Improved distance protection method for hybrid new energy station based on unified reference frame and transient linear impedance modeling

Abstract

The improved distance protection method for hybrid new energy station based on unified reference frame and transient linear impedance modeling includes: analyzing the fault transient characteristics and phase reference differences of grid-connected units and grid-constructing units, and clarifying the core problem that the currents of the two types of units cannot be directly algebraically superimposed; establishing a unified reference frame based on the grid-constructing virtual phase to complete the coordinate conversion of the current and the synthesis of the station current; establishing a line transient equivalent impedance model and analyzing the mechanism of impedance measurement distortion caused by transient additional terms; using the least square method to linearly fit the nonlinear line transient equivalent impedance model and extracting the fitting fault characteristics; constructing a distance protection criterion based on the fitting fault characteristics to complete the fault distance discrimination and the distinction between internal and external faults. The method solves the problem that the distance protection of the mixed connection station cannot act correctly; effectively prevents the measurement distortion, misoperation and refusal of the distance protection of the transmission line under the hybrid new energy grid connection.

Description

Technical Field

[0001] This invention relates to the field of relay protection for power transmission lines, specifically to an improved distance protection method for hybrid renewable energy power stations based on a unified reference system and transient linear impedance modeling. Background Technology

[0002] With the continuous advancement of the construction of new power systems, high-proportion renewable energy power plants are increasingly exhibiting mixed grid connection of various types of power plants. While improving the decarbonization level and power supply flexibility of the power system, the fundamental differences in the control modes and phase reference benchmarks of the two types of units have led to problems such as distortion in the transient characteristic analysis of the transmission system and inaccurate operation of traditional distance protection. During grid fault transient periods, the phase-locked loop of grid-connected units is prone to instability and severe phase disturbances, while the virtual phase of grid-connected units is stable, but the currents of the two cannot be directly superimposed, resulting in distortion of the line transient impedance measurement and exposing the transmission line distance protection to the risk of maloperation and failure to operate. Based on the transient characteristics of mixed renewable energy power plants, a unified reference system is constructed and transient impedance linearization modeling is performed, and then a distance protection improvement scheme with strong adaptability is constructed for fault transient scenarios.

[0003] In response to the phenomenon that distance protection cannot operate correctly, one method is to construct a direction criterion by using the correlation coefficient between voltage and memory voltage, as provided in reference [1]: Yu Chen, MinghaoWen, Liexiang Hu, et al. Fault direction identification for wind power integration system[J]. The Journal of Engineering, 2019, 2019(16): 2520-2524. However, this method requires additional calculation of the equivalent impedance of the new energy power source, which increases the calculation difficulty, and its applicability needs further verification.

[0004] Another approach is to analyze the dual-loop control structure of the grid-type power supply, obtain the characteristic changes at the moment of fault occurrence, and then extract the mapping relationship between the voltage transient frequency characteristics and the fault point to construct a new distance protection scheme adapted to phase-to-phase short circuits. However, this approach does not clearly analyze the impact of the grid-type power supply on traditional distance protection, nor does it explore the adaptability in hybrid substations, as recorded in reference [2]: Song Haoran, Wu Tonghua, Gao Houlei, et al. New distance protection for grid-type inverter power supply transmission lines with voltage and current inner loops [J]. High Voltage Engineering, 2025. Summary of the Invention

[0005] To address the issue of inaccurate operation of traditional distance protection caused by different phase reference standards for renewable energy units with varying fault transient impedances, this invention provides an improved distance protection method for hybrid renewable energy power plants based on a unified reference system and transient linear impedance modeling. This method analyzes and unifies the phase characteristics of the two types of power plants, utilizes the unified reference system to eliminate phase mixing, performs least-squares linear fitting on the transient impedance, and finally constructs the protection criterion based on the characteristics of the fitted function. This solves the problem of incorrect distance protection operation in hybrid power plants and effectively prevents the risks of measurement distortion, false tripping, and failure to trip of distance protection on transmission lines under hybrid renewable energy grid connection.

[0006] The technical solution adopted in this invention is as follows: An improved distance protection method for hybrid renewable energy power stations based on a unified reference frame and transient linear impedance modeling includes the following steps: Step 1: Analyze the fault transient characteristics and phase reference differences between grid-connected and non-grid-connected units to clarify the core issue that the currents of the two types of units cannot be directly algebraically superimposed; Step 2: Establish a unified reference system based on the network-type virtual phase, and complete the coordinate conversion of the current and the synthesis of the station current; Step 3: Establish a transient equivalent impedance model for the line and analyze the mechanism by which transient additional terms cause impedance measurement distortion; Step 4: Use the least squares method to perform linear fitting on the nonlinear line transient equivalent impedance model and extract the fitted fault features; Step 5: Construct distance protection criteria based on fitted fault features to complete fault distance discrimination and differentiation between faults inside and outside the zone.

