Mixed core structure for split-core inverter output transformers

Abstract

The invention relates to a mixed core that prevents extra temperature increases and vibrations that occur in transformers. The invention particularly relates to a mixed core structure, in which air gaps are removed, and which is created by adding core material with different properties to one of the cores produced from the same core material.

Description

[0001] MIXED CORE STRUCTURE FOR SPLIT-CORE INVERTER OUTPUT TRANSFORMERS

[0002] Technical field of the invention

[0003] The invention relates to a mixed core that prevents extra temperature increases and vibrations that occur in transformers.

[0004] The invention particularly relates to a mixed core structure, in which air gaps are removed, and which is created by adding core material with different properties to one of the cores produced from the same core material.

[0005] State of the Art

[0006] Static inverter / converter circuits are used to convert direct current (DC) electrical energy into alternating current (AC) electrical energy. Nowadays, these converters are widely used in different purpose applications where direct current is converted to alternating current, in transferring DC energy produced in photovoltaic systems and wind turbines to the AC grid, and in uninterruptible power supplies (UPS). In these applications, it is desired for the inverter output voltage to be compatible with the distribution grid voltage level and within a controllable range. On the other hand, in low- amplitude direct voltage applications such as renewable energy sources, the alternating current (AC) output voltage can be made compatible by increasing it with a transformer. In addition to transformers providing isolation between two sources, filtering of switched power signals can also be done with the help of these transformers with a proper design. For this reason, the design of transformers, which are used especially in uninterruptible power supplies (UPS) and have a great impact on device output performance and efficiency, is very important. These transformers undertake tasks such as voltage matching, isolation and preventing the DC component that may occur at the inverter output from being transferred to critical loads.

[0007] Considering the characteristics of the inverter transformers, such as saturation and input impedance, from standard transformers operating at grid frequency, it is seen that they have significant differences in terms of their structural features. In terms of their structural features, inverter transformers differ from standard transformers operating at grid frequency in terms of input impedance and taking precautions against the saturation effect that may occur due to switching errors. In addition, transformers that can be found in the inverter circuit not only provide isolation, but also undertake the task of filtering power signals. Inverter transformers differ from standard transformers in terms of features such as saturation and input impedance

[0001] . If the positive and negative alternances are not equal in the switching made with pulse width modulation (PWM) in power electronic circuits, the core parts of the transformer at the inverter output may become saturated and cause extra temperature increases and vibrations. This effect being excessive causes transformers to overheat even without any loads. Eliminating the saturation effect by balancing the alternances with PWM signals is a risky situation as it is not always easy to do. Negativities in the control system, delay of switches, and non-linear behavior increase this effect. However, the saturation effect can be prevented more effectively by making design changes in the core structure of the inverter transformer [2].

[0008] In classical designs, the reactance of transformers where the primary winding is at the bottom and the secondary winding is at the top is approximately 3-6%. For low reactance (<3%), the primary and secondary windings can be divided and interleaved. Design techniques with auxiliary iron cores have been developed to create high reactance (>10%) in inverter transformers. In this technique, two separate core structures are used. One of the cores is made continuously like a normal transformer core and is called the main core. The core made at intervals, like the cores of choke inductors used as filters, is called auxiliary core. Here, the secondary winding is wound only around the main core, and the primary winding is wrapped around both cores. Thus, the idea that split core design in transformers would be suitable for use in inverters is known in the current system [3].

[0009] In addition, partial precautions can be taken against the saturation effect with the split core structure. To obtain high value leakage reactance and prevent saturation in the core, a certain number of equal-sized air gaps can be placed in the second core. However, the production of such a structure is quite difficult. Additionally, when using classical core-structured transformers at the inverter output, additional filter inductances are required to obtain sinusoidal output voltage. In the state of the art, there are many studies, patents and / or utility model applications regarding the transformer core structures. One of these is the patent application numbered “US5376912A”. The invention relates to a combined transformer and inductor developed for power transfer between a source and a load. The developed apparatus includes a transformer core and an inductor core, which are magnetically independent from each other. A primary winding is wound around the transformer core and is electrically connected to the source via a primary connector. A secondary winding is wound around the transformer core and is electrically connected to the load via a secondary connector. Additionally, an inductor coil is wound around the inductor core and placed in series with one of the primary and secondary windings. Multiple air gaps are placed in the inductor core to provide an inductance effect in the primary winding and one of the secondary windings. An advanced combined transformer and inductor, which distribute the heat generated in the core more homogeneously in the inductor core than previous designs, have been developed. Manufacturing is difficult due to the presence of air gaps in the core. For this reason, there is a need for a new technology that does not have air gaps and can eliminate the fringe flux and the negative effects of fringe flux.

