WAZIPOINT Engineering Science & Technology: Electrical Substation Equipment: Grounding Network and Its Functions

Tuesday, August 1, 2023

Electrical Substation Equipment: Grounding Network and Its Functions

Ground Grid Substation
Fig-Electrical Grid Substation

Substation equipment grounding is a crucial aspect of electrical power systems to ensure safety and proper functioning. The main purpose of grounding is to provide a low-resistance path for fault currents, stray currents, and lightning surges, allowing them to flow to the ground rather than damaging equipment or posing a safety hazard to people.

Electrical Substation Equipment Grounding Networks Details

What do we need to know to understand the substation grounding properly following the local and international standards? The Basics of Substation Grounding must be known to any person who works in the substation area.

Here are some key points related to substation equipment grounding that will help to know the purpose of substation grounding and procedure:

Safety: Proper grounding helps protect personnel working in and around the substation from electric shocks and ensures the safety of the public.

Fault Current Path: Grounding provides a low-impedance path for fault currents to flow, facilitating the operation of protective devices like circuit breakers, which quickly isolate the faulty section and minimize damage.

Lightning Protection: Substations are vulnerable to lightning strikes. An effective grounding system helps dissipate the lightning surge into the ground, reducing the risk of equipment damage.

Stability of the Power System: A well-designed grounding system can help stabilize the power system by providing a reference point for voltage levels and improving the system's ability to handle fault conditions.

Ground Grid: Substations typically have an extensive ground grid, which is a network of interconnected conductors buried in the ground. This grid enhances the effectiveness of grounding and ensures the proper distribution of fault currents.

Grounding Electrodes: Grounding electrodes, such as rods or plates, are used to establish a connection between the ground grid and the earth.

Grounding of Equipment: All metallic structures and equipment in the substation, such as transformers, circuit breakers, switchgear, and control panels, need to be grounded to maintain equipotential bonding and ensure safety.

Substation Design Standards: Substation equipment grounding practices are governed by various international and national standards, including IEEE (Institute of Electrical and Electronics Engineers) standards, IEC (International Electrotechnical Commission) standards, and local regulations.

Grounding Resistance: The resistance of the grounding system is an important parameter that needs to be within specified limits to ensure adequate fault current dissipation.

Periodic Testing and Maintenance: Regular testing and maintenance of the grounding system are essential to ensure its continued effectiveness.

It is important to note that substation equipment grounding should only be performed by qualified personnel following the applicable standards and regulations to ensure the safety and reliability of the power system.

What are The Major Functions of the Substation Equipment Grounding System?

Need for a Grounding System in the Substation: Substation equipment grounding is an essential aspect of electrical safety and is designed to provide protection to personnel, equipment, and the electrical system itself. The grounding system serves several critical functions in a substation. Here are the main functions of substation equipment grounding:

Personal Safety: 

Substation Equipment Grounding will provide you personal safety. The primary purpose of grounding is to protect personnel working in and around the substation from electric shock. If a fault occurs, the grounding system provides a low-resistance path for the fault current to flow, thus minimizing the risk of electric shock to humans. Substation Equipment Grounding reduces the touch potential and step voltage accident.

Fault Current Path: 

Substation Equipment Grounding will ensure the easy fault current path. In the event of a fault, such as a short circuit, the grounding system provides a path for the fault current to return to the source. This facilitates the operation of protective devices (such as circuit breakers) to quickly interrupt the fault and isolate the faulty section from the rest of the system.

Equipment Protection: 

Substation Equipment Grounding will protect your valuable equipment from surges. Grounding protects substation equipment, such as transformers, circuit breakers, and switchgear, from damage caused by transient overvoltages or fault currents. By providing a low-resistance path, the grounding system helps to limit the voltage rise and dissipate fault currents, preventing equipment damage.

Lightning Protection: 

Substation Equipment Grounding will provide unexpected lightning surges. Substations are vulnerable to lightning strikes due to their elevated structures and extensive network of conductors. Proper grounding helps to divert the lightning's energy safely into the ground, protecting the substation equipment from damage.

