UNFOLDING INVERTER WITH 3 HF SWITCHES ON DC SIDE

Field of the Invention

The present invention relates to a three-phase, three-level, transformerless, step-down inverter that prevents the capacitive leakage currents drawn from the DC power source.

In particular, the present invention relates to a transformerless, high efficiency, three- level, high power density, three-phase DC/AC inverter used in renewable energy, military land/sea/air vehicles, rail systems, medical devices, electric vehicle applications.

State of the Art

Inverters are also called DC-AC converters. The inverter converts a fixed DC voltage to a symmetrical AC voltage with the desired frequency and amplitude. Inverters are also widely used in industrial applications such as uninterruptible power supplies (UPS), induction motors and variable-speed AC motor drives. Inverters can be divided into two groups as single-phase and three-phase inverters. The output of the single-phase inverter is a symmetric AC voltage with a square or sinusoidal wave form. The three- phase inverter can be thought of as three single-phase inverters, the output waveform of each phase is shifted by 120 degrees with respect to each other.

Inverters with or without transformer are used to convert DC input voltage to AC output voltage. Transformerless inverters, compared to transformer based inverters are advantageous in terms of size, volume, weight, cost and efficiency. The leakage capacitive currents can be prevented from being drawn from the input power source as transformer inverters can isolate input and output terminals galvanically. Transformerless inverters are disadvantageous in terms of leakage currents drawn from the input power source besides the advantages they have. Various power converter topologies, switching strategies and filtering methods so as to suppress the leakage currents drawn from the input power supply in transformerless inverters, are taken into consideration. As a result of the research made on the subject, US7411802B2 has been encountered. The application describes a method of converting a DC voltage from a DC voltage source, more specifically a photovoltaic DC voltage source, to an AC voltage. However, there is no mention of a transformerless inverter with high efficiency, low volume, low weight, easy to apply for control and modulation algorithms, reducing leakage currents for three-phase systems.

Single-phase inverter can produce solutions in applications where single-phase AC output voltages are needed. The single-phase inverter shown in Figure 2 is especially used due to its performance in reducing capacitive leakage currents. It is possible to produce a solution by using a single-phase inverter for each phase so as to adapt a single-phase inverter to a three-phase system. However, in this case, problems arise such as the necessity of transferring the neutral line to the output, controlling the phases with independent power converters, and controlling the current circulations between the phases. In addition, in the case of using three single-phase inverters, a total of nine switches operate at high frequency, and six switches are switched at high frequency in the same time interval.

Two series electrolytic capacitors are needed in the three-phase inverters used in the state of the art. Additional control algorithms must be applied to keep the voltages of the electrolytic capacitors in balance in the three-phase inverter shown in Figure 3. Electrolytic capacitors not only increase the size of the circuit, but also limit the lifetime of the product. This inverter cannot produce a natural solution for reducing capacitive leakage currents. The number of switches switched at high frequency is twelve in the inverter, and the number of switches switched at high frequency in the same time interval is six, thus resulting in high switching losses. The modulation algorithms created for switching twelve semiconductors in total are complex and involve difficulties in implementation.

As a result, due to the abovementioned disadvantages and the insufficiency of the current solutions regarding the subject matter, a development is required to be made in the relevant technical field.

Object of the Invention

The present invention aims to solve the abovementioned disadvantages by being inspired from the current conditions. The main object of the present invention is to provide a transformerless inverter with high efficiency, low volume, low weight, easy to apply for control and modulation algorithms, reducing leakage currents for three-phase systems.

Another object of the present invention is to reduce the capacitive leakage currents drawn from the input DC power supply.

Another object of the present invention is to eliminate the need for the use of electrolytic capacitors at the input.

