Active frequency control method for alternating-current and direct-current hybrid power system

Abstract

The invention discloses an active frequency control method for an AC-DC hybrid power system, and the method is realized based on a feedforward active control idea and a system state space model, and comprises the steps: firstly, designing an AC-DC hybrid power grid active coordination rapid frequency response hierarchical control framework; secondly, establishing a system frequency response state space model based on a grid structure of the AC-DC hybrid power system, and forming a two-stage frequency response control strategy taking the arrival time of the lowest point of the frequency as a demarcation point; and thirdly, through active coordination control of whole network resources of the hybrid system, the system frequency modulation potential under two interconnection modes of synchronization and asynchronization is fully mined. According to the invention, the system frequency control effect under the condition of high-power loss can be obviously improved, and the anti-interference capability of a power system is improved.

Description

Technical Field

[0001] The present invention relates to the field of frequency stability control of power systems, and in particular to an active frequency control method for an AC / DC hybrid power system. Background Art

[0002] As the frequency response capability of power systems with high penetration of clean energy continues to weaken, the control pressure for frequency stability has increased dramatically. After the system suffers a power disturbance, the frequency response of the unit in the fault area is activated first, and the areas with different electrical distances from the fault location respond in turn according to the local frequency difference changes. The traditional passive frequency response, namely Passive Frequency Response, PFR, has the frequency spatiotemporal distribution characteristics caused by the asynchronous status of frequency response, which makes the entire network have the greatest potential for improvement in the emergency control stage after the accident. Only by transforming the original passive control, improving the asynchronous status of wide-area power grid frequency response, and changing from passive to active to fully utilize the frequency spatiotemporal distribution characteristics of the emergency control stage, can the resource frequency regulation potential be fully tapped.

[0003] In current research, most of the systems interconnected by DC are equivalent to generators with fixed regulation capabilities, or the opposite system is equivalent to a system that operates according to the frequency control law of the AC system. In essence, they are all based on the feedback action of the frequency difference signal. Therefore, similar to the AC synchronous interconnected system, after the loss of large power, the frequency response capability of the opposite DC system is not fully utilized, which is not conducive to the mutual assistance advantage of the frequency resources of the interconnected power grid. At present, the frequency regulation resources of power generation types mainly include: thermal power units, namely Thermal Power Unit, TPU, hydropower units, namely Hydropower Unit, HPU, gas turbines, namely Gas Turbine Unit, GTU, energy storage devices, namely Energy Storage Devices, ESD. Existing studies have designed active frequency response control based on the feedforward idea, namely Active Frequency Response, AFR, based on model predictive control, namely Model Predictive Control, MPC. Based on active frequency response control, an active frequency response controller is added in the back-to-back DC transmission link, which realizes the synchronous response of the back-to-back converter station and the disturbance occurrence area based on event triggering. The application of active control ideas in DC interconnection systems has shown initial results, also known as back-to-back DC control mode, namely Back-to-backDC-AFR, BD-AFR. However, due to the spatial distribution of frequency, the frequency difference fluctuations between the converter station and the units in each area are not synchronized, resulting in a delayed response of the unit action based on feedback control. Therefore, applying active control measures only in the converter station link cannot completely solve the problem of asynchronous response of the units in each area of ​​the asynchronous interconnected power grid, and cannot fully utilize the frequency regulation potential of the units in the asynchronous interconnected area. Summary of the invention

[0004] Aiming at the deficiencies of the conventional frequency stability control method of AC / DC hybrid power system, the present invention proposes a frequency stability control framework and control method of AC / DC hybrid power system based on active control concept in the field of power system frequency stability control.

[0005] In order to achieve the above object, the present invention provides an active frequency control method for an AC / DC hybrid power system, which comprises:

[0006] S1: Establish a hierarchical control framework for fast frequency response in active coordination of AC / DC hybrid power grids;

[0007] S101: The upper layer of the control framework is a control decision layer, in which the decision controller performs global coordination of control targets, that is, when a certain level of power loss occurs in the AC / DC hybrid power grid, it is determined that the power loss caused by the fault is greater than P1, and active control is applied in the AC synchronous interconnected power grid; if the power loss caused by the fault is greater than P2, active control is applied in the AC / DC hybrid power grid; wherein P2 indicates that the disturbance zone can withstand a power loss of level P2 with the active support of the AC synchronous interconnected power grid; P1 indicates that based on the traditional passive control method, only a power loss of level P1 can be intercepted without triggering the low-frequency load reduction threshold set by the system;

