Method for dc voltage uplift under energy balance condition of hvdc station

Abstract

The present application relates to a kind of DC voltage lifting method under the condition of flexible direct current converter station energy balance, belong to power transmission and distribution technical field.The present application can adjust DC voltage by increasing or decreasing the total number of each phase bridge arm submodule input under the condition that the total number of each phase bridge arm bridge arm input submodule is variable, therefore the present application can increase the total number of submodule input under the condition that submodule voltage is kept unchanged, can maintain AC / DC power balance while lifting DC voltage, effectively reduce the effective value of bridge arm current in the case of maintaining the level of flexible direct current transmission power, improve the ability of direct current transmission power.The control strategy is proposed to increase the total number of submodule input to lift DC voltage under the condition that submodule voltage is kept unchanged, improve the utilization efficiency of bridge arm submodule, while avoiding the occupation of active power in the process of voltage boosting of flexible direct current system, reduce the frequency disturbance to receiving end grid.

Description

Technical Field

[0001] This invention belongs to the field of power transmission and distribution technology, and particularly relates to a method for DC voltage boosting under energy balance conditions in a flexible DC converter station and a method for selecting DC boost control parameters. Background Technology

[0002] With the continuous high proportion of renewable energy sources such as wind power and photovoltaics being connected to the grid, the system inertia level is declining and the frequency stability problem is becoming increasingly prominent. Actively supporting the grid frequency using flexible DC transmission (Voltage Source Converter Based HighVoltage DC, VSC-HVDC) technology has become a future development trend.

[0003] During active support, the increase in power transmitted by the flexible DC system leads to a rapid increase in the converter valve arm current, which can easily trigger the current limits of existing IGBTs, hindering the continuous and stable maintenance of power transmission levels. Regarding the method of using DC bus voltage to transmit frequency change information from the receiving-end AC grid to the sending end, most existing research adopts the method of reducing the DC voltage to reflect the grid frequency drop to the sending end. However, this approach requires transmitting more active power while reducing the DC voltage, further accelerating the rise in arm current and limiting its effectiveness in high-power scenarios. Simply increasing the DC voltage to transmit frequency drop information will cause the flexible DC system to crowd out active power during transients, leading to a further drop in the receiving-end grid frequency.

[0004] To address the aforementioned problems, this invention proposes a method for raising DC voltage under energy balance conditions in flexible DC converter stations. Summary of the Invention

[0005] The purpose of this invention is to propose a DC voltage boosting method under energy balance conditions in a flexible DC converter station to solve the problem mentioned in the background art where the converter valve arm current value increases rapidly during active support, easily triggering the current limit of existing IGBTs.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for raising DC voltage under energy balance conditions in a flexible DC converter station includes the following steps:

[0008] Step 1: Introduce submodule energy quota control loops in both the sending and receiving converter stations. The sending end's D and Q axis dual-loop control targets remain unchanged, and submodule energy quota control based on additional dimensions is added. The receiving end changes the D axis outer loop control target to submodule energy, while the Q axis control target remains unchanged, and DC voltage control based on additional dimensions is added.

[0009] Step 2: Introduce bridge arm voltage limiting circuits in both the sending and receiving end converter stations. The upper limit of the bridge arm voltage is determined by the positive level output capability of the bridge arm, and the lower limit of the bridge arm voltage is determined by the degree of DC voltage rise, so that each phase bridge arm can be equipped with a sufficient number of sub-modules to meet the DC voltage rise requirements.

[0010] Step 3: Perform Fourier decomposition on the bridge arm voltage expression during the DC boost process, calculate the effective value constraint of the bridge arm current and the constraint expression of the submodule capacitor voltage fluctuation rate, analyze the control effect under different control parameter values ​​under the condition of satisfying the above constraints, and select appropriate control parameters to achieve effective suppression of the bridge arm current.

[0011] Compared with the prior art, the present invention has the following beneficial effects:

[0012] This invention employs a method of raising DC voltage to transmit information about the frequency drop of the receiving-end power grid. Based on the variable number of sub-modules in each phase arm, the DC voltage is dynamically adjusted by increasing or decreasing the total number of sub-modules in each phase arm, thus overcoming the inherent dynamic characteristic limitations of the DC bus capacitor under conventional control architecture.

