Low voltage ride through control method and system for converting alternating current excitation into virtual inductance

Abstract

The invention discloses a low-voltage ride-through control method and system for converting AC excitation into virtual inductance, and relates to the technical field of variable-speed pumped storage AC excitation, and the method comprises the steps: calculating the iron loss of a doubly-fed motor through an impedance parameter conversion method; a rotor port is equivalent to a virtual inductor by controlling phases and amplitudes of output voltage and current of a rotor-side frequency converter; based on iron loss and virtual inductance calculation results, an alternating current excitation system model is established through real-time digital simulation; a doubly-fed motor model after iron loss conversion is set, and electrical parameters are determined; and controlling full-load and light-load operation of the doubly-fed motor under power generation and electric working conditions by using an alternating-current excitation system model, carrying out a variable-speed pumped storage low-voltage ride-through test, and testing the control performance when the voltage of the power grid is suddenly changed. According to the low-voltage ride-through control method for converting the virtual inductance through the alternating-current excitation, the low-voltage ride-through problem of a variable-speed pumped storage alternating-current excitation system is effectively solved on the premise that the authenticity and the accuracy of real-time simulation are guaranteed.

Description

Technical Field

[0001] The present invention relates to the technical field of variable-speed pumped storage AC excitation, and particularly to a low-voltage ride-through control method and system for an AC excitation converted virtual inductor. Background Art

[0002] Variable-speed pumped storage power stations have functions of peak shaving, valley filling, frequency modulation and emergency standby, and are an important way for large-capacity electric energy storage at present. With the continuous increase of the installed capacity of clean energy in China, the demand for variable-speed pumped storage units is becoming increasingly urgent. Considering from the perspective of power grid operation, variable-speed pumped storage units are conducive to quickly and accurately responding to the needs of the power grid and improving the stability and power quality of the power grid; considering from the perspective of the operation of the power station itself, variable-speed pumped storage units have the advantages of improving the efficiency and stability performance of the units, improving the cavitation performance, and having a power regulation function.

[0003] When operating at a suitable speed, the wear of variable-speed units can be greatly reduced, the annual average efficiency can be increased by 3% - 5%, and the overhaul period of the units can be extended by about one time. Since the AC excitation system of variable-speed pumped storage units includes an inverter, its output waveform contains rich harmonic components, which increases the iron loss of the motor. And variable-speed pumped storage AC excitation units must have the ability of low-voltage ride-through, that is, when a voltage dip fault occurs in the power grid, the units should be able to ensure continuous operation without tripping. Therefore, in order to improve the low-voltage ride-through control accuracy of the AC excitation system on the basis of considering the iron loss of the motor, and at the same time simplify the model, and then help the actual project, it is necessary to study the iron loss of variable-speed pumped storage units and its low-voltage ride-through control method. Summary of the Invention

[0004] In view of the above existing problems, the present invention is proposed.

[0005] Therefore, the technical problems solved by the present invention are: the existing low-voltage ride-through control methods for variable-speed pumped storage units have problems such as inaccurate calculation of motor iron loss, overly complex models, and insufficient low-voltage ride-through capabilities, and the optimization problem of how to improve the low-voltage ride-through control accuracy while considering the motor iron loss and simplify the calculation model.

[0006] To solve the above technical problems, the present invention provides the following technical solutions: a low-voltage ride-through control method for an AC excitation converted virtual inductor, including:

[0007] Calculating the iron loss of the doubly-fed motor through an impedance parameter conversion method;

[0008] By controlling the phase and amplitude of the output voltage and current of the rotor-side inverter, the rotor port is equivalent to a virtual inductor;

[0009] Based on the calculation results of the iron loss and the virtual inductor, an AC excitation system model is established through real-time digital simulation;

[0010] Set up the doubly-fed motor model with reduced iron loss in the simulation software and determine the electrical parameters;

[0011] Use the AC excitation system model to control the full-load and light-load operation of the doubly-fed motor under generating and motoring conditions, and conduct the variable-speed pumped storage low-voltage ride-through test to test the control performance when the grid voltage suddenly changes.

[0012] As a preferred scheme of the low-voltage ride-through control method for AC excitation reduced virtual inductor according to the present invention, wherein: the impedance parameter reduction method includes setting a transformation factor, multiplying the excitation branch of the equivalent circuit of the doubly-fed induction motor by the modulus of the transformation factor for approximation to obtain the equivalent Z parameter.

