IIDG-accessed power distribution network protection mismatch risk constant value identification and adjustment method and system

Abstract

The invention discloses an IIDG-accessed power distribution network protection mismatch risk constant value identification and adjustment method and system. The method comprises the following steps: calculating IIDG allowable output fluctuation boundaries corresponding to upstream and downstream protection constant values; when the output fluctuation of the IIDG accords with a set flow starting criterion, judging whether the output fluctuation of the IIDG is in a range of an IIDG allowable output fluctuation boundary corresponding to each protection constant value, and if so, determining that the corresponding protection constant value is a risk constant value; calculating short-circuit current on an upstream line and a downstream line of an IIDG grid-connected point when a short-circuit fault occurs in the current operation state, further adjusting a protection constant value judged as a risk constant value, and updating an IIDG allowable output fluctuation boundary corresponding to the protection constant value; and comparing the adjusted protection setting value with the protection setting value before adjustment, and if the difference exceeds a set allowable range, using the adjusted protection setting value. According to the method, the influence of the IIDG output fluctuation on the protection constant value is quantified, and the reliability of a protection system is remarkably improved.

Description

Technical Field

[0001] The invention belongs to the technical field of relay protection for distribution networks, and more specifically, relates to a method and system for identifying and adjusting the mis-match risk setting values of relay protection for an active distribution network with IIDG access. Background Art

[0002] Renewable energy is an important way and has been vigorously developed and promoted in various countries around the world in recent years. However, after distributed generation (DG) is connected to the distribution network, it changes the original single-source radial power supply network structure of the traditional distribution system, seriously affecting the selectivity and sensitivity of traditional current protection.

[0003] Distributed generation can be classified into two categories according to the difference in operating principles: one is the distributed generation (DG) based on traditional rotating machines, and the other is the inverter-based distributed generation (IIDG) connected to the distribution network through an inverter. Since the DG based on traditional rotating machines has similar operating characteristics to conventional generators, it can be regarded as a series model of impedance and voltage source. However, due to the intervention of power electronic components, the fault characteristics of IIDG are significantly affected by its control strategy. It is necessary to model according to the fault control strategy and study the fault characteristics.

[0004] Currently, the IIDG connected to the distribution network mainly includes distributed photovoltaic, wind power, and energy storage systems (ESS), etc. There are relatively complete studies on the control strategy, modeling, short-circuit current algorithm, etc. for photovoltaic and wind power connected to the distribution network. The energy storage power supply has three operating states: charging, discharging, and standby, and there is currently less research on the energy storage power supply in different operating states. Some scholars have analyzed the influence of the energy storage system in two working states, i.e., as a load and a power source, on the distribution network protection, but have not combined specific control strategies. There are also scholars who have analyzed the operating characteristics of distance protection after energy storage is connected to the distribution network from the perspective of measured impedance according to the operating characteristics of the energy storage power station, but have not considered the fault ride-through control strategy under different states of the energy storage.

[0005] CN109066622A discloses a protection method, device, medium, equipment and active distribution network for an active distribution network. The method comprises: obtaining a current value of a target protection device in a target active distribution network, and triggering a protection action when the current value is greater than a current setting value; wherein the current setting value is obtained by: creating an injected positive sequence current calculation model according to the provisions of the national standard on the low voltage ride-through process of an IIDG; determining an injected negative sequence current calculation model according to the instantaneous power theory using the injected positive sequence current calculation model; determining a voltage equation of each node using a node voltage method according to boundary conditions of a fault in the target active distribution network; calculating the branch current of each node using an iterative method according to the injected positive sequence current calculation model, the injected negative sequence current calculation model and the voltage equation; and calculating the current setting value of the target protection device in the target active distribution network according to the branch current. However, the patent only proposes a method for calculating the short-circuit current after the IIDG is connected to the distribution network, and calculates the protection setting value through the setting formula of the traditional three-stage current protection. It does not take into account the output fluctuation characteristics of new energy sources, and lacks a method for dynamically adjusting the protection setting value. When there is a risk of false operation or refusal to operate, the protection setting value cannot be updated in time, affecting the sensitivity and selectivity of the protection; and its adaptability to complex fault scenarios is limited. Summary of the invention

[0006] In order to solve the deficiencies in the prior art, the present invention conducts research with the three-stage current protection of the distribution network with IIDG access as the core, and proposes a solution method for the allowable output fluctuation boundary of IIDG. When the output fluctuation of IIDG exceeds the output fluctuation boundary corresponding to the protection setting value, the protection setting value is determined to be a risk setting value; then an online iterative solution algorithm for short-circuit current is proposed, and the risk setting value is dynamically adjusted according to the result. Finally, a distribution network protection mismatch risk setting value identification and adjustment method suitable for inverter-type distributed power generation access is formed to meet the high performance requirements of the distribution network protection system.

[0007] The present invention adopts the following technical solution.

[0008] The first aspect of the present invention proposes a method for identifying and adjusting the fixed value of protection mismatch risk of distribution network accessed by IIDG, characterized in that it includes:

[0009] During normal operation, the distribution network operation status is obtained to calculate the IIDG output fluctuation boundary corresponding to each upstream and downstream protection setting value; the protection setting value is the threshold for judging whether the distribution network has a fault and triggering the protection action, and the IIDG output fluctuation boundary is the critical range of the IIDG output when the protection is correctly operated. Exceeding this range will cause the protection action to fail;

[0010] Collect IIDG output data in real time. When the IIDG output fluctuation meets the set process start criteria, determine whether the IIDG output fluctuation is within the range of the IIDG allowable output fluctuation boundary corresponding to each protection setting. If so, the corresponding protection setting is a risk setting.

[0011] If there is a protection setting value that is a risk setting value, the short-circuit current on the upstream and downstream lines of the IIDG grid connection point when a short-circuit fault occurs in the current operating state is calculated, and then the protection setting value judged as the risk setting value is adjusted, and the IIDG output fluctuation limit corresponding to the protection setting value is updated at the same time;

[0012] Compare the adjusted protection setting with the protection setting before adjustment. If the difference exceeds the set allowable range, use the adjusted protection setting; otherwise, use the protection setting before adjustment.

