The difference between grounding transformer and transformer is pointed out, and it is hoped that the relevant personnel will pay attention when choosing grounding transformer.
Key words: structure type, capacity, impedance and voltage group, classification number, document identification code, document number and preface of grounding transformer. In the power system with small current grounding, when the capacitive current of the system reaches a certain level, the grounding current and its arc gap overvoltage caused by grounding fault will eventually fail to extinguish itself, endangering system safety, so it must be limited.
At present, there are two main measures to limit grounding current and its arc gap overvoltage. One is that the neutral point of the power transformer in the substation is grounded through the arc suppression coil, and the grounding current is compensated by induction, so that the grounding arc is extinguished instantly, thus limiting the arc gap overvoltage. The other is that the neutral point of power transformer in substation is grounded through grounding resistance, and resistive current is injected at the grounding point to change the phase of grounding current, accelerate the discharge of residual charge in the circuit, and promote the self-extinguishing of grounding arc, thus limiting arc gap overvoltage.
At the same time, this measure can also provide enough zero-sequence current and zero-sequence voltage to make the grounding protection work reliably.
However, in the power system with small current grounding, because the winding on one side of the power transformer in the substation is triangular, and there is no neutral point connected with the arc suppression coil or grounding resistance, it is necessary to use a special grounding transformer (equivalent to the transformer whose original winding is connected with OR, hereinafter referred to as grounding transformer) to manually connect the arc suppression coil or grounding resistance, and send the grounding compensation current generated by the arc suppression coil or grounding resistance to the power grid when the system is grounded.
Because the grounding transformer is similar to a transformer, its rated voltage capacity, structural type, positive sequence impedance (or impedance voltage), oil-immersed or dry no-load loss should be considered when selecting the grounding transformer.
However, grounding transformers and transformers are not exactly the same.
Grounding transformer is mainly used to transmit the grounding compensation current of arc suppression coil or grounding resistance, so there will be special requirements in structure type, capacity, impedance voltage and so on, so we must pay attention to it.
Structure type of grounding transformer The biggest function of grounding transformer is to transmit grounding compensation current.
The grounding compensation current is actually a zero-sequence current, which can only flow smoothly in the network with small zero-sequence impedance.
Therefore, in order to make the compensation current generated by the arc suppression coil or grounding resistance pass through the grounding transformer smoothly, the zero-sequence impedance must be very small, which is the primary condition for selecting the grounding transformer.
The magnitude of zero-sequence impedance has a great relationship with the structural type of transformer (mainly refers to the magnetic circuit system composed of the wiring group connected by the number of phase windings and the core type, etc.). ), so the selection of grounding transformer structure is very important.
Structural types of grounding transformers From the current situation of Youshi, there are many structural types of grounding transformers in common use, and their zero-sequence impedance is very small.
The structural types of these grounding transformers are different.
Among them, the three-phase transformer with the main winding of the core column as zigzag connection group is called formal grounding transformer, as shown in the figure.
The main winding of the stem is the revision date. Yu Jianshu (male, Wuhu, Anhui, senior engineer, engaged in electrical operation, construction and design.
When the second brother needs to bring a secondary load, it is also an aspect of it.
The last three-phase transformer connected with arc-suppression coil and grounding resistance connected with arc-suppression coil and grounding variable load is called grounding transformer, as shown in the figure of cyanogen connected with arc-suppression coil and grounding resistance connected with arc-suppression coil and load. Delta-shaped grounding variable precision opening has several structural types, one is composed of a single-phase transformer with four iron core columns and an open three-phase transformer group, the other is a three-phase transformer with more than one iron core column (called shell type), and its main winding is an open connection group.
This type is called open grounding transformer, as shown in the figure.
The zero-sequence impedance of grounding transformer of any structure is very small, mainly through grounding compensation (zero-sequence current), so according to the symmetrical component method, the grounding compensation (zero-sequence current) will flow evenly through the grounding transformer and connect to each primary winding of the system.
The multi-function of grounding transformer acts on the grounding transformers of the above-mentioned structural forms. Sawtooth precision grounding transformer (see the marked grounding transformer shown in Figure and Figure) can be used to connect arc suppression coil or directly carry secondary load.
If it is used in a substation, it can also be used as a substation transformer, playing a multi-functional role.
The starting grounding transformer can be used to connect the arc suppression coil, and it can also take the secondary load through the intermediate transformer, as shown in the figure.
