The present invention relates to switching power converters. More particularly, the invention relates to a method and circuitry for improving the magnitude and shape of the output current of switching power converters.
Currently, there are several types of power converters which are widely used for the DC-to-DC (Direct Current), DC-to-AC (Alternating current), AC-to-DC and AC-to-AC power conversion. In some applications, the purpose of the converter is to provide a regulated output voltage. In other applications, the purpose of the power conversion scheme is to regulate the output current independent of the load voltage. For example, in the case of a battery charger, the converter needs to feed the battery with a current rather than with a fixed output voltage. Another example is a grid-connected inverter for feeding energy into the power line. In this case, the required shape of the current is a sinusoidal waveform synchronized to the grid frequency having a magnitude that depends on the power capabilities of the source, which can be an array of photovoltaic cells, fuel cells, wind turbines and the like. Thus, a large family of applications requires that the switch mode converter behaves like a current source rather than a voltage source.
According to the prior art, two approaches have been suggested in the prior art to achieve the current source behavior. The first one is by adding a current feedback loop to a voltage source. The second approach is based on circuits that behave naturally as current sources. The advantage of the natural current source approach is that it does not rely on extra control loops that add to the cost of the units and reduce the reliability. The latter is due to the fact that current control loops of a voltage source converter are sensitive to deterioration of electrical components, spurious signals, noise and the like. Once the current loop is lost, the system may not only malfunction but could be severely damaged when the voltage sourcing converter is connected to a load which behaves like a voltage source. On the other hand, if the converter behaves naturally as a current source, there is no danger of runaway even without a dedicated current loop, since the converter keeps the current at a safe level by itself.
Currently, there is a growing use of converters that are required to behave like a current source in connection with the drive for alternative energy sources. Alternative energy sources such as solar cells and wind generators, can be used in an autonomous mode and grid-connected mode. In the first case, the generated energy is used to feed local loads. This mode of operation is rather limited in that it does not contribute to the general electrical energy needs. In the second case, in grid-connected mode, the energy is fed to power line when energy is available (during the day in case of solar cells or windy periods in the case of wind generators). Hence, the power grid serves, in a sense, as energy storage. When the alternative energy is available, it absorbs energy, and at that time, lowers the need for electrical energy generation by the grid power stations. When the alternative energy source is unavailable, the power line can still feed all loads connected to it.
The case of a grid-connected solar cells array is an example of a case in which electrical energy needs to be fed into a voltage source. The power line system behaves as a rigid voltage source with a low internal resistance. Hence, when connecting an external source to the grid, one has to make sure that energy flows from the source to the power line and not backward from grid to source. The latter not only constitutes a malfunction, but may cause severe damage to the feeding source. Another issue that needs to be carefully taken care of is the requirement of feeding the power line with a current of low harmonics content. That is, the shape of the injected current needs to be close to sinusoidal and in phase with the line voltage. This requirement is mandatory since the injection of high harmonic current is harmful; it increases power losses and could generate voltage interferences that might disturb other customers of the power line. Further, the recommended and mandatory standards, such as “EN61000-3-2”, set a limit to the high order harmonics that are allowed to be injected into the power line.
Most prior art solutions to the problem of current injection to the grid are based on generating a voltage source with a sinusoidal average output voltage and a filter inductor to reduce the ripple current. Typically, the sinusoidal voltage source is generated by a switch mode converter running at high switching frequency. The fundamental block diagram 100 of this prior art approach is depicted in
Another prior art approach for current injection is described in connection with electronic ballasts for HID (High Intensity Discharge) lamps. Discharge lamps need to be fed by a current source to maintain stable operation. For example, such an approach is described in U.S. Pat. No. 7,084,584, as illustrated in
According to the prior art, a number of approaches have been proposed to realize the inherent current source behavior. One approach is based on a buck-boost or flyback converter. In this case the magnetic element is charged by the primary energy source and discharges into the load or power line (in the case of a grid-connected system). Such approach is disclosed in U.S. Pat. No. 7,084,584, as illustrated on
Therefore, there is a continuous need for providing current sourcing soft-switched inverters based on “AC inductors” that can be used reliably to inject to the grid a sinusoidal current, synchronized to the power line voltage. It is further desirable that such converters will be capable of operating without a current loop, or if a current loop is used, not to go astray or be damaged, when the current feedback is lost.
