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Process and Analysis of Amorphous Alloy Core Step-up Transformer for Photovoltaic Inverter

Process and Analysis of Amorphous Alloy Core Step-up Transformer for Photovoltaic Inverter

  • Categories:Industry News
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  • Origin:
  • Time of issue:2020-07-24
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(Summary description)  introduction

  Solar energy is one of the most valuable renewable energy sources, and photovoltaic power generation has gradually become the main body of the world's energy supply. Solar energy is affected by factors such as weather conditions, seasonal changes, day and night alternation, and the uncertainty of sunshine intensity and duration, which makes the output power of photovoltaic power generation system discontinuous and unstable, and long-term low-load operation has become the norm. Reference [1] Point out that low-loss power transformers should be used. Amorphous alloy iron core transformer (abbreviated as amorphous) has extremely low no-load loss characteristics, suitable for long-term low-load operation environment, applied to photovoltaic power generation is conducive to reducing energy consumption and improving the conversion rate of solar energy to electricity.

  1. Photovoltaic power generation unit module

  At present, the inverters with mature technology, stable operation and large-scale applications on the market have a single capacity of 500 to 630kW. Restricted by the capacity of the inverter, photovoltaic power plants generally connect the photovoltaic module and the inverter to form a minimum power generation unit, and use a double-split step-up transformer to form a power generation unit module, that is, a step-up transformer is connected in parallel with two sets of inverters. The smallest power generation unit of the transformer, as shown in Figure 1, effectively reduces the number of transformers and limits the circulating current on the AC output side of the two inverters connected in parallel.


 


  Figure 1 Schematic diagram of photovoltaic power generation unit module

  Therefore, the capacity of the step-up transformer is 1000 ~ 1300kVA, and the output voltage of the inverter mainly has three specifications of 270, 315, and 400V. It is boosted to 10kV or 35kV on-site by a photovoltaic step-up transformer, and finally sent to the power transmission and distribution system to complete the integration. Net work. Photovoltaic step-up transformers are generally supplied as complete sets of combined transformers (Meibian) or pre-installed substations (European transformers). This article only analyzes the transformer itself.

  2. Step-up transformer for photovoltaic

  2.1 Analysis of winding split form

  The double split transformer is composed of 1 high-voltage winding and 2 low-voltage windings, and its electromagnetic working principle is similar to that of a three-winding transformer. The split form of the transformer can be split in the amplitude direction and split in the axial direction, and there are certain differences in the manufacturing process, as shown in Figure 2.


 


  Figure 2 Schematic diagram of transformer split form

  Figure 2a) shows the amplitude split 1: Two low-voltage windings are located on both sides of the high-voltage winding and have two main channels. The manufacturing cost is high, the hidden danger of insulation accidents increases, and it is difficult to ensure the half-passing of the two split windings. The impedance is the same.

  Figure 2b) shows the second amplitude split: the two low-voltage windings are located inside the high-voltage winding. This split method, in order to ensure that the impedance of the two sets of low-voltage windings are the same, the interleaved winding method can be used; in fact, the two sets of low-voltage windings are made into a double-layer foil winding form, but insulation must be provided between the copper foil and the copper foil. So as to become two independent windings. As shown in Figure 3.


 


  Figure 3 Schematic diagram of staggered winding

  If it is a wire-wound winding, it can be made into a multi-layer cylinder, but the turns need to be divided into two independent leads to lead out to make it into two sets of low voltage windings. The disadvantage is that the split impedance is small, the magnetic coupling of the two low-voltage windings is strong, and the mutual influence is greater during operation.

  Figure 2c) shows the axial split: the high and low voltage windings are split axially, as if they are symmetrical up and down. This ensures that the parameters are basically the same from the structure and manufacturing, the split impedance is relatively large, and the half-through impedance is almost equal. Therefore, the photovoltaic step-up transformer is recommended to adopt the axial split form shown in Figure 2c). The impedance calculation can refer to the reference [2].

