As the fault impedance is less than the load impedance, the fault current is more than load current. If a short circuit occurs, the circuit impedance is reduced to a low value and therefore a fault is accompanied by a large current.

Overcurrent protection is that protection in which the relay picks up when the magnitude of current exceeds the pickup level. The Overcurrent relays are connected to the system, normally by means of CTs.

• Difference between Overcurrent and Overload

Overcurrent: is a protection against excessive currents or current beyond the acceptable current rating of equipment. Short circuit is a type of overcurrent. Magnetic circuit breakers, fuses and overcurrent relays are commonly used to provide overcurrent protection.

Overload: is a protection against a running overcurrent that would cause overheating of the protected equipment. Hence, an overload is also type of overcurrent. Overload protection typically operates on an inverse time curve where the tripping time becomes less as the current increases. Overload relays as well as “slow blow” fuses are commonly used to provide overload protection.

• Overcurrent Protection Types

Non-Directional Overcurrent Protection: The relay responds for a fault current flow in both the directions through the relay either forward or reverse, depending upon fault location. In this case the Overcurrent protection tripping time shall be calculated and coordinated with other relays in order to avoid false tripping.

Directional Overcurrent Protection: The relay responds for a particular defined direction, so that proper discrimination is possible. This can be achieved by introduction of directional control elements. During the opposite flow of current the CT polarity reverses, the power measuring device in which the system voltage is used as a reference for establishing the relative phase of the fault current.

The Overcurrent protection is applied for different equipment in the power system as

a. Motor Protection

• Used against overloads and short circuits in stator windings of the motor.

• Inverse time and instantaneous overcurrent phase and ground

b. Transformer Protection

• Extensively also at power-transformer locations for external fault backup protection.

c. Line Protection

• On some sub-transmission lines where the cost of distance relaying cannot be justified.

• primary ground-fault protection on most transmission lines where distance relays are used for phase faults.

• For ground backup protection of most lines that having pilot relaying for primary protection.

d. Distribution Protection

• Overcurrent relaying is very well suited to distribution system protection for the following reasons:

• It is basically simple and inexpensive.

• Very often the relays do not need to be directional and hence no PT supply is required.

• It is possible to use a set of two O/C relays for protection against inter-phase faults and a separate Overcurrent relay for ground faults.

A. Instantaneous Characteristics

Definite current relay operates instantaneously when the current reaches a predetermined value.

• Operates in a definite time when current exceeds its Pick-up value.

• Its operation criterion is only current magnitude (without time delay).

• There is no intentional time delay around 0.1s or less

Application: This type is applied to the outgoing feeders.

B. Definite Time Characteristics

In this type, two conditions must be satisfied for operation (tripping), current must exceed the setting value and the fault must be continuous at least a time equal to time setting of the relay. Constant tripping time independent of in feed variation and fault location

Modern relays may contain more than one stage of protection each stage includes each own current and time setting.

Application

• Backup protection of distance relay of transmission line with time delay.

• Backup protection to the differential relay of the power transformer with time delay.

• Main protection to outgoing feeders and bus couplers with adjustable time delay setting.

C. Inverse Time Characteristics

In this type of relays, operating time is inversely changed with current. So, high current will operate overcurrent relay faster than lower ones. The operating time of an overcurrent relay can be moved up (made slower) by adjusting the ‘time dial setting’. The lowest time dial setting (fastest operating time) is generally 0.5 and the slowest is 10.

The inverse overcurrent characteristics can be described by

Also, some of the protection relays as ABB are providing a programmable curve with an equation that the user can define its parameters that would generate the required slope characteristics.

a. Normal Inverse Time Overcurrent Relay

The accuracy of the operating time may range from 5 to 7.5% of the nominal operating time as specified in the relevant norms. The uncertainty of the operating time and the necessary operating time may require a grading margin of 0.4 to 0.5 seconds.

Application:

• Most frequently used in utility and industrial circuits. especially applicable where the fault magnitude is mainly dependent on the system generating capacity at the time of the fault.

b. Very Inverse Time Overcurrent Relay

Gives more inverse characteristics than that of IDMT.

Application:

• Particularly effective with ground faults because of their steep characteristics.

• Suitable if there is a substantial reduction of fault current as the fault distance from the power source increases and independent of normal changes in generating capacity.

c. Extremely Inverse Time Overcurrent Relay

It has more inverse characteristics than that of IDMT and very inverse overcurrent relay. The operating time of a time overcurrent relay with an extremely inverse time-current characteristic is approximately inversely proportional to the square of the current

Application:

• Suitable for the protection of machines against overheating.

