A small disturbance stability analysis method and system for a grid-type converter station

Abstract

The present invention discloses a method and system for analyzing the small-disturbance stability of a station containing a grid-type converter, comprising: establishing an impedance model of a grid-following converter; establishing an impedance model of a grid-forming converter; constructing a grid-connected system model of a new energy station containing both a grid-following and a grid-forming type; establishing a simplified equivalent circuit of the station grid-connected system; establishing a Nyquist criterion for the small-disturbance stability of the grid-connected new energy station containing the grid-forming type; and analyzing the small-disturbance stability characteristics of the station grid-connected system using a Nyquist diagram. The present invention solves the problem in the prior art of the lack of quantitative and effective analysis of the small-disturbance stability of new energy stations containing grid-following and grid-forming converters. Based on the established small signal model, a station small-disturbance stability criterion is established to quantitatively analyze the small-disturbance stability characteristics of new energy stations containing grid-forming converters in low grid strength scenarios. Compared with traditional impedance modeling methods for new energy stations, the present invention can better reflect the impact of grid-forming access on the small-disturbance stability of new energy stations.

Description

Technical Field

[0001] The present invention relates to a stability analysis method and system for a transformer station, and in particular to a small disturbance stability analysis method and system for a station containing a grid-type converter. Background Art

[0002] When traditional renewable energy stations are connected to the main grid via long-distance lines and multi-stage transformers, the grid-following converter control system is susceptible to coupling with the grid impedance. This coupling can increase the risk of oscillations at low short-circuit ratios, seriously impacting the safe and stable operation of the system. As the installed capacity of traditional grid-following converters in renewable energy stations increases, the problem of subsynchronous oscillations in weak grid scenarios becomes increasingly prominent. Currently, most literature on small-disturbance stability research focuses on the grid-connected stability of a single type or single-device inverter in weak grid conditions. However, relatively little research has been conducted on the small-disturbance stability analysis of traditional grid-following renewable energy multi-stations connected to grid-forming converters.

[0003] Small-signal modeling methods for grid-connected converters can be primarily categorized into the impedance method and the state-space method. The state-space method establishes a linearized small-disturbance dynamic model of the system at its static operating point, calculates the system's state-space matrix, and then solves for the eigenvalues ​​and eigenvectors of the system's state matrix to determine system stability. However, the state-space method relies on a deep understanding of the system's internal mechanisms, including various electrical quantities and control parameters, which are often difficult to obtain directly or remain confidential. The impedance method, based on frequency-domain analysis, can be used to directly model the system using control structures and parameters, or to characterize the system's dynamic behavior by measuring the impedance characteristics between system ports. This method does not require a detailed understanding of the system's internal structure and parameters, and is therefore known as a "black-box" model. The application of the impedance method is not only of great theoretical value but also demonstrates its practicality in practical engineering applications. Summary of the Invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a small-disturbance stability analysis method and system for a station containing a grid-type converter, which can better reflect the small-disturbance stability characteristics of the station grid-connected system under a scenario with weak grid strength, and solve the problem in the prior art of lack of quantitative and effective analysis of the small-disturbance stability of new energy stations containing grid-type converters.

[0005] Technical solution: The present invention includes: establishing an impedance model of a grid-following converter; establishing an impedance model of a grid-forming converter; constructing a grid-connected system model of a new energy station including grid-following and grid-forming types; establishing a simplified equivalent circuit of the station grid-connected system: converting the system model into a circuit model, converting the grid-following converter into a controlled current source model, converting the grid-forming converter into a controlled voltage source model, performing circuit equivalent calculations, solving the equivalent circuit model of the station grid-connected system for small-disturbance stability analysis, aggregating the station impedance, and simplifying the station equivalent circuit; establishing the Nyquist criterion for small-disturbance stability of the grid-connected new energy station including the grid-forming type: combining the impedance models of the grid-following converter and the grid-forming converter with the line impedance, deriving the transfer relationship between the grid-connected point current and each power source, and solving the small-disturbance stability criterion of the system from the transfer relationship; using the Nyquist diagram to analyze the small-disturbance stability characteristics of the station grid-connected system.

