CN110994674A - Power electronic transformer fault ride-through method based on photovoltaic power supply support - Google Patents

Power electronic transformer fault ride-through method based on photovoltaic power supply support Download PDF

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CN110994674A
CN110994674A CN201911181756.XA CN201911181756A CN110994674A CN 110994674 A CN110994674 A CN 110994674A CN 201911181756 A CN201911181756 A CN 201911181756A CN 110994674 A CN110994674 A CN 110994674A
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voltage
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load
photovoltaic power
power supply
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CN110994674B (en
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肖凡
涂春鸣
葛钦
兰征
管亮
肖标
葛平娟
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Hunan University
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
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Abstract

The invention discloses a power electronic transformer fault ride-through method based on photovoltaic power supply supportsAfter tripping, the control is carried out according to the following conditions: when P is presentPV(0)≥Pload(0) When the voltage on the high voltage side rises gradually, the voltage on the low voltage side can still be kept stable, and the switch k1、k2、k3The closed state is kept, so that after the photovoltaic power supply responds, the redundant energy of the high-voltage side capacitor needs to be consumed through the load; when P is presentPV(0)<Pload(0) When the voltage on the high-voltage side is gradually reduced, the voltage on the low-voltage side can still be kept stable, and the photovoltaic power supply can not meet the load requirement, and the load needs to be cut off, namely the switch k1、k2Opening, k3Keeping a closed state, and after the photovoltaic power supply responds, a circuit breaker ksBefore closing, the photovoltaic power supply starts to compensate energy required by the high-voltage direct-current side capacitor. The method can effectively solve the problem of instantaneous impact current generated by the power electronic transformer in the reclosing process, and achieves the purpose of inhibiting the impact current.

Description

Power electronic transformer fault ride-through method based on photovoltaic power supply support
Technical Field
The invention belongs to the technical field of alternating current and direct current hybrid micro-grids, and relates to a power electronic transformer fault ride-through method based on photovoltaic power supply support.
Background
With the rapid development of future power grid technologies such as smart power grids and energy internet, power electronic transformers capable of achieving multiple functions such as voltage transformation, electrical isolation, power regulation and control, renewable energy access and the like are also called Solid State Transformers (SSTs) and are gaining more and more attention. The SST has a plurality of advantages, becomes important electric energy conversion equipment in a future smart grid and an energy internet, and is mainly applied to a vehicle-mounted converter system for electric locomotive traction, the smart grid/energy internet and a distributed renewable energy power generation grid-connected system. SST is used as a novel power transformer and a high-capacity power electronic converter applied to a power system, and stability and reliability are important indexes for evaluating performance of the SST. However, since the SST input stage is connected to the power grid, when the power grid fails, the stable operation of the SST is directly affected.
In the grid, power system operating experience shows that most faults in overhead lines are "transient", with permanent faults typically less than 10%. Therefore, after the short-circuit fault is removed by the relay protection action, the electric arc is automatically extinguished, and the insulation at the short-circuit part can be automatically recovered under most conditions, namely, the electric arc is automatically reclosed, so that the safety and the reliability of power supply are ensured. Simulation shows that under traditional control, the input stage feeder circuit breaker reclosing will cause the SST to generate a larger impact current. In the existing SST fault control method, the stability of the SST low-voltage direct-current bus voltage is maintained when the power grid voltage is interrupted through energy conservation by an energy storage ring, but the problems of instantaneous grid-connected impact current and quick start are ignored.
Disclosure of Invention
In order to achieve the purpose, the invention provides a power electronic transformer fault ride-through method based on photovoltaic power supply support, which effectively solves the problem of instantaneous impact current generated by the power electronic transformer in the reclosing process.
