Inverters change Direct Current (DC) to Alternating Current (AC).
Inverters are used to convert the dc power to ac power. Inverters are designed to be used as stand-alone system inverters or as utility-interactive inverters. Some utility-interactive inverters are capable of operating in a stand-alone mode. Any inverter used in a PV system shall be listed and identified for such use (690.4(D)).
Inverters that are suitable for use in utility-interactive PV systems must be listed and identified for that use (690.60). Modern inverters are capable of operating with peak efficiencies over 90% with very good power quality. Stand-alone inverter output waveforms vary in shape and/or power quality, as shown in Figure 1. Note that all three waveforms of Figure 1 have Vrms values of 120 V.
Figure 1. Three types of inverter output waveforms.
The simplest waveform to generate is the square wave of Figure 1a, which can be used in simple stand-alone systems where insensitive loads can tolerate the low quality power.
Square waves have high harmonic distortion; thus, they are not advisable for use with some electronic equipment, and they can cause motors and other magnetic components to overheat.
If a square wave inverter is used, it is important to check the instruction book to see what loads are not suitable for use with the unit.
The output of square-wave inverters is not suitable for connecting to the utility grid. Square wave inverters are now generally considered obsolete and are seldom found in stand-alone systems.
The next waveform for stand-alone inverters is the modified sine wave of Figure 1b. This waveform is commonly used in uninterruptible power supplies (UPS) for electronic equipment and is also used by many moderate-cost stand-alone PV inverters.
The harmonic distortion is less than the harmonic distortion of a square wave, and the waveform is suitable for use with more types of loads.
Inverters with modified sine-wave outputs (more accurately known as modified square-wave inverters) are in common use in stand-alone PV and recreational vehicle applications.
Common loads that are often incompatible with this waveform include light dimmers, laser printers, chargers for cordless tools, smoke detectors, and the electric igniter circuits on some gas ranges.
Modified-square wave inverters are not suitable for connection to the utility grid, even though the inverter may be used as a replacement for the utility when the inverter is used in a UPS application.
The ideal ac waveform is the sine wave shown in Figure 1c. A sine-wave inverter is suitable for either stand-alone or utility interactive applications. Stand-alone inverters that have a sine-wave output are suitable for all loads within their power and current ratings.
They have low harmonic current and voltage distortion. An inverter is considered utility-interactive provided that it meets the requirements of IEEE Standard 929-2000 and is listed to UL 1741.
These standards ensure that the inverter output waveform has less than 5% total harmonic distortion and that the inverter will disconnect from the grid if grid power is lost. Once disconnected, the inverter will continue to sample the grid voltage.
After the grid voltage has again stabilized, and after a required five-minute delay, the inverter will reconnect to the grid and deliver power from the PV system. Some sine-wave inverters are capable of multiple modes of operation.
The first of these modes is stand-alone in which the inverter operates independent from the utility grid. The second is hybrid which uses another form of generation as backup for the PV system. The third is utility-interactive in which the unit uses grid power to charge batteries if the PV system fails to do so but does not send power from the PV system to the grid while doing so.
In the utility-interactive mode, it is also able to send excess PV power to the grid after meeting the needs of the installation site. The installer typically must refer to the instruction manuals for the installation, operation, and maintenance details for these inverters.
The installer should be familiar with some of the key characteristics of inverters, regardless of the inverter being used. Inverters are characterized by input voltage and current limits, output voltage and current limits, waveform type, operational modes, and power rating.
Figure 2 shows a schematic diagram of a common utility-interactive PV system with one particular type of inverter that uses battery backup and an optional standby ac distribution panel.
Figure 2 is used to show the required calculations for wire size, system ratings, fusing, and disconnects.
Figure 2. Inverter - Utility-interactive PV system with battery backup.
Most of the same methods for the calculations that were done for the stand-alone system regarding wire sizes, fuse sizes, and disconnect sizes apply for utility-interactive systems.
Because each module is rated at 120 watts at STC, this system may be referred to as a 720-watt utility-interactive system with a UPS option as it has battery backup to supply emergency ac loads.
The installer must be aware that while it may be referred to as a 720-watt system, the maximum output of the PV array will fall short of 720-watts.
The reasons for the shortfall will be discussed in detail later in this Guide. The nominal dc voltage is 24 V, with each battery being a six-volt deep- discharge unit.
