A selective dynamic supplementary frequency control method for multi-terminal vsc-hvdc systems

Abstract

The application discloses a selective dynamic additional frequency control method for a multi-terminal HVDC system, when frequency disturbance occurs in the system, the frequency disturbance judgment module is set to selectively start the frequency control, and when each converter station participates in frequency modulation, the voltage droop coefficient is adjusted according to the system frequency change and the DC voltage change, so that each converter station can ensure the frequency stability of the local power grid and the stability of the system DC voltage while providing frequency support. In order to couple the frequency change to the DC voltage, the additional frequency control superimposes the frequency signal on the active power through the frequency droop coefficient, and then couples the frequency signal to the system DC voltage through the droop control, the application can effectively distinguish the frequency disturbance side power grid and the normal side power grid, selectively start the additional frequency control, and reduce the influence on the system DC voltage and the normal side frequency in the frequency modulation process.

Description

Technical Field

[0001] This invention relates to the field of power system transmission and distribution technology, and specifically to a selective dynamic additional frequency control method for multi-terminal flexible DC systems. Background Technology

[0002] In recent years, due to the application and development of renewable energy, minimizing line losses during long-distance power transmission has become a challenge, leading to the emergence of high-voltage direct current (HVDC) transmission. Traditional HVDC transmission systems typically connect two AC systems. With the construction of power grids, more and more new energy power plants are being connected to the grid, making traditional point-to-point double-ended HVDC transmission technology insufficient. Connecting multiple power grids through HVDC transmission lines has become a trend both domestically and internationally. Compared to double-ended HVDC systems, multi-ended HVDC transmission offers more flexible operation modes. At the same time, the system's control and protection strategies are more complex. Ensuring the frequency stability of multi-ended flexible HVDC systems is crucial for their stable operation. However, the decoupling effect of HVDC transmission systems causes AC power grids to lose their ability to mutually regulate frequencies.

[0003] To address the frequency stability issue in multi-terminal flexible DC systems, frequency control is typically added to the DC voltage droop control of converter stations. This couples the frequency deviation of the AC system to the power of the converter stations, which is then reflected in the DC voltage through droop control, causing each converter station to participate in frequency regulation. However, this method offers no selectivity during frequency disturbances. Even minor frequency disturbances in the AC grid can trigger frequency regulation control. Furthermore, the traditional fixed droop coefficient cannot be adjusted in real time according to the system's operating status, which can easily cause significant changes in the DC voltage and, in severe cases, affect the stable operation of the system. Patent document CN110808602B discloses an improved additional frequency for a multi-terminal flexible DC transmission system. It adjusts the frequency droop coefficient and voltage droop coefficient based on the frequency deviation, thereby reducing the impact on DC voltage during frequency regulation. Patent document CN110198045B utilizes virtual inertia technology to couple frequency changes with the AC system frequency signal, adjusting the voltage droop coefficient based on the frequency change to reasonably improve system frequency stability. Both methods adjust power distribution by changing the droop coefficient, but they do not consider that the participation of the normal-side converter station in frequency regulation can cause frequency shift on that side. Even minor frequency disturbances still lead to additional frequency control, resulting in frequent start-stop of frequency regulation control, meaning no selectivity is provided during frequency disturbances.

[0004] Therefore, the present invention provides an additional frequency control for an additional frequency disturbance judgment module to avoid frequent start and stop of frequency modulation control, and provides a dynamic additional frequency control method to improve the frequency modulation capability of the system while ensuring that the DC voltage of the system is stable within a safe range. Summary of the Invention

[0005] The technical problem to be solved by this invention is to provide a selective dynamic additional frequency control method for multi-terminal flexible DC systems. This method can effectively distinguish between the frequency disturbance side and the frequency support side, avoiding frequent activation of frequency modulation control caused by small fluctuations in system frequency or power. Simultaneously, it can avoid the damage to the multi-terminal flexible DC system caused by excessive DC voltage deviation due to additional frequency control, and improve the frequency stability of the AC system on the non-disturbance side.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0007] A selective dynamic additional frequency control method for a multi-terminal flexible DC system is disclosed. The multi-terminal flexible DC system includes four converter stations MMC1, MMC2, MMC3, and MMC4, and four AC power grids S1, S2, S3, and S4. AC power grids S3 and S4 convert electrical energy into DC power through converter stations MMC3 and MMC4, and transmit the electrical energy to converter stations MMC1 and MMC2 through parallel DC cables. Converter stations MMC1 and MMC2 convert the received electrical energy into AC power and transmit it to AC power grids S1 and S2.

