Communication-free power coordination control method for flexible interconnection system of two transformer areas

Abstract

The invention provides a communication-free power coordination control method for a two-transformer-area flexible interconnection system, and the method comprises the following steps: S1, measuring the output voltage and current of the low-voltage side of a transformer in each transformer area, and calculating the load rate of the transformer in each transformer area in real time, so as to obtain a DC voltage instruction value; s2, obtaining the direct current side voltage, the direct current transmission current and the equivalent impedance of a direct current transmission line of one transformer area, and obtaining a load rate prediction value of the other transformer area according to the direct current voltage instruction value; and S3, selecting one of the voltage source converters as a load rate secondary control end, and obtaining a secondary control superposed direct-current voltage instruction based on a load rate predicted value, thereby carrying out communication-free power coordination control on the flexible interconnection systems of the two transformer areas. And load rate secondary control is established based on a load rate prediction result, closed-loop adjustment is performed on the load rate of the transformer, power coordination control of the flexible interconnection system is realized, and power quantitative control of a power distribution area can be realized without depending on communication.

Description

Technical Field

[0001] This invention belongs to the field of power electronics engineering technology, specifically relating to a method for non-communication power coordination control of a flexible interconnection system of two substations. Background Technology

[0002] The power distribution network is a crucial link in the power system connecting power sources and users. With the increasing penetration of new energy sources and the continuous growth of load, traditional distribution networks face increasing operational pressure and uneven load distribution. In typical distribution substations, due to differences in power supply capacity, user types, and load variations, uneven load distribution often occurs, with some substations having higher load rates while others have lower load rates. This not only reduces equipment utilization but may also lead to accelerated aging of some equipment due to prolonged overload operation, affecting power supply reliability and operational economy.

[0003] To address the aforementioned issues, flexible interconnection technology has received widespread attention in distribution networks in recent years. By introducing power electronic interface devices (such as voltage source converters) between multiple distribution substations, a flexible interconnection system can be constructed, enabling flexible power flow adjustment and mutual assistance between substations, thereby effectively alleviating the problem of uneven load distribution. However, existing flexible interconnection systems mostly rely on communication systems for power coordination control, which has certain limitations in practical applications. On the one hand, communication failures may cause coordination failures, affecting the stable operation of the system; on the other hand, building highly reliable communication infrastructure requires additional costs, making it difficult to deploy widely in some distribution scenarios. Existing non-communication power coordination control technologies aim at balancing the load rate of distribution transformers, determining virtual resistance, and reducing the impact of DC line impedance voltage drop on the control effect to ensure system operation without communication, but this is limited to load rate balancing and cannot achieve flexible power mutual assistance between substations.

[0004] Therefore, there is a need to provide an improved technical solution that addresses the shortcomings of the existing technology. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art. This invention provides a method for communication-free power coordination control of a two-unit flexible interconnection system.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A method for non-communication power coordination control of a two-unit flexible interconnection system includes the following steps: Step S1: Measure the output voltage and current of the low-voltage side of each transformer in each distribution area, calculate the load rate of each transformer in real time, and multiply the transformer load rate and droop coefficient into the DC voltage control of the voltage source converter to obtain the DC voltage command value. Step S2: Obtain the DC side voltage, DC transmission current and DC transmission line equivalent impedance of one transformer substation, and obtain the load rate prediction value of the other transformer substation based on the DC voltage command value. Step S3: Select one of the voltage source converters as the secondary control terminal for load rate, obtain the DC voltage command superimposed on the secondary control based on the load rate prediction value, and thus carry out the non-communication power coordination control of the two-unit flexible interconnection system.

[0007] Preferably, in step S1, when the load rates of the transformers in each distribution area are approximately equal, the DC tie line does not transmit power.

[0008] Preferably, when determining the droop coefficient value for load rate droop control, each transformer in the distribution area is at a high load rate and the flexible interconnection system does not experience over-modulation.

[0009] Preferably, the DC voltage command value after introducing load rate droop control is used to generate a modulation reference voltage through voltage and current dual closed-loop control, and then a pulse width modulation signal is generated to drive the switching device after sinusoidal pulse width modulation.

