CN114118857B - Multi-DC feed-in receiving end power grid main grid frame bearing capacity assessment method - Google Patents
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Abstract
The invention provides a method for evaluating the bearing capacity of a main grid of a multi-DC feed-in receiving-end power grid, which comprises the following steps: firstly, constructing a main grid frame bearing capacity evaluation index system and collecting basic information of a power grid; secondly, carrying out power flow calculation under the condition of stable operation of the power grid to obtain a power grid strength evaluation index; then, setting an alternating current line fault mode, and performing stability simulation under an expected fault set of the power grid to obtain an evaluation index of the disturbance rejection capability of the power grid; simulating the power grid after the AC line faults are removed to obtain power grid elasticity assessment indexes; and finally, evaluating the bearing capacity of the main grid frame by using the power grid strength evaluation index, the power grid disturbance rejection capacity evaluation index and the power grid elasticity evaluation index respectively. According to the invention, a main grid frame bearing capacity assessment index system of the receiving end power grid is constructed by considering possible running states of the power grid, the capacity of the receiving end main grid frame for bearing large-scale direct current feed-in is comprehensively assessed, and a reference basis is provided for power grid planning and decision-making personnel to improve the main grid frame of the receiving end power grid.
Description
Technical Field
The invention relates to the technical field of power system safety, in particular to a method for evaluating the bearing capacity of a main grid frame of a multi-DC feed-in receiving-end power grid.
Background
Currently, the receiving-end region power grids such as the China east power grid, the China middle power grid and the Guangdong power grid form a multi-direct-current feed-in pattern. The east China power grid is used as the receiving end power grid with the most feeding of ultra-high voltage direct current transmission in the world, and has the functions of quasi east-Anhui nan, tin alliance-Taizhou, yanhui, and 7 times of high-capacity ultra-high voltage direct current to a home dam-Shanghai Lingzhou-Shaoxing-Zhejiang and jin Su Zhi, wherein the maximum feeding power of the direct current can reach 5770 kilokilowatts; according to the related planning, 5 high-capacity extra-high voltage direct currents and Yubei flexible back-to-back direct currents of Yuzhong, qi Shao, qinghai-Henan, shanxi-Hubei, yazhong-Jiangxi and the like are fed into the China in the future, and the direct current power receiving scale exceeds 4850 kilowatts; the Guangdong power grid is fed with 10 loops of direct current, and the direct current feed power reaches 3920 kilowatts. Unlike the foreign multi-DC feed-in receiving end power grid, the DC inversion stations in China have denser landing points, the delivered DC power is larger, and the characteristics of strong-DC weak-AC caused by the dense feed-in of the extra-high voltage DC group are obvious.
Although, the construction of the extra-high voltage direct current engineering solves the problem of unbalanced energy distribution in China, and is beneficial to the optimization configuration of resources in a large range. However, as the number and capacity of the direct current feeds increase in a step-by-step manner, the 'strong and weak intersection' characteristic of the receiving end power grid is prominent. Because the multi-feed direct current replaces the conventional thermal power generating unit at the receiving end in a large quantity, the equivalent inertia of the power grid at the receiving end is continuously reduced, the frequency adjusting capability and the voltage supporting capability are continuously reduced, and the problem of frequency and voltage stability is outstanding. Currently, the receiving end power grid has encountered practical problems affecting further improvement of the carrying capacity of the main grid, such as: the direct current dense feed-in leads to insufficient static voltage supporting capability of the power grid, reduced system frequency stability after mechanical inertia is greatly reduced, transient dynamic stability problem of the power grid, voltage stability characteristic deterioration caused by direct current continuous commutation failure, tide evacuation and safety problem of a direct current falling point near zone and the like. Therefore, in order to purposefully improve the weak links existing in the receiving-end power grid, it is necessary to consider the adaptability problem of the multi-direct-current feed receiving-end power grid in the normal state, the fault state and the fault recovery state.
Disclosure of Invention
Aiming at the defects in the background technology, the invention provides a multi-direct-current feed-in receiving-end power grid main grid frame bearing capacity assessment method which is used for improving weak links of a receiving-end power grid and providing reference for power grid planning and decision-making personnel to improve the receiving-end power grid main grid frame.