The technical scheme adopted by the invention is that the power electronic transformer fault ride-through method based on photovoltaic power supply support is carried out according to the following steps:
firstly, collecting input voltage u of alternating load in real time through a voltage transformerADDC load input voltage uDCInput voltage u of photovoltaic power supplyPVReal-time collection of AC load input current i by current transformerADDC load input current iDCInput current i of photovoltaic power supplyPVSolving the initial power consumption P of the load before reclosingload(0) Initial output power P of photovoltaic power supplyPV(0)；
Secondly, whether reclosing occurs or not can be judged by monitoring whether the three-phase input voltage is zero voltage or not in the operation process of the solid-state transformer SST;
thirdly, when the reclosing is judged to occur, the circuit breaker ksAfter tripping, the control is carried out according to the following conditions:
when P is presentPV(0)≥Pload(0) When the voltage on the high voltage side rises gradually, the voltage on the low voltage side can still be kept stable, and the switch k1、 k2、k3The closed state is kept, so that after the photovoltaic power supply responds, the redundant energy of the high-voltage side capacitor needs to be consumed through the load;
at photovoltaic power supply response time t1And then, the photovoltaic power supply adjusts the power as follows:
Figure RE-GDA0002399527940000021
wherein n is the number of SST input stage cascade H bridge chain link modules, CHMu is SST middle-level power transmission efficiency, v is the capacitance value of the high-voltage side capacitorH-refFor high-voltage DC voltage referenceA value; t is t1Response time for photovoltaic power supply, t2For reclosing time, tsIs a reclosing time interval; v'H(t2) The high-voltage direct current voltage is high-voltage direct current voltage during reclosing;
when P is presentPV(0)<Pload(0) When the voltage on the high-voltage side is gradually reduced, the voltage on the low-voltage side can still be kept stable, and the photovoltaic power supply can not meet the load requirement, and the load needs to be cut off, namely the switch k1、k2Opening, k3Keeping a closed state and responding to the photovoltaic power supply for a time t1Rear, breaker ksBefore closing the switch, the photovoltaic power supply starts to compensate energy required by a high-voltage direct-current side capacitor, and the required photovoltaic power supply adjusts the power as follows:
Figure RE-GDA0002399527940000022
the switch k1Controlling the AC load at the AC output of the SST output stage, the switch k2Controlling the DC load of the DC output end of the SST output stage, and the switch k3And the photovoltaic power supply controls the direct current output end of the SST output stage.
Further, the solving of the load initial consumed power P before reclosingload(0) The process is as follows:
Figure RE-GDA0002399527940000023
wherein, PAC-load(0) For the initial input power of the AC load, PDC-load(0) The power is initially input for the dc load.
Further, the solving of the initial output power P of the photovoltaic power supplyPV(0) The process is as follows:
PPV(0)＝upv×ipv。
further, the photovoltaic power supply adjustment power determination process includes:
to ensure the voltage on the low voltage DC side to be stable, the voltage on the low voltage DC side should be dropped before the voltage on the low voltage DC side drops, i.e. before the voltage on the high voltage DC side drops to 0Line power compensation, this time is denoted as t0Then, there are:
Figure RE-GDA0002399527940000031
in the formula
PO(0)＝Pload(0)-PPV(0) (2)
Wherein, POTo SST output power;
controlling before the high-voltage direct-current bus falls to 0;
(1) when reclosing occurs, if PPV(0)≥Pload(0) Then, then
Due to the fact that the SST is disconnected with the power grid, the response time t of the photovoltaic power supply is within1In the above, the energy overflowing will be stored in the high voltage dc side capacitor, resulting in the voltage on the high voltage dc side rising, when:
Figure RE-GDA0002399527940000032
wherein a, b and c are phases of a three-phase power grid, j is a, b and c have no practical meaning, and v is(a,b,c)HiRepresents the high-side direct-current voltage v of the i-th stage rectifier of a phase, b and c(a,b,c)Hi(t1) Is shown at t1The high-voltage side direct-current voltage of the ith-stage rectifier at the time points a, b and c;
at t1At the moment, the photovoltaic power supply reduces the power output to be less than the load power, and the energy on the high-voltage direct-current side capacitor is properly consumed, then
Figure RE-GDA0002399527940000033
In the formula
Figure RE-GDA0002399527940000034
Is obtained from the formulae (3) and (4) at t1At any moment, the power output of the photovoltaic power supply is adjusted as follows:
Figure RE-GDA0002399527940000035
(2) when reclosing occurs, if PPV(0)<Pload(0) Then at the photovoltaic power response time t1In, the energy of disappearance will be provided by high voltage direct current side electric capacity, leads to the fall of high voltage direct current side voltage, has this moment:
Figure RE-GDA0002399527940000041
t1at the moment, the load needs to be cut off, and meanwhile, the photovoltaic power supply compensates for the shortage of the energy of the high-voltage direct-current capacitor, then
Figure RE-GDA0002399527940000042
In the formula
Figure RE-GDA0002399527940000043
Is obtained from the formulae (3) and (4) at t1At any moment, the power output of the photovoltaic power supply is adjusted as follows:
Figure RE-GDA0002399527940000044
the size of the impact current is related to the size of the high-voltage direct-current side voltage at the closing time, and in order to ensure that the impact current generated by the SST can not cause the misoperation of the circuit breaker when the overhead line is closed, the impact current is ensured to be less than 1.2-1.3 times of rated current, namely
|id-max|＜k|idN|,k∈[1.2,1.3](11)
Wherein id-maxK is a proportionality coefficient between the allowable surge current value and the rated current value, idNIs a rated current value;
further, solving the maximum value i of the impact currentd-max；
The input stage cascade H-bridge state equation can be expressed as:
Figure RE-GDA0002399527940000045
