Characterization of Switching Transients in Low Voltage Power Systems of Tea Factories in Sri Lanka

Nowadays, intense technological development of electric utilities and associated power electronic gear controls is becoming increased concern about the quality of electric power. In contrary to this, they are also responsible for productivity losses due to their sensitivity to voltage disturbances including transients in a decidedly harsh environment. This mystify the scenario more, as some of electronic controls are also allies of this fortuitous power quality evils, like in variable frequency drives. In the long run, this demands great deal of screen, troubleshoot, and pre-empt power quality issues now more than ever before. The indirect effects of transient rooted damages such as increased maintenance costs, out-of-operation time in the industrial sectors due to damaged and temporary malfunctioning equipment will be much higher. As explained in the studies [1, 2], across broad consideration through many researchers in industrial power systems, the most identified and accepted consequence in starting process of motors is the voltage dip. At the same time, transient nature at the starting period of a power system has also gained great attention as they tend to make associated devices in to accelerated ageing. Whenever a load turned on or off through a switching action in the system, voltage transients possibly pierce to the specific electrical environment [3-5]. Therefore, a retrofit to equipment safety in an environmentally harsh situation can only succeed if sufficient attention is given to protecting associated electronic control gear or drivers against overvoltage conditions.

As reported in scientific literature [3, 5-14], on every occasion where the circuit conditions suddenly change, a transient is prone to be initiated, often follows with switching operations or owing to lightning. This can either be a change loading condition of the equipment or current demand predominantly. As they last only a short time and occurrence is random, it makes them little hard to capture. Consequently, a very precise measure and understanding of events occurred during disturbances are vital and can offer tremendous potential benefits for a fine analysis. A fair analysis supported by splendid measuring arrangement and capturing practices, transient disturbances will be able to investigate, understand and sometimes eliminate. Moreover, Pannila and Edirisinghe [9] and Greenwood [15] reported that sources external to the facility can give birth to transients but more commonly are grown internally. This study mainly focusing internally generated transients in tea factories located in Sri Lanka which undergoes different stages that requires machineries ranging from heavy electrical induction motors to sophisticated electronic colour separation machines. In fact, a tea factory is a place where it operates large electromechanical machineries prone to generate distressing transients while other classy electronic control elements and devices are greatly exposed to them in the same environment. It is not exaggerating to say that these internally generated frequent transients seep in to the efficiency of machines causing accelerated ageing or unplanned downtime confronting large financial loss [9, 16-18]. In Insurance like financial strategic matters, currently both the insurer and the insured party concern the vulnerability of the facility due to lightning, fire like visible threats only. However due to lack of understandings of internal transient influence on the equipment, it has not yet reckoned for calculating probable loss, making misunderstandings and losses for the both parties. This study may help to fill this knowledge gap which leads to consider vulnerability due to internally generated transients for loss analysis.

It is important to mention that the tea manufacturing industry involves various types of machineries and equipment which used at different stages in the tea production. The machine related tea processing undergoes the standard stages, Withering, Rolling, Fermentation, Drying and Grading which uses three-phase induction motors in the capacities of 2 kW to 15 kW. There are many methods in use to start three-phase induction motors such as; Direct On-Line starter (DOL), Star (Wye)-Delta starter(Y-Δ), Auto transformer starter, Reactor or resistor starter and Soft starting [1, 2, 19, 20]. During this study, it was observed that most tea factories in Sri Lanka use only first two methods of starting, depending on the power consumption of the motor.

Purpose of this study is to technically report and opening a sound awareness of transient nature in industrial site to the scientific community which possibly influence quality of power system. The influence of the electrical elements in factories, induction motors as well as capacitor banks to get VAR support on the supply system to be observed mainly. Detailing the diverse disturbances with the aftermath being sequential and disturbing the other equipment operation, such as transient bursts, high peaks, sags also expected. To analyze the impact of transient events quantitatively and qualitatively, characterizing transient parameters on a statistical basis and calculation of energy content also sought. The analysis where these transient events can be linked to possible power quality apocalypse in the facility, also planned in order to have complete power quality picture. Unveil the hidden danger of transient events and degree of nakedness of current power systems to these events which can cause even a slight detriment to the power quality, expected at the end. Reporting this, section 2 introduces the experimental setup and measurement system; section 3 introduces the methodology; section 4 elaborates results and key observations; section 5 makes a discussion and focuses on possible mitigation practices that can adopt and section 6 conclude the study mapping the objectives and findings.