It is an object of the present invention to provide a method and circuitry for injecting a current into an output voltage source, such as a power line, while presenting to the input electrical energy source, such as a solar cell panel, the desired characteristics, such as the optimum loading for achieving the maximum power.
It is another object of the present invention to provide a method and circuitry for connecting an alternative energy source to the power line by a current source circuitry to reduce the cost and increase the reliability of the system.
It is still another object of the present invention to provide economical and efficient method and circuitry for improving the performance and reliability of switch mode based systems that behave as sinusoidal current sources.
It is still another object of the present invention to provide a method and circuitry for tracking the maximum power point of a source and behaving as a current source with respect to its load.
It is still another object of the present invention to provide a method and circuitry for transferring energy from an electrical energy source to power line by using an AC inductor as a current limiter and adjuster.
It is a further object of the present invention to provide a method and circuitry of a switch-mode inverter suitable for grid connection applications, which has improved reliability and improved programmability features compared to prior art grid-connected inverters.
It is still a further object of the present invention to provide a method and circuitry, in which a current feedback loop is not required for the basic operation of injection of a current into a power line.
It is still a further object of the present invention to provide a method and circuitry, in which the injection of a current into a power line is not influenced by distortions of the current within said power line.
It is still a further object of the present invention to provide a method and circuitry, in which the soft switching is provided for maintaining the output current signal substantially undistorted.
It is still a further object of the present invention to provide a method and circuitry, which is relatively inexpensive.
Other objects and advantages of the invention will become apparent as the description proceeds.
Hereinafter, when the term “inherent current source” is mentioned, it should be understood that it refers to a source that can inject the desired current independently from the voltage of the load.
The present invention is directed to a method and circuitry (apparatus) that behaves as an inherent current source, for injecting a sinusoidal current into a power line. According to an embodiment of the present invention, a current feedback loop is not required for the basic injection operation. In addition, if a current feedback loop is not used, undesired currents will not be injected to the grid, and the circuitry will not be damaged by the grid when the current feedback loop is lost.
The apparatus of a grid-connected switching inverter for injecting a current into a power line comprises: (a) an electrical energy source for providing the substantially DC voltage to said apparatus; (b) a switching inverter connected to said electrical energy source for converting said substantially DC voltage of said electrical energy source to a high frequency alternating voltage; (c) a waveform generator for controlling the magnitude shape and frequency of said alternating high frequency voltage outputted from said switching inverter by means of a control signal fed into said switching inverter; (d) an inductor connected to an output of said switching inverter for generating an alternating current of variable magnitude and frequency from said alternating high frequency voltage, wherein the magnitude of said alternating current depends on a frequency of said alternating high frequency voltage; (e) a rectifier connected in series with said inductor for rectifying said alternating current and for outputting a rectified unipolar alternating current, wherein the rectified average value of said alternating current is proportional to the absolute magnitude of the power line voltage; and (f) a polarity commutator connected to an output of said rectifier for converting said rectified unipolar alternating current into a bipolar alternating current, and for injecting said bipolar alternating current into a power line, wherein said bipolar alternating current is substantially in phase with and of shape of said power line voltage of said power line.
According to an embodiment of the present invention, the apparatus further comprises a MPPT unit for tracking the maximum power point of the electrical energy source.
According to another embodiment of the present invention, the electrical energy source is one or more of the following: (a) a solar cell; (b) a fuel cell; (c) a wind turbine; (d) a battery or accumulator; and (e) a generator.
According to a particular embodiment of the present invention, the switching inverter is a H-bridge or half bridge switching inverter.
According to another particular embodiment of the present invention, the polarity commutator is a H-bridge or half bridge polarity commutator.
According to still another particular embodiment of the present invention, the switching inverter comprises two or more switches that are driven by means of high frequency control signals outputted from high frequency drivers.
According to a further particular embodiment of the present invention, the polarity commutator comprises two or more switches that are driven by means of low frequency signals outputted from low frequency drivers.
According to an embodiment of the present invention, the magnitude of the alternating current of the inductor depends on the duty cycle of the control signal fed to the inverter.
According to another embodiment of the present invention, the magnitude of the alternating current of the inductor is controlled by means of sequences of ON and OFF periods of the high frequency control signal fed to the inverter.
According to still another embodiment of the present invention, the magnitude of the alternating current of the inductor is controlled by means of the duty cycle of the control signal fed to the inverter.
According to still another embodiment of the present invention, the apparatus further comprises a pulse width modulation unit for keeping a signal switching the switches at a predefined maximal frequency.