[0013] As a preferred scheme of the low-voltage ride-through control method for AC excitation reduced virtual inductor according to the present invention, wherein: the transformation factor is expressed as

[0014]

[0015] wherein, Δ is the transformation factor, R Fe is the iron loss resistance, j is the imaginary unit, X m is the excitation reactance of the equivalent model before transformation;

[0016] The equivalent Z parameter is expressed as

[0017]

[0018] Let Δz1 = R s +jX s , Δz2 = R r +jX r , jX m |Δ| = jωL m

[0019] wherein, Z final is the parameter matrix of the final simplified equivalent model, z1 and z2 are the transition parameters respectively; R s , R r are the stator resistance and rotor resistance of the final simplified equivalent model respectively; L s , L r , L m are the stator inductance, rotor inductance and excitation inductance of the final simplified equivalent model respectively; X s , X r are the stator reactance and rotor reactance of the final simplified equivalent model respectively; ω is the grid frequency.

[0020] As a preferred solution of the low-voltage ride-through control method for the virtual inductor obtained by converting the AC excitation of the present invention, wherein: the equivalent of the rotor port to a virtual inductor includes, according to Kirchhoff's voltage law, assuming that the rotor-side port is equivalent to a virtual inductor L eq , writing the voltage equation of the rotor-side loop, expressed as

[0021]

[0022] wherein, L eq is the virtual inductor; L s , L r , L m are respectively the stator inductance, rotor inductance, and excitation inductance of the final simplified equivalent model; ψ s is the stator flux linkage; i r is the rotor-side current;

[0023] Ignoring the branch resistance, the set reference value of the rotor-side current is expressed as

[0024]

[0025] wherein, is the set reference value of the rotor-side current;

[0026] The set reference value of the rotor-side voltage is expressed as

[0027]

[0028] wherein, is the set reference value of the rotor-side voltage;

[0029] To prevent overvoltage and overcurrent on the rotor side, constraint conditions are set, expressed as

[0030]

[0031] wherein, I rm is the maximum current value of the rotor-side port; U rm is the maximum voltage value of the rotor-side port;

[0032] Determine the value range of the virtual inductor, expressed as

[0033]

[0034] wherein, ψ sl is the peak value of the stator flux linkage during low-voltage ride-through of the AC excitation system; E sl is the peak value of the rotor induced electromotive force during low-voltage ride-through of the AC excitation system.

[0035] As a preferred embodiment of the low-voltage ride-through control method for an AC-excitation converted virtual inductor according to the present invention, wherein: the AC-excitation system model includes a main circuit and a control circuit;

[0036] The main circuit includes a doubly-fed motor with AC excitation and a pumped-storage turbine;

[0037] The control circuit includes a monitoring system, an AC-excitation control system, and a governor.

[0038] As a preferred embodiment of the low-voltage ride-through control method for an AC-excitation converted virtual inductor according to the present invention, wherein: the doubly-fed motor model after converting iron loss and determining electrical parameters includes stator resistance, stator inductance, rotor resistance, rotor inductance, and excitation inductance parameters.

[0039] As a preferred embodiment of the low-voltage ride-through control method for an AC-excitation converted virtual inductor according to the present invention, wherein: the control performance when testing the sudden change of grid voltage is expressed as,

[0040]

[0041] wherein, is an intermediate state variable; Q s , P s are the actual values of stator reactive and active power; are the actual values of stator reactive and active power; u drc and u qrc are the feed-forward voltage compensation amounts; K p1 , K p2 , K p3 , K p4 are proportionality coefficients; K i1 , K i2 , K i3 , K i4 are integral coefficients; ω s is the synchronous speed; ω r is the rotor angular velocity; i dr and i qr are the d-axis and q-axis current values of the rotor-side converter, respectively; u dr and u qr are the d-axis and q-axis voltage values of the rotor-side converter, respectively.

[0042] Another object of the present invention is to provide a low-voltage ride-through control system for an AC-excitation converted virtual inductor, which can solve the problems of insufficient calculation accuracy of motor iron loss, complex model, and weak low-voltage ride-through ability in the existing low-voltage ride-through control methods by constructing a low-voltage ride-through control system for an AC-excitation converted virtual inductor.

[0043] To solve the above technical problems, the present invention provides the following technical solutions: A low-voltage ride-through control system for an AC-excited converted virtual inductor, comprising: an iron loss calculation module for calculating the iron loss of a doubly-fed motor by means of an impedance parameter conversion method; a virtual inductor control module for converting the rotor port into an equivalent virtual inductor by controlling the phase and amplitude of the output voltage and current of the rotor-side frequency converter; a simulation model establishment module for establishing an AC excitation system model through real-time digital simulation based on the calculation results of the iron loss and the virtual inductor; a motor model setting module for setting the doubly-fed motor model after converting the iron loss in the simulation software and determining the electrical parameters; and a low-voltage test module for controlling the doubly-fed motor to operate at full load and light load in the power generation and motor conditions using the AC excitation system model, conducting a variable-speed pumped storage low-voltage ride-through test, and testing the control performance when the grid voltage suddenly changes.

[0044] The impedance parameter conversion method includes setting a conversion factor, multiplying the excitation branch of the equivalent circuit of the doubly-fed induction motor by the modulus of the conversion factor for approximation, and obtaining the equivalent Z parameter.

