Networking STATCOM (static synchronous compensator) analysis model based on DC capacitance dynamics

Abstract

The invention relates to a network construction type STATCOM (static synchronous compensator) analysis model based on DC capacitance dynamics, and belongs to the technical field of power distribution networks. The invention aims to realize the simulation of the synchronization characteristic of a synchronous machine by utilizing the similarity between the dynamic state of the direct current capacitor and the dynamic state of a generator rotor, namely a grid-building STATCOM analysis model based on the dynamic state of the direct current capacitor for the self-synchronous control of the direct current capacitor. The control structure of the internal frequency of the STATCOM on the direct current voltage is adjusted, a voltage equation under a synchronous coordinate system is deduced according to the superposition theorem and a topological graph, and the equivalent synchronous machine is obtained according to the equivalent principle, so that an equivalent rotor motion equation of the equivalent synchronous machine is obtained. According to the control mode, the internal frequency of the equipment can be adjusted to realize synchronization with the power grid, and stable operation of the direct-current side is ensured by controlling the direct-current voltage.

Description

Technical Field

[0001] The present invention belongs to the technical field of distribution networks. Background Art

[0002] The topological structure of a typical new energy transmission system with a network-forming STATCOM (hereinafter referred to as "joint transmission system") is shown in the figure. Among them, new energy represents a wind farm or a photovoltaic power station. The network-forming STATCOM and the new energy station are connected to the grid through the same bus, and form a typical two-machine system with the remote grid. Figure 1 In it, X g 、X s and X v are the reactances of the corresponding branches, E s and δ s are the amplitude and phase of the transient electromotive force of the network-forming STATCOM, U v 、U p and δ v 、δ p are the amplitudes and phases of the voltages of the new energy station and the bus at the collection point respectively.

[0003] For the working principle of the network-forming STATCOM, the main circuit topology of the network-forming STATCOM is shown in Figure 2 It includes a converter and an LC filter circuit. Among them, the three-phase bridge arms of the converter are connected to the grid in a △ connection mode. Each phase-interphase bridge arm is composed of n single-phase H-bridge circuits connected in series. The DC side of each H-bridge power unit module uses a storage battery. The LC filter circuit includes a filter reactor and a filter capacitor. The filter reactor is connected to the head and tail of each phase converter, and the inductance value is 0.5L c and the filter capacitor value is C. Summary of the Invention

[0004] The purpose of the present invention is to utilize the similarity between the dynamics of the DC capacitor and the dynamics of the generator rotor to realize the simulation of the synchronization characteristics of the synchronous machine, that is, an analysis model of a network-forming STATCOM based on the dynamics of the DC capacitor with self-synchronization control.

[0005] The construction process of the model of the present invention is as follows: S1. Adjust the control structure of the DC voltage for the internal frequency of the STATCOM: In the formula: ω * is the internal frequency of the STATCOM, ω g is the frequency reference value, VDCref is the DC voltage reference value, V DC is the voltage of the DC bus capacitor, K J is the inertia simulation coefficient, K D is the damping coefficient, K T is the DC voltage tracking coefficient; Without considering the power loss of the capacitor itself, the active power output by the DC-side capacitor during the transient period is equal to the active power output by the STATCOM, and its dynamics are expressed as: From equations (1) and (2), we get: In the formula: the equivalent inertia coefficient \(K_{Jeq} = K_JC_DC / 2\); the equivalent damping coefficient \(K_{Deq} = K_DC_DC / 2\); the DC voltage coefficient \(K_{Teq} = K_TC_DC / 2\); after neglecting the voltage tracking term, equation (3) is simplified to: The rotor motion equation is: J SG sω SG +D SG (ω SG -ω Grid )=P M -P ESG (5) Where, ω SG is the rotor speed, ω Grid is the grid frequency, J SG is the inertia time constant, D SG is the equivalent mechanical damping coefficient of the machine, P M is the mechanical power provided by the prime mover, P ESG is the electromagnetic power output by the synchronous machine; After neglecting the resistance of the line and the filter, the active power output by the STATCOM is: Where, X Σ =X F +X s +X g ,U * is the internal electromotive force of the STATCOM, U g is the grid voltage, δ * is the phase of U * ,θ g is the phase of U g ; Considering \(sδ * =ω Base ω * ,sθ g =ω Base ω g and δ=δ * -θ g ,it can be further written as: Equivalent Modeling of the Combined Sending System: Based on the superposition theorem and the topological diagram, the voltage equation in the synchronous coordinate system is derived: Among them, A = X g X s + X g X v + X v X s ; The VSC current is determined by the power reference value, that is, I vd = P ref / U v , I vq = Q ref / U v ; Under the unity power factor control, P ref = P v , Q ref = 0, then the amplitude of the converter port voltage is expanded by the formula to get the formula: Considering the voltage distribution of the actual system, the voltage amplitude U v at the VSC port is: Among them, δ s and the relationship between δ v : According to the above equivalent principle, the P eqe , P meq of the equivalent synchronous machine are obtained: Among them, Therefore, the equivalent rotor motion equation of the equivalent synchronous machine is: Among them, ω B is the rated angular frequency; Δω sc is the relative angular frequency: Δω sc = ω s - ω g ; ω s is the angular frequency of the grid-forming SVG.

