CN110429643B - Interstage cooperative starting system and method for distributed photovoltaic direct current collection system - Google Patents
Interstage cooperative starting system and method for distributed photovoltaic direct current collection system Download PDFInfo
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- CN110429643B CN110429643B CN201910683290.7A CN201910683290A CN110429643B CN 110429643 B CN110429643 B CN 110429643B CN 201910683290 A CN201910683290 A CN 201910683290A CN 110429643 B CN110429643 B CN 110429643B
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M3/00—Conversion of dc power input into dc power output
- H02M3/02—Conversion of dc power input into dc power output without intermediate conversion into ac
- H02M3/04—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters
- H02M3/10—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M3/145—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M3/155—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
- H02M3/156—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators
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Abstract
The invention provides an interstage cooperative starting system and method for a distributed photovoltaic direct current collection system, which comprises the following steps: configuring a low-voltage direct current side; a charging step: controlling a unique master control converter in the preceding stage Boost converter to adopt a master control mode to charge the low-voltage direct-current bus, and controlling a slave control converter in the preceding stage Boost converter to adopt a slave control mode to charge the low-voltage direct-current bus; and a control step of the post step-up converter: judging whether the voltage of the low-voltage direct-current bus meets a first voltage condition or not by a control system of the post-stage step-up converter so as to determine control information of the post-stage step-up converter; a mode switching step: judging whether the voltage of the low-voltage direct-current bus meets a second voltage condition or not by a preceding-stage Boost converter control system so as to determine mode switching information of the preceding-stage Boost converter and control information of a starting resistance device; the invention can realize effective starting of the system.
Description
Technical Field
The invention relates to the field of cooperative start control and operation mode switching among multilevel converters, in particular to an interstage cooperative start system and method for a distributed photovoltaic direct current collection system.
Background
Photovoltaic power generation is one of the most main forms of new energy power generation at present, and is the main force for realizing renewable energy source substitution in the future in China. The distribution condition of illumination resources, a power system and a system pattern in China determine large-scale construction and centralized grid-connected access, and the centralized grid-connected access becomes an important form for future photovoltaic development and utilization. The traditional alternating current collection grid-connected technology based on the inverter faces two major technical bottlenecks in the construction of large photovoltaic power stations above the million kilowatts level, namely: the problem of stability of the parallel connection of a plurality of inverters under the weak synchronous support is prominent, and the phenomena of voltage out-of-limit and wide frequency domain oscillation are frequent; the loss of an alternating current collection line between stations in the station is large, and the overall efficiency of the system is low.
In order to solve the problems, a large photovoltaic power generation base is constructed based on a photovoltaic direct current collection system in the future, namely, low-voltage direct current output by a photovoltaic array is directly pumped to the voltage level of a direct current distribution network by a photovoltaic direct current boost converter, and after further collection, the low-voltage direct current is subjected to centralized inversion access to an alternating current large power grid by a VSC converter station or is further boosted to the HVDC level by a large boost converter station, so that the direct current photovoltaic power generation base is remotely sent out. The scheme is suitable for development and construction of large-scale photovoltaic power stations, has been widely concerned by academic circles and engineering circles at home and abroad at present, and has bright prospect. At present, various photovoltaic direct current collection system architecture schemes have been proposed in academic circles and industrial circles, and the schemes can be divided into two categories, namely parallel connection type and series connection type. The parallel direct current collection system may adopt a collection network topology similar to that of an existing alternating current collection and distribution type photovoltaic power station, as shown in fig. 8, so as to form a collection and distribution type photovoltaic direct current collection system. The system structure has the outstanding advantages of front-stage and back-stage power decoupling, wide operation range, easy modularization expansion and the like, and becomes a photovoltaic direct current collection system scheme with the best engineering popularization and application prospect under the current technical condition. At present, the direct current photovoltaic power station demonstration project of domestic start construction in China mainly adopts a convergence system architecture. As shown in fig. 9, in consideration of the unidirectional energy flow characteristic (source side to grid side) of the photovoltaic power generation system, in order to reduce the system hardware cost, a diode rectifier bridge having the unidirectional energy flow characteristic is generally employed on the high-voltage side of the DC-DC boost converter. This physical characteristic will present technical challenges to the start-up of the entire photovoltaic dc collection system.
For a traditional alternating current photovoltaic grid-connected system, because a bridge inverter topology has a bidirectional energy flow characteristic, a starting scheme from a network side to a source side can be adopted, a low-voltage direct current bus voltage is established by the inverter, and then each preceding-stage Boost converter at the preceding stage is directly started in a current source mode (MPPT mode). In a direct current collection system, energy cannot be injected into a front-end low-voltage direct current bus from a medium-voltage direct current network side due to the unidirectional energy flow characteristic of a Boost converter, so that a brand-new starting scheme from a source side to a network side is adopted, the front-stage Boost converter is started in a voltage source mode and establishes a low-voltage direct current bus voltage, then the Boost converter can be started, and finally the front-stage Boost converter is switched into an MPPT mode to operate as a current source. The starting process is obviously different from the traditional alternating current photovoltaic grid-connected system, and relates to the problem of coordinated switching of the operation modes of the front-stage converter and the rear-stage converter, and the starting process is one of the important technical problems which must be faced in the engineering application of the photovoltaic direct current boosting collecting system.
