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Condition monitoring of motor-operated valves in nuclear power plants

June 2011

Authors:

Grenoble Institute of Technology

This article deals with a new motor-operated valves (MOVs) condition monitoring method. MOVs mainly consist of an induction machine driving a valve through a mechanical actuator. Such systems need important maintenance policies since they are critical concerning the operational readiness of the safety related systems of nuclear power plants. Unfortunately, today's policies present a major drawback. Indeed, these monitoring techniques rely on the measurement of an internal mechanical quantity and require the presence of human operators in dangerous power plants areas. The aim of this research work is to develop a monitoring method relying exclusively on remote electrical measurements (MOV supply voltages and currents), without the need of any internal MOV quantity. In this article, the instantaneous active power absorbed by the induction machine is first shown to be an interesting electrical quantity to elaborate mechanical failure indicators. An efficient real-time algorithm developed to estimate this physical quantity is then detailed and mechanical failure indicators based on the estimated electrical power are proposed. Finally, their performance is illustrated through experimental data.

Schematic representation of a motor-operated valve

…

Time waveforms of MOV active power during steady-state operations

…

Power spectral densities of MOV active power during steady-state operations

…

Stem nut fault indicator based on low frequency components of P ˆ + ( t )

…

Figures - uploaded by Pierre Granjon

Content may be subject to copyright.

Content uploaded by Pierre Granjon

Content may be subject to copyright.

Condition monitoring of motor-operated valves in nuclear power plants

Pierre Granjon

Gipsa-lab / Grenoble University

Grenoble, France

33 4 76 82 71 32

pierre.granjon@grenoble-inp.fr

Abstract

This article deals with a new motor-operated valves (MOVs) condition monitoring method.

MOVs mainly consist of an induction machine driving a valve through a mechanical actuator.

Such systems need important maintenance policies since they are critical concerning the

operational readiness of the safety related systems of nuclear power plants. Unfortunately,

today’s policies present a major drawback. Indeed, these monitoring techniques rely on the

measurement of an internal mechanical quantity and require the presence of human operators

in dangerous power plants areas.

The aim of this research work is to develop a monitoring method relying exclusively on

remote electrical measurements (MOV supply voltages and currents), without the need of any

internal MOV quantity.

In this article, the instantaneous active power absorbed by the induction machine is first

shown to be an interesting electrical quantity to elaborate mechanical failure indicators. An

efficient real-time algorithm developed to estimate this physical quantity is then detailed and

mechanical failure indicators based on the estimated electrical power are proposed. Finally,

their performance is illustrated through experimental data.

1. Introduction

Nuclear power provides about 14% of the world’s electricity, with the United States of

America, France and Japan together accounting for about 50% of nuclear generated

electricity (1). This important production is realized through complex systems subject to

drastic safety constraints: nuclear power plants.

Key components of such systems are motor-operated valves (MOVs), whose condition has to

be continuously monitored in order to ensure power plants security, safety and performance

requirements. Nowadays, condition monitoring of MOVs is usually based on the joint

analysis of electrical supply and internal mechanical quantities. Unfortunately, these methods

require access to the valve and increases radiation exposure of operators, leading to long

overhaul outage and high maintenance costs.

A possible solution is to realise MOVs condition monitoring by using supply voltages and

currents only, without any mechanical measurements. Indeed, electrical quantities are

accessible outside radioactively contaminated areas and allow remote and nonintrusive MOV

testing. However, necessary mechanical information initially given by mechanical

measurements has now to be extracted from MOVs electrical quantities. One possibility is to

use the instantaneous fundamental positive-sequence active power delivered to the MOV.

Indeed, this electrical quantity is closely related to the instantaneous mechanical power

hal-00586633, version 1 - 18 Apr 2011

Author manuscript, published in "The Eighth International Conference on Condition Monitoring and Machinery Failure Prevention

Technologies (CM/MFPT 2011), Cardiff : United Kingdom (2011)"

produced by the induction motor of the MOV, and is therefore an image of internal MOVs

mechanical phenomena.

This paper describes a new MOVs condition monitoring method based exclusively on the

analysis of the fundamental positive-sequence active power absorbed by these systems.

Section 2 justifies this approach through a general description of MOVs and of their usual

monitoring techniques. The real-time active power estimation algorithm developed for this

specific application is described in section 3, followed by experimental results in section 4

and some concluding remarks in section 5.

