A method for suppressing grid voltage unbalance of a fixed-power static synchronous compensator

Abstract

The application discloses a method for suppressing grid-connected voltage unbalance of a static synchronous compensator with fixed active power, comprising the following steps: obtaining the current limit value of an inverter of the static synchronous compensator, a fixed active power instruction and grid parameters, and constructing a grid-connected voltage calculation model; on the basis, an optimization model with the minimum voltage unbalance degree as the target is established, the model simultaneously considers the inverter current constraint and the fixed active power constraint; for the non-convex optimization problem, the original problem is converted into a single-variable quasi-convex optimization problem about the positive sequence current amplitude by grid voltage orientation and geometric analysis, and the optimal double sequence current and power instruction are obtained; finally, the instruction is input into the static synchronous compensator controller for execution. The application can strictly meet the dispatching active instruction and the equipment safety limit value, coordinate and optimize the positive and negative sequence voltages, realize the global optimal suppression of the grid-connected point voltage unbalance degree, and improve the power quality and operation stability of the wind power grid-connected system.

Description

Technical Field

[0001] This invention belongs to the field of new energy grid connection control technology, specifically relating to a method for suppressing grid voltage imbalance of a static synchronous condenser under constant active power. Background Technology

[0002] With the advancement of the large-scale development of the desert and wasteland base, the construction of the supporting heavy-haul railway transportation system has entered its peak period. These railways typically employ high-power electric traction, and their loads are characterized by single-phase operation, strong impact, and enormous power. Since the traction power supply system generally draws power from the public grid through single-phase transformers, it injects a huge negative-sequence current into the three-phase grid, leading to severe voltage imbalance at the point of common coupling. In the desert and wasteland region, the power grid structure is relatively weak with limited short-circuit capacity, making this voltage imbalance caused by the single-phase load of the heavy-haul railway particularly prominent. This not only causes grid protection malfunctions and endangers the railway's own power supply safety and operational efficiency, but also leads to the disconnection of photovoltaic, wind power, and other new energy units connected to the same grid, causing overheating, reduced efficiency, and shortened lifespan of other surrounding three-phase electrical equipment, seriously threatening the safe and stable operation of the regional power grid.

[0003] Static synchronous condensers (SSCs) are considered an effective tool for mitigating grid voltage imbalances, particularly for compensating for single-phase load surges in heavy-haul railways, due to their flexible and rapid independent control capabilities of positive and negative sequence currents. Currently, control strategies for suppressing voltage imbalances using SSC inverters are mainly divided into two categories: The first type is based on preset voltage reference values. Its core is to use droop control or a proportional-integral controller to preset reference values ​​for the positive and negative sequence voltages, thereby generating an AC current command. However, this type of method typically regulates voltage only through the q-axis current, offering limited suppression of voltage imbalance and potentially causing inverter synchronism issues under certain operating conditions, posing a stability risk.

[0004] The second category is optimization-based methods that improve the control efficiency of negative-sequence voltage by optimizing the current command. Some studies generate an optimized current reference by minimizing the negative-sequence voltage at the PCC (point of common coupling) under inverter current constraints, such as in the literature [NEJABATKHAH F, LI YW, WU B. Control strategies of three-phase distributed generation inverters for grid unbalanced voltage compensation[J].IEEE Transactions on Power Electronics, 2016, 31(7): 5228-5241]; other studies further consider negative-sequence voltage suppression under active power constraints, such as in the literature [GUO Y, PAL BC, JABR R A. On the optimization of voltage unbalance attenuation by inverters[J]. IEEE Transactions on Sustainable Energy, 2022, 13(3): 1492-1506]. Although these methods effectively reduce negative-sequence voltage, the problem of positive-sequence voltage drop is becoming increasingly prominent with the increase in renewable energy penetration. Existing optimization methods often neglect coordination with positive-sequence voltage support during the optimization process. If negative-sequence voltage is prioritized, it will affect the net current and active power capacity available for positive-sequence control, potentially leading to insufficient suppression capability in the event of severe voltage problems. Furthermore, some methods fail to incorporate both active power and current constraints simultaneously to simplify the optimization model. Without active power constraints, power imbalance may occur between the DC and AC sides, resulting in DC-side voltage instability; without current constraints, it may lead to inverter overcurrent damage.