[0010] Preferably, in step S2, control error is not considered during the calculation of the load rate prediction value.

[0011] Preferably, in step S3, the load allocation coefficient of each transformer substation is defined, and the load allocation coefficient is flexibly adjusted according to the capacity configuration, load characteristics and operating conditions of the transformer substation.

[0012] Preferably, in step S3, when the load rate secondary control of the two-area flexible interconnection system tends to stabilize, the DC voltage command value is acquired secondary to control the DC side voltage of each area voltage source converter.

[0013] Beneficial effects: This invention establishes a load rate prediction method without communication based on load rate droop control; it establishes secondary load rate control based on the load rate prediction results, performs closed-loop regulation of transformer load rate, realizes power coordination control of flexible interconnection systems, and can achieve quantitative control of distribution area power without relying on communication, solving the problem of power imbalance in distribution areas. Furthermore, this invention directly controls the load rate of transformers in the distribution area, is not limited by transformer type or capacity, and has universal applicability. Attached Figure Description

[0014] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. Wherein: Figure 1 This is a diagram showing the result of the two-unit flexible interconnection system of the present invention; Figure 2 (a) is the block diagram of VSC1 load rate droop control; Figure 2 (b) Block diagram of VSC2 load rate droop control; Figure 3 Here is the control block diagram for VSC1 power coordination control; Figure 4 (a) shows the load rate droop control adopted by the two-unit flexible interconnection system; Figure 4 (b) shows that the flexible interconnection system between the two zones adopts communication-free power coordination control. Figure 5 Simulation waveform of load rate of transformer in interconnected distribution area for load rate droop control; Figure 6 (a) shows the simulated waveform of the load rate of the interconnected transformer in the power coordination control area without communication. Figure 6 (b) Comparison of actual and predicted load rates of transformer T2; Figure 6 (c) Simulation waveforms of output power of VSC1 and VSC2 without communication power coordination control; Figure 6 (d) Simulation waveform of DC voltage superposition of VSC1 under load rate secondary control; Figure 6 (e) Simulated DC voltage waveforms of VSC1 and VSC2 without communication power coordination control. Detailed Implementation

[0015] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention are within the scope of protection of the present invention.

[0016] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. It should be noted that, unless otherwise specified, the embodiments and features described herein can be combined with each other.

[0017] Two flexible interconnection systems in the area, such as Figure 1 As shown, the AC sides of the two voltage source converters are connected to the low-voltage sides of transformers I and II in distribution substation, respectively, while the DC sides are connected via transmission lines. In the diagram, VSC represents a voltage source converter. Voltage source converters (VSCs), with their core characteristics of using fully controlled devices, DC-side capacitor energy storage, PWM control, and the ability to independently and rapidly control active and reactive power, have fundamentally changed the way power is transmitted, controlled by the grid, and utilized by renewable energy. They have overcome many limitations of traditional converters in areas such as weak grid access, reactive power support, and commutation failure, becoming an indispensable core technology for building modern, intelligent, flexible, and reliable power grids. Figure 1 middle R 1 and R 2 represents the equivalent resistance of the filter reactances of VSC1 and VSC2, respectively.L 1 and L 2 represents the equivalent inductance of the filter reactances of VSC1 and VSC2, respectively. C 1. C 2 represents the DC-side filter capacitors of VSC1 and VSC2, respectively. R 3. L 3 represents the equivalent resistance and inductance of the DC cable. u dc1 , u dc2 These represent the DC-side voltages of VSC1 and VSC2, respectively. i dc For direct current transmission, u c1 , u c2 These represent the voltage vectors of the modulation outputs of VSC1 and VSC2, respectively. i s1 , i s2 These are the current vectors output from VSC1 and VSC2, respectively. P VSC1 , P VSC2 These represent the active power outputs of VSC1 and VSC2, respectively. P dc1 , P dc2 These represent the DC-side port power of VSC1 and VSC2, respectively.

[0018] Based on the established two-area flexible interconnection system, the communication power coordination control method for the two-area flexible interconnection system provided by this invention includes the following steps: Step S1: Measure the output voltage and current on the low-voltage side of each transformer area using voltage and current transformers, calculate the load rate of each transformer area in real time, multiply the transformer load rate by the droop coefficient, and introduce it into the DC voltage control of the voltage source converter to obtain the DC voltage command value, where K1 and K2 are the droop coefficients.