2. Motor-operated valves condition monitoring and problem statement

2.1 Motor-operated valves description

The purpose of motor-operated valves (MOVs) encountered in all nuclear power plants is to

control the flow of fluids in the whole plant fluid system. Their typical constitution is

illustrated in Fig. 1, and consists of three main parts:

- The first element is a three-phase induction machine that drives the system for opening or

closing, depending on its rotation direction.

- The second part is a mechanical actuator whose input is the shaft driven by the induction

machine, and whose output is the valve stem. Its purpose is to transform the output torque of

the induction machine into a vertical translation force applied to the valve stem, the stem

thrust. Indeed, the electrical motor drives a worm gear through a worm fixed on the machine

shaft. The stem nut, driven by the worm gear, then raises or lowers the valve stem depending

on its rotation direction.

- The third and last part of the MOV is a valve which is closed when the valve stem is

lowered and opened in the opposite case.

Figure 1. Schematic representation of a motor-operated valve

The time evolution of some MOV’s physical quantities can help to understand the different

physical phenomena occurring in such a system. Fig. 2 shows the waveforms of the stem

thrust and the electrical apparent power absorbed by the induction machine during opening

and closing strokes.

hal-00586633, version 1 - 18 Apr 2011

2.a. opening stroke

2.b. closing stroke

Figure 2. Time waveforms of MOV stem thrust and electrical apparent power

At the beginning of the opening stroke, the stem valve is under pressure in order to maintain

the valve correctly closed, the measured thrust is then strongly negative. Next, the electrical

power transient induced by the induction machine starting is clearly visible around t = 2 s.

Then, the motor drives the mechanical actuator, cancels the pressure in the stem valve (the

stem thrust tends to 0 kN just before t = 3 s) and unseats the valve (positive transient on stem

thrust just after t = 3 s). Afterwards, the valve stem rises up with a constant speed and the

whole MOV is in steady-state operation. The stem thrust and the electrical apparent power are

approximately constant during this long period of time. Just after t = 18 s, the valve stem

reaches its maximal height in the MOV and a sensor stops the induction machine and the

opening stroke ends.

The closing stroke contains similar events. Indeed, the electrical power transient generated by

the motor starting occurs just after t = 1 s and is followed by the long steady-state behaviour

when the stem lowers at constant speed. The valve finally seats around t = 17 s, and the valve

stem is put under pressure (the stem thrust becomes strongly negative) in order to correctly

close the valve. This last event necessitates an important motor torque, and implies an

increase of the electrical power visible on the corresponding curve. The closing stroke is

finally stopped by a torque switch mounted on the motor shaft.

The previous description of MOVs brings to light two types of behaviour for these systems:

- short dynamic behaviours at the beginning and at the end of strokes (motor starting,

actuator locking/unlocking, valve seating/unseating),

- long steady-state behaviours in the middle of strokes (stem rising and lowering).

The above information will be useful to understand MOVs condition monitoring techniques

described in the following paragraph.

2.2 Classical monitoring methods

Most of usual MOVs condition monitoring methods need at the same time stem thrust and

electrical supply quantities measurements in order to elaborate mechanical fault

indicators (2)(3). Unfortunately, stem thrust measurement requires access to the valve for

installation of mechanical transducers. Therefore, these ‘at-the-valve testing methods’ lead to

strong human problems (because of operators radiation exposure) and strong economic

problems (because of maintenance outage and costs). Other existing methods only use

electrical supply quantities to elaborate mechanical fault indicators (4)(5)(6), but they rely on an

hal-00586633, version 1 - 18 Apr 2011

electrical model of the induction machine present in the MOV. Consequently, such ‘model-

based methods’ lead to problems in terms of model validity, model identification and model

update usually connected to such approaches. Moreover, all these methods are based on

measurements acquired during dynamic behaviours of the MOVs because their mechanical

parts are then strongly excited. The previous paragraph has shown that such dynamic

behaviour events have very short duration, which most often leads to fault indicators with

strong variability.

Finally, these drawbacks limit the performance of the MOVs condition monitoring methods

usually employed in nuclear power plants.

2.3 Principle of the proposed method

In order to avoid the problems of the previously described classical methods, the method

proposed in this article has the following principles.

First, fault indicators are calculated by using exclusively electrical supply voltages and

currents of the MOV. This approach avoids any at-the-valve measurement and allows remote

testing since such quantities are accessible outside radioactively contaminated areas.

Second, mechanical fault indicators are obtained through the analysis of the instantaneous

fundamental positive-sequence active power absorbed by the induction machine contained by

the MOV. Indeed, the usage of this particular electrical quantity presents numerous

advantages:

- Apart from losses, it is equal to the mechanical power produced at the induction machine

output (7). Therefore, this electrical quantity is closely related to internal mechanical

phenomena occurring in the MOV.

