The current flowing to a balanced delta connected load through line 'a' is 15 A when the conductor of line 'b' is open. With the current in line 'a' as reference, compute the symmetrical components of the line currents. Assume the phase sequence of 'abc'.

For the given question,
Ib = 0
Ia = -Ic
Calculation:

Ia1 = 1/3*(Ia + a*Ib + a²*Ic)
= 1/3*(15 + 15∟-120°)
= 7.5 + j*4.33 A
Ia2 = 1/3*(Ia + a²*Ib + a*Ic)
= (1/3)*(15 + 15∟120°)
= 7.5 - j*4.33 A
Iao = (1/3)*(Ia + Ib + Ic)
= (Ia - Ia)
= 0

The single line diagram of the simple system is shown in figure the corresponding reactance diagram is

Simple logic of reactance diagram is that simplified equivalent circuit of power system in which the various components of power system are represented by their reactance.
The reactance diagram can be obtained from impedance diagram if all the resistive components are neglected.
So, from figure each of the component is represented by its reactance.

A transformer designed for operation at 60 Hz supply is working at 50 Hz supply system without changing its voltage and current ratings. Compared to the full load efficiency at 60 Hz, the full load transformer efficiency at 50 Hz.

Voltage dips cannot be caused by which of the following:

A voltage dip is a short temporary drop in the voltage magnitude in the distribution or customer's electrical system.
Common causes of voltage sags and under voltages are short circuits (faults) on the electric power system, motor starting, customer load additions and large load additions in the utility service area.
Sags can cause computers and other sensitive equipment to malfunction or simply shut off.

Three resistances 750 Ω, 600 Ω and 200 Ω are connected in parallel. The total current is 1 A. Determine the voltage applied.

Equivalent resistance of the circuit, Req = (750|| 600 || 200)
Req = 125 Ω
Current from source = 1 A
Source voltage = 1*125
Source voltage = 125 V

A grid interactive solar system:

A grid-interactive photovoltaic (PV) system uses solar energy to generate renewable power that charges batteries for use during power failures and feeds power into the electricity grid.
Grid-interactive systems are based on their grid-tied and off-grid counterparts.