[0007] In step 1, for both grid-connected and grid-connected power plants included in a hybrid renewable energy power plant, the phase reference of the fault current differs fundamentally due to the influence of their control systems. Since the received current is unprocessed, its phase will deviate from the true value. If the reference systems are inconsistent, the protection device will struggle to determine the change in the power angle, potentially leading to misjudgments of the system status. For grid-connected GFM units, their transient power angle It satisfies the following formula: ; In the formula: To match the phase of the terminal voltage of the grid-connected GFM generator unit; The phase of the PCC reference voltage at the grid connection point.

[0008] If the protection device uses the local reference frame of the grid-connected GFL unit as a reference, then at this time It will be expressed as follows: ; In the formula: The transient power angle of the grid-connected GFM unit is taken as the reference reference. The reference phase provided for the system's phase-locked loop.

[0009] Because phase-locked loops (PLLs) can oscillate or even lose lock during fault transients. and Significant discrepancies exist between these factors, leading to errors in the calculation of the power angle, which in turn affects the correct operation of the distance protection element.

[0010] In step 2, in the grid-connected GFL unit control loop, the phase angle of the generator terminal voltage is... It is a dynamically changing value with oscillations, and its characteristics are affected by the voltage at the grid connection point (PCC), the PLL parameter settings, etc. When a system fault occurs, the PLL may fail to track the grid phase, leading to instability. The generator terminal voltage phase angle... Uncertainty exists: ; In the formula: The phase angle before the fault; ζ represents the system phase error; ζ represents the damping ratio. The rated frequency; The frequency of the damped oscillation; The relevant characteristics of a grid-connected GFM unit are determined by the virtual damping in its control loop. D and inertia J The decision offers greater controllability. The rotor characteristics of a simulated synchronous generator in a grid-connected GFM unit are shown in the following equation: ; In the formula: , These are VSG analog mechanical torque and electromagnetic torque, respectively. D , J For VSG virtual damping and inertia; and The actual value and rated value of the virtual rotational speed.

[0011] Selecting a grid-type GFM unit As a unified benchmark reference system. The current is dynamically phase-transformed based on the following equation: ; In the formula: , For grid-connected GFL units that are converted using the grid-connected GFM phase as the reference frame d, q Axis current components; To determine the phase difference between grid-connected GFL units and the unified reference system; , For grid-connected GFL units, this is the situation before conversion. d, q Axis current components; at this time The output current of both grid-connected and grid-structured substations can be calculated using the unified reference system of the grid-structured GFM. At this time, the output current of the transmission lines... d, q Axial current can be expressed as the sum of the two current phasors: ; In the formula: , These are the d-axis and q-axis components of the total current on the line after phase transformation. , These are network-type GFM units d, q Axial current component.

[0012] In step 3, considering the transient process, the resistance-inductance characteristics of the line can be described by differential equations. The transient equivalent impedance during the transient period is defined. The quotient of the instantaneous voltage difference and the current is given by the following formula: (7); In equation (7): , For measuring point voltage d, q Axial components; , for of d, q Axial components.

[0013] This is the resistance value of the circuit itself; The imaginary unit; The system angular frequency is 2πf. This is the system frequency, typically 50Hz; This is the inductance value of the circuit itself; This is a transient additional term for the transient equivalent impedance.

[0014] Transient Additional Items The occurrence of transient equivalent impedance No longer a set impedance value: it was fixed at a certain point before the fault occurred; however, after the transient phase of the fault, it is subject to transient additional terms. The influence causes the impedance trajectory to no longer be a fixed value, but to change differently depending on the location of the fault, and to return to the fixed point before the fault after the transient process ends.

[0015] In step 4, based on the least squares method, equation (7) is linearly fitted to obtain... Functional expressions of the form a、b The approximate expression is: (13); In equation (13): , These represent the slope and intercept of the linear function obtained through linear fitting. This is the resistance value of the circuit itself; This is the inductance value of the circuit itself; The q-axis component of the current when no fault occurs; This represents the q-axis component of the current after a fault occurs. The system angular frequency is 2πf. This is the system frequency, typically 50Hz.

[0016] According to Equation (7), the transient equivalent impedance is a fixed value before the fault occurs, but changes during the transient phase after the fault occurs. After fitting it, the fitting function should have the following characteristics: when the fault does not occur, i.e., t=0, the value of b is a fixed value, but after the fault occurs, the values ​​of a and b will change differently with different fault distances, and thus the fitting fault characteristics can be extracted.

[0017] In step 5, when no fault occurs, the intercept... The set impedance value for the line; after a fault occurs, its value will change accordingly with the fault location, and can reflect the fault location to a certain extent; while the slope This is positively correlated with the fault distance; the closer the fault location is to the measurement point, the more severe the fault process, resulting in a greater slope. As the fault point moves further away from the protection installation location... The smaller the value.

