Online Condition Monitoring Scheme for a Grid-

Connected Wind Power Generation System

under Fault Conditions

Kyeong-Hwa Kim1,

1 Department of Electrical and Information Engineering,

Seoul National University of Science and Technology,

232 Gongneung-ro, Nowon-gu, Seoul, 139-743, Korea

{ Kyeong-Hwa Kim, k2h1@seoultech.ac.kr

Abstract. To analyze the responses under open fault conditions in switching

devices, an online condition monitoring scheme is presented through an

integrated simulation for a permanent magnet synchronous generator (PMSG)

based variable speed grid-connected wind power generation system. The open

fault in switching devices may arise due to the destruction of switches by an

accidental over current or the forced disconnection for protection. As an

effective way of the condition monitoring by online basis without requiring any

diagnostic apparatus, the estimation scheme of the generated voltage is

proposed and the validity of the proposed scheme is proved through

comparative simulations.

Keywords: Condition monitoring, Grid-connected wind turbine, PMSG, Open

fault, Integrated simulation.

1 Introduction

Over the last ten years, there has been an increasing interest in the renewable energy

sources because of the worldwide energy crisis created by the depletion of fossil

energy and the greenhouse gas emission limit. Among various renewable energy

resources connected to the grid, wind power generation is recognized as the most

competitive and economic one, which leads to a rapid growth in the global wind

power generation capacity [1]. While small wind power was connected to the grid in

the past, the level of wind energy penetration in the electrical power systems has been

considerably increased recently because the wind power equipment is being produced

on much larger scale. Therefore, the issue of improving the stability in the utility grid

through the reliability enhancement has become a major concern in the wind power

generation system. For this purpose, an advanced maintenance scheme based on a

fault diagnosis and condition monitoring has been reported for next-generation wind

power system [2]. Considering that the recent large-scale wind power systems have

been built in off shore or remote locations, leading to considerable times and costs for

maintenance or repair, this issue will become more important.

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While fault diagnosis schemes for a PMSG or inverter as a single unit have been

studied partly [3], [4], those under the direct connection to the wind turbine are not

sufficient. Thus, on-line basis fault diagnosis schemes that the controller can detect a

fault during the operation without any additional diagnostic equipment such as sensor

networks are much required. Since variable speed wind turbines often undergo a

severe mechanical and environmental stress, a minor fault may be continuously

propagated and magnified to adjacent system. Whereas considerable costs are

required to restore the system in an extreme situation, the loss for rehabilitation can be

minimized if a fault is detected at an initial stage of failure.

In this paper, response characteristics and condition monitoring scheme for a

PMSG-based variable speed grid-connected wind turbine system are presented

through an integrated simulation research when an open fault occurs in switching

devices. If the fault in the switching devices occurs in the generator-side converter,

the three-phase voltages calculated in the controller cannot be synthesized accurately

in the converter, which increases harmonic components in current and decreases the

generated power. To detect such faults and monitor the normal operation of system,

the no-load generated voltage of the PMSG is estimated using the observer. To verify

that monitoring this parameter is effective to detect the open fault in the switching

devices, integrated simulation results are presented.

2 PMSG-based grid-connected wind power generation systems

Fig. 1. Three-phase PWM converter.

Fig. 1 shows the PMSG and three-phase PWM converter. The speed and torque

controls of the PMSG are achieved through a current control of the generator-side

converter designed on the synchronous reference frame. With the PI decoupling

control, the voltage references are calculated as follows:

qemdemdrqemqem

im

pmqemeiLii

s

kkv ))((

** (1)

qemqrdemdem

im

pmdemiLii

s

kkv ))((

** (2)

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274

where pm

k and im

k represent the proportional and integral gains, respectively, and

s is the Laplace operator. When (1) and (2) are applied to the PMSG voltage

equations, the transfer function from *

qemi to

qemi can be obtained as follows:

impmsq

impm

qem

qem

kskRsL

ksk

sI

sIsT

)()(

)()(

2*. (3)

From the second-order transfer function in (3), the current controller bandwidth B

can be approximately determined using the damping ratio and natural frequency

n as

nB )85.1196.1( for 8.03.0 . (4)

From (3) and (4), the bandwidth of the current controller can be assigned. Also, the

transfer function and bandwidth for the d-axis current can be determined in a similar

way. In the generator-side converter, the q-axis and d-axis components of the current

control the generator torque and power factor, respectively, and the computed voltage

references are applied with the space vector PWM technique [5].

3 Condition monitoring of PMSG-based wind power systems

A parameter observation scheme that can detect a switch open fault in the three-phase

converter as well as monitor a normal operation is presented. Considering that the

three-phase converter is operated with the PMSG, the no-load generated voltage of

the PMSG is estimated through the observer for a condition monitoring. Monitoring

this parameter can be effectively used to detect a switch open fault in three-phase

converter.

The q-axis no-load generated voltage of the PMSG mrqem

e can be easily

estimated using a disturbance observer theory. Although qem

e is not a state variable,

the conventional observer can be easily extended to estimate this voltage. With the

assumption that the observer has much faster dynamics than the time variation of

qeme , 0

qeme holds during each sampling intervals [6]. From this assumption, the

state equation can be obtained as follows:

0

)/(

0

/1

00

/1/demqdr

qem

q

qem

qemqqs

qem

qem iLLv

L

e

iLLR

e

ip . (5)

Because (5) is observable, full states can be completely observed. To reduce the

computational load, a reduced-order observer is used to estimate qem

e .

Online Condition Monitoring Scheme for a Grid-Connected Wind Power Generation System under Fault Conditions

275

Fig. 2. Configuration of a PMSG-based grid-connected wind power generation system.

Fig. 3. Simulation results for the condition monitoring under a-phase upper switch open fault in

the generator-side three-phase converter.

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276

4 Simulation Result

The overall configuration of the wind power generation system employing a back-to-

back converter and PMSG is shown in Fig. 2. The overall system consists of a wind

turbine, a PMSG, a generator-side converter, a grid-side inverter, and a controller.

The entire simulation uses the PSIM software and the main controller is implemented

with the DLL block. For a current control in the converter and inverter, the

synchronous PI decoupling control is used with the sampling period of 100 [μsec].

Fig. 3 shows the simulation results for the condition monitoring when a-phase

upper switch in the generator-side three-phase converter has an open fault at 0.2 [sec].

The normal operation is changed to abnormally at 0.2 [sec]. The estimation of the no-

load generated voltage qem

e starts at 0.1 [sec]. Whereas the estimate of this

parameter reaches the nominal value within 0.01 [sec] under the normal operating

condition without a switch open fault, it shows unusual vibrating characteristics as

soon as the switch open fault occurs. From these results, it can be confirmed that

monitoring this parameter can be used effectively to detect an abnormal system

operation.

5 Conclusions

As an effective way to improve the reliability of the generation system and detect the

faults with an online basis without any additional equipment, a monitoring scheme for

the no-load generated voltage of the PMSG has been proposed. The effectiveness of

the proposed condition monitoring scheme has been verified through the integrated

simulation research.

Acknowledgements. This research was supported by Basic Science Research

Program through the National Research Foundation of Korea(NRF) funded by the

Ministry of Education, Science and Technology(2012R1A1A2042759)

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