CN113783164B - Control method for three-phase photovoltaic grid-connected inverter relay closing time sequence - Google Patents
Control method for three-phase photovoltaic grid-connected inverter relay closing time sequence Download PDFInfo
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- CN113783164B CN113783164B CN202111040594.5A CN202111040594A CN113783164B CN 113783164 B CN113783164 B CN 113783164B CN 202111040594 A CN202111040594 A CN 202111040594A CN 113783164 B CN113783164 B CN 113783164B
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/10—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers
- H02H7/12—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers
- H02H7/122—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers for inverters, i.e. dc/ac converters
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E10/00—Energy generation through renewable energy sources
- Y02E10/50—Photovoltaic [PV] energy
- Y02E10/56—Power conversion systems, e.g. maximum power point trackers
Abstract
The invention relates to the technical field of photovoltaic inverters, and particularly discloses a control method of a three-phase photovoltaic grid-connected inverter relay closing time sequence, wherein the control method comprises the following steps: acquiring total direct current bus voltage in a power loop, first voltage of a positive pole of the direct current bus to the ground before switching on and second voltage of the positive pole of the direct current bus to the negative pole of the direct current bus before switching on in real time; determining a switching-on control mode of a first successive electrical appliance according to the magnitude relation of the first voltage and the second voltage; if the first voltage is larger than the second voltage, determining the closing time point of the first successive electrical appliance according to the phase voltage peak value and generating a closing driving signal; if the first voltage is not greater than the second voltage, determining a closing time point of the first successive electrical appliance according to the phase voltage valley value and generating a closing driving signal; and controlling the closing of the second phase relay and the third phase relay. The control method of the three-phase photovoltaic grid-connected inverter relay closing time sequence can effectively reduce common mode impact and differential mode impact, and does not increase hardware cost.
Description
Technical Field
The invention relates to the technical field of photovoltaic inverters, in particular to a control method of a three-phase photovoltaic grid-connected inverter relay closing time sequence.
Background
In a photovoltaic grid-connected inverter circuit, each point has potential difference to the earth (casing earth), and all points have equivalent resistance and equivalent capacitance, such as ISO resistance and equivalent capacitance of a Photovoltaic (PV) board anode and cathode to the earth respectively, insulation resistance and Y capacitance of a direct current BUS (BUS) anode and cathode to the earth respectively, Y capacitance of an electromagnetic interference (EMI) filter to the earth, and equivalent resistance to the earth formed by a grid phase voltage sampling circuit and a BUS cathode to a divider resistance in an earth voltage sampling circuit. The resistors form a direct current loop, before the relay is switched on, the direct current loop determines the potential difference of each point in the circuit to the ground, and at the moment, the potential of the midpoint of the BUS is generally not equal to the ground potential. After the relay is switched on, the capacitance value of the X capacitor Cxa/Cxb/Cxc for filtering is larger, a low-impedance alternating current loop is formed, so that the neutral point potential of the BUS is clamped by three-phase voltage, and the neutral point potential of the BUS is approximately equal to the earth potential at the moment. The change of the potential before and after the closing forms a loop through the common-mode inductor and other equivalent capacitors at the closing moment, so that common-mode impact current of tens to hundreds of amperes can be generated, and the CPU halt or the false triggering of an MOS/IGBT driving circuit can be caused. Before the relay is switched on, the X capacitance Cxa/Cxb/Cxc for filtering has no voltage, and after the relay is switched on, the voltage of the X capacitance Cxa/Cxb/Cxc for filtering is about phase voltage. The voltage change before and after the closing forms a low impedance loop at the closing moment, so that differential mode impact current of tens of amperes to hundreds of amperes can be generated to charge Cxa/Cxb/Cxc, and the contact point of the relay can be ablated and even stuck. Therefore, a method is needed for controlling the closing timing of the relay so as to reduce the common mode impact and the differential mode impact to the minimum or even to approach zero.
Disclosure of Invention
The invention provides a control method for a switching-on time sequence of a relay of a three-phase photovoltaic grid-connected inverter, which solves the problem that the common mode and the differential mode can not be reduced in the related technology.
