A power quality monitoring method for active distribution network based on worst point analysis
By using a worst-case analysis-based approach, combined with impedance analysis and active distribution network characteristics, the configuration of power quality monitoring devices is optimized. This solves the problems of large number of monitoring devices, large data volume, and high cost in traditional methods, and achieves low-cost and efficient power quality monitoring.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- XI AN JIAOTONG UNIV
- Filing Date
- 2023-03-31
- Publication Date
- 2026-07-10
AI Technical Summary
Traditional power quality monitoring methods require the installation of numerous monitoring devices throughout the power distribution network, resulting in massive amounts of data, high costs, and making large-scale application difficult.
A low-data-dependency active distribution network power quality monitoring method based on worst-case analysis is adopted. By combining impedance analysis and active distribution network characteristics, the worst-case power quality point is determined and the power quality monitoring device is optimized.
It significantly reduces the number of monitoring devices, requiring only two devices to be installed, thus reducing data requirements, facilitating engineering implementation, and improving the feasibility of the monitoring system.
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Figure CN116365709B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of active distribution network power quality detection technology, specifically involving a low-data-dependency active distribution network power quality monitoring method based on worst-case analysis. Background Technology
[0002] With the increasing penetration of distributed energy resources, traditional distribution networks with unidirectional power flow have gradually evolved into active distribution networks with bidirectional power flow. Active distribution networks subsequently generate complex and diverse power quality issues, posing new challenges to the safe and reliable operation of the power grid. Therefore, power quality monitoring of distribution networks is of significant practical importance for the rational, efficient, and high-quality regulation and operation of active distribution networks. However, traditional power quality monitoring methods generally require the installation of numerous monitoring devices throughout the distribution network to acquire massive amounts of data for power quality analysis. This results in complex sensor and communication device installation processes, huge data transmission volumes, and extremely high costs, which is highly detrimental to the large-scale application of monitoring systems. Summary of the Invention
[0003] In order to overcome the problems existing in the prior art, the purpose of this invention is to provide a low data-dependent active distribution network power quality monitoring method based on worst-case analysis. This method can significantly reduce the number of monitoring devices for active distribution network power quality monitoring, and the configuration process only requires the grid topology, with little need for accurate data such as grid impedance, and is easy to promote in engineering.
[0004] The technical solution adopted in this invention is:
[0005] A low-data-dependent active distribution network power quality monitoring method was proposed. By combining impedance analysis with the inherent characteristics of the active distribution network, the distribution characteristics of the worst power quality points when a single distributed source or multiple distributed sources are connected were analyzed.
[0006] An active power quality monitoring method for distribution networks based on worst-case analysis, comprising the following steps:
[0007] S1. Based on the characteristics of the active distribution network, analyze and determine the main power quality problems of the active distribution network;
[0008] S2. Based on the main power quality indicators, combined with impedance modeling methods, perform worst-case analysis when a single distributed power source is connected.
[0009] S3. Based on the worst-case analysis results when a single distributed power source is connected, and combined with the characteristics of the active distribution network, perform the worst-case analysis when multiple distributed power sources are connected.
[0010] S4. Based on the worst-case analysis results, optimize the configuration of the power quality monitoring device.
[0011] In step S1, the active distribution network refers to a distribution network that comprehensively controls distributed energy sources, namely distributed power sources, energy storage devices, and flexible loads.
[0012] Considering the degree of impact on users and the importance attached to domestic and international standards, an analysis is conducted on the power quality indicators in Table 1. The definitions of each indicator and the relevant standard thresholds are also attached to Table 1:
[0013] Table 1. Key Power Quality Indicators and Their Thresholds
[0014]
[0015] The thresholds in Table 1 are sourced from GB / T12325, GB / T12326, and GB / T 14549.
[0016] In Table 1, r refers to the number of voltage changes per hour.
