News Column

Patent Issued for Starter of Grid-Connected Inverter and Control Method

August 27, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- According to news reporting originating from Alexandria, Virginia, by VerticalNews journalists, a patent by the inventors Zhan, Wen-Tao (Taoyuan Hsien, TW); Li, Liang (Taoyuan Hsien, TW); Wu, Hong-Yang (Taoyuan Hsien, TW); Huang, Heng (Taoyuan Hsien, TW); Wang, Bin (Taoyuan Hsien, TW), filed on July 9, 2012, was published online on August 12, 2014.

The assignee for this patent, patent number 8804383, is Delta Electronics, Inc. (Taoyuan Hsien, TW).

Reporters obtained the following quote from the background information supplied by the inventors: "The present disclosure relates to a grid-connected inverter. More particularly, the present disclosure relates to a starter of a grid-connected inverter and a control method thereof.

"With the ever-increasing seriousness of energy-related problems in recent times, the development of new ways in which to generate energy (e.g., wind energy, solar energy, etc.) is attracting the attention of those involved in research and development in various fields. In a power generation system, the frequency and phase of the AC (alternating current) voltage outputted by the power generator are often different from those of the AC electric grid. Thus, the AC voltage outputted by the power generator is generally converted into a DC (direct current) voltage through an inverter (or a converter), and subsequently the DC voltage is inverted to another AC voltage having the same frequency and phase as the AC electric grid, so as to realize grid-connected power generation. In the process of converting the AC voltage into the DC voltage and subsequently inverting the DC voltage into another AC voltage, electronic components (e.g., power switching valves) are PWM (pulse-width modulation)-controlled, and the AC-DC conversion and DC-AC conversion are realized by turning on or off these power switching valves.

"Furthermore, as such electronic components are widely applied, the problem of harmonic waves and asymmetry of the power system is becoming more and more serious, and this is greatly harmful to the operation safety, stability and reliability of the grid-connected inverter mentioned above. As a result, it becomes necessary to suppress the harmonic waves and perform harmonic compensation. One solution of the prior art involves using an active power filter (APF) to realize harmonic compensation. However, in such an APF, the inductance at the AC side is often small, and only a filter capacitor is arranged at the DC side. Thus if the APF is incorporated into an AC electric grid without a soft start process, a large start-up inrush current will be generated, which greatly threatens the safety of the power components, and may even cause failure of the APF grid connection.

"In view of this, many in the industry are endeavoring to find ways in which to design a starter of a grid-connected inverter and reasonably control the starter, so as to minimize the inrush current during start-up of the grid-connected inverter while avoiding increases in the capacity and cost of the system, and also so as to protect the power switch components and incorporate the inverter into the electric grid through a soft start process."

In addition to obtaining background information on this patent, VerticalNews editors also obtained the inventors' summary information for this patent: "In order to overcome the prior art disadvantages associated with when a grid-connected inverter is connected with an AC electric grid, a starter of a grid-connected inverter and a control method thereof is provided.

"An aspect of the present disclosure is related to a starter of a grid-connected inverter. The starter includes a first switch and a first resistor connected in parallel. The first switch is electrically connected between an AC electric grid and the grid-connected inverter. The starter further includes a controller. The controller includes an input end, a first output end and a second output end. The input end of the controller receives and inspects the DC voltage of an inverter. When the DC voltage exceeds a predetermined voltage threshold, the first output end of the controller sends a first control signal to turn on the first switch, and the second output end sends a second control signal to make the grid-connected inverter enter into the chopping mode. There is a delay period td1 between the send time of the first control signal and that of the second control signal, and td1 is larger than 0.

"In an embodiment, the second control signal is a PWM enable signal, and the PWM enable signal makes the grid-connected inverter enter into a boost mode. The send time of the second control signal is any time in the period from the time point T/24 before the zero-crossing point of the phase voltage of the electric grid to the time point T/24 after the zero-crossing point of the phase voltage. T is a power frequency cycle. Furthermore, the send time of the second control signal is the zero-crossing point of the phase voltage of the electric grid.

"In an embodiment, the starter further includes a second switch connected in series with the first resistor. The controller further includes a third output end, and the third output end outputs a third control signal to turn on or off the second switch. At the send time of the first control signal, the second switch is still turned on based on the third control signal having a first level. At the send time of the second control signal, the second switch is turned off based on the third control signal having a second level. There is a delay period td2 between the send time of the third control signal having the second level and the send time of the second control signal. td2 is larger than 0 and smaller than td1.

"In an embodiment, the send time of the first control signal is any time in the time period from the time point T/12 before the zero-crossing point of the phase voltage of the electric grid to the time point T/12 after the zero-crossing point of the phase voltage, and T is the power frequency cycle. In some embodiments, the send time of the first control signal is the zero-crossing point of the phase voltage of the electric grid.

"In an embodiment, the send time of the first control signal is any time in the time period from the time point (T/12+tc) before the zero-crossing point of the phase voltage of the electric grid to the time point (T/12+tc) after the zero-crossing point of the phase voltage. T is the power frequency cycle, and tc is the operation time of the first switch.