As an aspect of the present invention, a method for controlling a closing timing sequence of a relay of a three-phase photovoltaic grid-connected inverter is provided, where the method includes:
the method comprises the steps that total direct current bus voltage in a power loop, first voltage of a positive pole of a direct current bus to the ground before switching on and second voltage of the positive pole of the direct current bus to the negative pole of the direct current bus before switching on are obtained in real time, and the sum of the first voltage and the second voltage is equal to the total direct current bus voltage;
determining a switching-on control mode of a first successive electrical appliance according to the magnitude relation of the first voltage and the second voltage;
if the first voltage is greater than the second voltage, determining the closing time point of a first successive electric appliance according to the phase voltage peak value and generating a closing driving signal;
if the first voltage is not greater than the second voltage, determining a closing time point of a first successive electrical appliance according to a phase voltage valley value and generating a closing driving signal;
and controlling the closing of the second phase relay and the third phase relay.
Further, when a boost circuit in the power loop works, the first voltage and the second voltage both change along with the non-linear change of the total voltage of the direct current bus.
Further, the determining a closing control mode of the first successive electrical appliance according to a magnitude relation between the first voltage and the second voltage includes:
and when the booster circuit in the power circuit does not work in the initial state before the relay is switched on, judging the magnitude relation between the first voltage and the second voltage.
Further, if the first voltage is greater than the second voltage, determining a closing time point of a first successive appliance according to a phase voltage peak value and generating a closing driving signal, including:
if the first voltage is larger than the second voltage, determining the closing time point of the first successive electric appliance according to the comparison result of the first voltage difference between the relay contacts before closing and 0, and generating a closing driving signal,
and the first voltage difference between the relay contacts before closing = total voltage/2- (second voltage + peak value of phase voltage) of the direct current bus.
Further, if the first voltage difference between the relay contacts before switching on is greater than or equal to 0, determining that the optimal time point of switching on of the first successive electrical appliance is the time point corresponding to the peak position of the phase voltage waveform, and calculating switching-on advance action time according to the phase voltage waveform to generate a switching-on driving signal of the first successive electrical appliance.
Further, if the first voltage difference between the relay contacts before switching on is smaller than 0, controlling a booster circuit in the power loop to work, and when the first voltage difference between the relay contacts before switching on increases to be equal to 0 along with the increase of the total voltage of the direct current bus, determining the optimal time point of switching on of a first successive electric appliance as the time point corresponding to the peak position of the phase voltage waveform, calculating the switching on advance action time according to the phase voltage waveform, and generating a switching on driving signal of the first successive electric appliance;
if the first voltage difference between the relay contacts before switching on is smaller than 0, controlling a booster circuit in the power loop to work, when the total voltage of the direct current bus is increased to a preset highest voltage and the first voltage difference between the relay contacts before switching on is still smaller than 0, determining the optimal time point of switching on of a first successive electrical appliance according to the value of the total voltage of the direct current bus corresponding to the time when the first voltage difference between the relay contacts before switching on is closest to 0 in the process that the first voltage difference between the relay contacts before switching on follows the total voltage of the direct current bus, calculating the advance action time of switching on according to the phase voltage waveform, and generating a switching on driving signal of the first successive electrical appliance.
Further, if the first voltage is not greater than the second voltage, determining a closing time point of a first successive appliance according to a phase voltage valley value and generating a closing driving signal, including:
if the first voltage is not greater than the second voltage, determining the closing time point of the first successive electrical appliance according to the comparison result of the second voltage difference between the relay contacts before closing and 0, and generating a closing driving signal of the first successive electrical appliance,
and the second voltage difference between the relay contacts before switching on = the total voltage/2- (the second voltage-phase voltage peak value) of the direct current bus.
Further, if the second voltage difference between the contacts of the relay before closing is less than or equal to 0, determining that the optimal time point of closing is the time point corresponding to the trough position of the phase voltage waveform, and calculating the closing advance action time of the first successive electrical appliance according to the phase voltage waveform to generate a closing driving signal of the first successive electrical appliance.
Further, if the second voltage difference between the relay contacts before switching on is greater than 0, controlling a booster circuit in the power loop to work, and when the second voltage difference between the relay contacts before switching on is increased along with the increase of the total voltage of the direct current bus and is reduced to be equal to 0, determining the optimal time point of switching on of the first successive electrical appliance as the time point corresponding to the trough position of the phase voltage waveform, calculating switching-on advance action time according to the phase voltage waveform, and generating a switching-on driving signal of the first successive electrical appliance;
if the second voltage difference between the relay contacts before switching on is larger than 0, controlling a booster circuit in the power loop to work, when the total voltage of the direct current bus is increased to a preset highest voltage and the second voltage difference between the relay contacts before switching on is still larger than 0, determining the optimal time point of switching on of a first successive electrical appliance according to the value of the total voltage of the direct current bus corresponding to the time when the second voltage difference between the relay contacts before switching on is closest to 0 in the process that the second voltage difference between the relay contacts before switching on follows the total voltage of the direct current bus, calculating the advance action time of switching on according to the phase voltage waveform, and generating a switching on driving signal of the first successive electrical appliance.
