Earthing and protection element amplitude-phase compensation method for embedded AC-DC hybrid system

Abstract

The invention discloses a grounding and protection element amplitude-phase compensation method oriented to an embedded alternating current-direct current hybrid system. The method comprises the following steps: constructing a fault equivalent model of the embedded alternating current-direct current hybrid system; considering control characteristics of a direct current system, and analyzing weak feedback and phase controlled characteristics of a fault component close to a rectification side; in combination with a positive sequence voltage polarization type distance protection criterion, protection refusal action causes under different fault types are analyzed; amplitude-phase compensation schemes are provided for grounding and inter-phase protection elements respectively. According to the method, amplitude-phase compensation schemes are provided for grounding and inter-phase protection elements, it can be ensured that the protection elements can act correctly under different fault conditions, and the protection adaptability is remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of AC / DC hybrid system protection technology, specifically to a method for amplitude and phase compensation of grounding and protection elements for embedded AC / DC hybrid systems. Background Technology

[0002] With the large-scale integration of new energy sources, such as wind and solar power, into provincial power grids, the volatility of new energy output and the growth in electricity supply demand pose challenges to power transmission and absorption capabilities. Therefore, proposing solutions to enhance the transmission capacity of key sections of provincial power grids that are both economically viable and feasible has become a research focus. Compared to AC systems, DC systems offer greater transmission capacity and faster, more flexible power control. Converting reserved AC channels into DC systems, forming embedded DC systems where both the sending and receiving ends are within the same regional power grid, is an effective solution for improving the power transmission and absorption capabilities of provincial power grids and represents a future development trend. Taking the Yangzhen DC project, the first embedded DC project in China, as an example, after rigorous feasibility verification and comprehensive consideration, the project ultimately adopted self-adaptive statcom and line commutation converter (SLCC) technology, employing a symmetrical unipolar topology. This design scheme, based on precise control of the converter valves, effectively supports the commutation voltage, significantly reducing the risk of commutation failure on the inverter side. However, SLCC technology inherits the single-phase power transmission characteristics of line commutated converter (LCC) technology, which means that in AC / DC hybrid systems, if a fault occurs in the AC system near the rectifier side, the fault component has obvious weak feedback characteristics, and its phase is significantly affected by the DC system control process, causing the protection to fail to operate.

[0003] Existing research on the protection of AC / DC hybrid systems can be categorized according to the type of DC transmission system. Currently, the most widely used in engineering are DC transmission systems based on LCC and Modular Multilevel Converter (MMC) technologies. The core of MMC system relay protection research lies in deeply exploring the unique fault processes of MMC and developing new protection principles and algorithms that can overcome the limitations of traditional protection and adapt to the transient characteristics of DC systems. LCC system relay protection research is similar, but focuses more on the impact of inverter-side commutation failure on AC system protection, as well as the commutation failure mechanism and detection methods, while neglecting potential problems on the rectifier side. Therefore, there is a lack of in-depth analysis of the fault characteristics near the rectifier side in embedded AC / DC hybrid systems, and there is an urgent need to research adaptive improvement schemes for traditional protection technologies. Summary of the Invention

[0004] To improve the adaptability of near-rectifier side distance protection in embedded AC / DC hybrid systems, this invention proposes a method for amplitude and phase compensation of grounding and protection components in such systems. Based on an equivalent fault model of the embedded AC / DC hybrid system, this method theoretically analyzes the weak feed and phase-controlled characteristics of near-rectifier side fault components. Furthermore, by combining positive-sequence voltage polarization distance protection criteria, it explores the causes of protection failure under different fault types. Referring to actual engineering conditions and general system fault sequence network analysis results, amplitude and phase compensation schemes are proposed for both grounding and phase-to-phase protection components. Simulation results show that this method can ensure correct operation of protection components under different fault conditions, significantly improving the adaptability of the protection.

[0005] The technical solution adopted in this invention is as follows: A method for amplitude and phase compensation of grounding and protection components in an embedded AC / DC hybrid system includes the following steps: Step 1: Construct an equivalent fault model for an embedded AC / DC hybrid system; Step 2: Considering the control characteristics of the DC system, analyze the weak feed and phase-controlled characteristics of the fault component near the rectifier side; Step 3: Combine the positive sequence voltage polarization distance protection criteria to analyze the causes of protection failure to operate under different fault types; Step 4: Propose amplitude and phase compensation schemes for grounding and phase-to-phase protection components respectively.

[0006] In step 1, when a fault occurs in the AC system near the rectifier side, the converter bus voltage decreases, and the DC system responds rapidly. The DC system can be equivalently represented as a current source controlled by the converter bus voltage. The equivalent fault model of the embedded AC / DC hybrid system is as follows: Figure 1 As shown: in: Z s Equivalent impedance of AC system; Z c This is the equivalent impedance of the AC filter and reactive power compensation device; Z fm , Z fn These are the line impedances from the fault point to bus M and bus N, respectively; bus M is a regular bus, and bus N is a converter bus. R f For transition resistance; This is the equivalent current for a DC system fault. For inflow Z c The current; This refers to the fault current flowing into the converter bus N; This is the fault voltage of the converter bus.