Electromagnetic Interference (EMI) Mitigation: 

Substation Equipment Grounding safes other communication equipment from signal distortion. The grounding system helps reduce electromagnetic interference, which can affect communication systems, electronic devices, and sensitive equipment in the vicinity of the substation.

Static Charge Dissipation: 

Substation Equipment Grounding discharges any kind of static charge dissipation. Grounding ensures that static charges do not accumulate on equipment, preventing potential hazards and disruptions.

Stability and System Performance: 

Substation Equipment Grounding ensures the substation stability and system performance. Proper grounding enhances the stability of the power system and improves its overall performance, reducing the risk of voltage fluctuations and system disruptions.

Safety during Maintenance: 

Substation Equipment Grounding provides you safety when you work in a substation. Grounding provides a safe reference point for personnel working on substation equipment, allowing them to safely discharge any stored electrical energy and reduce the risk of accidental shocks.

It is crucial to design, install, and maintain the grounding system in accordance with relevant standards and best practices to ensure its effectiveness in providing these vital functions. The specific grounding design and requirements may vary depending on the substation's size, location, and type of equipment installed.

Which Equipment is Connected to Substation Grounding Network?

Substation grounding networks are designed to provide a low-impedance path to the ground for fault currents and to ensure safety for personnel working in and around the substation. The main equipment connected in a substation grounding network includes:

Substation Grounding Network
Fig- Substation Grounding Network

  1. Grounding Grid: The grounding grid is a network of buried conductors or rods placed in the ground around the substation. It is the primary component of the grounding system and provides a low-resistance path for fault currents to flow into the ground.
  2. Grounding Mat: The grounding mat is an extended metallic plate or a mesh of conductors buried below the substation's switchyard. It provides an equipotential surface and helps to distribute fault currents evenly.
  3. Grounding Electrodes: Grounding electrodes are metal rods or conductors driven into the earth at specific locations to enhance the grounding system's effectiveness. They provide additional paths for fault currents to reach the ground.
  4. Lightning Arresters: Lightning arresters are installed on transmission lines and substation equipment to protect against lightning strikes. They are also connected to the grounding network to divert lightning-induced currents safely into the ground.
  5. Equipment Grounding: All substation equipment, such as transformers, circuit breakers, disconnect switches, and other apparatus, are connected to the grounding network. This ensures that any leakage or fault current in the equipment is safely directed to the ground.
  6. Surge Arresters: Surge arresters protect the equipment from transient overvoltages and are connected between the equipment and the grounding network.
  7. Control Building Grounding: The control building or substation control room is connected to the grounding network to provide a safe path for fault currents and to protect personnel in case of a fault.
  8. Fence Grounding: The substation perimeter fence is connected to the grounding network to prevent the buildup of dangerous voltages on the fence and to provide additional safety for personnel.
  9. Grounding Conductors: Various conductors, such as busbars, cables, and grounding straps, are used to interconnect different grounding elements within the substation.

It is essential to design and maintain the substation grounding network properly to ensure the safety of personnel, protect equipment, and maintain reliable substation operations. The design of the grounding system may vary depending on the specific substation's size, location, and requirements.

Types of Substation Grounding Network Connection 

Substation grounding networks are essential to provide a safe path for fault currents and to protect equipment and personnel from electrical hazards. There are various types of substation grounding network connections, each with its advantages and specific applications. Some of the common types of substation grounding network connections include:

