2010 IEEE International Conference on Power and Energy (PECon2010), Nov 29 - Dec 1, 2010, Kuala Lumpur, Malaysia
Testing of Contactors under Voltage Sag and Non-sinusoidal Voltage Conditions Surya Hardi, I. Daut and M.Irwanto Laboratory of Electrical Energy and Industrial Electronic Systems Cluster Research at School of Electrical System Engineering, UniMAP, Kangar, Malaysia. Email:
[email protected] Abstract—Sensitivity of electromagnetic contactors for voltage sags and non-sinusoidal voltage supply has been carried out by laboratory testing. Effects of sag characteristics such as; sag magnitude, sag duration and point on wave of sag initiation to be considered on contactors behavior. Contactors are subjected to voltage sags of depth varying, sag duration varying 5 ms to 600 ms, different point on wave and under different non-sinusoidal conditions. A Schaffner Profline 2100 EMC has been used as sag generator to create sag characteristics. The results show that sensitivity curves of the contactors depend on sag magnitude, sag duration and point on wave, whereas influence of non-sinusoidal voltage is slightly different. Keywords—Contactors; Voltage sag; Point on wave; Nonsinusoidal supply; Sensitive curve.
Figure 1. Basic motor control circuit [4].
I. INTRODUCTION Industrial process equipment may be affected by a variety of different power quality disturbances (e.g., voltage sags, voltage swells, over voltages, interruptions, transients, voltage unbalance, voltage flickers, and harmonics). Voltage sags are one of them that the most important problem faced by many industrial customers [1, 2]. Therefore voltage sag can result in tripping the customer equipment and shutting down of production loss and expensive restart procedure [1, 3]. One of the most common pieces of the equipment used in industrial system is contactors. The voltage sags can cause electrically heldin contactors to drop out. The contactor most common is to control electric motor which prevents it from suddenly restarting when voltage recovered. Circuit diagram connection of a contactor and a motor is shown in Fig. 1. This figure is a basic motor control circuit. By pushing the start button (N/O) voltage is connected to coil. The contactor and its auxiliary contacts are pulled in and current can flow via the normally open bridging contact after the Start button is released. The Stop button (N/C) interrupts the circuit when pressed; the contactor drops out and stays out after the Stop button is released because the bridging contact has opened as well. An AC coil contactor is electromechanical devices, which act as a switch to connect and disconnect a variety of electric systems for both power and control purpose that have been identified as weak link to voltage sags and interruption [5, 6]. One manufacturer has provided data that indicates their line of motor contactors will drop out at 50% voltage if the condition lasts for longer than one cycle. This data should be expected to vary among manufacturers, and some contactors can drop out at 70% of normal voltage or even higher [1].
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A contactor drops out when the field strength of the magnetic field becomes smaller than spring presure that tries to push the yoke away from armature. During voltage sag, the magnetic circuit will disengage if the electromagnetic force Fmin is less or equal to the force provided by the spring. Acording to reference [5], the electromagnetic force Fmin can be designated as,
Fmin =
1 2μ o A
φ 2 min
(1)
Where, μ 0 is the free space permeability, A is the cross sectional area of the electromagnetic pole and φ min the minimum flux needed to prevent the electromagnet from disengaging. To obtain the voltage required to keep the contactor from dropping out, the coil self inductance is assumed to be constant and is dominant. The minimum rms hold in voltage can be derived as [5],
Vhold ,min =
N c ωφ min 2
(2)
Where Nc is the total number of turns in the electromagnet coil and ω is the fundamental frequency of supply voltage.
