Security and protection method and device suitable for flexible direct feed-in high-proportion power grid

Abstract

The invention provides a security and protection method and device suitable for a flexible direct feed-in high-proportion power grid, and the method relates to the control of three dimensions of power angle, frequency and voltage, and respectively constructs three defensive lines of power angle, frequency and voltage stability of a power system. Firstly, a power angle interval is judged through a synchronous power coefficient, and measures of optimizing a power grid structure, reducing flexible direct current power output or cutting off an out-of-step unit and the like are adopted respectively. And secondly, judging a frequency interval by combining an equivalent inertia lifting factor and a frequency deviation factor, adjusting flexible straight frequency modulation output, starting active power support and load adjustment, and cutting off a unit or a load when necessary. And finally, judging a voltage interval according to the voltage stiffness, optimizing reactive compensation, and adjusting load voltage or cutting off a load. According to the method, a multi-stable-dimension cooperative defense system is constructed, the flexible direct current regulation capability is deeply embedded into hierarchical control logic, cascading failures are avoided through three-interval data sharing, and the all-working-condition survivability of a high-proportion flexible direct current power grid is improved.

Description

Technical Field

[0001] This application belongs to the field of power grid security, and particularly relates to a security prevention method and device applicable to a high-proportion flexible DC (VSC-HVDC) fed power grid. Background Art

[0002] Traditional power grid operation divides into three levels of security and stability standards and proposes a three-line defense security defense system for power systems to ensure secure and stable operation. With the wide application of voltage source converter based high voltage direct current (VSC-HVDC) transmission technology in the grid connection of high proportion renewable energy, the power grid gradually shows the characteristics of "high proportion power electronics".

[0003] However, traditional security prevention strategies do not adequately consider the coordinated relationship among power angle, frequency, and voltage stability.

[0004] Moreover, the fast power regulation ability of VSC-HVDC is not incorporated into the hierarchical defense system. Summary of the Invention

[0005] The purpose of this application is to overcome the defects in the above-mentioned prior art and provide a security prevention method and device applicable to a high-proportion flexible DC (VSC-HVDC) fed power grid.

[0006] This application provides a security prevention method applicable to a high-proportion flexible DC (VSC-HVDC) fed power grid, including:

[0007] Obtaining a synchronous power coefficient, where the synchronous power coefficient is the ratio of the output increment of a unit to the change in power angle; judging the current power angle interval according to the synchronous power coefficient and performing: in the first power angle interval, optimizing the power grid structure to reduce the power angle deviation; in the second power angle interval, reducing the VSC-HVDC power output; if the power angle difference continues to increase, disconnecting the out-of-step unit; in the third power angle interval, disconnecting the regional power grid;

[0008] Obtaining an equivalent inertia improvement factor and a frequency deviation factor, where the equivalent inertia improvement factor is the ratio of the equivalent inertia time constant during the inertia support control of an asynchronous machine power source to that without inertia support control; the frequency deviation factor is the sum of the frequency regulation coefficient of the system active load, the equivalent frequency regulation coefficients of all asynchronous machine power sources, and the reciprocal of the equivalent droop rate of all synchronous generator governors; judging the current frequency interval according to the equivalent inertia improvement factor and the frequency deviation factor and performing: in the first frequency interval, optimizing the VSC-HVDC frequency modulation output to reduce the frequency deviation; in the second frequency interval, starting the active power support of the VSC-HVDC converter and the active regulation of the load power; if the frequency continues to increase, disconnecting a unit or a load; in the third frequency interval, disconnecting the regional power grid;

[0009] Obtain the voltage stiffness, where the voltage stiffness is the ratio of the voltage amplitude at the port of the grid-connected device after it is connected to the power grid to the no-load voltage amplitude at the connection point; judge the current voltage range according to the voltage stiffness and execute: in the first voltage range, optimize the reactive power compensation configuration to reduce the voltage deviation; in the second voltage range, perform active load voltage regulation and cut off the secondary load; if the voltage continues to deteriorate, cut off the main load in a preset number of rounds in a hierarchical manner; in the third voltage range, disconnect the regional power grid.

