WO2020146999A1

WO2020146999A1 - Pv power converter and control method and pv power plant using the same

Info

Publication number: WO2020146999A1
Application number: PCT/CN2019/071743
Authority: WO
Inventors: Xing Huang; Xiaobo Yang; Kai Liu; Hailian XIE
Original assignee: Abb Schweiz Ag
Priority date: 2019-01-15
Filing date: 2019-01-15
Publication date: 2020-07-23

Abstract

A PV power converter (4a, 4b, 4c, 4d), its control method and a PV power plant, the PV power converter includes: output terminals, input terminals being configured to be electrically coupled to output ends of PV array (2a, 2b, 2c, 2d), a power conversion circuit being configured to convert power supplied from the PV array (2a, 2b, 2c, 2d), and output the converted power at the output terminals, having at least one power switch being electrically inserted between the input terminals for bypassing a flow of the converted power around the output terminals, and a local controller (9a, 9b, 9c, 9d) being configured to close the power switch in response to an occurrence of a fault external to the PV power converter (4a, 4b, 4c, 4d) and keep the power switch closed for a predetermined time interval longer than a switching period of the power conversion circuit. This is helpful for decreasing the DC short circuit value while not relying on the conventional fault clear devices, such like fuses and circuit breaker. Consequently, this makes the system cheaper by not using the sorts of fault protection devices.

Description

PV POWER CONVERTER AND CONTROL METHOD AND PV POWER PLANT USING THE SAME

Technical Field

This invention relates generally to PV (photovoltaic) power conversion, and more particularly, to protection of a fault occurring to the PV power conversion device.

Background Art

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.

The key components inside a 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.

Conventionally, DC optimizer relies on fuses and circuit breakers for protection against the hazards arising from overcurrent, fault, and arcing. For example, it is disclosed in Patent Application CN 107 681 644 A that a PV station protection solution uses DC switches to cut-off DC short circuit current on locating the DC fault.

However, as DC protection should be rapid due to the fast growth of fault currents, the design of cost-effective high speed DC switches with better arc extinction capability are challenging.

Brief Summary of the Invention

According an aspect of present invention, it provides a PV power converter, including: output terminals, input terminals being configured to be electrically coupled to output ends of PV array, a power conversion circuit being configured to convert power supplied from the PV array and output the converted power at the output terminals, having at least one power switch being electrically inserted between the input terminals for bypassing a flow of the converted power around the output terminals, and a local controller being configured to close the power switch in response to an occurrence of a fault external to the PV power converter and keep the power switch closed for a predetermined time interval longer than a switching period of the power conversion circuit.

According to another aspect of present invention, it provides a PV power plant, including a first group of the PV power converters, a central power converter, a DC bus arranged at input side of the central power converter, a multiple of DC cables being arranged between output side of the respective PV power converters and the DC bus, and a plurality of DC switches, being inserted between two ends of the respective DC cables.

According to another aspect of present invention, it provides a method for controlling the PV power converter, including: monitoring if the fault occurs, and closing the controllable power switch in response to the occurrence of the fault.

Conventional power generation sources typically employ fuses or circuit breakers to protect against potential hazards arising from overcurrent and faults. When a fault occurs, conventional protection means typically rely on the power generation source, or a stored energy supply, for the current needed to open or clear a protection device. The maximum short circuit current of PV panel is almost the same as its normal operation current. In response to a fault occurring external to a PV power converter, for example a short circuit happens to its corresponding DC cable, rather than by opening the short circuit with fuses or circuit breakers, by turning on a power switch in the PV power converter, its output power flow can bypass around its power output side. In other words, the turned-on power switch can confine the PV array output power within a closed loop involving a part of the PV power converter and the PV array itself. This is helpful for decreasing the DC short circuit value while not relying on the conventional fault clear devices, such like fuses and circuit breaker. Consequently, this makes the system cheaper by not using the sorts of fault protection devices.

Preferably, the current rating of the power switch is selected above short-circuit current of the PV array. Therefore, the bypass operation will not damage the PV array and the power switch instantaneously, allowing plenty of time for clearing the fault.

Preferably, the power conversion circuit re-uses the at least one controllable power semiconductor switch. For example, where the power conversion circuit adopts Boost topology, its controllable power switch is used both in the switching-mode and the bypassing mode. The controller is further configured to control the at least one controllable power semiconductor switch to operate in a switching-mode such that the power conversion circuit operates in conversion of the PV array power without the occurrence of the fault, and when the fault occurs, the at least one power semiconductor forms a bypass circuit to power conversion circuit.

