阅读说明：本技术 一种双馈抽水蓄能机组低压穿越控制方法 (Low-voltage ride through control method for double-fed pumped storage unit ) 是由 张宇 刘加洪 雍丽英 吕艳玲 李凯 于 2021-09-17 设计创作，主要内容包括：本发明公开了一种双馈抽水蓄能机组低压穿越控制方法,属于双馈抽水蓄能机组控制领域。本发明为了解决双馈抽水蓄能机组使用现有低电压穿越策略无法合理应对不同程度的电压跌落工况的问题。本发明包括判断是否出现故障模式,若否,执行双馈抽水蓄能机组在正常工作模式下工作；若是,判断所述双馈抽水蓄能机组的电压跌落工况,所述电压跌落程度包括轻微跌落工况、中度跌落工况和严重跌落工况,根据所述电压跌落工况选择相应的工作模式,所述工作模式包括轻微跌落工作模式、中度跌落工作模式和严重跌落工作模式。本发明使得双馈抽水蓄能机组在不同电压跌落工况中均具有较好的低电压穿越能力。(The invention discloses a low-voltage ride-through control method for a double-fed pumped storage unit, and belongs to the field of control over the double-fed pumped storage unit. The invention aims to solve the problem that the doubly-fed pumped storage unit cannot reasonably cope with voltage drop working conditions of different degrees by using the conventional low-voltage ride-through strategy. Judging whether a fault mode occurs, if not, executing the operation of the double-fed pumped storage unit in a normal operation mode; if yes, the voltage drop working condition of the doubly-fed pumped storage unit is judged, the voltage drop degree comprises a slight drop working condition, a moderate drop working condition and a severe drop working condition, and a corresponding working mode is selected according to the voltage drop working condition, and the working mode comprises a slight drop working mode, a moderate drop working mode and a severe drop working mode. The double-fed pumped storage unit has better low-voltage ride through capability under different voltage drop working conditions.)

1. A low-voltage ride-through control method for a doubly-fed pumped storage unit comprises the following steps: the system comprises a water pump turbine, a double-fed motor, a rotor side converter, a change-over switch, a bidirectional DC/DC converter, a super capacitor, a grid side converter, a three-phase transformer and a power grid; the low-voltage ride through control method is characterized in that when the doubly-fed pumped storage unit is in a power generation mode, the low-voltage ride through control method comprises the following steps:

s11, judging whether a fault mode occurs, if not, executing a step S12, and if so, executing a step S13;

s12, the double-fed pumped storage unit works in a normal working mode;

s13, judging the voltage drop working condition of the doubly-fed pumped storage unit, wherein the voltage drop degree comprises 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 comprises a slight drop working mode, a moderate drop working mode and a severe drop working mode.

2. The low-voltage ride-through control method for the doubly-fed pumped-storage unit according to claim 1, wherein the normal operating mode is as follows:

s121, decoupling and controlling active power and reactive power by the rotor-side converter through vector control;

s122, the grid-side converter controls the voltage of the direct-current bus to be maintained within a normal interval through an active loop, and the active loop adjusts the output of reactive power;

and S123, comparing the SOC value of the super capacitor with a preset value SOC, when the SOC is more than the SOC, closing the change-over switch, releasing energy by the super capacitor in a BOOST mode of the bidirectional DC/DC converter, feeding back energy in a low-current mode and reserving accident reserve capacity, and when the SOC is less than or equal to the SOC, opening the change-over switch.

3. The low-voltage ride-through control method for the doubly-fed pumped-storage unit according to claim 1, wherein the light-drop operating mode is as follows:

s1311, keeping the switch-off state of the change-over switch;

s1312, decoupling and controlling active power and reactive power by the rotor side converter through vector control;

s1313, the grid-side converter controls the voltage of the direct-current bus to be maintained within a normal interval through an active loop, and the active loop adjusts reactive power output;

and S1314, storing energy through a transmission chain of the unit.