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

[0011]

[0012] where K syn is the synchronous power coefficient, P g is the unit output, δ g is the unit power angle.

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

[0014]

[0015] where is the equivalent inertia time constant without inertia support, is the equivalent inertia time constant when inertia support is put in.

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

[0017]

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

[0019] Optionally, optimizing the reactive power compensation device includes:

[0020] Put in the STATCOM device and dynamically adjust the reactive power output of the SVG.

[0021] This application also provides a security device applicable to a flexible DC-fed high-proportion power grid, including:

[0022] The power angle security protection module obtains the synchronous power coefficient, which is the ratio of the output increment of the unit to the power angle change; determines the current power angle interval according to the synchronous power coefficient and executes: 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 DC power output; if the power angle difference continues to increase, disconnect the out-of-step unit; in the third power angle interval, disconnect the regional power grid;

[0023] The frequency security protection module obtains the equivalent inertia improvement factor and the frequency deviation factor. The equivalent inertia improvement factor is the ratio of the equivalent inertia time constant during the inertia support control of the asynchronous machine power supply to that without inertia support control; the frequency deviation factor is the sum of the frequency regulation coefficient of the system active load, the equivalent frequency regulation coefficients of all asynchronous machine power supplies, and the reciprocals of the equivalent droop rates of all synchronous generator governors; determines the current frequency interval according to the equivalent inertia improvement factor and the frequency deviation factor and executes: in the first frequency interval, optimize the flexible DC frequency modulation output to reduce the frequency deviation; in the second frequency interval, start the active power support of the flexible DC converter and the active regulation of the load power; if the frequency continues to increase, disconnect the unit or the load; in the third frequency interval, disconnect the regional power grid;

[0024] The voltage security protection module obtains the voltage stiffness, which is the ratio of the voltage amplitude at the port after the grid-connected device is connected to the grid to the no-load voltage amplitude at the connection point; determines the current voltage interval according to the voltage stiffness and executes: 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 disconnect the secondary load; if the voltage continues to deteriorate, disconnect the main load in preset rounds; in the third voltage interval, disconnect the regional power grid.

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

[0026]

[0027] where 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.

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

[0029]

[0030] where is the equivalent inertia time constant without inertia support, is the equivalent inertia time constant when inertia support is put into use.

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

[0032]

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

[0034] Optionally, in the voltage security module, the optimized reactive power compensation device includes:

[0035] Put into the STATCOM device and dynamically adjust the reactive power output of the SVG.

[0036] The beneficial effects of this application are:

[0037] The present application provides a security and protection method applicable to a high-proportion flexible DC power-fed power grid, including: obtaining a synchronous power coefficient, where the synchronous power coefficient is the ratio of the output power increment of a unit to the change in power angle; judging the current power angle interval according to the synchronous power coefficient and performing: in the first power angle interval, optimizing the power grid structure to reduce the power angle deviation; in the second power angle interval, reducing the flexible DC power output; if the power angle difference continues to increase, disconnecting the out-of-step unit; in the third power angle interval, disconnecting the regional power grid; obtaining an equivalent inertia improvement factor and a frequency deviation factor, where the equivalent inertia improvement factor is the ratio of the equivalent inertia time constant during the inertia support control of an asynchronous machine power supply to that during the non-inertia support control; the frequency deviation factor is the sum of the frequency regulation coefficient of the system active load, the equivalent frequency regulation coefficients of all asynchronous machine power supplies, and the reciprocal of the equivalent droop rate of all synchronous generator governors; judging the current frequency interval according to the equivalent inertia improvement factor and the frequency deviation factor and performing: in the first frequency interval, optimizing the flexible DC frequency modulation output to reduce the frequency deviation; in the second frequency interval, starting the active power support of the flexible DC converter and the active regulation of the load power; if the frequency continues to increase, disconnecting the unit or the load; in the third frequency interval, disconnecting the regional power grid; obtaining a voltage stiffness, where the voltage stiffness is the ratio of the voltage amplitude at the port after the grid-connected equipment is connected to the power grid to the no-load voltage amplitude at the connection point; judging the current voltage interval according to the voltage stiffness and performing: in the first voltage interval, optimizing the reactive power compensation configuration to reduce the voltage deviation; in the second voltage interval, performing active voltage regulation of the load and disconnecting the secondary load; if the voltage continues to deteriorate, disconnecting the main load in preset rounds; in the third voltage interval, disconnecting the regional power grid. The present application constructs a multi-stability dimension collaborative defense system, deeply embeds the flexible DC regulation ability into the hierarchical control logic of the power angle, frequency, and voltage intervals, and avoids the cascading faults caused by the actions in traditional single intervals through data sharing in the three intervals, thereby improving the full-condition survival ability of the high-proportion flexible DC power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 is a security and protection schematic diagram applicable to a high-proportion flexible DC power-fed power grid in the present application;

[0039] Figure 2 is a schematic diagram of the three lines of defense for synchronous stability in the present application;

[0040] Figure 3 is a schematic diagram of the three lines of defense for frequency stability in the present application;

[0041] Figure 4 is a schematic diagram of the three lines of defense for voltage stability in the present application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0042] Exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present disclosure are shown in the drawings, it can be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, the embodiments are provided so that the present disclosure can be more thoroughly understood and the scope of the present disclosure can be fully conveyed to those skilled in the art.

[0043] Please refer to Figure 1 As shown, the present application provides a security method applicable to a high-proportion flexible DC power feeding power grid, and the method includes:

[0044] S101. Obtain the synchronization power coefficient, where the synchronization power coefficient is the ratio of the output increment of the unit to the change in power angle; judge the current power angle interval according to the synchronization power coefficient, and execute: in the first power angle interval, optimize the power grid network structure to reduce the power angle deviation; in the second power angle interval, reduce the flexible DC power output; if the power angle difference continues to increase, disconnect the out-of-step unit; in the third power angle interval, disconnect the regional power grid;

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

[0046]

[0047] where K syn is the synchronization 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 synchronization power coefficient by an analytical method. Therefore, the synchronization power coefficient is calculated by a simulation method: 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 change in power angle Δδ g of the unit, and the synchronization power coefficient is obtained by the ratio of the two.

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

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

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

[0052] In a normally operating receiving-end power grid, the critical fault clearing time after a three-phase metallic short-circuit fault occurs at the AC bus i is t criticali Its meaning is: when the three-phase metallic short-circuit fault at bus i lasts for more than t criticali When the system is in a state of power or voltage instability,

[0053] Taking into account the action time of the mechanical device, the critical removal time t of the three-phase metallic short-circuit fault corresponding to any AC bus in the receiving-end power grid is criticali Should be strictly greater than 0.1s.

[0054] At the same time, the longer the minimum critical fault clearing time in the system is, the smaller the risk of transient instability of the system as a whole is.

[0055] In order to solve the problem of synchronous stability of power systems with a high proportion of flexible direct current feed-in, corresponding measures are taken according to the grid operation status caused by the power angle difference of the units, and a "three-line defense" solution is proposed. Specifically, according to the value of the synchronous power coefficient, the power angle difference of the units is divided into three intervals, including the first power angle interval, the second power angle interval and the third power angle interval. The first, second and third lines of defense are set for the first, second and third power angle intervals respectively:

[0056] First line of defense: Synchronization stability assessment and out-of-step prevention control during system planning and normal operation;

[0057] The second line of defense: stable control after a step loss occurs;

[0058] The third line of defense: wide-area coordination and emergency control after power angle instability.

