Patent Publication Number: US-10784683-B2

Title: Method of controlling electrical power system and apparatus using the same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is based upon and claims priority to Chinese Patent Application No. 201710993085.1, filed on Oct. 23, 2017, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present application relates to the field of power electronics technology, and in particular, to a method of controlling an electrical power system and an apparatus of controlling the electrical power system using the method. 
     BACKGROUND 
     Wind power generation relies on meteorological conditions and gradually merges into the power grid in a form of large-scale wind farms. The penetration rate of the wind power generation is higher and higher, which brings various impacts on the power grid and leads to a reduction in the stability of the power grid. When a DC bus capacitor is introduced into the stability analysis of the electrical power system, it appears that the bus capacitor interacts with other wind turbines, DC transmission, synchronous machine shafts, etc., resulting in sub-synchronous oscillation of the wind power generation system, which seriously threatens the stability of the electrical power system. 
     Some scholars have proposed dynamic stability analysis of the amplitude and phase of a power electronized electrical power system containing a wind generating set, which introduces the DC bus capacitor into the stability analysis of the electrical power system. The power electronized equipment also has internal potential, whose stability problem is also shown as the voltage power angle stability problem, which mainly includes three aspects of dynamic analysis, namely, rotor speed control, DC voltage control and AC current control. Through the stability analysis, it is concluded that the greater a loop bandwidth of the DC bus voltage is, the greater the contribution to the system stability will be. Without changing the actual bandwidth, the smaller the bus capacitance is, the greater the contribution to the system stability will be. 
     SUMMARY 
     The present application aims to provide a method of controlling an electrical power system and an apparatus of controlling the electrical power system using the method, so as to overcome the stability problem of the electrical power system due to limitations and disadvantages of the related art to a certain extent. 
     Other features and advantages of the present application will be apparent from the following detailed description, or may be learned in part through the practice of the present application. 
     According to one aspect of the present application, a method of controlling an electrical power system is provided, wherein the electrical power system includes a DC bus and a DC bus capacitor connected to the DC bus, and the method includes: 
     a setting step, receiving a virtual DC bus capacitance value of the DC bus capacitor: 
     a detecting step, detecting a DC bus voltage: 
     a calculating step, calculating an expected value of a DC bus current based on the virtual DC bus capacitance value and the DC bus voltage; and 
     an adjusting step, adjusting the DC bus current, so that the DC bus current reaches the expected value and thus the DC bus capacitor is equivalent to the virtual DC bus capacitance value. 
     According to another aspect of the present disclosure, an apparatus of controlling an electrical power system is provided, wherein the electrical power system includes a DC bus and a DC bus capacitor connected to the DC bus, and the apparatus includes: 
     a setting module, configured to receive a virtual DC bus capacitance value of the DC bus capacitor; 
     a detecting module, configured to detect a DC bus voltage; 
     a calculating module, configured to calculate an expected value of a DC bus current based on the virtual DC bus capacitance value and the DC bus voltage; and 
     an adjusting module, configured to adjust the DC bus current, so that the DC bus current reaches the expected value and thus the DC bus capacitor is equivalent to the virtual DC bus capacitance value. 
     An actual DC bus capacitance value is fixed. Considering the high-frequency ripple, the capacitance should not be excessively reduced. Therefore, the present application proposes a virtual DC bus capacitor technology, by which, a DC bus capacitor is virtualized at a low frequency to change the actual bus capacitance value as required, thus further improving the grid-connection stability. On the other hand, if the stability problem is not considered, a positive capacitor may also be virtualized to reduce the cost of DC bus capacitor. 
     For a better understanding of features and technical contents of the present application, please refer to the following detailed description of the present application and the accompanying drawings, but the detailed description and drawings herein are merely used to illustrate the present application and not to limit the scope of claims of the present application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present disclosure will become more apparent by describing its exemplary embodiments in detail with reference to the accompanying drawings. 
         FIG. 1A  is a partial schematic diagram of an electrical power system  1000 ; 
         FIG. 1B  is a flowchart of one embodiment of a method of controlling an electrical power system of the present application; 
         FIG. 2A  is a schematic diagram of a three-phase electrical power system  2000 A according to an embodiment of the present application; 
         FIG. 2B  is a schematic diagram of the electrical power system of the present application being a wind power generation system  2000 B; 
         FIG. 2C  is a flowchart of another embodiment of a method of controlling an electrical power system of the present application; 
         FIG. 3A  is a schematic diagram of the electrical power system of the present application being a wind power generation system  3000 ; 
         FIG. 3B  is a flowchart of another embodiment of a method of controlling an electrical power system of the present application: 
         FIG. 4A  is a schematic diagram of the electrical power system of the present application being a wind power generation system  4000 ; and 
