A low pressure ride through control method for a double-fed pumped storage unit

Abstract

The present invention discloses a low voltage ride-through control method for a doubly-fed pumped storage unit, and belongs to the field of doubly-fed pumped storage unit control. The present invention is to solve the problem that the doubly-fed pumped storage unit cannot reasonably cope with different degrees of voltage drop conditions using the existing low voltage ride-through strategy. The present invention includes judging whether a fault mode occurs, and if not, executing the doubly-fed pumped storage unit to work in a normal working mode; if so, judging the voltage drop condition of the doubly-fed pumped storage unit, the voltage drop degree includes a slight drop condition, a moderate drop condition and a severe drop condition, and selecting a corresponding working mode according to the voltage drop condition, the working mode includes a slight drop working mode, a moderate drop working mode and a severe drop working mode. The present invention enables the doubly-fed pumped storage unit to have good low voltage ride-through capability in different voltage drop conditions.

Description

Technical Field

[0001] The invention belongs to the field of doubly-fed pumped storage unit control, and in particular relates to a low-pressure ride-through control method for a doubly-fed pumped storage unit. Background Art

[0002] The installed grid-connected capacity and power generation share of new energy generators such as wind power and photovoltaic power generation are increasing year by year. Since the input sources of these generators are intermittent and cannot guarantee stable output of electricity, if they are directly connected to the grid on a large scale, it will undermine the stability of the power system. Pumped storage is a relatively mature energy storage technology with the advantages of strong storage capacity, high conversion efficiency and fast load response. Using it as a "stabilizer" to absorb and coordinate other new energy power generation equipment can improve the quality of power generation and give full play to the advantages of green resources.

[0003] Pumped storage units switch between electric and generating modes by controlling the forward and reverse rotation of the pump turbine. When the load is low, the electric mode is enabled to store energy; when the load is high, it switches to generating mode to feed back energy. Doubly fed generators use some power converters, have low operating losses, can achieve variable speed and frequency modulation through AC excitation, and are widely used in pumped storage units. The unit control method usually uses traditional vector oriented control, which orients a single vector in the dq coordinate system and releases the coupling relationship between the vectors, achieving independent control and simplifying the process.

[0004] Since the stator winding of the doubly-fed generator is directly connected to the grid, and the control speed and output power of the rotor-side converter are limited, it cannot cope with the impact of voltage drop at the grid connection point. In the power generation mode, the converter is easily damaged by overcurrent and overvoltage. In order to protect the fragile power electronic devices, the unit will be disconnected. The disconnection and reconnection of large-capacity units is a major threat to the safe and stable operation of the entire power system, which may expand the harm. Therefore, with the increase of single-unit capacity and overall proportion, the doubly-fed pumped storage unit needs to enhance the low voltage ride-through capability to adapt to voltage drop faults.

[0005] Due to the complex control conditions of the doubly-fed units used in pumped storage, there is little research on the relevant low voltage ride-through strategies, especially in the electric working mode. The current research on the low voltage ride-through strategies of traditional doubly-fed pumped storage units can be divided into two categories: additional hardware protection devices and improvement and optimization based on control strategies. Among the additional hardware protection devices, the most commonly used is the crowbar circuit placed at the front end of the rotor-side converter, and the most effective is the dynamic voltage restorer (DVR) placed at the grid connection point. In addition, there are methods such as additional supercapacitors, dynamic resistors, and DC choppers; in the improvement based on control strategies, there are methods that replace traditional PI controllers with fuzzy controllers and add feedforward signals to increase the current loop tracking speed, there are stator current feedback methods that directly track stator current to suppress rotor current, and there are demagnetization control methods guided by demagnetization. In principle, adding hardware inside the unit can protect the converter from the perspective of energy management by absorbing surplus energy through hardware devices; adding hardware on the grid-connected point side can fundamentally solve the fault by supporting the terminal voltage. However, hardware protection devices will increase the size and cost of the entire unit, and there are certain limitations to simply adding hardware protection devices. For example, a single additional Crowbar circuit cuts off the input of the rotor-side converter control signal when working, making the unit unable to output reactive power, and because the structure is equivalent to a squirrel-cage asynchronous motor, it will also absorb reactive power in the reverse direction; a single supercapacitor auxiliary straight line cannot alleviate the impact of surplus capacity on the rotor-side converter in advance because it is installed at the DC bus. The optimized control strategy can be achieved by weakening the transient response of certain quantities, reasonably adjusting the voltage and current values, offsetting the rotor induced electromotive force (EMF) to a certain extent, and using the system itself to absorb the surplus energy. However, due to the limited energy storage capacity of the unit itself, it is difficult to complete low voltage ride-through under severe voltage drops by relying solely on control strategies. Such methods are only applicable to moderate voltage drops and below. Summary of the invention