- It is estimated by using the three voltages and currents of the induction machine, and

therefore realizes some kind of data fusion of the whole information contained in this three-

phase system.

- Its estimator is based on a physical definition and not on any induction machine model

contrary to classical methods.

Last, measurement and analysis of electrical supply quantities are done during steady-state

operations of the MOV. As shown in paragraph 2.1, such behaviours have long duration (for

example more than 10 s on Fig. 2) and lead to fault indicators with small variability.

Moreover, the measured physical quantities are stationary during these operations, and

classical signal processing tools such as spectral analysis can be used without any restriction.

Therefore, the proposed method consists of two main steps:

- the real-time estimation of the instantaneous fundamental positive-sequence active power

absorbed by the MOV,

- the analysis of this instantaneous electrical quantity in order to propose efficient

mechanical fault indicators.

These two points are further detailed in the following two sections.

3. Active power estimation algorithm

This section is dedicated to the real-time estimation algorithm of the instantaneous

fundamental positive-sequence active power in three-phase systems. The first paragraph deals

with the physical definition of this quantity, and the second paragraph describes the proposed

algorithm.

hal-00586633, version 1 - 18 Apr 2011

3.1 Power definition

A general expression for a three-phase voltage system is:

€

vat

( )

vbt

( )

vct

( )

Vacos 2

f1t+

( )

+˜

( )

Vbcos 2

f1t+

( )

+˜

( )

Vccos 2

f1t+

( )

+˜

( )

( 1 )

where

€

is the fundamental frequency and

€

v t

( )

are voltage components with frequency

different from

€

Such a system is completely characterized at the fundamental frequency

€

by the following

vector containing three complex phasors:

€

Vaej

Vbej

Vcej

( 2 )

Since Fortescue (8), it is known that such a system is equivalent to its ‘symmetrical

components’:

€

V+

V−

(

Va+aVb+a2Vc

( )

Va+a2Vb+aVc

( )

Va+Vb+Vc

( )

(

1a a2

1a2a

1 1 1

(

( 3 )

where

€

a=ej2

is a complex number, and

€

V+

€

V−

and

€

refer respectively to fundamental

positive-, negative- and zero-sequence components.

This transform is commonly used to analyse three-phase quantities in power networks

because it allows simple and intuitive physical interpretations. Indeed, the positive-sequence

component represents for example the amount of voltage contributing to the power flow from

generators to loads (9). Another useful interpretation is that the torque produced by an

induction motor is determined by the positive-sequence component of its supply voltages (9).

The three-phase voltage system given by Eq. ( 1 ) is now supposed to feed a three-phase

induction machine. Such a load absorbs a three-phase current system and transforms electrical

input power into mechanical output power. Several electrical powers have been defined for

three-phase systems (10), but the fundamental positive-sequence active power plays a very

important role. Indeed, it is known (11) that this power is the only one converted into

mechanical power by a three-phase motor. It is definition, given thanks to symmetrical

components of three-phase voltage and current systems, is the following one:

€

+=3

Re V+I+

{ }

V+I+cos

( )

( 4 )

hal-00586633, version 1 - 18 Apr 2011

where

€

{ }

is the real part and

€

the complex conjugate.

In this definition,

€

V+

and

€

I+

refer respectively to the positive-sequence components of voltage

and current systems, and

€

is the phase difference between them or equivalently the phase

angle of the complex number

€

V+I+

This electrical power being closely related to the mechanical power produced by the induction

machine of the MOV, its instantaneous value

€

( )

can be seen as the image of the internal

mechanical phenomena generated at each time in this system. This instantaneous electrical

quantity is clearly interesting to monitor mechanical phenomena and create efficient

mechanical fault indicators.

3.2 Power estimation algorithm

An elegant way to obtain a real-time estimation of

€

( )

is through the instantaneous

symmetrical components (12)(13). They are obtained thanks to a transformation similar to Eq. (

3 ), but directly applied to time-domain waveforms instead of the fundamental phasors. For

example, the instantaneous symmetrical components of the previous three-phase voltage

system are given by:

€

v+t

( )

v−t

( )

v0t

( )

(

vat

( )

+avbt

( )

+a2vct

( )

vat

( )

+a2vbt

( )

+avct

( )

vat

( )

+vbt

( )

+vct

( )

(

1a a2

1a2a

1 1 1

(

vat

( )

vbt

( )

vct

( )

(

( 5 )

By analogy with the symmetrical components defined in Eq. ( 3 ),

€

v+t

( )

€

v−t

( )

and

€

v0t

( )

refer

respectively to instantaneous positive-, negative- and zero-sequence components. This

transform thus leads to three time-domain signals among which

€

v+t

( )

and

€

v−t

( )

are complex-

valued, and

€

v0t

( )

is real-valued.

