Control of DPGS Under Grid Faults

Control of DPGS Under Grid Faults

Marco Liserre [email protected]


Marco Liserre

[email protected]



Outline

• Introduction• Crowbar protection for DFIG• Effect of voltage sag over the IG connected to grid• Effect of voltage sag over the wind farm• Solutions for dynamic reactive injections• LVRT with variable-speed wind turbines• Control strategies under unbalanced voltage• Conclusions



Introduction Control under grid faults has become very actual after new grid codes (Germany,

Spain) has requested not only ride-through but also reactive power injection under fault

Low voltage ride-through LVRT can be achieved by quickly limiting the active power using a crowbar and inject reactive power to support grid voltage restoring

Reactive power injection during asymmetrical fault requires a positive sequence angle extraction

Advanced control specially designed for unbalance voltage requires positive/negative sequence extraction and current controllers for both sequences

P+Resonant current controllers can handle both sequences Using the instantaneous power theory different control strategies can be designed in

order to minimize the dc voltage oscillations or output P and Q oscillations during unbalanced voltage

New grid codes are expected to require even more controllability under grid fault in order to support the grid recovering (for ex. Non-oscillating P or Q)



Grid Faults and their consequences In general when a fault occurs in the electric grid and the fault is detected, the

minimum possible portion of the grid containing the fault should be isolated and the neighboring parts should try to go back to normal operation. If a component fails then a larger area can be affected.

Then the fault is removed and the damage is repaired, finally the normal service is restored.

Faults can lead to both angle and voltage instabilities

The angle instability is cause by active power imbalance and by the excitation of mechanical generator dynamics. As a consequence the generators lose synchronism and they are disconnected.

Voltage instability arises due lack of reactive power and as a consequence leads to overload of power lines and as a consequence disconnection of loads.



Low Voltage Ride Through Two requirements can be defined for WTS during grid fault:

to remain connected even during severe under-voltage

to control reactive power to support the grid voltage

These two requirements are associated with:

Measure and maintain full control on the reactive and active power injected hence the capability of limiting it

Limit the power quality decrement in terms of power fluctuations and harmonic distortion that may have severe effects on the grid.

LVRT is strictly related to the capability of the WTS to preserve the converter safety and avoid overcurrent tripping. The semiconductor over-load capability can be increased only reducing the switching frequency



Crowbar protection

Rotor short-circuited Resistive crowbar

When the resistive crowbar is implemented, the stator and rotor transient current decay rapidly to value with amplitude lower than 1 p.u.

Amplitude of transient electromagnetic torque is reduced when the resistive crowbar is activated.

On the other hand the electromagnetic torque oscillates longer with the resistive crowbar.



Effect of voltage dips over the IG connected to grid

During the fault period: Voltage at the wind turbine terminal drops as well as the air-gap flux in the generator.

Reduction in the electromagnetic torque which causes the rotor to accelerate.

After the clearance of the fault: Reactive power is supplied by the power system to recover the air-gap flux.

This causes a high inrush current to be drawn by the wind turbine from the power system, which in turn causes a voltage drop at the wind turbine terminal.

Effect of a sustained voltage drop after fault clearance: If the electromagnetic torque is not strong enough in comparison with the aerodynamic torque, the rotor speed will continue to increase and the induction generator could draw high inrush current from the external power system until appropriate protection devices trip it.

In this condition, voltage at the wind turbine terminal dips and the output power of wind turbine drops.

The system loses stability and the wind turbine has to be disconnected.



Effect of voltage dips over the IG connected to grid

Electromagnetic torque Rotor speed Positive-sequence voltage at the stator

Active power delivered by the SCIG-WT

Reactive power delivered by the SCIG-WT

Negative sequence voltage at the stator



without STATCOM

Source: American Superconductor

Effect of voltage dips over the wind farm



Source: American Superconductor

Effect of voltage dips over the wind farmWith D-VAR STATCOM



LVRT retrofit solution for wind farms – D-VAR,Dynamic CapacitorsDynamic reactive power injection

Source:

American Superconductor



STATCOM is a potential technology to retrofit LVRT to existing wind farms based on fixed-speed generators

As LVRT is compulsory in some countries and will be spread worldwide a big market is open for STATCOM industry

Along with providing LVRT, the STATCOM is providing also voltage regulation especially important for weak grids

Reactive power grid support is an ancillary service that will be paid by TSO in the future making it more attractive

LVRT with STATCOM



DFIG stability and reactive power Conventional DFIG rotor current control based on synchronous d-q reference frames is prone to oscillations when reactive power is fed to grid from the rotor side under fault conditions

The generator will be unstable if:

This is only twice the rotor current that is required for operation at unity power factor

