A grid voltage feedforward device and control method based on PMU measurement

Abstract

Voltage feedforward control based on the common coupling point (PCC) is widely used in grid-connected inverters to suppress grid current harmonic interference caused by grid voltage distortion. However, since the voltage at the PCC contains a series of background harmonics under the condition of a weak grid, the introduction of the PCC voltage feedforward term in the control system will cause harmonic instability in the grid-connected system; to solve this problem, the present invention designs a grid voltage feedforward device and control method based on PMU measurement, which uses a phasor measurement unit (PMU) to sense the grid voltage, and uses the global positioning system (GPS) to sample the grid voltage at standard time, and then transmits the grid voltage to the control system through a data concentrator for feedforward control; at the same time, considering that the coupling between the phase-locked loop (PLL) and the grid impedance will cause system resonance or even instability, a new compensation term is introduced to eliminate the negative impact of the phase-locked loop; a method for improving the transient response of the system is proposed, so that the cutoff frequency is not affected by the grid impedance.

Description

Technical Field

[0001] The present invention relates to the field of grid-connected inverters, and in particular to a grid voltage feedforward device and a control method based on PMU measurement. Background Art

[0002] The grid-connected inverter is the interface between the photovoltaic power generation system and the large power grid, and is used to inject the electricity generated by photovoltaic power generation into the power grid. Therefore, it is very important to ensure that the grid-connected inverter can deliver stable electricity to the power grid under various working conditions. However, the stability of the grid-connected current is challenged by weak power grids. The grid-connected system under weak power grids contains a variety of background voltage harmonics and a large grid impedance value at the point of common coupling (PCC). This background harmonic will cause the grid-connected current to be distorted. There are usually two ways to suppress the interference of background harmonics. One method is to achieve infinite loop gain at the harmonic frequency. Another method to suppress background harmonics at the PCC is to adopt a PCC voltage feedforward strategy. Under normal circumstances, the voltage feedforward at the PCC can effectively suppress the harmonic interference in the grid current. However, under weak power grid conditions, the proportional feedforward of the PCC voltage will weaken the stability margin. Therefore, it is necessary to improve the PCC voltage feedforward strategy. For renewable power generation systems, in order to ensure that the frequency between the controller and the power grid is the same, a phase-locked loop (PLL) is usually used as a synchronization device in the system. Therefore, PLL plays an important role in new energy grid-connected systems. However, some literature points out that PLL may be one of the main reasons affecting the stability of the grid inverter system. Phase-locked loop is usually an effective method to detect the phase angle of the grid, but due to the existence of grid impedance, the coupling effect between the phase-locked loop and the grid impedance may cause the instability of the grid-connected system. Therefore, it is necessary to compensate for the negative impact of PLL on the system.

[0003] Based on the improvement of voltage feedforward technology, the existing technical methods are difficult to ensure that the harmonic interference at the common coupling point is suppressed while improving the stability of the grid-connected system under weak power grids. In other words, the existing control methods are difficult to simultaneously meet the requirements of the output impedance amplitude and phase angle of the grid-connected inverter. Based on the existing PLL compensation technology, if the influence of PLL on the grid-connected inverter is improved from the perspective of reshaping the output impedance, the implementation of the virtual impedance adjustment item adopted depends on the measurement of the grid impedance, and the measurement results of the grid impedance often have deviations, so it is difficult to achieve precise control. Solving the PLL problem by optimizing the controller parameters will make the controller design process relatively complicated. Summary of the invention

[0004] In view of the deficiencies in the prior art, the present invention designs a power grid voltage feedforward device and a control method based on PMU measurement.

[0005] A grid voltage feedforward device based on PMU measurement includes: PMU equipment, GPS system, grid-connected inverter, weak grid, bus, general dispatching center, data concentrator and inverter control module;

[0006] The inverter control module includes a current PI controller and a PLL controller;

[0007] The PMU device is installed on the busbar of the weak power grid, and one end of the PMU device is connected to the GPS system and the data concentrator at the same time; the inverter control module is installed on the grid-connected inverter, and one end of the grid-connected inverter is connected to the other end of the data concentrator; at the same time, the other end of the data concentrator is connected to the general dispatching center; the weak power grid is connected to the grid-connected inverter;