The technical scheme of the invention is realized as follows:
a method for evaluating the bearing capacity of a main grid frame of a multi-DC feed-in receiving-end power grid comprises the following steps:
S1: constructing a main grid frame bearing capacity evaluation index system and collecting basic information of a power grid; the main grid frame bearing capacity evaluation index system comprises a power grid strength evaluation index, a power grid disturbance rejection capacity evaluation index and a power grid elasticity evaluation index;
S2: according to the basic information of the power grid in the step S1, carrying out power flow calculation under the condition of stable operation of the power grid to obtain a power grid strength evaluation index;
s3: setting an alternating current line fault mode, and performing stability simulation under an expected fault set of the power grid to obtain an evaluation index of the disturbance rejection capability of the power grid;
s4: simulating the power grid after the AC line faults are removed to obtain power grid elasticity assessment indexes;
s5: and evaluating the bearing capacity of the main grid frame by using the power grid strength evaluation index, the power grid disturbance rejection capacity evaluation index and the power grid elasticity evaluation index respectively.
Preferably, the power grid strength evaluation index is a related index for evaluating the bearing capacity of the main grid frame in the normal running state of the power grid, and comprises a multi-feed short circuit ratio index, total rotational kinetic energy of a system, an equivalent inertial time constant of the system and a relative inertial constant of the system;
The power grid disturbance rejection evaluation index is a related index for evaluating the main grid frame bearing capacity of the power grid under the fault running state, and comprises a fault passing rate, a transient voltage stability index, a frequency stability index, a power angle stability index, a tide safety margin index and a short circuit current margin index;
The power grid elasticity evaluation index is a related index for evaluating the main grid frame bearing capacity when the power grid is recovered from a fault state to a normal level, and comprises a frequency recovery capacity index, a power angle recovery capacity index and a voltage recovery capacity index.
Preferably, the multi-feed short-circuit ratio index is used for evaluating the voltage supporting strength of the receiving ac power grid to the dc system, and the calculation formula is as follows:
Wherein: MSCR is a multi-feed-in short-circuit ratio index, S aci is a three-phase short-circuit capacity at an ith back direct current converter bus, P dNi is rated transmission power of the ith back direct current, and MIIF j,i is a ratio of the voltage variation of the jth back direct current converter bus to the voltage variation of the ith back direct current converter bus when reactive disturbance is applied to the ith back direct current converter bus;
the total rotational kinetic energy of the system refers to the sum of the rotational kinetic energy of the whole power grid starting machine set, and the calculation formula is as follows:
Wherein: h sys is total rotational kinetic energy of the system, H n is an inertial time constant of the unit N, S n is rated capacity of the unit N, N is the number of units, x n is an on-off state of the unit N, n=0 is an off state, and n=1 is an on state;
the equivalent inertia time constant is used for evaluating the inertia level of the system, and the calculation formula is as follows:
Wherein: h eq is the equivalent inertial time constant;
The system relative inertia constant is used for measuring the relative strength relation of mechanical inertia of an alternating current system after direct current is connected to a receiving end power grid, and the calculation formula is as follows:
Wherein: h dc is the system relative inertia constant, S ac is the system reference power, and P dc is the total dc feed capacity.
