ES2264871B1 - ADAPTATIVE SIGMA-DELTA MODULATOR FOR THE CONTROL OF THE POWER STAGE OF AN ALTERNATE CONTINUOUS CONVERTER. - Google Patents

Abstract

Adaptive sigma-delta modulator for controlling the power stage of a DC to AC converter. The invention that is recommended consists in the development of an adaptive sigma-delta modulator that allows, by varying its feedback signal as a function of the amplitude of the voltage signal at its input, the generation of control signals for tripping of the power semiconductors that are part of a multilevel inverter bridge which acts as a demodulation circuit together with its LC filter. This multilevel power stage, controlled by said adaptive modulator, can be adapted to the switching technology of a resonant link, belonging to a network connection circuit, for example, of a photovoltaic system. Implemented in an FPGA, the adaptive modulator blocks are: Frequency Divider (6), Signal Generator (7), Sigma-Delta Modulation (8) and Conformer (9). The implementation is simple because it only needs the timing of the modulator sampling clock and has a better spectral response compared to other modulators.

Description

Adaptive sigma-delta modulator for controlling the power stage of a converter Continue to alternate.

Object of the invention

The present invention, framed in the sector of electronics, adapts modulation techniques sigma-delta to the control of an investor for the connection to an electrical network and, in particular, to the network of a Foltovoltaic system

The invention described herein refers to an adaptive sigma-delta modulator that allows control a power stage of a current converter continues to alternate, through an inverter bridge, in particular, multilevel and more particularly, five levels of capacitors floating.

It has its field of application within the scope of electronic power conversion systems derived from renewable energy sources, specifically the Photovoltaic Solar Energy.

The object of the invention is to develop the switching and modulation technologies necessary to increase maximum power density of a current converter continuous to alternate (DC-AC) to be applied in the connection to an electrical network and, particularly, of a system to from photovoltaic solar energy.

It is also the object of the invention to decrease Significantly the size of the DC-AC converter of a photovoltaic solar network connection system.

A final object of the invention is to have an inverter choir capacity to handle a power of 5 KW, to the once its external dimensions allow its installation in any room in a home, without its visual impact being appreciable, within a system for foltovoltaic buildings of Small size, in general.

Background of the invention

In order to bring solar energy closer photovoltaic to a competitive situation with the other classes from energy sources, there are currently multiple initiatives to regulation, programs and incentives.

Today, the law that regulates the market Energy aims to strengthen the installation of less systems of 5 KW, mainly used in homes, providing the maximum  electric power subsidy produced by this type of installations.

Leaving the isolated systems apart (from telecommunications, independent housing, water pumping, signaling, etc.), systems are of special interest photovoltaic connected to the power grid, especially those of little size.

In photovoltaic buildings connected to the distribution network, when the generated electrical power exceeds the consumed, the excess energy is injected into the network, selling To the electric company.

In view of the above, it seems foreseeable to extend the use of small facilities power (<5 KW), many of them in homes.

In this type of building systems Photovoltaic is very interesting to minimize the size of the electric power converters.

The converter size reduction can be achieve by reducing the size of the components inductive and capacitive that make up the necessary filters in the power stage

To do this, in order to obtain a technique of switching that allows higher power density, making smaller the converter for the same power handled, is it is necessary to increase the switching frequency of the converter DC-AC

With the technique that has been used in the actuality in this type of systems, with hard commutations and Pulse Width Modulation (PWM), increasing the frequency of switching implies an increase in the power of losses in the converter.

This fact involves two problems: the most important, it causes a decrease in the overall performance of the converter; another associated drawback is that it makes necessary larger dissipative elements, which goes against of the initial objective.

In the scientific literature published until the date, it is recommended for this type of converters the use of other modulation techniques Among all of them, the most appropriate is the  Sigma-Delta Modulation (SDM), since its operation is based on its output changes in moments predetermined by the sampling period of the modulator.

The sigma-delta modulators they belong to the family of Pulse Coded Modulators (PCM). Its origin is the field of communications, where they were developed to digitize analog audio signals usually and normally used in a subsequent transmission of said signal to through any digital communication channel (radio, cable, etc...). In power electronics they have been used for more than a decade, fundamentally the first order versions.