  1. Ungrounded (Isolated) System: In this type of grounding, the entire substation is not connected to the earth's ground. It is rarely used in modern power systems due to safety concerns. Faults in an ungrounded system can lead to high transient overvoltages and pose a risk to personnel and equipment.
  2. Solid Grounding (Direct Grounding): In a solid grounding system, the neutral or the main earthing point of the substation is directly connected to the earth grid. This connection provides a stable reference point for the system voltage and allows fault currents to flow through the grounding network. Solid grounding is commonly used in low-voltage distribution systems.
  3. Resistance Grounding: In resistance grounding, a resistor is placed between the neutral point and the earth grid. The resistor limits the fault current magnitude, which can help in reducing equipment damage during faults. Resistance grounding is used in medium and high-voltage systems to limit ground fault currents and improve system stability.
  4. Reactance Grounding (Inductive Grounding): In reactance grounding, a reactor (inductor) is connected between the neutral point and the earth grid. The reactor limits the fault current but allows higher current flow during system faults compared to resistance grounding. Reactance grounding is used in some medium-voltage systems.
  5. Low-Impedance Grounding (Solidly Grounded with Low Resistance): This type of grounding provides a very low impedance path for fault currents. The goal is to quickly clear faults by allowing high currents to flow, which helps reduce equipment damage and system downtime. Low-impedance grounding is often used in industrial substations and critical facilities.
  6. High-Impedance Grounding: In high-impedance grounding, a grounding resistor with a relatively high value is used. This limits the fault current to a low level, allowing the system to continue operating under single line-to-ground fault conditions. High-impedance grounding is mainly used in systems where continuous operation is critical.
  7. Grounding through Arc Suppression Coil (Petersen Coil): Petersen Coil is a specialized type of grounding connection used in systems with high-ground capacitance, like long overhead transmission lines. The coil creates a compensating reactive current that balances the capacitive ground current, effectively limiting the fault current to low values.

The choice of substation grounding network connection depends on factors such as system voltage level, fault current considerations, equipment protection requirements, and safety considerations. It is essential to select the appropriate grounding system to ensure the reliable and safe operation of the substation and the connected power system.

How to Connect to the Earth in Substation Grounding System?

There are primarily three main methods to connect a substation grounding network to the Earth:

Ground Rods or Earth Rods Grounding System

This is the most common and basic method used for grounding a substation. Ground rods, also known as earth rods, are driven vertically into the ground. The number of ground rods required depends on factors such as soil resistivity and the desired level of grounding. The rods are usually made of copper or galvanized steel, and their length varies based on the depth needed to reach a low-resistance soil layer.

To create a ground grid, multiple ground rods are interconnected with conductors buried underground. The substation equipment, like the transformer's neutral point, is then connected to this ground grid.

Ground Grid or Mat Grounding System

A ground grid, also referred to as a ground mat or mesh, is a network of interconnected conductors buried in the earth around the substation. This method provides a more extensive and distributed grounding system than individual ground rods, reducing the grounding resistance and improving fault current dissipation.

The ground grid is typically made of copper or aluminum strips or cables. It surrounds the substation and is connected to the substation equipment, such as the transformer's neutral point, using grounding conductors.

Grounding Rings or Counterpoise Grounding System

Grounding rings, sometimes called counterpoise conductors, are circular conductors buried in the earth around the substation. They are placed concentrically around critical equipment and are often utilized in high-voltage substations or in substations with sensitive equipment.

The grounding rings are usually made of copper or aluminum and are connected to the equipment grounding system. They help in equalizing the potential around the critical equipment, reducing the risk of voltage gradients, and ensuring stable operation.

It's important to note that the choice of grounding method depends on various factors, including the substation's size, location, soil resistivity, and the level of fault current to be managed. Proper engineering analysis and adherence to safety standards are essential during the design and installation of the grounding system. Additionally, local regulations and codes may also play a role in determining the specific grounding requirements for a substation.

What Should Be the Ground Grid Substation Earth Resistance Value?

The ground grid substation earth resistance value depends on the specific requirements of the electrical system and the standards or regulations applicable in your region. The main purpose of a ground grid is to provide a low-resistance path for fault currents to flow into the ground, ensuring the safety of equipment and personnel and maintaining system stability.

Different standards and industries might have varying recommendations for acceptable ground grid resistance values. In general, lower resistance values are preferred because they offer better grounding and reduce the potential for dangerous voltage buildup.

