Nonlinear control method for eliminating low-frequency oscillation of regional large power grid in flexible direct current system

Abstract

The invention provides a nonlinear control method for eliminating regional large power grid low-frequency oscillation of a flexible direct current system. The nonlinear control method comprises rectification side control and inversion side nonlinear control. The rectification side control adopts constant-voltage double-closed-loop control; the inversion side nonlinear control performs interval control on active power and reactive power of the flexible direct current system according to real-time angular frequency deviation, frequency change rate and frequency integral quantity information of a power grid, and working modes comprise four operation modes of maximum suppression, rapid recovery, tracking control and normal operation. According to the method, the maximum utilization of the flexible direct current system is realized through active and reactive flexible control, the influence of the real-time power grid frequency state on the system is fully considered, and the capability of the flexible direct current system for suppressing the low-frequency oscillation of the large power grid in the region is greatly improved.

Description

Technical Field

[0001] The invention relates to the technical field of control and protection of a flexible direct current transmission system, and in particular to a nonlinear control method for a flexible direct current system to eliminate low-frequency oscillation of a regional large power grid. Background Art

[0002] With the increasing penetration of new energy sources such as photovoltaic and wind power in my country, the installed capacity of traditional synchronous generators has been continuously reduced, making the power grid more prone to oscillation when it is disturbed by external factors. Therefore, large-capacity flexible DC systems should play their role in power grid frequency regulation and assume the responsibility of maintaining the stable operation of the power grid.

[0003] At present, the control methods of flexible DC systems are mainly divided into grid-following type and grid-building type. The core of the grid-following flexible DC transmission system control method, such as direct power control, is to stably transmit power according to the needs of the power grid. However, this method lacks a response mechanism to the grid frequency and cannot effectively improve the stability of the grid. Grid-building flexible DC transmission system control methods, such as virtual synchronous generator control. This method simulates the large inertia and strong damping characteristics of traditional synchronous generator sets from the external characteristics, but it cannot significantly adjust the control parameters according to the real-time angular frequency deviation, frequency change rate and frequency integral information of the power grid, so it is difficult to achieve rapid suppression of low-frequency oscillations of regional large power grids.

[0004] In summary, in order to realize the rapid elimination of regional large power grid oscillations in the flexible DC system, there is an urgent need for a method with a simple parameter design process and the ability to quickly suppress oscillations based on the real-time frequency information of the power grid. Summary of the invention

[0005] In view of the above technical problems, the present invention provides a nonlinear control method for a flexible DC system to eliminate low-frequency oscillations of a large regional power grid, which can quickly eliminate power grid oscillations without the need for a complex parameter design process.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A nonlinear control method for eliminating low-frequency oscillation of a regional large power grid in a flexible direct current system comprises the following steps:

[0008] Step 1: Set the angular frequency deviation, frequency change rate and frequency integral control threshold of the inverter side regional large power grid;

[0009] Step 2: Use a current transformer (TA) and a voltage transformer (TV) to detect the three-phase current, DC capacitor voltage and three-phase voltage of the inverter side in real time, and 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: The three-phase current and the three-phase voltage on the inverter side are transformed by the Parker transformation matrix to obtain the current and voltage signals in the dq rotating coordinate system;

[0011] Step 4: The rectifier side control of the flexible DC system adopts constant voltage double closed-loop control, and the difference between the real-time detected DC capacitor voltage and the reference capacitor voltage is closed-loop controlled through a PI controller to achieve the DC capacitor voltage tracking the reference voltage without difference;

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

[0013] Furthermore, in the step 2, the real-time three-phase current of the inverter side is detected by a current transformer (TA), and the DC capacitor voltage and the three-phase voltage of 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 3, the three-phase current or voltage on the inverter side is expressed by the Park transformation matrix as follows:

[0015]

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

[0017] Furthermore, in step 4, the rectifier side control adopts constant voltage double closed loop control, that is,

[0018]