[0005] Voltage imbalance is a key indicator for assessing the degree of voltage imbalance. Although some studies have simplified power constraints when suppressing voltage imbalance, failing to incorporate all effective constraints, effectively reducing voltage imbalance while fully considering active power and current constraints remains a complex optimization problem with non-convex and nonlinear characteristics. Summary of the Invention

[0006] In view of the above, the present invention provides a method for suppressing grid-connected voltage imbalance under constant active power conditions for static synchronous condensers. This method can simultaneously coordinate and optimize positive and negative sequence voltages, and achieve optimal suppression of grid-connected voltage imbalance under the premise of strictly meeting the inverter current constraints of static synchronous condensers and the active power command of grid dispatch, thereby improving the system's operational stability.

[0007] A method for suppressing grid voltage imbalance of a stationary synchronous condenser under constant active power includes the following steps: (1) Obtain relevant information about inverter current limit, active power dispatch and grid connection lines from the static synchronous condenser, and use this information to construct a grid connection voltage calculation model for the inverter; (2) Based on the grid voltage calculation model, construct an optimization model for suppressing the grid voltage imbalance of a static synchronous condenser under a fixed active power command; (3) The positive and negative sequence injection current and positive and negative sequence active power are obtained by optimizing the grid connection point voltage imbalance suppression optimization model with the grid connection point far-end voltage orientation and geometric constraints. (4) The obtained positive and negative sequence injection current and positive and negative sequence active power are used as command values ​​and applied to the static synchronous condenser inverter control system. Command tracking is achieved through PI (proportional integral) control to dynamically suppress the voltage imbalance at the grid connection point.

[0008] Furthermore, the information obtained in step (1) includes the inverter's maximum allowable output current. I max Active power commands issued by the power grid dispatch center P ref The positive sequence voltage of the remote power grid sensed by the static synchronous condenser at the grid connection point. V g+ and the negative sequence voltage of the remote power grid V g- The equivalent line impedance parameters of the grid connection point include resistance. R and reactance X .

[0009] Furthermore, the grid-connected voltage calculation model is based on Kirchhoff's voltage law and describes the positive-sequence voltage at the grid connection point under the combined effects of line current, remote grid voltage, and line impedance. V + and negative sequence voltage V - The calculation relationship is expressed as follows: in: I d+ and I d- These represent the positive-sequence d-axis current and the negative-sequence d-axis current injected into the inverter at the grid connection point, respectively. I q+ and I q- These are the positive-sequence q-axis current and the negative-sequence q-axis current injected into the grid connection point of the inverter, respectively.

[0010] Furthermore, the grid connection point voltage imbalance suppression optimization model in step (2) takes minimizing the grid connection voltage imbalance VUF as the objective function, where VUF = V - / V + Decision variables include positive-sequence injection current. I + Negative sequence injection current I - Positive sequence active power P + Negative sequence active power P - The constraints include the total inverter current amplitude constraint, i.e. I + + I - ≤ I max Line positive and negative sequence power flow constraints, i.e., grid-connected voltage calculation model; fixed total active power output constraints, i.e. P + + P - = P ref .

[0011] Furthermore, the specific implementation of step (3) is as follows: S31: By analyzing the geometry of the solution space formed by the constraints of the optimization model, the coupling relationship between the positive sequence injection current and the positive sequence active power when VUF reaches its minimum value is determined. S32: Based on the aforementioned coupling relationship, combined with the total active power output constraint and the total current amplitude constraint, the original high-dimensional non-convex optimization problem involving multiple variables is simplified to a problem concerning only the positive-sequence injection current. I + A single-variable optimization problem; S33: Transform the objective function of the optimization model into a function about... I + The univariate function VUF( I + ), by analyzing the function in I + The properties within the feasible region are used to solve for the function VUF( I + The corresponding minimum case I + Then, the corresponding negative sequence injection current is calculated. I - Positive sequence active power P + Negative sequence active powerP - .

