A bus equalization method in the power-on static state

Abstract

The present application discloses a bus equalization method under power-on static state, which is applied to an inverter system and includes the following steps: constructing a symmetric compensation network between the positive and negative buses and the bus midpoint, and performing equalization compensation on the half-bus impedance of the positive and negative buses or performing equalization compensation on the half-bus voltage of the positive and negative buses through the symmetric compensation network. The beneficial effects of the present application are as follows: By compensating the half-bus voltage or half-bus impedance of the positive and negative buses, the equalization of the bus voltage can be achieved under the static state when the inverter is powered on but not started. Moreover, there are multiple ways to achieve equalization. Each way adds fewer components to the inverter system, has a low cost, and the implementation method is relatively simple.

Description

Technical Field

[0001] The present application relates to the technical field of inverters, and in particular, to a bus balance method under static power-on conditions. Background Art

[0002] Most existing inverters have a non-isolated structure. There are common systems where the bus negative is grounded with the power supply, and there are also power supply floating ground systems. Before the inverter is powered on and not yet started, the bus may become unbalanced. Since the inverter is in a non-started state, it is not possible to inject zero-sequence current through the DC / AC device to achieve bus voltage balance. Therefore, when the inverter is not started, the bus balance is actually a static balance, mainly determined by the impedance network of the system. In addition, DC / DC device failures may also cause bus imbalance during the system startup process. When the inverter starts, if the bus voltage is in an unbalanced state, it may cause the inverter to fail to start normally. Summary of the Invention

[0003] One of the objectives of the present application is to provide a bus balance method under static power-on conditions that can solve at least one of the defects in the above background art.

[0004] To achieve at least one of the above objectives, the technical solution adopted in the present application is as follows: A bus balance method under static power-on conditions, applied to an inverter system, includes the following steps: constructing a symmetric compensation network between the bus and the midpoint of the bus, and performing balanced compensation on the half-bus impedance of the positive and negative buses, or performing balanced compensation on the half-bus voltage of the positive and negative buses through the symmetric compensation network.

[0005] Preferably, the symmetric compensation network adopts an impedance network, and the impedance network is suitable for increasing the positive half-bus impedance, or simultaneously reducing the positive half-bus impedance and the negative half-bus impedance.

[0006] Preferably, the impedance network includes a compensation resistor connected in parallel between the positive bus and the midpoint of the bus, and the resistance value of the compensation resistor is equal to the impedance value of the midpoint of the bus.

[0007] Preferably, when the inverter system takes power with a floating ground power supply, the negative bus is connected to the signal ground through a control switch; when the impedance network is working, the control switch is in a closed and conducting state; when the symmetric compensation network is not working, the control switch is in an open and disconnected state.

[0008] Preferably, the impedance network adopts an insulation impedance detection unit equipped with an inverter system; the insulation impedance detection unit includes a pair of measuring resistors and a first bidirectional switch; the branch formed by the measuring resistors in series is connected in parallel to the positive and negative busbars, and the series midpoint of the measuring resistors is connected to the busbar midpoint and the ground through the first bidirectional switch; when performing insulation impedance detection, the first bidirectional switch connects the series midpoint of the measuring resistors to the ground; when performing bus balancing, the first bidirectional switch connects the series midpoint of the measuring resistors to the busbar midpoint.

[0009] Preferably, the impedance network includes a compensation resistor and an insulation impedance detection unit equipped with an inverter system; the compensation resistor is connected in parallel between the positive bus and the bus midpoint; the insulation impedance detection unit includes a pair of measuring resistors and a first bidirectional switch; the branch formed by the measuring resistors in series is connected in parallel to the positive and negative busbars, and the series midpoint of the measuring resistors is connected to the bus midpoint and the ground through the first bidirectional switch; the specific process of the symmetrical compensation network balancing the busbar is: calculating the voltage deviation of the positive and negative busbars, and comparing the calculated result with the set voltage deviation threshold value; if the calculated voltage deviation is less than the set voltage deviation threshold value, the first bidirectional switch connects the series midpoint of the measuring resistors to the ground; otherwise, the first bidirectional switch connects the series midpoint of the measuring resistors to the bus midpoint.

[0010] Preferably, the symmetrical compensation network adopts a power supply network, and the power supply network is suitable for compensating one of the half bus voltages so that the positive and negative bus voltages are kept balanced.