Further, the first successive appliance is any one phase of a three-phase relay.
According to the control method of the switching-on time sequence of the relay of the three-phase photovoltaic grid-connected inverter, the S1 group of relays in the power loop are independently controlled, common mode impact and differential mode impact can be effectively reduced, and hardware cost is not increased.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention and not to limit the invention.
Fig. 1 is a schematic diagram of a power circuit.
Fig. 2 is a flowchart of a control method of a switching-on timing sequence of a relay of a three-phase photovoltaic grid-connected inverter provided by the invention.
Fig. 3 is a voltage waveform diagram when the first voltage is greater than the second voltage and the first voltage difference between the relay contacts before closing is greater than or equal to 0.
Fig. 4 is a voltage waveform diagram when the first voltage is greater than the second voltage, the first voltage difference between the relay contacts before closing can be increased from less than 0 to equal to 0, and the boost circuit does not work.
Fig. 5 is a voltage waveform diagram after the first voltage is greater than the second voltage, the first voltage difference between the relay contacts can be increased from less than 0 to equal to 0 before the switch-on, and the boost circuit operates.
Fig. 6 is a voltage waveform diagram when the first voltage is greater than the second voltage and the first voltage difference between the relay contacts before closing is less than 0.
Fig. 7 is a voltage waveform diagram when the first voltage is not greater than the second voltage and the second voltage difference between the relay contacts before closing is less than or equal to 0.
Fig. 8 is a voltage waveform diagram when the first voltage is not greater than the second voltage, the second voltage difference between the relay contacts before closing can be reduced from greater than 0 to equal to 0, and the boost circuit does not work.
Fig. 9 is a voltage waveform diagram of the first voltage not greater than the second voltage, the second voltage difference between the relay contacts before closing can be reduced from greater than 0 to equal to 0, and the boost circuit operates.
Fig. 10 is a voltage waveform diagram when the first voltage is not greater than the second voltage and the second voltage difference between the relay contacts before closing is greater than 0.
Fig. 11 is a voltage waveform diagram of a first sequential electrical appliance provided by the present invention immediately after completion of closing.
Fig. 12 is a voltage waveform diagram of the inverter circuit after the first sequential electrical appliance is switched on and the inverter circuit starts to work, until the second phase relay and the third phase relay can be switched on.
Fig. 13 is an expanded view of the graph shown in fig. 12 between 0.2s and 0.4 s.
Detailed Description
It should be noted that the embodiments and features of the embodiments may be combined with each other without conflict. The present invention will be described in detail below with reference to the embodiments with reference to the attached drawings.
In order to make those skilled in the art better understand the technical solution of the present invention, the technical solution in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
It should be noted that the terms "first," "second," and the like in the description and claims of the present invention and in the drawings described above are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the data so used may be interchanged under appropriate circumstances in order to facilitate the description of the embodiments of the invention herein. Furthermore, the terms "comprises," "comprising," and "having," and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, system, article, or apparatus that comprises a list of steps or elements is not necessarily limited to those steps or elements expressly listed, but may include other steps or elements not expressly listed or inherent to such process, method, article, or apparatus.
