Sep 30, 2020
On how to apply UPS systems more environmentally friendly: UPS basic principles and pollution
Abstract: This article briefly introduces the basic structure of UPS, which is the advanced application of inverters, focuses on the analysis of the pollution caused by UPS to the power grid and the environment and its causes, and finally gives several more advanced methods to eliminate such pollution. Due to the long length of the environmental protection technical measures, it will be divided into basic principles and explanations of pollution types and the ways and differences of environmental protection measures.
Keywords: UPS, pollution, environmental protection, current harmonics, power factor, superconducting energy storage, flywheel energy storage, inverter
UPS means uninterruptible power supply system, which can provide continuous and stable power to the load. With the popularization of electrical equipment such as computers and precision electronic instruments, and the increasing requirements for power supply quality in important places such as telecommunications, hospitals, banks, stadiums, and airports, UPS has been widely used and has gradually developed into a highly reliable, High-performance, highly automated local power supply center. But with the massive use of UPS, the pollution caused by UPS to the power grid and the environment has gradually emerged. Today, with the increasingly strong awareness of environmental protection, people continue to research and develop new environmental protection technologies to replace the original technologies, making UPS gradually become a true environmental protection power source.
2. The basic principle of UPS
Generally speaking, UPS consists of five major parts: rectifier circuit, energy storage mechanism, inverter circuit, bypass switch circuit and measurement and control circuit.
Figure 2.1 UPS structure
Rectifier circuit: Convert alternating current to direct current to complete the charging of the energy storage mechanism and at the same time supply power to the load through the inverter
Energy storage mechanism: The energy storage mechanism is the core part of the UPS. When the mains power is normal, the energy storage mechanism absorbs energy from the grid and stores it; when the mains power is interrupted, the energy storage mechanism releases electrical energy for use by the inverter.
Inverter circuit: Convert the DC voltage obtained by the rectifier circuit or the voltage of the energy storage mechanism into AC voltage.
Bypass switch: It is an electrical conversion device for mains bypass power supply and inverter power supply.
Measurement and control circuit: It is the brain of the UPS, monitoring the input voltage and current level and controlling the output voltage and current accuracy; setting and controlling the rectifier and inverter; controlling the charging and discharging of the energy storage mechanism; controlling the connection between the main circuit and the bypass Conversion.
3. Pollution caused by UPS
3.1. Pollution to the grid
General UPS rectifier circuits often use thyristor phase-controlled rectifier circuits. Commonly used rectifier circuits include three-phase full-bridge six-pulse rectifier circuits, six-phase full-bridge twelve-pulse rectifier circuits, etc. The structure of the phase-controlled rectifier circuit is simple and the control technology is mature, but due to the low AC input power factor and the injection of a large amount of harmonic current into the grid, it will cause greater pollution to the grid.
3.1.1 High harmonic content
The phase-controlled rectifier circuit utilizes the conduction and cut-off effects of the rectifier element to short-circuit and cut off the current to achieve the purpose of changing the output voltage, which will generate harmonic current.
In addition, in UPS, AC mains input is generally rectified. After rectification, large-capacity capacitors are used for filtering to smooth the output voltage (batteries are also connected in parallel in UPS). Only when the voltage is higher than the voltage across the filter capacitor, filtering The capacitor only begins to charge, which forms a pulse current with a narrow width during the capacitor charging period. This current not only lags behind the power supply voltage seriously, but also has a large harmonic component.
3.1.2 Low input power factor
Medium and large UPS are generally dual-inverter on-line structure. The input rectifier adopts a three-phase full-bridge six-pulse controllable rectifier circuit, and its input power factor is determined by the commutation overlap angle γ and the control angle α. The commutation overlap angle γ refers to the time for the two-phase voltages in the three-phase rectifier circuit to conduct together; the control angle α represents the trigger delay time, that is, the thyristor does not conduct from the zero crossing of the sine wave to the triggering of the thyristor. time.
The power factor of the rectifier is mainly related to the cosine of the control angle. The smaller the control angle, the greater the power factor; otherwise, the lower the power factor.
In fact, in the rectifier circuit, in addition to the phase difference between the rectified voltage and the rectified current. There is also the problem of current waveform distortion caused by higher harmonic currents, which can be calculated by the current distortion coefficient µ.
