Direct current charging system of electric vehicle

Abstract

A direct current charging system for an electric vehicle comprises a combination of a plurality of power conversion groups connected in parallel, at least one charging output port and a switch switching matrix. The combination of the plurality of power conversion groups connected in parallel is used for providing direct current power. The at least one charging output port is at least electrically connected with one of the buses in a switchable manner. The switch switching matrix comprises a plurality of first parallel connection lines, a plurality of second parallel connection lines, a plurality of first parallel connection switches and a plurality of second parallel connection switches, and each first parallel connection line and the corresponding first parallel connection switch are electrically connected with any two buses in a switchable and non-repeated mode. The second parallel connection line and the second parallel connection switch can be electrically connected with a combined bus of any two different power conversion groups in a switchable manner.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a charging system, in particular to a direct current charging system for electric vehicles. BACKGROUND

[0002] The switching matrix in the prior art usually provides a number of switches m*n according to the number m of power conversion modules and the number n of charging output ports. The number of switches used is large and the architecture is complex, thus leading to an increase in the cost of the charging system.

[0003] In addition, as the number of charging guns and the number of power conversion modules increase, the number of switches required by the system will also further increase, thus causing considerable inconvenience in the expansion of the charging system. SUMMARY

[0004] To solve the above problems in the prior art, the purpose of the present disclosure is to provide a direct current charging system for electric vehicles, which can effectively solve the problem of cost increase caused by the large number of switching elements used in the prior art.

[0005] To achieve the above purpose, the present disclosure provides a direct current charging system for electric vehicles, comprising: a plurality of parallel-connected power conversion groups, at least one charging output port, and a switching matrix. The plurality of parallel-connected power conversion groups are used to provide direct current power, each power conversion group comprising a plurality of power conversion module groups, each power conversion module group comprising a plurality of power conversion modules and a bus for transmitting direct current. The at least one charging output port is switchably connected to at least one of the buses. The switching matrix comprises a plurality of first parallel lines, a plurality of second parallel lines, a plurality of first parallel switches, and a plurality of second parallel switches. Each first parallel line and first parallel switch is switchably and non-repeatedly connected to any two buses in the same power conversion group. The second parallel line and the second parallel switch are switchably connected to the buses of any two different power conversion groups.

[0006] In one embodiment of the present disclosure, any bus connected to the at least one charging output port through the second parallel line and the second parallel switch is connected to at most one different power conversion group.

[0007] In one embodiment of the present disclosure, only one position between the at least one charging output port and the bus connected switchably is provided with a third switch for controlling whether the at least one charging output port and the bus are connected switchably. The position is provided with only one third switch.

[0008] In one embodiment of the present disclosure, each power conversion module comprises at least one switching rectifier element.

[0009] In one of the embodiments of the present disclosure, the second parallel line is electrically connected to the busbar in the combination of the different power conversion groups without repeating.

[0010] In one of the embodiments of the present disclosure, the number of the power conversion module groups in each of the power conversion group combinations is the same, the number of the second parallel lines is the same as the number of the busbars in one of the power conversion group combinations, and the second parallel lines are provided with pluggable connectors between adjacent power conversion group combinations to adjust the number of the parallel power conversion group combinations.

[0011] In one of the embodiments of the present disclosure, one end of each second parallel line in each of the power conversion group combinations is provided with a male connector, and the other end is provided with a female connector, and the second parallel switch on each second parallel line is arranged between the male connector and the female connector of the second parallel line.

[0012] In one of the embodiments of the present disclosure, the number of the power conversion module groups in at least one of the power conversion group combinations is different from the number of the power conversion module groups in the other power conversion group combinations.

[0013] In one of the embodiments of the present disclosure, the number of the charging output ports is different from the number of the busbars.

[0014] The present disclosure also provides a direct current charging system for electric vehicles, which includes a plurality of parallel power conversion group combinations, at least one charging output port, and a switch matrix. The plurality of parallel power conversion group combinations are used to provide direct current power. Each power conversion group combination includes a plurality of power conversion module groups. Each power conversion module group has a plurality of power conversion modules and a busbar for transmitting direct current. The at least one charging output port is switchably electrically connected to at least one busbar. The switch matrix includes a plurality of parallel lines and a plurality of parallel switches. Each busbar is electrically connected to at least one parallel line, and the parallel line is switchably and non-repetitively electrically connected to different busbars through the corresponding parallel switch.

