ssr in dfig systems

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Presenter:

Hossein A. Mohammadpour

[email protected]

February 2014

Grid-Connected

Advanced

Power Electronics

Systems GRAPES

Supervised by:

Prof. Enrico Santi

[email protected]
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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1. H. A. Mohammadpour, E. Santi, Modeling and control of gate - controlled series capacitor interfaced with aDFIG-based wind farm, IEEE Transactions on Industrial Electronics, DOI:10.1109 /TIE.2014.2347007, Availableon-line: 12 August 2014.

2. H. A. Mohammadpour, E. Santi, Sub-synchronous resonance analysis in DFIG-based wind farms: definitionsand problem identification - Part I, IEEE Energy Conversion Congress and Exposition (ECCE) 2014, pp. 1 - 8,14 - 18 September, Pittsburgh, PA , USA.

3. H. A. Mohammadpour, E. Santi, Sub-synchronous resonance analysis in DFIG-based wind farms: mitigationmethods - TCSC, GCSC, and DFIG controllers - Part II, IEEE Energy Conversion Congress and Exposition (ECCE),pp. 1 - 8, 14 - 18 September, Pittsburgh, PA , USA.

4. H. A. Mohammadpour, Y. J. Shin, E. Santi,SSRanalysis of a DFIG-based wind farm interfaced with a gate-controlled series capacitor, IEEE Twenty-Ninth Annual Applied Power Electronics Conference and Exposition(APEC) 2014, pp. 3110 - 3117, 16 - 20 March, Fort Worth, TX, USA.

5. H. A. Mohammadpour, E. Santi, Sub-synchronous resonance mitigation in wind farms using gate-controlledseries capacitor, IEEE 4th International Symposium on Power Electronics for Distributed Generation Systems(PEDG) 2013, pp. 1 - 6, 8 - 11 July, Rogers, AR, USA.

Please ask for full paper at [email protected] , if you do not have

access to the paper.s Please do not hesitate to ask any question

regarding this presentation.

Thanks you.

Hossein


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Offshorewind farm

Long HVACtransmission line

Inf. bus

Fixed series capacitor

Maximum transmittable power is reduced by transmission line reactance.Indeed, the longer the line, the less maximum transmittable power.

Series compensation is the most economical way to increase maximumtransmissible power of a transmission line.


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Fixed series capacitor

Sub-synchronous resonance

(SSR) may appears in thesystem and make the systemunstable.

But:


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Control

Series FACTS

A well-designed FACTScontroller can damp the SSRand stabilize the wind farm.


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Gate-controlled

series capacitor (GCSC)

1. A SSR damping controller is used to stabilize the wind farm.

2. Eigenvalue analysis approach is used to design the SSRDC.

3. Residue-based analysis is used to find the optimum input control signal (ICS)

to SSRDC.

4. Root-locus approach is used to compute SSRDC gain.

5. PSCAD /EMTDC is used to validate the approach.


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Introduction to Wind Farms

Series Compensation Basics and FACTS Devices

Modeling of Series Compensated DFIG

Studied Power System Basics: Small-Signal Stability and abcto dqTransformation

Mathematical Modeling and Implementation in Matlab/Simulink

Sub-Synchronous Resonance (SSR)

Definition and Basics

Eigenvalue Analysis of DFIG

Detailed Time-Domain Simulation in PSCAD/EMTDC

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Gate-Controlled Series Capacitor (GCSC) Basic Structure

Power Scheduling Controller (PSC)

SSR Damping Controller (SSRDC)

SSR Damping Controller (SSRDC) Design Residue-Based Analysis for Optimal Input Selection to SSRDC

Rotor speed, line current, and voltage across GCSC

Root-Locus Diagram for Computing SSRDC Gain

Detailed Time-Domain Simulation in PSCAD Conclusion and Future Work

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Advantages :

No air pollution

No greenhouse gasses

Does not pollute water with mercury

No water needed for operations

Disadvantages:

Intermittent source of power

Only when the wind blows (night? day?) Transmission constraints

Offshore wind farms are far away from customers

Need long transmission lines

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Offshore Wind Farms vs Onshore Wind Farms:

Much bigger

Further distance from customers

Require a reliable transmission lines with high voltage

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Studies show that HVAC option is technically feasible for distances larger

than 250 km provided that capacitive series compensation is used.