The sensitivity of ac coil contactor to voltage sag is often expressed in terms of only their magnitude and duration which called rectangular voltage-tolerance (sensitivity) curve. Sensitivity of contactors will drop for sag to below 80% with duration 20 ms are classified as worst case and for sag about 40% with duration 80 ms and above are best case. The sensitivity contactors are most likely trip for between sag of 40% to 50%, 10 ms duration and above [7]. With the increasing use of electronic equipment in office buildings, household, industrial, etc that connected to supply system will create harmonics pollution. The equipments are non-linier load have been known as harmonic sources. Effects of harmonics supply on the equipment voltage tolerance have been investigated by Barros [8]. Test was carried out to computers. From the results that the voltage tolerance curve of the equipment used in the test depending on both the magnitude of voltage harmonics distortion and phase difference between harmonics. The equipment more sensitive to low order harmonics and lower voltage crest factors. This paper presents behaviours of contactors during voltage sags and interruption. Testing of different of the contactors was carried out for different characteristics of voltage sags such as magnitude, duration and point on wave. Sensitivity of the contactors was presented with sinusoidal and also under non-sinusoidal supply conditions. II.
CHARACTERISTIC OF VOLTAGE SAG
Voltage sag is defined in [1, 2] as being a decrease in the RMS voltage between (10-90) %, at the power frequency, for duration from a half cycle to one minute. If the duration is greater than 1 minute it is considered as under voltage. Voltage sag is classified as instantaneous when its duration ranges from 0.5 cycles to 30 cycles, momentary lasting between 30 cycles and 3 seconds, and temporary extending from 3 to 60 seconds. Voltage sag is normally characterized by magnitude and duration which influenced on behaviour of equipment [1, 2, 6]. The magnitude is determined by electrical distance to fault (impedance) and types of faults. The duration is given by the fault clearing time of protection system and the circuit breaker. When the faults are cleared, the voltages return to their normal values. The others characteristics such as balanced and unbalanced voltage sags, phase angle shift or phase angle jump, point on wave of initiation and recovery have been found to influence significantly the equipment’s sensitivity to the voltage sags [6]. A short circuit in a power system not only causes a drop in voltage in magnitude but also a change in the phase angle of the voltage [6, 7]. It is due to the change of the X/R ratio. Phase angle shift is the phase angle difference between voltages the pre-fault and during fault. The phase shift is generally assumed to be a factor of influence for power electronic converter [6]. The point on wave is the point where voltage sag occurs and it is
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also important aspect that affects behavior of contactors [5]. III.
REVIEW OF EXISTING STANDARD AND PREVIOUS RESEARCH
A. Voltage tolerance curve Generally, equipments are sensitive to magnitude and duration of sags. Away to characterize the behavior of equipment to voltage sag is to use voltage tolerance curves or sensitivity curve. Existing standard for testing equipment voltage sag immunity focus primarily on verifying a minimum immunity requirement response to voltage sags. Several popular standard equipment tolerances curve which usually used namely; the information technology industry council (ITIC) curve [10], the SEMI F47 curve [11] and IEC Standard 6100-411 curve [12]. Each point on the curve indicates how long this piece of equipment is able to ride through certain voltage sags. The first curve, ITIC curve, was formerly called the computer and business equipment manufacturer association (CBEMA) curve. It represents the voltage variation tolerance requirements of information technology equipment as defined by the information technology industry council, formerly known as CBEMA. On the other hand, the second curve specifies the voltage sag immunity of semiconductor manufacturing equipment. It is widely used by semiconductor vendors in evaluating their needs for protection against voltage sags. IEC 610004-11 is a standard that equipment must tolerate voltage dips on the AC mains. The standards specify the same depths and durations of voltage dips, and explain how to apply these dips to single-phase and three-phase equipment. 61000-4-11 applies to equipment rated up to 16 amps per phase. As these curves are characterized by the sag magnitude and duration, a proper representation of the system performance in terms of these parameters is needed in order to evaluate the consequences of voltage sags. Comparison of third voltage tolerance curves is shown in Fig. 2.
