Within power systems there are a majority of system faults that include the ground. In order to effectively and efficiently protect and manage a power system it is inherently necessary to understand and know the grounding methods and their importance to the power systems. First the basics of grounding are needed to understand on a larger and more complicated environment.
Objects that have an excessive charge, whether positive or negative, can have their excess charge removed through the process of grounding. Grounding, in its most basic sense, is removing this excess charge by transitioning the electrons between it and another object. The charge is then balanced between the previously overcharged object and the ground. The ground itself serves as the conduit for electrons to flow to and from to negate the overly charged object and to balance out the charge between the objects. Grounding is critical to electrical systems for a few reasons. The first is the increased safety of the electrical system. By connecting the electrical system to a ground the risk of shock and injury from leaking current or faulty insulating properties are mitigated to an acceptable level that does not present a hazardous environment. If the system is adequately grounded the fault current or leaking current is sent to the ground and neutralized through the electron distribution.
Within ground of the power systems there are three primary methods of grounding including the solidly-grounded system, ungrounded system and the impedance-grounded system. All three have their advantages and disadvantages and each will be ventured into further with a focus on solidly-grounded and impedance-grounded systems. The reason for this is that the ungrounded system has not intentional connection to a ground and does not establish a stable line-to-ground structure (Bridger, 23). This is basically establishing a scenario in which power systems can experience a system fault. The solidly-grounded system has an intentional connection to the ground and incorporates a stable line-to-ground frame of reference. The impedance-grounded system is based upon an arrangement for a ground through a high resistance element that ranges from one to ten amperes. This type of impedance-grounded system is referred to as a High-Resistance Grounded system. The method of grounding will vary according to the usage and types of implementations that will be present based upon the desired end state use of the system. Each grounding method could limit the ground fault completely or progressively mitigate the risk exposure based on the need.
In order to alleviate the problems associated with faulty grounding methods or misappropriated resources on the wrong configuration of grounding solutions it is imperative to understand the types of grounding solutions and their primary objectives. The grounding systems have an overall same primary function but how they achieve that purpose balances many factors such as expense, continuity, scalability and protection.
Grounding System Foundations
Overall the system grounding subject is vital to the performance and operation of power systems. The susceptibility of the system to the voltage variations, load types the system can handle and the ability for the system to perform are all based on the type of grounding system implemented with the power system. The grounding of the power source is based upon the type of power source which falls into basically four broad categories. These include utility services, generators, transformers and static power converters. The four categories are defined by the NEC as “separately-derived systems”. This is important to the application of the NEC requirements when determining the system grounding (NEC, 90.1A). In order to connect the relationship between system voltage and system grounding it must be understood that there are two common ways of connecting windings of the system. These include wye and delta. The wye arrangement includes four terminals and has a phase-to-neutral voltage established by the winding voltage and the end state of phase-to-phase voltage set by the vector. The delta configuration is differs from that of the wye in part due to the number of terminals which is three. The phase-to-phase voltage set by the winding voltage and the third terminal, neutral, is not defined. While neither of these has a formally established grounding system associated with it there are some commonly used grounding systems that can be utilized to best serve each type of system.
As mentioned previously, the solidly grounded system is by far the most practical and common grounding system. This is due in part to the versatility and flexibility of the uses for the system. The solidly grounded system is used in many commercial and industrial power systems. This is based on the NEC’s mandate for many applications but even outside of those mandates the solidly grounded system is utilized based upon the economics and balance between mitigation of failures and cost to implement. For the line-to-neutral loads, the neutral point of the wye-connection is connected to the body of the system. If the wye-connection is not properly affixed to the system the load would provide an environment that would produce unbalances between the ground and the connections as well as instability in the system. This would defeat the purpose of the ground and reduce the overall functionality of the system. A typical example of when a solidly grounded system is utilized includes a power system that uses 120 V loads. The diagram below denotes a 120/208 V Wye system. This is a solidly grounded depiction with line-to-ground configuration.
The ungrounded circuit is primarily used when service continuity is paramount and a vital necessity to the system functionality. The major purpose of grounded systems is to provide enhanced functionality as well as safety to the users of the power systems. With ungrounded systems there is also a level of safety that is involved in the system which is utilized based upon how the ungrounded system is serviced and implemented. When a system is receiving maintenance or service the equipment of the ungrounded system are electrically isolated between the main power supply and the user’s equipment. The operator or user may come in contact with the poles of the power system and does not conduct a path for the current to flow which does not harm the user. If the circuit was connected between a ground and the pole the user coming in contact would create a circuit and receive the full amount of voltage through his or her body. This would potentially provide a fatal implication. This is a prime example of how the setup of the power system determines the type of grounding system that is utilized to provide not only the most efficient use of the power system but also a safe environment for the users of the system.