  2.2 Connection group label

  The third harmonic can circulate in the D-connection winding, which can effectively reduce the impact of harmonics on the power grid. For the 10kV photovoltaic step-up transformer, the Dy11y11 form can be used, which is in line with the habit of ordinary distribution transformers. For 35kV photovoltaic step-up transformer, Yd11d11 form is more recommended. The high voltage is

Process and Analysis of Amorphous Alloy Core Step-up Transformer for Photovoltaic Inverter

(Summary description)  introduction

  Solar energy is one of the most valuable renewable energy sources, and photovoltaic power generation has gradually become the main body of the world's energy supply. Solar energy is affected by factors such as weather conditions, seasonal changes, day and night alternation, and the uncertainty of sunshine intensity and duration, which makes the output power of photovoltaic power generation system discontinuous and unstable, and long-term low-load operation has become the norm. Reference [1] Point out that low-loss power transformers should be used. Amorphous alloy iron core transformer (abbreviated as amorphous) has extremely low no-load loss characteristics, suitable for long-term low-load operation environment, applied to photovoltaic power generation is conducive to reducing energy consumption and improving the conversion rate of solar energy to electricity.

  1. Photovoltaic power generation unit module

  At present, the inverters with mature technology, stable operation and large-scale applications on the market have a single capacity of 500 to 630kW. Restricted by the capacity of the inverter, photovoltaic power plants generally connect the photovoltaic module and the inverter to form a minimum power generation unit, and use a double-split step-up transformer to form a power generation unit module, that is, a step-up transformer is connected in parallel with two sets of inverters. The smallest power generation unit of the transformer, as shown in Figure 1, effectively reduces the number of transformers and limits the circulating current on the AC output side of the two inverters connected in parallel.


 


  Figure 1 Schematic diagram of photovoltaic power generation unit module

  Therefore, the capacity of the step-up transformer is 1000 ~ 1300kVA, and the output voltage of the inverter mainly has three specifications of 270, 315, and 400V. It is boosted to 10kV or 35kV on-site by a photovoltaic step-up transformer, and finally sent to the power transmission and distribution system to complete the integration. Net work. Photovoltaic step-up transformers are generally supplied as complete sets of combined transformers (Meibian) or pre-installed substations (European transformers). This article only analyzes the transformer itself.

  2. Step-up transformer for photovoltaic

  2.1 Analysis of winding split form

  The double split transformer is composed of 1 high-voltage winding and 2 low-voltage windings, and its electromagnetic working principle is similar to that of a three-winding transformer. The split form of the transformer can be split in the amplitude direction and split in the axial direction, and there are certain differences in the manufacturing process, as shown in Figure 2.


 


  Figure 2 Schematic diagram of transformer split form

  Figure 2a) shows the amplitude split 1: Two low-voltage windings are located on both sides of the high-voltage winding and have two main channels. The manufacturing cost is high, the hidden danger of insulation accidents increases, and it is difficult to ensure the half-passing of the two split windings. The impedance is the same.

  Figure 2b) shows the second amplitude split: the two low-voltage windings are located inside the high-voltage winding. This split method, in order to ensure that the impedance of the two sets of low-voltage windings are the same, the interleaved winding method can be used; in fact, the two sets of low-voltage windings are made into a double-layer foil winding form, but insulation must be provided between the copper foil and the copper foil. So as to become two independent windings. As shown in Figure 3.


 


  Figure 3 Schematic diagram of staggered winding

  If it is a wire-wound winding, it can be made into a multi-layer cylinder, but the turns need to be divided into two independent leads to lead out to make it into two sets of low voltage windings. The disadvantage is that the split impedance is small, the magnetic coupling of the two low-voltage windings is strong, and the mutual influence is greater during operation.

  Figure 2c) shows the axial split: the high and low voltage windings are split axially, as if they are symmetrical up and down. This ensures that the parameters are basically the same from the structure and manufacturing, the split impedance is relatively large, and the half-through impedance is almost equal. Therefore, the photovoltaic step-up transformer is recommended to adopt the axial split form shown in Figure 2c). The impedance calculation can refer to the reference [2].

