Threat Jun06

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GENERATION

energize - June 2006 - Page 51

A better understanding of the prob lems

associated with voltage dips at the terminals

of these generators is necessary to ensure

rotor side converters are adequately rated

and protected.

Loss of generation during a critical period

of a voltage dip can also introduce stability

problems on the network and thus it isimportant that this problem is addressed. The

response of the rotor circuit of the generator

to a voltage dip is presented. The influence

of trapped flux in the generator at the instant

of voltage recovery appears to generate

rotor currents that can be sufficient to

damage converter devices. Measurements

and simulations are presented to further

analyse the threat of voltage dips on rotor

circuit converters.

The growing demand for dist r ibuted

generation and renewable energy resources

has seen an increase in the popularity of wind

power. Wind turbines are quickly becoming

cost effective sources of generation as

their power ratings increase with advances

in materials, power electronics and

control technologies.

Wind ene rgy conver ters (WECs) can be

found in various forms, the most popular of

which is the doubly fed induction generator

(DFIG) which has several advantages over its

fixed frequency/speed counterparts [1]. The

DFIG has a wound rotor that is connected

to the grid through back-to-back voltagesource converters (Fig. 1). These converters

decouple the mechanical and electrical

rotor frequencies and hence supply power

at the grid voltage and frequency [2]. This

also allows for increased efficiency as the

turbine speed can be adjusted to maximise

the output power of the generator for a

particular wind speed. The converters can

also control the flux and hence the torque

of the generator. A reduction in torque

pulsations and oscillations that are common

with wind turbines is achieved resulting in

better power quality and a longer gearboxlifespan [3].

The DFIG will act as an asynchronous

generator only if the rotor circuit allows

bidirectional power flow at both sub-

The threat that voltage dips impose on

wind power generationby Simon Davies and Dr. John van Coller, University of the Witwatersrand.

synchronous and super-synchronous speeds

(typically ±30% of synchronous speed [2]).

Only the power in the rotor circuit needs to

be converted which reduces the ratings of

the converters to approximately 25% of the

total power. The converters can also supply or

absorb reactive power to and from the gridand hence maintain the terminal voltage at

the generator. The generation/absorption of

reactive power is limited by the rating of the

converters connected to the rotor circuit.

One drawback of using DFIGs is the

vulnerability of the rotor converters to supply

disturbances such as voltage dips. Converter

failure resulting in the loss of generation

during a critical period of a voltage dip can

introduce stability problems on the network.

It is thus very important to understand the

threat that voltage dips impose on the

generators and their associated converters.

A significant amount of research aimed at

improving the control of the converters to

handle system disturbances such as voltage

dips has been published [2-4]. However, very

little information exists as to what is happening

from the generator’s perspective during a

voltage dip. The aim of this paper is to identify

and explain the transients that can cause

problems to rotor side converters used in

DFIG schemes. Measurements conducted ona simplified LV DFIG system identify how the

response of the generator to a voltage dip

can develop significant electrical transients

in the rotor circuit. Simulations performed

using the Alternative Transients Program (ATP)

confirm these results and allow for a more

detailed understanding of how the generator

behaves during a voltage dip.

Voltage dips

Voltage dips are classif ied as a sudden

reduction in the RMS voltage for a period of

between 20 ms and 3 s of any or all phase

voltages in a single or poly-phase supply.

The duration of a voltage dip is the time

measured from the moment the RMS voltage

drops below 0,9 per unit of the declared

Fig. 1: Doubly fed induction generator.

Fig. 2: Simplified DFIG test circuit.



GENERATION


voltage to when the voltage rises above 0,9 per unit of the declared voltage [5].

Voltage dips are generally caused by network faults. Large fault currents flowing

through the network result in large voltdrops across the network impedances

resulting in voltage reduction further down the network.

Doubly fed induction generator response to voltage dips

A voltage dip at the supply terminals of an induction generator will produce

transient currents and transient torques depending on the severity of the dip

and on the machine parameters [6]. The sudden reduction of voltage at the

supply terminals of the generator as a result of a three phase fault will have the

following consequences:

The generated real power is reduced [6].

There is still the ability for both rotor and stator currents to flow so torque can still be

produced. Large transient rotor and stator currents as a result of the voltage dip

will generate large transient torques [7]. Fatigue as a result of exposure to voltage

dips can reduce the lifetime of the gearbox connected to the turbine.

The theorem of constant flux linkage [6] states that the mutual flux linking the stator

and rotor windings in the machine cannot instantaneously change. At the instant

of a voltage dip, the stator voltage attempts to enforce a new flux condition inthe generator. This forces the rotor circuit to respond to maintain the mutual flux

at that instant. This effect results in significant transient currents developed in the

rotor circuit. Furthermore, because the rotor windings have rotational speed and

the flux linkage is not able to change instantaneously, the flux fixed to the stator

at this instant induces high voltages on the rotor side.

