Consider the power multi-quadrant mode under the coupling system static voltage stability evaluation method

Abstract

The application discloses a static voltage stability evaluation method of a coupled system in a power multi-quadrant mode, and is implemented according to the following steps: for a node in a simple two-node system, voltage rise and drop capability evaluation indexes and static voltage safety evaluation indexes are established in the power multi-quadrant mode; for a node in a simple multi-node system, voltage rise and drop capability evaluation indexes and static voltage safety evaluation indexes suitable for the power multi-quadrant mode are established according to power and voltage characteristics at a voltage collapse point; and static voltage stability evaluation results of the coupled system are obtained according to the voltage rise and drop capability evaluation indexes and the static voltage safety evaluation indexes of the multi-node complex power system. The evaluation method can improve the capability of supporting the stability of a power grid, and can provide important index basis for guiding the planning, operation and control of a power system containing a high proportion of new energy.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of voltage stability analysis and control of power systems containing a high proportion of renewable energy, and specifically relates to a static voltage stability evaluation method for a coupled system in a power multi-quadrant mode. BACKGROUND

[0002] With the continuous access of renewable energy to the power system and the continuous promotion of the "integration of wind, light, fire and storage", the boundary of the "source-load" attribute of the nodes in the power grid becomes blurred. In particular, the coupling system formed by thermal power and renewable energy at the same grid-connected point is widely used, the comprehensive active and reactive power of the nodes in the coupling system presents a "four-quadrant" mode, the static voltage stability level will produce uncertain fluctuations, and it will bring a great impact on the safe and stable operation of the system.

[0003] Static voltage stability evaluation is a most commonly used index for reflecting the static voltage stability of a power system, and it is of great significance for guiding the voltage safe operation of the power system.

[0004] At present, a large number of researches have been carried out on the static voltage security analysis and evaluation of traditional power systems. Ref. [Wang Y, Lu JP, Li WY, et al. Equivalent model based on local network voltage phasor and its voltage stability index [J]. Proceedings of the CSEE, 2008, 28(34): 52-58] and Ref. [Liu MS, Zhang BM, Yao LZ, et al. Voltage stability on-line monitoring based on PMU and improved Thevenin equivalent model [J]. Automation of Electric Power Systems, 2009, 33(10): 6-10] face the load nodes, establish the node Thevenin equivalent circuit through local measurement information, and put forward the voltage stability index based on the power transmission limit; Ref. [Mariana Kamel, Abdelrahman A. Karrar, Ahmed H. Eltom, Development and Application of a New Voltage Stability Index for On-Line Monitoring and Shedding [J]. IEEE Transaction on Power System, 2018, 33(2): 1231-1241] analyzes the relationship between the growth of node power and the drop of voltage, and puts forward the node voltage stability index based on the voltage-power sensitivity; Ref. [Zhang Qian, Zheng Huiping, Wang Jinhao, et al. Calculation method of power system static voltage stability margin based on AQ node [J]. Power System Technology, 2019, 43(2): 714-721] constructs the voltage stability index based on the singular value / eigenvalue of Jacobian matrix according to the relationship between the voltage solution of system power flow and Jacobian matrix. The above researches are all for the voltage stability evaluation under single load power mode, without considering the influence of renewable energy access and power multi-quadrant change, which cannot meet the demand of voltage security analysis under the wide access of new energy.

[0005] The document [Du Xiaoxiao, Zhou Lin, Guo Ke, et al. Large photovoltaic power station static voltage stability analysis [J]. Power grid technology, 2015, 39 (12): 3427-3434.] takes large photovoltaic power station as the research object, and analyzes the influence of factors such as light intensity and installed capacity on the voltage stability of the grid connection point and internal voltage of the power station by using the minimum eigenvalue of Jacobian matrix; The document [Xu Xiaoyan, Huang Yuehui, Liu Chun, et al. Influence of distributed photovoltaic power generation on distribution network voltage and solution scheme of voltage out-of-limit [J]. Power grid technology, 2010, 34 (10): 140-146.] combines the R / X ratio of the distribution network, analyzes the reason of voltage uplift caused by the reverse flow of photovoltaic access distribution network power flow, and puts forward the scheme to suppress voltage uplift. The above documents study the static voltage safety problem after the renewable energy access to the power grid, but only consider the single case of node power supply power growth, do not consider the voltage stability evaluation under the power multi-quadrant mode after the high proportion of renewable energy access, it is difficult to accurately describe the static voltage safety level of the coupled system, lack of analysis and adaptability of voltage stability state evaluation of voltage uplift or decline mechanism of multi-quadrant power mode and different grid properties, which may cause one-sided evaluation, and cannot make comprehensive, rapid, sensitive and fine response to the changing power grid state, and weaken the monitoring, optimization and control of the system. SUMMARY