Preferably, the failure passing rate is the proportion of the number of safe and stable operation of the power grid after the elements with the same voltage level fail to the total number of the elements, and the calculation formula is as follows:
wherein: alpha is the failure passing rate, N p is the total number of situations that the power grid can safely and stably run after failure, and N' is the total number of expected failures;
The transient voltage stability index refers to whether the voltage can be recovered to be more than 0.9pu of the voltage stability threshold within 10s after the fault is encountered, and the calculation formula is as follows:
wherein: t v is a transient voltage stability index, M is the number of 500kV buses of the receiving-end power grid, As an indicator of the severity of the voltage drop of bus m,T c is the fault occurrence time, U m (t) is the voltage amplitude of the bus m at the time t, and U S,m is the threshold value of the bus m for maintaining the transient voltage stability;
The frequency stability index comprises the transient extreme frequency of the system after the fault and the quasi-steady frequency of the system after the fault;
The power angle stability index adopts the maximum power angle difference after failure, and the calculation formula is as follows:
δ＝max|δi'-δj'|；
Wherein: delta i' is the power angle of the i 'th generator after the fault, delta j' is the power angle of the j' th generator after the fault;
the load flow safety margin index is used for monitoring the load flow safety of the line after the power grid faults, and the calculation formula is as follows:
Wherein: ρ is a tide safety margin index, P max is the thermal stability limit power of the transmission line, and P is the transmission power of the line after the fault;
The short-circuit current margin index represents the current carrying level of a 500kV bus of a receiving end power grid under a short-circuit fault, and the calculation formula is as follows:
Wherein: k is a short-circuit current margin index, I max is the upper limit of short-circuit current allowed to flow through the 500kV bus m, and I is the short-circuit current when the 500kV bus is subjected to three-phase metallic short-circuit.
Preferably, after the dc blocking fault is cleared, the system frequency will gradually rise to a quasi-steady-state frequency, and the system frequency recovery rate in this process is adopted to quantitatively analyze the frequency recovery capability of the system, and the calculation formula is as follows:
Wherein: f avg is the recovery rate of the system frequency after the fault, f (t) is the system frequency function curve, t 1 is the fault occurrence time, and t 2 is the time when the system frequency is recovered to the quasi-steady-state frequency;
the power angle recovery time of the generator after fault clearing is used as a power angle recovery capacity index, and the calculation formula is as follows:
tar＝tae-tac；
wherein: t ar is a power angle recovery capability index, t ac is a fault occurrence time, and t ae is a system power angle recovery stabilization time;
the voltage recovery time is used as a voltage recovery capacity index, and the calculation formula is as follows:
tur＝tue-tuc；
wherein: t ur is a voltage recovery capability index, t uc is a fault occurrence time, and t ue is a system power angle recovery stabilization time.
Preferably, the method for evaluating the bearing capacity of the main grid frame by using the grid strength evaluation index comprises the following steps: the system total rotational kinetic energy, the system equivalent inertia time constant and the system relative inertia constant index are used for reflecting the inertia level of the receiving-end power grid, wherein the inertia level refers to the capacity of the system for inhibiting the system frequency change under the condition of unbalanced active power; the larger the index value, the higher the inertia level and the stronger the ability to suppress frequency variation.
Preferably, the method for evaluating the main grid frame bearing capacity by using the grid disturbance rejection capacity evaluation index comprises the following steps: the higher the fault passing rate is, the stronger the disturbance rejection capability of the main grid frame to faults is; when the load flow safety margin index and the short circuit current margin index are positive values, the safety problem of the system load flow and the short circuit current is not existed; the transient voltage stability evaluation standard is that in the transient process of the power system after being greatly disturbed, the bus voltage is recovered to be more than 0.8p.u. within 10 s; the frequency stability requires that the transient extremum frequency offset after the system is greatly disturbed is not more than 1Hz; the power angle stability index takes the maximum power angle difference after the fault as a judgment standard, and if the power angle difference is continuously increased, the transient power angle instability of the system occurs.
Preferably, the method for evaluating the bearing capacity of the main grid frame by using the grid elasticity evaluation index comprises the following steps: the power grid elasticity evaluation index is used for evaluating the recovery capacity of the power grid, and the shorter the time is, the faster the speed is, the stronger the main grid bearing capacity is.
Compared with the prior art, the invention has the beneficial effects that: starting from possible running states of the power grid, searching factors restricting the bearing capacity of the main grid frame of the power grid at the receiving end in the normal state, fault occurrence and fault recovery processes, and constructing an evaluation index system of the bearing capacity of the main grid frame of the power grid at the receiving end based on the restricting factors; the index system can be used for revealing the power grid strength, the disturbance rejection capability and the elasticity level of the multi-direct current feed-in receiving end power grid in three operation scenes of normal state, fault occurrence and fault recovery processes, thereby comprehensively evaluating the capability of the receiving end main grid frame for bearing large-scale direct current feed-in, and providing reference for power grid planning and decision-making personnel to improve the receiving end power grid main grid frame.
