In this post we will be seeing , the Basic Concept of Overcurrent Protection, which includes, relay Characteristics, Relay Settings with Example.

What is an Overcurrent Relay?

  1. Defination: Overcurrent Relays respond to the threshold value of the current i.e. once the current through the relay goes, beyond the set Value, it operates
  2. ANSI Code: Its ANSI Code is 50.

    EHV system is an Interconnected Network System of Circuit Element, having more than one Voltage Source. Hence the Fault current may flow  to Fault point from both end of any meshed line element. Hence,   simple Overcurrent Relays cannot provide proper Co-ordination and Discrimination for Meshed Transmission System.

    Overcurrent/Earth Fault Setting Calculation:  

    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.