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| {{Menu_Photovoltaic_installations}}
| | #REDIRECT [[Design of electrical installations integrating solar production]] |
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| There are two main possibilities for connection and operation of PV systems installed on a building rooftop, car park, or integrated in the building structure – self-consumption and grid export.
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| In the export-to-grid operation, the PV installation is connected to the electrical distribution network and it does not interfere with the building electrical installation. Although physically linked, the PV system and the building installation are two independent and autonomous units.
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| In case of self-consumption, the PV system is connected to the building electrical installation where the PV production is used in priority to satisfy the electrical consumption needs of the local loads.
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| As there is a physical connection between the two installations, the self-consumption of PV production comes with specific installation rules and architecture requirements.
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| Electrical installations with local PV generation for self-consumption could be designed to operate:
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| * '''only grid-connected''' - In case of blackout at the electrical network side, the electrical installation is no longer supplied by the local generation.
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| :Current PV systems used for self-consumption operate mainly in this mode.
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| * '''grid-connected and islanded''' – the electrical installation operates connected to the grid, but can also operate in an off-grid mode supplying the totality of the loads or part of them with the local energy sources. Today PV installations cannot assure the operation of the electrical installation in off-grid mode standalone, as the produced PV energy is volatile, predictable, but unplannable and with limited control capabilities. To assure an off-grid operation, PV installations must be associated to another major and stable source such as Storage or Generator.
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| :Also, the operation of the electrical installation in both grid-connected and island mode is much more complex and requires a dedicated control. This kind of operation is rare, especially in countries where the electrical grid is stable and blackouts are abnormal.
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| This section focuses on electrical installations with local PV production operating grid-connected, which is the main use of PV systems in the case of self-consumption. The specific requirements for installations with PV production for self-consumption are explained and guidelines for sizing and equipment selection are provided.
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| == PV system connection to the electrical installation ==
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| The integration of PV system into the electrical installation can be done through a connection:
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| * to the main LV Switchboard
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| * to a distribution LV Switchboard
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| * upstream to the main LV switchboard
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| * to the MV system
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| The corresponding schemes, field of application, advantages and drawbacks for each of these possibilities are detailed hereafter.
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| === Connection at the main low voltage switchboard ===
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| With this configuration, the PV Installation architecture can include:
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| * a single PV inverter, directly connected to the main LV switchboard
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| * a group of PV inverters. The inverters’ outputs are gathered to a local generation switchboard and each in its turn feeds the main LV switchboard of the electrical installation – see {{FigRef|P26}}. (Another possibility is to connect each inverter individually to the main LV switchboard, but this configuration is not recommended for cost, installation complexity and maintenance reasons)
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| The PV inverter power range used for self-consumption in commercial and industrial buildings being typically between 20kW and 60kW, it can be considered that for installed power up to 30kW a single inverter is used, and above this value a group of inverters is preferred.
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| {{FigImage|DB431004|svg|P26|PV installation connected to the main LV Switchboard}}
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| A configuration where the PV installation is connected to the main LV switchboard is used in the following cases:
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| * PV system is located near the main LV switchboard
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| * PV production is used both for self-consumption and export to the grid of the excess PV energy
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| * PV system is associated with other local energy sources, e.g., storage or CHP. A connection at the main LV switchboard allows all local production to be gathered in a single area, which facilitates the maintenance and the operation of the installation. It is the ideal configuration for new electrical installations, or existing ones when the main LV switchboard is easily accessible (e.g., one- or two-floor buildings)
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| * PV production capacity is between 10% to 100% of the building installed power. For smaller scale PV installations, a connection to a closer secondary switchboard can be preferred.
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| For larger PV installations, a connection upstream to the LV switchboard can be more suitable for existing installations.
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| == Connection to a distribution switchboard ==
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| The PV system is connected to the closest switchboard.
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| This configuration is preferred in the following situations:
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| * PV production does not exceed the consumption downstream to the switchboard it is connected to
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| * the PV inverters are far from the main LV switchboard.
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| This configuration is typically used in multi-floor buildings characterized by:
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| * A PV production at rooftop area significantly lower than the building energy consumption needs
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| * a main LV switchboard usually located at the ground floor (utility incomer through ground cables).
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| {{FigImage|DB431005|svg|P27|PV installation connected to secondary LV switchboards}}
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| The connection to a secondary LV switchboard presents the following advantages:
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| * The cable length between the PV system and its connection point to the electrical installation is minimized
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| * The installation is easier and optimized.
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| Although, this configuration presents some limitations:
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| * '''More complex maintenance if the number of PV sources multiplies''' - When a feeder is under maintenance, all power sources potentially supplying the feeder must be isolated. When connecting PV installations to the closest switchboard, the number of connected power sources is potentially higher and much more dispersed, which makes the isolation more complex
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| * '''Limited upgradability''' – this configuration is much less evolutive than a connection to the main LV switchboard: an extension of the PV installation may require modifications in the existing building electrical installation (cables, switchboards and protections may need to be resized).
