How to match the frequency converter transformer

In modern industrial automation system, frequency converter, as an important equipment for motor speed control, is widely used in fans, water pumps, conveyor belts and other occasions. As one of the key equipments in power system, transformer is responsible for voltage transformation and isolation, and ensures the stable transmission of electric energy. In order to realize the efficient and safe operation of the system, the matching of frequency converter and transformer is very important. This paper will discuss the reasonable matching method of inverter and transformer from the aspects of power matching, harmonic influence, voltage adaptation and short circuit protection.

First, power matching

The power matching between inverter and transformer is the most basic and key link in the design. The rated capacity of the transformer should be slightly larger than the input power of the inverter, and it is generally recommended that the transformer capacity be 1.2~1.5 times of the total power of the inverter. This is because the inverter will produce certain harmonic current and reactive power during operation. If the transformer capacity is insufficient, it will easily lead to overheating, efficiency decline or even burning.

For example, for a frequency converter with rated power of 110kW, after considering the starting current, harmonic loss and other factors, it is suggested to choose a transformer with a capacity of about 160kVA to ensure the stable operation of the system.

Second, the voltage adaptation

The input voltage of frequency converter must be consistent with the output voltage of transformer. Common inverter input voltage levels are 380V, 400V, 440V, 690V, etc., so the secondary output voltage of the transformer needs to match it. At the same time, the fluctuation range of grid voltage should be considered to ensure that the inverter works within the allowable voltage range (usually 10%).

In addition, if the medium-voltage frequency conversion system (such as 6kV or 10kV) is used, the corresponding medium-voltage transformer needs to be configured for step-down processing to meet the input requirements of low-voltage frequency converters.

Third, harmonic influence and transformer selection

As a kind of nonlinear load equipment, frequency converter will produce a large number of harmonic currents during operation, which will increase the copper loss and iron loss of transformer, lead to the temperature rise of transformer and shorten its life. Therefore, the following points should be considered when selecting the transformer:

-Choosing a transformer with high short-circuit impedance is helpful to limit short-circuit current and reduce the influence of harmonics;
-Using K-coefficient transformers (e.g. K=4, K=13, etc.), specially designed to withstand harmonic loads;
-Install input reactor or filter device between transformer and frequency converter to reduce the influence of harmonic current on power grid and transformer.

Four, short circuit protection and cooperation

Inverter usually has certain over-current and short-circuit protection functions, but its protection range is limited. As the main power supply equipment of the system, the protection configuration of the transformer must be coordinated with the frequency converter to prevent the equipment from being damaged due to short circuit fault. It is usually suggested to install a circuit breaker on the low voltage side of the transformer and set the protection setting value reasonably, so that it can cut off the power supply in time when the inverter fails.

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To sum up, the matching between inverter and transformer is not only a simple correspondence of electrical parameters, but also involves the safety, stability and energy efficiency of system operation. Only by comprehensively considering the power, voltage, harmonic control and protection measures, can we ensure the efficient and reliable operation of the whole electric drive system. Therefore, in engineering design and practical application, scientific selection should be made according to specific working conditions, and the relationship between frequency converter and transformer should be rationally configured to provide a solid guarantee for industrial automation system.