[0006] The control method of the present invention can not only adjust the internal frequency of the device to achieve synchronization with the power grid, but also ensure the stable operation of the DC side by controlling the DC voltage. Description of the Drawings

[0007] Figure 1 It is a new energy transmission system diagram with a grid-forming STATCOM; Figure 2 It is a topological structure diagram of a grid-forming STATCOM; Figure 3a It is a control block diagram of the converter of a grid-forming STATCOM with outer-loop power control; Figure 3b It is a control block diagram of the converter of a grid-forming STATCOM with inner-loop voltage and current control; Figure 4 It is a control block diagram of a grid-forming STATCOM with a DC capacitor; Figure 5 It is an equivalent model diagram of a new energy transmission system; Figure 6 It is a grid-following converter with LVRT; Figure 7 It is a comparison diagram of output voltages in different ways; Figure 8 It is a comparison diagram of output currents in different ways; Figure 9 It is a comparison diagram of output powers in different ways. Specific implementation manners

[0008] The control block diagram of the converter of the grid-forming STATCOM is as above Figure 3a and Figure 3b , including outer-loop power control and inner-loop voltage and current control. The outer-loop power control includes an active loop and a reactive loop. As Figure 3a shown, the output of the active loop serves as the frequency and phase of the modulation wave of the converter output voltage, and the output of the reactive loop serves as the amplitude of the modulation wave of the converter output voltage. The active loop simulates the rotor motion equation of a synchronous generator to achieve the inertia characteristic and active power-frequency characteristic of the grid-forming STATCOM, and the reactive loop simulates the excitation system of a synchronous generator to achieve the primary voltage regulation characteristic. The formulas of the active loop and the reactive loop are In the formula, J is the virtual inertia; D p is the damping coefficient; D q is the reactive-voltage droop coefficient; K is the voltage integral coefficient; ω is the output angular frequency; U n , ω n are the rated voltage and rated angular frequency; P set , Q set are the active and reactive reference values.

[0009] To improve the control accuracy, the inner-loop controller of the converter adopts a double closed-loop control link of voltage and current to achieve decoupled control of active and reactive power, as shown in Fig. (b), where e abc 、u abc 、i abc 、i gabc are the arm voltage, grid-connected voltage, converter current, and grid-connected current respectively; e dq 、u dq 、i dq 、i gdq are the components of the arm voltage, grid-connected voltage, converter current, and grid-connected current in the dq-axis coordinate system respectively. The input voltage reference value vector is set in the d-axis direction, that is, u q = 0, u d = E. The output signal drives the cascaded H-bridge circuit through carrier phase-shifted SPWM modulation.

[0010] Analysis Model of Grid-Forming STATCOM Based on DC Capacitor Dynamics The core of virtual synchronous generator (VSG) control lies in simulating the swing equation of a synchronous generator, enabling power electronic devices to possess the inertial damping characteristics of a synchronous generator, while also determining the transmission of its active power and its steady-state value. This control method often assumes a constant DC-side voltage, so the existing VSG control is difficult to be directly applied to STATCOM. Utilizing the similarity between the DC capacitor dynamics and the generator rotor dynamics can achieve the simulation of the synchronous characteristics of a synchronous generator, that is, DC capacitor self-synchronization control. This control method can not only adjust the internal frequency of the device to achieve synchronization with the power grid, but also ensure the stable operation of the DC side by controlling the DC voltage.