         FIG. 4B  is a flowchart of another embodiment of a method of controlling an electrical power system of the present application. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments will now be described more fully with reference to the drawings. The exemplary embodiments, however, may be implemented in various forms, and should not be construed as been limited to the implementations set forth herein; instead, the implementations are provided such that the present disclosure will be thorough and complete, and will fully convey the concept of exemplary embodiments to those skilled in the art. In the drawings, the same reference signs denote the same or similar structures, thus their detailed description will be omitted. 
     In addition, the features, structures or characteristics described herein can be combined in one or more embodiments in any appropriate way. In the description hereinafter, many specific details are provided for fully understanding of the embodiments of the present disclosure. However, it will be appreciated by those skilled in the art that the technical solution of the present disclosure can be practiced without one or more of the specific details, or with other structures, components, steps or methods, etc. In addition, known structures, components or operations will not be illustrated or described in detail, to avoid obscuration of the aspects of the present disclosure. 
     In addition, in the following drawings, if there is a cross between solid line segments as connection wires or dotted line segments as signal lines, a black dot “●” at the cross indicates that the cross point is a wire connection point or a merge-split point of the signal line, no black dot “●” on the cross means that the cross point is not a wire connection point or a merge-split point of the signal line, but merely traverse over each other. Various arrows indicate the flow of the respective current, signal or step, respectively. The notation of each element or signal not only represents the element or signal itself, but also represents an algebraic sign of the capacity or size of the element or signal. 
     The present application intends to eliminate the existence condition of the sub-synchronous oscillation by changing operating parameters of the electrical power system in real time, according to the study of the above-described instability problem in the electrical power system involving wind power generation. This application intends to solve the above problems by controlling the operating parameters of the DC bus. Specifically, the capacitance value of the DC bus capacitor is changed in a virtual manner in real time, such that a virtual DC bus capacitor technology based on current control is proposed. The virtual DC bus capacitor technology of the present application is essentially a method of controlling an electrical power system, and an apparatus using the method to control the electrical power system. 
     A method of controlling the electrical power system and an apparatus of controlling the electrical power system using the method of the present application will be described in detail below with reference to  FIGS. 1A-4B . 
     First, one embodiment of a method of controlling the electrical power system of the present application is described with reference to  FIGS. 1A and 1B . 
       FIG. 1A  is a partial schematic diagram of an electrical power system  1000 . The electrical power system  1000  in  FIG. 1A  at least includes: a DC bus B and a DC bus capacitor C. As shown in  FIG. 1A , the DC bus capacitor C is connected to the DC bus B. 
     An actual electrical power system  1000  may further include other power electronic devices. However, under the premise that those skilled in the art can understand, descriptions of other power electronic devices are temporarily ignored in the first embodiment, in order to make the method of controlling the electrical power system of the present application easier be understood and not drown the essence of the present disclosure due to overly complicated description. The ignored power electronic devices are only indicated by ellipses. 
     When the electrical power system  1000  operates, the DC bus voltage V BUS  and the DC bus current I BUS  are generated on the DC bus. 
       FIG. 1B  is a flowchart of one embodiment of a method of controlling an electrical power system of the present application. As shown in  FIG. 1B , the method of controlling the electrical power system of this embodiment includes: a setting step  100 , a detecting step  200 , a calculating step  300 , and an adjusting step  400 . 
     In the setting step  100 , a virtual DC bus capacitance value C VIR  set for the DC bus capacitor C is received. 
     Herein, the virtual DC bus capacitance value C VIR  is both an expected value and an equivalent value. Specifically, in order to eliminate or suppress sub-synchronous oscillation in the electrical power system  1000 , it is desirable to change the capacitance value of the DC bus capacitor C to be the virtual DC bus capacitance value C VIR , thereby making the sub-synchronous oscillation away from resonance as far as possible. However, in an actual circuit, the capacitance value of the DC bus capacitor C will not change arbitrarily. Therefore, the present application expects to make the capacitance value of the DC bus capacitor C be equivalent to the virtual DC bus capacitance value C VIR  by controlling other operating parameters of the DC bus. 
     In the detecting step  200 , the DC bus voltage V BUS  is detected. 
     As another embodiment, the detected DC bus voltage V BUS  may be filtered by a first-order small inertia element prior to the use. 
     In the calculating step  300 , an expected value I EXP  of the DC bus current I BUS  is calculated based on the virtual DC bus capacitance value C VIR  and the DC bus voltage V BUS . 
     In the adjusting step  400 , the DC bus current I BUS  is adjusted, so that the DC bus current I BUS  reaches the expected value I EXP  and thus the DC bus capacitor C is equal to the virtual DC bus capacitance value C VIR . 
     Since the capacitor exhibits a low impedance to an abruptly changed voltage, the change of the DC bus current I BUS  caused by the change of the DC bus voltage V BUS  is mainly the change of current flowing through the DC bus capacitor C. In this application, by controlling the DC bus current I BUS , the DC bus current I BUS  reaches the expected value I EXP , so that the capacitance value of the DC bus capacitor C is equivalent to the virtual DC bus capacitance value C VIR , which is equivalent to changing the capacitance value of the DC bus capacitor C to be the virtual DC bus capacitance value C VIR . 
     Externally expressed characteristics of the DC bus capacitor C are reflected in the relationship between the capacitance value C of the DC bus capacitor C, a voltage u dc  applied across the DC bus capacitor C, and a current i dc  flowing through the DC bus capacitor C, which is shown in the following formula (1): 
     
