(™Never an easy subject but I'll give it a go ?)
AC POWER
Modern transport aircraft use 400 hertz alternating current to power much of their electrical equipment for several reasons.  Voltages are easily converted from high to low or low to high.  The higher frequencies used in aircraft electrical systems allow components to be smaller, but develop the same power as the 60 hertz devices normally found in the home and industry.

Power for the electrical system is supplied by the three engine driven generators, and as a backup on the ground, APU generator, or an external power source.  Normally all of the electrical power in the airplane is produced by the engine driven AC generators.
An AC generator must be rotated at a constant speed throughout the operating RPM range of the engine.  This is necessary to maintain the appropriate frequency output of the generator.  A generator drive unit, called a constant speed drive, or CSD, which is a hydro mechanical device between the engine drive pad and the generator, accomplishes this function for each generator.  Each generator drive unit contains its own integral oil supply and pumps.  So that the oil pressure within the unit can be monitored, a low oil pressure light for each unit is included on the flight engineers electrical panel.  The amber light will come on if the oil pressure in a unit is too low. To cool the generator drive oil. an air cooled heat exchanger is installed in each drive system.  Engine fan stage bleed air continuously provides the required cooling air flow for the heat exchangers.  There is an oil temperature gauge for each generator drive unit.  These gauges have two scales.  Oil "IN" temperature is indicated on the lower scale, which is calibrated from 40 degrees to 160 degrees Celsius.  The upper scale is calibrated from 0 degrees to 30 degrees Celsius, and indicates the rise in the temperature of the oil as it passes through the generator drive unit.  Above each temperature gauge is a toggle switch for selecting either the "IN" or "RISE" temperature scale.  The "IN" temperature is sensed downstream of the oil cooler as it enters the generator drive.  This gives an indication of oil cooler efficiency.  The rise temperature is the difference between oil in and oil out temperatures. and is an indication of heat generated within the drive unit in rotating the generator. or the workload of the generator drive unit. The rise temperature caution range is 20 degrees to 30 degrees C.  If the "IN" temperature reads between 127 degrees and 140 degrees C the operating time limit is two hours.  With the "IN" temperature between 140 degrees and 160 degrees C. Operating time limit is 50 minutes.

To the left of each low pressure light is the disconnect switch for the associated generator drive unit.  These switches are red guarded and safety wired.  The switch under the guard has two positions, Normal and Disconnect.  The Disconnect position is momentary contact and spring loaded to the normal position.  The generator drive unit can be disconnected by opening the guard and moving the disconnect switch to the disconnect position.  This action disengages the mechanical coupling for the generator drive unit from the engine drive pad.  Also this action trips the associated generator breaker, breaking the electrical connection between the generator and its load bus.  The generator drive can only be reconnected on the ground by maintenance personnel.

The generators each produce three phase, 400 hertz. 115 volt AC electrical power.  The voltage produced by a generator depends on there being a magnetic field in the generator.  Current flowing from the voltage regulator produces this field.  The generator's output voltage is sampled by the voltage regulator which adjusts the current to the field so that the generator's output, when not in paralle, will be 115 volts. plus or minus 5 volts.

Under certain abnormal or emergency conditions it is necessary to reduce the voltage output of the generator to a minimum.  The field relay performs this function by interrupting the current flow from the voltage regulator to the generator.  With the field relay open, only residual voltage of 10 to 17 volts will be produced if the generator is rotating.  Each generator has a field relay controlled by the field switch on the electrical panel.  The light next to the field switch will be on when the field relay is open.

When the generator is producing full voltage, it may be connected to its load bus by closing the generator breaker. 
The load bus is a distribution point for the power produced by the generator.  Heavy load items, such as air conditioning pack fans, galleys, and hydraulic "B" pumps are powered directly from the load busses.  Power is also sent to the various circuit breaker panels to power electrical equipment throughout the airplane.  Each generator breaker is closed with a switch labeled GEN.  When a generator is connected to its load bus through the generator breakers the generator breaker light next to that generator switch will be out.  When the generator is not connected to its load bus. the light will be on.

To equalise the loads on the generators, and to protect the electrical system if one generator should fail, the three load buses are connected together by a third set of relays.  These bus tie breakers are connected to a common circuit referred to variously as the "tie bus" or the synch bus.  The tie bus shuttles power among the three AC load buses as power requirements change, but only when the bus tie breakers are closed.  If a generator should fail. the tie bus will power the AC load bus associated with that generator through its bus tie breaker.

