KR20090074318A - Solar power generation system and method for the compenasation of harmonics and the prevention of islanding operation thereof - Google Patents

Abstract

A harmonic compensation and isolated operation prevention method are provided to detect the load current in the isolated operation and to extract harmonic component from the solar cell generating system and their system. The solar cell generating system comprises a solar cell array, a three phase inverter(20), a load(40), a first and second LC parallel filter, and a first and second detecting sensors(60,61) and a power control unit(70). The solar cell generating system more includes a third detecting sensor(62). The load current is transformed and is filtered in the power control unit. The load current is the sum current of output current of three phase inverter detected from second detecting sensor and the systematic current found from third detecting sensor. The filtered load current becomes the reverse conversion to extract the harmonic component.

Description

Solar power generation system and method for the compenasation of harmonics and the prevention of islanding operation

1 is a basic configuration of a general grid-distributed distributed power system.

2 is a view showing an embodiment of a photovoltaic power generation system according to the present invention.

3 is a flowchart illustrating an embodiment of a photovoltaic power generation system according to the present invention.

4 is a graph showing changes in grid current when the harmonics compensation function and the single operation prevention function are added.

FIG. 5 is a graph showing a correlation between time and frequency in a single operation of a photovoltaic system in a state in which power supply to a commercial grid is stopped.

6 is a flowchart illustrating a harmonic compensation and a stand alone operation prevention method in a photovoltaic power generation system according to the present invention.

Explanation of Signs of Major Parts of Drawings

10: solar cell array 20: three-phase inverter

30: commercial power supply unit 40: load

50, 51: first and second LC parallel filter

60, 61, 62: first to third detection sensors

70: power control unit 80, 81, 82: first to third power interruption device

The present invention relates to a photovoltaic power generation system, and more particularly, to a photovoltaic power generation system and a method having a function capable of preventing single operation of a photovoltaic power generation system while performing harmonic compensation.

The photovoltaic power generation system is a system for generating electric power using solar energy. There are two types of solar power generation systems, a standalone power generation system that operates solely with a solar power generation system, and a grid-connected power generation system that operates in conjunction with a solar power generation system and a commercial power system. Nowadays, grid-connected power generation systems are being adopted. 1 is a view showing a schematic configuration of a typical grid-connected power generation system.

In the power system of the grid-connected power generation system as shown in FIG. 1, nonlinear loads and a large amount of low-capacity power converters, which are increasing in construction, have a considerable impact on power quality during generation and transmission and distribution. Is going crazy. This is because the harmonics generated from the power converter distort the voltage on the power system, resulting in poor power quality on the power system. Therefore, in order to prevent the power quality deterioration on the power system, a solar power generation system has been developed that adds harmonic compensation to generate solar energy into electric power.

In addition, in the grid-connected power generation system, a method for preventing single operation that is operated only by the solar power generation system has been studied. This is because the safety of the power system construction personnel during the sole operation, re-closing and re-closing of the relay may cause problems such as damage to the equipment, because it is necessary to detect the single operation and block the single operation. In this case, the single operation means that the power grid-linked photovoltaic power generation system is operated alone in a state where power supply to the power system is stopped.

)과 거의 일치하면 전력 계통 연계점에서 양측의 전력의 전압과 주파수가 동일하게 유지되므로 다양한 계전기들로 태양광 발전 시스템의 단독 운전을 검출할 수 없는 문제가 있었다. 여기서 다양한 계전기들이란 과전압 계전기( over voltage Relay ), 부족전압 계전기( under voltage Relay ), 과주파수 계전기( over frequency Relay ), 및 부족주파수 계전기( under frequency Relay )를 말한다. 과전압 계전기는 설정된 기준전압이상의 전압을 검출하는 기기이고 부족전압 계전기는 설정된 기준전압이하의 전압을 검출하는 기기이다. 그리고 과주파수 계전기는 설정된 기준주파수(60HZ)이상의 주파수를 검출하는 기기이고 부족주파수 계전기는 설정된 기준주파수이하의 주파수를 검출하는 기기이다. However, the active and reactive power (P _load + jQ _load ) used in the load during the operation of the solar power generation system alone is supplied by the grid-connected power generation system (P _PV + jQ _pv +

), The voltage and frequency of the power on both sides are kept the same at the power system connection point, so that various operations of the solar power generation system cannot be detected by various relays. Here, the various relays refer to an over voltage relay, an under voltage relay, an over frequency relay, and an under frequency relay. An overvoltage relay is a device that detects a voltage above the set reference voltage and an undervoltage relay is a device that detects a voltage below the set reference voltage. The overfrequency relay is a device for detecting a frequency above the set reference frequency (60HZ) and the underfrequency relay is a device for detecting a frequency below the set reference frequency.