[0007] like Figure 5 As shown, the core of the embedded AC / DC hybrid system lies in deeply embedding the DC transmission system into a regional AC power grid, making it a backbone power channel. This system mainly consists of a sending / receiving end AC system and an embedded DC system. The embedded DC system includes core components such as sending / receiving end converter stations, DC lines, and control systems.

[0008] In terms of connectivity: the sending-end converter station obtains AC power from the sending-end AC system connected to the energy base, and transmits it through DC lines after rectification; the receiving-end converter station inverts the DC power into AC power and injects it into the receiving-end AC system at the load center. Throughout the process, both ends are tightly coupled in the same AC power grid through converter buses, realizing deep interaction and joint operation of AC and DC power. While this architecture greatly improves transmission capacity and power flow control flexibility, it also leads to strong coupling and mutual influence between AC and DC systems, making system stability, fault propagation mechanisms, and coordinated control strategies more complex.

[0009] like Figure 1 As shown, the left side of the fault equivalent model of the embedded AC / DC hybrid system is the AC system side, represented by a Thevenin equivalent power source that represents the internal potential of the system. and its internal impedance Z s The system is configured in series; the right side of the model represents the DC system side. Influenced by the DC system control method, its current is mainly controlled by the converter bus voltage, and therefore it is equivalent to a controlled current. The filter and reactive power compensation components are equivalent to... Z c The model's center section represents an AC power transmission system. R f For transition resistance, Z fm , Z fn These are the line impedances from the fault point to bus M and N, respectively, which together represent the fault state of the AC system.

[0010] In terms of connectivity: it depicts the process where the sending-end converter station obtains AC power from the sending-end AC system connected to the energy base, rectifies it, and then transmits it through DC lines. The receiving-end converter station's process of inverting DC power into AC power and injecting it into the receiving-end AC system at the load center is not the focus of this patent and therefore is not shown in the diagram. Physically, Z fm , Z fn , R fAll three systems are connected to the same electrical node, which is the theoretical fault point. A voltage drop at this node simultaneously affects both sides. The current flowing through this node is the superposition of the fault current provided by the AC system and the equivalent current injected by the DC system. This model clearly reveals how the AC system and the embedded DC system are strongly coupled through this common connection point under fault conditions, providing the theoretical basis for analyzing dynamic processes such as the interaction of fault currents.

[0011] In step 2, when the AC system near the rectifier side fails, the DC voltage... U d and current I d The relationship is shown in equation (1), which respectively represent current deviation control, low voltage current limiting control and minimum current control: (1); In formula (1): I d = k 1 U d + b 1 indicates current deviation control; I d = k 2 U d + b 2 indicates low voltage current limiting control; I d = I dmin Indicates minimum current control; I d Indicates direct current; U d Represents the DC voltage (d). I dmin This is the minimum current command value allowed for the safe and stable operation of a DC system. k 1. k 2 represents the rate or proportion of change of current command with DC voltage in current deviation control and low voltage current limiting control, respectively; b 1. b 2 represents the constant terms introduced in current deviation control and low voltage current limiting control to ensure the continuity of the current command curve at the segmentation point; U d1 The starting threshold for current deviation control; U d2 The starting threshold for low-voltage current limiting control; U d3 This is the start-up threshold for minimum current control.

[0012] Taking a three-phase fault as an example, for a 12-pulse converter, the DC voltage is expressed as: (2); In formula (2): This represents the effective value of the converter bus voltage. This is the firing angle on the rectifier side; This represents the commutation reactance of each phase on the rectifier side; This represents the average value of the DC current.

[0013] By combining equations (1) and (2), we can obtain: (3); It is evident that as the three-phase fault becomes more severe, the converter bus voltage... Gradually decrease, The active power gradually decreases. Ignoring the active power loss of the converter, the active power injected into the converter by the AC system is equal to the power transmitted by the DC system, as shown in equation (4): (4); In equation (4): φ for and The included angle, where: This is the equivalent current for a DC system fault. This refers to the fault voltage of the converter bus. Equivalent current for DC system faults Valid value.

[0014] visible, φ and U LL_rms They are inversely correlated. Therefore, during a fault... The phase is significantly affected by the DC system control method, and Usually smaller, Depend on It is dominant, but also exhibits phase-controlled characteristics. Specifically: like Figure 8 As shown, where, This represents the equivalent fault current in the DC system prior to the fault. This refers to the fault voltage of the converter bus before the fault occurred. This represents the equivalent fault current of the DC system before the fault. Before the fault, since the reactive power compensation of the DC system usually adopts a full compensation method, the current during normal operation is... and The phases are the same. The reactive power consumed on the rectifier side accounts for approximately 60% of the DC system's reactive power, that is... Lag The phase is approximately 30°. After the fault, the converter bus fault voltage... The decrease, at the same time Zc The amplitude is large, resulting in The amplitude compared to Smaller. Therefore, in synthesis The time is mainly composed of Dominant, both are in close phase, and Its own phase-controlled characteristics are then transmitted to Up, so It also exhibits phase-controlled characteristics.