According to still another embodiment of the present invention, the apparatus further comprises a line synchronizer for sensing the line voltage and generating corresponding synchronization commands.
According to still another embodiment of the present invention, the synchronization commands are fed into the low frequency drivers.
According to still another embodiment of the present invention, the synchronization commands are fed into the waveform generator.
According to a particular embodiment of the present invention, the apparatus further comprises an overvoltage protection unit for guarding said apparatus from dangerous DC voltage levels under no load conditions.
According to a further embodiment of the present invention, the apparatus further comprises a microcontroller for controlling the operation of said apparatus.
According to still a further embodiment of the present invention, the apparatus further comprises a transformer for galvanically isolating the electrical energy source from the power line.
According to still a further embodiment of the present invention, the apparatus further implements soft switching of the switches of the inverter for reducing the power and stresses of said switches.
According to still a further embodiment of the present invention, the apparatus further comprises a DC-DC converter, placed between the electrical energy source, and switching inverter for adjusting the voltage level of said electrical energy source to that required for said switching inverter.
According to still a further embodiment of the present invention, the alternating current is generated in the inductor due to imposing a bipolar high frequency voltage across said inductor.
According to still a further embodiment of the present invention, the alternating current in the inductor is controlled by varying the frequency of the bipolar high frequency voltage.
According to still a further embodiment of the present invention, the alternating current in the inductor is controlled by toggling the bipolar voltage signal ON and OFF.
According to still a further embodiment of the present invention, the alternating current in the inductor is controlled by varying the ON time within each switching period of the bipolar voltage.
The method for injecting a current into a power line comprises: (a) providing an inductor, a terminal of which is, connected in series to a first input of a rectifier; (b) generating an alternating current of variable magnitude and frequency in said inductor by applying a high frequency voltage between the other terminal of said inductor and a second input of said rectifier; (c) rectifying said alternating current by means of said rectifier, thereby obtaining the magnitude of the rectified average value of the alternating current that is proportional to the absolute magnitude of the power line voltage; (d) commutating the polarity of the rectified alternating current to be synchronized with the phase of the power line current; and (e) injecting the synchronized rectified alternating current into said power line.
The method for injecting a current into a power line comprises: (a) providing the substantially DC voltage by means of an electrical energy source; (b) converting said substantially DC voltage of said electrical energy source to a high frequency alternating voltage by means of a switching inverter connected to said electrical energy source; (c) controlling the magnitude shape and frequency of said alternating high frequency voltage outputted from said switching inverter by means of a control signal fed into said switching inverter from a waveform generator; (d) generating an alternating current of variable magnitude and frequency from said alternating high frequency voltage by means of an inductor connected to an output of said switching inverter, wherein the magnitude of said alternating current depends on a frequency of said alternating high frequency voltage; (e) rectifying said alternating current and outputting a rectified unipolar alternating current by means of a rectifier connected in series with said inductor, wherein the rectified average value of said alternating current is proportional to the absolute magnitude of the power line voltage; and (f) converting said rectified unipolar alternating current into a bipolar alternating current and injecting said bipolar alternating current into a power line by means of a polarity commutator connected to an output of said rectifier, wherein said bipolar alternating current is substantially in phase with and of shape of said power line voltage of said power line.
In the drawings:
The present invention is directed to a method and circuitry (apparatus) that behaves as an inherent current source, for injecting a sinusoidal current into a power line.
It should be noted that the alternating voltage Vinv imposed on inductor Lh, causes it to operate as an AC inductor. Such voltage generates an alternating current ILh with a temporal magnitude that depends on the switching frequency and the wave form of Vinv. These are controlled by signal Vc that is generated by the Waveform Generator 807. By changing this control voltage as a function of time, the current ILh can be increased or decreased. For example, if the frequency of Vc is decreased, the switching frequency of the inverter will decrease and the Vinv frequency will be lower. This will increase the current ILh due to the fact that the impedance of an inductor becomes lower at lower frequencies. Vc can thus change the output voltage Vinv such that the envelope of ILh will be sinusoidal. After passing a Rectifier 803, the average current is unipolar having the form of a rectified sinusoidal wave. It should be noted that the rectified average value of the alternating current is proportional to the absolute magnitude of the power line voltage. The unipolar current is then converted into a bipolar AC current Iout by a Polarity Commutator 804 that might include a filter to attenuate the high frequency ripple current.
