An Optimization Control Strategy for the Transient Characteristics of a Distributed Generation System

Abstract

The present invention discloses an optimized control strategy for the transient characteristics of a distributed generation system, including the following control structures: an output power measurement unit, a parameter controller, a voltage regulation controller, a primary frequency modulation controller, an output power response optimization controller, and SPWM modulation. The distributed generation system realizes functions such as output power monitoring, operating parameter setting, voltage control, primary frequency modulation, inertia and damping simulation, rapid oscillation suppression, and inverter shutdown control through the above control structures. The present invention will solve the problem that when a grid-connected inverter is connected to the end of the power grid, it is vulnerable to frequency fluctuations and generates serious power oscillations, and can effectively optimize the transient response of the system to grid connection and achieve rapid suppression of output oscillations.

Description

Technical Field

[0001] The present invention relates to the field of new energy distributed power generation, and in particular to an optimized control strategy for the transient characteristics of a distributed power generation system. Background Art

[0002] When distributed new energy is directly connected to the grid through a grid-connected inverter, it cannot provide inertia and damping for the system, resulting in a lack of inertia and damping in the power grid, which has a great impact on the stability of the power grid. In order to improve the stability of distributed power generation grid connection and establish inertia and damping support for the power grid, the research on the inertia and damping simulation technology of grid-connected inverters has received widespread attention. However, while the existing research results achieve the virtual inertia and damping control of grid-connected inverters, a new problem of power oscillation in the output power of grid-connected inverters when the grid frequency changes is introduced. The existing research has insufficient optimization effect on this problem, and there are still problems of large power oscillation amplitude and long dynamic process time. Summary of the Invention

[0003] In order to solve the above technical problems, the present invention provides an optimized control strategy for the transient characteristics of a distributed power generation system.

[0004] To achieve the above object, the present invention is implemented according to the following technical solutions:

[0005] 1) Step 1: Collect the output voltage and current information of the grid-connected inverter through the output power measurement unit. The voltage and current information are respectively the three-phase voltage U abci and the three-phase current I abci , and at the same time calculate the output active power P ei and the output reactive power Q ei ;

[0006] Step 2: Set the operating parameters of the grid-connected inverter according to the actual operating requirements: rated reactive power Q refi , rated active power P refi , rated electromotive force E 0 and rated angular frequency ω 0 ;

[0007] Step 3: Input the output reactive power Q ei , rated reactive power Q refi and rated electromotive force E 0 into the voltage regulation controller to obtain the output electromotive force E i ;

[0008] Step 4: Input the rated active power P refi , rated angular frequency ω 0 and output angular frequency ω i into the primary frequency modulation controller to obtain the mechanical power P mi ;

[0009] Step 5: According to the output active power P ei and the mechanical power P mi input into the output power response optimization controller to calculate the control phase angle θ i ;

[0010] Step 6: Input the control phase angle θ i and the output electromotive force E i of the grid-connected inverter into the SPWM modulation to obtain the on-off signals of the control inverter bridge;

[0011] Specifically, Step 3 is as follows. The voltage regulation controller includes a first comparator, a first proportional-integral operator, and a first adder. The first comparator compares the rated reactive power Q refi with the output reactive power Q ei to obtain the reactive power difference. After passing through the proportional-integral operator, the adder adds the obtained result to the rated voltage E 0 to obtain the output electromotive force E i . The expression of the output electromotive force E i is:

[0012] E i = E 0 + ∫k qi (Q refi - Q ei )dt (1)

[0013] In formula (1): i represents the numbers of different grid-connected inverters in the distributed generation system, i = 1, 2, 3, and k qi is the reactive power control coefficient;

[0014] Specifically, Step 4 is as follows. The primary frequency modulation controller includes a second comparator, a first proportional operator, and a third comparator. The difference obtained by comparing the output angular frequency ω i with the rated angular frequency ω 0 passes through the proportional operator. Then, the third comparator compares the rated active power P refi with the output value of the proportional operator to obtain the mechanical power P mi . The expression of the mechanical power P mi is:

[0015] P mi = P refi - k fi (ω i - ω 0 ) (2)

