How to limit the current of grounding transformer

In power system, grounding transformer is a common equipment, its main function is to provide a grounded neutral point for neutral ungrounded or high impedance grounding system, so as to provide a path for zero-sequence current when a single-phase grounding fault occurs in the system. However, in actual operation, the grounding transformer may also face the problem of excessive current during short circuit or grounding fault. Therefore, it is of great significance to take appropriate current limiting measures to protect the safety of equipment and ensure the stable operation of the system.

First, the basic principle of grounding transformer

Grounding transformer usually adopts Z-shaped wiring (also called zigzag wiring) structure, and each phase of it consists of two windings, which are wound on different iron core columns respectively. This structure makes it possible to form a zero-sequence magnetic flux path and generate a zero-sequence current at the neutral point when a single-phase grounding fault occurs, thus providing a signal for the protection device and realizing fault location and removal.

Second, the current limiting requirements in grounding transformer

In the power system, when a short circuit or grounding fault occurs, the fault current may quickly rise to several times or even ten times the rated current, which will not only damage the grounding transformer itself, but also affect the stability of the whole system. Therefore, in the design and application of grounding transformer, we must consider how to effectively limit short-circuit current.

Three, commonly used current limiting methods of grounding transformer

1. Series current limiting reactor
It is one of the most common current limiting measures to connect a current limiting reactor in series between the neutral point of the grounding transformer and the ground. The reactor can limit the growth speed and amplitude of zero-sequence current when a fault occurs, thus reducing the impact on system equipment. The choice of reactor should be accurately calculated according to the short-circuit capacity and protection requirements of the system.

2. Use high impedance grounding mode.
The grounding fault current can be effectively limited by connecting an appropriate resistor in the loop of the grounding transformer. This method is called high impedance grounding, which can reduce the damage of fault current to equipment and maintain the continuous operation ability of the system.

3. Adopt controllable current limiting device.
In recent years, with the development of power electronics technology, controllable current limiter (such as solid state current limiter) has been gradually applied to high voltage system. This kind of device can respond quickly and limit short-circuit current in millisecond time, and has the advantages of fast response speed and high control accuracy.

4. Optimize the system operation mode
In the system design stage, the location and capacity of grounding transformers should be reasonably selected to avoid multiple grounding transformers running in parallel on the same bus, which can also limit the short-circuit current to some extent.

Iv. Application examples of current limiting measures

Taking a 110kV substation as an example, the substation adopts Z-connected grounding transformer, and a combined device of current limiting resistor and reactor is installed at its neutral point. In a single-phase grounding fault, the current limiting device successfully limits the fault current to the tolerable range of the protection device, avoiding equipment damage and system tripping, which fully embodies the practical value of current limiting measures.

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To sum up, grounding transformer is an indispensable equipment in power system, and its current limiting ability directly affects the safety and reliability of the system. By reasonably selecting current limiting elements, optimizing design and operation strategy, the short-circuit current level can be effectively controlled, and the stability of the system and the service life of the equipment can be improved. In the future development of power system, with the progress of smart grid and flexible transmission technology, the current limiting technology of grounding transformer will also develop in a more efficient and intelligent direction.