PV System Selection and Sizing
The first step in designing a PV system is to decide whether to install a PV system that is connected to the local utility grid or a remote system that functions without a utility connection. For either PV system type, the amount of shade-free roof area available from roughly 9 a.m. to 3 p.m. for mounting the PV array must be determined. If a ground- or a pole-mount is considered rather than a roof-mount, these optional sites need to be shade-free also during the same time period. A remote PV system’s array size is determined after determining the building’s average daily electrical demand and sizing other components in the total system. For a grid-connected PV system the array size can be simply sized to fit within the amount of mounting area and the budget for the project. Most of the discussion in this lesson will focus on grid-connected PV systems, with some tidbits on remote site installations
At this point, an economic decision must be made. Using a cost of $8 to $10/Watt of the array’s peak output to install a PV system, a preliminary assessment between available funds and array size can be made. Only install modules that have a UL 1703 listing. The Underwriters Laboratory (UL) uses safety and performance standards specific to the equipment type and issues a listing for models that pass the safety and performance tests.
For most grid-connected PV system installations, the estimated peak array output is used as the basis for specifying the inverter needed for the PV system. In some cases, the building’s electrical demand may be determined and used to specify the inverter. In a remote or stand-alone PV system installation, the average daily electric load of the building needs to be calculated first. The building’s electric demand should include the Watt demand of all ac loads running at the same time, plus the wattage from the surge of starting motors, plus all dc loads operating at the same time; this demand is further increased by 1.2 to account for inverter losses. In both the grid-connected and remote site situations, the initial estimate of the inverter’s capacity may be changed by making a decision that at some point in the future you plan to increase the size of PV array.
There are two basic types of inverters to consider for this course – those that produce a modified sine wave and those that produce a true sine wave. Although modified sine wave inverters are less expensive than true sine wave models, they can not produce the quality waveform required by some equipment. Utility companies produce electricity that is a true sine wave. A modified sine wave inverter produces a slightly squared off electrical waveform, but computers, power tools, refrigerators and most all equipment can use this generated electricity. Pure sine wave inverters produce a true sine wave that is the same as utility generated waveforms and is needed by high-end audio equipment and other specialized equipment that are electrically sensitive such as life support equipment. All inverters should be UL 1741 listed. In grid-connected installations the inverter must shut down rapidly in situations where the utility goes down – this is called anti-islanding and is a safety function for utility personal and electricians who may be working in the area.
|Sine Wave and Modified Sine Wave Electric Power Forms
Inverters designed for remote site and grid-connected installations are available that are designed to use nominal 12-, 24-, or 48-volt DC electricity from the PV array and some grid-connected inverters are designed to operate with input voltages ranging from 139 up to nearly 600 volts DC. The higher-voltage strings carry low current levels; this allows the use of smaller diameter wire in the circuit between the inverter and the modules.
Once you have identified the type and the output capacity of the inverter for a grid-connected system, you can determine the PV modules you’ll need. Using module maximum working voltage and amperage values, you can use series and parallel calculations to match the inverter’s input electrical requirement with the proposed array output electrical characteristics. You will have to be aware of the open circuit voltage and amperage of the module strings to not exceed the normal input range for the inverter chosen. Matching modules with the inverter will most likely be an iterative process. Inverters using high voltage inputs are best matched with PV modules using a computer program provided by each inverter manufacturer. The computer programs use a database of specific module electrical characteristics to identify the appropriate number of modules in each string and the number of strings feeding the specified inverter. Programs for several brands can be found at the links below:
- Xantrex: www.xantrex.com/support/gtsizing/disclaimer.asp?lang=eng
- Sunnyboy: www.sma-america.com/stringsizing.html
- PV Powered: www.pvpowered.com/string_sizing.php
- Beacon Power: www.beaconpower.com/StringCalc/
- Fronius: www.fronius.com/solar.electronics/downloads/configurator.htm
Installing a PV system parallel to a sloped roof would have the following equivalent tilt angle:
|Roof slope||Slope or Tilt Angle (degrees)|
The National Electric Code (NEC) has a significant impact on the design of and the components used in a PV system. Sandia National Laboratories' Photovoltaic Center has posted the following wire coding and sizing information from The Stand-Alone PV System Handbook on its website.
