For Examples 8.1 and 8.2, point B is connected to point A with a 250 MCM feeder housed in a steel conduit. If the feeder length is estimated to be 500 ft, determine the per-unit feeder impedance for the feeder and the short-circuit current at point B. The feeder is part of a 480Y/277-V system.

Question

For Examples 8.1 and 8.2, point B is connected to point A with a 250 MCM feeder housed in a steel conduit. If the feeder length is estimated to be [latex]500 \ ft[/latex], determine the per-unit feeder impedance for the feeder and the short-circuit current at point B. The feeder is part of a 480Y/277-V system.

Accepted Answer

For a 250 MCM feeder housed in a steel conduit, the resistance and the reactance can be obtained:

From Table 8.1[latex]\left\{\begin{matrix} R=0.054 \ \Omega /1000 \ ft \ X=0.052 \ \Omega /1000 \ ft \end{matrix} ight.[/latex]

Since the length of the feeder is 500 ft, the actual resistance and reactance of the feeder can be estimated:

[latex]\left\{\begin{matrix} R=0.054 \ \Omega*500 \ ft/ 1000 \ ft ightarrow R_F=0.027 \ X=0.052 \ \Omega*500 \ ft/ 1000 \ ft ightarrow R_F=0.026 \end{matrix} ight.[/latex]

The reference impedance can be estimated using the rated phase values for voltage and current, as illustrated in Figure 8.12 for this example.

Using Equation 8.9, the reference impedance can be calculated:

[latex]Z_{ref}(\Omega )=\frac{E_{\Phi ,S}}{I_{\Phi ,S}}[/latex] (8.9)

[latex]Z_{ref}=\frac{277 \ V}{1202 \ A}=0.23 \ \Omega[/latex]

Thus, the per-unit resistance and reactance of the 250 MCM feeders are

[latex]R_F(pu)=\frac{0.027}{0.23}=0.0117[/latex]

[latex]X_F(pu)=\frac{0.026}{0.23}=0.0113[/latex]

Using Equation 8.8, the per-unit impedance at point B can be obtained from the impedance at point A and the feeder impedance:

[latex]Z_B=Z_A+Z_F[/latex] (8.8)

with

[latex]Z_A=R_A+jX_A[/latex]

In order to estimate the resistance and the reactance of the impedance at point A, it is typically assumed that at this point X/R = 4 or that the phase angle for point A is θ =76° .
Thus, the split of the magnitude of the impedance at point A to resistance and reactance can be obtained:

[latex]Z_A(pu)=\frac{I_{L,S}}{I_{SC,A}}=\frac{1202}{18,800}=0.042[/latex]

[latex]Z_A(pu)=0.042(\cos 76^\circ+j\sin 76^\circ )=0.0101+j0.0408[/latex]

Therefore, the per-unit impedance at point B can be calculated:

[latex]Z_B(pu)=(0.0101+0.0117)+j(0.0408+0.0113)[/latex]

[latex]Z_B(pu)=0.0218+j0.0521[/latex]

The magnitude of the impedance at point B is

[latex]\left|Z_B(pu) ight|=\sqrt{0.0218^2+0.0521^2}=0.0565[/latex]

The application of Ohm’s law using the per-unit value provides

[latex]I_{SC,B}(pu)=\frac{E(pu)}{Z_B(pu)}=\frac{1}{0.0565}=17.71[/latex]

Thus, the short-circuit at point B can be estimated in Amperes:

[latex]I_{SC,B}(A)=17.71*1202=21,300 \ A[/latex]

Question 8.3: For Examples 8.1 and 8.2, point B is connected to point A wi......

Related Answered Questions

Determine all the required sections of a unit substation for a radial distribution system shown in Figure 8.21. Then, size the electrical room assuming the unit substation is placed in the center of the room. ...

Verified Answer:

Estimate the symmetrical short-circuit current at the secondary side of a 13.8 kV-208Y/120-V transformer rated at 1000 kVA with Z = 5% and supplied by a utility having 15 kVA fault current capabilities. ...

Verified Answer:

Verified Answer:

Estimate the short-circuit at point A for a system with the following specifications for the main transformer and utility: PT T = = 1000 kVA, Z P 4%, SC,U ...

Verified Answer:

For the system considered in Example 8.1, estimate the motor contribution and determine the total short-circuit current at Point A. ...

Verified Answer:

Verified Answer: