A nonlinear control method for eliminating low-frequency oscillations in a regional large power grid in a flexible DC system

Abstract

This invention provides a nonlinear control method for eliminating low-frequency oscillations in a regional large power grid in a flexible DC system, comprising: rectifier-side control and inverter-side nonlinear control; the rectifier-side control adopts constant-voltage dual-closed-loop control; the inverter-side nonlinear control performs interval control of the active and reactive power of the flexible DC system based on the real-time angular frequency deviation, frequency change rate, and frequency integral information of the power grid, with four operating modes: maximum suppression, fast recovery, tracking control, and normal operation. This invention maximizes the utilization of the flexible DC system by utilizing flexible active and reactive power control, and fully considers the impact of real-time power grid frequency status on the system, significantly improving the ability of the flexible DC system to suppress low-frequency oscillations in a regional large power grid.

Description

Technical Field

[0001] This invention relates to the field of control and protection technology for flexible DC transmission systems, specifically a nonlinear control method for eliminating low-frequency oscillations in regional large power grids in flexible DC systems. Background Technology

[0002] With the increasing penetration of new energy sources such as photovoltaics and wind power in my country, the installed capacity of traditional synchronous generator units is continuously decreasing, making the power grid more prone to oscillations when subjected to external disturbances. Therefore, large-capacity flexible DC systems should play a role in power grid frequency regulation and assume the responsibility of maintaining the stable operation of the power grid.

[0003] Currently, flexible DC system control methods are mainly divided into grid-connected and grid-connected types. Grid-connected flexible DC transmission system control methods, such as direct power control, focus on stabilizing power transmission according to grid demand. However, this method lacks a response mechanism to grid frequency and cannot effectively improve grid stability. Grid-connected flexible DC transmission system control methods, such as virtual synchronous generator control, simulate the large inertia and strong damping characteristics of traditional synchronous generator sets in terms of external characteristics. However, it cannot significantly adjust control parameters based on real-time grid angular frequency deviation, frequency change rate, and frequency integral information, thus making it difficult to rapidly suppress low-frequency oscillations in large regional power grids.

[0004] In summary, in order to achieve rapid elimination of regional power grid oscillations in flexible DC systems, there is an urgent need for a method with a simple parameter design process that can quickly suppress oscillations based on real-time power grid frequency information. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a nonlinear control method for eliminating low-frequency oscillations in a large regional power grid using a flexible DC system. This method achieves rapid elimination of power grid oscillations without requiring a complex parameter design process.

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

[0007] A nonlinear control method for eliminating low-frequency oscillations in a regional large power grid in a flexible DC system includes the following steps:

[0008] Step 1: Set the angular frequency deviation, frequency change rate, and frequency integral control thresholds for the inverter-side regional power grid.

[0009] Step 2: Use current transformers (TA) and voltage transformers (TV) to detect the three-phase current, DC capacitor voltage and inverter three-phase voltage on the inverter side in real time. Then, obtain the real-time frequency information of the power grid through a phase-locked loop. The real-time frequency information of the power grid includes the power grid angular frequency deviation, frequency change rate and frequency integral information.

[0010] Step 3: Obtain the current and voltage signals in the dq rotating coordinate system by passing the three-phase current and three-phase voltage on the inverter side through the Parker transformation matrix;

[0011] Step 4: The rectifier side control of the flexible DC system adopts constant voltage dual closed-loop control. The difference between the real-time detected DC capacitor voltage and the reference capacitor voltage is controlled by a PI controller to achieve error-free tracking of the reference voltage by the DC capacitor voltage.

[0012] Step 5: Nonlinear control of the inverter side of the flexible DC system. Based on the comparison of the angular frequency deviation, frequency change rate, and frequency integral control threshold of the regional power grid set in Step 1 with the real-time frequency information of the power grid, the operating mode of the inverter-side converter is determined. The active and reactive power of the flexible DC system is controlled in intervals by controlling the current and voltage signal output in the dq rotating coordinate system to quickly eliminate regional power grid oscillations. The operating modes include four modes: maximum suppression, fast recovery, tracking control, and normal operation.

[0013] Furthermore, in step two, the real-time three-phase current on the inverter side is detected by a current transformer (TA), and the DC capacitor voltage and the three-phase voltage on the inverter side are detected by a voltage transformer (TV). Subsequently, the real-time angular frequency is obtained by a single synchronous coordinate system software phase-locked loop, and then the grid angular frequency deviation, frequency change rate and frequency integral information are calculated.

