Synchronous Machines
Synchronous generators or alternators are used to convert mechanical power derived from steam, gas, or hydraulic-turbine to ac electric power
Synchronous generators are the primary source of electrical energy we consume today
Large ac power networks rely almost exclusively on synchronous generators
Synchronous motors are built in large units compare to induction motors (Induction motors are cheaper for smaller ratings) and used for constant speed industrial drives
Operation Principle
The rotor of the generator is driven by a prime-mover
A dc current is flowing in the rotor winding which produces a rotating magnetic field within the machine
The rotating magnetic field induces a three-phase voltage in the stator winding of the generator
Equivalent Circuit_1
The internal voltage Ef produced in a machine is not usually the voltage that appears at the terminals of the generator.
The only time Ef is same as the output voltage of a phase is when there is no armature current flowing in the machine.
There are a number of factors that cause the difference between Ef and Vt:
The distortion of the air-gap magnetic field by the current flowing in the stator, called the armature reaction
The self-inductance of the armature coils.
The resistance of the armature coils.
The effect of salient-pole rotor shapes.
Three-phase equivalent circuit of a cylindrical-rotor synchronous machine
The voltages and currents of the three phases are 120o apart in angle, but otherwise the three phases are identical.
Determination of the parameters of the equivalent circuit from test data
The equivalent circuit of a synchronous generator that has been derived contains three quantities that must be determined in order to completely describe the behaviour of a real synchronous generator:
The saturation characteristic: relationship between If and f (and therefore between If and Ef)
The synchronous reactance, Xs
The armature resistance, Ra
The above three quantities could be determined by performing the following three tests:
Open-circuit test
Short-circuit test
DC test
Open-circuit test
The generator is turned at the rated speed
The terminals are disconnected from all loads, and the field current is set to zero.
Then the field current is gradually increased in steps, and the terminal voltage is measured at each step along the way.
It is thus possible to obtain an open-circuit characteristic of a generator (Ef or Vt versus If) from this information
Short-circuit test
Adjust the field current to zero and short-circuit the terminals of the generator through a set of ammeters.
Record the armature current Isc as the field current is increased.
Such a plot is called short-circuit characteristic.
DC Test
The purpose of the DC test is to determine Ra. A variable DC voltage source is connected between two stator terminals.
The DC source is adjusted to provide approximately rated stator current, and the resistance between the two stator leads is determined from the voltmeter and ammeter readings
Determination of Xs
For a particular field current IfA, the internal voltage Ef (=VA) could be found from the occ and the short-circuit current flow Isc,A could be found from the scc.
Then the synchronous reactance Xs could be obtained using
Short-circuit Ratio
Another parameter used to describe synchronous generators is the short-circuit ratio (SCR). The SCR of a generator defined as the ratio of the field current required for the rated voltage at open circuit to the field current required for the rated armature current at short circuit. SCR is just the reciprocal of the per unit value of the saturated synchronous reactance calculated by
Parallel operation of synchronous generators
There are several major advantages to operate generators in parallel:
Several generators can supply a bigger load than one machine by itself.
Having many generators increases the reliability of the power system.
It allows one or more generators to be removed for shutdown or preventive maintenance.