In voltage dips the stability limit is further lowered

However, generators that used Direct Torque Control (DTC) instead of vector current control turned out to be reasonable stable allowing the rated reactive current production down to very low grid voltages

v2 S

rd rqm o S o

Ri iL L

LVRT with DFIG. Stability issue



Control of reactive power injection on the synchronous d-q frame

Stable, but dampnig is low

Slow

Slow decay of the dc component

Source: ABB

LVRT with DFIG example



Very stable dc voltage

Fast100ms

Fast decay of the dc component

Control of reactive power injection using DTC active crow bar

Source: ABB




Control of reactive power injection using DTC active crow bar

Fast reactive power support

Boost, voltage level without reactive

injection

Source: ABB




Two technologies: Full-scale converter with SG, PMSG or IG Double-fed IG

Both are capable of injecting reactive power to the extent of the kVA rating Both are capable of quickly reduce the generated power (vector control or DTC of generator, chopper) Both can reduce the aerodynamic torque after fault by pitching control (few seconds) Full-scale converter can control faster the grid currents being directly connected

LVRT with Variable Speed WT



P&Q Control Strategies Under Grid Fault∙ Power oscillations occurs due to negative sequence∙ These translates into dc voltage oscillations trip∙ The aim of the control is to reduce these oscillations and

inject positive sequence reactive power∙ The following additional control issues are necessary to be

addressed to the conventional control structure:▫ Positive/negative sequence calculation

◦ in synchronous dq frame (LP.BP, Notch filtering) ◦ In stationary frame with (DSC, SOGI-FLL)

▫ Current control that can handle both sequences◦ Dual synchronous (PI-dq) reference frame◦ Single stationary reference frame (P+Resonant)

▫ Current reference calculator to address a specific control strategy like: ◦ minimization of dc voltage oscillations◦ maximizing the transferred power◦ minimization of oscillations in the instantaneous P&Q

▫ Power reference calculator (P* and Q*)



a a h

h b b h

c c h

v v vV V V v v v

v v v

v V thn

nn

n

0 0

1

s in ( ) V V

ttt

Vt

tt

nn

n

nn

nn

nn

n

nn

nnn

s in ( )

s in ( )s in ( )

sin ( )s in ( )sin ( )

23

23

23

23

1

Grid voltage under generic conditions:

I

iii

It

tt

It

tt

a

b

c

nn

n

nn

nn

nn

n

nn

nnn

sin ( )

s in ( )s in ( )

s in ( )s in ( )s in ( )

23

23

23

23

1

Generic current injected by the three-phase grid inverter:

P&Q Control Strategies Under Grid FaultCurrent Reference Calculation – instantaneous power theory



Instantaneous active power delivered to the grid:

31

3 cos( ) cos( )2

n n n n n n n n

n

p V I V I

31 1

1 1

1 1

1 1

3 cos(( ) )2

cos(( ) )

cos(( ) )

cos(( ) ) .

m n m nm n

m nm n

m n m nm n

m nm n

m n m nm n

m n

m n m nm n

m n

p V I t

V I t

V I t

V I t




Instantaneous reactive power delivered to the grid:

31

3 sin( ) sin( )2

n n n n n n n n

n

q V I V I

31 1

1 1

1 1

1 1

3 sin(( ) )2

sin(( ) )

sin(( ) )

sin(( ) )

m n m nm n

m nm n

m n m nm n

m nm n

m n m nm n

m n

m n m nm n

m n

q V I t

V I t

V I t

V I t

.




Sequencedetector

DC power source

PWMinverter

LCfilter

Dy

Localload

SVMmodulator

Current controller

Current reference

Sensors

Power reference

Transformer

Gridsimulator

Simulatedfault

*v

*i

*P *Q , v v

vi

.- Grid Code Requirements.- Fault strategy.- MPPT.- Protection

s

dcv pwmv ,v igreenP

POWER PROCESSOR CONTROL

P&Q Control Strategies Under Grid Fault

∙ Experimental evaluation

Current Reference Calculation – instantaneous power theory

Source: [4,5]



Instantaneous Active Reactive Control (IARC)

-400

-200

0

200

400

v [V

]

-10

-5

0

5

10

i [A

]

-1

0

1

2

p, q

[kW

, kva

r]

pq

p v i

*2;pPg g i vv

*pP v i

* 0pq v i

∙Reference constrains:

∙Control law:

∙Comments:

∙Basis:

• Distorted and unbalanced current • Instantaneous power perfectly controlled• Overcurrent trip risk

P&Q Control Strategies Under Grid FaultCurrent Reference Calculation – P control laws



Instantaneous Active Reactive Control (IARC)

p v i

*2

*2

;

;

p

q

Pg g

Qb b

i vv

i vv

*pP v i


∙Control law:

∙Basis:q v i v i

*qQ v i

is a 90-degrees leaded versionof the voltage vector

vv

-1

0

1

2

p, q

[kW

, kva

r]

pq

-10

-5

0

5

10

i [A

]