One of the main features differentiators of SDM modulators versus other types is that the frequency with which they operate is much greater than the theoretical defined by Nyquist for the reconstruction of a signal sine.

How can it be easily deduced from operation of a sigma-delta modulator, despite that your output changes in predetermined moments by the sampling frequency, the resulting signal is not periodic, because the level of the output signal depends on the input signal at modular and of the previous level of said output signal.

On the other hand, the spectrum of SDM modulation It contains components in a range greater than PWM modulation.

From this modulator Basic sigma-delta, various authors have developed variations that allow to improve the spectrum of output, to the extent that low order harmonics are seen dimmed

As mentioned before, the spectrum of the sigma-delta modulation presents harmonics in a bandwidth below the sampling rate of the modulator, which represents an inconvenience with respect to the PWM modulation. However the amplitude of these harmonics is much less than in that, which reduces, among other things, noise acoustic derived from its use. To make modulation competitive sigma-delta in front of the PWM becomes necessary expand bandwidth where there are no bass harmonics order.

Among some of the solutions offered by the SDM modulation, there are those of the following modulators:

i)

Sigma-Delta modulator Vector

ii)

Interpolative Modulator

iii)

Adaptive Modulator

The first two classes of SDM modulators (vector and interpolative) manage to shift the spectrum towards frequency zones well above the baseband, thinking that it will be the output filter of the power stage (an LC filter or the motor inductance) who eliminates that spectral noise.

In adaptive modulators, the foundation of adaptive quantification is to make the signal amplitude output vary to match the variation of the signal entry.

The adaptive modulators that are known up to now they are not directly applicable to the control of a stage of power, but used in signal digitalization Analog audio for transmission in digital format.

Given this consideration, there would be to adapt the adaptive modulator to the possible stages of power to use

In the case of the electronic power circuit used in a photovoltaic system, adapting the period of modulator sampling based on the amplitude of the voltage of entry not only does not bring any improvement to the operation of the modulator, but often makes it worse.

In the photovoltaic conversion, facing the obtaining electrical energy from solar radiation, it uses an electronic power circuit to get the alternating power generation from the continuous obtained from photovoltaic solar panels.

Figure 1 illustrates the block diagram of a photovoltaic system for generating electricity to network. In it you can see the layout of the converter continuous (DC-DC) that allows obtaining the point of work of the photovoltaic panels in order to get the point of maximum power. The control circuit must measure the voltage and current at the input of the converter for later calculate the input power and, applying the algorithm of proper tracking, get the operating point of the converter that allows to optimize the extracted power.

Next to the converter DC-DC, the converter is connected DC-AC, which performs the energy conversion electrical in continuous, produced by the panels, at a voltage and alternating current for later injection into the power grid. Spanish legislation requires that the photovoltaic field be is galvanically isolated from the power grid to which connect the system In this case, said isolation is achieved by using a transformer at the output of the converter DC-AC The control circuit must get the inverter behavior that allows it to inject current into the electrical network so that it is in phase with the voltage of the network, providing in that case the maximum transfer of Energy. To do this, the voltage and current of the converter output.

It is known that any power system in that components with variable frequency response are used, as is the case of an LC filter at the output of an inverter bridge, requires a closed loop control that ensures its stability against changes in any of the parameters that affect.

Inside the feedback loop of a circuit of control of an inverter, is the modulator used to generate the control signals of the stage semiconductors of power

Considering everything explained above with reference to the best modulation response sigma-delta versus others in this type of circuits, to be able to apply an SDM modulator to a circuit control as described, used in a foltovoltaic system of connection to the mains, you have to adapt the modulator sigma-delta to such a circuit in the stage of power.

In conclusion, the way to solve the dilemma raised is to improve the frequency spectrum of the modulation sigma-delta while the modulator is adapted in the connection circuit to the electrical network of a system Foltovoltaic, to give maximum power density adjusting to the small sizes of the required converters in buildings with solar energy.

