Ride Through Solutions for Doubly Fed Induction Generators

B. I. Naess J. Eek T. M. Undeland T. Gjengedal NTNU NTNU NTNU NTNU Trondheim, Norway Trondheim, Norway Trondheim, Norway Trondheim, Norway Ph: +47 73594246 Ph: +47 73594252 Ph: +47 73594244 Ph: +47 24067038 Bjarne.Ness@elkraft.ntnu.no Eek@elkraft.ntnu.no Tore.Undeland@elkraft.ntnu.no Terje.Gjengedal@elkraft.ntnu.no

Abstract This paper presents simulations of three different

methods for achieving ride through possibilities for wind turbines equipped with DFIG during a severe grid disturbance. The test case is a voltage drop, during 1 ms, down to 0.2 pu and remains at this voltage for 100 ms then the voltage increase, during 1 ms, up to 1 pu. Since the event sequence is of transient nature the transient simulation tool PSCAD is used which is a useful tool for this type of analyses. Introduction

The control strategy used for the DFIG is the same as described and analyzed in [1]. To prevent that oscillation in the flux linkage will disturbing the control system this is oriented to the stator voltage. This gives good results as long as the system is in steady state and the flux linkage and the stator voltage is almost equal. If a rapid voltage drop appears the flux linkage and the stator voltage will not follow each other and the controller will get a transfer angle which lead to torque oscillations. However, it is difficult to estimate the flux linkage during a rapid voltage change and if the DFIG should be controlled through the failure it is necessary to apply a large rotor voltage. When the voltage reestablishes the control system will damp the flux linkage oscillations more rapidly if the control system is oriented to the stator voltage than the stator flux linkage. Thus the controller in these simulations is oriented to the stator voltage.

The machine equations are shown in (0.1) and the parameters are listed in Tab 1. The simulated DFIG systems are shown in Fig 1, Fig 2, Fig 3. The speed is set constant to 1.2 pu. This is done for simplify the simulations and the results.

Sbase 2.28 MVA Vbase 690 V b 314.16 rad/s rs 0.01 pu rr 0.009 pu xs 4.58 pu xh 4.4 pu xr 4.47 pu nr/ns 3 Vdc 1100 V = 1 pu

1

1

ss s s s

n

rr r r r

n

s s s h r

r h s r r

dr jdt

dr jdt

x xx x

= + +

= + +

= += +

v i

v i

i i i i

rf (0.1)

Methods for Protecting the Converter and the Mechanical Equipment

Three methods for protecting the converter and the mechanical system are simulated; active crowbar, flux damper and thyristor bridge connected in series with the rotor windings. The currents in the rotor windings are three times smaller than the currents in the stator windings due to the winding ratio. It is assumed that the transistors in the converter can handle a transient current at 1.5 pu. The plotted rotor voltage shown in Fig 4, Fig 5 and Fig 6 is the amplitude of the voltage ordered from the controller. An ordinary PWM switching strategy is used which means that ordered voltage over 1 pu, which equal the dc voltage, results in over modulation. The controller is limited to 2 pu. Crowbar

The crowbar setup, shown in Fig 1, is based on what is presented in [2]. The crowbar will protect the converter for over current. To hold the dc link voltage under 1.2 kV a breaking copper is used. If the diodes in the converter connected to the rotor windings could tolerate the over current the breaking chopper could be used as the crowbar. For this example an extra diode bridge and chopper is added as the crowbar.

A resistance is connected in series with the crowbar chopper. This resistance will decrease the transient rotor current. However, if the resistance is too large the voltage over this resistance will increase over the dc voltage and a current will flow through the converter and may damage the converter.

Fig 4 shows the response of the system during a voltage drop. When the rotor currents reach the limit of 1.5 pu the crowbar is activated and the converter is turned off. The rotor current will then flow through the crowbar. A torque oscillating will develop since the flux linkage Tab 1. Machine Parameters

mailto:Bjarne.Ness@elkraft.ntnu.nomailto:Eek@elkraft.ntnu.nomailto:Tore.Undeland@elkraft.ntnu.nomailto:Terje.Gjengedal@elkraft.ntnu.noDr.HebaHighlight

still exists and a large rotor current is flowing. When the rotor currents have decreased below 1.5 pu the converter is turned on and the crowbar is turned off. The induced rotor voltage is still larger than the converter voltage. Thus the torque will still oscillate to the induce voltage begins to decrease below the converter voltage and the regulator gets control over the rotor currents.

Fig 1. Crowbar connected to the Rotor Windings

Flux Damping

This is based on the flux damping technique described in [3]. By connecting a diode bridge and a transistor, which is normally on, on the stator windings an extra stator resistance could be connected under failure. An illustration is shown in Fig 2. This extra stator resistance will decrease the induced rotor voltage and also rapidly damp the stator flux linkage. The disadvantage is the extra losses and cost of the diode bridge and chopper. The stator current will flow through the diode bridge and transistor in steady state. The induced rotor voltage is estimated and the extra stator resistance is turned on if the estimated induced rotor voltage exceeding the upper limit or the rotor currents increase over 1.5 pu. The transistor is open unto the estimated induced rotor voltage is below the lower limit. The response of the system during a failure is shown in Fig 5. The flux linkage is rapidly damped after the voltage drop, then the transistor is turned on and the controller will try to damp out the rest of the flux linkage oscillations. When the stator voltage reestablishes the transistor is turned off to damp the new oscillations in the flux linkage. When the induced voltage is below the lower limit the transistor is connected and the controller begins to ramp up the current.

