1. HOG - Also called Head-on Generation. In this case, the power from the OHE is directly used to supply power to the coaches with suitable stepping down of the OHE voltage. Traditionally, a pantograph is provided on a power car coach for that purpose. As our Indian OHE cannot support high speed with multiple pantographs, this facility is provided in locomotives. Very high efficiency.
EOG - End-on Generation. Two power cars at each end have DG (diesel generator) sets in which the thermal energy generated by the combustion of fossil fuel (diesel) is utilised to generate electricity. Although it pollutes the surroundings by the emission of smoke, its efficiency is considerably higher than that of self generation.
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2. WAP7 : Co-Co three phase passenger locomotive. Continuous power at wheel rims is 6120 HP (some sources say 6000 HP) in 71-140 km/h range. Maximum is said to be 6350?? (Only IRFCA mentions this value). A moderate speed high power loco which happens to be a re-geared version of WAG9. Design speed is 140 km/h and operated up to to 130 km/h.
WAP4 : Co-Co conventional (tap changing) passenger locomotive. Continuous power at wheel rims is 5060 HP (some sources say 5000 HP). CLW was manufacturing them from early 90's till December 2015. Max speed is 140 km/h (same as WAP7).
WAP5 : Bo-Bo three phase high speed passenger locomotive. Maximum power is 6000 HP according to IRFCA, continuous power at wheel rims is 5440 HP (some sources mention 5500 HP). Two variants are available - gear ratio of 67:35:17 for 160 km/h (max 180 km/h) & gear ratio of 59:35:19 for 200 km/h (max 225 km/h). First 3-phase locomotive in India and first 10 were imported from ABB/AdTranz unit in Switzerland and one (# 30008) was received in kit form (sorry IRFCA, you got this one wrong). No loco got damaged in any fire accident 😝😝

3. P7 and P7i :-
'i' stands for IGBT (Insulated Gate Bipolar Transistor). Earlier, GTO (Gate turn-off) thyristors were used as inverters and auxiliary converters which had the following issues :-
(I) Higher switching losses.
(II) Obsolescence
(III)...
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Group drive resulting in poor slip/slide control
(IV) More noise
(V) Larger size and weight
(VI) Lower switching frequency
IGBT is much smaller and lighter. Hence, it was possible to add more components without exceeding weight and space limitations. Older locos featured one inverter per bogie (set of two or three axles), newer locos have one per every traction motor. As a coolant, a mixture of water (70%) and ethylene glycol (30%) is used which is largely inorganic unlike the oil that is used in GTO. Switching frequency is 3-4 times higher than that of GTO and the absence of complex snubber circuits reduced the switching losses by ~50%. Noise could also be reduced by ~10%.
Newer P7's have IGBT based traction and auxiliary converters. Some sheds include the suffix 'i' (like ELS/BIA) while most of them don't.
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4. P5 and P5i :-
Same as P7 and P7i.
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5. Power conversion in 3-phase locomotives :-
This happens in 3 stages :-
(i) Input Converter : This rectifies the AC from the OHE to a specified DC voltage using GTO (gate turn-off) thyristors. A transformer section steps down the voltage from the 25kV input. It has filters and circuitry to provide a fairly smooth (ripple-free) and stable DC output, at the same time attempting to ensure that a good power factor presented to the electric supply. The power factor is around 0.9.
(ii) DC Link : This is essentially a bank of capacitors and inductors, or active filter circuitry, to further smooth the DC from the previous stage, and also to trap harmonics generated by the drive converter and traction motors.
(iii) Drive Converter : This is basically an inverter which consists of three components (GTO/IGBT) that switch on and off at precise times under the control of a microprocessor (pulse-width modulation). The three components produce 3 phases of AC (120 degrees out of phase with one another). Additional circuitry shapes the waveforms so that they are suitable for feeding to the traction motors. The microprocessor controller can vary the switching of the thyristors and thereby produce AC of a wide range of frequencies and voltages and at any phase relationship with respect to the traction motors.