[0018] The result can be obtained by fitting a function using equation (13) based on the data obtained from real-time monitoring. 、 The protection criterion is constructed based on the value, and the protection criterion is expressed as equation (14): (14); In equation (14): The slope value is obtained by function fitting under normal system operation, and it is usually close to 0. The set impedance amplitude for the line is generally taken as 80% to 85% of the line impedance; this invention takes 85%. a , b The fitting function is obtained by calculating the data collected in real time.

[0019] In the protection claimed by this invention The value was obtained by fitting the three-phase ground fault condition with a transition resistance of 50Ω. a The value is used as the protection threshold, as shown in equation (15): (15); In equation (15): To protect the reliability factor, the present invention uses 0.95. This is the fitted value when a three-phase ground fault occurs at the end of the protection range via a 50Ω transition resistor.

[0020] This invention presents an improved distance protection method for hybrid renewable energy power stations based on a unified reference frame and transient linear impedance modeling. The technical advantages are as follows: 1) This invention constructs a unified reference system based on the analysis, eliminates the distortion problem in transient electrical quantity analysis caused by the phase difference between the two types of units, and greatly improves the accuracy of transient calculation and fault analysis of hybrid new energy systems.

[0021] 2) This invention achieves transient impedance linearization fitting through the least squares method, accurately extracts fault characteristics, and solves the problems of measurement distortion, false operation and failure to operate caused by transient additional terms in traditional distance protection from the root cause.

[0022] 3) The improved distance protection method proposed in this invention has a clear principle and intuitive criteria, which can quickly and accurately identify the fault type and location, adapt to high-proportion hybrid new energy grid connection scenarios, and has high protection reliability and is easy to implement in engineering.

[0023] 4) This method has a clear principle and strong adaptability, and can effectively improve the distance protection performance of the transmission lines of hybrid new energy power plants, providing a guarantee for the reliable operation of relay protection in new power systems. Attached Figure Description

[0024] The present invention will be further described below with reference to the accompanying drawings and examples; Figure 1(a) is a schematic diagram of the power angle characteristics of a normal system before a fault occurs; Figure 1(b) is a schematic diagram of the power angle characteristics of a normal system after a fault occurs.

[0025] Figure 2(a) is a power angle curve when a fault occurs in the transmission line of a grid-connected unit; Figure 2(b) is a power angle curve when a fault occurs in the transmission line of a grid-type unit.

[0026] Figure 3 This is a schematic diagram showing the characteristics of electrical components in different reference frames.

[0027] Figure 4(a) is Change diagram (faults within the area); Figure 4(b) is Change diagram (outside the area fault).

[0028] Figure 5(a) shows the simulation of the current value when a fault occurs 1 km from the measurement point; Figure 5(b) shows the simulation of the current value when a fault occurs 50km from the measurement point; Figure 5(c) is a simulation diagram of the voltage value when a fault occurs 1 km from the measurement point; Figure 5(d) shows the simulated voltage value when a fault occurs 50km from the measurement point; in: .

[0029] Figure 6 Send out system topology diagrams for new energy power plants.

[0030] Figure 7(a) shows the simulation of the transient current value when a fault occurs at F1; Figure 7(b) is a simulation diagram of the transient current value when a fault occurs 1 km from the protection measurement point; Figure 7(c) shows the simulation of the transient current value when a fault occurs at F3; Figure 7(d) is a simulation diagram of the transient current value when a fault occurs outside the positive region.

[0031] Figure 8(a) is a simulation diagram of the current value when a phase A ground fault occurs without phase conversion; Figure 8(b) is a simulation diagram of the current value after phase conversion when a phase A ground fault occurs; Figure 8(c) is a simulation diagram of the phase angle difference before and after phase conversion when a phase A ground fault occurs.

[0032] Figure 9(a) shows the calculated values ​​and simulation waveforms of the dq axis components before phase conversion; Figure 9(b) shows the calculated values ​​and simulation waveforms of the dq axis components after phase conversion; in: .

[0033] Figure 10(a) shows the simulation of the current value when a fault occurs at F3; Figure 10(b) shows the situation at fault F3 and BC. Simulation diagram; Figure 10(c) is a simulation diagram of the current value when a fault occurs at F1; Figure 10(d) shows the situation at F1 when BCG fails. Simulation diagram; Figure 10(e) is a schematic diagram of the fitting function and action boundary when the BC fault occurs at F3; Figure 10(f) is a schematic diagram of the fitting function and action boundary when a BCG fault occurs at F1. Detailed Implementation

[0034] The transmission line distance protection strategy based on a unified phase reference and linear fitting of transient impedance specifically includes the following steps: Step 1: Transient characteristic analysis of hybrid power stations: Since hybrid renewable energy power plants include both grid-connected and grid-connected power plants, their fault current phase references differ fundamentally due to the influence of their control loops. GFL power plants can be equivalent to controlled current sources, and their fault response is highly dependent on the grid connection point voltage and their own control loop. Depending on the degree of voltage drop, when the voltage drop is deep, a low-voltage ride-through control mode is required, and the power supply aims to provide reactive power support. At this time, the inner loop current control dominates, and its transient current exhibits second-order system response characteristics, which can be expressed as shown in formula (1): (1); In formula (1): and The current command value is set according to the power grid regulations; , , , and and These are coefficients related to the PI parameters of the inner current loop, the filter inductance, and the instantaneous state of the fault.