Wire Types Commonly Used in the U.S.
|Note: The use of NMB (Romex) is not recommended except for ac circuits as in typical residential wiring. Although commonly available, it will not withstand moisture or sunlight.|
In the United States, the size of wire is categorized by the American Wire Gage (AWG) scale. The AWG scale rates wires from No. 18 (40-mil diameter) to No. 0000 (460 mil diameter). Multiple conductors are commonly enclosed in an insulated sheath for wires smaller than No. 8. The conductor may be solid or stranded. Stranded wire is easier to work with particularly for sizes larger than No. 8. Copper conductors are recommended. Aluminum wire is less expensive, but can cause problems if used incorrectly. Many different materials are used to make the sheath that covers the conductors. You must select a wire with a covering that will withstand the worst-case conditions. It is mandatory that sunlight resistant wire be specified if the wire is to be exposed to the sun. If the wire is to be buried without conduit it must be rated for direct burial. For applications such as wiring to a submersible pump or for battery inter-connections, ask the component dealer for recommendations. Often the dealer or manufacturer will supply appropriate wire and connectors.
More useful information is contained in NEC. It is recommended that any designer/installer review Article 300 before proceeding. This article contains a discussion of wiring methods and Table 310-13 gives the characteristics and recommended usage of different wire types. Table 310-16 gives temperature derate factors. Another useful reference available from the PVSAC at Sandia National Laboratories is Photovoltaic Power Systems and the National Electrical Code, Suggested Practices.
Selecting the correct size and type of wire for the system will optimize performance and increase reliability. The size of the wire must be capable of carrying the current at the operating temperature without excessive losses. It is important to derate the current carrying capacity of the wire if high temperature operation is expected. A wire may be rated for high temperature installations (60-90°C), but this only means that the insulation of the wire can withstand the rated temperature — it does not mean that ampacity is unaffected.
The current-carrying capability (ampacity) depends on the highest temperature to which the wires will be exposed when it is carrying the current. According to Table 310-16 in the NEC, a UF-type wire operating at 55°C can safely carry only 40 percent of the current, or 30°C — a significant derate. If the ampacity of the wire is exceeded, it could result in overheating, insulation break-down, and fires. Properly sized fuses are used to protect the conductors and prevent this kind of damage.
Loss in a DC circuit is equal to I2R, where I is the current and R is the resistance of the wire. For 100 ampere current, this means 10,000 times the loss in the circuit compared to a one amp load. It is easy to see why resistance must be kept small. Also, the voltage drop in the circuit is equal to IR. Voltage drop can cause problems, particularly in low-voltage systems. For a 12-volt system, a one-volt drop amounts to more than 8 percent of the source voltage. Avoid long wire runs or use larger wire to keep resistance and voltage drop low. For most applications, AWG No. 8, No. 10, and No. 12 are used.
An abbreviated wire sizing table for a 12-Volt DC system is shown below. The table indicates the minimum wire size that should be used if the voltage drop is to be limited to 3 percent for any branch circuit. (This table can be adjusted to reflect different voltage drop percentages or different system voltages by using simple ratios. For example, a 2-percent loss can be calculated by multiplying the values in the table by 2/3. For a 24-Volt DC system, the values can be multiplied by two. For a 120-volt system multiply by 10.) The calculations show one-way distance, taking into account that two wires, positive and negative, are used in an electrical circuit.
As an example, assume the array is 30 feet from the controller and the maximum current is 10 amperes. The table shows that No. 8-size wire can be used up to a one-way distance of 30 feet (no temperature derate included). While the general rule is to limit the voltage drop for any branch circuit to 3 percent, there may be some applications, particularly those operating at or below 12 Volts, where the loss should be limited to 1 percent or less. For the total wire run on any path from source to load, the loss should be no greater than 5 percent.
|One-way Wire Distance (feet) for 3% voltage drop - 12 volt system - copper wire|
|AWG Wire Size|
The NEC requires certain conventions for color of conductors and specifies requirements for disconnecting the power source (code reference for each condition is given in brackets). Specifically:
- The grounded conductor is to be white. [200-6]. Convention is for the first ungrounded conductor of a PV system to be red, and the second ungrounded conductor black (negative in a center tapped PV system).
