CN106786485B - Voltage ripple suppression method for direct-current micro-grid under unbalanced load - Google Patents
Voltage ripple suppression method for direct-current micro-grid under unbalanced load Download PDFInfo
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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
- H02J1/00—Circuit arrangements for dc mains or dc distribution networks
- H02J1/02—Arrangements for reducing harmonics or ripples
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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
- H02J1/00—Circuit arrangements for dc mains or dc distribution networks
- H02J1/10—Parallel operation of dc sources
- H02J1/102—Parallel operation of dc sources being switching converters
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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
- H02J1/00—Circuit arrangements for dc mains or dc distribution networks
- H02J1/14—Balancing the load in a network
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- H02J3/383—
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- H02J3/386—
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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
- H02J3/388—Islanding, i.e. disconnection of local power supply from the network
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E10/00—Energy generation through renewable energy sources
- Y02E10/50—Photovoltaic [PV] energy
- Y02E10/56—Power conversion systems, e.g. maximum power point trackers
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E10/00—Energy generation through renewable energy sources
- Y02E10/70—Wind energy
- Y02E10/76—Power conversion electric or electronic aspects
Abstract
The invention discloses a voltage ripple suppression method for a direct current micro-grid under unbalanced load, which comprises the steps of establishing the direct current micro-grid, establishing a direct current voltage control system based on a super capacitor, measuring and processing signals, determining a power expression of the super capacitor, designing parameters of a super capacitor energy storage device, and determining a control error eiDetermining a sliding mode surface S, determining a control rate D, judging whether the control target is reached, and performing PWM modulation. The invention has the advantages of simple structure, high control precision, strong robustness and the like; when the load is unbalanced, the control method can achieve the control target of restraining the voltage pulsation of the direct-current micro-grid under the unbalanced load.
Description
Technical Field
The invention relates to a voltage ripple suppression method, in particular to a voltage ripple suppression method for a direct-current micro-grid under an unbalanced load, and belongs to the technical field of power supply control.
Background
The direct-current micro-grid uses a direct-current power distribution mode, is beneficial to coordination control of various distributed power supplies, and can provide higher electric energy quality, so that the direct-current micro-grid becomes a new direction for micro-grid technology research. However, dc microgrid also has certain stability problems. Because an actual system often contains unbalanced load, fundamental negative sequence components are introduced to the alternating current load side, so that direct current voltage has frequency doubling pulsation, and the power supply quality of a direct current micro-grid is seriously influenced. In addition, the double frequency pulsating quantity of the inverter output power under the unbalanced load can generate secondary ripple current on the power supply and the inverter of the direct current microgrid, and the service life of the direct current microgrid is seriously influenced. Excessive ripple current can also damage the electrodes and electrolyte of the battery, reducing battery efficiency; the photoelectric conversion efficiency of the photovoltaic module is reduced, and the operation cost of the photovoltaic power station is improved; the on-state loss and the current stress of the switch tube of the converter are increased, and the capacity of the converter is wasted.
At present, research under unbalanced load mainly aims at various topologies and control strategies provided by a three-phase inverter on a load side, and aims at improving the output voltage waveform of the three-phase inverter, so that the balanced condition of three-phase voltage meets the requirement of electric energy quality. Since control of the output voltage waveform of the load-side inverter is indispensable and the problem of the dc microgrid voltage pulsation due to the unbalanced characteristic of the load is not negligible, it is necessary to consider controlling the dc voltage from another point of view.
In order to solve the above problem, some researchers have proposed to suppress the dc voltage ripple by absorbing the double frequency power with the energy storage device. Guoyi Xu et al, IEEE Transactions on Energy Conversion, 2012, 27 (4): 1036-1045, "Coordinated DC voltage control of wind turbine with embedded energy storage system" discloses a DC voltage ripple suppression method for a wind storage DC microgrid with an embedded energy storage system. The dc bus is connected to an ac power grid through a Line Side Converter (LSC). And the energy storage system and the LSC are respectively controlled by adopting droop control and double-ring PI. When the output power of the fan has pulsation, the natural frequency of the PI controller and the droop coefficient of the droop controller are adjusted to coordinate the current on the direct current bus, so that the energy storage system presents a band-pass characteristic to the current, the LSC presents a low-pass characteristic to the current, the pulsation component of the current can be absorbed by the energy storage system, namely the pulsation power flows into the energy storage system, and the direct current voltage is ensured to be stable. Dong Chen et al in IEEE Transactions on Power Systems, 2012, 27 (4): 1897-. The energy storage system and the grid-connected converter still adopt droop control and PI control respectively. The transfer function of the power is directly controlled by adjusting the droop coefficient and the time constant of the PI controller, so that the pulsating power flows into the energy storage system, and the direct-current voltage pulsation can be restrained. The above methods all achieve the control goal by controlling the frequency response characteristic of the current or power transfer function on the dc bus, and although a better control effect can be achieved, the control performance is greatly affected by parameters, and a plurality of parameters need to be coordinated during debugging, which makes the process complicated.