• Used when Fault current is dependent on fault location and independent of normal changes in generating capacity.

• Suitable for protection of distribution feeders with peak currents on switching in (refrigerators, pumps, water heaters and so on).

• Suitable for grading and coordinates with fuses and recloses

• For the protection of alternators, transformers. Expensive cables, etc.

d. Long Time Inverse Overcurrent Relay

• The main application of long-time overcurrent relays is as backup earth fault protection.

If fault current can flow in both directions through a relay location, it is necessary to add directionality to the overcurrent relays in order to obtain correct coordination. Typical systems which require such protection are

parallel feeders (both plain and transformer) and ring main systems, each of which is relatively common in distribution networks.

In order to give directionality to an overcurrent relay, it is necessary to provide it with a suitable reference, or polarizing, signal. The reference generally used is the system voltage, as its angle remains relatively constant under fault conditions.

A. Self-Polarizing Voltage [Faulted Phase]

The relay uses the phase voltage of the faulted phase as the polarizing quantity. However, for a fault condition that occurs close to the relaying point, the faulty phase voltage will reduce to a value close to zero volts so the following polarizing quantities could be used

B. Positive or Negative Sequence Voltages or Quadrature phase-phase voltages, as shown in the table below:

For single or double-phase faults, there will always be at least one healthy phase voltage present for polarization of the phase overcurrent elements. For example, a close-up A to B fault condition will result in the collapse of the A and B phase voltages. However, the A and B phase elements are polarized from VBC and VCA respectively. As such a polarizing signal will be present, allowing correct relay operation 

C. Memorized [Fictive] Voltage

For a closeup three-phase fault, all three voltages will collapse to zero and no healthy phase voltages will be present. For this reason, the protection relays offer a synchronous polarization feature that stores the pre-fault voltage information and continues to apply it to the directional overcurrent elements for a certain time period typically less than 3 Seconds. This ensures that either instantaneous or time-delayed directional overcurrent elements will be allowed to operate, even with a three-phase voltage collapse.

When the voltage falls below the Minimum operating voltage at a close fault, the fictive voltage is used to determine the phase angle. The fictive voltage is discarded if the measured voltage stays below Min operate voltage and hysteresis for longer than the Voltage Memorized time or if the fault current disappears while the fictive voltage is in use. When the voltage is below the Minimum operating voltage and hysteresis and the fictive voltage is unusable, the fault direction cannot be determined. The fictive voltage can be unusable for two reasons:

o   The fictive voltage is discarded after Voltage Mem time

o   The phase angle cannot be reliably measured before the fault situation.

The fictive voltage is calculated using the positive phase sequence voltage measured before the fault occurred, assuming that the voltage is not affected by the fault. The voltage memory cannot be used for the "Negative sequence voltage" polarization because it is not possible to substitute the positive sequence voltage for the negative sequence voltage without knowing the network's unsymmetric level. This is the reason why the fictive voltage angle and corresponding direction information are frozen immediately for this polarization mode when the need for a voltage memory arises, and these are kept frozen until the memorized time is elapsed.

Directionality

Under system fault conditions, the fault current vector will lag its nominal phase voltage by an angle dependent upon the system X/R ratio. It is therefore a requirement that the relay operates with maximum sensitivity for currents lying in this region. This is achieved by means of the relay characteristic angle (RCA) setting; this defines the angle by which the current applied to the relay must be displaced from the voltage applied to the relay to obtain maximum relay sensitivity.

Two common applications which require the use of directional relays are illustrated below:

a. Distribution system using parallel transformers or feeder

In such an application, a fault at “F” could result in the operation of both R1 and R2 relays and the subsequent loss of supply to the low voltage busbar. Hence, with this system configuration, it is necessary to apply directional relays at these locations set to 'look into' their respective transformers.

In such an application, relays R1 and R2 may commonly require nondirectional overcurrent protection elements to provide protection to the low voltage busbar, in addition to providing a back-up function to the overcurrent relays on the outgoing feeders.

b. Closed ring network topology

A particularly common arrangement within distribution networks is the ring main circuit. The primary reason for its use is to maintain supplies to consumers in the event of fault conditions occurring on the interconnecting feeders.

The normal grading procedure for overcurrent relays protecting a ring main circuit is to open the ring at the supply point and to grade the relays first clockwise and then anti-clockwise. The arrows shown at the various relay locations depict the direction for the forward operation of the respective relays, i.e. in the same way as for parallel feeders, the directional relays are set to look into the feeder that they are protecting.