From the literature found on the subject, it is concluded that to extend the power ranges of a high frequency sigma-delta modulation, you have to develop extended SDM modulation techniques (multilevel type or non-constant switching period or, in general, of type hybrid) to obtain a lower switching frequency with the Same behavior of the baseband.

This may lead to think that a possible way to improve the spectral behavior of the output of a Adaptive SDM modulator consists of adjusting the sampling period according to the variations of the amplitude of the input signal. But, it has been empirically proven that the adaptation of the Sample rate does not yield favorable results.

Description of the invention

In order to solve the problem previously exposed, in each and every one of the aspects commented, the proposed invention consists of a modulator adaptive sigma-delta for one stage control of power of a converter of continuous to alternating, used in a mains connection circuit, in which the amplitude of the feedback signal to the modulator varies adapting to the level of the input signal and is characterized because in the feedback A circuit that behaves like a demodulator is connected.

Said circuit, which reconstructs the signal of input demodulating the modulator output signal with the level maximum that has been modulated, is constituted by a converter of power and the load LC filter that is usually connected to your exit.

The right solution for controlling a mains connection inverter circuit, preferably used in a system of small photovoltaic buildings is to adapt the amplitude of the feedback signal, of the output from the modulator to its entrance.

The drawback is that by varying the feedback, increases the amplitude of the lower harmonics of the spectrum and adaptive modulator becomes saturated. To avoid this problem, the proposed solution lies in demodulation, so that when demodulating the adaptive modulator output signal, apply the maximum value with which it has been modulated so that the signal reconstruction be perfect.

In this way, the benefits of a sigma-delta modulation with frequency of high sampling and those of the adaptive modulator in terms of its best signal-to-noise ratio

This modification in the circuit of the stage of power to include a demodulator prevents the use of a bridge traditional inverter, but it is solved with an inverter bridge multilevel

There are basically two kinds of circuits of power stages that allow to obtain an output voltage of the multilevel type known as diode fixing and called floating condenser.

Either configuration power stage circuits is susceptible to be used as signal demodulator from the adaptive modulator. The second option is preferred because it requires less semiconductors

Therefore, a multilevel investor is used in in conjunction with the output LC filter of any bridge investor. In particular, a bridge is preferably used Five-level inverter with floating capacitors, without diodes blocking.

The improvement in the spectral behavior of the multilevel inverter bridge output, controlling it with the adaptive modulator of the invention, is reflected in a reduction of its amplitude within the band of interest by more than 50% with compared to other SDM modulators, such as interpolative, keeping the start of harmonics on the same frequency significant sigma-delta spectrum, but making the low order harmonics disappear.

To such improved bridge behavior inverter, the advantage that the use of modulation is added SDM, particularly an adaptive modulator, when control a multilevel inverter, since it only needs a fixed period timer to implement the period of sampling, while an equivalent PWM modulator uses more than one or requires a variable period timer, with the complexity that this implies in its implementation.

It is achieved, then, with the new modulator adaptive sigma-delta, a frequency spectrum enhanced with high simplicity of the modulator circuit.

The adaptive modulator described is the one that It also acts as a generator circuit for the control signals of the power stages in DC-AC converters for a connection to the power grid, for example, of a central Solar photovoltaic.

Said adaptive modulator is implementable in a programmable logic digital device, such as a microprocessor, a digital DSP signal processor, a microcontroller, an FPGA card, etc.

Description of the drawings

To complement the description that is being performing and in order to help a better understanding of the characteristics of the invention, according to a preferred example of practical realization of it, is accompanied as part member of that description, a set of drawings where with illustrative and non-limiting nature, what has been represented next:

Figure 1.- Shows a block diagram of a foltovoltaic system for connection to the distribution network electrical, according to the State of the Art of the systems of Photovoltaic Solar Energy.

Figure 2.- Shows a block diagram of the multilevel inverter circuit controlled with the modulator adaptive sigma-delta, according to the purpose of the invention and according to a preferred embodiment.

Figure 3.- Shows a circuit scheme printed card corresponding to the power stage of the multilevel bridge, controlled by the modulator adaptive sigma-delta of the invention, according to the preferred embodiment of the previous figure, with the bridge Five-level inverter of floating capacitors.

