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Control of Distributed Generation

Eugeniusz Rosołowski

Protection and Controlof Distributed Energy Resources

Chapter 4

[email protected]

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1. Introduction 4. Control of Distributed Generation

General Considerations

From the economical point of view, optimal controlmaximizes the financial return on DG system

investment.

The fundamental goal of the control scheme is to

determine whether or not the on‐site generation

should be operating during a particular hour.

Buyback priority is used in cases where the operator

wishes to produce electricity and sell any or all of the

produced power back to the utility.

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2. DC sources 4. Control of Distributed Generation

DC cources interconnetion to the Grid

A PV or Fuel Cell grid-connected DG system with

energy storage

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2. DC sources 4. Control of Distributed Generation

Control of DC sources interconnetion to the Grid

Grid-connected power flow control scheme

through output voltage regulation

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3. Wind Turbine 4. Control of Distributed Generation

Wind Turbine Structure

Nelson V., Wind Energy. CRC Press, 2009

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Turbine Regulation

Pitch controller objectives:

to regulate aerodynamic torque in above‐rated wind

speeds;

to minimize peaks in gearbox torque;

to avoid excessive pitch activity;

to minimize tower base loads as far as possible by

controlling tower vibration, and

to avoid exacerbating hub and blade root loads.

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Turbine Characteristics

The power transferred by the turbine blades is given by:

where: ρ – air density;

Ar – area swept by the rotor;

V w – wind velocity;C p – power coefficient;

β – pitch angle.

3

2

1w pr m V C AP ⋅⋅⋅= ρ

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Theoretical curves of torque versus rpm for different wind speeds

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Wind Turbine Control

A pitch control system is one method to control rpm, start up

(need high torque), and overspeed.

The pitch can be changed to maintain a constant rpm for

synchronous generators. For an induction generator, variable‐

speed generator that operates over a range of rpm in the run

position, over this range the tip speed ratio is constant, and

the unit operates at higher efficiency.

Doubly fed induction generators have a large rpm range,

around 50%, and are used because of the increased

aerodynamic efficiency with blades in the run position for

large wind turbines.

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Main Control Loop for a Fixed‐speed Pitch‐regulated Turbine

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Lightning Protection

Lightning is a significant potential hazard to wind turbines and thatappropriate protection measures need to be taken. Some years ago,

it was thought that as the blades of wind turbines are made from

non‐conducting material (i.e., glass or wood‐epoxy) then it was not

necessary to provide explicit protection for these types of blades.However, there is now a large body of site experience to show that

lightning will attach to blades made from these materials and can

cause catastrophic damage if suitable protection systems have notbeen fitted.

The main protective measures are good bonding, effective shielding

and the use of appropriate surge protection devices at the zone

boundaries.

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4. Wind Driven Generators 4. Control of Distributed Generation

Type of Rotating Generators

Fixed Speed Induction Generator (FSIG).

Synchronous Generator (SG).

Permanent Magnet Synchronous Generator (PMSG).

Doubly‐Fed Induction Generator (DFIG).

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Generator as three terminal unit

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Turbine Regulation

DFIG wind turbines with converters rated at about25–30% of the generator rating are becoming

increasingly popular.

These studies showed that DFIG‐based wind turbines

do not have an impact on oscillation, as it has modern

power electronic devices to perform reactive power

compensation and smoother grid connection besidesspeed control.

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Scheme of the grid‐connected variable speed wind

generator

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4.

Wind

Driven

Generators 4. Control of Distributed Generation

Scheme of the grid‐connected variable speed wind

generator

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5.

DFIG 4. Control of Distributed Generation

Doubly‐Fed Induction Generator

DFIG/DFIM

DFIG‐based wind turbines offer variable speed

operation, four‐quadrant active and reactive power

capabilities, lower converter cost, and reduced power

loss compared to wind turbines using fixed speed

induction generators or fully‐fed synchronous

generators with full‐sized converters.

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5.


