LESSON 1.2

INDUCTION DISC & TIME-OVERCURRENT RELAYS

INTRODUCTION The electromagnetic induction disc relay is

frequently used where the time of relay operation should depend upon the value of an overcurrent.

The relay is essentially a small induction motor or watt-hour meter.

The magnitude of current, which initiates disc movement, is set by choosing the tap on the current coil.

The resulting operation of the relay contact is dependent upon the tap and the time dial settings.

The relay timing can be varied from a few cycles to as long as 30 seconds.

The actual speed-torque characteristic of the relay can be controlled by the designer to give a wide variety of operating characteristics.

THEORY OF OPERATION

Fig. 1.2-1 Typical Electro-mechanical Time Overcurrent Relay

The operating coil in Fig. 1.2-1 has three poles on one side of the disc and a common magnetic member or keeper on the opposite side. The main coil is on the center leg. Current (I) in the main coil produces flux φ, which passes through the air gap and disc to the keeper. A small portion of the flux is shunted off through the side air gap. The flux, Φ divides as ΦL through the left-hand leg and as ΦR through the right-hand leg, where Φ = ΦL + ΦR

The amount of current or voltage that the induction coil sees is proportional to the primary system values.Electromagnet induces flux into the disc to cause rotation of the disc and closing for the relay contacts.

Fig. 1.2-2 Induction Disc Type Relay Construction

DRAG (DAMPING) MAGNET

Fig. 1.2-3 Induction Disc Relay Connections

SPIRAL SPRING

The spiral spring has three functions.

The first is to reset the disc once the abnormal condition has stopped or the circuit is tripped.

Another function is to provide the initial negative torque on the disc and to allow an adjustment of minimum pickup.

The last function is to provide a temporary

path for the DC tripping current from the external circuit to the moving contact.

The spiral spring is not designed to carry

the tripping current for very long; thus, a seal-in, bypass function is required.

TIME DIAL

The time dial provides this adjustment by changing the distance between the moving and stationary contacts

INDUCTION DISC UNIT APPLICATIONSThere are two types of induction disc unit applications:1- Single input quantity.2- Double input quantities.

SINGLE INPUT QUANTITYSingle input quantity applies to overcurrent, and over/under voltage protective relays.DOUBLE INPUT QUANTITIESDouble input quantities apply to directional unit protective relay.

INDUCTION DISC DIRECTIONAL UNIT

Fig. 1.2-5 Induction Disc Directional Unit

Vector Diagram of V and I

The voltage is used as a reference to the current. The torque equation is expressed as follow:

T = K VI cos θ Where: θ is the angle between V & IV: is the reference voltage from voltage transformer.I: is the relay current from current transformer.K: is a constant.

INDUCTION DISC INVERSE TIME O/C UNIT

Torque is produced by the reaction between two induced fluxes in the disc. Adjustment to current setting is done by coil tap selected and time setting is by contact travel.

There are four different time characteristic curves, as shown in Fig. 1.2-8, that the relay unit can be set for one of the following characteristics:

Inverse type relays: are likely to provide faster overall protection in applications characteristic is used for highly inductive loads such as large motors, where it takes long starting time for current to reach steady state

The very inverse O/C relay is particularly suitable if there is a substantial reduction of fault current as the distance from the power source increases. Very inverse characteristic is used for the loads subjected to transient operation or repeated switching

Extremely inversecharacteristic is used when the time of travel is approximately inversely proportional to the square of the current. This makes it suitable for the protection of distribution feeder circuits in which the feeder is subjected to peak currents on switching in, as would be the case in a power circuit supplying induction motors, with high starting currents.

TIME OVERCURRENT RELAY

The main coil has a range of discrete current settings

so that the sensitivity can be set for a particular

situation. The relay coil is given a nominal rating,

usually 5A, which is referred to as 100%.

a) 50% - 200% in seven steps for overcurrent relays

(2.5 A – 10 A).

b) 20% - 80% in seven steps for earth fault relays

(1A – 4 A).

The ratio of relay current to relay setting current

is called the Plug Setting Multiplier (PSM). where

Relay Current = Primary Current/ CT ratio.

Fig. 1.2-11 O/C Protection with Three O/C Relays and Three CTs

Fig. 1.2-12 Overcurrent Protection with Two Overcurrent Relays

Fig. 1.2-13 Tripping Circuit

TYPE IFC TIME O/C RELAY

When the seal-in unit picks up, it raises a

target into view, which latches up and

remains exposed until released by

pressing a button. Instantaneous unit is a

small hinge (clapper) type unit with

contacts normally connected in parallel

with the contacts of the time O/C unit and

its coil is connected in series with the time

O/C unit.

EXAMPLE 1.2-1

For the following circuit, using time-current characteristics, find the time that the overcurrent relay will delay before tripping the breaker if the primary current is 3000A and O/C relay tap setting is at 10A. The time-dial setting is at position (3).