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;

Figure 2 illustrates a PV power plant according to an embodiment of present invention;

Figure 3 illustrates V-I characteristics of a PV panel; and

Figures 4A, 4B and 4C each shows topology of the PV power converter 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" .

Figure 2 illustrates a PV power plant according to an embodiment of present invention. As shown in figure 1, the PV power plant 1 comprises a plurality of PV (photovoltaic)

arrays

2a, 2b, 2c, 2d, wherein each PV array 2a-d comprises a plurality of interconnected PV panels. Each PV panel consists of one unit comprising interconnected photovoltaic cells. Each PV panel is arranged to receive energy from sun light and transform the energy into electric DC energy. Each PV array should typically include many PV panels, for example hundreds or more than one thousand panels to produce DC power. For clarity purposes, the figure do not illustrate the PV panels in each PV array 2a-d, but the number of PV panels in each PV array 2a-d may be between 3000 to 10000 panels, for example 8000 PV panels in 320 parallel lines with 25 PV panels serially connected in each line. The PV panels of each PV array 2a-d are arranged in series and in parallel to produce the electric DC power at an output (3a-d) . By the interconnections of the PV panels into an array of PV panels, a first set of DC collection systems are provided, each of these DC collection systems being referred to as a PV array 2a-d.

The PV power plant 1 also comprises a plurality of

PV power converters

4a, 4b, 4c, 4d adapted for converting power supplied from the

PV arrays

2a, 2b, 2c, 2d. The skilled person should understand that by switching its power semiconductor switches, each of the

PV power converters

4a, 4b, 4c, 4d can operate behaving like a DC optimizer to realize maximum power point tracking of the

respective PV arrays

2a, 2b, 2c, 2d connected thereto.

Each PV power converter 4a-d has a respective input 7a-d connected to the output 3a-d of a respective PV array 2a-d, so that a respective interface between each PV power converter and each PV array is provided. The outputs of the PV power converters 4a-d are connected in parallel to provide a DC bus 5 via respective DC cables 8a-d. The total power outputted from the connected PV power converters 4a-d are provided to an input of a central power converter 6. The system includes arranging the central power converter 6 to receive the DC power from the DC bus 5 by means of its DC input. Thus, each PV power converter 4a-d is connected to a respective PV array 2a-d at its input, and the outputs of the PV power converters 4a-d are connected in parallel and provide a DC output connected to the input of the central power converter 6, so that the DC cables 8a-d and the DC bus 5 provide an interface between the outputs of the PV power converters 4a-d and the central power converter 6.

The output of the central power converter 6 provides AC power for subsequent transmission on an AC transmission system (not illustrated) , via a transformer 12, which transformer 12 provides galvanic insulation between the power conversion and collection system and the AC transmission system.

Thus, the PV power plant collects solar energy and converts the produced electrical DC power from the PV arrays to an AC power at the output of the central power converter 6 by means of a DC power conversion and collection system and the central power converter 6. This DC power conversion system comprises a set of PV power converters 4a-d with a DC input, and a DC voltage arrangement in the form of the DC bus 5 and the DC cables 8a-d arranged for feeding DC power to the central power converter 6. Thus, PV voltages are converted to DC cable voltage and DC bus voltage, which intern is converted into AC voltage suppled to the network.

The PV power plant 1 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 central power converter 6 is provided with a controller 10, and each of the PV power converters 4a-d is provided with a respective controller 9a-d. Each controller 9a-d of the PV power converters 4a-d is adapted for controlling the conversion of low voltage DC, from each respective array, into the medium voltage DC power that is collected by means of the serial interconnection of the DC bus 5 and fed to the central power converter 6. The central power converter 6 is provided with a local controller 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 controller 14, which is communicatively connected to each "local" controller 9a-d of each PV power converter 4a-d and the controller 10 of the central power converter 6.

It is possible to omit the central controller 14, and adapt one or more of the controllers 9a-d, 10 of the PV power converters 4a-d or the central power converter 6 to provide the central control functions. The central controller 14 is adapted to obtain, or receive, operating information of each PV power converter 4a-d and the central power converter 6, which operating information is obtained at the respective inputs and outputs of the converters 4a-d and inverter 6. The central controller 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 PV power plant.