in the formula, e(a,b,c)Representing the grid-side phase voltage, i(a,b,c)Represents the net side phase current, u(a,b,c)NRepresents the AC side voltage uNORepresenting a neutral-to-ground voltage; s(a,b,c)i，i(a,b,c)HiRespectively representing the switching function and the output current of the ith stage rectifier of the a/b/c phase; l represents a network side filter inductor, and R is a network side resistance value;
when the voltage of the power grid is balanced and the three-phase SST system is stable, u isNO0, in order to simplify the design of a control system, the sine quantity of the fundamental wave of the PWM rectifier in an abc three-phase stationary coordinate system is converted into a direct current quantity in a d-q synchronous rotating coordinate system for analysis:
Figure RE-GDA0002399527940000051
in the formula: e.g. of the typed，eqD and q axis components of the electric network electromotive force respectively; i.e. id，iqD and q axis components of the alternating side current respectively; u. ofd，uqD and q axis components of the alternating side voltage respectively; omega is the angular frequency of the voltage of the power grid;
the input stage also adopts a PI regulator with feedforward decoupling as a controller of a voltage outer ring and a current inner ring, and the corresponding decoupling control equation is as follows:
Figure RE-GDA0002399527940000052
wherein id *，iq *Reference values of d and q axes of the alternating side current respectively; v. ofHsumThe sum of three-phase high-voltage direct current voltages at all levels; kvP、KvIRespectively an input stage voltage outer ring proportional coefficient and an integral coefficient; kiP、KiIAre respectively an input stageA current inner ring proportional coefficient and an integral coefficient; 1/s represents an integral operation in the frequency domain;
substituting equation (14) into (13) can yield:
Figure RE-GDA0002399527940000053
the current i is realized by (15)dAnd iqThe decoupling control of (1); at the same time idThe rate of change of (d) is:
Figure RE-GDA0002399527940000054
when reclosing happens, the energy of a direct current bus capacitor is lacked, so that the direct current bus voltage is given to an expected value of 3nv when the reclosing happensH-refAnd a feedback voltage vHsumIf there is deviation, a larger d-axis current given value i is obtained through the first formula in the formula (14), namely proportional integration of the voltage outer ring PI regulatord *Then, the d-axis current change rate is large through the formula (16), and the impact current occurs; up to idExceeds id *And make ud＝ed－Rid+wLidStop growing, i.e. idWhen the rate of change of (2) is 0, the maximum value is reached;
since the rush current generation time is very short, neglecting the integral amount generated in the integration process, when equation (16) is equal to 0, the maximum value of the rush current can be found as:
Figure RE-GDA0002399527940000061
as can be seen from the equation (17), the closer the actual value of the voltage on the high-voltage direct-current side is to the reference value, the smaller the impulse current is;
by substituting formula (11) for formula (17)
Figure RE-GDA0002399527940000062
According to the formulas (6), (10) and (18), and the high-voltage side voltage phase of each stageWhen the surge current reaches the requirement, i.e. | id-max|≤|kidNThe photovoltaic power supply compensation power is:
Figure RE-GDA0002399527940000063
the invention has the beneficial effects that: according to the invention, through different initial working conditions, a proper control method is selected to regulate and control the energy on the high-low voltage side capacitor, the difference value between the actual value of the voltage at the high-voltage side and the reference value during reclosing is reasonably reduced, the instantaneous impact current generated by the power electronic transformer during reclosing is effectively solved, and the purpose of inhibiting the impact current is achieved.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.
Fig. 1 is a schematic diagram of the inrush current control according to an embodiment of the present invention.
Fig. 2 is a power flow pattern of an embodiment of the present invention.
Fig. 3 is a diagram of dc voltage waveform and input current waveform under conventional control.
Fig. 4 is a diagram showing a dc voltage waveform and an input current waveform when the inrush current control is adopted.
Fig. 5 is a diagram of dc voltage waveform and input current waveform under conventional control.
Fig. 6 is a diagram showing a dc voltage waveform and an input current waveform in the case of the inrush current control.
Fig. 7 is a diagram of dc bus voltage waveform and input current waveform in condition 1.
Fig. 8 is a diagram of the dc bus voltage waveform and the input current waveform in operating mode 2.
Fig. 9 is a graph showing the dc bus voltage waveform and the input current waveform with the rush current of 0.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The SST output stage of the embodiment of the invention is an AC/DC hybrid output port, and an AC output end passes through a switch k1Controlling AC load, DC output controlling parallel connection k2、k3The switch respectively controls the direct current load and the photovoltaic power supply. Breaker ksOne side is connected with a power grid bus, and the other side is connected with the SST. When instantaneous fault occurs in high-voltage alternating-current bus or line connected in parallel with SST, circuit breaker k between bus and large power gridsAn automatic reclosing phenomenon occurs, at the moment, the SST is temporarily separated from the power grid to operate for a period of time, namely, the circuit breaker ksTime t of automatic reclosings. Breaker ksAfter tripping, a response time t exists due to photovoltaic power regulation1At t1In time, no matter the redundant or missing energy in the system is processed by the high-voltage direct-current side capacitor, a large difference value is generated between the redundant or missing energy and a reference voltage, impact current is brought during reclosing, and in order to reduce the impact current, the voltage value of the high-voltage direct-current side capacitor needs to be controlled by processing the energy on the high-voltage direct-current side capacitor before the reclosing, and the main control idea is shown in fig. 1.