[0006] The purpose of the present application is to provide a coupled system static voltage stability evaluation method considering power multi-quadrant mode, which solves the problem of inaccurate evaluation results caused by not considering multi-quadrant mode in the existing voltage stability evaluation method, and can improve the ability of power grid stability support.

[0007] The technical scheme adopted by the present application is a coupled system static voltage stability evaluation method considering power multi-quadrant mode, which is implemented according to the following steps:

[0008] Step 1, for a simple two-node system, in consideration of power multi-quadrant mode, establish voltage uplift, decline capacity evaluation index and static voltage safety evaluation index;

[0009] Step 2, for a multi-node complex system, according to the power and voltage characteristics at the voltage collapse point, establish voltage uplift, decline capacity evaluation index and static voltage safety evaluation index suitable for power multi-quadrant mode;

[0010] Step 3, obtain the coupled system static voltage stability evaluation result according to the voltage uplift, decline capacity evaluation index and static voltage safety evaluation index of the multi-node complex power system.

[0011] The present application has the following characteristics:

[0012] The specific process of step 1 voltage uplift, decline capacity evaluation index is:

[0013] For the simple two-node system, the voltage rise and drop capability evaluation index is constructed considering the power multi-quadrant mode:

[0014]

[0015] where γ = dU B / dP net represents the sensitivity of voltage to power; the variation range of VRI is 0-1, and the variation range of VDI is -1-0.

[0016] The establishment process of step 1 static voltage safety evaluation index is:

[0017] Based on the PV curve formed by active power and voltage in the two-node system, the static voltage stability evaluation index suitable for the power multi-quadrant mode is established according to the characteristics of active power and voltage in the evolution process of PV curve, which is specifically:

[0018] For the simple two-node system, U A is the voltage amplitude of the head node; U B is the voltage amplitude of the tail node; a virtual conductance G is introduced at node B, P net represents the comprehensive active and reactive power injected at the tail node, then P net and U B satisfy:

[0019]

[0020] The full differential expression of formula (7) is:

[0021] △P net = (U B +△U B ) 2 △G+(2U B +△U B )G△U B (8)

[0022] When the change △G→0 and △U B →0, the voltage stability index VSI is defined as:

[0023]

[0024] where γ = dU B / dP net , μ = P net / U B .

[0025] The establishment process of step 1 static voltage safety evaluation index is:

[0026] Based on the QV curve formed by reactive power and voltage in two-node system, the static voltage stability evaluation index suitable for power multi-quadrant mode is established according to the characteristics of reactive power and voltage in the evolution process of QV curve, which is specifically:

[0027] For the simple two-node system, a virtual conductance G is introduced at node B, then Q net and U B satisfy:

[0028]

[0029] The total differential of formula (10) can be expressed as:

[0030] △Q net =(U B +△U B ) 2 △B+(2U B +△U B )B△U B (11)

[0031] When the change △B→0, △U B →0, the voltage stability index VSI is defined as:

[0032]

[0033] In the formula, γ=dU B / dP net , μ=P net / U B .

[0034] The specific process of step 2 is:

[0035] Step 2.1, the system sensitivity is obtained by the power flow correction equation, and the voltage lifting and lowering capacity evaluation index and static voltage safety evaluation index are extended to the multi-node complex power system;

[0036] Step 2.2, combined with the voltage lifting and lowering capacity evaluation index and static voltage stability index of the simple two-node coupled system, the voltage lifting and lowering capacity evaluation index and static voltage safety evaluation index of node i in the multi-node complex power system are constructed as:

[0037]

[0038]

[0039] In the formula, μ i =P i / U i , Wherein, ρij =P j / P i 、 NP represents the total number of PQ nodes and PV nodes in the system, and NQ represents the total number of PQ nodes in the system.