Transmission Line Options:

High voltage DC (HVDC)

High voltage AC (HVAC)

Expensive


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Type A: Conventional

Induction

Generator (fixed speed)

Type B: Wound-Rotor Induction

Generator w/variable Rotor

Resistance

Type C: Doubly-Fed Induction

Generator (variable speed)

Type D: Full-

Converter Interface

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Studied Power System

Basics: Small-Signal Stability and abcto dqTransformation






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The most economical way to increase the transmittable power

Disadvantage:

It can increase the risk of sub-synchronous resonance (SSR)

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Definition:

Alternating current transmission systems incorporating power

electronics-based and other static controllers to enhance

controllability and increase power transfer capability.

Advantages of FACTS Devices

Transient stability improvement

Inter-area oscillation damping

Greater flexibility in power network Deliver the optimum power

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Sub-synchronous resonance mitigation


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Static VAR

Compensator - SVC

Thyristor Controlled

Series Compensator - TCSC

Gate Controlled

Series Compensator - TCSC

Unified Power Flow

Controller (UPFC)

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Solid State Series

Compensator - SSSC

Static Synchronous

Compensator - StatCom


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Definition and Basics Eigenvalue Analysis of DFIG


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Any dynamic system can be expressed by a set of n first

order nonlinear ordinary differential equations

For small disturbances, the differential equations can be

linearized around operating points and can be expressed in

state-space form.

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Stationary circuit variables referred to a synchronously rotating reference frame.

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Typical abc-to-dq transformation


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RSC real power

GSC real power

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Matlab/Simulink Model

A first order DC-link model


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Maximum Power Point Tracking (MPPT)

The aim of the GSC and RSC are to enable theDFIG to work on the MPPT curve.

Rotor-side converter (RSC) controllers

Grid-side converter (GSC) controllers

MPPT is used in order to achieve high

efficiency in the DFIG wind farm.

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A 4thorder transmission line model


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A 3rdorder mechanical system model


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Generator rotor oscillates at frequency

(torsional frequency).

Armature voltage component is induced at sub-synchronous

frequency: =

If , the sub-synchronous torque, generated by sub-

synchronous voltage component is sustained.

This interplay between mechanical and electrical part of the

system is termed as TI-SSR, which may result in shaft failure.

Because of the low shaft stiffness of the wind turbine drive train,

the frequency of torsional modes in wind turbines is in the range

of 1 to 3 Hz so that in order to cause TI, a very high level of series

compensation is required.

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Equivalent circuit of system under sub-synchronous frequency

= ++

: The entire reactance seen from

infinite bus

:Frequency corresponding to rotorspeed

andIf

sum of resistancesof the armatureand the network

< 0

>

Then there will be a negativeresistance at the sub-synchronous

frequency, and the sub-synchronouscurrent will increase with time.

This phenomenon is called induction generator effect (IGE).

IGE is the major problem in wind farms.

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Wind

speed

(/)

Mode 1

SSR Mode

Mode 2

Sup-SR Mode

Mode 3

Shaft Mode

Mode 4

Elec. Mech. Mode

7

. . 5.3126 640.1460

1.0405 5.9975

18.4753 95.5005

8

0.7958 106.4835

6.1834 642.3497

1.5952 5.7523

7.3302 64.5295

9 4.0322 107.0110

7.1346 645.3255

3.2322 4.6631

2.4978 31.2250

( ) Mode 1

SSR Mode

Mode 2

Sup-SR Mode

Mode 3

Shaft Mode

Mode 4

Elec. Mech. Mode

50

5.3908 179.2258

4.9224 572.6882

0.9432 6.0249

4.8676 97.6290

75

. .