Drawings
In order to more clearly illustrate the embodiments of the invention or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described, it being obvious that the drawings in the following description are only some embodiments of the invention, and that other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 is a main grid rack load capacity assessment index system construction flow;
FIG. 2 is a primary grid load capacity assessment flow;
FIG. 3 is a schematic diagram of a near-end grid frame of an extra-high voltage DC receiving end of a provincial power grid;
Fig. 4 is a schematic diagram of an extra-high voltage ac station of a provincial power grid.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without any inventive effort, are intended to be within the scope of the invention. Taking the evaluation of the bearing capacity of the main grid of two planning schemes of a provincial power grid as an embodiment, the provincial power grid is fed with two extra-high voltage direct currents, and the near-area grid of the extra-high voltage direct current receiving end is shown in fig. 3. The planning scheme A does not process the main network frame of the receiving end, and an extra-high voltage alternating current station is added to the main network frame of the receiving end in the planning scheme B, and the specific scheme is shown in fig. 4.
As shown in fig. 1 and 2, the embodiment of the invention provides a method for evaluating the bearing capacity of a main grid frame of a multi-direct-current feed-in receiving-end power grid, which comprises the following specific steps:
S1: constructing a main grid frame bearing capacity evaluation index system and collecting basic information of a power grid; the main grid frame bearing capacity evaluation index system comprises a power grid strength evaluation index, a power grid disturbance rejection capacity evaluation index and a power grid elasticity evaluation index;
the power grid strength evaluation index is a related index for evaluating the bearing capacity of the main grid frame in the normal running state of the power grid, and comprises a multi-feed short circuit ratio index, total rotational kinetic energy of a system, an equivalent inertial time constant of the system and a relative inertial constant of the system.
The power grid disturbance rejection evaluation index is a related index for evaluating the main grid frame bearing capacity of the power grid under the fault running state, and comprises a fault passing rate, a transient voltage stability index, a frequency stability index, a power angle stability index, a power flow safety margin index and a short circuit current margin index.
The power grid elasticity evaluation index is a related index for evaluating the main grid frame bearing capacity when the power grid is recovered from a fault state to a normal level, and comprises a frequency recovery capacity index, a power angle recovery capacity index and a voltage recovery capacity index.
And determining the running mode of the power grid and the start-stop condition of the system unit, and acquiring the grid feed direct current, 500kV/1000kV receiving end grid frame information and system inertia information.
S2: according to the basic information of the power grid in the step S1, carrying out power flow calculation under the condition of stable operation of the power grid to obtain a power grid strength evaluation index; calculating the total rotational kinetic energy of the system, the equivalent inertial time constant of the system and the relative inertial constant of the system according to the obtained power grid information; and carrying out load flow calculation under the determined power grid operation mode, and calculating a multi-feed-in short circuit ratio index.
The multi-feed-in short-circuit ratio index is used for evaluating the voltage supporting strength of the receiving end alternating current power grid to the direct current system, and the calculation formula is as follows:
Wherein: MSCR is a multi-feed-in short-circuit ratio index, S aci is a three-phase short-circuit capacity at an ith back direct current converter bus, P dNi is rated transmission power of the ith back direct current, and MIIF j,i is a ratio of the voltage variation of the jth back direct current converter bus to the voltage variation of the ith back direct current converter bus when reactive power disturbance is applied to the ith back direct current converter bus.
The total rotational kinetic energy of the system refers to the sum of the rotational kinetic energy of the whole power grid starting machine set, and the calculation formula is as follows:
wherein: h sys is total rotational kinetic energy of the system, H n is an inertial time constant of the unit N, S n is a rated capacity of the unit N, N is the number of units, x n is an on-off state of the unit N, n=0 is an off state, and n=1 is an on state.
The equivalent inertia time constant is used for evaluating the inertia level of the system, and the calculation formula is as follows:
wherein: h eq is the equivalent inertial time constant.
The system relative inertia constant is used for measuring the relative strength relation of mechanical inertia of an alternating current system after direct current is connected to a receiving end power grid, and the calculation formula is as follows:
Wherein: h dc is the system relative inertia constant, S ac is the system reference power, and P dc is the total dc feed capacity.