Disclosure of Invention
The invention aims to provide a voltage ripple suppression method for a direct-current micro-grid under an unbalanced load.
In order to solve the technical problems, the invention adopts the technical scheme that:
a voltage ripple suppression method for a direct current micro-grid under an unbalanced load comprises the following steps:
step 1: establishing a general direct-current micro-grid: the direct current micro-grid comprises a wind power generation unit formed by sequentially cascading a wind generating set, a fan measuring element and an AC/DC converter, a storage battery energy storage unit formed by sequentially cascading a storage battery, a storage battery measuring element and a first DC/DC converter, a photovoltaic power generation unit formed by sequentially cascading a photovoltaic array, a photovoltaic measuring element and a second DC/DC converter, and a direct current measuring element, the system comprises a grid-connected inverter, a grid-connected unit, a direct current load unit, a load inverter, a second alternating current measuring element, an alternating current load unit, a control system of a wind power generation unit, a control system of a storage battery energy storage unit and a control system of a photovoltaic power generation unit, wherein the grid-connected unit is formed by sequentially cascading a grid-connected inverter, a first alternating current measuring element and an alternating current power grid; the direct-current micro-grid takes a direct-current bus as a center, and the wind power generation unit, the storage battery energy storage unit, the photovoltaic power generation unit, the direct-current load unit, the alternating-current load unit and the grid-connected unit are sequentially connected into the direct-current bus through first to fifth direct-current measuring elements respectively to form a radial structure, wherein the direct-current load unit and the alternating-current load unit are connected into the direct-current bus through a fourth direct-current measuring element after being connected in parallel; the input ends of the control system of the wind power generation unit, the control system of the storage battery energy storage unit and the control system of the photovoltaic power generation unit are respectively connected with the output ends of the fan measuring element, the storage battery measuring element, the photovoltaic measuring element and the direct current measuring element, and the output ends of the control system of the wind power generation unit, the control system of the storage battery energy storage unit and the control system of the photovoltaic power generation unit are respectively connected with the input ends of the AC/DC converter, the first DC/DC converter;
step 2: establishing a direct-current voltage control system based on a super capacitor: the direct-current voltage control system comprises a super capacitor, a super capacitor measuring element, a fourth DC/DC converter, a fourth direct-current measuring element and a control system of the super capacitor; the direct-current voltage control system is connected with an alternating-current load in a direct-current micro-grid in parallel and is connected into a direct-current bus through a fourth direct-current measuring element, the input end of the control system is respectively connected with the output ends of the super-capacitor measuring element and the direct-current measuring element, and the output end of the control system is connected with the input end of the fourth DC/DC converter;
and step 3: signal measurement and processing: measuring the voltage u of a load-side DC bus by means of a voltage sensor and a current sensordcA current i flowing from the DC microgrid to the AC loadrInput current i of load invertercDischarge current i of the supercapacitorscTerminal voltage u of super capacitorsc(ii) a Calculating power P provided for load by direct-current microgridrPower absorbed by the load PL；
And 4, step 4: determining a reference power for a DC voltage control system
Wherein: u is the square of the DC voltage, C is the DC side capacitance, and t is the time;
and 5: designing parameters of the super-capacitor energy storage device:
the capacity of the super capacitor is as follows:
in the formula, PmaxIs reference power
A peak value of (d); the charging and discharging period is T, and the charging process is the terminal voltage u of the super capacitor in T/2scBy usc_initIs raised to usc_fin，
Filter inductance LDCComprises the following steps:
wherein: dBuckIs the duty cycle in Buck mode; f. ofsRepresenting the switching frequency of the DC/DC converter; Δ iscThe maximum ripple current allowed for the circuit;
step 6: determining a control error ei
Wherein:
is the current reference value of the super capacitor,
S＝ei+Ki∫eidt (5)
in the formula: kiIs a positive real number;