The system is essentially the same as the stand-alone system discussed earlier, up to the inverter. This system does not have a peak-power tracking inverter and the charge controller is used to maintain the batteries at the correct state of charge. Other multimode inverters may have PV maximum power tracking circuits and exclude the charge controller.
The charge controller is generally set to charge the batteries to the bulk charge level, and the inverter is generally set so that when the batteries reach the bulk level, current from the charge controller is diverted to the inverter where it is converted to ac and used either to supply the emergency loads or to sell back to the utility.
If the inverter has a battery charging option from either the utility or from alternate generation means, the charge settings in the inverter should be set lower than the settings of the PV charge controller; otherwise, the PV system sensors will find the batteries to be fully charged and the PV charge controller may limit the current from array-to-charge-controller output.
Some inverters can be set to inhibit the charging of the batteries at night from the grid to allow maximum energy to be extracted from the PV array and not the grid
For a system programmed to sell to the utility, it is still important that the sell voltage setting on the inverter be below the bulk setting of the PV charge controller.
If it is not, the PV charge controller may limit the PV output and consequently limit the output from the inverter to the utility. Recall that at the beginning of the bulk charge mode, the charge controller is supplying as much current to the batteries as the PV array can generate.
It is important to note that the system of Figure 2 has the array configured for effective battery charging. It should also be noted that the use of different types of inverters having different multimode capabilities would be connected in significantly different ways.
In larger utility-interactive systems with 48V nominal inverter-input voltage and no battery backup, the inverter may track maximum array power over array voltages ranging from approximately 44 to 88Vdc.
For this range of input voltages, it is possible to use four, 12-volt modules in series to meet the input voltage requirements. For example, with four modules in series, Vmp for each will drop to 14.7 V at 60°C, resulting in Vmp for the series combination of 58.8 V.
Note, however, that if four modules are used and the module temperature falls very much below -25°C, it is possible that Vmax for the series connection of the four modules may exceed 100 V.
Thus, it is important to check the inverter input voltage range for maximum power tracking as well as the absolute limits of the inverter input voltage to be sure that the array and inverter are properly matched.
It is important to install the correct wire sizes at inverter inputs and outputs. The sizing process is straightforward, as the wire is calculated for 125% of the inverter input or output current at full-rated power and as specified in the inverter instruction manual. Full-rated ac currents are generally close to the full-rated power divided by the output voltage for the output current or by the input voltage for the input current.
For example, a 4000 W inverter that has a 120 V output specifies an output current at full rated power of 33 A. Dividing the inverter power by 120 gives 4000÷120 = 33 A. However, dividing the inverter power by 24 (the nominal dc voltage) gives 4000÷24 = 167 A, and the manual states that the maximum current is 200 A.
The discrepancy between the dc value in the manual and our calculated value at the input is an important one. The reason the values differ is because the value in the manual is calculated at the minimum inverter input voltage of 22 V and, at full power, the inverter operates well below peak efficiency – near 80% efficiency.
Article 690.8(A)(4) of the NEC requires that for standalone PV systems, the inverter input current “. . . shall be the continuous inverter input current rating when the inverter is producing rated power at the lowest input voltage.” Dividing the full rated power by the lowest input voltage and by the efficiency at full rated power (85%) results in 4000W÷22V÷0.85 = 214 A (slightly higher than the value found in the manual). The battery conductors must be calculated for 125% of the full rated current or 267 A (214 A × 1.25 = 267 A).
Regardless of the load connected to the inverter, the size of the input and output conductors of the inverter must be based on the inverter input and output currents at rated load if the system has batteries. Hence, even for the 720 W system of Figure 2, if the inverter is rated at 4000 W, then the conductors to the input must be sized for 125% of 214 A, or 267 A. This will also be the rating of the circuit over current protection and disconnect for the battery. At the output, the wire and disconnect needs to be sized for 125% of the rated inverter output current, which, in this case, would be 125% of 33.3 A, or 41.62 A.
The wiring from the PV array is sized according to 156% of the PV output circuit short-circuit current. In this case, the combined short-circuit current of the three source circuits is 7.2 A × 3 = 21.6 A. The PV output circuit conductors then must have an ampacity of at least 21.6 A × 1.56 = 33.7 A.
