Xichi APF applied to harmonic control in buildings

Active Power Filter Application

At present, the HVAC system, water supply and drainage system and other equipment of the building and office buildings all adopt frequency conversion control. In addition, the office building not only has a large number of office lighting, but also a large number of flood lighting. It produces a lot of harmonics. The power supply and distribution network polluted by harmonics has serious hidden dangers, such as: transformer overheating, center line overload, abnormal tripping of equipment, chaos of automatic control system, crash of computer equipment and so on.

Project Overview:

There are a large number of current harmonics on the four transformers and distribution lines in the power distribution room of a commercial building. The existence of current harmonics aggravates the voltage distortion, resulting in different degrees of damage to the capacitor cabinets used under each transformer.

Solution:

After a detailed analysis of the harmonic content on site by our engineers, we found that the on-site harmonics are mainly generated by LED lights, air conditioning units, freezers, elevators and other loads. After communicating with customers, a reasonable solution was determined, that is, the above transformers An active power filter is installed below each, the main purpose is to control the 3rd and above harmonics contained in the system.

According to our plan, 4 sets of 400A parallel Xichi active power filters were selected to automatically track and control the harmonic current generated by the load. After the current harmonic treatment, the 3rd and above current harmonics are also within the allowable value of the national standard, and the voltage harmonics are reduced, which effectively protects the safety of transformers, power distribution cabinets, and equipment, and prolongs the life of the equipment.

Results:

As can be seen from the figure, the waveform after treatment is a sine wave. After the filter is put into use, the system harmonics are well suppressed, which improves the reliability of the power supply system and eliminates the hidden danger of system harmonics affecting the communication system.

Waveform before harmonic treatment	Waveform after harmonic treatment
Total harmonic content before compensation	Total harmonic content after compensation

Common Harmonic Sources in Buildings and Their Hazards

The most common sources of harmonics in current architectural office buildings are: transformers, frequency iverters and electronic rectifiers. These nonlinear power supply devices generate harmonic current and harmonic power while absorbing the fundamental power.

1. Power transformer: 

The magnetic induction circulating current of the iron core increases, the current harmonics will increase the copper loss, and the voltage harmonics will increase the iron loss. The comprehensive effect is to increase the temperature of the transformer and increase the power consumption; accelerate the aging of the insulation, resulting in a decrease in the capacity margin, affecting the Equipment life; harmonics may also cause resonance between transformer windings and inter-line capacitors, and cause iron core magnetic flux saturation or skew, which can cause transformer lamps to burn in severe cases.

2. Power cable: 

Due to the high harmonic frequency, the skin effect and proximity effect will be greater, the heat loss will increase, the insulation aging will be accelerated, and the lifespan will be affected. Moreover, the 3rd harmonic will be superimposed on the neutral line, and the current of 3 times the phase line will pass through the neutral line, so that the neutral line current greatly exceeds its safe current value and cause overload. In this state, it may cause the wire to be overloaded. Overheating will cause the combustible decoration materials around the line to catch fire or the neutral line to fuse, causing the voltage of each phase to be unbalanced, burning out the electrical equipment connected to the line and causing a fire.

3. Electric motor: 

For a motor that is often started, a large amount of harmonics will cause the motor to heat up, resulting in an additional temperature rise of the motor; in addition, when the harmonic current in the motor is close to the natural frequency of a certain part, it will also cause the motor to produce mechanical damage. Vibrates and makes loud noises.

4. Power capacitor bank: 

In order to improve the power factor in the power system, a power capacitor bank is generally added, and harmonics will accelerate the aging of the capacitor, increase the loss coefficient of the capacitor, and increase the additional loss, which is easy to discharge faults and shorten the capacitor's life. life. When the impedance of the capacitor and the line reaches the resonance condition, the harmonic current will be amplified, so that the capacitor and the fuse cannot operate normally or even burn due to overheating, overcurrent, overvoltage, etc.

5. Switch and relay equipment: 

Harmonic current will cause additional losses in the switch, and increase the temperature rise to reduce the ability to carry the fundamental wave current, and the increase in temperature rise will reduce the service life of some insulation components. The harmonic current will also cause the relay protection equipment to generate extra torque, change the operating characteristics of the electrical equipment, and cause malfunctions. The solid-state trip device of the low-voltage circuit breaker acts according to the current peak value, and this type of trip The unit may trip abnormally due to feeders supplying non-linear loads.

6. Measuring instruments, lighting, communications and other equipment: 

the electric energy measuring instrument will cause the induction turntable to generate additional electromagnetic torque due to harmonics, causing errors and reducing the accuracy; harmonic currents will cause lighting flickering, causing visual fatigue and reducing work. Efficiency; for communication and other equipment, it will be coupled to the system through electromagnetic induction, electrostatic induction and transmission, which will interfere with the equipment and affect its normal work or reduce its service life.

Measures to suppress harmonics

There are many different ways to reduce the harm caused by harmonics in the construction industry:

1. Increase the number of rectifier phases of the converter

Structural changes were made in the device, including connecting some sensitive equipment to the clean part of the grid, and a twelve-pulse driver could be used instead of a six-pulse driver.

2. Set passive filter

Passive filters can only filter one harmonic component. If different harmonics are to be filtered, different filters should be connected respectively.

3. Set the active power  filter

APF can not only compensate various harmonics, but also suppress flicker and compensate reactive power; the filtering characteristics are not affected by system impedance, etc., which can eliminate the risk of resonance with system impedance; it has an adaptive function, which can automatically track and compensate for changing Harmonics, that is, have the characteristics of high controllability and fast response.