5.2066 617.1976

0.9221 5.9992

9.9111 99.9693

90

. . 5.3126 640.1460

1.0405 5.9975

18.4753 95.5005

I. High series compensation, e.g. 90 , and different wind speeds

II. Low wind speed, e.g. 7 m/s, and different series compensation levels

IGE-SSR in DFIG is affected by:

Wind speed

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At lower wind speed, i.e. 7 m/s, even at a very realistic compensation level, i.e. 75%,

the DFIG wind farm is unstable due to SSR Mode.

Time-domain simulation confirms the eigenvalue analysis.

Electric Torque

Rotor Speed

Terminal Voltage

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At higher wind speed, i.e. 9 m/s, even at a very high compensation level, i.e. 90%, the

DFIG wind farm is still stable

Electric Torque

Rotor Speed

Terminal Voltage

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A step-by-step comprehensive approach to modal analysis

A useful understanding of the grid-connected series compensated

DFIG

Valuable starting point for small-signal analysis of multi-machine

systems

At higher wind speeds the SSR mode is stable.

At lower wind speeds the SSR mode could be unstable; however,

this instability could be solved using FACTS devices.

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Detailed Time-Domain Simulation in PSCAD

Conclusion and Future Work

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Aaaa

aaa

Gate-controlled series capacitor (GCSC)

Effective reactance of the GCSC

By controlling the GTOs in GCSC, the effective reactance of this device could be

controlled.

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For a complex root , the residue is a complex number, which can beconsidered as a vector having a certain direction.

, Right and left eigenvectors

In a root locus diagram, is representation of the direction and magnitudeof the closed loop eigenvalue , which leaves the pole .

Basics:


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The above equations show that residue influences closed-loop systemroot, by determining the direction and magnitude of it.

Suppose dynamics of all eigenvalues are ignored, except one specific eigenvalue, .

If the magnitude of the residue is large enough, then a smaller gain isneeded for the feedback control system

=

=


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This will increase the difficulty of the

controller design.

The reason is that a simpleproportional controller chosen toincrease damping of the SSR mode willdecrease the damping of the SupSR

mode.

The residue magnitude of

the SSR mode is small.

A larger gain is needed for

the feedback control.

The residues of the SSR and

SupSR modes are in an opposite direction.


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However, since the residues of the SSR

and SupSR modes in this case point in

opposite directions, stabilizing the SSR

mode via a feedback gain will decrease

the SupSR mode damping.


the SSR mode is large.

A smaller gain is needed for



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The SSR and SupSR modes are in the same

directions. This make the design of the

feedback control simple so that a small

gain will be enough to force both the SSR

and SupSR modes to move to the left and

make the system stable.


the SSR mode is large.

A smaller gain is needed for



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Introducing an unstable mode while stabilizing the SSR mode.

Large gain is needed to stabilize the SSR mode.

SSR Mode

SupSRMode

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Destabilizing the SupSR mode, while stabilizing the SSR mode.

SSR Mode

SupSR Mode

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Detailed Time-Domain Simulation in PSCAD

Conclusion and Future Work

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Super-synchronous oscillations

Electric Torque Terminal Voltage

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1. Time-domain simulation verifies the designed controller.

2. A well-designed GCSC can damp the oscillation.

3. Voltage across the GCSC is an optimal input control signal to SSRDC.


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The GCSC can stabilize the SSR mode.

A control system design procedure is presented for GCSC.

Rotor speed, line current, and cap. Voltage tested as ICS to SSRDC block.

The best signal as ICS to SSRDC block is voltage across series capacitor.

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Supported by NSF I/U CRC for

Grid-Connected Advanced Power Electronics Systems

(GRAPES)


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ssr in dfig systems

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