Grid-connected system oscillation suppression strategy for multi-configuration network type converter based on power feedback and phase compensation

Abstract

The application discloses a multi-structure grid type converter grid-connected system oscillation suppression strategy based on power feedback and phase compensation, mainly comprising the following modules: a power calculation module, a virtual governor module, a virtual rotor module, a reactive power-voltage droop control module, and a voltage and current double closed loop module.In the virtual rotor module, the output active power and the output power angle are differentiated to construct a power feedback loop, and the difference between the virtual rotor output angle frequency and the rated grid frequency is used as the phase compensation amount.Through the power feedback and phase compensation strategy, the problem that the multi-structure grid type converter is prone to severe power oscillation after being connected to the grid due to grid frequency disturbance is solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of new power system stability, in particular to a grid oscillation suppression strategy for a multi-network type converter grid connection system based on power feedback and phase compensation. BACKGROUND

[0002] With the urgent demand for energy transformation, the traditional synchronous generator dominated power system is gradually transforming into a new power system dominated by new energy such as wind and solar energy. Among them, new energy is connected to the grid through power electronic converters, but power electronic converters cannot provide damping and inertia support for the power system, thus causing a serious decline in system stability. The network type converter represented by the virtual synchronous generator is designed through the control algorithm of the power electronic converter, so that the power electronic device which does not have the characteristics of synchronous generator has similar grid-connected external characteristics of synchronous machine, thereby improving the stability of the power system. However, the virtual inertia control strategy of the existing network type converter will produce severe power oscillation problems under the operating conditions of grid side frequency fluctuation and switching load. The control strategy of the traditional network type converter can suppress such oscillation by introducing virtual damping, but the introduction of virtual damping will slow down the dynamic response speed of the system, and if the damping coefficient is not properly set, it will cause a new oscillation mode. SUMMARY

[0003] To solve the above technical problems, the present application provides a grid oscillation suppression strategy for a multi-network type converter grid connection system based on power feedback and phase compensation.

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

[0005] Step 1: The DC power supply is connected to the public grid connection point through the network type converter, LC the filter, PCC and then through the grid side equivalent inductance, L g the equivalent resistance, R g and is connected to the power grid, wherein, L i and C i are the filter inductance and filter capacitance respectively, after the system is started, the three-phase output current of the first i network type converter, I abci the filtered current, I oabci the grid connection point voltage, U pcc and the grid connection point phase angle, θ vpcc are collected.

[0006] Step 2: In the power calculation module, the filtered current collected in Step 1 is... I oabci Grid connection point voltage U pcc The output active power of the grid-type converter is obtained after calculation, using it as an input. P ei and output reactive power Q ei ;

[0007] Step 3: In the virtual speed controller module, set the active power reference value. P refi Rated angular frequency ω 0. Virtual rotor output angular frequency ω i As input, the output obtained after passing through the virtual speed controller is mechanical power. P mi The specific process involves outputting the angular frequency of the virtual rotor. ω i With the rated angular frequency ω The difference of 0 is denoted as the change in angular frequency. ω Δ The active power reference value P refi Subtract the first frequency modulation coefficient k fi With angular frequency change ω Δ The product of these can be used to obtain the mechanical power. P mi Its expression is:

[0008] (1)

[0009] Step 4: In the virtual rotor module, based on the virtual rotor control of a traditional grid-type converter, power feedback and phase compensation control strategies are added, as follows:

[0010] Step 4-1: Output active power P ei The filtered power is obtained after low-pass filtering. P ei0 Output power angle δ i Cosine δ i Multiply by the power angle adjustment coefficient t i Then, it is compared with the filtered power. P ei0 The summation is performed after differentiation. s And multiply by the differential coefficientr i obtaining a power feedback amount P c , the mechanical power P mi and the filtered power P ei0 , the power feedback amount P c respectively, to obtain a power deviation amount P Δ , the power deviation amount P Δ passes through a virtual inertia integral element 1 / (Jv), a damping coefficient J i s , and a proportional coefficient D i to obtain an angular frequency variation amount ω Δ , wherein J i is the virtual inertia, and the angular frequency variation amount ω Δ is added to a rated angular frequency ω 0 to obtain a virtual rotor output angular frequency ω i , and a specific expression is as follows:

