A control method for frequency stabilization of multiple DC asynchronous interconnected power grids

Abstract

The present invention provides a control method for frequency stabilization in multiple asynchronously interconnected DC power grids. This method can quickly restore power fluctuations in a regional power grid after a fault disturbance to the stable operating state prior to the fault. It also boasts more stable coordination and control performance, effectively controlling the frequency stability between AC power grids and addressing frequency stability control issues caused by power imbalance between multiple asynchronously interconnected DC power grids.

Description

Technical Field

[0001] The present invention belongs to the technical field of power grid control and relates to a control method for stabilizing the frequency of multiple DC asynchronous interconnected power grids. Background Art

[0002] my country is rich in energy resources, but the spatial distribution of resource production centers and load consumption centers exhibits an extremely imbalanced and inverse relationship. This means that there are significant regional differences in the distribution of energy resource production and load consumption. Cross-regional, long-distance, and large-capacity HVDC transmission technology is a key research direction for my country to achieve the goal of transmitting electricity from west to east and north to south, ensuring electricity for people's daily lives. The demands of production and daily life have led to the emergence of new ideas, new technologies, and new applications in the theory and practice of HVDC transmission technology. my country is actively exploring multi-regional energy interconnection, coordinated control, and resource sharing within large power grids, leading to the development of asynchronous DC interconnection.

[0003] Based on the type of interconnecting lines between regional power grids, they can be divided into synchronous grids connected by AC channels and asynchronous grids without AC channels and connected solely by DC lines. Regional grids can be interconnected solely through multiple DC lines, achieving multi-DC asynchronous interconnection between regional grids. This approach effectively addresses the complementary support of energy resources between regional grids in complex topologies and effectively balances power fluctuations between multiple DC transmission lines, thereby stabilizing system frequency fluctuations. Asynchronous interconnected grids also enable the operation of multiple regional grids under conditions of varying frequencies and phases, and enable rapid control and recovery. With the increasing popularity of asynchronous grid interconnection projects, their advantages are becoming increasingly prominent and their application prospects are broad. HVDC transmission lines can independently connect to the AC grids at each end, effectively preventing large-scale power outages caused by a series of consecutive faults and mitigating the impact of extreme climate and unique geographical factors.

[0004] Typically, the power on the DC transmission lines of asynchronously interconnected power grids is very high, and a DC blocking fault can have a significant impact on the systems at both ends. The backup capacity and system inertia of the asynchronously interconnected multi-DC grids decrease to varying degrees, reducing the power support capability between the AC grids at both ends of the DC line. This makes it impossible to effectively dispatch and coordinate the power on the DC transmission lines of the asynchronously interconnected grids when faced with system disturbances caused by faults such as load shedding or DC blocking between the grids. This leads to power imbalances between the systems at both ends of their respective grids, causing unstable fluctuations in grid frequency and increasing frequency stability issues. Therefore, more effective control methods are urgently needed to maintain frequency stability in the asynchronously interconnected multi-DC grids. Summary of the Invention

[0005] The present invention addresses the problems existing in the prior art and provides a control method for stabilizing the frequency of a multi-DC asynchronous interconnected power grid, comprising the following steps:

[0006] Set AC grid parameters, rectifier / inverter station parameters, DC transmission parameters, and system control module parameters, and build a multi-DC asynchronous interconnected circuit model;

[0007] Construct a "master-pole-valve" hierarchical coordinated control model suitable for multiple DC asynchronous interconnected power grids;

[0008] Setting a calculation expression for the maximum value of the sum of frequency deviation factors allowed for multiple DC power systems after asynchronous interconnection;

[0009] Establish the optimal objective function to keep the system frequency safe and stable under disturbance conditions;

[0010] The underlying variables of the circuit model are configured to satisfy the objective function under the constraints, thus achieving effective control of the power mutual support between multiple DC systems and stabilizing the system frequency fluctuations.

[0011] On the basis of the above scheme, the multi-DC asynchronous interconnection circuit model is a simulation circuit model pre-built based on PSCAD for simulating real multi-DC.

