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101 renewable - dc array combiners

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Last Updated
7th of January, 2020

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MidNite Array Combiner type DISCO

DC combiner is a box containing several input conductors coming together on a common bus, the positive bus; that common bus, meanwhile, has one output conductor, the "Home Run" to the DC disconnect which in turn can be a DC disconnect box or the DC disconnect of a transform less inverter. Many conductors go in, but only one big conductor comes out of the home run. Normally DC combiners have provisions for an OCPD (OCPD = Over Current Protection Device) on each of the input conductors. They also have an internal DC disconnect switch on the output conductor with some of the brands, surge protection devices, or contain monitoring equipment.


Standard DC combiner box:-

A standard DC combiner refers to a negative-ground PV system and it has the following components:

>_Ground Bus
>_Negative Bus
>_Positive Bus
>_Multiple input terminals that accept a range of conductor sizes
>_One set of output terminals that accepts a range of conductor sizes and may allow parallel conductors
>_OCPDs on each input to the positive bus OCPD = Over Current Protection Device
>_Provisions for mounting combiner to a support structure.

NEMA-rate enclosures (NEMA 3, 3R, 4)

On a positive-ground PV system, the OCPDs are located on the negative inputs. On an ungrounded PV system, the OCPDs are located on both the positive and negative inputs.

DC Combiners are either 600VDC or 1000VDC rated. Some of the DC Array Combiners are rated for 600VDC and 1000VDC. In case we have an array with a big number of strings it used to segment the array. By segmenting the array into smaller segments we talk about segment DC disconnects and segment array combiners.

There are some NEC requirements for DC array combiners:

DC combiner with the dc disconnect. Some of the array combiners are equipped with a disconnect on the outgoing for servicing purposes. They have to conform to NEC 2011 Article 690.16(B). That one requires that DC disconnecting means be located within the line of sight of the fuses on the PV output circuit. DC disconnects that are provided within the array field specifically to make maintenance procedures safer.

DC combiner listing requirements. Starting with the 2008 NEC, Article 690.4(D) specifically requires “source-circuit combiners” to be listed and identified for the application. To be identified according to NEC Article 100, a device has to be “recognizable as suitable for the specific purpose, function, use, environment, application, and so forth, were described in a particular Code requirement.” This recognition is usually accomplished by having the appropriate product listing, in this case, UL 1741. In the past, some dc combiners were incorrectly listed under UL 508, the standard for industrial controls, but they should now all be listed under UL 1741. To be listed, a dc combiner must have been tested by a nationally recognized testing laboratory (NRTL) that has been recognized by OSHA to certify to the UL 1741 test—such as UL, ETL, CSA or TUV—and have received its mark.

Mounting location. When determining the mounting location for DC combiners, consider Code requirements, accessibility, and the thermal environment. DC combiners need to be mounted in compliance with NEC Article 110.26 if they are used in systems operating at 600 V or less. If the system operates at potentially greater than 600 V, as is increasingly the case on larger PV systems, refer to Article 110.32 and other requirements found in Section III of Article 110.

Terminal temperatures. While terminal temperature ratings are important in the conductor sizing process, as described in Article 110.14(C), they often are not listed clearly in dc combiner documentation. Frequently the 90°C rating of the output terminal is listed in the documentation, for example, but the temperature rating of the OCPD terminal on the input circuits is not. For the most part, the OCPD terminals in dc combiners are listed for 75°C and rarely more. Unless they are specifically marked otherwise, terminals for circuits rated for 100 A or less, or marked for #14 through #1 conductor sizes, are rated for 60°C. The terminal temperature rating should be verified if it is not listed on the datasheet.

Grounding. As with any other electrical equipment, dc combiners have to be grounded with an equipment-grounding conductor (EGC) that is landed on the product’s ground bus. The dc combiner is often a transition point from one or more EGCs on the input to a single EGC on the output. The dc combiner is also a common point to install lightning or voltage surge protection devices.

New additions to DC combiners:

Remote actuated disconnect for DC combiners. Besides DC combiners with manual DC disconnect incorporated for service and maintenance purpose we have remote activated DC combiner through a DC, contactor mounted on the outgoing. This is a service for DC contactor with the main role to disconnect DC supply from the inverter in case of AC power loss on the utility line or in a case of an inverter mall function. This remote activated DC contactor has to be installed conform to Article 690.16(B) of NEC 2011.

Intelligent remote control system designed for DC combiner boxes. The best example is the MidNite "Birdhouse" mounted at the ground level where it can be very easily accessed. That one communicates through a CAT 5 , 600VDC rated to a custom build shunt trip with positive feedback where it is sending a remote trip input. That is available to all MidNite DISCOS series type of dc combiners.

DC combiner and surge protection. The dc combiner is also a common point to install lightning or voltage surge protection devices SPDs'. Indicating the SPD’s status via an integral auxiliary set of contacts is an enhanced feature that can provide a signal to a tripping shunt and disconnect the PV DC source from the rest of the installation in case a lightning strike occurs. That limits the chance of fire hazards. The connection between surge protection and the tripping shunt must be done conform NEC Article 690.11 Arc-Fault Circuit Protection (dc) and the same the way the surge protection is installed relative to DC combiner box.

This is a solution for a segmented array in two sub-segments each provided with own fault protection each sub-array segment considered as a source circuit. Because they are feeding separate inverters we look at a single source circuit for each of them. Because they are in the same array combiner box and they may be combined together in a single circuit, even by mistake, we look at the possibility of two source circuits connecting together.

Therefore we have to analyze the resilient to a fault of the combiner circuits.

Single source circuit. In a PV system consisting of a single source circuit, where the conductor is connected to the PV string on one end and to the inverter on the other, a fault on the conductor has two possible sources of current, as shown in Diagram 1. One source is the maximum PV source-circuit current, which NEC Article 690.8(A)(1) defines as the short circuit current multiplied by 125%. Therefore the fault current (IFAULT) at Point A in Diagram 1 is calculated as follows:

The conductor at Point A is already sized to carry the maximum circuit current. The another potential source is the inverter back feed current, or I BACKFEED at Point B. Most transformer-based inverters have zero back feed current.

>_where IMAX is equal to 1.25 x ISC.

Assuming that IBACKFEED is zero, as it most often is, the conductor at Point B has to carry the only IMAX for which it is already rated, and no OCPD is required.

Note, however, that some transformerless inverters may have back-feed current. If IBACKFEED is greater than the conductor rating, then either an overcurrent protection device (OCPD) is required at the inverter connection or the conductor has to be sized for the back feed current. If IBACKFEED exceeds the module’s series fuse rating, then overcurrent protection is required regardless of how the conductors are sized. In this case, the OCPD is not required in order to meet Article 110.3(B) of the NEC, which requires that equipment is “installed and used in accordance with any instructions included in the listing or labeling.”

Two source circuits. If two PV source circuits are combined at a common point and a fault occurs in a current carrying conductor, then the fault current carried by the conductor to the fault from the PV string is once again the maximum circuit current (IMAX) at Point A.

At Point B, the available fault current through the common connection point is the sum of the current from points C and D:

>_where IMAX is equal to 1.25 x ISC.

Assuming that IBACKFEED is zero, as it most often is, the conductor at Point B has to carry the only IMAX for which it is already rated, and no OCPD is required.


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