[0011] Preferably, the power supply network adopts a PID unit equipped with an inverter system; the PID unit includes a PID power supply and a second bidirectional switch; one end of the PID power supply is connected to the midpoint of the bus, and the other end of the PID power supply is connected to the negative bus and the ground respectively through the second bidirectional switch; when PID suppression is performed, the second bidirectional switch connects the PID power supply to the ground; when bus balancing is performed, the second bidirectional switch connects the PID power supply to the negative bus, and then adjusts the negative half bus voltage to be equal to the positive half bus voltage through the PID power supply.

[0012] Preferably, the symmetrical compensation network adopts a compensation selection network including a power supply network and an impedance network; the compensation selection network is suitable for selectively connecting the power supply network and the impedance network according to the voltage deviation of the positive and negative buses, thereby performing balanced compensation for the half-bus voltage or half-bus impedance of the positive and negative buses.

[0013] Preferably, the impedance network adopts the insulation impedance detection unit equipped in the inverter system, and the power network adopts the PID unit equipped in the inverter system; the insulation impedance detection unit includes a pair of measuring resistors and a first bidirectional switch; the branch formed by the series connection of the measuring resistors is connected in parallel to the positive and negative buses, and the series connection midpoint of the measuring resistors is connected to the bus midpoint and the ground through the first bidirectional switch; the PID unit includes a PID power supply and a second bidirectional switch; one end of the PID power supply is connected to the bus midpoint, and the other end of the PID power supply is respectively connected to the negative bus and the ground through the second bidirectional switch; the specific process of the symmetric compensation network for balancing the bus is as follows: calculate the voltage deviation between the positive and negative buses, and compare the calculated result with the set first voltage deviation threshold value and the second voltage deviation threshold value; among them, the first voltage deviation threshold value is less than the second voltage deviation threshold value; when the calculated voltage deviation is less than the set first voltage deviation threshold value, the first bidirectional switch conducts the series connection midpoint of the measuring resistors to the ground, and at the same time the second bidirectional switch conducts the PID power supply to the ground; when the calculated voltage deviation is greater than the set second voltage deviation threshold value, the first bidirectional switch conducts the series connection midpoint of the measuring resistors to the ground, and at the same time the second bidirectional switch conducts the PID power supply to the negative bus; when the calculated voltage deviation is between the first voltage deviation threshold value and the second voltage deviation threshold value, if the voltage deviation calculated at the previous moment is less than the set first voltage deviation threshold value, the first bidirectional switch conducts the series connection midpoint of the measuring resistors to the bus midpoint, and at the same time the second bidirectional switch conducts the PID power supply to the ground; otherwise, the first bidirectional switch conducts the series connection midpoint of the measuring resistors to the ground, and at the same time the second bidirectional switch conducts the PID power supply to the negative bus.

[0014] Compared with the prior art, the beneficial effects of this application are as follows:

[0015] By compensating the half-bus voltage or half-bus impedance of the positive and negative buses, the balance of the bus voltage can be achieved in the static state when the inverter is powered on but not started. And there are various ways to achieve the balance. Each way adds fewer components to the inverter system, has a low cost, and the implementation method is also relatively simple. Description of the Drawings

[0016] Figure 1 It is a schematic diagram of the working process of this application.

[0017] Figure 2 It is a schematic diagram of the architecture of a typical photovoltaic system in the prior art.

[0018] Figure 3 It is a schematic diagram of the architecture when the bus and the signal are completely isolated during power extraction of the system.

[0019] Figure 4 Schematic diagram of the structure where the bus and signal share the same ground when power is taken for the system.

[0020] Figure 5 Schematic diagram of the photovoltaic system structure where the negative bus and signal share the same ground in this application.

[0021] Figure 6 In this application Figure 5 Schematic diagram of the equivalent circuit corresponding to the structure shown.

[0022] Figure 7 In this application Figure 5 Schematic diagram of the structure that uses a compensation resistor as a symmetric compensation network in the structure shown.

[0023] Figure 8 In this application Figure 7 Schematic diagram of the equivalent circuit corresponding to the structure shown.

[0024] Figure 9 In this application Figure 5 Schematic diagram of the structure that uses an insulation impedance detection unit as a symmetric compensation network in the structure shown.

[0025] Figure 10 In this application Figure 9 Schematic diagram of the equivalent circuit corresponding to the structure shown.

[0026] Figure 11 In this application Figure 5 Schematic diagram of the structure that simultaneously uses a compensation resistor and an insulation impedance detection unit as a symmetric compensation network in the structure shown.

[0027] Figure 12 In this application Figure 11 Schematic diagram of the working process of the structure shown.

[0028] Figure 13 Schematic diagram of the photovoltaic system structure where the negative bus and signal are isolated in this application.