Patent document CN109103921A discloses a start control method for a "dc collection-ac grid-connected" type photovoltaic power station. The basic idea is to establish similar voltage on two sides of a circuit breaker by utilizing a breakpoint formed by a boost converter outlet circuit breaker and adopting a mode of synchronous and independent starting of a source side and a network side, and then switch on by selecting a machine. In the scheme disclosed in the patent, aiming at the starting of a preceding-stage MPPT converter and a boost converter, the MPPT converter is started in a mode of controlling output voltage firstly and then is converted into an MPPT mode of controlling input side voltage. In the disclosed scheme, both the front-stage MPPT converter and the rear-stage boost converter need to perform mode switching, and a specific scheme for realizing mode switching is not given in the literature, and according to the principle, the mode switching needs to be realized by an intra-station real-time communication system. This solution has the following drawbacks: the front-stage MPPT device and the boost converter have two working modes, and need to be switched timely, so that the control complexity is high; the input side of a single boost converter is not considered to be connected with a distributed system structure comprising a plurality of MPPT converter branches in parallel, and the application range of the MPPT converter is indirectly limited; the specific implementation of the coordination control between the pre-MPPT converter and the boost converter is unclear, and depends on a real-time communication line in the station, which increases the hardware cost.
Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to provide an interstage cooperative starting system and method for a distributed photovoltaic direct current collection system.
The invention provides an interstage cooperative starting method for a distributed photovoltaic direct current collection system, which is characterized by comprising the following steps of: a low-voltage direct current side configuration step: -starting the pre-stage Boost converter control circuit while keeping the pre-stage Boost converter main circuit latched; -starting the post boost converter control circuit, monitoring the low voltage dc bus voltage, while keeping the post boost converter main circuit latched; -switching in one or more starting resistances; the preceding stage Boost converter cooperation step: -selecting a preceding Boost converter as the master control converter; -selecting all preceding stage Boost converters except the master control converter as slave control converters; a charging step: -controlling the main control converter to charge the low voltage dc bus in a main control mode; -controlling the slave control converter to assume a slave control mode for charging the low voltage dc bus; and a control step of the post step-up converter: judging whether the voltage of the low-voltage direct-current bus meets a first voltage condition or not by a control system of the post-stage step-up converter so as to determine control information of the post-stage step-up converter; a mode switching step: judging whether the voltage of the low-voltage direct-current bus meets a second voltage condition or not by a preceding-stage Boost converter control system so as to determine mode switching information of the preceding-stage Boost converter and control information of a starting resistance device; the first voltage condition is that the voltage of the low-voltage direct-current bus is greater than or equal to the lower limit value of the starting voltage before the post-stage step-up converter is started; the second voltage condition is that the voltage of the low-voltage direct-current bus accords with the normal operation voltage after the post-stage boost converter is started; the control information of the post-stage boost converter controls a main loop of the post-stage boost converter to control the starting behavior of a low-voltage direct-current bus voltage mode; the starting resistance device control information controls the switching behavior of the starting resistance device; the mode switching information of the preceding stage Boost converter controls the mode switching behavior of the preceding stage Boost converter; the pre-stage Boost converter mode comprises any one or more of the following modes: -a master control mode; -a slave control mode; -maximum power point tracking mode.
Preferably, the number of the preceding stage Boost converters is greater than or equal to one.
Preferably, the charging step includes: configuring the only preceding stage Boost converter in the master control mode to adopt a voltage source working mode; configuring the preceding stage Boost converter in the slave control mode to operate in a current source mode; -a pre-stage Boost converter in voltage source mode to control the output voltage of the pre-stage Boost converter; and a preceding stage Boost converter in a current source mode is adopted to control the output current of the preceding stage Boost converter.
Preferably, the mode switching step includes: switching a maximum power point tracking mode: and when the voltage of the low-voltage direct-current bus accords with the normal operation voltage, cutting off the starting resistor, and switching the mode of the preceding stage Boost converter into the maximum power point tracking mode.
Preferably, the rear-stage boost converter has a unidirectional energy flow characteristic; the rear-stage boost converter is a rear-stage direct-current boost converter; the starting resistor device comprises: -one or more starting resistances; -one or more start-up resistor-throw switches.