[0012] Based on the above scheme, the AC grid parameters include equivalent voltage source parameters and equivalent power supply impedance parameters, the rectifier / inverter station parameters include transformer parameters and six-pulse control bridge circuit parameters, the DC transmission parameters include smoothing reactor parameters and DC transmission line equivalent impedance parameters, and the system control module parameters include parameters related to the constant voltage control link, the constant current control link, and the fixed trigger angle control link.

[0013] Based on the above solution, the simulation circuit model pre-built based on PSCAD for simulating real multi-DC specifically includes the following parts:

[0014] Two-side AC grid model for asynchronous interconnection, bipolar parallel rectifier / inverter controller module, multiple DC transmission line module and system control module;

[0015] The two-side AC power grid model for asynchronous interconnection is used to simulate the regional AC power grids on both sides of the real multi-DC system;

[0016] The bipolar parallel rectifier / inverter controller module is used to simulate the rectifier station and inverter station of a real multi-DC system;

[0017] The multi-DC transmission line module is used to simulate the DC transmission lines of a real multi-DC system;

[0018] The system control module is used to simulate the frequency stabilization control link of a real multi-DC asynchronous interconnected system.

[0019] Based on the above scheme, the connection between the control command variables of each link of the "master-pole-valve" hierarchical coordinated control model and the calculation method thereof are as follows:

[0020] The calculation formula for the current variable on the DC transmission line is:

[0021]

[0022] The calculation formula for voltage variables on DC transmission lines is:

[0023]

[0024] The calculation formulas for active power and reactive power of DC lines are:

[0025] P m =U m I d (3)

[0026] P n =U n I d (4)

[0027]

[0028]

[0029] in:

[0030]

[0031]

[0032] β=π-α (9)

[0033] γ=β-μ=π-(α+μ) (10)

[0034] Where U dm and U dn is the ideal no-load DC voltage of the converter at both ends of the system, α and β are the lag angle of the DC system during rectification and the lead angle of the DC system during inverter operation, γ is the lead angle at which the arc on the inverter side of the DC system is extinguished, and μ is the system commutation angle; X m and X n is the commutation reactance represented by the leakage reactance of the converter transformers on both sides of the system, R d is the impedance of the DC transmission line, R m and R n is the equivalent impedance of the smoothing reactors on both sides of the DC line.

[0035] Based on the above scheme, the specific calculation expression of the optimal objective function for maintaining safe and stable system frequency under the disturbance state is as follows:

[0036]

[0037] in:

[0038]

[0039]

[0040]

[0041] The regional control deviation value is the sum of the power support of the system DC transmission line and the product of the system frequency deviation and the frequency deviation coefficient in each region. The calculation method is:

[0042]

[0043] Where m is the number of DC transmission lines in the asynchronous interconnected system, P dc_i is the power exchanged on the i-th DC transmission line, P out_i is the active power output on the i-th DC transmission line, K i is the frequency deviation coefficient of the ith DC transmission line, f i is the actual frequency of the power grid connected to the i-th DC transmission line;

[0044] Taking the maximum value of the sum of the frequency deviation factors allowed by the interconnected multi-DC power system as the objective function, the mathematical expression of the optimal objective function for maintaining safe and stable system frequency under disturbance is established as follows:

[0045]

[0046] in:

[0047] Δδ rj =δ dc -δ0 (17)

[0048] Where: δ(MW / Hz) is the frequency deviation factor, V eq (Hz / MW) is the similarity conversion rate of the generator set, K (MW / Hz) is the power adjustment coefficient; P Nm (MW) is the steady-state power of the mth generator, f N (Hz) is the steady-state frequency of the system; ω 0.max is the maximum speed of the unit when it is no-load, ω F.min is the minimum speed of the unit under rated state, ω0 is the rated speed; ΔP L(MW) is the increase or decrease of system load, Δf (Hz) is the increase or decrease of system frequency; f(ΔP dc_m ) is the objective function of the sum of the frequency deviation factors allowed for stable operation of the DC transmission system, K j is the frequency correction coefficient, Δδ rj and δ0 are the frequency deviation factor variation and the steady-state rated frequency deviation factor of the multi-DC asynchronous interconnected power grid.

