Patent Publication Number: US-2022239107-A1

Title: Photovoltaic energy storage system and control method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to China Patent Application No. 202110086850.8, filed on Jan. 22, 2021, the entire contents of which are incorporated herein by reference for all purposes. 
     FIELD OF THE INVENTION 
     The present disclosure relates to a photovoltaic energy storage system and a control method, and more particularly to a photovoltaic energy storage system and a control method for cutting off the connection between an inverter and an energy storage unit and the connection between the inverter and a photovoltaic array assembly when an emergency event occurs. 
     BACKGROUND OF THE INVENTION 
     Nowadays, the integrated photovoltaic energy storage household system is widely used. The integrated photovoltaic energy storage household system includes an inverter, an energy storage unit (e.g., a battery) and a photovoltaic array assembly. According to the standard regulations, the connection between the photovoltaic array assembly and the inverter and the connection between the battery and the inverter must be cut off when the emergency event (e.g., a fire disaster) occurs. Consequently, the electric safety is enhanced. 
     In a conventional pure photovoltaic system, the inverter issues a shutdown command to turn off a rapid shutdown switch. Consequently, the connection between the photovoltaic array assembly and the inverter is cut off. 
     In a conventional photovoltaic energy storage system, it is necessary to cut off the connection between the photovoltaic array assembly and the inverter and the connection between the battery and the inverter when the emergency event occurs. Consequently, the designing complexity is increased. 
     In the conventional pure photovoltaic system, the shutdown command is generated when the AC power grid is shut down. That is, if the inverter detects the AC power decreases, the inverter issues the shutdown command to the rapid shutdown switch. However, this method is not applicable to the photovoltaic energy storage system as the photovoltaic energy storage system can still work normally when the AC power grid is disconnected. In other words, the inverter is unable to discriminate the abnormal situation (i.e., the emergency situation) from the normal off-grid mode. 
     Therefore, there is a need of providing an improved control method for the photovoltaic energy storage system in order to detect the occurrence of the emergency event and cut off the connection between an inverter and an energy storage unit and the connection between the inverter and a photovoltaic array assembly in response to the emergency event. 
     SUMMARY OF THE INVENTION 
     The present disclosure provides a photovoltaic energy storage system and a control method for cutting off the connection between an inverter and an energy storage unit and the connection between the inverter and a photovoltaic array assembly when an emergency event occurs. 
     The preset disclosure provides a photovoltaic energy storage system and a control method for the photovoltaic energy storage system. Even if the photovoltaic energy storage system is not equipped with the additional hardware component, the photovoltaic energy storage system is capable of accurately determining the occurrence of the emergency event and cutting off the connection between the inverter and an energy storage unit and the connection between the inverter and a photovoltaic array assembly when the emergency event occurs. Consequently, the technology of the present disclosure can comply with the associated safety standards. 
     In accordance with an aspect of the present disclosure, a control method for a photovoltaic energy storage system is provided. The photovoltaic energy storage system includes an energy storage unit, a photovoltaic array assembly, an inverter and a rapid shutdown switch. The inverter is electrically coupled to the energy storage unit and the photovoltaic array assembly. The rapid shutdown switch is electrically coupled to the inverter, a power grid and a load. The inverter includes a first terminal, a second terminal and a common terminal. A voltage between the first terminal and the command terminal is a first voltage. A voltage between the second terminal and the command terminal is a second voltage. A voltage between the first terminal and the second terminal is a third voltage. The control method includes the following steps. In a step (S 1 ), at least two voltages of the first voltage, the second voltage and the third voltage are detected. Then, a step (S 2 ) is performed to determine whether the rapid shutdown switch is turned off according to the at least two detected voltages and a reference value. In a step (S 3 ), when it is determined that the rapid shutdown switch is turned off, a connection between the inverter and the energy storage unit and a connection between the inverter and the photovoltaic array assembly are cut off. 
     In accordance with another aspect of the present disclosure, a photovoltaic energy storage system is provided. The photovoltaic energy storage system includes an energy storage unit, a photovoltaic array assembly, an inverter and a rapid shutdown switch. The inverter is electrically coupled to the energy storage unit and the photovoltaic array assembly. The inverter includes a detecting and controlling circuit, and has a first terminal, a second terminal and a common terminal. A voltage between the first terminal and the command terminal is a first voltage. A voltage between the second terminal and the command terminal being a second voltage. A voltage between the first terminal and the second terminal is a third voltage. The rapid shutdown switch is electrically coupled to the inverter, a power grid and a load. The detecting and controlling circuit detects at least two voltages of the first voltage, the second voltage and the third voltage, and determines whether the rapid shutdown switch is turned off according to the at least two detected voltages and a reference value. When it is determined that the rapid shutdown switch is turned off, a connection between the inverter and the energy storage unit and a connection between the inverter and the photovoltaic array assembly are cut off. 
