Summary:This article analyzes the concept of harmonics and their hazards, categorizes harmonic sources, and highlights the severe impact of harmonics on the entire power supply and distribution system of modern buildings. It emphasizes the importance and urgency of harmonic suppression and provides several measures. It further elaborates on the application and working principle of active harmonic filters in suppression through specific case studies, and demonstrates the effectiveness of these measures.
KeywordsHarmonics, Disturbances, Measures, Active Filters
Introduction
In recent years, as the process of modernization deepens and the country promotes energy conservation and emission reduction, the construction industry has extensively adopted rectifiers and variable-frequency equipment. The use of these devices has significantly contributed to energy savings. However, the widespread and increasing use of these non-linear loads has severely degraded the quality of power supply and distribution. The large amounts of multiple harmonics generated by these non-linear devices can, under certain parameter conditions, form series or parallel resonances with the system's reactance and impedance in the power supply and distribution network, leading to equipment damage in the system. The harmful effects of harmonics on the entire power supply and distribution system have garnered widespread attention, and it is imperative to suppress harmonics and improve the quality of electricity in the power supply and distribution system.
2. The Concept of Harmonics
The term "harmonic" originates from acoustics. Mathematical analysis of harmonics was well established in the 18th and 19th centuries, and the harmonic analysis methods proposed by Fourier and others are still widely used today.
Harmonics refer to the electrical quantities containing frequencies that are integer multiples of the fundamental frequency. Generally, they are the results of Fourier series decomposition of periodic non-sinusoidal quantities, yielding components identical to the grid's fundamental frequency, as well as a series of components greater than the grid's fundamental frequency. This portion of electricity is known as harmonics. The ratio of harmonic frequency to fundamental frequency (n=fn/f1) is called the harmonic order. In the grid, non-integer multiple harmonics may also exist, referred to as non-harmonics or fractional waves. Harmonics are essentially interference quantities that pollute the grid. They are a key indicator of power quality and an important aspect of technical supervision over power quality, with the degree of pollution directly affecting the quality of electricity.
Harmonic Source
Harmonics are typically generated by nonlinear power supply equipment, such as transformers, rectifiers, and frequency converters, which, while absorbing fundamental power, also produce harmonic currents and harmonic power. Common harmonic sources in today's typical office buildings include: *
3.1 Transformer generates harmonics
Harmonics in power transformers are primarily caused by the nonlinear magnetization curve of the transformer core material. The magnetization curve of the transformer core, in addition to the general linear region, also includes three typical nonlinear regions: saturation, dead, and hysteresis zones. It is symmetric around the origin and, under sinusoidal excitation, the excitation current is an odd function of symmetry. Currently, when designing transformers, economic considerations are taken into account, so the operating magnetic flux density is chosen near the saturation section of the magnetization curve. This results in a peak-shaped magnetization current, thereby containing odd harmonics. The harmonic order is also influenced by the primary and secondary winding connections (Δ or Y), and the magnitude of harmonics is related to the magnetic circuit structure, the degree of core saturation, and other factors. The higher the core saturation, the further the transformer operating point deviates from the linear working area, and the larger the harmonic current generated.
3.2 Inverter generates harmonics
With the rapid development of the inverter industry and the enhanced awareness of energy conservation and environmental protection, variable frequency drives are increasingly being used in equipment such as fans, pumps, and air conditioners. The main circuit topology of our commonly used inverters is typically AC-DC, converting AC power to DC, and then to the required frequency and voltage for controlling AC motors. This type of low-voltage inverter produces harmonics that are both current-source and voltage-source types. According to some data, the harmonic components produced by such equipment can sometimes reach up to 50% to 60% of the fundamental frequency component. Due to the high power and large number of variable frequency devices, their impact on the entire power supply and distribution system is significant.
3.3 Electronic Ballast Generates Harmonics
Currently, almost all architectural lighting equipment utilizes electronic ballasts. The benefits of using electronic ballasts include high luminous efficiency, elimination of flicker, extended lamp life, high power factor, low noise, dimmability, and low power consumption by the lighting equipment itself, resulting in significant energy-saving effects. An electronic ballast is a converter that transforms power frequency AC power into high-frequency AC power. The harmonic components produced by such devices are all odd harmonics. According to some data, the harmonic components produced by electronic ballasts can sometimes reach up to 25% of the fundamental frequency component. With the massive number of lighting equipment, the impact on the power supply and distribution system is considerable.