[0049] Based on the above scheme, the constraints for frequency safety and stability of the multi-DC asynchronous interconnected power grid are as follows:

[0050] P min ≤ΔP≤P max (18)

[0051] Q min ≤ΔQ≤Q max (19)

[0052] The frequency fluctuation of the asynchronous interconnected power grid is limited according to the frequency characteristics of the line power mutual assistance process. The constraints based on the frequency characteristic equation of the asynchronous interconnected multi-DC system are as follows:

[0053]

[0054] in:

[0055] P dc =P M -P E (twenty one)

[0056] Where, P min 、P max and Q min , Q max are the minimum and maximum thresholds of active / reactive power transmission on the interconnected lines of multiple DC systems, ΔP and ΔQ are the real-time active / reactive power values ​​distributed on the interconnected lines of the DC systems, respectively; H t is the time inertia constant of the system, dδ dc is the unit rotation median frequency deviation factor, P dc is the total power increase of the DC transmission line generator, P M and P E is the total mechanical power and electromagnetic power of the system generator set.

[0057] Beneficial effects of the present invention:

[0058] The present invention provides a control method for frequency stabilization in multiple asynchronously interconnected DC power grids. This method can quickly restore power fluctuations in a regional power grid after a fault disturbance to the stable operating state prior to the fault. It also boasts more stable coordination and control performance, effectively controlling the frequency stability between AC power grids and addressing frequency stability control issues caused by power imbalance between multiple asynchronously interconnected DC power grids. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] The present invention has the following accompanying drawings:

[0060] Figure 1 A flow chart of a control method for frequency stabilization of multiple DC asynchronous interconnected power grids. DETAILED DESCRIPTION

[0061] The following combination Figure 1 The present invention is described in further detail.

[0062] Step A: Set AC grid parameters, rectifier / inverter station parameters, DC transmission parameters, and system control module parameters to build a multi-DC asynchronous interconnected circuit model. The multi-DC asynchronous interconnected circuit model is a simulation circuit model pre-built based on PSCAD for simulating real multi-DC systems.

[0063] The parameters of each module are set based on the PSCAD simulation circuit model. The AC grid parameters include equivalent voltage source parameters and equivalent power supply impedance parameters, the rectifier / inverter station parameters include transformer parameters and six-pulse control bridge circuit parameters, the DC transmission parameters include smoothing reactor parameters and DC transmission line equivalent impedance parameters, and the system control module parameters include parameters related to the constant voltage control link, the constant current control link, and the fixed trigger angle control link.

[0064] The simulation circuit model pre-built based on PSCAD is essentially used to facilitate the simulation of the operating characteristics of a real multi-DC power grid, quickly and accurately analyze the frequency fluctuations of the multi-DC circuit, and set the parameters of related system modules based on the optimal and reliable operating state, so that the multi-DC asynchronous interconnected system can maintain frequency stability and reduce unnecessary losses and computing costs caused by system failures and disturbances. The simulation circuit model pre-built based on PSCAD for simulating a real multi-DC system specifically includes the following components: a two-sided AC power grid model for asynchronous interconnection, a bipolar parallel rectifier / inverter controller module, a multi-DC transmission line module, and a system control module; the two-sided AC power grid model for asynchronous interconnection is used to simulate the regional AC power grids on both sides of the real multi-DC system, the bipolar parallel rectifier / inverter controller module is used to simulate the rectifier and inverter stations of the real multi-DC system, the multi-DC transmission line module is used to simulate the DC transmission lines of the real multi-DC system, and the system control module is used to simulate the frequency stability control link of the real multi-DC asynchronous interconnected system.

