A PV system designer must know A few electrical concepts, like series and parallel wiring, a set of electrical circuit laws, and IV curves must be understood before you can construct your system. We’ll go over each fundamental principle before giving you a few straightforward applications that you may use in your own analyses. Whether you’re designing for string inverters or microinverters, these fundamental concepts are crucial.
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Wiring in Series and Parallel:
There are two wiring configurations utilized for various components of a PV system: series wiring and parallel wiring. The series strings are made by wiring individual modules together in series. After that, the series strings are connected in parallel before being fed to the rest of the system. Both off-grid and grid-tied systems using string inverters are covered by this. Off-grid users will also wire the battery bank in series and parallel. With microinverter systems, parallel wiring is not a big deal. When you connect the positive (+) lead of one module to the negative (-) lead of the next, you are wiring modules in series. You have one additional negative lead and one additional positive lead at the end of this string.
Series wiring has the effect of increasing voltage while maintaining the same amperage. When 10 modules are connected in series and each has an output rating of 30 volts at 8 amps, the string as a whole will have a voltage rating of (30×10) 300 volts at 8 amps.
When wiring series strings of modules in parallel, you must connect the extra positive and negative leads of the series strings together. Parallel wiring has the effect of maintaining the same voltage while increasing the amperage. For instance, if you connect two of the aforementioned strings (each rated at 300 volts and 8 amps) in parallel, the circuit as a whole will have a rated output of 300 volts and 16 amps. The rated power output of this array is 300 x 16 = 4,800 watts (remember that volts x amps equal watts).
Due to equipment compatibility and capability, you should connect modules in series and series strings in parallel rather than the other way around. PV modules typically produce a relatively high current and low voltage (around 30 volts DC) (typically about 8 amps DC). Contrast that with a typical home electrical circuit, which uses 120 volts AC and can only handle 15 or 20 amps AC at most. The voltage is increased without increasing the current when the modules are wired in series. Additionally, 200 to 400 volts and 10 to 40 amps are commonly supported by string inverters. These voltage and current ranges can be reached by the PV array by wiring modules in series and series strings in parallel.
Rules for DC Electrical Circuits:
A PV system designer must know Basic DC electrical circuits wired in series and parallel are governed by three electrical circuit principles.
Rule 1: The voltages increase while the amperage remains constant when modules are coupled in series.
Rule 2: The voltages of each of the strings must match when a series of strings of modules are wired in parallel.
Rule 3: The total amperage of the strings increases but the voltage of each string remains constant when series strings of modules are wired in parallel.
When determining the output values for your modules and strings and making sure the values are compatible with your inverter’s input needs and standards, you will follow these three guidelines. They also have an impact on how you build the module strings, although string inverter and microinverter systems are subject to distinct restrictions.
Every series string in an array must have an equal number of modules in order to comply with rule 2. If you know that your array will have 18 modules in total, for instance, you can wire them into two strings of 9 modules or even three strings of 6 modules, but you shouldn’t wire them into one string of 10 and one string of 8. If you have uneven strings, one will be functioning over its optimal value and the other will be operating below its optimal level since rule 2 requires that both strings of the parallel circuit have the same voltage. As a result, neither string will be using its ideal voltage level. With the help of the discussion of I-V curves that follow, you will comprehend this better.
The aforementioned guidelines don’t apply since microinverters convert from DC to AC at the module. As a result, you don’t need to bother about assigning equal numbers to each group of your modules. One string of 10 modules, one string of 8, or even two strings of 7 and one string of 4 are acceptable configurations without degrading performance, for instance. You must abide by the manufacturer’s restriction on the number of microinverters that can be linked together. Some arrays may consist of a single string since some microinverters can be connected in long strings of 20 or more modules. However, the majority of microinverters have lower limits, leading to arrays with two or more strings. Branch circuits are the official name for module groupings in microinverter systems, possibly to avoid confusion with string-inverter strings. Both words describe a set of modules connected in series. The maximum number of units per string for microinverter systems must not be exceeded by strings (branch circuits).
A PV system designer must know The relationship between current, or amperage (I), and voltage (V), in an electrical device such as a solar cell or PV module, is depicted graphically by an I-V curve. An I-V curve illustrates how variations in either the current or the voltage affect electrical output since electrical power (P, expressed in watts) is a product of current and voltage:
P (power) = I (current) × V (voltage) and 1 watt = 1 amp × 1 volt
The I-V curve of a single PV module shows that the current is zero when the voltage is at its maximum (in this case, 40 volts). This equals 0 watts of power (40 volts x 0 amps = 0 watts). Conversely, when the voltage is at 0 and the current is at its maximum (9 amps), there is no power generated (9 amps x 0 volts = 0 watts). This indicates that you must balance the current and the voltage in order to generate usable electricity.
A PV system designer must know Voltage can be thought of as the pressure that pushes electricity across an electrical circuit. Consider the pressure of water in a garden hose. The flow of electrons in a circuit, which is referred to as current, is a function of pressure. Consider the quantity of water flowing via a garden hose. The positive (+) and negative (-) output wires of a photovoltaic module, when connected together, produce a short circuit with maximum current (high flow), but zero voltage (no pressure), as there is no resistance to the flow. The point where the I-V curve meets the Y (current) axis is known as the short-circuit current value (Isc) on the module’s specification sheet. When the two wires are disconnected, an open circuit is formed with maximum voltage (high pressure), but no current (no flow). Even if the solar cells are working, there is nowhere for the electrons to travel, thus no power is produced. The point on the IV curve where the curve meets the X (voltage) axis is identified as the open-circuit voltage value (Voc) on the module’s specification sheet.
Every point on this curve has a power value (in watts) that is equal to the current (amps) times the voltage (volts) at that point. When the IxV product, or the product of the values of the current and voltage along the IV curve, is largest, this is when you get the most electric power. The maximum power point (MPP, or Pmax ) is the name given to this sweet spot. This is always close to the IV curve’s “knee”; in this given curve above, P is at roughly 33 volts and 7.6 amps. This is the same as the module’s rated output (33 volts x 7.6 amps = 250.8 watts), which is also where we want the module to function.
DC-AC inverters (both string inverters and microinverters) sense the current and voltage in a module’s or string’s circuit and then modify the circuit’s level of resistance to alter the voltage and current as necessary to always hit the MPP. The inverter modifies resistance to modify the voltage and maintain the MPP at the lower current value if the current decreases. In order to run at the MPP, the inverter once again changes the resistance as the current increases, altering the voltage and raising the output. No matter the weather outside, the homeowner always receives the highest power output from their PV system. A PV system designer should thanks to this ongoing MPPT optimization technique.