Power Simulation Example-DOC(Excerpts from Power Simulation-Sim E-final)

Here we will see the application of Directional overcurrent relay and it's a simulation in ETAP,

1st we need to understand what is the function of Overcurrent Directional relay

The directional relay(DOC) detects the direction of a fault current in the circuit, this direction will be detected by the phase angle between source input quantities and polarizing quantity, DOC has two input sources, one of which is the polarizing source, in recent practice the polarizing input source is Voltage but we can use current also as polarizing input. Polarizing input is the input by which reference we are setting our main input quantity's phase angular movement.

now we have DOC with current input and voltage as polarising input, we have standard connections for DOC but we can also use other customized connection as per the system requirement.

30-degree connection

it means due to fault or abnormal condition, DOC will trigger signal when line current(Ia) leads phase voltage(Va) by 30 degrees and leg by 150 degrees, during fault or abnormal condition if Phase to phase voltage(Vbc) shifted by 30 degrees, then there is maximum torque or sensitivity of DOC-30-degree connection. maximum torque means where relay has the least delay for tripping signal as compared to the complete operating zone of DOC.

45-degree connection

As shown in the figure mention above, due to fault or abnormal condition, DOC will trigger signal when line current(Ia) leads phase voltage(Va) by 45 degrees and leg by 135 degrees, and during this abnormal or fault condition if Phase to phase voltage(Vbc) shifted by 45 degrees, then there is maximum torque or sensitivity of DOC-45-degree connection.

Now the question is why we use DOC when we have Overcurrent relay where we need to install only one input(economical solution), the reason is that when we have multiple power sources in our ring circuit we do not want to interrupt complete power sources, this false tripping or complete power interruption occurs when we used Overcurrent non-directional relay.

we module one circuit with two power sources for understanding this concept.

1st we used Overcurrent Non Directional Relay(OC) to analyze the results

please note our input to OC is only Current transformer, Voltage transformer shown in the circuit is not connected to OC, as OC require only Current input, now analyze what happened when a fault occurs in a circuit.

As you see fault occurs at any source bus, means complete cut off of power supply to the circuit, Solution is DOC

Now see results on our circuit, we used Reverse direction(Reverse biasing) so relay see all faults from the power source side

You can observe that if there is a fault on U1 supply, due to DOC, only U1 will be disconnected and U2 keep supplying power to the system.

(Excerpts from Power Simulation-Sim E-Final)