Grounding Transformer
Grounding transformer, can be divided into oil type and dry-type according to the filling medium; According to the number of phases, grounding transformer can be divided into three-phase grounding transformers and single-phase grounding transformers. We can provide quotations and technical support for the grounding transformer.
The function of a grounding transformer is to provide an artificial neutral point for systems where the neutral point is not grounded, facilitating the use of arc suppression coils or small resistance grounding methods to reduce the capacitance current to ground in the event of a grounding short circuit fault in the distribution network and improve the power supply reliability of the distribution system.
Background of use:
The 6kV, 10kV, and 35kV power grids in the power system generally adopt a neutral point ungrounded operation mode. The low-voltage side of the main transformer in the power grid is generally connected in a delta configuration, and there is no neutral point that can be grounded. When a single-phase grounding fault occurs in a neutral point ungrounded system, the line voltage triangle remains symmetrical. The power system can continue to supply power to users for 1 to 2 hours, and the capacitance current is relatively small (less than 10A), which will not cause intermittent arcs. Some instantaneous grounding faults can disappear on their own, which is very effective in improving power supply reliability and reducing power outages. But with the continuous expansion of urban power grids and the increasing number of cable outlets, the system's capacitance current to ground increases sharply, and the capacitance current flowing through the fault point after single-phase grounding is relatively large (more than 10A). The arc is difficult to extinguish, easily triggers ferromagnetic resonance overvoltage, and generates intermittent arc grounding overvoltage, which may cause insulation damage, trip the line, and expand the accident.
Specifically:
In order to reduce the ground capacitance current during single-phase grounding faults, compensation devices such as arc suppression coils need to be installed at the neutral point of the transformer. Therefore, a neutral point needs to be artificially established to connect the arc suppression coil at the neutral point, reduce the ground short-circuit breaking current, and improve the reliability of the system power supply.
Transformer classification:
This type of transformer adopts Z-type wiring (also known as tortuous wiring). The difference from ordinary transformers is that each phase coil is divided into two groups and wound in reverse on the magnetic column of that phase. The advantage of this connection is that the zero sequence magnetic flux can flow along the magnetic column, while the zero sequence magnetic flux of ordinary transformers flows along the leakage magnetic circuit. Therefore, the zero sequence impedance of Z-type grounding transformers is very small (about 10 Ω), while ordinary transformers are much larger. According to regulations, when using a regular transformer with an arc suppression coil, its capacity shall not exceed 20% of the transformer capacity. Z-type transformers can be equipped with 90% to 100% capacity arc suppression coils, while grounding transformers can also carry secondary loads in addition to arc suppression coils, which can replace station transformers and save investment costs.
Single phase grounding transformers are mainly used for neutral point grounding resistance cabinets of generators and Satons transformers with neutral points, in order to reduce the cost and volume of resistance cabinets.
Job characteristics:
Main technical parameters:
In order to meet the needs of arc suppression coil grounding compensation in the distribution network and also to meet the power and lighting loads of substations, transformers with Z-type wiring connections need to be selected, and the main parameters of the grounding transformer need to be reasonably set [6].
(1) Rated capacity:
The primary side capacity of the grounding transformer needs to be matched with the capacity of the arc suppression coil. Based on the current capacity specifications of the arc suppression coil, it is recommended to set the capacity of the grounding transformer to 1.05-1.15 times the capacity of the arc suppression coil. The grounding transformer equipped with one 200kVA arc suppression coil has a capacity of 215kVA.
(2) Neutral point compensation current:
The duration of neutral point compensation current should be the same as the continuous working time of the arc suppression coil, which is 2 hours according to regulations.
(3) Zero sequence impedance:
Zero sequence impedance is an important parameter of grounding transformers, which has a significant impact on limiting single-phase grounding short-circuit current and suppressing overvoltage in relay protection. For grounding transformers with zigzag (Z-shaped) and star/open delta connections without secondary coils, there is only one impedance, namely zero sequence impedance, so that the manufacturing department can meet the requirements of the power department.
(4) Loss:
Loss is an important performance parameter of grounding transformers. For grounding transformers with secondary coils, their no-load loss can be the same as that of double winding transformers of the same capacity. For load loss, during full load operation on the secondary side, due to the lighter load on the primary side, its load loss is smaller than that of a dual winding transformer with the same capacity as the secondary side.
(5) Temperature rise:
According to the national standard, there are the following regulations for the temperature rise of grounding transformers:
1) The temperature rise under rated continuous current should comply with the provisions of the national standard for dry-type transformers in general power transformers, but mainly applicable to grounding transformers with frequent loads on the secondary side;
2) When the duration of short-term load current is within 10 seconds (mainly occurring when the neutral point is connected to a resistor), its temperature rise should comply with the temperature rise limit under short-circuit conditions specified in the national standard for power transformers;
3) When the grounding transformer operates together with the arc suppression coil, its temperature rise should comply with the regulations for the temperature rise of the arc suppression coil:
For a winding temperature of 80K that continuously flows through the rated current, it is mainly suitable for grounding transformers with star/open delta connections;
For windings with a maximum rated current flow time of 2 hours and a specified temperature of 100K. This situation is in line with the working conditions of most grounding transformers;
For windings with a maximum circulation time of 30 minutes, a temperature of 120K is specified.
The starting point of the above regulations is based on the fact that the maximum temperature of the winding hotspot under the most severe conditions does not exceed 140 ℃ to 160 ℃, in order to ensure the safe operation of insulation and not seriously endanger the insulation life.
Advanced production technology ensures the reliability of products:
The transformer body adopts a high-temperature resistant mixed insulation system, which effectively solves the problem of temperature field differences in different types of transformer structures and ensures the safety and reliability of the insulation system.
Made of high-quality conductor winding and reinforced with long and short cut glass fiber felt, it is poured under vacuum using the world-class Heinrich vacuum casting system
The robot is fully automatic in laminating, adopting a fully inclined 45 degree 7-step ladder joint structure, which improves the efficiency and quality of transformer laminating.
All products strictly control the production process and quality links, and undergo strict testing and inspection according to national standards and customer customized requirements before leaving the factory, including partial discharge inspection, lightning impact resistance inspection, noise inspection and other necessary inspection items. Advanced testing equipment and a comprehensive inspection and testing system ensure the high quality and excellence of the products.
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