     The above contents of the present disclosure will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic functional block diagram illustrating the circuitry structure of a photovoltaic energy storage system according to an embodiment of the present disclosure; 
         FIG. 2  is a schematic circuit diagram illustrating the inverter of the photovoltaic energy storage system as shown in  FIG. 1 ; 
         FIG. 3  schematically illustrates a flowchart of a control method for a photovoltaic energy storage system according to an embodiment of the present disclosure; 
         FIG. 4  is a flowchart illustrates a first example of the step S 2  in the control method of  FIG. 3 ; 
         FIG. 5  is a flowchart illustrates a second example of the step S 2  in the control method of  FIG. 3 ; 
         FIG. 6  is a flowchart illustrates an example of the step S 3  in the control method of  FIG. 3 ; and 
         FIG. 7  is a schematic circuit diagram illustrating the circuitry structure of the automatic transfer switch in the photovoltaic energy storage system as shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present disclosure will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of preferred embodiments of this disclosure are presented herein for purpose of illustration and description only. It is not intended to be exhaustive or to be limited to the precise form disclosed. 
       FIG. 1  is a schematic functional block diagram illustrating the circuitry structure of a photovoltaic energy storage system according to an embodiment of the present disclosure.  FIG. 2  is a schematic circuit diagram illustrating the inverter of the photovoltaic energy storage system as shown in  FIG. 1 . 
     As shown in  FIG. 1 , the photovoltaic energy storage system includes a photovoltaic array assembly  101 , an energy storage unit  102  (e.g., a battery unit), an inverter  103  and a rapid shutdown switch  104 . The inverter  103  is electrically coupled to the energy storage unit  102  and the photovoltaic array assembly  101 . The rapid shutdown switch  104  is electrically coupled to the inverter  103 , a power grid  107  and a load  105 . The inverter  103  includes a detecting and controlling circuit  108 . The detecting and controlling circuit  108  can determine whether the rapid shutdown switch  104  is turned off. When it is determined that the rapid shutdown switch  104  is turned off, the connection between the inverter  103  and the energy storage unit  102  and the connection between the inverter  103  and the photovoltaic array assembly  101  need to be cut off. 
     As shown in  FIG. 2 , the inverter  103  includes a DC bus capacitor C 1 , a full-bridge circuit  201 , a filtering circuit  202  and at least one power consumption element. For example, the at least one power consumption element includes a fan and a relay. The AC side of the inverter  103  includes a first terminal L 1 , a second terminal L 2  and a common terminal N. The common terminal N is electrically coupled to a neutral line of the power grid  107 . The voltage U L1N  between the first terminal L 1  and the common terminal N is defined as a first voltage. The voltage U L2N  between the second terminal L 2  and the common terminal N is defined as a second voltage. The voltage U L1L2  between the first terminal L 1  and the second terminal L 2  is defined as a third voltage. It is noted that the first voltage, the second voltage and the third voltage comply with a specified relationship. For example, if the first voltage and the second voltage are known, the third voltage is calculated according to the first voltage and the second voltage. Similarly, if the first voltage and the third voltage are known, the second voltage is calculated according to the first voltage and the third voltage. The DC side of the full-bridge circuit  201  is connected with the DC bus capacitor C 1 . The AC side of the full-bridge circuit  201  is connected with the filtering circuit  202 . The filtering circuit  202  includes a first impedance network  203  and a second impedance network  204 . The first impedance network  203  is electrically coupled between the first terminal L 1  and the common terminal N. The second impedance network  204  is electrically coupled between the second terminal L 2  and the common terminal N. The impedance value Z 1  of the first impedance network  203  and the impedance value Z 2  of the second impedance network  204  are not equal. 
     In some embodiments, the filtering circuit  202  further includes a common mode inductor L. The common mode inductor L is electrically coupled between the full-bridge circuit  201  and the power grid  107 . For example, the common mode inductor L is connected between a midpoint of a first bridge arm of the full-bridge circuit  201  and the first terminal L 1  and connected between a midpoint of a second bridge arm of the full-bridge circuit  201  and the second terminal L 2 . 