There are passive and active methods to prevent single operation in such a photovoltaic power generation system. However, in the passive method in such a photovoltaic power generation system, single operation is determined when the measured value change is small when the system is separated. There is an undetectable area that cannot. In addition, the active method injects a micro disturbance signal into the system, which causes the system to become unstable, causing power quality degradation and current distortion.

Therefore, in order to solve the above problems, the present invention detects a load current during single operation in the above photovoltaic power generation system, increases the harmonic content of the load current, induces an increase in frequency, and increases the frequency overfrequency. It is a first object to provide a photovoltaic power generation system capable of preventing single operation without degrading power quality by detecting with a relay.

Another object of the present invention is to provide a photovoltaic power generation system having a harmonic compensation function by reducing a harmonic component of a grid current by using a plurality of LC parallel filters and a plurality of voltage and current detection sensors.

In particular, it is a third object of the present invention to provide a harmonic compensation and an independent operation prevention method using a detected load current in a solar power generation system.

The solar power generation system according to the present invention for achieving the above-described first and second objects is a solar cell array 10, three-phase inverter 20, load 40, the first and second LC parallel filter 50 A conventional solar power generation system including a power control unit 70 having a 51, a first and a second detection sensor 60, 61, a signal control circuit 71, and a digital signal processing module 72; In the system, a third detection sensor 62 is further provided to supply a load current that is a sum current of the grid current detected by the third detection sensor 62 and the three-phase inverter output current detected by the second detection sensor 61. The digital signal processing module 72 of the control unit 70 performs a dq conversion and a filtering process, and then reverses dq conversion to extract harmonic components from the load current to perform a harmonic compensation function and an independent operation prevention function. .

Further, the filtering processing in the digital signal processing module 72 leaves only the direct current component in the dq-converted component.

Furthermore, the digital signal processing module 72 subtracts the filtered direct current component from the dq transform component after the filtering process to extract the high frequency component.

In particular, the digital signal processing module 72 subtracts the filtered DC component from the dq transform component with respect to the q-axis value of the dq transform component.

Harmonic compensation and single operation prevention method according to the present invention for achieving the above-described third object is a solar cell array 10, three-phase inverter 20, load 40, the first and second LC parallel filter 50 ), A photovoltaic power generation system including a power control unit 70 having a first to third detection sensors 60, 61, 62, a signal control circuit 71, and a digital signal processing module 72. In the step S1 of detecting the system current by the third detection sensor 62 and the three-phase inverter output current by the second detection sensor 60, and the system current detected using the plurality of adders. Detecting the load current by adding the three-phase inverter output current (S2), converting the detected load current after detecting the load current (S3), and filtering and subtracting the dq converted current. (S4), and inverse dq conversion of the harmonic component which is the result of subtraction of step S4 to extract the harmonic component (S5) And generating a current command value for compensating harmonics by the harmonic components extracted in step S5.

Hereinafter, the operation of the solar power generation system according to the present invention will be described in detail with reference to the accompanying drawings.