Each of the

PV power converters

4a, 4b, 4c, 4d has

input terminals

7a, 7b, 7c, 7d and a current sensor CSI _a, CSI _b, CSI _c, CSI _d. The

input terminals

7a, 7b, 7c, 7d are configured to be electrically coupled to the output ends 3a, 3b, 3c, 3d of the

respective PV arrays

2a, 2b, 2c, 2d. Thus, electrical power generated from the PV array may be fed to the PV power converter. The current sensors CSI _a, CSI _b, CSI _c, CSI _d are electrically coupled on the respective DC cables 8a, 8b, 8c, 8d, for example being disposed close to its end at the side of the DC bus 5. The current sensor, for example, can use a hall sensor. The current sensors CSI _a, CSI _b, CSI _c, CSI _d are thus configured to measure electrical signals on the respective DC cables 8a, 8b, 8c, 8d, for example the current values.

Preferably, the PV power plant 1 includes a plurality of current sensors CSIO _a, CSIO _b, CSIO _c, CSIO _d arranged at the power output sides of the respective

PV power converters

4a, 4b, 4c, 4d, and the converted power flows from the output side to the central converter 6 via the DC cable 8a, 8b, 8c, 8d and the DC bus 5.

Preferably, a plurality of DC switches 11a-d are inserted in the respective DC cables 8a-d. The current sensors CSI _a, CSI _b, CSI _c, CSI _d, the DC switches 11a-d, the DC bus 5, and part of the DC cables 8a-d are disposed in a DC distribution box.

Figure 3 illustrates V-I characteristics of a PV panel. PV panels have V-I characteristics very different from conventional sources of voltage generation or electrical supply such as generators or batteries. Conventional power generation sources typically employ fuses or circuit breakers to protect against potential hazards arising from overcurrent and faults. When a fault occurs, conventional protection means typically rely on the power generation source, or a stored energy supply, for the current needed to open or clear a protection device. As shown in figure 3, the maximum short circuit current (I _{sc_max}) of PV panel is almost the same as its normal operation current. In response to a fault occurring external to a PV power converter, for example a short circuit happens to its corresponding DC cable, rather than by opening the short circuit with fuses or circuit breakers, by turning on a power switch in the PV power converter, its output power flow can bypass its power output side. In other words, the turned-on power switch can confine the PV array output power within a closed loop involving a part of the PV power converter and the PV array itself. This is helpful for decreasing the DC short circuit value while not relying on the conventional fault clear devices, such like fuses and circuit breaker. Consequently, this makes the system cheaper by not using the sorts of fault protection devices.

Preferably, current rating of the power switch is selected above short-circuit current of the PV array. Therefore, the bypass operation will not damage the PV array and the power switch instantaneously, allowing plenty of time for clearing the fault.

As an alternative, the PV power plant may include a first group of PV power converters each converting DC power from the respective PV array into an AC power and an AC bus being arranged to electrically coupled to the output terminals of the PV power converters and a primary winding of a transformer.

Figures 4A, 4B and 4C each shows topology of the PV power converter according to an embodiment of present invention. As shown in figures 4A, 4B and 4C, a power switch electrically inserted between the input terminals is common to the exemplified embodiments of a PV power converter.

As shown in figure 4A, each of the PV power converters 4a-d uses a power conversion circuit having a topology of Boost converter. It includes a power diode D ₁, a controllable power switch Q ₁, an inductorL ₁ and a capacitor C ₁, input terminals 40 and output terminals 41. The input terminals 40 are configured to be electrically coupled to output ends of the respective PV arrays 2a-d. The controllable power switch Q ₁ can use a power semiconductor switch, such like IGBT. The controllable power switch Q ₁ is electrically inserted between the input terminals 40. In this example, the PV panel, the power switch Q ₁ and inductor L ₁ can form a closed loop when the power switch is turned on and keeps ON state in response to an occurrence of fault, and this results in an increase of the inductor current (shown in arrow) until it reaches the maximum short circuit current (I _{sc_max}) of PV panel. This requires keeping the controllable power switch closed longer than at least one switching period of the power conversion circuit 4a-d. Switching period is equal to the inverse of the switching frequency as of one of the parameters of a power converter. It refers to a sum of time of ON state and that of Off during a switching cycle. The current is circulating through the PV panel, the inductor and the power switch, without flowing out of the PV power converter. In other words, the flow of the power converted and provided by the power conversion circuit can be bypassed around the output terminals 41 of the PV power converter. Thus, the power flow can be isolated from the fault point in the PV power plant. Besides, the skilled person shall understand that during normal operation, the PV power converter 4a-d can operate in switching-mode under control of its local controller 9a-d. For example, the PV power converter can operate to track the maximum power point of its PV array.