step 8, determining the control rate D of sliding mode control:
D＝Deq+ Δ D (6) formula: d is the duty cycle of the DC/DC converter, DeqIs equivalent control, Δ D is on-off control;
when in use
When the super capacitor is charged, the DC/DC converter works in a Buck mode:
D1＝D1eq+ΔD1(7)
in the formula: d1、D1eq、ΔD1Corresponding to the form of equation (6) in Buck mode, the equivalent control and the switch control are respectively
In the formula: k is a radical of11And k12Is a positive real number;
when in use
When the super capacitor is discharged, the DC/DC converter works in a Boost mode:
D2＝D2eq+ΔD2(10)
in the formula: d2、D2eq、ΔD2Corresponding to the form of the formula (6) in the Boost mode, the equivalent control and the switch control are respectively
In the formula: k is a radical of21And k22Is a positive real number;
and step 9: judging whether the control target is reached, if so, turning to the step 10, otherwise, turning to the step 7;
step 10: PWM modulation: and PWM modulating the duty ratio D to obtain a fourth DC/DC converter switching signal in the super capacitor energy storage device, and sending the fourth DC/DC converter switching signal into a fourth DC/DC converter for control.
The technical effect obtained by adopting the technical scheme is as follows:
1. aiming at the direct-current micro-grid under the unbalanced load, the invention controls the direct-current voltage by using the super capacitor on the premise of ensuring the power supply quality of the load, and can ensure the power supply quality of the direct-current micro-grid at the same time.
2. The controller of the invention has simple structure and parameter adjustment, high control precision and strong robustness.
3. The parameter design method of the super capacitor energy storage device provided by the invention is simple and reliable, avoids the capacity waste of the super capacitor and can reduce the system cost.
Drawings
The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.
FIG. 1 is a flow chart of the present invention;
FIG. 2 is a diagram of a DC microgrid architecture;
FIG. 3 is a block diagram of a supercapacitor based DC voltage control system;
fig. 4 is a control block diagram of a supercapacitor under unbalanced load.
Detailed Description
Example 1:
as shown in fig. 1, a method for suppressing voltage ripple of a dc micro-grid under an unbalanced load includes the following steps:
step 1: establishing a general direct-current micro-grid: the direct current micro-grid comprises a wind power generation unit formed by sequentially cascading a wind generating set, a fan measuring element and an AC/DC converter, a storage battery energy storage unit formed by sequentially cascading a storage battery, a storage battery measuring element and a first DC/DC converter, a photovoltaic power generation unit formed by sequentially cascading a photovoltaic array, a photovoltaic measuring element and a second DC/DC converter, and a direct current measuring element, the system comprises a grid-connected inverter, a grid-connected unit, a direct current load unit, a load inverter, a second alternating current measuring element, an alternating current load unit, a control system of a wind power generation unit, a control system of a storage battery energy storage unit and a control system of a photovoltaic power generation unit, wherein the grid-connected unit is formed by sequentially cascading a grid-connected inverter, a first alternating current measuring element and an alternating current power grid; the direct-current micro-grid takes a direct-current bus as a center, and the wind power generation unit, the storage battery energy storage unit, the photovoltaic power generation unit, the direct-current load unit, the alternating-current load unit and the grid-connected unit are sequentially connected into the direct-current bus through first to fifth direct-current measuring elements respectively to form a radial structure, wherein the direct-current load unit and the alternating-current load unit are connected into the direct-current bus through a fourth direct-current measuring element after being connected in parallel; the input ends of the control system of the wind power generation unit, the control system of the storage battery energy storage unit and the control system of the photovoltaic power generation unit are respectively connected with the output ends of the fan measuring element, the storage battery measuring element, the photovoltaic measuring element and the direct current measuring element, and the output ends of the control system of the wind power generation unit, the control system of the storage battery energy storage unit and the control system of the photovoltaic power generation unit are respectively connected with the input ends of the AC/DC converter, the first DC/DC converter;