Firstly, collecting input voltage u of alternating load in real time through a voltage transformerADDC load input voltage uDCInput voltage u of photovoltaic power supplyPVReal-time collection of AC load input current i by current transformerADDC load input current iDCInput current i of photovoltaic power supplyPVIf there is an initial power consumption P of the load before reclosingload(0) Comprises the following steps:
Figure RE-GDA0002399527940000071
wherein, PAC-load(0) For the initial input power of the AC load, PDC-load(0) The power is initially input for the dc load.
Initial output power P of photovoltaic power supplyPV(0) Comprises the following steps:
PPV(0)＝upv×ipv
and secondly, whether reclosing occurs can be judged by monitoring whether the three-phase input voltage is zero voltage or not in the operation process of the SST.
Thirdly, when the reclosing is judged to occur, the circuit breaker ksAfter tripping, the control is carried out according to the following conditions:
when P is presentPV(0)≥Pload(0) When the voltage on the high voltage side is gradually increased, the voltage on the low voltage side can still be kept stable, k1、k2、k3The closed state is kept, so that after the photovoltaic power supply responds, the redundant energy of the high-voltage side capacitor needs to be consumed through the load;
at photovoltaic power supply response time t1And then, the photovoltaic power supply adjusts the power as follows:
Figure RE-GDA0002399527940000081
when P is presentPV(0)<Pload(0) When the voltage on the high-voltage side is gradually reduced, the voltage on the low-voltage side can still be kept stable, and the load needs to be cut off because the photovoltaic power supply can not meet the load requirement, namely k1、k2Opening, k3Keeping a closed state, and after the photovoltaic power supply responds, a circuit breaker ksBefore closing the switch, the photovoltaic power supply starts to compensate energy required by a high-voltage direct-current side capacitor, and the required photovoltaic power supply adjusts the power as follows:
Figure RE-GDA0002399527940000082
the photovoltaic power supply power regulation refers to the power output by the photovoltaic power supply after the photovoltaic power supply responds and the photovoltaic power supply is controlled according to given power. In actual operation, a photovoltaic power supply is connected with a low-voltage direct-current bus through a DC/DC converter, a modulation signal of the DC/DC converter can be obtained through a PI controller by making a difference between actual power and reference power, and output power tracking control within a maximum output power range is achieved.
Further, the adjusting range of the power of the photovoltaic power supply is determined.
Setting SST output power as POThe capacitance value of the high-voltage side capacitor is CHThe capacitance value of the low-voltage side capacitor is CLSST middle level power transmission efficiency is mu, reclosing time interval is ts(ii) a Recording the reclosing start time as 0 at t1At the moment, the photovoltaic power supply starts to adjust the power to PPV(t1) (ii) a At t2And (6) switching on again at the moment. The power flow direction is shown in fig. 2.
In order to ensure the voltage on the low-voltage direct-current side to be stable, power compensation is carried out before the voltage on the low-voltage direct-current side drops, namely before the voltage on the high-voltage direct-current side drops to 0, and the time is recorded as t0Then, there are:
Figure RE-GDA0002399527940000083
in the formula
PO(0)＝Pload(0)-PPV(0) (2)
Wherein n is the number of SST input stage cascade H bridge chain link modules, vH-refIs a high-voltage direct-current voltage reference value.
Under the normal condition, a photovoltaic power supply in an AC/DC micro-grid is in the maximum output state, and the power output P of the SSTO(0) The photovoltaic power supply is small and the control response of the photovoltaic power supply by the power electronic device is generally fast, so that the control can be carried out before the high-voltage direct-current bus falls to 0.
(1) When reclosing occurs, if PPV(0)≥Pload(0) Then, then
Due to the fact that the SST is disconnected with the power grid, the response time t of the photovoltaic power supply is within1In the interior, the overflowed energy will be stored in the high voltage direct currentOn the side capacitance, the voltage on the high-voltage direct current side rises, and at this time:
Figure RE-GDA0002399527940000091
wherein a, b and c are phases of a three-phase power grid, j is a, b and c have no practical meaning, and v is(a,b,c)HiThe high-voltage side direct-current voltage of the i-th-stage rectifier of the a, b and c phases is shown.