The invention provides an interstage cooperative starting system for a distributed photovoltaic direct current collection system, which comprises: low voltage direct current side configuration module: -starting the pre-stage Boost converter control circuit while keeping the pre-stage Boost converter main circuit latched; -starting the post boost converter control circuit, monitoring the low voltage dc bus voltage, while keeping the post boost converter main circuit latched; -switching in one or more starting resistances; preceding stage Boost converter cooperation module: -selecting a preceding Boost converter as the master control converter; -selecting all preceding stage Boost converters except the master control converter as slave control converters; a charging module: -controlling the main control converter to charge the low voltage dc bus in a main control mode; -controlling the slave control converter to assume a slave control mode for charging the low voltage dc bus; the post-stage step-up converter control module: judging whether the voltage of the low-voltage direct-current bus meets a first voltage condition or not by a control system of the post-stage step-up converter so as to determine control information of the post-stage step-up converter; a mode switching module: judging whether the voltage of the low-voltage direct-current bus meets a second voltage condition or not by a preceding-stage Boost converter control system so as to determine mode switching information of the preceding-stage Boost converter and control information of a starting resistance device; the first voltage condition is that the voltage of the low-voltage direct-current bus is greater than or equal to the lower limit value of the starting voltage before the post-stage step-up converter is started; the second voltage condition is that the voltage of the low-voltage direct-current bus accords with the normal operation voltage after the post-stage boost converter is started; the control information of the post-stage boost converter controls a main loop of the post-stage boost converter to control the starting behavior of a low-voltage direct-current bus voltage mode; the starting resistance device control information controls the switching behavior of the starting resistance device; the mode switching information of the preceding stage Boost converter controls the mode switching behavior of the preceding stage Boost converter; the pre-stage Boost converter mode comprises any one or more of the following modes: -a master control mode; -a slave control mode; -maximum power point tracking mode.
Preferably, the number of the preceding stage Boost converters is greater than or equal to one.
Preferably, the charging module includes:
configuring the only preceding stage Boost converter in the master control mode to adopt a voltage source working mode;
configuring the preceding stage Boost converter in the slave control mode to operate in a current source mode;
-a pre-stage Boost converter in voltage source mode to control the output voltage of the pre-stage Boost converter;
and a preceding stage Boost converter in a current source mode is adopted to control the output current of the preceding stage Boost converter.
Preferably, the mode switching module includes: switching a maximum power point tracking mode module: and when the voltage of the low-voltage direct-current bus accords with the normal operation voltage, cutting off the starting resistor, and switching the mode of the preceding stage Boost converter into the maximum power point tracking mode.
Preferably, the rear-stage boost converter has a unidirectional energy flow characteristic; the rear-stage boost converter is a rear-stage direct-current boost converter; the starting resistor device comprises: -one or more starting resistances; -one or more start-up resistor-throw switches.
Compared with the prior art, the invention has the following beneficial effects:
1. each photovoltaic array or group string is provided with a Boost converter, so that the structures of a front-stage MPPT converter and a rear-stage direct-current Boost converter can be simplified, and the topological structure design and the operation control of the converters are facilitated;
2. the low-voltage direct-current collection bus structure can effectively avoid the power adaptation phenomenon among all photovoltaic arrays, can enhance the reliability of the system and is convenient for internal fault isolation;
3. the photovoltaic array is controlled to charge the low-voltage bus without absorbing power from an external power grid and depending on the external power grid, so that the starting from a source to a grid is realized;
4. adding a starting resistor on a low-voltage direct-current side to realize effective control of low-voltage bus voltage in the starting process;
6. the coordination starting and the working mode switching between the front-stage converter and the rear-stage converter can be realized under the condition of not depending on the real-time communication in the station;
7. except for an MPPT mode adopted in normal operation, each preceding stage Boost converter is additionally provided with two initial starting working modes (a master control mode and a slave control mode), and the two initial starting working modes are mutually used as backups in starting;
8. the rear-stage direct-current boost converter always operates in a constant voltage mode without switching;
9. the system operation mode judgment under the condition of no real-time communication is realized by setting different voltage reference values for the front-stage converter and the rear-stage converter.
Drawings
Other features, objects and advantages of the invention will become more apparent upon reading of the detailed description of non-limiting embodiments with reference to the following drawings:
FIG. 1 is a schematic flow diagram of the present invention;
FIG. 2 is a block diagram of the present invention;
FIG. 3 is a schematic diagram of a hardware scheme based on an additional starting resistor on a low-voltage direct-current side;
FIG. 4 is a flowchart illustrating a master-slave synchronization start method according to an embodiment of the present invention;
FIG. 5 is a schematic diagram of the time variation of the low voltage DC side bus voltage in accordance with an embodiment of the present invention;
FIG. 6 is a schematic diagram of the output current from the control Boost converter as a function of time in an embodiment of the present invention;
FIG. 7 is a graph illustrating power of a photovoltaic array over time in accordance with an embodiment of the present invention;
fig. 8 is a schematic view of the overall architecture of the distributed photovoltaic dc collection system in the background art of the present invention;
fig. 9 is a schematic diagram of a distributed photovoltaic dc collection system with a diode having a unidirectional energy flow characteristic according to the background art of the present invention.
Detailed Description
The present invention will be described in detail with reference to specific examples. The following examples will assist those skilled in the art in further understanding the invention, but are not intended to limit the invention in any way. It should be noted that it would be obvious to those skilled in the art that various changes and modifications can be made without departing from the spirit of the invention. All falling within the scope of the present invention.
As shown in fig. 1, fig. 2, fig. 3, fig. 4, fig. 5, and fig. 6, the inter-stage cooperative starting method for the distributed photovoltaic dc collecting system according to the present invention includes: a low-voltage direct current side configuration step: -starting the pre-stage Boost converter control circuit while keeping the pre-stage Boost converter main circuit latched; -starting the post boost converter control circuit, monitoring the low voltage dc bus voltage, while keeping the post boost converter main circuit latched; -switching in one or more starting resistances; the preceding stage Boost converter cooperation step: -selecting a preceding Boost converter as the master control converter; -selecting all preceding stage Boost converters except the master control converter as slave control converters; here, one preceding stage Boost converter is selected as the only master control converter.
A charging step: -controlling the main control converter to charge the low voltage dc bus in a main control mode; -controlling the slave control converter to assume a slave control mode for charging the low voltage dc bus; and a control step of the post step-up converter: judging whether the voltage of the low-voltage direct-current bus meets a first voltage condition or not by a control system of the post-stage step-up converter so as to determine control information of the post-stage step-up converter; a mode switching step: judging whether the voltage of the low-voltage direct-current bus meets a second voltage condition or not by a preceding-stage Boost converter control system so as to determine mode switching information of the preceding-stage Boost converter and control information of a starting resistance device; the first voltage condition is that the voltage of the low-voltage direct-current bus is greater than or equal to the lower limit value of the starting voltage before the post-stage step-up converter is started; the second voltage condition is that the voltage of the low-voltage direct-current bus accords with the normal operation voltage after the post-stage boost converter is started; the control information of the post-stage boost converter controls a main loop of the post-stage boost converter to control the starting behavior of a low-voltage direct-current bus voltage mode; the starting resistance device control information controls the switching behavior of the starting resistance device; the mode switching information of the preceding stage Boost converter controls the mode switching behavior of the preceding stage Boost converter; the pre-stage Boost converter mode comprises any one or more of the following modes: -a master control mode; -a slave control mode; -maximum power point tracking mode.
The invention provides a starting control method of a photovoltaic direct-current boosting collecting system, which aims at the problem of coordinated starting and operation mode switching among multi-level converters generated by the unidirectional energy flow characteristic of the photovoltaic direct-current boosting collecting system in practical engineering application, is used for solving the problem of pre-charging of a collecting system module, and realizes quick, stable and effective starting of the system without depending on an external power grid. In order to realize the starting control of the direct current collection system, the invention adopts a method based on the addition of a starting resistor on a low-voltage direct current side, and adopts a master-slave synchronous starting strategy facing a preceding stage Boost converter and a corresponding inter-stage cooperation strategy on the basis of the method so as to meet different application scenes.
Preferably, the number of the preceding stage Boost converters is greater than or equal to one.
Preferably, the charging step includes: configuring the only preceding stage Boost converter in the master control mode to adopt a voltage source working mode; configuring the preceding stage Boost converter in the slave control mode to operate in a current source mode; -a pre-stage Boost converter in voltage source mode to control the output voltage of the pre-stage Boost converter; and a preceding stage Boost converter in a current source mode is adopted to control the output current of the preceding stage Boost converter.
Preferably, the mode switching step includes: switching a maximum power point tracking mode: and when the voltage of the low-voltage direct-current bus accords with the normal operation voltage, cutting off the starting resistor, and switching the mode of the preceding stage Boost converter into the maximum power point tracking mode.
Preferably, the rear-stage boost converter has a unidirectional energy flow characteristic; i.e. energy can only be transferred from the low-pressure side to the high-pressure side; the rear-stage boost converter is a rear-stage direct-current boost converter; the starting resistor device comprises: -one or more starting resistances; -one or more start-up resistor-throw switches.
The low-voltage direct current side is provided with a starting resistor. Each photovoltaic array is provided with a Boost converter, each Boost converter independently realizes MPPT and voltage lifting functions, and 820V unipolar low-voltage direct-current buses are formed in parallel at the output side. The direct current bus is boosted by the direct current boost converter to form a +/-35 kV bipolar direct current bus, so that the direct current bus is matched with the operation mode of an external direct current power distribution network, can be connected with the external direct current power distribution network and is convenient to grid, and can also be connected into a medium-voltage alternating current power distribution network through a DC-AC converter station.