[0045] The conversion factor is expressed as

[0046]

[0047] where Δ is the conversion factor, R Fe is the iron loss resistance, j is the imaginary unit, and X m is the excitation reactance of the equivalent model before conversion;

[0048] The equivalent Z parameter is expressed as

[0049]

[0050] Let Δz1 = R s + jX s , Δz2 = R r + jX r , jX m |Δ| = jωL m

[0051] where Z final is the parameter matrix of the final simplified equivalent model, z1 and z2 are the transition parameters respectively; R s , R r are the stator resistance and rotor resistance of the final simplified equivalent model respectively; L s , L r , L m are the stator inductance, rotor inductance, and excitation inductance of the final simplified equivalent model respectively; X s , X r are the stator reactance and rotor reactance of the final simplified equivalent model respectively; ω is the grid frequency.

[0052] The equivalent of the rotor port as a virtual inductor includes, according to Kirchhoff's voltage law, assuming that the rotor-side port is equivalent to a virtual inductor L eq , writing the voltage equation of the rotor-side loop, expressed as

[0053]

[0054] where L eq is the virtual inductor; L s , L r , L m are the stator inductance, rotor inductance, and field inductance of the final simplified equivalent model respectively; ψ s is the stator magnetic flux; i r is the rotor-side current;

[0055] Ignoring the branch resistance, the reference value of the rotor-side current is set, expressed as

[0056]

[0057] where is the reference value of the rotor-side current setting;

[0058] The reference value of the rotor-side voltage setting is expressed as

[0059]

[0060] where is the reference value of the rotor-side voltage setting;

[0061] To prevent overvoltage and overcurrent on the rotor side, a constraint condition is set, expressed as

[0062]

[0063] where I rm is the maximum current value of the rotor-side port; U rm is the maximum voltage value of the rotor-side port;

[0064] The value range of the virtual inductor is determined, expressed as

[0065]

[0066] where ψ sl is the peak value of the stator magnetic flux during low-voltage ride-through of the AC excitation system; E sl is the peak value of the rotor induced electromotive force during low-voltage ride-through of the AC excitation system.

[0067] The AC excitation system model includes a main circuit and a control circuit;

[0068] The main circuit includes a doubly-fed induction generator with AC excitation and a pump-turbine.

[0069] The control circuit includes a monitoring system, an AC excitation control system, and a governor.

[0070] The model of the doubly-fed induction generator after converting iron loss and determining electrical parameters includes stator resistance, stator inductance, rotor resistance, rotor inductance, and excitation inductance parameters.

[0071] The control performance when testing the sudden change of grid voltage is expressed as

[0072]

[0073] where is an intermediate state variable; Q s , P s are the actual values of stator reactive and active power; are the actual values of stator reactive and active power; u drc and u qrc are the feed-forward voltage compensation amounts; K p1 , K p2 , K p3 , K p4 are proportionality coefficients; K i1 , K i2 , K i3 , K i4 are integral coefficients; ω s is the synchronous speed; ω r is the rotor angular velocity; i dr and i qr are the d-axis and q-axis current values of the rotor-side converter respectively; u dr and u qr are the d-axis and q-axis voltage values of the rotor-side converter respectively.

[0074] A computer device includes a memory and a processor. When the processor executes the computer program, the steps of the low-voltage ride-through control method for AC excitation converted virtual inductor as described above are implemented.

[0075] A computer-readable storage medium stores a computer program. When the computer program is executed by a processor, the steps of the low-voltage ride-through control method for AC excitation converted virtual inductor as described above are implemented.

[0076] Advantages of the present invention: The low-voltage ride-through control method for converting virtual inductance by AC excitation provided by the present invention takes into account the iron loss of the motor in the variable-speed pumped-storage AC excitation system, simplifies the Z-parameter conversion of the equivalent circuit of the doubly-fed motor, and uses the real-time simulation method while fully considering the iron loss of the motor, which will make the simulation more consistent with the actual situation. However, paralleling the equivalent resistance of iron loss at both ends of the excitation inductance will complicate the voltage equation, flux linkage equation, etc. of the motor. A complex motor model is disadvantageous for real-time digital simulation (RTDS), and convergence problems caused by the calculation step size may occur. Therefore, it is necessary to simplify the model while considering iron loss. Since the only parameters that can be set in the packaged motor model in the simulation software RSCAD are the stator resistance, stator inductance, rotor resistance, rotor inductance, and excitation inductance, and there is no setting for the iron loss resistance, this paper intends to make full use of the built-in packaged model of the software. Therefore, first, it is simplified through impedance parameter (Z-parameter) conversion. Then, for the low-voltage ride-through problem of the variable-speed pumped-storage AC excitation system, referring to the principle of realizing low-voltage ride-through by switching the Crowbar, by controlling the phase and amplitude of the output voltage and current of the rotor-side frequency converter, the rotor port is made equivalent to a virtual inductance externally, thereby improving the low-voltage ride-through ability of the unit. Finally, the main circuit model of the variable-speed pumped-storage high and low ride-through test is built and tested in RTDS. When the terminal voltage is within the range of 90% - 110%, the motor can maintain normal operation. When the terminal voltage is 40% - 120%, it can maintain grid connection for at least 0.3 s and provide a certain amount of reactive power compensation ability. The present invention effectively addresses the low-voltage ride-through problem of the variable-speed pumped-storage AC excitation system on the premise of ensuring the authenticity and accuracy of real-time simulation, which is helpful for practical engineering. Description of the Drawings

