The solution of the differential equation, for
$$t > 0,\,\,y''\left( t \right) + 2y'\left( t \right) + y\left( t \right) = 0$$ with initial conditions $$y\left( 0 \right) = 0$$ and $$y'\left( 0 \right) = 1,$$ is $$\left[ {u\left( t \right)} \right.$$ denotes the unit step function$$\left. \, \right]$$,

A full-bridge converter supplying an RLE load is shown in figure. The firing angle of the bridge converter is 120º. The supply voltage $$${v_m}\left( t \right) = 200\pi \sin \left( {100\pi t} \right)\,V,$$$ $$$R = 20 Ω, E = 800 V$$$The inductor L is large enough to make the output current I_L a smooth dc current. Switches are lossless. The real power fed back to the source, in kW, is __________.

A single-phase bi-directional voltage source converter (VSC) is shown in the figure below. All devices are ideal. It is used to charge a battery at 400 V with power of 5 kW from a source V_s = 220 V (rms), 50 Hz sinusoidal AC mains at unity p.f. If its AC side interfacing inductor is 5 mH and the switches are operated at 20 kHz, then the phase shift (δ) between AC mains voltage (V_s) and fundamental AC rms VSC voltage (V_C1), in degree, is _________.