Consider the five-bus power system whose single-line diagram is shown in Figure

Question

Consider the five-bus power system whose single-line diagram is shown in Figure 6.2. Machine, line, and transformer data are given in Tables 9.1, 9.2, and 9.3. Note that the neutrals of both transformers and generator 1 are solidly grounded, as indicated by a neutral reactance of zero for these equipments. However, a neutral reactance 0.0025 per unit is connected to the generator 2 neutral. The prefault voltage is 1.05 per unit. Using PowerWorld Simulator, the fault currents and voltages are determined for a bolted single line-to-ground fault at bus 1, then bus 2, and so on to bus 5 FIGURE 6.2 4 800 MVA 345/15 kV 520 MW Single-line diagram for B51 B41 B1 B3 345 kV Example 6.9 800 MVA 400 MVA 15 kV B42 15 kV 400 MVA 345 kV 40 Mvar 80 MW 15/345 kV 345 kV 100 m B21 280 Mvar. 800 MW TABLE 9.1 Neutral Reactance Xn Synchronous machine Bus per unit per unit per unit per unit data for Example 9.8 0.0125 0.045 0.045 0.005 0,0225 0,0025 0.0225 TABLE 9.2 Line data for Bus-to-Bus per unit per unit Example 9.8 0.3 0.1 0.05 4-5 0.075 0,025 TABLE 9.3 Low-Voltage High-Voltage Transformer data for (connection) (connection) Leakage Reactance Neutral Reactance Example 9.8 bus bus per unit per unit 1 (A) 0.02 3 (A) 4 (Y) 0.01 00 MVA v 15 kV at buses 1, 3 345 kV at buses 2, 4, 5 Open PowerWorld Simulator case Example 9 8 to see this example. Tables 9.4 and 9.5 summarize the PowerWorld Simulator results for each of the faults. Note that these fault

Explanation / Answer

1. In a single phase to ground fault, if the fault is on phase a the constraints are as follows:

Ia=If=3Ia0 where Ia0=zero sequence current

Ia0=Ia1=Ia2.

VA=ZfxIf and |Vb|=|Vc|, Ib=Ic=0

Va= Va0+Va1+Va2

Va0= -Ia0Z0

Va1=Ea-Ia1Z1

Va2= - Ia2Z2

Ia1 and Ia2 are equal and Z1 is equal to Z2 in most cases. So we are left with Va=Ea- V0

Since V0 and E0 both have a phase angle of 0° so Va is always zero degrees.

Where Zf= Fault impedance and If= Fault current

Since the fault in the question is a bolted fault hence Zf=0

So we get Va=0 and there is no phase angle associated with it since there is no impedance so there is no voltage drop due to the fault current If flowing through phase A.

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