[0006] The establishment of the grid-connected converter impedance model is specifically as follows: each link of the small disturbance transmission is deduced to obtain the grid-connected converter output impedance model that takes into account the phase-locked loop, the current inner loop, and the DC voltage outer loop. The analytical expression is as follows:

[0007]

[0008] Where,

[0009] Z1=G id (I+G del G ci G vdc G vd ) -1 G del

[0010] Z2=(G dei -G ci )G pll_i -G ci G vdc G ve +G pll_d +G pll_v

[0011] Among them: G ci is the PI link of the current loop, G vdc is the voltage outer loop PI link, G dei is the current loop decoupling matrix, G del is the transfer function matrix considering sampling delay; G pll_v , G pll_i and G pll_d is the transfer function of voltage, current and duty cycle from system coordinates to control system coordinates through the phase-locked loop; G id : small disturbance transfer matrix from duty cycle vector to grid-side current vector in system synchronous coordinates; G vd: small perturbation transfer matrix from duty cycle vector to DC voltage vector;

[0012] The output admittance is:

[0013] Y out =Z out -1 .

[0014] The specific method of establishing the impedance model of the grid-type converter is as follows: Based on the small signal model of the main circuit and the small signal transmission relationship under the controller coordinate, the analytical expression of the external impedance of the self-synchronous voltage source of the virtual synchronous control can be obtained:

[0015]

[0016] Among them: G voio and G vovc It is the main circuit transformation matrix;

[0017] G vio and G vvc is the small signal linearization transfer function of the reactive loop;

[0018] G ωio and G ωvc is the small signal linearization transfer function of the active loop;

[0019] G Lf , G ωLf , G Cf and G ωCf For each filtering link;

[0020] T1=G Lf ·G Cf

[0021] T2=-(G Cf ·G Lf -1 ·G ωLf +G Cf -1 ·G ωCf )

[0022] T3=-G Cf

[0023] K=(I-T1·G vvc -T2·G ωvc ) -1 ·(T1·G vio +T2·G ωio +T3)

[0024] T s is the rotation matrix; T v T cIt is the small signal linearization transfer matrix for transforming the voltage and current controller coordinates to the system coordinate system.

[0025] The system model consists of multiple grid-following converters connected via line impedance Z l At the point of common coupling, it is connected in parallel with the voltage source and connected to the grid via the line impedance Z g Connected in series with the ideal source; among them, the grid-following converter keeps synchronization with the main grid through a phase-locked loop, and the grid-building converter adopts a virtual synchronization control strategy.

[0026] The grid-following converter adopts unified parameters, and multiple grid-following converters form a converter string via a tie line. The tie line impedance can be set separately, and multiple strings of converters are connected to the infinite power grid via the line impedance Zg at the common grid connection point PCC; the grid-forming converter adopts unified parameters, and multiple grid-forming converters are connected to the infinite power grid via the line impedance Zg at the common grid connection point PCC via the tie line impedance.

[0027] The Nyquist criterion for the small-disturbance stability of the grid-connected new energy station is established as follows: based on the simplified equivalent circuit of the grid-connected system, V PCC and I PCC are the voltage and current at the grid connection point respectively; I GFM with I GFL are the output currents of the grid-forming converter and the grid-following converter respectively; Z g is the line impedance, V Grid is the grid-side power supply; calculate I PCC The transfer function relationship between each power supply can be obtained from the equivalent impedance network model topology:

[0028]

[0029] in,

[0030]

[0031] H2(s)=Y GFM

[0032] H3(s)=Y GFM +Y GFL

[0033] Among them, H1(s) is related to the equivalent impedance of the grid-following and grid-forming converters and the line impedance, H2(s) is only related to the impedance of the grid-forming converter itself, and H3(s) is related to the equivalent impedance of the grid-following and grid-forming converters. For H1(s) and H3(s), H1(s) and H1(s)·H3(s) can be regarded as two closed-loop transfer functions. and Its stability depends on its open-loop transfer function Z g ·(YGFM +Y GFL ).

[0034] The small perturbation stability criterion is: when H2(s) does not have a positive real part pole and Z g ·(Y GFM +Y GFL ) satisfies the Nyquist criterion: when Z = N + P = 0, the sending system is stable; where Z is the number of closed-loop positive real poles; N is the number of cycles of the Nyquist diagram clockwise around the point (-1,0); and P is the number of open-loop positive real poles.