2 shows an embodiment of a photovoltaic system according to the invention, the photovoltaic system according to the invention has two reference currents. One current is the reference current for sending the maximum power to the system through the maximum power point tracking control and the other current is the reference current for the reference current with harmonic compensation and anti-single operation. . Add reference current with maximum power point tracking control and harmonic compensation and anti-stationary reference. The combined reference current is output through a current controller (not shown). As shown in FIG. 2, the first detection sensor 60 coupled to the rear end of the first LC parallel filter 50 including the first inductor L1 and the first capacitor C1 connected in parallel has a maximum power. It is a sensor for detecting voltage and current of solar energy for point tracking control. The first detection sensor 60 includes a first current sensor CT1 for detecting a current of solar energy and a first voltage sensor PT1 for detecting a voltage of solar energy. The first current sensor CT1 and the first voltage sensor PT1 are connected in parallel with each other. The third phase inverter 20 is coupled between the rear end of the first voltage sensor PT1 of the first detection sensor 60 and the front end of the second LC parallel filter 51. The second LC parallel filter 51 is composed of second to fourth inductors L21, L22, and L23 and second to fourth capacitors C21, C22, and C23 connected in parallel with each other. . The first and second LC parallel filters 50 and 51 reduce the harmonic components output from the solar cell array 10 and the three-phase inverter 20, respectively. The second detection sensor 61 is coupled to the rear end of the second LC parallel filter 51. The second detection sensor 61 includes second to fourth current sensors CT21, CT22, and CT23, and second to fourth voltage sensors PT21, PT22, and PT23. The second to fourth current sensors CT21, CT22, and CT23 and the second to fourth voltage sensors PT21, PT22, and PT23 are coupled in parallel to each other. The second to fourth current sensors CT21, CT22, and CT23 serve to detect the current output from the second LC parallel filter 51, and the second to fourth voltage sensors PT21, ( PT22) and PT23 serve to detect a voltage across the output of the second LC parallel filter 51. The third detection sensor 62 is coupled between the rear end of the second detection sensor 61 and the commercial power supply unit 30. The commercial power supply unit 30 is composed of a plurality of commercial power supply sources. The third detection sensor 62 is composed of fifth to seventh current sensors CT31, CT32, and CT33. The fifth to seventh current sensors CT31, CT32, and CT33 of the third detection sensor 62 are coupled in a one-to-one correspondence with a plurality of commercial power supplies of the commercial power supply unit 30, respectively. The fifth to seventh current sensors CT31, CT32, and CT33 of the third detection sensor 62 are the second to fourth current sensors CT21 and CT22 of the second detection sensor 61. And one-to-one correspondence with (CT23) and the second to fourth voltage sensors (PT21), (PT22), (PT23). The second to seventh current sensors CT21, CT22, CT23, CT31, CT32, and CT33 are sensors for detecting currents for harmonic compensation and independent driving prevention functions. The fourth to fourth voltage sensors PT21, PT22, and PT23 are sensors for detecting a voltage for synchronizing with a plurality of commercial power supplies of the commercial power supply unit 30. The load 40 is coupled between the second detection sensor 61 and the third detection sensor 62 via a plurality of load lines. The first common node a includes the other end of the second current sensor CT21 of the second detection sensor 61, one end of the second voltage sensor PT21 of the second detection sensor 61, and the load 40. This is the point of common connection with any one of the plurality of load lines. The second common node b includes the other end of the third current sensor CT22 of the second detection sensor 61, the one end of the third voltage sensor PT22 of the second detection sensor 61, and the load 40. This is the point of common connection with one of the plurality of load lines. The third common node c includes the other end of the fourth current sensor CT23 of the second detection sensor 61, one end of the fourth voltage sensor PT23 of the second detection sensor 61, and the load 40. This is the point of common connection with another one of the plurality of load lines. The power controller 70 receives the output signals of the first to third detection sensors 60, 61, and 62, respectively, to control the three-phase inverter 20 after signal processing according to an algorithm. Is generated and supplied to the three-phase inverter (20). The power control unit 70 includes a signal control circuit 71 and a digital signal processing module 72. The digital signal processing module 72 incorporates a current controller (not shown). In addition, the first power cut-off device 80 is coupled between the solar cell array 10 and the first LC parallel filter 50, and the second power cut-off device 81 comprises a second detection sensor 61 and It is coupled between the three detection sensors 62 and the third power cut device 82 is coupled between the third detection sensor 62 and the commercial power supply unit 30.

3 is a view illustrating a process for explaining a harmonic compensation function and an independent driving prevention function in a photovoltaic power generation system according to the present invention, and FIG. 3 (a) illustrates the first to third common nodes through a plurality of load lines. Figures (a), (b) and (c) show a process for explaining the reference current flowing into the load 40, respectively, and (b) shows the dq conversion from the reference current flowing into the load 30 ( 3 is a diagram illustrating a process for explaining a reference current generated by two-phase conversion). The process for explaining the harmonic compensation function and the anti-islanding function of FIG. 3 is an algorithm calculated in the digital signal processing module 72 of the power control unit 70. The digital signal processing module 72 of the power control unit 70 includes a plurality of adders 72 ₁₁ , 72 ₁₂ , 72 ₁₃ , 72 ₁₄ , and a low pass filter 72 ₂ .

