Experimental Investigation of AC Contactor Ride Through Capability During Voltage Sag

Hussain Shareef Dept. of Electrical, Electronic and System Engineering

Universiti Kebangsaan Malaysia Selangor, Malaysia

Nazri Marzuki Dept. of Electrical, Electronic and Systems Engineering

Universiti Kebangsaan Malaysia Selangor, Malaysia

Azah Mohamed Dept. of Electrical, Electronic and System Engineering

Universiti Kebangsaan Malaysia Selangor, Malaysia

Khodijah Mohamed Dept. of Electrical, Electronic and System Engineering

Universiti Kebangsaan Malaysia Selangor, Malaysia

Abstract—This paper presents the effect of voltage sags on different types of ac contactors. Based on recent testing standards and utilizing a modern industrial power corrupter, experiments were conducted at different sag magnitude, duration and point on wave. The predefined malfunction criterion is the unwanted disengagement of the main contacts. The experimental results show that different contactor has diverse level of ride through capability which depends on point on wave, magnitude and duration of the voltage sag. Experimental Investigation also shows that increasing sag initiation angle from 0⁰ to 90⁰ allows contactor immunity level to decease in term of sag magnitude.

Keywords-ac contactor; voltage sag; ride through capability; voltage tolerence curves

I. INTRODUCTION Voltage sags are among the most important power quality

problems that cause frequent disruptions in modern industrial processes and thus, pose a serious power quality issue for the electric power industry. Voltage sag is defined as a decrease in voltage magnitude between 0.1 and 0.9 per unit in root mean square (rms) voltage or current at the power frequency for duration between 0.5 cycles to 1 min [1]. Voltage sags are commonly caused by network faults, motor starting, and capacitor switching. These events may cause sensitive equipment such as ac contactor that controls large industrial motors to disengage their contacts and cause expensive shutdown in industrial processes.

The practical approaches investigate the effects of voltage sag by monitoring, conducting experiments on customers’ sensitive loads, and performing pertinent surveys [2, 3, 4]. Equipment sensitivity to voltage sag can also be considered and presented in the form of power acceptability curves. These curves are plots of bus voltage deviation versus time duration which separate the bus voltage deviation - time duration plane into two regions namely, “acceptable” and “unacceptable” regions. The lower limb of the power acceptability curve relates to voltage sags and momentary outages. The latest

power acceptability standards are the SEMI F47 issued by the Semiconductor Equipment and Materials International (SEMI) [5] and ITIC curve of the Information Technology Industry Council (ITIC) [6]. These standards are generally applicable for semiconductor manufacturing equipment and other electronic devices.

As an effort to understand the voltage immunity level of ac contactors, some works have been reported in the past. The experiment described in [7] was performed to show the impact of point on wave of sag, duration, and amplitude on the performance of ac contactors. It was shown that voltage sag with a specific magnitude and duration can have different effects on a contactor depending on the point on wave where it originates. Similar experiment has been conducted and noted that the coil current is one of the limiting factors that causes unwanted contactor disengagement on top of the effect of sag initiation angle [8]. Additional research was conducted to observe the effect of phase shift during the sag, two-stage sags and sags due to the starting of large motors [9]. It was reported that the threshold voltage that affects tripping does not have a big impact on phase shifts and two stage sags. A dynamic model of ac contactors has been developed in which the model demonstrates a means to simulate the behavior of contactors during voltage sags for obtaining the contactor susceptibility [10, 11]. This paper focuses on investigating the sensitivity of ac contactors to voltage sags in the 240V/50Hz distribution system. Extensive laboratory tests were conducted for this purpose by analyzing the operation of different contactors during various events of voltage sag.

II. AC CONTACTORS An ac contactor utilizes a solenoid to cause one or more

pairs of electrical contacts to engage when an appropriate voltage is applied to solenoid’s coil as shown in Fig. 1. The solenoid consists of an electromagnet that attracts a moveable bar. The moveable bar is spring loaded so as to cause the bar to move away from electromagnet when the electrical signal is

This work was carried out with the financial support from the Universiti Kebangsaan Malaysia under the research grant UKM-GUP-BTT-07-25-151

not present on the coil. Electrical contacts are attached to the moveable bar and the movement causes the contacts to close or open depending on the strength of the magnetic field. The instantaneous flux, Ф, and the force, F, that tends to close the air gap in the contactor can be expressed respectively as [11]:

(1)

(2)

where:

N : number of winding in the coil I : current flow through the coil l : length of magnetic path μ0 : absolute permeability μ : permeability of the coil substance A : cross sectional area of the air gap ω : steady state frequency in radians β : angle of phase shift Then, assuming the coil self inductance is constant and dominant, the minimum voltage, Vhold , required to keep the contactor from dropping out is given by [12]:

(3)

From (3), the hold in voltage, Vhold, depends very much on the flux which is directly proportional to the instantaneous current applied to the coil. The angle of phase shift in (1) also have a dramatic effect on the sensitivity of the contactors. For instance, a voltage sag initiates at the peak of the voltage waveform may leave a very small amount of flux to produce enough force to hold the contacts while an event at zero crossing of voltage waveform will leave much higher flux to hold the contacts.

