RO130090A0 - Circuit and method for decoupling the oscillating power for single-phase inverters - Google Patents

Abstract

The invention relates to a circuit and method for decoupling the oscillating power for single-phase inverters in order to cancel the alternating component of the current on the direct current line of the inverter. The claimed circuit comprises an H-bridge inverter (), supplied with a DC voltage (), the bridge having four pulse width modulated (PWM) transistors, the median points of the two arms of the inverter bridge being connected to two filtering coils () and two alternating current non-polarized capacitors () being connected to the output terminals of the coils, the capacitors being meant to decouple the oscillating component of the power, the other two terminals of the capacitors being together connected to one of the DC terminals () of the inverter. The claimed method consists in controlling the inverter arms with the purpose of generating two voltages which contain harmonics of even order for transferring the oscillating power towards the decoupling circuit, so that the DC current line of the inverter contains only the average component of the power, and the voltage and the current at the output terminals () are not affected.

Description

The invention relates to a circuit and to a method of decoupling the oscillating power for single-phase inverters, in order to attenuate the alternating current component from the inverter input and to reduce the capacitor capacity required on the inverter DC line. The invention falls within the field of electronic power converters for c.c.-c.a. conversion. and AC, as well as their associated control (IPC classification: H02M).

Several solutions are known for attenuating the alternative component of the DC power of single phase inverters. At present, the passive filtering solution, which uses a high capacity electrolytic capacitor on the DC line. of the inverter, it predominates in all applications due to its simplicity. However, new solutions are being sought to eliminate the main disadvantage of the passive method, namely the short life span of the electrolytic capacitor, which affects the reliability of the entire system as described in S. Hârb, R.S. Balog, "Reliability of Candidate Photovoltaic Module-Integrated-Inverter (PV-MII) Topologies-A Usage Model Approach," IEEE Transactions on Power Electronics, vol.28, no.6, 2013, pp.3019-3027.

Existing active decoupling methods are based on additional circuits operating as active power filters that extract the oscillating component of power and divert it to a short-term storage element that may be a small capacity capacitor, or coil. Low capacity polypropylene film capacitors are preferred due to their high lifetime, thus eliminating the aforementioned problem of electrolytic capacitors. However, existing active solutions require additional components, such as: transistors, diodes, filters. Therefore, the complexity of the converters will increase, their cost will be higher and the overall efficiency will decrease. A recent analysis of the main existing decoupling methods, mainly those for photovoltaic (PV) panel inverters, is presented in the work of H. Hu, S. Harb, N. Kutkut, I. Batarseh, Z.J. Shen, "A Review of Power Decoupling Techniques for Micro-inverters with Three Different Decoupling Capacitor Locations in PV Systems," IEEE Transactions on Power Electronics, vol. 28, no. 6, 2013 pp. 2711 - 2726. It is found that the solutions analyzed in the technical literature require at least one additional semiconductor element and passive elements. Next, they will be discussed

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i have existing inventions worldwide, highlighting their main disadvantages compared to the proposed invention.

A general solution for minimizing the alternative component of power in single-phase converters is presented in Philip T. Krein, Robert S. Balog, "METHODS FOR MINIMIZING DOUBLE-FREQUENCY RIPPLE POWER IN SINGLE-PHASE POWER CONDITIONERS", US 8325499 B2, 2012. This consists of adding an energy storage system and a power interface that deflects the oscillatory component of power, which is transferred from a source to a load, or a single-phase network, to the energy storage unit (eg capacitor). . In the mentioned patent, a control method is proposed, which is based on the generation of a voltage on the offset storage element by 45 ° with respect to the alternative output voltage. The method involves the use of an additional two-way converter for the storage element, which is a disadvantage of the invention.

In Shangzhi Pan, Sayed Aii Khajehoddin, Praveen K. Jain, Alireza Bakhshai, "POWER CONVERTER FOR A POWER GENERATOR", US 2011/0261593 Al, an electronic power converter is proposed for interfacing low power generators (e.g. PV , wind turbines) with single phase network. The converter contains two stages of power conversion, at the entrance a DC-DC floor, respectively an inverter at the output. By controlling the two floors the elimination of the oscillating component from the entrance is obtained, which will be processed on the c.c. of the inverter where a capacitor of reduced capacity is used (two orders of magnitude smaller than in the case of passive filtering method). As a result, the voltage of c.c. it will not remain constant (or negligible ripple) as in the case of conventional inverters, but will contain a high alternative component. In order to eliminate the harmonic pollution of the current injected into the network due to the variation of the DC voltage, a correction is applied to the reference voltage applied to the PWM generator for inverter control. Although the invention provides for the decoupling of the alternative component of power by using a small capacity capacitor, its main disadvantage is that it can only be applied by adding a c.c.-c.c. Also, for applications that require two-way power transfer between input and output (eg in the case of an energy storage system), the complexity of the DC-DC converter it will grow considerably.