Because the high frequency switching rectifier power supply is in the form of peak rectification, its input current is a very narrow high current pulse wave, the harmonic component is large, and the current distortion coefficient µ is very low, so its power factor PF is also very low.
3.1.3 The harm of high harmonic content and low power factor
After a large amount of harmonic current surges into the power grid, it will increase the additional loss of the line, cause the line to overheat and accelerate the aging of the insulating medium, leading to insulation damage. In addition, the harmonic current will produce active power loss when passing through the grid, which is very detrimental to the economic operation of the grid. In addition, the capacitive reactance of the parallel capacitors installed in the power grid will decrease with the increase of the harmonic order, which will cause the overcurrent heating of the capacitors to increase the insulation breakdown failure.
There are distributed capacitances and power factor compensation capacitors in the power system, and harmonic currents may stimulate local series resonance or parallel resonance, which will directly damage the safe operation of the entire system.
When the diesel generator set is matched with the UPS, the large number of higher harmonics reflected by the UPS to the diesel generator set, especially the 5th and 11th harmonics, will cause serious harm to the diesel generator set, making the efficiency of the diesel generator set greatly reduce.
A large number of harmonics will use electrical equipment to run abnormally or fail to operate normally; harmonics will also interfere with communication systems, reduce signal transmission quality, disrupt signal transmission, and even damage communication equipment.
A low power factor will cause the voltage of the power grid to drop, and electrical equipment will not be fully utilized. A large amount of reactive current flowing on the line takes up line resources, reducing the line's ability to transmit active current, increasing additional losses, and reducing power generation, transmission and users Equipment efficiency.
3.2. UPS pollution to the environment
At present, batteries are widely used in UPS as a device for storing electric energy. The battery needs to be charged with a DC power supply first, and the electrical energy is converted into chemical energy for storage. When the mains power is interrupted, the UPS will rely on the energy stored in the battery to maintain the normal operation of the inverter. At this time, the battery will discharge chemical energy into electrical energy for use by the UPS.
There are three types of batteries used in UPS: open-type liquid lead-acid batteries, sealed maintenance-free lead-acid batteries, and nickel-cadmium batteries.
The positive electrode active material of the open type liquid lead-acid battery is lead peroxide, the negative electrode active material is spongy lead, and the electrolyte is concentrated sulfuric acid. During the charging process of the battery, the mixture of sulfuric acid vapor, water vapor, hydrogen, and oxygen generated inside the battery will escape and diffuse into the air. During the manufacturing process of lead-acid batteries, a large amount of solid waste, waste water containing sulfuric acid and heavy metals, and a large amount of exhaust gas containing lead dust and lead smoke are generated. Lead and lead oxides in lead-acid batteries enter the human body in the form of dust and smoke during the production and use of the battery through the respiratory tract and suction tract. Lead is the only trace element that is not needed by the human body. It is stable and non-degradable. It has certain effects on the human nervous system, digestive system, hematopoietic system and kidneys.
Although manufacturers of sealed, maintenance-free lead-acid batteries adopt various methods to reduce the escape of sulfuric acid vapor, water vapor, hydrogen and oxygen and other mixtures so that they can be digested inside the battery as much as possible, absolute control is impossible; the same is true for the sealed type The working principle of the maintenance-free lead-acid battery still continues the traditional lead-acid battery, and the pollution to the environment is inevitable by using the same reactive substance.
The positive polarity of the cadmium nickel battery is high-valent nickel hydroxide, and the negative polarity is the sponge metal cadmium, and an aqueous solution of potassium hydroxide or sodium hydroxide is used as the electrolyte. Cadmium is an important industrial and environmental pollutant, mainly from the smelting of zinc, copper, and lead ore, electroplating, storage batteries, alloys, paint and plastics and other industrial production. The main sources of cadmium contamination are food and inhalation. Cadmium is a non-essential and toxic element for the human body, or an IA-level carcinogen. It has carcinogenic, teratogenic and mutagenic effects. The biological half-life of cadmium in the body is as long as 10-30 years. It is known to be the most toxic substance that is easily accumulated in the body . The continuous accumulation of cadmium can cause various diseases in contacts, and can also cause tumors in the lungs, prostate and testes.