Sep 25, 2020
Harmonic analysis and suppression in the working process of soft starter
Abstract: Soft start technology has been developed in recent years. It is a new technology that organically combines power electronics technology, microprocessor technology and automatic control technology. Compared with traditional step-down start control technology, it has many advantages. This article starts with the concept of soft starter, analyzes the causes and hazards of soft starter harmonics, and proposes methods to suppress harmonics on this basis.
Keywords: soft starter; harmonics; harm; suppression
With the continuous development of industrial production machinery, higher and higher requirements are put forward for the starting performance of the motor. Three-phase squirrel cage asynchronous motors are widely used, but the large starting current is a prominent shortcoming. In order to improve the starting performance, reduce the starting current, and increase the starting torque, traditionally use series reactors, autotransformers and other step-down starting methods. Although these methods can reduce the starting current, they also reduce the starting torque of the motor, and the starting current is not continuous, and the amount of maintenance is large, which increases the cost and reduces the reliability.
Soft starter is a new type of motor control equipment that integrates soft start, soft stop, light load energy saving and multi-function protection. It can not only start the motor smoothly without impact during the entire starting process, but also adjust the parameters during the starting process, such as current limit value, starting time, starting time, etc., according to the characteristics of the motor load. In addition, it also has a variety of protection functions for the motor, which fundamentally solves the many drawbacks of traditional step-down starting equipment. ABB, Schneider, Siemens and other large companies have related products.
1 Principle and performance characteristics of soft starter
The main composition of the soft starter is the three-phase parallel thyristor and its electronic control circuit connected in series between the power supply and the controlled motor. Modern soft starters basically use power electronic technology and microcomputer control technology, with single-chip microcomputer as the central control To complete the measurement and various control algorithms. Therefore, the soft starter has strong functions and flexibility.
The entire startup process is automatic operation under the control of digital program software. Using the electronic switching characteristics of three pairs of thyristors, the trigger pulse is controlled by the single-chip microcomputer in the starter to change the trigger angle, thereby changing the on-time of the thyristor, the output voltage of the device rises according to a certain rule, and the voltage of the controlled motor is reduced from zero When the voltage is increased to full voltage, the speed will smoothly accelerate from zero to the rated speed. It is a synthesis of power electronic technology and automatic control technology, and a control technology that combines strong and weak currents.
The performance characteristics of the soft starter: ① The starting voltage is adjustable to ensure the minimum starting torque of the motor starting, and to avoid motor overheating and energy waste; ② Control the motor to start smoothly and reduce the starting current impact; ③ The starting current can be adjusted according to the load condition to reduce Start-up loss, to generate the best torque with the smallest current; ④Start-up time is adjustable, within this time range, the motor speed gradually rises to avoid speed impact; ⑤Protect the transmission machinery, clear the torque surge and reduce the impact current; ⑥Constant acceleration and deceleration, no tachometer is needed, even when the motor load changes; ⑦ Free stop and soft stop are optional, and the speed of soft stop can be adjusted; ⑧ Phase sequence, phase loss, overheating, overcurrent during startup, and running Process overcurrent and overload detection and protection, the overcurrent value and overload value are adjustable.
2 Starting method of soft starter
The current soft starters have the following starting methods:
① Constant current soft start. When starting, the motor starting current is kept constant (that is, the starting current is limited), and the current limit value is usually selected between 1.5 to 4.5 times the rated current of the motor.
②Ramp voltage soft start. As the name implies, the voltage ramps up linearly from small to large, which changes the traditional step-down starting from stepless to stepless. This starting method is the simplest. It does not have current closed-loop control. It only controls the conduction angle of the thyristor to increase the starting voltage at a set rate and then convert it to the rated voltage. The disadvantage is that the initial torque is small, the parabolic rise of the torque characteristic is not good for the drive system, and the long starting time is detrimental to the motor.
③Slope constant current soft start. In the initial stage of starting, the starting current gradually increases according to the set rising rate (slope), and after reaching the set value, it maintains a constant current state until the start is completed. The rising rate of the starting current can be adjusted according to the load requirements. The higher the rate of current rise, the higher the starting torque and the short starting time. Generally, the starting torque can be adjusted between 5% and 90% of the rated torque. The starting time can be adjusted from 2 to 30 s. This starting method is most suitable for fans and pump loads, and is the most practically used soft starting method.