[0017] In step S2, the worst-case analysis for single distributed power source access includes the following steps:
[0018] S21. First, establish an equivalent impedance model of the active distribution network with single-node access to distributed power sources. Each line segment is modeled as the line impedance Z. Fi The load at each node is modeled as the load impedance Z. Li The power grid nodes are modeled as voltage sources U. o and output impedance Z o In the series Thevenin equivalent circuit, the distributed source is modeled as a current source I. s and output impedance Z s Parallel Norton equivalent circuit;
[0019] S22. Under the equivalent impedance model of an active distribution network, the equivalent impedance from the distributed generation access node to the point of common coupling node is written as:
[0020]
[0021] in, and These are the equivalent impedances from the distributed power source access point to the point of common coupling and the equivalent impedances from node n to the point of common coupling, respectively. and These are the load impedance and line impedance of node n, respectively.
[0022] Due to the line impedance at each node Load impedance Equivalent impedance of nodes Both the real and imaginary parts of the are positive numbers, and the equivalent impedance relationship at each node is written as:
[0023]
[0024] in The equivalent impedance of the power grid;
[0025] From equations (1) and (2), we get
[0026]
[0027] in, Let i be the line impedance of node i.
[0028] Based on the current distribution relationship in the equivalent impedance model of the active distribution network and combined with equation (3), the current distribution is derived as follows:
[0029]
[0030] in, and These represent the components of the distributed power source output current flowing towards the point of common coupling and the components flowing towards the end of the line, respectively. The equivalent impedance from the distributed power source access point to the end of the line.
[0031] Rated voltage Distributed power access node voltage The following relationship exists between the impedance of each segment and the impedance of each segment:
[0032]
[0033] Where α is the voltage deviation, ω n The rated angular frequency of the power grid. and These represent the currents at node n, the common connection point, and node i, respectively.
[0034] because and The real part of all is positive, which can be derived from equation (5).
[0035]
[0036] Substituting into equation (4), the current distribution ratio on both sides of the connection point is:
[0037]
[0038] S23. Under the premise of reasonable active distribution network design, the voltage deviation α of the normally operating active distribution network is between -0.1pu and 0.1pu.
[0039]
[0040] The current distribution relationship in the harmonic frequency band higher than the rated frequency is as follows:
[0041]
[0042] Considering the current shunting effect of the load and the positive resistivity of the impedance, the following relationship is obtained:
[0043]
[0044] in, and These are the line current and load current of the node preceding the transformer on the access point's transformer side, respectively.
[0045] The relationship between the line currents at the nodes on both sides of the access point and the line currents at other nodes is written as follows:
[0046]
[0047] Combining equation (9), the relationship between the harmonic amplitudes of the distributed power supply access point and any other node is as follows:
[0048]
[0049] S24. From equation (12), the harmonic current, current deviation, and current transient have the following distribution characteristics.
[0050]
[0051] Among them, THDI A and THDI i The current distortion rates ΔI and i are the current distortion rates of the distributed power supply access node and node i, respectively. A ΔIt represents the current deviation at the distributed power source access node. A For the current transients at the distributed power supply access node;
[0052] The distribution relationship of voltage-related power quality indicators is written as follows:
[0053]
[0054] Among them, THDU A and THDU i The voltage distortion rates ΔU and ΔU are respectively the voltage distortion rates of the distributed power supply access node and other i-th nodes. A and ΔU i The voltage deviation ΔUt represents the voltage difference between the distributed power supply access node and other nodes i, respectively. A and ΔUt i These represent the voltage transients of the distributed power access node and other i-th nodes, respectively.
[0055] In step S3, the worst-case analysis when multiple distributed power sources are connected includes the following steps:
[0056] S31. Analyze the power quality under multiple distributed power source access using the approximate superposition theorem;
[0057] The current-related power quality index, namely the current harmonic distribution characteristic, when a single distributed power source is connected is expressed by equation (15).