[0008] S102: The lower layer of the control framework is a partition control layer. After a large disturbance occurs, according to the upper layer instruction, if the disturbance level is lower than P1, passive control based on the local frequency difference is performed on the primary frequency regulation side of the primary equipment in the AC synchronous interconnected power grid; if the disturbance level is lower than P2 and greater than P1, resource coordination of active control of the primary frequency regulation side of the primary equipment is performed in the AC synchronous interconnected power grid; if the disturbance level is higher than P2, active control is applied simultaneously on the DC modulation side and the primary frequency regulation side of the primary equipment in each area of ​​the asynchronous interconnected power grid in the AC / DC hybrid power grid;

[0009] S2: Establish the frequency response state space model of AC / DC hybrid power grid and the active coordinated control strategy;

[0010] S201: Set the AC power grids of region i and region j to achieve asynchronous interconnection through back-to-back DC modulation. Region i can be divided into synchronously interconnected region iA and region iB. The unit type and frequency regulation model of region j that is asynchronously interconnected with region i are similar to those of region i. Each region includes four types of frequency regulation resources, namely ESD, GTU, TPU, and HPU. The frequency regulation resources jointly participate in the active control of the primary frequency regulation side. TPU and HPU with slightly lower output speed in each region are set to provide secondary frequency regulation function and use conventional PID controller.

[0011] S202: Install an active controller MPC on the primary frequency regulation side of the frequency regulation resource in area iA iA , apply active control signal u to ESD, GTU, TPU, HPU 1,iA 、u 2,iA 、u 3,iA 、u 4,iA ; Install an active controller MPC on the primary frequency modulation side of the regional iB frequency modulation resource iB , apply active control signal u to ESD, GTU, TPU, HPU 1,iB 、u 2,iB 、u 3,iB 、u 4,iB ; Install an active controller MPC on the primary frequency regulation side of the frequency regulation resource in area j j , apply active control signal u to ESD, GTU, TPU, HPU 1,j 、u 2,j 、u 3,j 、u 4,j ; Install active controller MPC on the DC modulation side i ;

[0012] S203: Establish a frequency response state space model of the AC / DC hybrid power grid under the MPC algorithm, which is:

[0013]

[0014] In the formula, A i is the state matrix corresponding to each variable, B i is the control matrix corresponding to each variable, C i is the observation matrix corresponding to each variable, F i is the disturbance matrix corresponding to each variable; i (t) is the control target C i The output vector of x is different, and different control stages take different coefficients. i (t) is the state vector, w i (t) is the disturbance vector, is the time derivative of the state vector, To control the time derivative of the vector;

[0015] S204: According to the fast frequency response hierarchical control framework, an active coordination control strategy is designed, which is specifically: according to different levels of power loss, the whole network active control is triggered in the frequency drop stage. When the lowest frequency point is effectively contained and the wide area measurement system detects that the system frequency has rebounded, the first stage of active control ends; in the second stage, the traditional passive control frequency adjustment method is switched to restore the frequency of each area to the rated value through the differenceless adjustment of the secondary frequency modulation;

[0016] S3: When active control is performed in the first stage, an optimal quadratic performance index function is constructed according to the optimal control law of the MPC algorithm and an optimization result is obtained. The optimal quadratic performance index function is:

[0017]

[0018] In the formula, at the control time t = k, r s is the system control target matrix; Q is the system error weighting matrix; R is the system control weighting matrix, N p is the prediction time domain, N c is the control time domain, and Δu is the change in the system control sequence.

[0019] In the preferred mode, the power support between regions iA and iB of the AC synchronous interconnected power grid satisfies the following formula:

[0020]

[0021] In the formula, is the differential of the power change of the AC interconnection line, T i is the transmission coefficient of the AC tie line, Δf iA is the system frequency change in area iA, Δf iB is the system frequency change in area iB;

[0022] The power support between area i and area j of the asynchronous interconnected power grid satisfies the following formula:

[0023]

[0024] In the formula, is the differential of the power change of the DC interconnection line, T SD,i is the transmission coefficient of the DC tie line, u i is the active control signal of region i, u j is the active control signal of region j, ΔP dc,j is the power change of the DC interconnection line, K i is the DC controller gain of region i.