step 2: establishing a direct-current voltage control system based on a super capacitor: the direct-current voltage control system comprises a super capacitor, a super capacitor measuring element, a fourth DC/DC converter, a fourth direct-current measuring element and a control system of the super capacitor; the direct-current voltage control system is connected with an alternating-current load in a direct-current micro-grid in parallel and is connected into a direct-current bus through a fourth direct-current measuring element, the input end of the control system is respectively connected with the output ends of the super-capacitor measuring element and the direct-current measuring element, and the output end of the control system is connected with the input end of the fourth DC/DC converter;
and step 3: signal measurement and processing: measuring the voltage u of a load-side DC bus by means of a voltage sensor and a current sensordcA current i flowing from the DC microgrid to the AC loadrInput current i of load invertercDischarge current i of the supercapacitorscTerminal voltage u of super capacitorsc(ii) a Calculating power P provided for load by direct-current microgridrPower absorbed by the load PL；
And 4, step 4: determining a reference power for a DC voltage control system
Wherein: u is the square of the DC voltage, C is the DC side capacitance, and t is the time;
and 5: designing parameters of the super-capacitor energy storage device:
in case of uneven loadWhen the balance (unbalance is defined as the percentage of negative sequence current divided by positive sequence current) is maximum, the reference power of the super capacitor is
Peak value P ofmaxThe voltage at the super capacitor terminal is usc_initIs raised to usc_finAnd the charging and discharging period is T, the capacity of the super capacitor is
Equivalent resistance R of super capacitorscVery small, typically in the milliohm range.
Filter inductance LDCThe parameters of (A) can be selected according to the running state of the Buck circuit, and the formula is
Wherein: dBuckIs the duty cycle in Buck mode; f. ofsRepresenting the switching frequency of the DC/DC converter; Δ iscThe maximum ripple current allowed by the circuit is generally set to 15% of the peak value of the rated current.
Step 6: determining a control error ei
Wherein:
is the current reference value of the super capacitor,
S＝ei+Ki∫eidt (5)
in the formula: kiIs a positive real number;
step 8, determining the control rate D of sliding mode control:
D＝Deq+ΔD (6)
in the formula: d is the duty cycle of the DC/DC converter, DeqIs equivalent control, Δ D is on-off control;
when in use
When the super capacitor is charged, the DC/DC converter works in a Buck mode:
D1＝D1eq+ΔD1(7)
in the formula: d1、D1eq、ΔD1Corresponding to the form of equation (6) in Buck mode, the equivalent control and the switch control are respectively
In the formula: k is a radical of11And k12Is a positive real number;
when in use
When the super capacitor is discharged, the DC/DC converter works in a Boost mode:
D2＝D2eq+ΔD2(10)
in the formula: d2、D2eq、ΔD2Corresponding to the form of the formula (6) in the Boost mode, the equivalent control and the switch control are respectively
In the formula: k is a radical of21And k22Is a positive real number;
and step 9: judging whether the control target is reached, if so, turning to the step 10, otherwise, turning to the step 7;
step 10: PWM modulation: and PWM modulating the duty ratio D to obtain a fourth DC/DC converter switching signal in the super capacitor energy storage device, and sending the fourth DC/DC converter switching signal into a fourth DC/DC converter for control.
The dc voltage reference value of this embodiment is generally set as the rated voltage of the dc bus connected to the ac load, i.e. the rated voltage of the node 5 in fig. 2, and the specific value is related to the load power. In the super capacitor energy storage device, the capacity of the super capacitor is 5.9mF, the equivalent resistance is 2.1m omega, and the filter inductance is 0.2 mH.
The direct-current micro-grid mainly comprises four parts: the distributed power generation unit comprises a wind generating set and photovoltaic power generation, is respectively connected to a direct current bus through an AC/DC converter and a DC/DC converter and is responsible for providing electric energy for a direct current micro-grid; the storage battery energy storage unit is connected to the direct current bus through the DC/DC converter, is responsible for maintaining power balance in the system and stabilizing direct current voltage in an island mode; the load type of the direct-current micro-grid is direct-current load and alternating-current load, and the direct-current load and the alternating-current load are respectively connected to a direct-current bus through a DC/DC converter and a DC/AC converter; the direct current microgrid is connected into the alternating current main network through a DC/AC converter.
