19.IJAEST Vol No 7 Issue No 2 a New Approach to Diagnosis of Power Quality Problems Using Expert...

8/6/2019 19.IJAEST Vol No 7 Issue No 2 a New Approach to Diagnosis of Power Quality Problems Using Expert System 290 297

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A new approach to Diagnosis of Power Quality

Problems using Expert System

A.N.Malleswara Rao Dr. K. Ramesh Reddy Dr. B. V. Sanker RamResearch Scholar Dean & HoD, EEE Dept. Professor, EEE Dept.,

JNTUH, Hyderabad GNITS, Hyderabad JNTUH, [email protected] [email protected]

Abstract: In this paper, a new approach is presented to identify

power quality disturbances using fuzzy logic systems. Distributed

single phase power electronic loads are usually treated as fixed

harmonic current injectors in distribution system. Their current

harmonics are characterized by normalized phasors |Ih/I1|angle h,

where the fundamental current component I1 is varied

proportionally with load power. Fixed harmonic current injection

lead to an over estimation of resulting voltage harmonics because it

neglects phase angle, dispersion of individual current harmonics.

Harmonic currents generated by single phase power electronicloads too small to cause any appraisable distribution feeders.

However as the number of these loads increases and as larger

nonlinear loads the cumulative harmonics becomes very significant.

Fuzzy logic is used to diagnose all these problems. Detailed digital

simulation results involving various types of transient power

quality disturbances are presented to prove the ability of the new

approach in classifying these disturbances..

Index TermsFuzzy logic, power system harmonics, Power

quality, Fault analysis

I. INTRODUCTION

Harmonic is a core issue regarding Power Quality. It is

not only pollutes the power system but also have negative effects

on electrical equipments. With increasing use of power

electronics in industrial, commercial, and residential consumers,

the issue of harmonics and their effects on performance of

electrical installation and electronic equipment increases [1].

The objective of the electric utility is to deliver sinusoidal

voltage at fairly constant magnitude throughout their system.

This objective is complicated by the fact that there are loads on

the system that produce harmonic currents. These currents

results in distorted voltage and current that can adversely impact

the system performance in different ways. A number of

harmonic producing loads have increased over the years, it has become increased necessary to address their influence when

making any additional or changes to an installation.

The fundamental or basic power frequency is 50 Hertz. This

means the waveform of the electricity repeats itself 50 times

every second. The characteristic harmonics are based on number

of rectifiers (pulse number) used in a circuit &can be determined

by the following equation

h= (n*p) 1 ---- (1)

where n=an integer (1, 2, 3, 4, 5.)

p=number of pulse or rectifiers

A pure AC sine wave is ideal, such as shown in figure 1(a), and

is obtained if the load is an incandescent light bulb.

Fig 1(a) Ideal AC sine wave

In a three phase system, three sine waves are generated120

degrees apart, as shown in figure 1(b)

Fig1(b).Three phase system

Harmonic frequencies are multiples of the fundamenta

frequency. Any distorted voltage or current wave is the sum of

the fundamental frequency and a number of the above

harmonics. Figure 2(a) shows the fundamental and the third

harmonic. Figure 2(b) demonstrates the addition of the harmonic

and the fundamental frequency.

A.N.Malleswara Rao* et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIESVol No. 7, Issue No. 2, 290 - 297

ISSN: 2230-7818 @ 2011 http://www.ijaest.iserp.org. All rights Reserved. Page 290
mailto:[email protected]:[email protected]


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Fig 2(a).Fundamental and the third harmonic wave forms

Fig 2(b).Combined waveform of fundamental and third

harmonic component.

II. BACKGROUND HISTORY

The voltage pushing that current through the load

circuit is described in terms of frequency and amplitude. The

frequency of the current will be identical to the frequency of the

voltage as long as the load impedance does not change. In a

linear load, like a resistor, capacitor or inductor, current and

voltage will have the same frequency. As long as the

characteristics of the load components do not change, the

frequency component of the current will not change. But in case

of non-linear loads such as switching power supplies, saturated

transformers, charged capacitors, and converters used in drives,

the characteristics of the load are dynamic. As the amplitude of

the voltage changes and the load impedance changes, the

frequency of the current will change. The changing current and

resulting complex waveform is a result of these load changes.