[0058]
[0059] Among them, I k (i) represents the current generated at node i after the k-th distributed power source is connected, I Lk I represents the current between the connection point and the transformer node. Rk The current I between the access point and the end node Lk =I topk -K Lk (NumDG k -i), I Rk =I botk +K Rk (i-NumDG k ), I topk with I botk I Lk Maximum value and I Rk Minimum value, K Lk With K Rk These represent the rate of change of current from the connection point to the transformer node and the rate of change of current from the connection point to the terminal node, respectively, NumDG k Let I be the node number of the k-th distributed power source in the direction from the point of common connection to the end of the line, and I topk >I botk K Lk >0, K Rk >0;
[0060] Ignoring differences in light intensity, we assume I topk =I top I botk =I bot ;
[0061] According to the superposition theorem, when γ distributed power sources are connected simultaneously, the distribution relationship I(i) of current-based power quality indicators can be written as:
[0062]
[0063] From equation (15), we get I Lk >I Rk ,K Lk >0; therefore, when k = NumDG1, I(k) is the maximum value;
[0064] S32. The distribution characteristics of voltage-related power quality indicators when a single distributed power source is connected are expressed by equation (17).
[0065]
[0066] Among them, U k (i) represents the voltage generated by the k-th distributed power source connected at node i, U Lk U is the voltage between the connection point and the transformer node. Rk U is the voltage between the access point and the end node. Lk =U topk -K L ′ k (NumDG k -i), U Rk =U topk -K′ Rk (i-NumDG k ), U topk For U Lk Maximum value, K L ′ k With K′ Rk These represent the rate of change of current between the connection point and the transformer node, and the rate of change of current between the connection point and the terminal node, respectively, and K L ′ k >0, K′ Rk >0;
[0067] When γ distributed power sources are connected simultaneously, the distribution relationship of voltage-related power quality indicators U(i) is written as follows:
[0068]
[0069] From equation (18), we get U topk >U Lk , K′ Rk >0; therefore, when k = NumDG γ When U(k) is at its maximum value.
[0070] In step S4, the optimal configuration method for the power quality monitoring device is as follows: based on the principle of measuring the worst point, the optimal installation location for the current monitoring device is the distributed power access node k closest to the point of common coupling. i k i =NumDG1; The optimal installation location for the voltage monitoring device is the distributed power supply access node k closest to the end of the line. u k u =NumDG γ .
[0071] Compared with existing methods, the beneficial effects of the present invention are:
[0072] 1) According to the power quality monitoring method provided by the present invention, in a linear distribution network, regardless of its size, only two monitoring devices need to be installed, which is far fewer than the traditional method.
[0073] 2) The method provided by this invention does not require readily available and time-varying data such as line impedance, load capacity, and power supply capacity during optimization configuration. It only requires the line topology to determine the location of the line end and the common junction point for installing monitoring equipment. Its engineering feasibility is superior to other traditional methods. Attached Figure Description
[0074] Figure 1 This is a typical low-voltage distribution network containing distributed generation;
[0075] Figure 2 An equivalent model of active distribution network impedance when distributed generation is connected;
[0076] Figure 3a This describes the distribution of current-based power quality in the active distribution network after multiple distributed power sources are connected. Figure 3b This describes the distribution of voltage-related power quality in the active distribution network after multiple distributed power sources are connected. Detailed Implementation
[0077] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.
[0078] An active distribution network is a distribution network that comprehensively controls distributed energy resources (distributed power sources, energy storage devices, and flexible loads). It effectively manages power flow through flexible network technologies, enabling distributed energy resources to play a certain supporting role in the system under reasonable regulatory environment and access criteria.
[0079] Distributed energy resources are first connected to a low-voltage active distribution network, and then transmitted via transformer step-up. To monitor power quality issues at their source, this invention primarily focuses on the 0.4kV low-voltage active distribution network in practical engineering applications. It has the following characteristics: 1) In low-voltage distribution lines, the influence of distributed parameters is negligible, resistivity dominates the line impedance, and the load impedance is resistive-inductive under most operating conditions. 2) Without photovoltaic integration, the voltage deviation at each node will be limited to a certain range under normal operating conditions. Taking China as an example, the voltage deviation is between -10% and +7%. A typical low-voltage active distribution network containing distributed power sources is shown below. Figure 1 As shown.