4. The low-voltage ride-through control method for the doubly-fed pumped-storage unit according to claim 1, wherein the moderate drop operating mode is as follows:

s1321, controlling the rotor-side converter by adopting a virtual inductor;

s1322, the grid-side converter adopts reactive power priority control;

s1323, detecting whether the current voltage drop working condition of the doubly-fed pumped storage unit meets the slight drop working condition or not, if so, executing a slight drop working mode, and if not, repeatedly executing the step S1321-the step S1322.

5. The low-voltage ride-through control method for the doubly-fed pumped-storage unit according to claim 1, wherein the severe drop operating mode is as follows:

s1331, controlling the rotor-side converter by adopting a virtual inductor;

s1332, the grid-side converter adopts reactive power priority control;

s1333, switching on the change-over switch, and connecting an additional super capacitor branch;

and S1334, detecting whether the current voltage drop working condition of the doubly-fed pumped storage unit meets the medium drop working condition, if so, disconnecting the super capacitor branch, and if not, repeatedly executing the step S1331-the step S1333.

6. The low voltage ride through control method for the doubly-fed pumped storage group according to claim 1, wherein the voltage drop degree determination method comprises the following steps:

setting a slight fall limit value h₁Limit of moderate drop h₂Severe drop limit h₃When the dropping voltage h is less than the slight dropping limit value h₁In time, the working condition is a light falling working condition; when the drop voltage h is in the interval [ h₁，h₂]In the middle, the working condition is a moderate falling condition; when the drop voltage h is in the interval [ h₂，h₃]In time, the condition of severe drop is adopted.

7. The low-voltage ride-through control method of the doubly-fed pumped-hydro energy storage unit according to claim 1, wherein when the doubly-fed pumped-hydro energy storage unit is in an electric mode, the low-voltage ride-through control method comprises the following steps:

s21, judging whether a fault mode occurs, if not, executing a step S22, and if so, executing a step S23;

s22, the double-fed pumped storage unit works in a normal working mode;

s23, adding a feedforward instruction into the rotor-side converter;

s24, comparing the SOC value of the super capacitor with a preset value SOC, when the SOC is more than the SOC, closing the change-over switch, the super capacitor releases energy in a BOOST mode of the bidirectional DC/DC converter, feeding back energy in a low current mode and reserving accident reserve capacity, and when the SOC is less than or equal to the SOC, opening the change-over switch;

s24, repeating the step S21.

Technical Field

The invention belongs to the field of control over a double-fed pumped storage unit, and particularly relates to a low-voltage ride through control method for the double-fed pumped storage unit.

Background

In order to respond to the strategy of double carbon, the grid structure accelerates transformation, and the installed grid-connected capacity and the power generation proportion of new energy generating sets such as wind power generation and photovoltaic power generation rise year by year, so that the input source of the generating sets is intermittent, the stable output of electric energy cannot be ensured, and the stability of an electric power system can be damaged if the generating sets are directly connected to the grid in a large scale. Pumped storage is a mature energy storage technology, and has the advantages of strong storage capacity, high conversion efficiency and fast load response. The novel energy storage battery can be used as a stabilizer to absorb and coordinate other new energy power generation equipment, the high-quality rate of power generation can be improved, and the advantage of green resources is fully exerted.

The pumped storage unit switches the electric and power generation working modes by controlling the water turbine of the water pump to rotate positively and negatively. When the load is low and the peak is high, starting an electric mode and storing energy; and at the time of load peak, the power generation mode is switched to feed back energy. The double-fed motor uses a partial power converter, has low running loss, can realize variable speed and frequency modulation through alternating current excitation, and is widely applied to pumped storage units. The conventional vector directional control is usually adopted in the unit control method, and the coupling relation among all vectors is removed by orienting a single vector under a dq coordinate system, so that the effects of independent control and process simplification are achieved.

