Solution for arc-free switching based on arc-free on-load voltage regulation

Traditionally, there are a lot of problems and defects in the transformer on-load voltage tap changer in the state of combining mechanical components with electric mechanisms, among which the high failure rate and slow response action are the main ones. More importantly, during the transformer voltage regulation process, affected by the arc force factor, it is very likely to cause problems such as contact erosion, which will not only cause oil contamination problems, but also restrict the realization of transformer functions to a large extent. The introduction of high-power power electronic switches to on-load tap changers can meet the arc-free characteristics of voltage regulation operation, thereby achieving the problem of improving the operation quality and level of power transformers. This article analyzes and discusses the above problems in detail and provides corresponding solutions.

1. Research status

The research on improving the tap switching of on-load tap-changing transformers based on the current characteristics of power electronic switches began in the mid-1970s. However, due to factors such as manufacturing costs and reliability of operation, this technology could not be implemented in practical applications. However, with the development and improvement of power electronic technology, the application of related technologies in power systems has become more perfect, making the feasibility of the transformer arc-free on-load tap-changing scheme more prominent.

Compared with my country, the research on arc-free on-load tap-changers started relatively early in China, and the research results achieved are more prominent and typical. Among them, the TADS switching switch based on the auxiliary effect of thyristors developed and produced by the Austrian Ilin On-Load Tap Changer Company is the most prominent. Under the auxiliary effect of thyristors, the operation of combined mechanical contacts can meet the comprehensive advantages of tap switches. Traditionally, the functions corresponding to arc contacts can be replaced by thyristors as switching elements. During the operation of the entire line, the switching elements based on thyristors can achieve the purpose of breaking the current during the switching process, and form a contact system corresponding to the switching switch in combination with mechanical contacts, thereby meeting the operation function of arc-free on-load tap-changers.

In the TADS-based switch, each phase corresponds to a thyristor component, and the mechanical contacts are equivalent to the main contacts in the conventional circuit system. When the entire circuit is in a static state, the current can flow through the mechanical contacts. At the same time, during the switching action of the current path, the opening and closing actions of all mechanical contacts are based on the basic premise of operating without current. From this perspective, the quality problems such as contact burning and oil contamination can be avoided to the greatest extent during the operation of the circuit, and the loss caused by frequent line maintenance can be fundamentally avoided.

However, the system also has certain shortcomings, mainly manifested in: the on-load voltage regulation function is realized based on thyristor components and mechanical contacts. Although the "arc-free" requirements can be met during the implementation of arc-free switching, it still belongs to a hybrid structure based on mechanical and electronic devices, which makes the operation and control actions relatively complex and difficult.

2. Analysis of the voltage regulation scheme based on no-arc on-load

Based on the above analysis of the operation of distribution transformers based on high-power power electronic switches, the author believes that:

In order to meet the requirements of arc-free on-load voltage regulation, it can be based on the high-voltage winding of the transformer and realize the effect of arc-free switching according to different connection methods. There are two specific solutions:

Solution A: The key to achieving arc-free switching in this solution is that the high-power power electronic switch bidirectional thyristor is based on the high-voltage winding of the transformer to achieve joint processing with the conventional tap selector. The schematic diagram of the arc-free switching working principle under this solution is shown in the figure below (see Figure 1).

In Figure 1, X1 and X2 are taps corresponding to the high-voltage winding of the transformer. R represents the transition resistance during the operation of the circuit, which can play a current limiting role relative to the high current transmitted by the high-voltage winding side of the transformer. SCR1 and SCR2 are thyristors corresponding to contactless power electronic switches, which correspond to a bidirectional relationship. ST is a tap selector, which supports the selection of taps when the line is in a no-load operating state.

The main working principle of scheme A to achieve arc-free on-load voltage regulation operation of distribution transformer is: if the high-voltage winding of the transformer is in normal operation, the X1 tap is used as the working carrier, and the corresponding SCR1 thyristor in Figure 1 is in the on state, and no current is detected in the 1-ST-R section path connected to the transition resistor. At the same time, the SCR2 thyristor is in the blocking state. Under the above operating conditions, the system voltage changes. In order 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 operating scheme in this state is: the ST tap selector switches from "1" to "2" in the no-load operation state, and switches the SCR1 thyristor to the off state, forming the X2-2-ST-R section path as shown in Figure 1. The operation of this section path triggers the SCR2 thyristor and causes it to be in the on state, and the current path formed is the X2-SCR2 section, which meets the function of one-time tap conversion.

Solution B: The key to achieving arc-free switching in this solution is to replace the ST tap selector in Solution A with a high-power solid-state relay group device as the carrier for completing the tapping task. The following figure (see Figure 2) is a schematic diagram of the arc-free switching working principle corresponding to Solution B.

The advantages of this solution are:

It can eliminate the moving parts and electric mechanisms involved in the mechanically operated voltage regulating transformer, and avoid the frequent failure of the electric mechanism during circuit operation. At the same time, the tap selection and switching action under the operation of the circuit can also be realized by software control.

The main working principle of scheme A to achieve arc-free on-load voltage regulation operation of distribution transformers is: when the high-voltage winding of the transformer is in normal operation, with the X1 tap as the working carrier, the corresponding SCR1 thyristor in Figure 2 is in the on state, and the X1-SCR1 section in Figure 2 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 system voltage fluctuation, in order to ensure the stability and reliability of the power supply quality on the low-voltage side of the transformer, under the condition of logical judgment, the transformer operating condition needs to be converted to the X2 tap joint. The operating scheme in this state is: the solid-state relay SSR2 is triggered and SSR1 is turned off synchronously. The current path formed in this state is expressed as the X2-SSR2-R section. On this basis, SCR2 is triggered to be in the on state, and the current path formed in this state is expressed as the X2-SCR2 section, which meets the function of one-time tap conversion.

3. Conclusion

Based on the analysis of the theoretical key points of high-power power electronic switch terminology distribution transformer, this paper proposes two different arc-free switching working schemes that can meet the arc-free on-load voltage regulation requirements. Among them, Scheme A belongs to the mechanical and electronic hybrid voltage regulation operation mode. Although it can meet the working requirements of arc-free switching, the structural setting is relatively complex, and it is prone to action delays and various faults during operation. As for Scheme B, due to the introduction of solid-state relay groups to participate in the operation, the moving parts and electric mechanisms involved in the mechanical operation voltage regulation transformer can be eliminated, avoiding the frequent occurrence of electric mechanism failures during circuit operation. In actual operation, the action is fast and the structure is simple. Especially for distribution transformers below 35kV voltage level, the advantages are outstanding. Scheme B can be used as the basic scheme for realizing arc-free on-load voltage regulation. The above issues are expected to attract the attention and attention of all parties.