PARTIAL POWER CONVERTER (PPC) IN AN ELECTRICAL ENERGY SYSTEM

Abstract

A partial power converter (PPC) in an electrical power system, comprising an input capacitor connected in parallel to a power source vpv and connected to a primary winding of a transformer, wherein the primary winding is connected in series to a M1 transistor of the MOSFET (Metal Oxide Semiconductor Field Effect Transistor) type, wherein two secondary windings Ns1 and Ns2—both with the same number of turns, are connected, each one, in series by means of a terminal, with diodes D1 and D2, respectively, and said diodes D1 and D2 are connected to the respective ends of a capacitor Cdc output; the other terminal of the secondary winding Ns1 of the transformer is connected to one of the terminals of the primary winding, whereas the other terminal of the secondary winding Ns2 is connected to one of the terminals of transistor M1, and wherein the output capacitor Cdc serves as a link to connect to a next stage.

Description

FIELD OF THE INVENTION

The present invention relates to a static DC-DC converter that increases the input voltage, and further handles a reduced portion of the power delivered by an energy system as a power source. More specifically, it refers to a partial power converter (PPC) of the elevator type of an electric power system.

DESCRIPTION OF THE STATE OF THE ART

The patent application US2011215778 dated Sep. 8, 2011, entitled “Solar power converter with multiple outputs” by Chen Yu-kai describes a solar power converter with multiple outputs and conversion circuit thereof, wherein the solar power converter includes a power input terminal, a solar power unit and a solar power conversion circuit with multiple outputs including a primary circuit, a first output circuit, a second output circuit, and a transformer with a first auxiliary winding and a second auxiliary winding. An output terminal of the second output circuit is connected to the power input terminal in series for providing a third output voltage to a load unit. The third output voltage is a sum of an input voltage generated by the solar power unit and a second output voltage generated by the second output circuit.

The patent application US2011249474 dated Oct. 13, 2011, entitled “Method and apparatus for power conversion using a flyback converter” by Luo Yuhao describes a method and apparatus for performing power conversion using an interleaved flyback converter with alternating master and slave branches. The apparatus comprises a plurality of parallel connected flyback circuits; a controller is coupled to the switches within the flyback circuits to turn-on and turn-off the plurality of flyback circuits; a current monitor element at the output connected to the controller; a voltage monitor element connected to the controller; based on monitored current and voltage the controller controls the operation of flyback circuit.

The patent application US2009086514 dated Apr. 2, 2009, entitled “Flyback-type power converter” by Martin Fornage et al., describes a method and apparatus for converting DC input power to DC output power. The apparatus comprises a plurality of parallel connected flyback circuits. A controller is coupled to the switches within the flyback circuits to provide accurate timing and automatic current balancing amongst the plurality of flyback circuits.

None of the cited documents describes or teaches a static DC-DC converter that increases the input voltage, handling a reduced portion of the power delivered by an energy system as a power source.

SUMMARY OF THE INVENTION

An object of the invention is to develop a partial power converter (PPC) in an electrical energy system comprising an input capacitor connected in parallel to an energy source v_pv and connected to a primary winding of a transformer, wherein the primary winding is connected in series to a transistor M1 of the MOSFET (Metal Oxide Semiconductor Field Effect Transistor) type, wherein two secondary windings Ns1 and Ns2—both with the same number of turns, are connected, each one, in series by means of a terminal, with diodes D1 and D2, respectively, and said diodes D1 and D2 are connected to the respective ends of an output capacitor C_dc; the other terminal of the secondary winding Ns1 of the transformer is connected to one of the terminals of the primary winding, whereas the other terminal of the secondary winding Ns2 is connected to one of the terminals of transistor M1, wherein the output capacitor c_dc serves as a link to connect to a next stage; furthermore, a magnetization inductance Lm is present in the transformer, which is not physically connected, but it allows to represent the capacity of the transformer to store magnetic energy and transform it into voltage and current, where the energy source v_pv is a solar energy source, and the power converter is partial and of the elevator type, since it handles a portion of the total power.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 describes a partial power converter in a photovoltaic solar application.

FIG. 2 describes the partial-power ratio curve depending on the relation between output voltage and input voltage.

FIG. 3 describes the variation of power and voltage as a function of the solar irradiation variation.

FIG. 4 describes the time interval corresponding to the switching of the semiconductors.

FIG. 5 describes the operation principle of the power converter when the MOSFET M1 is activated and the diodes D1 and D2 are turned off.