[0077] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0078] Figure 1 Structural diagram of a variable-speed pumped-storage unit for a low-voltage ride-through control method of converting virtual inductance by AC excitation provided by an embodiment of the present invention;

[0079] Figure 2 Overall flowchart of a low-voltage ride-through control method of converting virtual inductance by AC excitation provided by an embodiment of the present invention;

[0080] Figure 3The equivalent circuit diagram of the doubly-fed motor in a low-voltage ride-through control method for an AC-excitation converted virtual inductor provided by an embodiment of the present invention;

[0081] Figure 4 The final simplified equivalent model diagram in a low-voltage ride-through control method for an AC-excitation converted virtual inductor provided by an embodiment of the present invention;

[0082] Figure 5 The control block diagram of the rotor-side converter low-voltage ride-through control in a low-voltage ride-through control method for an AC-excitation converted virtual inductor provided by an embodiment of the present invention. Detailed implementation manners

[0083] To make the above objects, features and advantages of the present invention more obvious and understandable, the following detailed description of the specific implementation manners of the present invention is made in conjunction with the accompanying drawings of the specification. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0084] In the following description, many specific details are set forth to fully understand the present invention. However, the present invention can also be implemented in other ways different from those described herein. Those skilled in the art can make similar extensions without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0085] Embodiment 1

[0086] Refer to Figures 1 - 5 , which is an embodiment of the present invention, and provides a low-voltage ride-through control method for an AC-excitation converted virtual inductor, including:

[0087] Step 1: Fully consider the iron loss of the doubly-fed motor, and simplify the calculation of the motor iron loss by means of impedance parameter (Z-parameter) conversion, so as to make the control more accurate and the simulation closer to the actual project;

[0088] Step 2: Refer to the principle of using a switched Crowbar to achieve low-voltage ride-through, and control the phase and amplitude of the output voltage and current of the rotor-side converter, so that the rotor port is externally equivalent to a virtual inductor, thereby improving the low-voltage ride-through ability of the unit;

[0089] Step 3: On the basis of considering iron loss and calculating the virtual inductor, establish a model of the AC-excitation control system based on real-time digital simulation (RTDS), such as Figure 1As shown in the figure. The main circuit includes a doubly-fed motor with AC excitation and a pump-turbine, and the control circuit includes a monitoring system (including a coordinated controller), an AC excitation control system, and a governor. After stepping down through a transformer, one outgoing line at the output end of the power grid is connected to the rotor winding of the doubly-fed motor through a transformer and a three-level inverter with a AC-DC-AC structure, and the other outgoing line is connected to the stator winding. Capacitors on the upper and lower half-bridges of the DC side of the inverter are connected in parallel with a DC voltage source. Set parameters in the packaged motor model in the simulation software RSCAD, including: stator resistance, stator inductance, rotor resistance, rotor inductance, and excitation inductance parameters (all the above are parameters after converting iron loss).

[0090] Step 4: During the low voltage ride-through test of variable-speed pumped storage, the AC excitation control system controls the doubly-fed motor to operate at full load and light load respectively in the power generation and motor modes, and tests the control performance when the grid voltage suddenly changes. The flow chart is as Figure 2 shown.

[0091] Specifically, fully consider the iron loss of the doubly-fed motor to make the control more accurate, and simplify the calculation of the motor iron loss by converting the impedance parameters (Z parameters).

[0092] The method for converting the Z parameters of the iron loss of the doubly-fed motor includes:

[0093] In the equivalent circuit of the traditional doubly-fed induction motor, the iron loss of the motor is represented as a resistor with a constant resistance value, which can accurately reflect the iron loss of the motor under rated conditions, as Figure 3 shown.

[0094] The purpose of simplifying the model is to convert the iron loss resistor connected in parallel at both ends of the excitation inductance in Figure 3 to the stator and rotor branches at steady state, so as to obtain the final simplified model.

