Safe and reliable power system network can be achieved through steady state and transient analysis. Healthiness of network during operational condition, contingency condition and fault condition can be visualized through unbalanced load flow study, short circuit study, protection coordination study, harmonics study, cable derating study, motor starting study, arc flash study, study of sub synchronous resonance, short circuit ratio study, etc. Whereas response of network to small disturbance and sustained disturbance during switching, lightning, power system fault is verified through transient analysis involving insulation coordination study, study of very fast operating transient, designing sheath voltage limiters, designing filters, ferro resonance study, transient grid potential rise study, etc.
Electrical protection is designed to achieve the best compromise between equipment damage and service continuity. One of the prime objectives of electrical system protection is to obtain selectivity to minimize the extent of equipment shutdown in case of a fault. Therefore, many protection engineers would prefer that faulted equipment be de-energized as soon as the fault is detected.
The circumstances causing system malfunction are usually unpredictable. However, sound design, system healthiness and preventive maintenance can reduce the likelihood of such occurrences. The objectives of electrical system protection and coordination are:
- Limit the extent and duration of service interruption whenever equipment failure, human error, or adverse natural events occur on any portion of the system
- Minimize damage to the system components involved in the failure
The principle of electrical protection system coordination is to guard against short circuits and overloads. Short circuits may be caused in many ways, including failure of insulation due to excessive heat or moisture, mechanical damage to electrical distribution equipment, and failure of utilization equipment because of overloading or other abuse. Circuits may become overloaded simply by connecting larger or additional utilization equipment to the circuit. Overloads may also be caused by improper installation and maintenance, such as misaligned shafts and worn bearings. Improper operating procedures (e.g., too frequent starting, extended acceleration periods, obstructed ventilation) are also a cause of equipment overload or damage.
The designer of electrical power systems has several techniques to minimize the effects of abnormalities occurring on the system or in the utilization equipment that the system supplies. One can design into the electric system features that:
- Quickly isolate the affected portion of the system and, in this manner, maintain normal service for as much of the system as possible. This isolation also minimizes damage to the affected portion of the system.
- Minimize the magnitude of the available short-circuit current and, in this manner, minimize potential damage to the system, its components, and the utilization equipment it supplies.
- Provide alternate circuits, automatic transfers, or automatic reclosing devices, where applicable, to minimize the duration or the extent of supply and utilization equipment outages.
System protection encompasses the above techniques. Accordingly, the function of system protection may be defined as the detection and prompt isolation of the affected portion of the system whenever a short-circuit or other abnormality occurs that might cause damage to, or adversely affect, the operation of any portion of the system or the load that it supplies. Coordination is the selection and/or setting of protective devices to isolate only the portion of the system where the abnormality occurs. Coordination is a basic ingredient of a well-designed electrical distribution protection system and is mandatory in certain health care and continuous process industrial systems.
The objectives of coordination are to determine the characteristics, ratings, and settings of protective devices that minimize equipment damage and interrupt short circuits as rapidly as possible. These devices are generally applied so that upon a fault or overload condition, only a minimum portion of the power system is interrupted.
Protective devices are applied to a power system as primary and backup protection. Primary protection is the first line of defense against further damage caused by a fault or other abnormal operating condition. These devices are generally set to operate faster and remove less of the power system from service than backup protection.
Primary Protection: Device Closest to the Fault or Main Protection
Backup protection takes over when the primary protection fails to clear the abnormal condition. Backup protective devices and settings are selected to operate at some predetermined time interval after the primary device operates. Thus, a backup device should be able to withstand the fault conditions for a greater time than the primary protective device. For most applications, the operation of the backup device isolates circuits in addition to the faulted or overloaded circuit. Therefore, a greater portion of the power system is interrupted with backup protection.
In applying protective devices, it is occasionally necessary to compromise between protection and selectivity. While experience may suggest one alternative over another, the preferred approach is to favor protection over selectivity. Which choice is made, however, is dependent on the equipment damage and the effect on the process.
Usually, the over current coordination study performs on:
- Maximum and minimum- three-phase and phase to phase short-circuit currents
- Maximum and minimum- ground-fault currents
Criteria for Discriminations between two protective devices are:
- Depends on inherent (minimum) operating time of protective relay (TP)
- Operating time of Master trip relay (TM)
- Opening time of circuit breaker of faulted section (TC)
Discrimination time between two protective devices = TP + TM + TC + Safety margin
Example: Minimum discrimination time between two protective devices = 45ms + 15ms + 40ms + 60ms = 160ms. Hence, the discrimination time between two protective devices should be more than 160ms.
Note: Operating time of protective relay should allow the motor starting on direct online. Operating time of protective relay should allow the energization of transformer by adopting the harmonic blocking or cold load pickup criteria in relay.
To conclude, safety and reliability of power system network can be achieved through a coordinated protection system that detects and isolates failed or faulted components as quickly as possible, thereby minimizing the disruption to the remainder of the electric system. Accordingly, the protection system should be dependable (operate when required), secure (not operate unnecessarily), selective (only the minimum number of devices should operate) and as fast as required. Without this primary requirement, protection system would be largely ineffective and may even become a liability.