WO2020133056A1

WO2020133056A1 - Central and distributed photovoltaic power plant and control system therefor

Info

Publication number: WO2020133056A1
Application number: PCT/CN2018/124328
Authority: WO
Inventors: Zhuoran LIU; Xiaobo Yang; Xing Huang
Original assignee: Abb Schweiz Ag
Priority date: 2018-12-27
Filing date: 2018-12-27
Publication date: 2020-07-02

Abstract

It is therefore an objective of the invention to provide a central and distributed photovoltaic power plant and its control system. The power plant incudes a plurality of DC to DC power conversion circuits, a DC to AC power conversion circuit, a DC bus arranged between outputs of the plurality of DC to DC power conversion circuits and input of the DC to AC power conversion circuit, a power meter being configured to measure power supplied by the DC to AC power conversion circuit, and a control system; the control system is configured to control the DC to AC power conversion circuit to change the DC bus voltage in a direction, selectively control the plurality of the DC to DC power conversion circuits such that the one with its PV string MPP voltage deviating from the DC bus voltage in a first direction performs MPPT of its PV string and the one with its PV string MPP voltage deviating from the DC bus voltage in a second direction opposite to the first direction suspends switching conducting current from its PV string to the DC bus, and monitor a trend of the power measurement from the power meter, and further suspend the change of the DC bus voltage in case that the trend changes. By using a solution according to present invention, DCOs can be determined based on the output power measurement to work to regulate PV string at MPPT or just be bypassed. So, a global optimal DC bus voltage can be achieved at the maximum energy yield of PV station. In this case, DC optimizers and inverter both do MPPT to maximize the energy harvesting, which is defined as dual MPPT here and employed.

Description

CENTRAL AND DISTRIBUTED PHOTOVOLTAIC POWER PLANT AND CONTROL SYSTEM THEREFOR

Technical Field

This invention relates generally to photovoltaic (PV) power plants, and more particularly, to a control system for coordinating the switching of central and distributed DC to DC power conversion circuits associated with PV modules to yield highly efficient photovoltaic power plants.

Background Art

The photovoltaic system is quite popular as a renewable source in many applications. Its PV module has the maximum power point (MPP) phenomenon, which means the PV module outputs the maximum power at a certain point that is not the end of the operation range. Moreover, the output power of the PV module can vary with the temperature and the irradiation. Figure 1A is a P-V curve of a PV module illustrating the MPP phenomenon. As show in figure 1A, an output power of PV module increases with an increase of the PV module output voltage in a direction towards the MPP in region A. In contrast, an output power of PV module decreases with an increase of the PV module output voltage in a direction away from the MPP in region B. Figure 1B schematically depicts different P-V curves of a PV module for various operational conditions. As shown in figure 1B, the location of MPP varies with the operational conditions of the solar panel, such as its temperature and the irradiation intensity. For this reason, photovoltaic systems typically comprise a control system that varies the match between the load and impedance of its converter circuit connected to the PV module in order to ensure a switching between modes of voltage source control and maximum power point track control. Figure 1A also indicate operating points, A and B, of a PV module, which operating points A, B, differ from the maximum power point (MPP) of the PV module. When tracking the MPP (MPPT) , voltage levels (such as A and B) that for the current state differs from the MPP are adjusted to match the MPP.

In recent years, central and distributed (C&D) PV station solution has been getting more attention due to its higher power generation yield. The key components inside the C&D PV station are DC optimizers. DC optimizer (DCO) is a DC to DC converter technology to realize maximum power point tracking (MPPT) of PV modules connected to the input of DC optimizer. The DC optimizer can be used for both PV module level (namely panel level DC optimizer) and PV string level. For both cases, there will be an input DC bus and an output DC bus formed at the input terminal and the out terminal of the DC optimizer. A C&D PV station with string DC optimizers is shown in figures 2A and 2B. Figure 2A illustrates a PV plant architecture that employs a string combiner distribution of dc-dc converters 22. Figure 2B illustrates a PV plant architecture that employs a string distribution of dc-dc converters 32. The DC bus voltage is regulated by central inverter and usually fixed at a specified value. The voltage difference between PV string input voltage and DC bus voltage determines if DC optimizers step up the input voltage or step down the input voltage.