- Single-conductor cable is allowed for module connections only. Sunlight resistant cable should be used if the cable is exposed. [690-31b]
- Modules should be wired so they can be removed without interrupting the grounded conductor of another source circuit. [690-4c]
- Any wiring junction boxes should be accessible. [690-34]
- Connectors should be polarized and guarded to prevent shock. [690-33]
- Means to disconnect and isolate all PV source circuits will be provided. [690-13]
- All ungrounded conductors should be able to be disconnected from the inverter. [690-15]
- If fuses are used, you must be able to disconnect the power from both ends. [690-16]
- Switches should be accessible and clearly labeled. [690-17]
The purpose of grounding any electrical system is to prevent unwanted currents from flowing (especially through people) and possibly causing equipment damage, personal injury, or death. Lightning, natural and man-made ground faults, and line surges can cause high voltages to exist in an otherwise low-voltage system. Proper grounding, along with over-current protection, limits the possible damage that a ground fault can cause. Consider the following and recognize the difference between the equipment grounding conductor and the grounded system conductor:
- One conductor of a PV system (>50 volts) must be grounded, and the neutral wire of a center tapped three wire system must also be grounded. [690-41]. If these provisions are met, this is considered sufficient for the battery ground (if batteries are included in the system). [690-73]. A ground is achieved by making a solid low resistance connection to a permanent earth ground. This is often done by driving a metallic rod into the earth, preferably in a moist location. [250-83].
- A single ground point should be made. [690-42]. This provision will prevent the possibility of potentially dangerous fault current flowing between separate grounds. In some PV systems where the PV array is located far from the load, a separate ground can be used at each location. This will provide better protection for the PV array from lightning surges. If multiple ground points are used, they should be bonded together with a grounding conductor.
- All exposed metal parts shall be grounded (equipment ground). [690-44]
- The equipment grounding conductor should be bare wire or green wire. [210-5b]
- The equipment grounding conductor must be large enough to handle the highest current that could flow in the circuit. [690-43]
Because the module frames are usually aluminum and bare copper wire is used for the ground conductor, you must use the module grounding location and the manufacturers specified hardware to assure a low-resistance connection to provide long-term protection from shocks and fire hazards. The grounding conductor must be sized to safely carry the current of the over-current device protecting the circuit.
It is important that the installation crew includes a certified electrician knowledgeable about applicable codes and a person knowledgeable about the equipment used in the PV system installation. Article 690 of NEC addresses electrical requirements and the equipment for installing a PV system. The electrician must know the codes and be present to answer questions during the electrical inspection.
- During the initial site visit to check a single story building’s acceptability for a PV system, you note that the asphalt-shingled roof has a 4/12 slope and is oriented 10 degrees to the west of true south. The south-facing roof is a rectangle that is 30 feet wide and 20 feet from the eaves to the roof top. Is this building a good candidate for a PV installation? If it is and given that the roof can support the PV system and a 3-person installation crew, what would you suggest to the building owner as the largest, safe array (peak output) to install?
- For the same building described in question 1, what conditions might you encounter that would make you reject the site for a system installation?
- What estimated cost would you tell the building owner for an installed PV system with a peak output of 3000 Watts?
- Why is an inverter needed in a grid-connected PV installation?
- Why is an inverter needed in a remote or stand-alone PV system?
- How would you size the inverter for a grid-connected PV system?
- What is the color of the grounded conductor in a PV installation and how is it sized?
- What is the color of the equipment/frame ground wire in a PV installation and how is it sized?
- What function does the equipment/frame ground perform?
- Given that a PV system uses modules outputting a nominal 12 volts at 5 amperes, the modules are 30 feet from a combiner box, and you can only tolerate a 2% voltage drop, what gauge of wire should be used to connect the modules with the combiner box? What gauge of wire if the modules strings were 24 volt at 5 amps?