In order to fulfil the above-described aims, the present invention is a transformerless inverter with high efficiency, low volume, low weight, ease of application for control and modulation algorithms, preventing leakage currents for three-phase systems, comprising transformerless three-phase three-level step-down inverter which provides AC output voltages in three-phase alternating current sine wave form from a DC input power source. The invention also provides DC output voltage from three-phase AC input power sources. The invention prevents leakage currents and includes three semiconductor switches that are instantaneously switched at high frequency and located in the regulation section of the inverter.

The structural and characteristic features of the present invention will be understood clearly by the following drawings and the detailed description made with reference to these drawings and therefore the evaluation shall be made by taking these figures and the detailed description into consideration.

Figures to Help Understand the Invention

Figure 1 is an illustrative view of the inventive inverter.

Figure 2 is an illustrative view of a single-phase inverter showing the state of the art.

Figure 3 is an illustrative view of a three-phase inverter showing the state of the art.

Figure 4 is an illustrative view of the graphical display of voltages at the output terminals of the regulation section (connections 3, 4 and 5).

Figure 5 is an illustrative view of the graphic display of the switching signals of the switches in the unfolding section. Description of the Part References

T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15: Semiconductor switch

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15: Terminal

V _a , V _b , V _c : Voltage

Lf1, Lf2, Lf3: Inductor

Cf1, Cf2, Cf3: Capacitor

Vj _n : Input power supply

401: Input power supply

402, 403, 404, 405, 406, 407, 408, 409, 410, 411, 412, 413, 414, 415, 416: Semiconductor switch

Detailed Description of the Invention

In this detailed description, the preferred embodiments of the inventive three-phase, three-level, transformerless, step-down inverter are described by means of examples only for clarifying the subject matter.

A transformerless three-phase three-level inverter is provided to obtain three-phase AC output voltages from the DC input power supply within the scope of the three-phase, three-level, transformerless, step-down inverter. The three-phase inverter, which is considered in a three-phase, three-level, transformerless, step-down inverter, is shown in Figure 1 together with the numbers of the materials used.

The input power supply (401) between the input terminals (1 , 2) provides regulated or unregulated DC voltage.

A transformerless inverter is developed that generates output voltages in a three-phase alternating current sine wave form between terminals (11 , 15), (12, 15) and (13, 15) and fed from a direct current source connected to terminals (1 , 2) within the scope of three-phase, three-level, transformerless, step-down inverter. Output filters are located between terminals (8, 11 , 14), (8, 12, 14) and (10, 13, 14). Active switches are located between terminals (1 , 2) and (8, 9, 10). The three-phase, three-level, transformerless, step-down inverter comprises only three high-frequency semiconductor switches (T1 , T2, T3) switched instantaneously. All other semiconductors (T4, T5, T6, T7, T8, T9, T10, T11 , T12, T13, T14 and T15), in the three-phase, three-level, transformerless, step-down inverter, are switched at low frequency. DC input terminals and AC output terminals are separated in the zero vector regions of the inverter, preventing instantaneous high leakage currents from the input power supply with the help of the T1 switch.

A three-level, three-phase, transformerless, T-type inverter is shown in Figure 3. This inverter can generate three-level output waveforms. Increasing the level of waveforms at the inverter output reduces the core and copper losses in the filter inductors. In addition, the reduction of voltages on semiconductor switches is provided by this topology. Three-level, three-phase, transformerless, T-type inverters are widely preferred due to these advantages. Although this topology is advantageous in terms of efficiency, it cannot produce a solution for the prevention or suppression of capacitive leakage currents. In addition, it is important that the voltages on the series capacitors (C1 and C2 in Figure 3) at the input in this inverter structure are balanced, so that the output AC voltage is stable and regulated. High currents flowing through the capacitors cause the capacitors to be large in size. Although the inverter in Figure 3 is a transformerless inverter, the capacitors in its structure cause the volume of the inverter to increase and the power density to decrease. The number of switches switched at high frequency in this inverter is twelve. Six keys are switched in the same time interval.

