Modern renewable generation plants – wind and photovoltaic – are the facilities equipped with the largest grounding systems, with dimensions reaching kilometers.
The two types of generating plants, in addition to their large size, share another common feature: their high exposure to lightning. Regarding the grounding system topology, however, it's worth noting that they present significant differences:
- Wind Farms – linear topology, defined by the layout of medium-voltage lines (usually 34.5kV) that interconnect groundings concentrated at the bases of the wind turbine towers;
- Solar Plants – matrix topology, with a wide network of low-voltage lines (direct and alternating current) and medium-voltage lines (usually 34,5 kV), with grounding distributed throughout the area.
Wind farms are located in a wide variety of environments, from beachfront to inland, on plateaus to hilltops, sometimes at high altitudes. They typically consist of individual wind farms, each with up to 15 wind turbines, forming part of a wind complex of several farms interconnected to a high-voltage collector substation.
There is a wide variety of installation conditions for photovoltaic generation plants, which can be classified as:
- GFV – Photovoltaic Generators: plants within consumer units and which supply them primarily, installed on the ground or on the roofs of buildings (residential, commercial and industrial), including on parking lot roofs;
- UFV – Photovoltaic Plants: dedicated generation plants, which deliver energy to a medium-voltage distribution network (UFV GD – Distributed Generation) or to a high-voltage substation in the case of larger UFVs (UFV GC – Centralized Generation), which can even integrate hybrid generation plants, sharing nearby areas with other generating plants, whether wind farms or hydroelectric plants (in the case of floating UFVs).
UFVs can also be classified according to their installed power:
- distributed microgeneration – GFV with up to 75 kW and connected to building installations (roofs of houses, buildings, industrial warehouses and parking lots), connected to the LV or MV distribution network through the installations of consumer units;
- distributed minigeneration (DG): with up to 2,5 MWp, occupying areas of up to 5 ha and connected to the medium-voltage distribution network and an energy distribution company;
- large-scale plants: usually with installed power above 100 MWp and occupying areas of hundreds of hectares, with a complex internal low-voltage and medium-voltage network and usually connected to the National Interconnected System (SIN) through a high-voltage collector substation (from 138 kV to 500 kV).
For these generating plants, it is important to establish two definitions, which are already included in standard NBR-7117/2020, which are terms used indiscriminately, but which have different interpretations:
- Grounding grid: a set of non-natural grounding electrodes, interconnected and buried in the ground in an area limited by the installation to be served, specifically designed to dissipate electric currents in the ground;
- Grounding system: set of all interconnected grounding electrodes and conductors, buried or not, as well as metal parts that act as the distribution and dissipation function of electric currents in the ground, such as tower feet, foundation reinforcement, metal stakes, etc.
In the case of the grounding grid, although the term mesh is used, this set of conductors does not necessarily need to have the grid topology typical of a substation grounding grid.
It is worth remembering that the grounding grid of a typical substation is on the order of one to two hectares, while the grounding grid of a UFV can reach several square kilometers.
Among the large-scale grounding systems, in addition to the UFV, with a wide grounding grid interconnected to hundreds or thousands of stakes in the structures that support the photovoltaic modules, it is worth highlighting:
- Transmission lines, where the grounding counterweights of the structures are interconnected by lightning rods;
- Wind farms, where the tower grounds are interconnected by lightning rods or by grounding conductors of medium voltage lines.
The performance of a ground can be characterized by its resistance/impedance, and by the potentials at the ground surface produced when a current injected into it is dissipated in the ground.
The gradients of these potentials on the ground surface will give rise to the well-known step and touch voltages, which establish the human safety conditions of the grounding system, when ground faults occur in the installation.
The response of the grounding system to a current injection can be analyzed from two points of view:
- Grounding resistance: for a low frequency event, such as a ground fault on the high-voltage bus of the collector substation, or seen by the tail of an atmospheric discharge, when the dominant frequencies have already fallen in relation to the impulsive wave front;
- Grounding impedance: response of a point in the grounding system to an impulsive event, such as a lightning strike or the actuation of a lightning rod or SPD.
It should be noted that the premise of a single grounding resistance, applicable to medium to small grounding grids (with a diagonal of less than 300 m), does not apply to UFV groundings, which, due to their large dimensions, have a grounding impedance that will vary depending on the current injection point in the grid.
For this reason, two aspects related to the design of grounding systems measuring hundreds of meters or kilometers should be highlighted:
- The need for deep soil models, with a depth compatible with the size of the system to be designed, and soil models up to 2000 m deep may be necessary (in the case of high resistivity soils);
- The need to simulate the grounding system, to calculate its performance, using a program that considers the non-equipotentiality of the grid, which means that it considers the voltage drops that will occur along the grid conductors.
These two aspects are especially emphasized in the IEEE-2778 standard, which establishes the basic design criteria for UFV grounding systems.
In general, IEC and IEEE standards emphasize that the design criteria applicable to substations, which have areas of the order of a few hundred square meters, cannot be applied in the same way to the design of facilities that have areas of hundreds to thousands of square meters.
The COBEI CE 102.01 grounding committee, which develops ABNT standards for the electrical sector, is working on developing a UFV grounding standard, which will reflect the criteria already established by international standardization, as well as the experience acquired here in Brazil with the many projects that have been implemented here.
Text creator
Electrical engineer with a master's degree in Power Systems from PUC-RJ and a PhD in Geosciences from Unicamp. He holds a specialization in grounding from SES (Montreal, Canada). He is a founding partner of Paiol Engenharia and has worked for over 40 years on grounding and lightning protection system projects. He is a member of CE 03:102 – the Study Committee on "Safety in Electrical Grounding of AC Substations."
The opinions and information expressed are the sole responsibility of the author and do not necessarily represent the official position of the author. Canal Solar.