Provided that the voltage drop is not a problem, 8 AWG conductors with a 40-A dc circuit breaker serving as a disconnect device and over current protection would be adequate. If the wiring to the inverter is located in a hot location, such as a garage or on the wall of a building, the temperature corrections of the wiring must also be considered. If voltage drop is a problem, which is nearly always the case in 24-V systems, then larger wire will need to be used.
The inverter bypass switch is shown in Figure 2. The inverter bypass switch allows for inverter maintenance while supplying the emergency loads directly from the utility line. This switch in this circuit consists of a double-pole circuit breaker ganged together with a single-pole circuit breaker in a manner such that both breakers cannot be simultaneously ON, but both can be simultaneously OFF. When the double-pole breaker is on, the hot lead of the ac input from the utility passes through one pole of the breaker and enters the ac in terminal of the inverter.
The ac out of the inverter passes through the other pole of the breaker from which it is connected to the optional standby load. When the double-pole breaker is in bypass mode, the utility line in is blocked from the inverter, but is connected through the single-pole breaker to the emergency load.
This is the inverter bypass position. When both breakers are off, power is disconnected from both the inverter and the emergency panel. Because the bypass breakers must be rated at a minimum of 125% of the inverter output current, the inverter bypass switch also serves as the main ac disconnect for the inverter output.
All circuit breakers that can handle inverter output currents should be sized at 125% of the inverter output current rating. The circuit breaker in the main panel provides over current protection from utility-supplied currents for the conductors between the main panel and the inverter bypass switch.
The circuit breakers in the inverter bypass switch provide over current protection from utility-supplied currents for the conductors between the bypass switch and the ac input of the inverter and the conductors from the bypass switch to the sub panel.
Normally, these circuit breakers do not trip on faults involving the inverter output because they are sized at 125% of the inverter output. A short-circuit on the inverter output generally will cause the inverter to shut down before the circuit breakers trip. The input circuit breaker on the sub panels serves only as a disconnect switch for the sub panel.
The NEC allows a utility-interactive PV system to be connected on either the line side or the load side of a customer’s service disconnecting means. If the PV system is owned by the utility, it will probably be connected on the line side upstream of the meter. Line-side connections must meet the requirements of NEC 230.82(6). If the customer owns the system, it will often be connected on the load side of the service disconnecting means.
If the output of an inverter is connected to the line side of the main breaker, it is important that an acceptable termination procedure be used. If two wires are to be attached to a single point, the lug's used must be approved for this application. When possible, it is generally most convenient to establish the point-of-utility connection at a main distribution panel. This can be done by connecting the inverter output to the load side of a dedicated circuit breaker in the main distribution panel. When connected in this fashion, the requirements of NEC 690.64(B) apply.
NEC 690.64(B) requires that
The bus or conductor rating shall be sized for the loads connected in accordance with Article 220 (to ensure that the panel board is not overloaded). A permanent warning label shall be applied to the distribution equipment with the following or equivalent marking:
The requirements should come as no surprise to an experienced electrician. The first requirement is that each inverter must be connected by means of its own dedicated circuit breaker or fuse.
There is an exception for this in NEC 690.6 for ac PV modules, where multiple micro-inverters can be wired in parallel before connecting to the dedicated branch circuit. In no case, can loads can be connected on the circuit.
A key change in the 2008 NEC is the allowance of the sum of the supply breakers to be 120% of the bus bar rating for all PV installation, not just residential systems, as the 2005 NEC and before was written.
This allows for small PV systems relative to the size of the panel to be connected without having to resize the panel.
Item (2) also clarifies that the breaker used to connect the PV inverter be used for all upstream calculations instead of the sub panel breaker size as is often mistakenly done.
The 120% allowance is somewhat subtle, and should be looked at carefully. It states that the sum is of the over current devices, NOT the sum of currents.
This means that if a 200A bus bar is fed by a 200A main circuit breaker, then a circuit breaker connecting a PV source may not exceed 40A, which is 20% of the bus bar rating.
But this also means that the output current of the inverter may not exceed 32A, as the circuit breaker at the point-of-utility connection is sized at 125% of the inverter rated output current, and 32A × 1.25 = 40A.
Note, however, that this current may be at 120V or at 240V, as at 240V, 32A is added to each bus bar, while at 120V, 32A is added to only one bus bar.
If the current is at 240V, the power rating of the inverter can be twice as large. The other possibility is two inverters, one connected to each bus bar through a double-pole breaker to ensure that both bus bars are used.