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

[0020] Furthermore, in step 5, the inverter side nonlinear control is adjusted in different intervals according to the angular frequency deviation, the frequency change rate and the frequency integral information. The specific implementation process is as follows:

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

[0022]

[0023] Among them, K 1p and K 1i Represents the proportional and integral gain of the active power in the maximum suppression operation mode; K 2p and K 2i Represents the proportional and integral gain of the reactive power in the maximum suppression operation mode; P z and Q z Indicates reference active and reactive power; P and Q indicate real-time active and reactive power; I d and I q Indicates the reference value of the inverter side current d-axis and q-axis;

[0024] The reference active and reactive power of the maximum suppression operation mode is expressed as:

[0025]

[0026] Where S represents the capacity of the flexible DC system; P mp Indicates the maximum output active power; P m Indicates the minimum output active power in the operating state;

[0027] 2) When the frequency information is between the maximum suppression mode and the fast recovery mode threshold (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 represents the maximum and minimum control thresholds of the angular frequency deviation in the fast recovery operation mode, the R m , R n Indicates the maximum and minimum control thresholds of 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 Represents the proportional and differential gain of the active power in fast recovery operation mode; K 4p and K 4d Represents the proportional and differential gain of reactive power in fast recovery operation mode; D 3p Represents the proportional gain of the angular frequency deviation in the fast recovery operation mode; P ref , Q ref and ω ref Indicates reference active power, reactive power and angular frequency;

[0031] 3) When the frequency information is between the fast recovery mode and the tracking control operation mode threshold (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 represents the maximum and minimum control thresholds of the angular frequency deviation in the tracking control mode, the R p0 , R n0 represents the maximum and minimum control thresholds of the frequency change rate in the tracking control mode, the I max ,I min Indicates the maximum and minimum control thresholds of the frequency integral in the tracking control mode. The inverter-side converter of the flexible DC system adopts the tracking control mode:

[0032]

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

[0034] 4) When the frequency information is less than the tracking control operation mode threshold (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 adopts normal operation mode:

[0035]

[0036] Among them, K 7p and K 7i Represents the proportional and differential gain of the active power in normal operating mode; K 8p and K 8i Represents the proportional and derivative gains of the reactive power in normal operation mode.

[0037] The advantages of the present invention are as follows: the present invention does not require a complicated parameter design process, is simple and easy to implement, and has a high engineering application value; according to the real-time angular frequency deviation, frequency change rate and frequency integral information of the power grid, the present invention utilizes active and reactive power flexible control to achieve maximum utilization of the flexible DC system, quickly and effectively suppress power grid oscillations, and ensure the stability of the power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 A schematic diagram of a simulated power grid of a flexible DC system in the present invention;

[0039] Figure 2 It is the control block diagram of the rectifier side in the present invention;

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

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

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

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

[0044] Figure 7 A schematic flow chart of a nonlinear control method for eliminating low-frequency oscillations of a regional large power grid in a flexible DC system provided by the present invention. DETAILED DESCRIPTION

[0045] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0046] See also Figure 1-7 The embodiment of the present invention provides a nonlinear control method for eliminating low-frequency oscillation of 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 threshold of the inverter-side regional large power grid.

[0048] The control threshold of angular frequency deviation can be divided into Δω according to its size max , Δω m , Δω p0 , Δω n0 , Δω n , Δω min ; The control threshold of the frequency change rate can be divided into R max , R m , R p0 , R n0 , R n , R min ; The control threshold of the frequency integral can be divided into I max ,I min The actual values ​​of the angular frequency deviation Δω, the frequency change rate R and the frequency integral value I control threshold value can be set according to the power grid conditions and relevant standards.

[0049] Step 2: Use current transformer (TA) and voltage transformer (TV) to detect the real-time three-phase current, DC capacitor voltage and three-phase voltage on the inverter side, and 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 grid angular frequency deviation, frequency change rate and frequency integral information.