Figure 1. Basic construction of ac contactor

III. METHODS AND MATERIALS This section illustrates the design of the experiment for

contactor testing and the procedures followed to obtain the results on the performance of the various contactors during voltage sags.

A. Contactor Testing The methodology that is used in the testing is generally

based on the guideline given in [3, 4]. Four contactors and two relay type contactors having different specifications are tested to study the effect of voltage sags on the performance of the contactors. The specifications of the tested ac contactors are shown in Table 1. The specifications of the test samples listed

in Table1 include three old contactors that are taken currently from equipment and three that are brand new.

All contactors are tested with a 2kW spot light load attached to their main electrical contacts. First the contactor is warm up to their normal operating temperature by applying nominal coil voltage for couple of minutes before initiating the sag event. In this case the malfunction condition was defined as the unwanted disengagement of the main contacts. The disengagement of contacts can be guaranteed with the test setup shown in Fig. 2 where one of the normally open contacts is used to energize the ac coil of the contactor.

The experimental set up consists of four components namely, sag generator, contactor under test, data acquisition system, and a computer to analyze the signals. In this case, an industrial power corruptor (IPC) from the Power Standards Lab is used, which is a voltage sag generator combined with built-in data acquisition system that is capable of producing and interrupting voltages up to 480V and current at 50A in single or three phase systems.

TABLE I. SPECIFICATION OF TESTED CONTACTORS

Contactor No

Manufacturer/Model Rated Current

Rated Voltage

C1 LG Industrial System / GMC-18

18 A (new) 240 V

C2 LG Industrial System / GMC-40

40 A (new) 240 V

C3 FUJI / SC-N2 S 50 A (used) 240 V

C4 ABB / A9-30-10 25 A (used) 240 V

R1 Omron / MY2 5 A (used) 240 V

R2 Omron / MK3P-I 10 A (new) 240 V

Figure 2. AC contactor test setup

A series of test results on ac contactors is obtained by following the pre-defined procedure given below.

• Using the terminal blocks available at the back of IPC, the conductors from mains panel and conductors to the contactor coil are connected and the IPC is powered on.

Load

• Pressing the push button, the contactor is allow to engage. Then leave contactor for few minutes to get warmed up.

• Starting from nominal voltage, voltage sags are initiated in steps of 2.5% down to zero volts. The sag initiation angle and the duration are kept constant. The initial sag duration and phase angle are set to 1 cycle and 0° respectively. The critical sag depth for the pre-defined malfunction criteria is determined by repeated testing for at least 3 times for a particular sag magnitude and duration. For each triggered sag event, voltage and current waveforms of the contactor coil under test are recorded.

• The duration of sag is adjusted in steps of 1 cycle up to 50 cycles and measurements outlined in Step 3 are repeated.

• Finally, the sag initiation angle adjusted in steps of 45° up to 90° and measurements outlined in Steps 3 and 4 are repeated.

IV. RESULTS The test findings of tested contactors to voltage sags are

presented as typical voltage tolerance curves. The upper region of these curves represents proper operation region while the lower region indicates unacceptable voltage conditions where unwanted disengagement was encountered.

Contactor testing procedure does not produce single voltage tolerance curve, but families of curves corresponding to different point on wave initiation. Typical effect of point on wave for the contactor C1 listed in Table 1 is shown in Fig. 3.

From Fig. 3 it can be noted that the voltage sags having 0° point on wave is more sensitive to sag magnitude compare to the curves obtained for sag initiation angle of 45° and 90°. At 0° point on wave, contactor C1 is starting to disengage as early as 1 cycle at 60 % remaining voltage. However, it is less sensitive to deeper sags that last for short durations. For an example, in case where the sag event leaves 5% remaining voltage and last for 3 cycles, the contactor does not cause any unwanted disengagement. This is expected because the remaining dc current component after offsetting with low magnitude ac current is sufficient to hold the contacts for short time. At 45° point on wave, similar variation is observed as in the case of 0° point on wave. Here the average minimum hold in voltage, Vhold decrease to 62.5% compare to 65% in the case of tests conducted with 0° point on wave.

Unlike the cases of 0° and 45° point on wave point events, 90° initiation angle show more tolerance in terms of sag magnitude, however they are more sensitive to short and deeper sags. For this reason, at 90°, the obtained voltage tolerance curve has rectangular shape with sharp knee which appear nearly at 1 cycle 55% remaining voltage. These results agree with the theoretical explanations given in Section 2.

Similar results are obtained for other tested contactors.

