Ground Fault Protection on Ungrounded and High-Resistance Grounded Systems

Ground fault protection is a crucial aspect of electrical safety, particularly for ungrounded and high-resistance grounded systems. In these systems, a fault to ground can go undetected and result in serious damage to equipment or even electrocution of personnel.

Ground fault protection is designed to detect such faults and quickly de-energize the circuit, preventing any further damage or harm to the electrical system.

In this context, ungrounded systems are those where no intentional connection exists between the system and the ground, whereas high-resistance grounded systems have a connection with high resistance. This article will explore the importance of ground fault protection in ungrounded and high-resistance grounded systems, the methods used for protection, and the different types of protection devices available.

Ungrounded Systems

An ungrounded system is defined as a system of conductors with no intentional connection to ground except through potential indicating and/ or measuring or other very high impedance devices. This type of system is in reality coupled to ground through the distributed capacitance of conductors and transformer or motor phase windings. In the absence of a ground fault the line-to -ground voltage of the three phases will be approximately equal because of the equally distributed capacitance of the system.

Effects of Ground Fault

Theoretically, in a balanced three-phase system, the currents in all three lines are equal and 120° apart (see figure 1). The vector sum of the three capacitive phase currents (IA, IB and IC) is equal to zero at the ground point which also results in the system neutral being held at ground potential by the balanced capacitive voltages to ground (VAG, VBG and VCG). Thus, although an ungrounded system does not have an intentional connection to ground, the system is actually capacitively coupled to ground. The ungrounded system can be regarded as a three-wire system only, thus the following discussion is valid for both wye and delta transformer secondaries.

Figure 1. Ungrounded System Under Normal Condition

If one system conductor, phase C for example, becomes faulted to ground, then phase C and ground are at the same potential, zero volts (see Figure 2). The voltages of the other two phases in the reference to ground are increased to the system phase-to-phase voltage. This represents an increase of 73% over the normal line to ground voltage. Furthermore, the voltages to ground are now only 60° out of phase.

Figure 2. Ungrounded system with fault on Phase C.

If the ground fault is intermittent such as arcing, restriking or vibrating type, then severe overvoltages can occur. Unless the fault disappears as the phase voltage passes through zero, a DC offset voltage will remain on the system capacitance to ground. When the fault reappears the system voltage to ground will equal the sum of the DC offset and the AC component, which will depend on the point of wave at which the fault is reestablished. In this manner, the intermittent fault can cause the system voltage to ground to rise to six or eight times the phase-to-phase voltage leading to a breakdown of insulation on one of the unfaulted phases and the development of a phase-to-ground-to-phase fault. An intermittent type of fault is a very real danger. Therefore, early detection of this condition is of primary importance.

Ground faults in electrical systems can have several harmful effects, including:

Electrical shock - A ground fault can create a potential difference between the electrical system and the ground, which can result in electrical shock if a person comes into contact with both. This can cause severe injury or even death.
Equipment damage - Ground faults can cause significant damage to electrical equipment, including motors, generators, and transformers. This is because the fault current can flow through the equipment and cause thermal or mechanical stress that can lead to failure.
Fire hazard - Ground faults can generate enough heat to start a fire, particularly if they occur in areas with combustible materials or high temperatures.
Production downtime - When a ground fault occurs, the affected circuit must be de-energized and inspected before it can be put back into service. This can result in significant downtime for production processes that rely on electrical power.
Power quality issues - Ground faults can cause power quality issues, such as voltage sags or surges, which can lead to disruptions in electrical equipment and processes.

Overall, ground faults are a serious safety and reliability concern in electrical systems. Implementing effective ground fault protection is critical for preventing harm to personnel and equipment and ensuring uninterrupted operation of electrical systems.

High Resistance Grounding

High resistance grounding can be achieved by the following:

Neutral Grounding Resistors
Artificial Grounding

Overvoltages caused by intermittent fault can be eliminated by grounding the system neutral through an impedance, which is generally a resistance which limits the ground current to a value equal to or greater than the capacitive charging current of the system. This can be achieved on a wye-connected system by a neutral grounding resistor, connected between the wye-point and ground (see Figure 3).

Figure 3. Wye System Grounding.

In Figure 4, a step down transformer may be used for medium voltage systems to allow the use of a low voltage resistor. In a medium voltage system, the voltage level is typically between 2.4 kV and 35 kV. To limit the magnitude of ground fault current in such systems, a resistor is typically placed between the neutral point of the transformer and ground. This creates a high-impedance path to ground, which limits the fault current and reduces the risk of electrical shock or damage to equipment.

Figure 4. Medium Voltage Wye System Grounding

On a delta-connected system, an artificial neutral (see Figure 5) is required since no star point exists. This can be achieved by use of a zig-zag transformer as shown, or alternatively, three single phase transformers can be connected to the system and Ground to provide the ground path, with secondaries terminated by a current limiting resistor (see. Figure 6).