[0016] In formula (2): k fi is the primary frequency modulation coefficient;

[0017] Step 5 Output power response optimization controller, including a fourth comparator, a first-order inertia link, a fifth comparator, a first integrator, a second proportional operator, and a second adder. The fourth comparator compares the output active power P refi with the rated active power P ei . The difference after passing through the first-order inertia link is the output angular frequency ω i . The fifth comparator compares the output angular frequency ω i with the grid-side angular frequency ω g . The difference after passing through the second integrator is the input 1 of the second adder. At the same time, the output angular frequency ω i is multiplied by the optimized control parameter m i of the second proportional operator to obtain the input 2 of the second adder. Then, the input 1 and input 2 are added by the second adder to obtain the control phase angle θ i , specifically as follows:

[0018] Step 5-1: Based on the synchronous rotor motion equation and introducing an optimization link to construct a control model, the specific expression is:

[0019]

[0020] In formula (3): D i is the virtual damping coefficient, J i is the virtual inertia coefficient, m i is the optimized control parameter, and the value range is 0.2 < m i < 0.5, s is the differential operator;

[0021] Step 5-2: According to the above control model, the output electromagnetic power P ei is:

[0022]

[0023] In formula (4): U is the grid connection point voltage, X i is the impedance from the i-th grid-connected inverter to the grid connection point, K i is the synchronous voltage regulation coefficient;

[0024] Step 5-3: After passing through the output power response optimization controller, the closed-loop transfer function of the output active power and mechanical power is:

[0025]

[0026] At this time, the closed-loop transfer function of the output active power and grid frequency is:

[0027]

[0028] In formula (6): ωg is the grid frequency;

[0029] Step 6 is specifically as follows. SPWM modulation is used to output the electromotive force E i and control the phase angle θ i are the amplitude and phase of the modulation wave, which are used as the signal input values for SPWM modulation. The three-phase modulation wave e abci has the following expression:

[0030]

[0031] Compared with the prior art, the principle and advantages of this solution are as follows:

[0032] The present invention discloses an optimization control strategy for the transient characteristics of a distributed generation system, including the following control structures: an output power measurement unit, a parameter controller, a voltage regulation controller, a primary frequency modulation controller, an output power response optimization controller, and SPWM modulation. The distributed generation system realizes functions such as output power monitoring, working parameter setting, voltage control, primary frequency modulation, inertia and damping simulation, oscillation rapid suppression, and inverter turn-off control through the above control structures. The present invention will solve the problem that when a grid-connected inverter is connected to the end of the grid, it is vulnerable to frequency fluctuations and generates serious power oscillations, and can effectively optimize the grid-connected transient response of the system and achieve rapid suppression of output oscillations. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 is the main circuit diagram of the distributed generation system in the embodiment of the present invention;

[0034] Figure 2 is the control block diagram of an optimization control strategy for the transient characteristics of a distributed generation system in the embodiment of the present invention;

[0035] Figure 3 is the voltage regulation controller structure diagram of the grid-connected inverter in the embodiment of the present invention;

[0036] Figure 4 is the primary frequency modulation controller structure diagram of the grid-connected inverter in the embodiment of the present invention;

[0037] Figure 5 is the output power response optimization controller structure diagram of the grid-connected inverter in the embodiment of the present invention;

[0038] Figure 6 is the output active power response result diagram of the grid-connected inverter before using the method of the present invention in the embodiment of the present invention;

[0039] Figure 7 is the output active power response result diagram of the grid-connected inverter after using the method of the present invention in the embodiment of the present invention;

[0040] Figure 8 Frequency response result diagram of the grid-connected inverter before adopting the method of the present invention in the embodiment of the present invention;

[0041] Figure 9 Frequency response result diagram of the grid-connected inverter after adopting the method of the present invention in the embodiment of the present invention. Specific embodiments

[0042] The present invention will be further described below in conjunction with specific embodiments:

[0043] Figure 1 The figure shows the main circuit diagram of the distributed generation system. The distributed generation system consists of 3 grid-connected inverters and 1 infinite grid. Each grid-connected inverter is powered by a DC power supply to ensure stable DC energy input. There are 11 lines and 5 loads in the system, and the 5th load is set as a variable load.