Because the stator winding of the doubly-fed motor is directly connected with a power grid, and the control speed and the output power of the rotor-side converter are limited, the impact caused by voltage drop of a grid-connected point cannot be responded. In a power generation mode, a converter is easy to damage due to overcurrent and overvoltage, a unit is disconnected for protecting fragile power electronic devices, disconnection and reconnection of a large-capacity unit are great threats to safe and stable operation of the whole power system, and damage is possibly enlarged. Therefore, with the improvement of the single-machine capacity and the whole occupation ratio, the double-fed pumped storage unit needs to enhance the low-voltage ride through capability adaptive to the voltage drop fault.

The double-fed unit applied to pumped storage has the defects that the control working condition is complex, the related low-voltage ride through strategy is rarely researched, particularly in an electric working mode, and the research related to the low-voltage ride through strategy of the traditional double-fed pumped storage can be formulated into two categories of an additional hardware protection device and improvement and optimization based on the control strategy at present. In the additional hardware protection device, the most used method is a Crowbar (Crowbar) circuit arranged at the front end of the rotor-side converter, and the most effective method is a Dynamic Voltage Restorer (DVR) arranged at a grid-connected point, besides, methods such as an additional super capacitor, a Dynamic resistor and a direct current Chopper (Chopper) are additionally arranged; in the improvement based on the control strategy, a fuzzy controller is used for replacing the traditional PI controller, a method for increasing the tracking speed of a current loop by adding a feedforward signal, a stator current feedback method for directly tracking the stator current to restrain the rotor current, a demagnetization control method using demagnetization as a guide and the like are available. In principle, hardware is added in the unit, and the converter can be protected by absorbing surplus energy through a hardware device from the aspect of energy management; and the additional hardware on the grid connection point side fundamentally solves the fault by supporting the terminal voltage. However, the hardware protection device can cause the volume increase and the cost increase of the whole unit, and the simple addition of the hardware protection device has certain limitations, such as a single additional Crowbar circuit, because the input of a control signal of a rotor side converter is cut off during the work, the unit can not output reactive power, and because the structure is equivalent to a squirrel-cage asynchronous motor, the reactive power can be reversely absorbed; the single super capacitor auxiliary straight circuit cannot relieve the impact of surplus capacity on the rotor side converter in advance due to the fact that the installation position is located at the position of the direct current bus. The optimal control strategy can be implemented by weakening transient response of a certain quantity, reasonably adjusting voltage and current values, offsetting rotor induced Electromotive Force (EMF) to a certain extent, and absorbing surplus energy by using the system. However, because the energy storage capacity of the unit is limited, the low-voltage ride-through under the severe drop degree is difficult to complete by a control strategy, and the method is only suitable for the drop degree with moderate voltage and below.

Disclosure of Invention

In order to solve the problem that the double-fed pumped storage unit cannot reasonably cope with voltage drop working conditions of different degrees by using the existing low-voltage ride-through strategy, the invention provides a low-voltage ride-through control method of the double-fed pumped storage unit, so that the double-fed pumped storage unit has better low-voltage ride-through capability in different voltage drop working conditions.

The invention provides a low-voltage ride through control method for a double-fed pumped storage unit, wherein the double-fed pumped storage unit comprises the following steps: the system comprises a double-fed motor, a rotor side converter, a change-over 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 the following steps:

s11, judging whether a fault mode occurs, if not, executing a step S12, and if so, executing a step S13;

s12, the double-fed pumped storage unit works in a normal working mode;

s13, judging voltage drop working conditions of the doubly-fed pumped storage unit, wherein the voltage drop degrees comprise a slight drop working condition, a medium drop working condition and a severe drop working condition, and selecting corresponding working modes according to the voltage drop working conditions, wherein the working modes comprise a slight drop working mode, a medium drop working mode and a severe drop working mode;

further, the normal operation mode is:

s121, decoupling and controlling active power and reactive power by the rotor-side converter through vector control;

s122, the grid-side converter controls the voltage of the direct-current bus to be maintained within a normal interval through an active loop, and the active loop adjusts the output of reactive power;

and S123, comparing the SOC value of the super capacitor with a preset value SOC, when the SOC is more than the SOC, closing the change-over switch, releasing energy by the super capacitor in a BOOST mode of the bidirectional DC/DC converter, feeding back energy in a low-current mode and reserving accident reserve capacity, and when the SOC is less than or equal to the SOC, opening the change-over switch.