Patent US 2018/0131321 A1 discloses a solution for arranging a central and distributed PV station with multiple photovoltaic components. DC side voltage of a photovoltaic inverter is fixed at MPP voltage of one of the DCOs. The other DCOs selectively operate in switching or by-passing while output power of the central inverter of the photovoltaic system is measured and monitored so as to arrive at the maximum power output under a certain configuration where some DCOs are by-passed and some are switching operating at MPPT. However, this solution applies a constraint where one of the DCOs is by-passed and its corresponding PV module supplies power at MPP. As compared with the same central and distributed PV station without such limit, it is likely that the central and distributed PV station does not operate at a maximum power point. In view of the foregoing, there is need for a solution operating distributed DC to DC converters associated with PV modules to yield highly efficient photovoltaic power plants that mitigate the effects of losses due to fixed clamping voltage at the outputs of the DCOs.

Brief Summary of the Invention

According an aspect of present invention, it provides a central and distributed photovoltaic power plant, including a plurality of DC to DC power conversion circuits, a DC to AC power conversion circuit, a DC bus arranged between outputs of the plurality of DC to DC power conversion circuits and input of the DC to AC power conversion circuit, a power meter being configured to measure power supplied by the DC to AC power conversion circuit, and a control system; the control system is configured to control the DC to AC power conversion circuit to change the DC bus voltage in a direction, selectively control the plurality of the DC to DC power conversion circuits such that the one with its PV string MPP voltage deviating from the DC bus voltage in a first direction performs MPPT of its PV string and the one with its PV string MPP voltage deviating from the DC bus voltage in a second direction opposite to the first direction suspends switching conducting current from its PV string to the DC bus, and monitor a trend of the power measurement from the power meter, and further suspend the change of the DC bus voltage in case that the trend changes.

According to another aspect of present invention, it provides control system for central and distributed photovoltaic power plant. The control system includes a first input being configured to receive a first input signal representing measurement of power supplied by a DC to AC power conversion circuit of the central and distributed photovoltaic power plant, a plurality of first outputs being configured to respectively send a plurality of first control signals to the respective DC to DC power conversion circuits, a second output being configured to send a second control signal to a DC to AC power conversion circuit of the central and distributed photovoltaic power plant, and a control unit system being configured to:control the DC to AC power conversion circuit via the second output to change the DC bus voltage in a direction, selectively control the plurality of the DC to DC power conversion circuits via the plurality of the first outputs such that the one with its PV string MPP voltage deviating from the DC bus voltage in a first direction performs MPPT of its PV string and the one with its PV string MPP voltage deviating from the DC bus voltage in a second direction opposite to the first direction suspends switching conducting current from its PV string to the DC bus, and monitor a trend of the power measurement via the first input from the power meter, and further suspend the change of the DC bus voltage in case that the trend changes.

For each DC to DC power conversion circuit (DC optimizer, DCO) , though input power from corresponding PV string is maximized at MPPT, the conversion losses during DCO regulation and inverter losses and DC bus losses may exceed the losses caused by mismatching and lead to an output power reduction. So, by using a solution according to present invention, DCOs can be determined based on the output power measurement to work to regulate PV string at MPPT or just be bypassed. The whole circuits losses are fully dependent on DC bus voltage when circuits are installed. So, a global optimal DC bus voltage can be achieved at the maximum energy yield of PV station. In this case, DC optimizers and inverter both do MPPT to maximize the energy harvesting, which is defined as dual MPPT here and employed.