[0019] Under the transformer load rate droop control, the DC voltage control command for the voltage source converter is as follows: (1) (2) In the formula, VSC after introducing load factor droop control n DC voltage command value, U dcref This is the DC voltage command value. K 1. K2 represents the droop coefficient for load factor droop control. β n For transformer T n load rate, P Ln This represents the total active power of the AC load on the transformer. P VSCn This refers to the output power of VSC (voltage source converter) 1 and VSC (voltage source converter) 2.

[0020] Step S2: Obtain the DC-side voltage, DC transmission current, and equivalent impedance of the DC transmission line for one of the transformer substations. Taking the prediction of the load factor of transformer substation II from substation 1 as an example, according to Kirchhoff's voltage law, the DC-side voltages of the two substations satisfy: (3) In the formula, u dc1 , u dc2 These are the DC-side voltages of VSC1 and VSC2, respectively. i dc It is a direct current transmission current. R 3 represents the equivalent resistance of the DC cable.

[0021] Combining formula (1), we can obtain the DC voltage command value and the DC side voltage of VSC2. u dc2 for (4) In the formula, U dcref This is the DC voltage command value. K 2 represents the VSC2 load factor droop. β 2 represents the load rate of transformer T2.

[0022] Neglecting control errors, combining equations (3) and (4), we can obtain the predicted load rate value for transformer area II. (5) In the formula, This is the predicted load factor for transformer area II.

[0023] Select one of the VSCs as the secondary control terminal for load rate, obtain the predicted load rate value of another transformer area based on the DC voltage command value, and establish a secondary control strategy for the load rate of the interconnected transformer area.

[0024] Step S3: Select one of the voltage source converters as the secondary control terminal for load rate, obtain the DC voltage command superimposed on the secondary control based on the load rate prediction value, and thus carry out the non-communication power coordination control of the two-unit flexible interconnection system.

[0025] Taking VSC1 as an example, the control block diagram for the secondary control of VSC1 load rate is as follows: Figure 3 As shown.

[0026] The DC voltage command superimposed by the secondary control of the load rate on the VSC1 side is: (6) In the formula, u offset This is the superimposed amount of the DC voltage command for the secondary load rate regulation VSC1. k p , k i These are the proportional and integral coefficients of the PI controller, which is the core component of the voltage source converter (VSC) control system. It achieves precise regulation by dynamically adjusting the proportional and integral components of the error, playing a key role in achieving high-precision dynamic regulation and stable operation.

[0027] When the load rate secondary control of the flexible interconnection system tends to stabilize... β 1. β 2 and The relationship between them is: (7) The DC voltage command value is obtained in this secondary manner to control the DC side voltage of each transformer area's voltage source converter. The DC side voltages of VSC1 and VSC2 are controlled by the load factor secondary control as follows: (8) in, The DC voltage command value of VSC1 after introducing load factor droop control. The DC voltage command value of VSC1 after introducing power coordination control. After voltage and current dual closed-loop control to generate a modulation reference voltage, SPWM (Sine Wave Pulse Width Modulation, the core modulation technology of power electronic converters, converts DC voltage into sinusoidal AC current with adjustable amplitude and frequency by controlling the on and off time of switching devices) modulation, a PWM (Pulse Width Modulation, the core technology of power electronic energy conversion, precisely adjusts the amplitude, frequency and waveform of output voltage / current by controlling the on and off time ratio / duty cycle of switching devices) signal is generated to drive the switching devices.

[0028] By constructing a communication-free load rate prediction method based on load rate droop control, the limitations of existing technologies that rely on communication to obtain power information of interconnected distribution areas are overcome. Load rate droop control and secondary load rate control methods are established to achieve precise closed-loop regulation of transformer load rates. This strategy helps alleviate the problem of uneven load distribution in distribution transformers and improves equipment utilization.

[0029] It enables the coordination and quantitative adjustment of power between transformer substations even without communication, thereby improving the system's autonomy and economy.

