A security method and device suitable for a high proportion of flexible direct current feeding power grid

Abstract

The application provides a security and protection method and device suitable for a high proportion power grid with flexible direct current feed-in. The method involves the control of three dimensions of power angle, frequency and voltage, and three defense lines of power system power angle, frequency and voltage stability are respectively constructed. First, the power angle interval is judged by the synchronous power coefficient, and measures such as optimizing the power grid structure, reducing the flexible direct current power output or cutting off the out-of-step unit are taken. Second, the frequency interval is judged in combination with the equivalent inertia improvement factor and the frequency deviation factor, the flexible direct current frequency regulation output, the start active power support and the load adjustment are adjusted, and the unit or the load is cut off if necessary. Finally, the voltage interval is judged according to the voltage stiffness, the reactive power compensation is optimized, the load voltage is adjusted or the load is cut off. The method constructs a multi-stability dimension cooperative defense system, deeply embeds the flexible direct current regulation capacity in the hierarchical control logic, avoids the chain failure through the three interval data sharing, and improves the survival ability of the high proportion flexible direct current power grid in all working conditions.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of power grid safety, and particularly relates to a security and protection method and device suitable for a high-proportion power grid fed by flexible direct current. BACKGROUND

[0002] Traditional power grid operation divides three safety and stability standards, and proposes a security and defense system of three lines of defense of the power system to ensure safe and stable operation. With the wide application of flexible direct current (flexible direct current) power transmission technology in high-proportion renewable energy grid connection, the power grid gradually presents the characteristics of "high proportion of power electronics".

[0003] However, the traditional security and protection strategy does not consider the synergistic relationship of power angle, frequency and voltage stability,

[0004] and does not integrate the flexible direct current fast power regulation capability into the hierarchical defense system. SUMMARY

[0005] The purpose of the present application is to overcome the defects in the prior art, and to provide a security and protection method and device suitable for a high-proportion power grid fed by flexible direct current.

[0006] The present application provides a security and protection method suitable for a high-proportion power grid fed by flexible direct current, comprising:

[0007] obtaining a synchronous power coefficient, the synchronous power coefficient being the ratio of the output increment of a unit to the power angle change; judging the current power angle interval according to the synchronous power coefficient, and performing: in a first power angle interval, optimizing the power grid structure to reduce the power angle deviation; in a second power angle interval, reducing the flexible direct current power output; if the power angle difference continues to increase, cutting off the out-of-step unit; in a third power angle interval, splitting the regional power grid;

[0008] obtaining an equivalent inertia promotion factor and a frequency deviation factor, the equivalent inertia promotion factor being the ratio of the equivalent inertia time constant when the non-synchronous electromechanical power source inertia support control is performed to the equivalent inertia time constant when the inertia support control is not performed; the frequency deviation factor being the sum of the frequency regulation coefficient of the system active load, the equivalent frequency regulation coefficient of all non-synchronous electromechanical power sources and the inverse of the equivalent regulation rate of all synchronous generator governors; judging the current frequency interval according to the equivalent inertia promotion factor and the frequency deviation factor, and performing: in a first frequency interval, optimizing the flexible direct current frequency regulation output to reduce the frequency deviation; in a second frequency interval, starting the active power support of the flexible direct current converter and the active regulation of the load power; if the frequency continues to increase, cutting off the unit or the load; in a third frequency interval, splitting the regional power grid;

[0009] A voltage stiffness is obtained, the voltage stiffness being a ratio of a voltage amplitude of a port of the grid-connected device after accessing the power grid and a no-load voltage amplitude of the access point; a current voltage interval is determined according to the voltage stiffness, and the following is performed: in a first voltage interval, an optimization of reactive power compensation configuration is performed to reduce voltage deviation; in a second voltage interval, active regulation of load voltage and removal of secondary load are performed; if voltage continues to deteriorate, primary load is removed in a preset round of grading; and in a third voltage interval, the regional power grid is split.

[0010] Optionally, the synchronous power coefficient calculation formula is:

[0011]

[0012] wherein K syn is a synchronous power coefficient, P g is a unit output, and δ g is a unit power angle.

[0013] Optionally, the equivalent inertia promotion factor calculation formula is:

[0014]

[0015] wherein, is an equivalent inertia time constant when there is no inertia support, is an equivalent inertia time constant when there is inertia support.

[0016] Optionally, the voltage stiffness calculation formula is:

[0017]

[0018] wherein K vtg is a voltage stiffness, Z th is a Thevenin equivalent impedance, Z device is an impedance of the grid-connected device, is a phase angle of the impedance of the grid-connected device, is a phase angle of the equivalent impedance of the power grid, λ SCR is a short-circuit ratio, U sys is a voltage amplitude after accessing, and U sys0 is a no-load voltage amplitude.