[0065] Step B: Construct a "master-pole-valve" hierarchical coordinated control model suitable for multiple DC asynchronous interconnected power grids:

[0066] The described "main-pole-valve" hierarchical collaborative control model adopts the hierarchical collaborative control technology principle of "power complementary balance instruction-voltage and current conversion and transmission instruction-converter bottom trigger angle pulse control instruction", which can quickly detect and report system fault problems in a timely manner; when a system fault occurs, the relevant control parameters change. The main control layer receives the power complementary balance instruction and responds to it in a timely manner, and then converts it into a voltage and current adjustment instruction and sends it to the pole control layer. The pole control layer converts the received instruction into a converter bottom trigger angle adjustment instruction to indirectly control the relevant actions of the valve control layer. The valve control layer generates a trigger pulse to change the power distribution of the multi-DC system, thereby stabilizing the frequency fluctuation of the system.

[0067] The purpose of the hierarchical collaborative control model is to provide a step-by-step channel for achieving power complementarity and support among multiple DC systems, as well as control command conversion, distribution, and feedback correction. Power balance is the primary manifestation of this model, effectively adjusting the various variable commands at different levels to balance the power among multiple DC systems. The connections between the control command variables at each link and their calculation methods are as follows:

[0068] The calculation formula for the current variable on the DC transmission line is:

[0069]

[0070] The calculation formula for voltage variables on DC transmission lines is:

[0071]

[0072] The calculation formulas for active power and reactive power of DC lines are:

[0073] P m =U m I d (3)

[0074] P n =U n I d (4)

[0075]

[0076]

[0077] in:

[0078]

[0079]

[0080] β=π-α (9)

[0081] γ=β-μ=π-(α+μ) (10)

[0082] Where U dm and U dn is the ideal no-load DC voltage of the converter at both ends of the system, α and β are the lag angle of the DC system during rectification and the lead angle of the DC system during inverter operation, γ is the lead angle at which the arc on the inverter side of the DC system is extinguished, and μ is the system commutation angle; X m and X n is the commutation reactance represented by the leakage reactance of the converter transformers on both sides of the system, R d is the impedance of the DC transmission line, R m and R n is the equivalent impedance of the smoothing reactors on both sides of the DC line.

[0083] Step C: Set the calculation expression for the maximum value of the sum of the frequency deviation factors allowed by the multi-DC power system after asynchronous interconnection, and establish the optimal objective function for maintaining safe and stable system frequency under disturbance conditions:

[0084] The frequency deviation of the transmitting and receiving end networks of a multi-DC asynchronous interconnected power grid is inversely correlated with the frequency deviation factor of the grid. The frequency deviation factor is used as an indicator to evaluate the frequency support capability of each end system of the DC asynchronous interconnected power grid. The larger the value of the frequency deviation factor, the stronger the system frequency support capability. Its specific calculation expression is as follows:

[0085]

[0086] in:

[0087]

[0088]

[0089]

[0090] The regional control deviation is the sum of the power support of the system's DC transmission lines and the product of the frequency deviation of each regional system and the frequency deviation coefficient. If the asynchronous interconnected system has m DC transmission lines, the system must simultaneously meet the power and frequency stability requirements on each DC line when allocating power. The regional control deviation is calculated as follows:

[0091]

[0092] Where, P dc_i is the power exchanged on the i-th DC transmission line, P out_i is the active power output on the i-th DC transmission line, K i is the frequency deviation coefficient of the ith DC transmission line, f i is the actual frequency of the power grid connected to the i-th DC transmission line, and the steady-state value of the power grid f e =50Hz.

[0093] Taking the maximum value of the sum of the frequency deviation factors allowed by the interconnected multi-DC power system as the objective function, the mathematical expression of the optimal objective function for maintaining safe and stable system frequency under disturbance is established as follows:

[0094]

[0095] in:

[0096] Δδ rj =δ dc -δ0 (17)

[0097] Where: δ(MW / Hz) is the frequency deviation factor, V eq (Hz / MW) is the similarity conversion rate of the generator set, K (MW / Hz) is the power adjustment coefficient; P Nm (MW) is the steady-state power of the mth generator, f N (Hz) is the steady-state frequency of the system; ω 0.max is the maximum speed of the unit when it is no-load, ω F.min is the minimum speed of the unit under rated state, ω0 is the rated speed; ΔP L (MW) is the increase or decrease of system load, Δf (Hz) is the increase or decrease of system frequency; f(ΔP dc_m) is the objective function of the sum of the frequency deviation factors allowed for stable operation of the DC transmission system, K j is the frequency correction coefficient, Δδ rj and δ0 are the frequency deviation factor variation and the steady-state rated frequency deviation factor of the multi-DC asynchronous interconnected power grid.

