Boosting voltage gear selection and checking method for ultra-high voltage direct current power system matched with thermal power access system

Abstract

The present disclosure belongs to the technical field of power systems, and provides a method for selecting and checking the voltage-rising transformer gear of a UHVDC supporting thermal power access system, comprising the following steps: obtaining the static excitation characteristic curve of a thermal power unit; judging whether the unit is in phase-advance operation or phase-delay operation based on the obtained static excitation characteristic curve, the determination condition of unit phase-advance and the determination condition of unit phase-delay, and selecting the voltage-rising transformer gear according to the gear selection condition; checking the selected voltage-rising transformer gear during the power plant transmission line reverse power transmission stage and the unit commissioning stage, and improving the reactive power voltage level in the UHVDC near area. The present disclosure analyzes the risks existing in the strong and weak alternating network structure, and verifies the effectiveness and authenticity of the proposed method combined with actual engineering, thereby ensuring the reactive power voltage level in the UHVDC near area and the safe and stable operation of the system.

Description

Technical Field

[0001] This disclosure belongs to the field of power system technology, specifically relating to a method for selecting and verifying the tap position of a step-up transformer in an ultra-high voltage direct current (UHVDC) system connected to a thermal power plant. Background Technology

[0002] The statements in this section are merely background information relating to this disclosure and do not necessarily constitute prior art.

[0003] my country's primary energy reserves and demand are inversely distributed. Ultra-high voltage (UHV) AC and DC power grids, suitable for long-distance, large-capacity transmission, are crucial for meeting clean energy consumption and energy conservation and emission reduction needs. With the construction of new power systems, UHV transmission has effectively promoted regional coordination and economic and social development. Currently, the power grid has formed a new UHV AC / DC hybrid structure, which the power engineering and academic communities have summarized as having a "strong DC, weak AC" characteristic. In a strong DC, weak AC system, the strength of the AC grid is related to the system's safety and stability, and its impact on system fault patterns, regulation capabilities, and stability is becoming increasingly complex. In recent years, the impact of power plant grid connection on the system has received considerable attention, and the rational configuration of the step-up transformer at power plants directly affects the voltage level of nearby power grids, making it an important consideration in formulating voltage operation curves.

[0004] According to the inventor, the selection of the step-up transformer gear position for the unit is currently mainly determined or optimized based on the constraint of system voltage stability. Related research mainly focuses on the impact of transformer gear adjustment on system voltage after the unit is put into operation. However, since the step-up transformer gear position of the thermal power unit is determined during the reverse power transmission stage, and when the voltage regulation mode of the unit's step-up transformer is no-load voltage regulation, gear adjustment requires power outage operation, and frequent gear adjustment is not advisable. Summary of the Invention

[0005] To address the aforementioned issues, this disclosure proposes a method for selecting and verifying the tap position of the step-up transformer in an ultra-high voltage direct current (UHVDC) grid connected to a thermal power plant. Based on the strong DC / weak AC grid structure, the risks associated with such a grid are analyzed. A method for rationally selecting the operating tap position of the main transformer at the step-up substation of an UHVDC-connected thermal power plant is proposed, and recommended tap positions are provided. The effectiveness and authenticity of the proposed method are verified through practical engineering projects, ensuring the reactive voltage level in the near-area of ​​the UHVDC system and its safe and stable operation.

[0006] According to some embodiments, the present disclosure provides a method for selecting and verifying the tap position of a step-up transformer in an ultra-high voltage direct current (UHVDC) system connected to a thermal power plant, employing the following technical solution:

[0007] A method for selecting and verifying the tap position of a step-up transformer in an ultra-high voltage direct current (UHVDC) system connected to a thermal power plant includes the following steps:

[0008] Obtain the static excitation characteristic curve of the thermal power unit;

[0009] Based on the obtained static excitation characteristic curve, the determination conditions for the unit's leading phase and the determination conditions for the unit's lagging phase, it is determined whether the unit is in leading phase operation or lagging phase operation, and the booster shift position is selected according to the shift selection conditions.

[0010] During the reverse power transmission phase of the power plant's outgoing lines and the commissioning phase of the generating units, the selected step-up transformer position is verified to improve the reactive voltage level of the UHVDC near-area.