[0014] In step three, the three-phase current or voltage on the inverter side is represented by the Parker transformation matrix as follows:

[0015]

[0016] Among them, Z d and Z q Z represents the inverter-side current and voltage signals in the dq rotating coordinate system; sa , Z sb and Z sc These represent phases A, B, and C of the three-phase current or voltage on the inverter side, respectively; θ represents the synchronous phase.

[0017] Furthermore, in step four, the rectifier-side control employs constant voltage dual closed-loop control, i.e.

[0018]

[0019] Among them, K p and K i U represents the proportional and integral gain of the constant voltage dual closed-loop control; dc Indicates the reference capacitor voltage; u dc Indicates the real-time DC capacitor voltage; i d This represents the d-axis reference value for the rectifier-side current.

[0020] Furthermore, in step five, the inverter-side nonlinear control performs interval-based adjustment based on angular frequency deviation, frequency change rate, and frequency integral information. The specific implementation process is as follows:

[0021] 1) When the frequency information is greater than the maximum suppression operating mode threshold (ω≥Δω) max Or ω≤Δω min or R≥R max or R≤R min ), the Δω max , Δω min R represents the maximum and minimum control thresholds for suppressing angular frequency deviation in the maximum operating mode. max R min This represents the maximum and minimum control thresholds for the rate of frequency change in the maximum suppression operating mode. The inverter-side converter of the flexible DC system adopts the maximum suppression operating mode.

[0022]

[0023] Among them, K 1p and K 1i K represents the proportional and integral gain of the active power in the maximum suppressed operating mode. 2p and K 2i P represents the proportional and integral gain of reactive power in the maximum suppressed operating mode. z and Q z P and Q represent reference active and reactive power; I represents real-time active and reactive power. d and I q Indicates the reference values ​​for the inverter-side current on the d-axis and q-axis;

[0024] The reference active and reactive power for the maximum suppression operation mode are expressed as follows:

[0025]

[0026] Where S represents the capacity of the flexible DC system; P mp P represents the maximum output active power; m This indicates the minimum output active power during operation.

[0027] 2) When the frequency information is between the thresholds of the maximum suppression mode and the fast recovery mode (Δω) m ≤ω<Δω max or Δω min <ω≤Δω n Or R m ≤R <R max Or R min <R≤R n ), the Δω m , Δωn R represents the maximum and minimum control thresholds for the angular frequency deviation in the fast recovery operating mode. m R n This indicates the maximum and minimum control thresholds for the frequency change rate in the fast recovery operation mode. The inverter-side converter of the flexible DC system adopts the fast recovery operation mode.

[0028] I d =(K 3p +sK 3d )[P ref -P+D 3p (ω ref -ω)]

[0029] I q =(K 4p +sK 4d (Q) ref -Q)

[0030] Among them, K 3p and K 3d K represents the proportional and differential gain of active power in fast recovery mode. 4p and K 4d D represents the proportional and differential gain of reactive power in the fast recovery mode. 3p P represents the proportional gain that indicates the angular frequency deviation of the fast recovery operation mode; ref Q ref and ω ref This represents the reference active power, reactive power, and angular frequency.

[0031] 3) When the frequency information is between the threshold values ​​of fast recovery mode and tracking control operation mode (Δω) p0 ≤ω<Δω m or Δω n <ω≤Δω n0 Or R p0 ≤R <R m Or R n <R≤R n0 or I≥I max or I≤I min ), the Δω p0 , Δω n0 R represents the maximum and minimum control thresholds for the angular frequency deviation in the tracking control mode. p0 R n0 The I represents the maximum and minimum control thresholds for the frequency change rate of the tracking control mode. max I min This indicates the maximum and minimum control thresholds for the frequency integral in tracking control mode. The inverter-side converter of the flexible DC system adopts tracking control mode.

[0032]

[0033] Among them, K 5i and K 6i D represents the integral gain of active and reactive power in the tracking control operation mode; 5p The proportional gain represents the angular frequency deviation of the tracking control operation mode.

[0034] 4) When the frequency information is less than the threshold of the tracking control operation mode (Δω) n0 <ω<Δω p0 And R n0 <R<R p0 And I min <I<I max The inverter-side converter of the flexible DC system operates in normal mode.

[0035]

[0036] Among them, K 7p and K 7i K represents the proportional and differential gain of active power in normal operating mode. 8p and K 8i This represents the ratio and differential gain of reactive power during normal operation.