[0015] In one of the embodiments of the present disclosure, only one position is provided between the at least one charging output port and the switchably electrically connected busbar to control whether the at least one charging output port and the busbar are switchably electrically connected, and the position is provided with only one switch.

[0016] In one of the embodiments of the present disclosure, each power conversion module includes at least one switching rectifier element.

[0017] In one of the embodiments of the present disclosure, the number of the power conversion module groups in at least one of the power conversion group combinations is different from the number of the power conversion module groups in the other power conversion group combinations.

[0018] In one embodiment disclosed herein, the number of charging output ports is the same as the number of buses.

[0019] In one embodiment of this disclosure, the plurality of parallel lines include a plurality of first parallel lines and a plurality of second parallel lines, and the plurality of parallel switches include a plurality of first parallel switches and a plurality of second parallel switches. Each first parallel line and the first parallel switch are electrically connected switchably and non-repeatingly to any two buses in the same power conversion group combination, and the second parallel line and the second parallel switch are electrically connected switchably to the buses of any two different power conversion group combinations.

[0020] Compared with existing technologies, the electric vehicle DC charging system disclosed herein uses fewer switches, has a simpler combination architecture, and has a more flexible power output combination. Furthermore, the smaller number of switches not only improves the ease of system expansion but also effectively reduces costs and avoids the problems in existing technologies. Attached Figure Description

[0021] Figures 1 to 7 This is a schematic diagram of the circuit structure of a DC charging system for an electric vehicle according to different embodiments of this disclosure.

[0022] Explanation of reference numerals in the attached figures

[0023] 1,1a,1b,1c,1d,1f,2: Electric Vehicle DC Charging System

[0024] 10, 10a, 10b, 10c: Combinations of power conversion groups

[0025] 11, G1, G2, G3, G4: Power Conversion Module Group

[0026] 110: Power Conversion Module

[0027] 12: Busbar

[0028] 20, C1, C2, C3, C4: Charging output ports

[0029] 30: Switching Matrix

[0030] 310: Parallel connection

[0031] 311: First parallel connection

[0032] 312: Second parallel connection

[0033] 320: Parallel switch

[0034] 321, K1, K2: First parallel switch

[0035] 322, K3, K4: Second parallel switch

[0036] 323, K5, K6, K7, K8: Third switches

[0037] 40: Connector

[0038] 41: Male connector

[0039] 42: Female connector

[0040] P: Location

[0041] S: Switch Detailed Implementation

[0042] To make the above and other objects, features, and advantages of this disclosure more apparent and understandable, preferred embodiments of this disclosure will be described in detail below with reference to the accompanying drawings. Furthermore, in the drawings, structurally similar elements are denoted by the same reference numerals.

[0043] Reference Figure 1 This illustration shows a circuit structure diagram of a DC charging system for an electric vehicle according to an embodiment of the present disclosure. The present disclosure provides a DC charging system 1 for an electric vehicle, comprising: a combination 10 of multiple parallel power conversion groups, at least one charging output port 20, and a switching matrix 30. The combination 10 of multiple parallel power conversion groups is used to provide DC power. Each combination 10 of power conversion groups includes multiple power conversion module groups 11, each power conversion module group 11 including multiple power conversion modules 110 and a busbar 12 for transmitting DC current. The at least one charging output port 20 is electrically connected to at least one of the busbars 12 in a switchable manner. The switching matrix 30 includes multiple first parallel lines 311, multiple second parallel lines 312, multiple first parallel switches 321, and multiple second parallel switches 322. Each first parallel line 311 is connected to a first parallel switch 321 (e.g., ...). Figure 1 The switches marked K1 and K2 are electrically connected in a switchable manner to any two buses 12 in the same power conversion group combination 10 without repetition. The second parallel connection 312 is connected to the second parallel switch 322 (e.g., Figure 1 The switches marked K3 and K4 can be switched to connect the busbar 12 of any two different power conversion groups 10.

[0044] Specifically, the first parallel switch 321 and the second parallel switch 322 are, for example, relays. The bus 12 includes at least one power line and one ground line. The power conversion module 110 is, for example, a rectifier. The charging output port 20 is, for example, a charging gun or charging socket for an electric vehicle.

