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Electronic Overcurrent Protection

Overcurrent relays and fuses are the most commonly applied form of distribution power system protection. Relays are activated from current transformer secondaries with typical secondary ratings of 1A or 5A. These relays were originally electromechanical devices but modern versions are microprocessor based and incorporate many features and refinements to increase versatility and improve coordination between relays and fuses on the same feeder. All characteristics shown in the following diagrams are plotted on log-log time-current scales.

The generic term relay is used for a collection of elements that are associated with phase or ground fault protection supplied from current transformers to provide instantaneous or inverse-time phase and ground fault protection, with or without time delay characteristics. These elements are programmable to produce characteristics that can replicate fuse curves or multi-function curves composed of definite-time, inverse time and instantaneous components. Inverse-time elements are available with inverse, moderately inverse, very inverse, and extremely inverse characteristics specified by IEEE or IEC standards. Settings suitable for phase or ground fault protection are available, sometimes along with breaker fail backup protection.

A common feature for both phase and ground fault elements is selection between definite-time and inverse-time characteristics for different current operating levels commonly defined as high set and low set. High set elements provide fast operation to clear high fault current levels thus minimizing plant damage. In most cases, coordination with downstream protection is not an issue. Low set elements are required to properly coordinate with upstream and downstream protection and ensure adequate time margins between operating characteristics over the range of feeder fault current levels. A range of independently set timing elements provides a great deal of flexibility in dealing with the requirements to provide these margins.

Relay element settings are described generically in time and multiples of primary current or relay rated current. These in turn, can be translated into time multipliers and relay tap settings by reference to the manufacturers literature. In practice, most utilities employ a limited number of device types.

Types of Element

For application purposes, overcurrent relay elements are separated into:

Instantaneous
Definite time
Inverse time

Instantaneous elements are simple devices that operate at a given level of current. In practice, the higher the fault current above the current setting the faster the operation. There is a minimum time of operation dictated by the rate at which the fault current rises. The operating current related to the element setting is not particularly accurate since the elements are intended for backup protection purposes or to achieve fast fault clearance where accuracy is not an issue.

A particular form of instantaneous element is designed to increase the setting accuracy by filtering out the dc component of fault current and is used where improved accuracy is an issue in ensuring correct discrimination between protective devices.

Definite time-overcurrent elements are simply instantaneous overcurrent elements assigned a given current setting and controlled by a timer set to operate at a designated time.

Inverse-time elements are designed to operate according to the inverse-time curve shown above. They have a definite minimum operating current, determined by the current setting, and a definite minimum time of operation, determined by the time setting. The curve is asymptotic to both the time and current axes. The shape of the curve is chosen according to whether an inverse-time, a very inverse-time, or an extremely inverse-time characteristic is desired.

Inverse-time elements have a range of current settings that determine the currents, defined in amps, supplied by the current transformer at which the element is designed to just operate. For example, a 1 amp rated phase fault element usually has tap settings of 0.5, 0.75, 1.0, 1.25, 1.5, 1.75, 2.0 amps, being the nominal currents at which the relay operates. In practice, a 1.0 amp setting requires typically 1.1 amps or more to operate the element in a finite time. The companion ground fault element is likely to have a setting range of 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 amps.

Detecting Different Types of Faults

Phase fault and ground fault detection requires separate elements designed to cover the range of prospective phase and ground fault currents. Typically, ground fault elements have lower current settings than phase faults because of the relative values of the fault magnitudes and the fact that ground fault element current settings need not account for peak loading currents on a feeder. They can be set at relatively low values of fault current for greater sensitivity. All the elements for phase and ground fault are incorporated in one relay for compactness. In addition to the overcurrent elements themselves, the relay will include targets or flags for each element that signifies physically or electronically that the element has operated in the event of a fault.

A common combination for overcurrent relays is an element supplied from A phase and another supplied from C phase. The ground fault element is supplied from the residual current from the three phases. At least one phase element will operate for any phase-phase fault. A ground fault energizes one phase and the ground fault element for faults on A or C phases and the ground fault element only for a B phase ground fault. Where greater security is desired, all three-phase fault elements are included in addition to the residually connected ground fault element. Now two fault elements operate for any phase-phase or phase-ground fault.

Improved Coordination

Some applications that take advantage of the enhanced functionality of modern overcurrent protection are discussed below.

Case (a) represents a common situation where the discrimination between an inverse-time overcurrent element (blue) and an upstream fuse (black) is lost for higher fault current levels because of the fuse characteristic shape relative to an inverse-time element. One solution is to use a very or extremely inverse characteristic for the relay (green) to prevent crossover of the fuse and overcurrent element characteristics. Another solution (b) is to use a multi-function characteristic comprised of inverse and instantaneous with a timer to produce a combined characteristic (blue) that prevents a crossover. The latter has the advantage of faster fault clearance for most fault levels and more latitude to accommodate a shift of the fuse characteristic to the left as a consequence of fuse aging and exposure to multiple faults.

Electronic relays have many additional features that include:

Programmable output contacts
Element blocking capability
More than one setting group
Directional features
Communications capability

Such features can be put to good use to provide inexpensive solutions to resolve situations where compromise is often accepted in the absence of any other economic alternative. One such situation is shown below where local generation modifies the total fault current. Without generation the relay may not operate or may operate too slowly for effective protection.

The overcurrent setting is normally R2 when the generator is connected and the total fault current is If1 but must be reset to R1 when the machine is not running and the fault current is reduced to If1. A similar situation arises when the supply is lost and the generator is capable of supplying the local load on its own. Again, the relay setting is adjusted to ensure operation for reduced fault level conditions. The relay settings are changed on operation of the generator breaker.

These cases are just two examples of where electronic relay flexibility permits significant improvements in protection coordination at little cost. Opportunities arise to add sophisticated features when the logic and timing functions of these relays are employed effectively.