What are harmonics?

1, What are harmonics? Harmonic is an unwanted interference waveform that directly affects the quality of the power grid and should be taken into account when the sum of harmonic currents is higher than the allowable limit. Harmonic current is current whose frequency is a multiple of the fundamental frequency. For example, a 250Hz line on a 50Hz grid is a 5th harmonic. A 250Hz current is an unusable energy flow with devices on the grid. Therefore, it will be converted to THERMAL form and cause DAMAGE.

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2, Harmonic harm Harmonics can cause cables to overheat, damage insulation. The motor can also overheat or cause noise and torque fluctuations on the rotor leading to mechanical resonance and vibration. Capacitors overheat and in most cases can lead to destruction of the dielectric. Electrical displays and lights may flicker, protective devices may shut off power, computers may fail (data network) and measurement equipment may give incorrect readings. 3, What causes harmonic currents? Harmonic currents and voltages are generated by nonlinear loads connected to the power distribution system. All electrical energy converters used in different forms in the power system can increase harmonic noise by injecting harmonic currents directly into the grid. Common nonlinear loads include motor starters, electric drive systems, computers and other electronic equipment, electronic lamps, welding power sources, etc. 4, Solutions to reduce the influence of harmonic emissions from units inverter? The converters themselves contain nonlinear elements that are the source of harmonics. However, the harmonic current more or less depends on the structure of the drive system and the load, if using a large motor (compared to the power transformer) or increasing the motor load will increase the harmonic current. Therefore, in order to reduce harmonics, industrial manufacturers are forced to use low-harmonic frequency inverters or use external filtering methods. In which, to reduce the harmonic current must increase the AC, DC inductance or increase the number of rectifier valves in the rectifier and reduce the harmonic voltage caused by the harmonic current must increase the transformer capacity, reduce the transformer impedance or increase the voltage. short circuit capacity of the source. – Using 6-pulse, 12-pulse and 24-pulse rectifier The rectifier circuit in 3-phase inverters uses pulse width modulation (PWM) technology, usually a 6-valve diode bridge. Such rectifiers are simple, rugged, and cheap, but the input component contains many low-order harmonics. A 12-valve diode rectifier bridge is made by connecting two 6-valve rectifiers in parallel, which produces a smoother current than a 6-valve bridge. Similarly, a 24-valve rectifier is made simply from combining four sets of six valves together. – Using IGBT bridge (Integrated Gate Bipolar Thyristor) An active converter IGBT can be used to rectify input AC voltage. It will help improve power factor, reduce harmonics and bring many benefits such as: Safety even when power is lost; precise full-range control in rectification and regenerative modes; allowing energy to be returned to the grid; The supply current has a near sine waveform with a small harmonic component. The IGBT has less low harmonics at low frequencies, but high gain at higher frequencies; capable of raising voltage. When the supply voltage is dropped, the DC voltage can be boosted to keep the motor voltage higher than the supply voltage. 5, Harmonic reduction method ABB’s harmonic reduction converter provides a simple harmonic lowering solution incorporated in the inverter. These inverters use harmonic reduction technology without the use of external filters or multi-pulse transformers. Harmonic-reducing inverters produce low-order harmonics on the input side with a total distortion current of less than 5%. Therefore, ABB’s harmonic reduction inverters provide a simple, low-cost solution to satisfy stringent power quality standards. 6, Improve grid power quality In inverters with 6-pulse diode bridge, the grid-side current is non-sine and contains bi-harmonic components, especially 5th and 7th order components. It is represented by the current pattern. distortion, can be up to 30-50%. In ABB’s harmonic reduction inverter, the use of DTC (Direct Torque Control) method and filter will reduce the harmonic current to less than 5%. The result is a sinusoidal current that makes the shape of the mains voltage almost distortion-free. 7, Harmonic Control Regulations Needed Soon. Ideally, the alternating current on the grid supplied by power companies to consumers should be sinusoidal. However, the existence of nonlinear elements on the supplier’s grid as well as on the load side causes harmonics to appear, affecting the operation of the grid and equipment. Typical nonlinear elements are the steel cores of transformers, motors (saturation properties of ferromagnetic materials), power semiconductor devices such as diodes, and thyristors of converters. Usually the 3rd harmonics are suppressed by the delta windings in the transformer (along with the power loss), but the odd harmonics Others (maximum values ​​are level 5 and level 7) still propagate along the line, causing power loss, adverse impact on the operation of equipment, especially three-phase motors… not to mention the waves Higher harmonics can cause electromagnetic waves to propagate through space, affecting radio transceivers. Therefore, when investing in construction, it is necessary to choose equipment that causes less harmonics and use additional means to minimize harmonics. At the same time, the state management agencies should also consider and soon have specific regulations to control harmonics.

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8, Harmonics: Identify hazards to electrical systems:

The power grid system can be damaged by many factors, in which a potential hazard that hinders operation and causes equipment wear but few people realize is harmonics – a potential hazard that is detected. present in the early 1890s.