Fig. 1 shows a power circuit and equivalent capacitors and resistors, the number of the photovoltaic panels (PV panels) can be any, only 2 are schematically shown in fig. 1, and the cathodes of the photovoltaic panels are connected together and connected with the cathode of the dc BUS (BUS) to form a power ground together, and the power ground is also the signal ground of the inverter sampling and control circuit. Each path of PV plate is provided with a boost circuit (boost circuit), when the boost circuit works, the output voltage of the equivalent voltage source is the difference between the BUS positive voltage and the PV voltage (the invention only discusses the problem of relay closing in real time, at the moment, the output power of the PV is very low, the voltage is about open-circuit voltage and can be similar to the voltage source), and when the boost circuit does not work, the output voltage of the equivalent voltage source is zero. Due to the action of the diode, except for the path with the highest PV voltage, the diodes of other paths PV are always subjected to back pressure, namely disconnection. When the boost circuit works, the BUS voltage is higher than the PV voltage, the diode bears the back voltage and is equivalently disconnected, and when the boost circuit does not work, the diode is conducted, and the BUS voltage is approximately equal to the PV voltage. When the diode is equivalently disconnected, because the PV cathodes and the BUS cathode are always communicated, the PV is used as an equivalent voltage source, and the potential difference of each point in a direct current circuit to the earth before the relay is switched on can still be influenced through the ISO resistance of the PV anode to the earth. The voltage that can directly measure among this circuit includes BUS voltage (total voltage, positive half voltage, negative half voltage totally 3, only use total voltage can), each way PV voltage, electric wire netting triphase voltage, 2 relay intermediate point pairs N line voltage (Rsample in the corresponding map, because the blocking capacitor has established ties, can only sample the alternating current component), the BUS negative pole is to earth voltage. Because the negative poles are connected in common and the voltage of the power grid has no direct current component, the earth potential is between the positive pole of the BUS and the negative pole of the BUS no matter before or after the switch-on. The relays are divided into 2 groups, the switching-on of the relays in the S1 group is independent control, the S2 group is switched on first before the operations (at the moment, the S1 group is disconnected, and no impact is generated when the S2 group is switched on), and when the S1 group of relays are switched on, one arbitrary phase is switched on first, and then the other two phases are switched on simultaneously. Therefore, it is critical how to determine the time node of the first phase of the first switch-on.
In this embodiment, a method for controlling a closing timing sequence of a relay of a three-phase photovoltaic grid-connected inverter is provided, and fig. 2 is a flowchart of a method for controlling a closing timing sequence of a relay of a three-phase photovoltaic grid-connected inverter according to an embodiment of the present invention, as shown in fig. 2, the method includes:
s110, acquiring total direct current bus voltage in a power loop in real time, a first voltage of a positive pole of the direct current bus to the ground before closing and a second voltage of the positive pole of the direct current bus to a negative pole of the direct current bus before closing, wherein the sum of the first voltage and the second voltage is equal to the total direct current bus voltage;
in the embodiment of the present invention, the total dc BUS voltage (i.e., the BUS voltage) is denoted as Ubus, the highest BUS voltage allowed by the circuit design is denoted as Umax, and due to the action of the voltage balancing circuit, the voltages of the positive half BUS and the negative half BUS are equal, which is denoted as Uhalf =0.5 × Ubus. Recording the voltage of the positive pole of the BUS to the ground before closing as a first voltage U1 (which is a positive value forever according to the above description), and after the boost circuit works, the U1 changes along with the non-linearity of the Ubus; the voltage of the earth to the negative pole of the BUS before closing is recorded as a second voltage U2 (which is a positive value forever according to the above description), and after the boost circuit works, the U2 changes along with the non-linearity of the Ubus, and whenever the U1+ U2= the Ubus. Note that the phase voltage waveform of the phase that is first switched on is Ua = Upk × cos (ω × t), (typical values are Upk =220v × 1.41421, ω =2 × pi × 50 hz =314.16 rad/S), and since the N line and the PE line are connected in the low-voltage distribution TN-S system, the voltage of the relative ground is Ua, and the voltage of the negative pole of the relative BUS before switching on is U2+ Ua. Because only a certain successive electrical appliance is switched on first, and the other two-phase relay is still disconnected, no differential mode impact exists during switching on, only common mode impact exists, and a method is used for controlling the switching-on time point, so that U2+ Ua approaches Uhalf during switching on, and the closer to the common mode impact, the smaller the common mode impact, and even the zero is approached.
It should be appreciated that when the boost circuit in the power loop is operating, the first voltage and the second voltage each follow the total dc bus voltage in a non-linear manner.
S120, determining a closing control mode of a first successive electric appliance according to the magnitude relation of the first voltage and the second voltage;
specifically, when a boost circuit in the power circuit does not work in an initial state before the relay is switched on, the magnitude relation between the first voltage and the second voltage is judged.
