Introduction to Basic Overcurrent Protection and Coordination Study

Overcurrent protection is the most basic protection against excessive currents resulting from system faults. In general, power system faults are indicated by a sudden and significant increase in current, thus the prevalence of this type of protection. Overcurrent protection is often the simplest and cheapest to employ but in most cases, its application tends to be difficult.

Overcurrent protection is mainly used in electric power distribution and industrial systems for the following reasons:

Overcurrent protection is also widely used as back-up protection especially for transmission lines and power transformers. Though, care should be taken considering its tendency to either overreach or underreach a protection zone.

Overcurrent Protection and Coordination

Consider for example the single line diagram below.

From the perspective of selectivity, in order to ensure a very high level of continuity of service, protective devices at each bus (A, B, C) should operate only on faults at their zone of protection. That is, for relay A at Bus A, the line AB, relay B at Bus B, the line BC, and relay C at Bus C, line C.

Short-circuit currents generally are higher in magnitude the closer they are to the source. In figure 1, the fault F₁ is higher in magnitude than the faults F₂ and F₃. However, distinguishing between faults F₂ and F₃ can be really tricky since these faults are very close to each other resulting in current magnitudes that are approximately equal.

The basic approach to this problem is to conduct a protection coordination study using time-current grading.

Zones of Protection

Protective devices are selected and assigned to protect a specific part of the power system. These part, known as the primary zone of protection, is the responsibility of the protective device.

While each zone is bounded by the location of the circuit breakers, the fault detection is dependent on the location of the current transformers (CTs). The location of the CTs is purposely identified in order to have overlapping zones. This is shown in the following figures.

Primary and Backup Protection

The overlapping of zones allows a protective device to provide backup protection to the adjacent zone, also referred to as the backup or overreached zone. Protective devices operate as fast as possible within their primary zone of protection while a delayed operation is expected in the backup zone. This is intentional in order to allow the protective devices to operate first in their primary zone of protection. Backup protection is intended to operate only when the primary protection fails to clear the fault.

In figure 1, the relay A at Bus A primary zone of protection is the line AB, while its backup zone is the line BC.

Backup is defined by IEEE as,

“protection that operates independently of specified components in the primary protective system”

IEEE 100

The selection of protective device primary and backup zones of protection is achieved through a protective device coordination analysis, also referred to as protection coordination study. This study is very important in order to ensure the least service interruption arising from a fault or short-circuit.

Biased Differential Protection for Transformers

Learn More Biased Differential Protection for Transformers

Vector Group of Transformer Dyn1|Dyn11|Ynd1|Ynd11

Learn More Vector Group of Transformer Dyn1|Dyn11|Ynd1|Ynd11

Local VS Remote Backup

Backup protection is designed to operate when the primary protection fails to clear the fault. Backup protection can either be local or remote.

Local Backup

A protective device at the same location as the primary protection device is referred to as local backup. The local backup protection is commonly fed from a different CT core but operates the same circuit breaker. It provides a degree of backup against failure DC supply and tripping circuit failure. However, local backups DO NOT PROVIDE backup against circuit breaker failure.

Remote Backup

A remote backup protection is located at the adjacent upstream zone of protection. This backup protection effectively protect against circuit breaker failure but results otherwise to a wider area of service interruption.

Objective of Overcurrent Coordination

The objective of every protection coordination study is to determine the characteristics, ratings and settings of OC relays in the system under study such that after a fault or overload

A coordination study also provides data useful for the selection of

Basic Considerations

The following considerations are integral in carrying-out a protection coordination study.

Short-circuit Currents

Overcurrent protection is designed to protect the system from the intolerable conditions associated with short-circuits. Information on the available fault duties is important when performing a protection coordination study. The following table shows a summary of fault duties and how they are used.

	½ Cycle	1.5 to 4 Cycles	> 30 Cycles
HV Circuit Breaker	Close and Latch Capability	Interrupting Capability	N/A
LV Circuit Breaker	Interrupting Capability	N/A	N/A
Fuse	Interrupting Capability	N/A	N/A
Switchgear	Bus Bracing	N/A	N/A
Relay	Instantaneous Settings	N/A	Time-delayed Settings

You may want to check out these topics on short-circuit study and circuit breaker sizing.

Load Flow Currents

Equipment selection and specification is a basic requirement in the design of any electric power system. Typically, this starts with the determination of the system steady-state operating conditions through a load flow study. The determination of normal and emergency load flow currents is very important in the configuration of protective relay settings.

Minimum Operating Criteria

Based on the available information on short-circuit and load flow currents. A minimum operating criteria for overcurrent protection can be established. The following figure shows the criteria for selecting overcurrent relay taps.

The minimum operating quantity is commonly referred to as the ‘pickup’ value and is defined as the minimum value of current that starts an action. For an overcurrent protective relay, the ‘pickup’ value is the minimum value of current that causes the relay to start timing and ultimately close its contacts.

Delta-Wye Transformers

Delta-Wye transformers are of great interest when doing a protection coordination study. Consider for example the figure below. For simplicity, let us consider a 1:1 transformer. Take note that by setting a 1:1 ratio, the winding ratio of a delta-wye transformer will be √3:1.

For a phase-to-phase fault on the wye side of the transformer, the per-unit secondary line current is approximately equal to 86.6% of the current magnitude resulting from a three-phase fault. However, on the delta side of the transformer, the per-unit primary line current magnitude in one phase reaches a value approximately equal to the three-phase fault current.

References

IEEE Std 242-2001 [The Buff Book]: IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems.(2001). S.I.: IEEE.

ETAP Enterprise Solution for Electrical Power Systems Online Help

Blackburn, J. (2014). Protective Relaying Principles and Application, 4th ed. Boca Raton, FL: CRC Press.

G. Pradeep Kumar (2006), Power System Protection, notes on Power System Protection Training, Visayan Electric Company, Cebu City, Philippines.