1. Add a ‘Circuit Breaker’ block to the model as illustrated.
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2. The breaker can be set to either OPEN (value: 0) or CLOSED (value: 1). Set the initial status of the breaker to OPEN.
3. Configure the breaker to be switched externally and add a ‘Step’ block to the model to switch the breaker to CLOSED after 1 second.
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4. By adding ‘RMS’ blocks to the model adapt our measurement circuits so that we can compare ‘U1_RMS’ to ‘U2_RMS’.

1. Within the Current Measurement scope:
   • Adjust the timespan to approx. 0.9 s to 4 s.
   • Using the ‘Peak Finder’ measurement tool; find the value of peak current imposed upon the line during energisation.
   • Using the ‘Cursor Measurement’ tool; find the time taken for the transient current to dissipate to within 10% of its new steady state.
2. Within the Voltage Measurement scope:
   • Adjust the timespan to approx. 0.95 s to 1.10 s.
   • Using the ‘Bilevel Measurements’ tool (‘Transitions’); find the Rise Time and Slew Rate for the rising edge of the transient voltage.
   • Using the ‘Bilevel Measurements’ tool (‘Overshoots / Undershoots’); find the Settling Time of the transient voltage. 

Add a ‘Three-Phase Fault’ block to the model as illustrated.

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1. Configure the block to simulate a fault between ‘Phase A’ and ‘Ground’ and set the Switching Time to 4 s using a ‘Step’ block.
2. Run the simulation and find the peak value of the fault current at nodes B1 and B2.
3. At the circuit breaker (Node B1), find the value of the fault current 80 ms after the fault is incurred.

These values could be used by an engineer to inform circuit breaker design.

Note: the ‘breaking capacity’ is the maximum fault current that the circuit breaker is able to interrupt without being destroyed or causing a dangerous electrical arc. Specified at the breaker operating time, which is commonly around 80 ms.

This image illustrates the simple protection circuit that we shall be integrating into our model.

• The current transformer (CT) is an instrumentation device that is used for measuring the current flowing through a circuit. In our system the CT will reduce the current by a ratio of 120/5.
• The relay coil operates to energise the trip coil once the secondary side of the CT is above a preselected threshold value. In our system this threshold will be set to when the current is 10x the normal full load current (FLC) of the circuit.
• The FLC of the circuit can be ascertained from our model when in pre-fault conditions. We shall assume here that the FLC is 240A.
• The relay’s threshold setting will therefore be:

            I_R = FLC x (1/CT_RATIO) x 10 = 100 A

We shall also integrate the circuit breaker operating time (assume 80 ms) into our model.

As your model increases in size and complexity, you can keep the model visually tidy by grouping blocks into subsystems.

We typically use subsystems to establish a hierarchical model structure, where several functionally related blocks are grouped together into a subsystem. This subsystem exists at a level below the main model. By reducing the number of blocks displayed in our main model we can improve ease of navigation.

We shall add a Subsystem block to the model, around the breaker and expand it’s functionality by including a CT and relay device.

1. Highlight the illustrated blocks, right click them and select ‘Create Subsystem from Selection’.
2. Double-click the new subsystem block to enter it.
3. The subsystem uses ‘Inport’ blocks to retrieve inputs from the parent model and ‘Outport’ blocks to pass signals back to the parent model.

This image illustrates the our protection circuit which we shall build within our new subsystem.

• A ‘Current Measurement’ block and a ‘Gain’ block have been used to represent the CT with a ratio of 120/5.
• A ‘Relational Operator’ block compares the RMS of the CT output against our relay’s threshold value (represented using a ‘Constant’ block).
• A ‘S-R Flip-Flop’ block is used to pass a OPEN signal (value: 0) to the circuit breaker when the relational operator (testing: input1 > input2) is found to be true. A constant 0 value input to the reset ensures that the Flip-Flop output is permanent.

Finally, an ‘On/Off Delay’ block is used to represent the circuit breaker operating time. This block is set to provide an off delay (i.e. pass the value: 0) to the control of the circuit breaker after a time delay of 80 ms. This 0 value shall switch the CB to OPEN.

1. Configure the ‘Three Phase Fault’ block to simulate a fault at 1 s.
2. Run the simulation and by monitoring the input signal to the circuit breaker demonstrate that our protection circuit adequately detects the fault current at nodes B1 and sends the OPEN signal.
3. By monitoring the fault current at both the circuit breaker (Node B1) and at the fault location (Node B2) demonstrate that the circuit breaker adequately acts to isolate the fault.

As should be apparent from (3), the circuit breaker acts to isolate the fault current as designed and thus protects the transmission system equipment to prolonged exposure to damaging fault currents.

Note that the current at B2 does not return to zero as fast as that at B1. This is due to the energy stored in the capacitive elements of our transmission system.