In a short-circuited squirrel cage generator these transient rotor currents and

voltages have little consequence; however in the case of DFIGs, with rotor

circuit converters, the resulting transient rotor voltages and currents can have

damaging effects if no protection exists (i.e. a crowbar) or if devices are not

rated sufficiently.

Doubly fed induction generator response to voltage recovery

The response of the generator to voltage recovery after a voltage dip can

generate even larger transients depending on the flux conditions within the

generator at the time of recovery. These can be identified as re-switching

transients similar to those developed when a machine changes from one steady

state operating condition to another (e.g. star-delta switching) [6]. Some of the

more important considerations due to voltage recovery are as follows:

The process of voltage recovery in the machine is complicated and depends

on the flux conditions within the machine. Effects such as skin effect in the rotor

windings, the reaction of the core due to the rapid rise in flux and mechanical

considerations inherent in the design of the shaft and windings complicate the

process [6, 8].

The theorem of constant flux linkage plays an important role in the understanding

of the transients generated during voltage recovery. At the instant of voltagerecovery the flux linkage must remain constant. The severity of the transients

generated is thus dependant on the flux conditions within the machine at that

instant.

It takes time for magnetic energy stored in a generator to dissipate. Trapped flux

will continue to induce both rotor and stator emfs dependant on the size and

parameters (time constants) of the generator. Larger generators tend to have

longer time constants, thus the effect of a voltage dip can be more severe.

Upon voltage recovery, phase differences may exist between the supply voltage

and induced emfs as a result of trapped flux. If phase opposition occurs, the

transient currents may be very severe (recorded currents similar to that of

direct-on-line start are not uncommon). Thus the amount of stored energy in

the generator and the position of the rotor which affects the flux linkage, willaffect the transients developed in both the stator and rotor circuits at the instant

of voltage recovery [6, 8].

Large transient torques are associated with the transient currents developed at

voltage recovery. These torques can be severe enough to damage mechanical

Fig. 3: 3 phase 0,4 p.u. voltage dip.

Fig. 4: Measured stator current

Fig. 5: Simulated stator current.

Fig. 6: Measured rotor current.



GENERATION


components such as the gearbox connected

to the turbine.

Simplified test circuit

Rotor transients developed by the generator

during a voltage dip will be injected into the

DC bus via the rotor side converter. In this

case, the converter is simplified to a three

phase diode bridge rectifier (the anti-parallel

diodes of the rotor side converter) connected

to a DC bus capacitor and a load resistor

(Fig. 2). Converter control circuitry can limit the

conduction of the IGBTs; however it cannot

control the response of the anti-parallel

diodes to the electrical transients developed

by the rotor during a voltage dip.

Measurements and simulat ions were

conducted on a 19 kW 4 pole wound rotor

induction generator. Balanced three phase

and single phase dips were applied using

the Eskom/Wits voltage dip test bed. A varietyof dips were chosen to show the effects of

different voltage dip parameters on rotor

circuit transients.

Results

Simulations were performed using the

Alternat ive Transients Program (ATP) [9] .

A standard d-q model of a wound rotor

induction generator is used to predict the

transient performance during a voltage dip.

The test circuit described in Fig. 2 was used

in all simulations. Saturation and skin effect in

the windings were neglected.

Figs, 3 - 9 show both the measured and

simulated results for a balanced three

phase 0,4 p.u. voltage dip. The induction

generator is fully loaded with a constant

load torque and operates at 0,2 p.u. slip.

These waveforms are representative of all

tests performed and give valuable insight

into what is happening from the generator’s

perspective during a voltage dip.

The waveforms show that there are two

distinct transients – at the start of the dip and

at the instant the voltage recovers. Measured waveforms are on the left and simulated

waveforms are on the right.

There is reasonable agreement between

measured and simulated waveforms

although it is very difficult to

simulate the same conditions

in the generator at the instant

of the start of the voltage dip

and at the instant of voltage

recovery. Analysing the stator

current (Figs. 4, 5) it is evident

that at the start of the voltage

dip, the stator voltage attemptsto enforce a new flux condition

in the generator. The rotor

circuit is forced to compensate

and maintain the mutual flux

producing an equal and

opposite MMF to that of the

stator current (Figs. 6, 7). This

rotor current surge is injected

into the DC bus through the

rotor side converter and is the

cause of the DC bus voltage

transient at the start of the dip

(Figs. 8, 9). Phase opposition ofthe rotor flux in the generator is

believed to be the cause of the

transient at voltage recovery.

This is explained in more detail

in the next section.

Analysis

The most important factor

influencing rotor transients

during a voltage dip is the

dip magnitude. Larger dips

produce larger electrical and

mechanical transients. TheIGBTs used in the rotor converters

are typically rated at 2 p.u.

voltage and current. Table 1 shows typical

p.u. transients for a selection of voltage

dips. Single phase and three phase dips

exhibit very similar transients and have been

grouped together. It is clear that voltage dips

are cause of concern to converter devices as

over-currents and over-voltages may exceed

the ratings of the devices.