Further, the slight falling working mode is as follows:

s1311, keeping the switch-off state of the change-over switch;

s1312, decoupling and controlling active power and reactive power by the rotor side converter through vector control;

s1313, the grid-side converter controls the voltage of the direct-current bus to be maintained within a normal interval through an active loop, and the active loop adjusts reactive power output;

and S1314, storing energy through a transmission chain of the unit.

Further, the moderate falling mode of operation is as follows:

s1321, controlling the rotor-side converter by adopting a virtual inductor;

s1322, the grid-side converter adopts reactive power priority control;

s1323, detecting whether the current voltage drop working condition of the doubly-fed pumped storage unit meets the slight drop working condition or not, if so, executing a slight drop working mode, and if not, repeatedly executing the step S1321-the step S1322.

Further, the severe drop working mode is as follows:

s1331, controlling the rotor-side converter by adopting a virtual inductor;

s1332, the grid-side converter adopts reactive power priority control;

s1333, switching on the change-over switch, and connecting an additional super capacitor branch;

and S1334, detecting whether the current voltage drop working condition of the doubly-fed pumped storage unit meets the medium drop working condition, if so, disconnecting the super capacitor branch, and if not, repeatedly executing the step S1331-the step S1333.

Further, the method for determining the voltage sag degree includes:

setting a slight fall limit value h₁Limit of moderate drop h₂Severe drop limit h₃When the dropping voltage h is less than the slight dropping limit value h₁In time, the working condition is a light falling working condition; when the drop voltage h is in the interval [ h₁，h₂]In the middle, the working condition is a moderate falling condition; when the drop voltage h is in the interval [ h₂，h₃]In time, the condition of severe drop is adopted.

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

s21, judging whether a fault mode occurs, if not, executing a step S22, and if so, executing a step S23;

s22, the double-fed pumped storage unit works in a normal working mode;

s23, adding a feedforward instruction into the rotor-side converter;

s24, comparing the SOC value of the super capacitor with a preset value SOC, when the SOC is more than the SOC, closing the change-over switch, the super capacitor releases energy in a BOOST mode of the bidirectional DC/DC converter, feeding back energy in a low current mode and reserving accident reserve capacity, and when the SOC is less than or equal to the SOC, opening the change-over switch;

s24, repeating the step S21.

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

the method is used for solving the problems that the pumped storage unit cannot realize low voltage ride through by using a traditional control strategy, the serious drop effect is poor based on the control strategy optimization, the additional hardware cost is high, and the strategy is not used in the electric mode. In a power generation mode, two methods of virtual inductance control and super capacitor energy storage branch attachment are adopted, and different control strategies are set under different dropping grades, so that the pumped storage unit can cope with voltage dropping faults of different degrees and different types; in the electric mode, the low-penetration is realized by utilizing the software and hardware strategies designed above for the requirements of rotating speed control and energy input. Meanwhile, a change-over switch is added, so that the reliability of a switching control strategy is improved, and the starting frequency and the standby capacity of the super-capacitor energy storage branch circuit are reduced, so that the purposes of reducing cost and being easy to popularize are achieved.

Drawings

Fig. 1 is a block diagram of the overall structure of a doubly-fed pumped storage unit system according to an embodiment of the present invention;

FIG. 2 is a schematic diagram of a DFIG rotor-side equivalent circuit model according to an embodiment of the present invention;

FIG. 3 is a block diagram of an overall doubly-fed pumped storage group system employing an improved strategy according to an embodiment of the present invention;

fig. 4 is a block diagram of a rotor-side converter virtual dynamic inductance control according to an embodiment of the present invention;

fig. 5 is a block diagram of the reactive power priority control of the grid-side converter according to the embodiment of the present invention;

FIG. 6 is a bi-directional DC/DC and super capacitor topology according to an embodiment of the present invention;

FIG. 7 illustrates a control strategy for a two-phase interleaved bi-directional half-bridge DC/DC converter according to an embodiment of the present invention;

FIG. 8 is an overall flowchart of the power generation mode according to the embodiment of the present invention;

fig. 9 is an overall flowchart of the electric mode according to the embodiment of the present invention.