[0015] Furthermore, because the SLCC technology used in DC systems has single-phase power transmission characteristics, therefore The amplitude exhibits weak feedback characteristics. Specifically: like Figure 9 As shown, where, This is the effective value of the fault current flowing into the converter bus N after the fault. Figure 9 This demonstrates the changing trend of the effective values ​​of the three-phase currents after a phase-A ground fault. Because the SLCC technology used in the DC system has single-phase power transmission characteristics, the DC system struggles to provide fault current to the fault point during a fault. Therefore, the amplitude of the phase-A fault current does not increase significantly due to the fault; instead, it exhibits weak feedback characteristics. The amplitude exhibits weak feedback characteristics.

[0016] like Figure 9 As shown, for the non-faulty phases, because the equivalent positive and negative sequence impedances of the DC system are not the same, the shunt coefficients of the positive and negative sequence currents on both sides of the line are not equal, resulting in a significant increase in the amplitude of the current in the non-faulty phases. This is in... Figure 9 This is reflected in the amplitude variation trend of the fault current in phases B and C.

[0017] For the fault voltage component, neglecting line impedance, the converter bus voltage after the fault is expressed as: (5); In equation (5): This is the equivalent voltage on the grid side; Z s The equivalent impedance of the system; Z c This is the equivalent impedance of the AC filter and reactive power compensation device; R f For transition resistance; It is evident that the converter bus voltage phase lags significantly after the fault. The fault phase current can either lead or lag the fault phase voltage, primarily depending on the power transmission and reactive power compensation after the fault; details are as follows: For example, during a three-phase fault, the DC transmission power is significantly reduced, and the reactive power consumed is also reduced. The AC system absorbs the excess reactive power for compensation. Lagging behind During a single-phase fault, the DC transmission power will not decrease significantly, and the AC system will still supply reactive power to the DC system. ahead of ;because Depend on The fault phase current is dominant, therefore it can either lead or lag the bus voltage.

[0018] In step 3, the grounding and phase-to-phase discrimination elements of the positive sequence voltage polarization distance protection are shown in equations (6) and (7), respectively: (6); (7); In the above formula: This is the zero-sequence current compensation coefficient; It is the zero-sequence current; The set impedance; , These are phase voltage and phase current, respectively, with subscripts. Phases A, B, and C can be selected; This represents the positive sequence component of the phase voltage. , These are line voltage and line current, respectively. This represents the positive sequence component of the line voltage.

[0019] Taking a three-phase fault as an example, the operation of the grounding protection and phase-to-phase protection components is shown in Figure 3. Among them, , , This refers to the voltage of the faulty phase. , , This refers to the fault phase current. , , This is the operating voltage. It is evident that phase control renders the original phase compensation mechanism ineffective, and the weak feed effect causes the current component to lose its dominant role in synthesizing the operating voltage. Ultimately, the synthesized operating voltage is dominated by the fault voltage and cannot be compared with the reference positive-sequence phase voltage. Reference positive sequence line voltage In the same phase, the subscript Φ indicates phases A, B, and C, which ultimately leads to the protection failing to operate.

[0020] In step 4, the reason for the protection failure to operate is that the phase and amplitude of the fault current are controlled. Therefore, amplitude and phase compensation is performed to make the original protection mechanism take effect again. Considering that the amplitude and phase of the fault component can be directly measured, compensation schemes are proposed for the grounding and phase-to-phase protection elements respectively. The overall operation process is as follows: Figure 4 As shown, S4.1: After the protection and detection device detects a fault, each grounding and phase-to-phase protection element is activated, and voltage and current data are collected in real time; S4.2: Compensate for the fault phase current identified by each grounding protection element. If the fault phase voltage leads the phase current, then compensate for the phase of the fault phase current. φ set +Δ φ 1. Otherwise, phase compensation for the fault phase current - φ set -Δ φ 1. Simultaneously multiply the fault phase current. K 1×( I N / | i ( t )|), multiply the identified non-faulty phase current I N / | i ( t )|, to achieve amplitude compensation.

[0021] S4.3: Compensate for the fault phase current identified by each phase-to-phase protection element, and reduce the fault phase current. Phase compensation is applied to the midpoint of the voltage phases of the two identified faulty phases. φ m The current of another faulty phase Phase compensation to Reverse phase; for protection elements reflecting phase-to-phase faults of phases AB, Φ1 and Φ2 are A and B respectively; for protection elements reflecting phase-to-phase faults of phases BC, Φ1 and Φ2 are B and C respectively; for protection elements reflecting phase-to-phase faults of phases CA, Φ1 and Φ2 are C and A respectively. Simultaneously multiply the fault phase current K 2×( I N / | i ( t In addition, the voltage of the identified non-faulty phases will be compensated to match... φ m Inverted phase.