[0037] The advantages of this invention are as follows: This invention does not require a complex parameter design process, is simple and easy to implement, and has high engineering application value; based on the real-time angular frequency deviation, frequency change rate and frequency integral information of the power grid, this invention utilizes active and reactive power flexible control to maximize the utilization of the flexible DC system, quickly and effectively suppresses power grid oscillations, and ensures the stability of the power system. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of a simulated power grid for the flexible DC system in this invention;

[0039] Figure 2 This is a block diagram of the rectifier-side control in this invention;

[0040] Figure 3 This is a control block diagram of the maximum suppression operation mode of the inverter-side nonlinear control method in this invention;

[0041] Figure 4 This is a control block diagram of the fast recovery operation mode of the inverter-side nonlinear control method in this invention;

[0042] Figure 5 This is a control block diagram of the tracking control operation mode of the inverter-side nonlinear control method in this invention;

[0043] Figure 6This is a control block diagram of the normal operation mode of the inverter-side nonlinear control method in this invention;

[0044] Figure 7 This is a flowchart illustrating a nonlinear control method for eliminating low-frequency oscillations in a regional large power grid in a flexible DC system, as provided by the present invention. Detailed Implementation

[0045] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0046] Please see Figure 1-7 This invention provides a nonlinear control method for eliminating low-frequency oscillations in a regional large power grid in a flexible DC system, comprising the following steps:

[0047] Step 1: Set the angular frequency deviation, frequency change rate, and frequency integral control thresholds for the inverter-side regional power grid.

[0048] The control threshold for angular frequency deviation can be divided into Δω based on its magnitude. max , Δω m , Δω p0 , Δω n0 , Δω n , Δω min The control thresholds for the rate of change of frequency can be divided into R from large to small. max R m R p0 R n0 R n R min The control threshold for the frequency integral can be divided into I... max I min The actual values ​​of the angular frequency deviation Δω, the rate of change of frequency R, and the frequency integral I control threshold can be set according to the power grid conditions and relevant standards.

[0049] Step 2: Use current transformers (TA) and voltage transformers (TV) to detect the real-time three-phase current, DC capacitor voltage, and inverter-side three-phase voltage. Then, obtain the real-time grid frequency information through a phase-locked loop. The real-time grid frequency information includes grid angular frequency deviation, frequency change rate, and frequency integral information.

[0050] The real-time three-phase current on the inverter side is detected using a current transformer (TA). The DC capacitor voltage and the three-phase voltage on the inverter side are detected using a voltage transformer (TV). Subsequently, the real-time angular frequency is obtained using a single-synchronous coordinate system software phase-locked loop, and the grid angular frequency deviation, rate of change of frequency, and frequency integral are then calculated.

[0051] Step 3: Obtain the current and voltage signals in the dq rotating coordinate system by using the Parker transformation matrix to obtain the three-phase current and voltage on the inverter side.

[0052] The three-phase current or voltage on the inverter side can be expressed using the Parker transformation matrix as follows:

[0053]

[0054] Among them, Z d and Z q Z represents the inverter-side current and voltage signals in the dq rotating coordinate system; sa , Z sb and Z sc These represent phases A, B, and C of the three-phase current or voltage on the inverter side, respectively; θ represents the synchronous phase.

[0055] Step 4: The rectifier-side control of the flexible DC system adopts constant voltage dual closed-loop control. The difference between the real-time detected DC capacitor voltage and the reference capacitor voltage is used for closed-loop control through a PI controller to achieve error-free tracking of the reference voltage by the DC capacitor voltage. Figure 2 As shown.

[0056] The rectifier-side control adopts constant voltage dual closed-loop control, i.e.

[0057]

[0058] Among them, K p and K i U represents the proportional and integral gain of the constant voltage dual closed-loop control; dc Indicates the reference capacitor voltage; u dc Indicates the real-time DC capacitor voltage; i d This represents the d-axis reference value for the rectifier-side current.

[0059] Step 5: Nonlinear control of the inverter side of the flexible DC system. Based on the comparison between the control threshold in Step 1 and the real-time frequency information of the power grid, the operating mode of the inverter-side converter (maximum suppression, fast recovery, tracking control and normal operation) is determined. The active and reactive power of the flexible DC system is controlled in intervals by controlling the current and voltage output in the dq rotating coordinate system, so as to quickly eliminate regional power grid oscillations.

[0060] The inverter-side nonlinear control performs interval-based adjustment based on angular frequency deviation, frequency change rate, and frequency integral information. Operating modes include: maximum suppression, fast recovery, tracking control, and normal operation. Figure 7 As shown. The specific implementation process is as follows:

[0061] 1) When the frequency information is greater than the maximum suppression operating mode threshold (ω≥Δω) max Or ω≤Δω min or R≥R max or R≤R min The inverter-side converter of the flexible DC system adopts the maximum suppression operation mode, such as... Figure 3 As shown.