[0035] The Nyquist diagram is used to analyze the small disturbance stability characteristics of the station grid-connected system. Specifically, the Nyquist diagram under different parameters is drawn using the stability criterion to analyze the small disturbance stability of the system.

[0036] The stability criterion: L = Z g ·(Y GFM +Y GFL ) must satisfy the Nyquist criterion and obtain its eigenvalues ​​l1 and l2:

[0037]

[0038] Draw the Nyquist plots for l1 and l2 respectively and judge the stability.

[0039] A small-disturbance stability analysis system for a station containing a grid-forming converter comprises: a grid-following converter impedance model, a grid-forming converter impedance model, a system model containing both the grid-following and grid-forming types, a simplified equivalent circuit model of a station grid-connected system, a Nyquist criterion model for small-disturbance stability of a grid-connected new energy station containing a grid-forming type, and a model for analyzing the small-disturbance stability characteristics of the station grid-connected system.

[0040] Beneficial effects: The present invention solves the problem in the prior art of lack of quantitative and effective analysis of the small-disturbance stability of new energy stations containing grid-following and grid-forming converters. Based on the established small signal model, a small-disturbance stability criterion for the station is established, which can quantitatively analyze the small-disturbance stability characteristics of new energy stations containing grid-forming converters under low grid strength scenarios; compared with the traditional impedance modeling method for new energy stations, it can better reflect the impact of grid-forming access on the small-disturbance stability of new energy stations. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 is a flow chart of the present invention;

[0042] Figure 2 This is the main circuit topology diagram of the present invention;

[0043] Figure 3 This is a diagram showing the main circuit topology and control links of the grid-connected converter of the present invention;

[0044] Figure 4 This is a block diagram of the output impedance model of the grid-connected converter of the present invention;

[0045] Figure 5 This is a diagram showing the main circuit topology and control links of the grid-connected converter of the present invention;

[0046] Figure 6 This is a block diagram of the output impedance model of the grid-connected converter of the present invention;

[0047] Figure 7 The impedance equivalent circuit model of the new energy station after equivalent replacement of the impedance model of the present invention;

[0048] Figure 8 The impedance equivalent circuit model of the new energy station of the present invention;

[0049] Figure 9 This is a frequency sweep test diagram of the output admittance of the grid-connected converter according to the present invention;

[0050] Figure 10 This is a frequency sweep test diagram of the output admittance of the grid-connected converter of the present invention;

[0051] Figure 11 The Nyquist diagram of the grid-type converter of the present invention with the lower characteristic root l2 connected;

[0052] Figure 12 The grid-connected point current I dq Simulation waveform. DETAILED DESCRIPTION

[0053] The present invention will be further described below with reference to the accompanying drawings.

[0054] like Figure 1 As shown, the small disturbance stability analysis method of the grid-type converter station of the present invention includes the following steps:

[0055] S1. Establishing the impedance model of grid-following converter;

[0056] The grid-connected angle in the grid-following converter control is detected by the phase-locked loop, and then generates a drive signal after passing through the current loop. Figure 3 As shown in the figure, the various links of small disturbance transmission are derived, and the output impedance model of the grid-connected converter considering the phase-locked loop, current inner loop and DC voltage outer loop can be obtained. The small signal block diagram is shown in Figure 4 As shown, the analytical expression is as shown:

[0057]

[0058] Where,

[0059] Z1=G id (I+G del G ci G vdc G vd ) -1 G del

[0060] Z2=(G dei -G ci )G pll_i -G ci G vdc G ve +G pll_d +G pll_v

[0061] Among them: G ci is the PI link of the current loop, G vdc is the voltage outer loop PI link, G dei is the current loop decoupling matrix, G del is the transfer function matrix considering sampling delay; G pll_v , G pll_i and G pll_d is the transfer function of voltage, current and duty cycle from system coordinates to control system coordinates through the phase-locked loop; G id : small disturbance transfer matrix from duty cycle vector to grid-side current vector in system synchronous coordinates; G vd : small perturbation transfer matrix from duty cycle vector to DC voltage vector;

[0062] The output admittance is:

[0063] Y out =Z out -1 (2)

[0064] S2. Establishing a grid-type converter impedance model;

[0065] The main circuit topology and control links of the grid-type converter are as follows: Figure 5 As shown in the figure, the grid-type converter considers the power outer loop, and the voltage amplitude and phase angle are generated by the power loop.