At t1At the moment, the photovoltaic power supply reduces the power output to be less than the load power, and the energy on the high-voltage direct-current side capacitor is properly consumed, then
Figure RE-GDA0002399527940000092
In the formula
Figure RE-GDA0002399527940000093
Is obtained from the formulae (3) and (4) at t1At any moment, the power output of the photovoltaic power supply is adjusted as follows:
Figure RE-GDA0002399527940000094
(2) when reclosing occurs, if PPV(0)<Pload(0) Then at the photovoltaic power response time t1In, the energy of disappearance will be provided by high voltage direct current side electric capacity, leads to the fall of high voltage direct current side voltage, has this moment:
Figure RE-GDA0002399527940000095
t1at the moment, the load needs to be cut off, and meanwhile, the photovoltaic power supply compensates for the shortage of the energy of the high-voltage direct-current capacitor, then
Figure RE-GDA0002399527940000096
In the formula
Figure RE-GDA0002399527940000101
Is obtained from the formulae (3) and (4) at t1At any moment, the power output of the photovoltaic power supply is adjusted as follows:
Figure RE-GDA0002399527940000102
the size of the impact current is related to the size of the high-voltage direct-current side voltage at the closing time, and in order to ensure that the impact current generated by the SST can not cause the misoperation of the circuit breaker when the overhead line is closed, the impact current is ensured to be less than 1.2-1.3 times of rated current, namely
|id-max|＜k|idN|,k∈[1.2,1.3](11)
Wherein id-maxK is a proportionality coefficient between the allowable surge current value and the rated current value, idNIs a rated current value.
Further, the maximum value of the impact current is solved.
The input stage cascade H-bridge state equation can be expressed as:
Figure RE-GDA0002399527940000103
in the formula, e(a,b,c)Representing the grid-side phase voltage, i(a,b,c)Represents the net side phase current, u(a,b,c)NRepresents the AC side voltage uNORepresenting a neutral-to-ground voltage; s(a,b,c)i，i(a,b,c)HiRespectively representing the switching function and the output current of the ith stage rectifier of the a/b/c phase; l represents the net side filter inductance.
When the voltage of the power grid is balanced and the three-phase SST system is stable, u isNOTo simplify the control system design, the fundamental wave sine quantity of the PWM rectifier in the abc three-phase stationary coordinate system is usually converted into a direct current quantity in the d-q synchronous rotating coordinate system for analysis:
Figure RE-GDA0002399527940000111
in the formula: e.g. of the typed，eqD and q axis components of the electric network electromotive force respectively; i.e. id，iqD and q axis components of the alternating side current respectively; u. ofd，uqD and q axis components of the alternating side voltage respectively; and omega is the angular frequency of the grid voltage.
The input stage also adopts a PI regulator with feedforward decoupling as a controller of a voltage outer ring and a current inner ring, and the corresponding decoupling control equation is as follows:
Figure RE-GDA0002399527940000112
wherein id *，iq *Reference values of d and q axes of the alternating side current respectively; v. ofH-refIs a high voltage direct current voltage reference value; v. ofHsumThe sum of three-phase high-voltage direct current voltages at all levels; kvP、KvIRespectively an input stage voltage outer ring proportional coefficient and an integral coefficient; kiP、KiIRespectively is an input-stage current inner ring proportional coefficient and an integral coefficient; s represents a differentiation operation in the frequency domain, and 1/s represents an integration operation in the frequency domain.
Substituting equation (14) into (13) can yield:
Figure RE-GDA0002399527940000113
the current i is realized by (15)dAnd iqThe decoupling control of (1). At the same time idThe rate of change of (d) is:
Figure RE-GDA0002399527940000114
when reclosing happens, the energy of a direct current bus capacitor is lacked, so that the direct current bus voltage is given to an expected value of 3nv when the reclosing happensH-refAnd a feedback voltage vHsumWith some deviation, through the first formula in equation (14), i.e.Proportional integration of the voltage outer ring PI regulator to obtain a larger d-axis current given value id *Then, the d-axis current change rate is increased by the formula (16), and a rush current occurs. Up to idExceeds id *And make ud＝ed－Rid+wLidWill stop growing, i.e. idWhen the rate of change of (2) is 0, the maximum value is reached.
Since the rush current generation time is very short, neglecting the integral amount generated in the integration process, when equation (16) is equal to 0, the maximum value of the rush current can be found as:
Figure RE-GDA0002399527940000121
as can be seen from equation (17), the closer the actual value of the high-voltage dc side voltage is to the reference value, the smaller the inrush current.
By substituting formula (11) for formula (17)
Figure RE-GDA0002399527940000122
Wherein v isHsumThe voltage is the sum of three-phase high-voltage direct current voltages at all levels, and R is a grid side resistance value.
