A fault transient current analysis method considering the saturation characteristics of inverter controller

Abstract

The present invention discloses a fault transient current analysis method taking into account the saturation characteristics of an inverter controller. First, the controller working state during the fault process is characterized according to the controller output of the current loop of a new energy power control system; then, according to the current loop control response during the fault transient process, the transient current of the controller saturation stage after the fault is analyzed based on the control response; finally, according to the obtained transient current analysis formula, the corresponding time parameter is introduced, and the fault full transient current analysis under the current loop controller saturation scenario is realized based on the segmented equivalent method. The present invention fully considers the saturation characteristics of the current loop controller of the inverter control system, and avoids a large number of complex calculations. The analysis accuracy can be improved when performing fault transient current analysis calculations on the inverter power supply, and the accurate analysis of the fault transient current when the inverter controller is saturated is realized.

Description

Technical Field

[0001] The invention relates to the technical field of power system analysis, and in particular to a fault transient current analysis method taking into account the saturation characteristics of an inverter power supply controller. Background Art

[0002] The access of new energy sources has led to fundamental changes in the fault characteristics of the power system. There is a necessary demand for protection configuration on the new energy transmission side, especially new principle-based protection based on the time-frequency information of the transient waveform of new energy. However, the existing fault analysis methods for conversion equipment focus on characterizing the steady-state fault component of the fundamental frequency, ignoring the transient state of the fault, and do not deeply involve the nonlinear problems of the control system, making it difficult to meet the protection needs of the new energy transmission side. The key problem is that during the transient state of the fault, the output current of the inverter power supply is in the control response time scale, and each control link jointly determines the time domain current characteristics. The existence of saturation links in complex control systems will cause the transient output of the converter to show significant nonlinear characteristics.

[0003] The current loop PI controller is an important part of regulating the closed-loop error. Its response during the fault transient period will play a decisive role in the inverter output current. However, the conventional short-circuit current analysis method based on linear analysis often ignores the influence of the saturation characteristics of the current loop controller on the transient current at the quantitative level during the equivalent process, which leads to an increase in the analysis error. In addition, there is a lack of general analytical results considering the existence of controller saturation, making it difficult to obtain an accurate current expression. Therefore, the analysis of the fault transient current characteristics of the inverter power supply is not sufficient, and it is impossible to provide a correct theoretical research basis and effective verification means for the new protection principle of the new energy transmission system. Therefore, it is urgent to study the analytical method for solving the fault transient current of the inverter power supply based on the saturation characteristics of the fault transient current loop controller.

[0004] Purpose of the Invention

[0005] The purpose of the present invention is to address the deficiencies in the prior art and provide a method for analyzing fault transient currents taking into account the saturation characteristics of an inverter controller. The present invention fully considers the saturation characteristics of the current loop controller of the inverter control system and avoids a large number of complex calculations. The analysis accuracy can be improved when performing fault transient current analysis calculations on the inverter, and the accurate analysis of the fault transient current taking into account the saturation of the inverter controller is achieved. Summary of the invention

[0006] The present invention provides a fault transient current analysis method taking into account the saturation characteristics of an inverter power supply controller, comprising the following steps:

[0007] Step 1: Describe the working state of the controller during the fault process according to the controller output of the current loop of the new energy power supply control system;

[0008] Step 2: according to the current loop control response in the transient process of the fault, the transient current of the controller in the saturation stage after the fault is analyzed based on the control response;

[0009] Step 3: According to the obtained transient current analytical formula, the corresponding time parameter is introduced, and the full transient current analysis of the fault in the current loop controller saturation scenario is realized based on the segmented equivalent method.

[0010] Preferably, in step 1, according to the PI controller output f and the saturation link limit ±M 0 The effect of the saturation link of the dq axis components of the current loop after a fault occurs is described as shown in formula (1):

[0011]

[0012] The grid connection point voltage range when the q-axis controller output is saturated is shown in formula (2):

[0013] U≤0.9-M 0 / (k ip KI N ) (2),

[0014] Where, U represents the per unit voltage at the grid connection point; k ip is the proportional coefficient of the current loop; K represents the reactive current compensation coefficient during the fault period; I N Indicates rated current;

[0015] when a 1 =k ip {{(AI N ) 2 -[K(0.9-U)I N ] 2} 0.5 -BI N}≥M 0 , where A represents the maximum current allowed by the inverter for I N multiples of I, B represents the d-axis current in normal operation N multiples of;