  2.2 Connection group label

  The third harmonic can circulate in the D-connection winding, which can effectively reduce the impact of harmonics on the power grid. For the 10kV photovoltaic step-up transformer, the Dy11y11 form can be used, which is in line with the habit of ordinary distribution transformers. For 35kV photovoltaic step-up transformer, Yd11d11 form is more recommended. The high voltage is

  • Categories:Industry News
  • Author:
  • Origin:
  • Time of issue:2020-07-24
  • Views:0
Information

  introduction

  Solar energy is one of the most valuable renewable energy sources, and photovoltaic power generation has gradually become the main body of the world's energy supply. Solar energy is affected by factors such as weather conditions, seasonal changes, day and night alternation, and the uncertainty of sunshine intensity and duration, which makes the output power of photovoltaic power generation system discontinuous and unstable, and long-term low-load operation has become the norm. Reference [1] Point out that low-loss power transformers should be used. Amorphous alloy iron core transformer (abbreviated as amorphous) has extremely low no-load loss characteristics, suitable for long-term low-load operation environment, applied to photovoltaic power generation is conducive to reducing energy consumption and improving the conversion rate of solar energy to electricity.

  1. Photovoltaic power generation unit module

  At present, the inverters with mature technology, stable operation and large-scale applications on the market have a single capacity of 500 to 630kW. Restricted by the capacity of the inverter, photovoltaic power plants generally connect the photovoltaic module and the inverter to form a minimum power generation unit, and use a double-split step-up transformer to form a power generation unit module, that is, a step-up transformer is connected in parallel with two sets of inverters. The smallest power generation unit of the transformer, as shown in Figure 1, effectively reduces the number of transformers and limits the circulating current on the AC output side of the two inverters connected in parallel.

光伏逆变用非晶合金铁心升压变压器工艺与分析

 

  Figure 1 Schematic diagram of photovoltaic power generation unit module

  Therefore, the capacity of the step-up transformer is 1000 ~ 1300kVA, and the output voltage of the inverter mainly has three specifications of 270, 315, and 400V. It is boosted to 10kV or 35kV on-site by a photovoltaic step-up transformer, and finally sent to the power transmission and distribution system to complete the integration. Net work. Photovoltaic step-up transformers are generally supplied as complete sets of combined transformers (Meibian) or pre-installed substations (European transformers). This article only analyzes the transformer itself.

  2. Step-up transformer for photovoltaic

  2.1 Analysis of winding split form

  The double split transformer is composed of 1 high-voltage winding and 2 low-voltage windings, and its electromagnetic working principle is similar to that of a three-winding transformer. The split form of the transformer can be split in the amplitude direction and split in the axial direction, and there are certain differences in the manufacturing process, as shown in Figure 2.

光伏逆变用非晶合金铁心升压变压器工艺与分析

 

  Figure 2 Schematic diagram of transformer split form

  Figure 2a) shows the amplitude split 1: Two low-voltage windings are located on both sides of the high-voltage winding and have two main channels. The manufacturing cost is high, the hidden danger of insulation accidents increases, and it is difficult to ensure the half-passing of the two split windings. The impedance is the same.

  Figure 2b) shows the second amplitude split: the two low-voltage windings are located inside the high-voltage winding. This split method, in order to ensure that the impedance of the two sets of low-voltage windings are the same, the interleaved winding method can be used; in fact, the two sets of low-voltage windings are made into a double-layer foil winding form, but insulation must be provided between the copper foil and the copper foil. So as to become two independent windings. As shown in Figure 3.

光伏逆变用非晶合金铁心升压变压器工艺与分析

 

  Figure 3 Schematic diagram of staggered winding

  If it is a wire-wound winding, it can be made into a multi-layer cylinder, but the turns need to be divided into two independent leads to lead out to make it into two sets of low voltage windings. The disadvantage is that the split impedance is small, the magnetic coupling of the two low-voltage windings is strong, and the mutual influence is greater during operation.

  Figure 2c) shows the axial split: the high and low voltage windings are split axially, as if they are symmetrical up and down. This ensures that the parameters are basically the same from the structure and manufacturing, the split impedance is relatively large, and the half-through impedance is almost equal. Therefore, the photovoltaic step-up transformer is recommended to adopt the axial split form shown in Figure 2c). The impedance calculation can refer to the reference [2].

  2.2 Connection group label

  The third harmonic can circulate in the D-connection winding, which can effectively reduce the impact of harmonics on the power grid. For the 10kV photovoltaic step-up transformer, the Dy11y11 form can be used, which is in line with the habit of ordinary distribution transformers. For 35kV photovoltaic step-up transformer, Yd11d11 form is more recommended. The high voltage is Y connection, the phase voltage is 1/3 of the line voltage, the winding partial discharge is easy to control, and the quality is more guaranteed.

