PV SYSTEM COMPONENTS AND BOS

As you read in our article “Types of PV Systems”, you now know there are 3 types of systems:

  1. Off Grid Systems
  2. Hybrid Systems
  3. Grid Tied Systems

This will discuss the different components for each type of system.

GRID TIED SYSTEMS

Grid tied systems are the most basic type of PV system.  There is no energy storage, so there are no batteries for the system. The common item among all PV systems are the PV modules themselves. The PV modules for grid tie systems will use FUSES (600VDC rated as opposed to BREAKERS rated at only 150VDC). The FUSES will go into FUSE HOLDERS housed in the COMBINER BOX. The COMBINER BOX will allow all the strings to be combined.

For instance, if you have 3 strings, you typically would run 7 wires (3 +, 3 -, and 1 ground). However, with the combiner box, only 3 wires are needed to run to the inverter (+, -, and ground).

After the COMBINER BOX, The PV modules will send a relatively high DC voltage (300-500VDC) to an inverter which will convert the DC voltage into AC power, which is then used for consumption or sold back to the utilities. Typically, the inverter will be isolated on each side by a DC DISCONNECT and AC DISCONNECT.

HYBRID AND OFF GRID SYSTEMS

Hybrid systems are very similar to off-grid systems, with the only difference being the inverter, and in some cases the same inverter can be used for either type of system. Both Hybrid and Off-Grid Systems use energy storage, so batteries are involved in both systems. The main difference is that in Hybrid systems, selling back to the utilities is an option.

Off grid systems can NEVER sell power back to the utilities

However they can still use the grid as auxiliary power and to charge the batteries. A common misconception is that OFF-GRID means no access to utility power. OFF-GRID simply means that no power will be exported back to the grid. Commonly, there is a grid presence in off grid systems.

After the PV module strings, there is still a combiner box just like the Grid-Tied systems.  Unlike grid tied systems, since the string voltage is much lower (150VDC max), BREAKERS are used instead of fuses.  As you read under the Grid Tie Systems section, the PV modules go from the COMBINER BOX to an inverter. In OFF GRID systems, the combiner box instead goes to a CHARGE CONTROLLER (sometimes called a voltage regulator). The charge controller then regulated how much current will go the BATTERY BANK. The BATTERY BANK consists of several batteries, configured in series, parallel, or series-parallel, depending on the system voltage.

Typical off grid and hybrid systems use 12, 24, and 48VDC for system voltages. As a rule of thumb, I typically design 1000W and under systems to 12V, 1000-2000W to 24V, and 2000W and up to 48V. There will of course be exceptions depending on the needs of the customer.  The batteries then will connect to the inverter, which serves the same function. In Grid Tied systems it typically converts a higher DC voltage (300-500V) with a low current into AC power. The Hybrid and Off Grid systems will converter the lower DC Voltage (12, 24, 48) with a much higher current into AC power.

As with other systems, a DC and AC disconnect is required on each side in case the inverter needs to be isolated. Typically a larger off-grid/hybrid inverter will also come with a LOAD CENTER, making all the connections easier and saving time and money on the installation.  The hybrid inverter can be programmed to sell back power at certain times or when the battery voltage hits a certain level.  The Off-Grid inverter can be programmed to allow the grid to charge the batteries when a certain voltage set point is reached.

We have a full staff of sales, electrical, and mechanical engineers who can design a system for you and discuss any of your requirements, such as the type of system that is best for you.  Sun Electronics has designed thousands of PV systems over the past 25 years for customers all over the world. Call us for any questions you may have.

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