[0044] Figure 2 The figure shows the control block diagram of an optimization control strategy for the transient characteristics of a distributed generation system, Figure 3 The figure shows the structure diagram of the voltage regulation controller of the grid-connected inverter, Figure 4 The figure shows the structure diagram of the primary frequency modulation controller of the grid-connected inverter, Figure 5 The figure shows the structure diagram of the output power response optimization controller of the grid-connected inverter, including the following steps:

[0045] Step 1: Collect the output voltage and current information of the grid-connected inverter through the output power measurement unit. The voltage and current information are respectively three-phase voltage U abci and three-phase current I abci , and at the same time calculate the output active power P ei and output reactive power Q ei ;

[0046] Step 2: Set the operating parameters of the grid-connected inverter according to the actual operating requirements: rated reactive power Q refi , rated active power P refi , rated electromotive force E 0 and rated angular frequency ω 0 ;

[0047] Step 3: Input the output reactive power Q ei , rated reactive power Q refi and rated electromotive force E 0 into the voltage regulation controller to obtain the output electromotive force E i ;

[0048] Step 4: Input the rated active power P refi , rated angular frequency ω 0 and output angular frequency ω iInput to the primary frequency modulation controller to obtain the mechanical power P mi ;

[0049] Step 5: According to the output active power P ei and the mechanical power P mi Input into the output power response optimization controller to calculate the control phase angle θ i ;

[0050] Step 6: Input the control phase angle θ i and the output electromotive force E i of the grid-connected inverter into the SPWM modulation to obtain the control signal for turning on and off the inverter bridge;

[0051] Specifically, step 3 is as follows. The voltage regulation controller includes a first comparator, a first proportional-integral operator, and a first adder. The first comparator compares the rated reactive power Q refi with the output reactive power Q ei to obtain the reactive power difference. After passing through the proportional-integral operator, the adder adds the obtained result to the rated voltage E 0 to obtain the output electromotive force E i . The expression of the output electromotive force E i is:

[0052] E i = E 0 + ∫k qi (Q refi - Q ei )dt (8)

[0053] In formula (8): i represents the number of different grid-connected inverters in the distributed generation system, i = 1, 2, 3, and k qi is the reactive power control coefficient;

[0054] Specifically, step 4 is as follows. The primary frequency modulation controller includes a second comparator, a first proportional operator, and a third comparator. The difference obtained by comparing the output angular frequency ω i with the rated angular frequency ω 0 passes through the proportional operator. Then, the third comparator compares the rated active power P refi with the output value of the proportional operator to obtain the mechanical power P mi . The expression of the mechanical power P mi is:

[0055] P mi = P refi - k fi (ω i - ω 0 ) (9)

[0056] In formula (9): k fiis the primary frequency modulation coefficient;

[0057] Step 5 Output power response optimization controller, including a fourth comparator, a first-order inertia link, a fifth comparator, a first integral operator, a second proportional operator, and a second adder. Among them, the difference obtained by comparing the output active power P refi with the rated active power P ei is passed through a first-order inertia link to obtain the output angular frequency ω i , and the difference obtained by comparing the output angular frequency ω i and the grid-side angular frequency ω g is passed through a second integral operator to obtain the input 1 of the second adder. At the same time, the output angular frequency ω i is multiplied by the optimization control parameter m i of the second proportional operator to obtain the input 2 of the second adder. Then, the input 1 and the input 2 are added by the second adder to obtain the control phase angle θ i , specifically as follows:

[0058] Step 5-1: Based on the synchronous rotor motion equation and introducing an optimization link to construct a control model, the specific expression is:

[0059]

[0060] In formula (10): D i is the virtual damping coefficient, J i is the virtual inertia coefficient, m i is the optimization control parameter, and the value range is 0.2 < m i < 0.5, s is the differential operator;

[0061] Step 5-2: According to the above control model, the output electromagnetic power P ei is:

[0062]

[0063] In formula (11): U is the grid connection point voltage, X i is the impedance from the i-th grid-connected inverter to the grid connection point, K i is the synchronous voltage regulation coefficient;