Harmonics and their effects: Harmonic currents are unwanted currents that overload lines and transformers, increase system temperatures (or even cause fires), and interfere with the electrical grid. In case of running multiple motors at the same time, if no harmonic control measures are taken, the power system can be overloaded, increasing the power demand and causing the machine to stop running (due to the power being overloaded). If you have to replace damaged equipment due to harmonics, this can increase investment costs by up to 15% and operating costs by 10%. In the industry, protecting profits is a top priority, while keeping equipment and operating costs under control is key. To achieve this goal, businesses and organizations need to pay attention and do well in limiting the harmful effects of harmonics. Harmful effects of harmonics to the grid: When the r.m.s. value and amplitude value of the voltage or current signal increases due to harmonics, it will lead to a series of harms occurring to the entire grid system such as: increased heating of electrical conductors, electrical equipment generates high heat, causing equipment damage, fire and the risk of fire and explosion; overheat the capacitor and in many cases can lead to dielectric destruction. The higher-order harmonics can also cause the impact torque of the relay to deform, causing the relay to jump, leading to the incorrect timing of the relay, causing false alarms of the UPS and causing copper loss and damage. The flux loss and iron loss increase the transformer temperature, leading to an increase in power loss. In addition, harmonics also cause losses on the coil and steel core of the motor to increase, distort the form of torque, reduce machine efficiency, and cause noise; affect the error of the measuring devices, causing the measurement results to be erroneous. More dangerously, the higher-order harmonic waves can also generate motor shaft torque or cause mechanical resonance oscillations that damage mechanical components in the engine; flickering of electrical equipment and lighting, affecting people, and causing electromagnetic waves to propagate in space, affecting transceivers. Five methods of reducing harmonics: Replacing damaged equipment that is caused by harmonics is an expensive solution, increasing investment costs by 15% and operating costs by 10%. Therefore, it is necessary to choose low-cost solutions with high efficiency in harmonic control. The following methods are commonly used to reduce harmonics: + Use AC reactor (line choke) or DC reactor (DC choke) for the inverter; + Rectification solution 12 pulses (12 pulses); + Passive filter (passive filter); + Active filter (active filter); + Use low harmonice drive type inverter. Pros and cons: – Using reactors is the best solution for applications where filtering for heavily noisy mains is required and harmonic reduction is not a priority. – The 12-pulse inverting solution gives the best performance in harmonic reduction but the process is the most complex. – The passive gain filter consists of multiple reactors and capacitors installed in a resonant circuit to eliminate the frequency of the harmonics. A system of many passive filters can remove several harmonics. – The solution of using active filter is applied to many inverters attached in parallel on the same distribution line (point of common coupling) with the main task of power compensation and harmonic compensation. voltage and current harmonic compensation. – In applications requiring high harmonic reduction, using inverters with low harmonics is the optimal solution. These inverters use harmonic reduction technology without the use of external filters or multi-pulse transformers with less than 5% total current harmonic distortion (THDi). In addition, the choice of solution for the power system depends on the characteristics of the load and the power demand of the connected devices, with this guide to help you choose the best method. . Expert breakdown Comparison criteria include compactness or space required for installation, simplicity of operation, harmonic reduction efficiency, energy efficiency, and return on investment – ​​price value, the results achieved compared to the costs spent.

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Note: Comparison of harmonic reduction methods based on scores: 1 = worst, 5 = best * Viewed from the medium voltage network side ** Harmonic reduction depends on installation and power *** Effectiveness depends on according to the harmonic reduction

As can be seen, harmonics are unwanted currents that overload lines and transformers, increase system temperature (or even cause fires) and cause interference to the power grid. In the case of running multiple motors at the same time, uncontrolled harmonics can overload the power system, increase power demand and stop the machine (due to overloaded power), causing equipment damage. or shut down the whole system. Therefore, minimizing harmonics is an important factor to help maintain the life of operating equipment. Some devices help to reduce harmonics Altivar 212 ATV212 inverter uses C-less technology to reduce THDi below 30% without the need to install additional filters, meeting IEC 61000 standard with the lowest investment cost.

Medium voltage inverter ATV1200 ATV1200 inverter with optimal design structure and the following specifications makes it friendly to the grid and motor – Capacity up to 16,200 KVA with 3-phase voltage motor output from 2.4 kV to 11 kV. Easy integration into existing or new systems. Simple and efficient cooling system with forced air cooling technology with dual airflow. – Reduced electrical energy consumption and 96% greater operating efficiency. – Harmonic reduction technique with 18-54 pulse rectifier technology and multi-step inverter structure, so THDi is 3% lower (in both input and output), meet IEEE519-1992 standard.

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Energy saving

Optimal power quality Meets IEEE 519, THDi 5% lower in all operating modes, built-in radio-frequency noise filter- class 3, sinusoidal power supply current, power factor ( power factor) and Cos-phi is close to 1.