A method for improving voltage stability of a flexible direct current power grid

Abstract

This invention discloses a method for improving the voltage stability of a flexible DC power grid, comprising the following steps: Before the DC voltage-active power droop-type outer loop controller of the flexible DC converter station; adding a specified lead-lag correction element to the DC voltage sampling signal; ensuring that the equivalent admittance introduced by the droop control approximately exhibits a conductance characteristic with a constant droop gain within the DC voltage control time scale; thereby suppressing overshoot oscillations of the DC voltage under power fluctuations and improving DC voltage stability. Preferably, this invention uses a correction coefficient to transform the equivalent impedance of the flexible DC converter station into a first-order form. Simultaneously, thanks to the increase in the correction coefficient, the sensitivity of the damping ratio to the power magnitude of the flexible DC converter station decreases. Furthermore, this method of correcting the droop admittance is simple and practical. Further, decoupling the droop admittance of the flexible DC converter station from the proportional-integral controller's proportional coefficient makes the tuning of the command feedback loop more flexible.

Description

Technical Field

[0001] This invention belongs to the field of multi-terminal flexible DC systems / DC grids, and specifically relates to a method for improving the voltage stability of flexible DC grids. Background Technology

[0002] Flexible DC transmission technology, leveraging the advantages of voltage source converters (VSCs) in four-quadrant control of active / reactive power, short-circuit current limiting, and the absence of commutation failure risk, is widely used in applications such as renewable energy integration, increasing renewable energy penetration, and enhancing the safety and stability of interconnected power grids. Among these, the modular multilevel converter (MMC), benefiting from its cascaded submodule structure, has become the preferred topology for flexible DC converters. Flexible DC projects, exemplified by the Zhangbei four-terminal flexible DC grid and the Nan'ao multi-terminal flexible DC system, have achieved significant development in China and worldwide.

[0003] In flexible DC power grids, master-slave control strategies are often used to achieve system voltage stability and power distribution. However, since the unbalanced power of the DC system is borne independently by the converter station controlled by the constant DC voltage, problems such as converter station overload, unreasonable power distribution, and even DC voltage instability are prone to occur under disturbances.

[0004] The droop control method achieves distributed control of DC voltage, that is, the control of a single constant DC voltage converter station is distributed to multiple droop-type converter stations, allowing all droop-type converter stations to participate in DC voltage regulation according to the actual system conditions. Nevertheless, considering the tuning of control parameters of flexible DC converter stations and the differential characteristics of voltage-power droop control, DC voltage is prone to large overshoot oscillations under power disturbances. Suppressing system voltage overshoot oscillations is of great significance for preventing the risk of DC voltage exceeding limits and maintaining DC voltage stability.

[0005] To limit DC voltage deviation and improve system disturbance power distribution, some researchers have proposed a variable droop coefficient method. This method adaptively adjusts the droop coefficient by considering both transient and steady-state factors, including power margin, voltage deviation, and AC grid frequency. Additionally, some researchers have achieved similar control effects by changing the setpoint of the droop control (power or DC voltage command point). To fundamentally improve the damping of flexible DC converter stations and suppress DC voltage, current, and power oscillations, some researchers have proposed a virtual impedance method. This method improves the inner-loop control or modulation control of the flexible DC converter station by using high-pass or band-pass filters to provide virtual impedance to the converter station. This enhances system damping and improves dynamic characteristics without affecting the steady-state power flow distribution.

[0006] However, the adaptive droop factor method redistributes disturbance power and DC voltage by changing the droop factor, but it does not fundamentally suppress DC voltage overshoot oscillation under disturbance. The virtual impedance method is relatively complex and requires improvements to the controller's internal mechanisms. To overcome these shortcomings, a simple and practical method to improve the voltage stability of flexible DC grids is needed. Summary of the Invention

[0007] This invention is proposed to address the problems existing in the prior art, and its purpose is to provide a method for improving the voltage stability of flexible DC power grids.

[0008] The technical solution of this invention is: a method for improving the voltage stability of a flexible DC power grid, comprising the following processes:

[0009] i. Determine the stabilization node. The stabilization node is determined to be before the DC voltage-active power droop-type outer loop controller of the flexible DC converter station.

[0010] ii. Add a specified lead-lag correction element to the DC voltage sampling signal;

[0011] iii. The lead-lag compensation stage enables the equivalent admittance introduced by droop control to approximately exhibit a conductance characteristic that is constant in magnitude and equal to the droop gain over the DC voltage control time scale.

[0012] iv. The constant droop gain conductivity characteristic is used to suppress overshoot oscillations of DC voltage under power fluctuations and improve DC voltage stability.

