The earth rail transportation system has many complicated and complicated electrical equipment, including electric locomotives, air-condition pumps and fans, elevators, computer room network equipment, UPS, lighting control systems, fire protection systems, monitoring systems and other power electronic equipment throughout the subway application site . Resulting in a harsh harmonic environment, to ensure the safe operation of power systems and equipment caused a great threat.
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Rail transit problems
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(1) The power supply system of rail transit is a special single-phase power supply, which leads to serious imbalance of three phases;
(2) The electric rail transit power supply system is rectified and output by the high-power rectifier bridge for locomotive traction, the current domestic electric locomotives are DC chopper DC motor traction power locomotive rectifier system, locomotive traction flow load Dramatic changes and rapid, resulting in a large number of 11 times, 13 times the harmonic current;
(3) Followed by a large number of stations with inverters, UPS, a large number of nonlinear switching power supply load (3 times, 5 times, 7 times, etc.), and with the energy needs, frequency load share increased year by year. Frequency conversion load increased year by year, the harmonic current generated by it is also a corresponding increase.
(4) frequent start-up of electric locomotives, acceleration and braking make the system reactive rapid change, and the presence of harmonics also increases the reactive power loss of the system, resulting in variable and low power factor.
(5) According to GB50157-2003 "Metro Design Code", the power supply system of metro power lighting should adopt parallel power capacitor as reactive power compensation device. In theory, the power capacitor behaves capacitively both in fundamental and in harmonics, so harmonics from either the high-voltage side of the distribution transformer or from the low-voltage VFD both Play amplification. Its amplification has been verified by the actual measurement results of several subway systems in China.
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Hazards generated by harmonics
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(1) Affect the normal operation of power electrical equipment, in addition to causing additional losses, but also make the transformer serious local overheating, so that capacitors, cables and other equipment overheating, insulation aging, shortened life expectancy will lead to serious transformer burned, cable blasting, capacitors Breakdown and other failures;
(2) will cause local parallel resonance and series resonance in the system, so that the harmonic amplification, so that the aforementioned hazards greatly increased, and even cause serious accidents;
(3) will lead to power system relay protection and automatic device misoperation, such as protection switch tripping error, resulting in some lines power down;
(4) to make the power system such as computers and other sophisticated equipment can not work properly;
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Rail Transit System Harmonic Solutions
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APF substation centralized compensation mode, the active filter with a dynamic response speed of less than 20ms, filter out 2-21 harmonics, real-time detection of the system harmonic current, the control command device issued the same reverse harmonic current balance system Harmonic current, filter out system harmonic current, remove system harmonic current, remove harmonic loss, stabilize system voltage, balance three-phase voltage and compensate reactive loss.
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Applications
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A city rail transit system, substation system harmonic control using centralized compensation methods.
It can be clearly seen from the current waveform of the grid before and after putting into the filter that most of the grid current harmonics are filtered out and the waveform is already a sine wave. The total current is reduced from 2343A to 2240A and the RMS current is reduced by 4.3%. The total current The distortion rate dropped from 25.2% to 3.3%, the compensation rate reached 89% and the power factor increased from 0.95 to 0.99. According to the allowable value of the harmonic current of the national standard grid, it can be clearly seen that each harmonic current before the compensation significantly exceeds the requirements of the national standard by several times, and each harmonic current after compensation can meet the requirements of the national standard. For rail transit power supply system, safe operation is the first place. Active filter input, the distribution system harmonics have been very good to suppress, improve the reliability of the power supply system to eliminate the impact of harmonics on the rail transit system to protect the safe operation of the urban rail system to improve the energy Quality, overall improve the operational efficiency of rail transit system equipment and the use of electricity and energy efficiency.