[0080] To perform mathematical analysis on the active distribution network, an impedance equivalent model of this typical distribution network is established, such as... Figure 2 As shown, Z FiLet be the impedance of each line segment, and let be the load impedance of each node. The grid nodes are modeled as Thevenin equivalent circuits with voltage sources and output impedances connected in series, and the distributed power sources are modeled as Norton equivalent circuits with current sources and output impedances connected in parallel.
[0081] The present invention proposes a low-data-dependency active distribution network power quality monitoring method based on worst-case analysis, which combines impedance analysis with the inherent characteristics of active distribution networks to analyze the distribution characteristics of the worst-case power quality when a single distributed source or multiple distributed sources are connected.
[0082] The low-data-dependent active distribution network power quality monitoring method based on worst-case analysis is implemented according to the following steps:
[0083] The first step is to analyze and identify the main power quality problems of the active distribution network based on its characteristics.
[0084] The second step is to conduct a worst-case scenario analysis for single distributed power source access based on key power quality indicators and impedance modeling methods.
[0085] The third step is to conduct a worst-case scenario analysis based on the results of the worst-case scenario analysis when a single distributed power source is connected, combined with the characteristics of the low-voltage active distribution network, when multiple distributed power sources are connected.
[0086] The fourth step is to optimize the configuration of the power quality monitoring device based on the worst-case analysis results.
[0087] The first step, based on the characteristics of active distribution networks, analyzes and identifies the main power quality problems. This analysis includes the following steps: An active distribution network refers to a distribution network that comprehensively controls distributed energy resources (distributed power sources, energy storage devices, and flexible loads). Analysis shows that compared to traditional distribution networks, the power quality problems in active distribution networks are more complex, mainly due to the following reasons:
[0088] The introduction of distributed generation enables bidirectional power flow in active distribution networks, which may lead to a significant increase in voltage at end nodes and serious voltage deviation problems.
[0089] Because distributed energy sources are intermittent, for example, the power output of a photovoltaic array will change when the light intensity and temperature change, which in turn causes voltage transient problems.
[0090] Distributed power sources contain a large number of power electronic converters. The switching process of the converters generates high-frequency harmonics, and the control structure of the converters also causes them to generate corresponding voltage and current harmonics under unbalanced and background harmonic conditions.
[0091] In summary, considering the impact on users and the importance attached to it by domestic and international standards, this invention mainly analyzes the power quality indicators in Table 1. The definitions of each indicator and the relevant standard thresholds are also attached in Table 1.
[0092] Table 1. Key Power Quality Indicators and Their Thresholds
[0093]
[0094] The thresholds in Table 1 are sourced from GB / T12325, GB / T12326, and GB / T 14549.
[0095] In Table 1, r refers to the number of voltage changes per hour.
[0096] The second step, based on key power quality indicators and combined with impedance modeling methods, performs worst-case analysis for single distributed power source integration, primarily following these steps:
[0097] 1) To perform mathematical analysis on the active distribution network, firstly, an impedance equivalent model of the active distribution network with a single node connected to a distributed power source is established, such as... Figure 2 As shown, Z Fi Z represents the impedance of each line segment. Li Assuming the load impedance at each node, the grid nodes are modeled as voltage sources U. o and output impedance Z o In the series Thevenin equivalent circuit, the distributed source is modeled as a current source I. s and output impedance Z s Parallel Norton equivalent circuit.
[0098] 2) Under the above equivalent impedance model of the active distribution network, the equivalent impedance from the distributed generation access node to the point of common coupling (transformer node) is:
[0099]
[0100] in, and These are the equivalent impedances from the distributed power source access point to the point of common coupling and the equivalent impedances from node n to the point of common coupling, respectively. and These are the load impedance and line impedance of node n, respectively.
