Solid-state transformer voltage supporting capability enhancement strategy based on energy storage system and load control

Abstract

The invention discloses a solid-state transformer voltage supporting capacity enhancing strategy based on an energy storage system and load control, and relates to the technical field of power electronics. The voltage supporting capacity of the solid-state transformer is limited by the rated capacity of the solid-state transformer, and when the active output of the solid-state transformer is large or deep voltage sag occurs, the reactive injection capacity of the solid-state transformer is small, and the voltage supporting capacity is weak. By utilizing the energy storage system at the low-voltage direct-current side of the solid-state transformer and controlling the voltage at the low-voltage alternating-current side, when asymmetric voltage sag occurs at the medium-voltage alternating-current side of the solid-state transformer, the current at the medium-voltage alternating-current side is ensured not to exceed a limit value, the active power fluctuation at the medium-voltage alternating-current side is inhibited, and the voltages at the medium-voltage direct-current side and the low-voltage direct-current side are kept stable; on the basis, the asymmetric voltage supporting capacity of the medium-voltage alternating current side is improved.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and more specifically to a strategy for enhancing the voltage support capability of solid-state transformers based on energy storage systems and load control. Technical Background

[0002] Voltage sag is a serious power quality problem that has plagued and affected normal production in recent years. It causes tens of thousands of yuan in economic losses annually and has become a focal point of power system power quality issues. Solid-state transformers (SMTs) are a potential method to improve voltage sag in power systems. The basic idea behind using SMTs to address voltage sag is to inject reactive power into nodes using SMTs, thereby achieving voltage support.

[0003] However, the reactive power injection capacity of solid-state transformers is limited by their rated capacity. When the load is heavy or a deep voltage sag occurs, the reactive power injection capacity of solid-state transformers is small and insufficient to support the voltage. Therefore, it is necessary to increase the reactive power injection capacity of solid-state transformers to increase their voltage support capability.

[0004] The invention described in patent number CN119891408A proposes an active voltage support method for solid-state transformers. This method adaptively calculates the reactive power-voltage droop coefficient at each port of the solid-state transformer and utilizes the residual reactive power capacity on the AC side of the transformer for voltage support. However, when the load is heavy or a deep voltage sag occurs, the residual reactive power capacity on the AC side of the solid-state transformer is relatively small, resulting in weak voltage support capability. Furthermore, this invention is not applicable to asymmetrical voltage sags, which are the most frequent type of voltage sag. Summary of the Invention

[0005] To address the shortcomings and problems of existing technologies, this invention provides a strategy for enhancing the voltage support capability of solid-state transformers based on energy storage systems and load control. The purpose is to ensure that the current on the medium-voltage AC side does not exceed the limit value when an asymmetrical voltage sag occurs on the medium-voltage AC side of the solid-state transformer, suppress the active power fluctuation on the medium-voltage AC side, maintain the voltage on the medium-voltage DC side and the low-voltage DC side, and on this basis, increase the asymmetrical voltage support capability on the medium-voltage AC side.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] Step 1: Calculate the positive sequence voltage amplitude on the medium-voltage AC side. If positive sequence voltage Greater than or equal to voltage threshold At that time, that is Then the solid-state transformer will operate normally; if Solid-state transformers start-up voltage support mode injects positive sequence reactive current into the medium-voltage AC side. Proceed to step 2.

[0008] Step 2, Medium-voltage AC power reference value Set as medium-voltage DC side power reference value Low-voltage AC side power Low-voltage DC side power The sum, that is Calculate the positive sequence active current amplitude and positive sequence reactive current amplitude Load control parameters Set to 1, power reference value for energy storage system Set to 0, calculate the maximum current amplitude on the medium-voltage AC side. ,like Less than or equal to the current limit value ,Right now Then calculate the reference value of the medium-voltage AC side current. and Calculate the reference value of the low-voltage AC side voltage. ;like Proceed to step 3.

[0009] Step 3: Calculate the maximum positive sequence active current amplitude. ,calculate Corresponding medium-voltage AC power reference value ,like ,but Calculate the positive sequence active current amplitude Calculate the maximum positive sequence reactive current amplitude. Calculate the positive sequence reactive current amplitude Load control parameters Set as Energy storage system power reference value Calculate the reference value of the medium-voltage AC side current. and Calculate the reference value of the low-voltage AC side voltage. ;like Proceed to step 4.

[0010] Step 4, if Calculate the positive sequence active current amplitude and positive sequence reactive current amplitude Load control parameters Set as Energy storage system power reference value Calculate the reference value of the medium-voltage AC side current. and Calculate the reference value of the low-voltage AC side voltage. ;like Proceed to step 5.