[0035] After the fault occurred, the output current of the GFL station... dq The shaft component needs to provide reactive power due to the low-voltage ride-through requirement, therefore, during the fault period, the output current of the station... q The axial component increases. d The axial component decreases.

[0036] GFM power stations are typically considered equivalent to controlled voltage sources, possessing virtual inertia and virtual damping characteristics. Due to the characteristics of their inverters, the output current amplitude is limited during faults, but their transient processes can still be considered equivalent to voltage sources. After a grid fault, a virtual impedance element is usually introduced to suppress overcurrent and simultaneously meet the requirements for low-voltage ride-through. After adding the virtual impedance, the expression for the GFM current after a fault can be derived as shown in formula (2): (2); In formula (2): , This represents the transient current amplitude. The attenuation coefficient; , The initial phase angle; , Reference values ​​set for low voltage ride-through of GFM units.

[0037] The transient quantity analysis of the above two control modes of the unit uses the relevant parameters of its own control loop as the reference phase reference. However, in the hybrid system considered in this patent, since the reference phases of the two types of units are different, the output transient current of the new energy power station under the two control modes is not the actual output value of each unit in the actual multi-station grid-connected system. Therefore, the protection strategy constructed based on this may fail to operate or maloperate in some cases. If the protection receives an unprocessed current, its phase will deviate from the true value. Let the actual current be as shown in formula (3): (3); The protection device receives the following: (4); This will cause the calculated impedance to differ from the actual measured impedance by a phase factor. ,Right now (5); In this situation, the operation of the protection system will be directly affected. Furthermore, in multi-source systems, the power angle is typically defined as the phase difference between the internal potential of the power source and the system reference voltage. If the reference systems are not unified, the protection device will have difficulty determining the changes in the power angle, potentially leading to misjudgments of the system state. Taking a GFM unit as an example, its transient power angle... Satisfying formula (6): (6); In the formula: For the phase of the GFM unit; This refers to the phase of the reference voltage at the grid connection point.

[0038] If the protection device uses the local reference frame of the GFL unit as a reference, then at this time It will be expressed as shown in formula (7): (7); In the formula: The reference phase provided for the system's phase-locked loop.

[0039] As can be seen from the above equation, the phase-locked loop (PLL) experiences oscillations or even loss of lock-on during fault transients. and Significant discrepancies exist between these factors, leading to errors in the calculation of the power angle, which in turn affects the correct operation of the distance protection element.

[0040] Considering the additional impedance and the effect of phase inconsistency, the resultant phase of the current is thus affected. This can lead to distortion of the impedance trajectory, causing the protection system to malfunction. The amplitude and phase of the measured impedance are affected by the composite phase. When the reference frame changes drastically due to inconsistency, the additional impedance will change significantly, which in turn will affect the trajectory of the measured impedance on the RX plane, resulting in irregular jitter, drift, or even erroneous entry / exit from the operating zone, leading to a decrease in protection performance. Step 2: Determining the unified reference frame: The key to achieving a unified system reference frame lies in determining what constitutes the reference frame. In both control modes, the phase tracking equation for the grid-connected unit can be expressed by equation (8): (8); In the formula: The phase generated for the internal control loop; ω is the angular velocity.

[0041] In the GFL control loop, By dynamically determining the phase-locked loop (PLL), when a system fault occurs and the system voltage changes, the PLL tracking equation represents a typical second-order system response: (9); In the formula: The phase angle before the fault; ζ represents the system phase error; ζ represents the damping ratio. The rated frequency; is the damped oscillation frequency.

[0042] During the fault It is a dynamically changing value with oscillations, and its characteristics are affected by the voltage at the PCC point of grid connection and the PLL parameter settings. Therefore, during a fault, the PLL may fail to track the grid phase, leading to instability. Uncertainty exists. As shown in Figures 1(a) and 1(b) before and after the fault, the power angle characteristics are as follows: the system-side and station-side voltages are respectively... and Measure voltage and current respectively and The system power angle is After a line fault occurs, the change in power angle is related to system impedance, measured voltage, and current. Before the fault, the power angles on both sides of the system are relatively small. However, after the fault, the measured voltage and current are further affected by the new energy control strategy, leading to changes in the measured impedance, which causes the power angle to increase and exceed the applicable range of normal distance protection.