[0025] In the preferred mode, the state vector x of region iA is iA (t) = [Δf iA ΔP SED,iA ΔX gE,iA ΔX gV,iA ΔP gR,iA ΔP g,iA ΔX t,iA ΔP tr,iA ΔP t,iA ΔX w,iA ΔP wt,iAΔP w,iA ΔP a,iA ΔP tie,AB ΔP dc,iA Δf iB ] T , where Δf iA is the system frequency change in area iA, ΔP SED,iA is the ESD output power change in area iA, ΔX gE,iA is the state change of the GTU valve positioner in area iA, ΔX gV,iA is the intermediate state change of the GTU in area iA, ΔP gR,iA is the intermediate state change of the GTU combustion chamber in area iA, ΔP g,iA is the GTU output power change in area iA, ΔX t,iA is the output change of the TPU speed regulator in area iA, ΔP tr,iA is the intermediate state change of the GTU reheat turbine in area iA, ΔP t,iA is the GTU output power change in area iA, ΔX w,iA is the output change of the HPU speed regulator in area iA, ΔP wt,iA is the output change of the HPU mechanical hydraulic governor in area iA, ΔP w,iA is the change in HPU output power in area iA, ΔP a,iA is the output change of the automatic generation controller in area iA, ΔP tie,AB is the transmission power change of the AC tie line in area iA, ΔP dc,iA is the change in the DC interconnection line transmission power in area iA, Δf iB is the system frequency change in area iB.

[0026] In the preferred mode, the control vector u of region iA is iA (t)=[u 1,iA u 2,iA u 3,iA u 4,iA u db,i ] T , the perturbation vector matrix w iA (t) = [ΔP L Δf j u db,j ] T ; Among them, u 1,iA 、u 2,iA 、u 3,iA 、u 4,iA are the active control signals applied by the active controller of area iA to ESD, GTU, TPU, and HPU; u db,iis the active control signal applied by the active controller of region i to the DC modulation; u db,i is the active control signal applied by the active controller of region j to the DC modulation; ΔP L is the power disturbance increment, Δf j is the system frequency change in region j.

[0027] The beneficial effects of the present invention are as follows: In view of the fact that active control research is limited to the AC interconnection system and a single DC converter station side, and the frequency spatiotemporal distribution characteristics of the AC / DC hybrid power grid are obvious after being disturbed under PFR, which leads to the problem of low utilization of frequency regulation resources, the present invention, based on the AFR control concept, constructs an AFR control model under extreme accidents, and proposes an active coordinated AC / DC hybrid system fast frequency response control method; through the system state space model and MPC algorithm solution, an AFR control strategy for the coordination of synchronous interconnection and asynchronous interconnection systems is proposed, which can effectively improve the frequency control effect of the wide-area power grid under large power loss. The active frequency control method of the AC / DC hybrid power system proposed in the present invention can significantly improve the system frequency control effect under large power loss and improve the anti-disturbance capability of the power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a schematic diagram of the content to be studied for active control of the present invention;

[0029] Figure 2 A hierarchical control framework for active frequency response of the present invention;

[0030] Figure 3 The frequency variation curve of the 1A zone under different control modes of the present invention;

[0031] Figure 4 The frequency variation curve of zone 1B under different control modes of the present invention;

[0032] Figure 5 It is the frequency variation curve of zone 2 under different control modes of the present invention. DETAILED DESCRIPTION Specific embodiment:

[0034] To this end, in view of the problem that existing studies have failed to fully utilize the frequency regulation potential of the entire unit of the wide-area power grid, this paper designs a hierarchical control framework for active coordination of AC / DC hybrid power grids based on the AFR control concept. Relying on the idea of ​​active coordinated control of the entire network, active control measures are applied simultaneously on the primary side of the AC system equipment and the DC power modulation side to tap the system frequency regulation potential under both synchronous and asynchronous interconnection modes and improve the control effect.

[0035] Specifically, an active frequency control method for an AC / DC hybrid power system is proposed, which includes:

[0036] S1: Figure 1 , 2 As shown, a hierarchical control framework for fast frequency response of active coordinated AC / DC hybrid power grid is established;

[0037] S101: The upper layer of the control framework is the control decision layer, in which the decision controller performs global coordination of the control target, that is, when a certain level of power loss occurs in the AC / DC hybrid power grid, it is judged that the power loss caused by the fault is greater than P1, and active control is applied in the AC synchronous interconnected power grid; if the power loss caused by the fault is greater than P2, the active control mutual assistance means of the synchronous power grid cannot prevent the system from low-frequency load shedding, then it is necessary to expand the active support means from the synchronous interconnection area to the entire AC / DC hybrid power system, and apply active control in the AC / DC hybrid power grid; wherein, P2 means that the disturbance area can withstand a power loss of level P2 under the active support of the AC synchronous interconnected power grid; P1 means that based on the traditional passive control method, only a power loss of level P1 can be intercepted without triggering the low-frequency load shedding threshold set by the system; in addition, the decision layer needs to switch the control target and control method according to whether the frequency is in the rising stage or the falling stage;