[0011] (2)

[0012] In formula (2), τ i is a filter time constant;

[0013] Step 4-2: The virtual rotor output angular frequency ω i is subtracted from a grid frequency ω g , and a difference result is multiplied by a phase angle compensation coefficient m i to obtain a phase angle compensation amount θ Δ , the virtual rotor output angular frequency ω i passes through an integral element to obtain an initial phase angle θ 0, and the initial phase angle θ 0 is added to the phase angle compensation amount θ Δ to obtain an actual output phase angle after phase angle compensation θ i , and the actual output phase angle θ i is added to a grid point voltage phase angle θ vpccAfter the difference, the output power angle is obtained δ i The specific expression is:

[0014] (3)

[0015] Step 4-3: Combine step 4-1, step 4-2 and output active power P ei and the approximate linear relationship expression between output power angle δ i , the complete closed-loop control model of the grid-connected converter virtual rotor module can be obtained, wherein the approximate linear relationship expression between output active power P ei and output power angle δ i is:

[0016] (4)

[0017] In formula (4), E i is the actual output voltage of the nth grid-connected converter, i X i is the equivalent reactance between the nth grid-connected converter and the grid point, i K i is defined as the synchronous voltage correlation coefficient;

[0018] Step 5: In the reactive-voltage droop control module, the output reactive power Q ei , the reference reactive power Q refi and the reference voltage E 0 are obtained after the reactive-voltage droop control link, and the actual output voltage E i is obtained. The specific expression of the reactive-voltage droop control link is:

[0019] (5)

[0020] In formula (5), k qi is the reactive-voltage droop control coefficient;

[0021] Step 6: The three-phase output current I abci , the grid point voltage U pcc and the actual output phase angle θ i ​​actual output voltage obtained in step 5 E i as an input quantity, a three-phase modulation wave signal obtained after voltage-current double closed-loop control U 0-abci , the three-phase modulation wave signal U 0-abci is modulated by an SVPWM module to generate a driving circuit signal g 1~ g 6, used to control the turn-on and turn-off of the grid-connected type converter, and finally stabilize the output voltage of the grid-connected type converter, so as to meet the grid access requirements.

[0022] Further, in step 4, the filter time constant τ i is in the range of 0.5 τ i <0.85, the power angle regulation coefficient t i is in the range of 0 t i <100, the differential coefficient r i is in the range of 0 r i <0.5, the phase angle compensation coefficient m i is in the range of 0.1 m i <0.4.

[0023] Compared with the prior art, the principles and advantages of the scheme are as follows:

[0024] The application discloses a multi-grid-connected type converter grid-connected system oscillation suppression strategy based on power feedback and phase compensation, mainly comprising the following modules: a power calculation module, a virtual governor module, a virtual rotor module, a reactive power-voltage droop control module and a voltage-current double closed-loop module. In the virtual rotor module, a power feedback loop is constructed by differentiating the output active power and the output power angle, and a difference between a virtual rotor output angle frequency and a rated grid frequency is taken as a phase compensation amount. By applying the power feedback and phase compensation strategy, the problem that the multi-grid-connected type converter is easily subjected to grid frequency disturbance to generate severe power oscillation after being grid-connected is solved. DETAILED DESCRIPTION

[0025] Figure 1 Fig. 1 is a schematic diagram of a multi-grid-connected type converter grid-connected system topology and an oscillation suppression strategy based on power feedback and phase compensation in an embodiment of the application;

[0026] Figure 2 Fig. 2 is a virtual governor control structure diagram of the grid-connected type converter in the embodiment of the application.