The nature of such signals can be understood thanks to their frequency contents. Indeed,

under the assumption that all Fourier transforms exist, the Fourier transform of Eq. ( 5 ) leads

to:

€

V+f

( )

V−f

( )

V0f

( )

(

3Vaf

( )

+aVbf

( )

+a2Vcf

( )

3Vaf

( )

+a2Vbf

( )

+aVcf

( )

3Vaf

( )

+Vbf

( )

+Vcf

( )

(

( 6 )

More particularly, at the fundamental frequency

€

, this relation becomes:

hal-00586633, version 1 - 18 Apr 2011

€

V+f1

( )

V−f1

( )

V0f1

( )

(

2Vaf1

( )

+a×2Vbf1

( )

+a2×2Vcf1

( )

2Vaf1

( )

+a2×2Vbf1

( )

+a×2Vcf1

( )

2Vaf1

( )

+2Vbf1

( )

+2Vcf1

( )

(

3Va+aVb+a2Vc

( )

3Va+a2Vb+aVc

( )

3Va+Vb+Vc

( )

(

V+

V−

(

( 7 )

Therefore, the spectral contents of the instantaneous symmetrical components at the

fundamental frequency

€

consist of the classical symmetrical components of the three-phase

system. More particularly, the classical fundamental positive-sequence component

€

V+

contained by the instantaneous positive-sequence component

€

v+t

( )

at frequency

€

Following this result, the output of a selective bandpass filter applied to

€

v+t

( )

whose only

passband is centered on the fundamental frequency

€

+f1

is given by

€

( )

=V+ej2

f1t

. It should

be noticed that this filter only passes spectral components located around

€

+f1

, its transfer

function being different from zero only around

€

+f1

and not around

€

−f1

. This transfer function

does not verify the so-called hermitian symmetry, and can only be obtained with complex-

valued filter coefficients. Such ‘complex filters’ are very useful for real-time

demodulation (14)(15) or real-time Hilbert transform (16) applications. Similarly, the same filter

applied to the instantaneous positive-sequence component

€

i+t

( )

of the three-phase current

system gives

€

( )

=I+ej2

f1t

A real-time estimator

€

( )

of the fundamental positive-sequence active power

€

defined by

Eq. ( 4 ) is finally obtained by using the two previous filtered signals in the following way:

€

( )

Re v+

( )

×i+

( )

{ }

( 8 )

Figure 3 gives a schematic representation of the proposed algorithm.

Figure 3. Real-time estimation of the fundamental positive-sequence active power

Clearly, each operation appearing in Fig. 3 (the linear combination of Eq. ( 5 ), the complex

bandpass filter or the product and real part of Eq. ( 8 )) can be easily real-time implemented.

In the next section, this algorithm is applied to realize MOVs condition monitoring.

hal-00586633, version 1 - 18 Apr 2011

4. Experimental results of MOVs condition monitoring

The algorithm described in the previous section is now applied to the electrical supply

quantities of a MOV. The aim is to estimate the instantaneous value of the fundamental

positive-sequence active power, and next to employ this estimate to monitor the mechanical

condition of the MOV.

4.1 Active power waveform

The proposed algorithm is applied to the same data used to obtain the curves shown in Fig. 2

and consisting of an opening and a closing stroke. The estimated value of the fundamental

positive-sequence active power

€

( )

appears in red in Fig. 4, while the apparent power first

shown in Fig. 2 now appears in black.

4.a. opening stroke

4.b. closing stroke

Figure 4. Time waveforms of MOV stem thrust, apparent and active powers

Naturally, the estimated active power

€

( )

remains lower than the apparent power absorbed

by the induction machine. However, as expected, important mechanical events appear much

more clearly in the active power. This is verified for the hammer blow and the valve unseating

(the two events following the starting transient at the beginning of the opening stroke), and for

the valve seating (at the end of the closing stroke). These results validate the idea that

€

( )

closely related to internal mechanical phenomena occurring into the MOV.