[0022] S4.4: Determine whether a zero-sequence component exists using the fault components before compensation. If not, set the zero-sequence component input to the grounding protection element to 0; otherwise, calculate the zero-sequence component using the fault components after compensation. Input each fault component after compensation into the protection device to determine whether a fault has occurred.

[0023] in: φ set Line impedance angle K 1. K 2 represents the amplitude compensation coefficient; The phase difference between the fault phase current and the fault phase voltage as identified by the grounding protection element; The midpoint of the phase between the fault phase current and the fault phase voltage as identified by the phase-to-phase protection element. φ m The phase difference between them; I N Rated current; i ( t () represents the fault phase current.

[0024] like Figure 4 As shown, the overall operation flowchart includes the following steps: Step 1: The real-time monitoring system collects voltage and current data for each phase and calculates the phase if the fault start-up conditions are met. Step 2: For grounding protection components, perform phase compensation on the identified fault phase current and amplitude compensation on the identified fault phase current and non-fault phase current. Then, use the data before compensation to determine whether zero-sequence compensation is necessary. Step 3: For phase-to-phase protection components, perform amplitude and phase compensation on the current of the identified faulty phase. Also, adjust the phase of the voltage of the non-faulty phases based on the voltage of the identified faulty phase. Step 4: Input the compensated fault data into the protection device to determine whether a fault has occurred.

[0025] This invention provides a method for amplitude and phase compensation of grounding and protection components in embedded AC / DC hybrid systems, with the following technical advantages: 1) Based on the fault equivalent model of the embedded AC / DC hybrid system, this invention analyzes the weak feed and phase-controlled characteristics of the fault component near the rectifier side, laying a theoretical foundation for protection adaptability analysis.

[0026] 2) This invention analyzes the adaptability of positive sequence voltage polarization distance protection based on fault characteristics, and points out that the fundamental reason for its failure to operate is that the phase control causes the original protection mechanism to fail, and the weak feed characteristic causes the current component to lose its dominant role when synthesizing the working voltage.

[0027] 3) This invention combines the results of general system fault sequence network analysis with actual engineering conditions, and proposes amplitude and phase compensation schemes for grounding and phase-to-phase protection elements respectively, which significantly improves the adaptability of protection. Attached Figure Description

[0028] The present invention will be further described below with reference to the accompanying drawings and examples; Figure 1 This is a fault equivalent model diagram for an embedded AC / DC hybrid system.

[0029] Figure 2 This is a schematic diagram of the fault control characteristics of a DC system.

[0030] Figure 3(a) Phasor diagram of positive sequence voltage polarization distance protection during three-phase fault (grounding protection element operation). Figure 3(b) Phasor diagram of positive sequence voltage polarization distance protection during three-phase fault (phase-to-phase protection element operation).

[0031] Figure 4 This is a flowchart for compensation of grounding and phase-to-phase protection components.

[0032] Figure 5 This is a topology diagram of an embedded AC / DC hybrid system.

[0033] Figure 6 This is the protection phasor diagram after amplitude-phase compensation for a single-phase ground fault.

[0034] Figure 7(a) shows the operation of the grounding protection element for a single-phase ground fault (phase A protection element). Figure 7(b) shows the operation of the grounding protection element for a single-phase ground fault (phase B protection element); Figure 7(c) shows the operation of the grounding protection element for a single-phase ground fault (C-phase protection element).

[0035] Figure 8 This is a schematic diagram illustrating the effect of phase control on the synthesis of fault components.

[0036] Figure 9 This is a schematic diagram showing the current variation trends of faulty and non-faulty lines under the influence of DC control. Detailed Implementation

[0037] To address the problem of malfunction of positive-sequence voltage polarized distance protection caused by the distorted fault characteristics near the rectifier side in embedded AC / DC hybrid systems, this invention proposes a method for amplitude and phase compensation of grounding and protection components in embedded AC / DC hybrid systems. This method is based on an equivalent fault model of embedded AC / DC hybrid systems, considering the control characteristics of the DC system, and theoretically analyzes the weak feed and phase-controlled characteristics of the fault component near the rectifier side. Furthermore, combining the positive-sequence voltage polarized distance protection criterion, it analyzes the protection operation under different fault types, pointing out that the fundamental reason for protection malfunction lies in the lack of the original phase compensation mechanism, causing the fault current to lose its dominant role in synthesizing the operating voltage. Considering practical engineering conditions and referring to the results of general system fault sequence network analysis, amplitude and phase compensation schemes are proposed for grounding and phase-to-phase protection components respectively. Simulation results show that the fault characteristics near the rectifier side under different fault conditions are consistent with the theoretical analysis, and the proposed adaptive compensation scheme can ensure that the protection components can operate correctly under various fault conditions, demonstrating significant application value. Specifically, the method includes the following steps: S1: When a fault occurs in the AC system near the rectifier side, the converter bus voltage drops, and the DC control system responds rapidly. The DC system can be equivalently represented as a current source controlled by the converter bus voltage. The equivalent fault model for an embedded AC / DC hybrid system is as follows: Figure 1 As shown.