Another invention, Dong Dong, Dushan Boroyevich, Ruxi Wang, Fred Wang, "TWOSTAGE SINGLE PHASE BI-DIRECTIONAL PWM POWER CONVERTER WITH DC LINK CAPACITOR REDUCTION", US 2012/0257429 Al, can provide substantial reduction of the DC filter capacitor, it can also be adapted for applications with two-way transfer a) / ^ -2014-00611

1 -00- 2014 power, and consists of adding an arm with two series transistors with active filter role on the line

D.C. of the inverter. Thus, the deviation of the oscillating component of the power towards this capacitor is ensured. An application is exemplified in which the capacitance reduction is about 30 times that of passive filtering. The main disadvantage of the method is the addition of two transistors and a filter inductor to ensure the decoupling function, increasing the complexity of the system which will be reflected at a higher cost.

In the Sayed Aii Khajehoddin, Praveen Jain, Alireza Bakhshai patent, "INVERTER FOR A DISTRIBUTED POWER GENERATOR", US 8670249 B2, 2014, a current inverter is proposed for connecting PV panels to the single phase network, where a DC inductor is used. and a DC-DC converter with a transistor and a diode which, in addition to adapting the voltage level at the input, also provides the decoupling function. In this case, no capacitors are used but the coil inside the current inverter is used as a short-term storage element for filtering the oscillating component of power. The coil will be driven by the DC current to which a high alternating component is added, which can lead to saturation of the magnetic circuit. In addition to this, the decoupling additionally requires a transistor and a diode, which represents the main disadvantages of the invention.

The object of the invention is to develop a circuit and a method of decoupling the oscillating component of the power for single phase inverters in H-bridge, without using additional semiconductor elements.

The technical problem solved by the invention consists in ensuring the decoupling of the oscillating component of power in the case of single phase inverters only by the control of the inverter and the use of non-polarized capacitors with polypropylene film, connected on each arm of the inverter bridge between the midpoint and one of the DC line terminals. As a result, the decoupling function is provided under similar conditions to the solutions presented above, but without the use of additional semiconductor elements, which represents the main advantage of the proposed invention. Another advantage of the invention is that it can be used in different applications that require a single phase inverter, which can operate unidirectionally (eg interfacing renewable energy sources to the grid, or supplying a single phase load), or bidirectional (eg. energy storage). Due to the small number of components required, the complexity of the system is reduced, which is another advantage of the invention.

Following is an example of embodiment of the invention in relation to Figures 1 ... 4, which shows:

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FIG. 1: Schematic of the single phase inverter with decoupling circuit, with the common point of the decoupling capacitors connected in two variants: a) to the negative terminal DC-; b) at the DC + positive terminal;

FIG. 2: The main waveforms characteristic of the single phase inverter with the invented decoupling method;

FIG. 3: General block diagram of the single-phase inverter control circuit with the invented method and decoupling circuit;

FIG. 4: Experimental results: a) idea with and without decoupling; b) Uj _c with and without decoupling; c) harmonic analysis of the current of ideas with and without decoupling; d) voltage (ucdi) and current (icdi) of a decoupling capacitor; e) voltage (w ₀ ) and current (z ₀ ) on load, current through the first arm of the inverter (G).