An external direct-current power distribution network is assumed to be a strong power network, random fluctuation of photovoltaic output is calculated, and a step-by-step forward constant voltage control mode is adopted for the direct-current boosting and collecting system during normal operation. For each stage of circuitry, its power injection side is defined as the front end. In this mode, the front stage circuit presents current source characteristics to the rear stage, and the system presents current source characteristics to the external power grid. Specifically, the method comprises the following steps: each preceding stage Boost converter independently controls the output voltage of the photovoltaic array, and the maximum power tracking operation is independently realized; the post-stage DC boost converter controls the voltage of the low-voltage DC bus; the output voltage of the photovoltaic direct-current boosting and collecting system is given by the support of an external strong power grid.
In order to stably establish the voltage of the low-voltage direct-current bus, a starting resistor is arranged at the outlet of each pre-stage Boost converter, and as shown in fig. 3, the pre-charging and the voltage control of the low-voltage direct-current bus are realized. Wherein R is1-RnTo start upResistance, S1-SnFor starting the resistance-switching switches, Io1-IonThe output current of the preceding stage Boost converter. Actual-input starting resistor R 'at outlet of each preceding-stage Boost converter'iCan be expressed as:
the equivalent starting resistance is RstAnd is provided with
Rst＝R′1||R′2||…||R′n
Defining the starting voltage VstThe voltage value which needs to be stably established in the starting stage of the low-voltage direct current bus is the equivalent starting resistance RstThe power consumed is starting power PstAnd has:
when the system normally operates, the DC boost converter controls the voltage of the low-voltage DC bus to be Vnorm。
Each preceding stage Boost converter is provided with three working modes (a master control mode, a slave control mode and an MPPT mode), and the purpose is that if the master control Boost converter which is pre-designated in the master-slave synchronous starting cannot perform master control work due to faults and the like, the other Boost converters can be selected as master control converters, and the rest of the Boost converters are still used as slave control converters, namely all the preceding stage Boost converters are mutually backup during starting, so that the reliability of the system is improved.
In the master-slave synchronous starting, a front stage Boost converter is selected as a master control converter, and constant direct current voltage control is adopted to balance the power fluctuation of a system; other preceding stage Boost converters are used as slave control converters and are controlled by constant direct current or direct current power, and therefore the no-difference control of bus voltage can be achieved. But have many limitations and high requirements on the capacity and voltage regulation performance of the main control converter.
As shown in fig. 4, taking the case that the Boost converter 1 adopts the master control mode and the other Boost converters adopt the slave control mode, the method includes the following steps:
1) and controlling the photovoltaic array to charge the low-voltage direct-current bus, putting each starting resistor into the photovoltaic array, and locking the direct-current boost converter.
All the Boost converters are started simultaneously, starting resistors are input at the outlets of all the converters, the master control Boost converter is configured to be in a voltage source working mode, the slave control Boost converter is configured to be in a current source working mode, and the low-voltage direct-current bus is charged. Because each preceding stage of the Boost converter can work in a voltage source mode and a current source mode, the Boost converter working in the voltage source mode controls the converter to output voltage (namely low-voltage bus voltage), and the Boost converter working in the current source mode controls the converter to output current. Therefore, the Boost converter 1 is in a voltage source mode and controls the bus voltage VbusThe other Boost converters are in a current source mode and control the output current Io。
2) And acquiring the voltage of the low-voltage direct-current bus, and judging whether a first voltage condition is met and the set time is continued so as to start the rear-stage direct-current boost converter, wherein the first voltage condition is whether the voltage of the bus reaches the lower limit value of the starting voltage before the rear-stage direct-current boost converter is started.
After the low-voltage direct-current bus is basically charged, the rear-stage direct-current Boost converter needs to be started to control the bus voltage, the bus voltage is kept constant, and a foundation is provided for mode switching of the front-stage Boost converter. Therefore, the bus voltage reaches the starting voltage lower limit value (V)busAnd-delta V), starting the post-stage direct-current boost converter through the starting time delay delta t, and regulating the bus voltage.
3) And continuously acquiring the voltage of the low-voltage direct-current bus, judging whether a second voltage condition is met so as to cut off all starting resistors and switch the mode of the front-stage Boost converter, wherein the second voltage condition is whether the voltage of the bus reaches the normal operation voltage after the rear-stage direct-current Boost converter is started.
In the process of regulating the bus voltage by the rear-stage direct-current Boost converter, each front-stage Boost converter still worksIn a voltage source or current source mode, the output power is small, the generated power of the photovoltaic array is related to the port voltage, and if the Boost converter does not work in a maximum power tracking mode, the port voltage of the photovoltaic array cannot be controlled, and normal generation and transmission of power cannot be realized. Therefore, mode switching needs to be performed on the preceding stage Boost converter in time, port voltage of the photovoltaic array is controlled to achieve maximum power tracking control, and power transmission is performed until the photovoltaic direct-current Boost collecting system can be started. Thus, when the bus voltage reaches the normal operating voltage VnormAnd then, cutting off each starting resistor, and simultaneously converting all Boost converters into a maximum power tracking mode. And after the system is started, the system is converted into a normal operation state, and photovoltaic grid-connected output can be performed.