[0038] S102: The lower layer of the control framework is a partition control layer, which is divided into a DC synchronous interconnection area and an AC asynchronous interconnection area according to the connection mode. After a large disturbance occurs, according to the upper layer instruction, if the disturbance level is lower than P1, the primary frequency modulation side of the primary equipment in the AC synchronous interconnection power grid is passively controlled based on the local frequency difference; if the disturbance level is lower than P2 and greater than P1, the primary frequency modulation side of the primary equipment in the AC synchronous interconnection power grid is actively controlled. Resource coordination; if the disturbance level is higher than P2, it is necessary to expand the scope of rapid frequency response control resource coordination from the local area to the entire network, and then in the AC / DC hybrid power grid, active control is applied simultaneously on the DC modulation side and the primary frequency modulation side of the primary equipment in each area of ​​the asynchronous interconnection power grid, so as to achieve active coordinated control of the entire network resources of the synchronous and asynchronous interconnection system, and send instructions according to different control stages of the decision-making layer to complete the feedforward AFR based on model predictive control or the PFR based on feedback of the local frequency difference;

[0039] S2: Establish the frequency response state space model of AC / DC hybrid power grid and the active coordinated control strategy;

[0040] S201: Set the AC power grids of region i and region j to achieve asynchronous interconnection through back-to-back DC modulation. Region i can be divided into synchronously interconnected region iA and region iB. The unit type and frequency regulation model of region j that is asynchronously interconnected with region i are similar to those of region i. Each region includes four types of frequency regulation resources, namely ESD, GTU, TPU and HPU. The frequency regulation resources jointly participate in the active control of the primary frequency regulation side. To ensure that the proposed active control strategy has good applicability, can be connected with the secondary frequency regulation and help the system frequency recover to the rated value, the TPU and HPU with slightly lower output speed in each region are set to provide the secondary frequency regulation function and use the conventional PID controller.

[0041] S202: To ensure the optimal active control performance of the entire network in extreme power disturbances such as large power loss, an active controller MPC is installed on the primary frequency regulation side of the regional iA frequency regulation resource. iA , apply active control signal u to ESD, GTU, TPU, HPU 1,iA 、u 2,iA 、u 3,iA 、u 4,iA ; Install an active controller MPC on the primary frequency modulation side of the regional iB frequency modulation resource iB , apply active control signal u to ESD, GTU, TPU, HPU 1,iB 、u 2,iB 、u 3,iB 、u 4,iB ; Install an active controller MPC on the primary frequency regulation side of the frequency regulation resource in area j j , apply active control signal u to ESD, GTU, TPU, HPU 1,j 、u 2,j 、u 3,j 、u 4,j ; Install active controller MPC on the DC modulation side i ; Through the active controller MPC iA 、MPC iB 、MPC i 、MPC j and the coordination of the upper decision controller to achieve active coordinated control of the AC / DC hybrid system. iA 、MPC iB 、MPC j , all are newly added controllers;

[0042] S203: Establish a frequency response state space model of the AC / DC hybrid power grid under the MPC algorithm, which is:

[0043]

[0044] In the formula, Ai is the state matrix corresponding to each variable, B i is the control matrix corresponding to each variable, C i is the observation matrix corresponding to each variable, F i is the disturbance matrix corresponding to each variable; i (t) is the control target C i The output vector of x is different, and different control stages take different coefficients. i (t) is the state vector, w i (t) is the disturbance vector, is the time derivative of the state vector, To control the time derivative of the vector;

[0045] Taking region iA as an example, the state vector x of region iA is iA (t) = [Δf iA ΔP SED,iA ΔX gE,iA ΔX gV,iA ΔP gR,iA ΔP g,iA ΔX t,iA ΔP tr,iA ΔP t,iA ΔX w,iA ΔP wt,iA ΔP w,iA ΔP a,iA ΔP tie,AB ΔP dc,iA Δf iB ] T , where Δf iA is the system frequency change in area iA, ΔP SED,iA is the ESD output power change in area iA, ΔX gE,iA is the state change of the GTU valve positioner in area iA, ΔX gV,iA is the intermediate state change of the GTU in area iA, ΔP gR,iA is the intermediate state change of the GTU combustion chamber in area iA, ΔP g,iA is the GTU output power change in area iA, ΔX t,iA is the output change of the TPU speed regulator in area iA, ΔP tr,iA is the intermediate state change of the GTU reheat turbine in area iA, ΔP t,iA is the GTU output power change in area iA, ΔX w,iA is the output change of the HPU speed regulator in area iA, ΔP wt,iA is the output change of the HPU mechanical hydraulic governor in area iA, ΔP w,iA is the change in HPU output power in area iA, ΔP a,iAis the output change of the automatic generation controller in area iA, ΔP tie,AB is the transmission power change of the AC tie line in area iA, ΔP dc,iA is the change in the DC interconnection line transmission power in area iA, Δf iB is the system frequency change in area iB;