[0019] Optionally, the optimization of reactive power compensation device comprises:

[0020] A STATCOM device is put into operation to dynamically adjust SVG reactive power output.

[0021] The application also provides a security and protection device suitable for a high-occupation power grid with flexible direct current feed-in, comprising:

[0022] The power angle security module obtains a synchronous power coefficient, which is the ratio of the output increment of the unit to the power angle change; according to the synchronous power coefficient, the current power angle interval is determined, and the following is executed: in the first power angle interval, the power grid structure is optimized to reduce the power angle deviation; in the second power angle interval, the flexible power output is reduced; if the power angle difference continues to increase, the out-of-step unit is removed; and in the third power angle interval, the regional power grid is split.

[0023] The frequency security module obtains an equivalent inertia promotion factor and a frequency deviation factor, the equivalent inertia promotion factor is the ratio of the equivalent inertia time constant when the non-synchronous electromechanical power inertia support control is performed to the equivalent inertia time constant when the non-synchronous electromechanical power inertia support control is not performed; the frequency deviation factor is the sum of the frequency regulation coefficient of the system active load, the equivalent frequency regulation coefficient of all non-synchronous electromechanical powers and the inverse of the equivalent regulation rate of all synchronous generator governors; according to the equivalent inertia promotion factor and the frequency deviation factor, the current frequency interval is determined, and the following is executed: in the first frequency interval, the flexible frequency regulation output is optimized to reduce the frequency deviation; in the second frequency interval, the active power support of the flexible converter and the active regulation of the load power are started; if the frequency continues to increase, the unit or the load is removed; and in the third frequency interval, the regional power grid is split.

[0024] The voltage security module obtains a voltage stiffness, which is the ratio of the voltage amplitude of the port of the grid-connected equipment connected to the power grid to the no-load voltage amplitude of the connection point; according to the voltage stiffness, the current voltage interval is determined, and the following is executed: in the first voltage interval, the reactive power compensation configuration is optimized to reduce the voltage deviation; in the second voltage interval, the active regulation of the load voltage and the removal of the secondary load are performed; if the voltage continues to deteriorate, the primary load is removed according to a preset round of grading; and in the third voltage interval, the regional power grid is split.

[0025] Optionally, the synchronous power coefficient calculation formula is:

[0026]

[0027] wherein K syn is the synchronous power coefficient, P g is the unit output, and δ g is the unit power angle.

[0028] Optionally, the equivalent inertia promotion factor calculation formula is:

[0029]

[0030] wherein, is the equivalent inertia time constant when the inertia support is not performed, is the equivalent inertia time constant when the inertia support is performed.

[0031] Optionally, the voltage stiffness calculation formula is:

[0032]

[0033] K is voltage stiffness, Z vtg is Thevenin equivalent impedance, Z th is Thevenin equivalent impedance, Z device is the impedance of the grid-connected device, is the phase angle of the grid-connected device impedance, is the phase angle of the grid equivalent impedance, λ SCR is short circuit ratio, U sys is the voltage amplitude after access, U sys0 is the no-load voltage amplitude.

[0034] Optionally, in the voltage security module, the optimization reactive power compensation device comprises:

[0035] The STATCOM device is put into operation, and the SVG reactive power output is dynamically adjusted.

[0036] The beneficial effects of the present application are:

[0037] The application provides a security and protection method suitable for a high proportion power grid with flexible direct current feed-in, comprising: obtaining a synchronous power coefficient, the synchronous power coefficient being a ratio of an output increment of a unit to a power angle change amount; judging a current power angle interval according to the synchronous power coefficient, and performing: in a first power angle interval, optimizing a power grid structure to reduce a power angle deviation; in a second power angle interval, reducing flexible direct current power output; if a power angle difference continuously increases, removing a step-out unit; and in a third power angle interval, splitting a regional power grid; obtaining an equivalent inertia promotion factor and a frequency deviation factor, the equivalent inertia promotion factor being a ratio of an equivalent inertia time constant when a non-synchronous electromechanical power source inertia support control is performed to an equivalent inertia time constant when no inertia support control is performed; the frequency deviation factor being a sum of a frequency regulation coefficient of a system active load, an equivalent frequency regulation coefficient of all non-synchronous electromechanical power sources and an inverse of an equivalent regulation rate of all synchronous generator governors; judging a current frequency interval according to the equivalent inertia promotion factor and the frequency deviation factor, and performing: in a first frequency interval, optimizing flexible direct current frequency regulation output to reduce a frequency deviation; in a second frequency interval, starting flexible direct current converter active power support and active load power regulation; if the frequency continuously increases, removing a unit or a load; and in a third frequency interval, splitting the regional power grid; obtaining a voltage stiffness, the voltage stiffness being a ratio of a voltage amplitude of a port of a grid-connected equipment after the grid-connected equipment is connected to a power grid to a no-load voltage amplitude of an access point; judging a current voltage interval according to the voltage stiffness, and performing: in a first voltage interval, optimizing reactive power compensation configuration to reduce a voltage deviation; in a second voltage interval, performing active load voltage regulation and removing a secondary load; if the voltage continuously deteriorates, removing a primary load according to a preset round of grading; and in a third voltage interval, splitting the regional power grid. The application embeds flexible direct current regulation capacity into grading control logic of power angle, frequency and voltage intervals through constructing a multi-stability dimension collaborative defense system, avoids cascading failures caused by traditional single-interval actions through three-interval data sharing, and improves the survival ability of a high proportion flexible direct current power grid in all working conditions. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 FIG. 1 is a security and protection schematic diagram suitable for a high proportion power grid with flexible direct current feed-in in the application;

[0039] Figure 2 FIG. 2 is a three-line defense schematic diagram of synchronous stability in the application;

[0040] Figure 3 FIG. 3 is a three-line defense schematic diagram of frequency stability in the application;

[0041] Figure 4 FIG. 4 is a three-line defense schematic diagram of voltage stability in the application. DETAILED DESCRIPTION

[0042] Exemplary embodiments of the present disclosure will be described in greater detail below with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it is understood that various forms of the present disclosure can be implemented without being limited by the embodiments set forth herein. Rather, the embodiments are provided so that the present disclosure can be more thoroughly understood, and so that the scope of the present disclosure can be conveyed to those skilled in the art.

[0043] Referring to Figure 1 The present application provides a security method suitable for a high proportion of flexible direct current feeding power grid, which comprises:

[0044] S101, obtain a synchronous power coefficient, which is the ratio of the output increment of a unit to the power angle change; determine the current power angle interval according to the synchronous power coefficient, and execute: in the first power angle interval, optimize the grid structure to reduce the power angle deviation; in the second power angle interval, reduce the flexible direct current power output; if the power angle difference continues to increase, remove the out-of-step unit; in the third power angle interval, split the regional power grid;

[0045] In the present application, the synchronous power coefficient is used as an evaluation index of synchronous stability, and the expression of the synchronous power coefficient is:

[0046]

[0047] wherein, K syn is the synchronous power coefficient, P g is the output of the unit, and δ g is the power angle of the unit.

[0048] In an actual large-scale power system, it is difficult to calculate the synchronous power coefficient by using an analytical method, so the synchronous power coefficient is calculated by simulation: in the simulation model of the actual large-scale power system, a small output increment ΔP g is added to the unit, and then a power flow calculation is performed to obtain the power angle change Δδ g of the unit, and the synchronous power coefficient is obtained by the ratio of the two.

[0049] Obviously, the synchronous power coefficient should be greater than 0, and the greater the value, the better the synchronous stability of the unit. If the synchronous power coefficient is close to 0, it indicates that the synchronous stability of the unit is close to the critical point, and the possibility of out-of-step is greater.

[0050] In the transient synchronous stability evaluation, the critical fault removal time is used to measure the transient synchronous stability of the system. On the one hand, short-circuit faults are serious faults in the power grid; and statistics show that three-phase short-circuit faults are the most serious short-circuit faults. On the other hand, the longer the fault duration, the more serious the impact on the power system.

[0051] Therefore, the critical clearing time after a three-phase metallic short-circuit fault can be used to assess the stability of a power system.

[0052] In a normally operating receiving-end power grid, the critical clearance time for a three-phase metallic short-circuit fault occurring at AC bus i is t. criticali Its meaning is: it indicates that when the duration of the three-phase metallic short-circuit fault at bus i exceeds t... criticali At this time, the system will experience power angle or voltage instability.

[0053] Considering the action time of mechanical devices, the critical clearing time t of a three-phase metallic short-circuit fault corresponding to any AC bus in the receiving-end power grid. criticali It should be strictly greater than 0.1s.

[0054] Meanwhile, the larger the minimum critical fault clearance time in the system, the smaller the risk of transient instability in the system as a whole.