Since we are dealing with alternating current, we must be certain that the voltages of the various sources we are joining in parallel are "in phase".  By this we mean that the positive and negative portions of the two voltages that we are connecting occur at the same time.  If we joined the voltage sources when they were not "in phase", serious damage could be done to a generator. In practice, the bus tie breakers are left closed so a single power source could power all three AC load buses.  To protect against connecting a generator out of phase, automatic protective circuits prevent the generator breakers from being closed unless the associated generator is in phase with the other generators already powering the system.  The bus tie breakers do not have this protective feature.

Some AC powered items are considered to be more critical to safe flight.  These are powered through the essential AC bus, which can be supplied by any of the three generators directly without the necessity of its generator breaker being closed. The selected generator's field relay must be closed so that the generator will be able to supply electrical power.  The essential power selector switch on the upper right side of the electrical panel controls the selection of a power source for the essential AC bus.  Normally generator three supplies the essential power, with the other two generators available.  The essential AC bus is also powered when external power or the APU is supplying the airplane. Preference for essential sources are  eng 3, 1, 2 in that order. It's due to the loads on each bus, 3 being the lightest load, 2 the heaviest

 Failure of the selected essential AC power source is shown by red warning lights.  There is a steady red light on the essential power selector panel and a flashing red light, labeled "WARN - PUSH TO RESET", on the pilot's center instrument panel.  The flashing red light can be extinguished by pressing the light cap, but the steady red light will not go out until the essential AC bus is powered from another source.

Certain of the captain's instruments are protected even further.  They are powered from the standby AC bus.  As long as the essential AC bus is powered, it powers the standby AC bus.  In the event of a failure of essential AC power in flight the standby bus will automatically be powered by a static inverter.  The static inverter is powered from the battery bus.

On the ground the standby AC bus may be powered from the static inverter, howeve,. it is necessary to select "STANDBY" with the essential power selector to do so.  This is normally done when standby AC power is required, but AC power is not available from the airplane's generators or an external power source.  The essential power selector must be depressed before it can be rotated to the "STANDBY" position.  When the selector is moved to the standby position on the ground or in flighty the essential AC bus will no longer be powered even if it were powered previously.

The last major AC bus is the AC transfer bus.  Normally this bus is powered from the number 3 AC load bus, but under certain conditions can be powered from an external power source.  Distribution of electrical power is through the various circuit breaker panels.  The lower portion of the P 6 panel is divided into three sections.  P6-11, P6-12, and P6-13.  These sections are associated with the three AC load buses. 1, 2. and 3 respectively.  On each panel is a power light which glows continuously when the associated bus is powered. The rest of the P6 panel and the P18 panel an the left side of the cockpit contain the systems sections with the circuit breakers for those systems. 
The upper portion of the P 6 panel is divided from top to bottom into four main sections, P6-1, P6-2, P6-3. and P6-4. The other main circuit breaker panel is P-18, which is located on the left sidewall above the first observer's seat.  The P18 panel is further subdivided into four main sections numbered from bottom to top.  In general these panels are: P 18-1. radio equipment; P18-2, light instruments, autopilot, and interphone; P18-3. passenger accommodation and P18-4. cockpit lighting, service lights, and exterior lighting.  Isolated groups at circuit breakers related to lighting are installed in several other cockpit locations.  There are also some circuit breakers which are inaccessible to the crew located in the electronic equipment compartment. 

On the right side of the flight engineers panel is an AC meters selector.  Each of the three engine driven generators, the APU generator, or the external power can be sampled as well as the voltage and frequency on the synch bus.  When a generator is rotating with its field relay open, it will produce 10 to 17 volts residual voltage.  This voltage can be read an the voltmeter lower scale by selecting that generator and pushing the residual volts button.  When the generator field relay is closed  the generator field is energised by the voltage regulator.  Now normal voltage can be read an the top scale of the voltmeter.  It should read 115 volts plus or minus 5 volts. Above the AC meters selector is a frequency meter.  This meter will indicate the frequency of the power source selected by the AC meters selector.  When selected to generators 1, 2, or 3. the frequency desired is 400 hertz plus or minus 9 hertz.  If the frequency is not 400 hertz, it can be adjusted using the frequency knob on the left panel.  The knob for each generator allows adjustment of the frequency within about a 15 hertz spread.  Frequency will be indicated only when the generator field relay is closed. 