Step S2 is performed on the planning scheme A in the embodiment, and the obtained power grid strength evaluation index is shown in table 1.
Table 1 grid strength evaluation index
As shown in table 1, the multi-feed short-circuit ratio index of each direct current in the planning scheme a is greater than 3, and the system belongs to the category of strong systems, and the inertia level meets the requirement of the bearing capacity of the main grid.
S3: setting an alternating current line fault mode, and performing stability simulation under an expected fault set of the power grid to obtain an evaluation index of the disturbance rejection capability of the power grid; setting an alternating current line fault mode, and calculating part of power grid disturbance rejection evaluation indexes: fault passing rate, tide safety margin index; setting a direct current line fault mode, and calculating part of power grid disturbance rejection capability assessment indexes: transient voltage stability index, frequency stability index and power angle stability index; and (3) carrying out three-phase short-circuit current verification on the 500kV and 1000kV buses, and calculating to obtain a short-circuit current margin index.
The fault passing rate is the proportion of the number of safe and stable operation of the power grid to the total number of elements after the elements (circuits and transformers) with the same voltage class have faults, and can be divided into N-1 fault passing rate and N-2 fault passing rate of the double-circuit lines with the same tower according to fault types. The higher the pass rate is, the stronger the disturbance rejection capability of the receiving end power grid for handling alternating current faults is. The calculation formula is as follows:
wherein: alpha is the failure passing rate, N p is the total number of situations that the power grid can safely and stably run after the failure, and N' is the total number of expected failures.
The transient voltage stability index refers to whether the voltage can be recovered to be more than 0.9pu of the voltage stability threshold within 10s after the fault is encountered, and the calculation formula is as follows:
wherein: t v is a transient voltage stability index, M is the number of 500kV buses of the receiving-end power grid, As an indicator of the severity of the voltage drop of bus m,T c is the fault occurrence time, Δt is generally set to 10s, U m (t) is the voltage amplitude of the bus m at time t, U S,m is the threshold for keeping the transient voltage stable for the bus m, and the threshold is set to 0.9pu.
The frequency stability index comprises the transient extreme frequency of the system after the fault and the quasi-steady frequency of the system after the fault;
The power angle stability index adopts the maximum power angle difference after failure, and the calculation formula is as follows:
δ＝max|δi'-δj'|；
Wherein: delta i' is the power angle of the i 'th generator after the fault, delta j' is the power angle of the j' th generator after the fault;
the load flow safety margin index is used for monitoring the load flow safety of the line after the power grid faults, and the calculation formula is as follows:
Wherein: ρ is a tide safety margin index, P max is the thermal stability limit power of the transmission line, and P is the transmission power of the line after the fault;
The short-circuit current margin index represents the current carrying level of a 500kV bus of a receiving end power grid under a short-circuit fault, and the calculation formula is as follows:
Wherein: k is a short-circuit current margin index, I max is the upper limit of short-circuit current allowed to flow through the 500kV bus m, and I is the short-circuit current when the 500kV bus is subjected to three-phase metallic short-circuit.
And (3) performing step S3 on the planning scheme A in the embodiment to obtain the power grid disturbance rejection evaluation index shown in tables 2-4.
TABLE 2
TABLE 3 tidal current safety margin index
TABLE 4 safety index of short-circuit current margin
As shown in the table above, the power flow safety margin index in the planning scheme a does not meet the requirement of the power grid for safe and stable operation, and the step S3 operation is performed on the planning scheme B to obtain the power flow safety margin index as shown in table 5.
TABLE 5 tidal current safety margin index
And the power flow safety margin index of the planning scheme B meets the safety and stability operation requirement of the power grid.
Tables 2-4 list the results of the evaluation index calculation of the disturbance rejection capability of the power grid under the planning scheme A, and the situation that the power flow safety margin index is smaller than 0 exists under the planning scheme, so that the requirements of safe and stable operation of the power grid are not met. For this purpose, a planning scheme B is provided, and the existing tidal current safety problem is solved in a targeted manner.