       
         
           
             
               
                 
                   
                     i 
                     
                       d 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       c 
                     
                   
                   = 
                   
                     D 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         du 
                         
                           d 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           c 
                         
                       
                       dt 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     In the present application, by approximating the voltage u dc  applied across the DC bus capacitor C in the above formula (1) as the DC bus voltage V BUS , and setting the capacitance value C of the DC bus capacitor C as the virtual DC bus capacitance value C VIR , the expected value I EXP  of the DC bus current is calculated, which corresponds to i dc  in formula (1). By controlling the DC bus current I BUS , the DC bus current I BUS  reaches the expected value I EXP , and then the externally expressed characteristics of the DC bus capacitor C may be equivalent to the case when the capacitance value is the virtual DC bus capacitance value C VIR . 
       FIG. 2A  is a schematic diagram of a three-phase electrical power system  2000 A according to an embodiment of the present application. As shown in  FIG. 2A , the three-phase electrical power system  2000 A includes a DC bus capacitor C, a three-phase inverter  30 , an energy storage module  50 , and an apparatus  500  of controlling the electrical power system. The DC bus capacitor C is connected to the three-phase inverter  30  through the DC bus B. The energy storage module  50  is connected in parallel with the DC bus B to store electrical energy E 50  provided by the DC bus B or provide the electrical energy E 50  to the DC bus B. The apparatus  500  of controlling the electrical power system is connected to the DC bus B and the energy storage module  50 . 
     The three-phase electrical power system  2000 A may further include auxiliary devices such as a contactor K 1 , an LC filter  31 , a soft start module  32 , an AC fuse F AC , and a DC fuse F DC . The reference numeral  40  indicates a grid. They are connected as shown in the figure. Since these devices themselves are not related to the present disclosure, their structures are not described. 
     The dashed arrows in  FIG. 2A  indicate the flow of current, electrical energy, or power. As shown in  FIG. 2A , the energy storage module  50  includes a charging/discharging unit  51 , an energy storage element  52 , and a circuit breaker K 5 . The charging/discharging unit  51  may be a bidirectional DC/DC converter of various topologies. When the energy storage module  50  is in operation, the charging/discharging unit  51  has a charging/discharging current I C/D , which charges the energy storage module  50  from the DC bus B or discharges the energy storage module  50  to the DC bus B. The DC bus B charges the energy storage module  50  or the energy storage module  50  discharges to the DC bus B in an active power P A50 . A person skilled in the art knows how to connect the internal elements of the energy storage module  50 , and therefore will not be described in details. 
     As shown in  FIG. 2A , the apparatus  500  of controlling the electrical power system includes: a setting module  510 , a detecting module  520 , a calculating module  530 , and an adjusting module  540 . 
     The setting module  510  is configured to receive a virtual DC bus capacitance value C VIR  of the DC bus capacitor C. The setting module  510  may be various input devices having knobs, joysticks, buttons, mice, keyboards, touch pads, touch screens, and the like. 
     The detecting module  520  is configured to detect the DC bus voltage V BUS , the DC bus current I BUS , and the charging/discharging current I C/D  of the energy storage module  50 . The detecting module  520  may be various sensors or detectors for voltage, current or power. 
     The calculating module  530  includes two calculating units which are a first reference current calculating unit  531  and a second reference current calculation unit  532 . The first reference current calculating unit  531  is configured to calculate an expected value I EXP  of a DC bus current based on the virtual DC bus capacitance value C VIR  and the DC bus voltage V BUS . The second reference current calculation unit  532  is configured to subtract the actual DC bus current I BUS  from the expected value I EXP  of the DC bus current, so as to obtain a virtual current value I VIR . The calculating module  530  may be various computing devices, software modules or firmware modules, such as an operating circuit, a processor or a computer. 
     The adjusting module  540  is configured to adjust the charging/discharging current I C/D , such that the charging/discharging current I C/D  reaches the virtual current value I VIR . An adjusting process of the charging/discharging current I C/D  essentially behaves as injecting a positive or negative current into the DC bus B, so that the DC bus current I BUS  reaches the expected value I EXP . Then the DC bus capacitor C presents a virtual DC bus capacitance value C VIR . The adjusting module  540  may be a variety of output devices, software modules or firmware modules, such as signal generators, signal controllers, signal amplifiers. 
     As another embodiment, the apparatus  500  of controlling the electrical power system may further include a first-order small inertia element A, so as to filter the detected DC bus voltage V BUS  by the first-order small inertia element. The first-order small inertia element may be integrated in the detecting module  520 . 
     In addition, the apparatus  500  of controlling the electrical power system may further include a driving module  541 , which converts a signal S 50  output by the adjusting module  540  into PWM signals to driving switches (such as transistors). The PWM signals output by the driving module  541  control power switches in the charging/discharging unit  51  so that the charging/discharging current I C/D  reaches the virtual current value I VIR . For example, the PWM signals control a duty cycle of each power switch to adjust the charging/discharging current I C/D , so that the charging/discharging current I C/D  reaches the virtual current value I VIR . The driving module  541  may also be a part of the electrical power system. 
       FIG. 2B  is a schematic diagram of the electrical power system of the present application being a wind power generation system  2000 B. As shown in  FIG. 2B , the wind power generation system  2000 B mainly includes: a wind generator  10 , a generator-side power converter  20 , a grid-side power converter  33 , a grid  40 , an energy storage module  50 , and an apparatus  500  of controlling the electrical power system. The DC bus capacitor C is connected between the generator-side power converter  20  and the grid-side power converter  33  through the DC bus B. The energy storage module  50  is connected in parallel with the DC bus B, to store electrical energy E 50  provided by the DC bus B or provide the electrical energy E 50  to the DC bus B. The apparatus  500  of controlling the electrical power system is connected to the DC bus B and the energy storage module  50 . 
     The wind power generation system  2000 B may further include auxiliary devices such as contactors K 1 -K 3 , a main breaker K 4 , an LC filter  31 , a soft start module  32 , an AC fuse F AC  and a DC fuse F DC , whose connection relationships are as shown in the figure. Since these devices themselves are not related to the present disclosure, their structures are not described. It should be noted that, the wind power generation system  2000 B shown in  FIG. 2B  is a doubly-fed wind power generation system, but the present application is not limited thereto. For example, the wind power generation system  2000 B may also be a full-power wind power generation system. 
     In an actual electrical power system, the generator-side power converter  20  may be various bidirectional AC-DC converters that can be controlled by power switching elements. The grid-side power converter  33  may be various bidirectional DC-AC inverters that can be controlled by power switching elements. However, internal composition details of the generator-side power converter  20  and the grid-side power converter  33  are not related to the present disclosure. Therefore, under the premise that those skilled in the art can understand, in order to avoid drowning the essence of the present disclosure due to overly complicated description, internal details of the generator-side power converter  20  and the power-side power converter  33  are omitted herein. 
     The energy storage module  50  includes a charging/discharging unit  51 , an energy storage element  52  and a circuit breaker K 5 . The charging/discharging unit  51  may be a bidirectional DC/DC converter of various topologies. The energy storage element  52  may be constituted by a super capacitor or a rechargeable battery, but not limited thereto. When the energy storage module  50  is in operation, the charging/discharging unit  51  charges the energy storage module  50  from the DC bus B or discharges the energy storage module  50  to the DC bus B with a charging/discharging current I C/D . That is to say, the DC bus B charges the energy storage module  50  or the energy storage module  50  discharges to the DC bus B in an active power P A50 . 
