The global demand for green energy sources is driving strong market growth for solar power systems. Although a great deal of development work is still focused on making photovoltaic (PV) energy conversion more efficient, there is also an acute need to make the delivery of solar power more efficient, reliable and cost-effective. This need has been recently highlighted by an announcement from the US Department of Energy (DOE) regarding its "SunShot" initiative, which is aimed at reducing the total costs of utility-scale PV energy systems by about 75 percent, and designed to make them cost competitive with other methods of electrical power generation.
Circuit design at the cutting edge
Responsibility for practical PV electrical system development eventually falls on of the shoulders of electrical circuit designers, including those who work for companies that create complete solar energy systems, system integrators who provide turnkey systems to end customers, and designers of various solar energy subsystems. Many of these designers are charged with creating electronics that optimize the performance and cost of PV installations. These engineers are typically involved with the design of electrical circuits for solar arrays, DC combiner boxes, or inverters.
Solar energy systems involve relatively new technology, so PV system designers often have greater experience in working on different types of electrical and electronic systems. For example, a company that is now producing small solar inverters may have previously focused on building power conversion or UPS systems. In their new positions, these new PV system designers may be called on to design solar energy circuits on a scale of 1MW DC for connection to the electrical grid. Designing circuits and specifying components for these high voltage solar energy applications is very different from the same tasks when applied to other DC power systems or even high power AC applications.
Basic circuit protection needs
The selection of circuit protection devices for solar energy circuits is one area where designers can get into trouble. These circuits may be used in systems ranging from residential-scale applications to those intended for large industrial facilities and grid-connected solar farms. On all of these systems, circuit protection devices are needed in many locations (Figure 1). Many application notes are available to provide guidance on selecting circuit protection devices for AC power and digital communication systems used for monitoring and control. Those areas are beyond the scope of this article, which focuses on the DC side of solar energy systems, where circuit designers are more likely to encounter unanticipated problems.
Figure 1. Places where circuit protection devices are needed in solar energy systems. For a larger (5M pdf) version of the image click here.
In a typical solar energy electrical system, individual solar panels or modules are connected in series to increase output voltage, which in turn increases efficiency. Multiple strings are connected in parallel to obtain the required output current and resulting power. Depending on the system size and design details, parallel strings can be connected in string-combiner boxes, which can be connected in parallel within array-combiner boxes, then connected to an inverter. Figure 2.
Figure 2: Typical solar energy electrical system
In most cases, multiple strings and arrays are connected using combiner boxes in accessible locations. These common connection points help simplify assembly and maintenance of the system. Wherever they are used, it is necessary to analyze the circuit to determine the available fault current (that is, the short-circuit current) of the system in comparison to the over-current capabilities of the components and then install appropriate circuit protection devices to prevent damage to PV modules, disconnects, wiring, and wiring devices.DC vs. AC circuit protection
Circuit breakers are often the preferred method of protection on the AC side of a solar energy system, and it may be tempting to try using the same circuit breakers on the DC side. Although the circuit breaker method is convenient, as a general rule, it is not always the best approach. The designer must carefully determine that the circuit protection device being used on the DC side of a solar energy system has been designed, tested and certified to a PV standard by an outside agency such as Underwriters Laboratories (UL) or VDE to be confident that the device will operate properly in the event of a fault. It is much more difficult for a circuit protection device to interrupt DC voltage than the equivalent RMS AC voltage. This is driven by the fundamental principle that AC circuit voltage reaches zero volts twice during each voltage cycle, which is a key factor affecting circuit protection devices’ ability to interrupt the voltage safely and isolate the troubled circuit.
Given that solar PV panels generate DC power, the current and voltage are constant for a given level of irradiance on the PV panels. With high voltage DC current, it is difficult for typical circuit protection devices to interrupt the circuit reliably under the range of operating conditions likely to occur in a solar energy system. In the worst case, a circuit protection device that’s not designed and certified for DC PV systems may fail violently, causing equipment damage, fire and possibly even injury to personnel. However, the most common problem will be that the devices don’t operate quickly enough under typical PV system overcurrent conditions.
For example, in a string, the short circuit current (ISC
) may not be much higher than the normal current. A typical solar string might output 4.2A in normal operation, and its forward ISC
will be around 4.5A. When combined with other strings in a small 450VDC 10kW system, the short circuit current that the properly sized 10A overcurrent protection device (OCPD) will be called on to interrupt in the event of a string fault will be approximately 20A. These high DC voltage, low overload conditions present a major challenge in designing a cost-effective OCPD that can interrupt a circuit over the appropriate range of voltage, current, and temperature.
For these reasons, the most common first line of defense is OCPDs in the form of fuses (Figure 3). Fuses, which are inherently passive devices, can be designed to be less costly than circuit breakers with the same performance characteristics. These PV system fuses and their testing are described in UL Standard 2579, Low-Voltage Fuses for Photovoltaic Systems, and IEC standard 60269-6. These fuse standards have been created in coordination with PV panel standards UL 1703 and IEC60129, as well as inverter standards UL1741 and IEC61727.Figure 3
. Diagram for a typical string combiner with fusing and other wiring devices.
Depending on the application and system design, the DC string voltage will typically be in the range of 300V to 1000V but may have the potential to go as high as 1500VDC in grid-connected systems. Fuses, disconnects, wiring devices, etc. for combiner boxes must be selected accordingly. In addition, UL and IEC standards have specific performance requirements for OCPDs used in these applications.
When the OCPD is a fuse, it must be selected to protect a PV source circuit operating at its short-circuit current rating, and also protect it in case of a fault on that circuit. NEC Article 690.8(A)(1) defines the fault current as 125 percent of the PV’s ISC
, plus any reverse or feedback current that could flow in the opposite direction from normal current flow.
Generally, the source of reverse current during a fault would be from back-feed current (IBACKFEED
) from the other strings in the affected array. Figures 4a and 4b
can be calculated to be approximately Isc x (n-1) where n equals the number of strings in the affected array. UL1703 and IEC60129 specifies PV panel testing to insure there is no dangerous overheating of the panel in the case of a back fed current equal to or less than Istring fuse x 135 percent for two hours. The UL PV fuse standard subsequently defined the PV fuse opening characteristic of not greater than Istring fuse x 135 percent for one hour. This guarantees proper coordination when using a UL or IEC panel with a UL fuse.Figure 4a
: Backfeed current caused by faultFigure 4a:
Backfeed current caused by fault