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Monday, 14 December 2015

How useful is a Combiner Box in a solar PV system?

We are fast adopting renewable energy (RE) sources to fulfil our energy requirements and solar energy is going to be the dominant RE source in future, particularly in India. Two different systems of solar energy are employed now-a-days viz. solar thermal plants, and solar PV plants. Solar PV system is gaining a swift popularity among the mass, particularly the roof top solar PV system.

Two diverse types of roof top solar PV systems are in use. They are the standalone i.e. grid independent PV system and the second one is the grid-tied roof top solar PV system. The standalone solar PV system is relatively small in capacity; varying from a few hundred watts to a couple of kilowatts. On the other hand grid-connected or the grid-tied roof top solar PV system is comparatively larger in size; starting from a couple of kilowatts to few hundred kilowatts.
To get the required power output, the different PV panels are connected in series and/ or parallel. How a panel is connected depends on the rating of the charge controller, battery and the inverter. Factors such as voltage drop and power loss are also taken into consideration while going for a particular connection.

Let’s understand this aspect considering an example:

Suppose that we have to install a 400 W solar PV system. Four panels of 100 W each with a voltage (at maximum power) of 17.2 V and maximum current of 5.8 A are considered for installation. Now as per the load calculation and needed back up time an inverter of 850 VA and battery of 150 Ah are to be employed. Usually an inverter of 850 VA is available in the 12 V rating and so is the battery. Thus we have to use a charge controller of 12 V rating and matching the required current capacity. 
All the four panels should be connected in parallel so that the DC output after the charge controller is of 12 volts (input to the charge controller is a bit higher than 12 V). In this way, by parallel connection of panels, we can match the voltage rating of all the equipments i.e. inverter, the battery and the charge controller. Now if the inverter available in the market is of 24 V, then we have to use two batteries of 12 V each in series so that their voltage adds up. Now you have to connect your PV panels in series parallel combination. I hope the connection is clear to you.

What is a Combiner Box? 

       
Combiner boxes are an integral part of solar PV installation. They serve as the junction point where the several parallel connections from the PV panels come and join. Panels are connected in series and / or parallel as per the requirement as mentioned above.
               
The combiner box contains the necessary over-current fuses and circuit breaker, the bus-bars and the terminals for the required connections. Many solar PV installers fabricate their own combiner boxes to cut down the cost and to promote their products; otherwise there are so many combiner box manufacturers with many variants. Custom build or tailor made combiner boxes are also available on demand. Smart combiner box with data monitoring capability are also available which allows easy installation of data monitoring system.

 Solar PV systems above 5 kW usually have more wires and their connection becomes a difficult job without the use of a combiner box. Some grid-tied inverters come with the fuse protection and arrangements for parallel input connections and hence separate combiner box is not required.

Fig 1: A combiner box with Fuses & Circuit Breakers


Location of a Combiner Box

The combiner box should be as close as possible to the PV panels so that the length of wire required is reduced and the trouble shooting becomes easy as one can easily locate/ identify different wires. Since most of the combiner boxes are installed near the PV array i.e. they are installed outdoor and hence must be adequately weatherproof.

Voltage rating of a Combiner Box

Each combiner box is rated for a specific DC voltage. Most of the combiner boxes compatible with standalone PV system can handle 150 V DC at the maximum, whereas the grid-tied combiner box is rated up to 600 V DC.

Number of Terminals in a Combiner Box

Combiner boxes usually have a fixed number of input and output terminals. Grid interactive system requires fewer input and output terminals, as they usually work at higher DC voltage, while the battery backed PV system has more number of parallel wires. Large battery backed system may have multiple charge controllers and each controller may have its own combiner box. For an off-grid system it is a smart decision to have enough terminals in the combiner boxes for future extensions. To make the future additions an easy job, the combiner box and its output wires should be adequately sized.

Protection in a Combiner Box

As stated earlier both fuses and DC circuit breakers are used as protective devices in a combiner box, depending upon the DC system voltage. The fuse used in a combiner box cannot be opened up under load and hence cannot be employed as DC disconnectors.