Therefore, it is necessary to choose whether the grounding transformer is connected with the arc suppression coil as shown in Figure△. The capacity of the grounding transformer is determined according to the type of grounding transformer, the capacity and nature of neutral point connection equipment and whether it is equipped with secondary load.
Because the capacity of neutral connection equipment (arc suppression coil or grounding resistance) of grounding transformer has been considered in the calculation, the calculation coefficient is not increased when calculating the capacity of grounding transformer.
Determination of the capacity of grounding transformer without secondary load The capacity of grounding transformer without secondary load is determined according to its arc suppression coil or grounding resistance capacity, and according to the same fixed time as the arc suppression coil or grounding resistance capacity, or according to the tripping time of grounding resistance grounding protection action.
The capacity of the grounding transformer determines that when the grounding transformer is connected with arc suppression coil or grounding resistance, the arc suppression coil will be the reactive load of the grounding transformer and the grounding resistance will be the active load of the grounding transformer.
When grounding fault occurs in the system, the grounding compensation current generated by arc suppression coil or grounding resistance will flow through the three-phase primary winding uniformly, so the calculation formula for determining the capacity of grounding transformer is or, where the rated capacity of a grounding transformer, an arc suppression coil and a grounding resistance are determined, and the capacity of an open grounding transformer determines that the open grounding transformer is a special grounding transformer because it is connected to the arc suppression coil or grounding resistance through the open end of the opening instead of the neutral point. Therefore, when this kind of grounding transformer adopts 1 single-phase transformer to form the arc suppression coil or grounding resistance, the rated capacity of each single-phase transformer is S-factory, the rated phase voltage of the secondary winding of each single-phase transformer is B-fly, the rated line current of the secondary winding of each single-phase transformer is changed from factory to factory, and the open-circuit grounding variable load is the rated capacity of the open-circuit grounding variable load in the formula. The rated voltage of open-circuit grounding transformer is variable, and the rated current of open-circuit grounding transformer is also variable, so it is the same. When an open shell grounding transformer is used, its rated capacity is 100. In this way, when the open-circuit grounding transformer is connected with arc suppression coil or grounding resistance, the rated capacity of its single-phase grounding transformer will be greater than that of arc suppression coil or grounding resistance, and the rated capacity of its three-phase grounding transformer will be greater than that of arc suppression coil or grounding resistance. . 2 2 Determination of grounding transformer capacity with secondary load The grounding transformer capacity with secondary load is mainly determined according to the capacity of arc suppression coil and secondary load, and according to the same fixed time as the capacity of arc suppression coil.
When the load is heavy and important, it can also be determined according to the continuous running time.
Among them, the capacity of arc suppression coil should be calculated according to reactive power, and the capacity of secondary load should be calculated according to calculated reactive load and calculated active load.
The calculation formula is as follows: bow slams the primary active load and mouth slams the secondary reactive load. When the grounding protection reflecting the active component of zero sequence current is adopted, in order to improve the sensitivity and selectivity of grounding protection, a certain amount of grounding resistance is connected in parallel with the primary side or the secondary side of the arc suppression coil, which consumes energy.
However, this kind of grounding resistor has a short service time and little added current, so there is no need to increase the capacity of grounding transformer.
For example, in the power system with 10 small current grounding, if arc suppression coil or W grounding resistance is used as the compensation equipment for grounding current, and some compensation points have secondary loads, and the load capacity is power factor, the calculation results of each grounding variable capacity are shown in the table. The rated capacity of grounding transformer when it is empty compensates the capacity of equipment. What is the single-phase grounding resistance of arc suppression coil with secondary load? Determination of grounding variable impedance voltage and transformer impedance.
The impedance voltage of transformer, also called short-circuit voltage, is the percentage of voltage applied outside the line end of primary winding when the short-circuit current at the line end of secondary winding of transformer is rated.
Its magnitude mainly depends on the reactive component of its impedance voltage, which depends on the reactance of the transformer, so the impedance voltage of the transformer is a quantity mainly related to the reactance of the transformer.
Grounding transformer is a special kind of transformer, and its impedance voltage is mainly determined by considering the influence of short-circuit current on the dynamic and thermal stability of grounding transformer.
If the grounding transformer has a secondary load, its requirements for the quality of load supply voltage should also be considered.