[0038] in: Z s The equivalent impedance of the system; Z c This is the equivalent impedance of the AC filter and reactive power compensation device; Z fm , Z fn These are the line impedances from the fault point to bus M and N, respectively; R f For transition resistance; This is the equivalent current for a DC system fault. For inflow Z c The current; This refers to the fault current flowing into bus N; This is the fault voltage of the converter bus.

[0039] S2: DC system fault control characteristics, such as Figure 2 As shown, for ease of analysis, the current deviation control is linearly equivalent. Since the minimum firing angle control operates in scenarios with very small voltage drops (i.e., large transition resistance), and distance protection primarily addresses metallic fault identification, this type of fault is not discussed. Therefore, when a fault occurs in the AC system near the rectifier side, the DC voltage... U d and current I d The relationship can be expressed as equation (1).

[0040] (1); in: k 1. b 1. k 2. b 2. U d1 , U d2 , U d3 , I dmin These are all control parameters for DC systems.

[0041] Taking a three-phase fault as an example, for a 12-pulse converter, the DC voltage can be expressed as: (2); in: U LL_rms This represents the effective value of the converter bus voltage. α This is the firing angle on the rectifier side; X rThis represents the commutation reactance of each phase on the rectifier side; I d This represents the average value of the DC current.

[0042] Combining (1) and (2), we can obtain: (3); It is evident that as the fault worsens, the converter bus voltage... U LL_rms Gradually decrease, U d / U LL_rms The active power is gradually reduced. Ignoring the active power loss of the converter, the active power injected into the converter by the AC system is equal to the power transmitted by the DC system, as shown in equation (4).

[0043] (4); in: φ for and The included angle.

[0044] visible, φ and U LL_rms They are inversely correlated, and during a fault... The phase is significantly affected by the DC system control method, and Usually smaller, Depend on The dominant characteristic is phase control. Furthermore, because the SLCC technology used in DC systems has single-phase power transmission characteristics, therefore... The amplitude exhibits weak feedback characteristics. For the non-faulty phase, since the equivalent positive and negative sequence impedances of the DC system are not the same, the shunt coefficients of the positive and negative sequence currents on both sides of the line are not equal, resulting in a significant increase in its amplitude.

[0045] For the fault voltage component, neglecting line impedance, the converter bus voltage after the fault can be expressed as: (5); in: This is the equivalent voltage on the grid side.

[0046] It is evident that the converter bus voltage phase lags significantly after a fault. The fault phase current can either lead or lag behind the fault phase voltage, depending primarily on the power transmission and reactive power compensation conditions after the fault.

[0047] S3: The grounding and phase-to-phase discrimination elements of the positive sequence voltage polarization distance protection are shown in Equation (6) and Equation (7), respectively.

[0048] (6); (7); Wherein: the subscript Φ represents the three phases A, B, and C.

[0049] Taking a three-phase fault as an example, the operation of the grounding protection and phase-to-phase protection elements is shown in Figures 3(a) and 3(b). Among them, , , This refers to the voltage of the faulty phase. , , This refers to the fault phase current. , , This is the operating voltage. It can be seen that the fault phase current of the protection device on the N side of the input bus is... φ Under the influence of angle, the phase shifts significantly, and simultaneously under the influence of weak feed, and The inability to control the synthesis of the working voltage results in the working voltage being almost in phase with the fault voltage, but out of phase with the reference positive sequence voltage, causing the protection to fail to operate. This indicates that phase control renders the original phase compensation mechanism ineffective, and the weak feed effect causes the current component to lose its dominant role in synthesizing the working voltage.

[0050] S4: The root cause of protection failure lies in the phase control and amplitude limitation of the fault current. Therefore, amplitude and phase compensation is required to reactivate the original protection mechanism. Considering that the amplitude and phase of the fault component can be directly measured, and referring to the general system fault sequence network analysis results and relevant engineering experience, the following compensation schemes are proposed for grounding and phase-to-phase protection components. The overall operation process is as follows: Figure 4 As shown, where, The phase difference between the fault phase current and the fault phase voltage as identified by the grounding protection element; The midpoint of the phase between the fault phase current and the fault phase voltage as identified by the phase-to-phase protection element. φ m The phase difference between them.

[0051] (1) After the protection and detection device detects the fault, each grounding and phase-to-phase protection element is activated and voltage and current data are collected in real time; (2) Compensate for the fault phase current identified by each grounding protection element. If the fault phase voltage leads the phase current, then compensate for the phase of the fault phase current. φ set +Δ φ 1. Otherwise, phase compensation for the fault phase current - φ set -Δ φ 1. Simultaneously multiply the fault phase current. K 1×( I N / |i ( t )|), multiply the identified non-faulty phase current I N / | i ( t )|, to achieve amplitude compensation.