[0095] Let the transformation factor Multiply the excitation branch of the equivalent circuit of the doubly-fed induction motor by the modulus of the transformation factor for approximate treatment, and the final equivalent Z parameter is

[0096]

[0097] Let Δz1 = R s +jX s ,Δz2 = R r +jX r ,jX m |Δ| = jωL m

[0098] Among them, Z final is the parameter matrix of the final simplified equivalent model; X m is the excitation reactance of the original equivalent model; z1 and z2 are transition parameters respectively; R s 、Rr are the stator resistance and rotor resistance of the final simplified equivalent model, respectively; L s , L r , L m are the stator inductance, rotor inductance, and field inductance of the final simplified equivalent model, respectively; X s , X r are the stator reactance and rotor reactance of the final simplified equivalent model, respectively; ω is the grid frequency; R Fe is the iron loss resistance; j is the imaginary unit; Δ is the transformation factor.

[0099] The final simplified model is obtained as Figure 4 shown.

[0100] It should be noted that the design of the virtual inductor parameters also includes: to minimize the voltage requirement at the rotor-side converter port, according to Kirchhoff's voltage law, the induced voltage at the port should be in the same direction as the voltage drop across the rotor inductor. Therefore, the rotor-side port being equivalent to a pure inductor load externally is the optimal choice for the low-voltage ride-through of the AC excitation unit. Further calculate the voltage and current controlled and output by the rotor-side converter, and accordingly determine the equivalent virtual inductor value.

[0101] Assume that the rotor-side port is equivalent to a virtual inductor L eq , and write the voltage equation for the rotor-side loop as:

[0102]

[0103] where L eq is the virtual inductor; L s , L r , L m are the stator inductance, rotor inductance, and field inductance of the final simplified equivalent model, respectively; ψ s is the stator flux linkage; i r is the rotor-side current.

[0104] Ignoring the branch resistance, the reference value of the rotor-side current setting is:

[0105]

[0106] where is the reference value of the rotor-side current setting.

[0107] The reference value of the rotor-side voltage setting is:

[0108]

[0109] where is the reference value of the rotor-side voltage setting.

[0110] To prevent overvoltage and overcurrent on the rotor side, the constraint conditions are as follows:

[0111]

[0112] Among them, I rm is the maximum current value of the rotor-side port; U rm is the maximum voltage value of the rotor-side port.

[0113] The reasonable value range of the virtual inductor is:

[0114]

[0115] Among them, ψ sl is the peak value of the stator magnetic flux during low-voltage ride-through of the AC excitation system; E sl is the peak value of the rotor induced electromotive force during low-voltage ride-through of the AC excitation system.

[0116] Furthermore, the mathematical differential equation model of the low-voltage ride-through control of the rotor-side frequency converter of the AC excitation system also includes:

[0117]

[0118] Among them, is the intermediate state variable; Q s , P s are the actual values of the stator reactive and active powers; are the actual values of the stator reactive and active powers; u drc and u qrc are the feed-forward voltage compensation amounts; K p1 , K p2 , K p3 , K p4 are the proportionality coefficients; K i1 , K i2 , K i3 , K i4 are the integral coefficients; ω s is the synchronous speed; ω r is the rotor angular velocity; i dr and i qr are the d-axis and q-axis current values of the rotor-side frequency converter respectively; u dr and u qr are the d-axis and q-axis voltage values of the rotor-side frequency converter respectively.

[0119] The control block diagram of the low-voltage ride-through control of the rotor-side frequency converter is as Figure 5 shown.

[0120] Example 2

[0121] An embodiment of the present invention provides a low-voltage ride-through control system for an AC-excited equivalent virtual inductor, comprising:

[0122] An iron loss calculation module, configured to calculate the iron loss of a doubly-fed motor by an impedance parameter conversion method;

[0123] A virtual inductor control module, configured to equivalent the rotor port to a virtual inductor by controlling the phase and amplitude of the output voltage and current of the rotor-side frequency converter;

[0124] A simulation model establishment module, configured to establish an AC-excitation system model through real-time digital simulation based on the calculation results of iron loss and virtual inductor;

[0125] A motor model setting module, configured to set a doubly-fed motor model with converted iron loss in a simulation software and determine electrical parameters; and,

[0126] A low-voltage test module, configured to control the doubly-fed motor to operate at full load and light load in power generation and motor conditions by using the AC-excitation system model, conduct a variable-speed pumped-storage low-voltage ride-through test, and test the control performance when the grid voltage suddenly changes.

[0127] The impedance parameter conversion method includes setting a conversion factor, multiplying the excitation branch of the equivalent circuit of the doubly-fed induction motor by the modulus of the conversion factor for approximation processing to obtain an equivalent Z parameter.