Brief Description of the Drawings

The subject matter of the invention will be explained in more detail in the following text with reference to preferred exemplary embodiments which are illustrated in the drawings, in which:

Figure 1A illustrates a P-V curve of a PV module illustrating the MPP phenomenon;

Figure 1B schematically depicts different P-V curves of a PV module for various operational conditions;

Figures 2A and 2B illustrate conventional C&D PV stations with string DC optimizers;

Figure 3 shows a central and distributed PV power plant according to an embodiment of present invention;

Figure 4A illustrate a step-up DC to DC power conversion circuit according to an embodiment of present invention;

Figure 4B illustrate a step-up DC to DC power conversion circuit according to another embodiment of present invention;

Figure 5A illustrate a step-down DC to DC power conversion circuit according to an embodiment of present invention;

Figure 5B illustrate a step-down DC to DC power conversion circuit according to another embodiment of present invention;

Figure 6 illustrates the changes of the DC bus voltage when the system converges to a global maximum power yield according to an embodiment of present invention;

Figure 7 illustrates the three zones from breaking down a PV curve according to an embodiment of present invention;

Figure 8 illustrates PV curve of a DCO according to an embodiment of present invention; and

Figure 9 illustrates power limitation control according to an embodiment of present invention.

The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.

Preferred Embodiments of the Invention

In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular circuits, circuit components, interfaces, techniques, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods and programming procedures, devices, and circuits are omitted so not to obscure the description of the present invention with unnecessary detail.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. Note, the headings are for organizational purposes only and are not meant to be used to limit or interpret the description or claims. Furthermore, note that the word "may" is used throughout this application in a permissive sense (i.e., having the potential to, being able to) , not a mandatory sense (i.e., must) . " The term "include" , and derivations thereof, mean "including, but not limited to" . The term "connected" means "directly or indirectly connected" , and the term "coupled" means "directly or indirectly connected" .

The embodiments described herein provide solution searching for maximum power point of a central and distributed PV power plant by changing clamping voltage at output side of each of a plurality of DC to DC power conversion circuits (DCOs) while monitoring trend of measurement of output power of the central and distributed PV power plant, for example the power output from central DC to AC power conversion circuit (central inverter) . There is no constraint on fixed clamping voltage at output of the DC to DC power conversion circuit. Consequently, the embodiment provides more operational flexibility due to the availability of the plurality of DC to DC power conversion circuits that can be controlled to operate with output voltage clamped at various voltages.

Figure 3 shows a central and distributed PV power plant according to an embodiment of present invention. As shown in Figure 3, the PV power plant 1 includes a plurality of DC to DC

power conversion circuits

4a, 4b, 4c, 4d. A plurality of

PV strings

2a, 2b, 2c, 2d are connected to the respective DC to DC

power conversion circuits

4a, 4b, 4c, 4d, each consisting of K PV modules connected in serial. Each PV module consists of one unit comprising interconnected photovoltaic cells.

Each PV module is arranged to receive energy from sun light and transform the energy into electric DC energy. Each PV string should typically include many PV modules. By the interconnections of the PV modules into a string of PV modules, a first DC collection system is provided, each of these DC collection systems being referred to as a PV string.

The central and distributed PV power plant 1 also comprises a plurality of DC to DC

power conversion circuits

4a, 4b, 4c, 4d adapted for transforming an input DC power having a lower voltage into DC power having a higher voltage, especially a high voltage or medium high voltage, or as an alternative, transforming an input DC power having a higher voltage into DC power having a lower voltage. Each DC to DC power conversion circuit has a respective input connected to the output of a respective PV string, so that a respective interface between each DC to DC power conversion circuit and each PV string is provided. The outputs of the DC to DC power conversion circuits are connected in parallel to provide a DC bus 5. The total power outputted from the connected DC to DC

power conversion circuits

4a, 4b, 4c, 4d are provided to an input of a DC to AC conversion circuit 6. By the interconnection of the DC to DC power conversion circuits to a common output, a second DC collection system, is provided. The system includes arranging the DC to AC conversion circuit to receive the DC power from the second collection system by means of its DC input. Thus, each DC to DC power conversion circuit is connected to a respective PV string at its input, and the outputs of the DC to DC power conversion circuits are connected in parallel and provide a DC output connected to the input of the DC to AC conversion circuit, so that the second DC collection system provides an interface between the outputs of the DC to DC power conversion circuits and the DC to AC conversion circuit.