Fig. 1 shows the diagram of the single phase inverter with a circuit for decoupling the alternative component of the power. The system consists of a voltage inverter 1 in bridge H with four transistors T1-T4, which is supplied by a DC line 2 with the voltage Ude. The Ti-T ₄ transistors are controlled by pulse width modulated (PWM). The midpoints A and B of the two arms of the inverter axle are connected to two filter coils 3, each having the inductance Ly. At the output terminals of the coils are coupled two nonpolarized capacitors 4 of equal capacity Cd, the other two terminals of the capacitors being connected together to one of the DC terminals of the inverter 5, DC + (Fig. A) or DC- (Fig. Lb ). Depending on the application in use, the output of inverter 6 in ac (with voltage w ₀ ) can be connected to a single-phase network, single-phase generator, or to a local load. According to this figure, it is indicated that the circuit and the corresponding control method ensure the transfer of the oscillating component of power (p) to the two decoupling capacitors, so that the DC line of the inverter will be traveled only by the average power component (P). ). at the same time the invented solution will not adversely affect the primary function of the inverter, so that the output sizes u ₀ and i ₀ will not be affected.

The principle of the inverter control method with the decoupling circuit consists in changing the control voltages of the two arms of the H-bridge so that the alternating component of the output power is transferred to the capacitors 4. Considering the case of linear load operation at the inverter terminals, the power Output p (f) snapshots can be expressed as follows:

p (t) = S cos φ - S cos (2 <ut - φ) (1) ^ -2014-00611 fr1 Ι '-ββ-2BU in which: P - represents the average power component; p - the oscillatory component, or so on, with a frequency equal to twice the output frequency;

By the invented method, it is required that the two capacitors of the decoupling circuit take over all the alternative component p = p _{C (} ii + Pcd2, that is:

p - C _d u _cdl du _{cdl tn} du _cd2

WVWV (2) at the same time, it is required that the output voltage (m _o ) is not affected by the decoupling circuit, that is:

u _o = Wcdi ~ ^u cd2 = V2i / _O sin (iut) (3)

Solving the system of equations given by (2) and (3), results the mathematical expression of the voltages at the terminals of the two capacitors, according to the invented method:

, V2t / _O ~ 2 sin (tat) +1 —— sin (2 <ut - φ) - 2 [/ ² sin ² (<ut) + 4t? -I --- _a sin (<p) (4) where: U _a - the effective value of the output voltage; 5 - the apparent output power; ω - output voltage pulse; φ - the phase difference between the voltage and the output current; C _d - the capacity of a decoupling capacitor; Uco - an initial voltage on capacitors, which is numerically determined.

For generalization, the above relationship can be written in reported units (u.r.) as follows:

'iS ^t 2 2S * —rsin (2mt - φ) - 2 sin ² (rut) -I- 4U _c0 -I - r-sin (^) (5) in which were taken as the basic sizes: U _b = U _o , S _b = S _n , C _b _ Sn <*> n ^v b with S _n - the nominal apparent power, and ω „- the nominal pulse.

Starting from equation (5), Fig. 2 shows the main waveforms characteristic to the inverter with the invented decoupling method. The voltages on the two decoupling capacitors, together with the output voltage, are shown in Fig. 2a, and the main currents in the circuit are represented in Fig. 2b. It should be noted that the DC line current i _dc component contains only the average Ufr- As a result of the inventive control method, the voltages on the two capacitors decoupling circuit will contain higher even order harmonics, the main one being the order of 2 but the output voltage is not affected. Also, the currents through the capacitors and the arms of the inverter will contain higher harmonics, as shown in

V 2 Ο Η - Ο Ο 6 1 1 3

1 -08- 2014

Fig. 2c. in FIG. 2d the instantaneous values of the main powers in the circuit were represented over a period of alternating voltage. It is observed that p = p _Cdl + Pcd2, so that p _dc - P. Therefore, the decoupling function is performed, as claimed, without the use of additional semiconductor elements.

Fig. 3 shows the general block diagram of the control system for the single phase inverter with decoupling circuit, according to the invented method. This includes a subsystem 7 with regulators for output voltage (w _o ) and output current (z _o ) that provide the inverter reference voltage u _c ab · Implementation of these regulators can be done in several ways depending on the application in which the inverter is used, but the decoupling method can be added regardless of the solution adopted. In order to obtain the decoupling according to the invented method, the regulator 8 was added, which amplifies the harmonic components (2 and 4 mainly) of the current i _dc in order to cancel the pulsating current, providing at the output the compensation voltage u _comp . It should be emphasized that the addition of the regulator 8 provides the decoupling function according to the invented method. The signal u _c ab, multiplied by 1/2 by the amplification block 9, is gathered by means of an adder 10 with the signal u _CO mp, resulting in the control voltage ua, which is divided by a division block 11 at the voltage on the line. cc U _dc provides the fill factor Yes for the first arm of the inverter. Similarly, the voltage u _c ab, multiplied by 1/2 through the amplification block 9, is subtracted from the voltage u _com p through the adder 12 and results in the control voltage ub, which is divided by a division block 13 to the voltage on the line of cc U _dc provides the filling factor Db for the second arm of the inverter. Further, the resulting filling factors Da and Db are transmitted to a PWM 14 pulse generator, which provides 4 control pulses for the T1-T4 transistors within the inverter bridge.

