0 Preface
Traditionally, there are a large number of problems and defects in the transformer on-load tap changer in the combined state of the mechanical component and the motor-driven mechanism, with a higher failure rate and a slower response action. More importantly: during the transformer voltage regulation process, it is affected by the arc force factor, which may cause problems such as ablation of the contacts, which will not only cause oil pollution problems, but also greatly To a certain extent, it restricts the realization of transformer function. However, the introduction of high-power electric electronic switches for on-load tap-changers can meet the arc-free characteristics of voltage-regulating operation, thereby improving the quality and level of power transformer operation. This article is to analyze and discuss the above problems in detail, and give corresponding solutions.
1. Research status
At home and abroad, research on the current characteristics of power electronic switch based on the improvement of the tap change of the on-load tap changer began in the mid-1970s, but it was affected by many factors such as manufacturing cost and operational implementation reliability. Technology cannot be implemented in practical applications. However, with the development and improvement of power electronics technology, the application of the power system to the related technology is more perfect, which makes the feasibility of the transformer arcless on-load voltage regulation scheme more prominent.
Compared with China, the domestic research on arc-free on-load voltage regulating switch has started relatively early, and the research results obtained are more prominent and typical. Among them, a TADS switch based on the thyristor developed by the Austrian Yilin on-load tap-changer company is the most prominent. Under the auxiliary action of the thyristor, the operation of the combined mechanical contact can meet the tap changer. Comprehensive advantages. Traditionally, the function of the arcing contact can be replaced by a thyristor as a switching element. During the operation of the whole line, the thyristor-based switching element can achieve the purpose of breaking current during the switching process, and the contact system corresponding to the switching switch is formed in combination with the mechanical contact, thereby satisfying the operation of the arcless on-load voltage regulation. Features.
In the TADS-based switch, each phase maintains a corresponding relationship with a thyristor component, which is equivalent to the main pass contact in a conventional circuit system. Current can flow through the mechanical contacts while the entire circuit is in a stationary state. At the same time, during the switching operation of the current path, the breaking action of all mechanical contacts is based on the operation without current. From this point of view, during the operation of the circuit, the quality problems of contact burning and oil body pollution can be avoided to the utmost, and the loss caused by frequent maintenance of the line is fundamentally avoided.
However, the system also has certain deficiencies, mainly as follows: based on the thyristor components and mechanical contacts, the on-load voltage regulation function is realized, although the "no arc" can be satisfied during the implementation of the arcless switching action. The requirements of the aspect, but still belong to the structure based on the mechanical and electronic mixing, which makes the operation and control actions relatively complicated and difficult.
2. Analysis based on arc-free on-load voltage regulation scheme
Based on the above analysis of the operation of distribution transformers based on the power of high-power electric electronic switches, the author believes that:
In order to meet the relevant requirements of arc-free load regulation, it can be built on the basis of the high-voltage winding of the transformer, and the effect of arc-free switching can be realized according to different connection modes. Specifically, there are two options:
Scheme A: The key to the arc-free switching effect of this scheme is that the high-power power electronic switch bidirectional thyristor is built on the high-voltage winding of the transformer to realize the joint processing with the conventional tap selector. The schematic diagram of the non-arc switching operation corresponding to this scheme is shown in the figure below (see Figure 1).
In Fig. 1, X1 and X2 are the taps corresponding to the high voltage winding of the transformer, and R represents the transition resistance during the operation of the circuit, which can exert a current limiting effect on the high current transmitted by the high voltage winding side of the transformer, SCR1 and SCR2. The thyristor corresponding to the non-contact power electronic switch corresponds to a bidirectional relationship, and the ST is a tap selector. The selector supports the selection of the tap under the unloaded operating state of the line.
The main working principle of the A scheme to realize the arc-free on-load voltage regulation operation of the distribution transformer is: if the X1 tap is used as the working carrier under the normal operation state of the transformer high-voltage winding, the corresponding SCR1 thyristor in Figure 1 is in the on state. No current is detected in the 1-ST-R section path such as the connection transition resistor. At the same time, the SCR2 thyristor is in a blocking state. Under the above operating conditions, the system voltage changes. To ensure the stability and reliability of the output voltage level on the low-voltage side, it is necessary to use the X2 tap as the working carrier for the distribution transformer. The operation scheme in this state is: ST tap selector switches from "1" to "2" in the no-load operation state, and switches the SCR1 thyristor to the off state to form X2-2- as shown in Fig. 1. ST-R segment path. The operation of the segment path triggers the SCR2 thyristor and causes it to be in a conducting state, and the formed current path is an X2-SCR2 segment, which satisfies the function of one-time tap conversion.
Scheme B: The key to achieve the arc-free switching effect of this scheme is to replace the ST tap selector in the A scheme with a solid-state relay group device based on high power as a carrier for completing the tapping task. The following figure (see Figure 2) is a schematic diagram of the arc-free switching principle corresponding to the B scheme.
The advantages of this program are:
It can eliminate the moving parts and motor-drive mechanism involved in the mechanically operated voltage-regulating transformer, and avoid the problem that the motor-drive mechanism malfunctions frequently during the operation of the circuit. At the same time, the tap selection operation and the switching action under the operation of the circuit can also be implemented by means of software control.
The main working principle of the A scheme to realize the arc-free on-load voltage regulation operation of the distribution transformer is as follows: under the normal operation state of the transformer high-voltage winding, the X1 tap is used as the working carrier, and the corresponding SCR1 thyristor in Figure 2 is in the conduction state, then In Figure 2, the X1-SCR1 segment is in the operating state of the current path. In the same case, SSR1S, SSR2, and SCR in Figure 2 are all in the off state. In other words, in the case of fluctuations in the system voltage, in order to ensure the stability and reliability of the power supply quality of the low-voltage side of the transformer, under the condition of logical judgment, it is necessary to convert the operating conditions of the transformer into the X2 tap joint. The operation scheme in this state is: the solid state relay SSR2 is triggered, and the SSR1 is synchronously turned off, and the current path formed in this state is represented by the X2-SSR2-R section. On this basis, the trigger SCR2 is in the on state, and the current path formed in this state is represented as the X2-SCR2 segment, which satisfies the function of one-time tap conversion.
3. Conclusion
Based on the analysis of the main points of the theory of power distribution transformers for high-power power electronic switches, this paper proposes two different arc-free switching schemes that can meet the requirements of arcless on-load voltage regulation. Among them, the A scheme belongs to the mechanical and electronic hybrid voltage regulation operation mode. Although it can meet the working requirements of arcless switching, the structure setting is relatively complicated, and the operation delay and various fault problems are prone to occur during operation. For the B scheme, since the solid-state relay group is introduced to participate in the operation, the moving parts and the motor-driven mechanism involved in the mechanically-operated voltage-regulating transformer can be eliminated, and the problem of frequent occurrence of the motor-drive mechanism during the operation of the circuit can be avoided. The operation is fast and the structure is simple. Especially for the distribution transformer below 35kV voltage level, the advantage is outstanding. The B scheme can be used as the basic scheme to realize the arc-free load regulation. The above problems are expected to attract the attention of all parties. With emphasis.
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