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

[0051] Step 3: The three-phase current and voltage on the inverter side are transformed through the Park transformation matrix to obtain the current and voltage signals in the dq rotating coordinate system.

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

[0053]

[0054] Among them, Z d and Z q represents the current and voltage signal of the inverter side in the dq rotating coordinate system; Z sa , Z sb and Z sc They represent the A, B, and C phases 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 double closed-loop control. The difference between the real-time detected DC capacitor voltage and the reference capacitor voltage is closed-loop controlled through a PI controller to achieve DC capacitor voltage tracking reference voltage without difference. Figure 2 shown.

[0056] The rectifier side control adopts constant voltage double closed loop control, that is,

[0057]

[0058] Among them, K p and K i Indicates the proportional and integral gain of the constant pressure double closed loop control; U dc Represents the reference capacitor voltage; u dc Indicates the real-time DC capacitor voltage; i d Indicates the d-axis reference value of 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 working mode of the inverter side converter (maximum suppression, fast recovery, tracking control and normal operation) is determined. By controlling the current and voltage output in the dq rotating coordinate system, the active and reactive power of the flexible DC system is controlled in different intervals to quickly eliminate the oscillation of the regional large power grid.

[0060] The inverter side nonlinear control is adjusted in different intervals according to the angular frequency deviation, frequency change rate and frequency integral information. The working modes include: maximum suppression, fast recovery, tracking control and normal operation mode, such as Figure 7 The specific implementation process is as follows:

[0061] 1) When the frequency information is greater than the maximum suppression operation 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 shown.

[0062]

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

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

[0065]

[0066] Where S represents the capacity of the flexible DC system; P mp Indicates the maximum output active power; P m Indicates the minimum output active power in the operating state.

[0067] 2) When the frequency information is between the maximum suppression mode and the fast recovery mode threshold (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 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 Represents the proportional and differential gain of the active power in fast recovery operation mode; K 4p and K 4d Represents the proportional and differential gain of reactive power in fast recovery operation mode; D 3p Represents the proportional gain of the angular frequency deviation in the fast recovery operation mode; P ref , Q ref and ω ref Indicates reference active power, reactive power and angular frequency.

[0071] 3) When the frequency information is between the fast recovery mode and the tracking control operation mode threshold (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 inverter-side converter of the flexible DC system adopts the tracking control mode, such as Figure 5 shown.

[0072]

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

[0074] 4) When the frequency information is less than the tracking control operation mode threshold (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 adopts the normal operation mode, such as Figure 6 shown.

[0075]

[0076] Among them, K 7p and K 7i Represents the proportional and differential gain of the active power in normal operating mode; K 8p and K 8i Represents the proportional and derivative gains of the reactive power in normal operation mode.

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

[0078] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A nonlinear control method for eliminating low-frequency oscillation of a regional large power grid in a flexible DC system, characterized in that: The steps include: Step 1: Set the angular frequency deviation, frequency change rate and frequency integral control threshold of the inverter side regional large power grid; Step 2: Use a current transformer (TA) and a voltage transformer (TV) to detect the three-phase current, DC capacitor voltage and three-phase voltage of the inverter side in real time, and 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: The three-phase current and the three-phase voltage on the inverter side are transformed by the Parker transformation matrix to obtain the current and voltage signals in the dq rotating coordinate system; Step 4: The rectifier side control of the flexible DC system adopts constant voltage double closed-loop control, and the difference between the real-time detected DC capacitor voltage and the reference capacitor voltage is closed-loop controlled through a PI controller to achieve the DC capacitor voltage tracking the reference voltage without difference; Step 5. Nonlinear control of the inverter side of the flexible DC system. The angular frequency deviation, frequency change rate and frequency integral control threshold of the inverter side regional power grid set in step 1 are compared with the real-time frequency information of the power grid to determine the working mode of the inverter side converter. The active and reactive power of the flexible DC system is controlled in partitions by controlling the output of current and voltage signals in the dq rotating coordinate system to quickly eliminate oscillations in the regional power grid. The working modes include four operating modes: maximum suppression, fast recovery, tracking control and normal operation.