The methodology followed here is similar to what was presented in one of the early researches of authors [9, 10]. As the study wishes to measure common mode transients of the power system in each phase, the probe linked to the 230 Vrms system line and ground clip has been connected to the chassis of the panel which was connected to the system ground [9]. All the measured voltage transient waveforms were captured directly at the switch contact or at the distribution panel of the equipment location, as this provides a better awareness of quality of the voltage provided to the equipment and then the transient produced [17, 18]. The 3-core cable of 1 m length which brings ac supply to probe base, HV Probe’s cable of 10 ft. length and PicoScope’s data and DC power cable all are laid without crossing each other guaranteeing no interference fed in any means to the data collected [9], as illustrated in Figure 2. In Figure 2, Star-Delta Electric Distribution Panel, Standard 3-core cable, Tektronix High Voltage Probe, PicoScope Oscilloscope and Notebook PC (on battery powered) are labeled as (a), (b), (c), (d) and (e) respectively. It is important to mention that in each and every possible place, it was managed to follow all the available standards and manufacturer specifications [9, 21]. Moreover, trigger threshold of the software interface was decided after measuring 5 to 10 test waveforms at each measuring attempt by observing the average noise level of the system.

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Figure 2. Depiction of test set-up

In characterizing transient parameters, mainly two viewpoints were considered; the motor starting technique and the tea processing stage. Hence, time domain transient parameters like amplitude, rise time and pulse duration were calculated and statistically interpreted to correlate the operated electrical machine to the key attributes of transients. In this study, a collection of more than five pulses together reckoned as a burst and such burst durations were also calculated. Furthermore, for the rise time calculations, time taken to 10% to 90% of the step height considered and pulse duration of transient was also calculated.

Transient disturbances in electric power systems almost perpetual problem and there are lot of misconceptions with transient analysis in which most cases mingled with voltage sags, swells and outages. As reported in numerous surveys at diverse specific environments by previous researchers, to gain absolute and definitive knowledge in transients [3, 9, 5, 22, 23]. Some of such studies confined to certain margins like power capacity, the type of induction motors [1, 4] and some studies have been carried out extensively with wide view in power quality phenomenon [2, 16, 19, 24-27]. Though there were few works carried out at installations and cabling [8, 9, 28, 29], no fitting pursuit in to tea factories has been done, to detect the veiled risk due to this silent killer of valuable equipment which create leeway of millions of moneys.

In most of power-quality literature, transients are mainly focused as an anomalous condition arise at the front end of power elements and steepness, amplitude and duration like single event characteristics are allocated much weight [16, 24, 25, 30]. This is true as the transient burden do not exhibit significant and clear affliction in short time, like other major voltage interruptions. The boundaries of bad consequences of transients are limited to local environment in many cases, but intimidating consequences of this silent but devastating threat emerges more complementary studies to fill the remnants of incomplete transient definitions. At the end this always demands for rigorous and watertight argument to deduce any idea due to unpredictability of transients.

Some transient peaks of very high frequency, formed by switching actions probably surpass the recommended voltage ratings even with arresters located at the motor terminals [6] while high-duration low-amplitude transients may hit with their high energy content to the connected electrical utensils. This is a real burden that industrial facilities have been facing as plenty of expensive electronic control devices are uncovered in a sophisticated electromechanical environment. Thus amplitude, duration, burst width, rise time and frequency are recognized as major transient parameters to report in the analysis. Even without a load the motor tends to draw greater currents across it if the motor started under Direct-On-Line scheme. In addition, these currents at starting stage typically 8 - 10 times as large as the rated current at full operation of the machine but may contain components with a wide range in frequency variation due to the change in the motor's speed. During this small age of starting transient, the machine is under the dreadful environment of severe mechanical and electrical stress. This nature is also valid for the star-delta starting even though the method controls the high starting currents, but mechanical transition switching from star to delta generates severe transients.