[0016] The grid connection point voltage range when the d-axis controller output is saturated is shown in formula (3):

[0017]

[0018] In the formula, a 2 =A 2 -[B+M 0 / (k ip I N )] 2 , a 3 =0.9-(a2 ) 0.5 / K;

[0019] when a 1 ≤-M 0 When the d-axis controller output is saturated, the grid connection point voltage range is as shown in formula (4):

[0020]

[0021] In the formula, a 4 =BM 0 / (k ip I N ), a 5 =a 2 +4BM 0 / (k ip I N ), a 6 =0.9-(a 5 ) 0.5 / K;

[0022] Preferably, in step 2, there exists a relationship between the electrical quantities between the inverter port and the grid connection point as shown in formula (5):

[0023]

[0024] In the formula, v, u, i represent the inverter port output voltage, grid-connected point voltage and grid-connected current respectively, the subscripts d and q represent the corresponding dq axis components, and the superscript "*" represents the corresponding command value; R and L represent the equivalent resistance and inductance between the inverter port and the grid-connected point respectively, and ω is the power frequency angular velocity;

[0025] If the q-axis controller is saturated by the upper boundary M after the fault occurs 0 Take the inverter as an example, at this time, the modulated voltage command and current of the inverter satisfy the relationship shown in formula (6):

[0026]

[0027] Eliminating the q-axis inverter port voltage in equations (5) and (6), the first-order dynamic response of the q-axis current loop is obtained, as shown in equation (7):

[0028]

[0029] Substitute the initial current conditions Solve equation (7), where represents the fault time t 0 The transient current expression of the q-axis controller saturation stage after the fault can be obtained as shown in formula (8):

[0030]

[0031] Preferably, in step 3, the transient current expression of the q-axis controller in the unsaturated stage can be solved by the second-order differential equation of the current loop, as shown in formula (9):

[0032]

[0033] In the transient process after a fault in the saturated output scenario of the PI controller, the PI link output will transition from the saturated working state, i.e., the saturated nonlinear region, to the unsaturated working state, i.e., the linear region. The current expression in the saturated nonlinear region is the same as that shown in equation (8), and the current expression in the linear region is the same as that shown in equation (9). By introducing the corresponding time parameters to combine the two working states, the controller performs equivalent substitution in different working regions, and the transient current expression shown in equation (10) is obtained:

[0034]

[0035] In the formula, the time parameter t kq is the time domain dividing point between the saturated nonlinear region and the linear region, that is, the moment of state change; the time parameter t mq The role is to ensure that t kq The current value does not suddenly change at any moment;

[0036] By solving t kq With t mq The final analytical expression of the fault full transient current is shown in formula (11):

[0037]

[0038] In the formula, q; Satisfies the equation shown in formula (12):

[0039]

[0040] The formula (11) is applicable to both the d and q axes. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 A schematic flow chart of a fault transient current analysis method taking into account the saturation characteristics of an inverter power supply controller provided by an embodiment of the present invention;

[0042] Figure 2 A schematic diagram of a typical inverter power delivery system and its control structure according to an embodiment of the present invention;

[0043] Figure 3 This is a schematic diagram of current loop control according to an embodiment of the present invention;

[0044] Figure 4 The comparison between the analytical current and the simulated current when the controller is saturated is considered according to the embodiment of the present invention;

[0045] Figure 5 This is a comparison between the analytical current and the recorded current when the controller is saturated as described in the embodiment of the present invention. DETAILED DESCRIPTION

[0046] The present invention will be described in detail below in conjunction with the accompanying drawings.

[0047] Figure 1 The present invention provides a flow chart of a fault transient current analysis method taking into account the saturation characteristics of an inverter controller. The present invention provides a fault transient current analysis method taking into account the saturation characteristics of an inverter controller, comprising the following steps:

[0048] Step 1: Describe the working state of the controller during the fault process according to the controller output of the current loop of the new energy power supply control system;

[0049] In this step, the working state of the inverter current loop controller is first analyzed as a theoretical basis for its influence on the inverter transient current. In order to describe the saturation nonlinear characteristics of the control system current loop from a quantitative perspective, the actual output of the PI controller of the dq axis component of the current loop after the fault is described using the grid voltage U. Figure 2 The figure shows a typical inverter power supply transmission system and its control structure schematic diagram according to an embodiment of the present invention, wherein the operation state of the current inner loop control system is affected by the following Figure 3 The figure shows the PI link output f and the upper and lower limit values ​​M of the saturation link in the current loop control schematic diagram according to the embodiment of the present invention. 0 and -M 0 (In practice, the upper and lower limit boundary values ​​are not required to be opposite to each other) and the effect of the saturation links of the dq axis components of the current loop after a fault occurs is explained.