The relay action delay time is long, usually several milliseconds, so the actual relay driving signal is sent out by the CPU in advance of a certain time. The action delay time of the relay has certain discreteness, each operation is not identical, and the action delay time can be further changed along with the lapse of the using time, so that the closing time point is selected as far as possible when the change rate of the Ua waveform is small, but not when the change rate is large, for the sine waveform, the derivative at the peak value and the valley value is zero, the derivative at the zero crossing point is maximum, and the closing time point is close to the peak value or the valley value.
In an initial state before the relay is switched on, the boost circuit does not work, and because the discharge resistor is connected in parallel, the voltages of the X capacitors Cxa/Cxb/Cxc for filtering are all zero. If U1> U2, step S130, otherwise step S140.
S130, if the first voltage is larger than the second voltage, determining a closing time point of a first successive electrical appliance according to a phase voltage peak value and generating a closing driving signal;
if the first voltage is larger than the second voltage, determining the closing time point of the first successive electric appliance according to the comparison result of the first voltage difference between the relay contacts before closing and 0, and generating a closing driving signal,
and the first voltage difference between the relay contacts before closing = the total voltage of the direct current bus- (the second voltage + the peak value of the phase voltage).
As shown in fig. 3, if the first voltage difference between the contacts of the pre-closing relay is greater than or equal to 0, determining that the optimal time point for closing the first successive electrical appliance is a time point corresponding to a peak position of the phase voltage waveform, and calculating a closing advance action time according to the phase voltage waveform to generate a closing driving signal of the first successive electrical appliance.
It should be understood that, noting the first differential pressure Δ U1= Uhalf- (U2 + Upk) between the relay contacts before closing, if Δ U1> =0, the optimal closing time point is at the peak of the phase voltage, the differential pressure Δ U1 (which cannot be closer to zero) between the relay contacts before closing, calculating the closing advance action time according to the sine waveform, and then driving the relay to act. As shown in fig. 3.
If the first pressure difference between the relay contacts before closing is smaller than 0, controlling a booster circuit in the power loop to work, determining the optimal closing time point of a first successive electrical appliance as the time point corresponding to the peak position of the phase voltage waveform when the first pressure difference between the relay contacts before closing increases to be equal to 0 along with the increase of the total voltage of the direct current bus, calculating closing advance action time according to the phase voltage waveform, and generating a closing driving signal of the first successive electrical appliance;
in this embodiment, if Δ U1<0, the boost circuit is operated to gradually increase Ubus, and since U1 and U2 change nonlinearly with Ubus, Δ U1 needs to be recorded gradually at the same time, and if Δ U1 increases to Δ U1=0 as Ubus increases, at this time, the optimal time point for closing is at the phase voltage peak, the differential pressure between the relay contacts before closing is zero (less affected by the relay operation), ubus is maintained, the closing advance operation time is calculated from the sinusoidal waveform, and then the relay is driven to operate. (as shown in fig. 4 and 5, when the boost circuit is not operating and after the boost circuit is operating, respectively).
If the first voltage difference between the relay contacts before switching on is smaller than 0, controlling a booster circuit in the power loop to work, when the total voltage of the direct current bus is increased to a preset highest voltage and the first voltage difference between the relay contacts before switching on is still smaller than 0, determining the optimal time point of switching on of a first successive electrical appliance according to the value of the total voltage of the direct current bus corresponding to the time when the first voltage difference between the relay contacts before switching on is closest to 0 in the process that the first voltage difference between the relay contacts before switching on follows the total voltage of the direct current bus, calculating the advance action time of switching on according to the phase voltage waveform, and generating a switching on driving signal of the first successive electrical appliance.
In this embodiment, if the Ubus is already increased to Umax, Δ U1<0 is maintained in the whole process, and the Ubus is controlled to the value again according to the recorded Ubus value when Δ U1 is closest to zero, at this time, the optimal time point of closing is not at the peak value of the phase voltage, but at a position before the peak value (or symmetrically at a position after the peak value), the differential pressure between the relay contacts before closing is zero (greatly influenced by the action of the relay), the advance action time is calculated according to the sine waveform, and then the relay is driven to act. As shown in fig. 6.
And S140, if the first voltage is not greater than the second voltage, determining a closing time point of the first successive electrical appliance according to the phase voltage valley value and generating a closing driving signal.
If the first voltage is not greater than the second voltage, recording and determining the closing time point of the first successive electrical appliance according to the comparison between the second voltage difference between the relay contacts before closing and 0, and generating a closing driving signal of the first successive electrical appliance,
and the second voltage difference between the relay contacts before closing = the total voltage of the direct current bus- (the peak value of the second voltage-phase voltage).