4. Harmonics' Dangers
Currently, due to the widespread use of equipment generating harmonics, harmonics are causing severe interference and degradation to the power supply and distribution systems in buildings, with the main hazards including:
4.1 Power Transformer
Core magnetic induction circulation increases, leading to higher copper losses due to current harmonics and increased iron losses due to voltage harmonics. The combined effect is an elevated transformer temperature and increased power consumption; it accelerates insulation aging, reduces capacity margin, and affects equipment lifespan. Harmonics may also cause resonance between transformer windings and inter-winding capacitance, as well as saturation or distortion of the core magnetic flux, which can severely damage transformer windings.
4.2 Power Cable
Due to the higher harmonic frequencies, the skin effect and proximity effect are more pronounced, leading to increased heat loss and accelerated insulation aging, which affects the lifespan. Moreover, the third harmonic overlays on the neutral wire, with three times the current of the phase line passing through it, significantly exceeding its safe current value, causing overloading. In this condition, it is possible for the wire to overheat, igniting flammable materials around the circuit or causing the neutral wire to melt, leading to unbalanced phase voltages, damaging electrical equipment connected to the circuit, and potentially causing a fire.
4.3 Electric Motor
For frequently-started motors, larger ones can cause the motor to heat up, resulting in additional temperature rise. Additionally, when harmonic currents in the motor are close to the natural frequency of a certain component, they can also induce mechanical vibrations in the motor, generating significant noise.
4.4 Power Capacitor Bank
In power systems, to improve power factor, it is common to add a bank of power capacitors. However, harmonics can accelerate the aging of capacitors, increasing their loss coefficient and additional losses, which can lead to more frequent faults and a shorter lifespan for the capacitors. When the capacitors and line impedance reach resonance, harmonic currents are amplified, causing capacitors and fuses to malfunction due to overheating, overcurrent, and overvoltage, potentially leading to damage.
4.5 Switches and Relay Equipment
Harmonic currents cause additional losses in switches and increase temperature rise, thereby reducing the capacity to carry fundamental frequency current. The increased rise can also shorten the lifespan of certain insulation components. Harmonic currents can also induce additional torque in relay protection devices, alter the operating characteristics of electrical appliances, leading to false operation. Solid-state trip devices in low-voltage circuit breakers operate based on current peak values, and this type of trip device may cause abnormal tripping due to the feeding of nonlinear loads.
4.6 Measuring Instruments, Lighting, Communication Equipment, etc.
Electrical measuring instruments can cause additional electromagnetic torque on the induction disk due to harmonics, leading to errors and reduced accuracy. Harmonic currents can cause lighting flickering, making people prone to visual fatigue and lowering work efficiency. For communication equipment and others, harmonics can couple into the system through electromagnetic induction, electrostatic induction, and transmission, thereby causing interference and affecting normal operation or reducing the lifespan of the equipment.
5. Measures to suppress harmonics
To control the proliferation of harmonics, purify the waveforms of the power supply and distribution systems, and enhance the quality and efficiency of electrical energy supply, the National Standard GB/T14549-93 "Power Quality - Harmonics in Public Power Systems" has been established. This standard specifies the voltage limits of harmonics in public power systems (Table 1) and harmonic current values (Table 2).
5.1 Increased Rectification Phase Count for Inverter Devices
Structural modifications have been made within the facility, including connecting some sensitive equipment to the clean section of the power grid. Additionally, a twelve-pulse driver can be optionally chosen in place of the six-pulse driver.
5.2 Set Passive Filters
When harmonic levels are high, traditional use of passive filters for harmonic filtering introduces some issues. Passive filters consist of a series of inductors, capacitors, and sometimes resistors as passive components. Due to their low impedance at resonance frequencies, harmonic currents of any magnitude pass through the circuit, making passive filters prone to overload. Overload can cause machines to shut down or be damaged. Overload may be caused by unforeseen harmonic sources in the power supply system, such as newly added drives. The degree of filtering by the passive filter depends on its impedance in relation to other impedances in the grid, making it difficult to control. Additionally, over time, component aging or changes in the distribution system can alter the resonance frequency, significantly reducing the filtering effectiveness. Moreover, crucially, passive filters can only filter one harmonic component; if different harmonics need to be filtered, separate filters must be connected.
5.3 Set Active Filters
Active Power Filters (APF) are a rapidly evolving technology in recent years. APF employs controllable power semiconductor devices and operates in a closed-loop with real-time detection to inject currents into the power grid that are equal in amplitude and opposite in phase to the harmonic source current, resulting in a total harmonic current of zero in the power supply. APF can dynamically suppress harmonic currents and compensate for them in real-time (with varying magnitudes and frequencies). Depending on the circuit configuration and grid connection methods, APFs can be categorized into voltage-source and current-source types, as well as series and parallel configurations. In practical applications, the appropriate type can be selected based on the specific conditions of the power supply and distribution system. The significant advantages of APFs make them a promising direction for future harmonic compensation, poised to replace traditional passive filters.