[0013] Preferably, the IIDG allowable output fluctuation boundary corresponding to each upstream and downstream protection setting value is calculated as follows:

[0014] The upstream and downstream protection settings include upstream I, II, III protection settings, and downstream I, II, III protection settings; the upstream and downstream are the upstream and downstream lines of the IIDG grid connection point respectively; the maximum current and minimum current when the upstream and downstream protection devices do not meet the selectivity requirements of the protection settings are set. The selectivity requirement is that after the IIDG is connected, when a three-phase short circuit occurs at the adjacent line outlet, the short-circuit current flowing through the line at this level is greater than the corresponding protection settings;

[0015] According to the following formula, combined with the set maximum current and minimum current, calculate the maximum value of the output fluctuation current corresponding to each protection setting. and minimum value

[0016]

[0017] In the formula, Short-circuit current provided to the distribution network; and For the components of the short-circuit current on the d and q axes of the IIDG output before the output fluctuation, set and output fluctuation ΔI DG In phase; I lim.max and I lim.min To set the maximum current and minimum current when each protection device does not meet the selectivity requirements of each protection setting;

[0018] The maximum and minimum values of the output fluctuation current are multiplied by the voltage at the IIDG grid connection point calculated at this time to obtain the upper and lower limits of the IIDG output fluctuation boundary allowed.

[0019] Preferably, when the maximum current at each protection device does not meet the selectivity requirement of each protection setting value, it is set to 0.95 times the corresponding protection setting value of the downstream protection device, and when the minimum current at each protection device does not meet the selectivity requirement of each protection setting value, it is set to 0.8 times the corresponding protection setting value of the downstream protection device.

[0020] Preferably, the starting criterion for the setting process is that the output fluctuation of the IIDG continuously increases or continuously decreases.

[0021] Preferably, calculate the short-circuit currents on the upstream and downstream lines of the IIDG connection point when a short-circuit fault occurs under the current operating state, specifically:

[0022] Detect the negative-sequence current to determine whether the fault type is a three-phase short circuit or a two-phase short circuit, so as to obtain the additional impedance Z related to the fault type Δ ;

[0023] Combined with this additional impedance, solve the following formula to obtain the voltage at the IIDG connection point

[0024] In the formula: are the short-circuit currents on the upstream and downstream of the IIDG connection point when a short-circuit fault occurs under the current operating state, Z s , Z1, and Z2 are the distribution network impedance, the impedance of the upstream line of the IIDG connection point, and the impedance of the downstream line, respectively, is the short-circuit current output before the IIDG output fluctuation calculated at this time, is the voltage at the IIDG connection point, is the distribution network voltage, is the current of the upstream line of the IIDG connection point;

[0025] According to the solved voltage at the IIDG connection point calculate the short-circuit currents on the upstream and downstream of the IIDG connection point when a short-circuit fault occurs under the current operating state

[0026] Preferably, detect whether there is negative-sequence current in the distribution network impedance, the impedance of the upstream line of the fault point, or the impedance of the line where the fault point is located. If there is negative-sequence current, it is a two-phase fault, otherwise it is a three-phase fault;

[0027] When it is a three-phase short circuit, Z Δ = 0, when it is a two-phase short circuit The superscript "-" represents the negative-sequence component, are the negative-sequence components of the distribution network impedance, the impedance of the upstream line of the IIDG connection point, and the impedance of the downstream line, respectively.

[0028] Preferably, the voltage at the IIDG connection point is obtained by solving, and the Gauss iteration method is used for solving. When iterating, the output current of the IIDG before the fault occurs is selected as the initial value of I DG during iteration.

[0029] Preferably, the protection setting value judged as the risk setting value is adjusted, specifically:

[0030] According to the short-circuit currents on the upstream and downstream lines of the IIDG connection point when a short-circuit fault occurs under the current operating state, the adjustment coefficients of each protection setting value are calculated, and the protection setting value judged as the risk setting value is multiplied by its corresponding adjustment coefficient;

[0031] The calculation formula for the adjustment coefficient is:

[0032]

[0033] In the formula, K1 and K2 are the adjustment coefficients of the upstream and downstream protection setting values respectively; are the short-circuit currents on the upstream and downstream of the IIDG connection point when a short-circuit fault occurs under the current operating state; P ref is the IIDG output value; K 01 , K 02 are the reference coefficients of K1 and K2 respectively; for the protection setting values of the first and second segments, the reference coefficients K 01 , K 02 take 1.0, while for the protection setting value of the third segment, K 01 , K 02 take 1.1 - 1.3.

[0034] Preferably, the set allowable range is 90% - 110% of the protection setting value before adjustment. If the adjusted setting value exceeds this range, the adjusted setting value is used.

[0035] The second aspect of the present invention proposes a system for identifying and adjusting the risk setting value of protection mismatch in a distribution network with IIDG access using the method described in the first aspect of the present invention, including a boundary calculation module, a risk setting value identification module, a setting value adjustment module, and a setting value modification judgment module: It is characterized in that:

[0036] Boundary calculation module: used to obtain the operating state of the distribution network during normal operation, so as to calculate the allowable output power fluctuation boundaries of the IIDG corresponding to each upstream and downstream protection setting value; the protection setting value is the threshold for judging whether a fault occurs in the distribution network and triggering a protection action, and the allowable output power fluctuation boundary of the IIDG is the critical range of the IIDG output power when the protection acts correctly. Exceeding this range will cause the protection action to fail;

[0037] Risk fixed-value identification module: It is used to collect the output data of IIDG in real time. When the output fluctuation of IIDG meets the set criterion for starting the process, it determines whether the output fluctuation of IIDG is within the range of the allowable output fluctuation boundary of IIDG corresponding to each protection fixed value. If so, the corresponding protection fixed value is the risk fixed value;

[0038] Fixed-value adjustment module: If there is a protection fixed value that is a risk fixed value, it is used to calculate the short-circuit currents on the upstream and downstream lines of the IIDG connection point when a short-circuit fault occurs under the current operating state, and then adjust the protection fixed value determined to be the risk fixed value. At the same time, it updates the allowable output fluctuation boundary of IIDG corresponding to this protection fixed value;

[0039] Fixed-value modification judgment module: It is used to compare the adjusted protection fixed value with the protection fixed value before adjustment. If the difference exceeds the set allowable range, the adjusted protection fixed value is used; otherwise, the protection fixed value before adjustment is still used.

