CN-122000902-A - Method for analyzing stability of network-structured converter based on inner and outer loop control coupling

CN122000902ACN 122000902 ACN122000902 ACN 122000902ACN-122000902-A

Abstract

The invention discloses a stability analysis method of a network-structured converter based on inner and outer loop control coupling, which is characterized in that a control coupling mechanism of the network-structured converter is clarified by establishing a single-input single-output model of the network-structured converter, a key coupling link which leads to instability of the network-structured converter is disclosed, and a stability criterion based on a Berde criterion shown in a formula (13) is provided according to the key coupling link, so that the critical stable short-circuit ratio and the oscillation frequency of the network-structured converter can be simply, conveniently and accurately obtained. Unlike available impedance analysis and eigenvalue analysis, the present invention solves the problem of the available method that qualitative analysis is only possible and quantitative analysis is impossible.

Inventors

SIMA WENXIA
JIAN PEIYI
YANG MING
FENG MOKE
YUAN TAO
SUN POTAO
GUO JINGMEI

Assignees

重庆大学

Dates

Publication Date: 20260508
Application Date: 20260114

Claims (10)

1. The method for analyzing the stability of the network-structured converter based on the control coupling of the inner ring and the outer ring is characterized by comprising the following steps: Step 1) feeding a grid-connected system of the grid-built converter according to the dq rotating coordinate system rotating speed Dividing lines to form two subsystems, wherein the two subsystems comprise a power grid, a main circuit system and a grid-structured converter system; Step 2) acquiring a small disturbance variable relation between a power grid and a main circuit system and between a constructed network type converter system; Step 3) establishing a power grid and main circuit system small disturbance model and a grid-structured transformer system small disturbance model under a corresponding rotation coordinate system; Step 4) based on the small disturbance variable relation and the small disturbance model, establishing a unified single-input single-output model of the grid-structured converter grid-connected system; step 5) determining an open loop transfer function of a key control coupling link which leads to system instability by using a single-input single-output model; step 6) analyzing the network configuration type change by utilizing an open loop transfer function of a key control coupling link And (5) the stability of the grid-connected system of the current transformer.
2. The method for analyzing the stability of the grid-built converter based on the inner and outer ring control coupling according to claim 1, wherein in the step 1), the dq rotating coordinate system rotating speed of the grid-built converter system is synchronous rotating speed; The dq rotating coordinate system of the grid-structured converter system is asynchronous in rotating speed.
3. The method for analyzing the stability of the grid-connected converter based on the internal and external loop control coupling according to claim 1, wherein in the step 2), the small disturbance variable relationship between the power grid and the main circuit system and the grid-connected converter system is as follows: (1) Wherein, the Representing the dq rotating coordinate system of vector x in the control system of the grid-built converter In (c) the dq component of the vector, The coordinate system adopts grid-connected point voltage orientation, d-axis is coincided with the d-axis vector of the grid-connected point voltage, q-axis is vertical to d-axis, positive direction is in anticlockwise direction of d-axis, and origin point is dq two-axis intersection point; Representing dq rotating coordinate system of vector x in power grid and main circuit system In (c) the dq component of the vector, The coordinate system adopts the voltage orientation of the power grid, the d axis is coincided with the d axis vector of the voltage of the power grid, the q axis is vertical to the d axis, the positive direction is in the anticlockwise direction of the d axis, the origin point is the intersection point of the dq two axes, and the sign is that Representing small disturbance components of the corresponding variables; And outputting disturbance for the network construction control algorithm. 、 Representing the dq component of the vector x in the reference state.
4. A method for analyzing stability of a network-structured converter based on coupling of inner and outer loop control according to claim 3, wherein the network-structured control algorithm outputs disturbance The following is shown: (2) in the formula, Representing a small disturbance component of the output active power of the converter; The synchronous angular velocity is represented, D p and J represent virtual synchronous control damping coefficients and rotational inertia, and s represents complex frequency variable.
5. The method for analyzing the stability of the network-structured converter based on the coupling of the inner loop and the outer loop control according to claim 1, wherein in the step 3), a small disturbance model of a power grid and a main circuit system is as follows: (3) (4) (5) Wherein L g 、R g represents the equivalent inductance and resistance of the power grid, L f 、R f 、C f represents the filter inductance, parasitic resistance and filter capacitance of the converter; 、 The d-axis and q-axis disturbance components of the inductor current of the converter; 、 The d-axis and q-axis disturbance components of the grid-connected current; 、 the d-axis and q-axis disturbance components of the grid-connected point voltage; 、 D-axis and q-axis disturbance components of the equivalent voltage of the power grid; 、 The disturbance components of the d and q axes of the output line voltage of the current transformer.
6. The method for analyzing the stability of the network-structured converter based on the coupling of the inner loop and the outer loop control according to claim 1, wherein the small disturbance model of the network-structured converter system is as follows: (6) (7) (8) Wherein m dq is the d and q axis components of the modulated signal, T d is the equivalent time delay, K pwm is the PWM modulation factor; 、 the current of the current transformer is d and q axis disturbance reference values; 、 The d and q axes disturbance reference values of the output voltage of the converter are obtained; 、 g VL 、G CL is the transfer function of a voltage loop and a current loop PI controller; 、 the d and q axis disturbance components of the current transformer inductance current; 、 The d and q axis disturbance components are modulated signals; 、 the disturbance components of the d and q axes of the output line voltage of the current transformer; For controlling the equivalent time delay of the system.
7. The method for analyzing the stability of the grid-connected transformer based on the coupling of the inner loop and the outer loop control according to claim 1, wherein in the step 4), a unified single-input single-output model of the grid-connected system of the grid-connected transformer is as follows: (9) (10) Wherein G VL 、G CL represents a transfer function of the voltage loop and the current loop PI controller; G q represents the virtual exciter integration link; And G 1 ,G 2 ,G GFM ,Z v is a simplified expression parameter.
8. The method for analyzing stability of a network-structured converter based on inner and outer loop control coupling according to claim 6, wherein the closed loop transfer function of the key control coupling link is as follows: (11) in the formula, Representing the closed loop transfer function of the critical control coupling element.
9. The method for analyzing stability of a network-structured converter based on inner and outer loop control coupling according to claim 1, wherein in step 5), an open loop transfer function G o of a critical control coupling link causing system instability is as follows: (12) in the formula, Representing grid-tie point voltage d-axis component initial value.
10. The method for analyzing the stability of a network-structured converter based on the coupling of inner and outer loop control according to claim 1, wherein in the step 6), if the open loop transfer function G o satisfies a stability criterion, the system is stable; The stability criteria are as follows: (13) in the formula, Is a phase margin; Is that Amplitude crossing frequency is the corresponding angular frequency when the amplitude of amplitude-frequency characteristic is 1; is the amplitude margin; Is that The phase crossing frequency is the angular frequency corresponding to the phase frequency characteristic phase of-180 degrees.

