CN107579530A - A low-frequency load shedding method and low-frequency load shedding control system for power grid - Google Patents

Abstract

The invention discloses a low-frequency load shedding method and a low-frequency load shedding control system for a power grid. The frequency at the generator end is obtained through a wide-area measuring device, and the frequency stability is monitored; when the system frequency drops rapidly due to a power shortage, the active power shortage is estimated; The electric vehicle management center calculates dispatching capacity and total dispatching capacity at all levels according to the state of electric vehicles, and reports to the control center; the control center formulates a load reduction plan and issues a load reduction order according to the power shortage and dispatching capacity; the management center sends a load reduction order according to the load reduction order The electric vehicle assigns the load shedding task; the control terminal controls the charging/discharging of the electric vehicle according to the load shedding task, and the online low-frequency load shedding relay assists the load shedding according to the instruction. The present invention dispatches electric vehicle resources in the process of low-frequency load shedding, through centralized management and decentralized control of electric vehicles, with the help of online low-frequency load shedding action, completes low-frequency load shedding, quickly suppresses frequency drop, and improves system stability while optimizing resource allocation sex and economy.

Description

A low-frequency load shedding method and low-frequency load shedding control system for power grid

technical field

Embodiments of the present invention relate to smart grid technologies, and in particular, relate to an under-frequency load shedding method and an under-frequency load shedding control system for a power grid.

Background technique

Frequency stability is an important guarantee for maintaining safe and stable operation of the system. A large-scale power shortage will lead to a rapid drop in system frequency, and low-frequency load shedding is an important control measure to prevent the rapid drop in frequency and prevent frequency collapse. Low-frequency load shedding enables the system to reach a new active power balance by reducing the load wheel by wheel, ensuring safe and stable operation of the system.

As a new type of transportation, electric vehicles are strongly supported by governments of various countries because they can alleviate energy crisis and environmental pollution. With the promulgation of a series of incentive policies in China, electric vehicles are being promoted continuously, and electric vehicles connected to the power grid are becoming more and more popular. Electric vehicles shoulder the dual characteristics of charging load and mobile energy storage unit. They have the characteristics of fast response, good regulation performance, and large dispatchable capacity. They can become important load-shedding resources on the demand side of the smart grid.

Traditional low-frequency load shedding achieves power balance by shedding loads, which fails to make full use of electric vehicle resources. Therefore, if the resources of electric vehicles are effectively utilized in the process of low-frequency load shedding, a reasonable low-frequency load shedding scheme is formulated, which is of great significance to the safe and stable operation of the power grid.

Contents of the invention

The purpose of the present invention is to address the above-mentioned defects in the prior art, to provide a low-frequency load-shedding method oriented to an active smart grid and taking electric vehicles as the main object,

For realizing above-mentioned purpose of the invention, the present invention adopts following technical scheme, and the concrete steps of this scheme have:

1) Measure the grid frequency, and calculate the active power deficit of the power system according to the generator terminal frequency and frequency change rate;

2) According to the type of electric vehicles and the order of investment priority, the dispatching capacity of electric vehicles is divided into several levels, and then according to the initial state of charge, expected state of charge, current charging/discharging power and maximum charging/discharging power of electric vehicles, calculate Dispatching capacity and total dispatching capacity of electric vehicles at all levels;

3) Based on the active power deficit in step 1) and the total dispatching capacity of electric vehicles in step 2), a low-frequency load shedding plan is formulated and a load shedding command is issued;

4) The electric vehicle management center dispatches a load shedding task to each electric vehicle according to the load shedding instruction, and the electric vehicle is charged/discharged according to the assigned task, and the online low-frequency load shedding relay assists the action to complete the low-frequency load shedding.

The present invention also provides the following subsidiary technical solutions:

Further, the expression for estimating the system power deficit according to the generator terminal frequency and frequency change rate in step 1) is shown as:

In the formula:

In the above formula, ΔP is the total power deficit in the system; f _c is the center frequency of inertia; H _i-sys is the inertia time constant of the i-th generator; S _i is the capacity of the i-th generator; S _b-sys is System rated capacity.