According to the formulas (6), (10) and (18), the high-voltage side voltages of all the stages are equal, and when the surge current reaches the requirement, i.e. | id-max|≤|kidNThe photovoltaic power supply compensation power is:
Figure RE-GDA0002399527940000123
wherein, v'H(t2) The high-voltage direct current voltage is high-voltage direct current voltage during reclosing; because SST intermediate/isolation level adopts Double Active Bridge (DAB), because DAB has voltage-sharing control, can guarantee that every grade of high-voltage direct-current voltage of every phase is equal, namely, v(a,b,c)Hi(t2)＝v′H(t2)。
In order to verify the effectiveness of impact suppression after reclosing, a Matlab/Simulink simulation model of a three-phase three-cascade SST containing a photovoltaic power supply is established, and system simulation parameters are shown in table 1.
TABLE 1 three-phase cascade SST simulation parameters
Figure RE-GDA0002399527940000124
Figure RE-GDA0002399527940000131
As can be seen from the equations (1) and (11), after the reclosure is started, the high-voltage DC side voltage falls to 0 for the shortest time
Figure RE-GDA0002399527940000132
Rated positive sequence current value
Figure RE-GDA0002399527940000133
Wherein Es、INThe amplitude of the voltage of the power grid and the amplitude of the rated current.
When the switch is switched on, according to the formula (11), the rated impulse current of 1.3 times can be obtained, and the voltage operating range of the high-voltage direct-current side is
3220(V)≤v′H(t2)≤3380(V)
At time 0.4s, reclosing occurs with a reclosing time interval of 1 s.
(1) Initial state: pPV(0)＝500kV·A，Pload(0) 400 kV. A, i.e. PPV(0)≥Pload(0) Time of flight
After tripping, the high-voltage direct-current side capacitor is in a charging state, the voltage value is gradually increased, and after 0.1s, if the power output of the photovoltaic power supply is adjusted to be PPV(t1) At this time, the high-low voltage dc-side voltage waveform and the current waveform are as shown in fig. 3, 400kV · a.
The surge current exceeds the maximum allowable current during reclosing, so that the photovoltaic power supply can adjust the power range to
386.1(kV·A)≤PPV(t1)≤394.9(kV·A)
Get PPV(t1) After power adjustment is performed at 386.1kV · a, the dc bus voltage waveform and the reclosing current waveform are as shown in fig. 4.
(2) Initial state: pPV(0)＝300kV·A，Pload(0) 400 kV. A, i.e. PPV(0)<Pload(0) Time of flight
After tripping, the high-voltage direct-current side capacitor is in a discharging state, the voltage value is gradually reduced, after 0.1s, the load needs to be disconnected because the photovoltaic power supply can not completely provide the power required by the load, and at the moment, if the output power P of the photovoltaic power supply is adjustedPV(t1) At this time, the high-low voltage dc-side voltage waveform and the current waveform are as shown in fig. 6.
As can be seen from fig. 6, the inrush current exceeds the maximum allowable current when switching on, so that the photovoltaic power supply has a regulated power range according to equation (19):
7.3(kV·A)≤PPV(t1)≤18.6(kV·A)
get PPV(t1) After power adjustment is performed at 18.6kV · a, the dc bus voltage waveform and the reclosing current waveform are as shown in fig. 5.
As is apparent from fig. 4, 5, and 6, after the power of the photovoltaic power supply is adjusted, at the closing time of 1.4s, the inrush current generated by the SST is kept within 1.3 times of the rated current, so that inrush current suppression is realized.
In order to further verify the correctness and the effectiveness of the inhibition strategy provided by the embodiment, low-pressure experiment verification is carried out on the provided control strategy. The experimental parameters are shown in table 2. Two working conditions are designed, namely working condition 1: pPV(0)＝4kV·A，Pload(0) 3 kV. A, i.e. PPV(0)≥Pload(0) (ii) a Working condition 2: pPV(0)＝2kV·A，Pload(0) 3 kV. A, i.e. PPV(0)<Pload(0). Adjusting the power of the photovoltaic power supply according to the control strategy in section 3, performing an experiment by using the boundary value of the modulation range of the photovoltaic power supply, and obtaining an experiment resultAs shown in fig. 7-8.
TABLE 2 three-phase Cascade SST test parameters
Parameter(s) Numerical value
Rated voltage/V of input stage 380
Filter inductor/mH 2
Number of module cascades per phase 3
High voltage direct current side capacitance/uF 500
Low-voltage DC side capacitor/uF 4000
High voltage direct current side voltage/V 150
Isolation stage high-frequency transformation ratio 1.6～1.7
Isolation stage each stage high frequency voltage transformation equivalent leakage inductance/mu H 40～60
Low voltage DC side voltage/V 90
Low-voltage direct-current side safety voltage/V 700
The transmission efficiency of each level of the isolation level is mu 0.92～0.94
Load power/kV. A 0～6
Photovoltaic power/kV & A 0～4
Photovoltaic power supply response time/s 0.1
Fig. 7-8 show the high-voltage direct-current voltage waveform and the input stage a-phase current waveform, respectively. As can be seen from fig. 7 and 8, after reclosure occurs, the voltage on the high-voltage direct-current side reaches the required range at the closing time through power adjustment of the photovoltaic power supply and the load, and therefore, the impact current generated by the input stage is within the allowable range (1.3 times of the rated current) during closing.
