Comparative Analysis of Measurements
Time-Varying Harmonics
Frederico M. Carvalho, Fernando N. Belchior, IEEE Member, Paulo F. Ribeiro, IEEE Fellow Member Federal University of Itajubá, Itajubá – MG, Brazil
Abstract This paper aims to evaluate the performance of commercial power quality (PQ) instruments. By using a high precision programmable voltage and current source, two meters from different manufacturers are analyzed and compared. Three-phase voltage signals are applied to PQ instruments of different classes, considering 3 situations of time varying harmonic distortions. This work is relevant considering that Brazil is deploying standardization in terms of power quality measurements aiming towards regulatory procedures and index limits.
Index Terms Electrical Engineering, Indexes, Monitoring, Power Engineering, Power Quality.
I.INTRODUCTION
Power Quality (PQ) is an important subject for electric utilities by several reasons, among them the need to develop rules that provide means to measure, in an appropriate way, the PQ indices. Poor PQ indices affect functioning of utilities, different industrial units, productions, customer services and other system performance and operating costs [1].
The increased requirements on supervision, control, and performance in modern power systems make power quality monitoring a required practice for utilities. Power Quality is related to a set of parameters about disturbances in voltage and current with goals of monitoring, mitigating, regulating. It makes possible the distribution of electricity to end users according to accepted requirements for normal operation of the electrical distribution system and characteristics of voltage (magnitude, frequency, symmetry, waveform etc.). Some electronic devices, such as microprocessors, micro-controllers, sensitive computerized equipment etc., use ground as the reference for all their internal operations and connect throughout the plant. This makes them susceptible to ground differences and to power quality problems. Power disturbances compromise product quality, increase downtime, and reduce customer satisfaction. By knowing the amount of harmonics, transient impulses and noise distortion in the system, it is possible to take appropriate actions to reduce the harmful effects [1], [2].
Consequently, there is an ever increasing need for power quality monitoring systems. This performance analysis is of particular interest because it has been found that different PQ instruments fully meeting characteristics prescribed in the standards may still disagree significantly in some specific actual measurements [3], [4], [5]. It is important to mention
that most references listed in this paper about PQ monitoring are consistent with the IEC framework [6], [7].1
In Brazil there is not yet a standardization of measurement protocols for PQ instruments and different values of the same phenomenon may be presented by instruments of different manufacturers [3].
Paper [4] moves into a direction of definition of a test protocol to perform a full metrological characterization of measurement instrument for power quality monitoring. These testing protocols aim to reproduce the PQ situations associated with voltage and current signals.
Papers [5], [8] describe voltage harmonic distortion measurements in a laboratory-based comparison of power quality class A analyzers. The tested measuring instruments were designed in accordance to the requirements of the IEC 61000-4-30 standard. [6]. Four tests have indicated the variation of ±5%, based on the measured value.
Paper [9] presents an implementation of a test system for advanced calibration and performance verification of flickermeter. This is justified because it can be shown that different digital flickermeter implementations that fully meet performance tests defined in IEC 61000-4-15 [10] can still disagree significantly in some actual measurements.
With the focus on energy meters, paper [11] proposes a methodology to guide the verification of single-phase energy meters under sinusoidal and nonsinusoidal conditions. A design of an experiment that allows the testing meters under a combination of disturbances showing positive and negative correlations are described. A hardware and software system to perform verification procedures in complex situations is also presented.
It is also important to consider the calibration on the instruments. In this context, paper [12] proposes an innovative procedure, based on the Monte Carlo method. Starting from the determination of the probability density function of the uncertainty contribution of every device from the signal input stage to the analog-to-digital conversion stage, this procedure estimates the probability density function of the measurement results and the measurement standard uncertainty as the standard deviation associated with this function. The result of the experimental work done on a prototype of a power quality measuring instrument is reported.
Considering PQ monitoring by virtual instrumentation, it is common the use of the LabVIEW platform. References [13],
1
[14], [15], [16], [17] and [18] give the development of a power quality monitoring system that is suitable for utilities and end-users. With virtual instrumentation, engineers and researchers can reduce development time, design higher quality products, lower their design costs, and keep close watch on waveforms. This helps in the efficient use of electric power, and systems reliability.
In this work, different PQ instruments were subjected to the same tests condition in a laboratory and their performance were evaluated comparing the obtained values, when compared to the data that are injected by a high accuracy programmable voltage and current source.