The complex current waveform can be described by defining

each component of the waveform. The frequencies that are

normally dealt with using drives are 50 or 60 Hz. By definition,

these frequencies are termed fundamental in their respective

distribution systems.

The drawing shown indicates how current follows the

frequency when the load is linear only a phase angle change isshown in Fig 3.

Fig 3. Current follows the frequency when the load is linear and

phase angle change.

When the load become non-linear, the current is not continuous

and will contain many frequencies. Figure 4 shows how the

current might look like in a non-linear circuit.

Fig 4.Line current in non-linear circuit

A. Source of Harmonics

Harmonic load currents are generated by all non-linear loads

These include single phase loads, e.g.

Switched mode power supplies (SMPS) Electronic fluorescent lighting ballasts Small uninterruptible power supplies (UPS) units

Three phase loads, e.g.

Variable speed drives Large UPS units Highly Fluxed Iron Cores

B.Problems caused by harmonics

Harmonic currents cause problems both on the supply and

within the installation. The effects and the solutions are very

different and need to be addressed separately; the measures that

are appropriate to controlling the effects of harmonics within the

installation may not necessarily reduce the distortion caused on

the supply and vice versa [2]. There are several common

problem areas caused by harmonics:

1) Overloading of neutrals

In a three-phase system the voltage waveform from each phase

to the neutral star point is displaced by 120so that, when each

phase is equally loaded, the combined current in the neutral is

zero. In the past, installers have taken advantage of this fact by

installing half-sized neutral conductors. However, although the

fundamental currents cancel out, the harmonic currents do not -




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in fact those that are an odd multiple of three times the

fundamental, the triple-N harmonics, add in the neutral.

2) Overheating of transformers

Transformers are affected in two ways by harmonics. Firstly, the

eddy current losses, normally about 10 % of the loss at full load,

increase with the square of the harmonic number. This results in

a much higher operating temperature and a shorter life. The

second effect concerns the triple-N harmonics. When reflected back to a delta winding they are all in phase, so the triple-N

harmonic currents circulate in the winding. The triple-N

harmonics are effectively absorbed in the winding and do not

propagate onto the supply, so delta wound transformers are

useful as isolating transformers.

3) False tripping of circuit breakers

Residual current circuit breakers (RCCB) operate by summing

the current in the phase and neutral conductors and, if the result

is not within the rated limit, disconnecting the power from the

load. False tripping can occur in the presence of harmonics for

two reasons. Firstly, the RCCB, being an electromechanical

device, may not sum the higher frequency components correctly

and therefore trips erroneously. Secondly, the kind of equipment

that generates harmonics also generates switching noise that

must be filtered at the equipment power connection.

4) Over-stressing of power factor correction capacitors

Power factor correction capacitors are provided in order to draw

a current with a leading phase angle to offset lagging current

drawn by an inductive load such as induction motors. The

impedance of the PFC capacitor reduces as frequency rises,

while the source impedance is generally inductive and increases

with frequency. The capacitor is therefore likely to carry quite

high harmonic currents and, unless it has been specificallydesigned to handle them, damage can result. A potentially more

serious problem is that the capacitor and the stray inductance of

the supply system can resonate at or near one of the harmonic

frequencies when it occurred, very large voltages and currents

can be generated, often leading to the catastrophic failure of the

capacitor system [4].

C.Problems caused by harmonic voltages:

1) Voltage distortion:

Because the supply has source impedance, harmonic load

currents give rise to harmonic voltage distortion on the voltage

waveform (this is the origin of flat topping). There are two

elements to the impedance: that of the internal cabling from the

point of common coupling (PCC), and that inherent in the

supply at the PCC, e.g. the local supply transformer. The

distorted load current drawn by the non-linear load causes a

distorted voltage drop in the cable impedance. The resultant

distorted voltage waveform is applied to all other loads

connected to the same circuit, causing harmonic currents to flow

in them - even if they are linear loads. Here separate circuits

feed the linear and non-linear loads from the point of common

coupling, so that the voltage distortion caused by the non-linear

load does not affect the linear load.