Taking the example that the Boost converter i adopts a master control mode and the other Boost converters adopt slave control modes, in order to balance the power consumed by the starting resistor and stably establish the starting voltage V for the low-voltage direct-current busstIn order to prevent the main control Boost converter from losing the direct-current voltage regulation capability due to reaching the power limit and prevent the low-voltage direct-current bus from being overcharged due to overlarge output current of the slave control Boost converter, the starting strategy needs to meet the following power and current constraints:
wherein P isi_maxIs the maximum output power of the ith photovoltaic array, Iref_jIs the output current reference value of the jth preceding stage Boost converter.
Specifically, in one embodiment, the photovoltaic dc concentration system startup architecture shown in fig. 3 employs 10 photovoltaic arrays as input sources, wherein each photovoltaic array comprises 15 parallel branches, each parallel branch comprises 11 photovoltaic cell modules connected in series, and the photovoltaic cell modules are of the type SunPower SPR-305E-WHT-D. The illumination intensity of all photovoltaic modules is 1000W/m2The temperature is 25 ℃, the maximum power of each photovoltaic array is 50kW, and the maximum power point voltage is 600V.
Low-voltage DC bus starting voltage Vst850V, 5V for lower limit deviation of starting voltage, 0.1s for starting delay, and low-voltage DC bus voltage V for normal operation of systemnorm820V, the voltage of the bipolar grid-connected side is +/-35 kV. The batch Start strategy employs a set Start Power, and Pst20kW, then equivalent starting resistance Rst36 Ω. In the master-slave synchronous starting, the front stage Boost converters 1-10 are started at the same time, and S is closed1～S10R is obtained by the formula1～R10360 omega, wherein Boost converter 1 adopts the master control mode, and Boost converter 2 ~ 10 adopts from the control mode, and its output reference current Io_ref＝2A。
In simulation, the low-voltage DC bus voltage V is obtained during startingbusThe change is as shown in fig. 5(a) and 5(b), it can be seen that before the DC/DC converter starts (0-1.27 s), the bus voltage slowly rises to about 850V, after the DC/DC converter starts, the bus voltage drops, reaches 820V at 1.30s, all the front-stage Boost converters are switched to the MPPT mode, the bus voltage is finally controlled at 820V, and the maximum fluctuation Δ V of the bus voltage in the process ismax＝74V。
Output current I from control Boost converteroAs shown in fig. 6, before entering the MPPT mode (0 to 1.3s), the output current of each slave control Boost converter is close to 2A.
Photovoltaic array output power P controlled by master control Boost convertermasterAnd the output power P of the photovoltaic array controlled by the slave control Boost converterslaveAnd total output power P of photovoltaic arraytotalAs shown in FIG. 7, it can be seen that the total output power of the photovoltaic array is close to 20kW before entering the MPPT mode (0-1.3 s). After entering the MPPT mode, the output power of each array is 50 kW.
The inter-stage cooperative starting method for the distributed photovoltaic direct current collecting system provided by the invention can be understood as an embodiment of the inter-stage cooperative starting system for the distributed photovoltaic direct current collecting system provided by the invention by those skilled in the art. Namely, the interstage cooperative starting system for the distributed photovoltaic direct current collecting system can be realized by executing the step flow of the interstage cooperative starting method for the distributed photovoltaic direct current collecting system.
As shown in fig. 2, the interstage cooperative starting system for the distributed photovoltaic direct current collecting system provided by the invention comprises: low voltage direct current side configuration module: -starting the pre-stage Boost converter control circuit while keeping the pre-stage Boost converter main circuit latched; -starting the post boost converter control circuit, monitoring the low voltage dc bus voltage, while keeping the post boost converter main circuit latched; -switching in one or more starting resistances; preceding stage Boost converter cooperation module: -selecting a preceding Boost converter as the master control converter; -selecting all preceding stage Boost converters except the master control converter as slave control converters; here, one preceding stage Boost converter is selected as the only master control converter.
A charging module: -controlling the main control converter to charge the low voltage dc bus in a main control mode; -controlling the slave control converter to assume a slave control mode for charging the low voltage dc bus; the post-stage step-up converter control module: judging whether the voltage of the low-voltage direct-current bus meets a first voltage condition or not by a control system of the post-stage step-up converter so as to determine control information of the post-stage step-up converter; a mode switching module: judging whether the voltage of the low-voltage direct-current bus meets a second voltage condition or not by a preceding-stage Boost converter control system so as to determine mode switching information of the preceding-stage Boost converter and control information of a starting resistance device; the first voltage condition is that the voltage of the low-voltage direct-current bus is greater than or equal to the lower limit value of the starting voltage before the post-stage step-up converter is started; the second voltage condition is that the voltage of the low-voltage direct-current bus accords with the normal operation voltage after the post-stage boost converter is started; the control information of the post-stage boost converter controls a main loop of the post-stage boost converter to control the starting behavior of a low-voltage direct-current bus voltage mode; the starting resistance device control information controls the switching behavior of the starting resistance device; the mode switching information of the preceding stage Boost converter controls the mode switching behavior of the preceding stage Boost converter; the pre-stage Boost converter mode comprises any one or more of the following modes: -a master control mode; -a slave control mode; -maximum power point tracking mode.