Here are some common ground grid resistance values suggested by different standards:

IEEE 80-2013 (IEEE Guide for Safety in AC Substation Grounding):

For substations, a typical maximum ground grid resistance value might be 1 ohm or lower.

IEC 60364-4-41 (International Electrotechnical Commission):

In some cases, IEC recommends a ground resistance of 10 ohms or less for electrical installations.

NEC (National Electrical Code - USA):

The NEC doesn't specify a specific resistance value but emphasizes that the grounding system should be of sufficiently low impedance to allow for proper fault current dissipation.

ANSI/IEEE Std 142-1991 (IEEE Green Book - Recommended Practice for Grounding of Industrial and Commercial Power Systems):

This standard suggests a maximum ground resistance of 1 ohm for systems up to 1 kV, and lower values for higher voltage systems.

Keep in mind that some industries or specific installations might require even lower resistance values to meet their safety and operational criteria. Moreover, soil resistivity, site conditions, and other factors will also influence the design of the ground grid and its resistance value.

To determine the appropriate ground grid earth resistance value for your specific case, it is essential to consult with a qualified electrical engineer or grounding specialist who can perform a site-specific analysis considering all relevant factors and standards. They can conduct soil resistivity tests and use various software tools to simulate the grounding system and ensure it meets the necessary requirements.

The Best Practice in Ground Grid Substation Design

Designing a ground grid for a substation is essential for ensuring the safety and reliability of the electrical system. A well-designed ground grid provides effective grounding and fault current dissipation, minimizing the risk of electrical hazards. Below are some best practices in ground grid substation design:

  • Soil Resistivity Survey: Conduct a thorough soil resistivity survey to determine the electrical properties of the soil at the substation site. Soil resistivity is a critical factor that affects the performance of the ground grid. Proper data collection and analysis help optimize the grid design.
  • IEEE and IEC Standards: Follow relevant standards, such as IEEE 80 (Guide for Safety in AC Substation Grounding) and IEC 61936-1 (Power installations exceeding 1 kV AC), which provide guidelines for designing effective ground grids and ensuring personnel safety.
  • Fault Current Analysis: Perform a comprehensive fault current analysis to determine the maximum fault current levels that the ground grid may encounter during faults. This analysis will help establish the required grid dimensions and conductor sizing.
  • Grid Layout and Geometry: Optimize the layout and geometry of the ground grid to achieve low impedance paths for fault current dissipation. Typically, a mesh or radial pattern is used for substation ground grids.
  • Grid Material Selection: Use high-quality, corrosion-resistant materials for ground grid conductors, such as copper or copper-coated steel. The material selection should consider the soil's corrosivity and the grid's expected service life.
  • Connection Techniques: Employ proper connection techniques to join grid conductors, ensuring low resistance and long-term reliability. Exothermic welding or compression connectors are commonly used for these connections.
  • Ground Rod Placement: Install ground rods at suitable locations within the grid to achieve a low-resistance grounding system. The number and spacing of ground rods depend on the soil resistivity and fault current magnitude.
  • Depth of Grid Conductors: Bury the ground grid conductors at a sufficient depth to protect them from physical damage and ensure effective grounding performance.
  • Surge Protection: Consider incorporating surge protection devices into the ground grid design to safeguard the substation equipment from transient voltage surges.
  • Maintenance and Testing: Establish a regular maintenance and testing program for the ground grid to ensure its ongoing effectiveness. Periodic testing should include measurements of ground grid resistance and continuity.
  • Safety Measures: Implement safety measures to protect personnel during maintenance activities near the ground grid. Clear markings and protective barriers should be used to prevent accidental contact.
  • Continual Improvement: Continually assess the performance of the ground grid and identify areas for improvement. Changes in the substation layout or increased fault currents may necessitate modifications to the ground grid design.

Remember that ground grid design is a complex process that requires expertise in electrical engineering and familiarity with local regulations and environmental conditions. It is advisable to involve experienced professionals in the design and implementation of substation ground grids.

Substation-related major equipment is discussed in another episode.

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