Most photovoltaic (PV) power system manufacturers sell their equipment through either specialized solar distributors or electrical distributors that have a solar
|These solar panels are being connected to a ballast-type racking system on a commercial rooftop.|
department. Then, this equipment is typically installed and maintained by companies that specialize in solar. However, general electricians are often called upon after the fact to work on the related wiring of these systems as part of a service call or renovation request.
If you receive this call, are you prepared to accept the job? Or are you going to have to pass this valuable lead on to someone else? Given the declining price of PV components, these systems are becoming much more common. Therefore, at minimum, electrical contractors need to know how to safely shut down PV systems in order to perform other electrical work. Savvy contractors looking for new revenue streams will take things a step further and gain new business.
This article provides a basic understanding of how these systems are designed and how they operate. It focuses on one of the basic types of PV systems — a string inverter-based system — that uses a single inverter to connect PV modules to the electric utility grid without a battery backup component.
Basic System Design
The Figure shown in the SIDEBAR: PV System Notes, shows the basic 3-line diagram of a string inverter PV system where one or more series strings of PV modules are used to generate a DC voltage output of 250V to 500V. The text next to the diagram describes the function of the components and some of the special considerations to be aware of when working with this type of system.
These systems function automatically and do not require any user or owner interaction. If the electric utility power is interrupted — or is not within the normal limits of voltage and frequency — the system will stop operating until these values return to stable limits. The inverters are designed to detect a ground fault within the PV array and its associated components and cease operation.
Troubleshooting a Non-Working System
Typically, you’ll be contacted by the owner of this type of system when it has stopped working. Most of the time, the call will come in because one of the following five problems has occurred:
- The inverter is no longer connected to AC power within the normal limits of voltage and frequency.
- The inverter may have detected a ground fault on the DC side of the system.
- The inverter may have failed.
- Either the inverter, disconnects in the AC circuit, or breakers in the AC circuit may have been turned off.
- A fuse in the system has opened.
Many string inverters have a simple digital display and a few LED indicators to show system status. Check these first, and correct any external problems. If the inverter appears to be non-functioning, check the AC power circuits, and measure the voltage across the output terminals of the inverter. An absence of AC voltage indicates a problem with the AC circuit. Fuses might have opened or breakers in the AC circuit may have been turned off. It is unlikely that the frequency of the power system has caused the outage. Inverters should be listed to UL 1741 and have a label that indicates the acceptable AC and DC voltage ranges the unit can operate within. If you do find AC supply voltages that are out of specification, notify the electric utility of the problem.
If the inverter indicates a ground fault condition, this situation is a little more difficult to address. It also requires you to follow safety procedures (see SIDEBAR: Safety Warning) to limit your exposure to the potential high voltages of the array. On one hand, the situation may be transient, due to moisture or pressure on wire insulation that is not normally present. However, oftentimes the 1A fuse in the inverter has opened. If so, replace the 600VDC fuse (verify proper size by referring to the inverter manual), and follow the reset procedure outlined per the manufacturer’s instructions.
If the system has more than two strings of PV modules, there should be fuses for each string. They will be located in the inverter or in a separate DC combiner enclosure. Turn off all switches (DC and AC) and use an insulated tool to snap open all DC fuse holders. (Warning: There is a possibility of a long arc appearing if a ground fault is located between the DC combiner and the inverter — or within the inverter itself.) If a long arc does appear during this operation, close the fuse holder quickly, and cover the entire array with opaque material so the system stops producing electricity. At this point, it is now safe to start opening the DC fuse holders again.
Once all fuses have been removed, go ahead and remove the opaque material you placed on top of the panels, and measure the voltage output of each string. A string with a ground fault will have a lower voltage. If a ground fault is not detected, use a high-voltage resistance tester to check all wiring and the PV modules. The general procedure is to isolate all wiring from ground and test all circuits. (Caution: The inverter must be isolated, because it may be damaged by the high voltages of resistance testing.)
Most UL-listed PV modules are rated for use in systems up to 600VDC and can withstand test voltages of twice that voltage. The typical test procedure is to connect the negative lead of the tester to ground and the positive lead of the tester to the negative lead of a string — and then test at 500V. A new system with good insulation should indicate more than 2,000 MΩ; however, readings greater than 200 MΩ are acceptable. Lower values should be investigated by disconnecting modules and testing parts separately until the source of the low reading is pinpointed.
You may have to consult the inverter manufacturer if there are other error codes displayed on the inverter. Some units have fans that can fail, which may be indicated by either a fan fail code or a high temperature code.
Handling Poor Performance Issues
It is often hard to distinguish between real and perceived performance issues. Many string inverters have displays that indicate
|Typical PV system with two inverters, AC combiner, AC and DC disconnects, and data system.|
power output in watts. You might expect that on a clear day at noon, for example, the display will equal the rating of their PV array. However, this is not the case, given the fact that the ratings are at 25°C, and the sunshine on the PV array will warm the PV array to well above 25°C.