     The energy storage module  50  may be disposed inside a converter which is a device including the generator-side power converter  20  and the grid-side power converter  33 , forming a wind storage integrated machine. The energy storage module  50  does not change original topology and control structures of the wind power generation system. The generator-side power converter  20  and the energy storage module  50  share the grid-side power converter  33 , the LC filter  31 , other converters, and the like (not shown in the drawings), to save cost. A person skilled in the art knows the internal connection of the energy storage module  50 , and therefore will not be described in details. 
     The AC side of the generator-side power converter  20  is connected to a rotor winding of the wind generator  10 , and the AC side of the grid-side power converter  33  is connected to the grid  40  through the LC filter  31 , the AC fuse F AC , the contactor K 1  and the main breaker K 4 . The stator winding of the wind generator  10  is connected to ground through the contactor K 3 , and to the grid  40  through the contactor K 2  and the main breaker K 4 . The dashed arrow in  FIG. 2B  indicates the flow of current, electrical energy, or power. 
     As shown in  FIG. 2B , the apparatus  500  of controlling an electrical power system of this embodiment includes: a setting module  510 , a detecting module  520 , a calculating module  530 , and an adjusting module  540 . 
     The setting module  510  is configured to receive a virtual DC bus capacitance value C VIR  of the DC bus capacitor C. The setting module  510  may be various input devices having knobs, joysticks, buttons, mice, keyboards, touch pads, touch screens, and the like. 
     The detecting module  520  is configured to detect the DC bus voltage V BUS , the DC bus current I BUS , and the charging/discharging current I C/D . The detecting module  520  may be various sensors or detectors for such as voltage, current or power. 
     The calculating module  530  includes two calculating units which are a first reference current calculating unit  531  and a second reference current calculation unit  532 . The first reference current calculating unit  531  is configured to calculate an expected value I EXP  of the DC bus current based on the virtual DC bus capacitance value C VIR  and the DC bus voltage V BUS . The second reference current calculation unit  532  is configured to subtract the actual DC bus current I BUS  from the expected value I EXP  of the DC bus current, to obtain a virtual current value I VIR . The calculating module  530  may be various computing devices, software modules or firmware modules of an operating circuit, a processor or a computer. 
     The adjusting module  540  is configured to adjust the charging/discharging current I C/D , such that the charging/discharging current I C/D  reaches the virtual current value I VIR . An adjusting process of the charging/discharging current I C/D  essentially behaves as injecting a positive or negative current into the DC bus B, so that the DC bus current I BUS  reaches the expected value I EXP . Then the DC bus capacitor C presents a virtual DC bus capacitance value C VIR . The adjusting module  540  may be a variety of output devices, software modules or firmware modules, such as signal generators, signal controllers, signal amplifiers. 
     As another embodiment, the apparatus  500  of controlling the electrical power system may further include a first-order small inertia element A, so as to filter the detected DC bus voltage V BUS  by the first-order small inertia element. The first-order small inertia element may be integrated in the detecting module  520 . 
     In addition, the apparatus  500  of controlling the electrical power system of the present application may further include a driving module  541 , which converts a signal S 50  output by the adjusting module  540  into PWM signals to driving a switches (such as transistors). The PWM signals output by the driving module  541  control power switches in the charging/discharging unit  51 . For example, the PWM signals control a duty cycle of each power switch to adjust the charging/discharging current I C/D , so that the charging/discharging current I C/D  reaches the virtual current value I VIR . The driving module  541  may also be a part of the electrical power system. 
     With reference to the descriptions of  FIGS. 2A and 2B , embodiments of the method of controlling an electrical power system of the present application are described below with further reference to  FIG. 2C . 
       FIG. 2C  is a flowchart of another embodiment of a method of controlling an electrical power system of the present application. As shown in  FIG. 2C , the method of controlling the electrical power system of this embodiment includes: a setting step  100 , a detecting step  201 , a calculating step  301 , and an adjusting step  401 . 
     The setting step  100 , the detecting step  201 , the calculating step  301 , and the adjusting step  401  in  FIG. 2C  are similar to the foregoing setting step  100 , detecting step  200 , calculating step  300 , and adjusting step  400  in  FIG. 1B . The same parts are not described repeatedly, and only the difference will be emphasized. 
     In the adjusting step  401  of this embodiment, the active power P A50  charged from the DC bus B to the energy storage module  50  or discharged from the energy storage module  50  to the DC bus B is controlled. The energy storage module  50  stores the electrical energy E 50  drawn from the DC bus B or provides the electrical energy E 50  to the DC bus B, so as to adjust the DC bus current I BUS . In the adjusting step  401 , the active power P A50  charged from the DC bus to the energy storage module is controlled, which makes the energy storage module draw electrical energy from the DC bus so as to adjust the DC bus current. Or in the adjusting step  401 , the active power P A50  discharged from the energy storage module to the DC bus is controlled, which makes the energy storage module provide electrical energy to the DC bus so as to adjust the DC bus current. 
     As another embodiment of the method of controlling the electrical power system of the present application, in the detecting step  201 , the DC bus current I BUS  and the charging/discharging current I C/D  of the charging/discharging unit  50  are further detected. 
     In the calculating step  301 , the DC bus current I BUS  is further subtracted from the expected value I EXP  to obtain a virtual current value I VIR . 
     In the adjusting step  401 , a closed-loop control is performed on the charging/discharging current I C/D  to form switch signals S 50 . The switch signals S 50  control operations of each power element in the charging/discharging unit  51  to adjust the charging/discharging current I C/D , such that the charging/discharging current I C/D  is equal to the virtual current value I VIR . Thus the DC bus current I BUS  reaches the expected value I EXP . 
     A further embodiment of the electrical power system of the present application is described below with further reference to  FIGS. 3A and 3B .  FIG. 3A  is a schematic diagram of the electrical power system of the present application being a wind power generation system  3000 . As shown in  FIG. 3A , the wind power generation system  3000  is a full-power power generation system, but not limited thereto. The wind power generation system  3000  mainly includes: a wind generator  10 , a generator-side power converter  20 , a grid-side power converter  33 , an apparatus  500  of controlling the electrical power system, and a grid  40 . The DC bus capacitor C is connected between the generator-side power converter  20  and the grid-side power converter  33  through the DC bus B. The apparatus  500  of controlling the electrical power system is connected to the DC bus B and the generator-side power converter  20 . 
     The wind power generation system  3000  may further include auxiliary devices such as a wind generator output inductor L, a contactors K 1 , an LC filter  31 , a soft start module  32 , and an AC fuse F AC , whose connection relationships are as shown in the figure. Since these devices themselves are not related to the present disclosure, their structures are not described. 
     The AC side of the generator-side power converter  20  is connected to the wind generator  10  through the wind generator output inductance L, and the AC side of the grid-side power converter  33  is connected to the grid  40  through the LC filter  31 , the AC fuse F AC  and the contactor K 1 . The solid arrows in  FIG. 3A  indicate the flow of current, electrical energy and power. 
     The apparatus  500  of controlling the electrical power system controls the active current I A20  between the generator-side power converter  20  and the DC bus B, such that the active current I A20  is a positive or negative current. Then the generator-side power converter  20  draws electrical energy E 20  from the DC bus B or provides the electrical energy E 20  to the DC bus B, to adjust the DC bus current I BUS . 
     According to a current model of the three-phase converter, a Q-axis voltage u sq  of the AC side of the converter in the DQ rotating coordinate system becomes zero after the AC side voltage of the converter is phase locked and the D axis is oriented in the DQ rotating coordinate system. Therefore, in the case of ignoring conduction loss and switching loss of the power switching devices in the converter, a D-axis current i dref  on the AC side of the converter in the DQ rotating coordinate system, a voltage u dc  applied to the DC bus (i.e., a voltage applied across the DC bus capacitor C), a D-axis voltage u sd  on the AC side of the converter in the DQ rotating coordinate system, and a current i dc  flowing through the DC bus (i.e., approximate to the current flowing through the DC bus capacitor C) have a proportional relationship, as shown in formula (2) below: 
     