[0059] Please refer to Figure 2 As shown, Figure 2 Demonstrates three lines of defense for synchronized stability.

[0060] The first line of defense is divided into two stages. The first stage is the short-term and long-term planning stage of the power grid; the second stage is the normal operation stage.

[0061] In the short-term and long-term planning stage of the power grid, the synchronous stability of the target system is evaluated through the synchronous power coefficient to select a reasonable power source and grid structure. At the same time, the low penetration parameters of the flexible direct current are optimized to reduce the power angle of the unit during the power angle swing process, thereby avoiding the system from losing step under the expected operating mode.

[0062] During the normal operation stage, aiming at frequency stability, the safe operation boundary is found based on the synchronous power coefficient, and the operation mode of the power grid is monitored and adjusted to ensure the normal operation of the system.

[0063] The second line of defense is divided into two stages. The first stage is during the rapid separation of the power angle; the second stage is after the out-of-step of individual units occurs.

[0064] Based on the synchronous power coefficient, the current state of the power angle is judged. During the rapid separation of the power angle, the potential out-of-step danger is sensed in advance, and the flexible DC power is reduced in advance. Since the flexible DC can quickly reduce its own active current through converter control, the problem of further widening of the power angle can be effectively improved.

[0065] After the out-of-step of individual units occurs, the synchronous machine has gone out of step. At this time, only the out-of-step synchronous machine can be disconnected to ensure the operation safety of the remaining units and the power grid.

[0066] The third line of defense is the out-of-step of a large number of units within the regional power grid.

[0067] To prevent the large-scale spread of out-of-step and the resulting low-voltage problems and the occurrence of power outage accidents, emergency protection measures such as disconnection are taken to restore the stable operation of the system and avoid the tripping of a large number of units and equipment damage.

[0068] S102. Obtain the equivalent inertia improvement factor and the frequency deviation factor. The equivalent inertia improvement factor is the ratio of the equivalent inertia time constant during the inertia support control of the asynchronous machine power supply to that during the no-inertia support control; the frequency deviation factor is the sum of the frequency regulation coefficient of the system active load, the equivalent frequency regulation coefficients of all asynchronous machine power supplies, and the reciprocal of the equivalent droop rate of all synchronous generator governors. According to the equivalent inertia improvement factor and the frequency deviation factor, judge the current frequency interval and execute: in the first frequency interval, optimize the flexible DC frequency modulation output to reduce the frequency deviation; in the second frequency interval, start the active power support of the flexible DC converter and the active regulation of the load power; if the frequency continues to increase, disconnect the unit or load; in the third frequency interval, disconnect the regional power grid;

[0069] Frequency stability can be measured from two aspects: inertia support ability and primary frequency modulation ability. The inertia support ability is used to describe the change rate of the frequency in the initial period after the power grid suffers from active power disturbance. The primary frequency modulation ability is used to describe the magnitude of the active power that the power grid can absorb and release when there is a frequency deviation in the power grid.

[0070] The direct reflection of the inertia size is the rate of change of frequency (RoCoF) of the system. In particular, the RoCoF in the initial stage of the disturbance is only related to the inertia size and the disturbance itself. Therefore, the calculation of the inertia size can be transformed into 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 RoCoF. For example, the maximum value of RoCoF is limited to not exceeding 1 Hz / s, etc.

[0071] Usually, the minimum requirement of the power grid for the inertia size is deduced based on the maximum limit value of RoCoF and the expected maximum active power disturbance. Different power grids have different regulations for the expected maximum active power disturbance. For example, the maximum active power disturbance specified by the European continental power grid is the loss of 3000 MW of generating power, and the maximum active power disturbance specified by the power grid in China is generally the bipolar blocking of a single maximum DC transmission line.