[0013] This invention avoids the squeezing out of active power by introducing energy quota control and bridge arm voltage limiting control at both ends of the DC boost process.

[0014] This invention controls the energy conservation of the bridge arms at both ends of the converter station and introduces a bridge arm voltage limiting circuit to obtain a sufficient number of sub-modules for DC boost. At the same time, this invention can effectively control the distortion of the bridge arm voltage and achieve a good suppression effect on the bridge arm current value by reasonably selecting the DC boost control parameters. Attached Figure Description

[0015] Figure 1 This is a block diagram of the DC boost control system for the receiving-end converter station; U in the diagram dcref U cref Q ref For DC voltage, submodule capacitor voltage, and reactive power setting values; K A For control parameters; U arm_max U arm_min U is the voltage limiting value for the bridge arm; dc_N f ref These are the rated values ​​for DC voltage and frequency.

[0016] Figure 2 This is the bridge arm voltage waveform during DC boost;

[0017] Figure 3 This is a block diagram of the grid control system for the sending-end converter station; u in the diagram sd_ref u sq_ref M represents the setting values ​​for the d-axis and q-axis components of the AC voltage.dc This is the DC modulation ratio, and its value is the actual DC voltage U. dc With rated DC voltage U dc_N Ratio; k p k i k pn k in These are the parameters for the PI controller. Detailed Implementation

[0018] The following will provide a detailed description of a DC voltage boosting method under energy balance conditions in a flexible DC converter station, as well as a method for selecting DC boost control parameters, which relates to the present invention. It should be emphasized that the following description is merely exemplary and not intended to limit the scope or application of the invention.

[0019] The technical problem this invention aims to solve is based on the existing method of transmitting frequency change information of the receiving-end AC grid to the sending end using DC bus voltage. By increasing or decreasing the total number of sub-modules in each phase bridge arm, dynamic adjustment of the DC voltage is achieved. Furthermore, sub-module energy quota control and bridge arm voltage limiting control are added to both converter stations to avoid the squeezing of active power by the flexible DC system during DC boosting. This invention also provides a reasonable method for selecting DC boosting control parameters. The following description, in conjunction with relevant figures and specific examples, illustrates the DC voltage boosting method under energy balance conditions for flexible DC converter stations and the method for selecting DC boosting control parameters proposed in this invention.

[0020] Example 1:

[0021] Methods for DC voltage boosting under energy balance conditions in flexible DC converter stations include:

[0022] Step 1:

[0023] The control architecture of the receiving-end converter station includes an outer-loop power control loop and an inner-loop current control loop. The active power outer-loop control target can be selected as the DC voltage U. dcm The square of the average capacitor voltage U of the submodule cm 2 The target of reactive power outer loop control is reactive power Q. m The DC voltage control circuit is used to control the DC voltage bias of the bridge arm; the bridge arm voltage limiting circuit is used to control the minimum value of the bridge arm voltage; CCSC is the circulating current suppression circuit. Figure 1The control strategy includes three selection switches. Switch 1 is the active power outer loop control target selection switch, switch 2 is the DC voltage bias selection switch, and switch 3 is the bridge arm voltage lower limit selection switch. These three switches switch according to two control modes. Mode I is conventional control, and Mode II is the DC boost control proposed in this invention. To prevent large fluctuations in DC voltage during receiving-end grid faults, considering that the allowable frequency fluctuation range of the large receiving-end grid under steady state is ±0.2Hz, a selection is made... Figure 2 ΔU dcmax 0.2K A ΔU dcmin -0.2K A Wherein: K A These are the control parameters for the DC voltage control circuit, which can be manually selected based on the DC boost level. Please refer to [link / reference]. Figure 3 Based on the grid control of the sending-end converter station, an additional dimension-based submodule energy control loop is introduced to maintain stable capacitor voltage in the sending-end submodules. The energy W of all submodule capacitors within the bridge arm... MMC The rate of change reflects the unbalanced power between AC and DC, which can be specifically expressed by equation (1):

[0024]

[0025] Where: AC power P ac Determined by the photovoltaic unit, the DC voltage U dc Controlled by the receiving-end converter station, therefore W MMC The adjustment is mainly achieved by changing the direct current I. dc The value of is used to achieve this. Since the total number of submodules remains constant, the equivalent capacitance of the bridge arm is constant, and its stored energy is positively correlated with the average voltage of the submodule. To simplify the calculation, the submodule energy-DC current outer loop control can be designed as Equation (2):

[0026]

[0027] In the formula: U c_avg U represents the average voltage of the submodule. * c_avg k is the average voltage reference value for the submodule. p and k i These are the proportional and integral coefficients in the PI controller, respectively.