[0055] To address the synchronization stability issue in power systems with a high proportion of flexible DC feeds, and based on the grid operation status caused by the widening power angle difference between generating units, a "three-line defense" solution is proposed. Specifically, based on the magnitude of the synchronization power coefficient, the power angle difference between generating units is divided into three intervals: a first power angle interval, a second power angle interval, and a third power angle interval. For each of these intervals, a first line of defense, a second line of defense, and a third line of defense are established.

[0056] The first line of defense: synchronous stability assessment and out-of-synchronization prevention and control during system planning and normal operation;

[0057] The second line of defense: stabilization control after a loss of synchronization occurs;

[0058] The third line of defense: wide-area coordination and emergency control after the angle of attack becomes unstable.

[0059] Please refer to the example below. Figure 2 As shown, Figure 2 It demonstrates three lines of defense for synchronization stability.

[0060] The first line of defense is divided into two phases: the first phase is the near-term and long-term planning phase of the power grid; the second phase is the normal operation phase.

[0061] In the near-term and long-term planning stages of the power grid, the synchronous stability of the target system is evaluated through the aforementioned synchronous power coefficient in order to select a reasonable power source and grid structure. At the same time, the low-pass parameters of the flexible DC are optimized to act as the power angle of the speed reducer during the power angle swing, so as to avoid the system losing synchronization under the expected operating mode.

[0062] In the normal operation stage, the frequency stability is taken as the target, the safe operation boundary is found based on the synchronous power coefficient, the power grid operation mode is monitored and adjusted, and the normal operation of the system is ensured.

[0063] The second defense line is divided into two stages. The first stage is in the process of rapidly opening the power angle. The second stage is after the loss of step of individual units.

[0064] The current state of the power angle is determined based on the synchronous power coefficient. In the process of rapidly opening the power angle, the loss of step risk that may occur is perceived in advance, and the flexible direct current power is reduced in advance. Since the flexible direct current can rapidly reduce the active current through the converter control, the problem of further increasing the power angle can be effectively improved.

[0065] After the loss of step of individual units occurs, the synchronous machine has lost step. At this time, only the loss of step synchronous machine can be removed to ensure the safe operation of the remaining units and the power grid.

[0066] The third defense line is the loss of step of a large number of units in the regional power grid.

[0067] To prevent the loss of step and the low voltage problem that may occur, emergency protection measures such as splitting are taken to make the system return to stable operation and avoid a large number of units being off the grid and equipment damage.

[0068] S102, an equivalent inertia promotion factor and a frequency deviation factor are obtained. The equivalent inertia promotion factor is the ratio of the equivalent inertia time constant when the non-synchronous power supply inertia support control is performed to the equivalent inertia time constant when the inertia support control is not performed. The frequency deviation factor is the sum of the frequency regulation coefficient of the system active load, the equivalent frequency regulation coefficient of all non-synchronous power supplies, and the inverse of the equivalent regulation rate of all synchronous generator governors. According to the equivalent inertia promotion factor and the frequency deviation factor, the current frequency interval is determined, and the following is performed: in the first frequency interval, the flexible direct current frequency regulation output is optimized to reduce the frequency deviation; in the second frequency interval, the flexible direct current converter active power support and the load power active regulation are started; if the frequency continues to increase, the unit or the load is removed; and in the third frequency interval, the regional power grid is split;

[0069] The frequency stability can be measured from two angles: the inertia support capability and the primary frequency regulation capability. The inertia support capability is used to describe the frequency change rate of the power grid in the initial period after the active disturbance. The primary frequency regulation capability is used to describe the size of the active power that the power grid can swallow when there is a frequency deviation.

[0070] The direct reflection of the inertia size is the frequency rate of change (RoCoF) of the system, especially the RoCoF in the initial stage of the disturbance, which is only related to the inertia size and the disturbance itself. Therefore, the calculation of the inertia size can be converted to the calculation of the RoCoF in the initial stage of the disturbance. It should be noted that for a specific power system, there is a maximum limit for the RoCoF, such as limiting the maximum value of the RoCoF to not more than 1 Hz / s.

[0071] Generally, the minimum requirement of the grid for the inertia size is inversely deduced according to the maximum limit of the RoCoF and the maximum active power disturbance expected. Different power grids have different regulations for the maximum active power disturbance, such as the maximum active power disturbance of 3000 MW power generation loss regulated by the European mainland power grid, and the maximum active power disturbance of the single-loop maximum DC transmission line bipolar blocking generally regulated by the power grid in China.