[0012] Furthermore, the coupling relationship between the positive-sequence injected current and the positive-sequence active power in step S31 is expressed as follows: Further, in step S33, when the function VUF( I + The corresponding minimum case I + Then, further I max minus I + That is, the negative sequence injection current is obtained. I - Then according to I + The positive-sequence active power was calculated based on the coupling relationship. P + Finally, P ref minus P + Thus, the negative-sequence active power is obtained. P - .

[0013] A computer device includes a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the above-described method for suppressing grid voltage imbalance of a stationary synchronous condenser under constant active power.

[0014] A computer-readable storage medium storing a computer program, which, when executed by a processor, implements the above-described method for suppressing grid voltage imbalance of a stationary synchronous condenser under constant active power.

[0015] Under the dual constraints of strict fixed active power and inverter current limits, this invention transforms a complex non-convex optimization problem into a solvable form through geometric analysis. The obtained solution is the global optimal solution under the current constraints, and can achieve a lower grid connection point voltage imbalance than existing methods. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of a three-phase grid-connected system based on static synchronous condenser resources and its control principle.

[0017] Figure 2 A schematic diagram of the VUF surface and its projection in the solution space for grid-connected control of a stationary synchronous condenser.

[0018] Figure 3 For quasi-convex functions of single-variable optimization problems, VUF(I + (Diagram) Detailed Implementation

[0019] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0020] This embodiment provides a method for suppressing grid voltage imbalance of a stationary synchronous condenser under constant active power. The specific implementation process includes the following steps: Step 1: Obtain system operating parameters in real time.

[0021] For example Figure 1 The three-phase grid-connected system based on static synchronous condenser resources shown acquires or determines the following necessary parameters in real time through the measurement unit, communication interface, and parameter identification module in the static synchronous condenser control system: Maximum allowable output current of static synchronous condenser inverter I max Fixed active power commands issued by the power grid dispatching authority P ref The positive sequence voltage amplitude of the remote power grid sensed at the point of grid connection of the static synchronous condenser. V g+ and negative sequence voltage amplitude V g- Equivalent line impedance parameters (resistance) of the static synchronous condenser grid connection point R and reactance X ).

[0022] Step 2: Construct a voltage calculation model and optimization framework for the grid connection point.

[0023] Based on circuit laws, the relationship between grid connection point voltage and injected current is established. In a synchronous rotating coordinate system oriented with the remote grid voltage vector as the reference, this relationship can be expressed as: In the formula: V + It is the grid-connected positive sequence voltage. V - It is the grid-connected negative sequence voltage; V g+ and V g- These are the positive-sequence voltage and the negative-sequence voltage of the remote power grid, respectively. R and X These are the equivalent resistance and inductance of the line connecting the static synchronous condenser to the remote power grid, respectively. I d+ It is the positive-sequence d-axis current.I d- It is a negative-sequence d-axis current; I q+ It is the positive-sequence q-axis current. I q- It is a negative-sequence q-axis current.

[0024] Based on the relationship between power and voltage / current, voltage can be further expressed as: This formula applies to both positive and negative sequence voltages, where I + , I - The positive and negative sequence current amplitudes are respectively , , P + , P - These represent the positive and negative sequence active power, respectively. .

[0025] Based on this, an optimization model for voltage imbalance suppression is constructed, including: Objective function: Minimize grid-connected voltage imbalance VUF = V - / V + .

[0026] Decision variables: I + , I - , P + , P - .

[0027] Current constraint: I + + I - ≤ I max .