Further, the dispatching capacity of electric vehicles at all levels and the total dispatching capacity in step 2) are calculated as follows:

Following the principle of minimizing the impact on users, the type of electric vehicles and the priority of investment are determined according to the network access requirements, and the dispatching capacity of electric vehicles is divided into four levels. The first level is the maximum power discharge of the discharge type electric vehicle, the second level is the charging type electric vehicle stop charging, the third level is the maximum power discharge of the maintenance type electric vehicle, and the fourth level is the maximum power discharge of the charging type electric vehicle; The priority of the first level is the highest, and the priority of other levels is in order.

The expressions of the above-mentioned four-level scheduling capacity and the total scheduling capacity are shown as:

P _EV,4 = n*P _max

P _EV ＝P _EV,1 +P _EV,2 +P _EV,3 +P _EV,4

In the above formula, P _max is the maximum discharge power of an electric vehicle, P _EV,1 is the dispatchable capacity of the first level; z is the number of electric vehicles in the discharge category; P _d,i is the current discharge power of the i-th electric vehicle in the discharge category; P _EV,2 is the second-level schedulable capacity; P _c,i is the current charging power of the i-th electric vehicle in the charging category; n is the number of charging electric vehicles; P _EV,3 is the third-level schedulable capacity; P _{m ,i} is the current charging/discharging power of the i-th electric vehicle in the maintenance category; k is the number of electric vehicles in the maintenance category; P _EV,4 is the dispatchable capacity of the fourth level; P _EV is the total dispatch capacity of electric vehicles.

Further, the formulation principles of the low-frequency load shedding plan in step 3):

In order to prevent over-cutting, the low-frequency load shedding cut-off amount is calculated as follows:

P _shed =1.05*(ΔP-P _thr )

In the formula: P _shed is the load that needs to be removed; P _thr is the minimum value of the system's allowable power shortage;

In order to make full use of electric vehicle resources, the load shedding tasks are assigned to electric vehicles as much as possible when formulating the load shedding plan. When P _shed ≤ P _EV , all load shedding tasks are assigned to electric vehicles. When P _shed > P _EV , electric vehicles All of them are put in, and the remaining parts are cut off with the assistance of low-frequency load shedding device; the load shedding method adopts wheel-by-wheel input, and the basic wheel is set to 5 rounds, the delay time is set to 0.35 seconds, the first and second rounds are set to 0.25P _shed , and the third and fourth rounds are set to 0.25P shed. The cut off amount is 0.2P _shed , the cut off amount of the fifth round is 0.1P _shed , the basic round will give priority to calling electric vehicle resources according to the round, if the electric vehicle resources are insufficient, then start low-frequency load shedding, set up 3 special rounds, and the cut off amount of each round Both are 0.1P _shed , the delay time is 15 seconds, and the difference is 5 seconds. The special wheel will give priority to electric vehicle resources according to the situation after the basic wheel is put in. If it is insufficient, then start the low frequency reduction.

Further, the dispatch rules of the electric vehicle load reduction task in step 4) are as follows:

The management center determines the load reduction task of electric vehicles and the removal amount of each round according to the load reduction instruction. If the dispatching capacity is less than the cutting amount of this round, all the dispatchable capacity of this level will be put into use, and the dispatchable capacity of the next level will be called; Loading task, load reduction task dispatch method is as follows:

In the above formula, is the maximum discharge power allowed by the i-th electric vehicle in level I; P _i is the current charging/discharging power of the i-th electric vehicle; P _i ^I is the schedulable capacity of the i-th electric vehicle in level I ; P _shed,I is the total amount of load shedding tasks dispatched to level I; P _s,i is the load shedding task of the i-th electric vehicle in level I.

The present invention also provides a low-frequency load shedding control system, which includes a grid control center, an electric vehicle management center, an online low-frequency load shedding relay, and an interface circuit, each interface circuit includes a communication module and a V2G control module, wherein the communication The module communicates bidirectionally with the electric vehicle management center and the V2G control module respectively, and is used to collect the current charging/discharging power, maximum charging/discharging power, current SOC, expected SOC, allowed maximum and minimum SOC of the electric vehicle, and determine the current charging/discharging power of the electric vehicle. The type and capacity that can be dispatched, and upload the information to the management center, and accept the tasks dispatched by the electric vehicle management center; the V2G control module is used to control the charging and discharging of electric vehicles according to the charging and discharging instructions; the electric vehicle management center also communicates with the control center Communication, used to calculate and upload dispatching capacity at all levels and total dispatching capacity, and dispatch load shedding tasks.