2) Induction motorsHarmonic voltage distortion causes increased eddy

current losses in motors in the same way as in transformers

However, additional losses arise due to the generation of

harmonic fields in the stator, each of which is trying to rotate the

motor at a different speed either forwards or backwards. High

frequency currents induced in the rotor further increase losses.

3) Zero-crossing noise

Many electronic controllers detect the point at which the supply

voltage crosses zero volts to determine when loads should be

turned on. This is done because switching reactive loads at zero

voltage does not generate transients, so reducing electromagnetic

interference (EMI) and stress on the semiconductor switching

devices. When harmonics or transients are present on the supply

the rate of change of voltage at the crossing becomes faster and

more difficult to identify, leading to erratic operation. There may

in fact be several zero-crossings per half cycle.

D.Problems caused when harmonic currents reach the supply

When a harmonic current is drawn from the supply it gives rise

to a harmonic voltage drop proportional to the source impedance

at the point of common coupling (PCC) and the current. Since

the supply network is generally inductive, the source impedance

is higher at higher frequencies[5]. Of course, the voltage at the

PCC is already distorted by the harmonic currents drawn byother consumers and by the distortion inherent in transformers,

and each consumer makes an additional contribution.

III. ELECTRICAL EQUIVALENT OF POWER

SYSYTEMS:

The equivalent circuit of a non-linear load is shown in Figure 5

It can be modeled as a linear load in parallel with a number of

current sources, one source for each harmonic frequency. The

harmonic currents generated by the load or more accurately

converted by the load from fundamental to harmonic current

have to flow around the circuit via the source impedance and all

other parallel paths. As a result, harmonic voltages appear across

the supply impedance and are present throughout the

installation. Source impedances influence the harmonic voltage

distortion resulting from a harmonic current.




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Fig 5. Equivalent circuit of Non-linear load

Whenever harmonics are suspected, or when trying to verify

their absence, the current must be measured.

The Fourier series represents an effective way to study

and analyze harmonic distortion. It allows inspecting the various

constituents of distorted waveform through decomposition.

Generally, any periodic wave form can be expanded in the form

of a Fourier series

----(2)

Measures of Harmonic distortion:

A distorted periodic current or voltage waveform expanded into

a Fourier series is expressed as follows

------ (3)

Accordingly, the following relationship for active power andreactive power supply are

----- (4)

Reactive power is defined as

----- (5)

Voltage distortion factor VDF, also known as voltage total

harmonic distortion is defined as

------ (6)

Analogously, Current distortion factor CDF, further known as

Current total harmonic distortion is defined as

------(7)

Where V1, I1 represent the fundamental peak voltage and curren

respectively. With

------ (8)




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the apparent power is

----(9)

Where S1 is the apparent power at fundamental frequency and

the power factor is

----(10)

A.Harmonic phase sequences

The phase sequences of harmonic waveforms at

different frequencies are compared with the fundamental

waveform and are classified as positive, zero and negative

sequence harmonics. Harmonics such as the 7th, which "rotate"

with the same sequence as the fundamental, are called positive

sequence. Harmonics such as the 5th, which "rotate" in the

opposite sequence as the fundamental, are called negative

sequence. Triplen harmonics which don't "rotate" at all because

they're in phase with each other are called zero sequence.

In a balanced three-phase power system, the currents in phases

a-b-c are shifted in time by +/-120 degrees of fundamental.

Therefore, since

----- (11)

then the currents in phases b and c lag and lead by 2/3 radians,

respectively. Thus

-----(12)

Expanding the above series,

By examining the current equations, it can be seen that

i) The first harmonic (i.e., the fundamental) is positive sequence

(a-b-c) because phase b lags phase a by 120, and phase c leads

phase a by 120,

ii) The second harmonic is negative sequence (a-c-b) because

phase b leads phase a by 120, and phase c lags phase a by 120,iii) The third harmonic is zero sequence because all three phases

have the same phase angle.

