The Voltage & current operation of photovoltaic solar panels has some properties that are not like the properties of other common forms of solar energy.
The origin of Photovoltaic Solar Panels currents
From the point of view of an electrician, Solar Panels installer or electrical inspector, currents start in the Solar Panels module, at least for the solar DC part of the system. Those interested in learning about the Solar Panels photovoltaic effect of converting photons from the sun into electrons should take a physics course at a local school, take an online course or get a good book on physics.
The electricity from common sources, such as domestic alternating current (AC) at 120 volts and 60 Hz, or the output of the battery at 12 volts direct current (DC) is relatively stable. In comparison, the output (voltage and current) of a Solar Panels cell, a Solar Panels module or a Solar Panels array varies with the solar light of the Solar Panels system, the temperature of the modules and the load connected to the system. A single silicon photovoltaic cell will produce approximately 0.5 volts under an optimum load. There are other Solar Panels materials (e.g., cadmium telluride, indium serenade and copper) used in photovoltaic modules that will have different characteristics.
Standard test conditions.
PV modules are rated for power, voltage and current output when exposed to a set of standard test conditions. Those ratings are printed on the back of each module and are available in the data sheets for each particular module. The standard solar intensity (called irradiation) is set at 1000 watts per square meter (W / m2). This is an international constant and is close to the average value of irradiation at sea level on the surface of the earth. The modules are also rated at a standard module/cell temperature of 25 degrees Celsius (C) [77 degrees Fahrenheit (F)]. These two values of irradiance and temperature are known as standard test conditions (STC).
When the module is exposed to these standard test conditions and connected to the correct load, during the safety assessment of the module, performed by a nationally recognized testing laboratory (NRTL), which leads to the module appearing as-as required by the National Electrical Code (NEC) 690.4 (D), the classifications of the module are also verified to be within some percentage of the values of the label. The tolerance on the values of the label is usually 10 percent but can be as low as 3 percent. A photovoltaic module, as a source of current, not a voltage source, can be short-circuited indefinitely without damage. And, as will be shown in subsequent articles, the wiring, switching equipment, and overcurrent protection are designed to allow full short circuits of photovoltaic panels without damage.
The real world environment.
Unfortunately, or fortunately, depending on your point of view, solar radiation and temperatures are generally not in the STC values. At night, there is no output from the photovoltaic module, and shadows, clouds, rain, fog, smoke, and dust will reduce the solar intensity and, at a lower temperature, the module temperature and output power will be lower. Below the STC classification.
And, on clear, sunny and cold days, the photovoltaic module can produce more than its nominal current, voltage, and power for three hours or more; usually during a period around solar noon, which should not be confused with the local standard noon. Due to the significant variation in the output of the photovoltaic system and due to environmental conditions, a single measurement of the voltage or current of a photovoltaic system is not particularly useful. To see how the output of a photovoltaic module.
Measurements of Current and Voltage.
The measurement of the output of the module or matrix under short-circuit conditions will allow the measurement of the short-circuit current (Isc), which will be used in the sizing of the photovoltaic system and in many calculations of the Code. A voltage measurement in short-circuit conditions will produce zero (0) volts.
If a voltmeter is used to measure the voltage output of a module or photovoltaic assembly that is not connected to any load, the voltage obtained will be the open circuit voltage (no load) (Voc). A current measurement would be zero (0) for this open circuit condition. If simultaneous measurements of voltage and current are taken in a photovoltaic module or a photovoltaic array and these measurements are represented for several loads, a graph showing the electrical characteristics of a photovoltaic module could be shown.
The graph would have current (I) on the vertical axis and voltage (V) on the horizontal axis. This graph or graph for a single photovoltaic module is shown in figure 1 and is called curve IV. Similar curves appear in the data sheets of the photovoltaic modules and are made in the laboratory in photovoltaic modules or in the field in modules, chains or photovoltaic matrices.