In order to inhibit direct-current voltage pulsation under unbalanced load, the super capacitor is added to be connected to a direct-current bus at the outlet of a load converter, as shown in fig. 3, the super capacitor is simplified and is connected to the direct-current bus through a non-isolated Buck-Boost bidirectional DC/DC converter. In the figure, C is the DC side capacitance udcIs the capacitor voltage, PrPower to the load for the dc microgrid, PLThe power absorbed for the load; pscAbsorbed power for the supercapacitor, S1、S2Denotes a switching tube, LDCIs an inductor, Csc、RscEquivalent capacitance and equivalent resistance, i, representing a simplified model of a supercapacitorscCharging current for the supercapacitor uscFor the terminal voltage of the supercapacitor, ucIs ultraThe stage capacitor is equivalent to the terminal voltage of the capacitance. The invention mainly aims at the control of a DC/DC converter, namely a direct-current voltage control system based on a sliding mode controller.
The mathematical model of the direct current side capacitance is:
reference power of the DC voltage control system
Comprises the following steps:
wherein u is a DC voltage udcSquare of (d).
Establishing a control target of the sliding mode controller; the main control objective of the DC/DC converter is to control the current i of the super capacitorscThe direct-current voltage ripple is restrained, and the dynamic response performance is good; the following tracking error is thus determined:
in the formula:
is the current reference value of the super capacitor,
determining a slip form surface, wherein the invention adopts an integral slip form surface:
S＝ei+Ki∫eidt (16)
in the formula: the integral term is introduced to eliminate the static error of the system, KiIs a positive real number;
determining a control rate; the invention adopts the following control rate structure:
D＝Deq+ΔD (17)
in the formula: deqThe equivalent control is used for enabling the system to move along the sliding mode surface in an ideal state, and can accelerate the response speed of the system and reduce the static error of the system. And deltad is a switching control which can make the system reach the sliding mode surface from an arbitrary initial state in a limited time. According to the control target of the present invention, the control rate is divided into two cases:
when in use
When the super capacitor is charged, the DC/DC converter works in a Buck mode:
D1＝D1eq+ΔD1(18)
in the formula: d1、D1eq、ΔD1Corresponding to the form of control rate in Buck mode, its equivalent control order
To obtain
The invention adopts the supercoiling algorithm in the sliding mode control to design the switch control; according to the design rule of the supercoiling algorithm, the switch control is designed as follows:
in the formula: k is a radical of11And k12Is a positive real number;
when in use
When the super capacitor is discharged, the DC/DC converter works in a Boost mode:
D2＝D2eq+ΔD2(21)
in the formula: d2、D2eq、ΔD2Corresponding to the form of the control rate in Boost mode, the equivalent control and the on-off control can be obtained as the same
In the formula: k is a radical of21And k22Is a positive real number;
establishing a control strategy taking suppression of direct-current voltage pulsation as a control target when the load is unbalanced; the sliding mode control can control the alternating current quantity, so that the direct current voltage and no static difference can be adjusted as long as the reference value of the controller is set as the direct current quantity, namely double-frequency pulse motion of the direct current voltage is eliminated; as can be seen from the direct current side mathematical model, the stability of the direct current voltage substantially reflects the power P provided by the direct current microgrid to the loadrPower absorbed by the load PLAnd the power P absorbed by the supercapacitorscBalancing the three components; as long as P is controlledsc2 frequency multiplication components in the equal-sign right-side power sum of the compensation formula (14) can effectively inhibit direct-current voltage pulsation;
fig. 4 shows a control block diagram of the control strategy.