In FIG. 3A, the first inverter output current i _{pv_out_a} is a current detected from the second current sensor CT21 of the second detection sensor 61 and the second inverter output current i _{pv_out_b} is the second. The current detected from the third current sensor CT22 of the detection sensor 61 and the third output current i _{pv_out_c} are currents detected from the third current sensor CT23. Also, the first grid current i _{utility_a} is a current detected from the fifth current sensor CT31 of the third detection sensor 62 and the second grid current i _{utility_b} is the _sixth of the third detection sensor 62. The current detected from the current sensor CT32 and the third system current i _{utility_c} is the current detected from the seventh current sensor CT33 of the third detection sensor 62. The first load current i _{load_a} is a reference current flowing from the first common node a to the load 40 by the first adder 72 ₁₁ and the first inverter output current i _{pv_out_a} and the first system. The sum of the current i _{utility_a} and the second load current i _{load_b} are reference currents flowing from the second common node b to the load 40 and are output by the second adder 72 ₁₂ by the second adder 72 ₁₂ . (i _{pv_out_b)} and a second current combined with the two-line current (i _{utility_b)} the third load current (i _{load_c)} comprises a third adder (72 ₁₃ as a reference current that flows to the load 40 from the third common node (c) ) Is the _{sum of} the third inverter output current i _{pv_out_c} and the third system current i _{utility_c} . On the other hand, as can be seen in Figure 3 (b), the first to the third load current (i _{load_a} ), (i _{load_b} ), (i _{load_c} ) is a dq conversion of each of the q-axis current component (i _q separated by the dq conversion) ) Is filtered with a low pass filter (72 ₂ ). Then, only the DC component remains among the first to third load currents i _{load_a} , i _{load_b} , and i _{load_c} . The fourth adder 72 ₁₄ subtracts the current component i _{q_1pf which} is a direct current component passing through the low pass filter 72 ₂ from the current component i _q of the separated q-axis. The current component i _{q_ref} subtracted by the fourth adder 72 ₁₄ serves as an anti-islanding function. The inverse dq conversion of the subtracted current component i _{q_ref} is performed. The reverse dq conversion of the subtraction current component (i _{q_ref)} the anti-is the reference current of ahyiseul landing capabilities _{(I a_ref), (I b_ref} ), (I c_ref). In other words, the first to third load currents (i _{load_a} ), (i _{load_b} ), and (i _{load_c} ) are converted to dq by subtracting the current component passing through the low pass filter (72 ₂ ) from the result of dq conversion. Harmonic components are extracted. The extracted harmonic components become a current command value for compensating harmonics. The current controller (not shown) in the digital signal processing module 72 then generates a pulse width modulated signal such that the current flowing through the digital signal processing module 72 can be compared with the current command calculated by detecting the current flowing.

Figure 4 is a graph showing the flow change of the grid current on the time axis in the solar power generation system according to the present invention.

As can be seen in Figure 4, in general, the system current is a sinusoidal signal. The harmonic current output from the three-phase inverter 20 in the grid current at this time is as shown in Equation (1).

= 6 c

1 , 여기서 c=1,2,3,.... (1)

= 6 c

1, where c = 1,2,3, .... (1)

Therefore, the harmonic components output from the three-phase inverter 20 are fifth, seventh, eleventh, thirteenth, .... As a result, the current i _out output from the three-phase inverter 20 is expressed as in Equation (2).

+ I₅

+ I₇

+ I₁₁

+I₁₃

.... (2)i _out = I ₁

+ I ₅

+ I ₇

+ I ₁₁

+ I ₁₃

.... (2)

In general, most of the lower harmonic currents flow from the three-phase inverter 20 to the light load side of the system. As a result, when about 0.1125 seconds have elapsed as shown in FIG. 4, the sinusoidal signal with harmonic components compensated normally flows. However, since the harmonic current flows to the load side when the system is cut off, the current (i _load ) flowing to the load 40 side becomes equal to the current (i _out ) output from the three-phase inverter 20. In order to detect the harmonic current flowing through the load 40, dq conversion of the load current i _load is performed as shown in Equation (3).

+(I₁₁ - I₁₃)

i _d = (I ₅ -I ₇ )

+ (I ₁₁ -I ₁₃ )

+(I₁₃ - I₁₁)

...(3)i _q = I ₁ + (I ₇ -I ₅ )

+ (I ₁₃ -I ₁₁ )

... (3)

When dq conversion of the load current (i _load ), the 60HZ component appears as a DC value. That is, the d-axis value becomes 0 and the q-axis value becomes a DC value when dq conversion of the load current i _load . In order to extract the harmonic components, the harmonic components can be extracted by cutting off the DC component of the q-axis. Accordingly, the value obtained by subtracting the direct current component I ₁ from Equation (3) becomes the reference current of the anti-islanding function. Therefore, the reference current is shown in equation (4).

+ (I₁₁ - I₁₃)

i _{d_ref} = (I ₅ -I ₇ )

+ (I ₁₁ -I ₁₃ )

+ (I₁₃ -I_{11 )}

...(4)i _{q_ref} = (I ₇ -I ₅ )

+ (I ₁₃ -I ₁₁₎

...(4)

Inverse dq conversion of equation (4) becomes the reference current of the high frequency compensation function.