Just above the meters selector are two white lights labeled "SYNCHRONIZED WHEN LIGHTS ARE OUT".  These lights are used when connecting generators in parallel with the bus tie breakers.  Automatic paralleling protection is provided when the generators are brought an the line normally since the generator breakers are used.  If the generator breakers cannot be used because an abnormal or emergency procedure requires the bus tie breakers to be used, the manual paralleling procedure outlined here must be used.  After one generator is connected to the synch bus. selection of another with the AC meters selector will cause the synch lights to flash in unison.  They are indicating the synchronisation of the selected generator in comparison to the synch bus.  Before closing the bus tie breaker, which places that generator in parallel with any other on the synch bus, the frequency of that generator is adjusted with its frequency knob to 400 hertz so that the lights are flashing slowly.  When the lights are out. the generator is synchronized and can be safely paralleled.

When an electrical load is sustained by an engine driven generator, the load is indicated in kilowatts on its kilowatt meter.  There is one for each engine driven generator.  The maximum continuous load for a single generator that is not operating in parallel is 36 kilowatts.  It can sustain an overload of 54 kilowatts for 5 minutes.  Two generators operating in parallel are limited to 54 kilowatts total load, and 3 generator in parallel may be operated continuously with a total load of 102.5 kilowatts. 

The loads on paralleled generators should be nearly equal, indicating that the generators are sharing the loads equally. Another electrical quantity which can be read on the meters is kilovolt-amperes reactive, or KVARS.  The KVAR button is shown in yellow.  When the KVAR button is pushed and held, it changes the three meters to read kilovolt amperes reactive. A measure of reactive power.  All three meters should show the same readings for reactive power.

When certain electrical system faults occurs lights an the electrical fault annunciator panel on the flight engineer's auxiliary panel will indicate the type of fault and the system involved.  The reset button on the panel is used to put out the annunciator lights when required.  The test button is used to test the annunciator lights.

OTHER POWER SOURCES
The system may be powered by the APU generator which in identical to the engine driven generators but is geared directly to the APU accessory drive.  When the APU is operating at 100% RPM, the APU generator will be providing 400 hertz power.  The controls for the APU generator are located on the flight engineers auxiliary panel.  There is a field switch and a generator breaker switch.  Both of these switches are three position lever lock switches.  The amber light associated with the field switches is a field off light.  The amber light associated with the generator breaker is a generator circuit open light.
The AC ammeter located on the APU control panel indicates the AC load on the APU generator in amps.  This ammeter will also indicate external power load in amps if an external power unit is being used.  The maximum electrical load when using the APU generator is limited to 165 amps.  APU voltage and frequency can be read on the AC meters with APU selected. When the APU is running at normal speed and its generator field relay is closed, closing its generator breaker will connect the APU generator directly to the airplane's synch bus.  The individual load buses will be powered from the synch bus if the bus tie breakers are closed.  In normal operation the bus tie breakers are left closed and the transfer of power sources is done with the generator breakers, or in the case of external power, the external power contactor.  With all bus tie breakers closed, AC buses 1,2, and 3 are now powered by the APU generator. 

An external power unit may be used to provide electrical power to the airplane systems.  An AC connected light on the flight engineers electrical panel comes on when external power is plugged into the nose of the airplane. This light signifies that power is available but it does not show whether the power is actually energising the airplane's AC buses.  By selecting external power with the AC meters selector, the voltage and frequency of the external power can be monitored.  Approximately 115 volts and 400 hertz should be indicated before external power is accepted. The external power source can be connected to the airplane electrical system by means of the external power switch.  The switch is held in the ON position by a solenoid.  A temporary loss AC power will allow the switch to return to the center OFF position.  Moving the switch to the ON position will connect the external power to the synch bus.  As with APU power, the bus tie breakers must be closed for the power from the synch bus to reach the three AC load buses.