[0040] The power flow correction equation expression is:

[0041]

[0042] Where ΔP and ΔQ are the small increments of active and reactive power at the system nodes; Δθ and ΔU represent the corrections of the phase and voltage at the system nodes, respectively. Pθ , J PU , J Qθ and J QU represents the corresponding submatrix of the system Jacobian matrix J.

[0043] The beneficial effects of the present invention are:

[0044] (1) For this generalized coupled system with a high proportion of renewable energy and thermal power, the present invention reveals the voltage evolution mechanism in different power quadrants from two aspects: voltage analytical expression and voltage component geometric characteristics;

[0045] (2) In view of the fact that the voltage amplitude of the coupled system node in the multi-quadrant power mode can show different voltage rise or fall trends in different power change intervals, the present invention proposes a sensitivity normalized voltage rise capability evaluation index and a voltage fall capability evaluation index, which can be used to evaluate the voltage trend characteristics of different nodes and different power changes;

[0046] (3) According to the PV / QV curve variation characteristics of the coupled system nodes in different power quadrants, the present invention constructs a voltage stability evaluation index based on the voltage-power variation characteristics of the voltage collapse point to evaluate the stability level of the coupled system node voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 This is a flow chart of the method for evaluating static voltage stability of a coupled system in a power multi-quadrant mode according to the present invention;

[0048] Figure 2 is the infinite power grid-generalized node system diagram;

[0049] Figure 3 PV curves under tanφ for different systems;

[0050] Figure 4 PV curves under different impedance parameter ratios;

[0051] Figure 5Figure for voltage deviation curve of node under power multi-quadrant;

[0052] Figure 6 Network topology graph of IEEE 14-node system;

[0053] Figure 7 PV curve graph of IEEE 14-node system under power multi-quadrant;

[0054] Figure 8 Voltage rise and drop capacity evaluation index of IEEE 14-node system;

[0055] Figure 9 Voltage stability evaluation index of IEEE 14-node system;

[0056] Figure 10 Coupling system graph of a certain region in Liaoning;

[0057] Figure 11 PV curve graph of key node of coupling system;

[0058] Figure 12 Voltage rise and drop capacity evaluation index graph of coupling system;

[0059] Figure 13 Voltage stability evaluation index graph of coupling system. DETAILED DESCRIPTION

[0060] The application will be described in detail below in combination with specific embodiments.

[0061] In the application, for the nodes of the coupling system, the voltage evolution mechanism is revealed in two aspects of voltage analytical expression and voltage deviation geometric characteristics under consideration of power multi-quadrant mode, specifically:

[0062] First, the analytical expression of the voltage of the node of the coupling system under the multi-quadrant power mode is established;

[0063] As shown in the infinite grid-generalized node system, Figure 1 node A is the bus of the infinite system, node B represents the generalized node of the grid, which can be a new energy import node, a load node, a comprehensive access node of new energy and load, the comprehensive power of node B is:

[0064]

[0065] In the formula, P net , Q net are the comprehensive active and reactive power of node B; P B,g , Q B,g , PB,l , Q B,l are the active and reactive power of renewable energy and the active and reactive power of load respectively; P A ,Q A are the active and reactive power of node A respectively. Assume that the power factor angle is There is a four-quadrant power mode, namely: ①P net >0, Quadrant 1 mode; ②P net <0, Quadrant 2 mode; ③P net <0, Quadrant 3 mode; ④P net >0, It is quadrant 4 mode.

[0066] The line impedance Z = R + jX, and the voltage relationship between nodes A and B can be expressed as:

[0067]

[0068] Further, establish about U B 2 The expression is:

[0069]

[0070] Solving for U B 2 , we can get:

[0071]

[0072] From formula (4), we can see that U B It's about P net 、 function of R and X. and R / X impedance ratio, U B Follow P net The power (unit value, based on 100MVA) changes to obtain different PV curves, such as Figure 3 、 4 shown.

[0073] For analysis Figure 2 、 3 The voltage change trend is calculated by U in equation (4). B P net The sensitivity of The positive and negative can reflect the U B Follow P net The evolution of change. Expressed as:

[0074]

[0075] where, represents the voltage rise in the current state; otherwise, the voltage drop. Among them, corresponding to the voltage maximum state, represents the power limit state.