FIG. 6 describes the operation principle of the power converter when the MOSFET M1 is turned off and the diodes D1 and D2 are activated.

FIG. 7 illustrates the waveforms of the above-detailed points for the analyzed switching instants, in addition to the mean value for the currents both in the MOSFET M1 as in the diodes D1 and D2.

FIG. 8 describes the Ripple of input current ipv in partial power converter (PPC) and traditional flyback converter (FPC).

FIG. 9 describes the overall efficiency of the partial power converter (PPC) and traditional flyback converter (FPC) based on the input power.

DESCRIPTION OF A PREFERRED EMBODIMENT

The present invention relates to a static DC-DC converter that increases the input voltage, and further handles a reduced portion of the power delivered by a power source. By managing a part of the power only, it is possible to increase the efficiency of the energy system, since the losses in the semiconductors are reduced. Due to the configuration, it is possible to improve the quality of voltage and current at the level of the input source, since the ripple in both variables is reduced. More specifically, it refers to a partial power converter (PPC) of the elevator type of an electric power system.

The applications of the invention are extended to those ones that require a direct-current voltage regulation. For the description of a preferred embodiment an application of the photovoltaic industry is described. The partial power converter (PPC) shown in FIG. 1 consists mainly of a transistor M1 of the MOSFET (Metal Oxide Semiconductor Field Effect Transistor) type and two diodes D1 and D2, which work depending on the operation to be executed, either allowing the passage of current or blocking the same. Furthermore, a transformer is used with a winding on the primary side with Np number of turns and two windings on the secondary side Ns1 and Ns2—both with the same number of turns. By having two secondary windings in the transformer, it is possible to have a first induced voltage v₂₁ and a second induced voltage v₂₂. In addition, it is considered that a magnetization inductance Lm is present in the transformer, which is not physically connected, but allows to represent the capacity of the transformer to store magnetic energy and transform it into voltage and current. The secondary windings, being directly connected to the primary one, allow to have a bypass path for a portion of the input current i_in, where said portion of current i_o is injected directly into the inverter stage, which is used regularly to connect the system to the electricity network. The current that enters the partial power converter i_pc is the difference between the current i_in minus the current i_o. This current—being less than the total current, leads the converter to process only a portion of the power. In addition, due to the serial connection in the windings, it is possible to increase the voltage at the output of the converter, preventing the inverter from carrying out the entire process of increasing the input voltage v_pv, which implies a stress reduction both semiconductors as in the transformer. Another advantage deriving from the presented configuration is the reduction of the ripple of both the input current i_pv as of the input voltage v_pv.

To define the concept of power partiality a variable P_pr is defined, which is a ratio that relates the power handled by the P_pc converter divided for the power delivered by the P_pv system. If this ratio is less than the unity, then the converter is defined as a partial power converter. The closer the ratio is to zero, the smallest possible amount of power is handled by the converter. The power delivered by the P_pv system is expressed as the multiplication of the input voltage v_pv by the input current i_pv, while the power handled by the P_pc converter is represented as the multiplication of the input voltage v_pv by the current that enters the converter i_pc. The output power of the converter P_o is expressed as the multiplication of the output voltage v_dc by the output current i_dc. Considering that the efficiency of the system η is equal to the ratio between the output power P_o divided for the input power P_in, and said efficiency is unitary, the partiality ratio is expressed with the following equation (1),

$\begin{array}{cc}{P}_{\mathrm{pr}}=\frac{1}{\eta }-\frac{1}{G_v}& \left(1\right)\end{array}$

FIG. 2 shows the graph that represents the power partiality ratio P_pr. It is appreciated that the partial power ratio depends on the voltage gain Gv, which is the ratio between the output voltage v_dc divided for the input voltage v_pv. Due to the configuration, it is considered as a partial power converter provided that the output voltage is greater than the input voltage, since the power only flows in one direction and must be positive. In addition, the figure shows that the power partiality ratio P_pr is inferior, as long as the voltage gain decreases.

It is known that the partial power converter must be able to increase the voltage to reach the required output value v_dc; furthermore, the input voltage at the solar panel level v_pv varies depending on the solar irradiation, as described in FIG. 3, where this voltage variation is shown as Δ_vmpp. In this case, the objective is to maintain the voltage of the solar panel at the point of maximum power mpp. By varying this value, the partial power converter must be able to compensate this variation by modifying the voltage v_pc. The lower the ratio of power partiality Ppr, less power is processed by the partial power converter, which improves its efficiency. While the higher the ratio of power partiality P_pr, more power is processed by the partial power converter, which guarantees the operation over a greater range of voltage variation Δ_vmpp. Therefore, the application of the invention will be the one defined by the point of operation and it is the one that serves as the basis for the design of the partial power converter.