Fig. 4 presents relevant experimental results to demonstrate the functionality of the single phase inverter with the decoupling circuit and the invented method. A laboratory stand with a single phase inverter was used which supplies a linear load of lkW at a voltage of 230V and a frequency of 50Hz. The DC supply voltage is 450V and the PWM switching frequency is 10kHz. The decoupling capacitors have the capacity C _d = 60pF each. Highlighting the reduction of the alternating current component i _dc was achieved by comparing the results obtained with the invented circuit and decoupling method, compared to the classic case of an inverter in bridge H without decoupling. Thus, FIG. 4a and FIG. 4b illustrates the current i _dc and the voltage v _dc in the two cases. A significant decrease of the oscillating component is observed. Also, the harmonic analysis of the current i _dc , from Fig. 4c shows a substantial attenuation of the main 100Hz alternative component. Fig. 4d and FIG. 4e shows the voltages on a capacitor

ον- 2 ο 14 - 0 0 6 1 1 ι ι-οι have decoupling also on the load, respectively the currents through the capacitor, filter and load. The waveform of the voltage on a decoupling capacitor is observed to be similar to that in Fig. 2a, and the current through the same capacitor contains, in addition to the low frequency harmonic components, a ripple produced by the switching of the transistors. It should be noted that the output voltage (v _o ) does not suffer from visible harmonic distortions, the total distortion factor of the voltage being about 2%, so the load supply is not affected by the circuit and the decoupling method invented.

Claims (3)

1. Circuit for decoupling the oscillating power for single phase inverters, characterized in that the inverter in bridge Η (1) is powered by a DC line (2) with the voltage Ude, the four transistors of the bridge being controlled with pulses modulated in width (PWM), the midpoints of the two arms of the inverter bridge are connected to two filter coils (3), and to the output terminals of the coils two unpolarized capacitors (4) are connected intended for decoupling the oscillating component (p) of the instantaneous output power (p), the other two terminals of the capacitors being connected together to one of the DC terminals. (5) of the inverter, and depending on the application the output in c.a. (6) of the inverter can be connected to a single-phase voltage network, to a generator, or to a load. 2. Method of decoupling the oscillating power for single phase inverters, characterized in that the two arms of the inverter with the decoupling circuit according to claim 1 are ordered to generate two voltages on the decoupling capacitors (4), according to equation 4, each voltage being composed. from half of the fundamental component from the output of the inverter, in antiphase for the two arms, to which is added a compensation component, containing harmonics of even-order voltage, which ensures the transfer of the oscillating power (p) to the decoupling capacitors, so that on the DC line of the inverter will find only the average component (/ ') of the power, and the voltage (w _o ) and the current (z _o ) at the output terminals (6) will not be affected. 3. The method of decoupling the oscillating power for single phase inverters according to claim 2, characterized in that the inverter control system includes in a block (7) regulators for the output voltage (w _o ) and the output current (z _o ). , which provides the reference voltage of the inverter u _c ab, and in order to obtain the decoupling according to claim 2, the controller (8) for the alternating component of the current id _{C has been added} , which amplifies only the harmonic components (two and four mainly) of the idc current. in order to cancel the pulsating current id _c , providing at the output a compensation voltage u _comp , and the signal u _c ab multiplied by 1/2 through the amplification block (9) is gathered by means of an adder (10) with the signal u _comp , resulting the control voltage u _A , which is divided by a division block (11) to the voltage on the DC line Ude provides the filling factor Yes for the first the arm of the inverter; similarly the voltage u _c ab multiplied by 1/2 through the amplification block (9) is subtracted from the voltage u _CO m _P through the adder (12) and results in the control voltage ub, which is divided by a division block (13) to the voltage from the DC line Ude provides the filling factor D _p for the second arm of the inverter, and subsequently the resulting filling factors D _A and Db are transmitted to a PWM generator (14), which provides control impulses for the transistors. from the inverter deck.