SimPowerSystems

Transient Performance for a Line Fault

In order to speed up the simulation, you need to discretize the network. The sample time is specified in the Powergui block as a variable Ts. This sample time Ts is also used in the integrator block of the MOV energy calculator controlling the gap.

In the MATLAB window, define the variable

• Ts = 50e-6;
  

Ensure that the simulation parameters are set as follows:

• Stop time : 0.2
  Solver options Type: Fixed-step; discrete (no continuous state)
  Fixed step size: Ts
  

Line-to-Ground Fault Applied on Line 1

Ensure that the fault breaker is programmed for a line-to-ground fault on phase A. Start the simulation and observe the waveforms on the three scopes. These waveforms are shown here:

Figure 2-5: Simulation Results for a 4 Cycle Line-to-Ground Fault at the End of Line 1

The simulation starts in steady state. At the t = 1 cycle, a line-to-ground fault is applied and the fault current reaches 10 kA (trace 3). During the fault, the MOV conducts at every half cycle (trace 5) and the energy dissipated in the MOV (trace 6) builds up to 13 MJ. At t = 5 cycles the line protection relays open breakers CB1 and CB2 (see the three line currents on trace 2) and the energy stays constant at 13 MJ. As the maximum energy does not exceed the 30 MJ threshold level, the gap is not fired. At the breaker opening, the fault current drops to a small value and the line and series capacitance starts to discharge through the fault and the shunt reactance. The fault current extinguishes at the first zero crossing after the opening order given to the fault breaker (t = 6 cycles). Then the series capacitor stops discharging and its voltage oscillates around 220 kV (trace 4).

Three-Phase-to-Ground Fault Applied on Line 1

Open the 3-Phase Fault block dialog box. Select the Phase B Fault and Phase C Fault, so that you now have a three-phase-to-ground fault.

Restart the simulation.The resulting waveforms are shown.

Figure 2-6: Simulation Results for a 4-Cycle Three-Phase-to-Ground Fault at the End of Line 1

Note that during the fault the energy dissipated in the MOV (trace 6) builds up faster than in the case of a line-to-ground fault. The energy reaches the 30 MJ threshold level after three cycles, one cycle before the opening of the line breakers. As a result, the gap is fired and the capacitor voltage (trace 4) quickly discharges to zero through the damping circuit.

Setting the Initial Load Flow and Obtaining Steady State

Frequency Analysis