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| === Connection upstream to the main low voltage switchboard ===
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| In this configuration, the PV installation is connected upstream to the main LV switchboard. A possible configuration is to connect all PV inverters and the utility incomer to a switchboard which feeds the main LV switchboard of the electrical installation. Another alternative, especially when the utility incomer and the local sources are not in proximity, is to connect a single output of the PV system to the utility incomer, before feeding the electrical installation.
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| This connection upstream to the main LV switchboard is used in existing buildings where the PV production capacity exceeds the load consumption for which the electrical installation is sized. In this case, a connection of the PV system to a secondary or main switchboard would overload the existing electrical infrastructure and will require its modification such as replacement of cables, switchboards and protection equipment. Coming as a better option, a connection upstream to the main switchboard does not require any modifications of the main LV switchboard or downstream to the installation.
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| The main advantage of this architecture is its capacity to integrate large-scale PV production without impact on the existing electrical installation infrastructure. It can be used both for self-consumption and export of PV production excess. A drawback can be the need for a dedicated, additional switchboard to gather all sources, which potentially increases the cost of the system.
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| {{FigImage|DB431006|svg|P28|PV installation connected upstream to the main LV switchboard}}
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| === Connection to the MV system ===
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| A connection to the medium voltage side of the electrical installation is extremely rare, as the PV production and the electrical installation loads are in geographical proximity.
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| A medium voltage connection of the local PV production will require additional costs and will be less energy efficient, except if the PV production is far from the loads and with important production capacity.
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| == Operation of electrical installation with local PV production ==
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| === Normal operating conditions ===
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| ==== Operating modes ====
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| In normal operating conditions, electrical installations with PV production for self-consumption are characterized by two operating modes:
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| * The electrical installation operates supplied by the grid only when there is no PV production (e.g., during the night)
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| : The PV installation may be disconnected - as an option - through its protection device
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| * The electrical installation operates supplied both by the grid and by the PV installation when there is a PV production. The output waveform of the PV inverter(s) is synchronized with the grid voltage and frequency. This function is ensured by the PV inverter(s) embedded control.
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| :The produced PV power goes to the loads, as the electricity takes in priority the path of least resistance. Consequently, there is no need of specific equipment redirecting the flow of electrons.
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| ==== Management of excess PV production ====
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| In case where the PV production exceeds the instantaneous consumption needs of the electrical installation, one of the following management strategies can be adopted:
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| * Inject the excess power in the grid where it is consumed by other users.
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| : The injected power can be remunerated at the wholesale price of electricity, or at other tariff, or not given value at all depending on the agreement with the energy provider
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| * Limit the PV production - Some energy providers do not allow the injection of excess PV power in the grid, or authorize a restricted injection only
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| * Use storage
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| * Share the PV excess with a community, with a private electrical network if allowed by the local regulation
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| ==== Generated disturbances ====
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| PV systems create some limited disturbances in the electrical installation, coming mainly from the operation of the PV inverters. Those are:
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| * '''Harmonics''': as most electronic equipment, PV inverters generate harmonics. The harmonics emission is specified by the PV manufacturers, generally they are below 3%THDI.
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| * '''DC residual currents''': In case of an earth fault on the DC part of the network, the feeding of a DC residual current at the AC side of the installation depends on the isolation between the DC and AC side of the PV system:
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| ** Galvanic isolation between DC and AC side of the PV installation guarantees that DC residual currents will not pass at the AC side.
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| ** When there is no galvanic isolation between the DC and the AC side, a DC residual current may be present at the AC side of the installation and must be eliminated, unless the PV inverter is designed to prevent, limit or avoid such situation.
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| When an electrical installation requires the installation of Residential Current Device (RCD) of the AC circuit, the type (AC, A or B) should be selected in function of the residual current that can be present. PV inverters manufacturer usually specify the injected by the PV inverters maximum DC residual current and the RCD type that should be used with this inverter.
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| === Fault conditions ===
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| ==== Utility supply loss ====
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| In installations operating grid-connected only, the PV inverters shut down automatically in the case of utility supply loss. They are not designed to provide back-up power during utility outages.
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| ==== Fault inside the electrical installation ====
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| In the case of fault in the electrical installation, a high short-circuit current flows from the utility incomer to the fault location. The PV inverters also contribute to the fault providing their maximal current, which usually does not exceed two times the nominal PV inverter current (The maximal short-circuit current value is provided by the PV inverter manufacturer).
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| Electrical faults in installations with PV systems for self-consumption are localized and isolated by overcurrent protections, and specific protection functions or devices are not required.