ESSENTIAL POWER
Essential power can be supplied on the ground when either the APU or an external power source is powering the AC load buses.  Power from the number 3 AC load bus is tapped off and supplied to a pair of relays which, when the proper conditions are met, will allow the essential power selector to supply AC power to the essential AC bus.  There are certain requirements which must be met before the essential AC bus can be powered by selecting APU or external power on the essential power selector.  When the number 3 AC bus is powered from any source. the essential AC bus will be energized from that bus with APU selected, only if the APU is running and its field relay is closed.

To power the essential AC bus from the external power position. External power must be powering the buses.  With the external power switch off, the external power position of the essential power selector is not powered.  The bus tie breaker between the synch bus and number 3 load bus must be closed to power the number 3 load bus and feed the external power position.
The external power switch has a third position which allows certain outlets and lights in the cabin to be powered without energising any other buses in the airplane.  This position of the external power switch is labelled GROUND SERVICE.  It was mentioned earlier that the AC transfer bus is normally powered from number 3 AC load bus.  The AC transfer bus provides the power for the outlets and lights for the passenger cabin.
If it is desired to energise these circuits without the necessity of powering any other buses in the airplane. The ground service switch position is used.  With external power plugged in. moving the external power switch to ground service powers the AC transfer bus without supplying power to any other AC bus.
To prevent damage to electrically powered airplane components, automatic protective features are incorporated into the electrical system.  Control of the AC electrical system is provided by latching relays. Which require electrical power for opening or closing.  This power is supplied by three units called generator control panels.  These control panels must be powered at all times to provide indicator lights, remote control from the second officer's panel, and protective circuits for its generator.  There are two power sources for each control panel.  If its generator is not operating, the battery powers the control panel with 24 volts DC.  The battery switch must be on to provide this power. If the generator associated with the control panel is operating with its field relay closed. It will power its own control panel through a rectifier. Changing 115 volt AC to 28 volt DC.
One feature of the protective circuitry is that not more than one source of power may be connected to the airplane electrical system at the same time.  For examples if the airplane generators are powering the airplane, the APU generator breaker open. If the APU generator breaker switch is moved to close. The airplane generator breakers open before the APU generator breaker closes.  A similar sequence occurs when connecting an external power source.  Conversely if an airplane generator breaker switch is moved to the close position, the APU generator breaker or the external power contactor will open and then the engine driven generator breaker will close.  This feature is commonly referred to as the "break before make" protection.
The generator control panel contains regulating control, and protection circuits to insure that proper power is delivered to the system.  The panel maintains constant voltage output from an isolated generator under varying loads and maintains equal sharing of the kilowatt and KVAR loads when the generators are operating in parallel.  Protective sensing circuits will cause the bus tie breakers, a generator field relay and generator breaker together. Or a generator breaker alone to open due to system or generator malfunction.
In general, the bus tie breaker may be opened by its control switch, it will trip automatically for an excitation fault or a phase unbalance.  An excitation fault is usually caused by a failure of the voltage regulator when the voltage regulator calls for too much or too little current to the field windings of a paralleled generator, or if the current is unstable.  When a high / low or unstable excitation current is sensed on a generator, the protective system opens the bus tie breakers an that generator to isolate that generator.
The synch bus is also monitored by the protective circuits.  A short or ground on one of the three phases on the synch bus will show an imbalance in loads on the three phases.  This fault, called a phase unbalance, will cause all three bus tie breakers to open. Isolating the generators from the faulty synch bus.
A generator field relay will open due to tripping the generator field switch,  pulling the engine fire switch, voltage faults, or a differential fault.  An excitation fault may be followed immediately by a voltage fault.  Once the generator is no longer paralleled, a voltage regulator fault can then be sensed as a high or low voltage.  The generator field will trip along with the generator breaker.

