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COBEI (Brazilian Electricity, Electronics and Lighting Committee) is the arm of ABNT (Brazilian Association of Technical Standards) that prepares standards for the electrical sector.
The NBR-5410 standard – Low Voltage Electrical Installations is one of the main standards in the electrical sector and the 1980 revision, which replaced the old NB 3, dated 1960, marked the beginning of the adoption of IEC standards (International Electrotechnical Commission).
In this text, we will adopt the concept of GFV (photovoltaic generator) for small generating plants, associated with consumer installations, such as GFVs on roofs or parking lots, attached to buildings (residential, commercial and industrial).
The term UFV (photovoltaic plant) is reserved for ground or floating plants, of medium or large size, that deliver the energy generated to the medium-voltage distribution network of an energy concessionaire or to the SIN (National Interconnected System). ), through a substation and a high-voltage transmission line.
UFVs are dedicated generation plants that can share transmission infrastructure (substation and transmission line) with other generating plants, usually wind farms or hydroelectric plants (floating UFVs).
The NBR 16690:2013 standard – Electrical Installations of Photovoltaic Arrays — Project Requirements, based on the technical specification IEC/TS 62548 – Photovoltaic (PV) Arrays – Design Requirements, is a “child” standard of NBR-5410 and refers to it several aspects related to GFV.
It is subject to the general requirements of the NBR 5410 standard with an emphasis (not declared) on rooftop Photovoltaic Generators, which are low-voltage installations (on the dc side), although the inverters can be connected to a medium-voltage substation.
The IEC/TS 62738:2018 standard – Ground-mounted photovoltaic power plants – Design guidelines and recommendations focuses on ground installations, not distinguishing whether they are part of a GFV or a UFV.
This standard addresses, in a more objective way, the grounding system of a soil plant, consisting of a grounding mesh that interconnects the support structures of the photovoltaic arrays, as well as other elements that make up the infrastructure of the generating plant.
Aspects related to GFV grounding are addressed by NBR 16690 in item 6.4 – Grounding and Equipotentialization, whose subitems are presented and discussed below. At the end of this article, a review of this item of the standard is proposed, in light of the comments presented here.
The NBR-16690 standard is a young standard, which is still in its 1st edition, and it can be said that a standard can be considered “mature” after at least two revisions.
Item 6.4 begins by listing some justifications for implementing a grounding and equipotentialization system for a GFV:
- functional grounding – necessary for the proper operation of the GVF, whether of one of the poles of the photovoltaic array or of non-energized metal parts, so that the inverter's DSI can detect dc leakage currents;
- grounding as an integral element of the protection system against atmospheric discharges;
- establishment of a common ground reference for all UFV components (equipment and structures), with a view to equipotentializing the GFV.
The term equipotentialization means interconnection, since effective equipotentialization across the entire frequency range of voltages and currents to which an installation of this type is subject is impossible.
Item 6.4.1 – Grounding Electrodes and Conductors
Item 6.4.1 refers to NBR-5410 the aspects related to GFV grounding conductors and electrodes. Interfaces with the grounding and lightning protection system (SPDA) of the consumer installation to which the GFV is interconnected are not mentioned.
It is important to note that a GFV must comply with several requirements established by other standards, in addition to NBR-5410.
The NBR-16254 standard is always applicable, as is the NBR-5419 standard, since photovoltaic arrays are necessarily external installations, exposed to the weather, which may or may not be protected against direct lightning strikes.
If the installation that houses the GFV has a medium or high voltage substation, then standards NBR-14039 and NBR-15751 will also apply.
Item 6.4.2 – Equipotentialization
Item 6.4.2 addresses, in an extremely succinct manner, the equipotentialization criteria, establishing only the minimum sections of copper cables to be used as equipotentialization conductors:
- 6 mm² copper cable for GFV with photovoltaic arrays separate from the installation's lightning protection system (spda); It is
- 16 mm² copper cable for projects in which the photovoltaic arrays are interconnected to the installation's spda, therefore subject to the circulation of portions of lightning currents.
Item 6.4.3 – Protective Conductors
Item 6.4.3 refers to NBR 5410 the aspects relating to protective conductors (PE), establishing an exception for item 6.4.3.4 of NBR 5410, relating to PEN conductors.
The IEC 60364-7-712 and DIN VDE 0100-712 standards allow only two shock protection measures for the dc side in GFV – extra-low voltage SELV or PELV (limited to 60 Vdc and adopted by microinverters) and/or insulation double or reinforced, in which the GFV components have class II insulation (modules, cables, boxes, connectors). It appears, therefore, that the concept of protective conductor for the dc side of a GFV does not apply, since double or reinforced insulation (class II) does not admit the protective conductor.
Item 6.4.4 – Equipotentialization Conductors
Item 6.4.4 returns to the theme of equipotentialization, establishing the need for equipotentialization conductors interconnecting the various components of the GFV, in addition to mentioning parts 2 and 3 of NBR-5419.
Figure 1 reproduces Figure 12 of NBR 16690, which illustrates the concept of grounding and equipotentialization of the conductive parts of a GFV, and which makes it very clear the need for equipotential connection between the exposed conductive parts of the photovoltaic arrays and the application circuit, which can be an inverter, a battery charger or a dc load
This item establishes that the functional equipotentialization of a GFV is carried out by conductors placed close to the dc pair of the photovoltaic array, in order to reduce the area of the loop between conductors and, thus, the overvoltages induced by direct or indirect atmospheric discharges.
It is important here to define what conductive parts are, according to NBR-5456:
- 7.5.64 – conductive part – part capable of conducting current, but not necessarily used to conduct it in normal service;
- 7.5.65 – exposed conductive part – conductive part, normally dead, that can be easily touched and that can be under voltage in the event of a defect or lack.