Moreover, dynamic and steady-state behaviours of the MOV are clearly visible in Fig. 4. In

what follows, the estimated active power is only analyzed during steady-state behaviours. The

zoom realized in Fig. 5 shows that during these particular periods, the active power is mainly

constituted by periodic components. Following previous considerations, these periodic

variations can be considered as the image of mechanical phenomena generated into the MOV

during steady-state behaviours, such as the rotation of the worm attached to the induction

machine shaft, the rotation of the stem nut, …

hal-00586633, version 1 - 18 Apr 2011

Figure 5. Time waveforms of MOV active power during steady-state operations

4.2 Active power spectral contents

Spectral analysis of

€

( )

during the steady-state behaviours of the MOV allows to quantify

the importance of each periodic components, and to link them to specific known mechanical

phenomena. Fig. 6 represents the power spectral densities obtained for the corresponding

signals of Fig. 5.

Figure 6. Power spectral densities of MOV active power during steady-state operations

These results show that the active power is constituted of the same periodic components for

the opening and closing stroke. A first group of important components is located around the

rotation frequency of the induction machine, just below 25 Hz. Therefore, the power of these

components represents the quantity of mechanical phenomena applied to the shaft of the

hal-00586633, version 1 - 18 Apr 2011

induction machine at each revolution. A second group of components is located in the low

frequency band with a fundamental frequency of 0.82 Hz, the rotation frequency of the stem

nut. Therefore, the power of these low frequency components is directly related to the

mechanical efforts generated in the stem nut, and can be used as an indicator of the stem nut

mechanical condition.

Finally, these results show that the spectral contents of

€

( )

can be used to monitor the

mechanical condition of different parts of the MOVs.

4.3 Fault indicators and experimental results

These results encourage the creation of mechanical fault indicators based on the analysis of

the estimated active power. One example is the condition monitoring of the stem nut through

the low frequency contents of

€

( )

. In that case, the indicator is simply obtained by summing

the power spectral density of

€

( )

from 0 to 10 Hz, and represents the quantity of low

frequency phenomena in

€

( )

, and therefore the stem nut mechanical condition. Figure 7

shows the results obtained with this particular indicator applied to 14 similar MOVs in

different French nuclear power plants. This indicator reaches a high value for MOVs number

2 and 12. These particular MOVs have been controlled, and it has been noticed that the

corresponding stem nuts present an important lack of grease. This first result is encouraging

and other efficient mechanical fault indicators should be proposed in the future to monitor

MOVs condition through electrical active power.

Figure 7. Stem nut fault indicator based on low frequency components of

€

( )

5. Conclusion

In this paper, a new condition monitoring technique dedicated to motor-operated valves in

nuclear power plants has been proposed. This method is based on the idea that the

fundamental positive-sequence active power absorbed by the MOV can be used to monitor the

mechanical condition of this system. As a consequence, a new real-time estimation algorithm

of this electrical quantity has been presented in section 3 (see Fig. 3). Moreover, an example

of one mechanical failure indicator elaborated from the previous estimated active power has

hal-00586633, version 1 - 18 Apr 2011

been described in section 4. Obtained results, shown in Fig. 4, are very encouraging and

allows to monitor the mechanical condition of one particular part of the MOV.

Futur works need to be addressed, however. For example, others fault indicators are needed to

monitor the condition of each part of the MOV independently.

Acknowledgements

The author would like to thank EDF-DTG for his assistance and financial support.

References and footnotes

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April 4, 2011, from:

http://en.wikipedia.org/w/index.php?title=Nuclear_power&oldid=422269454.

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3. S. Kang et al, ‘A study on the actuator efficiency behavior of safety-related motor-

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4. J-C. Jund and P-H. Seong, ‘Error analysis in improved motor control center method for

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5. S. Kang et al, ‘Motor control center based technology study for safety-related motor-

operated valves’, Nuclear Engineering and Technology, Vol 38, N° 2, pp 155-162, 2005.

6. J. Chai, ‘Non-invasive diagnostics of motor-operated valves’, Proceedings of the

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7. P. L. Alger, ‘Induction machines’, 2d edition, Gordon & Breach Science Pub, 1995.

8. Charles L. Fortescue, ‘Method of symmetrical co-ordinates applied to the solution of

polyphase networks’, AIEE Transactions, Vol 37, Part II, pp 1027-1140, 1918.

9. M. Bollen and I. Gu, ‘Signal processing of power quality disturbances’, Wiley IEEE

Press, pp 68-71, 2006.

10. A. E. Emanuel, ‘Summary of IEEE standard 1459: definitions for the measurement of

electric power quantities under sinusoidal, nonsinusoidal, balanced or unbalanced

conditions’, IEEE Transactions on Industry Applications, Vol 40, N° 3, pp 869-876,

May-June 2004.