[0045] In one embodiment of this disclosure, any bus 12 electrically connected to the at least one charging output port 20 is bridging with a combination 10 of different power conversion groups at most once via a second parallel connection 312 and a second parallel switch 322.

[0046] Specifically, refer to Figure 1 The switchable ground electrical connection (via switch number K5) to the bus 12 of at least one charging output port 20 (e.g., charging output port 20 numbered C1) via the second parallel connection 312 and the second parallel connection switch 322 (e.g., switch numbered K3) is only with combinations 10 of different power conversion groups (e.g. Figure 1 The power conversion group combination 10 in the lower middle section is bridging at most once. A switchable ground electrical connection (via switch K6) to the bus 12 of the at least one charging output port 20 (e.g., charging output port 20 of C2) via a second parallel connection 312 and a second parallel switch 322 (e.g., switch K4) is only connected to different power conversion group combinations 10 (e.g., ...). Figure 1 The combination of power conversion groups in the middle and lower part (10) can be bridging each at most once.

[0047] In one embodiment of this disclosure, only one position P is provided between the at least one charging output port 20 and the switchably electrically connected bus 12 to control whether the at least one charging output port 20 and the bus 12 are switchably electrically connected, and only one third switch 323 is provided at position P.

[0048] Specifically, refer to Figure 1 Only one position P (e.g., the position of the switch numbered K5) is provided between the at least one charging output port 20 (e.g., the charging output port 20 numbered C1) and the switchably electrically connected bus 12 to control whether the at least one charging output port 20 and the bus 12 are electrically connected to each other. Only one third switch 323 (e.g., the switch numbered K5) is provided at this position P.

[0049] In one embodiment disclosed herein, each power conversion module 110 includes at least one switching mode rectifier (SMR), also known as a high-frequency switching rectifier. It converts AC power into DC power, and in addition to powering battery packs, it can also be designed as a power supply suitable for electric vehicle charging equipment, with a wide range of applications. It provides quiet, highly reliable DC power and is characterized by its small size, light weight, and ease of operation. Utilizing advanced high-frequency switching power supply technology and a highly efficient design, it becomes a full-function high-frequency switching rectifier that can supply power individually or in parallel. In a parallel system, the central monitoring module (Control and Supervisory Unit, CSU) performs overall monitoring, featuring optimal current equalization, precise voltage stability, battery charging current limiting, automatic battery equalization, and battery temperature compensation.

[0050] In one embodiment of this disclosure, the second parallel connection 312 and the second parallel connection switch 322 are electrically connected non-repeatingly to the bus 12 in any two combinations 10 of different power conversion groups.

[0051] Specifically, refer to Figure 1 The second parallel connection 312 and the second parallel switch 322 (e.g., switch number K3) are electrically connected non-repeatingly to any two different power conversion group combinations 10 (e.g., Figure 1 The bus 12 in the combination 10 of the upper and lower power conversion groups (e.g., the combination 10 of the middle and upper power conversion groups) Figure 1 The second parallel connection 312 is connected to the bus 12 of the power conversion module group 11 numbered G1 and G3. The second parallel connection 312 and the second parallel connection switch 322 (e.g., switch numbered K4) are electrically connected without repetition to any two different power conversion group combinations 10 (e.g., Figure 1 The bus 12 in the combination 10 of the upper and lower power conversion groups (e.g., the combination 10 of the middle and upper power conversion groups) Figure 1 Bus 12 of the power conversion module group 11 connected to G2 and G4.

[0052] This disclosure presents an embodiment of an electric vehicle DC charging system 1. By switching the switches within the matrix 30 using a switching switch, the electrical energy from the power conversion module group 11 can be appropriately distributed to each charging output port 20 according to different charging conditions. Specifically, the following describes... Figure 1 Let's take an example to explain how the switch matrix 30 works. For ease of explanation, Figure 1 The multiple power conversion module groups 11 are numbered G1 to G4, the multiple first parallel switches 321 are numbered K1 to K2, the multiple second parallel switches 322 are numbered K3 to K4, the charging output ports 20 are numbered C1 to C4, and the third switches 323 are numbered K5 to K8.

[0053] When the charging condition is single-output full power, taking the charging output port C1 as an example: turn on the third switch K5 and the first parallel switch K1, and turn on any two of the first parallel switch K2 and the second parallel switches K3 to K4, so that the power of all power conversion modules G1 to G4 can be applied to the charging output port C1 at the same time.