[0029] Figure 14 In this application Figure 13 Schematic diagram of the equivalent circuit corresponding to the structure shown.

[0030] Figure 15 In this application Figure 13 Schematic diagram of the structure that uses a compensation resistor as a symmetric compensation network in the structure shown.

[0031] Figure 16 In this application Figure 13 Schematic diagram of the structure that uses an insulation impedance detection unit as a symmetric compensation network in the structure shown.

[0032] Figure 17For this application Figure 13 The architecture shown uses a PID unit as a symmetrical compensation network.

[0033] Figure 18 For this application Figure 13 The illustrated architecture uses both an insulation impedance detection unit and a PID unit as a symmetrical compensation network.

[0034] Figure 19 For this application Figure 18 Schematic diagram of the workflow of the shown architecture. DETAILED DESCRIPTION

[0035] Below, the present application is further described in conjunction with specific implementation methods. It should be noted that, in the description of this specification, the reference terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" and the like are intended to mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms should not be understood as necessarily referring to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification.

[0036] In the description of this application, it should be noted that for directional words, such as the terms "center", "horizontal", "longitudinal", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise" and so on, indicating the orientation and position relationship are based on the orientation or position relationship shown in the accompanying drawings, which is only for the convenience of describing this application and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and cannot be understood as limiting the specific scope of protection of this application.

[0037] It should be noted that the terms "first", "second", etc. in the description and claims of this application are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence.

[0038] In this application, unless otherwise clearly specified or limited, terms such as "installed", "connected", "linked", "fixed", etc. shall be understood in a broad sense. For example, it can be a connection, a detachable connection, or an integral body; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two components or the interaction relationship between two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0039] In this application, unless otherwise clearly specified or limited, the first feature being "on" or "under" the second feature may include the direct contact between the first and second features, or may include the situation where the first and second features are not in direct contact but in contact through other features therebetween. Moreover, the first feature being "above", "over" and "on top of" the second feature includes that the first feature is directly above and obliquely above the second feature, or merely means that the horizontal height of the first feature is higher than that of the second feature. The first feature being "under", "below" and "beneath" the second feature includes that the first feature is directly below and obliquely below the second feature, or merely means that the horizontal height of the first feature is lower than that of the second feature.

[0040] The terms "comprising" and "having" and any variations thereof in the description and claims of this application are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device that comprises a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these process, method, product or device.

[0041] One preferred embodiment of this application, as Figure 1 shown, a bus equalization method in the static power-on state, applied to an inverter system, includes the following steps: constructing a symmetric compensation network between the positive and negative buses and the bus midpoint, and performing equalization compensation on the half-bus impedance of the positive and negative buses through the symmetric compensation network, or performing equalization compensation on the half-bus voltage of the positive and negative buses.

[0042] It can be understood that when the inverter is powered on but not started, through the following analysis of the inverter architecture, it can be known that the bus voltage balance of the inverter system mainly depends on whether the impedances of the positive and negative half-buses are balanced. When the bus voltage is unbalanced, it is mainly due to the existence of the bus midpoint impedance that causes the imbalance of the positive and negative half-bus impedances. Therefore, an impedance network for equalizing the half-bus impedance of the positive and negative buses can be constructed as the symmetric compensation network. Of course, for the imbalance of the bus voltage, the most direct method can be used to compensate the half-bus voltage of the positive and negative buses, so as to achieve the balance of the bus voltage; that is, a power supply network for equalizing the half-bus voltage of the positive and negative buses is constructed as the symmetric compensation network.

[0043] It should be noted that there are various types of inverter systems to which the busbar balancing method of the present application can be applied, such as photovoltaic power generation systems, photovoltaic energy storage systems, and other forms of new energy power generation systems. For the convenience of understanding, the present application will take a photovoltaic system as an example for detailed description.

[0044] As Figure 2 shown, it is a schematic diagram of the architecture of a typical photovoltaic system in the prior art. The overall adopts a multi-string architecture. Each group of PV units is connected to the busbar through the corresponding DC / DC unit, and then is connected to the grid through the DC / AC unit. In this system, the power is generally taken from the busbar. The power-taking methods of the system can be mainly divided into two types. One is as Figure 3 shown. After taking power from the busbar, a galvanic isolation type DC / DC unit is used to obtain a weak power supply, and then the weak electrical signal ground is not connected to the negative busbar. The other is as Figure 4 shown. After taking power from the busbar, a galvanic isolation type DC / DC unit is used to obtain a weak power supply, and then the weak electrical signal ground is connected to the negative busbar. There are certain differences in the busbar balancing methods based on the above two power-taking methods. For the convenience of understanding, the busbar balancing processes under the above two power-taking methods will be described in detail through two embodiments below.