[0013] Furthermore, the flexible DC converter station is a grid-connected flexible DC converter station, which is connected to a strong AC power grid.

[0014] Furthermore, the DC voltage-active power droop control of the flexible DC converter station in step i is specifically as follows:

[0015] First, obtain the active power command value of the flexible DC converter station and the real-time transmission power on the AC side;

[0016] Then, the DC voltage command value of the flexible DC converter station is obtained and the DC voltage is measured in real time;

[0017] Finally, the expression relationship between DC voltage and active power droop control is obtained as follows:

[0018] (P ref -P ac )+β(V dcref -V dc ) = 0

[0019] In the formula,

[0020] Pref With P ac These represent the active power command value of the converter station and the real-time transmission power on the AC side, respectively.

[0021] V dcref With V dc These represent the commanded DC voltage value and the real-time measured DC voltage, respectively.

[0022] β is the droop gain.

[0023] Furthermore, in step ii, a specified lead-lag compensation element is added to the DC voltage sampling signal. The specific process of the lead-lag compensation element is as follows:

[0024] First, obtain the proportional coefficient and integral coefficient of the drooping outer loop controller of the flexible DC converter station;

[0025] Then, based on the above proportional coefficient and integral coefficient, the correction coefficient is obtained;

[0026] Finally, the relationship between the lead-lag correction links is established as follows:

[0027]

[0028] In the formula,

[0029] G c (s) is the transfer function of the correction element;

[0030] K P With K I These represent the proportional coefficient and integral coefficient of the drooping outer loop controller of the flexible DC converter station, respectively.

[0031] K pp This is a correction factor.

[0032] Furthermore, the correction coefficient K pp With the proportionality coefficient K P The specific preferred relationships are as follows:

[0033] K pp =1+K P .

[0034] Furthermore, combining the above-obtained relationship expression for the lead-lag correction element and the preferred correction coefficient, the equivalent admittance introduced by droop control is approximately equal to the conductance characteristic of a constant droop gain β.

[0035] Furthermore, step iv uses the constant droop gain conductance characteristic to suppress overshoot oscillations of DC voltage under power fluctuations and improve DC voltage stability, as follows:

[0036] By approximating the conductance characteristics of a constant droop gain β, the damping characteristics of the droop-type converter station within the DC voltage control time scale are significantly improved, thereby suppressing overshoot oscillations and improving DC voltage stability.

[0037] The beneficial effects of this invention are as follows:

[0038] This invention simplifies the equivalent impedance of the flexible DC converter station into a second-order transfer function with zeros. By adding a specified lead-lag correction element, it offsets the adverse effects of the original impedance zeros on the system dynamics. At the same time, this element only changes the damping ratio of the flexible DC converter station without affecting other parameters such as the natural frequency.

[0039] The present invention preferably uses a correction factor to transform the equivalent impedance of the flexible DC converter station into a first-order form. At the same time, thanks to the increase of the correction factor, the sensitivity of the damping ratio to the power of the flexible DC converter station also decreases. In addition, this method of correcting the droop admittance is simple and practical, without requiring modifications to the original dual-loop controller. Furthermore, it decouples the droop admittance of the flexible DC converter station from the proportional coefficient of the proportional-integral controller, making the tuning of the command feedback loop more flexible. Attached Figure Description

[0040] Figure 1 The basic architecture of a grid-type flexible DC converter station incorporating the method of the present invention;

[0041] Figure 2 This is a topology diagram of a four-terminal flexible DC transmission system / DC grid;

[0042] Figure 3 This refers to the DC voltage under power disturbance (the subscript -VSC represents the variable belonging to each converter station);

[0043] Figure 4 The converter station power under power disturbance (the subscript -VSC represents the variable belonging to each converter station);

[0044] Figure 5 Voltage and power under station failure (subscript -VSC represents the variable belonging to each converter station). Detailed Implementation

[0045] The present invention will now be described in detail with reference to the accompanying drawings and embodiments:

[0046] like Figures 1 to 5 As shown, a method for improving the voltage stability of a flexible DC power grid includes the following steps:

[0047] i. Determine the stabilization node. The stabilization node is determined to be before the DC voltage-active power droop-type outer loop controller of the flexible DC converter station.

[0048] ii. Add a specified lead-lag correction element to the DC voltage sampling signal;

[0049] iii. The lead-lag compensation stage enables the equivalent admittance introduced by droop control to approximately exhibit a conductance characteristic that is constant in magnitude and equal to the droop gain over the DC voltage control time scale.

