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ELECTRIC, WITH AN EDGE
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ELECTRIC, WITH AN EDGE
Solar sola power is an important form of solar energy utilisation and is a form of power generation that uses solar cells to convert sunlight energy into electrical energy. As solar sola power is not limited by energy resources, raw materials and application environment, and is green and environmentally friendly, it has a broad development prospect and is one of the renewable energy technologies that countries are currently striving to develop. In grid-connected sola power systems, transformers are naturally essential as intermediate conversion equipment.
At present, the main types of sola power transformers are the SC series epoxy insulated sola power transformers of 10 and 35 kV voltage level, which can be roughly divided into double-winding sola power transformers and double split sola power transformers. transformers, the selection of which is described in this article.
The structure of the double-wound sola power transformer for sola power is shown in Figure 1. There are no major differences between this type of double-winding sola power transformer and the traditional distribution sola power transformer in terms of product winding design, technology and manufacture, the difference being that the former is a step-up transformer.
Depending on the rated output voltage of the inverter and the voltage connected to the grid, a set of inverters is usually connected to the matching double-winding sola power transformer.
In view of the fact that the normal operation of the PV inverter does not allow the neutral point of the sola power transformer to be earthed and the presence of harmonic content, the coupling group of the double-wound sola power transformer for sola power is required to be generally Dy11.
In recent years, the use of split transformers has been increasing in order to limit short-circuit currents and save capital investment. A split transformer is a transformer in which one of the windings (usually the low-voltage winding) is split into several branches that are not connected electrically above and below each other.
The use of double split transformers is common in sola power projects, i.e. two separate inverter groups are connected to two branches of the double split winding, which can be operated separately or simultaneously.
Taking into account the harmonic component of the inverter system, the connection group of the sola power transformer for bifurcated sola power is generally chosen as D, y11y11 or Y, d11d11.
At present, bifurcated sola power transformers can be divided into axial bifurcated, amplitude bifurcated and special bifurcated structures according to their structure.
The structure of an axially bifurcated sola power transformer is shown in Figure 2.
The low-voltage winding consists of two branches distributed axially on the same core column, there is no electrical connection between the branches of the split winding, but there is a magnetic connection, the degree of connection depends on the structure of the transformer [2], this structure low-voltage winding can be made into a foil structure or wire-wound structure.
The high voltage winding is also made of two branches in parallel with the corresponding low voltage winding.
Usually the rated capacity and rated voltage of the two branch windings are the same, the conductor specifications, geometry and internal arrangement of each branch winding are basically the same, and the capacity of the two branches combined is equal to the total capacity of the transformer.
The axially bifurcated sola power transformer has good operating characteristics in both traversing and semi-traversing operation due to its completely symmetrical structure and uniform magnetic leakage distribution.
At the same time, the high impedance between the two independent axially split branches effectively reduces the short-circuit current in both systems, so that even if one branch fails, the other branch can still operate normally.
However, the high voltage winding of this structure is two windings connected in parallel, which inevitably results in the number of turns of the high voltage winding increasing by a factor of one compared to conventional products, while the wire cross section is only 1/2 that of conventional products.
If the user chooses a double split sola power transformer with a 35 kV voltage level and D-connected high voltage winding, this will certainly cause difficulties in the winding process of the high voltage winding, low production efficiency and, to a certain extent, the safety and reliability of the sola power transformer, taking into account the need to control the amount of product discharge.
In addition, the low-voltage windings are arranged up and down, and the temperature rise of the upper winding is significantly higher than that of the lower winding due to air convection. Test results show that the temperature difference between the upper and lower low-voltage windings of the axially split sola power transformer prototype is nearly 20 K. Therefore, when designing and manufacturing the conventional axially bifurcated sola power transformer, it is necessary to strengthen the calibration of the temperature rise and the choice of insulation materials.
The commonly used amplitude bifurcated sola power transformer structure is shown in Figure 3.
The low-voltage winding consists of two branches distributed in the amplitude direction, which are usually made into a wire-wound structure due to their structural peculiarities; the high-voltage winding is a single winding as a whole. The high voltage winding of a double split sola power transformer is a single winding.
The number of turns and the cross-section of the wires in this structure are selected as normal, so the winding process and the efficiency of the high-voltage winding in this structure are significantly better than in the axial bifurcated structure.
Due to the near complete symmetry of this structure, there is a good safety-turn balance in both traversing and semi-traversing operation.
In addition, the temperature rise of the amplitude-divided low-voltage windings is more uniform.
However, this type of bifurcated sola power transformer low-voltage winding is split in the amplitude direction, the splitting impedance between the two low-voltage windings is small and the coupling capacitance is large, which increases the interference between the two low-voltage split windings and affects the output power quality and the operational reliability of the relevant components of the inverter.
The structure of the special double split sola power transformer is shown in Figure 4. The low-voltage winding of this type of bifurcated sola power transformer is an axial bifurcated structure, which can be made into a foil or wire-wound structure; the high-voltage winding is a single winding as a whole.
The special bifurcated structure combines the axial bifurcated structure and the amplitude bifurcated structure, which is a rather special bifurcated structure.
The special double split structure solves both the defects of the amplitude split low-voltage winding and many of the problems of the axial double split high-voltage winding, and it reduces the product cost and improves the winding efficiency of the winding, which seems to be ideal.
However, in actual operation, due to environmental influences or inverter fault conditions, the input side of the photovoltaic transformer is not symmetrical up and down, asymmetrical operation occurs, or even half crossing operation (i.e. split winding a branch to the high voltage winding operation).
In this case, the high and low voltage windings have a more serious ampere-turn imbalance, resulting in a large amount of magnetic leakage at the end of the low voltage winding and a significant heat generation at the end of the low voltage winding, so this solution also has a greater risk.
The winding of a sola power transformer for grid-connected sola power is mainly divided into a double-winding structure and a double-split structure. Among them, the double winding is a step-up transformer, whose structure is not very different from the traditional distribution sola power transformer in the design and production process, and is generally selected as D,y11. while the double split PV sola power transformer should pay attention to the following points.
(a) In order to ensure power quality and the reliability of the operation of the relevant components, the split impedance of the two low-voltage windings must not be too small; there is a large temperature difference between the upper and lower low-voltage windings during the operation of an axially split transformer, and attention needs to be paid to the temperature rise and the choice of insulation materials.
For bifurcated sola power transformers of high voltage classes such as 35 kV, the use of Y, d11d11 coupling groups is recommended; special bifurcated sola power transformers with large defects in semi-crossing operation are not recommended in principle.
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