[0040] The beneficial effects of the present invention are as follows. Compared with the prior art,

[0041] 1. By analyzing the phasor relationship between short-circuit current and voltage, the allowable output fluctuation boundary of IIDG when the selectivity requirements corresponding to each protection fixed value are not met is solved. According to this boundary and the output fluctuation of IIDG, the risk fixed value is judged. It has dynamic adaptability, can respond to the output fluctuation of IIDG in real time, dynamically adjust its output fluctuation boundary according to the protection fixed value, and ensure the correct operation of the protection; it quantifies the influence of the output fluctuation of IIDG on the protection fixed value, avoids misoperation or refusal of protection caused by excessive output fluctuation of IIDG, and significantly improves the reliability of the protection system; only adjusts the risk fixed value that exceeds the boundary, reduces unnecessary fixed-value update operations, optimizes resource allocation, and reduces maintenance costs;

[0042] 2. The short-circuit current is solved online by the Gauss iterative method, which is applicable to complex distribution network topologies. Considering the short-circuit current output characteristics of IIDG, it avoids the errors of traditional linear methods and improves the calculation accuracy; it has a fast operation speed and few iteration times, meets the real-time requirements of online adjustment, and is suitable for the scenario of dynamically adjusting protection fixed values; it is compatible with various fault types, and adapts to the calculation requirements of different fault types by introducing negative-sequence current detection and additional impedance;

[0043] 3. Based on the iterative results of short-circuit current, the protection fixed values with risks are dynamically adjusted, which can effectively improve the sensitivity of the second and third sections of the upstream protection and ensure the selectivity of the first section of the downstream protection, and improve the performance of the traditional three-section current protection;

[0044] 4. A triggering condition is set, and the judgment and adjustment process of the risk fixed value is only started when the output fluctuation of the IIDG continuously increases or decreases, which can filter out the small - amplitude output fluctuations of the IIDG, avoid frequent identification and update of the risk fixed value, optimize the system resource occupation, and reduce the maintenance cost. Brief Description of the Drawings

[0045] Figure 1 is the equivalent circuit for the IIDG to access the distribution network;

[0046] Figure 2 is the phasor diagram of the constant - voltage vector control;

[0047] Figure 3 is the distribution network system accessed by the IIDG;

[0048] Figure 4 (a) shows the relationship between the short - circuit current at the downstream protection and the increase in the IIDG output;

[0049] Figure 4 (b) shows the relationship between the short - circuit current at the downstream protection and the decrease in the IIDG output;

[0050] Figure 5 is the flowchart of the method of the present invention. Detailed Embodiments

[0051] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. The embodiments described in this application are only a part of the embodiments of the present invention, rather than all embodiments. Based on the spirit of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the protection scope of the present invention.

[0052] As Figure 5 shown, Embodiment 1 of the present invention proposes a method for identifying and adjusting the risk fixed value of the protection mismatch in the distribution network accessed by the IIDG, including the following content:

[0053] Obtain the operation state of the distribution network during normal operation, and thus calculate the allowable output fluctuation boundaries of the IIDG corresponding to the protection fixed values of the upstream and downstream; the protection fixed value is the threshold for judging whether a fault occurs in the distribution network and triggering the protection action, and the allowable output fluctuation boundary of the IIDG is the critical range of the IIDG output when the protection operates correctly. Beyond this range, the protection action will fail;

[0054] Collect the IIDG output data in real - time. When the output fluctuation of the IIDG meets the set process start criterion, judge whether the output fluctuation of the IIDG is within the allowable output fluctuation boundaries of the IIDG corresponding to each protection fixed value. If so, the corresponding protection fixed value is the risk fixed value;

[0055] If there is a protection setting value equal to the risk setting value, calculate the short-circuit currents on the upstream and downstream lines of the IIDG connection point when a short-circuit fault occurs under the current operating state, and then adjust the protection setting value determined to be the risk setting value. At the same time, update the allowable output power fluctuation boundary of the IIDG corresponding to this protection setting value;

[0056] Compare the adjusted protection setting value with the pre-adjustment protection setting value. If the difference exceeds the set allowable range, use the adjusted protection setting value; otherwise, still use the pre-adjustment protection setting value.

[0057] It should be noted that among the currently grid-connected inverter-based distributed power sources, photovoltaic and energy storage are the most common. In addition, most inverter-based distributed power sources need to be connected to the distribution network through pulse-width modulation converters, and their fault characteristics are affected by control strategies. Therefore, this section mainly conducts research on the control strategies of these two typical IIDGs, namely photovoltaic and energy storage. The equivalent circuit of the IIDG connected to the distribution network through an inverter is as Figure 1 shown.

[0058] Photovoltaic is a typical inverter-based power source, and its different grid connection control strategies and low voltage ride-through strategies need to be considered, including:

[0059] 1) When a symmetrical short-circuit fault occurs in the distribution network and there is no negative sequence component in the voltage at the connection point, in order to enable the inverter-based distributed power source to have the low voltage ride-through ability during symmetrical faults, a voltage-oriented vector control strategy with independent decoupling control of active and reactive power is adopted;

[0060] 2) When an asymmetrical fault occurs in the distribution network, there is a negative sequence component in the voltage at the connection point. And when an asymmetrical grounding fault occurs, since the medium-voltage distribution network of 3 - 66 kV usually adopts the operation mode of non-effective grounding of the neutral point, the zero-sequence component is relatively small without considering the equivalent capacitance to the ground. Therefore, the inverter-based distributed power source needs to suppress the output of the negative sequence component while having the low voltage ride-through ability during faults. At this time, the inverter-based distributed power source adopts a positive and negative sequence dual IIDG current control strategy.

