Electrical BOS is the backbone of any power generation unit. In the today landscape a PV Power Plant or Wind Power Generation are part of the broader distributed power generation. The distributed power generation requires smart Electrical B.O.S. components thus they can be monitored and remote controlled to accommodate that PV or Wind power plant into the larger picture of utility grid system.
Electrical Distribution or Electrical B.O.S is related to PV Power Plants or Wind Power Generators and covers all the electrical harnessing components of the distribution chain: array combiners, disconnects, ground fault protection, critical load panels, surge protections, transfer switches, marking label, fusses, terminals and strain reliefs. All this components must come in terms and conditions with NEC section 705.12 (A), section 250.24 and section 250.92(B). They are specified to be either on the DC supply side connection or on the AC load connection towards the utility power line. The rest of components correspond to the PV core of elements on the DC side like PV modules, mounting frames for PV modules. The rest of this article refers to some of the NEC guidance for the Electrical BOS components associated to any of the sections of the PV Power Generation system.
Some general considerations:
>>PV system is an intermittent power source, dependent of the micro geography where it is installed, and that translates in level of irradiation and day night transitions. In case of an additional wind power generation system added to the system that one will try to compensate the day night intermittent power generation of the PV plant but it will depend of the wind conditions related to geographical environment.
>>Conventional PV systems operate at unity power factor, regardless of reactive power needs of the utility network.
>>Due to concerns regarding unintentional islanding, current interconnection standards require distributed PV resources to cease to export power during voltage and frequency disturbances, thereby reducing generation at times it is needed most.
>>Ability to absorb reactive power from the power generation plant and that mainly by the system inverter(s).
Considerations On Supply-Side Connections:-
Supply-Side Connections :-
apply to the utility service connection point from the main disconnect of the load panel to service panel and service connection point to utility line. Here are some of the elements included by this section and what NEC offers as guidance:
Over current protection devices (OCPDs).:-
NEC 2011 clarifies how much current PV systems can impose on the service conductors. The sum of the over current protection devices (OCPDs) from the PV system connected to the service conductors cannot exceed the service conductors’ rating. That comes down to breaker selection.
equires that PV system disconnecting means (PV plant load panel) must have no more than six switches or six circuit breakers in addition to the utility service disconnects. PV disconnecting means being installed under the same rules as service disconnects.
Grounded conductor bonding:-
The grounded current-carrying conductor on the supply side of service equipment for PV systems should be bonded to the PV disconnect in accordance with Section 250.24. This is to make sure that the grounded service conductor provides the effective ground fault current path from the power supply to ensure that dangerous voltages from a ground fault are quickly removed by opening the OCPD.
Raceways containing the supply side of service equipment conductors for PV systems should be bonded in accordance with Section 250.92(B). This helps ensure the electrical continuity at the service equipment, as intended by Code.
The conductors and methods used for supply-side connected PV systems should be limited to those identified in Section 230.43. Using these methods keeps the inverter output circuit consistent with the wiring methods used for the service conductors. This imposes a higher standard on the wiring methods, as compared to treating them as branch circuits, and results in a robust installation.
Considerations On Load-Side Connections:-
Load-Side Connections :-
applies from the output of the PV Plant Utility-Interactive Inverters, to the utility service connection point from the main disconnect of the load panel. Here are some of the elements included by this section and what NEC offers as guidance:
Load-side service connections are more common than supply-side connections, especially for residential and small commercial PV systems. Section 705.12(D), “Utility-Interactive Inverters,” is relatively straightforward and begins by stating that the load-side connection allowance applies to the output of utility-interactive inverters only.
705.12(D) (5), Suitable for Back feed:-
Code requires that circuit breakers used for the PV system interconnection be suitable for back feed. Conductors from the inverter can back feed dedicated circuit breakers that are not marked “Line” and “Load” as outlined in the informational note that follows this subsection. If the circuit breakers indicate a line and load connection, they have not been evaluated for back feeding conditions and are not appropriate for load-side connections.
705.12(D) (1), Dedicated Over current and Disconnect:-
Each inverter circuit must terminate to a dedicated circuit breaker or fusible disconnect. This requirement does not apply to micro inverters; all micro inverters on the same string are required to connect to a single breaker in accordance with the listed instructions as covered in Section 110.3(B).