[0006] In order to solve the problem that the doubly-fed pumped-storage unit cannot reasonably cope with voltage sag conditions of different degrees using the existing low voltage ride-through strategy, the present invention provides a low voltage ride-through control method for a doubly-fed pumped-storage unit, so that the doubly-fed pumped-storage unit has good low voltage ride-through capability in different voltage sag conditions.

[0007] The present invention provides a low voltage ride through control method for a doubly fed pumped storage unit, wherein the doubly fed pumped storage unit comprises: a doubly fed generator, a rotor side converter, a conversion switch, a bidirectional DC / DC converter, a super capacitor, a grid side converter, a three-phase transformer and a power grid; when the doubly fed pumped storage unit is in a power generation mode, the low voltage ride through control method comprises:

[0008] S11, determine whether a fault mode occurs, if not, execute step S12, if yes, execute step S13;

[0009] S12, the doubly-fed pumped storage unit operates in a normal working mode;

[0010] S13, judging the voltage drop condition of the doubly-fed pumped storage unit, the voltage drop degree includes a slight drop condition, a moderate drop condition and a severe drop condition, and selecting a corresponding working mode according to the voltage drop condition, the working mode includes a slight drop working mode, a moderate drop working mode and a severe drop working mode;

[0011] Furthermore, the normal working mode is:

[0012] S121, the rotor-side converter decouples and controls active power and reactive power through vector control;

[0013] S122, the grid-side converter controls the DC bus voltage to be maintained within a normal range through the active loop, and the active loop adjusts the output of reactive power;

[0014] S123. Compare the SOC value of the supercapacitor with the preset value SOC*. When SOC>SOC*, close the conversion switch. The supercapacitor releases energy in the BOOST mode of the bidirectional DC / DC converter, feeds back energy in the form of a small current and reserves emergency spare capacity. When SOC≤SOC*, open the conversion switch.

[0015] Furthermore, the slight drop working mode is:

[0016] S1311, the transfer switch remains in the off state;

[0017] S1312, the rotor-side converter decouples and controls active power and reactive power through vector control;

[0018] S1313, the grid-side converter controls the DC bus voltage to be maintained within a normal range through an active loop, and the active loop adjusts the output of reactive power;

[0019] S1314. Store energy through the unit's own transmission chain.

[0020] Furthermore, the moderate drop working mode is:

[0021] S1321, rotor side converter adopts virtual inductance control;

[0022] S1322, the grid-side converter adopts reactive power priority control;

[0023] S1323, detecting whether the current voltage drop condition of the doubly-fed pumped storage unit meets the slight drop condition, if so, executing the slight drop working mode, if not, repeating steps S1321-S1322.

[0024] Furthermore, the severe drop working mode is:

[0025] S1331, rotor side converter adopts virtual inductance control;

[0026] S1332, the grid-side converter adopts reactive power priority control;

[0027] S1333, the transfer switch is closed and the additional supercapacitor branch is connected;

[0028] S1334. Detect whether the current voltage drop condition of the doubly-fed pumped storage unit meets the moderate voltage drop condition. If so, disconnect the supercapacitor branch. If not, repeat steps S1331 to S1333.

[0029] Furthermore, the voltage drop degree determination method includes:

[0030] Set the light drop limit value h 1 , medium drop limit value h 2 , severe drop limit value h 3 , when the drop voltage h is less than the slight drop limit value h 1 When the drop voltage h is in the interval [h 1 ,h 2 ], it is a moderate drop condition; when the drop voltage h is in the interval [h 2 ,h 3 ] is a severe fall condition.