[0098] Step D: Setting frequency security and stability constraints for multiple DC asynchronous interconnected power grids, and constraining the power distribution and frequency operating characteristics on each DC line;

[0099] The power support allocation must meet the carrying capacity of multiple DC grids after asynchronous interconnection. The constraints given for safety reasons are as follows:

[0100] P min ≤ΔP≤P max (18)

[0101] Q min ≤ΔQ≤Q max (19)

[0102] The frequency fluctuation of the asynchronous interconnected power grid is limited according to the frequency characteristics of the line power mutual assistance process. The constraints based on the frequency characteristic equation of the asynchronous interconnected multi-DC system are as follows:

[0103]

[0104] in:

[0105] P dc =P M -P E (twenty one)

[0106] Where, P min 、P max and Q min , Q max are the minimum and maximum thresholds of active / reactive power transmission on the interconnected lines of multiple DC systems, ΔP and ΔQ are the real-time active / reactive power values ​​distributed on the interconnected lines of the DC systems, respectively; H t is the time inertia constant of the system, dδ dc is the unit rotation median frequency deviation factor, P dc is the total power increase of the DC transmission line generator, P M and P E is the total mechanical power and electromagnetic power of the system generator set.

[0107] Step E: Configure the underlying variables of the circuit model to satisfy the objective function under the above constraints, thereby achieving effective control of power mutual support between multiple DC systems and stabilizing system frequency fluctuations.

[0108] The underlying variables of the circuit model are the voltage, current and trigger angle adjustment instructions in the "main-pole-valve" hierarchical cooperative control model. According to the calculation method in step B, the optimal setting parameters of each variable when the multi-DC system power is balanced can be obtained, thereby constructing the optimal parameters of the control method of each link involved in the system control module in step A; the objective function established in step C based on the maximum value of the sum of frequency deviation factors needs to meet the two basic constraints given in step D, and participate in the "main-pole-valve" hierarchical cooperative control model in step B to perform power mutual adjustment on the multi-DC asynchronous interconnected power grid, thereby achieving frequency stability of the multi-DC system and ensuring the safe and reliable operation of the multi-DC asynchronous interconnected system under frequency stability.

[0109] The above embodiments are intended only to illustrate the present invention and are not intended to limit the present invention. Persons skilled in the art may make various changes and modifications without departing from the essence and scope of the present invention. Therefore, all equivalent technical solutions also fall within the scope of the present invention, and the scope of protection of the present invention shall be defined by the claims. Matters not described in detail in this specification constitute prior art known to those skilled in the art.

Claims

1. A control method for frequency stabilization of multiple DC asynchronous interconnected power grids, characterized in that: The steps include: Set AC grid parameters, rectifier / inverter station parameters, DC transmission parameters, and system control module parameters, and build a multi-DC asynchronous interconnected circuit model; Construct a "master-pole-valve" hierarchical coordinated control model suitable for multiple DC asynchronous interconnected power grids; Setting a calculation expression for the maximum value of the sum of frequency deviation factors allowed for multiple DC power systems after asynchronous interconnection; Establish the optimal objective function to keep the system frequency safe and stable under disturbance conditions; The underlying variables of the circuit model are configured to satisfy the objective function under the constraints, effectively controlling the mutual power support between multiple DC systems and thus stabilizing the system frequency fluctuations. The specific calculation expression of the optimal objective function for maintaining safe and stable system frequency under the disturbance state is as follows: ; in: ; ; ; The regional control deviation value is the sum of the power support of the system DC transmission line and the product of the system frequency deviation and the frequency deviation coefficient in each region. The calculation method is: ; Where m is the number of DC transmission lines in the asynchronous interconnected system, is the power exchanged on the i-th DC transmission line, is the active power output on the i-th DC transmission line, is the frequency deviation coefficient of the ith DC transmission line, is the actual frequency of the power grid connected to the i-th DC transmission line; Taking the maximum value of the sum of the frequency deviation factors allowed by the interconnected multi-DC power system as the objective function, the mathematical expression of the optimal objective function for maintaining safe and stable system frequency under disturbance is established as follows: ; in: ; Where: is the frequency deviation factor, V eq is the similarity conversion rate of the generator set, K is the power adjustment coefficient; P Nm is the steady-state power of the mth generator, is the system steady-state frequency; The maximum speed of the unit when it is no-load. is the minimum speed of the unit under rated conditions, is the rated speed; is the increase or decrease of system load, is the increase or decrease of the system frequency; is the objective function of the sum of the frequency deviation factors allowed for stable operation of the HVDC system, is the frequency correction coefficient, and It is the frequency deviation factor variation and steady-state rated frequency deviation factor of multiple DC asynchronous interconnected power grids.