The output of the DC to AC conversion circuit provides AC power for subsequent transmission on an AC transmission system (not illustrated) , via a transformer, which transformer provides galvanic insulation between the power conversion and collection system and the AC transmission system.

The central and distributed PV power plant also includes a number of control units (9a-d, 10, 14) being arranged and adapted for controlling the functioning of the power collection and power conversion. The DC to AC conversion circuit is provided with a control unit 10, and each of the DC to DC power conversion circuits is provided with a respective control unit 9a-d. Each control unit 9a-d of the DC to DC power conversion circuits 4a-d is adapted for controlling the conversion of low voltage DC, from each respective string, into the medium voltage DC power that is collected by means of the serial interconnection of the DC bus 5 and fed to the DC to AC conversion circuit 6. The DC to AC conversion circuit 6 is provided with a control unit 10 adapted for converting medium voltage DC power into AC power of a high, or medium, voltage level in accordance with the AC transmission grid. To provide an efficient overall power conversion system, the power system also includes a central control unit 14, which is communicatively connected to each local control unit 9a-d of each DC to DC power conversion circuit 4a-d and the control unit 10 of the DC to AC conversion circuit 6.

It is possible to omit the central control unit 14, and adapt one or more of the control units 9a-d, 10 of the DC to DC power conversion circuits 4a-d or the DC to AC conversion circuit 6 to provide the central control functions. However, this is not illustrated in figure 3.

The central control unit 14 is adapted to obtain, or receive, operating information of each DC to DC power conversion circuit 4a-d and the DC to AC conversion circuit 6, which operating information is obtained at the respective inputs and outputs of the converters 4a-d and inverter 6. The Control unit 14 is adapted to use the measurements and operating information provided from the local control units 9a-d, 10 to perform an overall system control to obtain an overall power efficient operation of the whole power conversion system and the central and distributed PV power plant.

Each DC to DC

power conversion circuits

4a, 4b, 4c, 4d has a voltage sensor VSI ₁, VSI ₂, VSI ₃, VSI ₄ and a current sensor CSI ₁, CSI ₂, CSI ₃, CSI ₄ at the input terminals to measure the corresponding PV string voltage and current and then calculate PV output power and also a voltage sensor VSO ₁, VSO ₂, VSO ₃, VSO ₄ at the output terminals to obtain the DC bus voltage. All the measurement data from sensors are sent to the respective second control units 9a-d, and each of has a digital control unit to collect all this measurement results and send PWM driving signals to semiconductors to do switching and regulation. DC bus 5 connects the output terminals of DC to DC

power conversion circuits

4a, 4b, 4c, 4d with DC input port of the DC to AC power conversion circuit 6 (central inverter) . A DC bus voltage sensor VSD is installed on the DC input port of central inverter 6 and the measured DC voltage is received by the central control unit 14. A power meter 8 is installed on the output cable of central inverter. The output cable is connected with an isolated step-up transformer and the output power is transmitted to a medium-voltage AC Bus via the transformer. The voltage and current of three phases are measured by the sensors and sent to central inverter control unit. Based on the data from, the central control unit 14 calculates the PWM duty ratio for each transistor and send the driving signals to the central inverter. Thus, the AC voltage sensors and AC current sensors may be used for power meter 8 for measuring power supplied by the DC to AC power conversion circuit 6.