P = V x I
The point to the right on the horizontal axis is the open voltage circuit (VOC) and the current at this point is zero (0). In the vertical current axis, the curve crosses the axis in the short-circuit current (Isc) where the voltage is zero.
A device called the IV curve plotter is used by the largest module manufacturers, test laboratories or photovoltaic installation organizations under a constant solar irradiation at a constant temperature to automatically register the voltage and current data for these IV curves. The irradiance and the temperature of the module are also recorded during the very short period of time (one second or two or less) required to take the data.
The multiplied by the current.
Each point on the IV curve represents a voltage value and a current value at a particular load. By multiplying the voltage (V) by the current (I), the power (P) produced by a module and delivered to the load will be calculated.
P = V x I
From this relationship, it can be seen that the module does not supply power either at the open-circuit voltage point or at the short-circuit current point because one of the energy factors is zero at these points. However, if other points on the curve are examined (producing power at different loads), it will be observed that the power is not zero for these points.
If the power output curve is added to curve IV, the graph shown in figure 2 is obtained, which includes curve IV in blue and the power curve in red. The horizontal axis for the combined graph is still volts, but the vertical axis (to the right) for the power curve is now marked in watts. The power curve shows that it reaches a peak for a certain load between Isc and Voc and this point is called the maximum power point (Pmp).
It should be noted that the output voltage of a photovoltaic module is not constant and varies with the load. This output is modified by several different external environmental conditions in addition to the connected load.
The current varies with the intensity of sunlight.
The current output of a photovoltaic module is directly proportional to the intensity (irradiance) of the sunlight falling on it. The nominal currents (both Isc and Imp) are emitted under standard irradiation test conditions of 1000 W / m2. However, photovoltaic modules are exposed to irradiation values from 0 (night) to 1500 W / m2 (cloud, water, snow or improved sand) and the current follows changes in the intensity of sunlight.
A 10% reduction in the irradiance value will result in a 10% reduction in Imp. However, the open circuit voltage (Voc) remains relatively unchanged with small variations in irradiance. Figure 3 shows the IV curves for a photovoltaic module since the intensity of sunlight varies from 1000 W / m2 to 500 W / m2. As can be seen, Its changes in direct proportion to changes in irradiance, but Voc and Vmp do not vary as much.
This is a significant fact. The voltage in a photovoltaic module or photovoltaic array will generally be present in very low levels of light, such as at dawn or dusk. solar panel array can have hundreds of volts in wiring at dawn and dusk, even when the sun does not directly illuminate the fronts of the modules. Dangerous voltages in terminals exposed in DC combiners, disconnections and input terminals to inverters will be present.
A second variation in the output of the module and the IV curve is caused by the temperature. The module current is relatively insensitive to temperature, but the voltages Voc and Vmp will be affected. In crystalline silicon photovoltaic modules, the Voc varies inversely with the temperature of approximately 0.5% per degree Celsius and the peak power voltage (Vmp) varies inversely to approximately 0.4% per degree Celsius.
Turn off the photovoltaic system.
The photovoltaic modules in the public utility systems are connected in series and the open circuit voltage can approach 600 volts (housing), 1000 volts (commercial) and 1500 volts (utility scale) in cold climates.
The only way to effectively shut off all the electricity from a photovoltaic module or a photovoltaic array is to cover it with an opaque material. Working at night in the matrix wiring is an option, but worker safety would be a concern, and the rays that light the far sky are known to illuminate the matrix enough to produce electric shocks.
Variations of voltage and current: why and how to deal with that.
In the photovoltaic design process, the output of the matrix must match the input of the grid inverter. The typical inverter will require voltages of several hundred to thousands of volts or more to operate efficiently. Designers and installers of photovoltaic systems want to maintain high voltage to reduce the size and costs of driver
This limit will often be as high as 600-1500 volts, depending on the design of the system and the equipment used. Similarly, as explained below, all conductors, overcurrent devices, and switching equipment must be able to handle the output current of the array in the worst conditions of high sunlight intensity. Exceeding the rated voltage in inverters, conductors, switching equipment or other equipment would be a violation of the Code and is known to have damaged such equipment.