[0011] Step 5, calculate load control parameters ,like Medium-voltage AC power reference value Calculate the positive sequence active current amplitude and positive sequence reactive current amplitude Load control parameters Set as Energy storage system power reference value Set to 0, calculate the reference value of the medium-voltage AC side current. and Calculate the reference value of the low-voltage AC side voltage. ;like Proceed to step 6.

[0012] Step 6: Calculate the positive sequence active current amplitude. and positive sequence reactive current amplitude Load control parameters Set as Energy storage system power reference value Calculate the reference value of the medium-voltage AC side current. and Calculate the reference value of the low-voltage AC side voltage. .

[0013] Specifically, in steps 2 to 6, the reference value of the medium-voltage AC side current is calculated. and for:

[0014] ,

[0015] in, This represents the negative sequence voltage amplitude on the medium-voltage AC side. , These represent the positive-sequence voltage and negative-sequence voltage on the AC side of the α-axis in the αβ coordinate system. , These represent the positive-sequence voltage and negative-sequence voltage on the AC side of the β-axis in the αβ coordinate system. This represents the amplitude of the negative sequence active current on the medium-voltage AC side. The amplitude of the negative sequence reactive current on the medium-voltage AC side is as follows:

[0016] .

[0017] Specifically, in steps 2 to 6, the reference value of the low-voltage AC side voltage is calculated. for:

[0018] ,

[0019] in, This is the initial reference value for the low-voltage AC side voltage.

[0020] Specifically, in step 1, positive-sequence reactive current is injected into the medium-voltage AC side. The expression is:

[0021] ,

[0022] in, This represents the maximum injectable positive-sequence reactive current. The voltage threshold that supports the start-up voltage. For injection Voltage threshold.

[0023] Specifically, in step 2, the maximum current amplitude on the medium-voltage AC side is calculated. for:

[0024] ,

[0025] in, .

[0026] Specifically, in step 2, the amplitude of the positive-sequence active current is calculated. and positive sequence reactive current amplitude for:

[0027] .

[0028] Specifically, in step 3, the maximum positive sequence active current amplitude is calculated. for:

[0029] .

[0030] Specifically, in step 3, the calculation is performed. Corresponding medium-voltage AC power reference value for:

[0031] .

[0032] Specifically, in step 3, the maximum positive sequence reactive current amplitude is calculated. for:

[0033] ;

[0034] Specifically, in step 3, the amplitude of the positive-sequence reactive current is calculated. for:

[0035] .

[0036] Specifically, in step 4, the positive-sequence active current amplitude is calculated. and positive sequence reactive current amplitude for:

[0037] .

[0038] Specifically, in step 5, the load control parameters are calculated. for:

[0039] .

[0040] In summary, through the above six steps, when an asymmetrical voltage sag occurs on the medium-voltage AC side of the solid-state transformer, the present invention ensures that the current on the medium-voltage AC side does not exceed the limit value by means of the energy storage system on the low-voltage DC side of the solid-state transformer and by controlling the load voltage on the low-voltage AC side. It also effectively avoids active power fluctuations on the medium-voltage AC side and voltage fluctuations on the medium-voltage DC side. On this basis, it increases the reactive current injection capacity on the medium-voltage AC side, thereby increasing the asymmetrical voltage support capability on the medium-voltage AC side. Attached Figure Description

[0041] Figure 1 This is a block diagram of the overall control of a solid-state transformer. Figure 2 Diagram showing the positive sequence reactive current injection on the medium-voltage AC side of a solid-state transformer; Figure 3 A strategy to enhance the voltage support capability of solid-state transformers based on energy storage systems and load control. Detailed Implementation

[0042] To make the basic principles, technical solutions, and advantages of this invention clearer, a detailed description of a solid-state transformer voltage support capability enhancement strategy based on energy storage systems and load control, as involved in this invention, is provided below. It should be understood that the following description is merely exemplary and not intended to limit the scope of protection or application of this invention.

[0043] Figure 1 This is a block diagram of the overall control of a solid-state transformer. , These are the reference and actual values ​​for medium-voltage DC. For the common coupling point voltage, This refers to the AC current on the medium-voltage side of the solid-state transformer. , These are the reference and actual values ​​for low-voltage DC voltage. This is the low-voltage AC side voltage. This refers to the low-voltage AC side current. The solid-state transformer consists of an input stage, an isolation stage, and an output stage. The input stage is an AC / DC converter, connected to the medium-voltage AC grid, controlling the input stage power transmission and stabilizing the medium-voltage DC side voltage. The isolation stage is a DC / DC converter, connecting the medium-voltage DC side and the low-voltage DC side, controlling the isolation stage power transmission and stabilizing the low-voltage DC side voltage. The output stage is a DC / AC converter, connected to the low-voltage AC grid, controlling the output stage power transmission, employing grid-type control to stabilize the low-voltage AC side voltage.