[0045] Embodiment 1: The scenario where the signal is grounded with the negative busbar.

[0046] For the convenience of understanding this embodiment, the busbar imbalance situation in the corresponding scenario of this embodiment can be analyzed first.

[0047] As Figure 5 shown, it is a schematic diagram of the architecture of a photovoltaic system where the negative busbar is grounded with the signal. Among them, the number of PV units is n, which are respectively marked as PV#1 to PV#n. The measurement resistors corresponding to PV#1 are R PV1+ and R PV1- . Similarly, it can be known that the measurement resistors corresponding to PV#n are R PVn+ and R PVn- . For the busbar, the impedance of the total busbar and the voltage of the half busbar are generally measured. Among them, the measurement resistors of the total busbar are R dc+ and R dc1- , and the measurement resistors of the negative busbar are R NP and R dc2- .

[0048] The above measurement resistors are all connected to the signal ground. The equivalent simplified circuit in the static state is as Figure 6 shown. It can be seen from Figure 6 that the signal ground GND is connected to the negative busbar Bus-, then the measurement resistors R PV1- connected to the negative busbar Bus-,..., RPVn- and R dc1- as well as R dc2- will all be bypassed and will not have any impact on the system voltage equalization. Additionally, the measuring resistors R PV1+ ,..., R PVn+ as well as R dc+ connected across the positive and negative buses will also not affect the balance at the midpoint of the bus. Furthermore, what truly determines the bus equalization is the impedance connected at the midpoint of the bus, i.e., the measuring resistor R NP .

[0049] In this embodiment, in order to suppress the impact of the midpoint impedance R NP on the midpoint equalization of the bus, an impedance network can be used as a symmetric compensation network for half-bus impedance equalization compensation. There are multiple ways to achieve half-bus impedance equalization. For example, the impedance of the positive half-bus can be increased, or the impedance of both the positive half-bus and the negative half-bus can be decreased simultaneously. For the convenience of understanding, the following will be described in detail through specific examples.

[0050] Example 1: As Figure 7 shown, since the negative bus is grounded with the signal, the midpoint impedance R NP is connected between the midpoint of the bus and the negative bus; then the impedance network can include a compensation resistor R NP ' connected in parallel between the positive bus and the midpoint of the bus. The resistance value of the compensation resistor R NP ' is equal to the value of the midpoint impedance R NP .

[0051] It can be understood that after adding the compensation resistor R NP ', from the Figure 8 equivalent simplified circuit diagram shown, the impedance between the positive bus and the midpoint of the bus and the impedance between the midpoint of the bus and the negative bus are symmetrically equal. At this time, the bus voltage is theoretically in an equalized state.

[0052] Example 2: Since an insulation impedance detection unit is generally equipped in the inverter system to detect the insulation impedance of the inverter system; then the insulation impedance network corresponding to the insulation impedance detection unit can be used for midpoint balance of the bus. As Figure 9 shown, the insulation impedance detection unit includes measuring resistors R1 and R2 and a first bidirectional switch S1; the branch formed by the series connection of the measuring resistors R1 and R2 is connected in parallel across the positive and negative buses, and the series midpoint of the measuring resistors R1 and R2 is connected to the midpoint of the bus and the ground through the first bidirectional switch S1; that is, a contact 1 is connected at the midpoint of the bus, and a contact 2 is connected at the ground position.

[0053] When the inverter system is performing insulation impedance testing, the first bidirectional switch S1 can connect contact 2, thereby connecting the series midpoint of the measuring resistors R1 and R2 to ground, thereby implementing insulation impedance testing of the inverter system. When performing bus balancing, the first bidirectional switch S1 can connect contact 1, thereby connecting the series midpoint of the measuring resistors R1 and R2 to the bus midpoint, thereby reducing the half-bus impedance corresponding to the positive and negative busbars to suppress the degree of imbalance at the bus midpoint.

[0054] It is understandable that the specific structure and working principle of the first bidirectional switch S1 are well known to those skilled in the art, so they will not be elaborated in detail here. Figure 9 The corresponding architecture can be simplified as follows Figure 10 The equivalent circuit shown in Figure 1 is Figure 10 It can be seen that the measuring resistor R1 is connected in parallel between the positive bus and the bus midpoint, and the measuring resistor R2 is connected in parallel between the bus midpoint and the negative bus. It should be known to those skilled in the art that in the insulation impedance network, the resistance of the measuring resistors R1 and R2 is much smaller than the bus midpoint impedance R NP This means that the balance of the bus midpoint will be determined by the measuring resistors R1 and R2.