11. S. Orts-Grau et al, ‘Discussion on useless active dans reactive powers contained in the

IEEE standard 1459’, IEEE Transactions on Power Delivery, to be appeared, 2011.

12. W. V. Lyon, ‘Transient analysis of alternating-current machinery’, Wiley, 1954.

13. G. C. Paap, ‘Symmetrical components in the time domain and their application to power

network calculations’, IEEE Transactions on Power Systems, Vol 15, N° 2, pp 522-528,

May 2000.

14. Y. T. Sheen, ‘A complex filter for vibration signal demodulation in bearing defect

diagnosis’, Journal of Sound and Vibration, Vol 276, N° 1-2, pp 105-119, 2004.

15. P. Granjon et al, ‘Surveillance du désalignement d’un moteur asynchrone par analyse du

vecteur d’espace courant’, GIS 3SGS workshop, Nancy France, June 2009.

16. A. Reilly et al, ‘Analytic signal generation – tips and traps’, IEEE Transactions on Signal

Processing, Vol 42, N° 11, November 1994.

hal-00586633, version 1 - 18 Apr 2011

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Sep 2016

In the context of the KAStrion European project, a complete solution was proposed in order to monitor wind turbines. The developed solution comprises both hardware and software sections of the condition monitoring system. In terms of software, modules for vibration analysis and electrical signal analysis have been developed. This paper presents the electrical analysis solution proposed in the context of the project. The electrical module is able to detect both mechanical and electrical faults in a wind turbine system. The goal of mechanical fault detection using electrical signals is to confirm the faults also detected by vibration analysis, while the main focus of the module remains the detection of electrical faults. Results showing the performance of mechanical fault detection are presented using electrical signals acquired on the test-bench developed for testing the KAStrion system. Moreover, results regarding the electrical unbalance are presented using signals acquired on a three-phase transformer. Since the end of 2014, the final solution has been implemented on two onshore wind turbines and the online condition monitoring results are presented at the end of the paper.

A Review of Prognostics and Health Management Applications in Nuclear Power Plants

Article

Full-text available

Jul 2015

The US operating fleet of light water reactors (LWRs) is currently undergoing life extensions from the original 40- year license to 60 years of operation. In the US, 74 reactors have been approved for the first round license extension, and 19 additional applications are currently under review. Safe and economic operation of these plants beyond 60 years is now being considered in anticipation of a second round of license extensions to 80 years of operation. Greater situational awareness of key systems, structures, and components (SSCs) can provide the technical basis for extending the life of SSCs beyond the original design life and supports improvements in both safety and economics by supporting optimized maintenance planning and power uprates. These issues are not specific to the aging LWRs; future reactors (including Generation III+ LWRs, advanced reactors, small modular reactors, and fast reactors) can benefit from the same situational awareness. In fact, many small modular reactor (SMR) and advanced reactor designs have increased operating cycles (typically four years up to forty years), which reduce the opportunities for inspection and maintenance at frequent, scheduled outages. Understanding of the current condition of key equipment and the expected evolution of degradation during the next operating cycle allows for targeted inspection and maintenance activities. This article reviews the state of the art and the state of practice of prognostics and health management (PHM) for nuclear power systems. Key research needs and technical gaps are highlighted that must be addressed in order to fully realize the benefits of PHM in nuclear facilities. © 2015, Prognostics and Health Management Society. All Rights Reserved.

Discussion on Useless Active and Reactive Powers Contained in the IEEE Standard 1459

Article

Full-text available

May 2011

This paper deals with the useless active powers and the reactive powers included in the unbalance power (S_U1), and in the nonfundamental effective apparent power (S_eN) defined in IEEE Standard 1459. Only the fundamental positive-sequence active power (P₁⁺) must be delivered to loads under ideal supply operating conditions. Fundamental negative- and zero-sequence active powers (P₁^- and P₁⁰ , respectively), and the harmonic active power (P_H) are considered useless active powers because they increase the power losses in the load and in the distribution lines, reducing the global efficiency of the electrical power systems. S_U1 and S_eN are calculated in this paper for two cases, resolving the effective apparent power (S_e) by means of the product between the terms of the effective voltage (V_e) and the effective current (I_e). It yields to expressions of some useless active powers and reactive powers that differ from the definitions included in IEEE Standard 1459. New definitions of V_e, I_e, and S_e are proposed to avoid the disagreements detailed in this paper. The new definitions are compared with the IEEE Standard 1459 definitions by means of one numerical example. Magnitudes calculated by means of the IEEE Standard 1459 definitions present a general overvaluation that arrives at a maximum of 83% for S_U1.