[0072] In this application, an equivalent inertia enhancement factor is used to characterize the inertia support strength of the non-synchronous machine power supply. The derivation process is as follows:

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

[0074]

[0075] where k const is a constant related to the system operation 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 considered operation mode.

[0076] Define the equivalent inertia time constant when all non-synchronous machine power supplies adopt inertia-free support control as follows:

[0077] is

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

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

[0080] is

[0081] Thus, define the multiple of the equivalent inertia enhancement caused by the input of the inertia support control of the non-synchronous machine power supply as the equivalent inertia enhancement factor H amp , as shown in the following formula:

[0082]

[0083] Among them, is the equivalent inertia time constant of the whole network when the inertia-free support control is adopted for all the asynchronous machine power supplies in the whole network, is the equivalent inertia time constant of the whole network when the inertia support control of all the asynchronous machine power supplies is put into operation.

[0084] Obviously, H amp is always greater than 1, reflecting the improvement effect of the inertia support control of the asynchronous machine power supply on the equivalent inertia of the whole network. Therefore, it can characterize the inertia support strength of the asynchronous machine power supply.

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

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

[0087]

[0088] Among them, 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 asynchronous machine power supplies, 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 common unit of the frequency deviation factor β is MW / 0.1Hz.

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

[0090]

[0091] For frequency stability, in the power grid with a high proportion of flexible DC feeding, there are two forms of frequency stability. One is the frequency instability caused by excessive frequency deviation during the traditional primary frequency regulation process, and the other is the phenomenon of excessive frequency deviation caused by the short-term power deficit caused by the flexible DC low voltage ride-through.

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

[0093] The phenomenon of excessive frequency deviation caused by the short-term power deficit during the flexible DC low voltage ride-through occurs during the power angle swing, rather than on the time scale of primary frequency regulation. In the system, the units cannot be regarded as synchronous frequency motion. The influence degrees of different units affected by the flexible DC low voltage ride-through are different, and the maximum frequency deviations are also different. For this type of frequency stability, the formulation of the stability control strategy is the result of local area optimization.

[0094] At present, the main problems faced by the power grid with a high proportion of flexible DC feed-in are the high-frequency problems during the operation of the local isolated grid caused by DC blocking, and the low-frequency problems caused by the flexible DC low voltage ride-through and disconnection from the grid. For such frequency problems, this application proposes a solution method of "three lines of defense". Specifically, according to the magnitudes of the equivalent inertia improvement factor and the frequency deviation factor values, the unit frequency deviation is divided into three intervals, including the first frequency interval, the second frequency interval, and the third frequency interval. A first line of defense, a second line of defense, and a third line of defense are respectively set for the first frequency interval, the second frequency interval, and the third frequency interval:

[0095] The first line of defense: Frequency stability assessment and frequency preventive control during the system planning and normal operation stages;

[0096] The second line of defense: Stable 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 as shown in Figure 3 which shows the three lines of defense for frequency stability in this application.

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

[0100] During the short-term and long-term planning stages of the power grid, the frequency stability of the target system is evaluated through the inertia support ability and the primary frequency regulation ability, a reasonable power supply and DC planning scheme is selected, and the flexible DC frequency modulation potential is fully explored to avoid excessive system frequency deviation under the pre-conceived operation mode.

[0101] During the normal operation stage, with the goal of frequency stability, the safe operation boundary is found based on the inertia support ability and the primary frequency regulation ability, and the power grid operation mode is monitored and adjusted based on this result to ensure the normal operation of the system.

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

[0103] Judge the current frequency state based on the inertia support ability and primary frequency regulation ability. When the frequency deviation just begins to exceed the limit, utilize the frequency regulation function of the flexible DC to adjust its own power, and activate the emergency power support function of the DC system. The large load adjusts its own power through demand-side response, and tries to avoid load shedding as much as possible.

[0104] When the frequency deviation further increases and the flexible DC, load, and DC emergency power support fail to limit the frequency within a reasonable range, load shedding or generator tripping can only be used to restore the frequency at this time.