[0031] Furthermore, when the doubly-fed pumped storage unit is in electric mode, the low voltage ride-through control method includes:

[0032] S21, determine whether a fault mode occurs, if not, execute step S22, if yes, execute step S23;

[0033] S22, the doubly-fed pumped storage unit operates in a normal working mode;

[0034] S23, adding a feedforward instruction to the rotor-side converter;

[0035] S24, compare the SOC value of the supercapacitor with the preset value SOC*. When SOC>SOC*, close the conversion switch, and the supercapacitor releases energy in the BOOST mode of the bidirectional DC / DC converter, feeds back energy in the form of a small current and reserves emergency spare capacity. When SOC≤SOC*, open the conversion switch;

[0036] S24. Repeat step S21.

[0037] As described above, compared with the prior art, the present invention has the following effects:

[0038] When pumped storage units cannot achieve low voltage ride-through using traditional control strategies, the effect of optimizing the control strategy to cope with severe drops is poor, the cost of additional hardware is high, and there is no strategy in electric mode. In power generation mode, two methods, virtual inductance control and additional supercapacitor energy storage branch, are adopted, and different control strategies are set at different drop levels, so that the pumped storage unit can cope with voltage drop faults of different degrees and types; in electric mode, the speed control and energy input requirements are met, and the software and hardware strategies designed above are used to achieve low ride-through. At the same time, additional conversion switches are added to improve the reliability of the switching control strategy, reduce the activation frequency and spare capacity of the supercapacitor energy storage branch, so as to achieve the purpose of reducing costs and facilitating promotion. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is an overall structural block diagram of a doubly-fed pumped storage unit system according to a specific embodiment of the present invention;

[0040] Figure 2 This is a diagram of an equivalent circuit model of a DFIG rotor side according to a specific embodiment of the present invention;

[0041] Figure 3 It is an overall block diagram of a doubly-fed pumped storage unit system adopting an improved strategy according to a specific embodiment of the present invention;

[0042] Figure 4 This is a block diagram of virtual dynamic inductance control of a rotor-side converter according to a specific embodiment of the present invention;

[0043] Figure 5 It is a reactive power priority control block diagram of a grid-side converter according to a specific embodiment of the present invention;

[0044] Figure 6 A bidirectional DC / DC and supercapacitor topology according to a specific embodiment of the present invention;

[0045] Figure 7 A two-phase interleaved bidirectional half-bridge DC / DC converter control strategy according to a specific embodiment of the present invention;

[0046] Figure 8 It is an overall flow chart of the power generation mode of a specific embodiment of the present invention;

[0047] Fig. 9 It is an overall flow chart of the electric mode in a specific embodiment of the present invention. DETAILED DESCRIPTION

[0048] The following describes the embodiments of the present invention by specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the following embodiments and features in the embodiments can be combined with each other without conflict.

[0049] It should be noted that the illustrations provided in the following embodiments are only used to illustrate the basic concept of the present invention in a schematic manner, and thus the illustrations only show components related to the present invention rather than being drawn according to the number, shape and size of components in actual implementation. In actual implementation, the type, quantity and proportion of each component may be changed arbitrarily, and the component layout may also be more complicated.

[0050] like Figure 1 As shown, in a specific embodiment, a low voltage ride-through control method for a doubly-fed pumped storage unit is provided, wherein the doubly-fed pumped storage unit includes: a water pump turbine, a gear box, a doubly-fed motor, a filter, a rotor-side converter, a transfer switch, a bidirectional DC / DC converter, a supercapacitor, a grid-side converter, a three-phase transformer and a power grid;

[0051] The DFIG mathematical model can be represented by a space vector as follows:

[0052]

[0053]

[0054] In the formula, are the stator and rotor flux vectors respectively; are the stator and rotor current vectors respectively; L s , L m and L r They are stator self-inductance, stator-rotor mutual inductance and rotor self-inductance respectively; are the stator and rotor voltage vectors respectively; R s , R r The superscripts "r" and "s" represent that the vector has been converted to the rotor and stator coordinate systems, respectively. The conversion relationship between the two is as follows:

[0055]

[0056] Substituting equation (1) into equation (2), we get the rotor voltage:

[0057]

[0058] Where σ is the leakage inductance coefficient, and σL r is the transient inductance on the rotor side.

[0059] From equation (4), we know that the rotor voltage can be regarded as the rotor induced electromotive force and the voltage drop across the rotor winding impedance The equivalent circuit model of the doubly-fed generator rotor side can be drawn from this formula as shown in the attached figure. Figure 2 shown.