"The starter further includes a knife switch arranged between the first switch and the AC electric grid.

"Furthermore, the predetermined voltage threshold is 0.8 times the line voltage peak value of the AC electric grid.

"Another aspect of the present disclosure is related to a starter of a grid-connected inverter. The starter includes a first switch, a precharge branch circuit and a controller. The first switch is electrically connected between an AC electric grid and the grid-connected inverter. The precharge branch circuit is connected with the first switch in parallel, and the precharge branch circuit includes a second switch and a first resistor connected in series. The controller includes an input end, a first output end and a second output end. The input end of the controller receives and inspects the DC voltage of the inverter. When the DC voltage exceeds a predetermined voltage threshold, the first output end of the controller sends a first control signal to turn on the first switch, and the second output end outputs a second control signal to turn on or off the second switch. There is a delay period td3 between the send time of the second control signal having a first level and the send time of the first control signal, and td3 is larger than 0.

"In an embodiment, at the send time of the first control signal, the second switch is still turned on based on the second control signal having a second level. The first level is a low level and is used to turn off the second switch. The second level is a high level and is used to turn on the second switch.

"In an embodiment, the send time of the first control signal is any time in the time period from the time point T/12 before the zero-crossing point of the phase voltage of the electric grid to the time point T/12 after the zero-crossing point of the phase voltage, and T is the power frequency cycle. In some embodiments, the send time of the first control signal is the zero-crossing point of the phase voltage of the electric grid.

"In an embodiment, the send time of the first control signal is any time in the time period from the time point (T/12+tc) before the zero-crossing point of the phase voltage of the electric grid to the time point (T/12+tc) after the zero-crossing point of the phase voltage. T is the power frequency cycle, and tc is the operation time of the first switch.

"Furthermore, the predetermined voltage threshold is 0.8 times the line voltage peak value of the AC electric grid.

"A further aspect of the present disclosure is related to a control method of a starter. The starter includes a first switch and a first resistor connected in parallel. The first switch is electrically connected between an AC electric grid and a grid-connected inverter. The control method includes inspecting a DC voltage of the grid-connected inverter, sending a first control signal to turn on the first switch when the DC voltage exceeds a predetermined voltage threshold, performing a delay for a time period td1 which is larger than zero, and sending a second control signal to make the grid-connected inverter enter into a boost mode.

"In an embodiment, the time period td1 is not smaller than the operation time used for completely turning on the first switch.

"In an embodiment, the send time of the second control signal is any time in the time period from the time point T/24 before the zero-crossing point of the phase voltage of the electric grid to the time point T/24 after the zero-crossing point of the phase voltage, and T is the power frequency cycle.

"In another embodiment, the send time of the first control signal is any time in the time period from the time point T/12 before the zero-crossing point of the phase voltage of the electric grid to the time point T/12 after the zero-crossing point of the phase voltage, and T is the power frequency cycle.

"Furthermore, the predetermined voltage threshold is 0.8 times the line voltage peak value of the AC electric grid.

"Still a further aspect of the present disclosure is related to a control method of a starter. The starter includes a first switch and a precharge branch circuit connected in parallel. The first switch is electrically connected between an AC electric grid and a grid-connected inverter. The precharge branch circuit includes a second switch and a first resistor connected in series. The control method includes inspecting the DC voltage of the grid-connected inverter, sending a first control signal to turn on the first switch when the DC voltage exceeds a predetermined voltage threshold, performing a delay for a time period td4 which is larger than zero, and sending a second control signal having a first level to turn off the second switch.

"In an embodiment, before performing the step of sending the first control signal, the control method further includes sending a second control signal having a second level to turn on the second switch, so that a DC bus capacitor at the DC side of the inverter is in a precharge state.

"In another embodiment, the send time of the first control signal is any time in the time period from the time point T/12 before the zero-crossing point of the phase voltage of the electric grid to the time point T/12 after the zero-crossing point of the phase voltage, and T is the power frequency cycle.

"Furthermore, the predetermined voltage threshold is 0.8 times the line voltage peak value of the AC electric grid.

"By adopting the starter of the grid-connected inverter and the control method thereof, the controller of the starter sends a first control signal to turn on a main loop switch, and after a delay period the controller sends a second control signal to make the grid-connected inverter enter into a chopping mode, so as to prevent the inverter from entering into an operation state when the main loop switch is not completely turned on, which otherwise may cause an excessively large loss of a precharge resistor. Furthermore, after the grid-connected inverter enters into a boost mode, the DC bus voltage value is gradually increased to a reference voltage value, so that while the total soft start process is completed, the power components are protected from being damaged due to an excessive start-up inrush current, thereby increasing the operation stability and reliability of the inverter."

For more information, see this patent: Zhan, Wen-Tao; Li, Liang; Wu, Hong-Yang; Huang, Heng; Wang, Bin. Starter of Grid-Connected Inverter and Control Method. U.S. Patent Number 8804383, filed July 9, 2012, and published online on August 12, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8804383.PN.&OS=PN/8804383RS=PN/8804383

Keywords for this news article include: Electronic Components, Delta Electronics Inc..

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Source: Electronics Newsweekly


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