[0008] The PMU device installed on the weak power grid side detects the amplitude and phase information of the grid voltage, and the PMU device transmits this information to the GPS system. At the same time, the GPS device transmits the time signal to the PMU device, so that it can be sampled at the standard time; the PMU device sends the detected grid voltage information to the data concentrator, and through the data concentrator, the grid voltage will be transmitted to the grid-connected inverter and the general dispatching center; the dispatching center will analyze and store the grid voltage information; and the grid-connected inverter will use the grid voltage for feedforward control; the grid voltage signal is introduced into the inverter control module through a feedforward function; and the compensation term G H Compensate into the inverter control module to offset the negative impact of PLL on the grid-connected inverter; design the improvement item G based on the dynamic characteristics of the device according to the open-loop transfer function of the grid-connected inverter x and introduce it into the inverter control module to improve the response speed of the device;

[0009] On the other hand, a control method for grid voltage feedforward based on PMU measurement specifically includes the following steps:

[0010] Step 1: The PMU device detects the amplitude and phase information of the grid voltage under the weak grid and sends this information to the data concentrator;

[0011] Step 2: The inverter control module receives the detected grid voltage information through the data concentrator;

[0012] Step 3: Create the feedforward function G in the inverter control module f 1 / K PWM , K PWM Represents the grid-connected inverter gain;

[0013] Step 4: Multiply the grid voltage by the feedforward function and feed forward to the output end of the current PI controller in the inverter control module;

[0014] Step 5: Based on the above grid voltage feedforward control, the influence of PLL on the system is further compensated; the output impedance Z of the inverter control module under grid voltage feedforward is calculated. out_H for:

[0015]

[0016] Where L represents the grid-connected inverter filter inductance; G ci represents the current PI controller in the inverter control module; G del Represents the delay of the inverter control module; Z g represents the grid impedance; Represents the voltage disturbance caused by the PLL in the inverter control module; represents the disturbance of duty cycle by PLL in inverter control module; represents the disturbance of the current by the PLL in the inverter control module;

[0017] The negative impact of the phase-locked loop (PLL) on the device is performance, thus compensating the inverter control module That is, it is necessary to set the corresponding compensation item in the inverter control module to offset Z out_H In item;

[0018] Step 6: Create the PLL controller parameter matrix G in the inverter control module P for:

[0019]

[0020] Where K ppll is the proportional gain of the PLL controller; K ipll is the integral gain of the PLL controller;

[0021] Step 7: Create the reference current matrix G of the PLL controller in the inverter control module I for:

[0022]

[0023] Where i qref represents the q-axis reference current of the grid-connected inverter; i dref represents the d-axis reference current of the grid-connected inverter;

[0024] Step 8: Convert the PLL controller parameter matrix G in the inverter control module P , reference current matrix G I and current PI controller G ci Connect in series and multiply by the adjustment coefficient β to form the compensation term G Hfor:

[0025] G H =G P ·G I ·G ci β (4)

[0026] In the formula, β is used to adjust the amplitude of the compensation term;

[0027] Step 9: Set the compensation term G H Inserted into the output end of the current PI controller in the inverter control module;

[0028] Step 10: Calculate the open-loop transfer function G of the inverter control module after PLL compensation open for:

[0029]

[0030] Step 11: Since the resistance component in the grid impedance has little effect on the system stability, only the inductance component in the grid impedance is considered, and the inductance component L in the grid impedance is established. g The relationship between the filter inductance L is: g =x·L; x is a constant;

[0031] Step 12: Create the function G of the improved term in the inverter control module x for:

[0032]

[0033] Where λ is the adjustment coefficient;

[0034] Step 13: Improve item G x Inserted into the feedback channel of the inverter control module;

[0035] Step 14: Calculate the open-loop transfer function at this time:

[0036]

[0037] According to the above formula, at this time, the open-loop transfer function of the device no longer includes the grid impedance, the cutoff frequency of the device is no longer affected by the grid impedance, and the dynamic characteristics of the grid-connected inverter are improved.