     Please refer to  FIG. 2  again. When the photovoltaic energy storage system is in a normal working state, the rapid shutdown switch  104  is in a on state and the inverter  103  is connected with the power grid  107 . Under this circumstance, the first voltage U L1N  and the second voltage U L2N  are maintained in the balanced state. When an emergency event occurs, the rapid shutdown switch  104  is turned off. For example, when one sees a fire, he will turn off the rapid shutdown switch  104  manually. As mentioned above, when an emergence event (e.g., a fire disaster) occurs, the connection between the photovoltaic array assembly  101  and the inverter  103  and the connection between the energy storage unit  102  must be immediately cut off according to the associated standards. In order to quickly respond to the emergency event, the inverter  103  is equipped with the detecting and controlling circuit  108  to determine whether the rapid shutdown switch  104  is indeed turned off. Especially, when the detecting and controlling circuit  108  confirms that the inverter  103  and the power grid  107  are disconnected, the inverter  103  is controlled to operate in a voltage source mode by the detecting and controlling circuit  108 . Under this circumstance, the relationships between the first voltage U L1N , the second voltage U L2N  and the voltage U L1L2  can be expressed by the following mathematic formulae: 
         U   L1N   −Z 1/( Z 1+ Z 2)× U   L1L2    (1)
 
         U   L2N   =Z 2/( Z 1+ Z 2)× U   L1L2    (2)
 
     After the first voltage U L1N  and the second voltage U L2N  are calculated and processed according to the mathematic formulae (1) and (2), the detecting and controlling circuit  108  acquires the degree of imbalance between the first voltage U L1N  and the second voltage U L2N . If the degree of imbalance exceeds a predetermined threshold, the detecting and controlling circuit  108  confirms that the rapid shutdown switch  104  between the inverter  103  and the power grid  107  is turned off. 
     In an embodiment, the energy storage unit  102  includes a battery pack  1021  and a battery pack switch  1022 , which are connected with each other in series. For example, the battery pack  1021  is a chargeable battery. The battery pack switch  1022  is electrically coupled between the battery pack  1021  and the inverter  103 . Moreover, the photovoltaic array assembly  101  includes a photovoltaic array  1011  and a photovoltaic array switch  1012 , which are connected with each other in series. The photovoltaic array switch  1012  is electrically coupled between the photovoltaic array  1011  and the inverter  103 . 
       FIG. 3  schematically illustrates a flowchart of a control method for a photovoltaic energy storage system according to an embodiment of the present disclosure. The control method is applied to the photovoltaic energy storage system as shown in  FIG. 1 . The steps S 1 , S 2  and S 3  of the control method are used to determine whether the rapid shutdown switch  104  is turned off. When it is determined that the rapid shutdown switch  104  is turned off, the connection between the inverter  103  and the energy storage unit  102  and the connection between the inverter  103  and the photovoltaic array assembly  101  are cut off. Consequently, the security of the photovoltaic energy storage system in the emergency situation is enhanced. 
     Firstly, in the step S 1 , the detecting and controlling circuit  108  detects at least two of the first voltage U L1N , the second voltage U L2N  and the third voltage U L1L2 . Since the third voltage U L1L2  is equal to the sum of the first voltage U L1N  and the second voltage U L2N , the detection of at least two of the first voltage U L1N , the second voltage U L2N  and the third voltage U L1L2  can acquire all of the first voltage U L1N , the second voltage U L2N  and the third voltage U L1L2 . 
     Then, in the step S 2 , the detecting and controlling circuit  108  determines whether the rapid shutdown switch  104  is turned off according to the at least two detected voltages and a predetermined reference value. 
     When it is determined that the rapid shutdown switch  104  is turned off, the step S 3  is performed. In the step S 3 , the connection between the inverter  103  and the energy storage unit  102  and the connection between the inverter  103  and the photovoltaic array assembly  101  are cut off. When the emergency event occurs, the rapid shutdown switch  104  is turned off. Consequently, the connection between the photovoltaic array assembly  101  and the inverter  103  and the connection between the energy storage unit  102  and the inverter  103  can be immediately cut off to comply with the associated standards. 
     In some embodiments, after the connection between the inverter  103  and the energy storage unit  102  and the connection between the inverter  103  and the photovoltaic array assembly  101  are cut off, a step S 4  is performed. In the step S 4 , the DC bus capacitor C 1  of the inverter  103  is discharged through the at least one power consumption element. By adjusting the discharge speed of the DC bus capacitor C 1 , the DC bus voltage can be quickly reduced to be within a safety range. 
       FIG. 4  is a flowchart illustrates a first example of the step S 2  in the control method of  FIG. 3 . In an embodiment, the step S 2  includes sub-steps S 202 A, S 204 A and S 206 A. 