[0060] Take the calculation of various parameters under normal power generation conditions as an example:

[0061] The grid voltage is:

[0062]

[0063] where ω s is the stator voltage angular frequency, V s is the rated stator voltage.

[0064] Substituting (5) into (2) and ignoring the small term containing stator resistance, the stator flux can be obtained as:

[0065]

[0066] Substitute into formula (6) The EMF expression obtained by the calculation formula is:

[0067]

[0068] Where λ is the slip rate, and λ=(ω s -ω r ) / ω s ,λ∈[-0.3,0.3].

[0069] Calculation of various parameters under power grid fault conditions:

[0070] Based on the symmetrical component method, the grid voltage under grid fault is composed of positive sequence, negative sequence and zero sequence components, which can be expressed as:

[0071]

[0072] Where V sp 、V sn and V s0 Represent the amplitudes of the positive-sequence, negative-sequence and zero-sequence components of the stator voltage respectively.

[0073] In most cases, the neutral point of the motor is not grounded and has no leakage inductance, so the zero-sequence component does not need to be analyzed. If the flux can follow the sudden change of the stator terminal voltage, it means that the energy stored in the winding can disappear out of thin air, which obviously does not conform to the law of conservation of energy. Therefore, in order to ensure the continuity of the flux, there will be a slowly decaying DC component in the stator flux. Combine (1), (2) and (8), and ignore the small R s The stator flux expression under fault conditions is obtained by adding the DC component:

[0074]

[0075] In the formula, Ψ st is the amplitude of the initial flux DC component, τ s is the decay time of the stator flux.

[0076] Substituting (9) into Calculate the formula and ignore the smaller τ s The EMF expression under power grid fault can be obtained as follows:

[0077]

[0078] Wherein, the first, second and third terms are the positive sequence component, negative sequence component and DC component of EMF respectively.

[0079] Symmetrical power grid faults with greater hazards include:

[0080]

[0081] Symmetrical fall:

[0082]

[0083] By comparison, the EMF under fault conditions is different from that under normal conditions. From the frequency point of view, the intervention of DC and negative sequence components makes the EMF exhibit the characteristics of coexistence of multiple frequency components. In terms of value, although each component will decay over time during the fault, the amplitude generated by the superposition of multiple components in the initial stage of the drop will far exceed the EMF amplitude under normal conditions. Taking the worst case, that is, full drop (h = 1) and λ = -0.3 as an example, the EMF amplitude will reach 1.3V s L m / L s , while the EMF amplitude is only 0.3V under normal working conditions s L m / L s , comparing the two, the former is 4.33 times of the latter.

[0084] In summary, after a power grid failure, the EMF frequency is high and the amplitude is high. In order to maintain grid-connected operation, the rotor voltage and current need to work together to offset the EMF. The double-fed pumped storage unit uses a partial power converter, and the output power is relatively small. The rotor-side converter is prone to out-of-control and over-current. At the same time, due to the weak power transmission capacity of the grid-side converter, the energy will be stored in the DC bus because there is no place to store it, resulting in overvoltage, which seriously endangers the safe operation of the unit. Therefore, the low voltage ride-through method of the double-fed pumped storage unit in this embodiment includes a low voltage ride-through control method in the power generation mode and a low voltage ride-through control method in the electric mode.

[0085] When the doubly-fed pumped storage unit is in a power generation mode, the low voltage ride-through control method includes:

[0086] S11, determine whether a fault mode occurs, if not, execute step S12, if yes, execute step S13;

[0087] This embodiment implements real-time monitoring of the current, voltage and voltage value of the rotor converter and the unit end according to the grid voltage state. If an overcurrent is detected in the rotor and a voltage drop at the unit end is detected, it is a fault mode.

[0088] S12, the doubly-fed pumped storage unit operates in a normal working mode;

[0089] The normal working mode is:

[0090] S121, the rotor-side converter decouples and controls active power and reactive power through vector control;

[0091] S122, the grid-side converter controls the DC bus voltage to be maintained within the normal range through the active loop, and the output of reactive power is fine-tuned by the reactive loop;

[0092] S123, comparing the SOC value of the supercapacitor with the preset value SOC*, when SOC>SOC*, closing the conversion switch, the supercapacitor releases energy in the BOOST mode of the bidirectional DC / DC converter, feeds back energy in the form of a small current and reserves emergency spare capacity, when SOC≤SOC*, opening the conversion switch;

[0093] S124. Repeat step S11.