[0038] Beneficial technical effects of the present invention:

[0039] The present invention installs a PMU device and a GPS system on the power grid side, and uses a data concentrator to transmit the information detected by the PMU device; uses the PMU to detect the amplitude and phase information of the power grid voltage for feedforward control; and converts the compensation term G based on the PLL into a HCompensate to the inverter control module; design improvement items based on the dynamic characteristics of the grid-connected inverter G x Solve the instability problem of the grid-connected system under weak power grid, and improve the stability of the grid-connected system under weak power grid from the perspective of voltage feedforward and phase-locked loop. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 A structural diagram of a device for grid voltage feedforward based on PMU measurement in an embodiment of the present invention;

[0041] Figure 2 Control block diagram of PCC voltage feedforward according to an embodiment of the present invention;

[0042] Figure 3 A grid voltage feedforward equivalent control block diagram according to an embodiment of the present invention;

[0043] Figure 4 A grid voltage feedforward equivalent control block diagram according to an embodiment of the present invention;

[0044] Figure 5 The Norton equivalent circuit of the inverter and the weak grid when the grid voltage is fed forward in the embodiment of the present invention;

[0045] Figure 6 Equivalent control block diagram of the grid-connected inverter after PLL compensation according to an embodiment of the present invention;

[0046] Figure 7 A control block diagram of an improved grid-connected inverter according to an embodiment of the present invention. DETAILED DESCRIPTION

[0047] The following further describes a device and control method for grid voltage feedforward based on PMU measurement in conjunction with the accompanying drawings and embodiments;

[0048] A grid voltage feedforward device based on PMU measurement, as shown in the attached Figure 1 As shown, it includes: PMU equipment, GPS system, grid-connected inverter, weak power grid, bus, general dispatching center, data concentrator and inverter control module;

[0049] The inverter control module includes a current PI controller and a PLL controller;

[0050] The PMU device is installed on the busbar of the weak power grid, and one end of the PMU device is connected to the GPS system and the data concentrator at the same time; the inverter control module is installed on the grid-connected inverter, and one end of the grid-connected inverter is connected to the other end of the data concentrator; at the same time, the other end of the data concentrator is connected to the general dispatching center; the weak power grid is connected to the grid-connected inverter;

[0051] The PMU device installed on the weak power grid side detects the amplitude and phase information of the grid voltage, and the PMU device transmits this information to the GPS system. At the same time, the GPS device transmits the time signal to the PMU device, so that it can be sampled at the standard time; the PMU device sends the detected grid voltage information to the data concentrator, and through the data concentrator, the grid voltage will be transmitted to the grid-connected inverter and the general dispatching center; the dispatching center will analyze and store the grid voltage information; and the grid-connected inverter will use the grid voltage for feedforward control; the grid voltage signal is introduced into the inverter control module through a feedforward function; and the compensation term G H Compensate into the inverter control module to offset the negative impact of PLL on the grid-connected inverter; design the improvement item G based on the dynamic characteristics of the device according to the open-loop transfer function of the grid-connected inverter x and introduce it into the inverter control module to improve the response speed of the device;

[0052] On the other hand, a control method of grid voltage feedforward based on PMU measurement is specifically as follows:

[0053] The control block diagram of the grid-connected inverter under PCC voltage feedforward control is as follows: Figure 2 shown. Figure 2 middle and Represents the disturbance of PLL to system voltage, current and duty cycle, G ci represents the PI controller, G del represents the delay in the control system, K PWM Represents the inverter gain, V PCC,dq is the voltage at PCC on the dq axis. V g,dq is the grid voltage under the dq axis. The amplitude and phase information of the grid voltage are transmitted to the inverter control module through the PMU device and the data transmission system under the action of GPS. g,dq is the grid-connected current under dq axis, i ref,dq is the reference value of the grid-connected current under the dq axis, ωLi g,qd is the decoupling term, Z g is the grid impedance, L is the grid-side filter inductance, G is f is the feedforward function. Next, the feasibility of the grid voltage feedforward control strategy is analyzed through the output impedance. PCC,dq =V g,dq +Z g ·i g,dq ,but:

[0054] V g,dq ·G f =V PCC,dq ·G f -Z g ·i g,dq ·Gf (1)

[0055] Therefore, according to the above formula, the grid voltage feedforward can be equivalent to a PCC voltage feedforward and a grid impedance voltage Z g ·i g Feed forward, such as Figure 3 As shown. Then the grid impedance voltage feedforward is equivalent to the grid current feedforward, and the feedback position is moved to the current controller G ci The feedback function is Z g G f / G ci , we get the grid voltage feedforward equivalent block diagram, as Figure 4 As shown. Figure 2 In comparison, it can be seen that when grid voltage feedforward is introduced, there is an additional feedback path related to the grid impedance in the inverter control module.

