using cross-linked polyethylene insulation is more suitable. The cross-linked polyethylene material has a low dielectric coefficient, 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 an insulation material is a more suitable choice.
Add a semi conductive layer outside the conductor to homogenize the electric field and reduce tip discharge. During the processing of conductors, defects (such as burrs) may appear on the surface. If there is no semi conductive layer outside the conductor, electric field distortion will occur at the defect location, which is prone to breakdown and insulation damage. If a semi conductive layer is applied, the surface electric field of the conductor is homogenized due to the presence of the semi conductive layer, which can effectively avoid 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 cores. If the four cores of the cable are directly connected to the cable, 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 cores are symmetrically connected to the cable.
Grounding measures for shielding layer. A 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 wrapped copper strip corrugated shielding needs to be grounded with a special fixture. 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 entirely ruled out to use polyvinyl chloride sheath, high-density polyethylene is more suitable.
Additional tests on cables. Generally, low voltage cables do not require pulse voltage testing, as 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 high-frequency wave amplitude can reach 1200~1900V, and the ringing frequency is about 100~2000 kHz. Pulse voltage test (type test) is conducted on the cable to verify its insulation level. The test can refer to the IEC 60502 standard, which applies ten pulse voltage tests, positive and negative. The test voltage can consider 40kV, but further verification is required. The factory can also decide whether it is necessary.
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 variable frequency power sources is also required to increase. However, due to the limitations of high current variable frequency components, further improvement of current capacity technology development is limited. On the other hand, increasing the output voltage of the variable frequency power supply is relatively easy. 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 clearly the preferred insulation material. If softness is required during laying, using ethylene propylene rubber insulation also has certain advantages. Due to the increase in working voltage, the emission ability of high-frequency electromagnetic waves is significantly enhanced, so the shielding structure is required to be more complete. Under the working conditions of variable frequency cables, coaxial cables are a suitable structure, so the three main cores of variable frequency cables adopt a coaxial structure, and the overall shielding structure is the same as that of low-voltage variable frequency cables.