[0055] Specifically, at this time, the resistance R2 and the busbar midpoint impedance R NP In parallel state; since the resistance of the measuring resistor R2 is much smaller than the busbar midpoint impedance R NP Based on the knowledge of parallel resistors, the total resistance of the parallel connection between the negative busbar and the busbar midpoint is significantly reduced to approach that of the measuring resistor R2. Since the resistances of measuring resistors R1 and R2 are generally equal, the insulation impedance network containing measuring resistors R1 and R2 can reduce the half-busbar impedance and make the positive and negative half-busbar impedances consistent, thus effectively achieving busbar voltage balance.

[0056] Example 3: Its essence can be seen as a combination of Example 1 and Example 2. Figure 11 As shown, the impedance network includes a compensation resistor R NP ´Insulation impedance detection unit equipped with inverter system; compensation resistor R NP Connected in parallel between the positive busbar and the busbar midpoint; the insulation impedance detection unit includes measuring resistors R1 and R2 and a first bidirectional switch S1. The branch formed by the series connection of measuring resistors R1 and R2 is connected in parallel to the positive and negative busbars. The midpoint of the series connection of measuring resistors R1 and R2 is connected to the busbar midpoint and ground via the first bidirectional switch S1.

[0057] Based on the above architecture, Figure 12As shown, the specific process of the symmetric compensation network for balancing the bus is as follows: First, calculate the voltage deviation ΔV of the positive and negative buses, ΔV = |V dc+ - V dc- |; where V dc+ represents the positive half-bus voltage, and V dc- represents the negative half-bus voltage. Then compare the calculated voltage deviation ΔV with the set voltage deviation threshold value V th . If the calculated voltage deviation ΔV is less than the set voltage deviation threshold value V th , it indicates that the imbalance degree of the bus voltage is relatively low at this time, and only the compensation resistor R NP ´ is needed to achieve balanced compensation; since the compensation resistor R NP ´ is already in an impedance balanced state, the first bidirectional switch S1 can conduct the series midpoint of the measuring resistors R1 and R2 to the ground at this time, so that the insulation impedance network operates in insulation impedance detection. If the calculated voltage deviation ΔV is greater than or equal to the set voltage deviation threshold value V th , it indicates that the imbalance degree of the bus voltage is relatively high at this time, and the compensation resistor R NP ´ and the insulation impedance network are required to jointly achieve rapid balancing of the bus voltage; since the compensation resistor R NP ´ is already in an impedance balanced state, the first bidirectional switch S1 can conduct the series midpoint of the measuring resistors R1 and R2 to the bus midpoint at this time.

[0058] Embodiment 2: The scenario where the signal and the negative bus are not grounded together, that is, the inverter takes power in a floating ground power supply mode.

[0059] As Figure 13 shown, it is a schematic diagram of the photovoltaic system architecture where the negative bus and the signal are not grounded together; the difference between this architecture and that of Embodiment 1 is that the signal ground and the negative bus are not connected, so the specific structure of the architecture will not be elaborated here.

[0060] Based on Figure 13 the architecture shown, the equivalent simplified circuit in the static state is as Figure 14 shown. As Figure 14 can be seen, only the diode corresponding to the PV unit with the highest PV voltage will conduct. Assuming that the voltage of PV#1 is the highest, the diode connected to the positive output end PV1+ of PV#1 will conduct. Therefore, the impedance between the positive bus Bus+ and the signal ground GND is R PV1+ / / R dc+ , and the impedance between the negative bus Bus- and the signal ground GND is R PV1- / / … / / R PVn- / / R dc1- / / R dc2-It can be seen from this that the static impedance of the positive bus and the negative bus are inconsistent, which easily leads to static imbalance of the bus voltage.

[0061] To mitigate the impact of impedance mismatch on bus midpoint voltage balancing, an impedance network can be used as a symmetrical compensation network for half-bus impedance balancing, or a power supply network can be used as a symmetrical compensation network for half-bus voltage balancing. For ease of understanding, this is explained in detail below using a specific example.

[0062] Example 4: Figure 15 As shown, the negative bus and the signal ground are connected through the control switch S2, and the symmetrical compensation network adopts an impedance network, which includes a compensation resistor R connected in parallel between the positive bus and the bus midpoint. NP ´, compensation resistor R NP The resistance value of ´ is equal to the busbar midpoint impedance R NP The value of .