[0072] In this application, the equivalent inertia boosting factor is used to characterize the inertia support strength of the non-synchronous machine power source, and the derivation process is as follows:

[0073] Suppose that under the specified maximum active power disturbance, the frequency rate of change of a certain node in the grid in the initial stage of the disturbance is

[0074]

[0075] where k const is a constant related to the system operating mode, P max is the unbalanced power under the specified maximum active power disturbance, and H eq is the equivalent inertia time constant of the system under the operating mode being considered.

[0076] The equivalent inertia time constant of all non-synchronous machine power sources when using inertia-free support control is defined as follows:

[0077] is

[0078] The inertia-free support control of the non-synchronous machine power source can be realized by setting H and Mnon to zero.

[0079] The equivalent inertia time constant of all non-synchronous machine power sources when the inertia support control is put into operation is defined as follows:

[0080] is

[0081] Therefore, the equivalent inertia boosting factor H amp is defined as the multiple of the equivalent inertia boosting caused by the inertia support control of the non-synchronous machine power source, which is shown as follows:

[0082]

[0083] wherein, is the equivalent inertia time constant of the whole grid when all non-synchronous machine power sources are controlled by inertia support, is the equivalent inertia time constant of the whole grid when all non-synchronous machine power sources are controlled by inertia support.

[0084] Obviously, H amp is greater than 1, reflecting the role of the non-synchronous machine power source inertia support in improving the equivalent inertia of the whole grid, so it can represent the inertia support strength of the non-synchronous machine power source.

[0085] The primary frequency regulation capability can be measured by the frequency deviation factor.

[0086] In the context of new power systems, the frequency deviation factor β can be defined as:

[0087]

[0088] wherein, R gen is the equivalent droop rate of all synchronous generator governors, with the unit of Hz / MW; K non is the equivalent frequency regulation coefficient of all non-synchronous machine power sources, with the unit of MW / Hz; D load is the frequency regulation coefficient of the system active load, with the unit of MW / Hz; the commonly used unit of the frequency deviation factor β is MW / 0.1 Hz.

[0089] The frequency deviation factor β describes the relationship between the active disturbance ΔP and the steady-state frequency deviation Δf∞ of the grid, as shown in the following formula.

[0090]

[0091] For frequency stability, in a high proportion of flexible DC power grid, there are two forms of frequency stability, one is the frequency instability caused by excessive frequency deviation in the traditional primary frequency regulation process, and the other is the excessive frequency deviation phenomenon caused by short-term power shortage caused by low penetration of flexible DC.

[0092] The frequency instability caused by excessive frequency deviation in the primary frequency regulation process, in this form, all units in the system can be regarded as the same frequency motion, and the whole grid can be described by a unified frequency model. For this type of frequency stability, the development of the stability control strategy is the result of the global optimization of the system.

[0093] The frequency deviation caused by the short-time power shortage of the flexible DC low penetration is too large. This frequency deviation is too large during the power angle swing, rather than the time scale of the primary frequency modulation. The units in the system cannot be regarded as the same frequency motion. The different units are affected by the flexible DC low penetration to different degrees. The maximum frequency deviation is also different. The development of the frequency stability control strategy is the result of the optimization of the local area.

[0094] At present, the main problem faced by the high proportion of DC feeding power grid is the high frequency problem of local area isolated network operation caused by DC blocking, and the low frequency problem caused by DC low penetration and off-grid. In view of this type of frequency problem, the application proposes a "three-line defense" solution. Specifically, according to the size of the equivalent inertia enhancement factor and the frequency deviation factor, the unit frequency deviation is divided into three intervals, including the first frequency interval, the second frequency interval and the third frequency interval. The first frequency interval, the second frequency interval and the third frequency interval are respectively set as the first line of defense, the second line of defense and the third line of defense:

[0095] The first line of defense: frequency stability evaluation and frequency prevention control in the system planning and normal operation stage;

[0096] The second line of defense: stability control after the frequency deviation is too large;

[0097] The third line of defense: wide-area coordination and emergency control after frequency instability.

[0098] Please refer to Figure 3 , which shows the three lines of defense of frequency stability in the application. Figure 3

[0099] The first line of defense includes two stages. The first stage is in the short-term and long-term planning stage of the power grid, and the second stage is in the normal operation stage.

[0100] In the short-term and long-term planning stage of the power grid, the inertia support capacity and the primary frequency modulation capacity are used to evaluate the frequency stability of the target system, select a reasonable power supply and DC planning scheme, and fully tap the potential of flexible DC frequency modulation to avoid excessive frequency deviation of the system in the expected operation mode.

[0101] In the normal operation stage, the inertia support capacity and the primary frequency modulation capacity are used to find the safe operation boundary based on the frequency stability. The results are used to monitor and adjust the power grid operation mode to ensure the normal operation of the system.

