[4-2] Synchronous Generator (1).ppt

UNIVERSITI

KUALA LUMPUR

British Malaysian Institute

Where Knowledge Is Applied

SYNCHRONOUS GENERATOR

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UNIVERSITI KUALA LUMPUR

Topic Contents

British Malaysian Institute

 Operating Principles  Commercial Synchronous Generator  Stationary Field and Revolving Field  Producing the DC Field, Number of Poles and Alternating   

 

Frequency Main Features of Stator and Rotor Field Excitation and Exciters SG at No load and SG under Load Phasor Diagram : Lagging PF, Leading PF Regulation Curves, Synchronization, Torque exerted and Active Power Delivered by Generator.

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SYNCHRONOUS GENERATOR

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 Primary source of all electrical energy.  Largest energy converters in the world  convert

mechanical energy into electrical energy in powers ranging up to 1500 MW.  Operates at synchronous speed  speed of the rotor

always matches supply frequency.

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OPERATING PRINCIPLES

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 The rotor is mounted on a shaft driven

by mechanical prime mover.  A field winding (rotating or stationary)

carries a DC current to produce a constant magnetic field.  An AC voltage is induced in the 3-phase

armature winding (stationary or rotating) to produce electrical power.  The electrical frequency of the 3-phase

output depends upon the mechanical speed and the number of poles. FAZH:SEM1/2013

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COMMERCIAL SYNCHRONOUS GENERATOR

KUALA LUMPUR

British Malaysian Institute

 Commonly, SG built with either stationary or rotating

dc magnetic field.

 There are TWO types :1. Stationary Field SG 2. Revolving Field SG

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STATIONARY FIELD SG

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 Field coil is stationary : Poles on the stator (field winding) are

supplied with DC to create a stationary magnetic field.  Armature coil rotates : Revolving armature cut the DC field.  As it rotates, a 3-phase voltage is induced whose value

depends on the speed of rotation and dc exciting current in the stationary poles.  The frequency of the voltage depends on speed and numbers of poles on the field.  Armature winding on rotor consists of a 3-phase winding

whose terminals connect to 3 slip-rings on the shaft. Brushes connect the armature to the external 3-phase load.  Rating : < 5 kVA. FAZH:SEM1/2013

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STATIONARY FIELD SG

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REVOLVING FIELD SG

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 Field coil rotates  Armature coil is stationary  3-phase stator winding is directly connected to the load

through large slip rings and brushes.  Rating : > 5 kVA.

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REVOLVING FIELD SG

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PRODUCING THE DC FIELD

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 For both stationary and revolving fields, DC supply is normally

produced by DC generator mounted on same shaft as rotor.  Permanent magnets can also produce DC field – used

increasingly in smaller machines as magnets get cheaper.

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NUMBER OF POLES & ALTERNATING FREQUENCY

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 The number of poles on a synchronous generator

depends upon the speed of rotation and desired frequency

Where f = frequency of the induced voltage (Hz) p = number of poles on the rotor n = speed of the rotor (rpm) FAZH:SEM1/2013

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MAIN FEATURES OF STATOR

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British Malaysian Institute

 From an electrical standpoint, the stator of a synchronous

generator is similar to 3-phase induction motor (cylindrical laminated core containing slots carrying a 3phase winding).  Winding is always most preferred connected in a star:1. Reduced voltage between stator conductor and ground stator core  reduce amount of insulation  increase the cross section of conductor  increase current and power output. 2. When SG under load, phase voltage becomes distorted due to third harmonics  thus, star connection effectively cancel distorting line-to-neutral harmonics that appear between lines. FAZH:SEM1/2013

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SYNCHRONOUS GENERATOR : STATOR

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MAIN FEATURES OF ROTOR

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 SG are built with TWO types of rotor :1.

Salient-Pole Rotor (usually driven by low-speed hydraulic turbines) 

2.

Cylindrical Rotor (usually driven by high-speed steam turbines)  



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Used for low speed applications (<300rpm) which require large number of poles to achieve required frequencies (e.g. hydroturbines)

Used for high-speed applications (steam/gas turbines). Minimum number of poles is 2, so for 50Hz the maximum speed is 3000rpm. High speed of rotation produces strong centrifugal forces, which impose upper limit on the rotor diameter.

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SYNCHRONOUS GENERATOR : ROTOR

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SYNCHRONOUS GENERATOR : ROTOR

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FIELD EXCITATION AND EXCITERS

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 The dc field excitation of a large synchronous generator is an

important part of its overall design.  The field must ensure not only a stable ac terminal voltage, but

must also respond to sudden load changes in order to maintain system stability.  Quickness of response is one of the important features of the

field excitation.  Two dc generators are used: a main exciter and a pilot exciter.