[0054] When the charging conditions are dual outputs at equal power, taking charging output ports C1 and C2 at equal power as an example: connect the third switches K5 and K6 and the second parallel switches K3 and K4. At this time, the output power of charging output port C1 is provided by power conversion module group G1 and G3. The output power of charging output port C2 is provided by power conversion module group G2 and G4.

[0055] When the charging conditions are unequal power outputs, taking the example where the power of charging output port C1 is greater than that of C2: Connect the third switches K5 and K6, the first parallel switch K2, and the second parallel switch K3. At this time, the output power of charging output port C1 is provided by the power conversion module group G1, G3, and G4. The output power of charging output port C2 is provided by the power conversion module group G2.

[0056] When the charging conditions are unequal power outputs, taking the power of charging output port C1 being less than that of C2 as an example: Connect the third switches K5 and K6, the first parallel switch K2, and the second parallel switch K4. At this time, the output power of charging output port C1 is provided by the power conversion module group G1. The output power of charging output port C2 is provided by the power conversion module groups G2, G3, and G4.

[0057] Specifically, the total number of the numbers of the first parallel switch 321, the second parallel switch 322, and the third switch 323 can be obtained by the following formula: Where N is the number of buses in a power conversion group combination 10, X is the total number of all power conversion module groups 11 in the electric vehicle DC charging system 1, and Y is the total number of charging output ports 20.

[0058] Reference Figure 2 and Figure 3 The diagram shows a circuit structure schematic of a DC charging system for an electric vehicle according to several embodiments of the present disclosure. Figure 2 Electric vehicle DC charging system 1a and Figure 1 The main difference between the DC charging systems for electric vehicles is that, Figure 2 The electric vehicle DC charging system 1a is equipped with two charging output ports 20, while Figure 1 The electric vehicle DC charging system 1 is equipped with four charging output ports 20. Figure 3 Electric vehicle DC charging system 1b and Figure 1The main difference between the DC charging systems for electric vehicles is that, Figure 3 The electric vehicle DC charging system 1b is connected in parallel to a combination of three power conversion groups 10, while Figure 1 The electric vehicle DC charging system 1 is connected in parallel to a combination of two power conversion groups 10.

[0059] Reference Figure 4 This diagram illustrates a circuit structure of a DC charging system for an electric vehicle according to one embodiment of the present disclosure. In one embodiment of the present disclosure, the number of power conversion module groups 11 in each power conversion group combination 10 is the same, the number of second parallel lines 312 is the same as the number of busbars 12 in one of the power conversion group combinations 10, and the second parallel lines 312 are provided with pluggable connectors 40 between adjacent power conversion group combinations 10 to adjust the number of parallel power conversion group combinations 10.

[0060] Reference Figure 4 In one embodiment of this disclosure, each second parallel line 312 of each power conversion group combination 10 is provided with a male connector 41 at one end and a female connector 42 at the other end, and the second parallel switch 322 on each second parallel line 312 is disposed between the male connector 41 and the female connector 42 of the second parallel line 312.

[0061] Specifically, a male connector 41 and a female connector 42 can be combined to form a pluggable connector 40. This disclosure does not limit the specific type of the male connector 41 and the female connector 42. Figure 4 The solid-line frame of the power conversion group assembly 10 shown can be considered as a plate, frame, or housing. Male connector 41 and female connector 42 are respectively disposed on the plate, frame, or housing of the power conversion group assembly 10 for quick installation or expansion.

[0062] In one embodiment of this disclosure, the number of power conversion module groups 11 in at least one power conversion group combination 10 is different from the number of power conversion module groups 11 in other power conversion group combinations 10. Specifically, this disclosure does not limit the number of power conversion module groups 11 in the power conversion group combination 10; the number of power conversion module groups 11 in the power conversion group combination 10 can be the same or different.