S4: simulating the power grid after the AC line faults are removed to obtain power grid elasticity assessment indexes; and (3) cutting off faults after setting the alternating current faults for a period of time, and calculating all power grid elasticity evaluation indexes: frequency recovery capability index, power angle recovery capability index, voltage recovery capability index; and after setting the direct current fault, cutting off the fault for a period of time, and calculating all power grid elasticity evaluation indexes: frequency recovery capability index, power angle recovery capability index, voltage recovery capability index.
After the direct current blocking fault is cleared, the system frequency is gradually increased to a quasi-steady-state frequency, and the frequency recovery capacity of the system is quantitatively analyzed by adopting the system frequency recovery rate in the process, wherein the calculation formula is as follows:
Wherein: f avg is the recovery rate of the system frequency after the fault, f (t) is the system frequency function curve, t 1 is the fault occurrence time, and t 2 is the time when the system frequency is recovered to the quasi-steady-state frequency;
the power angle recovery time of the generator after fault clearing is used as a power angle recovery capacity index, and the calculation formula is as follows:
tar＝tae-tac；
wherein: t ar is a power angle recovery capability index, t ac is a fault occurrence time, and t ae is a system power angle recovery stabilization time;
the voltage recovery time is used as a voltage recovery capacity index, and the calculation formula is as follows:
tur＝tue-tuc；
wherein: t ur is a voltage recovery capability index, t uc is a fault occurrence time, and t ue is a system power angle recovery stabilization time.
Step S4 is performed on the planning scheme A in the embodiment, and the obtained power grid elasticity evaluation index is shown in Table 6.
TABLE 6 Power grid elasticity assessment index
| Type of dc blocking | Frequency recovery rate Hz/s and time(s) | Power angle recovery time s | Voltage recovery time s |
| DC 1 monopole latch | 1.65(30) | 10.4 | 10.1 |
| DC 2 monopole latch | 1.33(36) | 9.8 | 12.5 |
| DC 1 bipolar latch | 1.82(37) | 10.8 | 10.6 |
| DC 2 bipolar latch | 1.88(42) | 11.1 | 11.6 |
As shown in table 6, the planning scheme a has strong recovery capability after the direct current blocking failure, and meets the requirement of the bearing capability of the main grid.
S5: and evaluating the bearing capacity of the main grid frame by using the power grid strength evaluation index, the power grid disturbance rejection capacity evaluation index and the power grid elasticity evaluation index respectively.
The method for evaluating the bearing capacity of the main grid frame by using the power grid strength evaluation index comprises the following steps: the system total rotational kinetic energy, the system equivalent inertia time constant and the system relative inertia constant index are used for reflecting the inertia level of the receiving-end power grid, wherein the inertia level refers to the capacity of the system for inhibiting the system frequency change under the condition of unbalanced active power; the larger the index value, the higher the inertia level and the stronger the ability to suppress frequency variation. According to practical operation experience, the receiving end system is a strong system when MSCR is more than or equal to 3, the receiving end system is a weak system when MSCR is less than or equal to 2, and the receiving end system is a weaker system when MSCR is less than or equal to 2 and less than or equal to 3.
The method for evaluating the main grid frame bearing capacity by using the power grid disturbance rejection evaluation index comprises the following steps: the higher the fault passing rate is, the stronger the disturbance rejection capability of the main grid frame to faults is; when the load flow safety margin index and the short circuit current margin index are positive values, the safety problem of the system load flow and the short circuit current is not existed; the transient voltage stability evaluation standard is that in the transient process of the power system after being greatly disturbed, the bus voltage is recovered to be more than 0.8p.u. within 10 s; the frequency stability requires that the transient extremum frequency offset after the system is greatly disturbed is not more than 1Hz; the power angle stability index takes the maximum power angle difference after the fault as a judgment standard, and if the power angle difference is continuously increased, the transient power angle instability of the system occurs.
The method for evaluating the bearing capacity of the main grid frame by using the power grid elasticity evaluation index comprises the following steps: the power grid elasticity evaluation index is used for evaluating the recovery capacity of the power grid, and the shorter the time is, the faster the speed is, the stronger the main grid bearing capacity is.