Determination of non-secondary load grounding variable impedance voltage The maximum short-circuit current that non-secondary load grounding transformer should bear is the steady-state value of a short-circuit current in several formulas, the dynamic thermal stability multiple of a transformer in a factory, and the rated current of one-to-one grounding transformer. The modified impedance voltage is regarded as the impedance voltage of grounding transformer without secondary load, and the calculated values in all ranges can meet the requirements of short-circuit current for thermal stability of grounding change. Therefore, in order to reduce the volume and cost, the maximum value can be taken, and the impedance voltage of grounding transformer without secondary load can be determined as the minimum value. When the grounding transformer with secondary load is equipped with secondary load, in order to reduce the cost, the capacity of the secondary load winding is generally smaller than that of the primary winding. Therefore, the capacity of primary and secondary windings of grounding transformer with secondary load is often unequal, and the calculation of its impedance voltage cannot be carried out by conventional methods. . 3 2.
Calculation of impedance voltage Because the capacities of primary and secondary windings of grounding transformer with secondary load are not equal, when considering the influence of short-circuit current on the dynamic and thermal stability of grounding transformer, the secondary load winding with smaller capacity should be able to withstand the sudden short circuit at its secondary terminal without damage.
The rated current and rated capacity of the secondary load winding of the grounding transformer in the two-state two-state one mode are identified, and the percentage of the impedance voltage of the grounding transformer to the secondary load is equal to the ratio of the rated capacity of the primary and secondary windings of the grounding transformer.
For example, when the rated capacity of the primary winding of grounding transformer is ingenious, if the rated capacity of the secondary load winding is selected as the calculation result of its impedance voltage, it is divided into impedance voltage check. In order to ensure that the grounding transformer with secondary load meets the requirements of load power supply voltage quality, it is necessary to calculate the loss of power supply voltage according to the impedance voltage of each load winding of the grounding transformer, and then determine the allowable value of voltage loss of the grounding transformer according to the allowable value of terminal voltage deviation of electrical equipment with secondary load and the specific situation of line voltage loss.
Finally, considering the allowable value of voltage loss of grounding transformer and the cost of grounding transformer under secondary load (including self-starting without large motor), the rated capacity of the optimal secondary load winding of grounding transformer is determined.
The voltage loss of grounding transformer can be calculated according to the following formula (only the percentage of voltage loss of squadron grounding transformer and the monthly, monthly and monthly grounding transformer load rate are recognized).
Load power factor.
Suppose the secondary load is that the load power factor is 0 and the grounding transformer (short circuit loss is 0. 4 0 k w) voltage loss calculation percentage.
According to the requirements of allowable value of terminal voltage deviation of secondary load electrical equipment and the specific situation of line voltage loss, the allowable voltage value of grounding transformer is determined as. Considering the secondary load and the reasonable cost of grounding transformer, it can be determined that the rated capacity of the best secondary load winding of grounding transformer is the rated capacity of the best secondary load winding of grounding transformer selected in the table. After meeting the voltage loss value and cost of grounding transformer under the condition of secondary load, try to choose the secondary load winding with smaller rated capacity.
Because the rated capacity of the secondary load meter load winding with smaller rated capacity of the grounding transformer has a larger impedance voltage when the voltage loss of the grounding transformer is determined as 4- 1 winding, which is beneficial to the selection of load equipment and economical at the same time.
Conclusion it is not difficult to find out the correct method of selecting grounding transformer by analyzing the structural capacity and impedance voltage of grounding transformer with L.
At the same time, it should be pointed out that although the grounding transformer is similar to the transformer, it is not a transformer after all, and they are different when connecting the arc suppression coil or grounding resistance, mainly in the following aspects: (l) the grounding transformer connects the arc suppression coil or grounding resistance with the neutral point of the primary winding or the open end of the triangle of the secondary winding, while the transformer connects the arc suppression coil or grounding resistance with the neutral point of the secondary winding.
The zero-sequence impedance of grounding transformer must be very small, but the transformer is not necessarily.
When the grounding compensation current passes through the grounding transformer, it is evenly distributed in its three-phase winding, but the transformer is not. It only passes through the grounding phase secondary winding, and does not uniformly generate induced current in each phase primary winding.
Arc suppression coil or grounding resistance is the main load in grounding transformer, but it is an additional secondary load in transformer, and its proportion in transformers with different wiring groups has different requirements. For professional knowledge, please refer to the information of China Transformer Trading Network.
Because of the difference of L, when selecting grounding transformer, we can't follow the selection principle of arc suppression coil or grounding resistance connected transformer.
In particular, the selection of structure type, capacity and impedance voltage of grounding transformer must be paid attention to.