DFIG Model

• Recent interest in distributed generator installation into

low voltage buses near electrical consumers, has

created some new challenges for protection engineers

that are different from traditional radially basedprotection methodologies.

• Therefore, typical protection configurations need to be

re-thought such as re-closures, out-of-step monitoring,

impedance relay protection zones with the detection of

unplanned islanding of distributed generator systems.

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5.


DFIG Model

General structure of DFIG based Generator

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5.


DFIG Model

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5.


Available Measurements

u ABC(s) – 3-phase stator voltage,

i ABC(s) – 3-phase stator current,

i ABC(r) – 3-phase stator current,

ω r – rotor angular velocity.

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5.


Machine Coordinates System

3-phase stator coordinates:u ABC(s) – 3-phase stator voltage

i ABC(s) – 3-phase stator current

0 stator coordinates:

iαβ0 – αβ0 stator currentuαβ0 – αβ0 stator voltage

dq0 rotor coordinates:i

dq0 – dq0 rotor current

udq0 – dq0 rotor voltage

xy stator flux oriented coordinates:

ixy(s), ixy(r) – xy stator or rotor currents

uxy(s), uxy(r) – xy stator or rotor voltages

3-phase rotor coordinates:

u ABC(r)

– 3-phase rotor voltage

i ABC(r) – 3-phase rotor current

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5.



Basic transformations:

⎥⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢⎢

⎣

⎡

−−

−=

2

3

2

11

2

3

2

11

011

C

⎥⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢⎢

⎣

⎡

−

−−=−

2

3

2

30

2

1

2

11

111

3

21C

,

ABC xCx 1

0

−=αβ

0αβ Cxx = ABC where:

Zero-sequence component may be omitted, then:

β α αβ x x x j+= for voltage, current or flux, stator reference.e

x xdqγ

αβ

j

e= for voltage, current or flux, rotor reference.sysx

-s xy x x x x sm je

j +== γ αβ )( for stator voltage, current or flux, stator flux reference.

( )ryrxdqr xy x x x x

sme

je

j

+== −γ γ

)( for rotor voltage, current or flux,stator flux reference.

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5.



ωsm ≈ ω t

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5.


DFIG – equivalent scheme in dq axis

( ) )()()()()()()()( jd

d

d

d r dqr dqmsdqsr dqrlsdqslr dqr sdqssdq ui Li Li

t

Li

t

Li Ri Ru ++−−−−−= ω

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5.

DFIG 4. Control of Distributed Generation,

Control Algorithm

• DFIG control algorithm is based on the observation thatactive and reactive stator power: Ps, Qs may be

independently defined as the functions of rotor currents

in xy – coordinates:

• In these equations rotor current components can be

forced by changing the rotor voltage.

( )

( ) ( )smrxsm

ssysxsxsys

rys

m

ssysysxsxs

iiU L LiuiuQ

iU L

LiuiuP

−≈−=

≈+=

32

32

3

2

3

2

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5.

DFIG 4. Control

of

Distributed

Generation,

Control AlgorithmThe algorithm is as follows:

1. Read the input measurements: u ABC(s), i ABC(s), i ABC(r) and

transform its to coordinates.

2. Read the rotor velocity ω r and calculate the

instantaneous rotor angle and slip:

3. Calculate the rotor current in dq coordinates.

4. Calculated the magnetizing current in xy coordinates:

0

0

d e

t

r e p γ τ ω γ += ∫ r ssl pω ω ω −=

β

α

s

m

srqsmq

s

m

srd smd

i L Lii

i L

Lii

−=

−=22

smqsmd sm ii I +=smd

smq

smi

iarctg=γ

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5.

