I'm doing an Online Electrical Engineering course and we're up to the last component of the course. We're currently doing Asymmetrical 3 phase, because it's online we haven't had much practice with these maths concepts compared to say university.

Here are the questions with my answers:

**1.**In a 3 phase circuit with resistance earthed and neutral and earth fault in phase “

**A**” which produces a fault current of 800 amps.

Load currents may be taken as zero.

What will be the positive, negative and zero sequence currents in this system?

**Answer**

I have Ia = 800 for Phase A, Ib = 0 and Ic = 0 and I assume this is a Phase-to-earth fault

According to this source https://www.selinc.com/WorkArea/DownloadAsset.aspx?id=100688, when Phase B and Phase C is zero (as suggested by "Load currents may be taken as zero"): the zero,

positive and negative sequence are all 1/3Ia.

Therefore Ia = 800A and I0, I1, I2 = 267A. Is this right?

**2.**In a three phase 4 wire system with phase to neutral voltage of 230V, a balanced set of resistive loads of 8 ohms are connected between each phase and neutral.

If the fuse cartridge of phase

**“C”**blows, what will be the currents in the corresponding phases and the symmetrical components of the load currents?

**Answer**

Because it's a 4 wire system with a neutral, the voltage difference through the resistors will utilize the phase to neutral voltage, and since they're both 8 ohms, the current magnitude will be the same.

Therefore |Ia| = |Ib| = V/R = 230/8 = 28.75 Amps

I turned Phase A and Phase B into their phasor values respectively to:

Ia = 28.75 arg(0)

Ib = 28.75 arg(240)

and because C is blown:

Ic = 0

I've calculated the Asymmetrical components using a calculator (under 012 Polar):

The printscreen is here:

I think Neutral current flows by adding Ia + Ib utilizing Kirchoff's current law, this equates to 28.75 arg(60), but I doubt I use this in my phase diagram.

Is this the right answer/approach?

**3.**In the circuit above in problem 2, if there is no neutral wire used and Line to Line voltage of 400Volts is given, and connected in Star configuration

*.*

Note (only 3 wire system now)

Note (only 3 wire system now)

If the fuse cartridge of phase

**“C”**blows, what will be the currents in the corresponding phases and the symmetrical components of the load currents?

**Please note:**

*(This is the known condition, when a motor is subjected to loss of a phase. Which is also known as “single-phasing or poling”).*

**Answer**

Because there is no neutral, the potential difference is 400V, and the star load becomes a series resistance of 8+8 = 16 ohms as Phase C is gone. The current is 400/16 = 25 amps.

Using Kirchhoff's voltage law, we can find the individual voltage drops of each resistor: 25*8 = 200V for both Ia and Ib resistors: this is consistent with the content in the bottom of http://www.allaboutcircuits.com/vol_2/chpt_10/5.html

I don't know the angle direction for each however, and I'm assuming Ic is = 0 because it's blown to begin with. I've been told it's 180degrees apart between Ia and Ib, but don't know how I can see it? anyone to clarify?

I'd really appreciate the feedback, I don't have a strong theoretical Elec Eng background compared to people who has been to University haha

Cheers!