Surveillance du désalignement d'un moteur asynchrone par analyse du vecteur d'espace courant

Article

Full-text available

Jun 2009

Recent advances have allowed the widespread use of electromechanical actuators with asynchronous machine in large systems. This article presents a new method of monitoring such actuators by analyzing their current supply. The proposed method is based on a theoretical signature model of mechanical defects in currents, specially developed for this work. The method also takes into account the three-phase nature of electrical quantities. The excellent performances achieved by this approach are illustrated with experimental data.

Analytic signal Generation—Tips and Traps

Article

Full-text available

Dec 1994

In this correspondence we discuss methods to produce the discrete analytic signal from a discrete real-valued signal. Such an analytic signal is complex and contains only positive frequencies. Its projection onto the real axis is the same as the original signal. Our use stems from instantaneous-frequency estimation and time-frequency signal analysis problems. For these problems the negative frequency component of real signals causes unwanted interference. The task of designing a filter to produce an approximation to the ideal analytic signal is not as simple as its formulation might suggest. Our result is that the direct methods of zeroing the negative frequencies, or using Hilbert transform filters, have undesirable defects. We present an alternative which is similar to the “quadrature“ filters used in modem designs

MOTOR CONTROL CENTER (MCC) BASED TECHNOLOGY STUDY FOR SAFETY-RELATED MOTOR OPERATED VALVES

Article

Jan 2006

It is necessary to monitor periodically the operability of safety-related motor-operated valves (MOVs) in nuclear power plants. However, acquiring diagnostic signals for MOVs is very difficult, and doing so requires an excessive amount of time, effort, and expenditure. This paper introduces an accurate and economical method to evaluate the performance of MOVs remotely. The technique to be utilized includes electrical measurements and signal processing to estimate the motor torque and the stem thrust, which have been cited as the two most effective parameters in diagnosing MOVs by the US Nuclear Regulatory Commission. The motor torque is calculated by using electrical signals, which can be measured in the motor control center (MCC). Some advantages of using the motor torque signature over other signatures are examined. The stem thrust is calculated considering the characteristics of the MOV and the estimated motor torque. The basic principle of estimating stem thrust is explained. The developed method is implemented in diagnostic equipment, namely, the Motor Operated Valve Intelligent Diagnostic System (MOVIDS), which is used to obtain the accuracy of and to validate the applicability of the developed method in nuclear power plants. Finally, the accuracy of the developed method is presented and some examples applied to field data are discussed.

A study on a characteristic of stem friction coefficient for motor operated flexible wedge gate valve

Article

Oct 2009
NUCL ENG DES

Stem friction coefficient is a coefficient that represents friction between thread leads of the stem and stem nut. It is an important factor to determine output thrust delivered from the actuator to the valve stem in assessing performance of motor operated valves. This study analyzes the effects of changes in differential pressure on stem friction coefficient, and determines the bounding value of stem friction coefficient. A dynamic test was conducted on multiple flexible wedge gate valves in various differential pressure conditions, and the test data was statistically analyzed to determine the bounding value. The results show that stem friction coefficient in middle and high differential pressure is influenced by fluid pressure, while stem friction coefficient in low differential pressure is almost not affected by fluid pressure. In addition, it is found that the bounding value of stem friction coefficient is higher in a closing stroke than in an opening stroke.

Method of Symmetrical Co-Ordinates Applied to the Solution of Polyphase Networks

Article

Aug 1918

C. L. Fortescue

In the introduction a general discussion of unsymmetrical systems of co-planar vectors leads to the conclusion that they may be represented by symmetrical systems of the same number of vectors, the number of symmetrical systems required to define the given system being equal to its degrees of freedom. A few trigonometrical theorems which are to be used in the paper are called to mind. The paper is subdivided into three parts, an abstract of which follows. It is recommended that only that part of Part I up to formula (33) and the portion dealing with star-delta transformations be read before proceeding with Part II. Part I deals with the resolution of unsymmetrical groups of numbers into symmetrical groups. These numbers may represent rotating vectors of systems of operators. A new operator termed the sequence operator is introduced which simplifies the manipulation. Formulas are derived for three-phase circuits. Star-delta transformations for symmetrical co-ordinates are given and expressions for power deduced. A short discussion of harmonics in three-phase systems is given. Part II deals with the practical application of this method to symmetrical rotating machines operating on unsymmetrical circuits. General formulas are derived and such special cases, as the single-phase induction motor, synchronous motor-generator, phase converters of various types, are discussed.