       
         
           
             
               
                 
                   
                     i 
                     dref 
                   
                   = 
                   
                     
                       2 
                       3 
                     
                     ⁢ 
                     
                       
                         u 
                         
                           d 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           c 
                         
                       
                       
                         u 
                         sd 
                       
                     
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                       i 
                       
                         d 
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                         c 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
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     The above coordinate transformation is performed on the AC side voltage and the AC side current of the generator-side power converter  20 , and then the D-axis current i dref  on the AC side of the converter in the above formula (2) corresponds to the active current I A20  in the present embodiment, the current i dc  flowing through the DC bus corresponds to the DC bus current I BUS  in the present embodiment. The active current I A20  and the DC bus current I BUS  have the same proportional relationship as the above formula (2) in three-phase applications. It is possible to indirectly control the DC bus current I BUS  by controlling the active current I A20 . For non-three-phase applications, for example, in single-phase, four-phase, six-phase, and eight-phase applications, a certain proportional relationship may also exist between the active current I A20  and the DC bus current I BUS , which will not be described herein, and the present application does not limit it, the same as below. 
     With continued reference to  FIG. 3A , as another embodiment of the apparatus  500  of controlling the electrical power system of the present application, similar to the structure of  FIG. 2B , only different parts will be described in detail. 
     The detecting module  520  of the present embodiment detects the AC side voltage V ABC  of the generator-side power converter  20 , the AC side current I ABC  of the generator-side power converter  20 , and the DC bus voltage V BUS . 
     The calculating module  530  of the present embodiment further includes a third reference current calculation unit  533 , a Clark converter  534  and a Park converter  535 . The third reference current calculation unit  533  is connected to the setting unit  510  and the detecting unit  520 , to receive the virtual bus capacitance value C VIR  and the DC bus voltage V BUS , and calculate the expected value I EXP  of the bus current according to the formula (1). In the calculating module  530  of the present embodiment, the Clark converter  534  and the Park converter  535  perform the aforementioned coordinate transformation on the AC side voltage V ABC  and the AC side current I ABC  to obtain the proportional relationship of the formula (2). The active current I A20  is calculated according to the proportional relationship and the DC bus current I BUS . Specifically, the AC side voltage V ABC  is phase locked to obtain an angle θ for the coordinate transformation. The AC side voltage V ABC  and the AC side current I ABC  are subjected to the Clark transformation, and then the Park transformation, to obtain the D-axis voltage u sd  on the AC side of the converter in the DQ rotating coordinate system, and the active current I A20  is calculated. The third reference current calculation unit  533  further receives the D axis voltage u sd , and calculates the reference value of the D axis current on the AC side of the converter based on the D axis voltage u sd  and the expected value I EXP  of the DC bus current I BUS , according to the aforementioned formula (2). The reference value i DREF  of the D axis current is the reference value of the active current I A20 . 
     The active current I A20  and the DC bus current I BUS  have the proportional relationship shown in formula (2). The reference value i DREF  of the active current and the expected value I EXP  also have the proportional relationship shown in formula (2). The proportional relationship of the formula (2) is obtained by the coordinate transformation described above. In the calculating module  530 , the active current I A20  is calculated according to the proportional relationship and the DC bus current I BUS , and the reference value i DREF  is calculated according to the proportional relationship and the expected value I EXP . 
     In the adjusting module  540  of this embodiment, the active current I A20  and its reference value i DREF  are received. Through the closed-loop control of the active current I A20 , switch signals S 20  are formed. The switch signals S 20  control operations of each power element in the generator-side power converter  20  to adjust the active current I A20 , such that the active current I A20  reaches the reference value i DREF . Because the active current I A20  and the DC bus current I BUS  have the proportional relationship shown by the formula (2), the active current I A20  reaches the reference value, and the DC bus current I BUS  also reaches the expected value I EXP . The control process behaves as that the generator-side power converter  20  injects a positive or negative current into the DC bus B, so that the DC bus current I BUS  reaches the expected value I EXP . 
     Accordingly,  FIG. 3B  is a flowchart of another embodiment of a method of controlling an electrical power system of the present application. As shown in  FIG. 3B , the method of controlling the electrical power system of the present embodiment includes: a setting step  100 , a detecting step  202 , a calculating step  302 , and an adjusting step  402 . 
     The setting step  100 , the detecting step  202 , the calculating step  302 , and the adjusting step  402  in  FIG. 3B  are similar to the foregoing setting step  100 , detecting step  200 , calculating step  300 , and adjusting step  400 . The same parts are not described repeatedly, and only the difference will be emphasized. 
     As another embodiment of the method of controlling the electrical power system of the present application, based on the detecting step  200  in  FIG. 1B , in the detecting step  202  of the present embodiment, the AC side voltage V ABC  and the AC side current I ABC  of the generator-side power converter  20  further need to be detected. 
     Based on the calculating step  300  in  FIG. 1B , in the calculating step  302  of this embodiment, the method further includes performing coordinate transformation on the AC side voltage V ABC  and the AC side current I ABC  to obtain an active current of the AC side current I ABC , i.e., the active current I A20  between the generator-side power converter  20  and the DC bus B. The reference value i DREF  of the active current I A20  is obtained according to the expected value I EXP  of the DC bus current I BUS  and the aforementioned formula (2). 