[0101] Due to the line impedance at each node Load impedance Equivalent impedance of nodes Both the real and imaginary parts of the number are positive. The following relationship can be obtained:
[0102]
[0103] Substituting equation (2) into equation (1) yields
[0104]
[0105] Based on the above approach, the equivalent impedance of each node can be derived as follows:
[0106]
[0107] in This is the equivalent impedance of the power grid.
[0108] From equations (1) and (4), we can obtain
[0109]
[0110] according to Figure 2 The current distribution can be derived from the current shunting relationship in the active distribution network impedance equivalent model combined with equation (5) as follows:
[0111]
[0112] in, and These represent the components of the distributed power source output current flowing towards the point of common coupling and the components flowing towards the end of the line, respectively. This is the equivalent impedance from the distributed power source access point to the end of the line.
[0113] The rated voltage can be determined from the definition of voltage deviation α. Distributed power access node voltage The following relationship exists between the impedance of each segment and the impedance of each segment:
[0114]
[0115] Where α is the voltage deviation, ω n The rated angular frequency of the power grid. and These represent the currents at node n, the common connection point, and node i, respectively.
[0116] because and The real part of all is positive, and we can deduce from equation (7) that
[0117]
[0118] Substituting into equation (6), we can obtain the current distribution ratio on both sides of the connection point as follows:
[0119]
[0120] 3) Since the analysis is conducted under the premise of a reasonable active distribution network design, the voltage deviation α of a normally operating active low-voltage distribution network is generally between -0.1 pu and 0.1 pu. Therefore, we can obtain...
[0121]
[0122] The above equation holds true at power frequency. Since the load impedance is mainly resistive-inductive, its magnitude increases with frequency. The line impedance is relatively large and mainly resistive, and its magnitude does not change much with frequency. Therefore, from equation (6), it can be seen that the current distribution relationship in the harmonic frequency band higher than the rated frequency is as follows:
[0123]
[0124] Considering the current shunting effect of the load and the positive resistivity of the impedance, the following relationship can be obtained:
[0125]
[0126] in, and These are the line current and load current of the node preceding the transformer on the access point's transformer side, respectively.
[0127] The relationship between the line currents at the nodes on both sides of the access point and the line currents at other nodes is written as follows:
[0128]
[0129] Combining equation (11), the relationship between the harmonic amplitudes of the distributed power supply access point and any other node is as follows:
[0130]
[0131] Therefore, when a single distributed power source is connected... The value of the current in each frequency band is the maximum value, meaning that the current harmonics at node A are the maximum values among all nodes.
[0132] Furthermore, due to load shunting, the current value at each point from the distributed power source access point to the common connection point will slowly decrease during the harmonic current conduction process, and the current value from the distributed power source access point to the end of the line will remain at a relatively small value.
[0133] Therefore, the worst point of current-type power quality (i.e., current harmonics) considered in this invention is the distributed power source access node. The distribution of current-type power quality indicators shows a stepped distribution characteristic of "slowly decreasing from the distributed power source access point to the common connection point, with the access point being much larger than the end side".
[0134] 4) Voltage-related power quality problems such as voltage distortion, voltage deviation, and voltage transients are all caused by the corresponding distorted current, current deviation, and current transient flowing through the line impedance. Since distorted current, current deviation, and current transient can all be expressed as the sum of various current harmonics, and the distribution characteristics of each current harmonic are known, the distribution characteristics of voltage-related power quality problems can be deduced from this.
[0135] From equation (14), it can be seen that harmonic current, current deviation (power frequency current generated by distributed power source), and current transient have the following distribution characteristics.
[0136]
[0137] Among them, THDI A and THDI i The current distortion rates ΔI and i are the current distortion rates of the distributed power supply access node and node i, respectively. A ΔIt represents the current deviation at the distributed power source access node. A This refers to the current transients at the distributed power supply access node.