Compared with the prior art, the present invention has the advantage of disclosing an under-frequency load shedding method and an under-frequency load shedding control system for a power grid. Control, assisted by online low-frequency load shedding action, completes low-frequency load shedding, quickly suppresses frequency drops, and improves system stability and economy while optimizing resource allocation.

Description of drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings that need to be used in the embodiments or related technical descriptions. Obviously, the drawings in the following description only relate to some implementations of the present invention example, not limitation of the present invention.

Fig. 1 is a frame structure diagram of an under-frequency load shedding system of an under-frequency load shedding method according to a preferred embodiment of the present invention.

Fig. 2 is a flow chart of an under-frequency load shedding method in a preferred embodiment of the present invention.

detailed description

In order to understand the above-mentioned purpose, features and advantages of the present invention more clearly, the technical solutions of the present invention will be described in further non-limiting detail below in conjunction with the accompanying drawings and specific embodiments.

The invention proposes a low-frequency load shedding method targeting the electric vehicle based on the characteristics of the decentralized grid connection of the electric vehicle, and establishes a control system for the electric vehicle to participate in the low-frequency load shedding. As shown in Figure 1, the principle framework of the system includes: grid control center, electric vehicle management center, low-frequency load shedding relay, and interface circuits, each interface circuit includes a communication module and a V2G control module, where the communication module is connected to the electric vehicle Two-way communication between the management center and the V2G control module is used to collect the current charging/discharging power, maximum charging/discharging power, current SOC, expected SOC, allowed maximum and minimum SOC of the electric vehicle, and determine the type of the current electric vehicle, which can be dispatched capacity, and upload the information to the management center, and accept the tasks dispatched by the electric vehicle management center; the V2G control module is used to control the charging and discharging of electric vehicles according to the charging and discharging instructions; And upload the scheduling capacity at all levels and the total scheduling capacity, and dispatch load shedding tasks. The management center is responsible for calculating and uploading the scheduling capacity at all levels and the total scheduling capacity, and dispatching load shedding tasks. The control center is responsible for calculating the shortfall, receiving schedulable capacity, formulating low-frequency load shedding plans, and issuing load shedding instructions.

In the above scheme, the principle of the low-frequency load shedding method for active smart grids with electric vehicles as the main target is: when a power shortage occurs and the frequency drops rapidly, the control center Estimate the power shortage with a constant, formulate a load shedding plan based on the schedulable capacity of electric vehicles, and issue load shedding instructions; the management center dispatches load shedding tasks according to the load shedding instructions; electric vehicles are charged/discharged according to the assigned load shedding tasks, and online low-frequency load shedding The auxiliary action of the relay completes the low frequency load shedding.

In conjunction with accompanying drawing 1 and accompanying drawing 2 again, the inventive method is as follows:

According to the needs of electric vehicle users and their network access characteristics, an electric vehicle cluster classification model is established, and a control framework for electric vehicles participating in low-frequency load shedding based on decentralized control and centralized management is established, and then the following steps are performed:

Step 1: Monitor the status of the power grid to determine whether the frequency is normal. If the frequency is within the normal range, do not start low-frequency load shedding; if the frequency is lower than the threshold value, start low-frequency load shedding, and perform step 2;

Step 2: Measure the grid frequency, and estimate the active power deficit of the power system according to the generator terminal frequency and frequency change rate;

Step 3: The electric vehicle management center calculates the dispatch capacity and total dispatch capacity of electric vehicles at all levels according to the initial state of charge, expected state of charge, current charging/discharging power, and maximum charging/discharging power of electric vehicles;

Step 4: The control center formulates a low-frequency load shedding plan according to the power shortage and the dispatching capacity of electric vehicles, and issues a load shedding command;

Step 5: The electric vehicle management center dispatches a load shedding task to each electric vehicle according to the load shedding instruction, and the electric vehicle is charged/discharged according to the assigned task, and the online low-frequency load shedding relay assists the action to complete the low-frequency load shedding;

Step 6: Perform step 1 again to monitor the grid status and determine whether the grid frequency is normal.