[0062]

[0063] Among them, K 1p and K 1i K represents the proportional and integral gain of the active power in the maximum suppressed operating mode. 2p and K 2i P represents the proportional and integral gain of reactive power in the maximum suppressed operating mode. z and Q z P and Q represent reference active and reactive power; P and Q represent real-time active and reactive power.

[0064] The reference active and reactive power for the maximum suppression operation mode can be expressed as:

[0065]

[0066] Where S represents the capacity of the flexible DC system; P mp P represents the maximum output active power; m This indicates the minimum active power output during operation.

[0067] 2) When the frequency information is between the thresholds of the maximum suppression mode and the fast recovery mode (Δω) m ≤ω<Δω max or Δω min <ω≤Δω n Or R m ≤R <R max Or R min <R≤R n The inverter-side converter of the flexible DC system adopts a fast recovery operation mode, such as... Figure 4 As shown.

[0068] I d =(K 3p +sK 3d )[P ref -P+D 3p (ωref -ω)]

[0069] I q =(K 4p +sK 4d (Q) ref -Q)

[0070] Among them, K 3p and K 3d K represents the proportional and differential gain of active power in fast recovery mode. 4p and K 4d D represents the proportional and differential gain of reactive power in the fast recovery mode. 3p P represents the proportional gain that indicates the angular frequency deviation of the fast recovery operation mode; ref Q ref and ω ref This indicates the reference active power, reactive power, and angular frequency.

[0071] 3) When the frequency information is between the threshold values ​​of fast recovery mode and tracking control operation mode (Δω) p0 ≤ω<Δω m or Δω n <ω≤Δω n0 Or R p0 ≤R <R m Or R n <R≤R n0 or I≥I max or I≤I min In flexible DC systems, the inverter-side converter adopts a tracking control mode, such as... Figure 5 As shown.

[0072]

[0073] Among them, K 5i and K 6i D represents the integral gain of active and reactive power in the tracking control operation mode; 5p This represents the proportional gain that indicates the angular frequency deviation of the tracking control operation mode.

[0074] 4) When the frequency information is less than the threshold of the tracking control operation mode (Δω) n0 <ω<Δω p0 And R n0 <R<R p0 And I min <I<I max In the flexible DC system, the inverter-side converter adopts normal operating mode, such as... Figure 6 As shown.

[0075]

[0076] Among them, K 7p and K 7i K represents the proportional and differential gain of active power in normal operating mode. 8p and K 8i This represents the ratio and differential gain of reactive power during normal operation.

[0077] Through the above control process, the utilization of the flexible DC system can be maximized, and the stability of the power system can be improved.

[0078] The above description is merely a specific 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 nonlinear control method for eliminating low-frequency oscillations in a regional large power grid in a flexible DC system, characterized in that, Includes the following steps: Step 1: Set the angular frequency deviation, frequency change rate, and frequency integral control thresholds for the inverter-side regional power grid. Step 2: Use current transformers (TA) and voltage transformers (TV) to detect the three-phase current, DC capacitor voltage and inverter three-phase voltage on the inverter side in real time. Then, obtain the real-time frequency information of the power grid through a phase-locked loop. The real-time frequency information of the power grid includes the power grid angular frequency deviation, frequency change rate and frequency integral information. Step 3: Obtain the current and voltage signals in the dq rotating coordinate system by passing the three-phase current and three-phase voltage on the inverter side through the Parker transformation matrix; Step 4: The rectifier side control of the flexible DC system adopts constant voltage dual closed-loop control. The difference between the real-time detected DC capacitor voltage and the reference capacitor voltage is controlled by a PI controller to achieve error-free tracking of the reference voltage by the DC capacitor voltage. Step 5: Nonlinear control of the inverter side of the flexible DC system. Based on the comparison of the angular frequency deviation, frequency change rate, and frequency integral control threshold of the regional power grid set in Step 1 with the real-time frequency information of the power grid, the operating mode of the inverter-side converter is determined. The active and reactive power of the flexible DC system is controlled in intervals by controlling the current and voltage signal output in the dq rotating coordinate system to quickly eliminate regional power grid oscillations. The operating modes include four modes: maximum suppression, fast recovery, tracking control, and normal operation.