[0043] In contrast, the characteristics of GFM units are determined by parameters within their own control loop, offering greater controllability. (The phase of a grid-connected unit...) Virtual damping in the control loop D and inertia JThe decision was made that, even during the fault, the oscillations of the GFL unit would not occur as typically in other GFL units, demonstrating better controllability and stability. Comparing the phase reference characteristics of the units under the two control modes, the dynamic range and rate of change of the output phase of the GFM unit are much smaller than those of the GFL unit. As shown in Figures 2(a) and 2(b), after the fault occurs, comparing the power angle change curves under the two control modes reveals that the grid-type unit exhibits superior power angle stability.

[0044] Figure 3 This demonstrates how the reference frame relies on a local phase-locked loop to track the grid-connected bus voltage. Figure 3 In sub-diagram (a), the system is in synchronous steady state. The inverter phase-locked loop of the GFL unit can effectively track the system voltage. The GFM current vector, the GFL current vector and the voltage vector at the measurement point maintain a normal relative phase relationship, and the phase of the synthesized total current vector is stable. Figure 3 In sub-diagram (b), the fault causes a voltage drop and phase jump, leading to instability in the dynamic response of the GFL phase-locked loop. At this time, The phase and amplitude of the current change drastically. This disturbance directly causes the phase shift of the line fault current to no longer simply reflect the characteristics of the fault circuit, but is mixed with the dynamic interference of the control system. Figure 3 Subgraphs (c) and (d) show the dynamic changes of the GFL current with the GFM as the reference phase. Under this reference system, the phase changes no longer change drastically, and the amplitude is not significantly affected by the current limiting strategy. Since the stability of the grid-type unit maintains the stability of the voltage and the system power angle, even if the phase of the line current shifts to a certain extent due to the fault, the impact is greatly reduced compared to the traditional reference system. This allows the line fault current to more accurately reflect the fault characteristics.

[0045] Step 3: Definition and function fitting of transient impedance: Traditional distance protection is based on the impedance concept under power frequency steady state. The operating voltage is usually used. To measure the electrical distance between the protection installation point and the fault point, among which , It is usually set as the total line impedance. 80%~85%. Considering transient processes, the resistance-inductance characteristics of the line can be described by differential equations. After establishing a phase reference system, the Park variation is used to... I m Transform to synchronous rotating coordinate system ( dq Under the reference single-phase RL series circuit model, the voltage-current relationship from the protection installation point to the protection critical point can be expressed as: (10); Based on this, the equivalent impedance of line protection during transient periods is defined. The quotient of instantaneous voltage difference and current: (11); Substituting equation (10) into equation (11), we can derive equation (12): (12); In equation (12): , For measuring point voltage d, q Axial components; , for of d, q Axial components.

[0046] Transient equivalent impedance From steady-state impedance And a time-varying transient additional term Composition. According to equation (12), its trajectory is fixed at a certain point before the fault occurs; while in the transient phase after the fault occurs, it is affected by... The influence of this causes the impedance trajectory to no longer be a fixed value, but to change differently depending on the location of the fault, and to return to the fixed point before the fault after the transient process ends. Meanwhile, as shown in Figures 4(a) and 4(b), the impedance offset trajectory caused by the transient additional term is not the same depending on the location of the fault. In the case of a forward fault, This increases the transient impedance, while the opposite occurs during reverse faults, providing a basis for constructing protection schemes.

[0047] Furthermore, as analyzed above, impedance exhibits time-varying characteristics, making it easily identifiable as a nonlinear variable, thus difficult to quantify precisely. Considering that when a fault has not occurred... To protect the set impedance, this fault generates... It can reflect the location of the fault to a certain extent. Based on this, in order to conduct a more digital analysis... Regarding the analysis of fault distance, we consider using the least squares method to linearize equation (12) into a function fit, so that the transient impedance change and time maintain a linear relationship in form, and obtain a simplified mathematical model of its change trend.

[0048] Step 4: Based on the function fitting results, construct the protection criterion: Based on step 3, a linear fit is performed on the transient impedance to obtain... Function expressions of the form. Because of the existence , Parameters, considering that the phase of the two units can be unified by determining a unified reference frame, are used here. Linear fitting is performed on the magnitude. Within a very short time after the fault occurs, it is assumed... , As a constant, we can obtain a、b The approximate expression is: (13); For the fitted Combining equation (13) and the relationship between fault location and fault current, it can be derived that when the fault has not occurred, the intercept... b The line impedance is set to a specific value. After a fault occurs, compared to the line impedance during normal operation, the fault current amplitude increases. However, due to the voltage support from the grid-connected generating units, the voltage does not change instantaneously. Therefore, the value of the fault impedance will change accordingly with the fault location, reflecting the fault location to a certain extent. The slope... a This is positively correlated with the fault distance; the closer the fault location is to the measurement point, the more severe the fault process, resulting in a greater slope. As the fault point moves further away from the protection installation location... a The smaller the value, the better. Figures 5(a) to 5(d) show the transient changes in current and voltage when a fault occurs at different locations.