[0030] In an optional embodiment, in step S1, the load factor droop control droop coefficient is determined according to a set rule. The setting rule is: when the load rates of transformers in each distribution area are approximately equal, the DC tie line does not transmit power.

[0031] Therefore, the design requirements K 1= K 2. Implement load factor droop control. K 1. K When selecting value 2, it is necessary to ensure that the flexible interconnection system does not experience overmodulation under high load conditions for the transformer in the distribution area.

[0032] In this embodiment, the DC voltage command value after introducing load droop control is used to generate a modulation reference voltage through voltage and current dual closed-loop control. The load droop control of the interconnected voltage source converter is shown in Figure 2. A pulse width modulation signal is generated after sinusoidal pulse width modulation to drive the switching devices. Among them, Figure 2(a) is the block diagram of load droop control for VSC1, and Figure 2(b) is the block diagram of load droop control for VSC2.

[0033] Furthermore, in step S2, the control error is not considered in the calculation of the load rate prediction value, which can significantly reduce the computational complexity and improve the robustness of the prediction algorithm, avoiding the risk of error propagation and amplification. Thus, the load rate prediction value of transformer area II can be obtained by combining equation (3) and equation (4).

[0034] In an optional embodiment, in step S3, the load allocation coefficient for each distribution area is defined, specifically defining the load allocation coefficients for distribution area I and distribution area II as follows: n； Right now n = β 1 / n The value of is [0,∞], and the specific value can be flexibly adjusted according to the capacity configuration, load characteristics and operating conditions of the transformer in the distribution area.

[0035] Based on the above embodiments, this application verifies the effectiveness of communication power coordination control in a two-unit flexible interconnection system using the following method: specifically... Figure 1 The simulation analysis and verification of the two flexible interconnection systems shown are presented in Table 1.

[0036]

[0037] The control block diagram of the two-zone flexible interconnection system is shown in Figure 4. In Figure 4(a), the load rate droop control of the two-zone flexible interconnection system is shown. Both VSC1 and VSC2 adopt constant droop coefficient load rate droop control. The constant droop coefficient refers to a fixed droop coefficient that is not adjusted with the change of equipment capacity. In Figure 4(b), the non-communication power coordination control of the two-zone flexible interconnection system is shown. VSC1 adopts load rate droop control and load rate secondary control, while VSC2 adopts load rate droop control.

[0038] In the picture, θ PLLn The phase angle of the grid voltage obtained through the phase-locked loop; u sdn and u sqn These are the voltages of the power grid on the low-voltage side of the transformer. u sn exist dq Components on the axis; i sdn and i sqn They are currents i sn exist dq Components on the axis; i sdrefn , i sqrefn These are the active current and reactive current command values, respectively. u dn * , u qn * They are respectively in dq Modulation reference voltage on the shaft. VSC power control outputs active current command and sets reactive current command value. i sqrefn =0, and a modulation reference voltage is generated through the inner current loop control.

[0039] Based on the above control framework, four operating conditions are set as shown in Table 2 to verify the effectiveness of the load droop control and non-communication power coordination control of the proposed two-unit flexible interconnection system. This simulation verification only uses... n Taking 1 as an example does not mean that this method is only applicable to n =1, n The value can be flexibly adjusted according to the capacity configuration, load characteristics and operating conditions of the transformer in the distribution area.

[0040]

[0041] During simulation verification, working condition 1 corresponds to a simulation time of 0~1s, working condition 2 corresponds to a simulation time of 1~2s, working condition 3 corresponds to a simulation time of 2~3s, and working condition 4 corresponds to a simulation time of 3~4s.

[0042] Figure 5 This is a simulation waveform of the transformer load rate under load droop control. Load droop control can effectively adjust the power distribution of flexible interconnection systems and alleviate the light and heavy load conditions of transformers in distribution substations, but it cannot achieve quantitative control of the load rate of transformers in two substations. Based on Figure 5 The records demonstrate the effectiveness of using the load factor droop control method for power control in interconnected transformer substations.