[0102] The second line of defense includes two stages. The first stage is when the frequency deviation just starts to exceed the limit, and the second stage is when the frequency deviation further increases.

[0103] ​Based on the inertia support ability and primary frequency modulation ability to determine the current frequency state, when the frequency deviation just starts to exceed the limit, the frequency regulation effect of the flexible direct current is used to adjust the power, and the emergency power support function of the direct current system is started, and the large load is adjusted by the demand side response to take the non-machine cutting means as much as possible.

[0104] When the frequency deviation further increases, the flexible direct current, load and direct current emergency power support cannot limit the frequency within a reasonable range, at this time only through machine cutting or load cutting to restore the frequency.

[0105] The third line of defense is that the grid frequency deviation is too large.

[0106] In order to prevent the large-scale spread of unit off-grid problems caused by frequency instability and cause power failure accidents, emergency protection measures such as splitting are taken to make the system return to stable operation.

[0107] S103, obtain the voltage stiffness, the voltage stiffness is the ratio of the voltage amplitude of the port of the grid-connected equipment after accessing the grid and the no-load voltage amplitude of the access point; according to the voltage stiffness, determine the current voltage interval, and execute: in the first voltage interval, optimize the reactive power compensation configuration to reduce the voltage deviation; in the second voltage interval, perform active load voltage regulation and remove the secondary load; if the voltage continues to deteriorate, remove the main load according to the preset round; in the third voltage interval, split the regional power grid.

[0108] In the voltage stability evaluation, the voltage amplitude of any bus in the grid will be affected by the type and capacity of the connected equipment, and the index corresponding to the influence degree is the voltage support strength.

[0109] The voltage stiffness is defined as the ability of any point in the grid to maintain the modulus of the access point voltage close to the no-load voltage of the access point, which can be expressed as:

[0110]

[0111] Where, K vtg is the voltage stiffness, U sys is the voltage amplitude of the port of the grid-connected equipment after accessing the grid, U sys0 is the no-load voltage amplitude of the access point, Z th = Z th ∠ φ th is the Thevenin equivalent impedance, Z device = Z device ∠ φ device is the impedance of the grid-connected equipment, λ SCR is the short circuit ratio.

[0112] Obviously, the value range of the voltage stiffness K vtg is [0, 1].

[0113] According to the voltage drop amplitude of the key node, corresponding measures are taken to maximize the safe operation of the power system. The application proposes a "three-line defense" method for the voltage stability problem of the power system with large loads. Specifically, according to the size of the voltage stiffness value, the voltage drop amplitude is divided into three intervals, including a first voltage interval, a second voltage interval, and a third voltage interval. For the first voltage interval, the second voltage interval, and the third voltage interval, a first line of defense, a second line of defense, and a third line of defense are set respectively:

[0114] The first line of defense is the voltage stiffness evaluation and voltage prevention control in the system planning and normal operation stage.

[0115] The second line of defense is the stable control of low voltage.

[0116] The third line of defense is the power grid splitting and load shedding after voltage collapse.

[0117] Please refer to Figure 4 , which shows the three-line defense of voltage stability in the application. Figure 4

[0118] The first line of defense includes two stages, the first stage is in the short-term and long-term planning stage of the power grid, and the second stage is in the normal operation stage.

[0119] In the short-term and long-term planning stage of the power grid, the voltage stiffness of the key nodes in the target system is evaluated through voltage stiffness, a reasonable source-network-load comprehensive planning scheme is selected, and automatic voltage control, multi-type reactive power source emergency voltage support, etc. are used to improve the voltage level during short-circuit fault and recovery period, avoiding voltage collapse in the expected operation mode.

[0120] In the normal operation stage, the safe operation boundary is found based on voltage stiffness with voltage stability as the target, and the power grid operation mode is monitored and adjusted based on the result to ensure normal operation of the system.

[0121] The second line of defense is divided into two stages, the first stage is when the voltage is low, and the second stage is during the voltage continuous reduction process.

[0122] According to the voltage stiffness, the current voltage state is judged, and when the voltage is low, the voltage collapse that may occur is perceived in advance, the demand side response of large loads is started in advance, and a part of unimportant loads is cut off. Through the power regulation capacity of large loads and the priority cutting of unimportant loads, the accident is avoided from further expanding as much as possible.

[0123] During the voltage continuous reduction process, the load is cut off according to the turn to ensure the safe operation of the system.

[0124] The third line of defense is the voltage collapse of part of the regional power grid.​

[0125] To prevent the large-scale spread of low voltage problems, cause power failure accidents, take emergency protection measures such as splitting, make the system resume stable operation, avoid a large number of units off-line and equipment damage, etc.