To conduct the analysis of the functioning, the ideal operation of the partial power converter is assumed. The number of turns in the secondary windings Ns1 and Ns2 is equal; therefore, Ns is defined as the number of turns of the secondary winding, which guarantees that the voltages v21 and v22 are similar and avoids imbalances in the currents reflected to the primary side. Under this criterion a new variable v₂ is defined that is equal to v₂₁ and equal to v₂₂. Therefore, the output voltage v_dc is equal to the sum of the input voltage v_pv plus twice the voltage v₂. In addition, the variable N is defined as the ratio of turns between the turns of the primary winding Np divided by the turns of the secondary winding Ns.

The operation of the partial power converter is deduced by the switching states of the semiconductors M1, D1 and D2. FIG. 4 shows the time interval in which the MOSFET M1 is turned on and off. During the time Ta the MOSFET M1 is activated, thus allowing the passage of current i_pc and the diodes D1 and D2 turn off, whereas the MOSFET M1 turns off during the time Tb, thus interrupting the passage of the current i_pc, and the diodes D1 and D2 turn on. The switching period is represented as T, which represents the sum of the time Ta plus Tb. The working cycle D is defined as the ratio between the time that Ta is activated divided for the switching period T. The working cycle varies between 0-1.

During the time interval between [0<t≤DT] the MOSFET M1 is activated, whereas the diodes D1 and D2 are turned off as shown in FIG. 5; during this interval, the voltage at the magnetization inductance v₁ is equal to the voltage at the input source v_pv. The current through the magnetization inductance Lm grows linearly with a slope as shown in the following equation (2),

$\begin{array}{cc}\frac{d_{{\mathrm{iL}}_m}}{\mathrm{dt}}=\frac{v_{\mathrm{pv}}}{L_m}& \left(2\right)\end{array}$

The current in the MOSFET M1 grows with the same slope described in equation (2), whereas the voltage is zero. During this time the diodes D1 and D2 are off; therefore, the current i_o is zero, whereas the voltage in each of the diodes is described as the primary voltage reflected to the secondary side plus half the difference between the output voltage and the input voltage (3),

$\begin{array}{cc}{v}_d=-\left(\frac{v_{\mathrm{pv}}}{n}+\frac{v_{\mathrm{dc}}-v_{\mathrm{pv}}}{2}\right)& \left(3\right)\end{array}$

During the time interval between [DT<t≤T] the MOSFET M1 turns off, whereas the diodes D1 and D2 are activated as shown in FIG. 6. During this interval, the voltage in the magnetization inductance v₁ is equal to the voltage in the output source v₂ reflected on the primary of the transformer with negative sign, due to the connection of the windings. The current through the magnetization inductance L_m decreases linearly with a slope as shown in the following equation (4),

$\begin{array}{cc}\frac{d_{{\mathrm{iL}}_m}}{\mathrm{dt}}=-\frac{N\left(v_{\mathrm{dc}}-v_{\mathrm{pv}}\right)}{2L_m}& \left(4\right)\end{array}$

The current in the MOSFET M1 is zero during the switching interval, whereas the voltage in the MOSFET M1 is the sum of the input voltage v_pv plus half the difference between the output voltage v_dc minus v_pv reflected on the primary of the transformer,

$\begin{array}{cc}{v}_m=v_{\mathrm{pv}}+\frac{N\left(v_{\mathrm{dc}}-v_{\mathrm{pv}}\right)}{2}& \left(5\right)\end{array}$

Assuming the analysis with ideal diodes, the voltage in the diodes is zero, the current in the diodes i_o decreases with the same slope described in equation (4) reflected on the secondary of the transformer.