[0050] iv. The constant droop gain conductivity characteristic is used to suppress overshoot oscillations of DC voltage under power fluctuations and improve DC voltage stability.

[0051] The flexible DC converter station is a grid-connected flexible DC converter station, which is connected to a strong AC power grid.

[0052] The DC voltage-active power droop control of the flexible DC converter station in step i is as follows:

[0053] First, obtain the active power command value of the flexible DC converter station and the real-time transmission power on the AC side;

[0054] Then, the DC voltage command value of the flexible DC converter station is obtained and the DC voltage is measured in real time;

[0055] Finally, the expression relationship between DC voltage and active power droop control is obtained as follows:

[0056] (P ref -P ac )+β(V dcref -V dc ) = 0

[0057] In the formula,

[0058] P ref With P ac These represent the active power command value of the converter station and the real-time transmission power on the AC side, respectively.

[0059] V dcref With V dc These represent the commanded DC voltage value and the real-time measured DC voltage, respectively.

[0060] β is the droop gain.

[0061] Step ii involves adding a specified lead-lag compensation element to the DC voltage sampling signal. The specific process of the lead-lag compensation element is as follows:

[0062] First, obtain the proportional coefficient and integral coefficient of the drooping outer loop controller of the flexible DC converter station;

[0063] Then, based on the above proportional coefficient and integral coefficient, the correction coefficient is obtained;

[0064] Finally, the relationship between the lead-lag correction links is established as follows:

[0065]

[0066] In the formula,

[0067] G c (s) is the transfer function of the correction element;

[0068] K P With K I These represent the proportional coefficient and integral coefficient of the drooping outer loop controller of the flexible DC converter station, respectively.

[0069] K pp This is a correction factor.

[0070] The correction coefficient K pp With the proportionality coefficient K P The specific preferred relationships are as follows:

[0071] K pp =1+K P .

[0072] Combining the above-derivative relationship expression of the lead-lag correction element and the preferred correction coefficient, the equivalent admittance introduced by droop control is approximately equal to the conductance characteristic of constant droop gain β.

[0073] Step iv: The constant droop gain conductivity characteristic is used to suppress overshoot oscillations of DC voltage under power fluctuations and improve DC voltage stability, as detailed below:

[0074] By approximating the conductance characteristics of a constant droop gain β, the damping characteristics of the droop-type converter station within the DC voltage control time scale are significantly improved, thereby suppressing overshoot oscillations and improving DC voltage stability.

[0075] Example 1

[0076] The following section will combine the basic control architecture of the flexible DC converter station and its DC side model, and use numerical simulation to provide a detailed explanation and verification of the simple and practical method for improving the voltage stability of the flexible DC grid proposed in this invention.

[0077] In such Figure 2 As shown, in the four-terminal flexible DC grid, the rated DC voltage of the DC system is ±200kV. Flexible DC converter stations VSC1 and VSC2 are connected to the strong AC grid and operate in droop control mode. In addition, VSC3 and VSC4 are connected to the wind farm and operate in island control mode.

[0078] In this invention, VSC1 and VSC2 both have a rated capacity of 600MVA, the proportional-integral coefficients of the outer loop controllers are 0.5 and 10 respectively, and the droop gain is 15MW / kV. Based on this, the method of this invention is applied to... Figure 2 In VSC1 and VSC2 shown, overshoot oscillations of DC voltage under power fluctuations in VSC3 and VSC4 are suppressed to improve DC voltage stability.

[0079] This invention proposes that, in front of the DC voltage-active power droop-type outer loop controller of a flexible DC converter station, a specified lead-lag correction element is added to the DC voltage sampling signal, so that the equivalent admittance introduced by the droop control approximately exhibits a conductance characteristic with a constant magnitude of droop gain within the DC voltage control time scale.

[0080] The specific basic control framework of the grid-connected MMC converter station, including the method of this invention, is as follows: Figure 1 As shown.

[0081] The DC voltage-active power droop control of flexible DC converter stations VSC1 and VSC2 has the following expression:

[0082] (P ref -P ac )+β(V dcref -V dc ) = 0

[0083] In the formula,

[0084] P ref With P ac These represent the active power command value of the converter station and the real-time transmission power on the AC side, respectively.

[0085] V dcref With V dc These represent the commanded DC voltage value and the real-time measured DC voltage, respectively, with β being the droop gain.

[0086] The active power command values ​​for both VSC1 and VSC2 are -360MW, and their DC voltage command values ​​are both 400kV.