  2.3 Core design

  2.3.1 Magnetic density

  In actual operation of the photovoltaic inverter, the waveform is usually in an asymmetrical state, that is, a DC bias is generated. It is usually required that the DC current component should not exceed 1% of its AC rating when it is connected to the grid. When DC bias occurs, the DC component is isolated by the step-up transformer. Although it will not flow into the power grid, it will cause the transformer core flux density to increase and the excitation current and noise will also increase. In severe cases, it can cause core saturation and Distortion of excitation current. Therefore, it is recommended to reduce the magnetic density by 0.05 to 0.1T compared with conventional products. If users have higher requirements for noise, refer to the reference [3] The relationship between magnetic density and noise (for each increase of magnetic density by 0.05T, the no-load noise of the iron core increases by about 2dB, and the transformer product can increase by about 5dB), and reduce it appropriately Magnetic density design.

  2.3.2 Core process coefficient

  Using the axially split amorphous transformation, the core window height is relatively high, and the amorphous core process coefficient is 3% to 5% lower than the conventional amorphous core process coefficient of the same capacity. It can be simply calculated as follows:

光伏逆变用非晶合金铁心升压变压器工艺与分析

 

  In the formula: KFe1, HW1 represent the process experience coefficient and window height of the conventional amorphous core with the same capacity; KFe2, HW2 represent the process coefficient and window height of the axially split amorphous core.

  The fractures of the iron core should be lapped in a distributed manner, with a lap length of 8-10mm, and no staggered layering. The outermost layer is made of silicon steel sheets to make steel buckles to ensure the best performance of the iron core.

  2.3.3 Core processing

  As the most critical annealing treatment in core production, the purpose is to eliminate the stress generated in the core forming process and restore the magnetic properties. According to experience, in addition to mastering the heating, holding and cooling process curves of amorphous strips from various manufacturers, the uniformity of the furnace temperature in the annealing furnace is also an important factor in improving the quality of the iron core. The relationship between the furnace temperature and the iron core temperature is shown in Figure 4. By improving the heating environment of the annealing furnace and reasonably arranging the hot air circulation system, the maximum deviation of the furnace temperature at each point is within 2.5 ℃, and at the same time, a better heat treatment effect can be obtained under the protection of a strict nitrogen atmosphere [4].

光伏逆变用非晶合金铁心升压变压器工艺与分析

 

  Figure 4 The relationship between furnace temperature and core temperature

  The surface coating of the iron core is beneficial to suppress the vibration of the iron core, reduce noise and reduce the generation of debris. For example, if the photovoltaic amorphous becomes an oil-immersed transformer (Meichang), the side of the iron core can be coated with glue, and the iron core is folded and connected as a whole, which can effectively prevent the amorphous debris from being scattered into the oil during the operation of the transformer and causing quality hazards; For transformers (European transformers), resin coating can be used, preferably with a coating thickness of 1.0 ~ 1.5mm.

  2.4 Temperature rise and insulation level

  Considering the influence of the inverter output current harmonics, the temperature rise design should accumulate the winding DC loss, electromagnetic wire eddy current loss, and harmonic influence loss. Precise calculations are conducive to cost control, while ensuring that the hottest spot temperature of the transformer does not exceed the allowable temperature of the insulating material. In particular, for dry-type transformers, because the axial split winding is relatively high, when the heat dissipation air flows from the bottom of the winding to the top, the temperature difference between the upper winding and the lower winding is often more than 10K, which requires a lower temperature rise design. Or the insulation material of the upper winding should be of higher heat resistance grade.

  Photovoltaic power generation is often installed in places with harsh climatic conditions. For high-altitude operating areas, the insulation level and temperature rise limit must be revised in accordance with the provisions of GB1094.2 and GB1094.11. When the ambient temperature exceeds the normal conditions specified in the standard, the temperature rise limit should also be reduced and corrected according to the exceeding part.