Claims (2)
1. A voltage ripple suppression method for a direct current micro-grid under an unbalanced load is characterized by comprising the following steps: the method comprises the following steps:
step 1: establishing a general direct-current micro-grid: the direct current micro-grid comprises a wind power generation unit formed by sequentially cascading a wind generating set, a fan measuring element and an AC/DC converter, a storage battery energy storage unit formed by sequentially cascading a storage battery, a storage battery measuring element and a first DC/DC converter, a photovoltaic power generation unit formed by sequentially cascading a photovoltaic array, a photovoltaic measuring element and a second DC/DC converter, and a direct current measuring element, the system comprises a grid-connected inverter, a grid-connected unit, a direct current load unit, a load inverter, a second alternating current measuring element, an alternating current load unit, a control system of a wind power generation unit, a control system of a storage battery energy storage unit and a control system of a photovoltaic power generation unit, wherein the grid-connected unit is formed by sequentially cascading a grid-connected inverter, a first alternating current measuring element and an alternating current power grid; the direct-current micro-grid takes a direct-current bus as a center, and the wind power generation unit, the storage battery energy storage unit, the photovoltaic power generation unit, the direct-current load unit, the alternating-current load unit and the grid-connected unit are sequentially connected into the direct-current bus through first to fifth direct-current measuring elements respectively to form a radial structure, wherein the direct-current load unit and the alternating-current load unit are connected into the direct-current bus through a fourth direct-current measuring element after being connected in parallel; the input ends of the control system of the wind power generation unit, the control system of the storage battery energy storage unit and the control system of the photovoltaic power generation unit are respectively connected with the output ends of the fan measuring element, the storage battery measuring element, the photovoltaic measuring element and the direct current measuring element, and the output ends of the control system of the wind power generation unit, the control system of the storage battery energy storage unit and the control system of the photovoltaic power generation unit are respectively connected with the input ends of the AC/DC converter, the first DC/DC converter;
step 2: establishing a direct-current voltage control system based on a super-capacitor energy storage device: the direct-current voltage control system comprises a super capacitor, a super capacitor measuring element, a fourth DC/DC converter, a fourth direct-current measuring element and a control system of the super capacitor; the direct-current voltage control system is connected with an alternating-current load in a direct-current micro-grid in parallel and is connected into a direct-current bus through a fourth direct-current measuring element, the input end of the control system is respectively connected with the output ends of the super-capacitor measuring element and the direct-current measuring element, and the output end of the control system is connected with the input end of the fourth DC/DC converter;
and step 3: signal measurement and processing: measuring the voltage u of a load-side DC bus by means of a voltage sensor and a current sensordcA current i flowing from the DC microgrid to the AC loadrInput current i of load invertercDischarge current i of the supercapacitorscTerminal voltage u of super capacitorsc(ii) a Calculating power P provided for load by direct-current microgridrPower absorbed by the load PL；
And 4, step 4: determining a reference power for a DC voltage control system
Wherein: u is the square of the DC voltage, C is the DC side capacitance, and t is the time;
and 5: designing parameters of the super-capacitor energy storage device:
the capacity of the super capacitor is as follows:
in the formula, PmaxIs reference power
A peak value of (d); the charging and discharging period is T, and the charging process is the terminal voltage u of the super capacitor in T/2scBy usc_initIs raised to usc_fin，
Filter inductance LDCComprises the following steps:
wherein: dBuckIs the duty cycle in Buck mode; f. ofsRepresenting the switching frequency of the DC/DC converter; Δ iscThe maximum ripple current allowed for the circuit;
step 6: determining a control error ei
Wherein:
is the current reference value of the super capacitor,
step 7, determining a sliding mode surface S:
S＝ei+Ki∫eidt (5)
in the formula: kiIs a positive real number;
step 8, determining the control rate D of sliding mode control:
D＝Deq+ΔD (6)
in the formula: d is the duty cycle of the DC/DC converter, DeqIs equivalent control, Δ D is on-off control;
when in use
When the super capacitor is charged, the DC/DC converter works in a Buck mode:
D1＝D1eq+ΔD1(7)
in the formula: d1、D1eq、ΔD1Corresponding to the form of equation (6) in Buck mode;
when in use
When the super capacitor is discharged, the DC/DC converter works in a Boost mode:
D2＝D2eq+ΔD2(8)
in the formula: d2、D2eq、ΔD2Corresponding to the form of equation (6) in Boost mode;
and step 9: judging whether the control target is reached, if so, turning to the step 10, otherwise, turning to the step 7;
step 10: PWM modulation: and PWM modulating the duty ratio D to obtain a fourth DC/DC converter switching signal in the super capacitor energy storage device, and sending the fourth DC/DC converter switching signal into a fourth DC/DC converter for control.
2. The voltage ripple suppression method for the dc microgrid under an unbalanced load of claim 1, characterized in that:
equivalent control D in said step 81eqComprises the following steps:
switch control Δ D1Comprises the following steps:
in the formula: k is a radical of11And k12Is a positive real number;
equivalent control D2eqComprises the following steps:
switch control Δ D2Comprises the following steps:
in the formula: k is a radical of21And k22Are positive real numbers.
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| CN112217219B (en) * | 2019-07-11 | 2023-11-10 | 华北电力大学（保定） | DC transient power quality control and recovery method based on super capacitor |
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