And if the second voltage difference between the relay contacts before the switching-on is less than or equal to 0, determining the optimal switching-on time point as the time point corresponding to the trough position of the phase voltage waveform, calculating the switching-on advance action time of the first successive electrical appliance according to the phase voltage waveform, and generating a switching-on driving signal of the first successive electrical appliance.
In the embodiment of the invention, a second pressure difference delta U2= Uhalf- (U2-Upk) between the relay contacts before closing is recorded.
If the delta U2< =0, the optimal closing time point is the phase voltage valley, the differential pressure between the relay contacts before closing is delta U2 (cannot be closer to zero), the closing advance action time is calculated according to the sine waveform, and then the relay is driven to act. As shown in fig. 7.
If the second voltage difference between the relay contacts before switching on is larger than 0, controlling a booster circuit in the power loop to work, when the second voltage difference between the relay contacts before switching on is increased along with the total voltage of the direct current bus and is reduced to be equal to 0, determining the optimal time point of switching on of a first successive electric appliance as the time point corresponding to the trough position of the phase voltage waveform, calculating switching on advance action time according to the phase voltage waveform, and generating a switching on driving signal of the first successive electric appliance;
in the embodiment of the invention, if the delta U2 is greater than 0, the boost circuit is enabled to work, the Ubus is gradually increased, because U1 and U2 change along with the Ubus in a nonlinear way, the delta U2 needs to be recorded gradually, if the delta U2 is reduced to the delta U2=0 along with the increase of the Ubus, at the moment, the optimal closing time point is at a phase voltage valley value, the differential pressure between relay contacts before closing is zero (the influence is small when the relay acts), the Ubus is maintained, the closing advance action time is calculated according to a sine waveform, and then the relay is driven to act. Fig. 8 and 9 show the operation of the boost circuit and the operation of the boost circuit, respectively.
If the second voltage difference between the relay contacts before switching on is larger than 0, controlling a booster circuit in the power loop to work, when the total voltage of the direct current bus is increased to a preset highest voltage and the second voltage difference between the relay contacts before switching on is still larger than 0, determining the optimal time point of switching on of a first successive electric appliance according to the value of the total voltage of the direct current bus corresponding to the time when the second voltage difference between the relay contacts before switching on is closest to 0 in the change process of the total voltage of the direct current bus, calculating the action advance time of switching on according to the phase voltage waveform, and generating a switching on driving signal of the first successive electric appliance.
In the embodiment of the invention, if the Ubus is increased to the Umax, the whole process is continued until the delta U2 is more than 0, and the Ubus is controlled to the value again according to the recorded Ubus value when the delta U2 is closest to zero, at the moment, the optimal closing time point is not at the phase voltage valley value, but at a certain position before the valley value (or at a certain position after the valley value symmetrically), the differential pressure between the relay contacts before closing is zero (the influence is larger when the relay acts), the advance action time is calculated according to the sine waveform, and then the relay is driven to act. As shown in fig. 10.
It should be noted that the first successive electrical device is any one phase of a three-phase relay.
And S150, controlling the closing of the second phase relay and the third phase relay.
After the first successive appliance is controlled to be switched on, the inverter circuit starts to work until the amplitude of the voltage waveform output by the inverter circuit reaches the amplitude of the voltage waveform of the power grid, and the second phase relay and the third phase relay are controlled to be switched on. It should be understood here that the second phase relay and the third phase relay may be switched on simultaneously, or the second phase relay and the third phase relay may be switched on sequentially, and a specific switching-on sequence is not limited.
According to the control method for the switching-on time sequence of the relay of the three-phase photovoltaic grid-connected inverter, the S1 group of relays in the power loop are independently controlled, common mode impact and differential mode impact can be effectively reduced, and hardware cost is not increased.