[0008] The frequency disturbance judgment module is added before the input terminal of the receiving-end converter station controller. The judgment module takes the following steps:

[0009] Step 1): Using frequency deviation and power deviation as inputs, compare them with the set frequency disturbance threshold to determine whether the frequency modulation control start-up conditions are met;

[0010] Step 2): Perform an AND operation between the product of the frequency signal and the power signal output after comparison and the frequency signal. The result is used as the output of the fault judgment module and connected to the input of the converter station frequency control controller.

[0011] When the controller of the aforementioned converter station is in operation, it performs the following steps:

[0012] First, the frequency input port of the converter station is controlled by the frequency droop factor K. f It converts frequency signals into power signals, which serve as the power input for the converter station.

[0013] Then, the power input terminal is connected through the voltage droop factor K. V The difference between the obtained signal and the set reference DC voltage is used to output a control signal through the PI controller to control the operation of the converter station.

[0014] The frequency control steps performed by the controller of the aforementioned converter station are as follows:

[0015] Step 1: Each converter station sets a frequency disturbance trigger threshold Δf based on its capacity and the strength of its AC grid connection. T When the frequency deviation Δf > Δf T When the triggering condition is met, the initial voltage droop coefficient K is set according to the load connected to each converter station. V0 ;

[0016] Step 2: Detect the frequency and power changes of each AC grid and converter station; if the frequency change reaches the trigger condition and the product of frequency deviation Δf and power deviation ΔP is greater than 0, proceed to Step 3; otherwise, the frequency control will not start and the frequency change will continue to be detected.

[0017] Step 3: Calculate the detected frequency deviation using the frequency droop coefficient K. f Coupled to the reference active power, under droop control, the remaining converter stations participate in frequency regulation; simultaneously, based on the detected AC grid frequency deviation and system DC voltage deviation, the voltage droop coefficient K is adjusted. V Adjust it so that it is no longer fixed at K. V0 While ensuring frequency regulation capability, it ensures that the DC voltage of the flexible DC system is within a safe operating range;

[0018] Step 4: Detect the system DC voltage. If the DC voltage deviation ΔU dc Greater than the set threshold ΔU T Then proceed to Step 3. If the DC voltage deviation ΔU dc Less than the set threshold ΔU T Then the AC power grid frequency returns to stability, and the control ends.

[0019] In Step 3 above, the voltage droop coefficient K V Adjust according to the following methods:

[0020] The expression for the DC voltage-active power characteristic of the receiving-end converter station under dynamic additional frequency control is:

[0021] (U dc0 -U dc )+K V (P0-K f (f0-f)-P)=0 (1)

[0022] Among them, U dc0 and U dc Here, P0 and P represent the setpoint and measured values ​​of the DC voltage, P0 and P represent the setpoint and measured values ​​of the active power of the converter station, f0 and f represent the setpoint and measured values ​​of the frequency, and K represents the frequency. V K fThese are the voltage droop coefficient and the frequency droop coefficient, respectively.

[0023] The change in power on the disturbance side, ΔP, can be expressed as:

[0024]

[0025] From equation (2), it can be seen that the frequency change ΔP on the disturbance side of the system is related to the voltage droop coefficient K on that side. V Negative correlation, K V The smaller the value, the greater the power support provided, and the better the frequency modulation effect;

[0026] Neglecting DC network losses and considering energy conservation in a multi-terminal flexible DC system, when the system provides frequency support, the total power of each converter station remains unchanged, and its expression is:

[0027]

[0028] In the formula, △P j Let n be the frequency change of the j-th converter station, and n be the number of converter stations in the system.