2. The nonlinear control method for eliminating low-frequency oscillation of a regional large power grid in a flexible DC system according to claim 1, characterized in that: In the step 2, the real-time three-phase current of the inverter side is detected by a current transformer (TA), and the DC capacitor voltage and the three-phase voltage of 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; In step 3, the three-phase current or voltage on the inverter side is expressed by the Park transformation matrix as follows: Among them, Z d and Z q represents the current and voltage signal of the inverter side in the dq rotating coordinate system; Z sa , Z sb and Z sc They represent the A, B, and C phases 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 oscillation of a regional large power grid in a flexible DC system according to claim 1, characterized in that: In step 4, the rectifier side control adopts constant voltage double closed loop control, that is, Among them, K p and K i Indicates the proportional and integral gain of the constant pressure double closed loop control; U dc Represents the reference capacitor voltage; u dc Indicates the real-time DC capacitor voltage; i d Indicates the d-axis reference value of the rectifier side current.

4. The nonlinear control method for eliminating low-frequency oscillation of a regional large power grid in a flexible DC system according to claim 1, characterized in that: In step 5, the inverter side nonlinear control is adjusted in different intervals according to the angular frequency deviation, the frequency change rate and the frequency integral information. The specific implementation process is as follows: 1) When the frequency information is greater than the maximum suppression operation mode threshold, that is, ω ≥ Δω max Or ω≤Δω min or R ≥ R max or R≤R min , the Δω max , Δω min represents the maximum and minimum control thresholds of the angular frequency deviation in the maximum suppression operation mode, the R max , R min Indicates the maximum and minimum control thresholds of the frequency change rate in the maximum suppression operation mode. The inverter-side converter of the flexible DC system adopts the maximum suppression operation mode: Among them, K 1p and K 1i Represents the proportional and integral gain of the active power in the maximum suppression operation mode; K 2p and K 2i Represents the proportional and integral gain of the reactive power in the maximum suppression operation mode; P z and Q z Indicates reference active and reactive power; P and Q indicate real-time active and reactive power; I d and I q Indicates the reference value of the inverter side current d-axis and q-axis; The reference active and reactive power of the maximum suppression operation mode is expressed as: Where S represents the capacity of the flexible DC system; P mp Indicates the maximum output active power; P m Indicates the minimum output active power in the operating state; 2) When the frequency information is between the maximum suppression mode and the fast recovery mode threshold, that is, 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 represents the maximum and minimum control thresholds of the angular frequency deviation in the fast recovery operation mode, the R m , R n Indicates the maximum and minimum control thresholds of 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 Represents the proportional and differential gain of the active power in fast recovery operation mode; K 4p and K 4d Represents the proportional and differential gain of reactive power in fast recovery operation mode; D 3p Represents the proportional gain of the angular frequency deviation in the fast recovery operation mode; P ref , Q ref and ω ref Indicates reference active power, reactive power and angular frequency; 3) When the frequency information is between the threshold of the fast recovery mode and the tracking control mode, that is, 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 represents the maximum and minimum control thresholds of the angular frequency deviation in the tracking control mode, the R p0 , R n0 represents the maximum and minimum control thresholds of the frequency change rate in the tracking control mode, the I max ,I min Indicates the maximum and minimum control thresholds of the frequency integral in the tracking control mode. The inverter-side converter of the flexible DC system adopts the tracking control mode: Among them, K 5i and K 6i Indicates the integral gain of active and reactive power in tracking control operation mode; D 5p Represents the proportional gain of the angular frequency deviation in the tracking control operation mode; 4) When the frequency information is less than the threshold of the tracking control operation mode, that is, 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 adopts normal operating mode: Among them, K 7p and K 7i Represents the proportional and differential gain of the active power in normal operating mode; K 8p and K 8i Represents the proportional and derivative gains of the reactive power in normal operation mode.

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