RESEARCH OF THE NON-SINUSOIDAL LOADS IMPACT ON THE OPERABILITY OF TAP-CHANGERS CONTACTS

ZІNOVKIN V.V. Doctor of Technical Science, Professor, Professor of the department “Electric drive and automation of industrial installations” of the National University ”Zaporizhzhia Polytechnic”, Zaporizhzhia, Ukraine, e-mail: super_znvvv@ukr.net; BLYZNIAKOV O.V. Candidate of Technical Science, associate professor, associate professor of the department “Electric and electronic apparatus” of the National University ”Zaporizhzhia Polytechnic”, Zaporizhzhia, Ukraine, e-mail: blizn1953@gmail.com;


I. INTRODUTION
Reliable functioning of power supply systems for energy-intensive technological electrical installations, such as arc steel-melting furnaces, rolling mills, transmission lines and back-to-back stations, rectifier and inverter substations, as well as facilities for converting the AC frequency 50 Hz to 60 Hz (Vyborg HVDC scheme), is an important technical problem [6]-[8], [10]- [12]. It is known that the key and most critical element of such systems are power (furnace) on-load tap-changing transform-ers, the reliability of which is largely determined by the reliability of the on-load tap-changers (OLTC), in particular of their contact systems [2], [6]. This is due to the fact that the maintenance of the directive task of the technological process is achieved by permanent voltage regulation using tap-changing devices and is confirmed by the data on the damageability of transformer equipment in the power supply systems of energy-intensive industries, presented in Table 1. The data presented in Table 1 show that the accident rate of transformer equipment due to failures of OLTC is relatively low compared to other components. However, a more detailed analysis shows that defects in OLTC, which are often latent in behavior, are the cause of failures on bushings, windings, other components of transformer equipment and the system as a whole [6].

II. ANALYSIS OF RECENT RESEARCHES
An essential feature of the OLTC operation is that, in the tap-changing process, they carry out multiple tap-changing operations under conditions of load  [15]. Such kind of research has also been carried out for OLTC [10]; however, they require further development due to the specific functioning of their contact systems. In particular, the influence of higher harmonics and unbalance in the current on the parameters of electrical contacts and their reliability has been insufficiently studied.

III. FORMULATION OF THE WORK PURPOSE
The purpose of this work is to analyze the operating conditions, to determine the cause-and-effect factors that lead to a decrease in the reliability of OLTC contact systems. The main task of the study is, first and foremost, to determine quantitative characteristics of the influence of load fluctuations on the parameters of the OLTC contacts. The task of the research is also the use of the derived scientific findings for the purpose to modernize operable devices and to develop new engineering and technical solutions.