[0052] (3) Compensate for the fault phase current identified by each phase protection element, and reduce the fault phase current. Phase compensation is applied to the midpoint of the voltage phases of the two identified faulty phases. φ m The current of another faulty phase Phase compensation to Inverse phase, the subscripts Φ1 and Φ2 can be taken as A, B, B, C, and C, A respectively, while multiplying by the fault phase current. K 2×( I N / | i ( t In addition, the voltage of the identified non-faulty phases will be compensated to match... φ m Inverted phase.

[0053] (4) Use the fault components before compensation to determine whether there is a zero-sequence component. If there is no zero-sequence component, set the zero-sequence component of the input grounding protection element to 0. Otherwise, calculate the zero-sequence component based on the fault components after compensation. Input each fault component after compensation into the protection device to determine whether a fault has occurred.

[0054] in: φ set The line impedance angle; K 1. K 2 represents the amplitude compensation coefficient; I N Rated current; i ( t () represents the fault phase current.

[0055] Example Analysis: Verification of the positive-sequence voltage polarization distance protection correction scheme was conducted, and a system was built in power system simulation software, such as... Figure 5 The simulation model of the embedded AC / DC hybrid system is shown. DCF and ACF represent the DC filter system and AC filter system, respectively, and SVG is the static var generator. The system has a rated voltage of ±200kV, a rated current of 3kA, and a rated transmission capacity of 1200MW.

[0056] 1. Verification of the amplitude and phase shift characteristics of the fault component: Analysis shows that after a fault in the AC system near the rectifier side, the phase of the fault current is affected by the DC system control strategy, resulting in a certain shift. However, the single-phase power transmission characteristics of the DC system itself limit the amplitude of the fault current, exhibiting weak feedback characteristics, ultimately leading to protection failure. The effective value of the converter bus neutral line voltage during normal operation is used as a reference. U LL_rms and the effective value of the current flowing into the converter bus I rms Based on this, we will explore different voltage drop conditions. and included angle φ And the change in the amplitude of the fault phase current. Taking a three-phase fault (A, B, C) as an example, since the voltage drop of the three phases is almost the same in a symmetrical fault, the resulting phase shift is approximately the same. Calculate the phase A... φ a Angle and RMS value of fault current As shown in Table 1.

[0057]

[0058] It is evident that as the severity of the fault gradually increases, for the faulty phase, and The included angle φ It also gradually increases, exhibiting strong phase-controlled characteristics. Furthermore, under the influence of single-phase power transmission characteristics, the fault phase current exhibits weak feedback characteristics. Even under conditions of high transition resistance, the fault current supplied by the opposite AC system increases to some extent due to the shunt effect, but it does not increase by a factor of several.

[0059] 2. Verification of the effectiveness of the protection scheme: Taking a phase A ground fault as an example, the effect of the amplitude phase compensation scheme proposed in this invention is demonstrated. The protection phasor diagram is as follows: Figure 6 As shown, by Figure 6 It can be seen that the proposed amplitude-phase compensation scheme restores the characteristics of the sudden increase in fault phase current while limiting the abnormal growth of non-fault phase current, thus restoring the compensation effect of zero-sequence current. Simultaneously, the adjustment of its phase also reactivates the original phase compensation mechanism. Under the combined compensation of amplitude and phase, the operating voltage of the fault phase is almost out of phase with the fault phase voltage, ensuring correct protection operation.

[0060] The operational status is shown in Figures 7(a) to 7(c). As can be seen from Figures 7(a) to 7(c), after amplitude-phase compensation, the phase ratio result corresponding to the fault quickly enters the operating zone, while the phase ratio result corresponding to the non-fault is basically consistent with the non-fault condition and does not enter the operating zone. Furthermore, numerous simulation results show that the phase ratio result calculated for the faulty phase under all fault types can enter the operating zone, while the phase ratio result calculated for the non-faulty phase fails to enter the operating zone, thus meeting the requirements for distance protection elements to identify the faulty phase.

[0061] It is evident that amplitude and phase compensation restores the fault characteristics of the fault phase current and voltage under fault conditions, enabling the original phase compensation mechanism to take effect again. The increase in the amplitude of the fault component also allows it to play a dominant role again in the synthesis of the working voltage, ensuring that the working voltage is in phase with the reference positive sequence voltage and that the protection operates correctly. Furthermore, numerous simulation results demonstrate that the proposed scheme has strong adaptability to different fault types. The operation of each phase-to-ground and phase-to-phase protection element meets the requirements of relay protection. For example, in the case of a two-phase-to-ground fault (A and B), the ground protection element and phase-to-phase protection element corresponding to the fault can accurately identify the fault and operate, while the protection element not corresponding to the fault does not operate.

Claims

1. A method for amplitude and phase compensation of grounding and protection components in an embedded AC / DC hybrid system, characterized in that... Includes the following steps: Step 1: Construct an equivalent fault model for an embedded AC / DC hybrid system; Step 2: Considering the control characteristics of the DC system, analyze the weak feed and phase-controlled characteristics of the fault component near the rectifier side; Step 3: Combine the positive sequence voltage polarization distance protection criteria to analyze the causes of protection failure to operate under different fault types; Step 4: Propose amplitude and phase compensation schemes for grounding and phase-to-phase protection components respectively.