[0076] Analysis of the geometric characteristics of the voltage deviation of the coupling system node in the multi-quadrant mode:

[0077] When establishing the analytical expression of the coupling system node voltage in the multi-quadrant power mode, the voltage U A and U B (effective solution) represent different size relationships. When the power changes to the power limit, there are always U B <U A , always U B >U A , and U B >U A →U B =U A →U B <U A . To reveal the mechanism of the above scenarios, for the system, the polar coordinate diagram is introduced to describe the voltage deviation geometric characteristics based on the mathematical expression of the voltage difference. Figure 2

[0078] The voltage relationship between nodes A and B can be expressed as:

[0079]

[0080] In the formula, ΔU Re and δU Im are the vertical and horizontal components of the voltage drop , and their expressions are:

[0081]

[0082]

[0083] According to formulas (6)-(8), the voltage polar coordinate diagram is drawn to represent the voltage size relationship between nodes A and B, as shown in Figure 5 . In the figure, the black curve represents the circular arc AA' with O as the center and OA as the radius, where OA=|U A |=1. The connecting line between any point and A point represents the polar radius, which corresponds to the amplitude |dU| of the voltage drop. The projection components of the polar radius in the direction of the 0° and 90° axes are-ΔU Re and-δU Im , respectively. Arcs AB', AB'', and AB'''represent the different voltage size relationships between nodes A and B in different power quadrants.​ The change track of the voltage difference as the power continues to change to the voltage collapse point The O point and The line connecting any point on the track is represented as

[0084] By Figure 5 The size relationship between U A And U B When the point on the arc AB' (or AB'', AB''') is located outside the circular arc AA', it represents U B > U A ; when the point on the arc AB' (or AB'', AB''') is located inside the circular arc AA', it represents U B <U A ; when the two arcs intersect, it represents U B = U A . The size relationship is expressed through the joint action of the vertical component and the horizontal component of the voltage drop , and the neglect of any component will make the result inaccurate, even produce completely different conclusions in size relationship. The voltage drop and the size and direction of active and reactive power transmission of the branch have a close relationship, and the branch transmission power is affected by the comprehensive influence of the power loss on the node B power and the branch impedance. For example, in the scenario of Figure 5 (a) , the increase of the power injected by node B and the change of the power loss on the branch impedance, the direction of the vertical component of the voltage deviation also changes, resulting in the change of the size and direction of the power of node A, and then showing the evolution state of U B > U A → U B = U A → U B <U A ; in Figure 5 (c), the power consumption of node B continues to increase, node A always sends power to meet the power consumption of node B and impedance branch, the direction of the vertical and horizontal components of the voltage deviation does not change, and there is always a state of U B <U A .

[0085] Based on this, the application proposes a coupling system static voltage stability evaluation method considering the power multi-quadrant mode, which is implemented according to the following steps:

[0086] Step 1, the voltage amplitude in different power variation intervals in the power multi-quadrant mode can present different voltage rising or falling trends, and the voltage rising or falling capacity needs to be determined in a given operating state, which plays an important role in analyzing whether the voltage evolution trend exceeds the limit. Therefore, for a simple two-node system, in the power multi-quadrant mode, the voltage rise and fall capacity evaluation index and the static voltage safety evaluation index are established to evaluate the voltage trend characteristics under different nodes and different power variations;

[0087] For a simple two-node system, it can be known from the analysis of the voltage evolution mechanism that the sensitivity can represent the evolution trend of U B with P net , which reflects the influence of node power variation on voltage, and only considers the node power variation, represented by dU B / dP net . Therefore, the voltage rise and fall capacity evaluation indexes can be constructed based on dU B / dP net . The voltage rise capacity evaluation index VRI (Voltage Rise Index) and the voltage drop capacity evaluation index VDI (Voltage Drop Index) are defined, and the expressions are as follows:

[0088]

[0089] In the formula, γ = dU B / dP net represents the sensitivity of voltage to power; the change range of VRI is 0-1, the larger the value of VRI is, the stronger the lifting capacity of the node voltage under the current power variation is, and vice versa, when VRI = 0, it means that the voltage is at the maximum value, and there is no lifting effect. The change range of VDI is -1-0. The larger the absolute value of VDI is, the stronger the falling capacity of the node voltage under the current power variation is, and vice versa.