[0029] Combining equations (2) and (3), we get the change in DC voltage of the system, ΔU. dc for:

[0030]

[0031] In the formula, △f j Let K be the frequency deviation of the j-th converter station. fj and K Vj These are the frequency droop coefficient and voltage droop coefficient of the j-th converter station, respectively;

[0032] Therefore, to dynamically adjust the voltage droop coefficient, taking into account both the system's DC voltage deviation and the disturbance-side frequency deviation, the expression for the droop coefficient is as follows:

[0033] K V =μK Vdc +(1-μ)K Vac (5)

[0034] In the formula, K Vdc To account for the droop factor of the DC voltage, K Vac To account for the voltage droop factor at AC grid frequency, μ is an adjustment factor, which is set using the hyperbolic tangent function:

[0035]

[0036]

[0037] In the formula, △U dcmax This represents the maximum DC voltage deviation.

[0038] The droop factor for DC voltage is set as follows:

[0039]

[0040] Among them, K Vdcmax DC voltage droop coefficient under maximum DC voltage deviation, ΔU dc This is the measured DC voltage deviation.

[0041] The voltage droop factor considering the AC grid frequency is set as follows:

[0042]

[0043] Where, △f max K represents the maximum frequency deviation of the AC power grid. Vacmin The DC voltage droop coefficient under the maximum frequency deviation, where Δf is the measured AC grid frequency deviation.

[0044] The present invention provides a selective dynamic additional frequency control method for multi-terminal flexible DC systems, which has the following beneficial effects:

[0045] (1) The present invention constructs a selective dynamic additional frequency control method for multi-terminal flexible DC systems. It uses the frequency change and DC voltage change as input parameters for logical judgment. Based on the set control module, it determines the frequency disturbance region, thereby selectively starting additional frequency control and avoiding frequent starting of additional control due to small frequency fluctuations.

[0046] (2) The dynamic additional frequency control proposed in this invention dynamically adjusts the voltage droop coefficient according to the frequency changes of the AC system and the DC voltage changes of the system, thereby ensuring that the AC system on the normal side provides power support to the faulty converter station while maintaining the frequency stability of its own side. Attached Figure Description

[0047] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0048] Figure 1 This is a schematic diagram of the grid connection structure of a four-terminal flexible DC system including an offshore wind farm;

[0049] Figure 2 A flowchart of a selective dynamic additional frequency control method for a multi-terminal flexible DC system provided in an embodiment of the present invention;

[0050] Figure 3 The frequency disturbance region judgment logic control block diagram provided by the present invention;

[0051] Figure 4The control block diagram for dynamic additional frequency control of the receiving-end converter station provided by the present invention;

[0052] Figure 5 The trend graph of the adjustment factor provided by the present invention;

[0053] Figure 6 This invention provides an output signal diagram of the disturbance side judgment module;

[0054] Figure 7 The simulation results of AC system S1 frequency when the load on the S1 side of the AC power grid increases by 50MW, as provided by the present invention;

[0055] Figure 8 The simulation results of the DC voltage of the AC power grid S1 side when the load is increased by 50MW are provided by the present invention.

[0056] Figure 9 The simulation results of AC system S2 frequency when the load on the S1 side of the AC power grid increases by 100MW, as provided by the present invention;

[0057] Figure 10 The simulation results of AC system S3 frequency when the load on the S1 side of the AC power grid increases by 100MW, as provided by the present invention;

[0058] Figure 11 The simulation results of the DC voltage of the AC power grid S1 side when the load is increased by 100MW are provided by the present invention; Detailed Implementation

[0059] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0060] Figure 1 This is a schematic diagram of the grid connection structure of a four-terminal flexible DC system, which mainly includes the sending-end converter station, the receiving-end converter station, and the AC power grid.

[0061] Figure 2 This is a flowchart of a frequency regulation method for a multi-terminal flexible DC grid-connected offshore wind power system based on adaptive reference power. The process specifically includes:

[0062] Step 1): Each converter station sets a frequency disturbance trigger threshold Δf based on its capacity and the strength of its connection to the AC power grid. T Based on the load connected to each converter station, set the initial voltage droop factor K. V0 ;

[0063] Step 2): Detect the frequency and power changes of each AC grid and converter station. If the frequency change meets the trigger condition and the product of the frequency deviation and power deviation is greater than 0, proceed to Step 3; otherwise, the frequency regulation control will not start, and the frequency change detection will continue.

