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THE SYNCHRONOUS GENERATOR (ALTERNATOR)

Synthetic notes

The synchronous generator (alternator) is a rotating electric machine which converts mechanical energy from the turbine into electric energy.

The turbine converts some kind of energy (steam, water, gas) into

mechanical energy

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The key principle is based on the electromagnetic induction phenomenon: a relative rotation between a magnetic inductor flux and a system of connected induced conductors. PERMANENT MAGNET GENERATOR GENERATOR WITH EXCITATION

N

S

N

S

+

Iex

In order to realize the relative movement between inductor and induced conductors the generator is constituted by a fixed part “the stator” and a rotating part “the rotor”. When the inductor conductor (rotor winding ) excited is turning at a fixed speed, an induced alternating voltage at the armature winding terminals (three phase winding) is produced: its frequency depends on the inductor rotating speed and its value ( RMS value) is proportional to the flux produced by the inductor according to the relationship

E =k Φ n

The generator speed is proportionall to the frequency according to the following relationship: N = 120 X F ________ P N is the speed (rpm) , F is the frequency ( Hz) and P the number of poles.

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A synchronous generator representation

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Current anf Power If the external circuit of the generator is closed on a load → a current will circulate in the armature windings. According to the phase angle between voltage and current, the generator can produce active power P and/or reactive power Q, in any case the rated power (nominal) is expressed in apparent power (MVA) which only depends from the product VOLTAGE X CURRENT. An=√3*Vn*In and it means the generator power plate in KVA. The nominal current value depends on the section of the conductors and on the cooling system and is equal to the maximum continous operating current. The nominal voltage value depends on the number of the induced conductors and on the dimensions of the magnetic circuits ( maximum flux).

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Excitation The inductor circuit (called field winding or rotor winding or field circuit) of the synchronous generator is fed by a continuous voltage whose value is inclusive into a range between 100V and 500 Volt (Excitation voltage). The excitation power can vary from around 0.3 to 1.5% the rated power according to the number of poles. The energy's source for the excitation is almost exclusively constituted by rectifier device that converts energy ( AC / DC converter )

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The excitation system These are the most important functions of the excitation system :

• Supply the necessary energy to fed generator field circuit converting it from an external source.

• Regulate automatically, through the control of the excitation, the

voltage at generator terminals .

• Limitate the generator operation inside its operating ranges;

• Optimize the dynamics of generator connected to the net

• Regulate the excitation current in the manual mode

• De- excite the generator after the stop request. (fast de- excitation)

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The generator “NO LOAD CHARACTERISTIC” : It represents the generator voltage at no load as a function of the excitation current

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Generator : mathematical representation of NO LOAD characteristic

0 175 350 525 7000

3.738

7.475

11.213

14.95Curva di saturazione a vuoto approx.

Ifd (Amp)

Vm

(kV

)

Vnom

IfolIfd0

.

Ifol = no load air gap current ( A) Ifd0= no load current (A) Vnom = Nominal voltage

.

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REACTIVE POWER EXCHANGE WITH THE GRID We suppose that the generator is connected to a infinite power grid (that has a voltage constant value= Vr through an uncoupling reactance X

• Case of inductive reactive power production to the grid. In this case, starting from an equilibrium condition whit Vr = Va (the voltages are represented as two vectors) and with the generator at no load, the value of the excitation current is increased ⇒ the generator voltage Va is greater than Vr: ⇒ The generator produces inductive reactive power to the grid ( Q >0)

Va ΔVt

G

Ia Va Vr

X ΔVt

+ Vr_

Grid

Ia Inductive load

• Case of absorption of inductive reactive power from the grid (or of production of capacitive reactive power to the grid)

In this case, starting from an equilibrium condition whith Vr = Va and with the generator at no load, the value of the excitation current is decreased ⇒ the generator voltage Va is smaller than Vr: ⇒ The generator absorbs inductive reactive power from the grid ( Q < 0)

G X

Grid

Ia Va Vr

+_ Va

ΔVt

Ia

ΔVt Vr

Capacitive load

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THE CAPABILITY DIAGRAM: is a diagram showing the operating limits of the generator

Max apparent

power of the generator

Active power Inaccessible area due to

the turbine power limitation

Inaccessible area due to stability and overheatin

the stator core ends g of

.

Nominal work. point

Max turbine power Inaccessible area

due to the rotor overheating

Under excitation limit Max rotor current limit

ϕ

0Reactive power

Under-excitation Over-excitation LEADING LAGGING

Motor

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THE STATIC EXCITATION SYSTEM A first classification of the EXCITATION systems is made on the basis of the power section that fed the generator field: the excitation system can be therefore static or rotating

the system is called called static because the field is fed from a static source without rotating parts, and statics rectifiers are used to product DC current. The static excitation system directly acts on the generator field using revolving contacts (brush-rings)

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STATIC EXCITATION SYSTEM

…….Actuallly the rotating excitation system ( brushless generator) are rarely used .

+

Excitation board

Generator Three phase

supply

Excitation transformer

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BRUSHLESS EXCITATION ROTATING DIODES

BRIDGE

EXCITATION CUBICLE

--

+

BRUSHLESS AUX

GENERATOR

PERMANENT MAGNET

GENERATOR

MAIN GENERATOR

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THE CONTROL INTERFACES Besides the power interfaces (see above) from the excitation transformer and to the rings, the excitation system must exchange signals with the external plant. Therefore a lot of control interfaces and therefore a lot of auxiliary connections with other plants equipments plant are realized

automation

automation

protections protections

plant

STATIC Excitation

plant

Plant means in general: interlocks, consents, commands to switchtes etc…

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ADVANTAGES OF THE STATIC EXCITATION SYSTEM RELIABILITY OF THE STATIC COMPONENTS REDUNDANCE HIGH DINAMIC RESPONSE LOW LOSSES FAST DE EXCITATION SHORT TIME TO REPAIR DAMAGES SMALL DIMENSION

……. while the only advantage of a rotating excitation system is the absence of revolving contacts ( not accepted in particular environmental conditions )

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Static excitation system: power and control sections

Gen brk

~

3

1

3

Control section

Line

Excitation feedback

G

Power section

Generator feedback

Line feedback (option)

Output signals

Commands

Excitation system

Power supply

The system is composed by 2 sections functionally separated

- the power section that is directly connected to the rotor field generator.

- the control section that guarantees the regulation of the main electrical greatness beginning from the measures of the greatness of the same generator.

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Static excitation system : Power and control sections details

G~crowbar

converter

==

controlsection

==

==

alpha

lineconverterpower supply

52G

power section

excitationcurrent

excitationvoltage

crow barcurrent

machine voltage

machine current

line voltage

synchronous voltages