The foregoing description of the preferred embodiments of the invention is not intended to be limiting, but rather is intended to cover all modifications, equivalents, alternatives, and improvements that fall within the spirit and scope of the invention.
Claims (6)
1. A method for evaluating the bearing capacity of a main grid frame of a multi-DC feed-in receiving-end power grid is characterized by comprising the following steps:
S1: constructing a main grid frame bearing capacity evaluation index system and collecting basic information of a power grid; the main grid frame bearing capacity evaluation index system comprises a power grid strength evaluation index, a power grid disturbance rejection capacity evaluation index and a power grid elasticity evaluation index;
S2: according to the basic information of the power grid in the step S1, carrying out power flow calculation under the condition of stable operation of the power grid to obtain a power grid strength evaluation index;
The power grid strength evaluation index is a related index for evaluating the bearing capacity of the main grid frame in the normal running state of the power grid, and comprises a multi-feed short circuit ratio index, total rotational kinetic energy of a system, an equivalent inertial time constant of the system and a relative inertial constant of the system;
The multi-feed-in short-circuit ratio index is used for evaluating the voltage supporting strength of the receiving end alternating current power grid to the direct current system, and the calculation formula is as follows:
Wherein: MSCR is a multi-feed-in short-circuit ratio index, S aci is a three-phase short-circuit capacity at an ith back direct current converter bus, P dNi is rated transmission power of the ith back direct current, and MIIF j,i is a ratio of the voltage variation of the jth back direct current converter bus to the voltage variation of the ith back direct current converter bus when reactive disturbance is applied to the ith back direct current converter bus;
the total rotational kinetic energy of the system refers to the sum of the rotational kinetic energy of the whole power grid starting machine set, and the calculation formula is as follows:
Wherein: h sys is total rotational kinetic energy of the system, H n is an inertial time constant of the unit N, S n is rated capacity of the unit N, N is the number of units, x n is an on-off state of the unit N, n=0 is an off state, and n=1 is an on state;
the equivalent inertia time constant is used for evaluating the inertia level of the system, and the calculation formula is as follows:
Wherein: h eq is the equivalent inertial time constant;
The system relative inertia constant is used for measuring the relative strength relation of mechanical inertia of an alternating current system after direct current is connected to a receiving end power grid, and the calculation formula is as follows:
Wherein: h dc is the relative inertia constant of the system, S ac is the reference power of the system, and P dc is the total dc feed-in capacity;
The power grid disturbance rejection evaluation index is a related index for evaluating the main grid frame bearing capacity of the power grid under the fault running state, and comprises a fault passing rate, a transient voltage stability index, a frequency stability index, a power angle stability index, a tide safety margin index and a short circuit current margin index;
the power grid elasticity evaluation index is a related index for evaluating the main grid frame bearing capacity when the power grid is recovered from a fault state to a normal level, and comprises a frequency recovery capacity index, a power angle recovery capacity index and a voltage recovery capacity index;
s3: setting an alternating current line fault mode, and performing stability simulation under an expected fault set of the power grid to obtain an evaluation index of the disturbance rejection capability of the power grid;
s4: simulating the power grid after the AC line faults are removed to obtain power grid elasticity assessment indexes;
s5: and evaluating the bearing capacity of the main grid frame by using the power grid strength evaluation index, the power grid disturbance rejection capacity evaluation index and the power grid elasticity evaluation index respectively.