The central control functions will be described in more detail with reference to Figures 4, and a central controller 14 is suggested for adapting the conversion system to function in accordance with the methods of the exemplifying embodiments of the invention. Figure 5 suggests implementing the central control functions in the control unit 10 of the DC to AC conversion circuit 6.

For each DCO, though input power from corresponding PV string is maximized at MPPT, the conversion losses during DCO regulation and inverter losses and DC bus losses may exceed the losses caused by mismatching and lead to an output power reduction. So, DCOs should be determined based on the output power measurement to work to regulate PV string at MPPT or just be bypassed. The whole circuits losses are fully dependent on DC bus voltage when circuits are installed. So, there is an expected global optimal DC bus voltage to achieve the maximum energy yield of PV station. In this case, DC optimizers and inverter both do MPPT to maximize the energy harvesting, which is defined as dual MPPT here and employed.

Returning to figure 1A operating point A illustrates a situation or state where the voltage is below the voltage of the MPP. Operating point B, on the other hand, illustrates a situation where the voltage level is above the voltage level of the MPP.

The conversion system is adapted to monitor the power output from the DC to AC conversion circuit 6 while adjusting the voltage of the DC bus 5. The adjustment is determined by the type of the DC to DC power conversion circuit 4a-d, which will be described with examples as below.

Example I

Each of the plurality of DC to DC power conversion circuits 4a-d uses a step-up topology, such like boost DC to DC converter.

Under the control of the central control unit 14, the DC bus voltage V _out is regulated to be V _out-1 (V _out-1 > MPP voltage of any of the

PV strings

2a, 2b, 2c, 2d) and then every DC to DC

power conversion circuits

4a, 4b, 4c, 4d can regulate its

PV string strings

2a, 2b, 2c, 2d voltage to respective MPP voltages V _MPP1, V _MPP2, V _MPP3, V _MPP4 by using technology of MPPT. In this embodiment, the local control units 9a-d control their respective DC to DC

power conversion circuits

4a, 4b, 4c, 4d to track the maximum power point of their

respective PV strings

2a, 2b, 2c, 2d, based upon their respective MPP voltages V _MPP1, V _MPP2, V _MPP3, V _MPP4 and the respective output voltage measurements of

PV strings

2a, 2b, 2c, 2d. Such measurements are provided from voltage sensor VSI ₁, VSI ₂, …VSI ₄. In this example, it is assumed that V _MPP1≤V _MPP2≤V _MPP3≤V _MPP4.

Next, under the control of the central control unit 14, the DC to AC conversion circuit 6 (central inverter) gradually decreases the DC bus voltage V _out. During the regulation, the output power on AC side of central inverter P _out keeps being sensed by the power meter 8. The DC bus voltage V _out is adjusted by the central inverter to vary between V _MPP1 and V _MPP4. The central control unit 14 selectively controls the DC to DC

power conversion circuits

4a, 4b, 4c, 4d such that the one with its PV string MPP voltage deviating from the DC voltage in a first direction performs MPPT of its PV string and the one with its PV string MPP voltage deviating from the DC voltage in a second direction opposite to the first direction suspends switching conducting current from its PV string to the DC bus. The different operating states of these converters are summarized in table I as below. Generally, each DC to DC

power conversion circuits

4a, 4b, 4c, 4d may operate in either of the two states:

State I:

Where the DC bus voltage V _out is below the MPP voltage of a PV string (the DC bus voltage deviating from the MPP voltage from a first direction) , suspending switching operation and making power direct flow from its PV string to DC bus. Its PV string output voltage V _in is clamped to DC bus voltage V _out (when voltage drop across diodes is neglected) . For example in case of a boost converter in Figure 4A, its switch Q ₁ keeps open and power is fed to the DC bus 5 via the diodeD ₁ and inductorL ₁ from the PV string. Another example in case of a partial power converter in Figure 4B. When the DC bus voltage V _out is below the MPP voltage of a PV string, the switches Q ₁, Q ₂, Q ₃, Q ₄ on the primary side of the isolated DC/DC converter keep open and power is fed to the DC bus 5 via the diodes D ₁, D ₂, D ₃, D ₄ and inductorL ₁ on the secondary sides from the PV string.