[0050] First, determine the current loop command of the control system. The dq axis control current command under normal operating conditions is as follows:

[0051]

[0052] The control current command during low voltage ride-through after a fault is as follows:

[0053]

[0054] Where i represents the grid-connected current, subscripts d and q represent the corresponding dq-axis components, and superscript “*” represents the corresponding command value; K represents the reactive current compensation coefficient during the fault period; U represents the grid-connected point voltage per unit value; I NIndicates rated current; A indicates the maximum current allowed by the inverter for I N multiples of I, B represents the d-axis current in normal operation N multiples of.

[0055] The specific working state of the current loop dq axis controller is shown in formula (1):

[0056]

[0057] Considering the fault time t 0 The current command value will change suddenly before and after, but the actual current cannot change suddenly due to the existence of inductance components in the electrical circuit, that is: in q, represents the fault time t 0 The moment before and after. It is the key to whether the current loop can trigger the saturation link of the PI controller. Taking the case where both the dq axis component PI controllers are not saturated after a fault as an example, the input of the current loop PI link is shown in the following formula when combined with equations (25) and (26):

[0058]

[0059] Where ΔPI represents the PI controller input.

[0060] Combining the specific working state of the current loop dq axis controller and the above expression of ΔPI, the output f of the PI link can be specifically expressed as shown in the following formula:

[0061]

[0062] Among them, k ip , k ii They are the proportional and integral coefficients of the current loop respectively. Since only At this moment, the integrator has not yet taken effect, so the integral term in the formula is 0. For the q-axis component, according to formula (16), the grid connection point voltage range when the saturation link is not in effect can be obtained as shown in formula (2):

[0063] U≤0.9-M 0 / (k ip KI N ) (2),

[0064] when a 1 =k ip {{(AI N ) 2 -[K(0.9-U)I N ] 2} 0.5 -BIN}≥M 0 The grid connection point voltage range when the d-axis controller output is saturated is shown in formula (3):

[0065]

[0066] In the formula, a 2 =A 2 -[B+M 0 / (k ip I N )] 2 , a 3 =0.9-(a 2 ) 0.5 / K.

[0067] when a 1 ≤-M 0 When the d-axis controller output is saturated, the grid connection point voltage range is as shown in formula (4):

[0068]

[0069] In the formula, a 4 =BM 0 / (k ip I N ), a 5 =a 2 +4BM 0 / (k ip I N ), a 6 =0.9-(a 5 ) 0.5 / K.

[0070] Step 2: according to the current loop control response in the transient process of the fault, the transient current of the controller in the saturation stage after the fault is analyzed based on the control response;

[0071] In this step, the short-circuit current analytical expression derived in this step fully considers the saturation characteristics of the control system current loop controller and can more accurately reflect the actual inverter fault characteristics. The transient characteristics of the inverter power supply fault depend on the response of the control system. Accurately characterizing the control response is the key to solving the inverter output current. The following relationship exists between the electrical quantities between the inverter port and the grid connection point:

[0072]

[0073] Where v and u represent the inverter port output voltage and grid-connected point voltage respectively; R and L represent the equivalent resistance and inductance between the inverter port and the grid-connected point respectively; ω represents the power frequency angular velocity.

[0074] If the q-axis component is saturated by the upper boundary M after the fault occurs 0 As an example (assuming that the system enters f q ≥M 0 Due to the characteristics of independent control of dq axis current, the method for analyzing the saturation of d axis current PI controller is the same. At this time, the output after the PI link and the saturation link are M 0 ,therefore Figure 2 The q-axis current equation in the inner loop control equation in the dotted box needs to be changed, that is, the PI link output is replaced by the saturation boundary M 0 Equivalent replacement:

[0075]

[0076] Eliminating the q-axis inverter port voltage in equations (33) and (34), the first-order dynamic response of the q-axis current loop is obtained:

[0077]

[0078] Substitute the initial current conditions Solving equation (35), we can obtain the transient current expression of the q-axis controller in the saturation stage after the fault:

[0079]

[0080] Step 3: According to the obtained transient current analytical formula, the corresponding time parameter is introduced, and the full transient current analysis of the fault in the current loop controller saturation scenario is realized based on the segmented equivalent method.

