Photovoltaic Ch 11 Electrical Integration

5. What is the voltage that determines PV systems for residential systems or commercial/utility-scale systems?

600V

29. What is a blocking diode? Explain why blocking diodes shall be used in PV systems.

A blocking diode is a diode used in PV source circuits to prevent reverse current flow. For example, in a simple self-regulated PV system, a fully charged battery may begin to discharge through the array when the array voltage falls below the battery volt-age at nighttime.

30. What is a bypass diode? Explain why bypass diodes shall be used in PV systems.

A bypass diode is a diode used to pass current around, rather than through, a group of PV cells that are shaded. In contrast to blocking diodes, bypass diodes are installed in parallel with modules or groups of cells.

28. What is a combiner box? Describe the use of combiner boxes in PV systems.

A combiner box is a junction box used as the parallel connection point for two or more PV array source circuits. Combiner boxes are used to combine parallel array source circuits into the PV output circuit. These boxes typically include terminal blocks, fuses, and fuse holders.

8. How to calculate the maximum current of PV output circuits.

For PV output circuits, the maximum cur-rent is the sum of the maximum currents of the parallel- connected source circuits. For example, a PV output circuit combining three parallel strings of modules, each with a maximum source circuit current of 6 A, has a maximum PV output circuit current of 18 A ( 3 × 6 A = 18 A).

9. How to determine the maximum inverter input current for interactive systems and stand-alone systems, respectively.

For an interactive inverter with the PV output circuit connected directly to the inverter input, the inverter input circuit is the same as the PV output circuit and, therefore, has the same maximum current. For stand- alone systems with batteries, the inverter input current depends on battery voltage. As battery voltage decreases, the inverter input current increases to provide the same power output. At low battery voltages and peak power output, this current can be considerably higher than the inverter input current rating at the nominal battery voltage. Thus, the highest possible input current is associated with the lowest inverter input operating voltage.

23. Explain how screw terminals are used to secure a conductor to a terminal.

Screw terminals use the compressive force of a screw to secure a conductor to a terminal and are highly reliable connections.

17. Explain how the use of conduits affects conduct's ampacity.

Since conduits exposed to direct sunlight on or above rooftops experience higher temperatures, a temperature adder is used to adjust the ambient temperature before determining the temperature- based correction factor.

15. Describe why the conductor ampacity must be derated for high temperatures.

Since temperature affects a conductor's ampacity, this nominal ampacity is derated (reduced) for ambient application temperatures higher than the nominal 30° C.

25. What are splices? Describe their uses in PV systems.

Splices are used in PV systems to connect or extend conductors, parallel ar-ray source circuits, or tap service- entrance conductors for supply- side interconnections. The splice and any exposed areas on the conductor must be insulated ( covered with appropriate tape or heat- shrink tubing) with a level of protection and rating equivalent to the conductor insulation.

39. What is DC grounding?

The DC grounding electrode conductor must be at least the size of the largest conductor supplied by the PV system, or 8 AWG copper, whichever is greater. However, when the DC grounding electrode conductor is the only connection to a rod, pipe, or plate electrode, the grounding electrode conductor is not required to be larger than 6 AWG copper.

12. What are stranded conductors?

made up of multiple smaller wires

19. Explain the purpose of conductor insulation.

Insulation protects a bare conductor from coming into contact with personnel or equipment.

22. What are the basic requirements for terminating electrical conductors?

• All terminating devices must be listed and identified for the proper conductor material and conditions of use and installed according to manufacturer's instructions. • Conductors or materials made of dissimilar metals must not be allowed to touch each other, and any solders or corrosion inhibitors used must be suitable for the application. • The ampacity of any connected conductors must be evaluated at the lowest termination temperature rating. • The proper crimping tool must be used to crimp lugs. • Terminals using setscrews must be torqued to the proper specifications. • Fine-stranded cables require special terminals intended for the use.

31. What conduit options can be available for PV systems? Explain when to use different type of conduit options.

• electrical metallic tubing ( EMT), • rigid nonmetallic conduit ( electrical PVC, schedule 40 or 80), and • electrical nonmetallic tubing ( ENT), assuming their use does not exceed their ratings. • Rigid metal conduit ( RMC) and intermediate metal conduit ( IMC) may be used when extra protection is needed.

20. What are the three critical properties of a conductor for their insulating materials?

• its maximum operating temperature, • its application and environmental resistance ( such as to sunlight, oil, or moisture), and • its permissible installation locations ( such as direct burial, in conduit, or exposed).

37. Why a disconnect must be provided in PV systems?

A disconnect is a device used to isolate equipment and conductors from sources of electricity for the purpose of installation, maintenance, or service. Article 690, Part III, addresses disconnect requirements for PV systems. Disconnects are required for both the DC and AC sides of a PV system. Disconnects are also required to isolate other power sources, such as batteries, and may be included at additional points to facilitate system testing or maintenance.

40. What is a ground fault?

A ground fault is the undesirable condition of current fl owing through the grounding conductor. Ground faults are typically caused by damage to the protective insulation of normally current- carrying conductors.

27. What is a junction box? Why to use junction boxes?

A junction box is a protective enclosure used to terminate, combine, and connect various circuits or components together.