State II:

Where the DC bus voltage V _out is above the MPP voltage of a PV string (the DC bus voltage deviating from the MPP voltage from a second direction opposite to the first direction) , keeping switching to conduct maximum power point tracking of its PV string.

Table I

The central control unit 14 keeps monitoring a trend of the power measurement, and further suspend the change of the DC bus voltage in case that the trend changes. Thus, the system converges to a global maximum power yield as shown in figure 6.

Each of the plurality of DC to DC power conversion circuits 4a-d uses a step-down topology, such like buck DC to DC converter

Under the control of the central control unit 14, the DC bus voltage V _out is regulated to be V _out-1 (V _out-1 < MPP voltage of any of the

PV strings

2a, 2b, 2c, 2d) and then every DC to DC

power conversion circuits

4a, 4b, 4c, 4d can regulate its

PV string strings

power conversion circuits

4a, 4b, 4c, 4d to track the maximum power point of their

respective PV strings

PV strings

Next, under the control of the central control unit 14, the DC to AC conversion circuit 6 (central inverter) gradually increases the DC bus voltage V _out. During the regulation, the output power on AC side of central inverter P _out keeps being sensed by the power meter 8. The DC bus voltage V _out is adjusted by the central inverter to vary between V _MPP1 and V _MPP4. The central control unit 14 selectively controls the DC to DC

power conversion circuits

4a, 4b, 4c, 4d may operate in either of the two states:

State I:

Where the DC bus voltage V _out is above the MPP voltage of a PV string (the DC bus voltage deviating from the MPP voltage from a first direction) , suspending switching operation and making power direct flow from its PV string to DC bus. Its PV string output voltage is clamped to DC bus voltage V _out (when voltage drop across diodes is neglected) . For example in case of a buck converter in Figure 5A, its switch Q ₁ keeps closed and power is fed to the DC bus 5 via the inductor L ₁ from the PV string, while the diode D ₁ is reverse-biased. Another example in case of a partial power converter in Figure 5B. When the DC bus voltage V _out is above the MPP voltage of a PV string, the switches Q ₅, Q ₆, Q ₇, Q ₈ on the secondary side of the isolated DC/DC converter keep closed while the switches Q ₁, Q ₂, Q ₃, Q ₄ on the primary side of the isolated DC/DC converter keep open and power is fed to the DC bus 5 via the inductorL ₁ from the PV string.

State II:

Where the DC bus voltage V _out is below the MPP voltage of a PV string (the DC bus voltage deviating from the MPP voltage from a second direction opposite to the first direction) , keeping switching to conduct maximum power point tracking of its PV string.

Table II

The central control unit 14 keeps monitoring a trend of the power measurement, and further suspend or reverse the change of the DC voltage in case that the trend changes. Thus, the system converges to a global maximum power yield as shown in figure 6.

Telecommunication between the central inverter and the DC to DC power conversion circuits

In the embodiment of present invention, it doesn’t require telecommunication between the central inverter and the DC optimizers. The central inverter is implemented by the DC to AC power conversion circuit 6 and the DC optimizers are implemented by the DC to DC power conversion circuits 4a-d. It is an autonomous control method in which the interaction between DC optimizers and central inverter is realized via the amplitude of DC bus voltage.

To realize the proposed power control method, firstly, the P-V characteristic curve of DC optimizer will be modified, which will be used for the coordination with power controller in the central inverter. Secondly, a new power controller is implemented at the central inverter side to regulate the power out of PV station.

