There are now 371MW of solar farms now connected to the grid with 2,000 acres are covered in panels that are capturing solar renewable energy and exporting it to the grid.
According to Irish Solar Energy Association’s ‘Scale of Solar Report, June 2023’, 599,131MWh of energy is now being generated by solar energy in Ireland. This is contributing to a saving of 202,127 tonnes of carbon annually. The 599,131MWh of electricity generation is the equivalent of generating enough electricity for 144,650 homes in Ireland.
With the large number of solar projects now in Ireland and an ambitious target of 8GW of solar capacity by 20230, solar farms and other solar devices must be protected against lightning strikes.
In his latest technical blog, Mr Hadi Beik Daraei, Engineering Manager, outlines the earthing requirements for solar PV farms which can protect against critical outages and help prevent fatal electrical shock for maintenance operatives.
This article will outline:
- The PV Farm Layout
- PV Circuit Component
- The role of an earthing and bonding system
- Functional Earthing
1. PV Farm Layout
In a Grid-connected PV solar farm, different levels of voltage exist including Direct Current (DC), Low voltage Alternating Current (AC), medium voltage AC, and high voltage AC. For distributing different levels of voltages there is a need to establish switchgear and substations in certain locations on-site.
The following figure 1 shows a typical Grid connection on a Solar PV farm. Substation locations are marked on the layout. These types of plants normally produce several Megawatts (MW) of electric power.
The interface between Electrode systems with different requirements is a challenging issue which must be investigated. Since a PV solar farm occupies a large area, employing a specialised design team is crucial in offering an optimal cost-effective design.
2. PV Circuit Components
A typical PV Solar circuit is composed of DC and AC circuits as shown in figures 2 and 3. These figures represent the DC and low-voltage AC circuits.
3. The Role of the Earthing and Bonding System
Earthing and /or bonding in a PV Solar Farm is addressed in the below four subjects:
1. Providing Safety to Some Extent
It is important to provide safety to some extent while a possible exceeded touch (contact) voltage could cause electrical shock.
Important Note 1.1
Earthing and bonding systems are not categorised as protection measures but are supplemental practices to provide safety when the main protection measures are functioning.
For example, if the protection measures are performed by automatic operation of the circuit breaker and one fault has happened, the earthing and bonding system should provide safety until the circuit breaker functions and clears the fault within the clearance time.
Important Note 1.2
The earthing system and its pertinent components do not have any role to provide protection against direct contact. As per IEC 60364-4-41, in terms of voltages, protection against direct contact shall be provided for the power circuit greater than 25v AC or 60v DC. This protection is performed by using insulation.
2. Making an effective path to discharge the primary lightning current
For this purpose, based on the type of external lightning protection system, an appropriate earthing system must be implemented.
Important Note 2.1
In most cases, the Keraunic value (number of thunderstorm days per year for a given installation location in Ireland) does not reach a level that causes a high level of system failure rate meaning that external protection against lightning is required.
Also, in most scenarios, an economic risk assessment shows that the installation of external protection would not make sense.
3. Making an effective path to discharge the Secondary Lightning Current (Surge Current)
This is where we have a Surge Protective Device (SPD) installed. As shown in Figure 4, the SPD could be installed at different points within the power circuit, therefore an accessible earth connection needs to be considered to ensure the appropriate functioning of the SPDs.
Important Note 3.1
The requirements for adequate earthing for discharging the lightning current have been elaborated in IECTR 63227, IEC 62305, and our previous technical blog about lightning protection for your solar panel system.
4. Functional Earthing
For functional reasons, certain photovoltaic module technologies require a polarity to be connected to the ground either directly or via a resistor.
The grounding of the polarity is carried out at a single point of the DC part of a PV generator, close to the DC input of the inverter or in the inverter itself if the latter allows it.
The earthing is preferably located immediately upstream of the cut-off and sectioning device of the DC input of the inverter (Figure 5). This is done to maintain the connection to the earth of the field even during the maintenance phases of the inverter.
4. Earthing and Bonding Requirements Attributed to Personal Safety
The below table represents the possible information and scenarios that are required to be known and investigated prior to commencing the earthing design for a PV solar farm.
Insulation failure happens in the power circuit between Array and LV Switchgear (DC and Low Voltage)
Please refer to the following Table 1 and Figure 6.
Insulation Breakdown Inside the Transformer (MV System Fault)
Please refer to Table 2 and Figure 7.
Fault Inside HV Substation
Please refer to table 3 and figure 8
To provide an adequate and optimum cost-effective design for an earthing system of a PV Solar Farm, the following subjects need to be thoroughly considered.
- Reviewing the Single Line Diagram of the power distribution to find out the level of the distributed voltages and the way of earth connection of the different power circuits. This will help to discover all possible paths of short circuit currents and the points at which the faults could initiate.
- Review the layout plan to find out the location of the Inverters, and MV and/ or HV substations. This is crucial to find the possible zone of influence areas and discover the interface between different types of electrode systems.
- Finding out about the possibility of the fence energisation points and considering an appropriate cost-effective Earthing Electrode System for fence earthing.
- Conduct an accurate investigation to find out whether an external lightning protection system is needed or not. For this purpose, a detailed economic risk assessment along with considering the accepted failure rate of the different parts of PV Arrays needs to be conducted. It is important to carry this out as installing any type of external lightning protection system will increase the cost of the Earthing Electrode System dramatically, as well as the project budget. There could be a possibility that an Earthing Electrode System may not be necessary from a technical point of view.