[0064] Step 3): The detected frequency deviation is converted to a frequency droop coefficient K. f Coupled to the reference active power, under droop control, the remaining converter stations participate in frequency regulation; simultaneously, based on the detected AC grid frequency deviation and system DC voltage deviation, the voltage droop coefficient K is adjusted. V While ensuring frequency regulation capability, it ensures that the DC voltage of the flexible DC system is within a safe operating range;

[0065] Step 4): Detect the DC voltage. If the DC voltage is greater than the set threshold, proceed to step 3). If the DC voltage deviation is less than the set threshold, the AC grid frequency will stabilize and the control will end.

[0066] Figure 3 This invention provides a block diagram for determining and controlling the frequency disturbance region; if the i-th side of the AC system experiences a load increase disturbance, resulting in a frequency drop, the frequency deviation Δf i If the value is positive, its additional frequency control will provide power support to the converter station on this side through the multi-terminal flexible DC system. Therefore, the active power deviation ΔP on this side will be positive. i If the value is positive, the product of the frequency deviation and the active power deviation is positive. According to the judgment logic, the frequency disturbance area identification output is 1, and this side is the disturbance area.

[0067] Conversely, when the AC system on the normal side of the j-th side provides power support to the disturbance side, its power deviation ΔP j The frequency deviation is less than 0 because the output power of the converter station decreases, resulting in a lower frequency of the AC system on that side, and the frequency deviation Δf is less than 0. j If the value is positive, the product of the frequency deviation and the active power deviation is negative. According to the judgment logic, the frequency disturbance area identification output is 0, and this side is the normal side area.

[0068] When the load reduction disturbance occurs in the i-th side AC system, its frequency deviation and power deviation are opposite to those mentioned above, and the positive and negative relationship of their product output remains unchanged. Therefore, through the set judgment logic, the frequency disturbance side region and the normal side region can still be accurately determined.

[0069] Figure 4 The expression for the DC voltage-active power characteristic of the receiving-end converter station under dynamic additional frequency control is:

[0070] (U dc0 -U dc )+K V (P0-K f (f0-f)-P)=0 (1)

[0071] Among them, U dc0 and U dcHere, P0 and P represent the setpoint and measured values ​​of the DC voltage, P0 and P represent the setpoint and measured values ​​of the active power of the converter station, f0 and f represent the setpoint and measured values ​​of the frequency, and K represents the frequency. V K f These are the voltage droop coefficient and the frequency droop coefficient, respectively.

[0072] The change in power on the disturbance side, ΔP, can be expressed as:

[0073]

[0074] From equation (2), it can be seen that the frequency change ΔP on the disturbance side of the system is related to the voltage droop coefficient K on that side. V Negative correlation, K V The smaller the value, the greater the power support provided, and the better the frequency modulation effect;

[0075] Neglecting DC network losses and considering energy conservation in a multi-terminal flexible DC system, when the system provides frequency support, the total power of each converter station remains unchanged, and its expression is:

[0076]

[0077] In the formula, △P j Let n be the frequency change of the j-th converter station, and n be the number of converter stations in the system.

[0078] Combining equations (2) and (3), we get the change in DC voltage of the system, ΔU. dc for:

[0079]

[0080] In the formula, △f j Let K be the frequency deviation of the j-th converter station. fj and K Vj These are the frequency droop coefficient and voltage droop coefficient of the j-th converter station, respectively;

[0081] Therefore, to dynamically adjust the voltage droop coefficient, taking into account both the system's DC voltage deviation and the disturbance-side frequency deviation, the expression for the droop coefficient is as follows:

[0082] K V =μK Vdc +(1-μ)K Vac (5)

[0083] In the formula, K Vdc To account for the droop factor of the DC voltage, K Vac To account for the voltage droop factor at AC grid frequency, μ is an adjustment factor, which is set using the hyperbolic tangent function:

[0084]

[0085]

[0086] In the formula, △U dcmax This represents the maximum DC voltage deviation.

[0087] The droop factor for DC voltage is set as follows:

[0088]

[0089] Among them, K Vdcmax DC voltage droop coefficient under maximum DC voltage deviation, ΔU dc This is the measured DC voltage deviation.

[0090] The voltage droop factor considering the AC grid frequency is set as follows:

[0091]

[0092] Where, △f max K represents the maximum frequency deviation of the AC power grid. Vacmin The DC voltage droop coefficient under the maximum frequency deviation, where Δf is the measured AC grid frequency deviation.