[0049] Based on the above analysis, it can be seen that when a fault occurs at different locations in the system, the amplitude and degree of change of the fault electrical components are not entirely the same. The proposed method using slope... a and intercept b All of them can correspond well to the presented features, therefore they can be fitted by a linear fitting function. Determine the fault location. Record the fitted function obtained during normal system operation. The slope in The slope is close to 0; the intercept during normal operation is denoted as... The numerical equivalent is the line impedance. .

[0050] The above function can be fitted based on the data obtained from real-time monitoring. a、b The construction of protection criteria for the value is based on the fact that... b The value can reflect the location of the fault, making it consistent with... The difference can reflect both forward and reverse faults, while a The value is significantly affected by transient processes and shows a clear trend with increasing fault distance. The protection criterion is: (14); In the formula: The slope value is obtained by function fitting under normal system operation, and it is usually close to 0. The set impedance amplitude for the line is generally taken as 80% to 85% of the line impedance; in this invention, 85% is used.a , b The fitting function is obtained by calculating the data collected in real time.

[0051] In the protection claimed by this invention Considering that three-phase short-circuit faults are the most severe type of fault in power systems, with significant transient processes and distinct electrical characteristics, the slope obtained by fitting under this condition can effectively represent the fault duration within the protected area. a The theoretical minimum value is used as the basis for setting, resulting in high reliability. However, considering that traditional distance protection generally has a weak tolerance to transition resistance, and may still fail to operate even with small transition resistance, this patent selects the value obtained by fitting the three-phase ground fault condition with a 50Ω transition resistance. a The value is used as the protection threshold, as shown in equation (15): (15); In the formula: To protect the reliability factor, we take 0.95. This is the fitted value when a three-phase ground fault occurs at the end of the protection range via a 50Ω transition resistor.

[0052] If the calculation result satisfies equation (14), it can be determined that the fault is located within the protected area and the protection needs to be activated; if one of the conditions is not met, it can be said that the protection is outside the protected area and the protection will not be activated.

[0053] From Figures 5(a) to 5(d), it is evident that the current on the line approaches a stable value within 20ms, and the transition time from the pre-fault stage to the post-fault stable stage in Figures 4-6 is very short, reaching steady state without requiring a full cycle. Furthermore, the fitting object primarily involves transient processes of impedance changes, and since this chapter employs dynamic phase conversion, only calculations are needed. dq Since the axis component does not require the window filling time of the Fourier algorithm, the entire protection action time is within 15ms.

[0054] Example verification: To verify the effectiveness of the proposed distance protection scheme, simulations were conducted for different fault locations and fault types. Figure 6 A hybrid renewable energy grid-connection model was established in PSCAD / EMTDC to verify the performance of the proposed protection system. The rated capacity of each power station is 100MW, the voltage level of the transmitting line is 220kV, and the length is 100km. Line parameters, power station main transformer parameters, and renewable energy parameters are detailed in Table 1. Fault location is set at... Figure 6The substation-side exit, internal exits at 75km, 50km, and 25km from the substation, and the system-side exit of the transmission line are denoted as F1, F2, F3, F4, and F5, respectively. The fault occurrence time is 2.5 seconds. The fault types are set as A-phase grounding (AG), BC two-phase short circuit (BC), BC two-phase short circuit to ground (BCG), and ABC three-phase short circuit (ABCG). Based on the line parameters and the previous analysis, the following can be obtained: The modulus of the protection setting impedance is 34.75. This is obtained through function fitting based on data from normal line operation. The value is -0.0661, while the reference value is the value when a three-phase fault occurs at the end of the protection. The value is -6.649, calculated using equation (15). The value is -6.31655. Considering other possible influencing factors, this invention takes... It is -6.32.

[0055]

[0056] Transient current characteristics and phase conversion verification: The transient current characteristics were verified using different line lengths at the distance measurement points as examples, as shown in Figures 7(a) to 7(d). From Figures 7(a) to 7(d), it can be clearly observed that when a three-phase ground fault occurs on the back side (F1), the transient current exhibits characteristics of the opposite power source (i.e., the large system), showing a larger amplitude. However, when a fault occurs in the forward direction with increasing distance, the fault current shows a gradually decreasing amplitude, consistent with the previous analysis, as shown in Figures 7(b), 7(c), and 7(d).

[0057] The feasibility of the protection criterion was verified in Figures 7(a) to 7(d). The magnitude of the current change during the fault period can distinguish the fault location to a certain extent. Furthermore, Figures 8(a) to 8(c) show the comparison of the A-phase current waveforms before and after phase conversion and the phase angle difference between the two, respectively. Figure 8(a) clearly shows that there is a significant phase angle difference between the two before and after the fault. The zero-crossing points and peak points of the two stations are misaligned. If the currents of the two are directly added together as the line current, phase confusion is highly likely to occur, leading to protection failure. However, after using the method proposed in this patent with the GFM generator phase as the reference phase, the current waveforms of the two are clearly synchronized, as shown in Figure 8(b), which solves the potential phase confusion problem. Figure 8(c) further reflects the phase angle difference between the GFM and GFL units before processing. The difference existed before the fault and increased further after the fault, indicating that even if the same phase angle is set in the control loop of the unit as before the fault, a difference will still occur after the fault.

