Oct 03, 2020
About how to apply UPS system more environmentally: the ways and differences of adopting environmental protection measures
4. Environmental protection measures
4.1. Reduce pollution to the grid
Modern UPS pays more and more attention to the awareness of environmental protection of the power grid. In terms of reducing harmonic pollution, reactive power loss, etc., different measures and methods can be adopted according to the difference of UPS power and circuit structure.
Traditional high-power UPS rectifiers mostly use thyristor phase-controlled rectification circuits, and passive filters are installed on the input side to absorb harmonics and improve power factor. However, due to the size and cost of the filter, the highest power factor can be achieved Increase to 0.9, current harmonic THD 5%, and passive filter suppression of harmonics is essentially a frequency domain processing method, that is, the non-sinusoidal periodic current is decomposed into Fourier series, and some harmonics are absorbed, so only It can suppress a fixed number of harmonics and compensate for a fixed reactive power. In view of the above shortcomings of passive filters, it is proposed to set up active filters on the UPS network side to compensate for harmonics and reactive power. Based on time domain analysis, the active filter tracks and compensates the distortion waveform in real time, so that the current waveform on the power supply side is consistent with the voltage waveform. Active filter has high controllability and fast response characteristics, and can compensate for each harmonic, automatically generate the required change of reactive power, its characteristics are not affected by the system, without adding capacitive components can avoid harmonic resonance in the system , Relative volume and weight are small.
The UPS circuit adopts high-frequency rectification technology, and through high-frequency PWM (PULS WIDTH MODULATION) control, the input current and the input voltage can be made the same phase, the grid-side power factor is 1, and the input harmonic current will also drop below 3%. The size of the high-frequency filter on the grid side is very small. As long as the frequency of the carrier is high enough, the stray inductance and small capacitance of the line can be used for filtering to achieve sinusoidal input current.
In addition, the UPS structure can be improved to avoid the traditional double-conversion on-line series cascade mode, and the advanced mode can be used to overcome the interference and influence of the larger power phase-controlled rectifier on the power grid, and it can also have a certain impact on the power grid. Regulation effect. For example, the use of high-frequency bidirectional conversion series-parallel compensation circuit structure.
The system consists of two inverters, both of which are high-frequency inverters capable of bidirectional conversion. The inverter (I) is actually a current source connected in parallel in the main circuit, which filters out reactive power and harmonics in the load current, and at the same time compensates for changes in the grid voltage. When the input voltage is higher than the output voltage, it absorbs power Form reverse polarity voltage compensation. When the input voltage is lower than the output voltage, the output power forms positive polarity compensation. The inverter (II) is a voltage source connected to both ends of the load to stabilize the output voltage and ensure that the load is provided with pure sine wave voltage. This function is completed together with the inverter (I). When the inverter (I) When the output power is compensated for positive polarity, the inverter (II) absorbs current from the grid and reversely transforms it to the inverter (I). When the inverter (I) absorbs power for reverse polarity compensation, the inverter The inverter (II) forwards the power absorbed by the inverter (I) to the load in the form of current. The inverter (II) also controls the voltage of the intermediate energy storage device, completes the charging of the energy storage device, and keeps the voltage at this point stable. In addition, the inverter (II) also compensates the reactive current and harmonic current at the load end to ensure that the current harmonic components at the load end are not transmitted to the input end. The high-frequency bidirectional conversion series-parallel compensation circuit can not only realize the sinusization of the input current, but also make the input power factor 1 or any value, and the operating efficiency of the system is also very high. This kind of UPS connection to the power grid will not only not cause the increase of the reactive power of the power grid, but also can make the grid reactive power regulation quantitatively.
In short, for low-power UPS, PFC rectifier and high-frequency PWM rectifier and their corresponding control technology can be used. For high- and medium-power UPS, it is more suitable to adopt high-frequency bidirectional conversion series and parallel compensation circuit structure, which has broad development and application prospects. .
4.2. Eliminate environmental pollution
In recent years, people have paid more and more attention to the environment. How to protect the environment while developing has become a major problem in social life. The large number of lead-acid and cadmium-nickel batteries used in UPS as energy storage devices has become a pollution source that damages the environment. The fundamental measure of UPS's pollution to the environment is to replace the original chemical batteries with environmentally friendly and pollution-free energy storage devices. Currently, there are two main emerging high-tech energy storage technologies: superconducting energy storage and flywheel energy storage.
4.2.1. Superconducting energy storage (SMES)
Superconducting materials have the characteristics of high current-carrying capacity and zero resistance, which can store large amounts of electrical energy for a long time without loss, and the stored energy can be continuously released when needed. On this basis, a superconducting energy storage system can be made. Superconducting energy storage devices generally consist of superconducting coils, cryogenic containers, refrigeration devices, rectifier inverter devices, and measurement and control systems.