[0081] In this step, the transient current expression of the q-axis controller in the unsaturated stage can be solved by the second-order differential equation of the current loop, specifically:

[0082]

[0083] In the PI controller output saturation scenario, the PI link output of the transient process after the fault will transition from the saturated working state (hereinafter referred to as the saturated nonlinear region) to the unsaturated working state (hereinafter referred to as the linear region), where the current expression in the saturated nonlinear region is formula (8), and the current expression in the linear region is the same as formula (9). By introducing the corresponding time parameters to combine the two working states, the controller is replaced by equivalent values ​​in different working areas, and the complete analytical formula is shown in formula (10):

[0084]

[0085] In the formula, the time parameter t kq is the time domain dividing point between the saturated nonlinear region and the linear region (the moment of state change); the time parameter t mqThe role is to ensure t kq The current value does not change suddenly at this moment.

[0086] The difficulty of the complete analytical formula (10) lies in the two time parameters t kq ,t mq The solution is, due to t kq The current will not change suddenly at the moment t, that is, the current expressions of the saturated nonlinear region and the linear region are kq The results at time t are the same, so consider changing time t kq Substitute into equation (10) and let (t 0 , t kq ] and [t kq ,∞) The results of the two-stage expressions are equal, so the time parameter t is solved kq ,t mq The functional relationship is t mq (t kq ) is expressed as follows:

[0087]

[0088] Due to t kq At this moment, the current cannot change suddenly, so t=t kq Substituting into formula (10) [t kq ,∞) stage, and the result is further substituted into equation (7) to obtain the time difference Δt q =t kq -t mq , as shown in the following formula:

[0089]

[0090] Compare the above expressions and eliminate the time parameter t mq , we can solve the time domain demarcation point t kq The specific expression is as follows:

[0091]

[0092] Get kq After combining the above expressions, we can get t mq The expression of the fault full transient current under the influence of the saturation characteristics of the control system current loop PI controller is finally completely analyzed (applicable to both d and q axes), as shown in formula (11):

[0093]

[0094] In the formula, q; Satisfies formula (12):

[0095]

[0096] Figure 4 This is a comparison diagram of the analytical current and the simulated current when the controller is saturated according to an embodiment of the present invention. Figure 4 The waveforms obtained by the traditional inverter power supply transient analysis method, the waveforms obtained by the transient current analysis method described in the present invention, and the transient current simulation waveforms are respectively plotted. It can be seen that, whether it is the dq axis current or the three-phase current, the results obtained by the traditional method have a large error with the simulation results, while the analysis results of the method described in the present invention are highly consistent with the simulation values, which confirms the feasibility of the analysis method. The main reason for its high accuracy is that the influence of the saturation of the current loop controller on the fault transient current in the transient process is taken into account. Due to the existence of the controller saturation characteristics, the fault transient transition time is increased.

[0097] Figure 5 This is a comparison diagram of the analytical current and the recorded current when the controller is saturated according to the embodiment of the present invention. According to the analytical method of the present invention, the current expression in the dq coordinate system is calculated, and further converted into the ABC three-phase coordinate system according to the phase angle data extracted from the recorded voltage. For comparison, the three-phase current waveform of the recorded current and the three-phase current waveform obtained by the traditional method are drawn in the same image. It can be seen that the algorithm proposed in this article can accurately depict the change law of the transient output current after the fault, and the calculated value is much more accurate than the traditional algorithm that does not consider the saturation characteristics of the current loop controller.

[0098] Thus, the present invention realizes the fault transient current analysis method taking into account the saturation characteristics of the inverter controller, and Figure 4 , Figure 5 It can be seen that the calculation accuracy of the transient current of the inverter power supply can be improved by the fault transient current analysis method taking into account the saturation characteristics of the inverter power supply controller of the present invention.