Detailed Description

The embodiments of the present invention are described below with reference to specific embodiments, and other advantages and effects of the present invention will be easily understood by those skilled in the art from the disclosure of the present specification. The invention is capable of other and different embodiments and of being practiced or of being carried out in various ways, and its several details are capable of modification in various respects, all without departing from the spirit and scope of the present invention. It is to be noted that the features in the following embodiments and examples may be combined with each other without conflict.

It should be noted that the drawings provided in the following embodiments are only for illustrating the basic idea of the present invention, and the drawings only show the components related to the present invention rather than the number, shape and size of the components in actual implementation, and the type, quantity and proportion of the components in actual implementation may be changed freely, and the layout of the components may be more complicated.

As shown in fig. 1, in a specific embodiment, a low voltage ride through control method for a doubly-fed pumped-hydro energy storage unit is provided, where the doubly-fed pumped-hydro energy storage unit includes: the system comprises a water pump turbine, a gear transmission case, a double-fed motor, a filter, a rotor side converter, a change-over switch, a bidirectional DC/DC converter, a super capacitor, a grid side converter, a three-phase transformer and a power grid;

the DFIG mathematical model can be characterized using space vectors as:

in the formula (I), the compound is shown in the specification,respectively are stator flux linkage vectors and rotor flux linkage vectors;respectively are stator current vectors and rotor current vectors; l is_s、L_mAnd L_rThe self inductance of the stator, the mutual inductance of the stator and the rotor and the self inductance of the rotor are respectively;respectively are stator voltage vectors and rotor voltage vectors; r_s、R_rRespectively a stator resistor and a rotor resistor. The superscripts "r" and "s" represent the vectors converted into the rotor and stator coordinate systems, respectively, as follows:

substituting equation (1) into (2) to obtain the rotor voltage:

wherein σ is a leakage inductance coefficient, andσL_ris the rotor side transient inductance.

From equation (4), the rotor voltage can be regarded as the rotor induced electromotive forceAnd voltage drop due to rotor winding impedanceThe method comprises two parts, and an equivalent circuit model of the rotor side of the doubly-fed motor can be drawn according to the formula and is shown in the attached figure 2.

Taking the calculation of each parameter under the normal power generation working condition as an example:

the grid voltage is:

wherein ω is_sIs stator voltage angular frequency, V_sThe stator rated voltage.

Substituting (5) into (2), neglecting the smaller stator-containing resistance term, the obtained stator flux linkage is as follows:

substitution of formula (6)The EMF expression obtained by the calculation formula is:

where λ is the slip, and λ ═ ω_s-ω_r)/ω_s，λ∈[-0.3,0.3]。

Calculating various parameters under the condition of power grid fault:

based on a symmetric component method, the grid voltage under the grid fault is formed by superposing positive sequence, negative sequence and zero sequence components, and is represented as follows:

in the formula V_sp、V_snAnd V_s0Representing the magnitudes of the positive, negative and zero sequence components of the stator voltage, respectively.

In most cases, the neutral point of the motor is not grounded, and no leakage inductance exists, so that the zero sequence component can not be analyzed. If the flux linkage can change suddenly along with the voltage of the stator terminal, the energy stored in the winding can disappear through empty space, and obviously the flux linkage does not accord with the law of energy conservation, so that a slowly-attenuated direct-current component exists in the stator flux linkage to ensure the continuity of the flux linkage. Simultaneous (1), (2) and (8), ignoring the smaller R-containing_sAnd (3) adding a direct current component to obtain a stator flux linkage expression under the fault working condition:

in the formula, Ψ_stIs the amplitude of the DC component of the initial flux linkage, τ_sIs the decay time of the stator flux linkage.