     The active current I A20  and the DC bus current I BUS  have the proportional relationship shown in formula (2). The reference value i DREF  of the active current and the expected value I EXP  also have the proportional relationship shown in formula (2). The proportional relationship of the formula (2) is obtained by the coordinate transformation described above. In the calculating step  302 , the active current I A20  is calculated according to the proportional relationship and the DC bus current I BUS , and the reference value i DREF  is calculated according to the proportional relationship and the expected value I EXP . 
     Based on the adjusting step  400  in  FIG. 1B , in the adjusting step  402  of the present embodiment, the switch signals S 20  are formed by performing the closed-loop control on the active current I A20 . The switch signals S 20  control operations of each power element in the generator-side power converter  20  to adjust the active current I A20 , such that the generator-side power converter  20  injects a positive or negative current into the DC bus B, and then the DC bus current I BUS  reaches the expected value I EXP . 
     In this embodiment, the active current I A20  between the generator-side power converter  20  and the DC bus B is controlled. The generator-side power converter  20  draws electrical energy E 20  from the DC bus B when the active current I A20  is a negative current. The generator-side power converter  20  provides the electrical energy E 20  to the DC bus B when the active current I A20  is a positive current. Then the DC bus current I BUS  is adjusted, which reaches the expected value I EXP . By controlling the active current I A20  between the generator-side power converter  20  and the DC bus B, the DC bus current I BUS  is the indirectly controlled, such that the externally expressed characteristics of the DC bus capacitor C may be equivalent to the case when the capacitance value is the virtual DC bus capacitance value C VIR . 
     In conjunction with the description of  FIGS. 1A-3B , another embodiment of the electrical power system of the present application is described below with further reference to  FIG. 4A . 
       FIG. 4A  is a schematic diagram of the electrical power system of the present application being a wind power generation system  4000 . As shown in  FIG. 4A , the wind power generation system  4000  mainly includes: a wind generator  10 , a generator-side power converter  20 , a grid-side power converter  33 , a grid  40 , an energy storage module  50 , a rectifier circuit  60 , and an apparatus  500  of controlling the electrical power system. The DC bus capacitor C 1  of the converter is connected between the generator-side power converter  20  and the grid-side power converter  33  through the DC bus B 1  of the converter. The AC side of the generator-side power converter  20  is connected to a rotor winding of the wind generator  10 , and the AC side of the grid-side power converter  33  is connected to the grid  40 . The DC side of the rectifier circuit  60  is connected in parallel to the DC bus capacitor C, and the AC side of the rectifier circuit  60  is connected between the stator winding of the wind generator  10  and the grid  40 . The energy storage module  50  is connected in parallel with the DC bus capacitor C through the DC bus B, to store electrical energy E 50  provided by the DC bus B or provide the electrical energy E 50  to the DC bus B. The apparatus  500  of controlling the electrical power system is connected to the AC side of the rectifier circuit  60  and the DC bus B. 
     The wind power generation system  4000  may further include auxiliary devices such as contactors K 1  and K 2 , a main breaker K 4 , an LC filter  31 , a soft start module  32  and an AC fuse F AC , whose connection relationship is shown in the figure. Since these devices themselves are not related to the present disclosure, their structures are not described. 
     As described above, the energy storage module  50  includes a charging/discharging unit  51  and an energy storage element  52 . The charging/discharging unit  51  may be a bidirectional DC/DC converter of various topologies. The energy storage element  52  may be constituted by a super capacitor or a rechargeable battery, but not limited thereto. When the energy storage module  50  is in operation, the charging/discharging unit  51  charges the energy storage module  50  from the DC bus B or discharges the energy storage module  50  to the DC bus B with a charging/discharging current I C/D , so that the DC bus B charges the energy storage module  50  or the energy storage module  50  discharges to DC bus B in an active power P A50 . 
     The AC side of the grid-side power converter  33  is connected to the grid  40  through the LC filter  31 , the AC fuse F AC , the contactor K 1  and the main breaker K 4 , and the stator winding of the wind generator  10  is connected to the grid  40  via the contactor K 2  and the main breaker K 4 . The dashed and solid arrows in  FIG. 4A  indicate the flow of current, electrical energy, or power. In an actual wind power generation system  4000 , the rectifier circuit  60  may be various bidirectional AC-DC converters controlled by power switching elements. However, since internal composition details of the rectifier circuit  60  are not related to the present disclosure, in order to avoid drowning the essence of the present disclosure due to overly complicated description, internal details of the rectifier circuit  60  are omitted herein under the premise that those skilled in the art can understand. When the wind power generation system  4000  is in operation, electrical power may flow between the wind generator  10  and the grid  40  through the rotor windings and stator windings simultaneously. Electrical power may also flow between the wind generator  10  and the energy storage module  50  through the rotor windings and stator windings simultaneously. The functions of the circuit composed of the generator-side power converter  20  and the grid-side power converter  33  have already been described above, and therefore will not be described again. An active current I A60  exists between the rectifier circuit  60  and the DC bus B, as indicated by the double-headed arrow in  FIG. 4A . The active current I A60  may be a positive current or a negative current, so that the rectifier circuit  60  extracts electrical energy E 60  from the DC bus B or provides the electrical energy E 60  to the DC bus B. 
     As another embodiment of the current control apparatus of the present application, as shown in  FIG. 4A , the apparatus  500  of controlling the electrical power system of the present embodiment controls the active current I A60  between the rectifier circuit  60  and the DC bus B, so that the active current I A60  reaches the expected value of the active current. The rectifier circuit  60  injects a positive or negative current to the DC bus B. The rectifier circuit  60  extracts electrical energy E 60  from the DC bus B when the active current I A60  is a negative current. The rectifier circuit  60  provides the electrical energy E 60  to the DC bus B when the active current I A60  is a positive current. Then the DC bus current I BUS  is adjusted. 