[0046] Control vector u of region iA iA (t)=[u 1,iA u 2,iA u 3,iA u 4,iA u db,i ] T , the perturbation vector matrix w iA (t) = [ΔP L Δf j u db,j ] T ; Among them, u 1,iA 、u 2,iA 、u 3,iA 、u 4,iA are the active control signals applied by the active controller of area iA to ESD, GTU, TPU, and HPU; u db,i is the active control signal applied by the active controller of region i to the DC modulation; u db,i is the active control signal applied by the active controller of region j to the DC modulation; ΔP L is the power disturbance increment, Δf j is the system frequency change in region j;

[0047] S204: According to the fast frequency response hierarchical control framework, an active coordination control strategy is designed, which is specifically: according to different levels of power loss, after the system suffers from severe power impact, the whole network active control is triggered in the frequency reduction stage, when the lowest frequency point is effectively contained and the wide area measurement system, i.e., Wide Area Measurement System (WAMS), detects that the system frequency has rebounded, the first stage of active control ends; in the second stage, since the lowest frequency value has been effectively intercepted under the first stage active frequency response control strategy, the active control as a control method for deeply tapping the potential of frequency modulation resources has a high control cost, so after the WAMS sampling system detects the lowest frequency point in the disturbance area, it should switch to the traditional passive control frequency adjustment method, and restore the frequency of each area to the rated value through the differenceless adjustment of the secondary frequency modulation;

[0048] S3: In the first stage of active control, the optimal quadratic performance index function is constructed according to the optimal control law of the MPC algorithm and the optimization result is obtained, that is, the deviation between the control matrix output value and the control target plus the optimal control target function of the control signal, which can be organized as:

[0049]

[0050] In the formula, at the control time t = k, r s is the system control target matrix; Q is the system error weighting matrix; R is the system control weighting matrix, N p is the prediction time domain, N c is the control time domain, and Δu is the change in the system control sequence.

[0051] Active coordination and control of the resources of the entire network, tapping the system frequency regulation potential under both synchronous and asynchronous interconnection modes. Among them, the power support between regions iA and iB of the AC synchronous interconnected power grid satisfies the following formula:

[0052]

[0053] In the formula, is the differential of the power change of the AC interconnection line, T i is the transmission coefficient of the AC tie line, Δf iA is the system frequency change in area iA, Δf iB is the system frequency change in area iB;

[0054] Among them, the power support between area i and area j of the asynchronous interconnected power grid satisfies the following formula:

[0055]

[0056] In the formula, is the differential of the power change of the DC interconnection line, T SD,i is the transmission coefficient of the DC tie line, u i is the active control signal of region i, u j is the active control signal of region j, ΔP dc,j is the power change of the DC interconnection line, K i is the DC controller gain of region i.

[0057] The differential of the frequency deviation vector of each region can be organized as:

[0058]

[0059] In the formula, is the differential of the power change of the DC interconnection line, T SD,i is the transmission coefficient of the DC tie line, ui is the active control signal of region i, u j is the active control signal of region j, ΔP dc,j is the power change of the DC interconnection line, K i is the DC controller gain of region i;

[0060]

[0061] In the formula, is the differential of the system frequency change in area iA, H iA is the inertia of the system in area iA, ΔP dc,A is the power change of the DC tie line in system A, D iA is the damping coefficient of area iA;

[0062]

[0063] In the formula, is the differential of the system frequency change in region iB, H iB is the inertia of the system in region iB, ΔP SED,iB is the ESD output power change in region iB, ΔP g,iB is the GTU output power change in area iB, ΔP t,iB is the GTU output power change in area iB, ΔP w,iB is the change in HPU output power in area iB, D iB is the damping coefficient of region iB.