[0058] Figures 9(a) and 9(b) show the phase transformation before and after. dqSimulated and calculated values ​​of the shaft components, based on the low-voltage ride-through strategy and the parameters of this patented system, show that after the fault... d , q The shaft current values ​​should be 52.5A and 210.62A. As shown in Figure 9(a), the electromagnetic simulation waveform and the calculated waveform basically coincide, verifying the correctness of the analysis of the fault transient current. The current waveform after unified reference system transformation is shown in Figure 9(b). By comparing the current curves in Figures 9(a) and 9(b), it can be intuitively seen that the GFL output current before and after transformation is... d, q The significant differences in the axial components further illustrate the necessity of performing dynamic phase conversion.

[0059] Verification of protection operation performance under different fault locations and types: Considering that different types of faults may occur at different locations in the transmission line, in order to verify whether the proposed protection can cope with various operating conditions, different types of faults were set at F1 to F5. Table 2 shows the operation of the protection.

[0060]

[0061] As shown in Table 2, the relevant judgment values ​​obtained when different types of faults occur inside and outside the protected area have significant distinguishability. For faults outside the protected area at locations F1 and F5... a The values ​​all meet the criteria. For F1, a The values ​​are all greater than And for b The value can further enhance the judgment of fault location, since the fault is outside the reverse zone. b The value should be less than the line's set impedance, hence the negative value. However, for point F5... a Value greater than The requirements are met. Furthermore, for faults within the protection zone, it can still operate accurately even near the boundary of the protection area. This verifies that the protection scheme proposed in this chapter can handle different types of faults in different locations.

[0062] To visually observe the performance of the protection system under different types of faults at different locations, we selected a BC phase-to-phase fault at F3 within the protection zone and a BCG phase-to-phase grounding fault at F1 outside the protection zone as examples. The changes in each component are shown in Figures 10(a) to 10(f). Figures 10(a) and 10(c) show that the farther the fault point is from the protection installation location, the smaller the measured current amplitude and the less drastic the transient phase changes. The calculated... Consistent with the previous analysis, the post-fault trends differ in different fault zones. The protection operation is shown in Figures 10(e) and 10(f). A linear function is plotted based on the calculated slope and intercept. The slope is within the set criteria, and the impedance satisfies the rule that faults within the zone are greater than a set value, while faults outside the zone are less than a set value. This verifies that the proposed protection can handle different fault types at different locations. Note that the horizontal axis in Figures 10(e) and 10(f) is not the actual fault time, but rather a representation of the fault duration. a , b The relationship between the two values ​​and the threshold needs to be plotted using a sequential timeline. Fault diagnosis is completed within 15ms of the action time.

Claims

1. An improved distance protection method for hybrid renewable energy power stations based on a unified reference frame and transient linear impedance modeling, characterized in that... Includes the following steps: Step 1: Analyze the transient fault characteristics and phase reference differences between grid-connected and grid-connected units; Step 2: Establish a unified reference system based on the network-type virtual phase, and complete the coordinate conversion of the current and the synthesis of the station current; Step 3: Establish a transient equivalent impedance model for the line and analyze the mechanism by which transient additional terms cause impedance measurement distortion; Step 4: Use the least squares method to perform linear fitting on the nonlinear line transient equivalent impedance model and extract the fitted fault features; Step 5: Construct distance protection criteria based on fitted fault features to complete fault distance discrimination and differentiation between faults inside and outside the zone.

2. The improved distance protection method for hybrid renewable energy power stations based on a unified reference frame and transient linear impedance modeling as described in claim 1, characterized in that: In step 1, for grid-connected GFM units, their transient power angle It satisfies the following equation: ； In the formula: To match the phase of the terminal voltage of the grid-connected GFM generator unit; The phase of the PCC reference voltage at the grid connection point; If the protection device uses the local reference frame of the grid-connected GFL unit as a reference, then at this time It will be expressed as follows: ； In the formula: The transient power angle of the grid-connected GFM unit is taken as the reference reference. The reference phase provided for the system's phase-locked loop; Because phase-locked loops (PLLs) can oscillate or even lose lock during fault transients. and The discrepancy between these factors leads to errors in the calculation of the power angle, which in turn affects the correct operation of the distance protection element.

3. The improved distance protection method for hybrid renewable energy power stations based on a unified reference frame and transient linear impedance modeling as described in claim 2, characterized in that: In step 2, in the grid-connected GFL unit control loop, the phase angle of the generator terminal voltage is... It is a dynamically changing value with oscillations, and its characteristics are affected by the voltage at the grid connection point (PCC) and the PLL parameter settings. When a system fault occurs, the PLL cannot track the grid phase, leading to instability, and the generator terminal voltage phase angle... Uncertainty exists: ； In the formula: The phase angle before the fault; ζ represents the system phase error; ζ represents the damping ratio. The rated frequency; is the damped oscillation frequency.