705.12(D) (2), Bus or Conductor Rating:-
Where distribution equipment is capable of supplying multiple branch circuits or feeders, the sum of the ampere rating of the inverter OCPDs and panel board OCPD must not exceed 120% of the panel board bus ampere rating. This is commonly referred to as “the 120% rule,” and it allows a PV system to interconnect to a panel board that has bus bars rated at the same ampere value as the panel board's over current protection.
705.12(D) (3), Ground-Fault Protection:-
Supply-side connections of electrical power production sources, as outlined in Section 705.12(A), are required for facilities incorporating ground-fault protection of equipment as required by Sections 215.9 and 230.95. If proper ground-fault protection for equipment from all ground-fault sources is provided, there is an exception to this requirement. The exception requires that if a connection is made to the load side of a ground-fault protection device, that device needs to be identified and listed for back feeding. This requirement often disqualifies load-side interconnections for large commercial PV systems with service disconnects rated at 1,000 A or more because few manufacturers can provide proof that their ground-fault protection device is identified and listed for back feeding.
705.12(D) (4), Marking:-
Equipment containing ac inverter OCPDs must be field-marked to indicate the presence of multiple AC power sources. This is to alert personnel working on that equipment to the multiple power sources and allow them the opportunity to properly protect themselves. The label must resist the environment for the system’s 25- to 40-year operational life, be suitable for the site’s specific environment and be installed in a manner so as not to void equipment listing as called out in Section 110.3(B).
705.12(D) (6), Fastening:-
Dedicated ac inverter circuit breakers that are back fed from identified and listed utility-interactive inverters do not need to be secured in place by an additional fastener as required by Section 408.36(D). Once the dedicated ac inverter circuit breaker has been removed from the panel board, the listed interactive inverter automatically powers down, and the terminals on that circuit breaker do not have any voltage present. You may need to direct your AHJ to Section 705.12(D) (6), as inspectors who have not worked with load-side–connected PV systems often incorrectly ask to have a fastening kit installed.
705.12(D)(7), Inverter Output Connection:-
Where distribution equipment is capable of supplying multiple branch circuits or feeders and the sum of the OCPDs supplying power exceeds the bus bar rating, the AC inverter circuit breaker must be located at the opposite end from the input feeder supply conductors. The inverter’s OCPD also needs a permanent warning label indicating that the OCPD is for the output of an inverter and cannot be relocated. The label should have language such as:WARNING—INVERTER OUTPUT CONNECTION; DO NOT RELOCATE THIS OVER CURRENT DEVICE.
This subsection also clarifies the calculations needed when connecting an inverter’s dedicated OCPD to a subpanel. It states that for panel boards connected in series, the rating of the first OCPD directly connected to the output of a utility-interactive inverter is the only one used for all bus bar and conductor calculations. Consequently, the inverter’s dedicated OCPD rating is used to evaluate all the conductors and bus bars connected in series, not the rating of the OCPD protecting the subpanel.
For example, consider a building that has a 400 a main distribution panel protected with a 400 AN OCPD. That main distribution panel feeds a 100 a subpanel protected at 100 A. Inside the subpanel; a 20 A dedicated inverter breaker is installed. The 20 A inverter breaker satisfies Section 705.12(D) (2) because the subpanel bus bar rating is not exceeded by more than 120%. The 400 A main panel can accept up to 80 A of additional power sources (400 an x 1.20 – 400 A). Section 705.12(D) (7) states that the first OCPD, the 20 a breaker in this example, is used for all upstream conductors and bus bars. Previous interpretations of the Code required that you use the subpanel’s 100 A OCPD when calculating the upstream conductors and bus bars, and would have prevented a small 20 A PV array from being connected to the subpanel in this example.
Considerations On power Generation-Side Connections:-
690.13, DISCONNECTING MEANS, ALL CONDUCTORS:-
Section 690.13 requires means to disconnect “all current-carrying dc conductors of a photovoltaic system from all other conductors in a building or other structure.” It also states, however, that the grounded current-carrying conductor cannot be switched if this leaves it energized and ungrounded. The portion of the grounded conductor on the array (line) side of a dc disconnect, for example, is energized and ungrounded with the switch in the open position.