[0027] Figure 3 a power feedback and phase compensation oscillation suppression strategy control structure diagram of the grid-forming converter in the embodiment of the application;

[0028] Figure 4 a reactive power-voltage droop control structure diagram of the grid-forming converter in the embodiment of the application;

[0029] Figure 5 an active power-time variation diagram of the multi-grid-forming converter grid-connected system before the method of the application is adopted in the embodiment of the application;

[0030] Figure 6 an active power-time variation diagram of the multi-grid-forming converter grid-connected system after the method of the application is adopted in the embodiment of the application;

[0031] Figure 7 a frequency-time variation diagram of the multi-grid-forming converter grid-connected system before the method of the application is adopted in the embodiment of the application;

[0032] Figure 8 a frequency-time variation diagram of the multi-grid-forming converter grid-connected system after the method of the application is adopted in the embodiment of the application. DETAILED DESCRIPTION

[0033] The application will be further described below in combination with specific embodiments:

[0034] Figure 1 shown is a multi-grid-forming converter grid-connected system topology structure and its oscillation suppression strategy based on power feedback and phase compensation, a direct current power supply accesses to a public grid-connected point through a grid-forming converter, LC a filter, PCC and then passes through a grid-side equivalent inductance, L g an equivalent resistance, R g and accesses to a power grid, wherein, L i and C i are a filter inductance and a filter capacitance respectively, the grid-forming converter oscillation suppression strategy mainly comprises a power calculation module, a virtual governor module, a virtual rotor module, a reactive power-voltage droop control module, a voltage and current double closed loop module, an SVPWM modulation module and a driving circuit.

[0035] Figure 2 shown is a virtual governor control structure diagram of the grid-forming converter, Figure 3 shown is a power feedback and phase compensation oscillation suppression strategy control structure diagram of the grid-forming converter, the control block diagram completely depicts the closed loop control between the virtual governor and the virtual rotor.Figure 4 The reactive-voltage droop control structure diagram of the grid-forming converter is shown, including the following steps:

[0036] Step 1: The DC power supply passes through the grid-forming converter, LC The filter is connected to the public grid connection point PCC , and then passes through the grid-side equivalent inductance L g , equivalent resistance R g is connected to the power grid, wherein, L i and C i are the filter inductance and filter capacitance respectively, after the system starts, the three-phase output current i of the first grid-forming converter is collected I abci , the filtered current I oabci , the grid connection point voltage U pcc and the grid connection point phase angle θ vpcc ;

[0037] Step 2: In the power calculation module, the filtered current I oabci , the grid connection point voltage U pcc collected in step 1 are taken as input quantities, and after calculation, the output active power P ei and the output reactive power Q ei of the grid-forming converter are obtained;

[0038] Step 3: In the virtual governor module, the active power reference value P refi , the rated angular frequency ω 0, and the virtual rotor output angular frequency ω i are taken as input quantities, and the output quantity is the mechanical power P mi after passing through the virtual governor, and the specific process is that the difference between the virtual rotor output angular frequency ω i and the rated angular frequency ω 0 is recorded as the angular frequency change ω Δ , and the active power reference value P refi is subtracted by the primary frequency modulation coefficient k fi and the angular frequency change ω ΔThe product of the mechanical power and the product of the mechanical power P mi The expression is:

[0039] (6)

[0040] Step 4: In the virtual rotor module, based on the virtual rotor control of the traditional grid-forming converter, the power feedback and phase compensation control strategy is added, which is as follows:

[0041] Step 4-1: The output active power P ei is low-pass filtered to obtain the filtered power P ei0 Step 4-2: The output power angle δ i is multiplied by the cosine of the power angle cos δ i , and then multiplied by the power angle adjustment coefficient t i , and then added to the filtered power P ei0 , and the sum is passed through a differentiation element s and multiplied by the differentiation coefficient r i to obtain the power feedback P c Step 4-3: The mechanical power P mi is subtracted from the filtered power P ei0 and the power feedback P c to obtain the power deviation P Δ Step 4-4: The power deviation P Δ is passed through a virtual inertia integral element 1 / ( J i s ), a damping coefficient D i , and a negative feedback element to obtain the angular frequency change ω Δ , wherein J i is the virtual inertia, and the angular frequency change ω Δ is added to the rated angular frequency ω 0 to obtain the virtual rotor output angular frequency ω i , and the specific expression is:

[0042] (7)

[0043] In formula (7),τ i is the filter time constant;

[0044] Step 4-2: the virtual rotor output angular frequency ω i is the grid frequency ω g After subtraction, the difference result is multiplied by the phase angle compensation coefficient m i to obtain the phase angle compensation amount θ Δ Step 4-3: the virtual rotor output angular frequency ω i is obtained through an integral element to obtain the initial phase angle θ 0, the initial phase angle θ 0 is added to the phase angle compensation amount θ Δ to obtain the actual output phase angle after phase angle compensation θ i Step 4-4: the actual output phase angle θ i is subtracted from the grid point voltage phase angle θ vpcc to obtain the output power angle δ i , and the specific expression is:

[0045] (8)

[0046] Step 4-3: combining step 4-1, step 4-2, and the approximate linear relationship expression between the output active power P ei and the output power angle δ i , the complete closed-loop control model of the grid-connected converter virtual rotor module can be obtained, wherein the approximate linear relationship expression between the output active power P ei and the output power angle δ i is:

[0047] (9)

[0048] In formula (9), E i is the actual output voltage of the nth grid-connected converter, i i is the equivalent reactance between the nth grid-connected converter and the grid point, X i is defined as the synchronous voltage correlation coefficient; i K

[0049] ​​Step 5: In the reactive-voltage droop control module, the output reactive power obtained in Step 2 is Q ei , the reference reactive power Q refi , and the reference voltage E 0 are inputted into the reactive-voltage droop control link to obtain the actual output voltage E i The specific expression of the reactive-voltage droop control link is:

[0050] (10)

[0051] In formula (10), k qi is the reactive-voltage droop control coefficient;

[0052] Step 6: The three-phase output current collected in Step 1 I abci , the grid point voltage U pcc , and the actual output phase angle obtained in Step 4-2 θ i , the actual output voltage obtained in Step 5 E i are inputted into the voltage-current double closed loop control to obtain the three-phase modulation wave signal U 0-abci The three-phase modulation wave signal U 0-abci is modulated by the SVPWM module to generate the driving circuit signal g 1~ g 6. The driving circuit signal is used to control the turn-on and turn-off of the grid-forming converter, and finally stabilize the output voltage of the grid-forming converter to meet the grid access requirements.

[0053] Further, in Step 4, the filter time constant τ i is in the range of 0.5 τ i <0.85, the power angle adjustment coefficient t i is in the range of 0 t i <100, the differential coefficient r i is in the range of 0 r i <0.5, the phase angle compensation coefficient m i is in the range of 0.1 m i <0.4.

[0054] Figure 5 and Figure 6 are active power-time variation diagrams of the multi-constructed grid-connected converter grid-connected system before and after using the method of the application. Two constructed grid-connected converters are simulated to run in parallel, the active power reference value of the constructed grid-connected converter 1 and the constructed grid-connected converter 2 is set to 20 kW, the grid rated frequency is 50 Hz, the grid frequency drops by 0.5 Hz at 4 s, and lasts until 9 s to recover to the rated frequency. Figure 5 and Figure 6 The conditions and parameter settings of the two groups of simulations are kept completely consistent. Among them, Figure 6 The respective adjustment parameters after adding the method of the application are: τ 1 = 0.85, τ 2 = 0.8; t 1 = t 2 = 100; r 1 = 0.25, r 2 = 0.2; m 1 = 0.4, m 2 = 0.35.