[0011] As a further technical limitation, the wiring of the transmission lines from the UHVDC-connected thermal power plants adopts a 3 / 2 wiring method.

[0012] As a further technical limitation, during the reverse power transmission phase, the unit steps up the voltage to supply power to the transformer; during the unit commissioning phase, the plant auxiliary load is transferred from the generator.

[0013] As a further technical limitation, the UHV-connected thermal power plant access system needs to consider the interaction between the tap position selection of the power plant step-up transformer and the system bus voltage fluctuation, generator terminal voltage, plant load voltage, unit leading phase capability, and unit lagging phase capability.

[0014] As a further technical limitation, the determination condition for the leading phase of the unit is as follows: Among them, Q in Q represents the leading phase capability of the generator set. inmax For the unit's maximum phase advance capability, U p The voltage for supplying power to the power plant's auxiliary load, U pmin U is the lower limit of the power supply voltage for plant loads. e U is the generator terminal voltage. emin U is the lower limit of the generator terminal voltage. emax U is the upper limit of the generator terminal voltage. g This is the system bus voltage.

[0015] As a further technical limitation, the determination condition for the unit's delayed phase is as follows: Among them, Q out Q represents the generator set's delayed phase capability. outmax For the unit's maximum late-phase capability, U p The voltage for supplying power to the power plant's auxiliary load, U pmax U is the upper limit of the power supply voltage for plant auxiliary loads. e U is the generator terminal voltage. emax U is the upper limit of the generator terminal voltage. g U is the system bus voltage. gmin This is the lower limit of the system bus voltage.

[0016] As a further technical constraint, to prioritize system voltage stability, data in the reference tables for different voltage levels that simultaneously meet the criteria for both unit phase advance and unit phase lag are considered valid data. Other data should be deleted or disregarded. A pairwise comparison should be performed from top to bottom. When comparing the two voltage levels separately, to ensure that the unit can maximize its phase advance capability, the minimum number of filter groups required to maintain unit phase advance at that voltage level should be as small as possible. At the same time, when comparing data in the reference tables with the same number of filter groups, if the generator terminal voltage reaches the upper limit or the lower limit, the voltage level with the higher system voltage should be selected as the power plant's step-up transformer voltage level.

[0017] As a further technical constraint, during the reverse power transmission phase of the power plant's outgoing lines, the recommended tap position of the power plant's step-up transformer is verified by using an empty charging plant outgoing line, a energized step-up transformer, and power supply from the plant's auxiliary load.

[0018] Furthermore, when the power plant's step-up transformer is in gear 4, the system voltage remains stable during the commissioning process.

[0019] As a further technical limitation, during the commissioning phase of the unit, when the system voltage rises, voltage regulation measures are taken to maintain the voltage within a reasonable range and meet the requirements of the operating curve. The power plant unit releases its leading-phase capacity to absorb excessive reactive power in the system. When the boost transformer is in the 4th gear and the system voltage rises, the unit releases its leading-phase capacity.

[0020] Compared with the prior art, the beneficial effects of this disclosure are as follows:

[0021] This disclosure comprehensively analyzes the impact of the access of near-area supporting power sources to the strong DC-weak AC power grid on system voltage fluctuations, studies the method and verification method for rationally selecting the tap position of the main transformer of the power plant step-up station, analyzes the impact of the power plant start-up combination on system voltage and the impact of different DC faults on AC system voltage fluctuations, and comprehensively considers the maximum phase advance capability of the near-area power plant units of the strong DC-weak AC power grid, the power supply voltage level of the plant auxiliary load and the system voltage level, recommends that the step-up transformer of the DC supporting power plant be operated at tap 4. Attached Figure Description

[0022] The accompanying drawings, which form part of this disclosure, are used to provide a further understanding of this disclosure. The illustrative embodiments of this disclosure and their descriptions are used to explain this disclosure and do not constitute an undue limitation of this disclosure.