The work has been based on the measurement procedures presented by IEC 61000-4-30 [6]. In addition, the recommendations of the Brazilian Electricity Regulatory Agency through the Distribution Procedures document - PRODIST [19], as well as the IEEE standard [20], which was taken into account.
This paper aims to show a comparative analysis of PQ instruments considering time harmonics varying [21, 22, 23]. For all cases, the results are compared with software developed at the LabVIEW platform.
II.DEFINITIONS TO VOLTAGE MEASUREMENTS
A. Voltage Magnitude
According to IEC 61000-4-30, the measuring method rms voltage must be done during a time interval of 10 cycles for 50Hz or 12 cycles for 60Hz. Each interval of 10/12 cycles must be contiguous and not overlap the adjacent intervals. This method of measurement is used for quasi-stationary signals and is not used for voltage dip, voltage swell, voltage interruption and others transients.
The rms value, by definition, includes fundamental frequency plus harmonics and inter-harmonics. The uncertainty of measurement should not exceed ±1% of the supply voltage.
B. Harmonic Voltage
The harmonic voltage measurements are defined in the IEC 61000-4-7 [24]. This standard defines a measurement of harmonics subgroups without discontinuity, composing 15 subdivisions of 10/12 cycles. Besides the harmonic rms value any instrument must be able to measure the individual harmonic voltage distortion of order h (Uh) and the total harmonic distortion of voltage (THD), as well as the phase angle of the harmonic component.
III.POWER QUALITY MEASUREMENTS
The voltage and current signals were generated by the OMICRON CMC 256-6 [25] (Fig. 1) programmable source and applied to the PQ instruments, with high-accuracy. For voltage and current the accuracy is less than 0.05%.
The electric powers and power factors showed in equations (1) to (7) were calculated using software developed at the LabVIEW platform. For this, the DAQ Acquisition NI USB-6212 (Fig. 2) [26] was used, with 16 bits of resolution, voltage transducer (0.2% of error) and a current transducer (0.15% of error).
Fig. 1. OMICRON CMC 256-6 programmable source.
Fig. 2. DAQ Acquisition NI USB-6212.
V.LABORATORY TESTS
Two commercial instruments, denoted as Class S and Class A [6], were used (Fig. 3).
Fig. 3. Setup to test the PQ where instruments were connected to the Programmable Source.
For this paper, three tests have made. Table I shows the amount of harmonics for all tests.
TABLE I.THD APPLIED IN ALL TESTS.
THD(%) U3(%) U5(%) U7(%) U11(%) U13(%)
2.9 0 0 2 1.5 1.5
10.9 3.5 10 2 1.5 1.5
15.6 3.5 15 2 1.5 1.5
6.7 3.5 5 2 1.5 1.5
The differences between each test are the duration of the harmonic’s application. Tables II, III and IV show the duration of tests I, II and III, respectively.
TABLE II.CHARACTERISTICS OF TIME-VARYING HARMONICS –TEST I.
THD (%) 2.9 0 10.9 0 15.6 0 6.7 0 2.9 0 2.9 0
Duration(s) 5 2 5 2 5 2 5 2 5 2 5 2
TABLE III.CHARACTERISTICS OF TIME-VARYING HARMONICS –TEST II.
THD (%) 2.9 0 10.9 0 15.6 0 6.7 0 2.9 0 2.9 0
TABLE IV.CHARACTERISTICS OF TIME-VARYING HARMONICS –TEST III.
THD (%) 2.9 0 10.9 0 15.6 0 6.7 0 2.9 0 2.9 0
Duration(s) 10 2 10 2 10 2 10 2 10 2 10 2
The Figs. In the next page show the results, comparing the total harmonic distortion (THD) and only the individual harmonics of 3rd and 5th orders that have changed. Figs. 4, 5 and 6 show the results of the test I. Figs. 7, 8 and 9 shows the
results of the test II. Figs. 10, 11 e 12 shows the results of the test III.
To facilitate the analysis, the tables V, VI and VII were created with the corresponding values to the Figs. 4 to12. It is important to notice that the ‘Ref’ at these tables is related to ‘Omicron results’.
Fig 4. Comparison of THD between the instruments - Test I.
Fig. 5. Comparison of U3 (%) between the instruments - Test I.
Fig. 6. Comparison of U5 (%) between the instruments - Test I.
Fig 7. Comparison of THD between the instruments - Test II.