The output of each generator is sensed at the generator and also at the load bus.  If a difference exists between the two. Then there is a ground or short in the feeder lines between the generator and the load bus.  This fault, called a differential fault or sometimes a feeder fault. It is serious enough so that when it trips the field relay and generator breaker, the field relay is locked out and may not be reset without resorting to an abnormal procedure to defeat the lockout.
A generator breaker will open due to tripping of the generator field relay by any means. Tripping of the generator control switch, or disconnecting the generator drive unit.  The generator breaker will also open due to a generator drive underspeed or overspeed, closing the APU generator breaker. Or by turning the external power switch to ON. A tendency for the constant speed drive on a generator to overspeed or under speed, if not corrected by that generator's load controller, will be indicated by a high or low kilowatt load if the generators are paralleled.  The KW loads should be monitored to determine if a load controller is  Failing. If the CSD overspeeds, the generator will assume a greater and greater portion of the airplane's electrical load until the generator is tripped automatically by its overload protection circuits.  This will isolate the generator since its bus tie breaker will open.  An unparalleled or isolated generator is protected from over or underspeed by a speed switch, which opens the generator breaker and prevents essential power from being supplied by that generator.
An electrical fault annunciator is located an the flight engineers auxiliary panel.  The series of five lights in the upper part of the first column will indicate an electrical fault in generator one's system. Column two's upper section for generator two, and Column three's upper section for generator three.  The bottom center light, phase unbalance, indicates a fault which affects all three systems. Under the annunciator panel is a test button for testing its lights.  On the left is a reset button which will put out any light remaining an after a fault has been corrected, with the exception of a differential fault.
DC POWER
The main DC electrical system is powered by transformer rectifiers.  These TR units convert 115 volts AC to 28 volts DC. The number 1 TR unit converts AC power from the number 1 AC load bus.  Likewise the number 2 TR converts power from the number 2 AC load bus.  The essential TR is powered from the essential AC load bus.  This DC power is delivered from the TR units through circuit breakers to the DC buses.  Then the DC buses feed the circuit breakers on the P 6 and P 19 panels to power equipment throughout the airplane. The number 1 and number 2 DC buses are connected by a current limiter to provide backup for loss of either TR 1 or TR 2. Either TR 1 or TR 2 is capable of carrying the load on both DC buses. 

To provide a backup source of power for the essential DC bus, there is a connection to the output from the number 1 TR.  Should the essential TR fail, either TR 1 or TR 2 could carry the load on all DC buses.  A blocking rectifier in the circuit prevents reverse current flow from the essential DC bus to the normal DC buses. The DC volt and ammeters and the DC meter selector are located an the lower right side of the flight engineers electrical panel.  With the DC meter selector in a TR position the voltmeter indicates voltage on the associated DC bus.  Amperage is sensed just downstream of the TR unit, so the ammeter indicates current flow through the TR unit that has been selected.  Thus, the amperage readout on the gauges is from the TR and the voltage from the DC bus.

BATTERY BUSES
The battery and battery transfer buses are normally powered from the essential DC bus.  If the essential DC bus is not powered, and the battery switch is on, the battery and battery transfer buses will be powered from the hot battery bus.  The battery transfer bus is powered any time the battery bus is powered.  The "transfer" designation implies that the source for that bus will change under certain circumstances.  When external power is plugged in, the battery transfer bus will be powered from a TR in the airplane's external power circuitry instead of the battery bus. The hot battery bus is powered by the airplane battery at all times that a serviceable battery is installed.  The hot battery transfer bus is powered any time the hot battery bus is powered.  When the airplane's electrical system is not energised, the hot battery transfer bus is powered from the hot battery bus.  When the essential DC bus is powered, the power source for the hot battery transfer bus becomes the battery bus.
 BATTERY
The Boeing 727 has a 24 volt nickel cadmium battery.  The battery provides an emergency power source for certain radio and instrument systems, and is required for starting the APU.  With the battery switch either on or off, battery voltage is indicated on the voltmeter with the battery selected.  The ammeter will show current flow from the battery as negative amperage and charging current as positive amperage. The battery charger is powered from the 115 volt AC transfer bus and is connected to the hot battery bus to charge the battery.  Whenever the AC transfer bus is powered, the battery charger will be operating. Charging the airplane's battery regardless of battery switch position.  After a high load, such as an APU start,  the battery will accept a high charging current.  The charger will supply this high current until the battery becomes sufficiently charged so that the current drops below a threshold value. At which time the charger will go into a pulsing mode for two minutes.  After the two minute period the charger will drop to a low steady charge that is barely discernible on the ammeter.
SUMMARY
The standby AC bus is powered whenever the essential AC bus is powered. and can be powered from the battery bus through an inverter.  The standby DC bus is powered whenever the essential DC bus is powered.  When the essential AC bus loses power in flight, or the essential power selector is moved to standby. Both the standby AC and standby DC buses will be powered from the battery bus.