[0087] The specified lead-lag correction element added to the DC voltage sampling signals of VSC1 and VSC2 has the following form:

[0088]

[0089] In the formula,

[0090] G c (s) is the transfer function of the correction element;

[0091] K P With K IThese represent the proportional coefficient and integral coefficient of the drooping outer loop controller (proportional-integral controller) in the flexible DC converter station, respectively.

[0092] K pp The preferred value for the correction factor is 1+K. P .

[0093] Specifically, considering that the outer loop scaling factor of both VSC1 and VSC2 is 0.5, their correction factor K PP The preferred value is 1.5.

[0094] Both VSC1 and VSC2 are connected to the strong AC power grid and operate in grid tracking mode.

[0095] also, Figure 1 Other variables in the code are defined as follows: PI is the proportional-integral controller, I... dref with I qref These represent the current commands generated by the dq-axis outer loop controller, I and I, respectively. d with I q These represent the d-axis and q-axis currents, respectively, with the rectification direction defined as the positive direction for both current and power. d With v q Representing the dq-axis voltages, R ac With L ac For the equivalent resistance and inductance on the AC side, C eq This is the equivalent capacitance on the DC side.

[0096] Specifically, the simplified average value model of the DC side of a flexible DC converter station is generally equivalent to a controlled current source connected in parallel with a capacitor, and its expression is:

[0097]

[0098] In the formula, I dc It is direct current.

[0099] The above equivalent model is based on the continuous switching function model and ignores the converter station loss and the dynamics of the inner and outer loop control.

[0100] Specifically, considering the aforementioned DC voltage-active power droop control, the small-signal model of the DC side of the flexible DC converter station, including the dynamics of the droop-type outer loop controller, is obtained under the DC voltage control time scale, as follows:

[0101]

[0102] In the formula, Δ represents the small-signal increment, the subscript 0 represents the initial steady-state point, and I eq With Y dc V represents the equivalent current source and the equivalent admittance introduced by droop control of the flexible DC converter station, respectively.pcc This refers to the voltage amplitude at the AC common coupling point.

[0103] After introducing the lead-lag element and optimizing the correction coefficient K PP Subsequently, the equivalent admittance Y introduced by droop control dc This will be transformed into:

[0104]

[0105] In the formula, since the system voltage always operates near its rated value, V pcc0 With V dc0 The per-unit value is approximately 1, which makes the equivalent admittance introduced by the droop control approximately exhibit a conductance characteristic with a constant magnitude of the droop gain β over the DC voltage control time scale.

[0106] As an application demonstration

[0107] Based on the above control parameters and system topology, a relevant model was built in PSCAD / EMTDC numerical simulation software, and the method of this invention was analyzed and explained by comparing simulations with conventional methods.

[0108] The design disturbance 1 is as follows: at 0.5s, the power of VSC3 and VSC4 suddenly increases to 600MW and 540MW respectively; at 1.5s, their power drops back to 420MW and 300MW respectively. The simulation results for voltage and power are as follows: Figure 3 and Figure 4 As shown, in conventional control, the outer loop proportional coefficient K of VSC1 and VSC2 is... P The value was changed from 0.5 to 3.0 at 1.3s.

[0109] The design disturbance 2 is as follows: at 0.2s, VSC3 is tripped without fault by a DC circuit breaker. The simulation results are as follows. Figure 5 As shown.

[0110] Figure 4 The dynamic DC voltage processes of VSC1 and VSC3 under power disturbances are shown. Since the waveforms of VSC2 and VSC4 are similar, they are omitted. The improved control method reduces the peak voltage deviations of VSC1 and VSC3 under a 0.5s power disturbance from 24kV and 26kV to 14kV and 15kV respectively, limiting the voltage deviations to within 20kV.

[0111] Under a 1.5s power disturbance, increasing K PWhile conventional methods effectively suppressed voltage overshoot oscillations, the peak voltage deviations of VSC1 and VSC3 still reached 17kV and 19kV respectively, while also prolonging the recovery process and increasing the settling time to 1 second. Thanks to the improved impedance of the droop converter station at low and medium frequencies, the method of this invention can quickly bring the disturbed DC voltage to a stable state, achieving a dynamic process similar to an inertial element. This method suppresses voltage overshoot oscillations and shortens the settling time while preserving the system's original speed.

[0112] Figure 5 The power dynamic processes of VSC1, VSC3, and VSC4 are illustrated. Since the waveform of VSC2 is similar to that of VSC1, it is omitted. With conventional control, the power dynamic process is relatively smooth, and the K value is relatively large. P It has the ability to suppress power overshoot. With the improved control, power overshoot is also avoided, and power changes more rapidly, but at the same time, there is a small oscillation process with amplitude.