[0066] The small signal model of the main circuit and the small signal transmission relationship under the controller coordinate are as follows: Figure 6 As shown, the analytical expression of the external impedance of the self-synchronous voltage source under virtual synchronous control can be obtained:

[0067]

[0068] Among them: G voio and G vovc It is the main circuit transformation matrix;

[0069] G vio and G vvc is the small signal linearization transfer function of the reactive loop;

[0070] G ωio and G ωvc is the small signal linearization transfer function of the active loop;

[0071] G Lf , G ωLf , G Cf and G ωCf For each filtering link;

[0072] T1=G Lf ·G Cf

[0073] T2=-(G Cf ·G Lf -1 ·G ωLf +G Cf -1 ·G ωCf )

[0074] T3=-G Cf

[0075] K=(I-T1·G vvc -T2·G ωvc ) -1 ·(T1·G vio +T2·G ωio +T3)

[0076] T s is the rotation matrix; T v T c It is the small signal linearization transfer matrix for transforming the voltage and current controller coordinates to the system coordinate system.

[0077] S3. Construct a grid-connected system model for new energy stations with both grid-following and grid-building types;

[0078] The system model consists of multiple grid-following converters connected via line impedance Z l At the point of common coupling (PCC), it is connected in parallel with the voltage source and connected to the grid via the line impedance Z g In series with an ideal source, the system topology is as follows Figure 2 The grid-following converter maintains synchronization with the main grid through a phase-locked loop, and the grid-forming converter adopts a virtual synchronization control strategy.

[0079] The grid-following converter adopts unified parameters. Multiple grid-following converters form a converter string via tie lines. Tie line impedance can be set separately. Multiple converter strings are connected to the infinite grid via line impedance Zg at the common grid connection point PCC. The grid-forming converter adopts unified parameters. Multiple grid-forming converters are connected to the infinite grid via line impedance Zg at the common grid connection point PCC via tie line impedance.

[0080] S4. Establish a simplified equivalent circuit for the station grid-connected system: convert the system model into a circuit model, apply Norton's theorem to convert the grid-connected converter into a controlled current source model, apply Thevenin's theorem to convert the grid-connected converter into a controlled voltage source model, perform circuit equivalent calculations, and solve the equivalent circuit model of the station grid-connected system for small disturbance stability analysis; aggregate the station impedance and simplify the station equivalent circuit.

[0081] The external characteristics of the grid-type converter are shown as a current source, and the external characteristics of the grid-type converter are shown as a voltage source. The grid-type converter uses a current source I S1 Parallel equivalent admittance Y GFL1 The Norton equivalent circuit is replaced by the grid-type converter using a voltage source V S1 With the series equivalent impedance Z GFM Thevenin equivalent circuit is replaced by Figure 7 As shown. Furthermore, the Norton circuit of multiple grid-type converters can be simplified to a Norton circuit consisting of a current source and its external output impedance in parallel. This circuit is then connected to the grid side to form a grid. The equivalent current source can be directly calculated by multiplying the number of converters N. The output impedance is calculated by opening the current source and performing series and parallel calculations. The impedance aggregation of the grid-type converter is shown as follows:

[0082]

[0083] S5. Establish the Nyquist criterion for small-disturbance stability of grid-connected new energy stations: Combine the dq output impedance model of the grid-following converter and the grid-connecting converter with the line impedance, derive the transfer relationship between the grid-connected point current and each power source, and solve the small-disturbance stability criterion of the system based on the transfer relationship.