The superconducting coil is the core component of the superconducting energy storage device, and it can be a toroidal coil or a toroidal coil. The spiral tube coil has a simple structure, but the surrounding stray magnetic field is relatively large; while the stray magnetic field around the toroidal coil is small, but the structure is more complicated. The superconducting coil directly stores electrical energy in the form of inductance. If the coil is wound by ordinary copper wire, the magnetic energy will be dissipated in the form of heat due to the existence of the coil resistance, but if the wire has superconducting properties (no resistance), the energy will last forever until needed. The ability to store current in the coil is determined by temperature and magnetic field strength. For most superconducting energy storage devices, the best operating temperature is 50-77K.
The operating principle of a complete superconducting energy storage system is very simple. First, the AC power provided by the grid is converted into DC power and added to the superconducting coil through the rectifier device. Therefore, when energy flows from the system into the coil, the DC voltage will affect the superconducting coil. The coil is charged and energy is stored in the coil. The amount of energy stored is determined by the design of the device. When the AC network needs to provide energy, the coil acts as a power source to release the stored energy, and the DC power is converted to AC power through the inverter.
Superconducting energy storage device is an advanced energy storage method. It stores electric energy in the magnetic field inside the superconducting coil. The magnetic field generated by the superconducting energy storage coil is very strong and the stored energy density is high. Energy storage and release The number of energy is basically unlimited. Because there is no chemical reaction in the superconducting energy storage system, no toxic substances will be produced during the operation, so it will hardly cause pollution to the environment. However, the realization of superconductivity is accomplished by reducing the temperature of the coil below its required temperature. As far as current technology is concerned, this temperature is very low. The use of niobium-titanium alloy superconducting coils requires keeping the temperature in the liquid. At the temperature of helium. Therefore, the maintenance cost for maintaining the low temperature required for the coil in the superconducting state is very expensive, which limits the popularization of superconducting energy storage applications. However, superconducting energy storage is still the research direction of many researchers.
4.2.2. Flywheel energy storage
A flywheel is a simple object rotating around an axis. The flywheel energy storage device is essentially a mechanical battery. The flywheel stores energy in the form of kinetic energy. Flywheel energy storage devices mainly include: flywheel, motor, shaft and bearing, vacuum container, rectifier, inverter, and measurement and control device. In the entire flywheel energy storage device, the flywheel is the core component, which directly determines the amount of energy stored in the entire device. The energy e stored in it is equal to the sum of the kinetic energy of the various parts of the flywheel, which is determined by the following formula.
E =1/2jω2 (1)
Where: j is the moment of inertia of the flywheel, which is related to the shape and weight of the flywheel;
ω is the angular velocity of the flywheel rotation
j = k*M*R2 (M: mass; R: radius); k = constant of inertia (determined by shape)
The inertia constants of different shapes are:
Rim k = 1
Solid disc with uniform thickness; k = 1/2
Solid sphere k = 2/5
Spherical shell k = 2/3
Thin rectangular bar k = 1/2
In order to reduce the loss during operation, improve the speed of the flywheel and the efficiency of the flywheel energy storage device, the bearings of the flywheel energy storage device generally use non-contact magnetic bearing technology, and the motor and flywheel are sealed in a vacuum container to reduce wind resistance. Usually the generator and the motor are realized by a single motor, which is directly connected to the flywheel through a bearing.
The most basic working principle of the flywheel energy storage device is to convert the electrical energy input from the grid into the kinetic energy of the flywheel rotation through the electric motor. When the measurement and control system senses that the grid-side power supply is abnormal or interrupted, it converts the kinetic energy of the flywheel through the generator. It is electric energy, which is output to an external load. The rectifier and inverter are bidirectional. When the energy is stored, the flywheel can be stored stably, and the output energy should meet the strict requirements of the load during the energy release process; when the grid-side power supply is restored When it is normal, the flywheel returns to the standby state, and the entire device can be operated with minimal loss.
Flywheel energy storage technology is relatively mature. Because of its safety, cleanliness, reliable work, high efficiency, long life, and low maintenance costs, it will gradually replace chemical energy storage devices and occupy a larger energy storage equipment market.
In summary, it is not difficult to see that the pollution of UPS to the power grid is mainly determined by its nonlinear characteristics. The several measures currently used to improve the power factor and reduce the harmonic current are all to correct or compensate the current waveform to make It presents linear load characteristics from the power supply side. However, due to the essential characteristics of UPS, various methods used today cannot fully realize the complete sinusoidal waveform of input current and input voltage. It is necessary to continuously research and develop new technical means; The main source of environmental pollution is heavy metals in its chemical energy storage batteries. The use of new superconducting energy storage and flywheel energy storage devices that do not contain pollution sources can fundamentally eliminate the environmental pollution of UPS.
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