[0138] Since voltage harmonics, voltage deviations, and voltage fluctuations are all formed by the superposition of voltage drops generated across the line impedance by the corresponding currents, it can be represented by the following equation:
[0139]
[0140] in, and These are the voltage at node i and the voltage at the point of common coupling, respectively.
[0141] Since resistivity dominates in line impedance, the distribution of voltage-related power quality problems can be deduced.
[0142]
[0143] Among them, THDU A and THDU i The voltage distortion rates ΔU and ΔU are respectively the voltage distortion rates of the distributed power supply access node and other i-th nodes. A and ΔU i The voltage deviation ΔUt represents the voltage difference between the distributed power supply access node and other nodes i, respectively. A and ΔUt i These represent the voltage transients of the distributed power access node and other i-th nodes, respectively.
[0144] Therefore, based on the stepped distribution characteristics of current-related power quality issues, the worst point for voltage-related power quality (voltage distortion rate, voltage deviation, voltage transient) is the distributed power source access node. After passing through the resistive line impedance, voltage-related power quality indicators exhibit a slope-like distribution characteristic: "gradually increasing from the point of common coupling to the distributed power source access point, and slowly decreasing from the access point to the end of the line."
[0145] The third step, based on the worst-case scenario analysis results when a single distributed power source is connected, and combined with the characteristics of the low-voltage active distribution network, performs the worst-case scenario analysis when multiple distributed power sources are connected. This is specifically implemented according to the following steps:
[0146] 1) Low-voltage active distribution networks exhibit resistive and relatively small line impedances, with node voltage phases being basically consistent and a high degree of linearity. The power quality under multiple distributed source connections can be analyzed using the approximate superposition theorem.
[0147] The current-related power quality indicators (current harmonics) distribution characteristics when a single distributed power source is connected are "slowly decreasing from the distributed power source connection point to the point of common coupling, with the connection point being much larger than the end side", which can be expressed by equation (18).
[0148]
[0149] Among them, I k (i) represents the current generated at node i after the k-th distributed power source is connected, I Lk I represents the current between the connection point and the transformer node. Rk The current I between the access point and the end node Lk =I topk -K Lk (NumDG k -i), I Rk =I botk +K Rk (i-NumDG k ), I topk with I botk I Lk Maximum value and I Rk Minimum value, K Lk With K Rk These represent the rate of change of current from the connection point to the transformer node and the rate of change of current from the connection point to the terminal node, respectively, NumDG k Let I be the node number of the k-th distributed power source in the direction from the point of common connection to the end of the line, and I topk >I botk K Lk >0, K Rk >0.
[0150] In practical engineering, large-scale regional photovoltaic systems are typically supplied by a single provider, with each household having a similar rated capacity. Furthermore, the area of a single low-voltage distribution network zone is limited, and differences in solar intensity can be ignored; therefore, I can be considered... topk =I top (k = 1, 2, 3, ...), I botk =I bot (k = 1, 2, 3, ...).
[0151] According to the superposition theorem, when γ distributed power sources are connected simultaneously, we have
[0152]
[0153] From equation (18), we can know that I Lk >I Rk ,K Lk >0. Therefore, I(k) reaches its maximum value when k = NumDG1. That is, when multiple distributed power sources are connected, the point with the worst current-type power quality (current harmonics) is the distributed power source node closest to the point of common coupling, such as... Figure 3a and Figure 3b As shown.
[0154] 2) The voltage-related power quality (current harmonics) distribution characteristics when a single distributed power source is connected are "gradually rising from the point of common coupling to the point of connection of the distributed power source, and slowly decreasing from the point of connection to the end of the line", which can be expressed by equation (20).