[0061] The energy storage system is the same as the photovoltaic in being an inverter-based distributed power source, but it has three operating states: discharging, charging, and standby states, and its control strategy is closely related to its operating state. Since the electrochemical energy storage system connected to the public power grid through a voltage level of 10(6) kV and above should have the low voltage ride-through ability, the control strategy of the energy storage during the discharging state can refer to that of the photovoltaic.

[0062] There are currently few documents proposing specific fault ride-through strategies for energy storage systems in the charging state. This invention draws on the requirements of IIDG fault ride-through and proposes a fault ride-through strategy for ESS. In the charging state, a fault voltage ride-through control strategy is adopted, giving priority to outputting the q-axis current to support the grid connection point voltage. Different from the single low-voltage ride-through strategy of photovoltaic, the ride-through strategies of ESS in charging and discharging modes are different: when ESS is in the charging mode, it injects reactive power into the system while absorbing active power. Therefore, in the fault ride-through strategy when ESS is in the charging mode, the main goal is to preferentially output / absorb reactive current. Correspondingly, the input active current is limited by the capacity of the power converter system (PCS).

[0063] Considering the control strategy described above, the short-circuit current output from the IIDG port changes with the change of the grid connection point voltage U. Taking the distribution network voltage U s as the reference phasor, setting the grid connection point voltage U to coincide with the d-axis, the short-circuit current output characteristics of IIDG can be expressed as:

[0064] (1) When the IIDG type is photovoltaic or energy storage in the discharging state:

[0065] 1) When U > 0.9U N where U N is the rated voltage, IIDG adopts voltage-dominated constant power control, outputs constant active power and current, and the reactive power output is set to 0, operating in a constant power state. The d-axis and q-axis currents I d and I q of IIDG are as shown in the following formula:

[0066]

[0067] In the formula: P ref is the reference value of active power, which is generally the maximum power tracked by the maximum power point tracking module.

[0068] 2) When 0.2U N ≤ U ≤ 0.9U N IIDG starts to output reactive current and power to suppress the grid voltage dip and switches to the voltage support operation state. The output current I d and I q of IIDG are as shown in the following formula.

[0069]

[0070] In the formula: k1 is the reactive power support coefficient of IIDG; k2 is the current limiting coefficient of IIDG, generally not greater than 1.2; U d is the d-axis component of the grid connection point voltage; Pmax is the maximum active power output of the IIDG.

[0071] 3) When U < 0.2U N , the IIDG only outputs reactive power and reactive current and operates in a constant current state. The output current satisfies:

[0072]

[0073] (2) When the IIDG is a storage energy in the charging state:

[0074] 1) When U > 0.9U N , the low voltage ride-through control mode is not adopted, and the IIDG absorbs active current. The d-axis and q-axis currents I d and I q output by the IIDG are as shown in the following formula:

[0075]

[0076] 2) When 0.2U N ≤ U ≤ 0.9U N , the IIDG starts to output reactive current and power to suppress the grid voltage dip. As the grid connection point voltage drops, the reactive current provided by the IIDG increases. Due to the limitation of the inverter current, the absorption of the active current by the IIDG will be suppressed. The output current I d and I q of the IIDG are as shown in the following formula.

[0077]

[0078] In the formula: U d is the d-axis component of the grid connection point voltage, kV; P max is the maximum active power output of the IIDG.

[0079] 3) When U < 0.2U N , the IIDG only outputs reactive power and reactive current, stops absorbing active current, and operates in a constant current state. The output current satisfies:

[0080]

[0081] After obtaining the d-axis and q-axis components of the IIDG output current, according to the phase relationship between the system voltage and the output current under the constant voltage vector control as Figure 2 shown, the output current of the IIDG can be calculated:

[0082]

[0083] In the formula: α is the phase angle of the grid-connected positive sequence voltage.

[0084] It can be seen that the output current of the IIDG is a step function of the grid-connected point voltage, and there is a correlation between the grid-connected point voltage and the inverter output current.

[0085] When a short-circuit fault occurs in the downstream line of the IIDG access point, the current flowing through the upstream line during the fault will be reduced, and the short-circuit current flowing through the downstream line will be increased. At the same time, it will cause the upstream protection of the access point to be insufficiently sensitive when used as a backup protection for the adjacent lower-level line and the downstream protection to malfunction when used as a quick-break protection for the current-level line. In order to solve such problems, the present invention proposes a method for identifying and adjusting the risk constant value of current protection mismatch: calculating the IIDG allowable output fluctuation boundary corresponding to the constant value, and when the IIDG output fluctuation exceeds the boundary, marking the constant value as a risk constant value; iteratively calculating the short-circuit current by obtaining the distribution network operation information online, and further correcting the risk constant value, thereby improving the sensitivity of the upstream current protection II and III sections and ensuring the selectivity of the downstream current protection I section.

[0086] The primary energy of IIDG is mostly renewable energy with large output fluctuations such as photovoltaic and wind power, so the output is highly intermittent and random. When the output of renewable energy fluctuates, the current flowing through the lower-level lines may be affected, resulting in malfunction or failure of downstream configuration protection in the event of a short-circuit fault.

[0087] To solve this kind of problem, this method proposes a calculation method to solve the allowable output fluctuation boundary of IIDG corresponding to the fixed value according to the phasor relationship between short-circuit current and voltage. When the output fluctuation of IIDG exceeds the boundary, it is judged that the fixed value may have the risk of false operation or refusal to operate.

[0088] by Figure 3 Taking the equivalent circuit of the IIDG distribution network shown as an example, when the IIDG output increases, the action range of the downstream line protection will increase. The maximum current when each protection device does not meet the selectivity requirements of each protection setting is set to 0.95 times the corresponding protection setting of the downstream protection device. The minimum current when each protection device does not meet the selectivity requirements of each protection setting is set to 0.8 times the corresponding protection setting of the downstream protection device.