[0056] according to Figure 2 The output impedance of the grid-connected inverter when the PCC voltage is fed forward under the action of the phase-locked loop is obtained as follows:

[0057]

[0058] according to Figure 4 The output impedance of the grid-connected inverter during grid voltage feedforward can be obtained as:

[0059]

[0060] Combining equations (3) and (4), the output impedance Z pll_02 It can also be expressed as:

[0061] Z pll_02 =Z pll_01 +Z gg Bar (4)

[0062] Where Z gg for:

[0063]

[0064] Therefore, according to the impedance analysis method, the Norton equivalent circuit between the grid-connected inverter and the weak power grid can be drawn as follows: Figure 5 As shown. At this time, the grid-connected current i' g 'for:

[0065]

[0066] Where i'0' is the inverter equivalent current source, which can be expressed as:

[0067]

[0068] Then the grid-connected current i' g ' can also be expressed as:

[0069]

[0070] Considering that the sampling period is very small, the time delay in the grid-connected inverter is also very small, and the grid impedance is also a very small value, so G in the first part of the above formula can be ignored. del ·Z g In the latter part of the above formula, due to Z g / Z pll_02 There is Z in the molecule g , so G in the denominator del ·Z g cannot be ignored, so the above formula can be simplified to:

[0071]

[0072] The stability of the grid-connected current can be determined according to the above formula. pll_01 It has nothing to do with the grid impedance, so the first term in the above formula is stable under weak grid conditions, and the stability of the grid-connected inverter mainly depends on Z pll_01 / Z g and Z gg / Z g . Where Z pll_01 / Z g It is related to the grid impedance and is the output impedance of the PCC voltage feedforward, reflecting the ability to eliminate harmonics. gg / Z g It is independent of the grid impedance and reflects the stability of the grid-connected inverter under weak grid conditions. In summary, using grid voltage feedforward instead of PCC voltage feedforward can not only maintain the ability of the grid-connected inverter to suppress harmonics, but also ensure stability under weak grid conditions.

[0073] The negative impact of the phase-locked loop on the grid-connected inverter is mainly through performance, so it mainly compensates for the inverter control module To offset The influence of the term on the system, the compensation term G is introduced into the control system H First, determine the compensation position and insert the compensation item into a forward channel in the inverter control module. The control block diagram of the grid-connected inverter after compensation is as follows: Figure 6 shown.

[0074] is the design compensation term G H , first according to Figure 6 The output impedance of the grid-connected inverter is:

[0075]

[0076] Offset The impact of G H It can be expressed as:

[0077] G H =G P ·G I ·G ci β (11)

[0078] Among them G P Parameters for the PLL controller:

[0079]

[0080] G I for:

[0081]

[0082] β is the adjustment coefficient, which is used to adjust the amplitude of the compensation signal. H After introduction, the output impedance of the grid-connected inverter is offset. This offsets the negative impact of PLL on the grid-connected inverter.

[0083] Next, in order to improve the dynamic characteristics of the grid-connected inverter, an improved control strategy based on grid impedance is designed. Figure 6 The open-loop transfer function of the system is obtained as follows:

[0084]

[0085] Since the resistance component in the grid impedance has little effect on the stability of the grid-connected inverter, only the inductance component in the grid impedance is considered, and the relationship between the inductance component in the grid impedance and the filter inductance is established as follows: L g =x·L. According to the above formula, it can be seen that the open-loop transfer function of the grid-connected inverter has nothing to do with the phase-locked loop. However, due to the adoption of the grid voltage feedforward control strategy, the denominator of the open-loop transfer function of the grid-connected inverter contains the grid impedance Z g , then as the grid impedance increases, the cutoff frequency of the grid-connected system gradually decreases, resulting in a decrease in the response speed of the system.

[0086] From the perspective of control structure, the improved item G is introduced in the inverter control module x , introduce G x The grid-connected inverter control box is as follows Figure 7 As shown. Figure 7 The improved open-loop transfer function of the grid-connected inverter is:

[0087]

[0088] At this time, in order to make the open-loop transfer function of the grid-connected inverter no longer include the grid impedance Z g , just need to order:

[0089]

[0090] Where λ is the adjustment coefficient. Substituting the above formula into formula (15) we can get:

[0091]

[0092] According to the above formula, at this time, the open-loop transfer function of the grid-connected inverter no longer includes the grid impedance, and the cut-off frequency of the system is no longer affected by the grid impedance.