[0063] Reference Figure 5 and Figure 6 The diagram illustrates a circuit structure of a DC charging system for an electric vehicle according to several embodiments of the present disclosure. In one embodiment of the present disclosure, the number of charging output ports 20 differs from the number of buses 12. Figure 5 Electric vehicle DC charging system 1d and Figure 1 The main difference between the DC charging systems for electric vehicles is that,Figure 5 The electric vehicle DC charging system 1d is connected in parallel to a combination 10 of three power conversion groups, and each combination 10 of power conversion groups contains four power conversion module groups 11. Figure 1 The electric vehicle DC charging system 1 is connected to a combination 10 of two power conversion groups, and each combination 10 of power conversion groups contains two power conversion module groups 11. Figure 6 Electric vehicle DC charging system 1f and Figure 5 The electric vehicle DC charging system 1d consists of three power conversion groups connected in parallel, each of which contains four power conversion module groups 11. The difference lies in... Figure 5 The electric vehicle DC charging system 1d is equipped with 4 charging output ports 20. Figure 6 The electric vehicle DC charging system 1f is equipped with 8 charging output ports 20.

[0064] Reference Figure 7 This illustration shows a schematic diagram of the circuit structure of a DC charging system for an electric vehicle according to one embodiment of the present disclosure. The present disclosure provides a DC charging system 2 for an electric vehicle, including a combination 10 of multiple parallel power conversion groups, at least one charging output port 20, and a switching matrix 30. The combination 10 of the multiple parallel power conversion groups is used to provide DC power. Each combination 10 of power conversion groups includes multiple power conversion module groups 11. Each power conversion module group 11 has multiple power conversion modules 110 and a bus 12 for transmitting DC current. The at least one charging output port 20 is electrically connected to at least one bus 12 in a switchable manner. The switching matrix 30 includes multiple parallel lines 310 and multiple parallel switches 320. Each bus 12 is electrically connected to at least one parallel line 310, and the parallel line 310 and the corresponding parallel switch 320 are switchably and non-repeatingly electrically connected to different buses 12.

[0065] Specifically, Figure 7 All connecting lines between any two busbars 12 are parallel connection lines 310. For the sake of simplicity, Figure 7 Only one parallel connection 310 is marked. The switch on parallel connection 310 is the parallel switch 320. Similarly, for the sake of simplicity, Figure 7 Only one parallel switch 320 is marked. For example... Figure 7 As shown, each busbar 12 is electrically connected to at least one parallel line 310, and the parallel line 310 is electrically connected to different busbars 12 without repetition.

[0066] In one embodiment of this disclosure, only one position P is provided between the at least one charging output port 20 and the switchably electrically connected bus 12 to control whether the at least one charging output port 20 and the bus 12 are switchably electrically connected, and only one switch S is provided at position P.

[0067] Specifically, such as Figure 7 As shown. The at least one charging output port 20 (e.g.) Figure 7 The uppermost charging output port 20) is connected to the switchable ground bus 12 (e.g. Figure 7 Only one position P is provided between the uppermost busbar 12 to control whether the at least one charging output port 20 is electrically connected to the busbar 12 in a switchable manner. Figure 7 The position of the topmost switch S is where only one switch S is installed.

[0068] In one embodiment of this disclosure, each of the power conversion modules 110 includes at least one switching rectifier element.

[0069] In one embodiment of this disclosure, the number of power conversion module groups 11 in at least one power conversion group combination 10 is different from the number of power conversion module groups 11 in other power conversion group combinations 10.

[0070] Specifically, such as Figure 7 As shown, the number of power conversion module groups 11 in the power conversion group combination 10c is 2, which is different from the number of power conversion module groups 11 4 in the other power conversion group combinations 10a or 10b.

[0071] In one embodiment disclosed herein, the number of charging output ports 20 is the same as the number of buses 12. Specifically, as... Figure 7 As shown, the number of charging output ports 20 is 10, the same as the number of busbars 12.

[0072] Reference Figure 7 and Figure 1 In one embodiment of this disclosure, the plurality of parallel lines 310 include a plurality of first parallel lines 311 and a plurality of second parallel lines 312, and the plurality of parallel switches 320 include a plurality of first parallel switches 321 and a plurality of second parallel switches 322. Each first parallel line 311 and the first parallel switch 321 are electrically connected in a switchable manner and non-repeatingly to any two buses 12 in the same power conversion group combination. The second parallel line 312 and the second parallel switch 322 are electrically connected in a switchable manner to the buses 12 of any two different power conversion group combinations.