Substitute (9) intoCalculated and neglect the smaller τ_sAnd (3) calculating an EMF expression under the grid fault as follows:

in the formula, the first term, the second term and the third term are respectively a positive sequence component, a negative sequence component and a direct current component of the EMF.

For the symmetrical grid faults with large harm, the following are provided:

symmetrical drop down:

compared with the normal working condition, the EMF under the fault is different from that under the normal working condition, and the intervention of the direct current and the negative sequence components leads the EMF to show the coexistence characteristic of the multiple frequency components from the aspect of frequency; in terms of value, although each component can be attenuated along with time during the fault, the amplitude generated by the superposition of a plurality of components in the initial stage of the fault is far beyond the EMF amplitude under the normal working condition. In the worst case, i.e., total roll-off (h ═ 1) and λ ═ 0.3, for example, the EMF amplitude will reach 1.3V_sL_m/L_sWhile the EMF amplitude under normal condition is only 0.3V_sL_m/L_sIn comparison, the former is 4.33 times that of the latter.

In conclusion, after the power grid fails, the EMF has high frequency and high amplitude, the rotor voltage and the rotor current need to be combined to offset the EMF in order to maintain grid-connected operation, the double-fed pumped storage unit adopts a partial power converter, the output power is relatively low, and the rotor-side converter is easy to lose control and overflow. Meanwhile, because the grid-side converter has weak power transmission capability, energy is stored and accumulated at a direct-current bus without place, so that overvoltage occurs to the grid-side converter, and the safe operation of the unit is seriously damaged, therefore, the low-voltage ride through method of the doubly-fed pumped storage unit of the embodiment comprises a low-voltage ride through control method in a power generation mode and a low-voltage ride through control method in an electric mode.

When the doubly-fed pumped storage unit is in a power generation mode, the low-voltage ride-through control method comprises the following steps:

s11, judging whether a fault mode occurs, if not, executing a step S12, and if so, executing a step S13;

in the embodiment, the current and the voltage of the rotor converter and the voltage value of the unit end are monitored in real time according to the voltage state of the power grid, and if the rotor is detected to be in an overcurrent state and the voltage of the machine end is detected to be in a fault mode.

S12, the double-fed pumped storage unit works in a normal working mode;

the normal working mode is as follows:

s121, decoupling and controlling active power and reactive power by the rotor-side converter through vector control;

s122, the grid-side converter controls the voltage of a direct-current bus to be maintained within a normal interval through an active loop, and the output of reactive power is finely adjusted through a reactive loop;

s123, comparing the SOC value of the super capacitor with a preset value SOC, when the SOC is more than the SOC, closing the change-over switch, releasing energy by the super capacitor in a BOOST mode of the bidirectional DC/DC converter, feeding back energy in a low-current mode and reserving accident reserve capacity, and when the SOC is less than or equal to the SOC, disconnecting the change-over switch;

and S124, repeatedly executing the step S11.

S13, judging voltage drop working conditions of the doubly-fed pumped storage unit, wherein the voltage drop degrees comprise a slight drop working condition, a medium drop working condition and a severe drop working condition, and selecting corresponding working modes according to the voltage drop working conditions, wherein the working modes comprise a slight drop working mode, a medium drop working mode and a severe drop working mode;

the method for determining the voltage sag degree in the embodiment includes:

setting a slight fall limit value h₁Limit of moderate drop h₂Severe drop limit h₃When the dropping voltage h is less than the slight dropping limit value h₁In time, the working condition is a light falling working condition; when the drop voltage h is in the interval [ h₁，h₂]In the middle, the working condition is a moderate falling condition; when the drop voltage h is in the interval [ h₂，h₃]When the voltage drops to h, the working condition is severe₃And when the time is less than or equal to h, the pumped storage unit is cut off from the power grid.

When judging that the current voltage drops the operating mode and is the slight working mode that falls, take slight falling mode, specifically include:

s1311, keeping the switch-off state of the change-over switch;

s1312, decoupling and controlling active power and reactive power by the rotor side converter through vector control;

s1313, the grid-side converter controls the voltage of the direct-current bus to be maintained within a normal interval through an active loop, and the active loop adjusts reactive power output;

and S1314, storing energy through a transmission chain of the unit.

