Three-Phase, Phase-to-Ground Fault and Single-Pole Reclosing of a 735-kV Transmission Line

This demonstration ilustrates the use of three-phase blocks to study phase-to-ground fault and single-pole reclosing of a 735-kV transmission line.

Pic : MODEL In SIMULINK MATLAB

Circuit Description

Two 735 kV parallel lines, 200 km long, transmit 3000 MW of power from a generation plant (12 generators of 350 MVA) to to an equivalent network having a short circuit level of 20 GVA. The generation plant is simulated with a simplified synchronous machine (subtransient reactance of 0.22 pu). The machine is connected to the transmission network through a 13.8 kV/ 735 kV Wye-Delta transformer.

The line models are distributed parameter lines. The lines are assumed to be transposed and their parameters R,L,C /km are specified in positive- and zero-sequence components. Each line is shunt compensated by two shunt reactors of 200 Mvars each, connected at line ends. Applying single pole reclosing at this voltage level is made possible by the use of neutral inductances for the two shunt reactances of line 2. Otherwise, the 'secondary arc current' which is induced into the fault ( mainly because of the capacitive coupling between the two sound phases and the faulted phase) would be too high to allow arc extinction after opening of line breakers on the faulted phase. If you open the two shunt reactance blocks of line 2, observe how the optimum neutral inductance is calculated.

Fault and Line Switching

A phase-to-ground fault is applied at the middle of line 2. In order to apply the fault along the line, this line is simulated in two sections of 100 km. As soon as the fault is detected by the protection relays (not simulated here) , an opening command is sent to the two line breakers of the faulted phase. The breaker are kept open during a certain 'dead time' , usually around 0.5 s, during which the arc normally extinguishes, then the two breakers are reclosed

When the two line breakers are tripped on the faulted phase, the fault current is interrupted but a small current will continue to flow through the arc. If this secondary arc current is too large (typically above 50 A), the arc cannot extinguish and the breaker will reclose on the fault.

Arc Model

The arc is modelled by a fixed or non-linear resistance R = f(Iarc_rms). The arc extinguishes when its rms current falls below a certain threshold value (typically 50 A) defined in the arc model block. Open the Arc Model block and look at the arc parameters. The mean arc resistance is programmed as an exponential function of the rms current. The mean arc resistance increases when the rms arc current decreases so that the time for arc current to decay below the threshold value is shortened . (With the specified parameters, Rarc = 0.1ohm and 30 ohms respectively for currents of 1kA and 100A). The Arc Model block is a masked block. Use 'Look under mask' to see how the arc model is implemented.

Open the blocks simulating the two line breakers. See how the line opening/reclosing sequence is programmed. The fault is applied at t = 1 cycle. Then, the opening command is sent to both breakers at t = 4 cycles (3 cycles detection + opening time). The two breakers are reclosed at t = 34 cycles after a dead time of 30 cycles, during which the arc creating the fault should extinguish.

Demonstration

Choose Simulation/Start and observe the voltage and current waveforms on the 4-trace oscilloscope. Observe the three phase-to-ground voltages and currents at sending end of line 2 and the current flowing into the fault. The line voltages and currents are measured with the 3-phase measurement bus of the Extras library. It allows to output voltages and currents directly in pu. The machine has been previously initialized to deliver 3000 MW of power at 1 pu voltage (with the Load Flow option of the Powergui). The line currents flowing into each line are therefore 15pu/100 MVA as observed on trace 2.

The fault current (I_arc) is measured in amperes. It reaches 22 kA during the first cycle, then it drops to a very small value after 3 cycles when the two line breakers open. Use Y-zoom to look at the secondary arc current. It contains a slowly decaying DC component and a fundamental component (12 A peak). As its rms value is below 50 A, the arc extinguishes at the first current zero crossing.

Now, open the Arc Model block menu and change the arc model to a fixed resistance (0.1 ohm). Restart the simulation. Notice that the DC component of the arc current prevents arc extinction, so that the line is now reclosed on a fault. You can also suppress the neutral reactance in the two shunt reactance blocks and see the impact on the secondary arc current.

Graphic



( SIBUEA MARK QUARK GLUON TAO )

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