Figure 4.- Shows a block diagram functional adaptive sigma-delta modulator to generate the control signals of the multilevel inverter bridge, according to the preferred embodiment to which figures 2 correspond and 3, in accordance with the object of the invention.

Figure 5.- Shows a representation of the waveforms of the output voltage of the multilevel inverter and after the LC filter, for the preferred embodiment with resistive load and the following parameters: load resistance (RL) =
100 \ Omega; input voltage (Vi) = 124 V; frequency of the reference signal at the output (f) = 50 Hz sampling frequency of the modulator (fs) = 200 kHz.

Figure 6.- Shows a detail of the waveforms represented in the previous figure, where appreciate the effect of adaptive modulation on the jumps of output voltage level.

Figure 7.- Shows a representation of the far spectrum, up to 200 kHz, of the output voltage of the multi-level inverter shown in figure 5.

Figure 8.- Shows a representation of the near spectrum, up to 10 kHz, of the output voltage of the multi-level inverter shown in figure 5.

Preferred Embodiment of the Invention

As illustrated in Figure 2, you can be described as one of the possible embodiments of the invention an adaptive sigma-delta modulator (1) that controls a multi-level inverter bridge (2) of five capacitor levels floating, to which it is connected.

The adaptive modulator (1) is implemented on the development platform of the FPGA EPF10K20RC240, which found on the University Program Design business card ALTERA Laboratory Package Board.

In turn, the inverter bridge (2) is connected to two power supplies (3, 3 ') and, at its output, to the LC filter (4) and the load (5) of the power stage.

Said five-level multi-level inverter bridge (2) levels, whose printed circuit is drawn in Figure 3, are It consists of eight Bipolar Transistors with Insulated Door (IGBT), which are controlled by the signals generated by the modulator (1) and fed by the two source cards (3.3 ').

The integrated FPGA circuit EPF10K20RC240 where the sigma-delta modulator is developed (1) thus constitutes a programmable logic device, in which produces the synthesis of control signals for eights trip switches representing transistors (IGBT) of the bridge (2). Said device allows to reduce significantly the construction costs of the modulator (1) of the invention. Your functional block diagram implemented in VHDL language for standard programming circuit description for FPGA, it is illustrated in Figure 4, where four are distinguished modules:

?

Divider Module Frequencies (6): It is responsible for generating a frequency (200 kHz) of sampling (CLK ') from the operating frequency of the circuit (25,175 MHz), which is the clock (CLK) of the FPGA card and that serves as input to the Conformer (9) of shots (DISP1- DISP8) towards the inverter bridge (2).

?

Signal Generator Module (7): It has the mission of generating a sequence of binary numbers, preferably 10-bit words, which correspond to the values of a 50 Hz frequency and amplitude sinusoid programmable, which is the sine reference signal to be modulated in the Sigma-Delta Modulator module (8).

The format of said reference sinusoidal signal allows, simply by a simple modification, use a sequence of data that it can come from an external control device, such as a microcontroller or a digital DSP signal processor. The data produced by this Signal Generator module (7) are extracted of a table saved and defined in a text file, where  They are previously calculated and measured.

In addition, the Signal Generator module (7) detects the situation within a window of the reference signal in each quadrant interval. These parameters, window and quadrant, are what determine the multilevel bridge levels (2), since the levels of voltage at the output of the power stage are defined with accuracy depending on the window, among the eight in which divide the sine of the reference signal, where it is located said signal.

The value of parameter that identifies the window where the sine signal of reference passes as input directly to the Conformer module (9), to give rise to the shot switching sequence (DISP1- DISP8), for each of the transistors (IGBT) of the bridge inverter (2), which produces the output voltage according to the corresponding level.

?

Modulator Module Sigma-Delta (8): It is the module that performs the adaptive modulation itself, as the object pursues of the invention, maintaining the load voltage in the multi-level inverter bridge floating capacitors (2).

The signal of feedback of the Sigma-Delta module (8) varies its amplitude depending on the window in which the signal is located input sine, adapting to amplitude levels determined for that window. With this the adaptation is achieved to the multilevel bridge (2), making the amplitude of feedback, depending on the quadrant in which the sinusoidal signal to modulate, to minimize the difference between the sine to modulate and the modulated signal, reducing the signal ratio at noise typical of sigma-delta modulation adaptive

?