Figure 2. Voltage tolerance curve (ITIC, SEMI F47 and IEC)
B. Previous work Previous work showed for testing of ac coil contactors to voltage sags in various references [4, 6, 13] give results for depth and magnitude influence only. This is also according to Ref. [2], which expressed the other parameters such as phase shift, point on wave initiation and point on wave recovery should not be considered
during voltage tolerance testing of low voltage equipment. Reference [4] presents results of drop off time for different contactors. The drop off time is shortest when the coil voltage is near peak value and longest at zero crossing. Reference [6], expressed result of contactors testing which the contactor tolerates any voltage sag down to about 70% of nominal voltage. When the sag magnitude is below 70% for longer than few cycles, the contactor drops out. Ref [13], describes that there are two conditions observed while a contactor subjected to voltage sags i.e., drop out and chattering that are being defined as follow; drop out voltage: is the voltage below the coil nominal voltage at which the contactor trip or drop out and chattering is a phenomenon that is observed when the voltage supplied to the contactor coil falls below a certain value. It refers to the distinct impact sound caused due to the repeated making and breaking of the armature circuit inside the contactor. Sag characteristics to achieve these conditions are different. Values of the sag characteristic that caused contactor chattering event is lower than drop out. For sag depth of 60%, there is chattering observed for sag duration greater than 30 cycles. In the case of 50% sag depth and 40% sag depth, the contactor trip for all sag duration. Investigation that considered the point on wave initiation found in references [5, 14]. Reference [5], which described for points in wave ranging between 00 and 900 in 150 increments. Since the contactor behaves symmetrically about the half cycle, a 00 and 900 range was sufficient to show its behaviour when subjected to voltage sags. The experimental results shown identify the point in wave where voltage sag occurs as a very important aspect of contactor behaviour. The results observation during this experiment was the contactor drop-out after voltage sag was over. Ref. [14], has investigated sensitivity contactors to voltage sags. Testing of different contactors was carried out for different of characteristics of voltage sags such as magnitude, duration and point in wave. Influence of point on wave of initiation for investigating only 0 and 90 degrees. Especially interesting is the deepest 00 sags and outage. A contactor may tolerate an outage of several hundred milliseconds but trips when exposed to 50% sag lasting only one cycle. For 900 of point on wave, the sag duration is not relevant. All contactors tripped during a 20 milli second (ms). IV.
TESTING FACILITY
Electrical Energy and Industrial Electronic Systems Cluster Research Laboratory at School of Electrical System Engineering UniMAP set up equipment for analyzing and testing power quality problems. A Profline 2100 EMC tester is capable of producing any arbitrary waveform and event at 3x3 kVA power rating and 0-300 V rms amplitude. Personal computer has been used for adjusting to produce sag characteristics desired. The equipment is shown in Fig. 3.
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Figure 3. Schaffner Profline 2100 EMC tester used for generating voltage sags.
V.
TESTING OF AC COIL CONTACTORS
A. Contactor tested Laboratory experiments were carried out to investigate effects of voltage sags. Three small contactors of different manufactures were tested. Their rating is such as in Table 1. TABLE 1. AC CONTACTORS USED IN EXPERIMENTAL TEST Specification of contactor
A
B
C
Coil Voltage (VAC)
240
240
240
Continuous Current Rating (A)
25
20
20
Frequency (Hz)
50
50
50
B. Test procedure Sags were applied in various depth, duration and point on wave of sag initiation. Each characteristic was reproduced repeatedly two to three times to avoid errors caused by in contactor performance. The sag magnitude was set to depth varying from 70% to 10% in step of 10% increments, whereas sag duration varying from 5 ms to 600 ms. Point on wave was adjusted from 00 to 900 in 150 increments. If the sags of certain magnitude, duration and point on wave causes the contactor to trip was assumed to be sensitive to this type of sags then were recorded. Based on the obtained data was plotted as voltage tolerance curve. Fig. 5 shows an example sag waveform of this process. One of the contactors was subjected to a sag initiation that occurs at a point on wave of 150 and sag of 30% with duration of 45 ms. In this condition the contactor did not trip. This figure shows the line voltage supply and current flow through in coil contactor.
Figure 4. Sag waveform of 30%, 150 point on wave, duration 45 ms
VI.
