Shortfalls of Existing Methods for Characterization of Voltage Sags from Recorded Waveform

Padmanabh Thakur Associate Professor

Department of Electrical and Electronics Engineering

Graphic Era University, Dehradun, India tonu_arth@rediffmail.com

Asheesh K Singh, Senior Member, IEEE Associate Professor

Department of Electrical Engineering MNNIT Allahabad, Allahabad, India

asheesh@mnnit.ac.in

Shashi Bhushan Singh Assistant Professor

Department of Electrical Engineering NIT, Kurukshetra, India shashidce@gmail.com

Abstract—This paper highlights the shortfalls, existing in the methods, currently used for the characterization of voltage sags, from recorded waveforms. Test waveform, available with IEEE data base, and data generated from Matlab®

simulations, are used to reveal their ambiguities. It is found that none of the existing methods give accurate and complete characteristics of the voltage sag event. Still large gaps exist between real and measured characteristics of the voltage sags. Hence, the need of holistic method, for characterization of voltage sags from recorded waveforms, is asserted.

Index Terms-Faults, power quality, phase voltage, symmetrical component, voltage sags.

I. INTRODUCTION Voltage sags have been regarded as one of the most

pressing issues of the industrial processes as they are frequent cause of malfunctioning of electrical equipment in the industries. Manufacturing facilities have cost ranging up-to millions of dollars attributed to a single disruption of the process due to voltage sags [1]-[10]. The two indices, namely, ‘magnitude’ and ‘duration’, have been highly recommended, in existing research and standards for their characterization. However, the ranges of these two indices vary from standard to standard [11]-[14]. Generally, reduction in root mean square (rms) voltage between 10% and 90% of the nominal voltage for the duration of half cycle to 1-minute has been considered as the voltage sag event [12]. Although, voltage sags are less severe than interruptions, but the total financial losses due to voltage sags are larger as the frequency of occurrence of voltage sags is much higher than the interruptions [1], [2], [15]. This explains the increasing concern towards accurate characterization and classification of voltage sags.

Numerous methods and standards have been reported for the characterization of the voltage sags from recorded waveforms. Taking into account the fault types, load types, and basic circuit behind the phenomenon, an institutive approach has been proposed in [2]. This approach divides that voltage sags in seven classes, namely, ABC classification. But

this method of characterization has not been recommended for characterization of the voltage sags from recorded waveforms [1], [4]-[6]. Subsequently, a systematic approach, based on symmetrical component (SC) analysis [4]-[6], the six phase (SP) algorithm [2], [7], [8], potential gradient method [9], three phase-three angle (TP-TA) algorithm [10], space vector method [16], recorded instantaneous voltage [17], [18], magnitude of rms voltage measured at high voltage network [19], [20], phase and positive sequence voltage [21], [22], voltage magnitude equality [23], sag score [24], lost energy in sag event [25], magnitude and duration severity index [26] have been reported in the literature. Additionally, several standards [11]-[14], [27], [28] were also documented for the characterization of the voltage sags. Nevertheless, gaps have been found between real and measured characteristics of voltage sags. Existing standards and methods are still found inadequate for the characterization of the voltage sags, precisely.

This paper highlights the shortfalls of the existing standards and methods that are highly recommended for the characterization of voltage sags from recorded waveforms. Test waveform and Matlab® simulations are used to reveal the impreciseness of existing characterization methods. It is shown that existing methods and standards is not suitable to provide complete and precise characteristics of voltage sags.

II. SHORTFALL OF EXISTING STANDARDS In the existing standards [11]-[14], [27]-[29], ‘magnitude’

and ‘duration’ of the voltage sags, have been strongly recommended as the most important indices for the characterization of voltage sags. The magnitude and duration as defined in these existing standards are depicted in Figure 1.

The magnitude of voltage sags is defined as the magnitude of minimum of all three–phase rms voltages during the disturbance, whereas the duration is defined as the time interval during which voltage of any phase falls below the threshold limit to the instant when all three phase voltages rise above threshold limit, as shown in Figure 1. This approach of

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characterization of the voltage sags has been denoted as minimum magnitude and total duration approach [30]. The shortfalls of this method of characterization are highlighted as follows:

a) The magnitude and duration as quantified by this approach are not found true for all phases. As it is obvious from Figure 1, the magnitude and duration of voltage sags of phase ‘a’ and ‘c’, are over-estimated by this method of characterization. Hence, the sensitivity of single-phase equipment, connected with the phase ‘a’ and ‘c’, cannot be assessed precisely as single-phase equipment responds according to phase voltage, to which it is connected.

b) Characterization with minimum magnitude and total duration results in same magnitude for symmetrical three-phase and single-phase voltage sag, as shown in Figures 2 and 3. However, the effects of these two voltage sags on three-phase equipment would be different.

