Operation, Special Features and System Benefits

Dr. Gaurang Vakil

Assistant Professor of Electrical Machines and Drives

Power Electronics, Machines and Control Group

gaurang.vakil@nottingham.ac.uk

Room A61 – Coates Building

Synchronous Generators

Content

• Introduction to PEMC

• Introduction to Synchronous Machine

• Fundamental concepts

• Construction

• PF control and VAR control

• Special features with PE converter

• System inertia capability

World Leading Research

• Transport electrification and energy

conversion

• Largest group world-wide, state of the art

facilities – 180 group members with 40-60

working in Electrical Machines/Drives

• National leader – EPSRC centre for power

electronics, APC electrical conversion

spoke, RAEng Electrical Machines

Innovation Centre

• 60% of research income linked to industry

Technology Demonstrators and Facilities

PMM

GCU SG 1

1geni

GCU SG 2

2geni

Other dc loads

iATRU2

ATRU2

HVDC2 540V

Other ac loads

vHVDC2

vHVAC2 HVAC2 230V

idcECS2

ECS2

AC ESS 230V

iATRU1

PMM

Other dc loads

HVDC1 540VvHVDC1

idcECS1

ECS1

ATRU1

HVAC1 230VvHVAC1

PMM

EMA2

PMM

EMA1

iCRU1

CRU

CIUCIU

CRU

idcEMA

SSG1 SSG2

SHVB

SESSSWIPSSATRU1 SATRU2

SECS1 SECS2

SEMA1 SEMA2

SDCL

SACL

F1

F2

F3

WIPS

DC Bus 1 DC Bus 2

AC Bus 1 AC Bus 2

AC Ess Bus

EMA development for primary and secondary actuation systems up to TRL4

Engine electrification development

Starter-Generator development up to TRL5

Conductor technology development

EHA development for primary actuation systems up to TRL5

Modelling, design and optimisation tools for networks and systems.

Limitation of DC and Transition from DC

• DC machines is generally are challenging

• Transmission of DC is efficient

• Safety related issues – breaking and protection is challenging

• Special converters need to generate DC if it is not available on board

• Good for battery operated applications

• Speed control is difficult (zero to max speed)

• Efficiency is low and has high maintenance

• Not for high speed applications

• AC in general is beneficial

• Rotating magnetic field – extremely helpful for the machines

• High efficiency

• Wide speed range (speed control is easy and efficient)

• Wide variety of applications (high speed/power to low speed/power)

• Decoupling of field and armature is difficult but possible

• Robust designs and low maintenance

• If brushless PMDC is driven by AC supply it is even more efficient but

control is having increased complexity compared to DC supply.

AC Machines in Generation

• Wind Turbines

• East Midlands Power

Station (Ratcliffe-on-Soar)

• Micro Generation

• Standby generation

2116 MW

8 to 9 MW

0.5 to 2 MW

50kW

AC Machine Principle of Operation

• In AC machines the production of electromagnetic torque is still a

result of current and a flux density distribution in the air gap of the

motor.

• In all machines we need a way to keep our currents orthogonal to

our field (to produce torque). So when the rotor is rotating, we

must have a way for our current position to change (rotate) to

keep in position.

Wound DC Machine Brushless DC Machine

Electrical SwitchingMechanical Commutation

AC Machine Principle of Operation

• As we have discussed, power supplies typically consist of 3

phases 1200 apart in time.

• We can arrange our three phases in a certain way to give an

inherent rotating current pattern, without electrical or mechanical

switching

𝐼𝑎 = I𝑚 𝑠𝑖𝑛 𝜔𝑡

𝐼𝑏 = I𝑚 𝑠𝑖𝑛 𝜔𝑡 − 1200

𝐼𝑐 = I𝑚 𝑠𝑖𝑛 𝜔𝑡 + 1200

AC Machine Principle of Operation

• In its most basic form an AC machine:

• Is supplied by sinusoidally varying (in time) currents and voltages

• Has a sinusoidally distributed (in space) winding configuration

• Has sinusoidally varying airgap fields (in time and space)

• As the current distribution rotates with the rotor due to the

inherent phase and winding distribution, there is no requirement

for other mechanical or electrical switching (although electrical

switching is often applied for control reasons).

AC Machines

• We will look at two main type of AC machines within this course

• AC-Synchronous Motors (Synchronous Machines) and.

• AC-Induction Motors (Asynchronous Machines)

• The stator windings of these machines are the same, wound and

supplied in order to generate a rotating sinusoidal field.

• We will look at winding configurations and how this rotating field

is developed first.

AC Machines: Coils and Slots

• AC Stator is generally a laminated magnetic steel structure, with

slots to hold the windings.

• Each slot contains one or more (usually two) copper coil sides.

• These must be insulated from other phases, the core and

between adjacent turns.