[0011] As known from Figure 4 , this control method actually controls the DC voltage by adjusting the internal frequency of STATCOM, and its control structure can be expressed as: where: ω * is the internal frequency of STATCOM, ω g is the frequency reference value, VDCref is the DC voltage reference value, V DC is the voltage of the DC bus capacitor, K J is the inertia simulation coefficient, K D is the damping coefficient, and K T is the DC voltage tracking coefficient.

[0012] The active power during the grid-connected transient of STATCOM comes from the energy stored in the DC-side capacitor. Without considering the power loss of the capacitor itself, the active power output by the DC-side capacitor during the transient period is equivalent to the active power output by STATCOM, and its dynamics can be expressed as:

[0013] Combining the above two equations, we can obtain: In the formula: the equivalent inertia coefficient KJeq = KJCDC / 2; the equivalent damping coefficient KDeq = KDCDC / 2; the DC voltage coefficient KTeq = KTCDC / 2.

[0014] It describes the active power and frequency characteristics under the self-synchronization control of STATCOM, and can be divided into the grid-connected synchronization term (the left side of the equation) and the DC voltage tracking term (the right side of the equation). Since this section is mainly for facilitating the explanation that the grid-forming STATCOM under self-synchronization control has similar active-power - frequency characteristics to the synchronous machine, and the DC voltage tracking dynamics often have a relatively fast time scale, the voltage tracking term can be ignored.

[0015] The above equation can be transformed into:

[0016] The rotor motion equation is: J SG sω SG +D SG (ω SG -ω Grid )=P M -P ESG (5) Where, ω SG is the rotor speed, ω Grid is the grid frequency, J SG is the inertia time constant, D SG is the equivalent mechanical damping coefficient of the machine, P M is the mechanical power provided by the prime mover, P ESG is the electromagnetic power output by the synchronous machine.

[0017] Comparing the above two equations, it can be seen that the grid-forming STATCOM under self-synchronization control has similar active-power - frequency characteristics to the synchronous machine.

[0018] When analyzing its synchronization principle, it is considered that the voltage and current loops can track their reference values well, so the dynamics of the control link are ignored. At this time, the device can be regarded as a controlled voltage source, whose phase is determined by the synchronization unit and the amplitude is determined by the reactive power control loop. After ignoring the resistance of the line and the filter, the active power output by STATCOM is: Where, X Σ =X F +X s +X g ,U * is the internal potential of STATCOM, Ug is the grid voltage, δ * is the phase of U * , θ g is the phase of U g .

[0019] Considering sδ * = ω Base ω * , sθ g = ω Base ω g and δ = δ * - θ g , it can be further written as:

[0020] It can be seen from the formula that at steady state, there is P S = P E . There are two equilibrium points in the system, but only one is a stable equilibrium point. The existence of this equilibrium point ensures that the STATCOM can be synchronized with the power grid. However, different from the synchronous machine, the STATCOM realizes the simulation of the swing equation and grid connection synchronization through the self-synchronization unit combined with the dynamic of the DC capacitor. And, from the above analysis, it can be known that this self-synchronization unit can simulate the inertia and mechanical damping, and adjusting the control parameters can change the magnitudes of the virtual inertia and damping.

[0021] Equivalent modeling of the combined transmission system The network-forming STATCOM adopts DC capacitor self-synchronization control, making it have synchronous characteristics similar to those of a synchronous machine. And due to the imbalance between the active power output by new energy and the grid power during a fault, active power flows back into the STATCOM. Therefore, from the grid side, new energy equipment can be regarded as the prime mover of the network-forming STATCOM. Thus, the combined transmission system can be equivalently modeled as an equivalent synchronous machine as shown in Figure 5 . The synchronous stability analysis method in the traditional power system can be applied to the combined transmission system.

[0022] Before deriving the equivalent rotor motion equation of the equivalent synchronous machine, it is necessary to obtain the equivalent electromagnetic power (P eqe ) and the equivalent mechanical power (P meq ). According to the superposition theorem and based on the topological diagram shown in the figure, the voltage equation in the synchronous coordinate system is derived as follows: Among them, A = X g X s + X g X v + X v X s .

[0023] In the combined power transmission system, the new energy power station is represented by a grid-following converter as shown in Figure 6 . The current reference value of the VSC is determined by the power outer loop during normal operation. When a fault occurs, a low-voltage ride-through control strategy is adopted, and the current reference value of the VSC is switched according to the operating conditions of the power grid, and its current amplitude and power factor angle also change accordingly.