DFIG 4. Control

of

Distributed

Generation,

Control Algorithm5. Calculate the current in xy coordinates:

6. Perform the PI algorithm for regulation of active and

reactive power:

7. Perform the PI algorithm for regulation of rotor voltage:

sm

dq xyγ j

eii =

QQQ

PPP

ref

ref

−=Δ

−=Δ

0

0

11

0

0

11

d

d

ryref

t

I Pryref

rxref

t

I Prxref

iPK PK i

iQK QK i

+Δ+Δ=

+Δ+Δ=

∫

∫

τ

τ

ryryref ry

rxrxref rx

iii

iii

−=Δ

−=Δ

0

0

22

0

0

22

d

d

pry

t

ry I ryP pry

prx

t

rx I rxP prx

uiK iK u

uiK iK u

+Δ+Δ=

+Δ+Δ=

∫

∫

τ

τ

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5.

DFIG 4. Control

of

Distributed

Generation,

Control Algorithm

8. Determine the rotor voltage in xy coordinates:

9. Determine the rotor voltage in 3-phase coordinates:

10. Apply for controlling the Electronic Converter:

prydryry

prxdrxrx

uuu

uuu

+=

+=

( )rxsmr sldry

ryr sldrx

i I Lu

i Lu

−=

−=

1σ σ ω

σ ω

where:

( )

)()(

j

)()( er r ABC

r xyr

esm

αβ

γ γ

αβ

Cuu

uu

==

−−

PWM)( ⇒r ABC u

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5.

DFIG 4. Control

of

Distributed

Generation,

Control Algorithm

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5.

DFIG 4. Control

of

Distributed

Generation,

Control Algorithm

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5.

DFIG 4. Control

of

Distributed

Generation,

Control Algorithm

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6.

DFIG

Protection 4. Control

of

Distributed

Generation

Comparison of synchronous (a) and induction (b) generators Short‐circuit currents

(a)

(b)

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6.

DFIG


of

Distributed

Generation

DFIG Short‐circuit at the Terminal

The stator voltage equation of the induction machine isdetermined as follows:

When the voltage drops to zero (in case of a fault at the

generator terminals), the stator flux space vector stops

rotating (dc component). This produces big current

oscillations in rotor windings and, consistently, high

voltage in supplying converter scheme.

Remedium is adequate protection.

sss

sss

t

i Ru ψ ω ψ

j

d

d ++=

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6.

DFIG


of

Distributed

Generation

Grid Code requirements

The latest grid code requirements specify the behaviourof wind generators when dips occure:

• under what type of dips the turbine must remain

connected to the grid ;• how much reactive power the wind generator must

provide to the grid in order to contribute to the fault

clearance.

Grid Code: a document establishing the rules governing

the operation, maintenance and development of the power

system and sets out the procedures for governing the

actions of all power system users.

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6.

DFIG


of

Distributed

Generation

Low Voltage Ride‐through (LVRT)

The LVRT requirements specify the above conditions (thewind generators connection to grid and demands of

reactive power).

The main requirement is the ability of wind generator toremain connected to the grid during and after faults. This

is known as low voltage ride through ability.

Different countries define their own Grid Coderequirements.

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6.

DFIG


of

Distributed

Generation

Low Voltage Ride‐through requirements

for different grid codes

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6.

DFIG


of

Distributed

Generation

DFIG protection methods

1. Software protection: adequate control of electronicconverters in case of:

• unbalance condition (unbalance load or unbalance

voltage);• small voltage dip.

2. Hardware protection:

• crowbar activation;

• braking chopper;

• replacement loads;

• series resistors.

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6.

DFIG


of

Distributed

Generation

Crowbar application

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6.

DFIG


of

Distributed

Generation

Breaking chopper application

Usually applied

with a

crowbar

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6.

DFIG


of

Distributed

Generation

Replacement loads application

Usually applied

with a

crowbar

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6.

DFIG


of

Distributed

Generation

Series resistors application

Usually installed in a

wind

farm

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6.

DFIG


of

Distributed

Generation

DFIG Short‐circuit Voltages

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6.

DFIG


of

Distributed

Generation

DFIG Short‐circuit Currents

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7.

Wind

Farm 4. Control

of

Distributed

Generation

Wind Farm Interconnection

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7.

Wind

Farm 4. Control

of

Distributed

Generation

Wind Farm Interconnection

HVDC Connection of the Off-shore Farm

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