In a broader context, considering the equipotentialization of metallic masses of the installation as a whole (including the ac and dc networks), as illustrated in Figure 2, it is worth highlighting the requirements established by the two standards:
- NBR-5410 – considering that the ac distribution network is of the TN type, all masses forming part of the low-voltage network must be connected to protective conductors (item 5.1.2.2.3.1), and all circuits must have a conductor protection in its entirety (5.1.2.2.3.6);
- NBR-16690 – item 6.4.4 – establishes the need for equipotentialization conductors interconnecting the various components of the GFV – structures, modules, junction boxes, inverters, etc.
On the AC side of the network, the protection conductors aim to guarantee a low impedance path for phase-to-ground fault currents, ensuring rapid protection action when the circuit is turned off.
It should be noted that on the ac side, equipotentialization and automatic disconnection measures are complementary – if dangerous contact voltages cannot be avoided through equipotentialization (as occurs in a substation), then the automatic disconnection feature must disconnect the circuit responsible for dangerous contact voltage.
On the dc side, the equipotentialization conductors aim to guarantee electrical continuity between the GFV components and the establishment of a common ground reference for the entire generating plant.
This guarantee of continuity is necessary because faults to ground are detected by the insulation supervision device (DSI), normally integrated into the inverter – the objective, in this case, is not protection against shock, but rather supervision of the integrity of the circuit. cc network
The standard does not address the issue of ground GFV, which can be equipotentialized through a grounding grid interconnected to the support structures of the photovoltaic arrays. In this case, the equipotentialization conductors launched together with the dc circuits may not be necessary, considering that the entire grounding system is capable of guaranteeing electrical continuity between the various components of the photovoltaic plant.
For a ground GFV interconnected to a grounding grid, the only risk condition for the occurrence of step and touch voltages is when the installation to which it belongs is connected to a medium-voltage network of a power utility or to a high-voltage substation.
In this case, when a ground fault occurs on the substation's medium or high-voltage bus, the system's contribution will return to the ground supply system, partially if the transmission line has lightning arrester cables. , and in the process of dissipation in the ground of a portion of the fault current through the GFV mesh, potential gradients will emerge in the ground.
These potential gradients must be properly controlled by the grounding system, within the limits of withstandable step and touch voltages.
Items 6.4.5 and 6.4.6 – Functional Grounding
Item 6.4.5 of NBR 16690 informs that the requirements specified in item 6.4.5 of NBR 5410 (functional equipotentialization) do not apply, with item 6.4.6 covering the concept of functional grounding of the GFV, which is the grounding of a of its poles.
This item begins by establishing that when the photovoltaic array has functional grounding, the grounding connection must be made at a single point, preferably inside or close to the inverter or the interface equipment with the application circuit.
If functional grounding of one of the poles of the photovoltaic array is adopted, the inverter must be provided with a device that supervises this grounding and controls its sectioning in the event of a fault to ground.
The item also establishes that “in terms of material and type, insulation, identification, installation and connections, grounding and equipotential conductors must comply with the provisions established in ABNT NBR 5410”, a recommendation that should be relocated to one of the initial sub-items of the item 6.4.
Inverters without galvanic isolation (transformerless) have an insulation supervisory device (DSI) that detects insulation faults. In the case of the 1st contingency, the DSI detects the failure of one of the poles to earth and issues a signal.
In the case of the 2nd contingency, which is the remaining fault for earth on the healthy pole, the DSI commands the inverter input to be turned off (MPPT). This shutdown does not eliminate the condition of double fault from the poles to the generating plant structures, which will persist as long as a maintenance team does not go to the field and arrange for its elimination, with the location of the damaged DC wiring points.
It is worth noting that in the 2nd contingency the equipotentialization system will provide a metallic return path for the fault DC. As there is no current circulation through the ground, there will be no step or touch voltages on exposed conductive parts originating from the dc network.
New structure suggested for NBR-16690 – Item 6.4
It is important that the NBR-16690 review committee decides whether this standard's applicability is restricted to GFV microgeneration/minigeneration, associated with low-voltage or medium-voltage electrical installations in buildings, or whether its applicability should be extended to UFV dedicated, larger and connected to medium and high voltage installations, which have several specific requirements. It is worth noting that both the IEC and the IEEE have developed specific standards for soil UFV.
It is suggested that a new item 6.4.1 addresses Grounding Conductors, referring the definition of these conductors to standards NBR-5410, NBR-5419 and NBR-16254.
If the installation has an interconnection with a medium-voltage substation, the definition of these conductors must also consider the criteria established in standards NBR-14039 and NBR-15751. In this case, a study is necessary to ensure that the installation's grounding mesh is capable of controlling step and touch voltages when earth faults occur on the medium-voltage bus.
A new item 6.4.2 could more fully address aspects relating to the equipotentialization of GFV elements, including a better characterization of aspects relating to their interfaces with the lightning protection system.
Item 5.7 of NBR-16690 addresses the functional grounding of one of the poles of the photovoltaic array, establishing that in this case there must be dc-ac galvanic separation by a transformer (internal or at the inverter output).
In this item 5.7.1 it is established, quite confusingly, that “When the CPU is an inverter with galvanic separation carried out on the alternating current side, the connection between inverter and transformer cannot be grounded if the grounding on the direct current side is required".
The concepts established in item 5.7, in addition to the confusing phrase reproduced above, need to be better coordinated with item 6.4, as both address the issue of functional grounding. A new item 6.4.3 could more fully address aspects related to Functional Grounding, combining the specifications of current items 6.4.5, 6.4.6 and item 5.7.