[0128] The conversion factor is expressed as

[0129]

[0130] wherein, Δ is the conversion factor, R Fe is the iron loss resistance, j is the imaginary unit, X m is the excitation reactance of the equivalent model before conversion;

[0131] The equivalent Z parameter is expressed as

[0132]

[0133] Let Δz1 = R s + jX s , Δz2 = R r + jX r , jX m |Δ| = jωL m

[0134] wherein, Z final is the parameter matrix of the final simplified equivalent model, z1 and z2 are transition parameters respectively; R s , R r are the stator resistance and rotor resistance of the final simplified equivalent model respectively; L s 、Lr , L m are respectively the stator inductance, rotor inductance, and excitation inductance of the final simplified equivalent model; X s , X r are respectively the stator reactance and rotor reactance of the final simplified equivalent model; ω is the grid frequency.

[0135] Equivalent the rotor port to a virtual inductor, including according to Kirchhoff's voltage law, assume the rotor-side port is equivalent to a virtual inductor L eq , and write the voltage equation of the rotor-side loop, expressed as

[0136]

[0137] where, L eq is the virtual inductor; L s , L r , L m are respectively the stator inductance, rotor inductance, and excitation inductance of the final simplified equivalent model; ψ s is the stator flux linkage; i r is the rotor-side current;

[0138] Ignore the branch resistance, then the reference value of the rotor-side current is set, expressed as

[0139]

[0140] where is the reference value of the rotor-side current set;

[0141] The reference value of the rotor-side voltage is set, expressed as

[0142]

[0143] where is the reference value of the rotor-side voltage set;

[0144] To prevent overvoltage and overcurrent on the rotor side, set the constraint conditions, expressed as

[0145]

[0146] where I rm is the maximum current value of the rotor-side port; U rm is the maximum voltage value of the rotor-side port;

[0147] Determine the value range of the virtual inductor, expressed as

[0148]

[0149] where ψ slis the peak value of the stator magnetic flux during the low voltage ride-through of the AC excitation system; E sl is the peak value of the rotor induced electromotive force during the low voltage ride-through of the AC excitation system.

[0150] The AC excitation system model includes a main circuit and a control circuit;

[0151] The main circuit includes a doubly-fed motor with AC excitation and a pumped-storage turbine;

[0152] The control circuit includes a monitoring system, an AC excitation control system, and a governor.

[0153] The model of the doubly-fed motor after converting the iron loss and determining the electrical parameters include stator resistance, stator inductance, rotor resistance, rotor inductance, and excitation inductance parameters.

[0154] Testing the control performance when the grid voltage suddenly changes is expressed as,

[0155]

[0156] Among them, is an intermediate state variable; Q s , P s are the actual values of the stator reactive and active powers; is the actual value of the stator reactive and active powers; u drc and u qrc are the feed-forward voltage compensation amounts; K p1 , K p2 , K p3 , K p4 are proportionality coefficients; K i1 , K i2 , K i3 , K i4 are integral coefficients; ω s is the synchronous speed; ω r is the rotor angular velocity; i dr and i qr are the d-axis and q-axis current values of the rotor-side converter respectively; u dr and u qr are the d-axis and q-axis voltage values of the rotor-side converter respectively.

[0157] Embodiment 3

[0158] An embodiment of the present invention, which is different from the previous two embodiments in that:

[0159] If the above-mentioned function is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or a part of this technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions for causing a computer device (which may be a personal computer, a server, or a network device, etc.) to execute all or part of the steps of the methods described in various embodiments of the present invention. The aforementioned storage medium includes: various media such as USB flash drives, mobile hard disks, read-only memories (ROMs), random access memories (RAMs), magnetic disks, or optical discs that can store program codes.

[0160] The logic and / or steps represented in the flowchart or otherwise described herein, for example, can be considered as a predefined sequence list of executable instructions for implementing a logical function, and can be specifically implemented in any computer-readable medium for use by an instruction execution system, apparatus, or device (such as a computer-based system, a system including a processor, or other systems that can fetch and execute instructions from the instruction execution system, apparatus, or device), or in combination with these instruction execution systems, apparatus, or devices. For the purposes of this specification, a "computer-readable medium" can be any device that can contain, store, communicate, propagate, or transport a program for use by or in combination with an instruction execution system, apparatus, or device.

[0161] More specific examples (non-exhaustive list) of computer-readable media include the following: an electrical connection portion (electronic device) having one or more wirings, a portable computer disk cartridge (magnetic device), a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), an optical fiber device, and a portable compact disc read-only memory (CDROM). Additionally, a computer-readable medium can even be paper or other suitable media on which the program can be printed, because the program can be obtained electronically, for example, by optically scanning the paper or other media, then editing, interpreting, or otherwise processing it as appropriate, and then storing it in a computer memory.

[0162] It should be understood that each part of the present invention can be implemented by hardware, software, firmware or a combination thereof. In the above embodiments, multiple steps or methods can be implemented by software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented by hardware, as in another embodiment, any one or a combination of the following techniques well known in the art can be used: discrete logic circuits having logic gate circuits for implementing logic functions on data signals, application specific integrated circuits having appropriate combinational logic gate circuits, programmable gate arrays (PGAs), field programmable gate arrays (FPGAs), etc.