[0028] Active power constraint: P + + P - = P ref .

[0029] Voltage model: as described above V + and V - The calculation formula.

[0030] Physical constraints:V + ≥0, V - ≥0.

[0031] The optimization problem of this model exhibits highly non-convex and nonlinear characteristics due to its fractional objective function and the nonlinear coupling of power and current.

[0032] Step 3: Based on geometric constraints, transform the high-dimensional non-convex problem into a single-variable quasi-convex optimization problem and solve it.

[0033] This step is the core of the method of this invention, and its purpose is to analytically transform the complex optimization model into a form that can be solved efficiently. The specific implementation process is as follows: S31: Introduce geometric constraints to determine the boundary where the optimal solution is located.

[0034] By analyzing the geometry of the solution space formed by the constraints, such as... Figure 2 The VUF surface and its projection are shown. It is found that the optimal solution that minimizes the global VUF lies on the boundary of the feasible region of the positive-sequence voltage. The mathematical condition corresponding to this boundary is that the expression for the positive-sequence q-axis current is non-negative within the square root. The positive-sequence current at the optimal solution is derived. I + With positive sequence power P + The coupling relationship that must be satisfied between them is as follows: This relationship is key to subsequent simplification.

[0035] S32: Elimination and Dimensionality Reduction.

[0036] The current constraint is taken as an equation when optimal, and then substituted into the expression of the objective function of the optimization model. Thus, the four decision variables of the original problem ( I + , I - , P + , P - It is transformed into a function that only considers a single variable. I + The optimization problem.

[0037] S33: Find the optimal solution for a univariate quasi-convex function.

[0038] The objective function corresponding to the optimization problem f ( I + )=VUF( I + It is a quasi-convex function within its domain, and its typical shape is as follows: Figure 3As shown, the solution process is as follows: (a) Determine the domain: Calculate the positive sequence current I + The feasible range is determined by the feasible region of the negative sequence current (ensuring that the negative sequence dq axis current is a real number).

[0039] (b) Finding the optimal solution: Calculate the stationary points of the function in its continuous domain (the points where the derivative is zero). Since the derivative equation is complex, it can be solved by numerical methods.

[0040] Based on the relationship between the stationary point and the domain, the final optimal positive sequence current is determined: if the stationary point value is less than 0, the optimal positive sequence current is taken as 0; if the stationary point value is greater than the upper limit of the positive sequence current, the optimal positive sequence current is taken as the upper limit of the positive sequence current; if the stationary point value is greater than 0 and less than the upper limit of the positive sequence current, the optimal positive sequence current is taken as the stationary point.

[0041] Step 4: Calculate and output the complete optimal double-order instruction set.

[0042] Based on the optimal positive sequence current obtained in step 3, the optimal positive sequence active power, optimal negative sequence current, and optimal negative sequence active power are calculated sequentially. Through inverse coordinate transformation, the optimal positive sequence current, optimal positive sequence active power, optimal negative sequence current, and optimal negative sequence active power are respectively solved into the components of the dual-sequence current in the dq coordinate system used for control.

[0043] Step 5: Instruction tracing and execution.

[0044] The final current command generated in step 4 is input into the dual-sequence decoupled current inner loop of the static synchronous condenser inverter. The inner loop controller uses a proportional-integral regulator to generate a voltage modulation signal to drive the power converter, so that the actual output current of the inverter can quickly and accurately track the optimal command, thereby achieving dynamic optimal suppression of voltage imbalance at the grid connection point while meeting the requirements of fixed active power generation.