[0126] The application also provides a security and protection device suitable for a high proportion of VSCF power grid, comprising:

[0127] The power angle security and protection module acquires a synchronous power coefficient, which is the ratio of the output increment of the unit to the power angle change; according to the synchronous power coefficient, the current power angle interval is determined, and the following is executed: in the first power angle interval, the power grid structure is optimized to reduce the power angle deviation; in the second power angle interval, the VSCF power output is reduced; if the power angle difference continues to increase, the out-of-step unit is removed; in the third power angle interval, the regional power grid is split.

[0128] The frequency security and protection module acquires an equivalent inertia promotion factor and a frequency deviation factor, the equivalent inertia promotion factor is the ratio of the equivalent inertia time constant when the non-synchronous power source inertia support control is performed to the equivalent inertia time constant when the inertia support control is not performed; the frequency deviation factor is the sum of the frequency regulation coefficient of the system active load, the equivalent frequency regulation coefficient of all non-synchronous power sources and the reciprocal of the equivalent regulation rate of all synchronous generator governors; according to the equivalent inertia promotion factor and the frequency deviation factor, the current frequency interval is determined, and the following is executed: in the first frequency interval, the VSCF frequency modulation output is optimized to reduce the frequency deviation; in the second frequency interval, the VSCF active power support and the load power active regulation are started; if the frequency continues to increase, the unit or the load is removed; in the third frequency interval, the regional power grid is split.

[0129] The voltage security and protection module acquires a voltage stiffness, which is the ratio of the voltage amplitude of the port of the grid-connected device connected to the power grid to the no-load voltage amplitude of the connection point; according to the voltage stiffness, the current voltage interval is determined, and the following is executed: in the first voltage interval, the reactive power compensation configuration is optimized to reduce the voltage deviation; in the second voltage interval, the load voltage active regulation and the removal of the secondary load are performed; if the voltage continues to deteriorate, the primary load is removed in a preset round of grading; in the third voltage interval, the regional power grid is split.

[0130] Further, the synchronous power coefficient calculation formula is:

[0131]

[0132] Wherein, K syn is the synchronous power coefficient, P g is the unit output, and δ g is the unit power angle.

[0133] Further, the equivalent inertia promotion factor calculation formula is:

[0134]

[0135] wherein, is the equivalent inertia time constant when there is no inertia support, is the equivalent inertia time constant when there is inertia support.

[0136] Further, the voltage stiffness calculation formula is:

[0137]

[0138] wherein, λ SCR is the short-circuit ratio, U sys is the voltage amplitude after access, U sys0 is the no-load voltage amplitude.

[0139] Further, in the voltage security module, the optimization reactive power compensation device comprises:

[0140] The STATCOM device is put into operation, and the SVG reactive power output is dynamically adjusted.

[0141] The above description of the embodiments is for facilitating the understanding and application of the present application by those of ordinary skill in the art. Those skilled in the art will easily make various modifications to the above embodiments, and apply the general principles described herein to other embodiments without having to go through creative labor. Therefore, the present application is not limited to the above embodiments, and the improvements and modifications made to the present application by those skilled in the art according to the disclosure of the present application should be within the scope of protection of the present application.

Claims

1. A security method suitable for a high proportion of flexible direct current feeding power grid, characterized in that, The method comprises the following steps: obtaining a synchronous power coefficient, the synchronous power coefficient being a ratio of an output increment of a unit to a power angle change amount; judging a current power angle interval according to the synchronous power coefficient, and performing: in a first power angle interval, optimizing a power grid structure to reduce a power angle deviation; in a second power angle interval, reducing a flexible power output; if the power angle difference continuously increases, cutting off a step-out unit; and in a third power angle interval, splitting a regional power grid; obtaining an equivalent inertia promotion factor and a frequency deviation factor, the equivalent inertia promotion factor being a ratio of an equivalent inertia time constant when inertia support control is performed on a non-synchronous electromechanical power source to a ratio when no inertia support control is performed, and the frequency deviation factor being a sum of a frequency regulation coefficient of a system active load, an equivalent frequency regulation coefficient of all non-synchronous electromechanical power sources and an inverse of an equivalent regulation rate of all synchronous generator governors; judging a current frequency interval according to the equivalent inertia promotion factor and the frequency deviation factor, and performing: in a first frequency interval, optimizing a flexible frequency modulation output to reduce a frequency deviation; in a second frequency interval, starting active power support of a flexible current converter and active regulation of a load power; if the frequency continuously increases, cutting off a unit or a load; and in a third frequency interval, splitting a regional power grid; obtaining a voltage stiffness, the voltage stiffness being a ratio of a voltage amplitude of a port of an access device after the access device accesses a power grid to a no-load voltage amplitude of an access point; judging a current voltage interval according to the voltage stiffness, and performing: in a first voltage interval, optimizing a reactive power compensation configuration to reduce a voltage deviation; in a second voltage interval, performing active regulation of a load voltage and cutting off a secondary load; if the voltage continuously deteriorates, cutting off a primary load in a preset round of grading; and in a third voltage interval, splitting a regional power grid; the equivalent inertia promotion factor calculation formula is: the voltage stiffness calculation formula is: ； wherein is the equivalent inertia time constant for the case of no inertia support, is the equivalent inertia time constant for the case of inertia support, f is the frequency, and t is the time. the synchronous power coefficient calculation formula is: ； wherein, is the voltage stiffness, is the Thevenin equivalent impedance, is the impedance of the grid-connected device, is the phase angle of the grid-connected device impedance, is the phase angle of the grid equivalent impedance, is the short circuit ratio, is the voltage amplitude after access, is the no-load voltage amplitude.