2. A control method for frequency stabilization of a multi-DC asynchronous interconnected power grid according to claim 1, characterized in that: The multi-DC asynchronous interconnection circuit model is a simulation circuit model pre-built based on PSCAD for simulating real multi-DC.

3. A control method for frequency stabilization of a multi-DC asynchronous interconnected power grid according to claim 1, characterized in that: The AC power grid parameters include equivalent voltage source parameters and equivalent power source impedance parameters, the rectifier / inverter station parameters include transformer parameters and six-pulse control bridge circuit parameters, the DC power transmission parameters include smoothing reactor parameters and DC transmission line equivalent impedance parameters, and the system control module parameters include parameters related to the constant voltage control link, parameters related to the constant current control link, and parameters related to the constant trigger angle control link.

4. A control method for frequency stabilization of a multi-DC asynchronous interconnected power grid according to claim 2, characterized in that: The simulation circuit model pre-built based on PSCAD for simulating real multi-DC specifically includes the following parts: Two-side AC grid model for asynchronous interconnection, bipolar parallel rectifier / inverter controller module, multiple DC transmission line module and system control module; The two-side AC power grid model for asynchronous interconnection is used to simulate the regional AC power grids on both sides of the real multi-DC system; The bipolar parallel rectifier / inverter controller module is used to simulate the rectifier station and inverter station of a real multi-DC system; The multi-DC transmission line module is used to simulate the DC transmission lines of a real multi-DC system; The system control module is used to simulate the frequency stabilization control link of a real multi-DC asynchronous interconnected system.

5. The control method for frequency stabilization of multiple DC asynchronous interconnected power grids according to claim 1, characterized in that: The connection between the control command variables of each link of the "master-pole-valve" hierarchical coordinated control model and the calculation method thereof are as follows: The calculation formula for the current variable on the DC transmission line is: ; The calculation formula for voltage variables on DC transmission lines is: ; The calculation formulas for active power and reactive power of DC lines are: ; ; ; ; in: ; ; ; ; Where, and is the ideal no-load DC voltage of the converters at both ends of the system, and It is the lag angle of the DC system during rectification and the lead angle of the DC system during inverter operation. is the lead angle at which the arc on the inverter side of the DC system is extinguished. is the system commutation angle; and It is the commutation reactance represented by the leakage reactance of the converter transformers on both sides of the system. is the impedance of the DC transmission line, and is the equivalent impedance of the smoothing reactors on both sides of the DC line.

6. A control method for frequency stabilization of multiple DC asynchronous interconnected power grids according to claim 1, characterized in that: The constraints for frequency stability of the multi-DC asynchronous interconnected power grid are as follows: ; ; The frequency fluctuation of the asynchronous interconnected power grid is limited according to the frequency characteristics of the line power mutual assistance process. The constraints based on the frequency characteristic equation of the asynchronous interconnected multi-DC system are as follows: ; in: ; Where, and are the minimum and maximum thresholds for active / reactive power transmission on the interconnected lines of multiple DC systems, and They are respectively the real-time active / reactive power values ​​distributed on the DC system interconnection lines; is the time inertia constant of the system, is the unit rotation median frequency deviation factor, is the total power increase of the DC transmission line generator, and is the total mechanical power and electromagnetic power of the system generator set.

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