[0064] The following scenario is set up below. Area iA and area iB transmit electric energy through AC synchronous interconnection, and both belong to area i. Area j is an asynchronous interconnected area interconnected with area i through back-to-back DC. The asynchronously interconnected areas i and j are called areas 1 and 2. The installed capacity ratio of the two areas is 4:3. Area 1A and area 2 are isomorphic. Assume that at t=5s, the system has a power shortage of 0.2pu, triggering active control of the entire network. Due to the large power shortage, the AFR control method of synchronous and asynchronous interconnected systems responding simultaneously is adopted. Combined with this specific embodiment and the accompanying drawings, the technical solution of the present invention is clearly and completely described.

[0065] According to the above embodiment, it can be obtained Figure 3 , 4 ,5. Frequency control results of each zone.

[0066] The disturbance occurrence area is the area of ​​greatest concern after the accident, such as Figure 3As shown in the figure, the lowest frequency value under the AFR control mode in the first stage is 49.73Hz, and the arrival time is 0.6s after the disturbance occurs; the lowest frequency value under the PFR control mode is 49.22Hz, and the arrival time is 1.5s after the disturbance occurs; the lowest frequency value under the BD-AFR control mode is 49.38Hz, and the arrival time is 1.5s after the disturbance occurs. In summary, when AFR control is applied simultaneously in the synchronous and asynchronous interconnected areas, the maximum frequency deviation of the system is 0.27Hz; the maximum frequency deviation of the system under the PFR control mode is 0.78Hz; and the maximum frequency deviation of the system under the BD-AFR control mode is 0.62Hz. In terms of the interception efficiency at the lowest frequency point in the disturbance occurrence area, the proposed AFR control scheme improves the control effect by 65.38% compared with the PFR control effect and by 56.45% compared with the BD-AFR control effect.

[0067] like Figure 4 As shown in the figure, in subsystem 1B, the PFR control mode is spontaneously transmitted by the regional AC interconnection line. In the frequency reduction stage, the system frequency reaches the minimum value of 49.80Hz 4.1s after the disturbance, resulting in a maximum frequency deviation of 0.20Hz in the system. In the BD-AFR control mode, the system frequency reaches the minimum value of 49.87Hz 3.1s after the disturbance in the frequency reduction stage, resulting in a maximum frequency deviation of 0.13Hz in the system. In the AFR control mode, active power support is provided by the speed regulator side of the whole network unit that is synchronously interconnected with the disturbance occurrence area. In the frequency reduction stage, the system frequency reaches the minimum value of 49.91Hz 4.4s after the disturbance, resulting in a maximum frequency deviation of 0.09Hz in the system. In summary, under the AFR control mode, in the AC system that is synchronously interconnected with the disturbance occurrence area, the control effect of the lowest frequency point in the frequency reduction stage is improved by 55% compared with the PFR mode and 30.77% compared with the BD-AFR mode.

[0068] like Figure 5 As shown in the figure, in area 2, which is asynchronously interconnected with the disturbance area, the frequency reaches the lowest point of 49.81Hz 3.7s after the disturbance occurs under the PFR control mode; in the BD-AFR control mode, the back-to-back DC modulation system performs active power transmission in the frequency reduction stage, and the frequency drops to 49.99Hz and 49.96Hz 0.6s and 3.8s after the disturbance occurs, respectively, of which the latter is the lowest frequency point under this control mode; in the AFR control mode, the back-to-back DC modulation system and the primary side of the whole network unit perform synchronous active control of feedforward, and the frequency drops to the lowest value of 49.98Hz 0.7s after the disturbance occurs. In the non-disturbance area of ​​asynchronous interconnection, the maximum frequency deviation control effect of the AFR control mode system is improved by 89.47% compared with PFR and 50% compared with BD-AFR.

[0069] In summary, the system frequency security protection effect under the AFR control mode is significantly improved compared with the PFR and BD-AFR control modes. In a multi-area interconnected system with back-to-back DC modulation, it can play a good frequency interception effect in the frequency reduction stage.