  2.5 Other process requirements

  The two sets of low-voltage input sources of the axial double-split step-up transformer usually operate in the same up and down state. When a ground fault occurs in one of the channels, the transformer will no longer be symmetrical up and down. At this time, the magnetic balance in the core will be lost, resulting in huge short-circuit mechanical stress in the axial direction and huge hidden dangers. The cross-section of the amorphous alloy core is usually rectangular, and the cross-section of the winding forms a rounded rectangle. The amplitude force of this winding has congenital defects; at the same time, the amorphous strip cannot be stressed, and any mechanical stress may affect its magnetic properties and noise [ 5].

光伏逆变用非晶合金铁心升压变压器工艺与分析

 

  Figure 5 Schematic diagram of dry-changed axial split winding

  Under the requirements of unattended or few guarded by photovoltaic power generation, the above problems must be carefully taken in the design and process of each link to improve the quality and reliability of the transformer.

  2.5.1 Low voltage winding

  The low-voltage winding is made of copper foil, and the upper and lower split windings are arranged symmetrically. The double-layer foil winding machine is integrated and synchronously wound. The end is flat and the size is easy to control. The inner side of the low-voltage winding of the oil-immersed transformer must be equipped with a hard epoxy skeleton, which is strong enough to withstand the amplitude of the short-circuit electromotive force and avoid winding deformation and core stress; the gap between the split windings is tightly filled with epoxy plate. Dry-type transformers are cast into a whole with resin, and the ability to withstand sudden short circuits is greatly improved.

  2.5.2 High voltage winding

  The high-voltage windings are also arranged symmetrically in the middle, and the turns are arranged reasonably to achieve ampere-turn balance as much as possible. The high and low voltage windings of oil-immersed transformers are sleeve-wound, and the overall dipping treatment can further improve the mechanical performance. The dry-type transformer casts two sets of split windings with integral resin to form a whole, as shown in Figure 5.

  2.5.3 Lead

  The leads of the axial double-split transformer are drawn symmetrically from the upper and lower parts. The lead wires of dry-type transformers are relatively simple, and the main considerations are sufficient safety distance and lead fixing.

  The lead wires of oil-immersed transformers are relatively complicated. Due to the increase in the number of lead wires and the large space layout range, pay special attention to reserve the fuel tank space for lead wire arrangement, increase enough lead wire clamps to fix the lead wires, and ensure the resistance balance of the low-voltage lead wires. Generally, the resistance unbalance rate of the finished product is required to be 1%, and it should be calculated accurately during product design, and copper bars or copper stranded wires with appropriate cross-sections should be selected.

  2.5.4 Grounding shield

  The effect of grounding shielding between the high and low voltage windings is limited. Because the harmonic content is much lower than the operating environment of the rectifier transformer, if the grounding screen is not handled well, it is likely to increase the potential for faults and increase the manufacturing cost.

  3. Conclusion

  In view of the characteristics of long-term low-load operation of photovoltaic power generation, an amorphous alloy core step-up transformer with extremely low no-load loss (design level code 15 and above) is undoubtedly the best choice for increasing the solar energy conversion rate. The product should adopt axial splitting, combined with the influence of inverter circuit DC bias, harmonic content, and split operation failure. Scientific corrections should be made in core design, impedance calculation, and temperature rise design. At the same time, the winding process and lead wire should be scientifically modified. The process and other aspects need to strengthen the mechanical properties. Only by fully mastering these details can we truly design and manufacture safe, reliable, and stable performance new energy products.

  references

  [1] China Electricity Council. Design specification for photovoltaic power station: GB50797—2012[S]. Beijing: China Planning Press, 2012: 31-37.

  [2] Jia Heqiang, Tan Lijun. Analysis of correction coefficient and splitting coefficient for impedance calculation of transformer with axial double split structure[J]. Transformer, 2005, 42(11): 1-4.

  [3] Zhao Xiaoying. Two structural forms and noise control of amorphous alloy dry transformation [J]. Mechanical and Electrical Engineering Technology, 2011, 40(3): 100-102.

  [4] Zhang Zhili, Ling Jian, Li Ran, et al. Development of a large-capacity dry-type amorphous alloy iron transformer [J]. Transformer, 2018, 55(3): 6-8.

  [5] Cui Lijun. Theory and design of special transformers [M]. Beijing: Science and Technology Literature Press, 1996.

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