7-1

Chapter 7

Conclusions and Future Work

7.1 Conclusions

In this thesis the operation of and winding short-circuit fault detection in a

Doubly-Fed Induction Generator (DFIG) based Wind Turbine Generator System

(WTGS) have been investigated. A review of the available wind turbine condition

monitoring systems in industry indicates that the focus is mainly on the gearbox

and bearing faults. However, according to surveys, generator faults are the fifth

most frequent cause of WTGS failure accounting for 9 % of downtime. Thus, it is

important to dedicate some effort on faults such as winding short-circuits, which

can develop into a failure quickly due to the presence of high current in the

shorted turns. To achieve this, it is necessary to understand the modelling detail

required, the operation of the DFIG and the design of the control system. To

validate the algorithms, a test bench is a valuable tool since it provides the

possibility to observe phenomena that do not appear in simulations due to the

trade-off between modelling detail and simulation speed.

The faultless operation of the DFIG has been discussed in Chapter-3. The

modelling requirements for the system have been investigated based on

literature review, which suggests that it is necessary to consider both the stator

and rotor flux dynamics to accurately determine the machine currents. A

lumped-mass model of the drive train can be used while wind speed can be

considered constant during the time frame of interest for machine electrical

faults, which is around one second. Such a model of the WTGS has therefore been

studied where the controllers have been designed for both the start-up and grid-

connected power production mode of operation. Two simulators have been

developed, one for each mode of operation. The first simulator of the WTGS that

models the generator, the converters and the grid in the dq domain provides the

possibility for fast simulations, at any operating point for the grid-connected

mode of operation, and has been used to compare the Proportional Integral (PI)

controller against the Linear Quadratic Gaussian (LQG) controller for both the

power and torque control strategy. It has been concluded that the torque control

strategy is better as compared to the power control strategy as it offers less

degradation in performance at operating points different from the one for which

the controller was tuned. The LQG control technique is superior to PI control

technique since it considers the Multiple Input Multiple Output (MIMO) nature of

the system while the design based on PI control breaks the system down into a

Single Input Single Output (SISO) subsystem for each of the d and q axis. The

second simulator uses the three-phase component models of

Matlab/SimPowerSystems and is used to simulate the start-up sequence of the

DFIG based WTGS. The procedure for correcting the phase error between the

generated stator voltage and the grid voltage has been simplified by using a

sample-and-hold technique thereby eliminating the need to design a separate

controller for this function. The modelled system corresponds to the laboratory

test bench and both the controllers and the developed phase difference

correction technique have been validated through experiments.

7-2

The faulted operation of the DFIG has been studied in Chapter-4 where the third

simulator developed for this purpose is used for winding turn-turn fault

simulation in the DFIG based WTGS. The procedure for modelling the machine

using the winding-function approach has been detailed. The developed machine

model is validated as a shorted-rotor singly-fed induction machine by observing

the harmonics in the stator currents, torque and the speed. It has been shown

that the model is able to reproduce the harmonics used extensively in literature

as indicators for squirrel-cage induction machine stator winding short-circuit

faults. The modelled machine is then used in a DFIG based WTGS with the control

system implemented. It has been concluded that it is necessary to consider the

effect of fault location as well as the control system. The current frequency

harmonic magnitudes have been shown to differ widely with the location of the

fault within the phase winding whereas the control system attenuates these

harmonic magnitudes altogether while introducing some new harmonics. This is

important when the presence of specific slip related harmonics is used as a fault

indicator and for the selection of a threshold value for the harmonic magnitudes

for alarm generation. The negative-sequence current component, as a fault

residual, has been shown to be less dependent on fault location and the number

of faulted turns (fault severity) in the presence of a control system. The effect of

noise, filter delays and stator voltage unbalance that also effect the detection

system performance has been taken into account.

The dimensioning of the major components for the test bench has been

discussed in Chapter-5. A number of sources have been consulted for the

required information. The necessary interface cards and protection circuits have

been developed and included for signal conditioning and protection against over-

voltage and over-current and over-temperature. The system has been used in

other applications and works satisfactorily.

The experimental results are provided in Chapter-6 where the tests to determine

the necessary parameters for simulation have been carried out. The operation of

the Grid-Side Converter (GSC) and the Rotor-Side Converter (RSC) has been

analyzed in the context of the DFIG based WTGS start-up.

7.2 Future Work

The WTGS model can be extended with a detailed model of the drive train and of the wind for simulations over a longer period of time to test the

control algorithms for power quality performance. The effect of

parameter variation should be included as well.

It is important for a fault detection system based on the negative-sequence current component to differentiate between the internal faults

of the generator and the current unbalance due to the non-symmetric grid

faults and sensor faults. The short duration grid faults are temporary as

opposed to winding short-circuits and do not require disconnection from

the grid but rather to ride through the fault. The effects of eccentricity can

be included by modifying the air-gap function in the developed generator

model thereby extending the range of faults can that be simulated.

7-3

The test bench can be modified to include a crowbar to study the Low Voltage Ride Through (LVRT) capability of the DFIG based WTGS for grid

faults. However, since the trend in WTGS is towards synchronous

generators with full-rated converters, to be able to comply with the

evolving grid codes, more effort can be placed on converter operation.

The model of a wind turbine for emulation on the test bench needs to be

implemented. An LCL filter can be designed which is becoming popular

for grid-interfaced converters.

R-1

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