The energy content of a transient is rather cannot be ignored as it makes subtle instant failures to fatigues in the associated electronics. Transient caused electrical overstress analysis is a critical and important process in developing and endorsing survival of equipment in the adverse electrical environment. The waveforms captured in this survey can be considered under Electrical Fast Transients (EFTs) [3, 31] as the devices under test were connected to mains [3]. These transients may cause device upsets, memory losses and insulation failures at long term exposure. Not only that, in modern microelectronics, it makes a reliability concern when single event transients (SET) occurs which may be produced by exited ionizing particle which reached to tenuous semiconductor node [7, 32]. Basically, the amount of charge fed from the radiation event yields a strike of current at the junction which appears as a transient. The frequency obviously depends on the type of ion, its energy, and the properties of the configuration of the power system [23, 33]. Therefore, it is essential component of this analysis to relate the energy content of transients in to the specific environment so as to address the suppression properly without placebos.

The readings were triggered in a setting such that root of the overvoltage is distant to the triggering point, but at the distribution Board as described in the study [9]. But as electrical installation Standards defines a separate sub circuit for each heavy inductive circuit, total energy (W) in the transient overvoltage is still of interest as a "worst-case" exposure for a protective device. The energy, W, transferred to the load, R_L, is given by Eq. (1) [9],

$W=\frac{1}{R_{\text {out}}+R_{L}} \cdot \frac{R_{L}}{R_{\text {out}}+R_{L}} \cdot \int_{0}^{\infty} V^{2}(t) \cdot d t$              (1)

where, W is energy transferred to the Load

R_L is Resistive Load

R_out is Output resistance of the circuit

V(t) is Voltage function

The value of Energy (W) to any load (R_L) can be estimated easily once the voltage integral is calculated for each case. As the computation does not account the of impedance (Z), for the consideration here, energy remains using V²s (volts square second) as its’ units. This method verified to be fair in cases where the source of the overvoltage is contiguous to the measurement point [3, 9, 31] but calculating Energy (W) with Z – 50 Ω could be uses as a rough estimate of W in watts where Z is the impedance of the mains, which is function of frequency and the voltage [9, 31].

Table 5. Energy generated through voltage Integral

Energy(W) calculated (as ∫ V² (t)⋅ dt) is presented in Table 5 which can be used to compute power associated with a transient, in the availability of appliance’s resistance and impedance of the mains at output source.

Therefore, the analysis reveals this energy content associated with voltage transients in terms of voltage integral, where that can then be easily utilized in the specific environment to calculate the real energy transferred applying circuit parameters (R_out and R_L).  

Voltage transients may present specially at places where the short circuit level of the supply is low. Such transients are addressed in the motor and wiring design but as a general resolution [34]. Many protective techniques are also available in the field for overvoltage aspects like amplitude, energy, yet the transient nature in power systems has not been particularly and precisely defined. Therefore, after proper characterization of transient phenomenon through an empirical analysis, such readily available transient suppression techniques can be adopted directly with appropriate technical supervision. Some researchers have revealed proper shielding the cables can severely mitigate and damp the current in the core letting only perturbations of low frequencies to circulate [34]. As proposed in [9] straightforward solid-state sensing arrangements can be set to trigger crowbarring mechanisms to protect sensitive electronic devices as a preparative defense. Nowadays, some of adjustable speed drives comes with these techniques inbuilt or as an option. Shunt RC snubbers, conventional series inductor arrangements or multilevel inverter mechanisms can also initially obstruct the unnecessary voltage disturbances [9]. Surge arresters of high-energy domain can use to limit the magnitude of the transient voltage at the end-user bus. Metal-oxide Varistor (MOV) based Surge protective devices specially designed for industrial level equipment are also available in the field. It is important to locate them near the load and not at the breaker [6]. There are quite literatures which reveal such easy to sophisticated transient elimination procedures in the field [3, 8, 15, 27, 35].

The facilities provided by the Department of Physics; University of Colombo is highly appreciated.