④ Pulse constant current soft start. This starting method has a larger starting impulse current in the initial stage of starting, and the current value is greater than the set constant current value, which can generate a larger starting impulse torque to overcome the larger static friction resistance torque and make the equipment Able to start, and then enter the constant current start phase, until the end of the start. Obviously, this starting method has a large starting torque and is suitable for heavy-load starting, such as belt conveyors and coal mills.
3. The generation of soft starter harmonics
The soft starter uses three pairs of anti-parallel thyristors to achieve AC voltage regulation. Because the thyristor is a typical non-linear device, a large number of harmonics will be generated during use, which will have an adverse effect on the stable operation of the equipment and the power grid.
The root cause of harmonics is caused by nonlinear loads. Nowadays, with a large number of applications of power electronic devices, it has become the most important source of harmonics.
Harmonics are all caused by equipment with nonlinear volt-ampere characteristics. The ideal sinusoidal voltage is applied to non-linear equipment, it will produce non-sinusoidal current, which becomes a harmonic source. When acting on both ends of the non-linear time-invariant resistance of the current-voltage characteristic, the current generated is:, it can be seen that the input voltage is a sine wave with frequency ω1, and the output current contains two frequency components of ω1 and 3ω1 Non-sine wave.
Harmonic is a sine wave component of a periodic electrical quantity, and its frequency is an integer multiple of the fundamental frequency. Because the harmonic frequency is always higher than the fundamental wave, it is called higher harmonic. The superimposition of higher harmonics on the fundamental wave will distort the waveform and make the sine wave become a non-sine wave. With the development of power electronics technology, various new types of electrical equipment have been continuously put into use, and various non-linear loads have increased day by day, resulting in a large number of harmonic currents being injected into the power grid. The existence of harmonics in the power grid will cause damage to a variety of electrical equipment, and in some places even directly threaten the safe operation of the power grid.
4. Harm of harmonics
For a system with a small capacity, the interference caused by harmonics cannot be ignored. The emergence of harmonic currents and harmonic voltages is a pollution to the public grid. The environment in which it uses electrical equipment deteriorates, which will affect the surrounding communication systems and public grids. Equipment other than that can cause harm. The seriousness of harmonic pollution to the power system is mainly manifested in:
(1) Harmonics cause additional harmonic losses to the power supply line.
(2) Harmonics affect the normal operation of various electrical equipment.
(3) Harmonics cause the capacitors in the power grid to generate harmonics. Under the power frequency, the circuit of the capacitors installed in the system for various purposes is much larger than the inductive reactance in the system, and will not produce resonance, but at the harmonic frequency, the inductive reactance value doubles and the capacitive reactance value doubles Reduce, it may appear resonance, resonance will amplify the harmonic current, causing the capacitor and other equipment to be burned.
(4) Harmonics cause local parallel resonance and series resonance in the public power grid, thereby amplifying the harmonics, which greatly increases the harm and even causes serious accidents.
(5) Harmonics will cause misoperation of relay protection and automatic devices, and cause large errors in meters and electric energy measurement. Harmonics are also very harmful to other systems and power users.
(6) The impact of harmonics on asynchronous motors is mainly to increase the additional loss of the motor, reduce the efficiency, and cause the motor to overheat in severe cases. In particular, the negative sequence harmonics generate a negative sequence rotating magnetic field in the motor, forming a torque opposite to the rotation direction of the motor, acting as a brake and reducing the output of the motor. In addition, the harmonic current in the motor, when the frequency is close to the natural frequency of a certain part, will also cause the motor to produce mechanical vibration and emit a lot of noise.
5. The significance of harmonic research
First of all, the harm of harmonics is very serious. The harmonics generated by various harmonic sources cause huge pollution to the power system and affect the operating environment of the entire power system.
Secondly, it also lies in its influence on the development of power electronics technology itself. Power electronic technology is an important pillar of the future development of science and technology, but the harmonic pollution generated by power electronic devices has become a major obstacle to the development of electronic technology.
However, it is difficult to eliminate all the generated high-order harmonics with the current technical level and economic conditions.
6. Suppression of harmonics
In order to suppress the flood of harmonics, purify the waveform of the power system, and improve the quality and efficiency of the system power, the State Bureau of Technical Supervision of China issued the national standard GB-T14549-93 "Power Quality Harmonics in Public Grids" in 1993. The attached table extracts the harmonic voltage (phase voltage) limits of the public grid in the standard (common voltage levels of low-voltage power distribution systems).