As an alternative shown in figure 4B, each of the PV power converters 4a-d uses a power conversion circuit having a topology of HB (half-bridge) converter. It includes two controllable power switches Q ₁, Q ₂ electrically coupled in series, a power diode D ₁, input terminals 40 and output terminals 41. The input terminals 40 are configured to be electrically coupled to output ends of the respective PV arrays 2a-d. The controllable power switches can use a power semiconductor switch, such like IGBT. The series-coupled power switches Q ₁, Q ₂ are electrically inserted between the output terminals 41 of the PV power converter. In this example, the PV panel and the power switch Q ₁, Q ₂ can form a closed loop when the power switches are turned on and keep ON state in response to an occurrence of fault, and this results in a jump of the current (shown in arrow) until it reaches the maximum short circuit current (I _{sc_max}) of PV panel. The current is circulating through the PV panel and the power switches, without flowing out of the PV power converter. In other words, the flow of the power converted and provided by the power conversion circuit can be bypassed around the output terminals 41 of the PV power converter. . This requires keeping the controllable power switch closed longer than at least one switching period of the power conversion circuit 4a-d. Thus, the power flow can be isolated from the fault point in the PV power plant. Besides, the skilled person shall understand that during normal operation, the PV power converter 4a-d can operate in switching-mode under control of its local controller 9a-d. For example, the PV power converter can operate to track the maximum power point of its PV array.

Still another alternative shown in figure 4C, each of the PV power converters 4a-d uses a topology of Buck converter and a controllable power switch. It a power diode D ₁, a first controllable power switch Q ₁, an inductor L ₁ and a capacitor C ₁, input terminals 40, output terminals 41, and a second controllable power switch Q ₂ electrically coupled between the input terminals 40. The input terminals 40 are configured to be electrically coupled to output ends of the respective PV arrays 2a-d. The controllable power switches can use a power semiconductor switch, such like IGBT. The series-coupled power switches Q ₁, Q ₂ are electrically inserted between the output ends of the PV panel. In this example, the PV panel, the second controllable power switch Q ₂ and inductor L ₁ can form a closed loop when the second controllable power switch Q ₂ is turned on and keeps ON state in response to an occurrence of fault, and this results in an increase of the inductor current (shown in arrow) until it reaches the maximum short circuit current (I _{sc_max}) of PV panel. The current is circulating through the PV panel, the inductor and the second controllable power switch, without flowing out of the PV power converter. In other words, the flow of the power converted and provided by the power conversion circuit can be bypassed around the output terminals 41 of the PV power converter. This requires keeping the controllable power switch closed longer than at least one switching period of the power conversion circuit 4a-d. Thus, the power flow can be isolated from the fault point in the PV power plant. Besides, the skilled person shall understand that during normal operation, the PV power converter 4a-d can operate in switching-mode under control of its local controller 9a-d. For example, the PV power converter can operate to track the maximum power point of its PV array.

Under the control of the central controller 14, fault protection can be performed and described with examples thereafter addressing DC short-circuit fault at DC cables.

Fault Protection Solution I

This method may be implemented using the embodiment shown in figure 2. Before beginning the fault protection method, under the control of the central controller 14, the PV power plant 1 operates in normal condition. In particular for example, both of the PV power converters 4a-d and the central power converter 6 operate in switching-mode, and the DC power generated by the PV arrays 2a-d from solar power is converted by the PV power converters 4a-d and supplied to the central power converter 6 via the DC cables 8a-d and the DC bus 5. The central power converter 6 converts the DC power and supply AC power through the transformer 12 to the grid. When the PV power converters 4a-d are operating in the switching-mode, the duty cycle of the controllable power switch for power bypassing is adjusted for regulation of the operation of the PV power converter, for example for the purpose of MPPT.

The current sensors CSI _a, CSI _b, CSI _c, CSI _d measure the currents on the respective DC cables 8a-d and send the measurements to the central controller 14. The central controller 14 can target the short-circuit fault point. For example, if the direction of current measurement from current sensor CSI _a is from the central power converter 6 to the PV power converter 4a and the current measurement is bigger than the pre-set protection level, then the central controller 14 can determine a short current fault occurs at the DC cable 8a. The short circuit fault determination scheme may apply to the other DC cables 8b, 8c, 8d, as well.