     Correspondingly,  FIG. 4B  is a flowchart of another embodiment of a method of controlling an electrical power system of the present application. As shown in  FIG. 4B , the method of controlling the electrical power system of this embodiment includes: a setting step  100 , a detecting step  203 , a calculating step  303 , and an adjusting step  403 . 
     The setting step  100 , the detecting step  203 , the calculating step  303 , and the adjusting step  403  in  FIG. 4B  are similar to the foregoing setting step  100 , detecting step  200 , calculating step  300  and adjusting step  400 . The same parts are not described repeatedly, and only the difference will be emphasized. 
     Based on the detecting step  200  in  FIG. 1B , in the detecting step  203  of the present embodiment, the AC side voltage V ABC  and the AC side current I ABC  of the rectifier circuit  60  further need to be detected. 
     Based on the calculating step  300  in  FIG. 1B , in the calculating step  303  of this embodiment, the method further includes performing coordinate transformation on the AC side voltage V ABC  and the AC side current I ABC  to obtain an active current of the AC side current I ABC  and a reference value of the active current. That is to say, the active current I A60  between the rectifier circuit  60  and the DC bus B and the reference value i DREF1  of the active current I A60  are calculated. 
     Similarly, in the present embodiment, the active current I A60  and the DC bus current I BUS  have the same proportional relationship as the above formula (2) in three-phase applications. The expected value i DREF1  of the active current I A60  is obtained according to the expected value I EXP  of the DC bus current and the proportional relationship as the above formula (2). The DC bus current I BUS  may be controlled indirectly by controlling the active current I A60 . When the active current I A60  is controlled to reach the expected value i DREF1 , the DC bus current I BUS  also reaches the expected value I EXP . 
     The active current I A60  and the DC bus current I BUS  have a proportional relationship of formula (2). The reference value i DREF1  of the active current and the expected value I EXP  also have the proportional relationship of formula (2). The proportional relationship of the formula (2) is obtained by the coordinate transformation described above. In the calculating step  303 , the active current I A60  is calculated according to the proportional relationship and the DC bus current I B US, and the reference value i DREF1  is calculated according to the proportional relationship and the expected value I EXP . 
     Based on the adjusting step  400  in  FIG. 1B , in the adjusting step  403  of this embodiment, switch signals S 60  are formed by performing the closed-loop control on the active current I A60 . The switch signals S 60  control operations of each power element in the rectifier circuit  60  to adjust the active current I A60 , such that the active current I A60  reaches the expected value i DREF1  of the active current. The rectifier circuit  60  injects a positive or negative current into the DC bus B, and thus the DC bus current I BUS  reaches the expected value I EXP . The adjustment process of the active current I A60  essentially presents as that the rectifier circuit  60  injects a positive current or a negative current to the DC bus B. 
     In this embodiment, the active current I A60  between the rectifier circuit  60  and the DC bus B is controlled. The rectifier circuit  60  draws electrical energy E 60  from the DC bus B when the active current I A60  is a negative current. The rectifier circuit  60  provides the electrical energy E 60  to the DC bus B when the active current I A60  is a positive current. Then the DC bus current I BUS  is adjusted, such that the DC bus current I BUS  reaches the expected value I EXP . That is, the externally expressed characteristics of the DC bus capacitor C may be equivalent to the case when the capacitance value is the virtual DC bus capacitance value C VIR . Because the DC bus B is connected with the DC bus B 1  of the converter, adjusting the parameters of the DC bus B may be equivalent to adjusting the parameters of the DC bus B 1 . Specifically, a sum of the DC bus capacitor C and the DC bus capacitor C 1  of the converter forms the DC bus capacitor of the wind power generation system  4000 . By making the DC bus capacitor C equivalent to the virtual capacitor value C VIR , the DC bus capacitor of the wind power generation system may be equivalent to the expected value. 
     By further reference to  FIG. 4A , as another embodiment of the apparatus  500  of controlling the electrical power system of the present application, in the detecting module  520  of the present embodiment, the AC side voltage V ABC  and the AC side current I ABC  of the rectifier circuit  60  are further detected. 
     In the calculating module  530  of this embodiment, the AC side voltage V ABC  and the AC side current I ABC  are further subjected to the aforementioned coordinate transformation to obtain the active current I A60  and its reference value i DREF1 . Specifically, reference may be made to the description of  FIG. 3A , and details are not described herein again. 
     The active current I A60  and the DC bus current I BUS  have the proportional relationship shown in formula (2). The reference value i DREF1  of the active current and the expected value I EXP  also have the proportional relationship shown in formula (2). The proportional relationship of the formula (2) is obtained by the coordinate transformation described above. In the calculating module  530 , the active current I A60  is calculated according to the proportional relationship and the DC bus current I BUS , and the reference value i DREF1  is calculated according to the proportional relationship and the expected value I EXP . 
     In the adjusting module  540  of this embodiment, through the closed-loop control of the active current I A60 , the switch signals S 60  is formed. The switch signals S 60  control operations of each power element in the rectifier circuit  60  to adjust the active current I A60 , such that the active current I A60  reaches the reference value i DREF1  of the active current I A60 . The rectifier circuit  60  injects a positive or negative current into the DC bus B, so that the DC bus current I BUS  reaches the expected value I EXP . 
     As another embodiment of the apparatus of controlling the electrical power system of the present application, as shown in  FIG. 4A , in the apparatus  500  of controlling the electrical power system of the present embodiment, an active power P A50  charged from the DC bus to the energy storage module  50  or discharged from the energy storage module  50  to the DC bus B is controlled, so that the energy storage module  50  stores the electrical energy E 50  drawn from the DC bus B or provides the electrical energy E 50  to the DC bus B, thus adjusting the DC bus current I BUS . 
     As another embodiment of the apparatus of controlling the electrical power system of the present application, as shown in  FIG. 4A , in the detecting module  520  of the present embodiment, the DC bus current I BUS  and the charging/discharging current I C/D  of the charging/discharging unit  50  are further detected. 