According to the formulas (17) and (19), when the impulse current is 0, the power of the photovoltaic power supply is adjusted, and an experiment is performed, wherein the obtained experimental waveform is shown in fig. 9.
In summary, the impulse current suppression strategy provided by the application can effectively solve the impulse current generated in the reclosing process SST on the premise of ensuring the voltage stability of the low-voltage direct-current bus, and achieves the purpose of impulse current suppression.
The above description is only for the preferred embodiment of the present invention, and is not intended to limit the scope of the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention shall fall within the protection scope of the present invention.

Claims (4)

1. The method for the fault ride-through of the power electronic transformer based on the photovoltaic power support is characterized by comprising the following steps of:
firstly, collecting input voltage u of alternating load in real time through a voltage transformerADDC load input voltage uDCInput voltage u of photovoltaic power supplyPVReal-time collection of AC load input current i by current transformerADDC load input current iDCInput current i of photovoltaic power supplyPVSolving the initial power consumption P of the load before reclosingload(0) Initial output power P of photovoltaic power supplyPV(0)；
Secondly, whether reclosing occurs or not can be judged by monitoring whether the three-phase input voltage is zero voltage or not in the operation process of the solid-state transformer SST;
thirdly, when the reclosing is judged to occur, the circuit breaker ksAfter tripping, the control is carried out according to the following conditions:
when P is presentPV(0)≥Pload(0) When the voltage on the high voltage side rises gradually, the voltage on the low voltage side can still be kept stable, and the switch k1、k2、k3The closed state is kept, so that after the photovoltaic power supply responds, the redundant energy of the high-voltage side capacitor needs to be consumed through the load;
at photovoltaic power supply response time t1And then, the photovoltaic power supply adjusts the power as follows:
Figure FDA0002291447950000011
wherein n is the number of SST input stage cascade H bridge chain link modules, CHMu is SST middle-level power transmission efficiency, v is the capacitance value of the high-voltage side capacitorH-refIs a high voltage direct current voltage reference value; t is t1Response time for photovoltaic power supply, t2For reclosing time, tsIs a reclosing time interval; v'H(t2) is the high voltage dc voltage at reclosing;
when P is presentPV(0)<Pload(0) When the voltage on the high-voltage side is gradually reduced, the voltage on the low-voltage side can still be kept stable, and the photovoltaic power supply can not meet the load requirement, and the load needs to be cut off, namely the switch k1、k2Opening, k3Keeping a closed state and responding to the photovoltaic power supply for a time t1Rear, breaker ksBefore closing the switch, the photovoltaic power supply starts to compensate energy required by a high-voltage direct-current side capacitor, and the required photovoltaic power supply adjusts the power as follows:
Figure FDA0002291447950000012
the switch k1Controlling the AC load at the AC output of the SST output stage, the switch k2Controlling the DC load of the DC output end of the SST output stage, and the switch k3And the photovoltaic power supply controls the direct current output end of the SST output stage.
2. The photovoltaic power supply support-based power electronic transformer fault ride-through method according to claim 1, wherein the solution of the load initial consumed power P before reclosingload(0) The process is as follows:
Figure FDA0002291447950000021
wherein, PAC-load(0) For the initial input power of the AC load, PDC-load(0) The power is initially input for the dc load.