[0089] Taking the current I-stage protection of the downstream protection device as an example, its maximum protection range should not be extended to the next-level line. After the IIDG is connected, when a three-phase short circuit occurs at the exit of the adjacent line, if the short-circuit current flowing through the line protection of this level is greater than its I-stage set value, it can be considered that the protection has lost selectivity. In order to retain a certain margin, the maximum current when the downstream protection does not meet the requirements is:

[0090]

[0091] When the output of the IIDG decreases, the operating range of the downstream line protection will be reduced. Affected by the reliability coefficient of the first section of the current, the short-circuit current of a three-phase short circuit at the end of the line is generally 0.7 - 0.8 times the protection setting value. To maintain a certain margin, the minimum current when the protection does not meet the requirements can be set as:

[0092]

[0093] That is, the short-circuit current flowing through the downstream protection when the output of the IIDG fluctuates When the following conditions are met, the downstream protection can operate correctly.

[0094]

[0095] Next, analyze the relationship between the output fluctuation of the IIDG, the short-circuit current, and the protection setting value: The short-circuit current flowing through the downstream protection is the superposition of the short-circuit current provided by the system and the short-circuit current provided by the IIDG. Without considering the current limiting requirements, the relationship between the output fluctuation of the IIDG and the two is as Figure 4 (a) and Figure 4 (b) shown. Figure 4 (a) shows the relationship between the increase in the output of the IIDG and the short-circuit current at the downstream protection; Figure 4 (b) shows the relationship between the decrease in the output of the IIDG and the short-circuit current at the downstream protection; in the figure is the short-circuit current provided by the system; is the voltage at the connection point of the IIDG obtained from this iterative calculation; is the short-circuit current output before the output fluctuation of the IIDG obtained from this iterative calculation; and are the components of the short-circuit current provided by the IIDG on the d and q axes. Since the output fluctuation of the IIDG mainly affects the active current, it is assumed that is in phase with the output fluctuation ΔI DG ; I lim.max and I lim.min are the maximum and minimum currents corresponding to the condition that the protection meets the requirements.

[0096] From Figure 4 (a), it can be seen that the increase in the output of the IIDG will increase the component of the short-circuit current in the d-axis direction. Assume that when the output of the IIDG increases by ΔI DG.max , the current flowing through the downstream protection lim.max reaches I, that is, the protection does not meet the selectivity requirement at this time. From Figure 4 (b), it can be seen that without considering the current limiting condition, the decrease in the output of the IIDG will reduce the component of the short-circuit current in the d-axis direction. Assume that when the output of the IIDG decreases by ΔI DG.min , the current flowing through the protection Reach I lim.min , at this time, the protected range is too small, and the downstream protection will refuse to operate.

[0097] To sum up, to make the downstream protection operate correctly, it can be obtained from the phasor diagram that:

[0098]

[0099] When the above formula is an equal sign, the maximum value of the output fluctuation current corresponding to each protection setting value can be calculated and the minimum value

[0100] Since the output of IIDG is generally measured by power, expressing the output fluctuation in the form of active power, multiplying the maximum and minimum values of the output fluctuation current by the voltage at the connection point of IIDG obtained at this time, the upper and lower bounds of the allowable output fluctuation boundary of IIDG can be obtained, and the allowable output fluctuation boundary of IIDG corresponding to this setting value can be obtained:

[0101]

[0102] In the formula, ΔP DG.max , ΔP DG.min are the upper and lower bounds of the allowable output fluctuation boundary of IIDG. When the IIDG output fluctuation meets the following requirements, the protection setting value is risk-free and can operate correctly; when the IIDG output fluctuation does not meet this requirement, it is determined that the protection setting value has a risk.

[0103] By detecting the negative sequence current to judge the fault type as three-phase short circuit or two-phase short circuit, the additional impedance Z related to the fault type can be obtained Δ ;

[0104] Detect whether there is negative sequence current in the distribution network impedance, the impedance of the upstream line of the fault point or the impedance of the line where the fault point is located. If there is negative sequence current, it is a two-phase fault, otherwise it is a three-phase fault;

[0105] When there is a three-phase short circuit, Z Δ =0, when there is a two-phase short circuit The superscript "-" represents the negative sequence component, are the negative sequence components of the distribution network impedance, the impedance of the upstream line of the IIDG connection point, and the impedance of the downstream line respectively.

[0106] According to Figure 3 topological relationship, combined with this additional impedance, solve the following formula to obtain the voltage at the connection point of IIDG

[0107]

[0108] In the formula: are the short - circuit currents upstream and downstream of the IIDG connection point during a short - circuit fault in the current operating state, respectively, Z s , Z1, and Z2 are the distribution network impedance, the impedance of the upstream line of the IIDG connection point, and the impedance of the downstream line, respectively, is the short - circuit current output before the IIDG output power fluctuation calculated at this time, is the voltage calculated at the IIDG connection point, is the distribution network voltage, is the current of the upstream line of the IIDG connection point;

[0109] According to the voltage calculated at the IIDG connection point, calculate the short - circuit currents upstream and downstream of the IIDG connection point during a short - circuit fault in the current operating state

[0110] It should be noted that if the topology diagram of the power grid is Figure 3 a different topology diagram from, the voltage at the IIDG connection point, the short - circuit currents upstream and downstream of the IIDG connection point during a short - circuit fault in the current operating state can be calculated according to the connection relationship of the topology diagram, the short - circuit currents upstream and downstream.

[0111] From the analysis of the short - circuit current output characteristics of the IIDG, can be expressed as a piece - wise function of the voltage calculated iteratively at the connection point Therefore, there is a strong correlation between the inverter output current and the access point voltage, and it is very difficult to accurately calculate the short - circuit current directly by solving a linear equation.

[0112] Therefore, the present invention selects to use the Gauss iteration method to solve the following formula, and the iteration process is as follows:

[0113] The impedance matrix Z is:

[0114]

[0115] The correction equation is:

[0116]

[0117] The convergence criterion is:

[0118]

[0119] where k is the number of iterations; is the voltage at the connection point calculated in the k - th iteration; is the short - circuit current upstream of the IIDG connection point calculated in the k - th iteration during a short - circuit fault in the current operating state, are the short-circuit currents output before the IIDG output fluctuation calculated in the k-th and (k + 1)-th iterations respectively, and ε is the set threshold value.

[0120] Repeat the above iteration until the convergence criterion is met.