[0094] S13, judging the voltage drop condition of the doubly-fed pumped storage unit, the voltage drop degree includes a slight drop condition, a moderate drop condition and a severe drop condition, and selecting a corresponding working mode according to the voltage drop condition, the working mode includes a slight drop working mode, a moderate drop working mode and a severe drop working mode;

[0095] The voltage drop degree determination method in this embodiment includes:

[0096] Set the light drop limit value h 1 , medium drop limit value h 2 , severe drop limit value h 3 , when the drop voltage h is less than the slight drop limit value h 1 When the drop voltage h is in the interval [h 1 ,h 2 ], it is a moderate drop condition; when the drop voltage h is in the interval [h 2 ,h 3 ], it is a severe drop condition. When the drop voltage meets h 3 When ≤h, the pumped storage unit is cut off from the grid.

[0097] When the current voltage drop condition is determined to be a slight drop condition, the slight drop working mode is adopted, which specifically includes:

[0098] S1311, the transfer switch remains in the off state;

[0099] S1312, the rotor-side converter decouples and controls active power and reactive power through vector control;

[0100] S1313, the grid-side converter controls the DC bus voltage to be maintained within a normal range through an active loop, and the active loop adjusts the output of reactive power;

[0101] S1314. Store energy through the unit's own transmission chain.

[0102] S1315: Determine whether the grid voltage has returned to normal. If so, execute step S12.

[0103] When it is determined that the current voltage drop condition is a moderate drop condition, the moderate drop working mode is adopted, which specifically includes:

[0104] S1321, rotor side converter adopts virtual inductance control;

[0105] In the power generation mode, the PQ power loop is cut off, the stator flux is tracked, and the given transient current reference value is calculated by the stator flux value and the preset virtual inductance value. This value is used as the input signal of the rotor-side converter current loop to control the rotor-side converter port impedance characteristic to be equivalent to the preset inductance value L. * , so that the rotor current and voltage values ​​are kept below the limit values.

[0106] The specific control principle and the preset inductance value are given as follows:

[0107] By the attached Figure 2 And formula (4), it is concluded that the rotor side converter is equivalent to the impedance Z RSCThe equivalent circuit diagram is shown in the attached figure. Figure 3 As shown, the circuit expression can be obtained:

[0108]

[0109] It can be concluded that the best way to offset the interference caused by EMF is to make the right side of expression (14) equal to E in the vector direction. r In the opposite direction. Ignore the smaller rotor resistance R r When the rotor-side converter port impedance Z RSC Equivalent to pure inductance L RSC In the form of r Just the opposite.

[0110] After determining that the rotor side port is equivalent to a pure inductor, the rotor side voltage and current expressions can be obtained according to (3):

[0111]

[0112]

[0113] Under the basic constraint condition L RSC ≥0;

[0114] Current Constraints

[0115] Voltage Constraints The lower value is selected as L * That's it.

[0116] While performing the above operations, a rotor current feedforward instruction is added.

[0117] If the current loop cannot accurately track the given reference signal, the effect of the virtual inductance control strategy will not be reflected. In the event of a fault, the current loop feedback signal will present multi-frequency characteristics due to the intervention of DC components and negative sequence components. In the dq coordinate system, the intervention quantity will be converted into 50Hz and 100Hz AC quantities. The bandwidth of the current control loop is limited, and the gain for the AC component is low, making it impossible to accurately adjust the AC quantity. Therefore, it is necessary to reduce the interference caused by the AC error by adding a feedforward command method to the rotor current loop, thereby improving the transient capability of the strategy. The virtual inductance control block diagram of the rotor-side converter is shown in the attached figure. Figure 4 shown.

[0118] S1322, the grid-side converter adopts reactive power priority control;

[0119] Figure 5This is the reactive power priority control diagram of the grid-side converter. In the case of moderate or severe faults in the power grid, the grid-side converter adopts the reactive power priority principle and determines the reactive current to be injected according to the voltage drop level as the reference signal of the reactive power control current loop to support the grid connection point voltage and assist in recovery. The residual margin is calculated based on the maximum current that the grid-side converter can output. After comparing it with the reference signal given by the DC bus, the smaller value is selected as the current loop i dg Reference signal.