[0043] Figure 6 shows the simulation waveforms of the flexible interconnected system under power coordination control without communication. Figure 6(a) shows the simulation waveform of the transformer load rate under power coordination control. From Figure 6(a), it can be seen that power coordination control without communication can achieve quantitative control of the transformer load rate in the distribution area. Figure 6(b) shows the comparison between the actual and predicted values ​​of the load rate of transformer T2. From Figure 6(b), it can be seen that the load rate prediction method of this invention can accurately predict the load rate of transformer T2 under conditions without communication. Figure 6(c) shows the simulation waveforms of the output power of VSC1 and VSC2, Figure 6(d) shows the simulation waveform of the DC voltage superposition of VSC1 under secondary load rate adjustment, and Figure 6(e) shows the simulation waveforms of the DC voltage of VSC1 and VSC2 under power coordination control. Taking operating condition 1 as an example, after power coordination control, the load rates of transformers T1 and T2 are... β 1. β 2 remains at 0.46. Actual value of transformer T2 load factor. β 2 and predicted value The waveforms are shown in Figure 6(b), representing the predicted load rate of transformer T2. The value is approximately 0.46, which is basically consistent with the actual value. VSC1 and VSC2 output power. P VSCn As shown in Figure 6(c), P VSC1 It is 135.5kW. P VSC2 The value is -125.9kW. The waveform of the DC side voltage superposition of the load factor secondary control VSC1 is shown in Figure 6(d). u offset The voltage is 15V. From equation (8), the DC side voltages of VSC1 and VSC2 can be obtained. u dc The theoretical values ​​are 769V and 754V respectively, and the DC side voltages of VSC1 and VSC2 are... u dcn The waveforms are shown in Figure 6(e), representing the DC side voltages of VSC1 and VSC2. u dcThe actual values ​​are approximately 770V and 755V respectively, which are within the allowable error range. Figure 6 demonstrates that the non-communication power coordination control method can quantitatively control the load rate of transformers in interconnected distribution areas, thereby achieving power coordination control in interconnected distribution areas.

[0044] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention shall be within the scope of protection of the pending claims of the present invention.

Claims

1. A method for communication-free power coordination control of a two-unit flexible interconnection system, characterized in that, Includes the following steps: Step S1: Measure the output voltage and current of the low-voltage side of each transformer in each distribution area, calculate the load rate of each transformer in real time, and multiply the transformer load rate and droop coefficient into the DC voltage control of the voltage source converter to obtain the DC voltage command value. Step S2: Obtain the DC side voltage, DC transmission current and DC transmission line equivalent impedance of one transformer substation, and obtain the load rate prediction value of the other transformer substation based on the DC voltage command value. Step S3: Select one of the voltage source converters as the secondary control terminal for load rate, obtain the DC voltage command superimposed on the secondary control based on the load rate prediction value, and thus carry out the non-communication power coordination control of the two-unit flexible interconnection system.

2. The method for coordinated control of communication power in a flexible interconnection system between two districts according to claim 1, characterized in that, In step S1, when the load rates of the transformers in each distribution area are approximately equal, the DC tie line does not transmit power.

3. The method for coordinated control of communication power in a flexible interconnection system between two districts according to claim 2, characterized in that, When determining the droop coefficient value for load rate droop control, each transformer area is at a high load rate and the flexible interconnection system does not experience over-modulation.

4. The method for coordinated control of communication power in a flexible interconnection system between two districts according to claim 1, characterized in that, After the introduction of load factor droop control, the DC voltage command value is used to generate a modulation reference voltage through voltage and current dual closed-loop control. After sinusoidal pulse width modulation, a pulse width modulation signal is generated to drive the switching device.

5. The method for coordinated control of communication power in a flexible interconnection system between two districts according to claim 1, characterized in that, In step S2, control error is not considered during the calculation of the load rate prediction value.

6. The method for coordinated control of communication power in a flexible interconnected system of two districts according to claim 1, characterized in that, In step S3, the load distribution coefficient for each transformer substation is defined. The load distribution coefficient is flexibly adjusted according to the capacity configuration, load characteristics and operating conditions of the transformer substation.

7. The method for coordinated control of communication power in a flexible interconnection system between two districts according to claim 1, characterized in that, In step S3, when the load rate secondary control of the two-area flexible interconnection system tends to stabilize, the secondary acquisition of DC voltage command value controls the DC side voltage of each area voltage source converter.

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