[0023] Figure 1 This is a flowchart of the method for selecting and verifying the step-up transformer gear position in the UHVDC-supported thermal power grid connection system according to an embodiment of this disclosure;

[0024] Figure 2 This is a schematic diagram of the unit access system in an embodiment of this disclosure;

[0025] Figure 3This is a schematic diagram of a typical topology within an ultra-high voltage-supported thermal power plant in an embodiment of this disclosure.

[0026] Figure 4 This is a schematic diagram illustrating the specific working principle of the step-up transformer range selection and verification method for the UHVDC-supported thermal power grid connection system in this embodiment of the present disclosure.

[0027] Figure 5 This is a schematic diagram of the strong vertical and weak cross-section grid structure in the embodiments of this disclosure;

[0028] Figure 6 This is a schematic diagram of the system voltage in an embodiment of this disclosure;

[0029] Figure 7 This is a schematic diagram of the reactive power of the unit in an embodiment of this disclosure. Detailed Implementation

[0030] The present disclosure will be further described below with reference to the accompanying drawings and embodiments.

[0031] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this disclosure. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains.

[0032] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this disclosure. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms “comprising” and / or “including” are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0033] Where there is no conflict, the embodiments and features described herein can be combined with each other.

[0034] Example

[0035] This disclosure describes a method for selecting and verifying the step-up transformer gear in an ultra-high voltage direct current (UHVDC) system connected to a thermal power plant.

[0036] like Figure 1 The method for selecting and verifying the tap position of a step-up transformer in an ultra-high voltage direct current (UHVDC) system connected to a thermal power plant includes the following steps:

[0037] Obtain the static excitation characteristic curve of the thermal power unit;

[0038] Based on the obtained static excitation characteristic curve, the determination conditions for the unit's leading phase and the determination conditions for the unit's lagging phase, it is determined whether the unit is in leading phase operation or lagging phase operation, and the booster shift position is selected according to the shift selection conditions.

[0039] During the reverse power transmission phase of the power plant's outgoing lines and the commissioning phase of the generating units, the selected step-up transformer position is verified to improve the reactive voltage level of the UHVDC near-area.

[0040] Specifically, this embodiment illustrates the impact of power source access on the system, analyzes the coupling relationship between the tap position of the power plant's step-up transformer and reactive power voltage, proposes a method for selecting the tap position of the power plant's step-up transformer during the reverse power transmission stage, and, in conjunction with the "strong DC, weak AC" grid structure in Northwest China, calculates and analyzes the reasonable tap position of the step-up transformer for DC near-area thermal power units connected to the system. The effectiveness and practicality of this method are verified through actual engineering verification. This is of great significance for fully utilizing the voltage support capability of thermal power units for the system, ensuring the reactive power voltage level in the near-area of ​​UHVDC, and ensuring the safe and stable operation of the system.

[0041] 1. Voltage fluctuations in the power supply system

[0042] When there are relatively few DC power supply points in the vicinity, and conventional thermal power units are connected to the UHV transmission system, the operating level of the main transformer at the step-up substation directly affects the voltage level at the grid connection point. An excessively high operating level will strongly raise the system voltage level, causing the converter station bus voltage to operate close to the upper limit. In the event of a DC system fault, the power transfer will have a huge impact on the sending-end AC system, which can easily cause line tripping. Since the DC system in operation needs to absorb a large amount of reactive power, the DC converter station needs to be equipped with a large number of reactive power compensation devices.

[0043] Within 200ms after a DC circuit is blocked due to a fault, the reactive power compensation is not completely removed. It generally takes about 200ms for it to be completely removed. A large amount of reactive power surplus will impact the AC system and cause voltage stability problems. The voltage rise of the sending-end bus after a DC circuit is blocked is shown in equation (1):

[0044]

[0045] Where ΔQ represents the reactive power fluctuation before and after the fault; and S represents the AC system capacity.

[0046] It is evident that the larger ΔQ is, the more severe the transient voltage rise in the system after a DC blocking fault. A reasonable selection of the booster shift position in the power plant can maximize the release of the unit's leading phase capability, thereby absorbing the reactive power surplus generated by the DC fault, reducing the ΔQ value, and helping to suppress system voltage fluctuations.