 Static exciters that involve no rotating parts at all are also

employed. Brushless excitation systems employ power electronics (rectifiers) to avoid brushes / slip ring assemblies. FAZH:SEM1/2013

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FIELD EXCITATION AND EXCITERS

Stationary field SG (low power)

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Revolving field SG (high power)

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SG : NO LOAD

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No Load Saturation Curve

Electric Circuit of SG (No Load)

 Ix–current to produce a flux in the air gap  Ix gradually increased  small value of Ix, Eo changes proportionally  as the iron begins to saturate, the voltage rises much less FAZH:SEM1/2013

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SYNCHRONOUS REACTANCE

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 XS = Synch. Reactance ;  Its value is 10 to 100 times

greater than R. Notes : Consequently, we can always neglect the resistance unless we are interested in efficiency or heating effects.

 N1 and N2 is not connected as

the load is balanced. FAZH:SEM1/2013

Electric Circuit of SG (With Load)

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SYNCHRONOUS REACTANCE

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 The field carries an exciting

current, produces flux as the field revolves, the flux induces in the stator  each phase of the stator possesses a resistance R and inductance L

where

Xs = synchronous reactance f L

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= generator frequency = apparent inductance of the stator 21

EQUIVALENT CIRCUIT OF SG (SHOWING ONLY ONE PHASE)

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 R (winding resistance) is neglected  Ix : produces the flux which induces the internal voltage Eo

 E : voltage at the terminal of the generator depend on Eo and Z  E and Eo : line to neutral voltage  I : line current FAZH:SEM1/2013

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EXAMPLE 1

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A 3-phase synchronous generator produces an open circuit line voltage of 6928 V when the dc exciting current is 50 A. The ac terminals are then short circuited, and the three line currents are found to be 800 A. Calculate the synchronous reactance per phase ii. the terminal voltage if three 12 Ω resistors are connected in wye across the terminals i.

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SG : UNDER LOAD

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 Types of load applied to the generator :1. Isolated loads 2. Infinite bus

Equivalent Circuit of SG (Under Load) FAZH:SEM1/2013

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PHASOR DIAGRAM : LAGGING PF

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1. Current I lags behind terminal voltage E by an angle θ. 2. Cosine θ = power factor of the load. 3. Voltage Ex across the synchronous reactance leads current I by 90o. It is given by the expression Ex = jIXs. 4. Voltage EO generated by the flux Ф is equal to the phasor sum of E plus Ex. 5. Both Eo and Ex , are voltages that exist inside the synchronous generator windings and cannot be measured directly. 6. Flux Ф is that produced by the dc exciting current Ix. FAZH:SEM1/2013

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PHASOR DIAGRAM : LEADING PF

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•

Capacitive load. Current I leads behind terminal voltage E by an angle θ.

•

What affect does this have on phasor diagram?

- The terminal voltage, E is greater than the induced voltage, EO. - If the load is entirely capacitive, a very high nominal voltage can be produced with small exciting current. - However, such undesirable.

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under-excitation

is

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REGULATION CURVES

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• When a single synchronous generator feeds a variable load, we are interested in knowing how the terminal voltage E changes as a function of the load current I. • The relationship between E and I is called the regulation curve.

• Regulation curves are plotted with the fixed for a given load and power factor.

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REGULATION CURVES

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• The change in voltage between no-load and full-load is expressed as a percent of the rated terminal voltage. • The percent regulation is given by the equation: % regulation = ( ( Enl – Eb) / Eb ) x 100

where

Enl = no load voltage Eb = rated voltage

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SYNCHRONIZATION OF GENERATOR

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• We often have to connect two or more generators in parallel to supply a common load. For example, as the power requirements of a large utility system build up during the day, generators are successively connected to the system to provide the extra power. • Later, when the power demand falls, selected generators are temporarily disconnected from the system until power again builds up the following day. • Synchronous generators are therefore regularly being connected and disconnected from a large power grid in response to customer demand. Such a grid is said to be an infinite bus because it contains so many generators essentially connected in parallel that neither the voltage nor the frequency of the grid can be altered.

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SYNCHRONIZATION OF GENERATOR

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• Before connecting a generator to an infinite bus (or in parallel with another generator), it must be synchronized. A generator is said to be synchronized when it meets all the following conditions:-

1. The generator frequency is equal to the system frequency. 2. The generator voltage is equal to the system voltage. 3. The generator voltage is in phase with the system voltage.

4. The phase sequence of the generator is the same as that of the system.

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SYNCHRONIZATION OF GENERATOR

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How synchronization can be done? • adjust the speed regulator of the turbine so that the generator frequency is close to the system frequency • adjust the excitation so that the generator voltage Eo is equal to the system voltage E • observe the phase angle between Eo and E using synchroscope • the line circuit breaker is closed – connecting the generator to the system

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TORQUE EXERTED ON GENERATOR

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ACTIVE POWER DELIVERED BY GENERATOR

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British Malaysian Institute

where P = active power Eo = induced voltage E = terminal voltage Xs = synchronous reactance δ = torque angle between Eo and E

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EXAMPLE 2

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A 36 MVA, 21kV, 1800 r/min, 3-phase generator connected to a power grid, has a synchronous reactance of 9 Ω per phase. If the exciting voltage is 12kV (line to neutral) and the system voltage is 17.3 kV (line to line), calculate: i. ii.

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active power which the machine delivers when the torque angle is 30o the peak power that the generator can deliver before it falls out of step (loses synchronization)

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