Power System Blockset    

Description of the Transmission Network

The single diagram shown in Figure 2-1 represents a three-phase, 60 Hz, 735 kV power system transmitting power from a power plant consisting of six 350 MVA generators to an equivalent network through a 600 km transmission line. The transmission line is split in two 300 km lines connected between buses B1, B2, and B3.

Figure 2-1: Series and Shunt Compensated Network

In order to increase the transmission capacity, each line is series compensated by capacitors representing 40% of the line reactance. Both lines are also shunt compensated by a 330 Mvar shunt reactance. The shunt and series compensation equipment is located at the B2 substation where a 300 MVA-735/230 kV transformer feeds a 230 kV-250 MW load through a 25 kV tertiary winding.

Each series compensation bank is protected by metal oxide varistors (MOV1 and MOV2). The two circuit breakers of line 1 are shown as CB1 and CB2.

We want to study the transient behavior of this circuit when faults are applied on line 1 and at bus B2.

This network is available in the psb3phseriescomp.mdl demonstration file. Load the psb3phseriescomp system and save it in your working directory as case1 in order to allow further modifications to the original system.

Compare the circuit modeled in the Power System Blockset (Figure 2-2) with the schematic diagram of Figure 2-1. The generators are simulated with a Simplified Synchronous Machine block. A Three-Phase Transformer (Two-Windings) block and a Three-Phase Transformer (Three-Windings) block are used to model the two transformers. Saturation is implemented on the transformer connected at bus B2.

B1 and B3 blocks are 3-phase V-I Measurement blocks taken from the Measurements library of powerlib_extras. B2 is a similar block that has been modified to accommodate two 3-phase inputs and one 3-phase output.These blocks have been reformatted and given a black background color to give them the appearance of bus bars. They output the three line-to-ground voltages multiplexed on output 4 and the three line currents multiplexed on output 5. Open the dialog boxes of B1 and B2. See how the blocks are programmed to output voltages in p.u. and current in pu/100 MVA.

The fault is applied on line 1, on the line side of the capacitor bank. Note that the fault and the two line circuit breakers are simulated with blocks from the three-phase library. Open the dialog boxes of the 3-Phase Fault block and of the 3-Phase breakers CB1 and CB2. See how the initial breaker status and switching times are specified. A line-to-ground fault is applied on phase A at t=1 cycle. The two circuit breakers that are initially closed are then open at t=5 cycles, simulating a fault detection and opening time of 4 cycles. The fault is eliminated at t=6 cycles, one cycle after the line opening.

Figure 2-2: Series Compensated Network (psb3phseriescomp.mdl)

Series Compensation1 Subsystem

Now, open the Series Compensation1 subsystem of the psb3phseriescomp system. The three-phase module consists of three identical subsystems, one for each phase. A note indicates how the capacitance value and the MOV protection level have been calculated. Open the Series Compensation1/ Phase A subsystem. You can see the details of the connections of the series capacitor and the Surge Arrester block (MOV block). The transmission line is 40% series compensated by a 62.8 µF capacitor. The capacitor is protected by the MOV block. If you open the dialog box of the MOV block, you will notice that it consists of 60 columns and that its protection level (specified at a reference current of 500 A/column or 30 kA total) is set at 298.7 kV. This voltage corresponds to 2.5 times the nominal capacitor voltage obtained at a nominal current of 2 kA rms.

A gap is also connected in parallel with the MOV block. The gap is fired when the energy absorbed by the surge arrester exceeds a critical value of 30 MJ. In order to limit the rate of rise of capacitor current when the gap is fired, a damping RL circuit is connected in series. Open the Energy & Gap firing subsystem. It shows how the energy dissipated in the MOV is calculated by integrating the power (product of the MOV voltage and current). When the energy exceeds the 30 MJ threshold, a closing order is sent to the breaker block simulating the gap.

Figure 2-3: Series Compensation Module

Three-Phase Saturable Transformer Model

Open the 300 MVA 735/230 kV Transformer dialog box and notice that the current-flux saturation characteristic has been set at:

This data is the current and flux values at points 1, 2, and 3 of the piece-wise linear approximation to the flux linkage curve shown in Figure 2-4.

Figure 2-4: Saturable Transformer Model

The flux-current characteristic is approximated by two segments (see Figure 2-4). The saturation knee point is 1.2 p.u. The first segment corresponds to the magnetizing characteristic in the linear region (for fluxes below 1.2 pu). At 1 p.u. voltage, the inductive magnetizing current is 0.0010/1.0= 0.001 pu, corresponding to 0.1% reactive power losses.

The iron core losses (active power losses) are specified by the magnetization resistance Rm=1000 pu, corresponding to 0.1% losses at nominal voltage.

The slope of the saturation characteristic in the saturated region is 0.25 p.u. Therefore, taking into account the primary leakage reactance (L1=0.15 pu), the air core reactance of the transformer seen from the primary winding is 0.4pu/300 MVA).


 Series Compensated Transmission Network Setting the Initial Load Flow and Obtaining Steady-State