Following the example assuming a short circuit fault is detected on DC cable 8a, the central controller 14 informs the local controller 9a of the fault occurring on the DC cable 8a and the local controller 9a controls the PV power converter 2a to bypass the power flow around the output side of the corresponding PV panel 2a, for example according to the solutions as described to figures 4A, 4B, 4C.

It can be noticed that, the power injection from PV panel 2a is bypassed. After the fully discharging of DC capacitors (C) in the PV power converter 4a, the current flow through the DC switch 11a will fall to zero. In this case, the central controller 14 can order the DC switch 11a to be turned-off with ZCS condition, which means cost effective DC breaker with low arc extinction capability.

Once the short circuit fault is isolated from the rest of

PV power converters

4b, 4c, 4d, the central controller 14 will order these

PV power converters

4b, 4c, 4d to be restarted with normal operation in order to increase the availability of PV panels.

Fault Protection Solution II

The current sensor CSIO _a, CSIO _b, CSIO _c, CSIO _d keep measuring current on the output sides of the respective PV power converters 4a-d and send the measurements to the respective local controllers 9a-d. For example, the voltage CSIO _a detects over current at the output side of the PV power converter 4a and sends the message to the corresponding local controller 9a. Based on the over current information, the local controller 9a controls the PV power converter 2a to bypass the power flow around the output side of the corresponding PV panel 2a, for example according to the solutions as described to figures 4A, 4B, 4C.

In parallel, all of the current sensor CSI _a, CSI _b, CSI _c, CSI _d send their current measurements to the central controller 14. With certain protection methods, the central controller 14 can target the short-circuit fault point. For example, if the direction of current measurement from CSIO _a is from the central power converter 14 to the PV power converter 4a, and the current value is bigger than the pre-set protection level, the central controller 14 can target short current fault point on the DC cable 8a.

As the PV power converter with output over current faults have been protected locally based on the monitoring of their own local current sensor, the fault detection speed of central controller 14 and the communication speed between central controller 14 and the respective PV power converter can be low. And this low speed communication will not cause damage problem to the devices.

Fault Protection Solution III

The current sensor CSIO _a, CSIO _b, CSIO _c, CSIO _d keep measuring current on the output sides of the respective PV power converters 4a-d and send the measurements to the respective local controllers 9a-d. For example, the voltage CSIO _a detects over current at the output side of the PV power converter 4a and sends the message to the corresponding local controller 9a. Based on the over current information, the local controller 9a controls the PV power converter 2a to bypass the power flow around the output side of the corresponding PV panel 2a, for example according to the solutions as described to figures 4A, 4B, 4C. Besides, the fault message is sent to the central controller 14, as well.

Being aware of a fault occurring in the PV power plant, the central controller 14 sends commands to each of the local controllers 9a-9d to control their respective PV power converters 4a-d to stop operation and sends commands to open the DC switches 11a-d. Then, the central controller 14 send restart command sequentially to the local controllers 9a-d for sequentially restarting the respective PV power converters 4a-d and send close command to the corresponding one of the DC switches 11a-d.

In the first step, the central controller 14 send restart command to the local controller 9a for to restart the PV power converter 4a and send close command to the corresponding the DC switch 11a. Since there is short circuit fault on the DC cable 8a, the central controller 14 receives over current measurement from the current sensor CSIO _a. Based on that, the fault is targeted at DC cable 8a, and then the central controller 14 sends commands to the local controller 9a to control the PV power converter 4a to stop operation and sends command to open the DC switches 11a.

In the second step, the central controller 14 send restart command to the local controller 9b for to restart the PV power converter 4b and send close command to the corresponding the DC switch 11b. Since there is no fault on the DC cable 8b, the central controller 14 receives no over current measurement from the current sensor CSIO _b. Based on that, the DC cable 8b is judged free of fault, and the PV power converter 4b keeps operation.

In the third and fourth steps, the fault point target process is performed respectively for the DC cables 8c, 8d. Similarly, depending on if there is a fault occurring there, the corresponding PV power converters 4c, 4d are controlled to operate to stop.

1) Comparison with Typical Solution

Table 1 shows that, typical solution uses high speed expensive DC switch to detect &cut off the DC short current. In this way, the protection control method is simple. In some cases, the communication system between DC switches and central controller is not required.