[0047] 2. Voltage adjustment principle with boost and gear shifting

[0048] Typically, power plant thermal power units step up the voltage to a two-winding transformer. By changing the transformer tap position, the turns ratio of the primary and secondary windings is correspondingly altered, thereby achieving voltage regulation. Figure 2 The diagram shown is a schematic of a thermal power unit connected to the system via a step-up transformer.

[0049] Let the voltage on the low-voltage side of the step-up transformer be the terminal voltage U. e The high-voltage side voltage is the system voltage U. g When the generator outputs power P+Qj, considering the voltage drop of the step-up transformer, the generator terminal voltage and the system voltage have the following relationship:

[0050]

[0051] Among them, R T X T Let be the transformer branch impedance. If the transformer branch resistance is ignored, the above equation can be rewritten as the relationship between voltage and reactive power output of the generator unit:

[0052]

[0053] When only the relationship between reactive power and voltage is considered, equation (3) can be further simplified to:

[0054]

[0055] Let the transformer ratio of the unit be k = U g / U e ,but:

[0056]

[0057] As can be seen from formula (5), there is a coupling relationship between the unit's reactive power output, the generator terminal voltage, and the system voltage. When the system voltage or the unit voltage remains constant, adjusting the voltage ramp will correspondingly change the unit's reactive power output. In engineering practice, adjustments should be made according to actual needs. The following will further explain the method for adjusting the unit's voltage ramp.

[0058] 3. Selection of main transformer tap position in power plant step-up substation

[0059] Typical wiring configurations for UHV transmission lines from thermal power plants include: Figure 3 As shown, the wiring adopts a 3 / 2 connection method. The power plant unit's step-up transformer is connected to bus #1 and bus #2 via side and center switches. The standby transformer wiring, high-voltage parallel reactor wiring, and power plant outgoing lines (outgoing line I and outgoing line II) are led out via the side and center switches. The plant service transformer is connected through the low-voltage side of the unit's step-up transformer. During the reverse power feeding phase, the unit's step-up transformer supplies power to the plant service transformer, while during the unit's commissioning phase, the plant service load can be transferred by the generator. Therefore, the tap position of the unit's step-up transformer directly affects the power quality of the plant service transformer.

[0060] According to GB / T40427-2021 "Technical Guidelines for Voltage and Non-Stop Power Supply in Power Systems", under normal operating conditions, the highest operating voltage of a 500kV busbar should not exceed +10% of the system's nominal voltage, and the lowest operating voltage should not affect the synchronous stability, voltage stability, normal use of plant auxiliary power, or regulation of the next-level voltage in the power system. Therefore, the selection of the tap position of the step-up transformer must comprehensively consider its impact on system voltage and plant auxiliary power.

[0061] The specific working principle diagram of the step-up transformer tap selection and verification method for UHVDC-supported thermal power grid connection system is as follows: Figure 4 As shown, the specific process is as follows:

[0062] Thermal power units typically possess the capability for both leading-phase and lagging-phase operation. This capability can be assessed by examining the static excitation characteristic curve (Q curve) in the generator's factory test report. Due to the unique structure of the UHVDC-AC grid, the impact of power plant grid connection on system voltage must be considered, especially in the UHVDC near-field region. Faults such as DC blocking can easily trigger transient overvoltages in the system. In such cases, thermal power units must fully utilize their voltage support capability for the AC system, releasing their maximum leading-phase capability to absorb excessive reactive power generated during system fluctuations and suppress system voltage fluctuations. Simultaneously, to ensure stable power supply voltage to the plant's auxiliary loads, the voltage fluctuation on the low-voltage side of the auxiliary transformer must be controlled to not exceed its allowable range (typically ±5%), and the generator terminal voltage must be within the unit's tolerance range. Thermal power plant step-up transformers typically have multiple taps, and the selection of these taps directly affects the constraints imposed on generator grid connection under the aforementioned conditions.

[0063] Therefore, when connecting ultra-high voltage power plants to the grid, it is necessary to consider the interaction between the selection of the tap position of the power plant's step-up transformer and the system bus voltage fluctuation, generator terminal voltage, plant load voltage, and the unit's leading and lagging phase capabilities.