+ I₇

+ I₁₁

+ I₁₃

+ ...I _{a_ref} = I ₅

+ I ₇

+ I ₁₁

+ I ₁₃

+ ...

+ I₇

+ I₁₁

+ I₁₃

+...I _{b_ref} = I ₅

+ I ₇

+ I ₁₁

+ I ₁₃

+ ...

+ I₇

+ I₁₁

+ I₁₃

+ ... (6)I _{c_ref =} I ₅

+ I ₇

+ I ₁₁

+ I ₁₃

+ ... (6)

As described above, the photovoltaic power generation system according to the present invention compensates the harmonic currents detected by the harmonic compensation function and the single operation prevention function when the system is not blocked. However, when the system is cut off, the current (i _out ) output from the three-phase inverter 20 as shown in Equation (2) flows to the load 40, and this current is detected by the anti-islanding function and becomes a reference current. Harmonic currents will increase further. The increased harmonic current is detected again and becomes the reference current, so the harmonic current continues to increase. At this time, since there is no system in the photovoltaic power generation system, as the harmonics of the voltage increase as shown in FIG. 5, the measured frequency also increases. As a result, the operation of the photovoltaic system is interrupted by the operation of the overfrequency relay.

The harmonics compensation and the single driving prevention method will be described in detail with reference to FIG. 6 showing the flow for explaining the harmonic compensation and the single driving prevention process in the solar power generation system according to the present invention.

In the photovoltaic power generation system as described above, harmonics compensation and standalone operation prevention method includes detecting a grid current by the third detection sensor 62 and detecting a three-phase inverter output current by the second detection sensor 60 ( S1). Then, the load current is detected by adding the detected system current and the three-phase inverter output current using the plurality of adders (step S2). After detecting the load current, the load current is converted dq (step S3). The dq-converted current is filtered and subtracted (step S4). In step S4, the DC component is left while filtering the dq converted current, and the DC component is subtracted from the dq converted current. In order to extract the harmonic components, the harmonic components which are subtraction results are converted to inverse dq (step S5). The current command value for compensating harmonics is generated by the harmonic components extracted in step S5 (step S6). By the current command value, the three-phase inverter 20 is controlled in comparison with the current actually flowing. As a result, single operation can be prevented due to extraction of harmonic components while compensating for harmonics of the system current.

According to the photovoltaic power generation system according to the present invention as described above, the harmonic components are compensated for when the grid is energized, and when the grid is cut off, the power quality is improved by preventing the operation alone. It is effective in preventing safety accidents.

Claims (6)

Solar cell array 10, three-phase inverter 20, load 40, first and second LC parallel filters 50 and 51, first and second detection sensors 60 and 61, signal conditioning In the solar power generation system including a circuit 71, a power control unit 70 having a digital signal processing module 72, and the like, The photovoltaic power generation system further includes a third detection sensor 62, which is a sum current of the three-phase inverter output current detected by the grid current detected by the third detection sensor 62 and the second detection sensor 60. The load current is converted by the digital signal processing module 72 of the power control unit 70 and filtered, and then the reverse dq is converted to extract the harmonic components from the load current. A photovoltaic power generation system, characterized by being operable. The method of claim 1, The filtering process in the digital signal processing module (72) leaves only the direct current component in the dq-converted component. The method of claim 2, The digital signal processing module (72) subtracts the filtered DC component from the dq conversion component after the filtering process to extract the high frequency component. The method of claim 2 or 3,  And the digital signal processing module (72) subtracts the filtered DC component from the dq transform component with respect to the q-axis value of the dq transform component. Solar cell array 10, three-phase inverter 20, load 40, first and second LC parallel filters 50, 51, first to third detection sensors 60, 61, 62 In the solar power generation system comprising a power control unit 70 having a signal control circuit 71, and a digital signal processing module 72, Detecting the system current by the third detection sensor 62 and detecting the three-phase inverter output current by the second detection sensor 60 (S1); Detecting a load current by adding a detected system current and a three-phase inverter output current using a plurality of adders (S2); Dq converting the detected load current after detecting the load current (S3), filtering and subtracting the dq-converted current (S4), Inverse dq conversion of harmonic components, which is a result of subtraction of step S4, to extract harmonic components (S5); And generating a current command value for compensating the harmonics by the harmonic components extracted in the step S5. The method of claim 1, Harmonic compensation and single operation prevention method characterized in that the DC component is left while filtering the dq converted current in the step S4 and the DC component is subtracted from the dq converted current.

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