2. What are the nine parts of NEC Article 690?

I. General II. Circuit Requirements III. Disconnecting Means IV. Wiring Methods V. Grounding VI. Marking VII. Connection to Other Sources VIII. Storage Batteries IX. Systems Over 600 Volts

42. What is a surge arrestor?

A surge arrestor is a device that protects electrical devices from transients ( voltage spikes). It does this by either limiting or shorting to ground all voltage above a certain threshold.

14. What is Ampacity?

Ampacity is the current that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.

18. Explain why the conductor ampacities must be further derated when more than three current-carrying conductors are installed together in a conduit or raceway longer than 24'.

If more than three wires are on a cable the wire cannot cool as well, so the max current has to be divided by 0.8 to account for the loss of cooling.

11. Describe American Wire Gauge (AWG) numbers.

Larger diameter conductors have smaller AWG numbers. Larger conductors have greater current-carrying capacity and less resistance. However, solid (single wire) conductors can be stiff and difficult to work with, especially for larger sizes. Therefore, conductors are also available stranded (made up of multiple smaller wires), which makes them more flexible. Solid and stranded conductors of the same AWG size have the same cross- sectional area, though stranding makes the diameter slighter larger. At size 6 AWG and larger, conductors are generally only available in stranded versions.

21. What is the purpose of color codes for conductor insulation?

The color of a conductor's insulation is used to identify the purpose of the conductor in the circuit and to match the ends of a conductor. The insulation may be a solid color or may consist of mostly one color with one or two stripes of another color. Most conductors are available in many colors. For conductors larger than 6 AWG, which are usually only black, colored tape is wrapped around the ends for identification.

38. What is AC grounding?

The grounding connection is typically made at the main electrical distribution panel, where the grounded conductor and the grounding electrode conductor are connected to the same bus bar. In PV systems, this section applies to AC- only systems, such as systems with AC modules and no field-installed or accessible DC circuits, or to the AC side only of a PV system with both AC and DC circuits.

36. What is neutral loading?

A particular overcurrent problem arises when one stand- alone inverter with a 120 V output supplies a 120/ 240 V distribution panel where the L1 and L2 bus have been connected together to create a single 120 V supply. If the system includes a multiwire branch circuit with two 120 V circuits sharing the neutral conductor, the neutral conductor can become dangerously overloaded. See Figure 11- 26. Since current on the two ungrounded conductors will be in- phase, instead of out-of- phase as with a normal 240 V split- phase supply, currents in a multiwire branch circuit will add when they return on the shared neutral conductor. Therefore, the neutral conductor may carry twice its rated circuit current. ( A similar problem can occur with interactive systems when PV systems are added to buildings that are already wired for standard 120/ 240 V service.)

33. What are the possible causes of overcurrent condition?

An overcurrent condition can be the result of an overload, ground fault, or short circuit.

1. What NEC article addresses requirements for PV installations?

Article 690

24. Describe the use of lugs in installing PV systems.

Lugs are used to terminate conductors with special connectors. In most cases, screw terminals are preferable to lugs because they are easier to install correctly and involve one fewer connection. Lug types include fork, ring, and disconnect ( spade or pin) lugs. Fork and ring lugs are used for connections made by threaded screws or nuts and the lug is sized according to screw diameter designations, such as 1/ 4 and # 8. Fork terminals can be easily removed by loosening the terminal screw or nut, while ring terminals require complete removal of the fastener. For this reason, ring lugs are usually used for large conductors, so they cannot accidentally slip out from under the screw or nut due to their weight or movement. Spade- or pin- type lugs have both male and female elements that connect only to each other and can be disconnected without tools.

7. Describe 125% maximum current factor.

This 125% factor accounts for the fact that PV devices can deliver currents higher than their rated short-circuit current under enhanced irradiance, possibly for more than 3 hr around solar noon.

26. Describe plug and receptacle configuration for DC only, and AC only, respectively.

the plugs and receptacles must be listed for DC power with a rating of at least 15 A, must have a separate equipment- grounding terminal, and must be different from any other receptacles used on the premises.

34. What are the two common types of overcurrent protection devices?

• A non- current-limiting device operates slowly, allowing dam-aging short- circuit currents to build up to full values before opening. • A current- limiting device opens the circuit in less than one- quarter cycle of short- circuit current, before the current reaches its highest value, limiting the amount of destructive energy allowed into the circuit. Overcurrent protection devices include fuses and circuit breakers.

32. Describe the purpose of an overcurrent protection device.

An overcurrent protection device is a component that protects conductors from higher-than- rated currents, which prevents damage from high temperatures. High temperatures can damage conductor insulation, causing electrical shock and fire hazards.

4. How to determine the maximum DC voltage of a PV source circuit or output circuit? Explain why this voltage is important.

According to Section 690.7, the maximum DC voltage of a PV source circuit or output circuit is determined by the sum of the rated open- circuit voltages of the series- connected modules corrected for the lowest expected ambient temperature. This voltage dictates the minimum voltage ratings, which must be less than the maximum voltage limits of all components on the DC side of the system, including the modules, inverter, charge controller, disconnects, and conductors. For single family dwellings, this cannot exceed 600V.

13. What sizes of AWG conductors are generally only available in stranded versions?

At size 6 AWG and larger, conductors are generally only available in stranded versions.