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

[0106] To prevent the large-scale spread of generator tripping problems caused by frequency instability and avoid power outages, emergency protection measures such as network disconnection are taken to restore the stable operation of the system.

[0107] S103. Obtain the voltage stiffness, where the voltage stiffness is the ratio of the voltage amplitude at the port of the grid-connected device after it is connected to the grid to the no-load voltage amplitude at the connection point; judge the current voltage range according to the voltage stiffness and execute: in the first voltage range, optimize the reactive power compensation configuration to reduce the voltage deviation; in the second voltage range, perform active load voltage regulation and cut off the secondary load; if the voltage continues to deteriorate, cut off the main load in preset rounds; in the third voltage range, disconnect the regional grid.

[0108] In the voltage stability assessment, the voltage amplitude of any bus in the grid will be affected by the types and capacities of the connected devices, and the index describing the degree of influence is the voltage support strength.

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

[0110]

[0111] where K vtg is the voltage stiffness, U sys is the voltage amplitude at the port of the grid-connected device after it is connected to the grid, U sys0 is the no-load voltage amplitude at the connection 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 device, and 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 reduction amplitude of the key nodes, corresponding measures are taken to ensure the safe operation of the power system to the greatest extent. For the problem of voltage stability in the power system with large load access, this application proposes a "three - line defense" method. Specifically, according to the magnitude of the voltage stiffness value, the voltage reduction amplitude is divided into three intervals, including the first voltage interval, the second voltage interval, and the third voltage interval. The first - line defense, the second - line defense, and the third - line defense are set for the first voltage interval, the second voltage interval, and the third voltage interval respectively:

[0114] The first - line defense: voltage stiffness evaluation and voltage preventive control during the system planning and normal operation stages;

[0115] The second - line defense: stable control of too - low voltage;

[0116] The third - line defense: grid disconnection and load shedding after voltage collapse.

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

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

[0119] During the short - term and long - term planning stages of the power grid, the voltage stiffness of the key nodes in the target system is evaluated through voltage stiffness, a reasonable comprehensive planning scheme of the source - grid - load is selected, and methods such as automatic voltage control and emergency voltage support of multiple types of reactive power sources are used to improve the voltage level during short - circuit faults and recovery periods, and to avoid voltage collapse in the system under the pre - planned operation mode.

[0120] During the normal operation stage, with the goal of voltage stability, the safe operation boundary is found based on voltage stiffness, and the grid operation mode is monitored and adjusted according to this result to ensure the normal operation of the system.

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

[0122] According to the voltage stiffness, the current voltage state is judged. When the voltage is relatively low, the possible voltage collapse is sensed in advance, the demand - side response of large loads is started in advance, and a part of the unimportant loads are cut off. Through the power adjustment ability of large loads and the priority cutting of unimportant loads, the further expansion of the accident is avoided as much as possible.

[0123] During the continuous voltage reduction process, at this time, the loads are cut off in rounds to ensure the safe operation of the system.

[0124] The third - line defense is that the voltage of some regional power grids collapses.

[0125] To prevent the large-scale spread of low-voltage problems and cause power outage accidents, emergency protection measures such as islanding are taken to restore the stable operation of the system and avoid the tripping of a large number of units and equipment damage, etc.

[0126] This application also provides a security device applicable to a high proportion of flexible DC power feeding into the grid, including:

[0127] A power angle security module, which obtains the synchronous power coefficient, and the synchronous power coefficient is the ratio of the output increment of the unit to the power angle change amount; according to the synchronous power coefficient, the current power angle interval is judged, and the following is executed: 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 DC power output; if the power angle difference continues to increase, trip the out-of-step unit; in the third power angle interval, island the regional grid;

