Relay Setting of IDMT and Instantaneous over current and earth fault Relay

Relay is a protecting device which detects any kind of abnormal happenings(fault) in power system or in power system elements (like transformer, generator etc.) and sends a signal to circuit breaker to isolate the fault or to the alarm circuit to raise an alarm.

RELAY IDENTIFICATION:

51- Over current IDMT relay

51N-IDMT earth fault relay

50-Over current definite time relay

50N-Definite time earth fault relay

49-Thermal overload relay

46-Negetive sequence relay

48-Motor starting supervision

66-Restart inhibit for motor

27-Under voltage relay

87-Differential relay

IDMT stands for Inverse Definite Minimum Time relay

SETTING METHODS:

51 & 51N

51 and 51N are both inverse time relay and used to detect phase fault and earth fault respectively. There setting methods are same but the only difference is that pick up of 51 is higher than that of 51N.
Please check our article CT connection to sense earth fault/Earth Faults Detection Philosophy to know earth fault is detected.

The following parameters are required to calculate the settings of 51 and 51N.

       a.       PICK UP: this is the value of current through relay for which the relay picks up.

       b.     PSM (Plug Setting Multiplier): this is the ratio of fault current through relay coil and pick up           current. (it is relays inherent property)

       c.   TSM (Time Setting Multiplier): it is the ratio of required relay operating time and actual relay           operating time.

Basically if the TSM=1 then the relay operating time is very high, that’s why a fraction given as TSM to decrease the relay operating time as per requirement or to co-ordinate with the up and downstream feeders.

Sample calculation:

Pick Up Calculation:

Fig 1: IDMT relay setting/
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The single diagram shows a very simple and typical line, through which 90A current is flowing, a CT of rating 120/5A is connected via which the 51 and 51N relays are connected.

Let say, the relay 51 will pick up for 120% of full load current.

So current seen by the relay will be,

=120% X FLC X CT Sec. /CT primary

=1.2 X 90 X 5/120

=4.5 A

So pick up setting or pick-up current of the relay is 4.5 Amp.

Now remember one thing, that some relay manufacturers make their relay pick up range in the following manner, say 0.5-10 X CT sec. That means the CT secondary will be multiplied within the relay program. So here multiplication with CT secondary will not be applicable.

For this type of relay the pick-up will be

=1.2 X 90/120=0.9

Relay will display the following setting- 0.9 X CT sec (In)  (In general In is referred as the relay rated current, which is in general same as the CT secondary current. That is CT secondary and Relay rated current shall be same. However there is exception for earth fault relay, which is beyond scope of this article)

So, it is strictly required to read the relay catalogue and know the setting range.

Method of calculating pick up of 51N is same as mentioned above, but the pick-up is considered as 5-10% of full load current, if the feeder is being fed from a transformer which has NGR then pick-up may be considered as 5-10% of NGR rating.

Suppose a transformer has an NGR of 400A for 10 sec i.e. limiting earth fault current upto 400 A. with NCT having ratio of 200/1A.

Then pick-up will be set as

=10% X 400 X CT sec/CT prim.

Here also see the relay catalogue to know if the CT sec is required to be multiplied or it will be multiplied internally.

Following the above mentioned process pick up of inverse time over current and earth fault relay is calculated.

However for earth fault protection is better to select minimum available pick up to ensure maximum sensitivity.

PSM calculation:

PSM=Fault current in relay coil/pick up current through the relay coil.

        = (Fault Current X CT secondary/CT primary)/ Pick up current through the relay coil.

Suppose fault current is 1000A.

So, PSM will be

= ((1000X5)/120)/4.5

=41.66/4.5

=9.26

There are several types of IDMT relay characteristics available now, according to the characteristics the relay operating time are the determined. The formulas are as follows:

Fig 2: IEC relay characteristics/
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I_r the PSM value which has been calculated earlier.

Now,

Suppose we have to operate the relay at 0.75 sec (say).

So, TMS will be (for standard inverse curve)= 0.75X((9.260^0.02)-1)/0.14

                                                                                    =0.24

So settings are –

 1. Pick up= 4.5 or 0.9 X In(it will be according to the relay)

  2. TMS=0.24

Some typical settings-

PHASE FAULT

1. B/C AND I/C OF MCC: 80% OF THE BUSBAR RATING OR 120% OF THE CONNECTED LOAD (TAKE THE AVAILABLE DATA).

2. UPSTREAM RELAY OF SL NO. 1 ( i.e. O/G OF PCC): SELECT PICK UP HIGHER THAN SELECTED FOR SL NO. 1

3. B/C AND I/C OF PCC: 120% OF TRANSFORMER FULL LOAD CURRENT.

4. AT THE H.V SIDE OF THE TRANSFORMER (51 which is back up of the downstream or LV side 51 relay): 130% OF TRANSFORMER HV SIDE FULL LOAD CURRENT.

GROUND FAULT

1. B/C AND I/ OF MCC: 15-30% OF BUSBAR RATING.

2. UPSTREAM RELAY (O/G OF PCC):30% OF BUS BAR RATING

3. B/C AND I/C OF PCC: 20% OF TRANSFORMER FULL LOAD CURRENT.

For 51N always use minimum available setting, that will ensure maximum percentage of protection.

TIME GAP

TRY TO MAINTAIN TIME GAP OF 0.25 SEC BETWEEN RELAYS.

50 & 50N

These can be used as definite time relay or instantaneous relay.

For relay 50 connected at H.V side of the transformer: For such connection  the pickup shall be more than the inrush current of the transformer. Otherwise the relay will trip during transformer starting.

Inrush current is generally 10-15 times of transformer FLC.

Fig 3: Instantaneous relay setting/
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Say transformer H.T FLC is 90 A.

Then inrush will be 10x90=900 A. we set pick up of the relay as 910 A.

So setting is=910x5/120=37.9 A or (910/120) X In=7.58 X In as per the available Relay functions.

Here also consider the relay catalogue as mentioned earlier.

If the 50 relay is connected with motor feeder for motor protection then pickup will be higher than the starting current of the motor.

Time delay may be or may not be given depending upon the requirement of instantaneous or time delayed operation.

50N setting method is also same, but use minimum pick up setting, generally 5-10% of NGR rating can be used. If NGR is not there then the same % of FLC can be used, but try to use minimum pick up setting for the relay to ensure maximum percentage of protection.

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