Grounding systems also include a low resistance grounding system. Basically a low resistance grounding system is utilized when protection of a high valued asset is essential and unexpected or prolonged downtime of the system would cause significant financial burden. If this is the case for the high value asset the low resistance grounding system would be utilized. This system employs a resistor that is connected at the system’s neutral point to the ground which allows only a slight amount of ground fault. This amount typically falls within the range of 200A to 1200A (Barrios, Skibinski, Wood, and Nichols, 134). The reason for this is the fact that there must be enough current to flow through the system to detect if the system needs to trip the system off-line but limit the amount of current so that it does not damage the system in any way. High resistance grounding is similar to low resistance grounding but there is a difference in the level of resistance employed. High resistance grounding is normally utilized in systems that include circuits of 480V with upper thresholds of 2400V and 4160V. The ground fault current that a high resistance grounding system can manage limits the damage to the power system but the system does not trip the system off but mitigates the level of current through resistance (Beltz, Peacock, and Vilcheck, 56). The downside to the system is the fact that the ground fault’s locations are not normally known and the system acts in that manner just as the ungrounded system. The significant difference in this type of grounding system is that the charge captured by the resistor is dissipated as heat. The critical component of this system is the type and size of the resistor designed for the application. The benefits of the high resistance system include the maximum continuity similar to the ungrounded system. The setup and usage is relatively inexpensive in comparison to other methods. This method also limits the damage to the electric motors and mitigates extraneous costs associated with replacement of equipment. The disadvantages include the inability to establish and remedy the ground faults unless there is a pre-existing schema for ground fault detection incorporated into the grounding system. Otherwise this type of grounding system is flexible, reliable, efficient and effective in protecting equipment and providing longevity to the power systems.
Grounding System Importance
Grounding systems have multiple requirements for their use and implementation. Understanding why a power system needs a grounding system may at first seem confusing because of the diverse requirements and sometimes seemingly conflicting requirement of their needs. These considerations fall into multiple categories. The first is system performance which grounding provides the ability for the power system to provide reliable, sustainable and predictable outputs. The next area is the safety consideration for the personnel utilizing the power system. Normally safety is limited to the personnel involved in using the system but grounding also provides protection to the equipment being grounded as well as the facilities in which the power system is incorporated into. Grounding minimizes and mitigates the potential for hazardous shock. Other considerations for ground range from AF noise and RF noise emissions and susceptibility as well as the immunity to ESD and its impact on the power systems.
The comparison provided by MATLAB is a comparison between the options of providing a ground directly to the earth, its composition and its diameter. This provides a basis for determining the type of ground system the engineers should employ when developing the appropriate level of continuity, performance and protection. This information provides the facts that the choice of selection is based upon the resistance and resistivity of the ground as well as the diameter and length. Simulating the variables on length and diameter provides key data points for ground system solutions. The simulation also takes into account the soil that is utilized from the grounding. The soil composition based upon how deep and the density would also impact the grounding necessary to ensure safety, reliability and performance of the grounding system. The conventional method established by IEEE Std. 80-2000 is the baseline for the simulation methods. The output from the simulations is based completely on the balance between balancing safety with performance. This balance occurs when a perfect harmony between safety and performance is met. The optimization is calculated between the shock factor and the performance factor. The two scenarios are based upon providing the safety as a paramount objective and the other is proving that the performance is optimized. The clarity that became apparent while running the simulations occurred when the determination was found regarding the safety optimization was achieved. The level of safety determined the level of performance optimization. This was determined by evaluating a system that had reached its safety threshold. Once a system was deemed safe the simulation could then further optimize the performance and eventually optimized to reduce cost based upon the variables introduced to the simulation. The best scenario established by limiting the cost of the system, maximizing performance while meeting the threshold for safety.
For grounding protection, there are multiple scenarios in which protection is required including protection from the electricity generated by the power system, electricity from other power systems in the same vicinity and potential acts in nature such as storms or other occurrences. Overall the NEC defines the minimal viable requirements for a system to be grounded. While these requirements have progressed and changed over the years, the basic requirements have remained stable and consistent. When a single building needs to be grounded there are specific requirements for that single building. In a single building with a power system there are three components. There is the transformer as the power service, the main panel for circuit breakers and a sub-panel. The dangers to both personnel and equipment come from the short circuit through the flow back to the transformer. If the resistance is high it may reduce the ability for the circuit breakers to work properly. The energy absorbed by the resistance will either limit the circuit breaker from working at all or would take an extended amount of time to trip the breakers. This would result in the energy flowing through the resistor to become heat and potentially cause damage to the transformer or cause a fire due to the excessive transfer of heat energy. This is why proper establishment of the grounding system is imperative and the proper resistance is correlated to the system needs. This is reflected in the drawing from the NEC.