[0010] Another document in the state of the art is the patent application numbered "US6650217B1 ". The invention describes a system in which a planar magnetic winding structure, such as a transformer or inductor, has an air gap in the core, a region 2 to 3 times the gap height without windings, and high-frequency winding losses are reduced by 35 percent or more. An aim of the invention is to provide a low-profile magnetic component having a planar magnetic winding structure with reduced winding losses. Another aim of the invention is to provide a component containing air gaps in the core with low winding losses. A new system is needed that eliminates the need for an air gap and eliminates the negative effects and limitations caused by the air gap.

[0011] The invention that is the subject of the patent numbered "US5783984A" in the state of the art relates to methods and tools designed to match the output impedance of an electrical device with a load impedance and generally eliminate the capacitive reactance of the load at the operating frequency. The main aim of the method and means of the present invention is to adjust any magnetic flux amplitude, phase, or waveform required for two independent components. Another aim of the present invention is to eliminate the need for low-pass filtering of non-sinusoidal drive signals coming from the output of the switching power amplifier. To achieve aforementioned aims, an air gap is left in the middle leg of the single-phase transformer core and the primary and secondary windings are placed on the outer legs. Thus, an inductor behavior is achieved. This method can only be applied for a single phase. For this reason, a new technology that can be developed as single-phase, three-phase or n- phase is needed.

[0012] As can be seen from the content of the patent and utility model applications, examples of which are given above, the air gaps are placed in one piece or two pieces between the auxiliary core legs and the yoke parts in order to partially take precautions against the saturation effect with the split core structure. However, in this case, fringe flux effects are very large, increasing losses and affecting the output impedance. In addition, it may cause premature aging due to extra temperature increases and vibrations in the parts where air gaps are located. In addition, negativities in the control system, variation in the delay times of the switches and non-linear behaviors also increase this effect. For this reason, a new technology is needed in the relevant technical field.

[0013] As a result, due to the negativities described above and the inadequacy of existing solutions on the subject, a new technology is needed in the relevant technical field.

[0014] Brief Description and Aims of the Invention

[0015] The invention relates to a mixed core that meets the aforementioned requirements, eliminates all disadvantages, and brings some additional advantages, eliminating the need for air gaps and preventing extra temperature increases and vibrations.

[0016] The most important aim of the invention is to provide a mixed core that does not require air gaps. For this purpose, air gaps were removed and instead, core material with lower magnetic permeability was added to the split core. Thus, the performance of the power electronics circuit increases, and a more compact system design becomes possible. At the same time, extra temperature increases and vibrations that occur when using an air gap can also be prevented. Another aim of the invention is to provide a hybrid core that increases the performance of the power electronics circuit and has a more compact structure.

[0017] Description of Drawings

[0018] FIGURE-1 is a drawing which is showing the isometric view of the mixed core in the transformer that is the subject of the invention.

[0019] FIGURE-2 is a drawing which is showing the isometric view of the mixed core in the transformer that is the subject of the invention.

[0020] FIGURE-3 is a drawing which is showing the rear view of the mixed core in the transformer that is the subject of the invention.

[0021] FIGURE-4 is a drawing which is showing the rear view of the mixed core in the transformer that is the subject of the invention.

[0022] FIGURE-5 is a drawing which is showing the isometric view of the mixed core in the transformer that is the subject of the invention.

[0023] FIGURE-6 is a drawing which is showing the isometric view of the mixed core in the transformer that is the subject of the invention.

[0024] FIGURE-7 is a drawing which is showing the rear view of the mixed core in the transformer that is the subject of the invention.

[0025] FIGURE-8 is a drawing which is showing the rear view of the mixed core in the transformer that is the subject of the invention.

[0026] Definition of Elements / Parts Composing the Invention

[0027] In order to better explain the mixed core developed with this invention, the elements in the figures are numbered and the description of each number is given below:

[0028] 1 . Core A

[0029] 2. Core B

[0030] 3. Core material Detailed Description of the Invention

[0031] The invention relates to a mixed, hybrid core where air gaps are not needed, and the extra temperature increase and vibrations are prevented. With the mixed core structure proposed by the invention, there is no need for air gap and the resulting extra temperature increases and vibrations are prevented. In addition, since air gaps are not needed and, the production difficulties in the core structure are eliminated, the performance of the power electronics circuit increases, and a more compact system has been developed.