0 2 4 6 8 10 12 14 16 18
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115
T
H
D
(%
)
TIME(s)
CLASS A CLASS S OMICRON
0 0,5 1 1,5 2 2,5 3 3,5 4
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115
U
3
(%
)
TIME(s)
CLASS A CLASS S OMICRON
0 2 4 6 8 10 12 14 16
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115
U
5
(%
)
TIME(s)
CLASS A CLASS S OMICRON
0 5 10 15 20
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125
T
H
D
(%
)
TIME(s)
Fig 8. Comparison of U3 (%) between the instruments - Test II.
Fig. 9. Comparison of U5 (%) between the instruments - Test II.
Fig. 10. Comparison of THD between the instruments - Test III.
Fig 11. Comparison of U3 (%) between the instruments - Test III. 0
1 2 3 4 5
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125
U
3
(%
)
TIME(s)
CLASS A CLASS S OMICRON
0 2 4 6 8 10 12 14 16
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125
U
5
(%
)
TIME(s)
CLASS A CLASS S OMICRON
0 5 10 15 20
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115
T
H
D
(%
)
TIME(s)
CLASS A CLASS S OMICRON
0 0,5 1 1,5 2 2,5 3 3,5 4
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115
U
3
(%
)
TIME(s)
Fig 12. Comparison of U5 (%) between the instruments - Test III.
TABLE V.RESULTS FROM TEST I.
THD(%) U3(%) U5(%)
Time (s)
Class S
Class A Ref
Class S
Class A Ref
Class S
Class A Ref
0 0.1 0.41 0.91 0 0.13 0.06 0 0.04 0.10
5 0.2 0.15 0.66 0.1 0.04 0.10 0 0.01 0.05
10 0.2 0.01 2.91 0 0 0.05 0 0 0.03
15 3 0.49 0.31 0 0 0.03 0 0 0.09
20 0.2 2.9 10.96 0.1 0 3.59 0 0 9.90
25 11 4.41 15.62 3.5 1.24 3.47 10 3.57 15.00
30 15.7 9.03 0.71 3.5 2.37 0.20 15 8.47 0.18
35 0.2 13.06 6.76 0 2.91 3.53 0 12.49 5.00
40 6.8 4.5 0.59 3.5 2.32 0.24 5 3.33 0.08
45 0.1 3.43 2.89 0 1.77 0.06 0 2.51 0.06
50 2.9 2.43 0.44 0 0.01 0.04 0 0 0.04
55 0.2 0.51 0.35 0.1 0 0.04 0 0 0.03
60 0.2 0.02 0.75 0 0 0.05 0.1 0 0.07
65 0.2 0.01 3.03 0.1 0 0.22 0 0 0.26
70 2.9 1.46 0.81 0 0 0.12 0.1 0 0.23
75 0.2 1.94 10.99 0 0 3.65 0 0 9.95
80 11 5.64 15.60 3.4 1.79 3.72 10 5.13 14.78
85 15.7 11.71 0.37 3.5 2.79 0.08 15 11.12 0.09
90 0.2 7.84 6.77 0.1 1.74 3.56 0 7.5 4.79
95 6.8 5.63 2.92 3.5 2.91 0.05 5 4.16 0.06
100 2.9 1.98 0.32 0.1 0.71 0.16 0 1 0.22
105 0.1 2.43 0.54 0 0 0.06 0.1 0 0.07
110 0.2 0.03 0.59 0 0 0.12 0.1 0 0.16
115 0.2 0.01 0.58 0 0 0.19 0.1 0 0.19
TABLE VI.RESULTS FROM TEST II.