1) The modified P-V curve of string DC optimizer (DCO)

During normal operation, the DCO will always operate under MPPT mode. Therefore the output power vs. output voltage characteristic curves (P-V curves) of every DCOs under certain condition can be plotted as shown in figure 6. The operation range of the output voltage is determined by the global MPPT voltage of the central inverter. In this example, the minimum and maximum DC voltages are denoted as V _min and V _max respectively. If the power losses of the DCOs are considered, the P-V curves are a cluster of parallel curved lines with maximum power points. The specific value of each P-V curve is determined by the operation condition of respective PV string connected to the DCO (irradiation, shading, installation angles and so forth) as well as the DCO’s efficiency.

In order to realize the power limitation control, the P-V curves of DC optimizers are modified. Each PV curve is segmented in to three zones, as shown in Figure 7.

Zone 1: A narrow voltage range between V _min and V _monL is defined for the start-up of the PV station system, in Which V _nomL is the lowest voltage limitation for MPPT operation. The DC bus voltage V _out is set up by the central inverter at the beginning of the PV station energization. When the DC bus voltage increases to V _min, All the DC optimizers begin to to operate. Further, when the DC bus voltage increases above V _nomL, DC optimizers turn to MPPT mode. In Zone 1, the power output of each DC optimizer will vary from 0 to its own P _mppt Continuously when DC bus voltage increases from V _min to V _nomL.

Zone 2: Zone 2 is voltage range for global MPPT operation. All the DC optimizers will operate under MPPT mode within Zone 2. At the same time, central inverter will control the DC bus voltage to a certain value based on the control method illustrated above Optionally, when the variable DC bus voltage control is employed, the central inverter can operate at global MPPT mode, by which the DC bus voltage level will be adjusted to an optimal value thus the maximum generation yield of the whole PV station is realized.

Zone 3: Zone 3 range from V _nomH to V _max, in which V _nomH is the highest voltage limitation for MPPT operation. When DC bus voltage is greater than V _nomH, all the DC optimizers start to decrease the power output. In Zone 3, the power output of each DC optimizer will vary from its own P _mppt to 0 continuously when DC bus voltage increases from V _nomH to V _max.

2) Power limitation control at central inverter

The power limitation control is realized by central inverter.

During normal operation, the DC bus voltage value V _out is controlled at a certain value between V _nomL and V _nomH.. When the power limitation order P _lim is received by the central inverter, the DC bus voltage controller of the central inverter will compare the power generation of central inverter P _gen with P _lim, if P _lim <P _gen, the DC bus voltage is ordered to increase higher than V _nomH. Once the DC bus voltage value is greater than V _nomH, all the DC optimizers will start to decrease the power output, as explained in 1) via the modified P-V curves. With proper feedback control of P _gen, the DC bus voltage will be stabilized at certain value between V _nomH and V _max. The power limitation control can be illustrated by Figure 8, in which V _A is the DC bus voltage for nominal MPPT operation. When the power limitation P _lim is ordered, the DC bus voltage is regulated from V _A to V _B. The sum of the power output of all DCOs are decreased from

to

and finally

By integrating the solution into the embodiment according to example I, the second control units 9a-d each are configured to control corresponding one of the DC to DC power conversion circuits 4a-d such that the DC to DC power conversion circuit stops performing the maximum power pint tracking at a DC bus voltage above the first initial value.

By integrating the solution into the embodiment according to example II, the second control units 9a-d each are configured to control corresponding one of the DC to DC power conversion circuits 4a-d such that the DC to DC power conversion circuit stops performing the maximum power pint tracking at a DC bus voltage below the first initial value.

Though the present invention has been described on the basis of some preferred embodiments, those skilled in the art should appreciate that those embodiments should by no way limit the scope of the present invention. Without departing from the spirit and concept of the present invention, any variations and modifications to the embodiments should be within the apprehension of those with ordinary knowledge and skills in the art, and therefore fall in the scope of the present invention which is defined by the accompanied claims.

