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  • PV Circuit Protection 

    PV Systems have multiple active points either on DC side or on the AC side which require protection.

    PV Circuit Protection Components

    Some of the protection elements most used are: Blocking Diodes, Photovoltaic Fuses, DC Circuit Breakers, Photovoltaic SPDs, Safety (Disconnect) Switches, Rapid Disconnects.

    PV Protection on circuits must comply with IEC 60269 and UL 1699B, respective NEC Article 690 NEC and UL1741 for the PV safety design. PV Circuit protection is different on the DC side from the AC side but they do interfere. The International Electrotechnical Commissions (IEC) recognize the protection of PV systems is different from standard electrical installations. This is reflected in IEC 60269-6 (gPV) which defines specific characteristics for protection at the string, respective array level;

    #PV Circuit Protection on the DC Side:~

    Particular characteristics of PV generators are their DC voltage levels and the fact they cannot be shut off as long as PV modules are sun exposed. The short-circuit current produced by the PV module is too low to trigger the power supply’s automatic disconnect. The most frequently used protective measures do not apply directly to PV systems. Because a fault can occur any time on the DC side, and because the DC faults are hard to detect, the chances to feed into a fire hazard on the roof are very high. Therefore the DC side protection and the interference with AC side are very important.

    [Tech Support]  Knowledge Base Information for:   PV Circuit Protection  
    Read from Knowledge Base and download a pdf file @  Article:   101 RENEWABLE - PV CIRCUIT PROTECTION AND DISCONNECTS 
    Fold | Unfold [ click ]  Read more...  

    Some general considerations:~

    >_PV system as a power plant is an intermittent power source, dependent of a set of factors like the place eco system. That translates into different levels of irradiation and day-night transition intervals. In some of the cases, we have added an additional wind power generator or a conservation power system, to compensate the day-night transition, and intermittent power generation. All those new additions will revolve around a specific ecosystem and a microenvironment pinned by geographical location.
    >_Conventional PV systems operate at unity power factor, regardless of reactive power needs of the utility network. PV System offers the ability to absorb reactive power from the power generation plant and that mainly by the system inverter(s);
    >_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;

    #Overcurrent protection, per NEC 690.9, Article 240:~

    Overcurrent protection, when used, protects PV cells against reverse current and cables against overload. Short-circuit current depends of solar irradiance, but it may be lower than the trip value of overcurrent protection. Although this is not an issue for cables as the current is within current-carrying capacity, the inverter will detect a voltage drop and stop producing power. It is therefore recommended that the maximum trip current should be significantly lower than Isc stc MAX. Protection against overcurrent can be done at String Level per IEC 60364 or at Sub-Array Level per IEC 60364. Circuit breakers or fuses are used to provide overcurrent protection. Overcurrent protection is required in only one leg of a PV circuit [NEC 690.9(C)] see element 3 in the picture.

    #Short Circuit Protection, protecting people against electric shock, per IEC 60364-7-712:~

    IEC 60364-7-712 stipulates that PV systems whose maximum UOC MAX (UOC = Open Circuit Voltage) is higher than 120V DC should use « double or reinforced insulation » as a protection against electric shock. This protective measure is intended to prevent the appearance of dangerous voltage on the accessible parts of electrical equipment through a fault in the basic insulation. Circuit breakers or fuses can be used to provide direct short circuit protection, and they are selective in acting on the fault.

    Short Circuit Protections

    #Protection against thermal effects and the risk of fire, per NFPA 70 and NESC 2012:~

    There are three internal system situations that can lead to abnormally high temperatures and the risk of fire in a PV system: insulation fault, a reverse current in a PV module, and overloading cables or equipment. And those are beside external fire hazards. Here are the outcome scenarios in case of a fire by PV systems: Building destroyed or damaged; PV system affected; System components damaged; PV system destroyed. If a PV electrical plant is designed and put in place according to national, European or international electrical code recommendations (i.e., NFPA 70, IEC 60364-7-712, CEI 64-8) the fire risk due to overload and a short circuit is well addressed and mitigated. Beside fusses and circuit breakers, rapid disconnect systems must be used to prevent the DC source feeding into the isolation fault. A ground-fault detector interrupter (GFDI) is a safety device especially used for PV solar arrays. A ground fault at the PV generator will trigger the GFDI, interrupting the leakage and preventing damage to the system, and feeding the fire.