     In the calculating module  530  of the present embodiment, the DC bus current I BUS  is further subtracted from the expected value I EXP  to obtain a virtual current value I VIR . 
     In the adjusting module  540  of the present embodiment, a closed-loop control is performed on the charging/discharging current I C/D  to form the switch signals S 50 , and the switch signals S 50  control operations of each power element in the charging/discharging unit  50  to adjust the charging/discharging current I C/D . Finally, the charging/discharging current I C/D  is equal to the virtual current value I VIR  and thus the DC bus current I BUS  reaches the expected value I EXP . 
     Correspondingly, as another embodiment of the method of controlling an electrical power system of the present application, based on the detecting step  200  in  FIG. 4B , in the detecting step of the present embodiment, the DC bus current I BUS  and the charging/discharging current I C/D  of the charging/discharging unit  60  are further detected. 
     Based on the calculating step  300  in  FIG. 4B , in the calculating step of this embodiment, the DC bus current I BUS  is further subtracted from the expected value I EXP  to obtain a virtual current value I VIR . 
     Based on the adjusting step  403  in  FIG. 4B , in the adjusting step of this embodiment, a closed-loop control is performed on the charging/discharging current I C /D to form the switch signals S 50 , and the switch signal S 50  control operations of each power element in the charging/discharging unit  50  to adjust the charging/discharging current I C/D , such that the charging/discharging current I C/D  is equal to the virtual current value I VIR  and thus the DC bus current I BUS  reaches the expected value I EXP . 
     As another embodiment of the apparatus of controlling the electrical power system of the present application, the DC bus capacitance of the wind power generation system  4000  may be equivalent to the expected value by making the DC bus capacitance C 1  of the converter equivalent to its corresponding virtual capacitance value. As shown in  FIG. 4A , in the apparatus  500  of controlling the electrical power system, the active current I A20  between the generator-side power converter  20  and the DC bus B 1  of the converter is controlled, such that the active current I A20  is a positive or negative current, and then the generator-side power converter  20  draws electrical energy E 20  from the DC bus B 1  or provides the electrical energy E 20  to the DC bus B 1 . The current of the DC bus B 1  of the converter is adjusted. 
     It should be noted that, the DC bus capacitance of the wind power generation system  4000  may be equivalent to the expected value by making the DC bus capacitance C 1  of the converter be equivalent to its corresponding virtual capacitance value and simultaneously making the DC bus capacitance C be equivalent to the virtual capacitor value C VIR . 
     In this embodiment, the DC bus current of the wind power generation system  4000  can be adjusted, by controlling the active current between the generator-side power converter  20  and the converter DC bus B 1 , the active current I A60  between the rectifier circuit  60  and the DC bus B, and the charging/discharging current I C/D  Of the energy storage module  50 , at the same time. The DC bus current of the wind power generation system  4000  can be adjusted, by controlling at least one of the active current between the generator-side power converter  20  and the converter DC bus B 1 , the active current between the rectifier circuit  60  and the DC bus B, and the charging/discharging current I C/D  of the energy storage module  50 . Finally, the DC bus current of the wind power generation system  4000  reaches the expected value. For example, the DC bus current I BUS  can be adjusted, only by controlling the active power P A50  for charging the energy storage module  50  from the DC bus B or discharging the energy storage module  50  to the DC bus B. The energy storage module  50  stores the electrical energy E 50  extracted from the DC bus B or provides the electrical energy E 50  to the DC bus B, so that the DC bus current I BUS  reaches the expected value I EXP . The externally expressed characteristics of the DC bus capacitor C may be equivalent to the case when the capacitance value is the virtual DC bus capacitance value C VIR . According to the capacity of the energy storage module  50 , the rectifier circuit  60  and the generator-side power converter  20  and actual requirements, the adjustment function of the DC bus in the wind power generation system  4000  may be flexibly configured, such that the bus capacitance of the wind power generation system  4000  may present its expected equivalent capacitance value. 
     In addition, the rectifier circuit  60  may not only be used to adjust the bus current to virtualize the bus capacitance, but also be used as a dual mode switch to switch the wind power generation system  4000  between the full power generation mode and the doubly fed power generation mode. Further, when a wind speed is less than a preset wind speed, the rectifier circuit  60  switches the wind power generation system  4000  to the full power generation mode. On the contrary, when the wind speed is greater than or equal to the preset wind speed, the rectifier circuit  60  switches the wind power generation system  4000  to the doubly fed power generation mode. In this way, the wind power generation system of the present embodiment can perform the full power generation mode at low wind speeds without increasing the cost, so as to still have good power generation efficiency at low wind speeds and increase the range of power generation operation. 
     The energy storage module  50  is disposed between the main breaker and the wind generating set. The energy storage module  50  may be used not only to adjust the bus current to virtualize the bus capacitance, but also to suppress events unfavorable to the grid  40  and the wind power generation system  4000 , such as grid frequency fluctuations, output power fluctuations of the wind generator and the like, by performing charging or discharging operation. In some embodiments, the energy storage module  50  and the grid-side power converter  33  may actually be integrated together in a cabinet (not shown), to save the cost of the wind power generation system. 
     An actual DC bus capacitance value is fixed. Considering the high-frequency ripple, the capacitance should not be excessively reduced. Depending on a virtual DC bus capacitor technology proposed by the present application, a DC bus capacitor is virtualized at a low frequency to change the actual bus capacitance value as required, thus further improving the grid-connection stability. On the other hand, if the stability problem is not considered, a positive capacitor may also be virtualized to reduce the cost of DC bus capacitor. 
     The present disclosure has been described by the above-described related embodiments. However, the above-described embodiments are merely examples of the present application. It is to be noted that the disclosed embodiments do not limit the scope of the present disclosure. Rather, changes and modifications without departing from the spirit and scope of the present disclosure all belong to the patent protection of the present disclosure.