[0090] The establishment process of the static voltage safety evaluation index is as follows:

[0091] Based on the evolution process of the P-V curve, the static voltage stability evaluation index suitable for the power multi-quadrant mode is established:

[0092] For a simple two-node system, U A is the voltage amplitude of the head node; U B is the voltage amplitude of the tail node, a virtual conductance G is introduced at node B, and P net represents the comprehensive active and reactive power injected at the tail node, then P net and UB satisfy: The total differential can be expressed as:

[0093] △P net =(U B +△U B ) 2 △G+(2U B +△U B )G△U B (10)

[0094] Where, the first term represents the virtual conductance change ΔG to P net The second term represents the node voltage change ΔU B P net The first term is the virtual conductance trying to get more power by increasing ΔG, and the second term is the voltage drop ΔU. B It will cause the node power to decrease. When the system reaches the voltage collapse state, the interaction between the two is exactly offset, P net Therefore, the voltage stability index can be constructed based on the ratio of the two items, considering the change ΔG→0, ΔU B →0, the voltage stability index VSI (Voltage Stability Index) is defined as:

[0095]

[0096] In the above formula, dU B / dG can be expressed as:

[0097]

[0098] Further, according to dP net / dG is expressed as:

[0099]

[0100] Combined equations (12) and (13), dU B / dG can be expressed as:

[0101]

[0102] Formula (14) and Substituting into formula (11), we can get:

[0103]

[0104] According to γ=dU B / dP net , and let μ=P net / U B , then:

[0105]

[0106] From equation (16), the smaller the VSI value, the higher the node voltage stability level, and vice versa. When the voltage is at the collapse point, VSI reaches the maximum value 1. The voltage stability evaluation index established by equation (16) is based on the characteristics of active power and voltage in the PV curve evolution process.

[0107] To make the coupling system node voltage evaluation index more complete and reflect the superiority of the application, the QV curve variation law will be derived below, and another way of establishing a static voltage safety evaluation index will be adopted. The specific process is as follows:

[0108] Based on the characteristics of reactive power and voltage in the QV curve evolution process, the static voltage stability evaluation index suitable for the power multi-quadrant mode is established as follows:

[0109] For a simple two-node system, a virtual conductance G is introduced at node B, so that Q net and U B satisfy: Taking the total differential, it can be expressed as:

[0110] △Q net = (U B +△U B ) 2 △B+(2U B +△U B )B△U B (17)

[0111] In the equation, the first term represents the influence of the change amount ΔB of the susceptance on Q net , and the second term represents the influence of the change amount ΔU B of the node voltage on Q net . In the first term, more reactive power is obtained by increasing ΔB, and in the second term, the decrease of ΔU B will cause the reactive power to decrease. When the system reaches the voltage collapse state, the two interact and cancel each other out, and Q net reaches the limit state. Therefore, the voltage stability index VSI can be established according to the ratio of the two terms. When the change amounts ΔB→0 and ΔU B →0 are considered:

[0112]

[0113] Where dU B / dB can be expressed as:

[0114]

[0115] According to dQ net / dB can be expressed as:

[0116]

[0117] Simultaneous equations (19) and (20) dU B / dB can be expressed as:

[0118]

[0119] Equation (21) and into equation (18), and after rearrangement, we have:

[0120]

[0121] According to Equation (22) can be converted to:

[0122]

[0123] Comparing equation (23) with equation (16), although equation (16) is derived from the change law of PV curve, and equation (23) is derived based on the change law of QV curve, both of them are for describing the voltage stability under the same power growth, and finally have uniformity in the form of index.

[0124] Step 2, in order to prove the universality of the application, the voltage rise and drop capacity evaluation index and the voltage stability evaluation index proposed in the application are generalized to a multi-node complex system. For the multi-node complex system, according to the power and voltage characteristics at the voltage collapse point, the voltage rise and drop capacity evaluation index and the static voltage safety evaluation index suitable for the multi-node complex power system under the power multi-quadrant mode are established;

[0125] The specific process is as follows:

[0126] The system sensitivity is derived through the power flow correction equation, and the voltage rise and drop capacity evaluation index and the static voltage safety evaluation index are generalized to the multi-node complex power system;

[0127] Considering that the active and reactive power of the system node is a small increment of the current state, ΔP and ΔQ, based on the current operating state, according to the power flow correction equation, the following expression is obtained:

[0128]

[0129] In the formula, ΔP and ΔQ are the small increments of the active and reactive power of the system node; Δθ and ΔU represent the correction amounts of the phase and voltage of the system node respectively, JPθ , J PU , J Qθ and J QU denote the corresponding sub-matrix of the system Jacobian matrix J.