[0120] In power generation mode, SOC≤SOC in severe drop and normal working conditions * In this case, the conversion switch is closed, and the additional supercapacitor auxiliary branch is connected. The charging and discharging of the supercapacitor is controlled by the bidirectional DC / DC converter to maintain the DC voltage stability.

[0121] Its bidirectional DC / DC and supercapacitor topology is shown in the attached Figure 6 The bidirectional DC / DC converter adopts a two-phase interleaved structure, and multiple switch tubes are connected in parallel to enhance the overall high current tolerance, thereby improving the power level of the converter to speed up energy conversion. Figure 7 This is a bidirectional DC / DC control block diagram. It adopts a dual current inner loop and voltage outer loop control method. The difference between the output voltage signal and the reference signal Vref is adjusted by PI to form a negative feedback. After conditioning, the output current reference signal i ref ,i ref The signal U is subtracted from the inductor current flowing through the DC / DC and adjusted by the PI controller of the current inner loop. c Entering the pulse width modulation generator PWM, the generated pulse signal controls the power device IGBT.

[0122] S1323, detecting whether the current voltage drop condition of the doubly-fed pumped storage unit meets the slight drop condition, if so, executing the slight drop working mode, if not, repeating steps S1321-S1322.

[0123] When the current voltage drop condition is determined to be a severe voltage drop condition, the severe voltage drop working mode is adopted, specifically including:

[0124] S1331, rotor side converter adopts virtual inductance control;

[0125] S1332, the grid-side converter adopts reactive power priority control;

[0126] The specific process of step S1331 and step S1332 can refer to the above-mentioned step S1321 and step S1322.

[0127] S1333, the transfer switch is closed and the additional supercapacitor branch is connected;

[0128] S1334. Detect whether the current voltage drop condition of the doubly-fed pumped storage unit meets the moderate voltage drop condition. If so, disconnect the supercapacitor branch. If not, repeat steps S1331 to S1333.

[0129] The control method of this embodiment is coordinated by software and hardware. In the power generation mode, a reasonable control method is selected according to the drop level. In the case of a slight drop, the double-fed generator's own transmission chain is used to store energy; in the case of a moderate drop, starting from the optimization control strategy, based on the stator flux transient compensation, the rotor-side converter adopts virtual inductance control technology, and switches the current outer loop from the PQ power control loop to a given current control signal, where the given value is calculated by tracking the stator flux with an equivalent virtual inductance, making full use of the converter margin, and the grid-side converter is converted to a reactive priority control mode to support grid voltage recovery; in the case of a severe drop, the above software control strategy is maintained, and based on energy management, a supercapacitor energy storage branch is added for coordinated control, and the SCES absorbs the accumulated energy caused by the fault, so that the DC bus voltage is stabilized within a certain range, protecting fragile power electronic devices;

[0130] When the double-fed pumped storage unit is in electric mode, a feedforward instruction is added to improve the control capability of the PI control loop on the speed, and the supercapacitor feedback energy is fully utilized to support the power supply. At the same time, a transfer switch is added, which is closed in normal working conditions and severe drop working conditions, and is disconnected in other working conditions to prevent the supercapacitor energy storage branch from flowing sideways.

[0131] The specific low voltage ride-through control methods in electric mode include:

[0132] S21, determine whether a fault mode occurs, if not, execute step S22, if yes, execute step S23;

[0133] S22, the doubly-fed pumped storage unit operates in a normal working mode;

[0134] S23, adding a feedforward instruction to the rotor-side converter;

[0135] S24, compare the SOC value of the supercapacitor with the preset value SOC*. When SOC>SOC*, close the conversion switch, and the supercapacitor releases energy in the BOOST mode of the bidirectional DC / DC converter, feeds back energy in the form of a small current and reserves emergency spare capacity to keep the pump turbine running. When SOC≤SOC*, open the conversion switch;

[0136] S24. Repeat step S21.