Figure 3. Voltage tolerance curve of contactor C1 at different point on wave

Overall results of all experimented contactors for different point on wave of sags are plotted in Figs. 4 to 6. Fig. 4 shows the voltage tolerance curves of all contactors at 0° point on wave. From Fig. 4, contactor C1 and R2 happen to be the most sensitive contactors to voltage sags in terms of both sag magnitude and duration. These contactors experience unwanted disengagement for voltage sag events which leaves 65% remaining voltage. Meanwhile the contactor R1 is observed to be the least sensitive contactor to voltage sags depths. Contactor R1 can tolerate for sags depths up to 52.5% remaining voltage. In Figs. 4 to 6, the SEMI F47 standard curve is also merged to compare its applicability to sensitivities of ac contactors during voltage sags. If one compares the SEMI F47 curve with the voltage acceptability curves of the contactors it can be verified that SEMI F47 is not a suitable standard to compare the tolerance levels of contactors as none of the contactor’s tolerance curve falls within the SEMI F47 curve. A more suitable standard to compare the performance of contactors could be IEC standard 60947-4-1 [13]. According to it, the limits between which the contactors should drop out and open fully are 75% to 20% of their rated control supply voltage for contactors.

By observing the other voltage tolerance curves shown in Figs. 5 and 6 for sag events initiating at 45° and 90° point on wave, it can be noted that contactor C1 and R2 are still the most sensitive contactor while the contactor R1 is least sensitive. Therefore it can be concluded that if a contractor is sensitive to a particular point of wave on sag in a system having many contactors, it will also be equally sensitive to sags having different initiation angle. Another conclusion that can be obtained from the experiments is that the minimum hold in voltage, Vhold, for an ac contactor should not be a single rms voltage value but a range of values depending upon the point on wave of the sag.

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Figure 4. Voltage tolerance curve of various contactor at 0° point on wave

Figure 5. Voltage tolerance curve of various contactor at 45° point on wave

Figure 6. Voltage tolerance curve of various contactor at 90° point on wave

V. CONSLUSIONS An extensive experimental study has been performed to

determine the effect of voltage sag on ac contactors. The test results on ac contactors clearly show that the magnitude and the duration of voltage sag are not the only parameters that

influence the sensitivity of a contactor during voltage sags. The sag initiation angle has significant influence on the behavior of ac coil contactors. The experiments also reveals that the minimum hold in voltage, Vhold, for an ac contactor should not be a single rms voltage value but a range of values depending upon the point on wave of the sag. Finally, it also showed that SEMI F47 is not a suitable standard to compare the tolerance levels of contactors as none of the contactor’s tolerance curve falls within the SEMI F47 curve. A more suitable standard to compare the performance of contactors could be IEC standard 60947-4-1.

ACKNOWLEDGMENT This work was carried out with the financial support from

the Universiti Kebangsaan Malaysia under the research grant UKM-GUP-BTT-07-25-151.

REFERENCES [1] IEEE Standard 1159 1995. IEEE Recommended Practice For

Monitoring Electric Power Quality. Institute of Electrical and Electronics Engineers. 1995.

[2] M. H. J Bollen, Understanding Power Quality Problems: Voltage Sags and Interruptions, 1st ed, IEEE Press: New York, 2000.

[3] IEC 61000-4-11, Electromagnetic Compatibility (EMC), Part 4: Testing and Measurement Techniques, Section 11: Voltage Dips, Short Interruptions and Voltage Variations Immunity Tests, International Electrotechnical Commission, 1994.

[4] IEC 61 000-4-30, Electromagnetic Compatibility (EMC), Part 4: Testing and Measurement Techniques, Section 30: Power Quality Measurement Techniques, International Electrotechnical Commission, 2003.

[5] SEMI F47-0200, Specification for Semiconductor Processing Equipment Voltage Sag Immunity, Semiconductor Equipment and Materials International, 2000.

[6] ITI(CBEMA) Curve and Application Note. Information Technology Industry Council, 2000. [Online]. Available from: Available: http://www.itic.org.

[7] P. Pohjanheimo, and M. Lehtonen, “Equipment sensitivity to voltage sags – test results for contactors, PCs and gas discharge lamps,” 10th Int. Conf. on Harmonics and Quality of Power, Brazil, p. 559- 564, October 2002.

[8] E. R. Jr. Collins, and M. A. Bridgwood, “The Impact of power system disturbances on AC-Coil contactors,” Annual Conf. Textile, Fiber Film Industry, USA, p. 1-6, May 1997.

[9] S. Z. Djokic, J. V. Milanovic, and D.S. Kirschen, “Sensitivity of AC coil contactors to voltage sags, short interruptions, and undervoltage transients,” IEEE Trans. Power Del., USA, vol. 19,pp. 1299-1307.

[10] J. Pedra, F. Corcoles, and L. Sainz, “Study of ac contactors during voltage sags,” 10th Int. Conf. on Harmonics and Quality of Power, Brazil, p. 565-570, October 2002.

[11] M. Hasmaini, and M.N. Khalid, “Evaluation on sensitivity of AC contactor during voltage sag.” IEEE TENCON Region 10 Conf., Thailand, p. 295-298, November 2004,.

[12] E. B. Kushare, and A. A. Ghatol, “Investigation of cost effective method to improve voltage sag ride through capability of ac coil contactors”, IET , Int. Conf. Information and Communication Technology in Electrical Sciences, India, p. 452-457, December 2007.

[13] IEC 60947-4-1, Low-Voltage Switchgear and Controlgear – Part 4-1: Contactors and Motor-Starters – Electromechanical Contactors and Motor-starters, International Electrotechnical Commission, 2009.

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