[0064] Step 5-3: After passing through the output power response optimization controller, the closed-loop transfer function of the output active power and the mechanical power is:

[0065]

[0066] At this time, the closed-loop transfer function of the output active power and the grid frequency is:

[0067]

[0068] In formula (13): ω g is the grid frequency;

[0069] Step 6 is specifically as follows. SPWM modulation is used to output the electromotive force E i and control the phase angle θ i as the amplitude and phase of the modulation wave, which are used as the signal input values for SPWM modulation. The three-phase modulation wave e abci has the following expression:

[0070]

[0071] Figure 6 The figure shows the output active power response result diagram of the grid-connected inverter before adopting the method of the present invention. Figure 7 The figure shows the output active power response result diagram of the grid-connected inverter after adopting the method of the present invention. In the distributed generation system, the capacities of the 3 grid-connected inverters are different. The capacity of grid-connected inverter 1 is 50% larger than that of grid-connected inverter 3, and the capacity of grid-connected inverter 2 is 50% larger than that of grid-connected inverter 1. After startup, the 3 grid-connected inverters all operate at 8.5 kW. The grid frequency is set to drop to 49.95 Hz within 4 - 10 s and recover to 50 Hz after 10 s. And in the two groups of experiments, the operating environments and parameter settings of the 3 grid-connected inverters in the distributed generation system remain unchanged. Only the method of the present invention is added, and the optimized control parameter m i of the method of the present invention is set to 0.35. As can be seen from Figure 6 , when the traditional method is adopted, within 4 s after startup, the output active powers of the 3 grid-connected inverters all have long-time oscillations with different magnitudes. When the grid frequency drops from 4 - 10 s, the demand power in the distributed generation system rises, and each grid-connected inverter increases its output power according to its capacity. There is a large power impact at the first wave peak after 4 s, and the recovery time is long. As can be seen from Figure 7 , when the method of the present invention is adopted, within 4 s after startup, the output active powers of the 3 grid-connected inverters start without oscillation and quickly enter the rated operation of 8.5 kW. When the grid frequency drops from 4 - 10 s, the demand power in the distributed generation system rises, and each grid-connected inverter also increases its output power according to its capacity. There is basically no overshoot at the first wave peak after 4 s, and it quickly enters a stable state. Obviously, when the traditional method is adopted, the stability of the distributed generation system is poor, it is extremely vulnerable to grid frequency disturbances, generates large power impacts, and the recovery time of the dynamic process is long, which has a great impact on the safe and stable operation. When the method of the present invention is adopted, when the distributed generation system is affected by grid frequency disturbances, the original large power impact is suppressed, and the recovery time of the dynamic process is significantly shortened.

[0072] Figure 8 The figure shows the frequency response result diagram of the grid-connected inverter before adopting the method of the present invention.Figure 9 The figure shows the frequency response results of the grid-connected inverter after adopting the method of the present invention. From Figure 8 it can be seen that when the traditional method is adopted, within 4 s after startup, the frequencies of the three grid-connected inverters oscillate for a long time. When the grid frequency drops from 4 s to 10 s, the frequencies of the grid-connected inverters follow the grid frequency drop under the action of the primary frequency modulation controller. The lowest frequency point drops to around 49.94 Hz. After 10 s, the distributed generation system resumes operation at 50 Hz, and there are still large oscillations in each grid-connected inverter. From Figure 9 it can be seen that when the method of the present invention is adopted, within 4 s after startup, the frequencies of the three grid-connected inverters fluctuate for a short time. When the grid frequency drops from 4 s to 10 s, the frequencies of the grid-connected inverters follow the grid frequency drop under the action of the primary frequency modulation controller. The lowest frequency point does not exceed 49.95 Hz. After 10 s, the distributed generation system resumes operation at 50 Hz, and each grid-connected inverter quickly enters a stable state. According to the above analysis, when the method of the present invention is adopted, the stability of the distributed generation system is greatly enhanced. It is significantly found that the recovery time of the dynamic process is significantly shortened, the frequency drop is avoided, the operation safety and stability of the distributed generation system are improved, and the transient characteristics of the three processes of inverter startup, disturbance generation and recovery are optimized to a large extent.