[0163] Embodiment 4

[0164] An embodiment of the present invention provides a low voltage ride-through control method for an AC-excited converted virtual inductor. In order to verify the beneficial effects of the present invention, scientific demonstration is carried out through economic benefit calculation and simulation experiments.

[0165] In this embodiment, a low voltage ride-through control model for an AC-excited converted virtual inductor is first established, and the setting of each typical parameter is as follows:

[0166] Doubly-fed motor parameters: rated capacity is 333.3 MVA; rated voltage is 18 kV; stator resistance (per unit value) is 0.00143; stator inductance (per unit value) is 0.119; rotor resistance (per unit value) is 0.00165; rotor inductance (per unit value) is 0.1853; excitation inductance (per unit value) is 2.559; moment of inertia is 6300 t·m 2 ;

[0167] Power supply parameters: line voltage effective value is 500 kV; internal resistance is 0.0001 Ω;

[0168] Step-down transformer parameters: rated capacity is 400 MVA; turns ratio is 500 / 18 kV; reactance (per unit value) is 0.1;

[0169] Excitation transformer parameters: rated capacity is 60 MVA; turns ratio is 18 / 4.5 kV; reactance (per unit value) is 0.1;

[0170] Resistance, inductance and capacitance parameters: stator side grid-connected resistance is 0.0001 Ω; grid side converter AC side resistance is 0.0001 Ω; grid side converter AC side inductance is 400 μH; machine side converter AC side resistance is 0.0001 Ω; machine side converter AC side inductance is 400 μH; converter DC side capacitance is 18 mF.

[0171] The test steps only for the power generation condition are as follows:

[0172] In the RTDS model, the motor selects the speed model, and the speed is set to 1.07 p.u. The AC excitation system controls the active power output of the motor stator to be 100 MW and 300 MW respectively. In the RTDS model, the terminal voltage of the machine is set to suddenly change from the rated value to 90%, 70%, 40%, 110%, and 120%.

[0173] In the RTDS model, the motor selects the torque model, and the torques are set to 0.3 p.u. and 0.9 p.u. (power factor 0.9, full load) respectively. The AC excitation system controls the motor speed to be 1.07 p.u. In the RTDS model, the terminal voltage of the machine is set to suddenly change from the rated value to 90%, 70%, 40%, 110%, and 120%.

[0174] The test conclusions are as follows: Light load: When the terminal voltage of the machine is 90%, the motor can maintain normal and stable operation; when the terminal voltage of the machine is 70%, the motor can maintain grid-connected operation within 1 s and provide at least 0.3 p.u. of positive-sequence reactive current to the power grid. When using the torque model, the motor speed is maintained within 0.9 - 1.1 p.u.; when the terminal voltage of the machine is 40%, the motor can maintain grid-connected operation within 0.3 s and provide at least 0.8 p.u. of positive-sequence reactive current to the power grid. When using the torque model, the motor speed is maintained within 0.9 - 1.1 p.u.

[0175] Full load: When the terminal voltage of the machine is 90%, the motor can maintain normal and stable operation; when the terminal voltage of the machine is 70%, the motor can maintain grid-connected operation within 1 s and provide at least 0.3 p.u. of positive-sequence reactive current to the power grid. When using the torque model, the motor speed is maintained within 0.9 - 1.1 p.u.; when the terminal voltage of the machine is 40%, the motor can maintain grid-connected operation within 0.3 s and provide at least 0.8 p.u. of positive-sequence reactive current to the power grid. When using the torque model, the motor speed is maintained within 0.9 - 1.1 p.u.

[0176] The results of the numerical example show that the proposed low-voltage ride-through control method for AC excitation equivalent virtual inductor can effectively solve the low-voltage ride-through problem of the AC excitation system of variable-speed pumped-storage units.

[0177] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered by the scope of the claims of the present invention.

Claims

1. A low-voltage ride-through control method for an AC-excited equivalent virtual inductor, characterized in that Including: Calculating the iron loss of the doubly-fed motor by the impedance parameter conversion method; Equivalently converting the rotor port into a virtual inductor by controlling the phase and amplitude of the output voltage and current of the rotor-side frequency converter; Based on the calculation results of the iron loss and the virtual inductor, establishing an AC excitation system model through real-time digital simulation; Setting the doubly-fed motor model after converting the iron loss in the simulation software and determining the electrical parameters; Using the AC excitation system model to control the full-load and light-load operations of the doubly-fed motor under generating and motoring conditions, conducting a variable-speed pumped-storage low-voltage ride-through test, and testing the control performance when the grid voltage suddenly changes.

2. The low-voltage ride-through control method for an AC-excited equivalent virtual inductor according to claim 1, characterized in that: The impedance parameter conversion method includes setting a conversion factor, multiplying the excitation branch of the equivalent circuit of the doubly-fed induction motor by the modulus of the conversion factor for approximation, and obtaining the equivalent Z parameter.

