The grounding transformer was selected by Yu Shushu from the Nanjing Power Supply Bureau in Jiangsu Province. Through an analysis of the structure type, capacity, and impedance voltage of the grounding transformer, a proper method for selecting this type of transformer was identified. It is essential to note the differences between a grounding transformer and a regular transformer, particularly when choosing the appropriate grounding transformer.
Key words: grounding transformer, structure type, capacity, impedance voltage, grouping, classification number, document identification code, article number.
Preface:
In a small current grounding power system, when the system's capacitive current reaches a significant level, the grounding fault current and the arc gap overvoltage may fail to self-extinguish, posing risks to system safety. Hence, it is crucial to limit these currents. Currently, there are two primary measures to achieve this. One involves grounding the neutral point of the power transformer in the substation through an arc suppression coil, providing inductive compensation to extinguish the grounding arc instantly. The other method grounds the neutral point of the power transformer through grounding resistance, injecting resistive current at the grounding point to alter the phase of the ground current and accelerate the discharge of residual charges, promoting the extinction of the grounding arc.
Additionally, this approach ensures sufficient zero-sequence current and voltage, enabling reliable grounding protection. However, in a small current grounding system, since the secondary winding of the power transformer in the substation is delta-connected, there is no neutral point to connect the arc suppression coil or grounding resistance. Consequently, a dedicated grounding transformer is required (referred to as a grounding change), creating an artificial neutral point to connect either the arc suppression coil or grounding resistance. During a system grounding fault, the compensation current generated by the arc suppression coil or grounding resistance is delivered to the power grid.
When selecting a grounding transformer, it is necessary to consider its high and low voltage side rated voltages, capacity, structure type, number, positive sequence impedance (or impedance voltage), oil immersion or dry type, and no-load loss, much like a regular transformer. However, the grounding transformer is not entirely identical to a regular transformer.
The primary function of a grounding transformer is to transmit the grounding compensation current of the arc suppression coil or grounding resistor. Thus, it requires unique specifications regarding structure type, capacity, and impedance voltage, which must be taken seriously. The most critical function of the grounding transformer lies in transmitting the grounding compensation current. This current is essentially a zero-sequence current, which can only flow freely in networks with very low zero-sequence impedance. To ensure smooth passage of the compensation current generated by the arc suppression coil or grounding resistor through the grounding transformer, the zero-sequence impedance must be minimal, making it the first condition for selecting a grounding transformer.
The size of the zero-sequence impedance is significantly influenced by the structure type of the transformer (primarily the wiring group connected by the number of phase windings and the magnetic circuit system formed by the core type). Therefore, choosing the appropriate grounding transformer structure type is crucial.
Grounding Transformer Structure Types:
Historically, various structural types of grounding transformers have been utilized, all featuring very low zero-sequence impedances. These grounding transformers include types such as open delta configurations. A three-phase transformer where the main winding of the iron core column is a zigzag wiring group is referred to as a zigzag grounding transformer, as depicted in the figure.
The introduction of the eight types of iron core column main winding is dated. Yu Shushu (a male from Wuhu, Anhui Province, senior engineer, engaged in electrical operation construction and design work) proposed one of the two aspects of the need for a second load.
A three-phase transformer connected to the arc-extinguishing coil and the load pattern grounding variable wiring group is called a grounding transformer. The D-connected arc-extinguishing coil is connected to the grounding resistor to connect the arc-extinguishing coil and load. There is a structure type of the â–³-shaped grounding band opening, one is a single-phase transformer with a single iron core column, and the three-phase transformer group connected to the opening constitutes another type of iron core column (shell type). The main winding is an opening three-phase transformer of the wiring group.
This design is referred to as an open grounding as shown. Gigi connection arc suppression coil connection grounding resistance diagram opening â–³ grounding transformer. The zero-sequence impedance of any structure type grounding is very small, and mainly through ground compensation (zero-sequence current), so according to the symmetrical component method, ground compensation (zero) the current will flow uniformly through the ground to each phase of the original winding connected to the system.
The multifunctionality of the grounding transformer is evident in several structural types of the aforementioned grounding transformers, where the zigzag type grounding transformer (see the figure and the grounding type of the grounding transformer diagram) can connect the arc-extinguishing coil simultaneously and can also directly carry two secondary loads. If used in a substation, it can serve as both a station transformer and play a multifunctional role.