[0073] The above embodiments are only the preferred embodiments of the present invention, and do not limit the scope of implementation of the present invention. Therefore, all changes made according to the shape and principle of the present invention should be covered within the protection scope of the present invention.

Claims

1. A transient characteristics optimization control strategy for a distributed generation system, characterized in that: The following steps are involved: Step 1: Collect the output voltage and current information of the grid-connected inverter through the output power measurement unit. The voltage and current information are the three-phase voltage U abci and three-phase current I abci At the same time, the grid-connected inverter output active power P is calculated ei And output reactive power Q ei ; Step 2: Set the grid-connected inverter operating parameters according to actual operating requirements: Rated reactive power Q refi , Rated active power P refi , rated electromotive force E0 and rated angular frequency ω0; Step 3: Output reactive power Q ei , Rated reactive power Q refi And the rated electromotive force E0 is input to the voltage regulation controller to obtain the output electromotive force E i ; Step 4: Set the rated active power P refi , rated angular frequency ω0 and output angular frequency ω i Input to the primary frequency modulation controller to obtain the mechanical power P mi ; Step 5: According to the output active power P ei and mechanical power P mi Input into the output power response optimization controller and calculate the control phase angle θ i ; Step 6: Set the control phase angle θ of the grid-connected inverter i and output electromotive force E i Input into SPWM modulation to obtain the on-off signal for controlling the inverter bridge; Step 3 is as follows: the voltage regulation controller includes a first comparator, a first proportional integral operator, and a first adder, wherein the first comparator converts the rated reactive power Q refi With output reactive power Q ei The reactive power difference is obtained by comparison. After passing through the proportional-integral operator, the adder adds the result to the rated voltage E0 to obtain the output electromotive force E i , output electromotive force E i The expression is: E i =E0+∫k qi (Q refi -Q ei )dt (1) In formula (1), i represents the number of different grid-connected inverters in the distributed generation system, i = 1, 2, 3, k qi is the reactive power control coefficient; Step 4 is as follows: the primary frequency modulation controller includes a second comparator, a first proportional operator, and a third comparator, wherein the second comparator outputs an angular frequency ω i The difference compared with the rated angular frequency ω0 passes through the proportional operator, and the third comparator converts the rated active power P refi Compare with the output value of the proportional operator to get the mechanical power P mi , mechanical power P mi The expression is: P mi =P refi -k fi (oh i -ω0) (2) In formula (2): k fi is the primary frequency modulation coefficient; Step 5 outputs a power response optimization controller, including a fourth comparator, a first-order inertia link, a fifth comparator, a first integral operator, a second proportional operator and a second adder, wherein the fourth comparator outputs the active power P refi With rated active power P ei After the difference is compared and passes through the first-order inertia link, the output angular frequency ω is obtained. i , the fifth comparator will output the angular frequency ω i and the grid side angular frequency ω g The compared difference is passed through the second integrator to obtain the second adder input 1, and the output angular frequency ω is i The optimized control parameter m of the second proportional operator i After multiplication, the second adder input 2 is obtained, and then the second adder adds input 1 and input 2 to obtain the control phase angle θ i , as follows: Step 5-1: Based on the synchronous rotor motion equation and introducing the optimization link, a control model is constructed. The specific expression is: In formula (3): D i is the virtual damping coefficient, J i is the virtual inertia coefficient, m i To optimize the control parameters, the value range is 0.2 <m i <0.5, s is the differential operator; Step 5-2: According to the above control model, the output electromagnetic power P is obtained ei for: In formula (4), U is the grid voltage, X i is the impedance from the i-th grid-connected inverter to the grid-connected point, K i is the synchronous voltage regulation coefficient; Step 5-3: After the output power response optimization controller, the closed-loop transfer function of the output active power and mechanical power is obtained as follows: At this time, the closed-loop transfer function of the output active power and grid frequency is: In formula (6): g is the grid frequency; Step 6 is as follows: SPWM modulation to output electromotive force E i and control phase angle θ i is the modulation wave amplitude and phase, as the signal input value of SPWM modulation, its three-phase modulation wave e abci The expression is:

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