The third item under 690.64(B) requires that the point of interconnection be on the line side of ac ground fault protection equipment.
This provision is necessary because the function of the ac ground-fault protection equipment can be damaged by feeding power to the load side once it trips unless it is designed for such conditions.
When the point of interconnection is on the line side, then the ground-fault equipment will continue to provide protection to all circuitry on its load side.
Item (4) requires equipment fed by multiple sources to be marked as such. Item (5) in the 2008 NEC 690.64(B) uses slightly different language than the previous code revisions by stating more accurately that circuit breakers must be “suitable” for back feed, not “identified” as previously stated.
It also clearly explains in the FPN that suitability is determined by the lack of “line” and “load” markings on the breaker. Item (6) states that the additional fastening means, required in NEC 408.36(F) for back-fed devices, can be omitted.
This provision recognizes that a listed utility-interactive inverter is inherently safe if the interconnecting breaker is inadvertently removed from the bus bar since the inverter automatically shuts down when loss of voltage is sensed.
MAINTENANCE OF POWER CIRCUIT BREAKERS
The maintenance of circuit breakers deserves special consideration because of their importance for routine switching and for protection of other equipment. Electric transmission system breakups and equipment destruction can occur if a circuit breaker fails to operate because of a lack of preventive maintenance. The need for maintenance of circuit breakers is often not obvious as circuit breakers may remain idle, either open or closed, for long periods of time. Breakers that remain idle for 6 months or more should be made to open and close several times in succession to verify proper operation and remove any accumulation of dust or foreign material on moving parts and contacts.
Overcurrent Protection and Devices, Short-Circuit Calculations, Component Protection, Selective Coordination, and Other Considerations
In order for an over current protective device to operate properly, the over current protective device ratings must be properly selected. These ratings include voltage, ampere and interrupting rating. Of the three of the ratings, perhaps the most important and most often overlooked is the interrupting rating. If the interrupting rating is not properly selected, a serious hazard for equipment and personnel will exist. Current limiting can be considered as another over current protective device rating, although not all over current protective devices are required to have this characteristic
Photovoltaic System Over current Protection
As the installations and demand for PV systems increases so does the need for effective electrical protection. PV systems, as with all electrical power systems, must have appropriate over current protection for equipment and conductors.
Inverters brands and types -
Sunny Boy Inverter - This series includes the popular SMA SB 3000US, SB 4000US, SB 5000US, SB 6000US, SB 7000US and SB 8000US grid-tie inverters. Also available is the Sunny Tower ST 36 a 36 kW inverter, and the ST 42, a 42 kW inverter.
Fronius Inverter - The IG + Series features the IG Plus 3.0, IG Plus 3.8, IG Plus 5.0, IG Plus 6.5, IG Plus 7.5, IG Plus, 10.0, IG Plus 11.4 UNI, IG Plus 11.4 Delta. and The IG Inverters Series is a reliable and affordable choice for grid-tie systems. Available in 2000W - 5100W models. Built-in display on all models.
PV Powered Inverter - American Made inverters with built-in disconnects from PV Powered include PVP1100, PVP2000, PVP2500, PVP2800, PVP3000, PVP3500, PVP4600, PVP4800 and PVP5200 models.
Solectria Inverter - Residential, Industrial and Commercial Inverters with built-in disconnects from Solectria include PVI3000, PVI4000, PVI5000 and PVI5300 Residential models and PVI13KW and PVI15KW commercial and industrial models.
Enphase Inverter - This grid-tie inverter has the unique advantage of using one inverter per solar panel so you can place modules on multiple roof faces, great for systems with shade problems. Models include the M190, M210 and D380 series.
Xantrex Inverter - Now Schneider Electric, this manufacturer provides GT Series inverters including the GT2.8, GT3.3N, GT3.8, GT4.0N, GT5.0. The XW Inverter series includes the XW4024, XW4548 and XW6048 hybrid inverter systems. Benefits include MPPT charging and grid-tie with battery backup.
SatCon PowerGate Inverter - The ideal inverter choice for large commercial grid-tie solar systems. 30kW – 1 megawatt models available. All models can be run in parallel for larger or upgradeable solar power systems. Current Models include the PowerGate Plus PVS-30, PVS-50, PV-50-S, PVS-75, PVS-100, PVS-110-S, PVS-135, PVS-210-S, PVS-250, PVS-375, PVS-500, and PVS-1000.