690.7, MAXIUM VOLTAGE:-
PV system designers know that having accurate cold temperature data is critical for calculating maximum array open-circuit voltage and designing PV source circuits (series strings) of the correct length. Array voltage should never exceed inverter dc voltage parameters or equipment voltage ratings. There is no standard dataset for these temperatures, and many designers resort to weather.com or other sources of almanac data for record low temperatures. However, the record low temperature for a location is extremely conservative for design purposes. Reasons for this include module voltage degradation over time and the very low likelihood of significant irradiance occurring at the same time as the record low temperature.
690.16, FUSES, Section 690.16(A):-
Requires that a disconnecting means be provided for fuses energized from both directions, and that it must be possible to disconnect “all sources of supply.” In addition, when fuses are installed in PV source circuits, that must be possible to disconnect each one independently of any other fuses. Previously this section required disconnecting means only if the fuses were accessible to unqualified persons.
690.31(B), PV Wire:-
Has a new Informational Note addressing Photovoltaic (PV) Wire, also known as PV Cable. Because PV Wire has a nonstandard outside diameter, designers must consult NEC Chapter 9, Table 1 for calculating conduit fill, as specifications are not contained in Informative Annex C. UL 4703 dictates the standards for PV Wire, which has a wet rating of 90°C and a dry rating of 90°C to 150°C. PV Wire can have a 1,000 or 2,000 volt rating and is the only single conductor that can be used for PV source circuits in ungrounded PV systems per 690.35(D).
690.11, ARC-FAULT CIRCUIT PROTECTION (DIRECT CURRENT):-
Likely the least-understood new provision of the 2011 NEC is the requirement for a listed arc-fault protection device on dc source and output circuits on or penetrating a building, where PV system maximum voltage is above 80 VDC as calculated per 690.7(A). This requirement follows in the footsteps of requirements for arc-fault protection devices on ac circuits and dc ground-fault protection, which have both been expanded in recent Code cycles. NEC provisions are focused on safety and fire prevention, and arcing faults and ground faults in PV arrays are primary causes of PV related fires. According to Brooks: “690.11 is by far the most contentious change in NEC 2011. Arc-fault detectors will definitely improve system safety eventually, but it will take some time to iron out operating issues that are likely to happen in early field applications.
690.9, OVERCURRENT PROTECTION:-
Section 690.9 covers the requirements for over current protection for PV circuits and equipment, referencing Article 240. The Exception to 690.9(A) allows conductors in very specific situations to be exempted from the Article 240 requirements for over current protection, such as where the ampacity of the conductor cannot be exceeded by short-circuit currents from all sources. The 2011 Code includes a terse but critical change in the 690.9 Exception, adding the words PV source. Specifying source circuits means that there are now no clear exceptions for over current protection for PV output circuits. PV output circuits could be mandated to follow all requirements of over current protection in 240, including 240.15(A), which states that each ungrounded conductor must have an over current device connected in series.
690.43, EQUIPMENT GROUNDING:-
Section 690.43 offers additional clarification on equipment grounding, particularly as it applies to PV arrays. The majority of the 2011 section was contained in the previous version, but now it has been broken out into a list of requirements (A) through (F). Requirement (C) addresses the use of mounting structures—typically the mounting rails or a metal racking system—as the equipment-grounding conductor (EGC). It specifically states: “Metallic mounting structures, other than building steel, used for grounding purposes shall be identified as equipment-grounding conductors or shall have identified bonding jumpers or devices connected between the separate metallic sections and shall be bonded to the grounding system.
690.8(B) (2), Conductor Ampacity:-
Section 690.8(B)(2) clarifies that conductors be sized to carry whichever is greater: 125% of the maximum current without correcting for conditions of use, which is the same value as the minimum over current device size; or the maximum current after applying conditions-of-use correction factors—such as the number of current-carrying conductors in the conduit, ambient temperature and elevated temperatures due to proximity to the rooftop.
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