Why Grounding and Earthing Are Essential
In solar PV systems, grounding ensures that all exposed conductive parts of electrical equipment are properly connected to the ground, while earthing ensures that any leakage current or fault current is safely dissipated into the earth. The two processes help mitigate the risk of overvoltage, protect personnel from electric shock, and stabilize voltage levels within the system. Moreover, they assist in meeting regulatory standards for system safety and compliance with grid interconnection requirements.
Types of Grounding in Solar Power Systems
- AC Grounding: Involves connecting the inverter’s AC side to the ground to ensure that the system's alternating current components are grounded in the event of a fault.
- DC Grounding: Involves grounding the negative or positive side of the solar array to the ground, depending on the system configuration. In many systems, the negative terminal of the DC circuit is grounded.
- Equipment Grounding: Ensures that all metallic parts such as racking, frames, and enclosures are connected to the ground to prevent electrocution.
Technical Aspects of Grounding and Earthing
1. Grounding the Solar Array
The grounding of a solar array involves connecting the photovoltaic (PV) module frames and racking to the grounding electrode conductor (GEC). In a grid-tied system, this is done to reduce potential differences between the array components and the ground. Grounding of each PV module can be done using grounding lugs or bonding jumpers, which ensure electrical continuity between the metallic components and the earth.
2. Grounding the Inverter
Most modern grid-tied inverters have integrated grounding terminals for easy connection to the system ground. The AC and DC sides of the inverter are grounded separately. It is important to connect the inverter's grounding terminal to the grounding electrode system to prevent potential hazards such as ground faults. The inverter’s internal protection circuits will trip in the event of grounding faults, ensuring system and personnel safety.
3. Grounding Electrode System
A grounding electrode system is crucial for dissipating fault currents into the earth. This system is created by driving grounding electrodes such as ground rods deep into the earth, creating a low-resistance path. The resistance of the earth connection should be less than 25 ohms for effective dissipation. Multiple rods can be used to reduce ground resistance further. Copper-bonded steel rods are the most commonly used grounding electrodes due to their corrosion resistance and durability.
4. Bonding Conductors and Grounding Grid
Bonding conductors connect various metallic components to ensure that they are at the same electrical potential, which prevents dangerous voltage differences between equipment. Bonding connects the array frame, racking system, and inverter casing to the grounding grid or earth electrode. The bonding conductor should be sized appropriately to handle fault currents, and its installation should meet local electrical codes and standards like NEC Article 250.
Earth Fault Detection and Protection
One of the critical functions of grounding and earthing is to enable the detection of ground faults. Ground Fault Protection Devices (GFPDs) are employed to monitor fault currents and automatically disconnect the system in case of fault conditions. GFPDs in inverters typically detect any current imbalances caused by earth faults and trip the system to isolate the fault. This is essential to prevent damage to both the equipment and the grid.
Steps for Grounding and Earthing Installation
- Planning and Site Survey: Perform a site survey to determine the soil resistivity and the most appropriate location for grounding electrodes. High-resistivity soils may require chemical treatment or deeper electrodes to achieve the required resistance levels.
- Installing Ground Rods: Drive ground rods into the earth, ensuring they penetrate a moisture-rich layer for better conductivity. Multiple rods may be spaced and interconnected to form a grounding grid.
- Connecting Bonding Conductors: Use bonding jumpers to interconnect all metallic components of the solar array and inverter system. Ensure that the bonding connections are secure and comply with local grounding standards.
- Connecting to the Grounding Electrode Conductor (GEC): The GEC is the main conductor that links the equipment to the grounding electrode. This connection should be low-resistance, secure, and sized appropriately to handle potential fault currents.
- Testing Ground Resistance: After installation, test the ground resistance using a ground resistance tester to ensure it meets the required values (typically below 25 ohms).
- Earthing Solar Inverter: Connect the inverter’s AC and DC grounding terminals to the common grounding point. The AC side should follow local grid regulations, while the DC side should be in accordance with system configuration.
Regulatory Standards for Grounding and Earthing
Adherence to local and international standards is critical for ensuring safety and compliance. The National Electrical Code (NEC), International Electrotechnical Commission (IEC), and IEEE standards provide guidelines for proper grounding in solar PV installations:
- NEC Article 690: Covers solar photovoltaic systems' grounding and bonding requirements.
- IEC 60364: Provides standards for electrical installations, including earthing and grounding of PV systems.
- IEEE 142: Covers grounding for industrial and commercial power systems.
Conclusion
Grounding and earthing are fundamental aspects of ensuring the safety and reliability of a grid-tied solar power plant. Proper planning, design, and execution of grounding systems reduce the risks associated with electrical faults, protect sensitive equipment, and safeguard personnel from electrical shock. By adhering to best practices and regulatory standards, solar power plant operators can achieve optimal safety and performance in their systems.