Over the past two decades, the application of variable frequency drive (VFD) systems has become increasingly widespread. A market research report on Chinese VFDs shows that the Chinese VFD market has maintained a high growth rate of 15%-20% in the past few years. Due to the rapid development of the industrial and construction sectors and substantial investments across various industries, the year-on-year growth rate reached nearly 40%, with the market size exceeding 8.5 billion RMB. With the widespread application of VFD devices, the demand for VFD cables used in conjunction with them is also increasing daily, with annual market demand growing at a rate of 30%. As specialized cables for power and signal transmission between VFDs and loads, VFD cables must be designed and used to meet the special requirements of VFD operating conditions.
2. Working principle of frequency converter
The working principle of a frequency converter is to convert 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 AC power with variable voltage and frequency. Due to the use of a sinusoidal pulse width modulation method programmed by a microprocessor, the output waveform is made approximately sinusoidal, which is used to drive an asynchronous motor to achieve stepless speed regulation. The above two conversions can be simplified to an AC-DC-AC (Alternating Current-Direct Current-Alternating Current) frequency conversion method.
Current frequency converters regulate voltage using power semiconductor devices, significantly altering waveform characteristics and introducing new problems for motors and cables. Frequency converters typically use high-power self-turn-off switching devices (BJTs, IGBTs, etc.) for rectification, followed by PWM inversion of the DC voltage. This results in high-order harmonics in the input and output circuits, interfering with the power supply system, loads, and other nearby electrical equipment, especially the I/O signals of the control system. Furthermore, the presence of these high-order harmonics necessitates higher insulation safety margins in frequency converter cables. In practical applications, interference from high-order harmonics in frequency converters is frequently encountered. The following is a brief introduction to the mechanism of harmonic generation and propagation paths. The main circuit of a frequency converter is generally composed of AC-DC-AC. An external 380V/50Hz power supply is uncontrolled rectified into DC voltage by a three-phase bridge circuit, then filtered by a filter capacitor and inverted into a variable-frequency AC voltage by a high-power thyristor switching element. In the rectifier circuit, due to the presence of irregular rectangular waves, the waveform is decomposed into a fundamental wave and various harmonics according to the Fourier series. 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 a PWM carrier signal. For GTR high-power inverter elements, the PWM carrier frequency is 2-3kHz, while the PWM carrier frequency of IGBT high-power inverter elements can reach up to 15kHz. Similarly, the output circuit current can also be decomposed into a fundamental wave containing only sine waves and other harmonics. Higher harmonic currents radiate into space through the cable, interfering with nearby electrical equipment. Therefore, considering the operating characteristics of frequency converters, frequency converter cables should focus on solving the following problems: electromagnetic waves emitted by the cable itself, suppressing interference from higher harmonics through the cable; and the impact of pulse voltage on insulation, preventing the effects of pulse voltage on the cable. Addressing the anti-interference capability and the safety and reliability of insulation in the cable structure design of frequency converter cables is particularly important.
3. Operating characteristics of frequency conversion cables
Understanding the operating characteristics of frequency converters, the design of frequency converter cables should focus on controlling the following aspects:
Cables emit electromagnetic waves. Typical household frequency converters are single-phase powered, short in length, and have low power. During design, the frequency converter, connecting cable, and motor are housed within a metal casing, suppressing electromagnetic wave emissions. However, in industrial applications, motors have higher power, and the cables connecting the frequency converter and power supply are longer. During operation, these cables become effective carriers of high-frequency electromagnetic waves, interfering with nearby communication devices and amplitude modulation receivers. This can sometimes be severe, constituting electromagnetic pollution. International standards have been established for such cables, and we have also proposed relevant EMC testing and control methods. While there are currently no national regulations specifying assessment indicators for environmental pollution caused by cable electromagnetic wave emissions, suppressing high-frequency interference is essential. To effectively suppress high-frequency interference, the shielding structure of the frequency converter cable is crucial. Shielding is the best method for suppressing high-frequency interference, and shielding structures are divided into copper wire braided shielding and copper tape shielding. When using copper wire braided shielding, the shielding suppression coefficient increases with the braiding density; the higher the braiding density, the better the shielding effect. When using copper tape braided shielding for cables, the shielding effect is only comparable to that of copper tape shielding when the braiding density reaches 90% or higher. Therefore, frequency converter cables should preferably use copper tape shielding to ensure effective shielding. Manufacturers often use copper wire braided shielding, but this is not the best method due to high material consumption, slow processing speed, and less than ideal shielding performance. Using copper tape overlapping, wrapping, and corrugating is a more advanced structure and process, forming a fully enclosed metal layer that achieves effective shielding.