3. The low-voltage ride-through control method for an AC-excited equivalent virtual inductor according to claim 2, characterized in that: The conversion factor is expressed as where Δ is the transformation factor, R Fe is the iron loss resistance, j is the imaginary unit, X m is the magnetizing reactance of the equivalent model before transformation; The equivalent Z parameter is expressed as Let Δz1 = R s +jX s Let Δz2 = R r +jX r jX m |Δ| = jωL m Among them, Z final is the parameter matrix of the final simplified equivalent model, and z1 and z2 are the transition parameters respectively; R s , R r are the stator resistance and rotor resistance of the final simplified equivalent model respectively; L s , L r , L m are the stator inductance, rotor inductance and field inductance of the final simplified equivalent model respectively; X s , X r are the stator reactance and rotor reactance of the final simplified equivalent model respectively; ω is the grid frequency.

4. The low-voltage ride-through control method of the AC-excitation converted virtual inductor according to claim 3, characterized in that: The equivalent of the rotor port as a virtual inductor includes setting the rotor-side port equivalent to a virtual inductor L according to Kirchhoff's voltage law eq , and writing the voltage equation of the rotor-side loop, expressed as Among them, L eq is the virtual inductor; L s , L r , L m are respectively the stator inductance, rotor inductance, and excitation inductance of the final simplified equivalent model; ψ s is the stator magnetic flux linkage; i r is the current on the rotor side; Neglecting the branch resistance, the set reference value of the rotor-side current is expressed as Among them, is the reference value of the rotor side current setting; The set reference value of the rotor-side voltage is expressed as Among them, is the reference value of the rotor-side voltage setting; To prevent overvoltage and overcurrent on the rotor side, a constraint condition is set, which is expressed as Among them, I rm is the maximum current value of the rotor side port; U rm is the maximum voltage value of the rotor side port; Determining the value range of the virtual inductor, which is expressed as Among them, ψ sl is the peak value of the stator magnetic flux during the low-voltage ride-through of the AC excitation system; E sl is the peak value of the rotor induced electromotive force during the low-voltage ride-through of the AC excitation system.

5. The low-voltage ride-through control method for an AC-excited equivalent virtual inductor according to claim 4, characterized in that: The AC excitation system model includes a main circuit and a control circuit; The main circuit includes a doubly-fed motor with AC excitation and a pump-turbine; The control circuit includes a monitoring system, an AC excitation control system, and a governor.

6. The low-voltage ride-through control method for an AC-excited equivalent virtual inductor according to claim 5, characterized in that: The doubly-fed motor model after converting the iron loss and determining the electrical parameters include stator resistance, stator inductance, rotor resistance, rotor inductance, and excitation inductance parameters.

7. The low-voltage ride-through control method of the AC-excitation converted virtual inductor according to claim 6, characterized in that: The control performance when testing the sudden change of the grid voltage is expressed as Among them, is the intermediate state variable; Q s , P s are the actual values of the stator reactive and active powers; P s * are the actual values of the stator reactive and active powers; u drc and u qrc are the feed-forward voltage compensation amounts; K p1 , K p2 , K p3 , K p4 are proportionality coefficients; K i1 , K i2 , K i3 , K i4 are integral coefficients; ω s is the synchronous speed; ω r is the rotor angular velocity; i dr and i qr are the d-axis and q-axis current values of the rotor-side frequency converter respectively; u dr and u qr are the d-axis and q-axis voltage values of the rotor-side frequency converter respectively.

8. A low voltage ride-through control system for an AC-excited equivalent virtual inductor, characterized in that, Including: An iron loss calculation module for calculating the iron loss of the doubly-fed motor by the impedance parameter conversion method; A virtual inductor control module for equivalently converting the rotor port into a virtual inductor by controlling the phase and amplitude of the output voltage and current of the rotor-side frequency converter; A simulation model establishment module for establishing an AC excitation system model through real-time digital simulation based on the calculation results of the iron loss and the virtual inductor; A motor model setting module for setting the doubly-fed motor model after converting the iron loss in the simulation software and determining the electrical parameters; and A low-voltage test module for using the AC excitation system model to control the full-load and light-load operations of the doubly-fed motor under generating and motoring conditions, conducting a variable-speed pumped-storage low-voltage ride-through test, and testing the control performance when the grid voltage suddenly changes.

9. A computer device, comprising a memory and a processor, the memory storing a computer program, characterized in that, When the processor executes the computer program, it implements the steps of the low-voltage ride-through control method for AC excitation conversion virtual inductor according to any one of claims 1 to 7.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by the processor, it implements the steps of the low-voltage ride-through control method for AC excitation conversion virtual inductor according to any one of claims 1 to 7.

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