[0084] The simplified equivalent circuit of the grid-connected system obtained based on S4 is as follows Figure 7 As shown, V PCC and I PCC are the voltage and current at the grid connection point respectively; I GFM with I GFL are the output currents of the grid-forming converter and the grid-following converter respectively; Z g is the line impedance, V Grid is the grid-side power supply. Calculate I PCCThe transfer function relationship between each power supply can be obtained from the equivalent impedance network model topology:

[0085]

[0086]

[0087] in,

[0088]

[0089] H2(s)=Y GFM (7)

[0090] H3(s)=Y GFM +Y GFL (8)

[0091] Among them, H1(s) is related to the equivalent impedance of the grid-type and grid-forming converters and the line impedance, H2(s) is only related to the impedance of the grid-forming converter itself, and H3(s) is related to the equivalent impedance of the grid-type and grid-forming converters. For H1(s) and H3(s), H1(s) and H1(s)·H3(s) can be regarded as two closed-loop transfer functions and Its stability depends on its open-loop transfer function Z g ·(Y GFM +Y GFL ). Therefore, the small disturbance stability criterion of the station is: when H2(s) does not have a positive real part pole and Z g ·(Y GFM +Y GFL ) satisfies the Nyquist criterion: Z = N + P = 0, the output system is stable. Here, Z is the number of closed-loop positive real poles; N is the number of clockwise cycles of the Nyquist plot around the point (-1, 0); and P is the number of open-loop positive real poles.

[0092] S6. Use Nyquist diagram to analyze the small disturbance stability characteristics of the station grid-connected system: Using the stability criterion in S5, draw Nyquist diagrams under different parameters to analyze the small disturbance stability of the system.

[0093] Stability criterion obtained from S5: L = Z g ·(Y GFM +Y GFL ) must satisfy the Nyquist criterion,

[0094] Find its eigenvalues ​​l1 and l2:

[0095]

[0096] Draw the Nyquist plots for l1 and l2 respectively and judge the stability.

[0097] Example

[0098] The small disturbance stability analysis method for a grid-connected converter station of this embodiment is applicable to grid-connected and grid-connected converter new energy stations, specifically including:

[0099] Step 1: Construct a grid-following converter impedance model;

[0100] The open-loop output impedance of the converter without considering the phase-locked loop:

[0101]

[0102] Where L, R are the line impedances before the grid connection point, and ω is the system angular frequency;

[0103] The transfer function matrix between the duty cycle vector and the grid-side current vector is:

[0104]

[0105] Where V dc is the DC side voltage;

[0106] The output angle formula under small disturbance of the phase-locked loop PLL is:

[0107]

[0108] for

[0109] Output angle small disturbance, is the small disturbance of q-axis voltage;

[0110] in:

[0111]

[0112] where k p_PLL 、k i_PLL is the phase-locked loop PI control parameter;

[0113] The transfer function relationship between the phase-locked loop output angle and the system q-axis voltage is shown in Equation 2-1-15.

[0114]

[0115] Definition G PLL for:

[0116]

[0117] but

[0118]

[0119] Substitute it into formula 2-1-11,

[0120]

[0121] Get G pll_v :

[0122]

[0123] Similarly, we get G pll_i and G pll_d :

[0124]

[0125]

[0126] Each control link:

[0127]

[0128]

[0129]

[0130] G ci The current loop PI link, where K p_I , K i_I is the current loop PI control parameter, G dei is the current loop decoupling matrix, G del G is the transfer function matrix considering sampling delay, LPF is the first-order low-pass filter of the feedforward voltage, where T del is the unit sampling time.

[0131] Considering the non-ideal source of the DC side power supply, when there is disturbance:

[0132]

[0133] in,

[0134] The small disturbance transfer matrix from the duty cycle vector to the grid-side current vector in the system synchronous coordinates is:

[0135]

[0136] The small perturbation transfer matrix from the duty cycle vector to the DC voltage vector is shown in Equation 2-1-34.

[0137]

[0138] By deducing each link of small disturbance transmission, we can obtain the output impedance model of the grid-connected converter taking into account the phase-locked loop (PLL), the current inner loop, and the DC voltage outer loop:

[0139]

[0140] Where,

[0141] Z1=G id (I+G del G ci G vdc G vd ) -1 G del

[0142] Z2=(G dei -G ci )G pll_i -G ci G vdc G ve +G pll_d +G pll_v

[0143] Y out =Z out -1 (28)

[0144] Substitute the grid-following converter parameters into the table 1.