Non-invasive diagnostics of motor-operated valves

Conference Paper

Aug 1994

This paper is concerned with the development of data analysis methods to be used in a model-based approach to the online monitoring and diagnosis of motor-operated valves (MOVs). The technique to be utilized will include the extensive integration of mechanical and electrical measurements and signal processing. A torque estimator is developed and tested to obtain electric torque of the induction motors which are attached to the MOV system. Transfer functions between the actuator housing vibration and gear meshing force are measured and inverse filters are designed to recover the source waveforms. To monitor the operating condition, amplitudes and frequencies of the recovered signal are examined. Various frequency demodulation techniques are studied to find the most robust method. An adaptive linear phase bandpass filter is developed to improve signal-to-noise ratio and to track the important frequency components for diagnostic purposes during the operation. Finally, stand-alone valve experiments are carried out to validate the developed estimation scheme. Faults are introduced into the experiment set-up by placing obstructions in the path of the closing valve. The results of this study will be directly applied in the monitoring of MOVs and the methods developed can be applied to other diagnostic system as well. An additional benefit of this study will be a reduction in reliability problems associated with torque switch failures in existing MOVs.

A complex filter for vibration signal demodulation in bearing defect diagnosis

Article

Sep 2004

Yuh-Tay Sheen

In this paper, a complex filter for Hilbert transform is proposed to apply in the real-time vibration signal demodulation. The filter could provide a complex signal directly, as a function of both frequency and time, and the envelope could be derived from the absolute value of the complex signal. Three parameters, the scaling factor, center frequency and passband width, are designated to achieve the satisfactory properties of fast waveform convergence, constant passband gain and little phase distortion. Thus, a finite waveform interval of the proposed filter could be possibly applied in the vibration signal demodulation. From theoretical analysis and experimental studies, it is shown that the proposed filter could be effectively applied in the real-time vibration signal demodulation for a roller bearing system.

A Study on the Actuator Efficiency Behavior of Safety-Related Motor Operated Gate and Globe Valves

Chapter

Sep 2011
NUCL ENG DES

Periodically, the operability of the safety-related motor-operated valves (MOVs) in nuclear power plants must be verified. Because the actuator efficiency is one of the most important factors in the determination of the actuator output, it should be considered in ensuring the operability of MOVs during the verification duration. In particular, special consideration should be paid to its potential degradation, but the design efficiency provided by manufacturers is usually used because the actuator efficiency calculation is difficult and requires considerable time and effort. In this paper, a method is introduced to calculate actuator efficiency by using diagnostic signals acquired in field tests. The actuator efficiency was calculated from the estimated motor torque, the stem thrust measured in field tests, and overall gear ratio provided by manufactures. The motor torque was estimated by using an algorithm, which can calculate electric torque from the three phases of currents and voltages, resistances between phases acquired in field tests. The validation of the design efficiencies was evaluated by comparing those efficiencies with the calculated actuator efficiencies. And, the age-related degradation was analyzed through the behavior analysis over time of the calculated actuator efficiencies. Most of the actuator efficiencies were found not to be degraded over time and kept efficiency greater than the design efficiency. However, two actuator efficiencies with lower motor speed, overall gear ratio, and maximum motor torque rating are susceptible to be lower than the design efficiencies. For the two actuators, threshold efficiencies were calculated and provided to replace their design efficiencies.

Signal Processing of Power Quality Disturbances

Book

Sep 2006

Math Bollen

Bridging the gap between power quality and signal processing This innovative new text brings together two leading experts, one from signal processing and the other from power quality. Combining their fields of expertise, they set forth and investigate various types of power quality disturbances, how measurements of these disturbances are processed and interpreted, and, finally, the use and interpretation of power quality standards documents. As a practical aid to readers, the authors make a clear distinction between two types of power quality disturbances: Variations: disturbances that are continuously present Events: disturbances that occur occasionally A complete analysis and full set of tools are provided for each type of disturbance: Detailed examination of the origin of the disturbance Signal processing measurement techniques, including advanced techniques and those techniques set forth in standards documents Interpretation and analysis of measurement data Methods for further processing the features extracted from the signal processing into site and system indices The depth of coverage is outstanding: the authors present and analyze material that is not covered in the standards nor found in the scientific literature. This text is intended for two groups of readers: students and researchers in power engineering who need to use signal processing techniques for power system applications, and students and researchers in signal processing who need to perform power system disturbance analyses and diagnostics. It is also highly recommended for any engineer or utility professional involved in power quality monitoring. © 2006 Institute of Electrical and Electronics Engineers, Inc. All rights reserved.