Figure 5. Delta system grounding

In a medium voltage delta grounding system (see Figure 6), the neutral point of the transformer is not grounded, but a grounding resistor is connected between the neutral point and ground. This creates a high-impedance path to ground, which limits the magnitude of the ground fault current and reduces the risk of electrical shock or damage to equipment. One of the advantages of a medium voltage delta grounding system is its ability to provide continuous power supply to the load, even in the event of a single line-to-ground fault. This is because the fault current can be limited, and the healthy phases can continue to supply power to the load.

Figure 6. Medium Voltage Delta grounding system

Maximum Let- Through Current Value

The maximum let-through current value refers to the highest current level that a protective device, such as a circuit breaker or fuse, will allow to pass through it without tripping or opening. Let-through current is the amount of current that passes through the protective device during the time it takes to trip or open and is typically measured in amperes or kiloamperes.

The let-through current is the maximum controlled current which may flow in a neutral grounding Resistor during line-to-ground fault, for wye or delta systems, and its value can be calculated as follows:

Figure 7. Formula of let-through current value

The maximum let-through current value is an important consideration in selecting and sizing protective devices for electrical systems. It is important to choose devices that are capable of withstanding the expected level of current and that will not allow excessive current to pass through in the event of a fault. Devices that have a low maximum let-through current value are typically more sensitive and will trip or open more quickly, providing better protection for the equipment and personnel in the system.

The maximum let-through current value may also be referred to as the let-through energy, which takes into account both the magnitude and duration of the current passing through the device. Let-through energy is a measure of the thermal stress that the protective device and the connected equipment will be subjected to in the event of a fault. Protective devices with a low let-through energy rating are generally considered to provide better protection for equipment and reduce the risk of damage or failure due to thermal stress.

Second Ground Fault Protection

As continuity of service is a major advantage of the high resistance grounded systems, they are often operated for a long periods of time with a single fault. Even though overvoltages are controlled with properly sized grounding equipment, the possibility of a second fault always exists. If the second fault occurs before the first one is cleared, the ground current is no longer controlled by the grounding resistor, but it will be limited by the supply impedance and the ground impedance between the two faults. If the second fault occurs on the same feeder as the first, the phase-to-ground fault changes to phase-to-phase, which cannot be detected by the ZSCT (it is effectively a load current).

Figure 8. Double Fault Protection

Thus, high current will flow and must be cleared by the overcurrent protection. On the other hand, if the second fault occurs on a different feeder some distance away, or the fault develops into an arcing fault, the ground impedance between the two faults will limit the fault, which cannot be cleared quickly by the overcurrent devices and it will cause severe damage. Protection against second ground faults can be provided when each feeder is equipped with a zero-sequence current sensor. Utilizing the sensor outputs through current relays, the protecting feeder breakers can be tripped when the fault current exceeds a predetermined level of say, 10 to 20 times the system charging current level.

The Design and Implementation Process

The ground fault protection system is an important step in the protection design, and it should be fully incorporated to form the total protection scheme. Therefore, it is required that all the necessary information be available before the design.

A complete single line diagram, containing the transformer data, type and size of the interrupters, the type and current rating of the overcurrent devices, the cable size, type and length of all feeders, load types and sizes, etc., is required for the ground fault protection system design. Additional information, such as operating modes and interlocking systems, special switching arrangement, etc., may influence the design if it is known. The level of supervision can also be a major factor: unattended systems may require fully automatic protection schemes, while selective indication may be sufficient for attended ones, where preventative and corrective maintenance can be scheduled.

Implementing ground fault protection on ungrounded and high-resistance grounded systems typically involves the following steps:

Conduct a system analysis - The first step in implementing ground fault protection is to conduct a detailed analysis of the electrical system to determine the appropriate protection methods and devices to use. This analysis should take into account the system configuration, operating conditions, and other factors that may impact the effectiveness of the protection.
Choose the appropriate protection devices - Based on the system analysis, choose the appropriate ground fault protection devices, such as ground fault relays, zero-sequence current transformers, or ground fault sensors. The selection of devices will depend on the specific system requirements and the level of protection needed.
Install the protection devices - Install the ground fault protection devices at appropriate locations in the electrical system. This may include the transformer neutral point, distribution panels, or other critical points in the system.
Test the protection system - Once the protection devices are installed, it is important to test the system to ensure that it is functioning properly. This may involve conducting fault simulations, testing the sensitivity of the protection devices, and verifying that the devices are tripping or opening as intended.
Maintain the protection system - Ground fault protection systems require regular maintenance to ensure that they are functioning properly. This may include testing the devices, replacing worn or damaged components, and updating the system as needed to reflect changes in the electrical system.

Implementing ground fault protection on ungrounded and high-resistance grounded systems can be complex and requires specialized knowledge and expertise. It is recommended that a qualified electrical engineer or other trained professional be involved in the design, installation, and maintenance of the protection system to ensure its effectiveness and reliability.

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Title: Ground Fault Protection on Ungrounded and High-Resistance Grounded Systems
Source: Post Glover

Electrical Axis