2. The nonlinear control method for eliminating low-frequency oscillations in a regional large power grid in a flexible DC system as described in claim 1, characterized in that: In step two, the real-time three-phase current on the inverter side is detected by a current transformer (TA), and the DC capacitor voltage and the three-phase voltage on the inverter side are detected by a voltage transformer (TV). Then, the real-time angular frequency is obtained by a single synchronous coordinate system software phase-locked loop, and then the grid angular frequency deviation, frequency change rate and frequency integral information are calculated. In step three, the three-phase current or voltage on the inverter side is represented by the Parker transformation matrix as follows: Among them, Z d and Z q Z represents the inverter-side current and voltage signals in the dq rotating coordinate system; sa , Z sb and Z sc These represent phases A, B, and C of the three-phase current or voltage on the inverter side, respectively; θ represents the synchronous phase.

3. The nonlinear control method for eliminating low-frequency oscillations in a regional large power grid in a flexible DC system as described in claim 1, characterized in that: In step four, the rectifier-side control employs constant voltage dual closed-loop control, i.e. Among them, K p and K i U represents the proportional and integral gain of the constant voltage dual closed-loop control; dc Indicates the reference capacitor voltage; u dc Indicates the real-time DC capacitor voltage; i d This represents the d-axis reference value for the rectifier-side current.

4. The nonlinear control method for eliminating low-frequency oscillations in a regional large power grid in a flexible DC system as described in claim 1, characterized in that: In step five, the inverter-side nonlinear control performs interval-based adjustment based on angular frequency deviation, frequency change rate, and frequency integral information. The specific implementation process is as follows: 1) When the frequency information is greater than the maximum suppression operating mode threshold, i.e., ω ≥ Δω max Or ω≤Δω min or R≥R max or R≤R min The Δω max , Δω min R represents the maximum and minimum control thresholds for suppressing angular frequency deviation in the maximum operating mode. max R min This represents the maximum and minimum control thresholds for the rate of frequency change in the maximum suppression operating mode. The inverter-side converter of the flexible DC system adopts the maximum suppression operating mode. Among them, K 1p and K 1i K represents the proportional and integral gain of the active power in the maximum suppressed operating mode. 2p and K 2i P represents the proportional and integral gain of reactive power in the maximum suppressed operating mode. z and Q z P and Q represent reference active and reactive power; I represents real-time active and reactive power. d and I q Indicates the reference values ​​for the inverter-side current on the d-axis and q-axis; The reference active and reactive power for the maximum suppression operation mode are expressed as follows: Where S represents the capacity of the flexible DC system; P mp P represents the maximum output active power; m This indicates the minimum output active power during operation. 2) When the frequency information is between the thresholds of the maximum suppression mode and the fast recovery mode, i.e., Δω m ≤ω<Δω max or Δω min <ω≤Δω n Or R m ≤R <R max Or R min <R≤R n The Δω m , Δω n R represents the maximum and minimum control thresholds for the angular frequency deviation in the fast recovery operating mode. m R n This indicates the maximum and minimum control thresholds for the frequency change rate in the fast recovery operation mode. The inverter-side converter of the flexible DC system adopts the fast recovery operation mode. I d =(K 3p +sK 3d )[P ref -P+D 3p (ω ref -ω)] I q =(K 4p +sK 4d )(Q ref -Q) Among them, K 3p and K 3d K represents the proportional and differential gain of active power in fast recovery mode. 4p and K 4d D represents the proportional and differential gain of reactive power in the fast recovery mode. 3p P represents the proportional gain that indicates the angular frequency deviation of the fast recovery operation mode; ref Q ref and ω ref This represents the reference active power, reactive power, and angular frequency. 3) When the frequency information is between the threshold values ​​of fast recovery mode and tracking control operation mode, i.e., Δω p0 ≤ω<Δω m or Δω n <ω≤Δω n0 Or R p0 ≤R <R m Or R n <R≤R n0 or I≥I max or I≤I min The Δω p0 , Δω n0 R represents the maximum and minimum control thresholds for the angular frequency deviation in the tracking control mode. p0 R n0 The I represents the maximum and minimum control thresholds for the frequency change rate of the tracking control mode. max I min This indicates the maximum and minimum control thresholds for the frequency integral in tracking control mode. The inverter-side converter of the flexible DC system adopts tracking control mode. Among them, K 5i and K 6i D represents the integral gain of active and reactive power in the tracking control operation mode; 5p The proportional gain represents the angular frequency deviation of the tracking control operation mode. 4) When the frequency information is less than the threshold of the tracking control operation mode, i.e., Δω n0 <ω<Δω p0 And R n0 <R<R p0 And I min <I<I max The inverter-side converter of the flexible DC system operates in normal mode: Among them, K 7p and K 7i K represents the proportional and differential gain of active power in normal operating mode. 8p and K 8i This represents the ratio and differential gain of reactive power during normal operation.

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