-400

-200

0

200

400

v [V

]

P&Q Control Strategies Under Grid FaultCurrent Reference Calculation – P&Q control laws



Positive- Negative-Sequence Compensation (PNSC)

-400

-200

0

200

400

v [V

]

-10

-5

0

5

10

i [A

]

-1

0

1

2

p, q

[kW

, kva

r]pq

p v v i i

* *p p P v i v i * * 0p p

v i v i

*2 2;pP

g g

i v v

v v

*

* * * *

0

p

p p p p

q

q

v i

v i v i v i v i


∙Control law:

∙Comments:

∙Basis:

• Sinusoidal unbalanced currents • Oscillations in the instantaneous reactive power• Maximun value of current is a known function of power and sag deep




Positive- Negative-Sequence Compensation (PNSC)

p v v i i

* *p p P v i v i * * 0p p

v i v i

*2 2

*2 2

;

;

p

q

Pg g

Qb b

i v vv v

i v vv v


∙Control law:

∙Basis:

q v v i i

* *q q Q

v i v i * * 0q q v i v i

-1

0

1

2

p, q

[kW

, kva

r]

pq

-10

-5

0

5

10

i [A

]

-400

-200

0

200

400

v [V

]




Balanced Positive Sequence (BPS)

-400

-200

0

200

400

v [V

]

-10

-5

0

5

10

i [A

]

-1

0

1

2

p, q

[kW

, kva

r]pq

*2;p

PG G

i v

v

*p P v i * 0p

v v i *p p v i

p v v i

* * *

0p p pq

q

v i v i v i


∙Control law:

∙Comments:

∙Basis:

• Sinusoidal balanced current • Oscillations in both instantaneous powers• Maximun value of current is easily calculated




Balanced Positive Sequence (BPS)

*2

*2

;

;

p

q

PG G

QB B

i vv

i vv

*p P v i * 0p

v v i *p p v i

p v v i


∙Control law:

∙Basis:

q v v i

*q Q

v i * 0q v v i *

q q v i

-40 -30 -20 -10 0 10 20 30 40 50 60-1

0

1

2

t [ms]

p, q

[kW

, kva

r]

pq

-40 -30 -20 -10 0 10 20 30 40 50 60-10

-5

0

5

10

t [ms]

i [A

]

-40 -30 -20 -10 0 10 20 30 40 50 60-400

-200

0

200

400

t [ms]

v [V

]




∙ IARC allows full control of the power delivered to the grid at the expense of the current quality. This strategy should be used when the control of the dc-bus voltage is the main issue. ∙ PNSC cancels instantaneous active power oscillation keeping unbalanced sinusoidal currents. However no full power can be delivered since oscillations exist in the reactive power and the maximum value of the injected current should be limited.∙ BPS achieves sinusoidal balanced currents and gives rise to oscillations in the instantaneous powers. This strategy should be used when current quality is the main issue.

0

1 0T

dtT

x y x y




Active Power Delivery - APD

∙Control law:

∙Comments:∙V+-RMS value of pos seq. voltage∙IN – RMS value of nominal current

• PBS – sinusoidal, balanced, limited. Oscillations in P and Q• IARC – distorted hard to predict peak. No P and Q oscillations•PNSC – unbalanced,sinusoidal. Oscillations in Q only


* *3 0NP V I Q

∙Goal: deliver max. available P and no Q without exceeding nominal current

Power Reference Calculation

Source: [6]

PBS

IARC

PNSC



Grid Voltage Supporting - GVS

∙Control law:

∙Comments:∙V+-RMS value of pos seq. voltage∙IN – RMS value of nominal current∙GVS: ON when V+ <0.9VN, OFF V+ ≈ VN• PBS – sinusoidal, balanced, limited. Oscillations in P and Q• IARC – distorted hard to predict peak. No P and Q oscillations•PNSC – unbalanced,sinusoidal. Oscillations in P only


* *0 3 NP Q V I

∙Goal: deliver max. available Q and no P without exceeding nominal current


Source: [6]

PBS

IARC

PNSC



Active Reactive Power Delivering- ARPD

∙Control law:

∙Comments:∙V+-RMS value of pos seq. voltage∙IN, VN – RMS value of nominal current(voltage∙ARPD: ON when V+ <0.9VN, OFF V+ ≈ VN

• PBS – sinusoidal, balanced, limited. Oscillations in P and Q• IARC – distorted hard to predict peak. No P and Q oscillations•PNSC – unbalanced,sinusoidal. Oscillations in P and Q


2 2* * *3 , 6 1N NN

VP V I Q Q V I

V

∙Goal: deliver P and Q according to E.ON guidlines. Deliver P & supporting grid with Q


Source: [6]

PBS

IARC

PNSC



Experimental results

IARC

PNSC BPS

Analyzing the reference current expressions it is possible to determine the maximum power to be delivered to the grid during unbalanced voltage sages without reaching the overcurrent limit in any of the phases at any time



Unbalance small (<2-5%) medium (5-15%) high (>15 %)Output power

capability Full Full PartialInput currents Should be

sinusoidal & balanced

May be sinusoidal but unbalanced

May be not sinusoidal & unbalanced

Possible fault-tolerant

scenario

Sinusoidal balanced currents: High quality in the injected current

Maximum current at the ac-side can be readily predicted

Oscillations in injected active power moderated dc-voltage oscillations

Sinusoidal unbalanced currents: Acceptable quality in the injected current, no high order harmonics.