IV. PRESENTATION OF THE BASIC MATERIAL AND ANALYSIS OF THE OBTAINED RESULTS
The object of the study was the contact system of the contactor of the switching device of the RNOA-110/1250 type, the appearance of which is shown in Fig. 1, and its electrical characteristics are given in Table. 2. To execute the experimental research, a specialized installation has been developed. Its structural diagram is shown in Figure 2.
The following designations are accepted in Figure  2: 1 -switch; 2 -non-sine waveform generator; 3current THD regulator; 4,5,6 -measuring transducers of power, voltage and current, respectively; 7 -test object (arcing contacts); 8 -transformer; 9 -electric drive of the OLTC; 10 -load; 11 -movie camera; 12movie camera drive; 13 -block for coordination of starting the all mechanisms drives; 14 -short-circuiter; 15 -chromel-copel thermocouples; 16 -thermal imaging camera.  The research were performed using a single-phase model of the on-load tap-changer contactor, which was connected to phase A of the physical model of an electric furnace transformer. Supply voltage was applied to the installation from an AC power source using a switch 1. The current THD was adjusted using a shaper 2 controlled by unit 3. The parameters to be researched were measured with devices 4, 5, 6, 15. A movie camera 11 made filming to visualize the contact opening process and further analysis. The contact heating temperature was measured using chromel-copel thermocouples 15 and thermal imaging camera 16. Unit 13 was used to coordinate the operation of short-circuiter 14, movie camera 11, thermocouples 15 and the transformer physical model 8. After each series of tapchanging operations, the contact resistance of the contacts being investigated was measured. The experimental research findings for six values of the THD current are shown in Table 3.
It is known that the contact resistance is a random variable [1], [4], [5], therefore, Table 3 presents the mathematical expectations of the contact conductance, derived from the probabilistic-statistical analysis of several parallel measurements [3]. The mathematical expectation of the contact conductance at certain values of the number of tap-changing operations is selected as the arithmetic mean of the results of parallel measurements: where i is the serial number of the measurement; n is the number of parallel measurements; σ i is the contact conductance at the i-th measurement.
The sample values of the general variance and the root-mean-square error of a single measurement of the contact conductance were calculated using the following formula:  The correlation function of the contact conductance, depending on the conditional numbers of the measurement series and the number of tap-changing operations, was determined in accordance with the following relationship: where l is the serial number of the measurement The probability densities of the contact conductance values (in the form of discrete values of the probability of their falling into the given intervals) were determined according to the expression: where k j is the number of conductance values falling into the interval ; k is the total number of conductance values obtained in the experiments; h is the interval width The mathematical expectation and variance of the contact conductance, calculated from experimental findings, depending on the number of tap-changing operations, are shown in Figure 3. An analysis of the invariance indicates that the largest deviations in the contact conductance occur in the range of 10 to 24 thousand tap-changing operations (region II, Figure 3). This can be explained as follows. In the initial stage of testing (region I, Figure 3), the contacts surfaces wear in less degree due to erosion processes. As the number of tap-changing operations increases, the contact wear effect due to arc erosion increases significantly (region II, Figure 3). However, when a certain level of wear is reached, their influence begins to decrease (region III, Figure 3).   Figure 5 shows the probability distribution density of the contact conductance, which in this case complies with a uniform law. The presented dependence makes it possible to estimate the number of conductance values that fall into the corresponding intervals. The analysis of the findings derived made it possible to establish certain regularities of decreasing the reliability of the OLTC in systems with load fluctuations in comparison with general-purpose systems. This can be explained by the fact that, due to electric arc erosion, the alloy structure of the metals used for making the contacts of the OLTC is partially destroyed. In the process, the contact resistance after each series of tap-changing operation increases resulted from a change in the state of the contact surfaces. Ultimately, the contact surfaces take a shape that is significantly different from the initial one.
From 40% to 80% of the contact surface is destructed, depending on the duration of the arc discharge during the tap-changing process. Contacts are destroyed most intensively at small distances between them. A significant role plays the cumulative effect when the contact surfaces are repetitively exposed by an electric arc discharge. It is well-known that the shorter the arc during the switching process, the less wear and the higher the reliability of the contact system and the tap-changer as a whole [1], [2], [4], [5]. As an example, Figure 6 shows the contact surfaces exposed ISSN 2521-6244 (Online) Розділ «Електротехніка» by arcing after 10,000 tap-changing operations. The results of the studies of the contacts make it possible to formulate technical requirements and technical specifications for the design of new standard versions of OLTC for special purposes and the reconstruction of general-purpose developments.

IV. CONCLUSION
1. Based on the analysis of the accident rate of transformer equipment in the power supply systems of energy-intensive industrial enterprises, it was found that one of the possible root causes of the development of accidents is damage to switching devices. At the same time, they refer to hidden factors, since when investigating accidents, primary attention is paid to obvious consequences, such as failures bushings, systems for exciting a magnetic field, magnetic circuits, electrodynamic effects, etc.
2. Indicators of accident rates of transformer equipment due to OLTC failures in power supply systems of energy-intensive industries, especially at load fluctuations, are 52% higher compared to generalpurpose systems. However, if the hidden behavior of the OLTC failures is taken into account, then the given indicator can increase significantly.
3. In the existing literary sources, there are practically no investigations of switching processes in OLTC for the purpose to determine the cause-and-effect factors of reducing their functional reliability in systems, where the power quality characteristics do not comply with the standard requirements. 4. A specialized device was proposed and a technique was developed for the experimental study of switching processes in tap-changers at variable THD of the exciting current, which made it possible to obtain specific results that explain the cause-and-effect factors of contact damage.
5. The contact resistance directly increases with the increase in the number of tap-changing operations, as well as the THD of the exciting current. It is shown that the largest dispersion in the contact resistance occurs in the range from 10 to 24 thousand tap-changing operations.
The article was received 05.06.2020