Power System Protection/Introduction

1.1 Objectives

The objectives of electrical system protection and coordination are to — 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 circumstances causing system malfunction are usually unpredictable; however, sound design and preventive maintenance can reduce the likelihood of system problems. The electrical system, therefore, should be designed and maintained to protect itself automatically.

1.2 Safety

Prevention of human injury is the most important objective when designing electrical systems. Interrupting devices should have adequate interrupting capability. Energized parts should be sufficiently enclosed or isolated to avoid exposing personnel to explosion, fire, arcing, or shock. Safety should always take priority over service continuity, equipment damage, or economics. These fundamental principles of safety have always been adhered to by responsible engineers engaged in the design and operation of electrical systems. The essential electrical infrastructure in industrial and commercial establishments employs protective devices, many of which are addressed in this recommended practice, that function to de-energize the electrical system in the event of a malfunction. However, electric shock can result in serious injury far more quickly than the available technology of interruption can perform its task. Therefore, the limitation of shock hazard, such as the step and touch potentials in electrical substations, depends upon the speed of interrupting the fault. Rapid fault isolation is important. Therefore, the most efficient form of electrical shock protection is to avoid shock altogether, and such avoidance is best accomplished through proper system design and operation, and through effective maintenance.

1.3 Equipment damage versus service continuity

Whether minimizing the risk of equipment damage or preserving service continuity is the more important objective depends upon the operating philosophy of the particular industrial plant or commercial business. Some operations can afford limited service interruptions to minimize the possibility of equipment repair or replacement costs, while others would regard such an expense as small compared with even a brief interruption of service. In most cases, electrical protection should be designed for the best compromise between equipment damage and service continuity. One of the prime objectives of 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.

However, for certain continuous process industry plants, high-resistance grounding systems that allow the first ground fault to be alarmed instead of automatically cleared are employed.

1.4 Economic and reliability considerations

The cost of system protection determines the degree of protection that can be feasibly designed into a system. Many features may be added that improve system performance, reliability, and flexibility, but incur an increased initial cost. However, failure to design into a system at least the minimum safety and reliability requirements inevitably results in unsatisfactory performance, with a higher probability of expensive downtime. Modifying a system that proves inadequate is more expensive and, in most cases, less satisfactory than initially designing these features into a system.

The system should always be designed to isolate faults with a minimum disturbance to the system and should have features to give the maximum dependability consistent with the plant requirements and justifiable costs. Evaluation of costs should also include equipment maintenance requirements. In many instances, plant requirements make planned system outages for routine maintenance difficult to schedule. Such factors should weigh into the economic versus- reliability decision process. When costs of downtime and equipment maintenance are factored into the protection system cost evaluation, decisions can then be based upon total cost over the useful life of the equipment rather than simply the first cost of the system. There are many protective systems now

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