[0028] The inner loop DC current control is shown in equation (3):

[0029]

[0030] Where: M dc This is the DC modulation ratio, and its value is the actual DC voltage U. dc With rated DC voltage U dc_N Ratio; kpn and k in These are the proportional and integral coefficients in the inner-loop PI controller, respectively; I * dc This is the reference value for DC current. M dc with U dc_N Multiplying by 2 yields the DC voltage bias U of the sending-end bridge arm. dcref / 2, by adjusting U dcref The difference between the received and received DC voltages enables the control of I. dc Control.

[0031] Step 2:

[0032] To ensure that each phase arm can deploy a sufficient number of submodules to meet the DC voltage rise requirement, during the DC voltage boost instant (e.g. Figure 2 (As shown in stage 2) Minimum bridge arm voltage U arm_min Generated by equation (4), sufficient submodules are deployed in exchange for a certain degree of weakening the AC side voltage support capability, in order to meet U dc Increased demand reduces the frequency drop depth during moments of disturbance.

[0033] U arm_min =U dc -N0U cref (4)

[0034] In the formula: U arm_min Indicates the minimum value of the bridge arm voltage; U cref Indicates the submodule capacitor voltage setting value; N0 represents the total number of submodules engaged in each phase arm when the DC voltage is at its rated value; in U dc After rising to its highest level, the energy within the bridge arm is sufficient to sustain the subsequent U-wave. dc level. Figure 1 Select switch maintains selection mode II, maintaining the corresponding U based on the degree of grid frequency recovery. dc The bridge remains horizontal, maintaining continuous power transmission to support the frequency. At this point, the voltage distortion of the bridge arms decreases, gradually restoring the ability to support the voltage on the AC side.

[0035] Step 3:

[0036] Under the strategy proposed in this invention, a low DC boost level has limited impact on improving the power transmission capability of flexible DC, while an excessively high boost level will lead to severe voltage distortion in the bridge arms. Therefore, controlling the DC voltage amplitude during the boost process is crucial. Under the receiving-end frequency support requirement, the bridge arm DC voltage bias U during the boost process... dc The result is obtained from equation (5):

[0037] U dc =U dc_N +K A (f ref-f m (5)

[0038] Taking phase A as an example, the voltage U of the upper bridge arm during the boost process... arm_pa It can be expressed by equation (6):

[0039]

[0040] It can be seen that due to the limitation of the positive level output capability of the bridge arm, U dc After rising to a certain level, the bridge arm voltage distortion becomes a flat-top wave. This is based on the control parameter K. A Fourier decomposition of equation (6) under different values ​​yields the harmonic components of the bridge arm voltage. The effective value of the bridge arm current I shown in equation (7) is obtained. arm_RMS Within the constraints and the voltage fluctuation rate constraint of the submodule capacitor shown in Equation (8), when the power delivery level remains constant, I arm_RMS The smaller the value, the better the suppression effect on the increase in bridge arm current.

[0041]

[0042] In the formula: I arm_RMS It must not exceed 75% of the IGBT's rated current; IGBT_max This refers to the rated current of the IGBT.

[0043]

[0044] In the formula: ΔU c (t) represents the offset of the actual value of the submodule capacitor voltage relative to the base value; ε0 represents the maximum allowable fluctuation rate of the submodule capacitor voltage. The economic value of ε0 under full-load reactive power conditions is usually 10% to 15%.

[0045] It should be emphasized that steps 1, 2 and 3 as a whole constitute the invention content, enabling the flexible DC system to avoid crowding out the transmitted active power while raising the DC voltage to suppress the effective value of the bridge arm current, increasing the upper limit of the flexible DC transmission power and reducing the frequency disturbance to the receiving end grid. The three steps are an organic and inseparable whole.