Winding – MMF due to one coil

3 Coil – Slot Current and Flux

Phase 1 Peak Phase 2 Peak Phase 3 Peak

3 Phase Windings

• Consider a set of three phase currents:

• And assume a sinusoidal winding distribution (ideal)

𝐼𝑎(𝑡) = I𝑚 𝑠𝑖𝑛 𝜔𝑡

𝐼𝑏(𝑡) = I𝑚 𝑠𝑖𝑛 𝜔𝑡 −2 ∙ 𝜋

3

𝐼𝑐(𝑡) = I𝑚 𝑠𝑖𝑛 𝜔𝑡 −4 ∙ 𝜋

3

𝑁𝑎(𝜃) = 𝑁 𝑐𝑜𝑠 𝜃

𝑁𝑏(𝜃) = 𝑁 𝑐𝑜𝑠 𝜃 −2 ∙ 𝜋

3

𝑁𝑐(𝜃) = 𝑁 𝑐𝑜𝑠 𝜃 −4 ∙ 𝜋

3

Rotating Magnetic Field Phasors

MMF𝑇 𝑡, 𝜃 =3𝑁𝐼

2𝑠𝑖𝑛 𝜔𝑡 − 𝜃

Rotating Field Example

Rotating Field Example

Synchronous Speed – Number of Poles

• As more pole pairs are added with the same airgap length,

• the rotating MMF wave travels more slowly.

• In general for a p pole pair motor the speed of the rotating

• field ωs (synchronous speed) is related to the electrical

• speed ωe by:

• And to electrical frequency fe by:

𝜔𝑠 =𝜔𝑒

p=2𝜔𝑒

2p=2𝜔𝑒

𝑃= 𝑁𝑠

2 ∙ 𝜋

60

𝜔𝑒 = 2 ∙ 𝜋 ∙ 𝑓𝑒 𝑁𝑠 =𝜔𝑒

𝑃∙60

2 ∙ 𝜋=120 ∙ 𝑓𝑒

𝑃

Poles Pole Pairs 𝜔𝑠 (rad/s) ns (rev/s) Ns (RPM)

2 1 314 50 3000

4 2 157 25 1500

6 3 105 16.7 1000

8 4 78.5 12.5 750

10 5 62.8 10 600

Types

• Three more types based on challenges it faced and evolution

• Ignore the

rotor

• Wound field

with no PM

• So, it would

need DC

excitation

• How can we

supply it?

Single Stage Generator

Types

• Three more types based on challenges it faced and evolution

• How can we

supply DC

excitation?

• How can we

remove slip rings?

• Consider the

synchronous

generator on the

same shaft

• Rectified AC to DC

• Supply of Exciter?Two Stage/Brushless Generator

Construction

Types of Rotor

• Inherently of two types

1. Salient pole rotor

2. Cylindrical rotor

Rotor Construction

• As always, the machine consists of a stator and a rotor.

AC Wound Stator Rotor

• Creates a travelling field rotating

around the airgap (rotating

magnetic field – space and time)

• Gives the electrical loading A

• Designed to create a multi-pole

magnetic field with permanent

magnets or DC windings

• Gives the magnetic loading B

T = 2VBA (ideally, with orthogonal B and A) (where is λf and λp??)

Damper Windings to Increase Stability

• A squirrel cage winding can be added to the rotor to damp

oscillations from transients.

• This is referred to as a damper windings / damper bars

• In steady state stator field rotation and rotor motion are

synchronised so no change in flux across bars, no induced EMF,

current or torque

Control of Power Factor

• Considering operation on constant voltage supplies :

• If the speed EMF E is lower than the supply voltage then

• The armature draws a lagging power factor (for a motor) current to boost the

air-gap flux until the combination of the excitation and armature fluxes

produces approximately the terminal voltage Vs in the winding

• If the speed EMF E is greater than the supply voltage then

• The armature draws a leading power factor (for a motor) current to oppose

the air-gap flux until the combination of the excitation and armature fluxes

produces approximately the terminal voltage Vs in the winding

Inverted V-curve

• V curve is the graph showing

the relation of stator

(armature) current as a

function of field current in

synchronous machines.

• If we plot the power factor

instead of the stator

(armature) current the curves

still have the same shape.

• Relation between stator

(armature) current and

excitation voltage.

Special Features to Focus on…

• Ability of machine to generate/consume VARs (Control of phase

and voltage level)

• Rotor excitation

• Over excitation

• Under excitation

• Purity of voltage waveform (1.5% THD or less)

• Winding arrangement

• Skewing

• Control

• Ability of machine to provide fault currents when there is an

external short circuit fault

• Ability of machine to cope up with harmonic distortion from other

sources

• Better than batteries and PE converter for system inertia

• Machine inertia and cost implication

• Smooth out imbalances between mechanical power

Construction

PE

Converter

(VSI)

Controlled

Rectifier