[0130]

[0131] By rearranging equation (25), the relationship between ΔU and ΔP is obtained as:

[0132]

[0133] △U = J' UP △P + J' UQ △Q (27)

[0134] where,

[0135] By equation (27), the voltage correction of node i can be expressed as:

[0136]

[0137] where NP denotes the total number of PQ nodes and PV nodes in the system, and NQ denotes the total number of PQ nodes in the system. Divide both sides of the above equation by ΔP i , and consider ΔU i / ΔP i → dU i / dP i , which can be expressed as:

[0138]

[0139] where ρ ij = ΔP j / ΔP i = P j / P i , ρ ij tanφ j = P j / P i · Q j / P j = Q j / P i = ΔQ j / ΔP i , that is, the ratio of the system power to the current state of the same small increment can be expressed as the ratio of the corresponding power.

[0140] By combining equation (29) with equations (9) and (16), we can construct the voltage raising and lowering capability evaluation index and the static voltage stability evaluation index under the multi-node system. The voltage raising and lowering capability evaluation index and the static voltage safety evaluation index of node i are:

[0141]

[0142]

[0143] Where: μ i =P i / U i 、 in, ρ ij =P j / P i 、 NP represents the total number of PQ nodes and PV nodes in the system, and NQ represents the total number of PQ nodes in the system.

[0144] Step 3: Obtain the static voltage stability assessment result of the coupled system based on the voltage rise and fall capability assessment index and the static voltage safety assessment index of the multi-node complex power system.

[0145] Equations (30) and (31) can be used to evaluate the safe operation status of the multi-node system voltage. The nodes with larger absolute values ​​of VRI or VDI indicate that their voltage raising or lowering capabilities are stronger. The nodes with larger VSI values ​​are more likely to have voltage approaching the collapse point and thus voltage instability. By calculating the relevant indicators of VRI, VDI and VSI, the key node set that affects the system voltage safety can be determined, which provides an important basis for guiding the voltage regulation of the coupled system in the power multi-quadrant mode.

[0146] In the static voltage stability assessment method of the coupled system under the power multi-quadrant mode, the present invention adopts two methods, voltage analytical expression and voltage component geometric characteristics, to reveal the evolution mechanism of the coupled system node voltage under different power quadrants. Based on the above two aspects, the voltage evolution mechanism analysis and corresponding formula are revealed, and the following is obtained: Figure 3 and Figure 4 PV curve diagram shown.

[0147] Figure 3 Indicates the same impedance parameters (R = 0.054, X = 0.233), different The PV curve under the above equation. The upper half of the curve is the high-pressure solution of equation (4), and the lower half is the low-pressure solution of equation (4). The upper half is the voltage effective solution, and the “nose point” of the PV curve is defined as the system’s limit power P max , which corresponds to the voltage collapse point of the saddle-node bifurcation. Figure 3It can be seen that for the four quadrants of power, U B The changes show different change properties, and their characteristics are: P net Greater than zero and When the voltage is greater than a certain value (-0.2 in the figure), the effective voltage solution increases with P net Growth shows a process of first rising and then falling; P net Greater than zero and When the voltage is less than a certain value (-0.2 in the figure), the effective voltage solution increases with P net Growth shows a continuous downward trend; P net Less than zero and When the voltage is greater than a certain value (-0.2 in the figure), the effective voltage solution increases with P net The absolute value growth shows a continuous downward trend, which includes the traditional load node power growth scenario; net Less than zero and When the voltage is less than a certain value (-0.2 in the figure), the effective voltage solution increases with P net The absolute value growth shows a process of first rising and then falling. In addition, in different power quadrant modes, the limit power level (absolute value) corresponding to the voltage collapse point also varies greatly. net >0 and When the power limit is about 5.13 pu (based on 100MW), net <0 and When P max About 2.4pu, in P net <0 and When , the power limit is about -1.14pu.