0 5 10 150

0.2

0.4

0.6

0.8

1

Time ( in cycles)

Mag

nitu

de (i

n p.

u.)

Phase aPhase bPhase c

Threshold Voltage= 0.9 p.u.

Total Duration=6.47 cycles

Min. Mag.=0.56 p.u.

Figure 1. Three- phase unbalanced voltage sags

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5-1

-0.5

0

0.5

1

Time ( in cycles)

Inst.

Vol

tage

(in

p.u.

)

Figure 2. Artificially generated three-phase symmetrical voltage sags of

magnitude 0.7 p.u.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5-1

-0.5

0

0.5

1

Time (in cycles)

Inst.

Vol

tage

(in

p.u.

)

Figure 3. Artificially generated single-phase voltage sag of magnitude 0.7

p.u.

c) The importance of other dimensions of voltage sags, viz., phase angle jump, point-on-wave characteristics, missing voltage, etc., were outlined in numerous studies [31]-[35]. But least attentions have been paid by the existing standards to incorporate these dimensions for the complete and accurate characterization of the voltage sags. Indeed, these dimensions have been recommended to ignore in [29].

d) This method of characterization assumes rectangular voltage sag profile. Usually, this assumption is not true in case of voltage sags associated with the starting of induction motor (IM) [2], [30], [36]. The voltage sag profile as shown in Figure 4 is obtained, using Matlab® simulation, at load terminals due to starting of IM load connected with same bus.

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10.7

0.8

0.9

1

1.1

Time (sec.)

R.M

.S V

olta

ge (p

.u.)

t2=0.1910 sec.

t1= 0.2576 sec.

Threshold Voltage = 0.9 p.u.

Figure 4. Voltage sags at load due to starting of induction motor

As shown in Figure 4, the starting of IM results in the non-rectangular voltage profile at load terminals. The duration of voltage sags (t2) as estimated by this method of characterization is nearly 0.1910 sec, due to assumption of sharp drop and sharp rise at start and ends of the voltage sags, while the actual duration (t1) is 0.2576 sec for 90% threshold voltage. Thus the duration, nearly equal to 4 cycles or ∆t = 0.067 sec, is over-estimated by this method of characterization. This limitation has been discussed in [2] and [36], also.

e) The types of faults associated with the voltage sags and the effect of transformer on the propagation of voltage sags have not been reflected in this method of the characterization [6], [37].

Therefore, the minimum magnitude and total duration approach, supplies limited information of voltage sags, hence not sufficient for accurate assessment of equipment sensitivity. For the proper assessment of equipment sensitivity against the voltage sags, the modifications or updates in existing standards are still required.

III. SHORTFALLS OF EXISTING METHODS

A. Methods of Characterization Based on Symmetrical Component (SC) Analysis To avoid the anomalies existing in the minimum

magnitude and total duration approach, a more logical and informative algorithm to extract the characteristics of voltage

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0.6

0.8

30

210

60

240

90

270

120

300

150

330

180 0

Cb

Cc

Db

Da

Dc

CaT=0

T=1T=2

T=3

T=5

T=4

sags from recorded waveforms, has been proposed, namely SC analysis, in [4]-[6]. This analysis uses only one phasor, namely, characteristic voltage, for the characterization of two-phase voltage sags of type C and single-phase voltage sag of type D. Furthermore, this approach divides the voltage sags in six classes, based on the angle between negative-sequence (V2) and drop in positive-sequence voltage (1-V1). The types of voltage sags have been determined with the help of (1) [8].

)1

(601

1

2

VVangleT−

= (1)

The value of ‘T’ decides the types of voltage sags, as shown in Figure 5.