[0046] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for raising DC voltage under energy balance conditions in a flexible DC converter station, characterized in that, The method achieves dynamic adjustment of DC voltage without relying on the imbalance power between AC and DC at the converter station, thus avoiding crowding out the active power of flexible DC transmission; the method includes the following steps: Step 1: Introduce sub-module energy quota control loops in both the sending and receiving end converter stations to maintain the AC / DC power balance of the converter stations during DC boosting. Step 2: Introduce bridge arm voltage limiting circuits in both the sending and receiving converter stations to obtain a sufficient number of sub-modules to achieve DC voltage boost. The minimum bridge arm voltage is determined based on the degree of DC voltage rise. Step 3: Based on the suppression effect of the DC voltage rise on the bridge arm current and the resulting bridge arm voltage distortion, calculate the control parameters for the DC boost, specifically including the following: Under the requirement of receiving-end frequency support, the DC voltage bias of the bridge arm during DC voltage boosting process U dc It can be calculated using the following formula: (5) In the formula, U dc_N This is the rated DC voltage. K A Indicates control parameters; f ref This is the frequency rating; f m This represents the actual value of the receiving-end power grid frequency. Taking phase A as an example, the voltage of the upper bridge arm during the voltage boosting process... U arm_pa It can be expressed by the following formula: (6) Due to the limitation of the bridge arm's positive level output capability U dc After rising to a certain level, the bridge arm voltage distortion becomes a flat-top wave. Based on control parameters K A Fourier decomposition of equation (6) under different values ​​yields the harmonic components of the bridge arm voltage. The effective value of the bridge arm current shown in equation (7) is then obtained. I arm_RMS Within the constraints and the voltage fluctuation rate constraint of the submodule capacitor shown in Equation (8), when the power delivery level remains constant I arm_RMS The smaller the value, the better the suppression effect on the increase in bridge arm current. (7) In the formula, I arm_RMS Not exceeding 75% of the IGBT's rated current; I IGBT_max This refers to the rated current of the IGBT. (8) In the formula, U c ( t The value represents the offset of the actual value of the submodule capacitor voltage relative to the base value. ε This indicates the voltage fluctuation rate of the submodule capacitor; ε 0 indicates the maximum allowable voltage fluctuation of the submodule capacitor.

2. The DC voltage boosting method under energy balance conditions in a flexible DC converter station according to claim 1, characterized in that, After introducing the submodule energy quota control link, the rate of change of the energy WMMC of all submodule capacitors inside the bridge arm reflects the unbalanced power between AC and DC, specifically expressed by the following formula: (1) In the formula, P ac Indicates AC power; P dc Indicates DC power; U sd Represents the d-axis component of AC voltage; I sd Represents the d-axis component of alternating current; U dc Indicates DC voltage; I dc Indicates direct current; The submodule energy-DC current outer loop control design is as follows: (2) In the formula, U c_avg The average voltage of the submodule; U * c_avg This is the average voltage reference value for the submodule; k p and k i These represent the proportional and integral coefficients in the PI controller, respectively. The submodule energy-DC current inner loop control design is as follows: (3) In the formula, M dc This represents the DC modulation ratio, and its value is the actual DC voltage. U dc With rated DC voltage U dc_N ratio; k pn and k in These represent the proportional and integral coefficients in the inner-loop PI controller, respectively. I * dc This is a reference value for DC current. M dc and U dc_N Multiplying by 2 yields the DC voltage bias of the sending-end bridge arm. U dcref / 2, by adjusting U dcref The difference between the received and received DC voltages enables the control of... I dc Control.

3. The DC voltage boosting method under energy balance conditions in a flexible DC converter station according to claim 2, characterized in that, The formula for calculating the minimum bridge arm voltage in step 2 is as follows: (4) In the formula, U arm_min This indicates the minimum voltage of the bridge arm; U cref This indicates the capacitor voltage setting value of the submodule; N 0 indicates the total number of submodules engaged in each phase arm when the DC voltage is at its rated value.

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