THD(%) U3(%) U5(%)
Time (s)
Class S
Class A Ref
Class S
Class A Ref
Class S
Class A Ref
0 0.3 0.01 0.40 0.1 0 0.01 0.1 0 0.08
5 0.3 0.01 0.43 0 0 0.02 0.1 0 0.02
10 0.3 0.49 2.91 0.1 0 0.02 0 0 0.08
15 2.9 2.9 0.46 0 0 0.16 0.1 0 0.12
20 0.3 0.5 11.0 0.1 0 3.52 0 0 9.99
25 11 9.15 0.28 3.5 2.9 0.13 10 8.33 0.07
30 0.3 1.84 15.6 0.1 0.58 4.01 0 1.66 14.5
35 15.7 10.45 0.94 3.5 2.32 0.28 15 9.99 0.17
THD(%) U3(%) U5(%)
Time (s)
Class S
Class A Ref
Class S
Class A Ref
Class S
Class A Ref
40 0.3 5.23 6.76 0 1.16 3.72 0 5 4.94
45 6.8 3.39 0.39 3.5 1.75 0.05 5 2.5 0.02
50 0.4 3.38 2.89 0.1 1.75 0.08 0.1 2.5 0.07
55 2.9 1.16 0.52 0.1 0 0.09 0 0 0.08
60 0.3 1.93 0.66 0 0 0.09 0 0 0.02
65 0.3 0.01 0.65 0.1 0 0.09 0.1 0 0.12
70 0.2 0.01 0.39 0.1 0 0.10 0 0 0.08
75 0.2 0.01 0.31 0 0 0.05 0 0 0.04
80 0.3 0.01 2.92 0.1 0 0.22 0 0 0.31
85 2.9 2.28 0.61 0.1 0 0.36 0 0 0.23
90 0.4 0.52 10.9 0.1 0.01 3.69 0 0 10.0
95 11 8.57 0.83 3.5 2.73 0.20 10 7.79 0.21
100 0.3 3.67 15.6 0.1 1.17 3.97 0 3.33 14.6
105 15.7 7.85 0.76 3.5 1.75 0.29 15.1 7.49 0.35
110 0.2 7.84 6.93 0 1.75 3.71 0 7.5 5.04
115 6.8 3.39 1.26 3.5 1.74 0.33 5 2.5 0.33
120 0.3 4.5 3.17 0 2.33 0.36 0 3.33 0.31
125 0.3 0.97 1.12 0 0 0.23 0 0 0.11
TABLE VII.RESULTS FROM TEST III.
THD(%) U3(%) U5(%)
Time Class S
Class
A Ref
Class S
Class A Ref
Class S
Class A Ref
0 3 1.94 2.96 0.1 0 0.09 0 0 0.16
5 3 2.9 0.96 0 0 0.09 0.1 0 0.06
10 0.3 1.95 11.10 0.1 0 3.48 0.1 0 9.95 15 11 7.33 11.00 3.5 2.33 3.54 10 6.67 10.08 20 10.9 10.98 15.70 3.4 3.49 3.67 9.9 10 14.94 25 15.7 9.85 15.70 3.6 2.55 3.50 15.1 9.24 15.02 30 15.7 15.04 0.86 3.5 3.35 0.00 15 14.4 0.23 35 0.3 10.45 6.80 0 2.33 3.61 0 10 4.89 40 6.8 5.63 6.91 3.4 2.91 3.52 5.1 4.16 5.11 45 6.9 6.76 3.02 3.6 3.49 0.10 5.1 5 0.02 50 3 2.78 2.98 0.1 1.18 0.11 0 1.67 0.07 55 2.9 2.91 0.97 0 0 0.09 0.1 0 0.03 60 0.3 2.79 3.03 0.1 0 0.08 0 0 0.22 65 2.9 2.01 3.01 0 0 0.07 0.1 0 0.20 70 2.9 2.9 11.00 0.1 0 3.58 0.1 0 10.04 75 11 2.8 10.90 3.6 0.58 3.61 10 1.66 9.97 80 11 10.98 0.93 3.5 3.49 0.11 10 10 0.19
0 2 4 6 8 10 12 14 16
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115
U
5
(%
)
TIME(s)
THD(%) U3(%) U5(%)
Time Class S
Class
A Ref
Class S
Class A Ref
Class S
Class A Ref 85 0.4 9.16 15.70 0.1 2.91 3.60 0.1 8.33 15.02 90 15.7 7.88 15.60 3.6 1.76 3.55 15 7.5 15.13 95 15.7 15.67 0.91 3.5 3.49 0.10 15 14.99 0.14 100 0.3 9.81 6.80 0.1 2.7 3.56 0 9.01 4.96 105 6.8 6.45 6.82 3.5 3.34 3.60 5.1 4.77 4.92 110 6.8 5.63 3.03 3.6 2.91 0.25 5 4.16 0.05 115 3 1.46 3.02 0.1 0 0.13 0.1 0 0.14
IV.DISCUSSIONS AND CONCLUSIONS
The results show that under time-varying conditions different instruments (Class A and Class S) give different results due to a number of factors including differences in protocol and synchronization issues. Therefore, special attention must be paid when dealing with time-varying harmonics, particularly in high distortion cases.
This paper evaluated the performance of commercial power quality (PQ) instruments under time-varying conditions. Using a high precision programmable voltage and current source, two meters from different manufacturers were analyzed and compared. The results demonstrated significant differences between different instruments when measuring harmonic distortions under time-varying conditions, which can lead to wrong conclusions and decisions.
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