2. The amplitude and phase compensation method for grounding and protection elements in an embedded AC / DC hybrid system according to claim 1, characterized in that: The embedded AC / DC hybrid system consists of a sending / receiving end AC system and an embedded DC system. The embedded DC system includes a sending / receiving end converter station, a DC line, and a control system. In terms of connectivity: the sending-end converter station obtains AC power from the sending-end AC system connected to the energy base, and transmits it through DC lines after rectification; the receiving-end converter station inverts DC power into AC power and injects it into the receiving-end AC system of the load center; throughout the process, both ends are closely coupled in the same AC power grid through the converter bus, realizing deep interaction and joint operation of AC and DC power.

3. The amplitude and phase compensation method for grounding and protection elements in an embedded AC / DC hybrid system according to claim 2, characterized in that: In step 1, the left side of the fault equivalent model of the embedded AC / DC hybrid system is the AC system side, represented by a Thevenin equivalent power source that represents the potential within the system. and its internal impedance Z s Composed of series connection; The right side of the fault equivalent model for an embedded AC / DC hybrid system represents the DC system side, whose current is controlled by the converter bus voltage and is therefore equivalent to a controlled current. The filter and reactive power compensation components are equivalent to... Z c ; The fault equivalent model of the embedded AC / DC hybrid system has the AC transmission system in the middle. R f For transition resistance, Z fm , Z fn These are the line impedances from the fault point to bus M and N, respectively, which together represent the fault status of the AC system. Z fm , Z fn , R f All three are connected to the same electrical node, which is the theoretical fault point. The voltage drop at this node affects both sides simultaneously. The current flowing through this node is the superposition of the fault current provided by the AC system and the equivalent current injected by the DC system.

4. The amplitude and phase compensation method for grounding and protection elements in an embedded AC / DC hybrid system according to claim 3, characterized in that: In step 2, when the AC system near the rectifier side fails, the DC voltage... U d and current I d The relationship is shown in equation (1), which respectively represent current deviation control, low voltage current limiting control and minimum current control: （1）； In formula (1): I d = k 1 U d + b 1 indicates current deviation control; I d = k 2 U d + b 2 indicates low voltage current limiting control; I d = I dmin Indicates minimum current control; I d Indicates direct current; U d Represents the DC voltage (d). I dmin This is the minimum current command value allowed for the safe and stable operation of a DC system. k 1. k 2 represents the rate or proportion of change of current command with DC voltage in current deviation control and low voltage current limiting control, respectively; b 1. b 2 represents the constant terms introduced in current deviation control and low voltage current limiting control to ensure the continuity of the current command curve at the segmentation point; U d1 The starting threshold for current deviation control; U d2 The starting threshold for low-voltage current limiting control; U d3 This is the start-up threshold for minimum current control.

5. The amplitude and phase compensation method for grounding and protection elements in an embedded AC / DC hybrid system according to claim 4, characterized in that: For a three-phase fault, the DC voltage is expressed as: （2）； In formula (2): This represents the effective value of the converter bus voltage. This is the firing angle on the rectifier side; This represents the commutation reactance of each phase on the rectifier side; This represents the average value of the DC current. By combining equations (1) and (2), we can obtain: （3）； It is evident that as the three-phase fault becomes more severe, the converter bus voltage... Gradually decrease, Gradually decrease; neglecting the active power loss of the converter, the active power injected into the converter by the AC system is equal to the power transmitted by the DC system, as shown in equation (4): （4）； In equation (4): φ for and The included angle, where: This is the equivalent current for a DC system fault. This refers to the fault voltage of the converter bus. Equivalent current for DC system faults The effective value; visible, φ and U LL_rms They are inversely correlated; therefore, during a fault... The phase is significantly affected by the DC system control method, and Usually smaller, Depend on It is dominant, but also exhibits phase-controlled characteristics.

6. The amplitude and phase compensation method for grounding and protection elements in an embedded AC / DC hybrid system according to claim 5, characterized in that: set up This represents the equivalent fault current in the DC system prior to the fault. This refers to the fault voltage of the converter bus before the fault occurred. This represents the equivalent fault current of the DC system before the fault. Before the fault, since the reactive power compensation of the DC system usually adopts a full compensation method, the current during normal operation is... and The phases are the same; the reactive power consumed on the rectifier side accounts for about 60% of the DC system, that is... Lag The phase is approximately 30°; after the fault, the converter bus fault voltage The decrease, at the same time Z c The amplitude is large, resulting in The amplitude compared to Smaller; therefore in synthesis The time is mainly composed of Dominant, both are in close phase, and Its own phase-controlled characteristics are then transmitted to Up, so It also exhibits phase-controlled characteristics; furthermore, because the SLCC technology used in DC systems has single-phase power transmission characteristics, therefore The amplitude exhibits weak feedback characteristics; For the non-faulty phase, since the equivalent positive and negative sequence impedances of the DC system are not the same, the positive and negative sequence current shunting coefficients on both sides of the line are not equal, resulting in a significant increase in the amplitude of the non-faulty phase current.