[0137] The present invention is used in situations where the pumped-storage unit cannot achieve low voltage ride-through using traditional control strategies, the effect of coping with severe drops based on control strategy optimization is poor, the cost of additional hardware is high, and there is no strategy in electric mode. In the power generation mode, two methods, virtual inductance control and additional supercapacitor energy storage branch, are adopted, and different control strategies are set at different drop levels, so that the pumped-storage unit can cope with voltage drop faults of different degrees and types; in the electric mode, the speed control and energy input requirements are met, and the software and hardware strategies designed above are used to achieve low ride-through. At the same time, a conversion switch is added to improve the reliability of the switching control strategy, reduce the activation frequency and spare capacity of the supercapacitor energy storage branch, so as to achieve the purpose of reducing costs and facilitating promotion.

[0138] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A method for controlling low pressure ride-through of a doubly-fed pumped storage unit, the doubly-fed pumped storage unit comprising: A pump turbine, a doubly-fed generator, a rotor-side converter, a transfer switch, a bidirectional DC / DC converter, a supercapacitor, a grid-side converter, a three-phase transformer and a power grid; characterized in that when the doubly-fed pumped storage unit is in a power generation mode, the low voltage ride-through control method includes: S11, determine whether a fault mode occurs, if not, execute step S12, if yes, execute step S13; S12, the doubly-fed pumped storage unit operates in a normal working mode; S13, judging the voltage drop working condition of the doubly-fed pumped storage unit, wherein the voltage drop working condition includes a slight drop working condition, a moderate drop working condition and a severe drop working condition, and selecting a corresponding working mode according to the voltage drop working condition, wherein the working mode includes a slight drop working mode, a moderate drop working mode and a severe drop working mode; The slight drop working mode is: S1311, the transfer switch remains in the off state; S1312, the rotor-side converter decouples and controls active power and reactive power through vector control; S1313, the grid-side converter controls the DC bus voltage to be maintained within a normal range through an active loop, and the active loop adjusts the output of reactive power; S1314, storing energy through the unit's own transmission chain; The moderate drop working mode is: S1321, rotor side converter adopts virtual inductance control; S1322, the grid-side converter adopts reactive power priority control; S1323, detecting whether the current voltage drop condition of the doubly-fed pumped storage unit meets the slight drop condition, if so, executing the slight drop working mode, if not, repeating steps S1321-S1322; The severe drop working mode is: S1331, rotor side converter adopts virtual inductance control; S1332, the grid-side converter adopts reactive power priority control; S1333, the transfer switch is closed and the additional supercapacitor branch is connected; S1334. Detect whether the current voltage drop condition of the doubly-fed pumped storage unit meets the moderate voltage drop condition. If so, disconnect the supercapacitor branch. If not, repeat steps S1331 to S1333.

2. A low pressure ride through control method for a doubly fed pumped storage unit according to claim 1, characterized in that: The normal working mode is: S121, the rotor-side converter decouples and controls active power and reactive power through vector control; S122, the grid-side converter controls the DC bus voltage to be maintained within a normal range through the active loop, and the active loop adjusts the output of reactive power; S123. Compare the SOC value of the supercapacitor with the preset value SOC*. When SOC>SOC*, close the conversion switch. The supercapacitor releases energy in the BOOST mode of the bidirectional DC / DC converter, feeds back energy in the form of a small current and reserves emergency spare capacity. When SOC≤SOC*, open the conversion switch.

3. A method for controlling low pressure ride-through of a doubly-fed pumped storage unit according to claim 1, characterized in that: The voltage drop condition determination method comprises: Set the slight drop limit value h1, the moderate drop limit value h2, and the severe drop limit value h3. When the drop voltage h is less than the slight drop limit value h1, it is a slight drop condition; when the drop voltage h is in the interval [h1, h2], it is a moderate drop condition; when the drop voltage h is in the interval [h2, h3], it is a severe drop condition.

4. A low pressure ride through control method for a doubly fed pumped storage unit according to claim 1, characterized in that: When the double-fed pumped storage unit is in electric mode, the low voltage ride through control method includes: S21, determine whether a fault mode occurs, if not, execute step S22, if yes, execute step S23; S22, the doubly-fed pumped storage unit operates in a normal working mode; S23, adding a feedforward instruction to the rotor-side converter; S24, compare the SOC value of the supercapacitor with the preset value SOC*. When SOC>SOC*, close the conversion switch, and the supercapacitor releases energy in the BOOST mode of the bidirectional DC / DC converter, feeds back energy in the form of a small current and reserves emergency spare capacity. When SOC≤SOC*, open the conversion switch; S24. Repeat step S21.

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