The first pioneers in the Solar Panels sector, including manufacturers of Solar Panels, Jet Propulsion Laboratories (space programs), Underwriters Laboratories (UL) and the National Fire Protection Association (NFPA) realized that, due to these variations in performance of the solar panels module, special consideration would be needed. To meet the electrical installation requirements of these systems in the NEC.
Standard UL 1703, Standard for flat-plate photovoltaic modules and panels, was drafted to establish the safety requirements (mechanical and electrical) that photovoltaic modules must meet. Until about 2012, the requirements were written in the module instruction manuals that modified the values of the standard test condition parameters printed on the back of the module. In the latest editions of the standard, the safety requirements are forwarded to the NEC to establish how the outputs of the solar panels module must meet the requirements of electrical safety in the installation of the system.
Three hours per day represent a continuous service in the Code and it was decided in the first days of the Solar Panels systems that the calculations of the Code for Solar Panels systems should be based on the worst possible results and that said products would be considered continuous 24 hours a day / 7 days per week / 52 weeks per year and do not vary in a daily cycle with the sun. Therefore, all voltages and currents used in the calculations for photovoltaic systems in the Code are adjusted from standard test state measurements to ensure that the electrical system complies with the safety requirements of the Code and that all equipment operates within the limits established by both the Code and the UL Standards.
Settings – Open circuit voltage.
As noted above, the open circuit voltage (Voc) varies inversely with temperature. Instruction manuals for photovoltaic modules made before 2012 had the following statement or its equivalent:
“Multiply the open circuit voltage (Voc) marked on the back of the module by 125% before applying any requirement established by the NEC.”
For crystalline silicon, a factor of 125% in Voc represents the open circuit voltage of a module at approximately -40 ° C (-40 ° F). Temperatures below -40 ° are found in some parts of the country and additional adjustments to Voc may be required where these temperatures are expected.
This factor of 125% associated with a temperature of -40 ° C (-40 °) is a conservative safety factor. However, that 125% multiplier requirement is no longer found in the PV module instruction manuals for modules created after 2012 and the NEC requirements now state the various correction factors that will be used during the installation. Table 690.7 in the NEC is also somewhat conservative and shows the open circuit voltage multiplication factors for various low temperatures. The NEC allows using this table or the open circuit voltage temperature coefficients of the module data sheet can be used for the calculation of VOC at low temperatures.
Designers and installers of photovoltaic systems will try to use as many photovoltaic modules in a series as possible because when temperatures are high and the temperatures of the modules are higher, the voltage of the open circuit drops and it may be too low for the inverter to turn on or to operate properly.
It is important that the AHJ requires the installer / designer of photovoltaic systems to indicate in the drawings that an expected low temperature suitable for the location of the installation has been used and that the Voc of the Solar Panels system calculated at this low temperature does not exceed the rated voltage of the Investor or any associated equipment in the CC part of the system. The author has damaged an inverter rated for 600 volts on the DC input with an input voltage of 604 volts.
Current corrections: another 125 percent.
As indicated above, the output current of the module, the string or the matrix varies directly with the irradiance and while the modules are rated in a standard test condition of 1000 W / m2, the irradiance on sunny days may exceed that value significantly for three hours or more.
This new current value is called the maximum current and is used in most other calculations in the Code that involve dc DC currents. This factor of 125 percent is equivalent to an irradiance of 1250 W / m2, a value of irradiance that is higher than what the Solar Panels matrix should experience for three hours or more.
The improved irradiations of clouds, water, sun, and snow are higher (up to 1500 W / m2), but they are transient in nature and last only a few minutes before the conditions (angle of the sun, movement of the clouds) that caused the change. The Code does not address these conditions directly, but the designer of the photovoltaic system must take them into account. The Maximum power point tracking (MPPT) technology can be used.
Solar Panels modules as current sources powered by sunlight have different electrical characteristics from other electrical sources. The output of the Solar Panels module is significantly affected by environmental conditions.