[0063] It is understandable that the specific structure and working principle of the control switch S2 are well-known technologies to those skilled in the art, so they will not be elaborated in detail here. When bus static balancing is required, the control switch S2 can be closed to change the power supply floating ground of the inverter system to the signal and negative bus common ground scenario corresponding to the first embodiment. At this time, whether the bus midpoint of the inverter system is balanced or not will depend on the bus midpoint impedance R NP , and then through the compensation resistor R NP Impedance balancing of the positive and negative half-busbars can be achieved. After completing static busbar balancing and starting the inverter, the control switch S2 can be opened to open the circuit. After that, the busbar voltage can be balanced by zero-sequence injection.

[0064] Example 5: The symmetrical compensation network uses an impedance network. Since the inverter system is generally equipped with an insulation impedance detection unit to detect the insulation impedance of the inverter system, the insulation impedance network corresponding to the insulation impedance detection unit can be used to balance the busbar midpoint. Figure 16 As shown, the insulation impedance detection unit includes measuring resistors R1 and R2 and a first bidirectional switch S1; the branch formed by the series connection of the measuring resistors R1 and R2 is connected in parallel to the positive and negative busbars, and the midpoint of the series connection of the measuring resistors R1 and R2 is connected to the midpoint of the busbar and the ground through the first bidirectional switch S1; that is, the midpoint of the busbar is connected to contact 1, and the ground is connected to contact 2.

[0065] When the inverter system is performing insulation impedance testing, the first bidirectional switch S1 can connect contact 2, thereby connecting the series midpoint of the measuring resistors R1 and R2 to ground, thereby implementing insulation impedance testing of the inverter system. When performing bus balancing, the first bidirectional switch S1 can connect contact 1, thereby connecting the series midpoint of the measuring resistors R1 and R2 to the bus midpoint, thereby reducing the half-bus impedance corresponding to the positive and negative busbars to suppress the degree of imbalance at the bus midpoint.

[0066] It is understandable that when the first bidirectional switch S1 connects to contact 1, the measuring resistors R1 and R2 are connected in parallel to the positive half-bus and the negative half-bus, respectively. Since the resistance values of the measuring resistors R1 and R2 are much smaller than the resistance value of the measuring resistor of the inverter system, the balance of the bus midpoint will be determined by the measuring resistors R1 and R2. Specifically, based on the knowledge of resistor parallel connection, the total resistance value of the measuring resistors between the positive bus and the bus midpoint is significantly reduced to approach the measuring resistor R1 after being connected in parallel. The total resistance value of the measuring resistors between the negative bus and the bus midpoint is significantly reduced to approach the measuring resistor R2 after being connected in parallel. Since the resistance values of the measuring resistors R1 and R2 are generally equal, the insulation impedance network including the measuring resistors R1 and R2 can reduce the half-bus impedance and the reduced positive and negative half-bus impedances tend to be consistent, thereby effectively achieving bus voltage balance.

[0067] Example 6: A symmetrical compensation network uses a power supply network that can compensate for one of the half bus voltages to keep the positive and negative bus voltages balanced.

[0068] It is understandable that there are various specific structures for power networks capable of achieving half-bus voltage compensation. Adding an additional power network may affect the normal operation of the inverter system and increase the cost of the inverter system. Considering that photovoltaic systems are generally equipped with a PID unit to suppress the PID effect, which can be achieved by applying a reverse bias voltage, the PID unit equipped with the inverter system can be used as a power network to balance the static bus midpoint voltage.

[0069] Specifically, such as Figure 17As shown in the figure, the PID unit includes a PID power supply and a second bidirectional switch S3; one end of the PID power supply is connected to the midpoint of the bus, and the other end of the PID power supply is connected to the negative bus and the ground respectively through the second bidirectional switch S3; that is, a contact 1 is connected to the position of the negative bus, and a contact 2 is connected to the position of the ground. When the inverter system performs PID suppression, the second bidirectional switch S3 is connected to the contact 2 to make the PID power supply conduct with the ground to achieve the suppression of the PID effect. When performing bus balancing, the second bidirectional switch S3 is connected to the contact 1 to make the PID power supply conduct with the negative bus; at this time, the PID power supply is connected in parallel between the negative bus and the midpoint of the bus, and then the output of the PID power supply can be controlled according to the positive half-bus voltage, so that the negative half-bus voltage is adjusted to be equal to the positive half-bus voltage through the PID power supply.

[0070] Example 7: The symmetric compensation network adopts a compensation selection network including a power network and an impedance network, that is, Example 7 can be regarded as a combination of Example 5 and Example 6. The compensation selection network can selectively access the power network and the impedance network according to the voltage deviation between the positive and negative buses, and then perform balanced compensation on the half-bus voltage or half-bus impedance of the positive and negative buses.