[0044] Figure 2 The positive sequence reactive current on the medium-voltage AC side of the solid-state transformer Injection diagram, as shown in the figure, positive sequence reactive current The injection is caused by positive sequence voltage. This is determined when the positive sequence voltage is less than the voltage threshold. At that time, the solid-state transformer injects positive-sequence reactive current into the medium-voltage power grid. To support positive sequence voltage When the positive sequence voltage Less than the voltage threshold At that time, the positive-sequence reactive current reaches its maximum value. ,when hour, Therefore, positive sequence reactive current is injected into the medium-voltage AC side. The expression is:

[0045]

[0046] Figure 3 The following details the strategies for enhancing the voltage support capability of solid-state transformers based on energy storage systems and load control.

[0047] ① Calculate the positive sequence voltage on the medium-voltage AC side ,judge .

[0048] If it is false, then it is case 1, which is the normal operating mode, as indicated by the formula. Calculate the reference value for medium-voltage AC power :

[0049]

[0050] Through formula Calculate the reference value for medium-voltage AC power The corresponding calculated positive-sequence active current amplitude :

[0051]

[0052] Set the positive sequence reactive current amplitude =0, through formula Calculate the reference value of the medium-voltage AC side current. and :

[0053]

[0054] in,

[0055]

[0056] Set load control parameters Set to 1, using the formula Calculate the reference value of the low-voltage AC side voltage. :

[0057]

[0058] Energy storage system power reference value Set as At this time, the solid-state transformer charges the energy storage system.

[0059] If true, proceed to step ②.

[0060] ② Solid-state transformer start-up voltage support mode, according to formula Injecting positive sequence reactive current into the medium-voltage AC side Set the reference value for medium-voltage AC power. According to the formula Calculate the positive sequence active current amplitude At this time, the positive sequence reactive current amplitude Through the formula Calculate the maximum current amplitude on the medium-voltage AC side. :

[0061]

[0062] judge .

[0063] If true, then it is case 2, obtained through the formula. Calculate the reference value of the medium-voltage AC side current. and According to the formula Calculate the reference value of the low-voltage AC side voltage. Set load control parameters Set to 1, power reference value for energy storage system Set to 0.

[0064] If false, proceed to step ③.

[0065] ③ Through-type Computational injection Maximum active current amplitude after :

[0066]

[0067] Through calculate Corresponding medium-voltage AC power reference value :

[0068]

[0069] judge .

[0070] If true, then it is case 3, set Through the formula Calculate the positive sequence active current amplitude Through the formula Calculate the maximum positive sequence reactive current :

[0071]

[0072] Set the positive sequence reactive current amplitude Maximum positive sequence reactive current Through the formula Calculate the reference value of the medium-voltage AC side current. and Load control parameters Set as minimum load control parameter Through the formula Calculate the reference value of the low-voltage AC side voltage. Set the power reference value for the energy storage system. At this point, the energy storage system begins to supply power to the low-voltage side load.

[0073] If false, proceed to step ④.

[0074] ④Judgment .

[0075] If true, then it is case 4, setting the positive sequence active current amplitude. Maximum active current amplitude Set the positive sequence reactive current amplitude for Through the formula Calculate the reference value of the medium-voltage AC side current. and Load control parameters Set as minimum load control parameter Through the formula Calculate the reference value of the low-voltage AC side voltage. Set the power reference value for the energy storage system. .

[0076] If false, proceed to step ⑤.

[0077] ⑤ According to the formula Calculate load control parameters :

[0078]

[0079] judge .

[0080] If true, then it is case 5, setting the reference value for medium-voltage AC power. According to the formula Calculate the positive sequence active current amplitude Set the positive sequence reactive current amplitude for Through the formula Calculate the reference value of the medium-voltage AC side current. and Load control parameters Set as style Calculated Through the formula Calculate the reference value of the low-voltage AC side voltage. Energy storage system power reference value Set to 0.

[0081] If false, proceed to step 6.

[0082] ⑥ This is case 6; set the positive sequence active current amplitude. Maximum active current amplitude Set the positive sequence reactive current amplitude for Through the formula Calculate the reference value of the medium-voltage AC side current. and Load control parameters Set as minimum load control parameter Through the formula Calculate the reference value of the low-voltage AC side voltage. Set the power reference value for the energy storage system. .