Perhaps the most interesting result is that

shorter dips produce larger transients thanlonger dips. This implies that the effect

of remnant flux in the machine plays an

important role in the rotor transients at the

instant of voltage recovery. Trapped flux

within the machine during a dip will induce

EMFs in both the stator and rotor windings.

If at the point of voltage recovery a phase

difference exists between the induced emfs

in the generator and the grid voltage, very

large currents will result. If phase opposition

occurs, currents may even approach that of

direct-on-line starting. This effect is well known

during supply switching or reconnection

(star-delta switching) and appears to occur

for both three phase and single phase dips.

Large generators will store large amounts ofenergy for longer periods of time as they have

longer time constants. The effect of trapped

flux in a larger machine can thus be more

significant. Preliminary simulations of a larger

generator confirm this fact.

Dip magnitude (p.u.) Dip duration (ms) DC bus voltage/current (p.u.)

0,8 100 2,0

0,6 100 2,7

0,4 100 3,5

0,8 50 2,6

0,6 50 3,1

0,4 50 3,6

Table 1: Selection of dip results.

Fig. 7: Simulated rotor current.

Fig. 8: Measured DC bus voltage.

Fig. 9: Simulated DC bus voltage.



GENERATION


The point on the 50 Hz cycle at the start

of the dip and at voltage recovery has an

effect on the rotor transients developed by

the generator. This is clearly evident for single

phase dips which may be very severe at

one point in the cycle, but quite insignificant

at another.

The size of the DC bus capacitance also

affects the transients in the rotor circuit during

a dip. A larger capacitance reduces the

rotor transient considerably. The tendency

to use smaller capacitance in the DC bus

of modern converters limits the effect of

the capacitance in the reduction of rotor

transients during a voltage dip.

The torque transients as a result of voltage

dips cannot be neglected. The interaction of

both large stator and rotor currents during a

voltage dip will produce significant transient

torques. Shorter dips appear to be more

severe in terms of the mechanical response

of the generator with torques approaching

direct-on-line start values. Gearbox failure is

a possibility for more severe dips if adequate

measures are not taken to limit over-current

transients in the generator.

Conclusions

This paper has identified the threat that

voltage dips impose on DFIGs. The response

of the induction generator to a dip has

been discussed with specific regard to the

rotor transients. The mechanism of constant

flux linkage plays an important role in the

magnitude of the currents and voltages

produced in the rotor circuit. Trapped flux

present in the generator during a voltage

dip appears to play a significant role in the

production of transient currents, especially

for shorter dips. The above analysis assumes

that the equivalent circuit of Fig. 2 is valid

during the dip. If the converters maintain

some control over the rotor current during the

dip then a more detailed investigation would

be required. Voltage dips are a common

occurrence on the utility networks. The effect

of these dips on wind power generators

such as DFIGs must be well understood to

ensure equipment is adequately rated and

protected.

Acknowledgement

This paper was first presented at Cigré’s

5th Southern Africa Regional Conference

in October 2005and is reproduced with

permission.

References

[1] V. T. Ranganathan, R. Datta, Variable-speed wind power generation usingdoubly fed wound rotor induction machine- A comparison with alternative schemes,IEEE Transactions on energy conversion, v17 n 3 Sept 2002 pp 414-421.

[2] R. Pena, J. C. Clare, G. M. Asher, Doublyfed induction generator using back-to-back PWM converters supplying anisolated load from a variable speed windturbine. IEE Proceedings: Electric Power Applications, v 143, n 5, Sep, 1996, p380-387.

[3] I. Cadirci, M. Ermis, Double-output induction

generator operating at subsynchronousand supersynchronous speeds: steady-state performance optimisation and wind-energy recovery, IEE Proceedings,Part B: Electric Power Applications, v 139,n 5, Sep, 1992, pp 429-442.

[4] J. B. Ekanayake, L. Holdsworth, X. Wu, N.Jenkins, Dynamic modeling of doubly fedinduction generator wind turbines, IEEETransactions on Power Systems, v18, n2,May 2003, pp803-809

[5] NRS. Minimum standards, NRS 048-2(Electricity Supply – Quality of Supply). NRS,2003.

[6] M. G. Say, Alternating Current Machines4th ed, Pitman, London, 1976.

[7] J. C. Das, Effects of momentary voltagedips on the operation of induction andsynchronous motors, IEEE Transactions onIndustry Applications, v 26, n 4, Jul-Aug,1990, pp 711-718.

[8] P. K. Kovacs, Transien t Phenomena inElectrical machines, Elsevier, New York,1984.

[9] Leuven EMTP Centre, Alternative TransientsProgram Rule Book, Leuven, Belgium,

1992.

Contact Simon Davies,

[email protected]

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