4. The improved distance protection method for hybrid renewable energy power stations based on a unified reference frame and transient linear impedance modeling as described in claim 3, characterized in that: The relevant characteristics of a grid-connected GFM unit are determined by the virtual damping in its control loop. D and inertia J The rotor characteristics of the simulated synchronous generator for the grid-connected GFM unit are determined as follows: ； In the formula: , These represent VSG simulations of mechanical torque and electromagnetic torque, respectively. D , J For VSG virtual damping and inertia; and The actual value and rated value of the virtual rotational speed.

5. The improved distance protection method for hybrid renewable energy power stations based on a unified reference frame and transient linear impedance modeling as described in claim 4, characterized in that: Selecting a grid-type GFM unit As a unified benchmark reference system; by The current is dynamically phase-transformed based on the following equation: ； In the formula: , For grid-connected GFL units that are converted using the grid-connected GFM phase as the reference frame d, q Axis current components; To determine the phase difference between grid-connected GFL units and the unified reference system; , For grid-connected GFL units, this is the situation before conversion. d, q Axis current components; at this time The output current of both grid-connected and grid-structured substations will be calculated according to the unified reference system of the grid-structured GFM; at this time, the output current of the transmission lines... d, q Axial current is expressed as the sum of the two currents in direct phasor form: ； In the formula: , These are the d-axis and q-axis components of the total current on the line after phase transformation. , These are network-type GFM units d, q Axial current component.

6. The improved distance protection method for hybrid renewable energy power stations based on a unified reference frame and transient linear impedance modeling as described in claim 5, characterized in that: In step 3, considering the transient process, the resistance-inductance characteristics of the line can be described by differential equations; the transient equivalent impedance during the transient period is defined. The quotient of the instantaneous voltage difference and the current is given by the following formula: （7）； In equation (7): , For measuring point voltage d, q Axial components; , for of d, q Axial components; This is the resistance value of the circuit itself; The imaginary unit; The system angular frequency, For system frequency; This is the inductance value of the circuit itself; This is a transient additional term for the transient equivalent impedance.

7. The improved distance protection method for hybrid renewable energy power stations based on a unified reference frame and transient linear impedance modeling as described in claim 6, characterized in that: Transient Additional Items This leads to transient equivalent impedance No longer a set impedance value: it was fixed at a certain point before the fault occurred; however, after the transient phase of the fault, it is subject to transient additional terms. The influence causes the impedance trajectory to no longer be a fixed value, but to change differently depending on the location of the fault, and to return to the fixed point before the fault after the transient process ends.

8. The improved distance protection method for hybrid renewable energy power stations based on a unified reference frame and transient linear impedance modeling as described in claim 7, characterized in that: In step 4, based on the least squares method, equation (7) is linearly fitted to obtain... Functional expressions of the form a、b The approximate expression is: （13）； In equation (13): , These represent the slope and intercept of the linear function obtained through linear fitting. This is the resistance value of the circuit itself; This is the inductance value of the circuit itself; The q-axis component of the current when no fault occurs; This represents the q-axis component of the current after a fault occurs. The system angular frequency, This refers to the system frequency.

9. The improved distance protection method for hybrid renewable energy power stations based on a unified reference frame and transient linear impedance modeling as described in claim 8, characterized in that: According to Equation (7), the transient equivalent impedance is a fixed value before the fault occurs, but changes during the transient phase after the fault occurs. After fitting it, the fitting function should have the following characteristics: when the fault does not occur, i.e., t=0, the value of b is a fixed value, but after the fault occurs, the values ​​of a and b will change differently with the different fault distances, so as to extract the fitting fault characteristics.

10. The improved distance protection method for hybrid renewable energy power stations based on a unified reference frame and transient linear impedance modeling as described in claim 9, characterized in that: In step 5, when no fault occurs, the intercept... Set the impedance value for the line; After a fault occurs, its value changes accordingly with the location of the fault, and can reflect the location of the fault to a certain extent; while the slope This is positively correlated with the fault distance; the closer the fault location is to the measurement point, the more severe the fault process, resulting in a greater slope. As the fault point moves further away from the protection installation location... The smaller the value; The result can be obtained by fitting a function using equation (13) based on the data obtained from real-time monitoring. 、 The protection criterion is constructed based on the value, and the protection criterion is expressed as equation (14): （14）； In equation (14): The slope value is obtained by function fitting under normal system operation; Set the impedance amplitude for the line; a , b The fitting function is obtained by calculating the real-time acquired data. The value was obtained by fitting the three-phase ground fault condition with a transition resistance of 50Ω. a The value is used as the protection threshold, as shown in equation (15): （15）； In equation (15): To protect the reliability coefficient; This is the fitted value when a three-phase ground fault occurs at the end of the protection range via a 50Ω transition resistor.

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