Inhibition of harmonics means to reduce or even eliminate the harmonic components generated by the harmonic wave source, so that the harmonics injected into the power grid fall below the specified value of the national standard.
The measures to suppress harmonics are generally divided into compensation and elimination: compensation is to set up devices to absorb harmonics; elimination is to change the working mode and working characteristics of the harmonic source to reduce or even not produce harmonics. Commonly used methods and measures are:
(1) Install LC filter
LC filter is a traditional harmonic compensation device, which is still widely used in engineering. It has a simple structure, low investment, high reliability, and low operating costs.
LC filter, also known as FM filter, is a series resonant circuit composed of inductance, capacitance and resistance of appropriate values. Its working principle is essentially to provide a release path for the harmonics in the circuit, that is, to retain the fundamental wave and short-circuit the harmonics. , So that harmonics are not injected into the grid. Generally use three-phase star connection. But its disadvantage is that it can only compensate the harmonics of a fixed frequency.
1. Single Tuned Filter
LC filters are divided into single-tuned filters, high-pass filters and double-tuned filters. Several groups of single-tuned filters and a group of high-pass filters are commonly used in practical applications to form a filtering device.
Single-tuned filter is constructed by the principle of series L and C resonance, and the harmonic order is X. The filter exhibits low impedance characteristics, and the n-th harmonic current is mainly shunted and rarely flows into the grid. For other orders of harmonics, which are much larger than those, the filter shows high impedance and the filter shunts very little.
Therefore, as long as the resonance order of the filter is set to the same order of the harmonics to be filtered out, most of the harmonics will be filtered out.
2. High pass filter
The high-pass filter is also called the damping filter. It presents low impedance for all frequencies after a certain frequency, forming a low-impedance path to the higher harmonics, so that most of these harmonic currents flow into the filter to achieve Purpose of filtering. The high-pass filter is divided into four forms: first-order, second-order, third-order and C-type.
3. High pass filter
The capacitance required by the first-order high-pass filter is too large, and the fundamental wave loss is too large, so it is generally not used.
The filtering performance of the second-order high-pass filter is the best, but compared with the third-order, its fundamental wave loss is higher.
The third-order high-pass filter has one more capacitance than the second-order, which improves the impedance of the filter to the fundamental frequency, thereby greatly reducing the fundamental wave loss. This is the main advantage of the third-order high-pass filter.
The performance of the C-type high-pass filter is between the second and third order. The disadvantage is that it is more sensitive to fundamental frequency detuning and component parameter drift.
4. Double tuned filter
Among the above four high-pass filters, the most commonly used is the second-order high-pass filter.
(1) Double-tuned filter, which has two resonant frequencies and absorbs the harmonics of two frequencies at the same time. Its function is equivalent to two parallel single-tuned filters. Compared with two single-tuned filters, the fundamental wave loss of the double-tuned filter is smaller. In normal operation, the fundamental wave impedance of the series circuit is much greater than the fundamental wave impedance of the parallel circuit, so the parallel circuit bears more power frequency voltage than the series The circuit is much lower. However, the structure of the double-tuned filter is complicated, the tuning is difficult, and the application is still less.
(2) Active Power Filter (APF)
Active Filter (APF) is a new technology that has developed rapidly in recent years. It is connected in series or parallel in the main circuit, and adopts real-time detection closed-loop operation mode to detect the high-order harmonics in the current in real time. According to the detection result, input the current with the same amplitude and opposite phase as the high-order harmonic components to achieve The purpose of real-time compensation of harmonic currents, so that the grid current contains only the fundamental current.
Compared with the traditional passive filter, APF has the following characteristics: ①It can not only compensate various harmonics, but also suppress flicker, compensate for reactive power, and have the characteristics of one machine and multiple functions; ②Green wave performance is not affected by system impedance It can eliminate the danger of resonance with the system impedance; ③With adaptive function, it can automatically track and compensate the changing harmonics, that is, it has the characteristics of high controllability and rapid response.
But the high cost of APF limits its popularization.
In short, compared with the traditional step-down starting equipment, the motor soft starter can better solve the problems caused by the motor starting and stopping, and has a wide range of application prospects. However, the harmonic interference generated by the soft starter also brings potential threats to the stable operation of the equipment. How to suppress the harmonics generated during the operation of the soft starter is still a subject that needs to be studied.
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