And S1315, judging whether the grid voltage is recovered to be normal or not, and if so, executing step S12.

When judging that the current voltage drop working condition is a moderate drop working condition, adopting the moderate drop working mode, which specifically comprises:

s1321, controlling the rotor-side converter by adopting a virtual inductor;

in the power generation mode, cutting off a PQ power loop, tracking stator flux linkage, calculating a given transient current reference value through a stator flux linkage value and a preset virtual inductance value, using the given transient current reference value as a current loop input signal of the rotor-side converter, and controlling the impedance characteristic of a port of the rotor-side converter to be equivalent to a preset inductance value L^*The rotor current and voltage values are kept below the limit values.

The specific control principle and preset inductance value are given as follows:

from FIG. 2 and equation (4), the equivalent impedance Z of the converter at the rotor side is obtained_RSCThe equivalent circuit diagram of time is shown in fig. 3, and a circuit expression can be obtained:

it follows that to counteract the interference caused by EMF, it is best to have the right term of expression (14) in the vector direction with E_rIn the opposite direction. Neglecting the smaller rotor-containing resistance R_rTerm, when the rotor side converter port impedance Z_RSCEquivalent to pure inductance L_RSCForm, overall vector direction and E_rThe opposite is true.

After determining that the rotor side port is equivalent to a pure inductance form, the rotor side voltage and current expressions obtained according to (3) are:

after satisfying the basic constraint condition L_RSC≥0；

Current constraint condition

Voltage constraint conditionLower value as L^*And (4) finishing.

And adding a rotor current feedforward command while performing the operation.

If the current loop cannot accurately track a given reference signal, the action of the virtual inductance control strategy cannot be reflected. When a fault occurs, the current loop feedback signal has a multi-frequency characteristic due to the intervention of a direct-current component and a negative sequence component. The intervention quantity is converted into 50Hz and 100Hz alternating current quantities in a dq coordinate system, the bandwidth of a current control loop is limited, the gain of the alternating current component is low, and the alternating current quantity cannot be accurately adjusted. Therefore, the interference caused by the alternating current error needs to be reduced by adding a feedforward instruction method in the rotor current loop, so as to improve the transient capability of the strategy, and a virtual inductance control block diagram of the rotor-side converter is shown in fig. 4.

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

fig. 5 is a grid-side converter reactive priority control diagram, and in the case of a moderate or severe fault in the power grid, the grid-side converter uses a reactive priority principle, determines a reactive current to be injected according to a voltage drop level as a reactive control current loop reference signal, supports a grid-connected point voltage, and assists recovery. Calculating the residual margin according to the maximum current output by the grid-side converter, comparing the residual margin with a reference signal given by a direct-current bus, and selecting a smaller value as a current loop i_dgA reference signal.

In the power generation mode, the SOC is less than or equal to the SOC under the severe falling and normal working conditions^*In case of closed transfer switch, additional super-capacitor auxiliary branchAnd the circuit is connected, the super capacitor is controlled to charge and discharge energy through the bidirectional DC/DC converter, and the stability of the direct-current voltage is maintained.

The bidirectional DC/DC and super capacitor topological structure is shown in figure 6. The bidirectional DC/DC converter adopts a two-phase staggered structure, and the plurality of switch tubes enhance the whole large-current endurance capacity through shunt in parallel, so that the power level of the converter is improved to accelerate the energy conversion. FIG. 7 is a bidirectional DC/DC control block diagram, which adopts a control mode of double current inner ring and voltage outer ring, the difference value of the output voltage signal and the reference signal Vref forms negative feedback through PI regulation, and the output current reference signal i is obtained through regulation_ref，i_refThe signal U is obtained by subtracting the inductive current flowing through the DC/DC and then adjusting the inductive current by a PI controller of a current inner loop_cAnd entering a Pulse Width Modulation (PWM) generator to generate a pulse signal to control a power device IGBT.