The inverter shown in Figure 1 can be considered in two parts. The first section is the regulation section between the input terminals (1 , 2) and the terminals (3, 4, 5). The second section is the unfolding section between the terminals (3, 4, 5) and the terminals (8, 9, 10).

The voltage waveforms formed at the terminals (3, 4, 5) located between the two sections are given in Figure 4. The waveforms given in this way are produced for balanced, 220Vrms phase-neutral and 50 Hz, three-phase AC voltages. The waveform of the envelope comprising highest values of the three-phase AC voltages is named with the TOP label in Figure 4, the waveform of the envelope comprising lowest values is named with the BOTTOM label in Figure 4, and the waveforms in the middle region are named with the MID label.

The waveforms shown in Figure 4 are generated at terminals (3, 4 and 5) when T1 (402), T2 (403) and T3 (404) semiconductor switches in the regulation section are switched at high frequency (1kHz - 1 MHz band).

In Figure 4, the semiconductor switches T1 (402), T2 (403) and T3 (404) located in the regulation section are controlled to be formed at the waveform terminal (3) named as TOP, the waveform terminal (4) named as MID, and the waveform terminal (5) named as BOTTOM.

Regulated waveforms at terminals (3, 4, 5) are separated and three-phase AC voltages are obtained at terminals (8, 9, 10) by controlling the semiconductor switches (T4 - T15) in the unfolding section.

The switching waveforms of the unfolding section are shown in Figure 5.

When Va (423) voltage, Vb (424) voltage, Vc (425) voltage or reference values are compared to each other, in regions where Va (423) voltage is the highest, T4 (405) switch is in conduction, T5 (406) and T6 (407) switches and T7 (408) switches are in cut-off. In regions where the Vb (424) voltage is greatest, T8 (409) switch is in conduction, T9 (410) and T10 (411) switches and T11 (412) switches are in cut-off. In regions where Vc (425) voltage is greatest, T12 (413) switch is in conduction, T13 (414) and T14 (415) switches and T15 (416) switches are in cut-off.

When voltage Va (423), voltage Vb (424), voltage Vc (425) or reference values are compared with each other, in regions where Va (423) voltage is the lowest, T7 (408) switch is in conduction, T5 (406) and T6 (407) switches and T4 (405) switches are in cut-off. In regions where the Vb (424) voltage is the lowest, T11 (412) switch is in conduction, T9 (410) and T10 (411) switches and T8 (409) switches are in cut-off. In regions where Vc (425) voltage is lowest, T15 (416) switch is in conduction, T13 (414) and T14 (415) switches and T12 (413) switch are in cut-off.

When voltage Va (423), voltage Vb (424), voltage Vc (425) or reference values are compared with each other, in regions where Va (423) voltage is in the middle, T5 (406) and T6 (407) switches are in conduction, T4 (405) switches and T7 (408s) switches are in cut-off. In the regions where the Vb (424) voltage is in the middle, T9 (410) and T10 (411) switches are in conduction, T8 (409) switch and T11 (412) switch are in cut-off. In regions where Vc (425) voltage is in the middle, T13 (414) and T14 (415) switches are in conduction, T12 (413) switch and T15 (416) switch are in cut-off.

In three-phase grid-connected applications; the voltage Va (423) between the terminals (11 , 15), the voltage Vb (424) between the terminals (12, 15) and the voltage Vc (425) between the terminals (13, 15) are measured and compared with each other and arranged to be the highest, medium, and lowest. These waveforms constitute the reference waveforms for the output voltages (3, 4, 5) of the regulation section.

In three-phase applications operating independently of the mains; reference signs are obtained for the voltages to be generated at the (3, 4, 5) terminals by comparing the reference voltages for the voltage Va (423) between terminals (11 , 15), the voltage Vb (424) between terminals (12, 15), and the voltage Vc (425) between terminals (13, 15) with each other.