    Fault Circuit Protection

    #Insulation fault protection, Ground Protection, per NEC 690.2, 690.43 and NEC 100:~

    DC insulation fault could be more dangerous as arc has less chance to extinguish itself as it does in AC circuit. When an insulation fault is detected whatever the solution is, the inverter is stopped and disconnected from AC side, but the fault is still present on DC side and the voltage between poles is the open circuit voltage of PV generator as long as the sun is shining. Through the arc we have a transport of energy from one poll to another which will look for the minimum resistive path, and choose to go to the ground. this type of fault brings very high temperature into equipment and PV modules, most of the time generating fire.

    In case of DC side base ground fault, Double Earth Fault it can be a ground fault in which current leaks to the ground. If this fault is not cleared, it may spread to the healthy pole and give rise to a hazardous situation where fire could break out. The fault level could be low (e.g. two insulation faults or a low short-circuit capability of the generator in weak sunlight) and below the tripping value of overcurrent protection (circuit breaker or fuses).

    Courtesy of Home Power magazine we picture the logic scheme for grounding on the DC side:

    Grounding DC Side

    Usually, a PV array has several exposed noncurrent-carrying conducting parts that do not carry any current during normal operation (module frames, mounting racks, metal enclosures, distribution panels, the chassis of end-use appliances and power converters). There is a potential risk of electric shock hazard from these conductors when an electrical connection is established between the current carrying conductors and noncurrent-carrying conductors due to a fault (e.g., corrosion, loss or melting of insulation, wire cutoff, and wrong wiring). Therefore, all of these conductors are connected together to the ground or earth through a conductor (equipment grounding conductor – EGC).

    USA National Electrical Code (NEC), Article 690.43, requires equipment grounding to protect people and animals from being electrocuted. Similarly, any accidental connection between a current carrying conductor and EGC/earth can cause significant current flow to the ground circuit (ground fault). For this reason, proper grounding is required for any electrical system to provide adequate personnel and system safety in case of one or multiple ground faults. Article 100 of the NEC defines solidly grounded as “connected to ground without inserting any resistor or impedance device.”

    #Protection of PV modules against reverse current per IEC 61730-2.:~

    This occurs if the open-circuit voltage of one string is significantly different from the open voltage of parallel strings connected to the same inverter. The current flows from the healthy strings to the faulty one instead of flowing to the inverter and supplying power to the AC network. Reverse current can lead to dangerous temperature rises and fires in the PV module, or PV system. Most of the time protection against reverse currents in PV installations is achieved by series fuses The rating of protection fuses must fulfill the condition requested by PV modules to withstanding 1.35 times this rating for two hours. In this way, the selected PV fuses will protect the conductor and the PV source circuit against reverse current faults.

    #PV System Disconnects and Rapid Shutdown Disconnect per NEC 690.15.:~

    PV system circuits installed on or in buildings shall include a rapid shutdown function to reduce shock hazard for emergency responders in accordance with 690.12(A) through (D). Disconnects must disconnect both positive and negative conductors on the DC side of the PV system. 690.15 stipulates for Disconnection of Photovoltaic Equipment: "Means shall be provided to disconnect equipment. such as inverters, batteries. and charge controllers, from all ungrounded conductors of all sources. If the equipment is energized from more than one source, the disconnecting means shall be grouped and identified." A PV system usually provides two type of disconnects, one at the DC level, mostly used in conjunction with inverter main disconnect, and another disconnect at the AC connection to the grid before the metering. Both must be visible placed, easily accessible and labeled.

    Grounding DC Side

    We provide RS (rapid shutdown) in one of the following situations:~

    >_For string inverter located at the edge of the array with dc-to-dc converters or shutoff switching devices at each module;
    >_For Building-Integrated PV array with no exposed metal or wiring and installed more than 8’ from grounded metal;
    >_For PV system conductors of more than 1.5 m (5 ft) in length inside a building, or more than 3 m (10 ft) from a PV array;
    >_For conductors which are energized with more as 30 volts and respective more as 240 volt-amperes within 10 seconds of rapid-shutdown initiation;
    >_The rapid-shutdown initiation methods shall be labeled in accordance with 690.56(B);

    The information posted herein has been compiled by Clean Energy Brands from OEM product data and reputable publications. All rights reserved!

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