While the star-type grounding is used to connect the arc-extinguishing coil, it can also be loaded with secondary loads through the intermediary transformer, as shown in the figure. Therefore, whether the grounding transformer connects the arc-extinguishing coil and carries a load depends on the grounding type of the neutral connection equipment capacity and nature and whether there is a secondary load.
Since the grounding transformer serves as a neutral point connection device (considering the capacity of the arc-extinguishing coil or grounding resistance), a large calculation margin has been considered in the calculation; hence, the calculation coefficient is no longer increased when calculating the capacity of the grounding transformer.
For a grounding transformer without a secondary load, the grounding transformer capacity is based on the capacity of the arc-extinguishing coil or grounding resistance it carries and equals the rated capacity of the arc-extinguishing coil or grounding resistor. The grounding protection resistor is determined by the tripping time.
When the grounding transformer is equipped with an arc-extinguishing coil or grounding resistance, the arc-extinguishing coil becomes the reactive load of the grounding, and the grounding resistance becomes the active load of the grounding. When a grounding fault occurs in the system, the grounding compensation current generated by the arc-extinguishing coil or grounding resistance will uniformly flow through the three-phase primary windings. Therefore, the calculation formula for determining the grounding transformer capacity is the rated capacity of the grounding transformer, the arc-extinguishing coil's rated capacity, the grounding resistor's rated capacity, and the open-grounding transformer capacity.
To determine the open-grounding transformer, it is a special type of grounding transformer because it is connected by the open end of the opening to the arc-extinguishing coil or grounding resistor, not the neutral point. Therefore, when the grounding transformer uses a single-phase transformer to form an open grounding band with the arc-extinguishing coil or grounding resistor, the rated capacity of each single-phase transformer is the rated capacity of each single-phase transformer in the open circuit of the S factory.
The secondary winding's rated phase voltage and the secondary winding's rated line current are determined accordingly. The open-grounding transformer load is an open-grounding transformer load rated capacity, and the open-grounding transformer load's rated voltage and rated current are determined similarly. When using an open-shell earth transformer, its rated capacity is either Y100 or 丫100. Hence, when the open-grounding transformer is connected to the arc-extinguishing coil or grounding resistance, its single-phase grounding transformer rated capacity will be better than the arc-extinguishing coil or grounding resistance's rated capacity. The rated capacity of the three-phase grounding transformer will also be larger than the rated capacity of the arc-extinguishing coil or grounding resistor.
Determining the Secondary Load Grounding Transformer Capacity:
When there is a secondary load, the grounding transformer capacity is mainly determined based on the capacity of the arc-extinguishing coil and the secondary load capacity, determined by the same rated time as the arc-extinguishing coil capacity. When the load is large and important, it can also be determined by continuous running time. Here, the capacity of the arc-extinguishing coil should be calculated based on reactive power, while the secondary load capacity should be calculated separately based on the calculated reactive load and active load. The calculation formula is as follows:
Active load calculation of the second load and reactive power calculation load of the second load. When using the active component of the reactive zero-sequence current for grounding protection, the sensitivity and selectivity of the grounding protection are improved. A certain amount of grounding resistance is connected in parallel with the primary or secondary side of the arc coil, consuming energy when used. However, this type of grounding resistance is used for a short time, with little increase in current, and it is not necessary to additionally increase the capacity of the grounding.
Example of Grounding Transformer Capacity:
If the power supply system with 10 small currents is grounded, the arc-extinguishing coil used or the grounding resistance of w is used as the compensation device for the ground current, and some compensation points are grounded with secondary loads, and the load capacity is calculated. The calculation results for each grounding transformer capacity are shown in the table. The calculation results of the grounding transformer capacity. Grounding transformer rated capacity Compensation equipment capacity without load Secondary with load Single-phase where three-phase arc-extinguishing coil grounding resistance is grounded.
Determination of Impedance Voltage:
The meaning of the grounding transformer's impedance voltage is the same as that of the transformer's impedance voltage. The transformer's impedance voltage, also known as the short-circuit voltage, is the percentage of the voltage applied externally to the line end of the original winding when the short-circuit current at the line end of the transformer's secondary winding is rated. Its size is primarily determined by its impedance voltage reactive component, and the size of the reactive component is determined by the reactance of the transformer, so the transformer's impedance voltage is mainly related to the reactance of the transformer.
Grounding is a special type of transformer. The determination of the impedance voltage is mainly based on the impact of the short-circuit current on the dynamic thermal stability of the grounding. If the grounding transformer has a secondary load, it must also consider its requirements for the quality of the load supply voltage.