[0145] Table 1 Parameters of grid-type converter

[0146]

[0147] Figure 8 The output impedance theory and frequency sweep test diagram of a grid-connected converter, which takes into account the phase-locked loop (PLL), the inner current loop, and the outer DC voltage loop. The solid line represents the theoretically calculated value, while the scattered dots represent the results of the frequency sweep calculation. The frequency sweep range is 20Hz to 600Hz. The frequency sweep results show that the theoretical impedance calculation of the grid-connected converter is consistent with the frequency sweep results, proving the accuracy of the impedance model.

[0148] Step 2: Constructing a grid-type converter impedance model;

[0149] Main circuit transformation matrix:

[0150]

[0151]

[0152] Where R C , L C is the impedance of the tie line to the grid connection point, ω sis the grid side angular frequency.

[0153] Reactive loop small signal linearization transfer function:

[0154]

[0155]

[0156] Among them, V Cdqc and I odqc is the steady-state output voltage and current of the converter under the controller rotation coordinate, K is the virtual inertia, D q is the virtual damping.

[0157] Active loop small signal linearization transfer function:

[0158]

[0159]

[0160] Among them, V Cdqc and I odqc is the steady-state output voltage and current of the converter under the rotating coordinate of the controller, J is the virtual inertia, D p is the virtual damping.

[0161] The filtering process is shown as follows:

[0162]

[0163]

[0164]

[0165]

[0166] Among them, Ω c Represents the steady-state value of the controller coordinate rotation angular frequency, I Ldq is the steady-state current value of the filter inductor under the controller coordinate. and and The transfer relationship can be expressed by formula 2-2-13.

[0167]

[0168] in:

[0169] T1=G Lf ·G Cf

[0170] T2=-(G Cf ·G Lf -1 ·GωLf +G Cf -1 ·G ωCf )

[0171] T3=-G Cf

[0172] Furthermore, by connecting the power outer loop, we can get the controller rotation coordinate and The transitive relationship:

[0173]

[0174] K=(I-T1·G vvc -T2·G ωvc ) -1 ·(T1·G vio +T2·G ωio +T3) (41)

[0175] The rotation matrix is ​​shown as:

[0176]

[0177] v Cdq and i odq The transformation relationship between the two coordinate systems is:

[0178] v Cdq s =T s ·v Cdq c (43)

[0179] i odq c =T s -1 ·i odq s (44)

[0180] Perform small signal linearization on the above two equations:

[0181]

[0182]

[0183] In the formula, Δ is the steady-state value of the phase angle difference between the two coordinates. In summary, by substituting the small signal model of the main circuit and the small signal transfer relationship under the controller coordinate, the analytical expression of the external impedance of the self-synchronous voltage source under virtual synchronous control can be obtained:

[0184]

[0185] Substitute the grid-type converter parameters into Table 2:

[0186] Table 2 Parameters of grid-type converter

[0187]

[0188] Figure 9 The following diagram shows the output impedance sweep test of a grid-connected converter with only the active and reactive power outer loop. The solid line represents the theoretically calculated value, and the scattered dots represent the results of the sweep calculation. The sweep range is 10Hz to 800Hz. The sweep results show that the theoretical impedance calculation of the grid-connected converter is consistent with the sweep results, verifying the accuracy of the impedance model.

[0189] Step 3: Establish a system model that includes both grid-following and grid-building new energy stations connected to the infinite power grid;

[0190] The system model consists of multiple grid-following converters connected via line impedance Z l At the point of common coupling (PCC), it is connected in parallel with the voltage source and connected to the grid via the line impedance Z g The grid-following converter is synchronized with the main grid through a phase-locked loop, while the grid-forming converter adopts a virtual synchronization control strategy.

[0191] Step 4: Establish a simplified equivalent circuit of the station grid-connected system;

[0192] The external characteristics of the grid-type converter are shown as a current source, and the external characteristics of the grid-type converter are shown as a voltage source. The grid-type converter uses a current source I S1 Parallel equivalent admittance Y GFL1 The Norton equivalent circuit is replaced by the grid-type converter using a voltage source V S1 With the series equivalent impedance Z GFM The Thevenin equivalent circuit is replaced. Furthermore, the Norton circuit of multiple grid-connected converters is simplified to a Norton circuit consisting of a current source and its output impedance in parallel, which is then connected to the grid. The equivalent current source can be directly calculated by multiplying the number of converters N. The output impedance is calculated by opening the current source and performing series and parallel calculations. The aggregate impedance of the grid-connected converter is shown in the equation below.