In the above embodiments, the method for estimating the system power deficit is as follows:

In the formula:

In the above formula, ΔP is the total power deficit in the system; f _c is the center frequency of inertia; H _i-sys is the inertia time constant of the i-th generator; S _i is the capacity of the i-th generator; S _b-sys is System rated capacity.

The calculation method of step dispatch capacity and total dispatch capacity of electric vehicles is as follows:

Following the principle of minimizing the impact on users, the type of electric vehicles and the priority of investment are determined according to the network access requirements, and the dispatching capacity of electric vehicles is divided into four levels. The first level is the maximum power discharge of the discharge type electric vehicle, the second level is the charging type electric vehicle stop charging, the third level is the maximum power discharge of the maintenance type electric vehicle, and the fourth level is the maximum power discharge of the charging type electric vehicle; The priority of the first level is the highest, and the priority of other levels is in order. The following is the calculation method of the scheduling capacity of the four levels and the total scheduling capacity:

P _EV,4 = n*P _max

P _EV ＝P _EV,1 +P _EV,2 +P _EV,3 +P _EV,4

In the above formula, P _max is the maximum discharge power of an electric vehicle, P _EV,1 is the dispatchable capacity of the first level; z is the number of electric vehicles in the discharge category; P _d,i is the current discharge power of the i-th electric vehicle in the discharge category; P _EV,2 is the second-level schedulable capacity; P _c,i is the current charging power of the i-th electric vehicle in the charging category; n is the number of charging electric vehicles; P _EV,3 is the third-level schedulable capacity; P _{m ,i} is the current charging/discharging power of the i-th electric vehicle in the maintenance category; k is the number of electric vehicles in the maintenance category; P _EV,4 is the dispatchable capacity of the fourth level; P _EV is the total dispatch capacity of electric vehicles.

Principles for formulating the low frequency load shedding plan:

In order to prevent over-cutting, the low-frequency load shedding cut-off amount is calculated as follows:

P _shed =1.05*(ΔP-P _thr )

In the formula: P _shed is the load that needs to be removed; P _thr is the minimum value of the system's allowable power shortage;

In order to make full use of electric vehicle resources, the load shedding tasks are assigned to electric vehicles as much as possible when formulating the load shedding plan. When P _shed ≤ P _EV , all load shedding tasks are assigned to electric vehicles. When P _shed > P _EV , electric vehicles All of them are put in, and the remaining parts are cut off with the assistance of low-frequency load shedding device; the load shedding method adopts wheel-by-wheel input, and the basic wheel is set to 5 rounds, the delay time is set to 0.35 seconds, the first and second rounds are set to 0.25P _shed , and the third and fourth rounds are set to 0.25P shed. The cut off amount is 0.2P _shed , the cut off amount of the fifth round is 0.1P _shed , the basic round will give priority to calling electric vehicle resources according to the round, if the electric vehicle resources are insufficient, then start low-frequency load shedding, set up 3 special rounds, and the cut off amount of each round Both are 0.1P _shed , the delay time is 15 seconds, and the step difference is 5 seconds. The special wheel will give priority to electric vehicle resources according to the situation after the basic wheel is put into use. If it is insufficient, then start the low-frequency load shedding.

The dispatch rules for electric vehicle load shedding tasks are as follows:

The management center determines the load reduction task of electric vehicles and the removal amount of each round according to the load reduction instruction. If the dispatching capacity is less than the cutting amount of this round, all the dispatchable capacity of this level will be put into use, and the dispatchable capacity of the next level will be called; Loading task, load reduction task dispatch method is as follows:

In the above formula, is the maximum discharge power allowed by the i-th electric vehicle in level I; P _i is the current charging/discharging power of the i-th electric vehicle; P _i ^I is the schedulable capacity of the i-th electric vehicle in level I ; P _shed,I is the total amount of load shedding tasks dispatched to level I; P _s,i is the load shedding task of the i-th electric vehicle in level I.