[0055] It can be seen that when the traditional control method is used, the constructed grid-connected converter 1 and the constructed grid-connected converter 2 have power oscillation between 0-4 s, cannot be quickly adjusted to the reference power, and during 4-9 s when the grid frequency drops to 49.5 Hz, the two converters have severe power oscillation, the overshoot of the constructed grid-connected converter 1 reaches 41.22%, and the overshoot of the constructed grid-connected converter 2 reaches 39.69%. In addition, after the grid frequency recovers to 50 Hz at 9 s, there is still obvious power oscillation, and until the simulation ends at 15 s, the output power has not been restored to the reference value 20 kW. In comparison Figure 5 After adding the strategy proposed in the application, during 0-4 s when the grid is normally operated at 50 Hz, the two converters can adjust the output power to the reference power 20 kW at 1.2 s, during 4-9 s when the grid frequency drops to 49.5 Hz, the power oscillation suppression effect of the two converters is obviously greatly improved, the overshoot of the constructed grid-connected converter 1 is only 3.04%, the overshoot of the constructed grid-connected converter 2 is only 4.81%, and the two converters only take 2.4 s to maintain the output power stable. In addition, after the grid frequency recovers to 50 Hz at 9 s, the two converters restore the output power to 20 kW at 11.2 s. Through Figure 6 and Figure 5 intuitive comparison, obviously, compared with the traditional control strategy, the control strategy proposed in the application can quickly and effectively suppress power oscillation when the constructed grid-connected converter is affected by grid frequency disturbance. Figure 6

[0056] Figure 7 and​Figure 8 are frequency-time variation diagrams of the multi-structure grid-connected converter grid-connected system before and after using the method of the application. Figure 7 , Figure 8 The frequency variation shown in Figure 5 , Figure 6 corresponds to the power variation shown in Figure 7 It can be seen from Figure 8 that between 0-4s after starting, the frequency of the two grid-connected converters always oscillates, and in the process of the grid frequency falling to 49.5Hz between 4-9s, both of the two grid-connected converters have serious frequency drop, the grid-connected converter 1 drops to 49.32Hz at the most serious time, and the grid-connected converter 2 drops to 49.42Hz at the most serious time. In addition, after the grid restores to 50Hz at 9s, the two grid-connected converters still have severe frequency oscillation, and cannot be stabilized at the rated frequency of 50Hz until the simulation ends at 15s. In contrast Figure 8 , after adding the strategy proposed in the application, during the normal operation of the grid at 50Hz between 0-4s, the two grid-connected converters only take 2s to restore to 50Hz after starting, and keep synchronous operation with the grid, and in the process of the grid frequency falling to 49.5Hz between 4-9s, the two grid-connected converters keep synchronous operation with the grid to 49.5Hz at 6.7s, and the two grid-connected converters also do not have oscillation phenomenon during this period. In addition, after the grid restores to 50Hz at 9s, the two grid-connected converters restore to the rated frequency at 11.4s, and the frequency steadily rises between 9s-11.4s, and does not have oscillation. In combination with Figure 5 , Figure 6 power response comparison and the above analysis, the following conclusions are drawn: the method of the application can effectively suppress the power oscillation of the multi-structure grid-connected converter grid-connected system caused by the grid frequency drop, and ensure the safe and stable operation of the power system.

[0057] The above-described embodiments are only preferred embodiments of the application, and do not limit the scope of the application, so that any changes made according to the shape and principle of the application should be covered within the protection scope of the application.