The impact of pulse voltage on insulation. Variable frequency power supplies have a wide frequency adjustment range, but regardless of the frequency, they possess a dominant frequency waveform profile containing many higher harmonics. As a traveling wave, these harmonics undergo multiple reflections, and their amplitudes can superimpose to several times the operating voltage. The longer the cable, the higher the amplitude. If the cable insulation safety factor is not high, it may be damaged. Therefore, to ensure cable safety, we address the following three aspects:
Increasing the insulation thickness improves the insulation's withstand voltage capability, while selecting materials with better insulation properties. The cable insulation thickness can be determined according to the specifications for the corresponding voltage level; appropriately increasing the thickness naturally improves reliability, which is particularly beneficial for frequency conversion cables. In general land-based applications, using PVC insulation is not ideal because its dielectric constant is relatively high, resulting in significant dielectric loss under alternating electric fields. Cross-linked polyethylene (XLPE) insulation is more suitable. XLPE has a low dielectric constant and low dielectric loss, and its temperature resistance and mechanical properties are better than PVC, combining excellent organic, electrical, and thermal properties. Therefore, using XLPE as the insulation material is a more appropriate choice.
Adding a semiconductive layer to the outside of a conductor homogenizes the electric field and reduces tip discharge. During the manufacturing process, defects (such as burrs) may be generated on the surface of the conductor. Without a semiconductive layer, electric field distortion occurs at the defects, which can easily lead to breakdown and damage to the insulation. However, by applying a semiconductive layer, the electric field on the conductor surface is homogenized due to the presence of the semiconductive layer, which can effectively prevent insulation breakdown.
The cable adopts a symmetrical structure to achieve a homogenized electric field and phase balance. For a four-core low-voltage cable, the first step is to improve the arrangement of the insulated cores. If the four cores of the cable are directly bundled together, it is an asymmetrical structure. However, if the fourth core is decomposed into three insulated cores with smaller cross-sections, the three large and three small cores are bundled together in a symmetrical structure.
Grounding measures for the shielding layer. Proper grounding of the shielding layer is a necessary condition for suppressing the emission of electromagnetic waves. The grounding method for copper wire braided shielding is relatively easy to solve, while longitudinally wrapped copper strip corrugated shielding requires a special clamp for grounding. The contact surfaces of the clamp and the corrugated copper tube should match, and the grounding wire should be led out from the tail end of the clamp.
Outer sheath. This type of cable is mostly laid indoors and generally does not require armor. Although the use of PVC sheath is not completely ruled out, high-density polyethylene is more suitable.
Additional Cable Tests. 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 situation is slightly different for the connecting cables of variable frequency motors, which need to withstand high-frequency pulse voltage. High-frequency wave amplitudes can reach 1200–1900V, with ringing frequencies of approximately 100–2000kHz. Pulse voltage testing (type testing) of cables is performed to verify the cable's insulation level. The test can refer to the IEC 60502 standard, which involves applying ten positive and ten negative pulse voltages. The test voltage can be considered as 40kV, but further verification is required, and the factory can decide whether it is necessary.
The development of 3.6/6～6/10kV medium-voltage variable frequency cables is driven by the increasing size of mechanical equipment, which necessitates a corresponding increase in motor capacity and output current of the variable frequency power supply. However, limitations imposed by high-current variable frequency components restrict further advancements in current capacity technology. On the other hand, increasing the output voltage of the variable frequency power supply is relatively easy, allowing for a significant increase in the power of the medium-voltage variable frequency motor. Consequently, the cable voltage rating must also be adjusted accordingly. Currently, 3.6/6～6/10kV medium-voltage variable frequency cables are already in use. In terms of insulation structure and electrical, mechanical, and physical properties, they are comparable to power cables. Cross-linked polyethylene is clearly the preferred insulation material. If flexibility is required during installation, ethylene propylene rubber insulation also offers advantages. Due to the increased operating voltage, the emission capability of high-frequency electromagnetic waves is significantly enhanced, necessitating a more robust shielding structure. Under the operating conditions of variable frequency cables, coaxial cables are a suitable structure. Therefore, 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.
As a type of dedicated frequency converter cable, cross-linked polyethylene insulated cable for frequency converter motors is a new series of products. Although the total market demand is not very large, the development of this type of cable is very promising. Medium and large-sized frequency converter motors should use this type of dedicated cable. As for frequency converter cables for small frequency converter motors, it is also acceptable to include them in this category.