To sum up, the present invention proposes a low-frequency load shedding method for electric vehicles based on the characteristics of decentralized network access of electric vehicles, and establishes a control framework for electric vehicles to participate in low-frequency load shedding; the framework includes electric vehicle batteries, communication modules, V2G control , management center, control center and online low frequency load shedding relay. In the event of a power shortage or a rapid drop in frequency, the control center estimates the power shortage based on the generator terminal frequency, frequency change rate, and generator inertial time constant, and formulates a load reduction plan based on the schedulable capacity of the electric vehicle, and issues a load reduction command; The management center dispatches the load shedding task according to the load shedding instruction; the electric vehicle controls the auxiliary action of the charging/discharging online low-frequency load shedding relay according to the assigned load shedding task, and completes the low-frequency load shedding. In the process of low-frequency load shedding, the present invention prioritizes scheduling of electric vehicle resources, and calls the online low-frequency load shedding relay auxiliary action to complete the low-frequency load shedding, which can reduce the amount of load shedding while quickly suppressing the frequency drop, optimize resource allocation, and improve system efficiency. stability and economy.

It should be pointed out that the above-mentioned preferred embodiments are only to illustrate the technical conception and characteristics of the present invention, the purpose of which is to enable those familiar with this technology to understand the content of the present invention and implement it accordingly, and cannot limit the scope of the present invention. protected range. All equivalent changes or modifications made according to the spirit of the present invention shall fall within the protection scope of the present invention.

Claims (7)