[0193]

[0194] Step 5: Establish the Nyquist criterion for small-disturbance stability of grid-connected new energy stations;

[0195] Based on the simplified equivalent circuit of the grid-connected system obtained in step 4, V PCC and I PCC are the voltage and current at the grid connection point respectively; I GFMwith I GFL are the output currents of the grid-forming converter and the grid-following converter respectively; Z g is the line impedance, V Grid is the grid-side power supply. Calculate I PCC The transfer function relationship between each power supply can be obtained from the equivalent impedance network model topology:

[0196]

[0197] I GFL =I S2 -V PCC ·Y GFL (50)

[0198] V PCC =I PCC ·Z g +V Grid (51)

[0199] Substituting the above formula into the combination, we can get:

[0200]

[0201] in,

[0202]

[0203] H2(s)=Y GFM (54)

[0204] H3(s)=Y GFM +Y GFL (55)

[0205] Among them, H1(s) is related to the equivalent impedance of the grid-type and grid-forming converters and the line impedance, H2(s) is only related to the impedance of the grid-forming converter itself, and H3(s) is related to the equivalent impedance of the grid-type and grid-forming converters. For H1(s) and H3(s), H1(s) and H1(s)·H3(s) can be regarded as two closed-loop transfer functions and Its stability depends on its open-loop transfer function Z g ·(Y GFM +Y GFL ). Therefore, the small disturbance stability criterion of the station is: when H2(s) does not have a positive real part pole and Z g ·(Y GFM +Y GFL ) satisfies the Nyquist criterion: Z = N + P = 0, the output system is stable. Here, Z is the number of closed-loop positive real poles; N is the number of clockwise cycles of the Nyquist plot around the point (-1, 0); and P is the number of open-loop positive real poles.

[0206] Step 6: Use Nyquist diagram to analyze the small disturbance stability characteristics of the station grid-connected system;

[0207] The stability criterion obtained from step 5: L = Z g ·(Y GFM +Y GFL ) must satisfy the Nyquist criterion,

[0208] Find its eigenvalues ​​l1 and l2:

[0209]

[0210] Draw the Nyquist plots for l1 and l2 respectively and judge the stability.

[0211] For a system with five grid-following converters connected in series with one grid-forming converter, the Nyquist diagram is as follows: Figure 10 shown.

[0212] Figure 11 PCC current I dq Simulation waveform. At 1.75s, the line impedance Lg switches from 0.03mH to 0.05mH, and at 3.5s it switches to 0.06mH. It can be seen that I dq When switching from steady state to a lower grid strength, the grid stabilizes after the damping oscillation caused by the switching, which is consistent with the Nyquist stability analysis results.

[0213] The small-disturbance stability analysis system of a station containing a grid-forming converter of the present invention comprises: a grid-following converter impedance model, a grid-forming converter impedance model, a system model containing the grid-following type and the grid-forming type, a simplified equivalent circuit model of the station grid-connected system, a Nyquist criterion model for the small-disturbance stability of the grid-connected station containing the grid-forming type new energy station, and a model for analyzing the small-disturbance stability characteristics of the station grid-connected system.

