What is resistance grounding?
Resistance grounding is a type of neutral grounding in which the neutral point of an electrical power system is connected to ground through one or more resistors. The purpose of resistance grounding is to limit fault currents and protect the system from transient overvoltages, as well as to reduce the risk of arcing grounds and enable ground-fault protection.
The value of the resistance used in a resistance grounding system should be carefully chosen to avoid both under- and over-grounding the system. If the resistance is too low, the system will be solidly grounded, which can result in high fault currents and potentially damaging transients. On the other hand, if the resistance is too high, the system will be ungrounded, which can impair the operation of ground-fault protection and create safety hazards.
In general, the value of the resistance is chosen such that the ground-fault current is limited to a level that is sufficient to permit the operation of ground-fault protection, but not so high as to create other problems. For example, the ground-fault current may be limited to 5% to 20% of the three-phase fault current. This helps to protect the system components and reduce the stress on the voltage during a fault condition.
The advantages of resistance grounding include:
- Minimization of arcing grounds: By adjusting the connected resistance to a suitable value, resistance grounding can help to minimize arcing grounds, which can reduce the risk of equipment damage and improve power quality.
- Improved stability: Resistance grounding can improve the stability of the system by reducing the power dissipation in the grounding resistance, which can help to reduce the accelerating power and improve the overall performance of the system.
- Reduced ground-fault current: The ground-fault current in a resistance-grounded system is generally smaller compared to a solidly grounded system, which can help to reduce the inductive interference with neighboring circuits and protect system components.
- Control of transient ground faults: In a resistance-grounded system, transient ground faults can be converted into controlled current faults, which can help to improve the overall stability and reliability of the system.
- Use of discriminative protective gear: A low value of resistance in resistance grounding can permit the use of discriminative types of protective gear, which can help to improve the accuracy and effectiveness of the protective system.
The disadvantages of resistance grounding include:
- Increased cost: Resistance grounded systems are generally more expensive than solidly grounded systems due to the need for lightning arrestors and higher-voltage insulation on equipment.
- Energy loss: In a resistance grounded system, there is a significant amount of energy loss due to the dissipation of fault energy, which can decrease the overall efficiency of the system.
What is reactance grounding?
Reactance grounding is a method of grounding electrical systems that involves the use of an inductor (also called a reactor) connected between the neutral point of the system and ground. The purpose of this arrangement is to limit the fault current flowing through the system in the event of a ground fault.
The reactance of an inductor is a measure of its opposition to the flow of electric current. The higher the reactance of an inductor, the more it will oppose the flow of current. In a reactance grounding system, the inductor is chosen to have a specific reactance value that will limit the fault current to a safe level.
The selection of the reactance value for the inductor is important because it determines the amount of fault current that will flow through the system. If the reactance is too small, the fault current will be too high and could cause damage to the system. On the other hand, if the reactance is too large, the fault current will be too low, and the system may not be adequately protected.
Reactance grounding is most effective for long transmission lines and cable systems because these systems are more prone to high levels of fault current. In order to minimize transient over-voltages, the current through the reactance during a fault should be within 25% of the three-phase fault current.
Necessity of Reactance Grounding:
Reactance grounding is a method used in electrical systems to limit fault currents and provide phase opposition between capacitive ground currents and fault currents. This is achieved by adding additional reactance (impedance) to the system, which increases the lagging current and neutralizes capacitive currents. Reactance grounding is used in transmission lines with operating voltages between 3.3 kV and 33 kV, and is particularly useful in circuits with high charging currents, such as underground cables, synchronous condensers, and lightly loaded transmission lines.
The effectiveness of reactance grounding is determined by the ratio of zero sequence reactance (X0) to positive sequence reactance (X1). If this ratio exceeds three, the system is considered a reactance grounded system. If the ratio is less than three, the system is considered a solidly grounded system. If the neutral of the system is effectively grounded and the ratio exceeds three, the system is still considered a reactance grounded system.
Reactance grounding is preferred over resistance grounding in some cases because it provides both fault current limiting and phase opposition, while resistance grounding only limits fault current. Reactance grounding also has the added benefit of ensuring satisfactory relay operation, partially grading equipment insulation, and reducing interference with communication circuits compared to solid grounding.
Advantages of reactance grounding:
- Satisfactory relay operation: Reactance grounding can provide reliable operation of protective relays, which are used to detect and respond to fault conditions in the system.
- Avoidance of arcing grounds: Reactance grounding can help prevent the formation of arcing grounds, which are dangerous electrical discharges that can occur when a fault occurs in the system.
- Conversion of transient ground faults to controlled current faults: In reactance grounding, transient ground faults (which are brief, high-current faults that can cause damage to the system) can be converted into controlled current faults, which are less damaging and easier to manage.
- Partial grading of apparatus insulation: In a reactance-grounded system, it is only necessary to partially grade the insulation of electrical apparatus (such as transformers and motors), rather than fully grading it as in a solidly grounded system. This can be more cost-effective.
- Reduced interference with communication circuits: In a reactance-grounded system, there is less interference with communication circuits (such as those used for telephone and data transmission) compared to a solidly grounded system.
Disadvantages of reactance grounding:
- Higher fault current required for relaying device operation: In a reactance-grounded system, the relaying device (such as a protective relay) may require a higher level of fault current to operate compared to a resistance-grounded system.
- High transient voltages under fault conditions: During fault conditions in a reactance-grounded system, very high transient voltages may appear, which could potentially cause damage to the system.