[0064] When the unit is operating in the leading phase, conditions (6) to (9) must be met:

[0065]

[0066] When the unit operates in a delayed phase, conditions (10) to (13) must be met:

[0067]

[0068] Among them, Q in Q represents the leading phase capability of the generator set. inmax Q represents the unit's maximum phase advance capability. out Q represents the generator set's delayed phase capability. outmax U represents the unit's maximum late-phase capability, measured in MVA. p The voltage for supplying power to the power plant's auxiliary load, Upmin U is the lower limit of the power supply voltage for plant loads. pmax The upper limit of the power supply voltage for plant auxiliary loads, in kV; U e U is the generator terminal voltage. emin U is the lower limit of the generator terminal voltage. emax U represents the upper limit of the generator terminal voltage, in kV. g U is the system bus voltage. gmin U is the lower limit of the system bus voltage. gmax This represents the upper limit of the system bus voltage, in kV.

[0069] Ultra-high voltage direct current converter stations are typically equipped with a certain number of filters, capacitors, and reactors. Besides filtering, the filters are primarily used for voltage regulation. When using filters for voltage regulation, let N be the number of filter banks in operation. f ∈{1,2,3,...,k}, where k is the maximum number of filter groups in the converter station, and let the step-up transformer position of the power plant be T. i ∈{1,2,3,...,n max At this point, the Q value under the boost shift position can be obtained by power flow calculation when conditions (6) to (13) are met. in (or Q) out ), U p U e U g Calculate and record different N values ​​sequentially. f and T i Given the parameters under the given conditions, the boost shifter selection table can be obtained. i , i∈{1,2,3,...,n max As shown in Table 1 below:

[0070] Table 1 i Schematic table

[0071]

[0072] According to TABLE i The unit can be selected for voltage boosting and shifting. To prioritize system voltage stability, the TABLE... i Only data sets that simultaneously meet conditions (6) to (13) are considered valid data, and must also meet the following condition (14):

[0073] Condition (14): Compare the front and back pairs from top to bottom. To ensure that the unit can maximize its leading phase capability, the minimum number of filter groups required to maintain the unit's leading phase at this setting should be as small as possible. At the same time, TABLE i When the number of filter banks is the same, when the terminal voltage U eWhen the upper limit (corresponding to the unit being late) or the lower limit (corresponding to the unit being advanced) is reached, the system voltage U is selected first. g The higher gear is the power plant's boost converter gear.

[0074] 4. Simulation Case Analysis

[0075] YKZHLZ, as the sending-end converter station of the ±800kV ZY DC transmission project, is connected to the Northwest main grid via three 750kV lines through the SH UHV substation, forming a regional DC cluster together with the YZ and LS DC lines. However, the ZY DC line has limited installed capacity, insufficient distribution, and weak voltage support from nearby coal-fired power plants. Currently, the supporting power supply is only 2 million kW, significantly less than the 10 million kW transmission capacity of the ZY DC line, making it a typical "strong DC, weak AC" system. Its typical grid structure in the near-region is as follows: Figure 5 As shown.

[0076] This embodiment takes the calculation of the tap position of a step-up transformer in the near-area planning of a power plant in the YKZHLZ power grid of Northwest China as an example to study the applicability of the tap position calculation method. The rated capacity of the unit's step-up transformer is 1140MVA, and the transformation ratio is 525±(1±2×2.5%) / 27kV, with a total of 5 tap positions. The plant auxiliary load is considered as 40MW, and the rated capacity of the plant auxiliary transformer is 80 / 50-50MVA, with a transformation ratio of 27×(1±2×2.5%) / 10.5-10.5kV. It is connected from the 27kV side of the power plant's step-up transformer to supply power to the plant auxiliary load.

[0077] To fully utilize the power plant units' voltage support capacity for the AC system, release the units' maximum phase advance capacity, absorb excessive reactive power generated during system fluctuations, suppress system voltage fluctuations, and ensure that the power plant's auxiliary load voltage fluctuation does not exceed ±5%, the minimum auxiliary load supply voltage is 9.975kV, and the power plant unit terminal voltage is maintained within the range of 27×(1±5%)kV.