T4 (405), T7 (408), T8 (409), T11 (412), T12 (413), T15 (416) semiconductor switches are switched via the Va (423), Vb (424) and Vc (425) voltage waveforms formed between the output terminals (11 , 15), (12, 15) and (13, 15) (Figure 5).

T5 (406), T6(407), T9 (410), T10 (411), T13 (414), T14 (415) semiconductor switches are switched at twice the frequency of the voltage waveforms Va (423), Vb (424) and Vc (425) formed between the output terminals (11 , 15), (12, 15) and (13, 15) (Figure 5).

Lf1 (417), Lf2 (418), Lf3 (419) inductors and Cf1 (422), Cf2 (421), Cf3 (420) capacitors are used to filter the transfer of high-frequency components produced by the inverter discussed within the scope of the invention to the output.

In order to create zero vectors in the inverter in Figure 3, T4, T6 and T8 semiconductor switches or T5, T7, T9 semiconductor switches must be in conduction at the same time. At the moments when these zero vectors is formed, the common mode voltage reaches its highest value and high leakage currents are drawn from the input DC source.

The need for zero vectors is eliminated with the help of the present invention. In Figure 1 , there is no need for T4 (405), T8 (409), T12 (413) switches or T7 (408), T11 (412), T15 (416) switches to be in conduction simultaneously. DC input voltage and three- phase AC voltages can be separated due to the fact that there is no need for a zero vector and T1 (402) switch connected in series on the main branch is in cut-off during free circulation time intervals. Thus, the problem of capacitive leakage current drawn from the input DC power source, which occurs in transformerless inverters, is suppressed by a three-phase, three-level, transformerless, step-down inverter.

Since the invention has three semiconductor switches switched at high frequency in a switching period, this ensures that the thermal power losses due to switching are low.

There is no need to connect a single or two electrolytic capacitors in series to the input terminals of the transformerless inverter structure with the help of the three-phase, three-level, transformerless, step-down inverter. Thus, the total volume of the inverter decreases, and the power density increases with the deactivation of the electrolytic capacitors. In general, the fact that electrolytic capacitors do not have a long lifespan also causes a short lifespan of power converters that need electrolytic capacitors. In this context, the inverter introduced with the present invention, which does not need electrolytic capacitors, has a long life.

Semiconductor switches (T1 - T15) in three-phase, three-level, transformerless, stepdown inverter structure can be used as MOSFET, IGBT in Silicon or Silicon Carbide technology. The reverse parallel diode, which can be found in the switch, can be added externally.

The inverter, which is a three-phase, three-level, transformerless, step-down inverter, can operate in two directions. The inverter, which can convert input DC voltage to output AC voltages, can also convert three-phase AC voltages to regulated DC voltage.

A serial switch can be connected between terminals (2) and (5) with the power flow from (5) to (2). This serial switch can be programmed to have the switching characteristics of the T1 (402) switch and the number of switching can be shared among the switches and it may be possible to reduce the thermal load on switch T1 (402).

High frequency signals at the output of the regulation section can be filtered with a filter comprising an inductor between terminals (3) and (6), an inductor between terminals (5) and (7), capacitors with one end each at terminals (6), (4) and (7) and the other ends in star point. Thus, the load of the output filter consisting of Lf1 (417), Lf2 (418), Lf3 (419) inductors and Cf1 (422), Cf2 (421), Cf3 (420) capacitors can be reduced and the size of this filter can be reduced.

The three-phase, three-level, transformerless, step-down inverter converts the DC input voltage taken from the solar panel or DC power source or battery to three-phase AC voltages, allowing it to be used for feeding electronic equipment or for transferring energy to the grid.

Three-phase AC voltages obtained from the network or from the alternator or turbine can be used to charge batteries or feed DC electronic loads.

Three-phase, three-level, transformerless, step-down inverter can be used in battery charging units of electric vehicles, in converting non-regulated AC voltages obtained from wind turbines to regulated DC output voltage due to its two-way operation.