Preferably, the number of the preceding stage Boost converters is greater than or equal to one.
Preferably, the charging module includes:
configuring the only preceding stage Boost converter in the master control mode to adopt a voltage source working mode;
configuring the preceding stage Boost converter in the slave control mode to operate in a current source mode;
-a pre-stage Boost converter in voltage source mode to control the output voltage of the pre-stage Boost converter;
and a preceding stage Boost converter in a current source mode is adopted to control the output current of the preceding stage Boost converter.
Preferably, the mode switching module includes: switching a maximum power point tracking mode module: and when the voltage of the low-voltage direct-current bus accords with the normal operation voltage, cutting off the starting resistor, and switching the mode of the preceding stage Boost converter into the maximum power point tracking mode.
Preferably, the rear-stage boost converter has a unidirectional energy flow characteristic; the rear-stage boost converter is a rear-stage direct-current boost converter; the starting resistor device comprises: -one or more starting resistances; -one or more start-up resistor-throw switches.
Each photovoltaic array or group string is provided with a Boost converter, so that the structures of a front-stage MPPT converter and a rear-stage direct-current Boost converter can be simplified, and the topological structure design and the operation control of the converters are facilitated; the low-voltage direct-current collection bus structure can effectively avoid the power adaptation phenomenon among all photovoltaic arrays, can enhance the reliability of the system and is convenient for internal fault isolation; the photovoltaic array is controlled to charge the low-voltage bus without absorbing power from an external power grid and depending on the external power grid, so that the starting from a source to a grid is realized; adding a starting resistor on a low-voltage direct-current side to realize effective control of low-voltage bus voltage in the starting process; the coordination starting and the working mode switching between the front-stage converter and the rear-stage converter can be realized under the condition of not depending on the real-time communication in the station; except for an MPPT mode adopted in normal operation, each preceding stage Boost converter is additionally provided with two initial starting working modes (a master control mode and a slave control mode), and the two initial starting working modes are mutually used as backups in starting; the rear-stage direct-current boost converter always operates in a constant voltage mode without switching; the system operation mode judgment under the condition of no real-time communication is realized by setting different voltage reference values for the front-stage converter and the rear-stage converter.
Those skilled in the art will appreciate that, in addition to implementing the system and its various devices, modules, units provided by the present invention as pure computer readable program code, the system and its various devices, modules, units provided by the present invention can be fully implemented by logically programming method steps in the form of logic gates, switches, application specific integrated circuits, programmable logic controllers, embedded microcontrollers and the like. Therefore, the system and various devices, modules and units thereof provided by the invention can be regarded as a hardware component, and the devices, modules and units included in the system for realizing various functions can also be regarded as structures in the hardware component; means, modules, units for performing the various functions may also be regarded as structures within both software modules and hardware components for performing the method.
The foregoing description of specific embodiments of the present invention has been presented. It is to be understood that the present invention is not limited to the specific embodiments described above, and that various changes or modifications may be made by one skilled in the art within the scope of the appended claims without departing from the spirit of the invention. The embodiments and features of the embodiments of the present application may be combined with each other arbitrarily without conflict.
Claims (4)
1. An interstage cooperative starting method for a distributed photovoltaic direct current collection system is characterized by comprising the following steps:
a low-voltage direct current side configuration step:
-starting the pre-stage Boost converter control circuit while keeping the pre-stage Boost converter main circuit latched;
-starting the post boost converter control circuit, monitoring the low voltage dc bus voltage, while keeping the post boost converter main circuit latched;
-switching in one or more starting resistances;
the preceding stage Boost converter cooperation step:
-selecting a preceding Boost converter as the master control converter;
-selecting all preceding stage Boost converters except the master control converter as slave control converters;
a charging step:
-controlling the main control converter to charge the low voltage dc bus in a main control mode;
-controlling the slave control converter to assume a slave control mode for charging the low voltage dc bus;
and a control step of the post step-up converter: judging whether the voltage of the low-voltage direct-current bus meets a first voltage condition or not by a control system of the post-stage step-up converter so as to determine control information of the post-stage step-up converter;
the control information of the post-stage step-up converter controls a main loop of the post-stage step-up converter to control the voltage of the low-voltage direct-current bus;
a mode switching step: judging whether the voltage of the low-voltage direct-current bus meets a second voltage condition or not by a preceding-stage Boost converter control system so as to determine mode switching information of the preceding-stage Boost converter and control information of a starting resistance device;
the mode switching information of the preceding stage Boost converter controls the mode switching behavior of the preceding stage Boost converter;
the first voltage condition is that the voltage of the low-voltage direct-current bus is greater than or equal to the lower limit value of the starting voltage before the post-stage step-up converter is started;
the second voltage condition is that the voltage of the low-voltage direct-current bus accords with the normal operation voltage after the post-stage boost converter is started;
the starting resistance device control information controls the switching behavior of the starting resistance device;
a starting resistor is arranged at the outlet of each preceding stage Boost converter;
all Boost converters are started simultaneously, and starting resistors are input at the outlets of all the converters;
a mode switching step: continuously acquiring the voltage of the low-voltage direct-current bus, judging whether a second voltage condition is met so as to cut off each starting resistor and switch the mode of the preceding stage Boost converter;
after the bus voltage reaches a normal operation voltage Vnorm, cutting off each starting resistor, and simultaneously converting all Boost converters into a maximum power tracking mode;
the pre-stage Boost converter mode comprises any one or more of the following modes:
-a master control mode;
-a slave control mode;
-maximum power point tracking mode;
the charging step includes:
configuring the only preceding stage Boost converter in the master control mode to adopt a voltage source working mode;
configuring the preceding stage Boost converter in the slave control mode to operate in a current source mode;
-a pre-stage Boost converter operating in voltage source mode to control the output voltage of the pre-stage Boost converter;
and the preceding-stage Boost converter adopts a current source working mode and controls the output current of the preceding-stage Boost converter.