S1323, detecting whether the current voltage drop working condition of the doubly-fed pumped storage unit meets the slight drop working condition or not, if so, executing a slight drop working mode, and if not, repeatedly executing the step S1321-the step S1322.

When judging that the current voltage drop working condition is a severe drop working condition, adopting the severe drop working mode, which specifically comprises the following steps:

s1331, controlling the rotor-side converter by adopting a virtual inductor;

s1332, the grid-side converter adopts reactive power priority control;

the specific flow of step S1331 and step S1332 can be referred to step S1321 and step S1322.

S1333, switching on the change-over switch, and connecting an additional super capacitor branch;

and S1334, detecting whether the current voltage drop working condition of the doubly-fed pumped storage unit meets the medium drop working condition, if so, disconnecting the super capacitor branch, and if not, repeatedly executing the step S1331-the step S1333.

The control method of the embodiment is characterized in that software and hardware are cooperated, a reasonable control method is selected according to the falling grade in a power generation mode, and energy is stored by using a self transmission chain of the double-fed motor when the double-fed motor slightly falls; when the voltage drops moderately, starting from an optimization control strategy, based on stator flux transient compensation, a rotor-side converter adopts a virtual inductance control technology to switch a current outer ring from a PQ power control ring into a given current control signal, wherein the given value is obtained by tracking stator flux and calculating with equivalent virtual inductance, the margin of the converter is fully utilized, and a grid-side converter is converted into a reactive priority control mode to support the voltage recovery of a power grid; when the direct current bus voltage drops seriously, the software control strategy is kept, a super capacitor energy storage branch circuit is additionally arranged for cooperative control based on energy management, and the SCES absorbs accumulated energy caused by faults, so that the direct current bus voltage is stabilized in a certain range, and fragile power electronic devices are protected;

when the double-fed pumped storage unit is in an electric mode, a feedforward instruction is added, the control capability of a PI control loop on the rotating speed is improved, and the feedback energy of the super capacitor is fully utilized to support power supply. Meanwhile, a change-over switch is added, the change-over switch is closed under a normal working condition and a severe falling working condition, and other working conditions are in an off state, so that the lateral flow of the energy storage branch of the super capacitor is prevented.

The low-voltage ride-through control method in the specific electric mode comprises the following steps:

s21, judging whether a fault mode occurs, if not, executing a step S22, and if so, executing a step S23;

s22, the double-fed pumped storage unit works in a normal working mode;

s23, adding a feedforward instruction into the rotor-side converter;

s24, comparing the SOC value of the super capacitor with a preset value SOC, when the SOC is more than the SOC, closing the change-over switch, the super capacitor releases energy in a BOOST mode of the bidirectional DC/DC converter, the energy is fed back in a small current mode, an accident reserve capacity is reserved, so that the water turbine of the water pump is kept running, and when the SOC is less than or equal to the SOC, the change-over switch is opened;

s24, repeating the step S21.

The method is used under the conditions that the pumped storage unit cannot realize low voltage ride through by using a traditional control strategy, the serious drop effect is poor based on the control strategy optimization, the additional hardware cost is high, and the strategy is not in a power-driven mode. In a power generation mode, two methods of virtual inductance control and super capacitor energy storage branch attachment are adopted, and different control strategies are set under different dropping grades, so that the pumped storage unit can cope with voltage dropping faults of different degrees and different types; in the electric mode, the low-penetration is realized by utilizing the software and hardware strategies designed above for the requirements of rotating speed control and energy input. Meanwhile, a change-over switch is added, so that the reliability of a switching control strategy is improved, and the starting frequency and the standby capacity of the super-capacitor energy storage branch circuit are reduced, so that the purposes of reducing cost and being easy to popularize are achieved.

The foregoing embodiments are merely illustrative of the principles and utilities of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or change the above-mentioned embodiments without departing from the spirit and scope of the present invention. Accordingly, it is intended that all equivalent modifications or changes which can be made by those skilled in the art without departing from the spirit and technical spirit of the present invention be covered by the claims of the present invention.