[0121] Since the maximum short-circuit current that the IIDG can output is usually limited by the converter to about 1.2 times the rated current, the output current of the IIDG before the fault occurs can be selected as the initial value for the iterative calculation. Since the impedance matrix Z is a positive definite matrix, the iterative method adopted will surely achieve convergence.

[0122] According to the voltage at the connection point of the IIDG obtained by solving Calculate the short-circuit currents at the upstream and downstream of the connection point of the IIDG when a short-circuit fault occurs under the current operating state

[0123] The short-circuit currents at the upstream and downstream of the connection point of the IIDG

[0124]

[0125] According to the short-circuit currents on the upstream and downstream lines of the connection point of the IIDG when a short-circuit fault occurs under the current operating state, calculate the adjustment coefficients of each protection setting value, and multiply the protection setting value judged as the risk setting value by its corresponding adjustment coefficient;

[0126] The calculation formula for the adjustment coefficient is:

[0127]

[0128] In the formula, K1 and K2 are the adjustment coefficients of the upstream and downstream protection setting values respectively; are the short-circuit currents at the upstream and downstream of the connection point of the IIDG when a short-circuit fault occurs under the current operating state; P ref is the IIDG output value; K 01 、K 02 are the reference coefficients of K1 and K2 respectively; for the protection setting values of the first and second segments, the reference coefficients K 01 、K 02 take 1.0, while for the protection setting values of the third segment, K 01 、K 02 take 1.1 to 1.3.

[0129] The numerator of the above formula is the result of the online iterative calculation of the short-circuit current after the IIDG is connected to the distribution network. And the denominator represents Figure 3When a short circuit occurs at point f2 at the end of the medium line L2 and the access of IIDG to the distribution network is not considered, the three-phase or two-phase short-circuit current flowing through the upstream and downstream lines is obtained through off-line calculation. Considering factors such as certain errors that may exist in short-circuit current calculation, a certain margin needs to be left for the protection range, and the sensitivity of the backup protection needs to be improved, etc., combined with actual simulation data, reference coefficients are set for the I and II section protections.

[0130] It should be noted that parameters such as the system rated voltage, system impedance, and line impedance in the above algorithms can be set in the protection device. And distribution network operation parameters such as the connection point voltage and the output current of the IIDG before the fault need to be obtained through the communication device, so as to realize the transmission between the dispatching side and the protection device. The telecontrol communication device based on the MMS (manufacturing message specification) message specification service can be applied to the transmission of distribution network operation information, and its maximum delay does not exceed 0.5 seconds. Considering that the primary energy output of the IIDG has a large time constant and the output current of the IIDG will not change suddenly in a short time, the use of MMS messages can meet the requirements of distribution network operation information transmission without the need for clock synchronization. In summary, when the distribution network containing IIDG is operating normally, the communication device can be used to enable the dispatching side to receive the operation parameters of the IIDG and the operation information of the distribution network online, and dynamically adjust the setting values of each section of the upstream and downstream protections online.

[0131] In this embodiment, a distribution network line model containing IIDG as shown in Figure 3 is built on the PSCAD / EMTDC platform. Protection 1 is installed upstream of the IIDG access point, that is, Protection 1 is the upstream protection device, and Protection 2 is installed downstream of the IIDG access point, and Protection 2 is the downstream protection device. The lengths of lines L1, L2, and L3 are 8 km, 3 km, and 4 km respectively, the unit impedance of the lines is 0.069 + j0.099 Ω / km, the rated voltage of the system equivalent power supply is 11 kV, and the system equivalent impedance is Z s = 0.00911 + j0.729 Ω.

[0132] On the MATLAB / Simulink platform, a program is written in combination with the PSCAD simulation data to implement the algorithm for solving the allowable output power fluctuation boundary of the IIDG corresponding to the protection setting value in Section 2.1. Taking the IIDG output power of 10 MW as an example, the selectivity of the downstream protection is verified under different output power fluctuations of the IIDG.

[0133] When setting a three-phase short circuit at the position f3 at the end of line L3 to simulate the situation where the output power of the IIDG is 10 MW and a three-phase short circuit fault occurs at the outlet of the adjacent line of Protection 2 and the output power of the IIDG fluctuates, the corresponding allowable output power fluctuation boundary of the IIDG of Protection 2 is calculated by combining the method proposed in Section 2.1, and the results and simulation data are shown in Table 1.

[0134] Calculation Results and Simulation Data when the Output of IIDG is 10 MW

[0135]

[0136] According to the data analysis in Table 1, when the output fluctuation of IIDG does not exceed the allowable output fluctuation range, the short - circuit current value flowing through Protection 2 is within the range, and the protection setting value is risk - free and can reliably not operate. When the output fluctuation of IIDG increases to exceed the corresponding IIDG output boundary of Protection 2, the short - circuit current flowing through Protection 2 is greater than I lim.max , and the first - stage current of Protection 2 will lose its corresponding selectivity and malfunction; when the output fluctuation of IIDG decreases to be lower than the minimum allowable output fluctuation value, the current of Protection 2 will be lower than I lim.min and cause the protection to refuse to operate. At this time, it is determined that the setting value of Protection 2 is a risk setting value, and communication should be made to the dispatching side, and immediately switch to the process of adjusting the setting value, update the protection setting value, and prevent Protection 2 from losing selectivity.

[0137] In summary, it is verified that the method of this embodiment can determine the allowable output fluctuation boundary of IIDG. When the output of IIDG fluctuates within this boundary, it can prevent the downstream protection from malfunctioning or refusing to operate, and effectively ensure the selectivity and reliability of the downstream protection in the access of IIDG to the distribution network.

[0138] To verify the dynamic adjustment method of protection mismatch risk setting value proposed in the present invention, based on the simulation operation data of IIDG, the iterative calculation process of short - circuit current is realized through programming on the MATLAB platform, and the calculated value of the short - circuit current flowing through Protection 1 when a three - phase short - circuit occurs at f2 at the end of Line L2 is obtained. The calculated results are compared with the corresponding simulation data obtained in PSCAD, as shown in Table 2.