Claims

1. A grid oscillation suppression strategy for a multi-configuration grid-connected converter system based on power feedback and phase compensation, characterized in that, Comprising the following steps: Step 1: DC power is connected to the grid through a grid-connected converter, LC The filter is connected to the common grid point PCC , and then through the grid-side equivalent inductance L g , equivalent resistance R g , and is connected to the grid, wherein L i and C i are the filter inductance and filter capacitance respectively, after the system is started, the three-phase output current of the first i grid-connected converter is collected I abci , the filtered current I oabci , the grid point voltage U pcc , and the grid point phase angle θ vpcc ; Step 2: In the power calculation module, the filtered current I oabci , and grid point voltage U pcc are inputted to calculate the output active power P ei and output reactive power Q ei of the grid-forming converter. Step 3: In the virtual governor module, the active power reference value P refi , the rated angular frequency ω 0, the virtual rotor output angular frequency ω i As an input quantity, the output quantity after the virtual governor is the mechanical power P mi The specific process is that the difference between the virtual rotor output angular frequency ω i and the rated angular frequency ω 0 is recorded as the angular frequency change ω Δ The active power reference value P refi is subtracted by a primary frequency modulation coefficient k fi The product of the angular frequency change ω Δ can obtain the mechanical power P mi The expression is: (1) Step 4: In the virtual rotor module, based on the virtual rotor control of the traditional networked type converter, the power feedback and phase compensation control strategy is added, as follows: Step 4-1: Output active power P ei The filtered power is obtained after low-pass filtering. P ei0 Output power angle δ i Cosine δ i Multiply by the power angle adjustment coefficient t i Then, it is compared with the filtered power. P ei0 The summation is performed after differentiation. s And multiply by the differential coefficient r i Obtain power feedback quantity P c mechanical power P mi With filtered power P ei0 Power feedback quantity P c The power deviation is obtained by subtracting the values ​​of each. P Δ Power deviation P Δ After the virtual inertia integration stage 1 / ( J i s Damping coefficient D i The negative feedback loop constitutes the change in angular frequency. ω Δ ,in, J i For virtual inertia, then the change in angular frequency ω Δ With the rated angular frequency ω The virtual rotor output angular frequency is obtained by adding 0s together. ω i The specific expression is: (2) In formula (2), τ i is the filter time constant; Step 4-2: the virtual rotor output angle frequency ω i Subtracting the grid frequency ω g After the subtraction, the difference result is multiplied by the phase angle compensation coefficient m i The phase angle compensation amount is obtained θ Δ The virtual rotor output angle frequency ω i The initial phase angle is obtained through the integration element θ 0, the initial phase angle θ 0 is added to the phase angle compensation amount θ Δ The actual output phase angle after phase angle compensation is obtained θ i The actual output phase angle θ i is subtracted from the grid point voltage phase angle θ vpcc The output power angle is obtained after the subtraction δ i , and the specific expression is: (3) Step 4-3: Combining Step 4-1, Step 4-2, and the output active power P ei With output power angle δ i From the approximate linear relationship between them, a complete closed-loop control model of the virtual rotor module of the grid-type converter can be obtained, where the output active power... P ei With output power angle δ i The approximate linear relationship between them is expressed as follows: (4) in formula (4), E i is the i actual output voltage of the grid-connected inverter, X i is the i equivalent reactance between the grid-connected inverter and the point of common coupling, K i is defined as the correlation coefficient of the synchronous voltage; Step 5: In the reactive-voltage droop control module, the output reactive power obtained in Step 2 is Q ei , the reference reactive power Q refi , and the reference voltage E 0 are input into the reactive-voltage droop control link to obtain the actual output voltage E i The specific expression of the reactive-voltage droop control link is: (5) In formula (5), k qi is a reactive power-voltage droop control coefficient; Step 6: the three-phase output current collected in Step 1 I abci the grid point voltage U pcc and the actual output phase angle obtained in Step 4-2 θ i the actual output voltage obtained in Step 5 E i as input quantities, the three-phase modulation wave signals obtained after voltage-current double closed loop control U 0-abci the three-phase modulation wave signals U 0-abci are modulated by an SVPWM module to generate drive circuit signals g 1~ g 6. used to control the turn-on and turn-off of the grid-connected converter, and finally stabilize the output voltage of the grid-connected converter to meet the grid access requirements.

2. The power feedback and phase compensation based multi-configuration grid connected converter system oscillation suppression strategy of claim 1, wherein, In step 4, the filter time constant τ i has a value in the range 0.5 τ i <0.85, power angle regulation factor t i has a value in the range 0 t i <100, derivative factor r i has a value in the range 0 r i <0.5, phase angle compensation factor m i has a value in the range 0.1 m i <0.4.

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