Electrical systems, such as those which power industrial equipment and plants and those used in power distribution, often are grounded to prevent damage to the system components during ground faults due to overvoltages or phase-to-phase faults. Outages caused by ground faults in ungrounded systems are particularly damaging and costly in continuous manufacturing processes, where an uncontrolled shutdown of the system can be particularly problematic. To prevent such damage and problems, such systems have been grounded, such as by providing a solid connection from the system neutral to ground. Relays can then be utilized to isolate the defective part of the system during ground fault occurrences.
However, even with solid grounding of a system, damage can still occur during ground fault conditions, and the damage at the point of the fault can still be excessive. Accordingly, systems have been developed which place a low resistive impedance (e.g., a resistor of low resistance) between the neutral and ground. This practice reduced fault damage to acceptable levels by lowering the ground fault current passed during a ground fault condition.
However, the ground fault current in such low resistance grounded systems could still remain high enough to effectively shut off the defective portion of the system via relays. Yet some users still prefer to maintain electrical service if possible even with a ground fault present on the system. If a shutdown of the system or portions of the system is necessary, it can then be conducted in a controlled fashion, rather than in an abrupt haulting of the process or the equipment. Moreover, low voltage solidly grounded systems can present flash hazards to those who work on the systems and solidly grounded systems can also pose the risk of sustained destructive arcs without initiating an automatic trip of the protection relays.
To overcome these problems, high resistance grounding of electrical systems was developed in which the system neutral was grounded through a high resistance resistor to limit the ground-fault current flow to a value equal to or slightly greater than the capacitive charging current of the system. This value of the resistance is chosen because it is the lowest level of ground-fault current flow at which system overvoltages can be effectively limited, thereby providing overvoltage protection. (Typically, a system is considered to be a high resistance grounded system if the initial current is limited by the resistance to about 25 amps or less, and often the resistance is chosen to limit the current to no more than about 10 amps.) Thus, such high resistance grounded systems can allow for continuous operation or controlled shutdown of the process equipment during a ground fault, while also providing overvoltage protection and point-of-fault damage protection.
Control and detection devices have also been developed for use with resistance grounded systems, such as the high resistance grounded systems described above. These devices have provided fault detection warnings, such that the ground fault cause and location can be investigated and corrected, potentially without shutting down the equipment. In particular, the current through or voltage across the grounding resistor can be monitored. When the normal current or voltage is detected, normal operation is indicated such as by using a green light, but when a non-normal current or voltage is detected for a predetermined amount of time, an alarm signal is activated.
Moreover, such devices have also provided fault location tracking through the use of a switch which provides current pulses into the system. To locate the ground fault, a pulsing circuit has been utilized which shorts out a portion of the grounding resistor and provides current pulses into the electrical system. A portable ammeter could then be used to test various nodes in the electrical system and when the pulses are not detected by the ammeter, the location of the ground fault has been located.
While such control and detection devices have been advantageous for use with resistance grounded systems, some disadvantages remain. For instance, such devices are generally based upon analog circuitry and thus can require time and expense in wiring several discrete components. Moreover, such devices can take up significant space, can suffer from accuracy problems, and/or can be subject to reverse engineering. Data logging and connection capability have also not typically been provided in such devices, and upgrades to the system require the time and expense of upgrading system components. Moreover, such devices can be subject to false ground fault alarms due to high frequency distortion (i.e., harmonics) in the electrical system monitored. Finally, typical ground fault control devices do not detect or provide an indication of high harmonic levels. Accordingly, it is desirable to provide improved ground fault control devices for resistance grounded systems which overcome one or more of these drawbacks.
USPTO Backgrounds
USPTO Backgrounds