To determine if a PV system is performing as intended requires the use of either special equipment, which can measure the solar irradiance on the PV array and the module operating temperature, or a study of the available performance data. The study method requires the measurement of the energy (kilowatt hours) produced over a specific period and comparison to system energy specifications. Many inverters display the accumulated energy delivered to the electric utility system, and systems may have a kilowatt meter on the inverter output. The recording of these readings over a period of time will provide the energy produced per day by the PV system. This can be compared to either the system’s energy specifications provided by the original installer or to energy calculations made over the same period, using either custom software or the National Renewable Energy Laboratory’s publicly available PVWatts calculator (www.nrel.gov/rredc/pvwatts). Even during a poor solar weather month, the output of the PV system should be within 15% of the predicted output for the location and system specific hardware.
Direct measurement of irradiance on a PV array requires a calibrated pyranometer or reference cell and a means of measuring operating cell temperature. The use of a handheld infrared temperature sensor typically produces an acceptable measurement value. Measure operating DC voltage and current with a clamp-on DC ammeter. Calculate the DC power output. Then, correct for irradiance (1,000 W/m² is reference irradiance), correct for cell temperature (-0.5% per degree Celsius from 25°C is a good estimate if the PV module temperature coefficient is not available). Compare results to the PV array power rating.
If the above calculations indicate a performance problem, then you should turn your attention to one or more of the following areas:
- Dirt or shadows on the array are limiting PV module performance. Generally, the least illuminated section of a module or string of modules affects the total string output.
- Inspect for module problems, such as cracked cells, open cell-to-cell connections, and shorted bypass diodes. Be aware that these problems can be difficult to identify in the field. Most PV modules use special connectors that do not expose the live parts for measurements while connected, making it difficult to test individual PV modules. If a PV module failure is suspected, contact the manufacturer and inquire about any long-term warranties that might be in effect.
- Look for melted plastic in the junction boxes. Some modules have been recalled due to connection problems in these boxes.
- The PV array may be incorrectly wired. Make sure all strings have the same number of modules, or check voltages in the DC combiner enclosure with the system off and the fuseholders open.
- Inspect for open fuses or loose connections in the DC combiner enclosure.
Inspect the inverter and focus on these two areas:
- The inverter may have multiple power sections and one may have failed in a manner not detected by the main inverter controller. This type of inverter will operate correctly at lower power levels but limit total operation of the system.
- Inverter fan or ventilation failures (check for blocked filters) may also limit operation at high power levels.
- Look for conduit fittings that may have become separated due to thermal cycling and wind.
- Check for loose connections that may allow water to enter the conduits or other system components.
It is also recommended that you leave a copy of any measurements taken at the site as an aid to others who may have to service the installation at a later date.
The bottom line is that most electrical contractors and technicians can handle the first level of troubleshooting for a PV power system, provided they follow the proper safety precautions and understand the basics of PV system operation. Take advantage of this niche market by educating yourself and your employees.
Kaszeta is the president of Photovoltaic Resources International in Mesa, Ariz. He can be reached at [email protected]
SIDEBAR: PV System Notes
All module frames must be bonded — lugs and wire shown — but other methods, such as special clips, may be used. The allowed number of series modules depends on the inverter design and local low temperature values.
Most inverters have the ability to display operational data and fault information on a digital display. A typical string inverter design is shown in this example. Note: There are other designs available, such as microinverters. The inverter may be marked for positive or negative ground (some PV modules perform better with a specific polarity). Note: No polarity is shown on this diagram. The inverter will likely have a fuse for detecting an array wiring ground fault. Some inverters do not ground reference either polarity of the array and use other methods to detect a ground fault.
Wiring exposed to sunshine must be rated for this exposure. Conduits and junction boxes must be bonded if containing conductors rated 250V (applies to both AC and DC systems) or greater. Note: Exposure to sunshine affects required wire size.
More than two module strings in parallel must have fuse protection per module listing. Fuses may be located in a DC combiner enclosure or in the inverter. DC disconnects may be separate, or part of the combiner or inverter.
The drawing illustrates a 3-phase system, but a single-phase system follows the same general design.
A separate kWh meter may be included in the design to record the energy generated by the PV system.
Many electric utilities require the inclusion of a separate lockable AC disconnect switch that is readily accessible to utility crews. These disconnects typically do not have fuses, and access may be locked by an electric utility padlock. Some utilities require a separate GEC bond from the inverter to the main service ground.
The drawing shows a load side connection of the inverter circuit to the service entrance. The circuit breaker is typically required to be on the opposite end of the distribution buss from the main circuit breaker and must be marked.
Labels specifying voltage and current values are required on many components in the system.
Refer to NEC Art. 690 for additional requirements.
SIDEBAR: Safety Warning
The systems outlined in this article use high DC voltages that cannot be simply turned off. This discussion is limited to test techniques that do not require disassembly of the PV array, but there is some exposure to terminals within the equipment that will normally have high voltages. Any work on roofs requires fall protection for safety.