Question 1

What are the Disadvantages of Time Only Grading ?

Accepted Answer

In this type of Relays, Pick up current is the same and the time setting of a series of relays is such that the Breaker nearest to the fault opens first.
Major disadvantage of such type of grading is that, the Fault nearest to the Source (which has maximum current) is cleared after the longest delay.

Question 2

What are the Disadvantages of Current Only Grading ?

Accepted Answer

In this type of Relays, Relay Operating time is the same and the Pick up of a series of Overcurrent relaysis chosen based on the assumed fault current which varies with the position of the Fault because of the difference of the Impedance values between the source and the Fault.
Major disadvantage of such type of grading is applicable only if there is large difference in magnitude of Fault current between the two Overcurrent relay locations, which is almost impossible to achieve.

Question 3

What are the Overcurrent/Earth Fault Setting Calculation ?

Accepted Answer

1) Connected CT Ratio (CTR):

Lets say, CTR = 200/1

2) Maximum Load Current in Secondary (Imax sec)

It is the Maximum Current flowing through the Transmission Line , for the last Six Months or Full Load Current in case of Transformer .
Lets say, Imax = 150 A
So, Imax (sec) = 150/200 = 0.75 A

3) Relay Pickup Current, Ipu:

It is the current which when flows through the Relay Coil, the Relay Operates.
It is generally set at 120% of Maximum Load Current.

Ipu = 1.2 x Imax sec= 1.2 x 0.75 = 0.9 A

4) Relay Nominal Current (In):

Should be kept equal to CT Secondary Rated Current. Generally, it is 1A or 5A.
Lets say In = 1 A

5) Current Setting/Plug Setting (PS)

It is the Percentage of the Relay Nominal Current (In), which when flows through the Feeder, the Relay should Operates.
PS = Ipu sec / In = 0.9 / 1 = 0.9

6) Fault Current in Secondary (Ifsec):

For Overcurrent Setting we consider Three Phase Fault Current and for Earth Fault we consider , Single Phase Fault Current.
Say, If = 4000 A
If sec = If / CTR = 4000/200 = 20

7) Plug Setting Multiplier (PSM):

It is the Fault current seen by the relay in the Multiples of Plug Setting.

PSM= If sec / PS = 20/ 0.9 = 22.22

Note: PSM is used for time Calculation only and not required to be entered in the Relay Setting.

8) Time Margin (TM):

For 10C3 IDMT Curve, Time Margin is given by the Formula,

TM = 3 / Log PSM = 3 / Log 22.22 = 2.23

9) Relay Opersting Time (ROT):

It is the Time taken by the Relay to Operate and is given by the Formula,

ROT = TM x TMS

Where, TM is the Time Margin, that we have Calculated in Step 8.

10) Time Multiplier (TMS):

TMS is the Time Multiplier Setting which needs to be entered in the Relay Settings.

TMS = ROT/ TM

Lets say we want Relay to Operate in 450 ms I.e ROT = 450 ms

Then, TMS = 0.45 / 2.23 = 0.202, which needs to be entered in the Relay as the Time Setting.

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