2. The security method suitable for a high proportion of flexible AC transmission power grid according to claim 1, characterized in that, the optimized reactive power compensation device comprises: ； wherein, is the synchronization power coefficient, is the unit output, is the unit power angle.

3. The security method for a high proportion of flexible AC transmission power grid according to claim 1, characterized in that, putting in a STATCOM device to dynamically adjust an SVG reactive power output. The method comprises the following steps:

4. A security device suitable for a high penetration power grid with flexible AC transmission, characterized in that, a power angle security module obtains a synchronous power coefficient, the synchronous power coefficient being a ratio of an output increment of a unit to a power angle change amount; judges a current power angle interval according to the synchronous power coefficient, and performs: in a first power angle interval, optimizes a power grid structure to reduce a power angle deviation; in a second power angle interval, reduces a flexible power output; if the power angle difference continuously increases, cuts off a step-out unit; and in a third power angle interval, splits a regional power grid; a frequency security module obtains an equivalent inertia promotion factor and a frequency deviation factor, the equivalent inertia promotion factor being a ratio of an equivalent inertia time constant when inertia support control is performed on a non-synchronous electromechanical power source to a ratio when no inertia support control is performed, and the frequency deviation factor being a sum of a frequency regulation coefficient of a system active load, an equivalent frequency regulation coefficient of all non-synchronous electromechanical power sources and an inverse of an equivalent regulation rate of all synchronous generator governors; judges a current frequency interval according to the equivalent inertia promotion factor and the frequency deviation factor, and performs: in a first frequency interval, optimizes a flexible frequency modulation output to reduce a frequency deviation; in a second frequency interval, starts active power support of a flexible current converter and active regulation of a load power; if the frequency continuously increases, cuts off a unit or a load; and in a third frequency interval, splits a regional power grid; ​ ​ The voltage security module acquires voltage stiffness, which is the ratio of the voltage amplitude of the port of the grid-connected device to the no-load voltage amplitude of the access point after the grid-connected device accesses the grid; According to the voltage stiffness, the current voltage interval is determined, and the following is executed: in the first voltage interval, the reactive power compensation configuration is optimized to reduce voltage deviation; in the second voltage interval, active load voltage regulation and removal of secondary load are performed; if the voltage continues to deteriorate, the primary load is removed in a preset round of hierarchical removal; in the third voltage interval, the regional grid is split. The equivalent inertia promotion factor calculation formula is: ； wherein is the equivalent inertia time constant for the case of no inertia support, is the equivalent inertia time constant for the case of inertia support, f is the frequency, and t is the time. The voltage stiffness calculation formula is: ； wherein, is the voltage stiffness, is the Thevenin equivalent impedance, is the impedance of the grid-connected device, is the phase angle of the grid-connected device impedance, is the phase angle of the grid equivalent impedance, is the short circuit ratio, is the voltage amplitude after connection, is the voltage amplitude at no load.

5. The security device of claim 4, wherein the security device is suitable for use in a high penetration grid with flexible AC transmission entry (FACTS) devices. The synchronous power coefficient calculation formula is: ； wherein, is the synchronous power coefficient, is the unit output, is the unit power angle.

6. The security device of claim 4, wherein the security device is suitable for use in a high penetration grid with flexible AC transmission entry. In the voltage security module, the reactive power compensation optimization device comprises: The STATCOM device is put into operation to dynamically adjust the SVG reactive power output.

Images