Claims

A central and distributed photovoltaic power plant, including:

a plurality of DC to DC power conversion circuits;

a DC to AC power conversion circuit;

a DC bus arranged between outputs of the plurality of DC to DC power conversion circuits and input of the DC to AC power conversion circuit;

a power meter being configured to measure power supplied by the DC to AC power conversion circuit; and

a control system being configured to:

control the DC to AC power conversion circuit to change the DC bus voltage in a direction;

selectively control the plurality of the DC to DC power conversion circuits such that the one with its PV string MPP voltage deviating from the DC bus voltage in a first direction performs MPPT of its PV string and the one with its PV string MPP voltage deviating from the DC bus voltage in a second direction opposite to the first direction suspends switching conducting current from its PV string to the DC bus; and

monitor a trend of the power measurement from the power meter, and further suspend the change of the DC bus voltage in case that the trend changes.
The central and distributed photovoltaic power plant according to claim 1, wherein:

the respective outputs of the plurality of DC to DC power conversion circuits are electrically coupled in parallel.
The central and distributed photovoltaic power plant according to claim 1 or 2, wherein: each of the plurality of DC to DC power conversion circuits uses a step-up topology.
The central and distributed photovoltaic power plant according to claim 3, wherein:

the DC bus voltage is changed from a first initial value which is above maximum power point voltage of each of the plurality of PV strings.
The central and distributed photovoltaic power plant according to claim 1 or 2, wherein:

each of the plurality of DC to DC power conversion circuits uses a step-down topology.
The central and distributed photovoltaic power plant according to claim 5, wherein:

the DC bus voltage is changed from a second initial value which is below maximum power point voltage of each of the plurality of PV strings.
The central and distributed photovoltaic power plant according to claim 4, wherein:

the control system includes:

a first control unit being configured to control the DC to AC power conversion circuit; and

a plurality of second control units each being configured to control corresponding one of the plurality of the DC to DC power conversion circuits such that the DC to DC power conversion circuit stops performing the maximum power pint tracking at a DC bus voltage above the first initial value.
The central and distributed photovoltaic power plant according to claim 6, wherein:

the control system includes:

a first control unit being configured to control the DC to AC power conversion circuit; and

a plurality of second control units each being configured to control corresponding one of the plurality of the DC to DC power conversion circuits such that the DC to DC power conversion circuit stops performing the maximum power point tracking at a DC bus voltage below the second initial value.
A control system for central and distributed photovoltaic power plant, including:

a first input being configured to receive a first input signal representing measurement of power supplied by a DC to AC power conversion circuit of the central and distributed photovoltaic power plant;

a plurality of first outputs being configured to respectively send a plurality of first control signals to the respective DC to DC power conversion circuits;

a second output being configured to send a second control signal to a DC to AC power conversion circuit of the central and distributed photovoltaic power plant; and

a control unit system being configured to:

control the DC to AC power conversion circuit via the second output to change the DC bus voltage in a direction;

selectively control the plurality of the DC to DC power conversion circuits via the plurality of the first outputs such that the one with its PV string MPP voltage deviating from the DC bus voltage in a first direction performs MPPT of its PV string and the one with its PV string MPP voltage deviating from the DC bus voltage in a second direction opposite to the first direction suspends switching conducting current from its PV string to the DC bus; and

monitor a trend of the power measurement via the first input from the power meter, and further suspend the change of the DC bus voltage in case that the trend changes.
The control system according to claim 9, wherein:

the control unit system includes:

a first control unit being configured to control the DC to AC power conversion circuit; and

a plurality of second control units each being configured to control corresponding one of the plurality of the DC to DC power conversion circuits such that the DC to DC power conversion circuit stops performing the maximum power pint tracking at a DC bus voltage above the first initial value.
The control system according to claim 9, wherein:

the control unit system includes:

a first control unit being configured to control the DC to AC power conversion circuit; and

a plurality of second control units each being configured to control corresponding one of the plurality of the DC to DC power conversion circuits such that the DC to DC power conversion circuit stops performing the maximum power point tracking at a DC bus voltage below the second initial value.