     In the sub-step S 202 A, a first ratio Q 1  is obtained after the absolute value of the difference between the first voltage U L1N  and the second voltage U L2N  is divided by the third voltage U L1L2 . The first ratio Q 1  can be expressed by the following mathematic formula: 
         Q 1=| U   L1N   −U   L2N / U   L1L2    (3)
 
     In other words, after the at least two of the first voltage U L1N , the second voltage U L2N  and the third voltage U L1L2  are detected, the first ratio Q 1  can be obtained according to the mathematic formula ( 3 ). For example, in case that the first voltage U L1N  and the second voltage U L2N  are detected by the detecting and controlling circuit  108 , the third voltage U L1L2  is calculated according to the first voltage U L1N  and the second voltage U L2N  and the first ratio Q 1  is obtained according to the mathematic formula (3). For example, in case that the first voltage U L1N  and the third voltage U L1L2  are detected by the detecting and controlling circuit  108 , the second voltage U L2N  is calculated according to the first voltage U L1N  and the third voltage U L1L2  and the first ratio Q 1  is obtained according to the mathematic formula (3). Alternatively, it is unnecessary to directly detect the at least two of the first voltage U L1N , the second voltage U L2N  and the third voltage U L1L2 . Other electrical parameters (e.g., voltages or currents) in the circuitry can also be detected to calculate the first ratio Q 1  as long as these other electrical parameters can reflect at least two of the first voltage U L1N , the second voltage U L2N  and the third voltage U L1L2 . In other words, the method of obtaining the first ratio Q 1  is not restricted. 
     In the sub-step S 204 A, the first ratio Q is compared with a predetermined first threshold value. According to the comparing result, the detecting and controlling circuit  108  acquires the degree of imbalance between the first voltage U L1N  and the second voltage U L2N . In this embodiment, the predetermined reference value in the step S 2  contains the first threshold value. 
     In the sub-step S 206 A, if the first ratio Q is greater than the first threshold value, it means that the degree of imbalance between the first voltage U L1N  and the second voltage U L2N  is very serious because the inverter  103  and the power grid  107  are disconnected. Under this circumstance, the detecting and controlling circuit  108  confirms that the rapid shutdown switch  104  is turned off. Generally, in the normal off-grid mode, there is also certain imbalance between the first voltage U L1N  and the second voltage U L2N  because of the load imbalance. For discriminating the abnormal situation (i.e., the emergency situation) from the normal off-grid mode, the first threshold value for determining whether the rapid shutdown switch  104  is turned off in the emergency situation should be specially determined. For example, the first threshold value is not smaller than 25% (e.g., 25%). 
       FIG. 5  is a flowchart illustrates a second example of the step S 2  in the control method of  FIG. 3 . In an embodiment, the step S 2  includes sub-steps S 202 B, S 204 B and S 206 B. In  FIG. 4 , the degree of imbalance between the first voltage U L1N  and the second voltage U L2N  is determined by the difference between the first voltage U L1N  and the second voltage U L2N . In the embodiment of  FIG. 5 , the degree of imbalance between the first voltage U L1N  and the second voltage U L2N  is determined by the ratio of the first voltage U L1N  to the second voltage U L2N  or the ratio of the second voltage U L2N  to the first voltage U L1N . 
     In the sub-step S 202 B, the first voltage U L1N  is compared with the second voltage U L2N , wherein after a larger one of the first voltage U L1N  and the second voltage U L2N  is divided by a smaller one of the first voltage U L1N  and the second voltage U L2N , a second ratio Q 2  is obtained. The second ratio Q 2  can be expressed by the following mathematic formula: 
         Q 2=Max( U   L1N   , U   L2N )/Min( U   L1N   , U   L2N )   (4)
 
     In the mathematic formula (4), Max(U L1N , U L2N ) is the larger one of the first voltage U L1N  and the second voltage U L2N , and Min(U L1N , U L2N ) is the smaller one of the first voltage U L1N  and the second voltage U L2N . 
     In the sub-step S 204 B, the second ratio Q 2  is compared with a predetermined second threshold value. According to the comparing result, the detecting and controlling circuit  108  acquires the degree of imbalance between the first voltage U L1N  and the second voltage U L2N . In this embodiment, the predetermined reference value in the step S 2  contains the second threshold value. 
     In the sub-step S 206 B, when the second ratio Q 2  is greater than the second threshold value, the detecting and controlling circuit  108  confirms that the rapid shutdown switch  104  is turned off. Similarly, in the normal off-grid mode, there is also certain imbalance between the first voltage U L1N  and the second voltage U L2N  because of the load imbalance. For discriminating the abnormal situation (i.e., the emergency situation) from the normal off-grid mode, the second threshold value for determining whether the rapid shutdown switch  104  is turned off in the emergency situation should be specially determined. For example, the second threshold value is not smaller than  8  (e.g.,  8 ). 