Claims

1. A small disturbance stability analysis method for a grid-type converter station, characterized in that: include: Establishing a grid-connected converter impedance model: By deducing each link of small disturbance transmission, we obtain the output impedance model of the grid-connected converter that takes into account the phase-locked loop, the inner current loop, and the outer DC voltage loop. Its analytical expression is as follows: ； Where, ； in: is the PI link of the current loop, is the voltage outer loop PI link, is the current loop decoupling matrix, is the transfer function matrix considering sampling delay; 、 and is the transfer function of voltage, current and duty cycle from system coordinates to control system coordinates through the phase-locked loop; : The small disturbance transfer matrix from the duty cycle vector to the grid-side current vector in the system synchronous coordinates; : small perturbation transfer matrix from duty cycle vector to DC voltage vector; The output admittance is: Y out =Z out -1 ; Establish an impedance model of a grid-connected converter; construct a grid-connected system model of a new energy station including grid-following and grid-connecting types; establish a simplified equivalent circuit of the station grid-connected system: convert the system model into a circuit model, convert the grid-connected converter into a controlled current source model, and convert the grid-connected converter into a controlled voltage source model, perform circuit equivalent calculations, solve the equivalent circuit model of the station grid-connected system for small-disturbance stability analysis, aggregate the station impedance, and simplify the station equivalent circuit; establish the Nyquist criterion for small-disturbance stability of the grid-connected new energy station including grid-connecting type: combine the impedance models of the grid-following converter and the grid-connecting converter with the line impedance, derive the transfer relationship between the grid connection point current and each power source, and solve the small-disturbance stability criterion of the system from the transfer relationship; use the Nyquist diagram to analyze the small-disturbance stability characteristics of the station grid-connected system.

2. A small disturbance stability analysis method for a grid-type converter station according to claim 1, characterized in that: The specific method of establishing the impedance model of the grid-type converter is as follows: Based on the small signal model of the main circuit and the small signal transmission relationship under the controller coordinate, the analytical expression of the external impedance of the self-synchronous voltage source of the virtual synchronous control can be obtained: ； in: and It is the main circuit transformation matrix; and is the small signal linearization transfer function of the reactive loop; and is the small signal linearization transfer function of the active loop; 、 、 and For each filtering link; ； ； ； ； is the rotation matrix; It is the small signal linearization transfer matrix for transforming the voltage and current controller coordinates to the system coordinate system.

3. The small disturbance stability analysis method of a grid-type converter station according to claim 1 is characterized in that: The system model consists of multiple grid-following converters connected via line impedance Z l At the point of common coupling, it is connected in parallel with the voltage source and connected to the grid via the line impedance Z g Connected in series with the ideal source; among them, the grid-following converter keeps synchronization with the main grid through a phase-locked loop, and the grid-building converter adopts a virtual synchronization control strategy.

4. A small disturbance stability analysis method for a grid-type converter station according to claim 3, characterized in that: The grid-following converter adopts unified parameters, and multiple grid-following converters form a converter string through the interconnecting line. The interconnecting line impedance can be set separately, and the multiple strings of converters are connected to the infinite power grid through the line impedance Zg at the common grid connection point PCC; the grid-forming converter adopts unified parameters, and multiple grid-forming converters are connected to the infinite power grid through the line impedance Zg at the common grid connection point PCC through the interconnecting line impedance.

5. The small disturbance stability analysis method of a grid-type converter station according to claim 1 is characterized in that: The Nyquist criterion for the small-disturbance stability of the grid-connected new energy station is established as follows: based on the simplified equivalent circuit of the grid-connected system, I PCC is the current at the grid connection point; Z g is the line impedance, V Grid is the grid-side power supply; calculate I PCC The transfer function relationship between each power supply can be obtained from the equivalent impedance network model topology: ； ； ； in, ； ； ； in, It is related to the equivalent impedance of the grid-following and grid-forming converters and the line impedance. Only related to the impedance of the grid-type converter itself, Related to the equivalent impedance of grid-following and grid-forming converters; , , you can and Considered as two closed-loop transfer functions and , its stability depends on its open-loop transfer function .

6. A small disturbance stability analysis method for a grid-type converter station according to claim 5, characterized in that: The small disturbance stability criterion is: There is no positive real part extreme, and Satisfies the Nyquist criterion: ; Where Z is the number of closed-loop positive real poles; N is the number of cycles of the Nyquist plot clockwise around the point (-1,0); and P is the number of open-loop positive real poles.

7. The small disturbance stability analysis method of a grid-type converter station according to claim 1 is characterized in that: The Nyquist diagram is used to analyze the small disturbance stability characteristics of the station grid-connected system. Specifically, the Nyquist diagram under different parameters is drawn using the stability criterion to analyze the small disturbance stability of the system.

8. A small disturbance stability analysis method for a grid-type converter station according to claim 7, characterized in that: The stability criterion: The Nyquist criterion must be met to obtain the eigenvalues ​​l1 and l2: ； Draw the Nyquist plots for l1 and l2 respectively and judge the stability.

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