[0078] The DC transmission power is constrained according to local UHVDC operation and management regulations. Considering the different number of filter switching groups in the converter station, the tap positions of the step-up transformer are calculated separately, and the results are shown in Table 2:

[0079] Table 2 Calculation of Boost Gear Shifting

[0080]

[0081]

[0082] As shown in Table 2, when 10 sets of filters are connected to the YKZHLZ transformer, under the condition of ensuring normal power supply voltage for the power plant's auxiliary load, if the tap position of the power plant's step-up transformer is at level 5, the phase-leading capacity of the power plant units can be maximized, with a maximum phase-leading capacity of 339.82 Mvar. However, when the units operate with a delayed phase, the terminal voltage has reached its upper limit to ensure that the system voltage remains at a normal level. If the tap position of the power plant's step-up transformer is at level 4, while ensuring normal power supply to the auxiliary load, the power plant units can achieve a maximum phase-leading capacity of 238.33 Mvar. When the units operate with a delayed phase, and the terminal voltage reaches its operating upper limit, the YKZHLZ... The Z500kV side voltage can be stabilized at a normal level. If the power plant's step-up transformer tap is in position 3, the unit's maximum phase advance capability is 137.91Mvar. When the converter station has 7 sets of filters in operation, the unit needs to lag by 67.56Mvar to ensure the normal power supply to the power plant's auxiliary load and the stability of the system voltage. If the power plant's step-up transformer tap is in position 2, the power plant's maximum phase advance capability is only 34.37Mvar. In order to ensure the normal power supply to the auxiliary load, the system voltage has reached the upper limit of operation. If the power plant's step-up transformer tap is in position 1, the power plant's unit no longer has the phase advance capability.

[0083] In conclusion, it is recommended that the step-up transformer of this power plant be set to gear 4.

[0084] To further verify the rationality of the selected step-up transformer tap position, the recommended tap position of the power plant step-up transformer was checked under three scenarios during the reverse power transmission phase of the power plant's outgoing lines: no power plant outgoing lines, step-up transformer energized, and power supply from plant auxiliary loads. The results are shown in Table 3.

[0085] Table 3. Verification of Boost Shifting

[0086]

[0087] As shown in Table 3, when the power plant's step-up transformer is in gear 4, the system voltage can be kept stable during the commissioning process.

[0088] During the commissioning phase of the generating units, in order to ensure system voltage stability, voltage regulation measures must be taken during periods of system voltage rise to maintain the voltage within a reasonable range and meet the requirements of the operating curve. The power plant units need to release their leading-phase capacity to absorb excess reactive power in the system. When the voltage booster is in position 4, the actual system operating voltage and the waveforms of the unit's reactive power leading-phase are as follows: Figure 6 and Figure 7 As shown, when the boost converter is in gear 4, the unit can release its phase-advancing capability when the system voltage increases, and the calculation results are consistent with the actual engineering results.

[0089] This embodiment introduces a reasonable method for selecting the tap position of the step-up transformer when connecting to the power plant system, explains the impact of power supply access on the system, analyzes the coupling relationship between the tap position of the power plant step-up transformer and reactive power voltage, proposes a method for selecting the tap position of the power plant step-up transformer, and calculates and analyzes the reasonable tap position of the step-up transformer for DC near-area thermal power units connected to the system in conjunction with the "strong DC weak AC" grid structure in Northwest China. Taking into account the maximum phase advance capability of the near-area power plant units in the strong DC weak AC grid, the power supply voltage level of the plant auxiliary load, and the system voltage level, it is calculated that the ZY DC supporting power plant step-up transformer operates at tap 4, which is consistent with the actual project. At the same time, during the reverse power transmission stage of the power plant's outgoing line, the recommended tap position of the power plant step-up transformer is checked using three scenarios: no-load charging of the power plant outgoing line, step-up transformer energized, and power supply of the plant auxiliary load, verifying the rationality and effectiveness of the method.

[0090] While the specific embodiments of this disclosure have been described above in conjunction with the accompanying drawings, this is not intended to limit the scope of protection of this disclosure. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of this disclosure are still within the scope of protection of this disclosure.