After the first-phase relay is closed, the sum of the capacitances of the filtering X capacitor Cxa (assuming that the phase a is the first phase) is much larger than the capacitances Ceq _ bus (+) and Ceq _ bus (-) so that the filtering X capacitor approximates to an ac short circuit, the ac circuit component is almost zero, and the phase voltage is almost entirely applied to the equivalent capacitor. When the switch-on is just completed, the direct current voltage component on the filtering X capacitor is zero, and due to the resistance voltage division effect of the direct current loop, the direct current voltage component on the filtering X capacitor slowly changes until the direct current voltage component is stable (the time constant is in a second level). Therefore, after the relay of the first phase is switched on, the inverter circuit should work immediately, the SPWM mode is used for emitting modulation waveforms, the amplitude value is gradually increased from zero until the modulation waveforms are equal to Upk, and when the inverter circuit works (the inverter circuit is a strong power supply and changes a direct current loop), direct current voltage components on the X capacitor for filtering can be kept to be zero all the time, so that the inverter circuit is switched on immediately after the relay is switched on, and extra impact current cannot be generated. According to the above analysis, when the inverter circuit is just turned on, the ac voltage component and the dc voltage component on the X capacitor for filtering are both close to zero, so the voltage of the BUS midpoint to the ground is approximately equal to the phase voltage of the first phase and changes in a sinusoidal waveform, and the BUS filter capacitor Cbp/Cbn is much larger than the equivalent capacitors Ceq _ BUS (+) and Ceq _ BUS (-), so the voltage of the BUS positive electrode and the BUS negative electrode to the ground also changes in a sinusoidal waveform with almost the same amplitude (as shown in the following figure), as the amplitude of the SPWM modulation waveform becomes larger, the ac component of the voltage waveform of the BUS midpoint to the ground is equal to the phase voltage waveform of the first phase minus the ac voltage component on the Cxa for filtering (i.e., the SPWM modulation waveform), until the amplitude of the SPWM modulation waveform reaches Upk, the voltage waveform of the BUS midpoint to the ground is close to zero (both the dc component and the ac component are close to zero), at this time, the voltage of the third phase filtering X capacitor is approximately equal to the phase voltage of the second phase and the phase contact is close to zero (the ac relay is not needed).
After all the three-phase relays are switched on, the three-phase relays can be switched to SVM for modulation at a proper phase, and then active power and reactive power are sent to a power grid.
The first phase can be any phase, three relays of the S1 group can be alternately used according to a certain rule, if a voltage sampling circuit (corresponding to a diagram Rsample) is connected between the X capacitor for filtering and the S2 group, and each relay of the S2 group can be independently controlled, a control mode can be exchanged with the relays of the S1 group, so that all six relays can be alternately used, and ablation on relay contacts is realized during switching on in a balanced mode.
According to the analysis, when the resistance values of the PV anodes and cathodes of all paths and insulation resistors of all points in the inverter circuit to the ground are large and uneven (namely, when the resistance values deviate from the ideal condition to be large), the point potential of the BUS is far away from the earth potential in the initial state, and the method has a very obvious effect.
It will be understood that the above embodiments are merely exemplary embodiments taken to illustrate the principles of the present invention, which is not limited thereto. It will be apparent to those skilled in the art that various modifications and improvements can be made without departing from the spirit and substance of the invention, and these modifications and improvements are also considered to be within the scope of the invention.
Claims (10)
1. A control method for a three-phase photovoltaic grid-connected inverter relay closing time sequence is characterized by comprising the following steps:
the method comprises the steps that total direct current bus voltage in a power loop, first voltage of a positive pole of a direct current bus to the ground before switching on and second voltage of the positive pole of the direct current bus to the negative pole of the direct current bus before switching on are obtained in real time, and the sum of the first voltage and the second voltage is equal to the total direct current bus voltage;
determining a switching-on control mode of a first successive electrical appliance according to the magnitude relation of the first voltage and the second voltage;
if the first voltage is greater than the second voltage, determining the closing time point of a first successive electric appliance according to the phase voltage peak value and generating a closing driving signal;
if the first voltage is not greater than the second voltage, determining a closing time point of a first successive electrical appliance according to a phase voltage valley value and generating a closing driving signal;
and controlling the closing of the second phase relay and the third phase relay.
2. The method according to claim 1, wherein when a boost circuit in the power loop is operated, the first voltage and the second voltage both vary non-linearly with the total dc bus voltage.
3. The method for controlling the switching-on timing sequence of the three-phase photovoltaic grid-connected inverter relay according to claim 1, wherein the determining of the switching-on control mode of the first sequential electrical appliance according to the magnitude relation between the first voltage and the second voltage comprises:
and when the booster circuit in the power loop does not work in the initial state before the relay is switched on, judging the magnitude relation between the first voltage and the second voltage.