According to an embodiment of the present invention, MPPT unit 806 measures by means of a sensor 805 the magnitude of ILh, which in turn reflects the current injected to the grid (Power Line 107), and adjusting signal Vc (the Duty cycle of signal Vc) outputted from Waveform Generator 807 such that ILh has the maximum possible value. For a given Vline, a maximum Iout (when ILh also has the maximal value) is, by definition, the maximum power point.
It should be noted that Electrical Energy Source 101 can be any electrical source, such as one or more solar, fuel cells, wind turbines, batteries, accumulators, (diesel) generators, etc.
The generation of the required current to be injected to AC Power Line 107 requires the generation of control signals Vc by the waveform generator, such that the average waveform of the current IR (ILh after rectification) and, in turn, the average waveform of the output current Io, will be sinusoidal. According to an embodiment of the present invention, this requirement is met by controlling the inverter according to the mathematical relationship between the output of the inverter 802, the inductor current ILh and the outputted current to the grid. This relationship is shown in reference to
In
It is assumed that circuit 1200 of
During time interval t1-t2 (
During the time interval t1-t2, the peak inductor current Ipk is:
where L is the inductance of inductor Lh. Similarly, during the time interval t2-t3, Ipk is expressed as:
From (1) and (2), taking into account that (t3-t1) is half a switching cycle, yields:
where F is the switching frequency. Rearranging (3) back for Ipk, leads to:
where Īout is the output current averaged over the switching cycle.
It should be noted that the equation (5) is obtained considering some time instant t. Thus, output current Īout of inverter 802 (
According to an embodiment of the present invention, grid-connected inverter 802 delivers the current which is in phase with and of shape of the line voltage VLine. The line voltage is considered as:
V
Line(t)=Vout(t)=Vrms√{square root over (2)} sin 2πflinet (7)
where Vrms is the RMS (Root Mean Square) line voltage, and fline is the line frequency.
Consequently, the inverters output current is given by:
where Pout is an average power delivered to the line. Substituting (8) into (6) yields:
As a result, if the switching frequency F(t) is set (programmed) according to (7), output current Īout will follow the line voltage. In addition, it follows from (7) that when the line voltage is low, the frequency can go too high. Therefore, according to another embodiment of the present invention, this problem is overcome by providing a PWM (Pulse Width Modulation) unit. That is, the control signal is kept at some predefined maximum frequency Flim awhile; the pulse width (ton) of the control signal is varies to generate the desired current level. It can be shown that the average output current will follow the line voltage, if ton is programmed according to (8):
The signal VLine(t) can be obtained by sampling the output voltage, from a local oscillator locked to the power line frequency or from a table stored in a digital memory. As a result, the magnitude and shape of the output current of circuitry 1100 is improved.
According to still another embodiment of the present invention, the drive frequency F(t) and ton(t) are generated by a circuit that compares current ILh of inductor Lh and the waveform of VLine. The deviations are translated into the excitation of voltage Vc (
As would be clear to a person trained in the art, the grid connected inverter according to an embodiment of the present invention, adds an advantage of soft switching of all power components. Zero Voltage Switching (ZVS) of inverter 802 is achieved by the fact that the inductor Lh current is switching direction every half cycle of the switching cycle. Consequently, during the dead time following the turn OFF of one transistor in each of the half bridges of the inverter (switches Q1, Q2 and switches Q3, Q4), the current of Lh causes self commutating of the voltage of the mid point of the half bride, and the complementary switch will be turned ON under ZVS. The soft switching has the advantage of lowering the switching losses (of reducing the power and stresses of switches), and hence improving efficiency. Another advantage of the soft switching is the lowering of the spurious signals that may cause electromagnetic interferences. Polarity Commutator 804 is switching under zero voltage and current conditions—at the zero crossing of the line voltage. Furthermore, the rectifying diodes assembly 803, exhibits lower reverse recovery losses due to the fact that the current is controlled by the inductor Lh.
While some embodiments of the invention have been described by way of illustration, it will be apparent that the invention can be put into practice with many modifications, variations and adaptations, and with the use of numerous equivalents or alternative solutions that are within the scope of persons skilled in the art, without departing from the spirit of the invention or exceeding the scope of the claims.
Number | Date | Country | Kind |
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184358 | Jul 2007 | IL | national |
Number | Date | Country | |
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Parent | PCT/IL2008/000897 | Jun 2008 | US |
Child | 12645066 | US |