What You Will Find In This Chapter

2.1 PHOTOVOLTAIC ELECTRICITY

2.2 BASIC SYSTEM CONFIGURATIONS

2.3 COMPONENT OPERATION

2.4 TYPICAL APPLICATIONS

2.5 SYSTEM COMPONENT OPERATION
2.1.1

 

 

 Introduction.   This section describes the similarities and differences between photovoltaic and "conventional" AC (Alternating Current) electricity. Knowing this information will help you to better understand the systems and the service operations.

For this section, you need to know that a photovoltaic module is the smallest unit of solar electricity-producing equipment you will normally work on. A photovoltaic array is a group of one or more modules.
2.1.2 Impact of Load. The amount of electricity produced by a photovoltaic system to operate lights, motors, electronics, and other loads is not infinite. For this reason, an oversized load, or one which operates too many hours per day, will cause problems. These problems range from an interruption of the load to damage to the photovoltaic system or the load.

Loads are described in more detail in Section 2.4
2.1.3 DC Electricity.  Photovoltaic electricity is DC (Direct Current). The current has a polarity, that is, it flows in one direction. This has an impact on wiring methods and equipment.   In photovoltaic systems, grounding methods must be complete and correct.  Wire color conventions are critical, not only to protect equipment from reverse polarity, but also to protect service personnel and system users. 

Section 2.5.7 has more information on polarity and color conventions.
2.1.4 Current-Limited System.  When the electrical lines from a utility company's AC power supply are crossed, the resultant short circuit causes an almost infinite current flow. For this reason, fuses and circuit breakers are used to provide over-current protection.

Photovoltaic modules are current-limited. A short-circuited photovoltaic module will produce current only up to a certain level. In fact, a common check of system performance is to deliberately short-circuit the photovoltaic modules and measure the current flow. This does not damage the modules (Figure 2-1).
 

 

 

FIGURE 2-1
Short Circuits in AC
Circuits (Top)
and Photovoltaic Module
Circuits (Bottom)

 figure2-1.gif (30232 bytes)

WARNING!

Photovoltaic modules can be short-circuited without damage. However, a short circuit in other system components may cause component damage and dangerous, even lethal, conditions. This is particularly true of storage batteries. Always be sure to open a disconnect switch between the short circuit and the batteries.

Never attempt to short-circuit storage batteries!

 

2.1.5 Low Voltage Does Not Mean Harmlessness.   Whenever working on or around photovoltaic systems remember three very important points:

1) Even at low voltages, photovoltaic systems may be able to deliver substantial current. The amount of available current may be high enough to kill you.

2) Photovoltaic systems can have two power supplies, not just one. Both the batteries and the modules in a system can deliver current.

3) Small "harmless" shocks can still injure you. For example, an arc created when making a wiring connection can ignite the hydrogen gas given off by storage batteries, causing an explosion. Likewise, a small shock can startle you, resulting in a fall from a ladder.
2.1.6 Voltage Drops.  Unlike most AC systems, photovoltaic systems can suffer from a substantial voltage drop between the power source and the load. Good design practices minimize this drop.

As an extreme example, the available voltage at the photovoltaic array might be 16 volts. After traveling through hundreds of feet of undersized wire, it could be as low as 11 volts (Figure 2-2).

The system would not be able to recharge a 12 volt storage battery. This is because the available voltage is not higher than the voltage of the battery.
FIGURE 2-2
Voltage Drop in a
Photovoltaic System		 figure2-2.gif (22765 bytes)
 

 

FIGURE 2-2

Voltage Drop in a

Photovoltaic System
Wire runs must be kept as short as possible. Wire must be large enough to minimize the voltage drop. Use the charts in Appendix C to determine these sizes. Notice that the wire sizes in photovoltaic systems are much larger than those in AC systems.
2.1.7 Connect/Disconnect Sequences.  Connect/Disconnect Sequences. Unlike AC systems, the sequence of connection and disconnection is critical to many photovoltaic system components.

It should be noted that explosive hydrogen gas may be present near batteries. Making the last connection at the battery may create a spark which could result in an explosion. The best sequence of battery terminal connection might be as follows and as shown in Figure 2.3:

1) positive connection at battery.

2) positive connection at load.

3) negative connection at battery.

4) negative connection at load.
FIGURE 2-3
Battery Terminal
Connection Sequence		 figure2-2.gif (22765 bytes)
Other components are equally sensitive to connect/disconnect sequences. Charge controllers, in particular, may need to be connected in the correct sequence to prevent damage.