Over Current & Earth fault Protection

Overcurrent IEC characteristics

  

Types of O/L Relays:-

1)         Inverse definite minimum type relays (IDMT):-

            a) Normal Inverse

            i) 3.0 sec relays - i.e. 3.0 sec. at ten times pickup with T.L of 1.0

            ii) 1.3 sec relays - i.e. 1.3 sec. at 10 times pickup

            b) Very Inverse relays

            c) Extremely Inverse relays

2)         Definite Time Relays

Instantaneous highest O/L relay supplementing the above O/C relays. By providing a timer, the required time delay can be obtained.

The O/L, E/L relays are used for line protection (for 11KV to 132KV) and for Transformer Protection.

O/C relaying is very well suited to distribution system protection for the following reasons:-

1.      It is basically simple and inexpensive

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

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

Pick-up Setting

For coordination of the inverse time O/C relays, the pickup current and time dial setting are to be choosen. The pickup of the relays must be choosen such that it will operate for all short circuits in its own line and provide backup for adjoining lines, keeping in view of maximum full load current.

O/C relay

Pickup setting = I max.load

E/F Relay

Pickup setting = 20% of rated current.

For the E/F relay, the load current is not a factor in the selection of pickup settings and is normally set at 20% of rated current.

Time Settings

            The actual operating time of the O/C & E/F relays can be varied by proper selection of the ‘Time Dial Setting’ which is selectable from 0.1 to 1.0.

            Time dial settings are to be chosen by having proper coordination and gradation in the system. Gradations between successive relays are obtained by ‘Selective time interval’ which is usually set between 0.3 to 0.4 Sec.

The operating time of various types of IDMT relays are in the sketches. Also can be obtained by the formulae: -

            Normal inverse            :           t = 0.14 x TL

                                                                 (PSM)^0.02 - 1

            Very inverse                :           t = 13.5 x TL

                                                                 PSM - 1

            Extremely inverse       :           t = 80    x   TL

                                                                 (PSM)² - 1

where PSM = Fault Current/(C.T.Ratio x Plug Setting)

Calculation example for O/L & E/L relay on line/Transformer:

For remote bus fault, fault current through the protected element

=          3-Phase : 3000 A (Assume)

            SLG      : 2500 A (Assume)

O/L relay:

Adopted C.T.Ratio on protection line =  600/1 A (Assume)

Pickup Setting for O/L relay               =  1 A (Plug Setting)

PSM for O/L relay                              =  3000

                                                               ---------- = 5

                                                               (600/1 x 1)

Actual time of operation for O/L & E/L relays is generally set to grade with the down side system.

Assume time setting required                                     =   0.4 Sec.

                                                                               0.14

Actual Time of Operation (ATO) : 0.4                       =   ------- x TL (For Normal Inverse)

                                                                                         (5)^0.02-1

     0.4 x 0.0327

                                                         T.L                     =   ------------- = 0.093

                                                                                          0.14

Set Time Dial (TL) for O/L relay                                = 0.1

Actual Time of Operation (ATO) for O/L relay                   0.14

(With T.L:0.1)                                                       T   =  ---------- x 0.1 = 0.42 Sec.

                                                                                       (5)^0.02-1

E/L relay:-

Pickup setting for E/L relay                =  0.2 A

                                                              

        2500

PSM for E/L relay                               =  ------------  = 20.8

                                                                600/1 x 0.2

If PSM exceed 20, set PSM = 20

Actual Time of Operation (ATO)

                                                                                    0.14

for E/L relay                                        = 0.4  =            ---------- x T.L

                                                                                    (20)^0.02-1

                                                               T.L  =           0.4 x 0.0617

                                                                                    -------------- = 0.176

                                                                                    0.14

Set T.L = 0.2

Actual Time of Operation for E/L relay with T.L = 0.2

0.14

                                                                         T =      ------------ x 0.2 = 0.45 Sec.

                                                                                    (20)^0.02-1                     

Selective time interval:

The time interval between two successive breakers to provide the required selectivity is termed as selective time interval.

The operating time of the relay at 1

i.e., t1 = t2 + b2 + 01 + f

where t2 = operating time of relay at 2

           b2 = breaker operating time at 2

           f   = factor of safety time

           01 = overtravel time of relay at 1

The selective time interval      S = b2+01+f = 0.3 to 0.4 Sec.