Figure 7. Sensitivity curves for Contactor C
RESULTS AND DISCUSSION
From Fig. 5, the performance of contactor A, when subjected to 30% sag depth has a significant effect of sags on them. Contactor trips for all point on wave at this voltage level, although different in sag duration. The contactor become sensitive when point on wave of 900 initiation sag and trips for 20 ms and above. The result test on contactor B is presented in Fig. 6. This figure shows the contactor trips in different voltage sag level. The contactor is less sensitive to sag duration if it is initiated at the zero crossing (00 on point wave) rather than others. It trips for 40% depth and 70 ms sag duration. Sensitivity of the contactor is decrease when the point on wave of 900 and 450 initiations sag. Threshold of voltage in this case of 40% sag depth. Fig. 7 represents result test on contactor C. Influence of point on wave is slightly different in sags duration that cause the contactor trips. 00 on point wave is more sensitive to sags magnitude, but less sensitive in sag duration on lower sag magnitude level. Comparison behaviors of different contactors while subjected to different point on wave of sag initiations are illustrated in Fig. 8 to Fig. 11. The sensitivity of each contactors performance is different. The sensitivity threshold to sag magnitude for contactor C is definite namely 50% sag depth, whereas for contactors A and B are indefinite and have sensitivity threshold of 40% sag depth for point on wave of sag initiations of 300 and 600 . It’s means that the contactors trip depend on the point on wave of sag initiation. Contactor C is most sensitive to this condition. From these figures can be shown that the contactors will trip faster when voltage sag initiation occurs at point wave 750 (Fig. 11). Value of the sensitivity threshold of sag duration for contactor B is 20 ms and contactor C is lowest i.e.,10 ms. The sensitivity threshold of contactor A is decrease when the sag initiation occurs at point wave 750 and it is starting to trip for sag of 20% depth or less.
The testing results of contactors A, B, and C are represented in the form of sensitivity curves such as in Fig. 5 to Fig. 7. The figures show the influence of the point on wave of sag initiation on each tested contactor for point on wave of 00, 450, and 900, respectively. There was no effect on the performance of the contactors for sags of depths 60% and above for all sag durations.
Figure 5. Sensitivity curves for Contactor A
Figure 6. Sensitivity curves for Contactor B
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supply conditions: first using a pure 50 Hz sinusoidal voltage sag supply and second when the voltage sag supply content harmonics distorted of third, fifth and seventh at point on wave of 00 and 750, respectively. From Fig. 12, the contactor is faster to trip when was supplied by non-sinusoidal for voltage sags of 50% sag depth or less. Harmonics 5rd and 7th have same influence on the contactor. Fig. 13, the contactor trips in 5 ms duration for depth sag 20% of nominal voltage. Presense harmornic distorted to be slight faster to trip but the contactor trip at lower sag depth i.e., has range 50% to 30% of nominal voltage. Figure 8. Sensitivity curves at point on wave 150.
Figure12. Sensitivity curves on contactor C in different supply at point on wave 00.
Figure 9. Sensitivity curves at point on wave 300.
Figure 10. Sensitivity curves at point on wave 600.
Figure 13. Sensitivity curves on contactor C in different supply at point on wave 750.
VII. CONCLUSION The behavior of the electric contactors for voltage sags is discussed. When the contactors trip, they are affected by three parameters i.e., depth of sag, duration of sag and point on wave initiation of voltage sag. Laboratory tests were carried out to three contactors having different manufacture. The test results clearly show that the magnitude and the duration of voltage sag are not the only parameters that influenced on the sensitivity of a contactor to voltage sag. The point on wave initiation has also significant influence on the behavior of AC contactor. Supply non-sinusoidal voltage has only a slight influence on the contactor.
Figure 11. Sensitivity curves at point on wave 75 degrees
Influence of input non-sinusoidal voltage on contactor C can be shown in Fig. 12 and Fig. 13. These figures show sensitivity curves under sinusoidal and harmonics
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[8]
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