[0070] The present invention discloses an active frequency control method for an AC / DC hybrid power system. After the system is subjected to a power disturbance, the frequency response of the unit in the fault-occurring area is started first, and the areas with different electrical distances from the fault-occurring area respond in turn according to the local frequency difference change. Existing studies have added an active frequency response controller in the back-to-back DC transmission link based on the active control concept, and realized the synchronous response of the back-to-back converter station and the disturbance-occurring area based on event triggering. The application of the active control concept in the DC interconnection system has initially shown results, also known as the back-to-back DC AFR control method. However, due to the spatial distribution of frequency, the frequency difference fluctuations between the converter station and the units in each area are not synchronized, resulting in a delayed response of the unit action based on feedback control. Therefore, only applying active control measures in the converter station link cannot completely solve the problem of asynchronous response of the units in each area in the asynchronous interconnected power grid, and cannot fully utilize the frequency regulation potential of the units in the asynchronous interconnected area. Based on the AFR control concept, the present invention designs a hierarchical control framework for active coordinated fast frequency response of the AC / DC hybrid power grid. Relying on the idea of ​​active coordinated control of the entire network, active control measures are applied simultaneously on the primary side of the AC system equipment and the DC power modulation side to tap the system frequency modulation potential under both synchronous and asynchronous interconnection modes and improve the control effect. This method can effectively improve the system frequency control effect in extreme situations such as large power loss. The specific work is mainly divided into three steps:

[0071] In the first step, based on the AFR control idea, a hierarchical control framework for active coordinated fast frequency response of AC / DC hybrid power grids is designed.

[0072] The second step is to establish a frequency response state space model of the AC / DC hybrid power system and formulate a control strategy based on the idea of ​​active coordinated control of the entire network.

[0073] The third step is to apply active control measures on both the primary side of the AC system equipment and the DC power modulation side to tap the system frequency modulation potential under both synchronous and asynchronous interconnection modes and improve the control effect.

[0074] The results of the frequency control effect of the large power loss system in a certain scenario show that: the original active control based only on back-to-back DC modulation has significantly improved the frequency control effect compared with the passive control method, but there is still room for improvement; the frequency control effect of the AC / DC hybrid power system full network active control scheme proposed in this application is more significant. Therefore, the active frequency control method proposed in this application can more fully tap the frequency regulation potential of the AC / DC hybrid power system, the system frequency control effect is better, and it is more helpful to maintain the system frequency stability under large power loss.

[0075] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. An active frequency control method for an AC / DC hybrid power system, characterized in that: It includes: S1: Establish a hierarchical control framework for fast frequency response in active coordination of AC / DC hybrid power grids; S101: The upper layer of the control framework is a control decision layer, in which the decision controller performs global coordination of control targets, that is, when a certain level of power loss occurs in the AC / DC hybrid power grid, it is determined that the power loss caused by the fault is greater than P1, and active control is applied in the AC synchronous interconnected power grid; if the power loss caused by the fault is greater than P2, active control is applied in the AC / DC hybrid power grid; wherein P2 indicates that the disturbance zone can withstand a power loss of level P2 with the active support of the AC synchronous interconnected power grid; P1 indicates that based on the traditional passive control method, only a power loss of level P1 can be intercepted without triggering the low-frequency load reduction threshold set by the system; S102: The lower layer of the control framework is a partition control layer. After a large disturbance occurs, according to the upper layer instruction, if the disturbance level is lower than P1, passive control based on the local frequency difference is performed on the primary frequency regulation side of the primary equipment in the AC synchronous interconnected power grid; if the disturbance level is lower than P2 and greater than P1, resource coordination of active control of the primary frequency regulation side of the primary equipment is performed in the AC synchronous interconnected power grid; if the disturbance level is higher than P2, active control is applied simultaneously on the DC modulation side and the primary frequency regulation side of the primary equipment in each area of ​​the asynchronous interconnected power grid in the AC / DC hybrid power grid; S2: Establish the frequency response state space model of AC / DC hybrid power grid and the active coordinated control strategy; S201: Set the AC power grids of region i and region j to achieve asynchronous interconnection through back-to-back DC modulation. Region i can be divided into synchronously interconnected region iA and region iB. The unit type and frequency regulation model of region j that is asynchronously interconnected with region i are similar to those of region i. Each region includes four types of frequency regulation resources, namely ESD, GTU, TPU, and HPU. The frequency regulation resources jointly participate in the active control of the primary frequency regulation side. TPU and HPU with slightly lower output speed in each region are set to provide secondary frequency regulation function and use conventional PID controller. S202: Install an active controller MPC on the primary frequency regulation side of the frequency regulation resource in area iA iA , apply active control signal u to ESD, GTU, TPU, HPU 1,iA 、u 2,iA 、u 3,iA 、u 4,iA ; Install an active controller MPC on the primary frequency modulation side of the regional iB frequency modulation resource iB , apply active control signal u to ESD, GTU, TPU, HPU 1,iB 、u 2,iB 、u 3,iB 、u 4,iB ; Install an active controller MPC on the primary frequency regulation side of the frequency regulation resource in area j j , apply active control signal u to ESD, GTU, TPU, HPU 1,j 、u 2,j 、u 3,j 、u 4,j ; Install active controller MPC on the DC modulation side i ; S203: Establish a frequency response state space model of the AC / DC hybrid power grid under the MPC algorithm, which is: In the formula, A i is the state matrix corresponding to each variable, B i is the control matrix corresponding to each variable, C i is the observation matrix corresponding to each variable, F i is the disturbance matrix corresponding to each variable; i (t) is the control target C i The output vector of x is different, and different control stages take different coefficients. i (t) is the state vector, w i (t) is the disturbance vector, is the time derivative of the state vector, To control the time derivative of the vector; S204: According to the fast frequency response hierarchical control framework, an active coordination control strategy is designed, which is specifically: according to different levels of power loss, the whole network active control is triggered in the frequency drop stage. When the lowest frequency point is effectively contained and the wide area measurement system detects that the system frequency has rebounded, the first stage of active control ends; in the second stage, the traditional passive control frequency adjustment method is switched to restore the frequency of each area to the rated value through the differenceless adjustment of the secondary frequency modulation; S3: When active control is performed in the first stage, an optimal quadratic performance index function is constructed according to the optimal control law of the MPC algorithm and an optimization result is obtained. The optimal quadratic performance index function is: In the formula, at the control time t = k, r s is the system control target matrix; Q is the system error weighting matrix; R is the system control weighting matrix, N p is the prediction time domain, N c is the control time domain, and Δu is the change in the system control sequence.