[0071] Specifically, as Figure 18 shown, the impedance network adopts an insulation impedance detection unit equipped in the inverter system, and the power network adopts a PID unit equipped in the inverter system. The insulation impedance detection unit includes measuring resistors R1 and R2 and a first bidirectional switch S1; the branch formed by the series connection of the measuring resistors R1 and R2 is connected in parallel to the positive and negative buses, and the series midpoint of the measuring resistors R1 and R2 is connected to the midpoint of the bus and the ground through the first bidirectional switch S1. The PID unit includes a PID power supply and a second bidirectional switch S3; one end of the PID power supply is connected to the midpoint of the bus, and the other end of the PID power supply is connected to the negative bus and the ground respectively through the second bidirectional switch S3.

[0072] Based on the above architecture, as Figure 19 shown, the specific process of the symmetric compensation network for balancing the bus is as follows: First, calculate the voltage deviation ΔV between the positive and negative buses, ΔV = |V dc+ -V dc- |; where, V dc+ represents the positive half-bus voltage, and V dc- represents the negative half-bus voltage. Then compare the calculated voltage deviation ΔV with the set first voltage deviation threshold V th1 and the second voltage deviation threshold V th2 ; where, the first voltage deviation threshold V th1 is less than the second voltage deviation threshold V th2 .

[0073] When the calculated voltage deviation ΔV is less than the set first voltage deviation threshold V th1 When the first bidirectional switch S1 connects the midpoint of the series connection of measuring resistors R1 and R2 to ground, and the second bidirectional switch S3 connects the PID power supply to ground. At this point, the insulation impedance network operates in insulation impedance detection mode, and the PID power supply operates in PID effect suppression mode. In other words, neither the insulation impedance network nor the PID power supply responds to bus balancing.

[0074] When the calculated voltage deviation ΔV is greater than the set second voltage deviation threshold V th2 When the voltage is equalized, the first bidirectional switch S1 connects the midpoint of the series connection of the measuring resistors R1 and R2 to ground, while the second bidirectional switch S3 connects the PID power supply to the negative bus. At this point, the insulation impedance network operates in insulation impedance detection mode, and the PID power supply balances the bus midpoint voltage. The PID power supply's equalization of the bus midpoint voltage can be seen in Example 6 above.

[0075] When the calculated voltage deviation ΔV is within the first voltage deviation threshold value V th1 and the second voltage deviation threshold V th2 When the voltage deviation is between , the hysteresis loop selection method can be used. That is, the voltage deviation of the previous moment is calculated. If the voltage deviation ΔV calculated at the previous moment is less than the set first voltage deviation threshold value V th1 At this time, the first bidirectional switch S1 connects the series midpoint of the measuring resistors R1 and R2 to the bus midpoint, and the second bidirectional switch S3 connects the PID power supply to the ground; so that the insulation impedance network balances the bus midpoint voltage, and the PID power supply operates in the PID effect suppression mode; the insulation impedance network's balancing process of the bus midpoint voltage can refer to the above example 5. If the voltage deviation ΔV calculated at the previous moment is greater than the set second voltage deviation threshold value V th2 At this time, the first bidirectional switch S1 connects the midpoint of the series connection of the measuring resistors R1 and R2 to ground, and the second bidirectional switch S3 connects the PID power supply to the negative bus; so that the insulation impedance network operates in the insulation impedance detection mode, and the PID power supply balances the bus midpoint voltage. The balancing process of the PID power supply on the bus midpoint voltage can refer to the above example 6.

[0076] The above describes the basic principles, main features, and advantages of the present application. Those skilled in the art should understand that the present application is not limited to the above-described embodiments. The above-described embodiments and the specification merely illustrate the principles of the present application. Various changes and improvements may be made to the present application without departing from the spirit and scope of the present application. These changes and improvements fall within the scope of the present application for which protection is sought. The scope of protection claimed by the present application is defined by the appended claims and their equivalents.