2. The interstage cooperative starting method for the distributed photovoltaic direct current collection system according to claim 1, wherein the post-stage step-up converter has a unidirectional energy flow characteristic;
the rear-stage boost converter is a rear-stage direct-current boost converter;
the starting resistor device comprises:
-a starting resistance;
-a starting resistance switch arranged in correspondence with the starting resistance;
the number of the starting resistor and the starting resistor switching switch is two.
3. An interstage cooperative starting system for a distributed photovoltaic direct current collection system, comprising:
low voltage direct current side configuration module:
-starting the pre-stage Boost converter control circuit while keeping the pre-stage Boost converter main circuit latched;
-starting the post boost converter control circuit, monitoring the low voltage dc bus voltage, while keeping the post boost converter main circuit latched;
-switching in one or more starting resistances;
preceding stage Boost converter cooperation module:
-selecting a preceding Boost converter as the master control converter;
-selecting all preceding stage Boost converters except the master control converter as slave control converters;
a charging module:
-controlling the main control converter to charge the low voltage dc bus in a main control mode;
-controlling the slave control converter to assume a slave control mode for charging the low voltage dc bus;
the post-stage step-up converter control module: judging whether the voltage of the low-voltage direct-current bus meets a first voltage condition or not by a control system of the post-stage step-up converter so as to determine control information of the post-stage step-up converter;
the control information of the post-stage step-up converter controls a main loop of the post-stage step-up converter to control the voltage of the low-voltage direct-current bus;
a mode switching module: judging whether the voltage of the low-voltage direct-current bus meets a second voltage condition or not by a preceding-stage Boost converter control system so as to determine mode switching information of the preceding-stage Boost converter and control information of a starting resistance device;
the mode switching information of the preceding stage Boost converter controls the mode switching behavior of the preceding stage Boost converter;
the first voltage condition is that the voltage of the low-voltage direct-current bus is greater than or equal to the lower limit value of the starting voltage before the post-stage step-up converter is started;
the second voltage condition is that the voltage of the low-voltage direct-current bus accords with the normal operation voltage after the post-stage boost converter is started;
the starting resistance device control information controls the switching behavior of the starting resistance device;
a starting resistor is arranged at the outlet of each preceding stage Boost converter;
all Boost converters are started simultaneously, and starting resistors are input at the outlets of all the converters;
a mode switching module: continuously acquiring the voltage of the low-voltage direct-current bus, judging whether a second voltage condition is met so as to cut off each starting resistor and switch the mode of the preceding stage Boost converter;
after the bus voltage reaches a normal operation voltage Vnorm, cutting off each starting resistor, and simultaneously converting all Boost converters into a maximum power tracking mode;
the pre-stage Boost converter mode comprises any one or more of the following modes:
-a master control mode;
-a slave control mode;
-maximum power point tracking mode;
the charging module includes:
configuring the only preceding stage Boost converter in the master control mode to adopt a voltage source working mode;
configuring the preceding stage Boost converter in the slave control mode to operate in a current source mode;
-a pre-stage Boost converter operating in voltage source mode to control the output voltage of the pre-stage Boost converter;
and the preceding-stage Boost converter adopts a current source working mode and controls the output current of the preceding-stage Boost converter.
4. The interstage cooperative starting system for the distributed photovoltaic direct current collection system according to claim 3, wherein the post-stage step-up converter has unidirectional energy flow characteristics;
the rear-stage boost converter is a rear-stage direct-current boost converter;
the starting resistor device comprises:
-a starting resistance;
-a starting resistance switch arranged in correspondence with the starting resistance;
the number of the starting resistor and the starting resistor switching switch is two.
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