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| The presence of PV inverters does not affect earth fault protection in the AC side of the installation.
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| PV inverters and DC side shall be protected against earth fault according to IEC 712.421.101.
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| == Architecture and equipment requirements ==
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| When operating only grid-connected, PV systems used for self-consumption are simply connected to a switchboard of the electrical installation.
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| Loop configurations used to increase the energy availability in multi-source installations do not bring any benefit, as PV inverters stop operating in case of utility supply loss.
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| === Automatic Transfer Operation ===
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| As described in the previous section, PV inverters operate in parallel with the grid. There is no transfer of operation from one source to another, thus Automatic Transfer Switch is not required for installations operating only grid-connected.
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| === Earthing ===
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| As the PV system for self-consumption is part of the electrical installation, the earthing arrangement of the electrical installation applies also to the AC side of the PV inverter. If the PV inverter is with distributed neutral, this one is connected to the same earth reference as the transformer neutral. The creation of two earth references inside the same building is forbidden as currents may circulate between
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| both earths.
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| The earth reference remains the same as far as the installation operates gridconnected.
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| Regarding the DC side of the PV inverter(s), there are two options:
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| * galvanic isolation between the DC and the AC side of the PV system: the earthing at the DC side does not depend on the earthing system of the AC electrical installation
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| * no galvanic isolation (most common case): the system earthing at the DC side must be compliant with the earthing of the AC electrical installation
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| === Grid interconnection ===
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| In case of utility supply loss, installations with local generation are required to guarantee that they do not inject power into the grid for utility workers’ safety.
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| For installations with PV production operating only grid-connected, this safety feature can be met by:
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| * the PV inverter (s) – most PV inverters integrate anti-islanding protection, which disconnects the PV inverter in case of power outage. The presence of this protection and its standard compliance is provided by the PV manufacturer
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| * a dedicated protection device installed at local sources feeders or at the utility incomer of the electrical installation.
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| === Reverse power protection ===
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| Reverse power protection for PV inverters is not required as they are unidirectional devices and do not pass reverse current.
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| Reverse power protection may be required in the case of the presence of other local sources, such as diesel generators, where the flow of the current back to the generator may cause its failure.
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| Reverse power protection is usually not required to prevent the power injection into the grid in normal operating conditions, this function is assured through control of local sources.
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| == Guidelines for sizing ==
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| Installations with PV production for self-consumption, even operating only grid-connected, present specific requirements on load flow calculation, placement and selection of the protection devices, and sizing of equipment.
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| === Load flow and short-circuit currents ===
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| Installations with PV production for self-consumption are characterized by at least two operating modes (supplied by the grid, and supplied by the grid and the PV sources). The load flow and the short-circuit currents flow should be evaluated for each operating mode, and the electrical installation components and equipment should be sized accordingly, taking the worst-case constraints.
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| If storage is present in the electrical installation, it should be taken into account in the load flow and short-circuit currents calculation twice – once as a load (when charging) and once as a source (discharge).
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| === Switchboard sizing ===
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| In single source installations, switchboards are sized in function of the maximal current drawn by the downstream load.
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| When a switchboard is fed by two or more sources, it should be sized in function of the maximal current that can transit through it, which can be:
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| * the current drawn by the load
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| * the current delivered by the local sources
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| === Cable sizing ===
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| Cables’ current ampacity must be higher than the maximal expected current, which should be evaluated for each operating configuration of the electrical installation.
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| === Transformer sizing ===
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| In single source installations, transformers are sized in function of the installed load power.
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| In installations with PV production for self-consumption where the injection of PV production to the grid is possible, the transformer should be sized in function of the installed local sources power capacity.
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| === Circuit breaker location ===
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| When a single inverter is connected to a switchboard of the electrical installation, a circuit breaker should be installed downstream to the cable connecting the PV inverter to the switchboard to isolate electrical faults or overloads occurring on the PV feeder.
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| When several PV inverters are regrouped to a switchboard before they interface with the electrical installation, it is recommended to install protection devices at both ends of the connection between the two switchboards:
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| * the role of the downstream breaker (CB2) is to isolate faults on the connection between the PV system and the building’s electrical installation (in case of fault at C2, a high short-circuit current will flow on the path from the grid to the fault location. A tripping of CB2 will isolate the fault allowing the electrical installation to continue to operate supplied by the grid). Selectivity between CB1 and CB2 should be assured. Otherwise, a fault in the PV installation may trip the main breaker and interrupt the supply from the grid.
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| * the role of the upstream circuit breaker (CB3) is to isolate electrical faults on the PV switchboard, the selectivity between the downstream and the upstream breaker on the PV feeder should be met.
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| {{FigImage|DB431007|svg|P29|PV installation connected upstream to the main LV switchboard}}
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