[0155]
[0156] Among them, U k (i) represents the voltage generated by the k-th distributed power source connected at node i, U Lk U is the voltage between the connection point and the transformer node. Rk U is the voltage between the access point and the end node. Lk =U topk -K′ Lk (NumDG k -i), U Rk =U topk -K′ Rk (i-NumDG k ), U topk For U Lk Maximum value, K′ Lk With K′ Rk These represent the rate of change of current between the connection point and the transformer node, and the rate of change of current between the connection point and the terminal node, respectively, and K′ Lk >0, K′ Rk >0.
[0157] When γ distributed power sources are connected simultaneously, the distribution relationship of voltage-related power quality indicators U(i) is written as follows:
[0158]
[0159] From equation (21), we can know that U topk >U Lk , K′ Rk >0. Therefore, when k = NumDG γ When U(k) is at its maximum value, the point with the worst voltage-related power quality (voltage distortion rate, voltage deviation, voltage variation) when multiple distributed power sources are connected is the distributed power source connection point closest to the end of the line, as shown in Figure 3.
[0160] In the fourth step, based on the worst-case analysis results, the optimal configuration method for the power quality monitoring device is as follows: based on the principle of measuring the worst point and combined with the analysis results of the second and third steps, in order to minimize the number of sampling devices installed, the optimal installation location for the current monitoring device is the distributed power supply access node closest to the point of common coupling, and the optimal installation location for the voltage monitoring device is the distributed power supply access node closest to the end of the line.
[0161] This invention employs worst-case scenario analysis to optimize power quality monitoring. In a linear distribution network, regardless of size, only two monitoring devices are required, significantly fewer than traditional methods. Furthermore, the optimized configuration of the monitoring devices eliminates the need for data such as line impedance and load capacity, which are difficult to obtain accurately over time. Only the line topology is needed to determine the locations of the line ends and the point of common coupling (PCC), allowing the installation of monitoring devices at the nearest distributed power source access points. This results in superior engineering feasibility compared to other traditional methods.
Claims
1. A method for monitoring power quality in an active distribution network based on worst-case analysis, characterized in that, The method includes the following steps: S1. Based on the characteristics of the active distribution network, analyze and determine the main power quality problems of the active distribution network; S2. Based on the main power quality indicators, combined with impedance modeling methods, perform worst-case analysis when a single distributed power source is connected. S3. Based on the worst-case analysis results when a single distributed power source is connected, and combined with the characteristics of the active distribution network, perform the worst-case analysis when multiple distributed power sources are connected. S4. Based on the worst-case analysis results, optimize the configuration of the power quality monitoring device; In step S2, the worst-case analysis for single distributed power source access includes the following steps: S21. First, establish an equivalent impedance model of the active distribution network with single-node access to distributed power sources. Each line segment is modeled as line impedance. The load at each node is modeled as load impedance. The power grid nodes are modeled as voltage sources. and output impedance In the series Thevenin equivalent circuit, the distributed source is modeled as a current source. and output impedance Parallel Norton equivalent circuit; S22. Under the equivalent impedance model of an active distribution network, the equivalent impedance from the distributed generation access node to the point of common coupling node is written as: (1) in, and These are the equivalent impedances from the distributed power source access point to the point of common coupling and from... n The equivalent impedance from node number 1 to the common connection point. and They are respectively n The load impedance and line impedance of node number 1; Because of the line impedance at each node Load impedance Equivalent impedance of nodes Both the real and imaginary parts of the are positive numbers, and the equivalent impedance relationship at each node is written as: (2) in The equivalent impedance of the power grid; From equations (1) and (2), we get (3) in, for i Line impedance at node number 1; Based on the current distribution relationship in the equivalent impedance model of the active distribution network and combined with equation (3), the current distribution is derived as follows: (4) in, and These represent the components of the distributed power source output current flowing towards the point of common coupling and the components flowing towards the end of the line, respectively. The equivalent impedance from the point of access to the distributed power source to the end of the line. Rated voltage Voltage of distributed power source access nodes The following relationship exists between the impedance of each segment and the impedance of each segment: (5) in, For voltage deviation, The rated angular frequency of the power grid. , and They are