(1) Field of the Invention
The present invention relates to an electrical architecture for an aircraft, to an aircraft, and to a method performed by the electrical architecture. 
The present invention thus relates to the field of aircraft, and in particular aircraft having a rotary wing.  More particularly, the invention relates to aircraft having a so-called “hybrid” power plant for driving rotation of at least one rotor of a rotary wing via a main power transmission gearbox of the aircraft.  Such a main power transmission gearbox is referred to more simply below as the “power transmission gearbox”. 
(2) Description of Related Art
A “hybrid” power plant designates a power plant comprising at least one fuel burning engine and at least one electrical member for driving movement of a power transmission gearbox or for transmitting power to the engine during transient engine power stages. 
The electrical member may be an electric motor, i.e. an electrical device that performs a motor function only. 
Nevertheless, the electrical member may also be an electrical machine, i.e. an electrical device that performs both a motor function and an electricity generator function.  An electrical machine can thus operate either in an electric motor mode in order to drive rotation of mechanical members, or else in an electricity generator mode in order to generate electricity.  An electrical machine is sometimes said to be “reversible” insofar as the electrical machine can switch from electric motor mode to electricity generator mode, and back again. 
An electricity generator is an electrical device that performs solely a function of generating electricity. 
The term “electrical member” is used herein to cover an electric motor, an electrical machine, and an electricity generator. 
In addition, a rotary wing aircraft may have one or more rotors for providing lift and possibly also propulsion, referred to as “main” rotors.  Such a rotary wing aircraft may have one or more auxiliary rotors for controlling movement in yaw of the aircraft, for example. 
The power plant conventionally then includes at least one fuel burning engine for driving rotation of the main rotors via the power transmission gearbox. 
Such an engine may co-operate with an electrical machine.  The electrical machine then operates in electric motor mode on starting the aircraft in order to start the engine. 
For example, the engine may be a turboshaft engine having a gas generator.  The first electrical machine then serves to rotate the gas generator on starting. 
Once the engine has started, the first electrical machine can then be operated in electricity generator mode in order to supply electricity to the on-board network of the aircraft. 
The electrical machine then transforms electrical energy into mechanical energy in order to start the engine in a first function, and it transforms mechanical energy coming from the engine into electricity for the on-board network in a second function. 
Document FR 2 993 243 describes a hybrid architecture for supplying mechanical power to a rotor that is managed from the on-board network of a rotorcraft. 
According to that Document FR 2 993 243, the electrical machine connected to an engine can assist the engine in flight.  Such assistance is provided by delivering mechanical energy to the engine from the electrical machine. 
Furthermore, rotorcraft are incorporating more and more electrical functions to replace hydraulic systems, and for example to perform new functions.  A manufacturer tends to make use of electrical members, given the flexibility made available by electricity in terms of conversion or transformation.  Furthermore, electricity is available on board the aircraft via generators or batteries while the aircraft is in flight, or via batteries and electrical connectors for use on the ground. 
Nevertheless, the various electrical members of an aircraft have different requirements, and they are connected in different manners. 
Some electrical members operate on low-voltage direct current (DC) electricity, or else on high-voltage direct current electricity, or indeed on high-voltage alternating current (AC) electricity.  For example, direct current may be distributed at a low voltage of about 28 volts (V) direct current.  Direct current can also be distributed at a high voltage of about 270 Vdc, and alternating current may be distributed in a three-phase voltage system with a phase voltage of 115 Vac and a frequency of 400 hertz (Hz). 
A voltage higher than 60 V may be referred to as “high” voltage in contrast to a voltage lower than 60 V, which is referred to as “low” voltage. 
Certain electrical members produce electricity, whereas other electrical members consume electricity. 
Under such circumstances, a manufacturer tends to provide an electricity converter upstream for each electrical member in order to adapt the characteristics of the electricity network to the operation of the member. 
Except during stages in which the electrical member is operating, such converters represent on-board deadweight that has a negative influence on the weight breakdown of the aircraft. 
The technological background includes in particular the following documents: CN 102 201 744; EP 2 122 271; EP 0 911 515; IN 2010/MU03358; JP 2011/004507; U.S. Pat.  No.  6,954,366; U.S. Pat.  No.  7,050,312; U.S. 2008/0123375; U.S. 2010/0165673; U.S. 2013/0039104; WO 2012/137210; U.S. Pat.  No.  7,923,865; EP 2 404 775; FR 2 961 767; EP 2 703 292; EP 2 327 625; and WO 2014/022316.