7. The amplitude and phase compensation method for grounding and protection elements in an embedded AC / DC hybrid system according to claim 6, characterized in that: For the fault voltage component, neglecting line impedance, the converter bus voltage after the fault is expressed as: （5）； In equation (5): This is the equivalent voltage on the grid side; Z s The equivalent impedance of the system; Z c This is the equivalent impedance of the AC filter and reactive power compensation device; R f For transition resistance; It is evident that the converter bus voltage phase lags significantly after a fault; while the fault phase current either leads or lags the fault phase voltage, primarily depending on the power transmission and reactive power compensation after the fault; details are as follows: During a three-phase fault, the DC transmission power is significantly reduced, and the reactive power consumed is also reduced; the AC system absorbs the excess reactive power for compensation. Lagging behind During a single-phase fault, the DC transmission power will not decrease significantly, and the AC system will still supply reactive power to the DC system. ahead of ;because Depend on The fault phase current is dominant, therefore it can either lead or lag the bus voltage.

8. The amplitude and phase compensation method for grounding and protection elements in an embedded AC / DC hybrid system according to claim 7, characterized in that: In step 3, the grounding and phase-to-phase discrimination elements of the positive sequence voltage polarization distance protection are shown in equations (6) and (7), respectively: （6）； （7）； In the above formula: This is the zero-sequence current compensation coefficient; It is the zero-sequence current; For setting impedance; , These are phase voltage and phase current, respectively, with subscripts. Phases A, B, and C can be selected; This represents the positive sequence component of the phase voltage. , These are line voltage and line current, respectively. This represents the positive sequence component of the line voltage.

9. The amplitude and phase compensation method for grounding and protection elements in an embedded AC / DC hybrid system according to claim 8, characterized in that: In the case of a three-phase fault, the operation of the grounding protection and phase-to-phase protection components... , , This refers to the voltage of the faulty phase. , , This refers to the fault phase current. , , The operating voltage is shown to be [value]. It is evident that phase control renders the original phase compensation mechanism ineffective, and the weak feed effect causes the current component to lose its dominant role in synthesizing the operating voltage. Ultimately, the synthesized operating voltage is dominated by the fault voltage and cannot be compared with the reference positive-sequence phase voltage. Reference positive sequence line voltage In the same phase, the subscript Φ indicates phases A, B, and C, which ultimately leads to the protection failing to operate.

10. The amplitude and phase compensation method for grounding and protection elements in an embedded AC / DC hybrid system according to claim 9, characterized in that: In step 4, the reason for the protection failure is that the phase of the fault current is controlled and the amplitude is limited. Therefore, amplitude and phase compensation is performed to make the original protection mechanism take effect again. Considering that the amplitude and phase of the fault component can be directly measured, compensation schemes are proposed for grounding and phase-to-phase protection components respectively. S4.1: After the protection and detection device detects a fault, each grounding and phase-to-phase protection element is activated, and voltage and current data are collected in real time; S4.2: Compensate for the fault phase current identified by each grounding protection element. If the fault phase voltage leads the phase current, then compensate for the phase of the fault phase current. φ set +Δ φ 1. Otherwise, phase compensation for the fault phase current - φ set -Δ φ 1; Simultaneously multiply the fault phase current K 1×( I N / | i ( t )|), multiply the identified non-faulty phase current I N / | i ( t To achieve amplitude compensation; S4.3: Compensate for the fault phase current identified by each phase-to-phase protection element, and reduce the fault phase current. Phase compensation is applied to the midpoint of the voltage phases of the two identified faulty phases. φ m The current of another faulty phase Phase compensation to Inverted; For protection elements that reflect phase-to-phase faults in phases AB, Φ1 and Φ2 are A and B respectively; for protection elements that reflect phase-to-phase faults in phases BC, Φ1 and Φ2 are B and C respectively; for protection elements that reflect phase-to-phase faults in phases CA, Φ1 and Φ2 are C and A respectively. Simultaneously multiply the fault phase current K 2×( I N / | i ( t In addition, the voltage of the identified non-faulty phases will be compensated to match... φ m Inverted; S4.4: Use the fault component before compensation to determine whether there is a zero-sequence component. If it does not exist, set the zero-sequence component of the input grounding protection element to 0. Otherwise, calculate the zero-sequence component based on the fault component after compensation. The compensated fault components are input into the protection device to determine whether a fault has occurred. in: φ set Line impedance angle K 1. K 2 represents the amplitude compensation coefficient; The phase difference between the fault phase current and the fault phase voltage as identified by the grounding protection element; The midpoint of the phase between the fault phase current and the fault phase voltage as identified by the phase-to-phase protection element. φ m The phase difference between them; I N Rated current; i ( t () represents the fault phase current.

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