[0113] Unlike the power surges in VSC3 and VSC4, when VSC3 disconnects without fault, the power injected into the DC system immediately drops to zero. Due to the power deficit in the DC system, the DC voltage of the droop converter station rapidly decreases to match the system power balance. With the improved control, the DC voltage and AC power of VSC1 exhibit noticeable oscillations. However, the method of this invention still reduces the peak DC voltage deviations of VSC1 and VSC3 from 28kV and 31kV to 14kV and 17kV, respectively. This method achieves better voltage response through faster power response, allowing the DC voltage to quickly and smoothly enter a stable state.

[0114] In summary, simulations of new energy output power fluctuations and station failures validated the correctness and effectiveness of the method presented in this invention. Adding a specified lead-lag correction element at the DC voltage signal input of the droop-type outer loop controller significantly improved the damping characteristics of the droop-type converter station within the DC voltage control timescale. This invention utilizes a simple and practical method to suppress DC voltage overshoot oscillations under disturbances and prevent the risk of DC voltage deviation exceeding limits, thereby improving the voltage stability of the DC system.

[0115] This invention simplifies the equivalent impedance of the flexible DC converter station into a second-order transfer function with zeros. By adding a specified lead-lag correction element, it offsets the adverse effects of the original impedance zeros on the system dynamics. At the same time, this element only changes the damping ratio of the flexible DC converter station without affecting other parameters such as the natural frequency.

[0116] The present invention preferably uses a correction factor to transform the equivalent impedance of the flexible DC converter station into a first-order form. At the same time, thanks to the increase of the correction factor, the sensitivity of the damping ratio to the power of the flexible DC converter station also decreases. In addition, this method of correcting the droop admittance is simple and practical, without requiring modifications to the original dual-loop controller. Furthermore, it decouples the droop admittance of the flexible DC converter station from the proportional coefficient of the proportional-integral controller, making the tuning of the command feedback loop more flexible.

Claims

1. A method for improving voltage stability of a flexible HVDC grid, characterized by: The method comprises the following steps: (i) determining a stabilizing node, and determining the stabilizing node in front of a DC voltage-active power droop outer loop controller of the flexible DC converter station; (ii) adding a specified lead-lag correction link to a DC voltage sampling signal; (iii) the lead-lag correction link makes an equivalent admittance introduced by the droop control approximately present a conductance characteristic with a constant droop gain in a DC voltage control time scale; (iv) the conductance characteristic with the constant droop gain is used to suppress overshoot oscillation of the DC voltage under power fluctuation, and improve DC voltage stability; In the step (ii), the specific process of the lead-lag correction link is as follows: Firstly, obtain a proportional coefficient and an integral coefficient of the droop outer loop controller of the flexible DC converter station; Then, obtain a correction coefficient based on the proportional coefficient and the integral coefficient; Finally, establish a relationship expression of the lead-lag correction link, and the relationship expression is as follows: In the formula, G c (s) is the transfer function of the correction element; K P With K I respectively represent the proportional coefficient and the integral coefficient of the droop type outer loop controller of the flexible DC converter station. K pp is a correction factor.

2. The method for improving voltage stability of a flexible HVDC grid according to claim 1, characterized in that: The flexible DC converter station is a grid-connected flexible DC converter station, and the grid-connected flexible DC converter station is connected with a strong AC power grid.

3. The method for improving voltage stability of a flexible HVDC grid according to claim 1, characterized in that: In the step (i), the DC voltage-active power droop control of the flexible DC converter station is as follows: Firstly, obtain an active power instruction value of the flexible DC converter station and a real-time transmission power on the AC side; Then, obtain a DC voltage instruction value of the flexible DC converter station and a real-time measured DC voltage; Finally, obtain an expression relationship of the DC voltage-active power droop control, and the expression relationship is as follows: (P ref -P ac )+β(V dcref -V dc )=0 In the formula, P ref With P ac respectively represent the active power instruction value of the converter station and the real-time transmission power on the AC side; V dcref With V dc respectively represent the direct current voltage instruction value and the real-time measured direct current voltage; β is a droop gain.

4. The method for improving voltage stability of a flexible HVDC grid according to claim 1, wherein: Said correction factor K pp With the specific preferred relationship of the proportionality factor K P as follows: K pp = 1 + K P .

5. The method for improving voltage stability of a flexible HVDC grid according to claim 4, characterized in that: In combination with the relationship expression of the lead-lag correction link and the optimized correction coefficient, the equivalent admittance introduced by the droop control is approximately a conductance characteristic with the constant droop gain β.

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