Advantages of Active Filters: Not only can they compensate for various harmonics, but also suppress flicker and reactive power compensation. The filter characteristics are not affected by system impedance, thereby eliminating the risk of resonance with system impedance. They feature adaptive functionality to automatically track and compensate for varying harmonics, demonstrating high controllability and rapid responsiveness.
6. Application Case of Active Filters in Office Buildings
6.1 Working Principle of Active Filters
Active filters and passive filters operate on entirely different principles. Active filters detect load current through current transformers, then use an internal DSP to calculate and extract harmonic components from the negative sequence current. Subsequently, PWM signals are sent to the internal IGBT to control the inverter, injecting a current into the grid that is equal in magnitude but opposite in direction to the load harmonic current, thereby achieving the purpose of filtering. As shown in the following diagram:
BESTSINE's active filter can provide compensation for three-phase harmonic currents, consisting of the following components:
A pulse current filter module. Its primary function is to absorb high-frequency pulse currents and compensate for a certain amount of reactive power.
An electromagnetic contactor module for smooth start-up. Designed to help reduce the amplitude of the surge current during the charging of DC capacitors.
A high-frequency inductor capacitor module, serving as an interface unit for the transmission of electrical energy between power rectifiers and power systems.
An IGBT (Insulated Gate Bipolar Transistor) power rectifier module. It is designed to convert harmonic energy in power systems and generate harmonic currents of equal magnitude but opposite direction to the harmonics in the grid, which are then injected into the grid. This process effectively cancels out harmonics in the grid.
A direct current capacitor module designed to store energy from the power system, then providing that energy to an IGBT rectifier to generate a compensating harmonic current with the same magnitude but opposite direction as the harmonic components in the power grid.
1) Switch
2) Fast-blow fuse
Pulse Current Filter Module
4) Smooth Start of Electromagnetic Contactor Module
5) High-frequency Inductor and Capacitor Model
6) IGBT Power Module
7) DC Capacitor Module
6.2 Application Examples
The HVAC systems, water supply and drainage systems, and other equipment in the current office buildings have extensively adopted variable frequency control. In addition to abundant office lighting, there is also a significant amount of flood lighting. The characteristics of these devices generate a large amount of harmonic distortion. The power supply and distribution networks affected by harmonic pollution pose severe safety risks, such as overheating transformers, abnormal tripping of overloaded equipment, chaotic automatic control systems, and computer equipment crashes. In response to this, we have implemented harmonic mitigation for the variable frequency refrigeration units within the building. Below is a comparison of the values before and after the treatment:
Comparison of the Wave Generator before and after operation:
Equipment operational; filters halted.
Equipment operational, filters functioning
2) Test Data and Analysis (Data see attached page)
From the above measurement data, it can be observed that, upon installation of the active filter.
Following the implementation, significant improvements in power quality were achieved: the total harmonic distortion (ITHD) rate dropped from the original 77% to around 11%; the 5th harmonic current decreased from nearly 20% to around 1.5%; the 7th harmonic current reduced from 12% to approximately 1.4%; and both power factor and voltage effective value were enhanced.
7. Ankorri APF Active Power Filter Product Selection
7.1 Product Features
DSP + FPGA control method, short response time, all-digital control algorithm, stable operation.
(2) Multi-functional, capable of both harmonic compensation and reactive power compensation; it can provide full compensation for harmonics from the 2nd to the 51st order or targeted compensation for specific harmonic orders.
(3) Equipped with comprehensive bridge arm overcurrent protection, DC overvoltage protection, and device overtemperature protection functions.
(4) Modular design, compact size, easy installation, and convenient for expansion.
(5) Equipped with a 7-inch large color touchscreen for parameter settings and control, offering convenience, ease of operation, and maintenance.
(6) Installed filtering devices at the output end to reduce the impact of high-frequency ripple on the power system.
(7) Parallel multi-machine setup achieves a higher level of current output.
7.2 Model Description
7.3 Size Description
7.4 Product实物展示
ANAPF Active Power Filter
8. Closing Remarks
This article provides a comprehensive explanation of harmonics, including their concept, generation, hazards, and applications in building structures. Its aim is to raise awareness about the dangers posed by harmonics. In an era of energy scarcity and with the nation strongly advocating for energy conservation and emission reduction, it is even more crucial to recognize the hazards harmonics present and to implement effective management at their root.
References
Gao Xiang. The Application of Active Filters in Building Office Buildings [J]. Installation, 2013(03): 52-55.
[2] Ankorree Enterprise Microgrid Design and Application Manual, 2022.05 Edition.