Claims

1. A busbar equalization method in the power-on static state, applied to an inverter system, characterized in that It includes the following steps: constructing a symmetric compensation network between the positive and negative busbars and the midpoint of the busbars, and performing equalizing compensation on the half-busbar impedance of the positive and negative busbars or equalizing compensation on the half-busbar voltages of the positive and negative busbars through the symmetric compensation network; The symmetric compensation network adopts an impedance network, and the impedance network is suitable for increasing the positive half-busbar impedance or simultaneously reducing the positive half-busbar impedance and the negative half-busbar impedance; The impedance network includes a compensation resistor and / or an insulation impedance detection unit equipped in the inverter system; The compensation resistor is connected in parallel between the positive busbar and the midpoint of the busbars; The insulation impedance detection unit includes a pair of measuring resistors and a first bidirectional switch; the branch formed by the series connection of the measuring resistors is connected in parallel between the positive and negative busbars, and the series midpoint of the measuring resistors is connected to the midpoint of the busbars and the ground through the first bidirectional switch; The specific process of the symmetric compensation network equalizing the busbars through the insulation impedance detection unit is as follows: Calculating the voltage deviation between the positive and negative busbars and comparing the calculated result with the set voltage deviation threshold value; If the calculated voltage deviation is less than the set voltage deviation threshold value, the first bidirectional switch conducts the series midpoint of the measuring resistors to the ground; Otherwise, the first bidirectional switch conducts the series midpoint of the measuring resistors to the midpoint of the busbars.

2. The bus balance method under the power-on static state according to claim 1, wherein, The resistance value of the compensation resistor is equal to the impedance value of the midpoint of the busbars.

3. The bus balance method in the power-on static state according to claim 2, wherein When the power supply of the inverter system adopts a power supply floating ground system, the negative busbar is connected to the signal ground through a control switch; When the impedance network is working, the control switch is in a closed conduction state; when the symmetric compensation network is not working, the control switch is in an open circuit state.

4. The bus equalization method under power-on static state according to claim 1, characterized in that, Or the symmetric compensation network adopts a power supply network, and the power supply network is suitable for compensating the voltage of one of the half-busbars to keep the voltages of the positive and negative busbars balanced.

5. The bus equalization method under the power-on static state according to claim 4, characterized in that, The power supply network adopts a PID unit equipped in the inverter system; The PID unit includes a PID power supply and a second bidirectional switch; one end of the PID power supply is connected to the midpoint of the busbars, and the other end of the PID power supply is respectively connected to the negative busbar and the ground through the second bidirectional switch; When performing PID suppression, the second bidirectional switch conducts the PID power supply to the ground; When performing busbar equalization, the second bidirectional switch conducts the PID power supply to the negative busbar, and then adjusts the voltage of the negative half-busbar to be equal to the voltage of the positive half-busbar through the PID power supply.

6. The bus equalization method in the power-on static state according to claim 1, wherein Or the symmetric compensation network adopts a compensation selection network including a power supply network and an impedance network; The compensation selection network is suitable for selectively accessing the power supply network and the impedance network according to the voltage deviation between the positive and negative busbars, and then performing equalizing compensation on the half-busbar voltage or half-busbar impedance of the positive and negative busbars.

7. The bus balance method under power-on static state according to claim 6, wherein, The impedance network adopts an insulation impedance detection unit equipped in the inverter system, and the power supply network adopts a PID unit equipped in the inverter system; The insulation impedance detection unit includes a pair of measuring resistors and a first bidirectional switch; the branch formed by the series connection of the measuring resistors is connected in parallel to the positive and negative buses, and the series midpoint of the measuring resistors is connected to the bus midpoint and the ground through the first bidirectional switch; The PID unit includes a PID power supply and a second bidirectional switch; one end of the PID power supply is connected to the bus midpoint, and the other end of the PID power supply is connected to the negative bus and the ground respectively through the second bidirectional switch; The specific process of the symmetric compensation network for equalizing the bus is as follows: Calculate the voltage deviation between the positive and negative buses, and compare the calculated result with the set first voltage deviation threshold value and second voltage deviation threshold value; among them, the first voltage deviation threshold value is less than the second voltage deviation threshold value; When the calculated voltage deviation is less than the set first voltage deviation threshold value, the first bidirectional switch conducts the series midpoint of the measuring resistors to the ground, and at the same time the second bidirectional switch conducts the PID power supply to the ground; When the calculated voltage deviation is greater than the set second voltage deviation threshold value, the first bidirectional switch conducts the series midpoint of the measuring resistors to the ground, and at the same time the second bidirectional switch conducts the PID power supply to the negative bus; When the calculated voltage deviation is between the first voltage deviation threshold value and the second voltage deviation threshold value, if the voltage deviation calculated at the previous moment is less than the set first voltage deviation threshold value, the first bidirectional switch conducts the series midpoint of the measuring resistors to the bus midpoint, and at the same time the second bidirectional switch conducts the PID power supply to the ground; Otherwise, the first bidirectional switch conducts the series midpoint of the measuring resistors to the ground, and at the same time the second bidirectional switch conducts the PID power supply to the negative bus.

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