1. a low-frequency load-shedding method of power grid, is characterized in that, described method is oriented to active smart grid, and is object with electric vehicle, and it comprises the steps: 1) Measure the grid frequency, and calculate the active power deficit of the power system according to the generator terminal frequency and frequency change rate; 2) According to the type of electric vehicles and the order of investment priority, the dispatching capacity of electric vehicles is divided into several levels, and then according to the initial state of charge, expected state of charge, current charging/discharging power and maximum charging/discharging power of electric vehicles, calculate Dispatching capacity and total dispatching capacity of electric vehicles at all levels; 3) Based on the active power deficit in step 1) and the dispatching capacity of electric vehicles at all levels and the total dispatching capacity in step 2), a low-frequency load shedding plan is formulated and a load shedding command is issued; 4) The electric vehicle management center dispatches a load shedding task to each electric vehicle according to the load shedding instruction, and the electric vehicle is charged/discharged according to the assigned task, and the online low-frequency load shedding relay assists the action to complete the low-frequency load shedding. 2. The low-frequency load shedding method of power grid according to claim 1, characterized in that, the expression of estimating the system power deficit according to generator terminal frequency and frequency change rate in said step 1) is shown as: <mrow><mi>&amp;Delta;</mi><mi>P</mi><mo>=</mo><munderover><mo>&amp;Sigma;</mo><mrow><mi>i</mi><mo>=</mo><mn>1</mn></mrow><mi>N</mi></munderover><msub><mi>&amp;Delta;P</mi><mi>i</mi></msub><mo>=</mo><mfrac><mn>2</mn><msub><mi>f</mi><mi>n</mi></msub></mfrac><munderover><mo>&amp;Sigma;</mo><mrow><mi>i</mi><mo>=</mo><mn>1</mn></mrow><mi>N</mi></munderover><msub><mi>H</mi><mi>i</mi></msub><mo>&amp;times;</mo><mfrac><mrow><msub><mi>df</mi><mi>c</mi></msub></mrow><mrow><mi>d</mi><mi>t</mi></mrow></mfrac><mo>,</mo><mrow><mo>(</mo><mi>i</mi><mo>=</mo><mn>1...</mn><mi>N</mi><mo>)</mo></mrow></mrow> In the formula: <mfenced open = "{" close = ""><mtable><mtr><mtd><mrow><msub><mi>f</mi><mi>c</mi></msub><mo>=</mo><munderover><mo>&amp;Sigma;</mo><mrow><mi>i</mi><mo>=</mo><mn>1</mn></mrow><mi>N</mi></munderover><msub><mi>H</mi><mrow><mi>i</mi><mo>-</mo><mi>s</mi><mi>y</mi><mi>s</mi></mrow></msub><msub><mi>f</mi><mi>i</mi></msub><mo>//</mo><munderover><mo>&amp;Sigma;</mo><mrow><mi>i</mi><mo>=</mo><mn>1</mn></mrow><mi>N</mi></munderover><msub><mi>H</mi><mrow><mi>i</mi><mo>-</mo><mi>s</mi><mi>y</mi><mi>s</mi></mrow></msub></mrow></mtd></mtr><mtr><mtd><mrow><msub><mi>H</mi><mrow><mi>i</mi><mo>-</mo><mi>s</mi><mi>y</mi><mi>s</mi></mrow></msub><mo>=</mo><msub><mi>H</mi><mi>i</mi></msub><mo>&amp;times;</mo><mfrac><msub><mi>S</mi><mi>i</mi></msub><msub><mi>S</mi><mrow><mi>b</mi><mo>-</mo><mi>s</mi><mi>y</mi><mi>s</mi></mrow></msub></mfrac></mrow></mtd></mtr></mtable></mfenced> In the above formula, ΔP is the total power deficit in the system; f _c is the center frequency of inertia; H _i-sys is the inertia time constant of the i-th generator; S _i is the capacity of the i-th generator; S _b-sys is System rated capacity. 3. The low-frequency load-shedding method of the power grid according to claim 2, wherein the step 2) determines the type and input priority of electric vehicles according to network access requirements, and divides the dispatching capacity of electric vehicles into several levels including: The dispatching capacity of electric vehicles is divided into four levels, the first level is discharging electric vehicles with maximum power, the second level is charging electric vehicles stop charging, the third level is maintaining electric vehicles discharging at maximum power, The first stage is for charging electric vehicles to discharge at maximum power. 4. the low-frequency load-shedding method of power grid according to claim 3, is characterized in that, the expression of the electric vehicle dispatching capacity of all levels and total dispatching capacity of described step 2) is shown as: <mrow><msub><mi>P</mi><mrow><mi>E</mi><mi>V</mi><mo>,</mo><mn>1</mn></mrow></msub><mo>=</mo><munderover><mo>&amp;Sigma;</mo><mrow><mi>i</mi><mo>=</mo><mn>1</mn></mrow><mi>z</mi></munderover><mrow><mo>(</mo><msub><mi>P</mi><mrow><mi>m</mi><mi>a</mi><mi>x</mi></mrow></msub><mo>-</mo><msub><mi>P</mi><mrow><mi>d</mi><mo>,</mo><mi>i</mi></mrow></msub><mo>)</mo></mrow><mo>;</mo></mrow> <mrow><msub><mi>P</mi><mrow><mi>E</mi><mi>V</mi><mo>,</mo><mn>2</mn></mrow></msub><mo>=</mo><munderover><mo>&amp;Sigma;</mo><mrow><mi>i</mi><mo>=</mo><mn>1</mn></mrow><mi>n</mi></munderover><mrow><mo>(</mo><mo>-</mo><msub><mi>P</mi><mrow><mi>c</mi><mo>,</mo><mi>i</mi></mrow></msub><mo>)</mo></mrow><mo>;</mo></mrow> <mrow><msub><mi>P</mi><mrow><mi>E</mi><mi>V</mi><mo>,</mo><mn>3</mn></mrow></msub><mo>=</mo><munderover><mo>&amp;Sigma;</mo><mrow><mi>i</mi><mo>=</mo><mn>1</mn></mrow><mi>k</mi></munderover><mrow><mo>(</mo><msub><mi>P</mi><mrow><mi>m</mi><mi>a</mi><mi>x</mi></mrow></msub><mo>-</mo><msub><mi>P</mi><mrow><mi>m</mi><mo>,</mo><mi>i</mi></mrow></msub><mo>)</mo></mrow><mo>;</mo></mrow> P _EV,4 = n*P _max ; P _EV ＝P _EV,1 +P _EV,2 +P _EV,3 +P _EV,4 ; In the above formula, P _max is the maximum discharge power of an electric vehicle, P _EV,1 is the dispatchable capacity of the first level; z is the number of electric vehicles in the discharge category; P _d,i is the current discharge power of the i-th electric vehicle in the discharge category; P _EV,2 is the second-level schedulable capacity; P _c,i is the current charging power of the i-th electric vehicle in the charging category; n is the number of charging electric vehicles; P _EV,3 is the third-level schedulable capacity; P _{m ,i} is the current charging/discharging power of the i-th electric vehicle in the maintenance category; k is the number of electric vehicles in the maintenance category; P _EV,4 is the dispatchable capacity of the fourth level; P _EV is the total dispatch capacity of electric vehicles. 5. the low-frequency load-shedding method of power grid according to claim 4, is characterized in that, the expression of the electric vehicle low-frequency load-shedding removal amount of described step 3) is shown as: P _shed =1.05*(ΔP-P _thr ) In the above formula, P _shed is the load that needs to be removed; P _thr is the minimum value of the allowable power deficit of the system. 6. The low-frequency load-shedding method of power grid according to claim 5, characterized in that, the step 4) dispatching load-shedding tasks to each electric vehicle comprises: When P _shed ≤ P _EV , all load shedding tasks are assigned to electric vehicles; When P _shed > P _EV , all electric vehicles are put into operation, and the remaining part is cut off with the assistance of low-frequency load shedding device; If the current schedulable capacity of this level is less than the removal amount of this round, all the schedulable capacity of this level will be put into use, and the schedulable capacity of the next level will be called; If the current schedulable capacity of this level is greater than the switching amount of this round, the load shedding task will be dispatched according to the dispatching capacity of the electric vehicle. The dispatching method of the load shedding task is as follows: <mfenced open = "{" close = ""><mtable><mtr><mtd><mrow><msubsup><mi>P</mi><mi>i</mi><mi>I</mi></msubsup><mo>=</mo><msubsup><mi>P</mi><mrow><mi>m</mi><mi>a</mi><mi>x</mi><mo>,</mo><mi>i</mi></mrow><mi>I</mi></msubsup><mo>-</mo><msub><mi>P</mi><mi>i</mi></msub></mrow></mtd></mtr><mtr><mtd><mrow><msub><mi>P</mi><mrow><mi>s</mi><mo>,</mo><mi>i</mi></mrow></msub><mo>=</mo><msub><mi>P</mi><mrow><mi>s</mi><mi>h</mi><mi>e</mi><mi>d</mi><mo>,</mo><mi>I</mi></mrow></msub><mo>*</mo><mfrac><msubsup><mi>P</mi><mi>i</mi><mi>I</mi></msubsup><msub><mi>P</mi><mrow><mi>E</mi><mi>V</mi><mo>,</mo><mi>I</mi></mrow></msub></mfrac></mrow></mtd></mtr></mtable></mfenced> In the above formula, is the maximum discharge power allowed by the i-th electric vehicle in level I; P _i is the current charging/discharging power of the i-th electric vehicle; P _i ^I is the schedulable capacity of the i-th electric vehicle in level I ; P _shed,I is the total amount of load shedding tasks dispatched to level I; P _s,i is the load shedding task of the i-th electric vehicle in level I. 7. The under-frequency load shedding control system according to any one of claims 1 to 6, wherein the system includes a power grid control center, an electric vehicle management center, an under-frequency load shedding relay, and an interface circuit, Each interface circuit includes a communication module and a V2G control module, Among them, the communication module communicates bidirectionally with the electric vehicle management center and the V2G control module respectively, and is used to collect the current charging/discharging power, maximum charging/discharging power, current SOC, expected SOC, allowed maximum and minimum SOC of the electric vehicle, and determine The type and dispatchable capacity of the current electric vehicle, upload this information to the management center, and accept the tasks dispatched by the electric vehicle management center; The V2G control module is used to control the charging and discharging of electric vehicles according to the charging and discharging instructions; The electric vehicle management center also communicates with the control center two-way, which is used to calculate and upload the dispatching capacity at all levels and the total dispatching capacity, and dispatch load shedding tasks.