respectively n Node #, common connection point i The current at node number 1; because , and The real part of all is positive, which can be derived from equation (5). (6) Substituting into equation (4), the current distribution ratio on both sides of the connection point is: (7) S23. Under the premise of reasonable active distribution network design, the voltage deviation of the active distribution network under normal operation at this time Between -0.1 pu and 0.1 pu, we get (8) The current distribution relationship in the harmonic frequency band higher than the rated frequency is as follows: (9) Considering the current shunting effect of the load and the positive resistivity of the impedance, the following relationship is obtained: (10) in, and These are the line current and load current of the node preceding the transformer on the access point's transformer side, respectively. The relationship between the line currents at the nodes on both sides of the access point and the line currents at other nodes is written as follows: (11) Combining equation (9), the relationship between the harmonic amplitudes of the distributed power supply access point and any other node is as follows: (12) S24. From equation (12), the harmonic current, current deviation, and current transient have the following distribution characteristics. (13) in, and Distributed power access nodes and i Current distortion rate at node number 1 For the current deviation of the distributed power source access node. For the current transients at the distributed power supply access node; The distribution relationship of voltage-related power quality indicators is written as follows: (14) in, and Distributed power access nodes and others i Voltage distortion rate at node number 1 and Distributed power access nodes and others i Voltage deviation at node number 1 and Distributed power access nodes and others i Voltage transient at node number 1; In step S3, the worst-case analysis when multiple distributed power sources are connected includes the following steps: S31. Analyze the power quality under multiple distributed power source access using the approximate superposition theorem; The current-related power quality index, namely the current harmonic distribution characteristic, when a single distributed power source is connected is expressed by equation (15). (15) in, For the first k After a distributed power source is connected, at the node i The current generated at that point, This refers to the current between the connection point and the transformer node. The current between the access point and the end node , , and They are respectively Maximum value and Minimum value, and These represent the rate of change of current between the connection point and the transformer node, and the rate of change of current between the connection point and the terminal node, respectively. The first in the direction from the common connection point to the end of the line k The node number of each distributed power source, and , , ; Ignoring differences in light intensity, it is assumed that , ; According to the superposition theorem, when Distribution of current-related power quality indicators when multiple distributed power sources are connected simultaneously. Written as (16) From equation (15) we get , ; therefore, when hour, It is the maximum value; S32. The distribution characteristics of voltage-related power quality indicators when a single distributed power source is connected are expressed by equation (17). (17) in, For the first k A distributed power source is connected to the node. i The voltage generated at that point, The voltage between the connection point and the transformer node. The voltage between the access point and the end node. , , for Maximum value and These are the current change rates between the connection point and the transformer node, and the current change rates between the connection point and the terminal node, respectively. , ; when Distribution of voltage-related power quality indicators when multiple distributed power sources are connected simultaneously. Written as (18) From equation (18) we get , ; therefore, when hour, This is the maximum value.
2. The active distribution network power quality monitoring method based on worst-case analysis according to claim 1, characterized in that, In step S1, the active distribution network refers to a distribution network that comprehensively controls distributed energy sources, namely distributed power sources, energy storage devices, and flexible loads. Considering the degree of impact on users and the importance attached to domestic and international standards, an analysis is conducted on the power quality indicators in Table 1. The definitions of each indicator and the relevant standard thresholds are also attached to Table 1: Table 1. Key Power Quality Indicators and Their Thresholds The thresholds in Table 1 are sourced from GB / T12325, GB / T12326, and GB / T 14549. In Table 1, r refers to the number of voltage changes per hour.
3. The active distribution network power quality monitoring method based on worst-case analysis according to claim 1, characterized in that, In step S4, the optimal configuration method for the power quality monitoring device is as follows: based on the principle of measuring the worst point, the best installation location for the current monitoring device is the distributed power supply access node closest to the point of common coupling. , The optimal installation location for the voltage monitoring device is the distributed power supply access node closest to the end of the line. , .