4. The method for controlling the switching-on timing sequence of the three-phase photovoltaic grid-connected inverter relay according to claim 3, wherein if the first voltage is greater than the second voltage, determining a switching-on time point of a first successive appliance according to a phase voltage peak value and generating a switching-on driving signal comprises:
if the first voltage is larger than the second voltage, determining the closing time point of the first successive electric appliance according to the comparison result of the first voltage difference between the relay contacts before closing and 0, and generating a closing driving signal,
and the first voltage difference between the relay contacts before closing = total voltage/2- (second voltage + peak value of phase voltage) of the direct current bus.
5. The method for controlling the closing timing of the relay of the three-phase photovoltaic grid-connected inverter according to claim 4,
and if the first voltage difference between the relay contacts before the switching-on is greater than or equal to 0, determining that the optimal switching-on time point of the first successive electrical appliance is the time point corresponding to the peak position of the phase voltage waveform, calculating switching-on advance action time according to the phase voltage waveform, and generating a switching-on driving signal of the first successive electrical appliance.
6. The method for controlling the closing timing of the relay of the three-phase photovoltaic grid-connected inverter according to claim 4,
if the first pressure difference between the relay contacts before closing is smaller than 0, controlling a booster circuit in the power loop to work, determining the optimal closing time point of a first successive electrical appliance as the time point corresponding to the peak position of the phase voltage waveform when the first pressure difference between the relay contacts before closing increases to be equal to 0 along with the increase of the total voltage of the direct current bus, calculating closing advance action time according to the phase voltage waveform, and generating a closing driving signal of the first successive electrical appliance;
if the first voltage difference between the relay contacts before switching on is smaller than 0, controlling a booster circuit in the power loop to work, when the total voltage of the direct current bus is increased to a preset highest voltage and the first voltage difference between the relay contacts before switching on is still smaller than 0, determining the optimal time point of switching on of a first successive electrical appliance according to the value of the total voltage of the direct current bus corresponding to the time when the first voltage difference between the relay contacts before switching on is closest to 0 in the process that the first voltage difference between the relay contacts before switching on follows the total voltage of the direct current bus, calculating the advance action time of switching on according to the phase voltage waveform, and generating a switching on driving signal of the first successive electrical appliance.
7. The method according to claim 3, wherein if the first voltage is not greater than the second voltage, determining a closing time point of a first successive appliance according to a phase voltage valley and generating a closing driving signal includes:
if the first voltage is not greater than the second voltage, determining the closing time point of the first successive electrical appliance according to the comparison result of the second voltage difference between the relay contacts before closing and 0, and generating a closing driving signal of the first successive electrical appliance,
and the second voltage difference between the relay contacts before closing = total voltage/2- (second voltage-phase voltage peak value) of the direct current bus.
8. The method for controlling the closing timing of the relay of the three-phase photovoltaic grid-connected inverter according to claim 7,
and if the second voltage difference between the relay contacts before the switching-on is less than or equal to 0, determining the optimal switching-on time point as the time point corresponding to the trough position of the phase voltage waveform, calculating the switching-on advance action time of the first successive electrical appliance according to the phase voltage waveform, and generating a switching-on driving signal of the first successive electrical appliance.
9. The method for controlling the relay closing timing of the three-phase photovoltaic grid-connected inverter according to claim 7,
if the second voltage difference between the relay contacts before switching on is larger than 0, controlling a booster circuit in the power loop to work, when the second voltage difference between the relay contacts before switching on is increased along with the total voltage of the direct current bus and is reduced to be equal to 0, determining the optimal time point of switching on of a first successive electric appliance as the time point corresponding to the trough position of the phase voltage waveform, calculating switching on advance action time according to the phase voltage waveform, and generating a switching on driving signal of the first successive electric appliance;
if the second voltage difference between the relay contacts before switching on is larger than 0, controlling a booster circuit in the power loop to work, when the total voltage of the direct current bus is increased to a preset highest voltage and the second voltage difference between the relay contacts before switching on is still larger than 0, determining the optimal time point of switching on of a first successive electrical appliance according to the value of the total voltage of the direct current bus corresponding to the time when the second voltage difference between the relay contacts before switching on is closest to 0 in the process that the second voltage difference between the relay contacts before switching on follows the total voltage of the direct current bus, calculating the advance action time of switching on according to the phase voltage waveform, and generating a switching on driving signal of the first successive electrical appliance.
10. The method according to claim 1, wherein the first successive electrical device is any one phase of a three-phase relay.
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