[0045] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. Those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A method for suppressing grid voltage imbalance of a stationary synchronous condenser under constant active power, characterized in that, Includes the following steps: (1) Obtain relevant information from the static synchronous condenser regarding inverter current limits, active power dispatch, and grid connection lines, including the maximum allowable output current of the inverter. I max Active power commands issued by the power grid dispatch center P ref The positive sequence voltage of the remote power grid sensed by the static synchronous condenser at the grid connection point. V g+ and the negative sequence voltage of the remote power grid V g- The equivalent line impedance parameters of the grid connection point include resistance. R and reactance X And based on this, a grid-connected voltage calculation model for the inverter is constructed; (2) Based on the grid voltage calculation model, construct an optimization model for suppressing the grid voltage imbalance of a static synchronous condenser under a fixed active power command; The grid connection point voltage imbalance suppression optimization model takes minimizing the grid connection voltage imbalance VUF as the objective function, where VUF = V - / V + , V + and V - These are the positive-sequence voltage and negative-sequence voltage at the grid connection point, respectively. Decision variables include positive-sequence injection current I + Negative sequence injection current I - Positive sequence active power P + Negative sequence active power P - The constraints include the total inverter current amplitude constraint, i.e. I + + I - ≤ I max Line positive and negative sequence power flow constraints, i.e., grid-connected voltage calculation model; fixed total active power output constraints, i.e. P + + P - = P ref ; (3) The positive and negative sequence injection currents and positive and negative sequence active power are obtained by optimizing the grid connection point voltage imbalance suppression optimization model with the grid connection point far-end voltage orientation and geometric constraints. The specific implementation method is as follows: S31: By analyzing the geometry of the solution space formed by the constraints of the optimization model, the coupling relationship between the positive sequence injection current and the positive sequence active power when VUF reaches its minimum value is determined. S32: Based on the aforementioned coupling relationship and combining the total active power output constraint and the total current amplitude constraint, the original high-dimensional non-convex optimization problem involving multiple variables is simplified to a problem concerning only the positive-sequence injection current. I + A single-variable optimization problem; S33: Transform the objective function of the optimization model into a function about... I + The univariate function VUF( I + ), by analyzing the function in I + The properties within the feasible region are used to solve for the function VUF( I + The corresponding minimum case I + Then, the corresponding negative sequence injection current is calculated. I - Positive sequence active power P + Negative sequence active power P - ; (4) The obtained positive and negative sequence injection current and positive and negative sequence active power are used as command values ​​and applied to the static synchronous condenser inverter control system. Command tracking is achieved through PI control to dynamically suppress the voltage imbalance at the grid connection point.

2. The method for suppressing grid voltage imbalance of a stationary synchronous condenser under constant active power as described in claim 1, characterized in that: The grid-connected voltage calculation model is based on Kirchhoff's voltage law and describes the positive-sequence voltage at the grid connection point under the combined effects of line current, remote grid voltage, and line impedance. V + and negative sequence voltage V - The calculation relationship is expressed as follows: in: I d+ and I d- These represent the positive-sequence d-axis current and the negative-sequence d-axis current injected into the inverter at the grid connection point, respectively. I q+ and I q- These are the positive-sequence q-axis current and the negative-sequence q-axis current injected into the grid connection point of the inverter, respectively.

3. The method for suppressing grid voltage imbalance of a stationary synchronous condenser under constant active power as described in claim 1, characterized in that, The coupling relationship between the positive-sequence injected current and the positive-sequence active power in step S31 is expressed as follows: 。 4. The method for suppressing grid voltage imbalance of a stationary synchronous condenser under constant active power as described in claim 1, characterized in that: In step S33, when the function VUF( I + The corresponding minimum case I + Then, further I max minus I + That is, the negative sequence injection current is obtained. I - Then according to I + The positive-sequence active power was calculated based on the coupling relationship. P + Finally, P ref minus P + Thus, the negative-sequence active power is obtained. P - .

5. A computer device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that: The processor is used to execute the computer program to implement the method for suppressing grid voltage imbalance of a stationary synchronous condenser under constant active power as described in any one of claims 1 to 4.

6. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by the processor, it implements the method for suppressing grid voltage imbalance of a stationary synchronous condenser under constant active power as described in any one of claims 1 to 4.

Images