[0032] The mixed core structure of the transformer, which provides electrical energy transfer between two or more circuits by electromagnetic induction, comprises core A (1 ), as shown in Figure 1 , and core B (2), to which a core material (3) with different electromagnetic properties and with low magnetic permeability is added. Core A (1 ) is made of silicon alloy steel and its magnetic flux value is approximately 1 .7 tesla.

[0033] The core material in core B (2) and the core material in core A (1 ) are of the same type and properties. In the mixed core structure developed with the invention, a core material (3) with different electromagnetic properties than the core material in core A (1 ) has been added to core B (2). The core material (3) used has lower magnetic flux and permeability values than core A (1 ) and core B (2). In other words, the magnetic flux value of the core material (3) used is less than 1 .7 tesla. The core material (3) can be composite powder. In this way, a mixed core structure was developed.

[0034] Generally, in split-core transformers, core A (1 ) and core B (2) are made of the same core material. Although it varies according to the design need, to obtain high value leakage reactance such as 10% and to prevent saturation in core B (2), air gaps in the required number and equal sizes are available in the design in core B (2). In the mixed core structure developed with the invention, these air gaps were removed to eliminate these air gaps and their negative effects. Instead of air gaps, a core material (3) with different electromagnetic properties has been added to core B (2).

[0035] As in the existing split-core systems, the primary winding to which alternating current is applied and the secondary winding from which alternating current is received are wound on core A (1 ) and core B (2). In this way, a transformer has been developed that raises or lowers the voltage of electrical energy without causing a change in frequency.

[0036] When using classical core structured transformers at the inverter output, additional filter inductances are required to obtain sinusoidal output voltage. By means of the mixed structure developed with the invention, the need for additional inductance as well as the production difficulties in split-core structures are eliminated. In addition, in the inverter transformer with core A (1 ) and core B (2) in split form, the split-core serves as an inductor connected to the primary side (inverter output) and eliminates the need for a filter inductor. Additionally, this helps minimize higher order harmonic components.

[0037] By means of the invention, by eliminating the air gaps in the state of the art, negative effects in terms of energy efficiency such as power loss increases due to saturation and temperature increase in the B core (2) as well as negative mechanical problems such as design difficulties and vibration effects are prevented. To provide mixed core structure electromagnetic performance, a core B (2), in which a core material (3) with lower flux and different permeability values is added between the yoke and leg parts, in the number (generally at least two on each core leg) determined during the sizing phase depending on the transformer power value, can also be developed.

[0038] Mixed, hybrid core structured split core transformers developed with the invention can be used in many sectors including power electronics applications such as renewable energy resources grid integration, electric vehicles and charging stations, power quality improvement, uninterruptible power supplies, high voltage direct current (HVDC) converters. Thus, the saturation effect is eliminated without using any additional equipment. This also reduces both production and maintenance costs. In addition, improvements are made in parameters such as repair time and time between two errors. REFERENCES

[0039] [1] Sefa L, Balci S., Altin N., and Ozdemir, S. “Core Losses of PWM Excited Inverter Transformers with Finite Element Method”, 7th International Conference on Technical and Physical Problems of Power Engineering, 2011 .

[0040] [2] Balci, S., “Evirici iki§ Transformatdrlerinin Modellenmesi ve Analizi”, Gazi Universitesi Fen Bilimleri Enstitusu, Yuksek Lisans Tezi, (2010).

[0041] [3] Battal, F., Sefa, I. & Balci, S. “Dogrusal Olmayan Yuk Ko§ullannda Qah§an Kuru Tip Transformatdrlerin Titre§im Etkileri Uzerine Bir Analiz”, Gazi University Journal of Science Part C: Design and Technology, 7 (3), 729-740. 2019, DOI: 10.29109 / gujsc.580521.

Claims

CLAIMS1. A transformer that provides electrical energy transfer between two or more circuits by electromagnetic induction, comprising:• Core A (1 ), which has a separate structure with core B (2), and• Core B (2) to which core material (3) with lower magnetic flux value and permeability value than core A (1 ) is added intermittently.

2. A transformer according to Claim 1 , comprising core A (1 ) made of silicon alloy steel with a magnetic flux value of 1 .7 tesla.

3. A transformer according to Claim 1 , comprising core material (3) with magnetic flux value less than 1 .7 Tesla.

4. A transformer according to Claim 1 , comprising Core B (2) to which core materials (3) are added that can be changed to have lower magnetic flux value and material compared to core A (1 ) and core B (2) for different applications according to parameters such as number of phases, current, voltage and frequency.

5. A transformer according to Claim 1 , comprising core B (2), whose number and size can be determined according to the core saturation characteristic and the flux value to be formed, forming a hybrid core by adding a core material (3) with a lower flux and different permeability value between the yoke and leg parts.

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