[0128] A frequency security module, which obtains the equivalent inertia improvement factor and the frequency deviation factor, and the equivalent inertia improvement factor is the ratio of the equivalent inertia time constant during the inertia support control of the asynchronous machine power supply to that without inertia support control; the frequency deviation factor is the sum of the frequency regulation coefficient of the system active load, the equivalent frequency regulation coefficients of all asynchronous machine power supplies and the reciprocal of the equivalent droop rate of all synchronous generator governors; according to the equivalent inertia improvement factor and the frequency deviation factor, the current frequency interval is judged, and the following is executed: in the first frequency interval, optimize the flexible DC frequency modulation output to reduce the frequency deviation; in the second frequency interval, start the active power support of the flexible DC converter and the active regulation of the load power; if the frequency continues to increase, trip the unit or load; in the third frequency interval, island the regional grid;

[0129] A voltage security module, which obtains the voltage stiffness, and the voltage stiffness is the ratio of the voltage amplitude at the port after the grid-connected equipment is connected to the grid to the no-load voltage amplitude at the connection point; according to the voltage stiffness, the current voltage interval is judged, and the following is executed: in the first voltage interval, optimize the reactive power compensation configuration to reduce the voltage deviation; in the second voltage interval, perform the active regulation of the load voltage and trip the secondary load; if the voltage continues to deteriorate, trip the main load in preset rounds by grading; in the third voltage interval, island the regional grid.

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

[0131]

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

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

[0134]

[0135] Among them, is the equivalent inertia time constant without inertia support, is the equivalent inertia time constant when inertia support is put in.

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

[0137]

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

[0139] Furthermore, in the voltage security module, the optimized reactive power compensation device includes:

[0140] Put in the STATCOM device and dynamically adjust the reactive power output of the SVG.

[0141] The above description of the embodiments is to enable those of ordinary skill in the art of the present technology to understand and apply the present invention. It is obvious that those skilled in the art can easily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative labor. Therefore, the present invention is not limited to the above embodiments, and the improvements and modifications made by those skilled in the art to the present invention should be within the protection scope of the present invention according to the disclosure of the present invention.

Claims

1. A security method applicable to a high-proportion flexible DC power feeding power grid, characterized in that, Including: Obtain the synchronous power coefficient, which is the ratio of the output increment of the unit to the change in power angle; judge 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 DC power output; if the power angle difference continues to increase, disconnect the out-of-step unit; in the third power angle interval, disconnect the regional power grid; Obtain the equivalent inertia improvement factor and the frequency deviation factor. The equivalent inertia improvement factor is the ratio of the equivalent inertia time constant during the inertia support control of the asynchronous machine power supply to that without inertia support control; the frequency deviation factor is the sum of the frequency regulation coefficient of the system active load, the equivalent frequency regulation coefficients of all asynchronous machine power supplies and the reciprocal of the equivalent droop rate of all synchronous generator governors; judge the current frequency interval according to the equivalent inertia improvement factor and the frequency deviation factor and execute: in the first frequency interval, optimize the flexible DC frequency modulation output to reduce the frequency deviation; in the second frequency interval, start the active power support of the flexible DC converter and the active regulation of the load power; if the frequency continues to increase, disconnect the unit or load; In the third frequency interval, disconnect the regional power grid; Obtain the voltage stiffness, which is the ratio of the voltage amplitude at the port after the grid-connected equipment is connected to the grid to the no-load voltage amplitude at the connection point; Judge the current voltage interval according to the voltage stiffness 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 regulation of the load voltage and disconnect the secondary load; if the voltage continues to deteriorate, disconnect the main load in preset rounds; in the third voltage interval, disconnect the regional power grid.

2. The security method applicable to a flexible DC feeding high proportion power grid according to claim 1, wherein The calculation formula of the synchronous power coefficient is: Among them, K syn is the synchronizing power coefficient, P g is the output of the unit, and δ g is the power angle of the unit.