Claims

1. A method for selecting and checking the voltage and gear of a booster system for ultra-high voltage direct current (UHVDC) power supply to a thermal power plant, characterized in that, Includes the following steps: Obtain the static excitation characteristic curve of the thermal power unit; Based on the obtained static excitation characteristic curve, the determination conditions for the unit's leading phase and the determination conditions for the unit's lagging phase, it is determined whether the unit is in leading phase operation or lagging phase operation, and the booster shift position is selected according to the shift selection conditions. The conditions for determining the leading phase of the unit are as follows: ,in, For the leading phase capability of the generator set, This represents the unit's maximum phase advance capability. Supply voltage to the power plant's auxiliary loads. The lower limit of the power supply voltage for plant auxiliary loads. This refers to the generator terminal voltage. This is the lower limit of the generator terminal voltage. This is the upper limit of the generator terminal voltage. This refers to the system bus voltage. System bus voltage upper limit; The determination condition for the unit's delayed phase is as follows: ,in, For the generator set's delayed phase capability, This represents the unit's maximum late-phase capability. Supply voltage to the power plant's auxiliary loads. The upper limit of the power supply voltage for plant auxiliary loads. This refers to the generator terminal voltage. This is the upper limit of the generator terminal voltage. This refers to the system bus voltage. This is the lower limit of the system bus voltage; When using filters for voltage regulation, let the number of filter banks be... Where k is the maximum number of filters in the converter station, and the step-up transformer speed at the power plant is assumed to be... At this point, through power flow calculations, under the condition that the unit's leading and lagging phase determination conditions are met, different results are obtained. and Under the conditions , , , You can then obtain the boost shift selector table. , ; To prioritize system voltage stability, in the step-up transformer selection table for different gear levels, each set of data that simultaneously meets the criteria for both unit leading and lagging phases is considered valid data. Other data should be deleted or disregarded. A pairwise comparison should be performed from top to bottom. To ensure that the unit can maximize its leading phase capability, the minimum number of filter groups required to maintain unit leading phase at this gear level should be as small as possible. At the same time, with the same number of filter groups, when the generator terminal voltage reaches the upper limit or the generator terminal voltage reaches the lower limit, the gear level with the higher system voltage should be selected as the power plant's step-up transformer gear level. During the reverse power transmission phase of the power plant's outgoing lines and the commissioning phase of the generating units, the selected step-up transformer position is verified to improve the reactive voltage level of the UHVDC near-area.

2. The method for selecting and verifying the step-up transformer range in an ultra-high voltage direct current (UHVDC) system connected to a thermal power plant as described in claim 1, characterized in that... The wiring of the transmission lines from the ultra-high voltage direct current (UHVDC) power plants to the thermal power plants adopts a 3 / 2 wiring method.

3. The method for selecting and verifying the step-up transformer range in an ultra-high voltage direct current (UHVDC) system connected to thermal power plants as described in claim 1, characterized in that... During the reverse power transmission phase, the unit steps up the voltage to supply power to the transformer; during the unit commissioning phase, the plant auxiliary load is transferred to the generator.

4. The method for selecting and verifying the step-up transformer range in an ultra-high voltage direct current (UHVDC) system connected to thermal power plants as described in claim 1, characterized in that... During the reverse power transmission phase of the power plant's outgoing lines, the recommended tap position of the power plant's step-up transformer is verified by using the power plant's outgoing lines under no-load conditions, the step-up transformer under energized conditions, and the plant's auxiliary load power supply.

5. The method for selecting and verifying the step-up transformer range in an ultra-high voltage direct current (UHVDC) system connected to a thermal power plant as described in claim 4, characterized in that... When the power plant's step-up transformer is in gear 4, the system voltage is stable during the commissioning process.

6. The method for selecting and verifying the step-up transformer range in an ultra-high voltage direct current (UHVDC) system connected to a thermal power plant as described in claim 1, characterized in that... During the commissioning phase of the unit, when the system voltage rises, voltage regulation measures are taken to maintain the voltage within a reasonable range and meet the requirements of the operating curve. The power plant unit releases its leading-phase capacity to absorb excessive reactive power in the system. When the boost transformer is in the 4th gear and the system voltage rises, the unit releases its leading-phase capacity.

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