2. The active frequency control method for an AC / DC hybrid power system according to claim 1, characterized in that: The power support between areas iA and iB of the AC synchronous interconnected power grid satisfies the following formula: In the formula, is the differential of the power change of the AC interconnection line, T i is the transmission coefficient of the AC tie line, Δf iA is the system frequency change in area iA, Δf iB is the system frequency change in area iB; The power support between area i and area j of the asynchronous interconnected power grid satisfies the following formula: In the formula, is the differential of the power change of the DC interconnection line, T SD,i is the transmission coefficient of the DC tie line, u i is the active control signal of region i, u j is the active control signal of region j, ΔP dc,j is the power change of the DC interconnection line, K i is the DC controller gain of region i.

3. The active frequency control method for an AC / DC hybrid power system according to claim 1, characterized in that: The state vector x of region iA iA (t) = [Δf iA ΔP SED,iA ΔX gE,iA ΔX gV,iA ΔP gR,iA ΔP g,iA ΔX t,iA ΔP tr,iA ΔP t,iA ΔX w,iA ΔP wt,iA ΔP w,iA ΔP a,iA ΔP tie,AB ΔP dc,iA Δf iB ] T , where Δf iA is the system frequency change in area iA, ΔP SED,iA is the ESD output power change in area iA, ΔX gE,iA is the state change of the GTU valve positioner in area iA, ΔX gV,iA is the intermediate state change of the GTU in area iA, ΔP gR,iA is the intermediate state change of the GTU combustion chamber in area iA, ΔP g,iA is the GTU output power change in area iA, ΔX t,iA is the output change of the TPU speed regulator in area iA, ΔP tr,iA is the intermediate state change of the GTU reheat turbine in area iA, ΔP t,iA is the GTU output power change in area iA, ΔX w,iA is the output change of the HPU speed regulator in area iA, ΔP wt,iA is the output change of the HPU mechanical hydraulic governor in area iA, ΔP w,iA is the change in HPU output power in area iA, ΔP a,iA is the output change of the automatic generation controller in area iA, ΔP tie,AB is the transmission power change of the AC tie line in area iA, ΔP dc,iA is the change in the DC interconnection line transmission power in area iA, Δf iB is the system frequency change in area iB.

4. The active frequency control method for an AC / DC hybrid power system according to claim 1, characterized in that: Control vector u of region iA iA (t)=[u 1,iA u 2,iA u 3,iA u 4,iA u db,i ] T , the perturbation vector matrix w iA (t) = [ΔP L Δf j u db,j ] T ; Among them, u 1,iA 、u 2,iA 、u 3,iA 、u 4,iA are the active control signals applied by the active controller of area iA to ESD, GTU, TPU, and HPU; u db,i is the active control signal applied by the active controller of region i to the DC modulation; u db,i is the active control signal applied by the active controller of region j to the DC modulation; ΔP L is the power disturbance increment, Δf j is the system frequency change in region j.

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