Determination of Grounding Transformer Impedance Voltage without Secondary Load:
The maximum short-circuit current that the grounding transformer can withstand without a secondary load is the steady-state value of a short-circuit current in a few people, and the dynamic thermal stability multiple of the transformer. One-to-one grounding transformer current, after transformation, the impedance voltage is the impedance voltage that is recognized by the grounding without the secondary load. The calculated value in the range can meet the requirements of the short-circuit current to the grounding thermal stability, so the volume is reduced and the cost, the maximum value can be taken so that the impedance voltage without the secondary load grounding can be determined as the minimum value with the secondary load, the grounding transformer impedance voltage, the grounding transformer, the secondary load, because the secondary load is generally small, so in order to reduce the cost, the capacity of the secondary load winding is generally selected to be smaller than the original winding. Therefore, the capacity of the primary and secondary windings with the secondary load grounding tends to be unequal, and the calculation of the impedance voltage cannot be performed by a conventional method.
Impedance Voltage Calculation:
Because the capacity of the secondary winding with the secondary load is not equal, when considering the influence of the short-circuit current on the dynamic thermal stability of the grounding, the secondary winding with a smaller capacity can withstand the secondary line terminal short-circuit without damage. A few of the six knives and two continents into the sound type, one grounding transformer's secondary load winding rated current, one grounding transformer is the rated capacity of the secondary load winding than the grounding, and when it is the second, the impedance with the secondary load is grounded. The voltage percentage value is equal to the ratio of the rated capacity of the grounding sub-winding.
For example, when the rated capacity of the grounding transformer winding is clever, if the rated capacity of the secondary load winding is selected, the calculation result of the impedance voltage is separately determined as the resistance of the stop voltage to ensure that the grounding of the secondary load is satisfied. The requirements of the load supply voltage quality must be calculated according to the impedance voltage of each load winding of the grounding transformer, and then according to the allowable value of the voltage offset of the terminal of the secondary load electrical equipment and the specific situation of the line voltage loss, determine the allowable value of the voltage loss due to grounding.
Finally, considering the secondary load situation (including no large motor self-starting) grounding transformer voltage loss allowable value and grounding cost and other factors, determine the rated capacity of the best secondary load winding for grounding. The voltage loss of the grounding can be calculated by the following formula: (only the squadron of the squad is sighed by the grounding voltage loss percentage value month-to-ground load rate, Yue Lu. A load power factor.
Assuming that the secondary load is the load power factor, the grounding value (short-circuit loss is .40 kw) is calculated as the percentage value of the voltage loss. If the allowable value of the terminal voltage offset of the secondary load electrical equipment and the specific condition of the line voltage loss are determined, the allowable value of the grounding voltage is determined. 2 5 Considering the factors of secondary load and reasonable grounding cost, it is determined that the rated capacity of the best secondary load winding of the grounding transformer is the rated capacity of the selected secondary grounding optimal secondary load winding in the table, which should be satisfied. After the factors such as the grounding voltage loss value and the grounding cost of the secondary load, try to select the secondary load winding with a smaller rated capacity.
Because the grounding band has a smaller rated capacity of the secondary load meter, the rated capacity of the load winding, the grounding voltage loss, and the loss of the core 4, have a large impedance voltage, which is beneficial to the selection of the load equipment and at the same time economical.
Conclusion:
By analyzing the grounding transformer’s structure type, capacity, and impedance voltage, it is not difficult to find a way to correctly select the grounding transformer. At the same time, it should be noted that although the grounding transformer is similar to a regular transformer, the grounding transformer is not a transformer after all. They differ when connecting the arc suppression coil or grounding resistance. The main performance is that (l) the grounding transformer connects the neutral point of the original winding or the open end of the auxiliary winding to the arc suppression coil or grounding resistor, whereas the transformer connects the secondary winding neutral point to the arc suppression coil or grounding resistor. The zero-sequence impedance of the grounding transformer must be small, but the transformer is not necessarily. When the ground compensation current is changed by grounding, it is evenly distributed in its three-phase windings, whereas the transformer only passes through the grounding phase and the secondary winding and does not uniformly generate induced current in the primary winding of each phase. The arc suppression coil or grounding resistor is the main load in the grounding transformer, whereas the additional secondary load in the transformer varies depending on the wiring group, with different requirements for transformers in different wiring groups. Since it is distinguished by L, when selecting the grounding transformer, the selection principle when the arc suppression coil or grounding resistance is connected to the transformer cannot be applied.
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