Description

Method for analyzing stability of network-structured converter based on inner and outer loop control coupling Technical Field The invention relates to the field of stability analysis of a network-structured converter, in particular to a stability analysis method of a network-structured converter based on inner and outer loop control coupling. Background With the new energy power generation taking the converter as an interface to replace a synchronous unit on a large scale, the inertia and the voltage supporting capability of the system are obviously weakened. The reason is that the current converter generally adopts the following net type control, so that necessary inertia response and dynamic reactive support cannot be provided for the system, and the safety of the system is seriously threatened. The grid-structured control simulates the electromagnetic-mechanical dynamic characteristics of the synchronous generator through an algorithm, so that the converter has the capability of autonomously constructing and maintaining the voltage and the frequency of the alternating current power grid. The control paradigm enables the converter to not only provide virtual inertia damping frequency fluctuation, but also realize power angle stabilization and voltage support through autonomous adjustment, becomes a necessary stability foundation of a novel power system, and is a core solution for guaranteeing safe operation of the system. However, the grid-structured converter has serious stability problem under the strong power grid, if the power grid strength exceeds the critical stable value, the output quantity of the grid-structured converter can generate divergent low-frequency oscillation, so that serious overvoltage and overcurrent are caused, and the new energy unit is disconnected from the power grid, thereby seriously affecting the safety and stability of the novel power system. At present, the grid connection of large-scale new energy obviously increases the operation difficulty of the power grid. The inherent intermittence and fluctuation of the power generation can cause the severe change of the power grid intensity caused by the switching of any power source or load. Therefore, a method for analyzing the stability of the grid-structured converter needs to be provided, so that the critical stable operation power grid strength of the grid-structured converter is found, the grid-structured converter is prevented from operating in a power grid exceeding the strength, and low-frequency oscillation is prevented. The existing stability analysis for the grid-structured current transformer is mainly developed along two directions. One aspect is based on the study of the external characteristics of the port impedance, which regards the converter as a whole, and analyzes the interaction of the converter with the impedance of the power grid by establishing an impedance model of the converter or an impedance model taking frequency coupling into account. Such studies reveal that as the strength of the power grid increases, the impedance resonance peak of the grid-structured converter shifts to low frequency, and even presents negative resistance-capacitance characteristics in certain frequency bands, so that low frequency or subsynchronous oscillation is easy to induce, and techniques for improving stability such as impedance remodeling are developed. However, this analysis method based on external characteristics has fundamental limitations, because the impedance characteristics of the current transformer are not physical entities, but merely external manifestations of the control algorithm thereof, the intrinsic mechanism of instability cannot be ascertained from the external characteristics, and it is difficult to mine core factors strongly associated with the intrinsic stability thereof. To reveal the nature of destabilization, another class of research has turned to the internal control of the coupling mechanism. Such studies have attempted to go deep into the internal control loop of a current transformer to analyze its dynamic process by deriving its closed loop transfer function or applying damping torque methods, complex torque coefficient methods, etc. These approaches have successfully indicated that grid-formed converters exhibit a negative damping torque characteristic under a strong grid that is proportional to the grid strength, and the underlying source of this negative damping is the coupling between the power and voltage loops in their internal control. Although the method touches the root of the problem, the model established by the analysis method is usually an extremely complex multi-input multi-output system, so that the transfer function of each link is highly coupled, the stability criterion obtained by the method is also extremely complex, and the complexity makes it difficult for researchers to clearly and intuitively grasp the specific interaction mechanism and coupling form between the power r