11.5 Solar Photovoltaic Systems

11.5 Solar Photovoltaic Systems

Quick Answer: Series-connected modules add voltage; parallel strings add current. Calculate maximum system voltage at the lowest expected temperature and maximum circuit current from short-circuit current under Article 690. Treat illuminated array conductors as energized even when the inverter ac breaker is open. Use Article 705 for the connection to other sources and follow Massachusetts licensing policy for installation of modules and associated support components.

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1. PV Circuit Behavior

A PV module produces dc whenever sufficient light reaches the cells. A standard breaker on the inverter ac output does not stop the array from producing voltage. Module-level electronics or a rapid-shutdown system can reduce voltage in specified conductors, but the array and equipment must be treated according to the actual shutdown state and listing.

Two module values drive calculations:

• Voc: open-circuit voltage, used for maximum-voltage calculation;
• Isc: short-circuit current, used as the starting point for maximum-current calculation.

Maximum-power voltage and current describe the operating point, but they do not replace Voc and Isc in the Code calculations that call for those values.

2. Series and Parallel Calculations

For identical modules in series:

[ V_{string}=N imes V_{module} ]

Current remains approximately the module current. For identical strings in parallel:

[ I_{array}=N_{strings} imes I_{string} ]

Voltage remains the string voltage.

Example: ten identical modules, each Voc 49 volts and Isc 11 amperes, form one series string. The uncorrected string Voc is 490 volts and Isc remains 11 amperes. Four parallel strings remain 490 volts but contribute 44 amperes of summed Isc before the Article 690 maximum-current factor.

Mismatched modules, shading, optimizers, and multiple MPPT inputs can change real performance. Code calculations still use the specified nameplate and design conditions.

3. Cold-Temperature Voltage

PV voltage rises as cell temperature falls. Maximum system voltage is the sum of series Voc multiplied by the correction factor or calculated from the listed temperature coefficient at the lowest expected ambient temperature. Compare the result with inverter input limits, conductor and equipment voltage ratings, connector ratings, and maximum system voltage.

Never use annual average temperature for the cold-voltage check. The design source supplies the lowest expected temperature for the location. If the corrected value exceeds equipment rating, reduce modules per string or select equipment with an adequate listing.

4. Maximum Current, Conductors, and OCPD

Article 690 circuit maximum current commonly starts at 125% of Isc for a PV source or output circuit. Conductors and overcurrent devices can then require continuous-current treatment, often creating the familiar 156% result:

[ 1.25 imes1.25=1.5625 ]

Do not apply 156% mechanically to every component. The edition-specific rule, listed assembly, conductor condition of use, terminal rating, and equipment maximum series-fuse value all matter. Apply temperature and bundle adjustment and ensure final ampacity satisfies the rule after those factors.

Overcurrent protection may be unnecessary for a single string when no source can drive damaging reverse current, but parallel strings can backfeed a faulted string. Compare the number of parallel sources and maximum reverse current with the module series-fuse rating and the Article 690 provisions.

PV connectors must be listed and matched. Physically compatible connectors from different manufacturers are not automatically a listed mating pair. Use identified tools and torque values.

5. Disconnects, Labels, and Rapid Shutdown

PV disconnecting means must open the required conductors, be accessible as specified, have an adequate voltage and current rating, and be suitable for dc where used on dc circuits. DC arcs do not pass through a natural zero crossing each half-cycle, so an ac-only switch can fail catastrophically on dc.

Markings identify power sources, disconnects, maximum voltage, current, rapid-shutdown function, and other hazards. Use exact wording, location, durability, and color from the tested edition; do not invent a universal placard.

Rapid shutdown limits voltage in controlled conductors within the defined array boundary and outside it after initiation. It is intended to reduce responder exposure, not to prove every module or conductor is at zero volts. Know the initiating device, controlled conductors, time limit, and voltage limit in the tested code edition and listed system.

6. Grounding, Bonding, and Arc-Fault Protection

Bond module frames, rails, enclosures, raceways, and other metal parts through listed methods. Follow the racking and module instructions; a listed bonding clip or washer works only in the identified combination and installation.

Transformerless inverters can use ungrounded or functionally grounded arrays and electronic ground-fault detection. Do not add an old-style grounded conductor bond that conflicts with the listed topology. PV dc arc-fault protection is distinct from branch-circuit AFCI protection and applies within its own scope.

7. Interconnection under Article 705

A grid-interactive inverter is another power source. At a load-side connection, apply the busbar and overcurrent-device rule in the tested edition, including conductor and panel markings. Positioning of the backfed breaker, sum of source ratings, panel rating, and power-control system can change the permitted design.

At a supply-side connection, service rules and utility coordination apply. Never energize an islanded system onto utility conductors unless the system is specifically designed and permitted for that operating mode. Interactive equipment must cease energizing or control its output according to its listing when the utility source is lost.

8. Massachusetts Licensure

The Massachusetts Board has published policy that a solar photovoltaic system, including associated frames, racks, rails, and modules within the defined installation, must be assembled and installed by a licensed electrician. This is a state scope-of-practice point, not a national Article 690 calculation.

For an exam problem: draw the strings, mark Voc and Isc, apply cold voltage and current factors, check conductors and OCPD, locate disconnect/rapid shutdown, then trace grounding and the Article 705 connection.