[0093] Figure 5 This graph shows the trend of the adjustment factor for the voltage droop coefficient. As the DC voltage deviation increases, the adjustment factor approaches 1. In the DC voltage droop coefficient, K... Vdc As the weighting increases, the voltage droop coefficient tends to increase, which helps to reduce unbalanced power in the system and thus reduce DC voltage deviation.

[0094] To verify the effectiveness of the method of the present invention, a system was built in PSCAD / EMTDC as follows. Figure 1 The simulation model of the four-terminal flexible straight-line system is shown in Table 1; the parameters are shown in Table 1.

[0095] Table 1

[0096]

[0097] Operating condition 1: At t = 4.5s, the load of AC power grid S1 increases by 50MW.

[0098] like Figure 6 , Figure 7 and Figure 8 As shown, when the load on AC system S1 increases by 50MW, the system frequency decreases; under conventional control strategies, due to the linear coupling between frequency and active power reference values, K... f and K VThe value cannot be adjusted in real time according to the DC voltage deviation and frequency deviation, so that the lowest frequency on the S1 side reaches 49.32Hz and the system DC voltage deviation is 387.2kV.

[0099] like Figure 6 As shown, a load disturbance occurs on the S1 side of the AC power grid at 4.5s, causing a frequency change. The frequency disturbance judgment module on the S1 side outputs a value that jumps from 0 to 1 at 4.5s, triggering additional frequency control. Compared to conventional control, this invention adjusts K based on frequency deviation and DC voltage deviation. V Under the proposed control, the lowest frequency of the frequency disturbance side system is increased to 49.72Hz, which improves the frequency stability of the AC power grid. At the same time, due to the increased frequency regulation effect, the unbalanced power in the system increases, and the maximum deviation of DC voltage increases, reaching 383.7kV, but it is still within the stable operating range.

[0100] The results show that, under the proposed control strategy, for small load disturbances in the AC power grid, the proposed control strategy can more effectively improve the frequency of the AC system compared with the traditional control strategy, while the DC voltage is only slightly increased and is still within the safe operating range.

[0101] Condition 2: At t = 4.5s, the load of AC power grid S1 increases by 100MW.

[0102] like Figure 9 , Figure 10 and Figure 11 As shown, when the load on AC system S1 increases by 100MW, the system frequency decreases. Under the conventional control strategy, the frequency of AC system S1 changes significantly, reaching a minimum of 48.85Hz. Simultaneously, the normal-side system participates in frequency regulation, and the system DC voltage deviation reaches 364.6kV, exceeding the system's safe operating range and threatening system stability. Meanwhile, the normal-side AC system S2, also participating in frequency regulation, experiences a frequency deviation of 49.75Hz, exceeding its stable operating range and reducing the frequency stability of the S2-side AC system.

[0103] Compared to conventional control, this invention adjusts K based on frequency deviation and DC voltage deviation. V Under the proposed control, the maximum DC voltage deviation was increased from 364.6kV to 381.5kV, which improved the operation of the DC grid. At the same time, the minimum frequency on the normal side was increased to 49.82Hz, which reduced the impact on the frequency stability of the AC grid.

[0104] The results show that when the system experiences a large frequency disturbance, the additional frequency control of the multi-terminal flexible DC system causes significant changes in both the frequency deviation on the disturbance side and the system DC voltage. Under the control proposed in this invention, the voltage droop coefficient K increases with the increase of the DC voltage deviation.V This increases the amount of unbalanced power in the system, preventing instability of the DC grid and reducing the impact of local AC system instability on the entire system.