All harmonic multiples of three (i.e., the triplens) are zero

sequence. The next harmonic above a triplen is positive

sequence; the next harmonic below a triplen is negative

sequence[5],[6],[7].

IV. FUZZY BASED CLASSIFICATION

Fuzzy logic (FL) can be defined as a problem-solving

control system methodology that lends itself to implementation

in systems ranging from simple, small, embedded micro-controllers to large, networked, multi-channel PC or

workstation-based data acquisition and control systems. Fuzzy

based classification technique employs a simple, rule-based IF X

AND Y THEN Z approach to a solving control problem rather

than attempting to model a system mathematically.

In the case of Neural Networks, considerable amount of

training time under different operating conditions is required for

good performance of the system. The fuzzy logic based fault

classification scheme does not require training and so it is

computationally much faster when compared to fault

classification by Artificial Neural Network (ANN) methods

This fuzzy logic based scheme is capable of accurately

predicting the exact type of fault under wide range of operating

conditions [9].




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The Fault Classification is based on Angular

differences among the sequence components of the fundamental

during fault current as well as on their relative magnitudes. The

phasor diagram of a phase a to ground fault is shown in the

Figure 6. The zero, positive and negative sequence components

of the post fault currents relative to phase a are denoted as Iaof,

Ia1f

and Ia2f

respectively.

Fig 6. Phasor diagram for L-G fault

The angles between the positive and negative sequence

components of phase a, b and c are given as

---(13)

The magnitudes of Iaof,

Ia1f

and Ia2f

are related by

Rof

=| Iaof,

/ Ia1f

| = 1 and R2f

=| Ia2f,

/ Ia1f

| = 1 ----(14)

Similarly, the magnitudes and angle between the positive and

negative sequence components are obtained for other types of

asymmetric faults.

For every type of fault, there exists a unique set of these

five parameters. So it is possible to formulate simple logic base

for determining the fault type from the values of the five inputs.

The different inputs are represented by a corresponding fuzzy

variable. Now a fuzzy rule was developed using these five

variables to detect the type of fault.

For example:

If arg_A is approximately 300

and arg_B is approximately

1500

and arg_C is approximately 1500

and Rof

is high and

Rsf

is high then fault type is a-g

In this method, only 3 parameters are sufficient and it

identifies 10 types of short-circuit faults accurately. But the main

disadvantage with this method is that it is applicable to only

asymmetric faults and it is not very effective if you are looking

to classify not just by the type of fault.

V. SIMULATION RESULTS

A. POWER QUALITY AND POWER SYSTEM EVENTS

The term power quality refers to a wide variety of

electromagnetic phenomena that characterize the voltage and

current at a given time and at a given location on the power

system. A power system event is a recorded (or observed)

current or voltage excursion outside the predetermined

monitoring equipment thresholds. A power disturbance is a

recorded (or observed) current or voltage excursion (event)

which results in an undesirable reaction in the electrica

environment or electronic equipment or systems. The term

power problem refers to a set of disturbances or conditions that

produce undesirable results for equipment, systems or a facility.

The term event is typically used to describe significant and

sudden deviations of voltage or current from its normal or idealwaveform (like in Fig 7.1) unlike the term variation which is

used to describe small deviations from the nominal values. The

monitoring of events is done using certain triggering thresholds.

Voltage or current variations are obtained by continuous

monitoring as shown in Fig 7.2.

Fig 7.1: Measurement of current during a fault

Fig 7.2: RMS voltage measurement for one day




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B. CASE STUDY: INDUCTION MOTOR STARTING

Starting of large induction motors is one more cause of voltage

dips. It has been concern for designers of industrial power

systems. During start-up an induction motor takes current five to

six times larger than normal. This current remains high until the

motor reaches its nominal speed. This lasts between several

seconds to one minute. The characteristics of the correspondingvoltage dip depend on the characteristics of the induction motor

(size, starting method, load, etc) and the strength of the system at

the point where the motor is connected. The magnitude of the

dip depends strongly on the system parameters. For the system

in Fig.8, Z0 is the source impedance and ZM the motor

impedance during starting. The voltage experienced at PCC is

found from the voltage divider equation:

where E is the source voltage. The magnitude of voltage dips

due to motor starting is rarely deeper than 0.85 pu.