[0099] By adopting the fault transient current analysis method taking into account the saturation characteristics of the inverter power supply controller described in the present invention, the following beneficial effects can be obtained:

[0100] (1) The influence of the transient saturation characteristics of the current loop controller fault in the inverter control system on the inverter output current is fully considered, which improves the accuracy of the transient current analysis of the inverter. It has a good reference value for the refined characterization of the transient fault characteristics of the inverter, and is conducive to the setting and verification of the relay protection principle;

[0101] (2) A large number of complex calculations are avoided, and the established transient current dynamic equation satisfies the low-order characteristics, ensuring that not too much extra time is added on the basis of improving the accuracy of short-circuit current analysis.

[0102] It should be pointed out that the above specific implementation modes are only preferred implementation modes of the present invention and should not be understood as limiting the scope of protection of the present invention. Any minor changes and modifications made to the present invention without departing from the concept of the present invention shall fall within the scope of protection of the present invention.

Claims

1. A fault transient current analysis method taking into account the saturation characteristics of an inverter power supply controller, characterized in that: The following steps are involved: Step 1: Describe the working state of the controller during the fault process according to the controller output of the current loop of the new energy power supply control system; Step 2: according to the current loop control response in the transient process of the fault, the transient current of the controller in the saturation stage after the fault is analyzed based on the control response; Step 3: According to the obtained transient current analytical formula, the corresponding time parameter is introduced, and the full transient current analysis of the fault in the current loop controller saturation scenario is realized based on the segmented equivalent method; In step 1, the effect of the saturation link of the dq axis components of the current loop after the fault occurs is described according to the PI controller output f and the saturation link limit ±M0, as shown in formula (1): As shown: The grid connection point voltage range when the q-axis controller output is saturated is shown in formula (2): U≤0.9-M0 / (k ip KI N ) (2), Where, U represents the per unit voltage at the grid connection point; k ip is the proportional coefficient of the current loop; K represents the reactive current compensation coefficient during the fault period; I N Indicates rated current; When a1=k ip {{(AI N ) 2 -[K(0.9-U)I N ] 2 } 0.5 -BI N }≥M0, where A represents the maximum current allowed by the inverter for I N multiples of I, B represents the d-axis current in normal operation N multiples of; The grid connection point voltage range when the d-axis controller output is saturated is shown in formula (3): Where a2 = A 2 -[B+M0 / (k ip I N )] 2 , a3=0.9-(a2) 0.5 / K; When a1≤-M0, the grid connection point voltage range when the d-axis controller output is saturated is as shown in formula (4): Where a4 = B-M0 / (k ip I N ), a5=a2+4BM0 / (k ip I N ), a6=0.9-(a5) 0.5 / K; In step 2, there is a relationship between the electrical quantities between the inverter port and the grid connection point as shown in formula (5): In the formula, v, u, i represent the inverter port output voltage, grid-connected point voltage and grid-connected current respectively, subscripts d and q represent the corresponding dq axis components; R and L are the equivalent resistance and inductance between the inverter port and the grid-connected point respectively, and ω is the power frequency angular velocity; After the fault occurs, the q-axis controller is affected by the saturation upper boundary M0. At this time, the modulated voltage command and current of the inverter satisfy the relationship shown in formula (6): The superscript "*" indicates the corresponding command value. Eliminating the q-axis inverter port voltage in equations (5) and (6), the first-order dynamic response of the q-axis current loop is obtained, as shown in equation (7): Substitute the initial current conditions Solve equation (7), where Represents the moments before and after the fault time t0, and the transient current expression of the saturation stage of the q-axis controller after the fault can be obtained as shown in formula (8): In step 3, the transient current expression of the q-axis controller in the unsaturated stage can be solved by the second-order differential equation of the current loop, as shown in formula (9): In the transient process after a fault in the saturated output scenario of the PI controller, the PI link output will transition from the saturated working state, i.e., the saturated nonlinear region, to the unsaturated working state, i.e., the linear region. The current expression in the saturated nonlinear region is the same as that shown in equation (8), and the current expression in the linear region is the same as that shown in equation (9). By introducing the corresponding time parameters to combine the two working states, the controller performs equivalent substitution in different working regions, and the transient current expression shown in equation (10) is obtained: In the formula, the time parameter t kq is the time domain dividing point between the saturated nonlinear region and the linear region, that is, the moment of state change; the time parameter t mq The role is to ensure that t kq The current value does not suddenly change at any moment; By solving t kq With t mq The final analytical expression of the fault full transient current is shown in formula (11): In the formula, q; Satisfies the equation shown in formula (12): The formula (11) is applicable to both the d and q axes.

Images