Power System Protection Explained

Power System Protection

An important consideration in power system design is power system protection. Without system protection, the power system itself, which is intended to be of benefit to the facility in question, would itself become a hazard.

The major concern for power system protection is protection against the effects of destructive, abnormally high currents. These abnormal currents, if left unchecked, could cause fires or explosions resulting in risk to personnel and damage to equipment. Other concerns, such as transient overvoltages, are also considered when designing power system protection although they are generally considered only after protection against abnormal currents has been designed.

Characterization of power system protection faults

Any current in excess of the rated current of equipment or the ampacity of a conductor may be considered an overcurrent. Overcurrents can generally be categorized as overloads or faults. An overload is a condition where load equipment draws more current that the system can safely supply. The main hazard with overload conditions is the thermal heating effects of overloaded equipment and conductors. Faults are unintentional connections of the power system which result in overcurrents much larger in magnitude than overloads.

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Faults can be categorized in several different ways. A fault with very little impedance in the unintended connection is referred to as a short circuit or bolted fault (the latter term is used due to the fact that a short circuit can be thought of as a bus bar inadvertently bolted across two phase conductors or from phase to ground). A fault to ground is referred to as a ground fault. A fault between all three phases is referred to as a 3 phase fault. A fault between two phases is referred to as a phase-to-phase fault. A fault which contains enough impedance in the unintentional connection to significantly affect the fault current vs. a true short circuit is known as an impedance fault. An arcing fault has the unintentional connection made via an electrical arc through an ionized gas such as air. All of these terms are used in practice to characterize the nature of a fault.

In order to quantitatively characterize a fault, it is necessary to calculate how much fault current could be produced at a given location in the system. In most cases this will be the three-phase short-circuit current, which is the current produced if all three phases were shorted to each other and/or to ground. The simplest method for illustrating this is to reduce the power system at the point in question to its Thevenin equivalent. The Thevenin equivalent is the equivalent single voltage source and impedance that produce the same short-circuit results as the power system itself. The Thevenin equivalent voltage Vth is the open-circuit voltage at the point in question, and the Thevenin equivalent impedance Zth is the impedance of the power system at the point in question with the source voltage equal to zero. If a further simplification is made such that the system can be reduced to its single-phase equivalent, then a simple 3-phase fault current calculation for the three-phase fault current If3ø can be performed as shown in Figure 1:

Fig. 1 - Simplified 3-phase fault calculation

The Thevenin impedance for a power system at a given point in the system is referred to as the short-circuit impedance. In the great majority of power systems the short-circuit impedance is predominately inductive, therefore one simplification that is often made is to treat the impedance purely as inductance. This has the effect of causing the fault current to lag the system line-to-neutral voltage by 90°. If the system is an ungrounded delta system the equivalent line-to-neutral voltage can be obtained by performing a delta-wye conversion of the source voltage.

The phase-to-phase fault value can be calculated from the three-phase fault value if it is remembered that the line-to-line voltage magnitude is equal to the line-to-neutral voltage magnitude multiplied by √3, and that there will be twice the impedance in the circuit since the return path must be considered. These two facts, taken together, allow computation of the line-to-line fault current magnitude.

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