Claims

1. A selective dynamic additional frequency control method for multi-terminal flexible DC systems, characterized in that, The multi-terminal flexible DC system includes four converter stations: MMC1, MMC2, MMC3, and MMC4, and four AC power grids: AC grid S1, AC grid S2, AC grid S3, and AC grid S4. AC grids S3 and S4 convert electrical energy into DC power through converter stations MMC3 and MMC4, and then transmit the electrical energy to converter stations MMC1 and MMC2 through parallel DC cables. Converter stations MMC1 and MMC2 convert the received electrical energy into AC power and transmit it to AC grids S1 and S2. The frequency disturbance judgment module is added before the input terminal of the receiving-end converter station controller. The judgment module takes the following steps: Step 1): Using frequency deviation and power deviation as inputs, compare them with the set frequency disturbance threshold to determine whether the frequency modulation control start-up conditions are met; Step 2): Perform an AND operation between the product of the frequency signal and the power signal output after comparison and the frequency signal. The result is used as the output of the fault judgment module and connected to the input of the converter station frequency control controller. The frequency control steps performed by the controller of the converter station are as follows: Step 1: Each converter station sets a frequency disturbance trigger threshold based on its capacity and the strength of its connected AC power grid. When frequency deviation When the triggering condition is met, the initial voltage droop coefficient is set according to the load connected to each converter station. ; Step 2: Detect changes in the frequency of each AC power grid and the power of the converter station; if the frequency change reaches the trigger condition and the frequency deviation is also... and power deviation If the product is greater than 0, proceed to Step 3; otherwise, frequency modulation control will not start, and frequency change detection will continue. Step 3: Calculate the detected frequency deviation using the frequency droop coefficient. Coupled to the reference active power, under droop control, the remaining converter stations participate in frequency regulation; simultaneously, based on the detected AC grid frequency deviation and system DC voltage deviation, the voltage droop coefficient is adjusted. Adjust it so that it is no longer fixed. While ensuring frequency regulation capability, it ensures that the DC voltage of the flexible DC system is within a safe operating range; Step 4: Detect the system DC voltage. If the DC voltage deviation is... Greater than the set threshold Then proceed to Step 3. If the DC voltage deviation is... Less than the set threshold Then the AC power grid frequency returns to stability, and the control ends; System DC voltage deviation for: ； In the formula, Let j be the frequency deviation of the j-th converter station. and Let be the frequency droop coefficient and voltage droop coefficient of the j-th converter station, respectively. n The number of converter stations in the system; Therefore, to dynamically adjust the voltage droop coefficient, taking into account both the system's DC voltage deviation and the disturbance-side frequency deviation, the expression for the droop coefficient is as follows: ； In the formula, To account for the droop factor of DC voltage, To account for the voltage droop factor at AC grid frequency, The adjustment factor is set using the hyperbolic tangent function: ； ； In the formula, This represents the maximum DC voltage deviation. The droop factor for DC voltage is set as follows: ； in, DC voltage droop coefficient under maximum DC voltage deviation This is the measured DC voltage deviation. The voltage droop factor considering the AC grid frequency is set as follows: ； in, This represents the maximum frequency deviation of the AC power grid. DC voltage droop coefficient at maximum frequency deviation To obtain the measured frequency deviation of the AC power grid.

2. The selective dynamic additional frequency control method for a multi-terminal flexible DC system as described in claim 1, characterized in that, When the controller of the converter station is in operation, it performs the following steps: First, the frequency input port of the converter station is controlled by the frequency droop factor. It converts frequency signals into power signals, which serve as the power input for the converter station. Then the power input terminal is connected via the voltage droop factor. K V The difference between the obtained signal and the set reference DC voltage is used to output a control signal through the PI controller to control the operation of the converter station.

3. The selective dynamic additional frequency control method for a multi-terminal flexible DC system as described in claim 2, characterized in that, In Step 3, the voltage droop coefficient Adjust according to the following methods: The expression for the DC voltage-active power characteristic of the receiving-end converter station under dynamic additional frequency control is: ； in, and These are the set and measured values ​​of the DC voltage. and These are the setpoint and measured values ​​of the active power of the converter station. and These are the set and measured values ​​for the frequency. , These are the voltage droop coefficient and the frequency droop coefficient, respectively. Its disturbance-side power change It can be represented as: ； From the above equation, we can see that the frequency change on the disturbance side of the system is... With the voltage droop factor on this side Negative correlation The smaller the value, the greater the power support provided, and the better the frequency modulation effect; Neglecting DC network losses and considering energy conservation in a multi-terminal flexible DC system, when the system provides frequency support, the total power of each converter station remains unchanged, and its expression is: ； In the formula, For the first j Frequency variation of each converter station n This refers to the number of converter stations in the system.

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