[0083] The above description illustrates a specific embodiment and advantages of the present invention, but the scope of protection of the present invention is not limited thereto. For those skilled in the art, variations and modifications can be made to the above embodiments without fundamentally departing from the technical spirit and principles described herein, and these variations and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A strategy for enhancing the voltage support capability of solid-state transformers based on energy storage systems and load control, characterized in that, When an asymmetrical voltage sag occurs on the AC side of a solid-state transformer, the method ensures that the current on the AC side does not exceed the limit, suppresses active power fluctuations on the AC side, maintains the voltage on the DC side and the low-voltage DC side, and increases the asymmetrical voltage support capability on the AC side. This method includes the following steps: Step 1: Calculate the positive sequence voltage amplitude on the medium-voltage AC side. ,like Solid-state transformers start-up voltage support mode injects positive sequence reactive current into the medium-voltage AC side. ; Step 2, Medium-voltage AC power reference value Set as medium-voltage DC side power reference value Low-voltage AC side power Low-voltage DC side power The sum, that is Calculate the positive sequence active current amplitude and positive sequence reactive current amplitude Load control parameters Set to 1, power reference value for energy storage system Set to 0, calculate the maximum current amplitude on the medium-voltage AC side. ,like Less than or equal to the current limit value ,Right now Then calculate the reference value of the medium-voltage AC side current. and Calculate the reference value of the low-voltage AC side voltage. ;like Proceed to step 3; Step 3: Calculate the maximum positive sequence active current amplitude. ,calculate Corresponding medium-voltage AC power reference value ,like ,but Calculate the positive sequence active current amplitude Calculate the maximum positive sequence reactive current amplitude. Calculate the positive sequence reactive current amplitude Load control parameters Set as Energy storage system power reference value Calculate the reference value of the medium-voltage AC side current. and Calculate the reference value of the low-voltage AC side voltage. ; Proceed to step 4; Step 4, if Calculate the positive sequence active current amplitude and positive sequence reactive current amplitude Load control parameters Set as Energy storage system power reference value Calculate the reference value of the medium-voltage AC side current. and Calculate the reference value of the low-voltage AC side voltage. ;like Proceed to step 5; Step 5, calculate load control parameters ,like Medium-voltage AC power reference value Calculate the positive sequence active current amplitude and positive sequence reactive current amplitude Load control parameters Set as Energy storage system power reference value Set to 0, calculate the reference value of the medium-voltage AC side current. and Calculate the reference value of the low-voltage AC side voltage. ;like Proceed to step 6; Step 6: Calculate the positive sequence active current amplitude. and positive sequence reactive current amplitude Load control parameters Set as Energy storage system power reference value Calculate the reference value of the medium-voltage AC side current. and Calculate the reference value of the low-voltage AC side voltage. .

2. The strategy for enhancing the voltage support capability of a solid-state transformer based on an energy storage system and load control as described in claim 1, characterized in that, In steps 2 to 6, the reference value of the medium-voltage AC side current is calculated. and Specifically: ， in, This represents the negative sequence voltage amplitude on the medium-voltage AC side. , These represent the positive-sequence voltage and negative-sequence voltage on the AC side of the α-axis in the αβ coordinate system. , These represent the positive-sequence voltage and negative-sequence voltage on the AC side of the β-axis in the αβ coordinate system. This represents the amplitude of the negative sequence active current on the medium-voltage AC side. The amplitude of the negative sequence reactive current on the medium-voltage AC side is as follows: ； In steps 2 to 6, the reference value of the low-voltage AC side voltage is calculated. Specifically: ， in, This is the initial reference value for the low-voltage AC side voltage.

3. The strategy for enhancing the voltage support capability of a solid-state transformer based on an energy storage system and load control as described in claim 1, characterized in that, In step 2, the maximum current amplitude on the medium-voltage AC side is calculated. Specifically: ， in, ; In step 2, the positive sequence active current amplitude is calculated. and positive sequence reactive current amplitude Specifically: 。 4. The strategy for enhancing the voltage support capability of a solid-state transformer based on an energy storage system and load control as described in claim 1, characterized in that, In step 3, the maximum positive sequence active current amplitude is calculated. Specifically: ； In step 3, the calculation Corresponding medium-voltage AC power reference value Specifically: ； In step 3, the maximum positive sequence reactive current amplitude is calculated. Specifically: ； In step 3, the positive sequence reactive current amplitude is calculated. Specifically: 。 5. The strategy for enhancing the voltage support capability of a solid-state transformer based on an energy storage system and load control as described in claim 1, characterized in that, In step 4, the positive sequence active current amplitude is calculated. and positive sequence reactive current amplitude Specifically: 。 6. The strategy for enhancing the voltage support capability of a solid-state transformer based on an energy storage system and load control as described in claim 1, characterized in that, In step 5, the load control parameters are calculated. Specifically: 。

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