[0148] Figure 4 As shown, in When 0.1 is 0.2 and 0.3, the PV curves under different R / X ratios reflect the properties of the transmission and distribution lines at different voltage levels. P net When P > 0, as the R / X ratio increases, the voltage effective solution rises more and the power limit level becomes higher; net <0, as the R / X ratio increases, the voltage effective solution decreases more and the power limit level becomes lower; P net >0, when the R / X ratio is greater than 0.4, the effective voltage solution shows a change characteristic of first rising and then falling. When the R / X ratio is less than 0.4, the effective voltage solution continues to fall. P netWhen R / X ratio is less than 0.4, the effective solution of voltage presents the change characteristics of lifting first and then falling, and when R / X ratio is greater than 0.4, the effective solution of voltage continuously falls. and P net When R / X ratio is greater than 0, the voltage lifting amplitude is large and the power limit level is high, and when R / X ratio is less than 0, the voltage falling amplitude is large and the limit power level is low. and P net When R / X ratio is less than 0, the voltage falling amplitude is large and the limit power level is low, and under other power change forms, the voltage lifting / falling presents more complex change forms and different power limit levels.

[0149] The feasibility and effectiveness of the method are illustrated by taking two specific examples of IEEE14 node system and a regional new energy and traditional power coupling grid system in Liaoning Province.

[0150] Embodiment 1

[0151] The IEEE14 node system diagram is shown in Figure 6 , and for the active and reactive power of original load nodes 12, 13 and 14, four-quadrant power modes of [-P0, -Q0], [P0, -Q0], [P0, Q0] and [-P0, Q0] are obtained based on the original values [P0, Q0] to analyze the voltage safe operation level of the power continuously changing node under different quadrants. When nodes 12, 13 and 14 increase synchronously according to the initial power under different power quadrants, the node PV curve, voltage lifting and falling capacity index and static voltage stability index are shown in Figure 7 , 8 and 9. Wherein, k represents the power change rate, that is, the ratio of the difference between the current power and the initial power to the initial power.

[0152] It can be seen from Figure 7 that the voltage evolution state of the node is different under the power continuously changing in different quadrant modes, and the power limit and voltage amplitude corresponding to the voltage collapse point have great differences. When the node presents the power attribute, the voltage has a relatively obvious lifting effect during the continuous increase of k, the voltage corresponding to the voltage collapse point is large, and the power limit is also large; when the node presents the load attribute, the voltage maintains a downward trend, the voltage amplitude corresponding to the voltage collapse point is small, and the power limit is also small. However, the voltage collapse point depending on the PV curve cannot accurately judge the voltage stability of the system.

[0153] It can be seen from Figure 8 that during the increase of k, the VRI and VDI indexes can depict the voltage lifting and falling process under the corresponding quadrant state in Figure 7 , and the size of the corresponding index reflects the strong and weak relationship of the voltage lifting and falling capacity of different nodes.

[0154] Figure 9The VSI of the nodes 12, 13 and 14 and a voltage stability index based on the minimum eigenvalue λ of the system Jacobian matrix are given. min When the system voltage is stable, λ min > 0, indicating that there is an effective solution for the voltage, that is, the index 1-λ min < 1; when the system is at a voltage collapse point, λ min = 0, that is, the index 1-λ min = 1. The VSI index can accurately reflect the static voltage stability level of the node in the k growth process in different quadrant modes, wherein the VSI value is negative, corresponding to the voltage lifting stage in the corresponding PV curve. Compared with 1-λ min , the VSI index is significantly reduced in the calculation scale, is more efficient in calculation, and is convenient for determining the weak node of the system voltage.

[0155] Embodiment 2

[0156] A new energy and traditional power coupling grid system diagram of a certain area in Liaoning Province is shown as Figure 10 indicated. The voltage level of the 20kV thermal power unit of the regional power grid is 20kV, the wind power unit is 0.7kV, and the photovoltaic unit is 0.4kV; the total capacity of the thermal power unit is 1200MW, and the total capacity of the new energy station is 300MW. Node 1, 10 and 11 are the grid connection points of the coupling system, the wind power station and the photovoltaic power station, respectively, and nodes 21 and 74 are the internal end nodes of the wind power station and the photovoltaic power station.