2. The method for evaluating the carrying capacity of a main grid of a multi-direct-current feed-in receiving-end power grid according to claim 1, wherein the failure passing rate is a proportion of the number of safe and stable operation of the power grid after the failure of the elements with the same voltage level to the total number of the elements, and the calculation formula is as follows:
wherein: alpha is the failure passing rate, N p is the total number of situations that the power grid can safely and stably run after failure, and N' is the total number of expected failures;
The transient voltage stability index refers to whether the voltage can be recovered to be more than 0.9pu of the voltage stability threshold within 10s after the fault is encountered, and the calculation formula is as follows:
wherein: t v is a transient voltage stability index, M is the number of 500kV buses of the receiving-end power grid, As an indicator of the severity of the voltage drop of bus m,T c is the fault occurrence time, U m (t) is the voltage amplitude of the bus m at the time t, and U S,m is the threshold value of the bus m for maintaining the transient voltage stability;
The frequency stability index comprises the transient extreme frequency of the system after the fault and the quasi-steady frequency of the system after the fault;
The power angle stability index adopts the maximum power angle difference after failure, and the calculation formula is as follows:
δ＝max|δi'-δj'|；
Wherein: delta i' is the power angle of the i 'th generator after the fault, delta j' is the power angle of the j' th generator after the fault;
the load flow safety margin index is used for monitoring the load flow safety of the line after the power grid faults, and the calculation formula is as follows:
Wherein: ρ is a tide safety margin index, P max is the thermal stability limit power of the transmission line, and P is the transmission power of the line after the fault;
The short-circuit current margin index represents the current carrying level of a 500kV bus of a receiving end power grid under a short-circuit fault, and the calculation formula is as follows:
Wherein: k is a short-circuit current margin index, I max is the upper limit of short-circuit current allowed to flow through the 500kV bus m, and I is the short-circuit current when the 500kV bus is subjected to three-phase metallic short-circuit.
3. The method for evaluating the carrying capacity of a main grid of a multi-direct-current feed-in receiving-end power grid according to claim 1, wherein after the direct-current blocking fault is cleared, the system frequency is gradually increased to a quasi-steady-state frequency, and the frequency recovery capacity of the system is quantitatively analyzed by adopting the system frequency recovery rate in the process, and the calculation formula is as follows:
Wherein: f avg is the recovery rate of the system frequency after the fault, f (t) is the system frequency function curve, t 1 is the fault occurrence time, and t 2 is the time when the system frequency is recovered to the quasi-steady-state frequency;
the power angle recovery time of the generator after fault clearing is used as a power angle recovery capacity index, and the calculation formula is as follows:
tar＝tae-tac；
wherein: t ar is a power angle recovery capability index, t ac is a fault occurrence time, and t ae is a system power angle recovery stabilization time;
the voltage recovery time is used as a voltage recovery capacity index, and the calculation formula is as follows:
tur＝tue-tuc；
wherein: t ur is a voltage recovery capability index, t uc is a fault occurrence time, and t ue is a system power angle recovery stabilization time.
4. The method for evaluating the main grid capacity of a multi-dc-feed-in receiver grid according to claim 1, wherein the method for evaluating the main grid capacity by using the grid strength evaluation index is as follows: the system total rotational kinetic energy, the system equivalent inertia time constant and the system relative inertia constant index are used for reflecting the inertia level of the receiving-end power grid, wherein the inertia level refers to the capacity of the system for inhibiting the system frequency change under the condition of unbalanced active power; the larger the index value, the higher the inertia level and the stronger the ability to suppress frequency variation.
5. The method for evaluating the main grid bearing capacity of the multi-direct-current feed-in receiving-end power grid according to claim 1 or 2, wherein the method for evaluating the main grid bearing capacity by using the power grid disturbance rejection evaluation index is as follows: the higher the fault passing rate is, the stronger the disturbance rejection capability of the main grid frame to faults is; when the load flow safety margin index and the short circuit current margin index are positive values, the safety problem of the system load flow and the short circuit current is not existed; the transient voltage stability evaluation standard is that in the transient process of the power system after being greatly disturbed, the bus voltage is recovered to be more than 0.8p.u. within 10 s; the frequency stability requires that the transient extremum frequency offset after the system is greatly disturbed is not more than 1Hz; the power angle stability index takes the maximum power angle difference after the fault as a judgment standard, and if the power angle difference is continuously increased, the transient power angle instability of the system occurs.
6. The method for evaluating the main grid capacity of the multi-dc-feed-in receiving-end power grid according to claim 1 or 3, wherein the method for evaluating the main grid capacity by using the power grid elasticity evaluation index is as follows: the power grid elasticity evaluation index is used for evaluating the recovery capacity of the power grid, and the shorter the time is, the faster the speed is, the stronger the main grid bearing capacity is.
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