The magnitude of three-phase fault currents at buses A and B of a power system are 10pu and 8pu, respectively. Neglect all resistances in the system and consider the pre-fault system to be unloaded. The pre-fault voltage at all buses in the system is 1.0pu. The voltage magnitude at bus B during a three-phase fault at bus A is 0.8pu. The voltage magnitude at bus A during a three-phase fault at bus B, in pu, is_______________.

A 50MVA, 10kV, 50Hz, star-connected, unloaded three-phase alternator has a synchronous reactance of 1 p.u. and a sub-transient reactance of 0.2 p.u.. If a 3-phase short circuit occurs close to the generator terminals, the ratio of initial and final values of the sinusoidal component of the short circuit current is______________.

A sustained three-phase fault occurs in the power system shown in the figure. The current and voltage phasors during the fault (on a common reference), after the natural transients have died down are also shown. Where is the fault located?

Three – phase to ground fault takes place at locations  in the system shown in the figure         

If the fault takes place at location , then the voltage and the current at bus A are  and  respectively. If the fault takes place at location , then the voltage and the current at bus A are  respectively. The correct statement about voltages and currents during faults at  is

Two generator units G1 and G2 are connected by 15kV line with a bus at the mid-point as shown below.

G1 = 250MVA, 15kV, positive sequence reactance X = 25% on its own base G2 = 100MVA, 15kV,  positive sequence reactance X for the line = 0.225

In the above system, the three-phase fault MVA at the bus 3 is

Consider a power system shown below:

Given that:

The positive sequence impedances are

 and

3-phase Base MVA = 100

Voltage base = 400 kV (Line to Line)

Nominal system frequency = 50 Hz

The reference voltage for phase ‘a’ is defined as

A symmetrical three phase fault occurs at centre of the line. i.e. point 'F’ at time . The positive sequence impedance from source  to point 'F' equals 0.004+ j0.04pu. The waveform correspond phase 'a' fault current from bus X reveals that decaying dc offset current is negative and in magnitude at its maximum initial value. Assume that the negative sequence impedances are equal to positive sequence impedances, and the zero sequence impedances are three times positive sequence impedances.

The instantof the fault will be

Consider a power system shown below:

Given that:

The positive sequence impedances are  and

3-phase Base MVA = 100

Voltage base = 400 kV (Line to Line)

Nominal system frequency = 50 Hz

The reference voltage for phase ‘a’ is defined as

A symmetrical three phase fault occurs at centre of the line. i.e. point 'F’ at time . The positive sequence impedance from source  to point 'F' equals 0.004+ j0.04pu. The waveform correspond phase 'a' fault current from bus X reveals that decaying dc offset current is negative and in magnitude at its maximum initial value. Assume that the negative sequence impedances are equal to positive sequence impedances, and the zero sequence impedances are three times positive sequence impedances.

The RMS value of the ac component of fault currentwill be

Consider a power system shown below:

Given that:

The positive sequence impedances are  and

3-phase Base MVA = 100

Voltage base = 400 kV (Line to Line)

Nominal system frequency = 50 Hz

The reference voltage for phase ‘a’ is defined as

A symmetrical three phase fault occurs at centre of the line. i.e. point 'F’ at time . The positive sequence impedance from source  to point 'F' equals 0.004+ j0.04pu. The waveform correspond phase 'a' fault current from bus X reveals that decaying dc offset current is negative and in magnitude at its maximum initial value. Assume that the negative sequence impedances are equal to positive sequence impedances, and the zero sequence impedances are three times positive sequence impedances.

Instead of the three phase fault, if a single line to ground fault occurs on phase 'a' at Point 'F' with zero fault impedance, then the RMS value of the ac component of fault current for phase 'a’ will be

A 3-phase generator rated at 110MVA, 11 kV is connected through circuit breakers to a transformer. The generator is having direct axis sub-transient reactance, transient reactance 26% and synchronous reactance =130%. The generator is operating at no load and rated voltage when a three phase short circuit fault occurs between the breakers and the transformer. The magnitude of initial symmetrical rims current in the breakers will be

A 500 MVA, 50 Hz, 3-phase turbo-generator produces power at 22 kV. Generator is Y-connected and its neutral is solidly grounded. Their sequence reactance are  and . It is operating at rated voltage and disconnected from the rest of the system (no load). The magnitude of the sub-transient line current for single line ground fault at the generator terminals in p.u will be

A 50 Hz alternator is rated 500 MVA, 20 kV, with  per unit and  per unit.  It supplied a purely resistive load of 400 MW at 20 kV.  The load is connected directly across the generator terminals when a symmetrical fault occurs at the load terminals. The initial RMS current in the generator in per unit is

For a fault at the terminals of a synchronous generator, the fault current is maximum for a

The positive sequence component of the voltage at the point of fault in a power system is zero for a 3 – phase fault. (True=1/False=0)

In a power-system, the 3-phase fault MVA is always higher than the single-line-to-ground fault MVA at a bus. (True=1/False=0)