Maximum current at the ac-side can be predicted

Depending on the selected strategy, active power oscillations can be cancelled when no reactive power is injected no dc-voltage oscillations

Nonsinusoidal currents: Poor current quality in the injected current

Maximum value of the injected currents can not be easily predicted

No oscillations in the injected active power independently of the reactive power no dc-voltage oscillations




LVRTcan be implemented with resistive crow-bar and reactive power injection to existing wind farms.

Variable speed wind turbines can do LVRTas they are capable of quickly reduce the torque of the generator and inject reactive power.

The capability of the system to allow ride-through grid disturbances is strictly related to the inverter rating (maximum voltage and current at the ac-side) as well as its dc-link energy storage (maximum/minimum voltage at the dc-side).

A robust control strategy for the grid inverter should ride-through transient faults avoiding under-/over-voltage trip in the dc-bus and output over-currents due to distortion and imbalance

Reactive power injection is helping the grid voltage to recover Several strategies for reference current generations. Overcurrent and

under/over voltage should be avoided to ride-through transient faults Different power delivery strategies under fault are also presented New grid codes with even more sestrictions during grid faults are expected in

the future an more advanced control strategies have to be developed.

Conclusions



Bibliography1. S. Seman, J. Niiranen, S. Kanerva, and A. Arkkio, ”Analysis of a 1.7 MVA Doubly Fed Wind-Power Induction Generator

during Power Systems Disturbances”,

2. Molinas, Marta; Suul, Jon Are; Undeland, Tore , “Improved grid interface of induction generators for renewable energy by use of STATCOM” , International Conference on Clean Electrical Power, 2007. ICCEP '07., Vol., Iss., 21-23 May 2007 Pages:215-222

3. G. Saccomando, J. Svensson, “Transient Operation of Grid-connected Voltage Source Converter Under Unbalanced Voltage Conditions,” 2001 IEEE Industry Applications Conference, 36th IAS Annual Meeting, September 30 - October 5, 2001, Chicago, USA, 2001.

4. Rodriguez, P.; Timbus, A. V.; Teodorescu, R.; Liserre, M.; Blaagjerg, F, “Independent PQ Control for Distributed Power Generation Systems under Grid Faults”, IEEE Industrial Electronics, IECON 2006 - 32nd Annual Conference on, Vol., Iss., Nov. 2006 Pages:5185-5190

5. P. Rodriguez, A.V. Timbus, R. Teodorescu, M. Liserre, F. Blaabjerg, “Flexible Active Power Control of Distributed Power Generation Systems During Grid Faults”. Industrial Electronics, IEEE Transactions on Volume 54, Issue 5, Oct. 2007 Page(s):2583 – 2592

6. P. Rodriguez, Luna, A., Teodoresc R., Blaabjerg F., - “Fault Ride-through Capability Implementation in Wind Turbine Converters Using a Decoupled Double Synchronous Reference Frame PLL” – Proceedings of EPE 2007, 2-5 sep. 2007 (on CD)

7. A. Luna, P. Rodriguez, R. Teodorescu and F. Blaabjerg, “Low voltage ride through strategies for SCIG wind turbines in distributed power generation systems,” in Proc. IEEE Power Electron. Conf. PESC 2008. Jun. 2008

8. F. Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus, “Overview of Control and Grid Synchronization for Distributed Power Generation Systems”, IEEE Trans. on Ind. Electronics, Vol. 53, Oct. 2006 Page(s):1398 – 1409

9. P. Rodríguez, A. Timbus, R. Teodorescu, M. Liserre and F. Blaabjerg, “Reactive Power Control for Improving Wind Turbine Systems Behaviour under Grid Faults,” IEEE Trans. on Power Electronics, Aug. 2008 (in press)



Acknowledgment

Part of the material is or was included in the present and/or past editions of the

“Industrial/Ph.D. Course in Power Electronics for Renewable Energy Systems – in theory and practice”

Speakers: R. Teodorescu, P. Rodriguez, M. Liserre, J. M. Guerrero,

Place: Aalborg University, Denmark

The course is held twice (May and November) every year

Control of DPGS Under Grid Faults

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