[0024] When the system is in a steady state, the VSC can be regarded as a power source, and its current is determined by the power reference value, that is, I vd = P ref / U v , I vq = Q ref / U v . Also, under the unity power factor control, P ref = P v , Q ref = 0, then the amplitude of the converter terminal voltage can be expanded by the formula to get the formula

[0025] Considering the voltage distribution of the actual system, solving the formula shows that the voltage amplitude U v at the VSC terminal is: Among them,

[0026] In addition, the relationship between δ s and δ v is deduced as:

[0027] Then, according to the above equivalent principle, the P eqe , P meq of the equivalent synchronous machine are obtained: Among them,

[0028] Therefore, the equivalent rotor motion equation of the equivalent synchronous machine is: Among them, ω B is the rated angular frequency; Δω sc is the relative angular frequency: Δω sc = ω s - ω g ; ω s is the angular frequency of the grid-forming SVG.

[0029] Analysis of Simulation Results To verify the characteristics of the combined power transmission system described above, simulations are carried out in the MATLAB / Simulink environment. The working conditions selected in this invention are as follows: Assume that a single-phase grounding short-circuit fault occurs at the A-phase of the rectifier side (sending end) AC bus at t = 5s, and the fault duration is 0.1s. Compare the output voltage, current, and active power waveforms of the grid-forming STATCOM, STATCOM, and synchronous condenser, and compare the fault recovery time.

[0030] It can be seen from the analysis of the simulation results that when a single-phase short-circuit fault occurs at the sending end of the system, the time for the grid-forming STATCOM voltage and current to recover to stability is significantly less than that of the other two reactive power compensation devices, and its active power stability is significantly higher than that of the other two reactive power compensation devices. Thus, it is concluded that the reactive power compensation method proposed in this invention can improve the system stability and has good practical application value.

Claims

1. A grid-type STATCOM analysis model based on DC capacitor dynamics, characterized by: S1. Control structure of adjusting the internal frequency of STATCOM to DC voltage: Where: * is the internal frequency of STATCOM, ω g is the frequency reference value, VDCref is the DC voltage reference value, V DC is the voltage of the DC bus capacitor, K J is the inertia simulation coefficient, K D is the damping coefficient, K T is the DC voltage tracking coefficient; Without considering the power loss of the capacitor itself, the active power output by the DC side capacitor during the transient period is equivalent to the active power output by the STATCOM, and its dynamic expression is: From formula (1) and formula (2), we can get: Where: equivalent inertia coefficient KJeq = KJCDC / 2; equivalent damping coefficient KDeq = KDCDC / 2; DC voltage coefficient KTeq = KTCDC / 2; after ignoring the voltage tracking term, equation (3) is simplified to: The rotor motion equation is: J SG sω SG +D SG (ω SG -ω Grid )=P M -P ESG (5) Among them, ω SG is the rotor speed, ω Grid is the grid frequency, J SG is the inertia time constant, D SG is the equivalent electromechanical damping coefficient, P M is the mechanical power provided by the prime mover, P ESG is the electromagnetic power output by the synchronous machine; After ignoring the resistance of the line and the filter, the active power output by STATCOM is: Among them, X Σ =X F +X s +X g , U * is the internal potential of STATCOM, U g is the grid voltage, δ * It's U * The phase, θ g It's U g The phase of Considering sδ * =ω Base ω * , sθ g =ω Base ω g and δ = δ * -θ g , further written as: Equivalent modeling of the joint delivery system: According to the superposition theorem and topological diagram, the voltage equation in the synchronous coordinate system is derived: in, A=X g X s +X g X v +X v X s ; The VSC current is determined by the power reference value, that is, I vd =P ref / U v , I vq =Q ref / U v ; Under unity power factor control, P ref =P v , Q ref =0, then the voltage amplitude of the converter port is expanded by the formula: Considering the voltage distribution of the actual system, the voltage amplitude at the VSC port is U v for: in, δ s and δ v The relationship between: According to the above equivalent principle, the P of the equivalent synchronous machine is obtained eqe , P meq : in, Therefore, the equivalent rotor motion equation of the equivalent synchronous machine is: where ω B is the rated angular frequency; Δω sc is the relative angular frequency: Δω sc =ω s -ω g ;ω s is the angular frequency of the meshed SVG.

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