Fig. 8: Voltage divider model for the calculation of voltage dip

during motor starting

The duration of the voltage dip due to motor starting

depends on a number of motor parameters. The most important

of them is the motor inertia. The duration of the dip is prolonged

if other motor loads are connected to the same bus bar, as they

will further keep the voltage down. Fig.9(a) and 9(b) shows the

fundamental frequency magnitude of the three phases during the

connection of a 500 HP induction motor on a 480 V bus as

simulated in MATLAB.

The short circuit level of the bus bar is 30 MVA. The

initial drop in voltage is 0.09 pu and it takes approximately 400

msec for voltage to reach its steady state value. This voltage dipis symmetrical, all three phases drop equally and then recover

gradually in a similar way because the starting current of the

motor is the same for all three phases. A measurement of a

voltage dip due to induction motor starting is shown in Fig10(a)

and 10.(b). The measurement comes from a low voltage

network. The three phases present exactly the same

characteristics. The voltage recovers within 7-8 cycles

Summarizing, voltage dips due to induction motor starting are:

Non-rectangular: voltage recovers gradually.

Symmetrical: all phases present the same behavior.

Figure 9 (a)

Figure 9 (b)

Fig. 9: Induction motor starting (a) Voltage waveforms (b)

Voltage magnitude (measurement in a 400 V network)

Figure 10(a)




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Figure 10 (b)

Fig. 10: Influence of induction motor load during a fault (a)

Non-rectangular (b) Symmetrical

V. CONCLUSION

This paper gives an overall view to diagnose the of power

quality problem. A brief discussion on the harmonic distortion

and their impacts on electric power quality are mentioned.

Harmonic distortion has a harmful effect on both distribution

system equipment and on loads. Because of this, harmonic

distortion is a main cause of supply quality degradation. If

harmonic distortion exceeds some limits, then equipment is

needed for its suppression. Equipment for harmonic suppression

should also compensate the reactive current thereby improving

both power factor and supply quality.

The main idea of this approach is to map the power quality

disturbance features in to real valued number though extended

fuzzy reasoning, in terms of which the PQ disturbance

waveforms are identified according to boundary values.

A study was conducted using MATLAB to investigate

the response of induction motor to voltage dips. Motor response

to the applied voltage dips show that the motor can support most

commonly encountered voltage dips. It has been demonstratedthat the dynamic behavior of induction motors is sensitive to the

values of the motor parameters.

VI. REFERENCES

[1] L. M. Tolbert, Harmonic Analysis of Electrical Distribution

Systems, Oak Ridge National Laboratory, ORNL-6887, March

1996.

[2] L. M. Tolbert, H. Hollis, P. S. Hale, Survey of Harmonics

Measurements in Electrical Distribution Systems, Conf. Record

of the 1996 IAS Annual Meeting.

[3] IEEE Task Force, Effects of harmonics on equipment,

IEEE Trans. Power Delivery, vol. 8, pp. 672-680, Apr. 1993.

[4] IEEE Emerald Book, IEEE Recommended Practice for

Powering and Grounding Sensitive Electronic Equipment, IEEE

Std 1100-1992.

[5] National Electrical Code, NFPA Std 70-1996.

[6] IEEE Recommended Practice for Establishing Transformer

Capability When Supplying Non sinusoidal Load Currents

ANSI/IEEE Std. C57.110-1986.

[7] Arrillaga.J., Bradley, D.A., and Bodger, P.S.: Power System

Harmonics, John Wiley,1985

[8] Guideline on Electrical Power for ADP Installations, Federa

Information Processing Standard Publications 94, NationalBureau of Standards, 1983.

[9] P.K.Dash, M.M.A.Salama, S.Mishra, and A.C. Liew

Classification of power system disturbances using a fuzzy

expert system and a Fourier linear combiner, IEEE

Transactions on Power Delivery, vol. 15, no. 2, pp. 472

477,April 2000.



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