Conformer Module (9): Your function is to apply the appropriate trigger control signals (DISP1- DISP8) to the eight transistors (IGBT) of the inverter bridge multilevel (2), as well as the relay trigger that activates the bridge initialization sequence (2).

The signal modulated generated by the Modulator module Sigma-Delta (8) is modified to be converted into the trigger signal of the inverter power stage (2) To this is unfolded into as many signals as devices Power semiconductors presents the inverter bridge (2); in this preferred embodiment, eight signals for transistors (IGBT), complementary with a dead time between each signal, adequate to the switching speed of such transistors (IGBT), preferably 120 ns, guaranteeing the balance of floating capacitor charges.

The voltage of inverter bridge output (2) and resistive load voltage (5) connected after the LC filter (4), which allows the circuit output the required sine voltage, whose values In the case of the example, they are L = 3 mH and C 50 µF. Over the signal reference sine, with an input amplitude of 124 V, can appreciate the five levels of tension achieved with the bridge multilevel (2).

By showing in detail the waveforms of the previous figure. you can see, in Figure 6, the effect of adaptive modulation, appreciating the zero crossing of the tension and the change caused in the adaptation of the feedback of the sigma-delta modulator (8). Modulation index The example is m = 1.

Figures 7 and 8 show the oscillogram of the output voltage spectrum of the multilevel inverter bridge (2) for the far spectrum (up to 200 kHz) and the near spectrum (up to 10 kHz) respectively, considering the spectra more unfavorable obtained from a high number of successive sweeps made with an oscilloscope.

In view of this description and game of figures, a person skilled in the art will understand that the invention has been described according to some preferred embodiments of the same, but it will be obvious to the person skilled in the art that multiple variations can be introduced in said preferred embodiments without leaving the object of the invention claimed.

Claims (6)

1. Adaptive sigma-delta modulator for the control of a power stage of a continuous-to-alternating converter, used in a circuit of connection to the electrical network, in which the amplitude of the feedback signal to the modulator varies adapting to the level of the input signal characterized in that in the feedback a circuit is connected that behaves like a power converter and a load LC filter, which reconstructs the input signal to the modulator, demodulating the output signal of said modulator. 2. Adaptive sigma-delta modulator for controlling a power stage of a DC to AC converter, according to claim 1, characterized in that said power converter of the demodulator circuit is a multi-level inverter bridge. 3. Adaptive sigma-delta modulator for controlling a power stage of a DC to AC converter, according to claim 2, characterized in that the multi-level inverter bridge is of floating capacitors. 4. Adaptive sigma-delta modulator for controlling a power stage of a continuous to alternating converter, according to claim 3, characterized in that the multi-level inverter bridge is five levels with floating capacitors. 5. Adaptive sigma-delta modulator for controlling a power stage of a DC to AC converter, according to claims 2 to 4, characterized in that it is implemented in a programmable logic device comprising: a Frequency Divider module (6) that generates the sampling frequency of the modulator from the FPGA clock frequency; a Signal Generator module (7), connected to the previous one, which generates a sequence of binary numbers corresponding to the values of an amplitude sinusoid programmable, which is the sine reference signal to modulate and detects the situation of said sinusoid within a window in each quadrant interval in which the signal is divided reference; a Sigma-Delta module (8) that the amplitude of the modulated signal varies, between the levels of amplitude dice of the multilevel inverter bridge, depending on the window in which the reference sine signal, of so that the difference between the signal to be modulated and the modulated signal, reducing the signal to noise ratio of the adaptive sigma-delta modulation; a Conformer module (9) that generates the signals of control of the multilevel inverter bridge, unfolding the signal modulated produced by the Sigma-Delta module (8) in as many signals as power semiconductor devices presents the multilevel inverter bridge. 6. Adaptive sigma-delta modulator for controlling a power stage of a DC to AC converter, according to claim 5, characterized in that the programmable logic device is selected from an FPGA card, a microcontroller or a digital DSP signal microprocessor .