[0139] Table 2 Comparison of Calculation Results and Simulation Results of the Three - Phase Short - Circuit Current of Protection 1

[0140]

[0141] According to the test results in Table 2, the average error of the iterative calculation results of the short - circuit current flowing through Protection 1 is 2.00%, which is basically equal to the simulation results, and the maximum number of iterations is 8 times. It can be seen from this that the dynamic adjustment algorithm of the setting value proposed in the present invention has a small error and a high calculation accuracy, and when the output of IIDG is different, the maximum number of iterations does not exceed 10 times, and the calculation convergence speed is relatively fast. Therefore, this algorithm has a certain feasibility in the dynamic adjustment of protection setting values.

[0142] The risk setting values verified this time are the second - stage protection setting value of Protection 1 current and the first - stage of Protection 2 current. Based on the short - circuit current calculation values provided in Table 1, the risk setting values are adjusted, where the K of the first and second stages of protection01 = K 02 = 1.0, the operating current before adjustment of the first segment of protection 2 current is The operating current before adjustment of the second segment of protection 1 current is

[0143] Under different IIDG output conditions, the setting values of the second segment of protection 1 current protection are updated, and the corresponding sensitivities are calculated, as shown in Table 3. It can be seen that the sensitivity of the method is greater than 1.3 under different outputs, meeting the sensitivity requirements, and can effectively improve the sensitivity of the second segment of the current protection upstream of the IIDG access point. Similarly, it can be obtained that this method can also improve the sensitivity of the third segment of the current and improve the protection performance.

[0144] Table 3 Setting values and sensitivities of the second segment of protection 1 current after correction

[0145]

[0146] When a three-phase short-circuit fault occurs at f2 at the end of line L2, the fault location exceeds the operating range of the instantaneous cut-off protection of protection 2 current. The short-circuit current flowing through protection 2 and the setting values after protection adjustment under different IIDG outputs are shown in Table 4. It can be seen from the data in Table 4 that when a fault occurs at the end of line L2 under different IIDG outputs, the first segment of protection 2 current protection will not operate. Therefore, this method can effectively prevent misoperation of the protection after IIDG access and ensure the selectivity of the downstream instantaneous cut-off protection after IIDG access.

[0147] Table 4 Revised setting values of the first segment of protection 2 current and short-circuit current values flowing through protection 2

[0148]

[0149] Embodiment 2 of the present invention proposes a system for identifying and adjusting the risk setting values of protection mismatch for a distribution network with IIDG access using the method described in Embodiment 1 of the present invention, including a boundary calculation module, a risk setting value identification module, a setting value adjustment module, and a setting value modification judgment module: It is characterized in that:

[0150] Boundary calculation module: used to obtain the operating state of the distribution network during normal operation, so as to calculate the allowable output fluctuation boundaries of IIDG corresponding to the setting values of upstream and downstream protections; the protection setting value is the threshold for judging whether a fault occurs in the distribution network and triggering protection actions, and the allowable output fluctuation boundary of IIDG is the critical range of IIDG output when the protection operates correctly. Beyond this range, the protection action will fail;

[0151] Risk fixed-value identification module: It is used to collect the output data of IIDG in real time. When the output fluctuation of IIDG meets the set criterion for starting the process, it judges whether the output fluctuation of IIDG is within the range of the allowable output fluctuation boundary of IIDG corresponding to each protection fixed value. If so, the corresponding protection fixed value is the risk fixed value;

[0152] Fixed-value adjustment module: If there is a protection fixed value that is the risk fixed value, it is used to calculate the short-circuit currents on the upstream and downstream lines of the IIDG connection point when a short-circuit fault occurs under the current operating state, and then adjust the protection fixed value judged to be the risk fixed value. At the same time, it updates the allowable output fluctuation boundary of IIDG corresponding to this protection fixed value;

[0153] Fixed-value modification judgment module: It is used to compare the adjusted protection fixed value with the protection fixed value before adjustment. If the difference exceeds the set allowable range, the adjusted protection fixed value is used; otherwise, the protection fixed value before adjustment is still used.

[0154] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: It is still possible to modify the specific implementation manners of the present invention or make equivalent replacements. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the protection scope of the claims of the present invention.

Claims

1. A method for identifying and adjusting the risk setting value of protection mismatch in a distribution network with IIDG access, characterized in that, include: During normal operation, the distribution network operation status is obtained and the IIDG allowable output fluctuation boundary corresponding to each upstream and downstream protection setting is calculated; The IIDG output fluctuation limit is the critical range of the IIDG output when the protection is in correct action. Exceeding this range will cause the protection action to fail. Collect IIDG output data in real time. When the IIDG output fluctuation meets the set process start criteria, determine whether the IIDG output fluctuation is within the IIDG allowable output fluctuation boundary range corresponding to each protection setting value. If so, the corresponding protection setting value is a risk setting value. If there is a protection setting value that is a risk setting value, the short-circuit current on the upstream and downstream lines of the IIDG grid connection point when a short-circuit fault occurs in the current operating state is calculated, and then the protection setting value judged as the risk setting value is adjusted, and the IIDG output fluctuation limit corresponding to the protection setting value is updated at the same time; Compare the adjusted protection setting with the protection setting before adjustment. If the difference exceeds the set allowable range, use the adjusted protection setting; otherwise, use the protection setting before adjustment.

2. The method for identifying and adjusting the fixed value of the protection mismatch risk of the distribution network connected to the IIDG according to claim 1 is characterized in that: Calculate the IIDG allowable output fluctuation boundary corresponding to each upstream and downstream protection setting value, specifically: The upstream and downstream protection settings include upstream I, II, III protection settings and downstream I, II, III protection settings; the upstream and downstream are the upstream and downstream lines of the IIDG grid connection point respectively; Set the maximum current and minimum current when each upstream and downstream protection device does not meet the selectivity requirements of each protection setting. The selectivity requirement is that after the IIDG is connected, when a three-phase short circuit occurs at the adjacent line outlet, the short-circuit current flowing through the current level line is greater than the corresponding protection setting; Calculate the maximum value of the output fluctuation current corresponding to each protection setting value according to the following formula, in combination with the set maximum current and minimum current and the minimum value In the formula, is the short-circuit current provided for the distribution network; and are the d-axis and q-axis components of the short-circuit current output before the IIDG output fluctuation calculated at this time. It is set that is in phase with the output fluctuation ΔI DG ; I lim.max and I lim.min are the maximum current and the minimum current when the selectivity requirements of each protection setting value are not met at each protection device. The maximum and minimum values of the output fluctuation current are multiplied by the voltage at the IIDG grid connection point calculated at this time to obtain the upper and lower limits of the IIDG output fluctuation boundary allowed.