Claims

1. A power grid voltage feedforward device based on PMU measurement, characterized in that: Specifically include: PMU equipment, GPS system, grid-connected inverter, weak power grid, busbar, general dispatching center, data concentrator and inverter control module; The PMU device is installed on the busbar of the weak power grid, and one end of the PMU device is connected to the GPS system and the data concentrator at the same time; the inverter control module is installed on the grid-connected inverter, and one end of the grid-connected inverter is connected to the other end of the data concentrator; at the same time, the other end of the data concentrator is connected to the general dispatching center; the weak power grid is connected to the grid-connected inverter; The inverter control module includes a current PI controller and a PLL controller; The PMU device installed on the weak power grid side detects the amplitude and phase information of the grid voltage, and the PMU device transmits this information to the GPS system. At the same time, the GPS device transmits the time signal to the PMU device, so that it can be sampled at the standard time; the PMU device sends the detected grid voltage information to the data concentrator, and through the data concentrator, the grid voltage will be transmitted to the grid-connected inverter and the general dispatching center; the dispatching center will analyze and store the grid voltage information; and the grid-connected inverter will use the grid voltage for feedforward control; the grid voltage signal is introduced into the inverter control module through a feedforward function; and the compensation term G H Compensation is added to the inverter control module to offset the negative impact of PLL on the grid-connected inverter; Design the improvement item G based on the dynamic characteristics of the device according to the open-loop transfer function of the grid-connected inverter x and introduce it into the inverter control module to improve the response speed of the device.

2. A control method for grid voltage feedforward based on PMU measurement, implemented based on a grid voltage feedforward device based on PMU measurement according to claim 1, characterized in that: The specific steps include: Step 1: The PMU device detects the amplitude and phase information of the grid voltage under the weak grid and sends this information to the data concentrator; Step 2: The inverter control module receives the detected grid voltage information through the data concentrator; Step 3: Create the feedforward function G in the inverter control module f 1 / K PWM ; Step 4: Multiply the grid voltage by the feedforward function and feed forward to the output end of the current PI controller in the inverter control module; Step 5: Based on the above grid voltage feedforward control, the influence of PLL on the system is further compensated; the output impedance Z of the inverter control module under grid voltage feedforward is calculated. out_H for: Where L represents the grid-connected inverter filter inductance; G ci represents the current PI controller in the inverter control module; G del Represents the delay of the inverter control module; Z g represents the grid impedance; Represents the voltage disturbance caused by the PLL in the inverter control module; represents the disturbance of duty cycle by PLL in inverter control module; represents the disturbance of the current by the PLL in the inverter control module; Step 6: Create the PLL controller parameter matrix G in the inverter control module P for: Where K ppll is the proportional gain of the PLL controller; K ipll is the integral gain of the PLL controller; Step 7: Create the reference current matrix G of the PLL controller in the inverter control module I for: Where i qref represents the q-axis reference current of the grid-connected inverter; i dref represents the d-axis reference current of the grid-connected inverter; Step 8: Convert the PLL controller parameter matrix G in the inverter control module P , reference current matrix G I and current PI controller G ci Connect in series and multiply by the adjustment coefficient β to form the compensation term G H for: G H =G P ·G I ·G ci ·β(4) In the formula, β is used to adjust the amplitude of the compensation term; Step 9: Set the compensation term G H Inserted into the output end of the current PI controller in the inverter control module; Step 10: Calculate the open-loop transfer function G of the inverter control module after PLL compensation open for: Step 11: Since the resistance component in the grid impedance has little effect on the system stability, only the inductance component in the grid impedance is considered; Step 12: Create the function G of the improved term in the inverter control module x for: Where λ is the adjustment coefficient; Step 13: Improve item G x Inserted into the feedback channel of the inverter control module; Step 14: Calculate the open-loop transfer function at this time: According to the above formula, at this time, the open-loop transfer function of the device no longer includes the grid impedance, the cutoff frequency of the device is no longer affected by the grid impedance, and the dynamic characteristics of the grid-connected inverter are improved.

3. The control method of power grid voltage feedforward based on PMU measurement according to claim 2, characterized in that: Step 3 K PWM Represents the grid-connected inverter gain.

4. The control method of power grid voltage feedforward based on PMU measurement according to claim 2, characterized in that: Based on step 5, it can be seen that the negative impact of the phase-locked loop PLL on the device is performance, thus compensating the inverter control module That is, it is necessary to set the corresponding compensation item in the inverter control module to offset Z out_H In item.

5. The control method of power grid voltage feedforward based on PMU measurement according to claim 2, characterized in that: In step 11, the inductance component L in the grid impedance is established. g The relationship between the filter inductance L is: g =x·L, x is a constant.

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