       FIG. 6  is a flowchart illustrates an example of the step S 3  in the control method of  FIG. 3 . In an embodiment, the step S 3  includes sub-steps S 302 , S 304 , S 306  and S 308 . 
     In the sub-step S 302 , when it is determined that the rapid shutdown switch  104  is turned off, the inverter  103  issues a control signal to the energy storage unit  102 . 
     In the sub-step S 304 , the energy storage unit  102  receives the control signal from the inverter  103 , and the battery pack switch  1022  is turned off through a battery management system (BMS) of the energy storage unit  102 . Consequently, the connection between the battery pack  1021  of the energy storage unit  102  and the inverter  103  is cut off. 
     In the sub-step S 306 , the inverter  103  stops issuing a communication signal to the photovoltaic array assembly  101 . 
     In the sub-step S 308 , the photovoltaic array switch  1012  of the photovoltaic array assembly  101  is turned off after the photovoltaic array assembly  101  has not received the communication signal from the inverter  103  for a specified time period. Consequently, the connection between the inverter  103  and the photovoltaic array  1011  of the photovoltaic array assembly  101  is cut off. 
     It is noted that numerous modifications and alterations may be made while retaining the teachings of the disclosure. For example, in another embodiment of the sub-step S 306 , the inverter  103  is further disabled to stop outputting the AC power. 
     As shown in  FIG. 1 , the photovoltaic energy storage system further includes an automatic transfer switch (ATS) device  106 .  FIG. 7  is a schematic circuit diagram illustrating the circuitry structure of the automatic transfer switch in the photovoltaic energy storage system as shown in  FIG. 1 . The automatic transfer switch device  106  is electrically coupled between the rapid shutdown switch  104  and the power grid  107 . The automatic transfer switch device  106  includes a transformer T, a first switch  301 , a second switch  302 , a third switch  303  and a neutral line switch  304 . The transformer T includes a first winding T 1 , a second winding T 2  and a center tap. The center tap is connected to the common node between the first winding T 1  and the second winding T 2 . In addition, the center tap of the transformer T is also connected with the common terminal N of the inverter  103 . The first switch  301  is electrically coupled with the rapid shutdown switch  104 , the first winding T 1  and the second winding T 2 . 
     When the photovoltaic energy storage system is in a grid-connected mode, the first switch  301  is turned off. When the photovoltaic energy storage system is in an off-grid mode, the first switch  301  is turned on and thus the first voltage U L1N  and the second voltage U L2N  are substantially balanced. The second switch  302  is electrically coupled between the rapid shutdown switch  104  and the load  105 . The third switch  303  is electrically coupled between the power grid  107  and the load  105 . A first terminal of the neutral line switch  304  is electrically coupled to the common terminal N. A second terminal of the neutral line switch  304  is electrically coupled to the first terminal L 1  or the second terminal L 2  of the inverter  103  through the rapid shutdown switch  104 . While the photovoltaic energy storage system is switched from the off-grid mode to the grid-connected mode, the neutral line switch  304  is turned on. When the neutral line switch  304  is turned on, the first voltage U L1N  or the second voltage U L2N  is slightly greater than zero and the different between the first voltage U L1N  and the second voltage U L2N  is large. 
     For discriminating the normal working state from the emergency situation, the step S 3  may optionally include a sub-step of comparing the smaller one of the first voltage U L1N  and the second voltage U L2N  with a predetermined third threshold value, and the reference value in the step S 2  contains the third threshold value. When the first ratio Q 1  is greater than the first threshold value and the smaller one of the first voltage U L1N  and the second voltage U L2N  is greater than the third threshold value, the detecting and controlling circuit  108  confirms that the rapid shutdown switch  104  is turned off. When the impedance in the circuit is taken into consideration, the third threshold value is not smaller than 30V (e.g., 30V). 
     From the above descriptions, the present disclosure provides a photovoltaic energy storage system and a control method for the photovoltaic energy storage system. Even if the photovoltaic energy storage system is not equipped with the additional hardware component, the photovoltaic energy storage system is capable of accurately determining the occurrence of the emergency event and cutting off the connection between the inverter and an energy storage unit and the connection between the inverter and a photovoltaic array assembly when the emergency event occurs. Consequently, the technology of the present disclosure can comply with the associated safety standards. 
     While the disclosure has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the disclosure needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.