3. The method for identifying and adjusting the fixed value of the protection mismatch risk of the distribution network connected to the IIDG according to claim 2 is characterized in that: The maximum current when each protection device does not meet the selectivity requirements of each protection setting is set to 0.95 times the corresponding protection setting of the downstream protection device, and the minimum current when each protection device does not meet the selectivity requirements of each protection setting is set to 0.8 times the corresponding protection setting of the downstream protection device.

4. The method for identifying and adjusting the fixed value of the protection mismatch risk of the distribution network connected to the IIDG according to claim 1, characterized in that: The set process start criterion is that the IIDG output fluctuation continuously increases or continuously decreases.

5. The method for identifying and adjusting the fixed value of the protection mismatch risk of the distribution network connected to the IIDG according to claim 1, characterized in that: Calculate the short-circuit current on the upstream and downstream lines of the IIDG grid connection point when a short-circuit fault occurs in the current operating state, specifically: By detecting the negative-sequence current to determine the fault type as three-phase short circuit or two-phase short circuit, the additional impedance Z related to the fault type can be obtained Δ ; Combined with the additional impedance, the voltage at the IIDG grid connection point is obtained by solving the following formula: Wherein: are the short-circuit currents upstream and downstream of the IIDG connection point during a short-circuit fault in the current operating state, respectively, and Z s , Z1, and Z2 are the distribution network impedance, the upstream line impedance of the IIDG connection point, and the downstream line impedance, respectively, is the short-circuit current output before the IIDG output fluctuation calculated at this time, is the voltage at the IIDG connection point calculated by iterative calculation, is the distribution network voltage, is the upstream line current of the IIDG connection point; According to the voltage at the connection point of the IIDG obtained by solving Calculate the short-circuit currents upstream and downstream of the connection point of the IIDG when a short-circuit fault occurs under the current operating state 6. A method for identifying and adjusting the risk fixed value of protection mismatch in a distribution network with IIDG access according to claim 5, characterized in that: Detect whether there is negative sequence current in the distribution network impedance, the upstream line impedance of the fault point or the line impedance where the fault point is located. If there is negative sequence current, it is a two-phase fault; otherwise, it is a three-phase fault. When a three-phase short circuit occurs, Z Δ = 0. When a two-phase short circuit occurs The superscript "-" represents the negative-sequence component, which are the negative-sequence components of the distribution network impedance, the upstream line impedance of the IIDG connection point, and the downstream line impedance, respectively.

7. A method for identifying and adjusting the risk fixed value of protection mismatch in a distribution network with IIDG access according to claim 5, characterized in that: Solve for the voltage at the connection point of the IIDG, and use the Gauss iteration method for the solution. When iterating, select the output current of the IIDG before the fault occurs as the initial value during the iterative calculation. of the initial value.

8. A method for identifying and adjusting the risk fixed value of protection mismatch in a distribution network with IIDG access according to claim 7, characterized in that: Adjust the protection fixed value determined to be the risk fixed value, specifically: According to the short-circuit current on the upstream and downstream lines of the IIDG connection point when a short-circuit fault occurs under the current operating state, calculate the adjustment coefficient of each protection fixed value, and multiply the protection fixed value determined to be the risk fixed value by its corresponding adjustment coefficient. The calculation formula for the adjustment coefficient is: Wherein, K1 and K2 are the adjustment coefficients of the upstream and downstream protection setting values respectively; are the short-circuit currents upstream and downstream of the IIDG connection point during a short-circuit fault in the current operating state; P ref is the output value of the IIDG; K 01 and K 02 are the reference coefficients of K1 and K2 respectively; for the protection setting values of Sections I and II, the reference coefficients K 01 and K 02 take 1.0, while for the protection setting value of Section III, K 01 and K 02 take 1.1 to 1.

3.

9. A method for identifying and adjusting the risk fixed value of protection mismatch in a distribution network with IIDG access according to claim 1, characterized in that: The set allowable range is 90% - 110% of the protection fixed value before adjustment. If the adjusted fixed value exceeds this range, the adjusted fixed value is used.

10. A system for identifying and adjusting the risk fixed value of protection mismatch in a distribution network with IIDG access using the method according to any one of claims 1-9, including a boundary calculation module, a risk fixed value identification module, a fixed value adjustment module, and a fixed value modification judgment module: characterized in that: The boundary calculation module: used to obtain the operating state of the distribution network during normal operation, so as to calculate the allowable output fluctuation boundary of the IIDG corresponding to each protection fixed value upstream and downstream; the protection fixed value is the threshold for judging whether a fault occurs in the distribution network and triggering the protection action, and the allowable output fluctuation boundary of the IIDG is the critical range of the IIDG output when the protection operates correctly. Exceeding this range will cause the protection action to fail. The risk fixed value identification module: used to collect the IIDG output data in real time. When the IIDG output fluctuation meets the set process start criterion, judge whether the IIDG output fluctuation is within the allowable output fluctuation boundary of the IIDG corresponding to each protection fixed value. If so, the corresponding protection fixed value is the risk fixed value. The fixed value adjustment module: used to calculate the short-circuit current on the upstream line and the downstream line of the IIDG connection point when a short-circuit fault occurs under the current operating state if there is a protection fixed value that is the risk fixed value, and then adjust the protection fixed value determined to be the risk fixed value, and at the same time update the allowable output fluctuation boundary of the IIDG corresponding to this protection fixed value. The fixed value modification judgment module: used to compare the adjusted protection fixed value with the protection fixed value before adjustment. If the difference exceeds the set allowable range, the adjusted protection fixed value is used; otherwise, the protection fixed value before adjustment is still used.

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