Introduction to the working principle and characteristics of rectifier transformer

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Introduction to the working principle and characteristics of rectifier transformer [Copy link]

Principle of rectifier transformer The principle of rectifier transformer is the same as that of ordinary transformer. Transformer is a device that converts AC voltage based on the principle of electromagnetic induction. Transformer generally has two independent windings, primary and secondary, which share a common iron core. When the primary winding of the transformer is connected to the AC power supply, the alternating current flows through the winding to generate magnetic potential, so there is an alternating magnetic flux in the closed iron core. The primary and secondary windings cut the magnetic lines of force, and the secondary can induce the same frequency of AC. The turn ratio of the primary and secondary windings of the transformer is equal to the voltage ratio. For example, the primary winding of a transformer is 440 turns and the secondary is 220 turns. The primary input voltage is 220V, and the output voltage of 110V can be obtained at the secondary of the transformer. Some transformers can have multiple secondary windings and taps, so that multiple output voltages can be obtained. Characteristics of rectifier transformers A transformer that forms a rectifier device with a rectifier to obtain DC power from an AC power source. Rectifier devices are the most commonly used DC power sources in modern industrial enterprises and are widely used in DC transmission, electric traction, steel rolling, electroplating, electrolysis and other fields. The primary side of the rectifier transformer is connected to the AC power system, called the grid side; the secondary side is connected to the rectifier, called the valve side. The structural principle of the rectifier transformer is the same as that of an ordinary transformer, but because its load rectifier is different from the general load, it has the following characteristics: (1) Each arm of the rectifier is turned on alternately in one cycle, and the conduction time only occupies a part of one cycle. Therefore, the current waveform flowing through the rectifier arm is not a sine wave, but is close to an intermittent rectangular wave; the current waveforms in the primary and secondary windings are also non-sinusoidal waves. The figure shows the current waveform when the three-phase bridge Y/Y connection is used. When using thyristor rectification, the larger the lag angle, the greater the steepness of the current fluctuation, and the more harmonic components in the current, which will increase the eddy current loss. Since the conduction time of the secondary winding only occupies a part of a cycle, the utilization rate of the rectifier transformer is reduced. Compared with ordinary transformers, under the same conditions, the volume and weight of the rectifier transformer are larger. (2) The primary and secondary power of ordinary transformers are equal (ignoring losses), and the capacity of the transformer is the capacity of the primary winding (or secondary winding). However, for rectifier transformers, the power of the primary and secondary windings may be equal or unequal (when the primary and secondary current waveforms are different, such as half-wave rectification), so the capacity of the rectifier transformer is the average value of the primary and secondary apparent powers, which is called equivalent capacity, that is, S1 is the primary apparent power and S2 is the secondary apparent power. (3) Compared with ordinary transformers, the ability of rectifier transformers to withstand short-circuit electric power must strictly meet the requirements. Therefore, how to make the product have short-circuit dynamic stability is an important issue in design and manufacturing. Electrochemical industry----This is the industry that uses rectifier transformers the most. It electrolyzes non-ferrous metal compounds to produce aluminum, magnesium, copper and other metals; electrolyzes salt to produce chlorine alkali; and electrolyzes water to produce hydrogen and oxygen. DC power supply for traction----DC power grid used for mines or urban electric locomotives. Since the valve side is connected to the overhead line, there are many short-circuit faults, the DC load changes greatly, and the electric locomotive is often started, resulting in varying degrees of short-term overload. For this reason, the temperature rise limit and current density of this type of transformer are both low. The impedance is about 30% larger than that of the corresponding power transformer. DC power supply for transmission----mainly used to power DC motors in electric transmission, such as the armature and excitation of rolling mills. DC power transmission----the voltage of this type of rectifier transformer is generally above 110kV, and the capacity is tens of thousands of kilovolt-amperes. Special attention should be paid to the superposition of AC and DC insulation to the ground. In addition, there are DC power supplies for electroplating or electrical processing, DC power supplies for excitation, DC power supplies for charging and electrostatic dust removal, etc. Reasons for the use of rectifier transformers In the chemical industry where rectifier transformers are used the most, high-power rectifier devices also have low secondary voltages and large currents, so they are similar to electric furnace transformers in many aspects, that is, the structural characteristics mentioned above, and rectifier transformers also have the same characteristics. The biggest feature of the rectifier transformer is that the secondary current is not a sinusoidal alternating current. Due to the unidirectional conduction characteristics of the subsequent rectifier elements, each phase line no longer flows with load current at the same time, but conducts soft current. The unidirectional pulsating current is converted into direct current through the filter device. The secondary voltage and current of the rectifier transformer are not only related to the capacity connection group, such as the commonly used three-phase bridge rectifier circuit and the double-reverse rectifier circuit with a balancing reactor. For the same DC output voltage and current, the secondary voltage and current of the rectifier transformer are different. Therefore, the parameter calculation of the rectifier transformer is based on the rectifier circuit. Generally, the parameter calculation is calculated from the secondary side to the primary side. Since the rectifier transformer winding current is non-sinusoidal and contains many high-order harmonics, in order to reduce the harmonic pollution to the power grid and to improve the power factor, the pulse number of the rectifier equipment must be increased, which can be solved by the phase shift method. The purpose of phase shift is to make a phase shift between the same-name end line voltages of the secondary winding of the rectifier transformer. Phase shifting method of rectifier transformer The simplest phase shifting method is to use two windings connected in magnitude and angle on the secondary side, which can double the pulse number of the rectifier furnace. For high-power rectifier equipment, more pulses are required. The number of pulses is 18, 24, 36, etc., which is increasing. Therefore, it is necessary to set a phase shifting winding on the primary side of the rectifier transformer for phase shifting. There are three ways to connect the phase shifting winding with the main winding, namely, zigzag line, hexagon and extended triangle. The voltage regulation range of the rectifier transformer used in the electrochemical industry is much larger than that of the electric furnace transformer. For chemical salt electrolysis, the voltage regulation range is usually 55%--105%, and for aluminum electrolysis, the voltage regulation range is usually 5%--105%. Commonly used voltage regulation methods include variable flux voltage regulation, series transformer voltage regulation and auto-coupling voltage regulation, just like the electric furnace transformer. In addition, due to the characteristics of the rectifier element, the phase angle of the silicon rectifier element can be directly controlled on the valve side of the rectifier furnace, and the average value of the rectifier voltage can be smoothly adjusted. This voltage regulation method is called phase-controlled voltage regulation. To achieve phase-controlled voltage regulation, one is to use a crystal valve tube, and the other is to use a self-saturated reactor. The self-saturated reactor is basically composed of an iron core and two windings. One is the working winding, which is connected in series between the secondary winding of the rectifier transformer and the rectifier, and the load current flows through it; the other is the DC control winding, which is provided by another DC power supply. The main principle is to use the nonlinear change of ferromagnetic materials to make the reactance value of the working winding change greatly. By adjusting the DC control current, the phase control angle α can be adjusted, thereby adjusting the average value of the rectifier voltage.