In the embodiments of present invention, low speed low cost DC switches can be applied in the system, because ZCS condition is realized during the turning-off of DC switches. But the coordination of multiple components in PV power plant is required, thus the protection control method is more complex than typical solution, and communication system is necessary. This will increase the capital cost.

As large scale DC optimizer based PV power plant requires thousands of DC switches, by having the solutions according to present invention, it can bring significant cost saving for PV power plant, even though complex protection control and installation of current sensors &communication system require some capital cost.

2) Comparison among Fault Protection Solutions I, II, III

As low speed low cost DC switches can be applied in three solutions, and the number of DC switches is not changed, thus the capital cost of DC switches in these three solutions are the same.

As there’s no current sensors installed in DC distribution box, the number of current sensors in Solution III is smallest, which means the least capital cost on current sensors. And DC optimizer and DC distribution box requires the installation of CSs in Solution II, thus the capital cost of current sensors in Solution II is the biggest.

As mentioned above, Solution I has high requirements on the communication speed, but it has advantage of the fastest fault restoration speed. On the other hand, Solution III has very low requirement on the communication speed, but it requires more complex protection control method in the central controller, and the fault restoration speed is low. During its restoration process, part of DC optimizers cannot operate normally, thus the power generation of the PV station might be influenced.

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 PV power converter, including:

output terminals;

input terminals being configured to be electrically coupled to output ends of PV array;

a power conversion circuit being configured to convert power supplied from the PV array and output the converted power at the output terminals, having at least one power switch being electrically inserted between the input terminals for bypassing a power flow from the PV array around the output terminals; and

a local controller being configured to close the power switch in response to an occurrence of a fault external to the PV power converter and keep the power switch closed for a predetermined time interval longer than a switching period of the power conversion circuit.
The PV power converter according to claim 1, wherein:

current rating of the power switch is selected above short-circuit current of the PV array.
The PV power converter according to claim 1 or 2, wherein:

the at least one power switch each uses a controllable power semiconductor switch; and

the power conversion circuit re-uses the at least one controllable power semiconductor switch.
The PV power converter according to any of claims 1 to 3, wherein:

the controller is further configured to control the at least one controllable power semiconductor switch to operate in a switching-mode such that the power conversion circuit operates in conversion of the PV array power without the occurrence of the fault.
The PV power converter according to claim 1 or 2, wherein:

the at least one power semiconductor forms a bypass circuit to power conversion circuit.
A PV power plant, including:

a first group of PV power converters each according to any of claims 1 to 5;

a central power converter;

a DC bus, arranged at input side of the central power converter;

a multiple of DC cables, being arranged between output side of the respective PV power converters and the DC bus; and

a plurality of DC switches, being inserted between two ends of the respective DC cables.
The PV power plant according to claim 6, further including:

a plurality of first sensors being arranged for measuring electrical signals on the respective DC cables; and

a central controller being configured to determine if the fault occurs in consideration of the measurements of the first sensors and inform the local controllers of the same.
The PV power plant according to claim 6, wherein:

each of the PV power converters includes a second sensor for measuring electrical signal on its power output side; and

each of the local controllers being configured to determine if the fault occurs in consideration of the measurement of the second sensor of the respective PV power converter.
The PV power plant according to claim 8, further includes:

a central controller being configured to

identify which of the DC cables has fault by sequentially changing the PV power converters into operation mode and closing the respective DC switch;

for the DC cables free of fault, keep the operation of the respective PV power converters and close the respective DC switches; and

for the DC cables with the fault, keep suspending the operation of the respective PV power converters and opening the respective DC switches.
The PV power plant according to claim 8, further includes:

a plurality of first sensors being arranged for measuring electrical signals on the respective cables; and

a central controller being configured to:

determine which of the DC cables has fault in consideration of the measurements of the first sensors; and

for the DC cables free of fault, start the operation of the respective PV power converters and close the respective DC switches.
The PV power plant according to any of claims 6 to 10, further including:

a power diode electrically inserted between one end of the first DC bus and the input of the central power converter.
A PV power plant, including:

a first group of PV power converters each according to any of claims 1 to 5; and

an AC bus, being arranged to electrically coupled to the output terminals of the PV power converters.
A method for controlling a PV power converter according to any of claims 1 to 5, including:

monitoring if the fault occurs; and

closing the power switch in response to the occurrence of the fault.