[0073] In one embodiment of this disclosure, the plurality of parallel lines include a plurality of first parallel lines and a plurality of second parallel lines, and the plurality of parallel switches include a plurality of first parallel switches and a plurality of second parallel switches. Each first parallel line and the first parallel switch are electrically connected switchably and non-repeatingly to any two buses in the same power conversion group combination, and the second parallel line and the second parallel switch are electrically connected switchably to the buses of any two different power conversion group combinations.

[0074] Compared with existing technologies, the electric vehicle DC charging system disclosed herein uses fewer switches, has a simpler combination architecture, and has a more flexible power output combination. Furthermore, the smaller number of switches not only improves the ease of system expansion but also effectively reduces costs and avoids the problems in existing technologies.

[0075] The above description of the preferred embodiments illustrates the features of this disclosure, and is intended to enable those skilled in the art to understand the content of this disclosure and implement it accordingly, rather than to limit the claims. Therefore, any equivalent modifications or alterations made without departing from the technical concept disclosed in this application should still be included in the following claims.

Claims

1. A direct current charging system for electric vehicles, comprising: a plurality of parallel power conversion clusters for providing direct current power, each power conversion cluster comprising a plurality of power conversion module clusters, each power conversion module cluster comprising a plurality of power conversion modules and a bus bar for transmitting direct current; at least one charging output port switchably connected to at least one of the bus bars; and a switch matrix comprising a plurality of first parallel lines, a plurality of second parallel lines, a plurality of first parallel switches and a plurality of second parallel switches, each first parallel line and first parallel switch switchably and non-repeatedly connecting any two bus bars in the same power conversion cluster, the second parallel lines and the second parallel switches switchably connecting bus bars in different power conversion clusters.

2. The electric vehicle DC charging system of claim 1, wherein, Any bus bar switchably connected to the at least one charging output port is connected to at most one bus bar in a different power conversion cluster through a second parallel line and a second parallel switch.

3. The electric vehicle DC charging system of claim 1, wherein, There is only one position for controlling the switchable connection between the at least one charging output port and the bus bar, and the position is provided with only one third switch.

4. The electric vehicle DC charging system of claim 1, wherein, Each power conversion module comprises at least one switching rectifier element.

5. The electric vehicle DC charging system of claim 1, wherein, The second parallel lines and the second parallel switches non-repeatedly connect bus bars in different power conversion clusters.

6. The electric vehicle DC charging system of claim 1, wherein, The number of power conversion module clusters in each power conversion cluster is the same, the number of second parallel lines is the same as the number of bus bars in one of the power conversion clusters, and the second parallel lines are provided with pluggable connectors between adjacent power conversion clusters for adjusting the number of parallel power conversion clusters.

7. The electric vehicle DC charging system of claim 6, wherein, Each second parallel line is provided with a male connector at one end and a female connector at the other end, and the second parallel switch on each second parallel line is arranged between the male connector and the female connector of the second parallel line.

8. The electric vehicle DC charging system of claim 1, wherein, The number of power conversion module clusters in at least one power conversion cluster is different from the number of power conversion module clusters in other power conversion clusters.

9. The electric vehicle DC charging system of claim 1, wherein, The number of charging output ports is different from the number of bus bars.

10. A direct current charging system for electric vehicles, comprising: a plurality of parallel power conversion clusters for providing direct current power, each power conversion cluster comprising a plurality of power conversion module clusters, each power conversion module cluster comprising a plurality of power conversion modules and a bus bar for transmitting direct current; at least one charging output port switchably connected to at least one of the bus bars; and a switch matrix comprising a plurality of parallel lines and a plurality of parallel switches, wherein each bus bar is connected to at least one parallel line, and the parallel lines and the corresponding parallel switches are switchably and non-repeatedly connected to different bus bars.

11. The electric vehicle DC charging system of claim 10, wherein only one position is provided between the at least one charging output port and the bus bar to which the at least one charging output port is switchably electrically connected, and the position is provided with only one switch.

12. The electric vehicle DC charging system of claim 10, wherein each of the power conversion modules comprises at least one switching rectifier element.

13. The electric vehicle DC charging system of claim 10, wherein, The number of power conversion module groups in the combination of at least one power conversion group is different from the number of power conversion module groups in the combination of other power conversion groups.

14. The electric vehicle DC charging system of claim 10, wherein, The number of charging output ports is the same as the number of bus bars.

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