It is possible to relate the behavior of the dynamics of the current in the inductance L_m for both switching intervals with the following equation (6),

$\begin{array}{cc}\frac{d_{{\mathrm{iL}}_m}}{\mathrm{dt}}=\frac{v_{\mathrm{pv}}}{L_m}D-\frac{N\left(v_{\mathrm{dc}}-v_{\mathrm{pv}}\right)}{2L_m}\left(1-D\right)& \left(6\right)\end{array}$

In steady state, i.e., when there are no variations around the point of operation, the value of the derivative of the current with respect to time is zero. Then equation (6) is worded as follows:

$\begin{array}{cc}0=\frac{v_{\mathrm{pv}}}{L_m}D-\frac{N\left(v_{\mathrm{dc}}-v_{\mathrm{pv}}\right)}{2L_m}\left(1-D\right)& \left(7\right)\end{array}$

By ordering the equation (7), there is the equation (8) existing between the output voltage v_dc and the input voltage v_pv,

$\begin{array}{cc}\frac{v_{\mathrm{dc}}}{v_{\mathrm{pv}}}=G_v=\frac{2D}{N\left(1-D\right)}+1& \left(8\right)\end{array}$

This equation (8) is also known as the voltage gain Gv of the partial power converter.

FIG. 7 illustrates the ideal current and voltage waveforms of the above-detailed points for the switching instants analyzed, along with the mean value for the currents both in the M1 as in the diodes D1 and D2: (a) trigger signal for the MOSFET M1; (b) voltage in the magnetization inductance Lm; (c) current in the magnetization inductance Lm; (d) current in the MOSFET M1; (e) voltage in the MOSFET M1; (f) current in diodes D1-D2; (g) voltage in diodes D1-D2.

Simulation tests have been conducted in order to validate the proposed partial power converter, in addition to comparing it with a traditional flyback converter. For this experiment, both converters have been designed to work in the same operation points. A photovoltaic panel was used as an input source, and a DC-AC converter was used to connect to a single-phase network. For the design, it has been considered that the working cycle is equal to half of the commutation period D=0.5, which allows a symmetrical magnetic flow in the transformer, and therefore the losses by magnetic transfer are reduced. The current ripple in the solar panel Δ_ipv and the overall efficiency of the DC-DC conversion stage have been considered as comparison measures. The first parameter is related to the design of the input capacitor and the efficiency of the tracking of the maximum power point. By presenting a lower current ripple value, then the size of the capacitor can be reduced. Furthermore, the point of maximum power shows oscillations of smaller amplitude. FIG. 8 shows the results obtained from the current ripple, wherein the partial power converter has a lower current ripple than the traditional flyback converter (FPC). FIG. 9 shows the results obtained from the overall efficiency of the partial power converter, i.e., the efficiency associated with the tracking of the maximum power point and the conversion efficiency. In the same way as in the previous case, they have been compared in both converters and the overall efficiency of the proposed partial power converter is greater due to the above-mentioned advantages, which are the lower elevation ratio and the smaller current ripple.

From the constructive point of view, the converter consists of a capacitor connected in parallel to the power source; a transformer with a primary winding and two secondary windings is required. The primary winding is connected in series to a MOSFET M1 and this is connected in parallel with the input capacitor. The secondary windings—both with the same number of turns, are connected in series with the diodes and the output capacitor. A terminal of one of the secondary windings of the transformer is connected to one of the terminals of the primary winding, whereas from the other secondary winding, one of the terminals is connected to one of the terminals of the MOSFET M1. The output capacitor serves as a link to connect to a next stage, which will depend on the application.

Claims

1. A partial power converter (PPC) in an electric power system, CHARACTERIZED in that it comprises an input capacitor connected in parallel to a power source vpv and connected to a primary winding of a transformer, wherein the primary winding is connected in series to a M1 transistor of the MOSFET (Metal Oxide Semiconductor Field Effect Transistor) type, wherein two secondary windings Ns1 and Ns2—both with the same number of turns, are connected, each one, in series by means of a terminal, with diodes D1 and D2, respectively, and said diodes D1 and D2 are connected to the respective ends of a capacitor output Cdc; the other terminal of the secondary winding Ns1 of the transformer is connected to one of the terminals of the primary winding, whereas the other terminal of the secondary winding Ns2 is connected to one of the terminals of transistor Ml, and wherein the output capacitor Cdc serves as a link to connect to a next stage.

2. The partial power converter according to claim 1, CHARACTERIZED in that it further comprises a magnetization inductance Lm presented in the transformer, which is not physically connected, but allows to represent the capacity of the transformer to store magnetic energy and transform it into voltage and current.

3. The partial power converter according to claim 1, CHARACTERIZED in that the energy source vpv is a solar energy source.

4. The partial power converter according to claim 1, CHARACTERIZED in that it is of the elevator type.

5. The partial power converter according to claim 1, CHARACTERIZED in that it handles a portion of the total power.