Power transformer; On-load voltage regulation; Technical analysis
The definition of on-load voltage regulation technology of power transformer is a transformer that can adjust the ratio under load. Transformers with on-load voltage regulation are all static electrical devices, which convert AC voltage in a certain range into another or several different digital voltages.
1 traditional on-load voltage regulation method
In the traditional sense, the on-load voltage regulating device of transformer adopts mechanical tap changer, taking double transition resistance as an example. When the tap is selected, the transfer switch switches from right to left or from left to right. The action of mechanical switch transmission gear is easy to cause operation accidents, which will weaken the reliability of transformer and bring certain safety hazards to work. In addition, when the mechanical switch moves, an arc can be formed, and a certain arc will cause chronic ablation of the mechanical switch contacts, so when the operation reaches a certain number of times, the contacts must be replaced. Another problem that we can't ignore is that the generated arc will lead to the decrease of transformer oil quality, thus weakening the insulation capacity of windings in the transformer, resulting in phase-to-phase short circuit or turn-to-turn short circuit. Some research data show that in the period of traditional on-load voltage regulation, tap-changer accidents and failures account for 10% to 20% of the total number of transformer accidents every year, and the failure rate of 500 kV transformer contact switches was as high as 25% at one time, with high frequency of accidents and failures. Because the reaction time of mechanical switch action is generally about 5 seconds, it takes a long time, so the transformer with traditional on-load voltage regulation technology can only be used for voltage regulation in stable state.
2 new on-load voltage regulation method
Due to the above shortcomings of traditional mechanical switches, countries are actively developing new on-load voltage regulating devices, which can be divided into three types according to the types of taps: mechanical improvement, electronic switch type and auxiliary coil type.
(A) Mechanical improvement of on-load voltage regulation technology
This kind of transformer is transformed from the traditional transformer and switching electronic circuit. Its tap-changer only needs a few thyristors and a transition resistor, and the mechanical switch and the electronic switch cooperate with each other to limit the generation of arc in operation. Thyristors are connected to both sides of the traditional switch. When a branch is disconnected, the contact voltage triggers the thyristor. The gate current provided by the diode flows through the branch at the other end. Because the current crosses zero and the thyristor is turned off, no arc will be generated. When the switch branch is closed, the current of the original branch is reduced due to the shunt at the thyristor side, which can also play the role of arc limiting. The advantage of this kind of device is that it does not need time control loop, and the capacity requirement of thyristor is also very low. If the thyristor is out of control, it will not damage the tap of the transformer, but the disadvantage is that the running speed is not fast.
(B) auxiliary coil on-load voltage regulation technology
In fact, as early as the 1980s, some researchers suggested that the three-phase transformer and the step-up transformer could be connected with each other by controlling the conduction angle of the thyristor and superimposing an adjustable voltage on the transformer. At the same time, the reverse thyristor can be used as the connecting medium. If there is no delay in triggering the thyristor, it will trigger after zero crossing, and the voltage will be loaded on the load in the same phase. If the trigger of thyristor is delayed, the preset short-circuit switch can prevent the boost transformer from opening. Later, on the basis of this theoretical research, other scholars reformed the transformer and added a certain auxiliary voltage to ensure that the superimposed voltage is in phase with the original voltage. Almost at the same time, someone in Siemens proposed another on-load voltage regulation method based on auxiliary coil to realize arc-free operation. The specific method is to use a coil with adjustable rated voltage to couple into the transformer, and the voltage regulation range is 0.63%.
(3) On-load voltage regulation technology of electronic switch
Due to the development and progress of electronic power technology, the capacity of thyristor has been raised to a new development level. The electronic power switch of thyristor can be directly controlled by microprocessor, and it can be assisted without mechanical switch. Instead, the power consumption of the thyristor is reduced as much as possible by properly selecting the trigger time. At present, this technology is only in the experimental stage, and the problems such as thyristor damage and short circuit caused by thyristor fault still need to be studied and solved.
(4) Comparative analysis of three new voltage regulating technologies for power transformers.
Mechanical improvement of on-load voltage regulation is characterized by high economy, low harmonic content, slow speed, arc limitation and no influence on transformer work when thyristor is out of control. On-load voltage regulation of auxiliary coil is characterized by many harmonics and unlimited arc; On-load voltage regulation of electronic power switch has the characteristics of high speed, great influence of thyristor out of control, low harmonic content and high price. It can be seen that the three on-load voltage regulation methods have their own advantages and disadvantages. At present, there is no systematic research on the on-load voltage regulation technology of new power transformers in China. If we can learn from some advanced foreign research results and improve the on-load voltage regulation technology in combination with the actual situation of local science and technology and economy, the working performance of power transformers will be greatly improved.
3 summary:
The main circuit design of on-load tap-changer should take care of heat dissipation, protection and trigger as much as possible to ensure the reliability of the device. In addition, we should pay attention to the cost requirements when adopting this technology.
References:
[1] Marcheshi Power supply and distribution project [M]. Tsinghua University Publishing House, 2009, (08).
[2] Ministry of Human Resources and Social Security Textbook Office. Maintenance electrician [M]. China Labor and Social Security Press, 2008, (1 1).
Zhang Pinxiu, Liang Chunying, Zhao Bin. Design of on-load tap-changer based on thyristor anti-parallel connection [J]. Coal Science and Technology, 20 10, (0 1).
, Cui, Chen Xiao. Study on Non-Surge Current in the Transition Process of Contactless On-Load Voltage Regulation [J]. Electrician and Electricity, 20 10(0 1)