[0157] As can be seen from Figure 11 , compared with the high voltage level, the voltage lifting problem of the internal nodes of the low voltage level new energy station in the coupling system is more prominent; as can be seen from Figure 12 , 13 , the VRI and VDI proposed in the application reflect the voltage lifting and dropping capacity of different nodes, and the VSI can accurately reflect the voltage stability level of different nodes.

[0158] As can be seen from Embodiment 1 and Embodiment 2, the VDI and VRI two indexes are used to represent the node voltage evolution state when the power continuously changes in different power quadrant modes; the VSI index can easily find the key node causing the voltage instability of the system.

[0159] In summary, the model provided in the application can further mine the maximum allowable new energy power carrying capacity in static stability, improve the ability to support the stability of the power grid, and can provide important index basis for guiding the planning, operation and control of the power system containing a high proportion of new energy.

Claims

1. A method for evaluating the static voltage stability of a coupled system in a power multi-quadrant mode is proposed, characterized in that: Please follow the steps below to implement: Step 1: For a simple two-node system, establish voltage rise and fall capability evaluation indicators and static voltage safety evaluation indicators in consideration of the power multi-quadrant mode; The voltage raising and lowering capability evaluation indicators are VRI and VDI, respectively, which are expressed as follows: （6） Where, γ = dU B / dP net Indicates the sensitivity of voltage to power; VRI ranges from 0 to 1, and VDI ranges from −1 to 0; U B is the voltage amplitude of the end node, P net represents the integrated active power injected into the end node; The static voltage safety assessment index is the voltage stability index VSI, which is expressed as follows: （9） Where, γ = dU B / dP net , μ = P net / U B ; Step 2: For complex multi-node power systems, establish voltage rise and fall capability evaluation indicators and static voltage safety evaluation indicators applicable to the power multi-quadrant mode based on the power and voltage characteristics at the voltage collapse point; Step 3: Obtain the static voltage stability assessment result of the coupled system based on the voltage rise and fall capability assessment index and the static voltage safety assessment index of the multi-node complex power system.

2. The method for evaluating the static voltage stability of a coupled system in a multi-quadrant power mode according to claim 1, wherein: The process of establishing the static voltage safety assessment index in step 1 is as follows: Based on the PV curve formed by the active power and voltage in the two-node system, a static voltage stability evaluation index suitable for the power multi-quadrant mode is established according to the active power and voltage characteristics during the evolution of the PV curve. Specifically, For a simple two-node system, U A is the voltage amplitude of the first-end node; U B is the voltage amplitude of the terminal node, and a virtual conductance is introduced at node B. G , P net is the comprehensive active power injected into the terminal node, then P net and U B satisfy: P net = （7） The total differential expression of formula (7) is: （8） When the change Δ G 0, Δ U B When 0, the voltage stability index VSI is defined as: （9） Where, γ = dU B / dP net , μ = P net / U B .

3. The method for evaluating the static voltage stability of a coupled system in a multi-quadrant power mode according to claim 1, wherein: The specific process of step 2 is: Step 2.1: Derived from the power flow correction equation, the system sensitivity is obtained, and the voltage rise and fall capability evaluation index and the static voltage security evaluation index are extended to multi-node complex power systems. Step 2.2: Combine the simple two-node coupled system voltage raising and lowering capability evaluation index and the static voltage safety evaluation index to construct a node in a multi-node complex power system. i The voltage raising and lowering capability evaluation index and the static voltage safety evaluation index are: （13） （14） Where: μ i = P i / U i 、 ;in, 、 、 ρ ij = P j / P i 、 ρ ij tan φ j = Q j / P i ; NP Indicates the total number of system PQ nodes and PV nodes, NQ Indicates the total number of PQ nodes in the system; J Pθ , J PU , J Qθ and J QU represents the system Jacobian matrix J The corresponding sub-matrix of .

4. The method for evaluating the static voltage stability of a coupled system in a power multi-quadrant mode according to claim 3 is characterized in that: The power flow correction equation expression is: （12） Where, Δ P and Δ Q It is a small increment of active and reactive power of system nodes; Δ θ and Δ U They represent the correction values ​​of the system node phase and voltage respectively, J Pθ , J PU , J Qθ and J QU represents the system Jacobian matrix J The corresponding sub-matrix of .

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