3. The security method applicable to a flexible DC feeding high proportion power grid according to claim 1, wherein, The calculation formula of the equivalent inertia improvement factor is: Among them, is the equivalent inertia time constant without inertia support, is the equivalent inertia time constant when inertia support is put in, f is the frequency, and t is the time.

4. The security and protection method applicable to a high-proportion flexible DC feeding power grid according to claim 1, wherein, The calculation formula of the voltage stiffness is: Among them, K vtg is the voltage stiffness, Z th is the Thevenin equivalent impedance, Z device is the impedance of the grid-connected device, is the phase angle of the impedance of the grid-connected device, is the phase angle of the equivalent impedance of the power grid, λ SCR is the short-circuit ratio, U sys is the voltage amplitude after connection, U sys0 is the no-load voltage amplitude.

5. The security method applicable to a flexible DC feeding high proportion power grid according to claim 1, wherein Optimizing the reactive power compensation device includes: Put into the STATCOM device and dynamically adjust the reactive power output of the SVG.

6. A security device applicable to a flexible DC feeding high proportion power grid, characterized in that Including: A power angle security module that obtains the synchronous power coefficient, which is the ratio of the output increment of the unit to the change in power angle; judges the current power angle interval according to the synchronous power coefficient and executes: 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 DC power output; if the power angle difference continues to increase, disconnect the out-of-step unit; in the third power angle interval, disconnect the regional power grid; A frequency security module that obtains the equivalent inertia improvement factor and the frequency deviation factor. The equivalent inertia improvement factor is the ratio of the equivalent inertia time constant during the inertia support control of the asynchronous machine power supply to that without inertia support control; the frequency deviation factor is the sum of the frequency regulation coefficient of the system active load, the equivalent frequency regulation coefficients of all asynchronous machine power supplies and the reciprocal of the equivalent droop rate of all synchronous generator governors; judges the current frequency interval according to the equivalent inertia improvement factor and the frequency deviation factor and executes: in the first frequency interval, optimize the flexible DC frequency modulation output to reduce the frequency deviation; in the second frequency interval, start the active power support of the flexible DC converter and the active regulation of the load power; if the frequency continues to increase, disconnect the unit or load; In the third frequency interval, disconnect the regional power grid; A voltage security protection module obtains voltage stiffness, where the voltage stiffness is the ratio of the voltage amplitude at the port of an in- grid device after connecting to the power grid to the no- load voltage amplitude at the connection point. Based on the voltage stiffness, the current voltage range is determined and the following operations are performed: in the first voltage range, the reactive power compensation configuration is optimized to reduce the voltage deviation; in the second voltage range, active load voltage regulation and secondary load shedding are carried out; if the voltage continues to deteriorate, the main loads are shed in graded rounds according to a preset number of rounds; in the third voltage range, the regional power grid is disconnected.

7. The security device applicable to a flexible DC power feeding high proportion power grid according to claim 6, wherein, The calculation formula for the synchronous power coefficient is: Among them, 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.

8. The security device applicable to a flexible DC feeding high proportion power grid according to claim 6, wherein The calculation formula for the equivalent inertia boosting factor is: Among them, is the equivalent inertia time constant without inertia support, is the equivalent inertia time constant when inertia support is put in.

9. The security device for a flexible DC feeding high proportion power grid according to claim 6, wherein The calculation formula for the voltage stiffness is: Among them, K vtg is the voltage stiffness, Z th is the Thevenin equivalent impedance, Z device is the impedance of the grid-connected device, is the phase angle of the impedance of the grid-connected device, is the phase angle of the equivalent impedance of the power grid, λ SCR is the short-circuit ratio, U sys is the voltage amplitude after connection, U sys0 is the no-load voltage amplitude.

10. The security device applicable to a flexible DC feeding high proportion power grid according to claim 6, characterized in that In the voltage security protection module, the reactive power compensation device optimization includes: Putting into operation the STATCOM device and dynamically adjusting the reactive power output of the SVG.

Images