3. A power electronic transformer fault ride-through method based on photovoltaic power support according to claim 1, wherein the solving of the initial output power P of the photovoltaic power is carried outPV(0) The process is as follows:
PPV(0)＝upv×ipv。
4. a power electronic transformer fault ride-through method based on photovoltaic power support according to claim 1, wherein the photovoltaic power adjustment power determination process is as follows:
in order to ensure the voltage on the low-voltage direct-current side to be stable, power compensation is carried out before the voltage on the low-voltage direct-current side drops, namely before the voltage on the high-voltage direct-current side drops to 0, and the time is recorded as t0Then, there are:
Figure FDA0002291447950000022
in the formula
PO(0)＝Pload(0)-PPV(0) (2)
Wherein, POTo SST output power;
controlling before the high-voltage direct-current bus falls to 0;
(1) when reclosing occurs, if PPV(0)≥Pload(0) Then, then
Due to the fact that the SST is disconnected with the power grid, the response time t of the photovoltaic power supply is within1In the above, the energy overflowing will be stored in the high voltage dc side capacitor, resulting in the voltage on the high voltage dc side rising, when:
Figure FDA0002291447950000023
wherein a, b and c are phases of a three-phase power grid, j is a, b and c have no practical meaning, and v is(a,b,c)HiRepresents the high-side direct-current voltage v of the i-th stage rectifier of a phase, b and c(a,b,c)Hi(t1) Is shown at t1The high-voltage side direct-current voltage of the ith-stage rectifier at the time points a, b and c;
at t1At the moment, the photovoltaic power supply reduces the power output to be less than the load power, and the energy on the high-voltage direct-current side capacitor is properly consumed, then
Figure FDA0002291447950000024
In the formula
Figure FDA0002291447950000031
Is obtained from the formulae (3) and (4) at t1At any moment, the power output of the photovoltaic power supply is adjusted as follows:
Figure FDA0002291447950000032
(2) when reclosing occurs, if PPV(0)<Pload(0) Then at the photovoltaic power response time t1In, the energy of disappearance will be provided by high voltage direct current side electric capacity, leads to the fall of high voltage direct current side voltage, has this moment:
Figure FDA0002291447950000033
t1at the moment, the load needs to be cut off, and meanwhile, the photovoltaic power supply compensates for the shortage of the energy of the high-voltage direct-current capacitor, then
Figure FDA0002291447950000034
In the formula
Figure FDA0002291447950000035
Is obtained from the formulae (3) and (4) at t1At any moment, the power output of the photovoltaic power supply is adjusted as follows:
Figure FDA0002291447950000036
the size of the impact current is related to the size of the high-voltage direct-current side voltage at the closing time, and in order to ensure that the impact current generated by the SST can not cause the misoperation of the circuit breaker when the overhead line is closed, the impact current is ensured to be less than 1.2-1.3 times of rated current, namely
|id-max|＜k|idN|,k∈[1.2,1.3](11)
Wherein id-maxK is a proportionality coefficient between the allowable surge current value and the rated current value, idNIs a rated current value;
further, solving the maximum value i of the impact currentd-max；
The input stage cascade H-bridge state equation can be expressed as:
Figure FDA0002291447950000041
in the formula, e(a,b,c)Representing the grid-side phase voltage, i(a,b,c)Represents the net side phase current, u(a,b,c)NRepresents the AC side voltage uNORepresenting a neutral-to-ground voltage; s(a,b,c)i，i(a,b,c)HiRespectively representing the switching function and the output current of the ith stage rectifier of the a/b/c phase; l represents a network side filter inductor, and R is a network side resistance value;
when the voltage of the power grid is balanced and the three-phase SST system is stable, u isNO0, in order to simplify the design of a control system, the sine quantity of the fundamental wave of the PWM rectifier in an abc three-phase stationary coordinate system is converted into a direct current quantity in a d-q synchronous rotating coordinate system for analysis:
Figure FDA0002291447950000042
in the formula: e.g. of the typed，eqD and q axis components of the electric network electromotive force respectively; i.e. id，iqD and q axis components of the alternating side current respectively; u. ofd，uqD and q axis components of the alternating side voltage respectively; omega is the angular frequency of the voltage of the power grid;
the input stage also adopts a PI regulator with feedforward decoupling as a controller of a voltage outer ring and a current inner ring, and the corresponding decoupling control equation is as follows:
Figure FDA0002291447950000043
wherein id *，iq *Reference values of d and q axes of the alternating side current respectively; v. ofHsumThe sum of three-phase high-voltage direct current voltages at all levels; kvP、KvIRespectively an input stage voltage outer ring proportional coefficient and an integral coefficient; kiP、KiIRespectively is an input-stage current inner ring proportional coefficient and an integral coefficient; 1/s represents an integral operation in the frequency domain;
substituting equation (14) into (13) can yield:
Figure FDA0002291447950000051
the current i is realized by (15)dAnd iqThe decoupling control of (1); at the same time idThe rate of change of (d) is:
Figure FDA0002291447950000052
when reclosing happens, the energy of a direct current bus capacitor is lacked, so that the direct current bus voltage is given to an expected value of 3nv when the reclosing happensH-refAnd a feedback voltage vHsumIf there is deviation, a larger d-axis current given value i is obtained through the first formula in the formula (14), namely proportional integration of the voltage outer ring PI regulatord *Then, the d-axis current change rate is large through the formula (16), and the impact current occurs; up to idExceeds id *And make ud＝ed－Rid+wLidStop growing, i.e. idWhen the rate of change of (2) is 0, the maximum value is reached;
since the rush current generation time is very short, neglecting the integral amount generated in the integration process, when equation (16) is equal to 0, the maximum value of the rush current can be found as:
Figure FDA0002291447950000053
as can be seen from the equation (17), the closer the actual value of the voltage on the high-voltage direct-current side is to the reference value, the smaller the impulse current is;
by substituting formula (11) for formula (17)
Figure FDA0002291447950000054
According to the formulas (6), (10) and (18), the high-voltage side voltages of all the stages are equal, and when the surge current reaches the requirement, i.e. | id-max|≤|kidNThe photovoltaic power supply compensation power is:
Figure FDA0002291447950000055
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