Fig 2. Flux Damping Strategy

Disconnecting the Rotor Windings By connecting a thyristor bridge in series with the converter, the rotor windings can be disconnected from the converter during a failure. Thus, because of the zero rotor currents, no torque will be developed and the converter is protected during the failure. When the induced rotor voltage increase over the available converter voltage currents with frequency equal to the rotor speed will begin to flow through the converter. This makes the thyristor bridge to commutate relatively fast and the torque ripples is small. However, due to the zero rotor currents during the failure the flux will not be damped and this results in large rotor currents when the converter is reconnected. To limit these transient currents inductors are added in series with the converter. The disadvantage of this is that the converter needs a higher voltage to control the currents. Thus a trade off has to be done between the control possibilities and the short circuit currents. And due to the undamped flux linkage it takes some time before the regulator will take over the control of the rotor currents. The response of the system during the voltage drop is shown in Fig 6.

By connecting the thyristor bridge in series the converter is disconnected through the failure. The flux linkage will not be damped and when the converter is reconnected large currents will flow through the converter. By connecting large (2 mH) inductors in series with the rotor windings the transient currents are damped. However, this solutions influence on the controllability of the rotor currents because of the extra voltage drop over the inductances. And there are extra losses due to the fact that the thyristor bridge is connected in series with the converter and the rotor currents has to flow through the bridge.

Reference Fig 3. Thyristors in series with the Rotor Windings.

[1] A. Petersson, Analysis, Modeling and Control of Doubly-Fed Induction Generators for Wind Turbines, P.h.D. dissertation, Chalmers University of Technology, Gtenborg, Sweden 2005.

Concluding Remarks

The crowbar solution is the most energy efficient since the crowbar is only connected during the failure. The drawback is the large rotor currents and the large torque oscillations which occur when the rotor windings is short circuited over a low resistance. This solution looks like the most secure way to protect the converter for over currents.

[2] J. Niiranen, Voltage dip ride through of a

doubly-fed generator equipped with an active crowbar, Nordic Wind Power Conference, 1-2 March, 2004. The flux damping strategy has the disadvantage that

the stator current has to flow through the diode bridge and the transistor. However, the transistor is not switched in steady state so there are no switching losses. This solution damps the flux linkage oscillations very rapidly and gives the opportunity to control the rotor currents very quickly after the disturbance.

[3] C. R. Kelber, W. Schumacher, Active damping

of flux oscillations in doubly-fed AC machines using dynamic variation of the systems structure, EPE 2001 Graz.

Appendix

Genrator System

Time 1.00 1.10 1.20 1.30 1.40 1.50 1.60

-1.0

0.0

1.0

kV

Vab Vbc Vca

-6.0 -3.0 0.0 3.0 6.0

kA

Isa Isb Isc

-2.0

-1.0

0.0

1.0

2.0

kA

Ira Irb Irc

-1.0

0.0

1.0

pu

Phi_d Phi_q

-3.0

-2.0

-1.0

0.0

pu

Te

-2.0

-1.0

0.0

1.0

2.0

pu

Ird Irq

0.0

1.0

2.0

pu

Vr

Fig 4. Response of the Generator System with a Crowbar Connected to the Rotor Windings

Genrator System

Time 1.00 1.10 1.20 1.30 1.40 1.50 1.60

-1.0

0.0

1.0 kV

Vab Vbc Vca

-6.0

0.0

6.0

kA

Isa Isb Isc

-2.0

-1.0

0.0

1.0

2.0

kA

Ira Irb Irc

-1.0

0.0

1.0

pu

phi_d phi_q

-3.0

-2.0

-1.0

0.0

pu

Te

-2.0

-1.0

0.0

1.0

2.0

pu

Ird Irq

-2.0

-1.0

0.0

1.0

2.0

pu

Vr

Fig 5. Response of the Generator System with a Flux Damper Connected to the Stator Windings.

Genrator System

Time [s] 1.00 1.10 1.20 1.30 1.40 1.50 1.60

-1.0

0.0

1.0 kV

Vab Vbc Vca

-6.0 -3.0 0.0 3.0 6.0

kA

Isa Isb Isc

-2.0

-1.0

0.0

1.0

2.0

kA

Ira Irb Irc

-1.0

0.0

1.0

pu

Phi d Phi q

-3.0

-2.0

-1.0

0.0

pu

Te

-2.0

-1.0

0.0

1.0

2.0

pu

Ird Irq

0.00

0.50

1.00

1.50

2.00

pu

Vr

Fig 6. Response of the Generator System with a Thyristor Bridge Connected in series with the Rotor Windings.