The other implications with building scenarios include the setup of multiple buildings or structures utilizing the power system. The setup of multiple buildings relies on grounding to properly function but offers a significantly greater variability in performance and reliability due to the inherent nature of additional variables such as the buildings themselves. The grounding system must still find a way to ensure the path returning to the transformer in order to trip the breaker system in case of system overload. The entire system must be tied off between each of the buildings to ensure the entire system is congruent and integrated. Each building must also incorporate a ground conductor between each of the buildings. The following figure from the NEC illustrates the multiple building scenarios.
The single and multiple building scenarios are important because not only does it keep the people safe as they work on, in and around the power systems but it also provides the ability to protect the equipment from unnecessary damage and promotes the longevity of the power system. Grounding is also important due to its protective properties against acts of nature such as lightning strikes. Lightning strikes are unpredictable and not significantly understood regarding the exact nature, causes, impacts and mitigations for the strikes. The main focus of grounding and lightning strikes revolves around the tremendous amount of energy the lightning contains and how to limit the impact of the energy from the strike impacting the power system and other equipment that is grounded. Lightning strikes cause a tremendous spike in current which overloads the system. The formation of lightning is based upon the electrical charge that is gained by storm clouds moving across the surface of the earth. As the moisture of the moves the electricity builds up and ultimately needs to be dissipated. The bottom of the formation is negatively charged and the surface of the earth is then positively charged. The distance between the clouds and the earth acts as a conductor of sorts and when the voltage exceeds the capacity of the resistance of the air the lightning strike happens. This is highly unpredictable and has a high level of uncertainty on what would be hit by the lightning strike and when it would occur. To mitigate the risk the philosophy of avoidance could not be implemented (Thomas, 202). The grounding system that is built for lightning protection is acting as a lightning attractor. This allows for the variability of when and where the lightning will strike to be reduced and directed to a location that would not harm the power system, people or other equipment protected by the grounding system. The charges associated with the lightning strike also impact the multi-building scenario. The buildings could potentially have multiple charges sent through them during a strike but the grounding system, acting as an attractor, short circuits the multiple charges in a manner that controls the overall charge pulsed into the system. The energy is dissipated before it can cause damage to the system or the people within the facilities. When a grounding system is implemented to limit lightning strike damage there is a high level of confidence needed in properly establishing the system. If the grounding system is improperly established the lightning rod will act only as an attractor and result in a magnet for lightning strikes without the ability to prevent damage. This type of error would result in an exponential growth in risk to the system based on the inability for the system to work properly. A properly established grounding system to prevent lightning strike damage would attract and dissipate the charge build up and ultimately control the voltage of a potential lightning strike by pushing the voltage away from key components and eliminating the charge through safe methods.
Lightning strikes involve a high level of voltage that would cause a significant level of damage in an expeditious manner. There is also the potential hazard to sensitive equipment due to static electricity buildup. Grounding is required for this type of risk due to the nature of utilizing high value assets that are sensitive to static electricity buildup and discharge. The ability to eliminate static electricity is required in the construction of buildings and was only found to be an issue when specific cost saving techniques was used when building structures. These structures were built without the inherent metal infrastructure, which maintained the same level of structural integrity but did not provide the same level of grounding protection against static electricity buildup. Static electricity is harmful to computing systems, phones, and other information technology infrastructures.
The grounding solutions work through removing the electrons from the over charged area and ensuring the balance of charge between all areas of the circuit. Grounding systems can eliminate the potential hazards of voltage spikes, lightning strikes and other electrical implications of running a power system. The overall objective of choosing the appropriate grounding system is based on balancing the advantages and disadvantages of each of the grounding methods and choosing the appropriate level of balance between protection, continuity, efficiency and effectiveness of the systems. The protective nature of the grounding system provides a vital an imperative purpose in power systems. The protection afforded by grounding allows for the mitigation of threat to the personnel working with the power system as well as the protection of the assets within the power system. The grounding system provides a reliable and predictable environment for the power system to operate. This increase reliability and predictability could not be achieved without the implementation of the appropriate grounding system based upon the requirements of the power system. The grounding system also eliminates variables that impact the system such as single and multi-building setups and mitigates natural occurrences that could damage the power system.
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