Figure 5. Phasor diagram for type detection of voltage

In Figure 5, type Da, Db, and Dc represent voltage sags in phase a, b, and c, whereas type Ca, Cb, and Cc represent voltage sags in two-phase, b-c, c-a, and a-b, respectively. Although, this method has been highly recommended for the characterization of the voltage sags from recorded waveforms, but it suffers from few shortfalls, simultaneously. The shortfalls of this method of characterization are highlighted as follows:

a) This approach does not provide any information about the types of faults associated with voltage sags [1], [37]. For example, T = 5 indicates the single- phase voltage sag in phase ‘b’, only. The identification of the faults associated with voltage sags is not possible [37].

b) The phase angle jump, point on wave of sag initiation, sag recovery, etc., associated with voltage sags have been ignored. Hence, this characterization approach becomes far from the real characterization.

c) The precise estimation of T is not possible at the start and end of the voltage sags [6], [8]. The value of T is determined at the start and end of the single voltage sags in phase ‘a’ for test waveform, available in [38], to reveal inaccuracy of this method of

characterization. As seen from Figure 6, the values of T at the point of initiation and recovery of voltage sags fall below ‘2’, and hence representing type Cb, whereas the actual type is Da. This limitation has been discussed in detail in [8], also, for voltage sags in two-phase.

1 1.5 2 2.5 3 3.5 4 4.5 51.6

1.8

2

2.2

2.4

2.6

2.8

3

Time (in cycles)

T

Figure 6 The variation of T with respect to time

d) Further, this method of characterization gives erroneous result at large phase angle jump conditions [1], [8], [10], [37].

Further contribution toward characterization of voltage sags from recorded waveforms and based on symmetrical component analysis has been made in [9]. But this method is limited to single-phase voltage sag, only.

B. Methods of Characrecterization Based on Phase-to-Phase and Phase-to-Neutral Voltage In several studies, attempts were made to characterize

voltage sags, using phase-to-neutral (PN) or phase-to-phase (PP) voltages, or both. In SP algorithm [7], PN and PP voltages, after subtracting zero sequence component of voltage, have been extracted from recorded waveforms and rms values of these six voltages were used to characterize the voltage sags. This method of characterization is simple but gives erroneous results under large phase angle jump conditions [8], [37], [39].

In [10], remaining voltage, inverse remaining voltage, and angle between the phase voltages have been used to extract the characteristics of voltage sags from recorded waveforms. Although this algorithm gives good result under large phase angle jump conditions, but not found suitable to identify the types of faults associated with the voltage sags. Further, attempt has been made to characterize the voltage sags from space vector derived from PN voltage of the recorded waveforms [16]. But this method has not been found suitable for the noisy conditions. Further, characterization of three phase unsymmetrical voltage has not been found suitable with space vector methodology.

In [17], rms value of minimum PN and PP voltages has been extracted from recorded waveforms to characterize the voltage sags. Although, the method presented in [17] is simple and fast to detect the types of voltage sags along with associated faults, but it gives erroneous characterization in case of type A and E, as both of these types are represented by same characteristic curve. To resolve this inconsistency,

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maximal PP and minimal PN voltage have been used in [18], for these two types of voltage sags, but the characterization method becomes two steps, hence, complex. Further contribution toward characterization of voltage sags were made in [19]-[23]. The characterization methods, presented in [19], [20], [23] have not been derived for characterization from recorded waveforms, hence these methods are not considered here for discussion. In [21] and [22], the positive sequence component of voltage and phase voltage has been proposed to characterize the voltage sags from recording. As this study is based on minimum magnitude and total duration approach, for the characterization of voltage sags, hence suffers with the same limitations, as discussed in Section II.

Additionally, numerous issues exist in precise estimation of the magnitude and duration of the voltage sags but these issues have not been highlighted in these methods. As this paper discusses the characterization method of the voltage sags, so these issues are not incorporated in this discussion. However, details of the various shortfalls of the methods of estimation of the magnitude and duration of voltage sags have been discussed properly in [2], [40]. In conclusion, it can be said that there are number of methods and standards, documented to characterize the voltage sags from recording waveforms, but none of them are adequate. Still, there is a need to improve the method of characterization for the precise assessment of the voltage sags.

IV. CONCLUSIONS This paper highlights the various shortfalls of the methods

and standards, highly recommended for characterization of voltage sags, from recorded waveforms. Three approaches, namely, minimum magnitude and maximum duration, symmetrical component, and PN/PP voltages, for characterization of voltage sags, are considered for discussion. Using Matlab® simulation and recorded waveforms, available with IEEE data base, it is revealed that, none of the existing approaches are suitable to characterize the voltage sags, precisely. It is found that, minimum magnitude and total duration approach gives erroneous characterization for unsymmetrical voltage sags whereas symmetrical component approach gives erroneous result under large phase angle jump conditions. Additionally, the methods, based on PN/PP voltage, are rigorously reviewed and it is shown that none of the methods give accurate characteristization of voltage sags. Still the gaps are found between real and measured characteristics of voltage sags. Hence, the need of holistic algorithm is required to characterize the voltage sags, accurately.

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