Comparative Analysis of Windings and Vector Groups in 20/0.4 kV Distribution Transformers at CV. Centrado Prima

— Turn ratio test is a comparison test of the number of secondary turns with the primary turns on the transformer. The turns ratio and group vector testing is one of the routine transformer tests. This test generally uses the Transformer Turns Ratio (TTR) test tool, the test equipment used sometimes experiences technical and non-technical problems so a comparative test method is needed to get a good analysis. The research method used is observational research and literature study, namely analyzing the transformer to be studied by conducting TTR testing in two ways, namely testing with test equipment and three-phase low voltage sources, aiming to obtain a very good level of accuracy. The test results were analyzed and compared with theory and calculations, with reference to the IEC 60076-1 standard. The results of the comparison of windings and group vectors using TTR 100 test equipment and a 3-phase low voltage source (TVR) show the same conclusions as the results of the analysis, where the first transformer sample is in normal condition and meets the requirements. SPLN D3.002-1:2007 or IEC Standard 60076-1. While the second sample transformer is in an abnormal condition in the W winding where the difference value is above 0.5%.


I. INTRODUCTION
In an electrical energy distribution system that plays an important role is the distribution transformer. Where electrical equipment is very vital in the distribution of electrical energy to consumers or the public, for that reliability must be maintained and in good condition for the smooth distribution of electrical energy. In the transformer there will be a failure, both thermal and electrical which results in the disruption of the electric power distribution system, to prevent this it is necessary to carry out preventive, predictive or corrective maintenance, these steps are to prevent total failure [1], [2]. To keep the distribution transformer in good condition, a test is needed to determine the state or condition of the transformer, because without good maintenance it will have an impact on decreasing the performance of a transformer [3]- [5].
In carrying out the test as a basic reference, it is carried out according to SPLN D3.002-1:2007 or IEC 60076-1 by going through three kinds of tests, namely: routine testing, type testing and special testing [6]. Turning ratio testing generally uses the Transformer Turns Ratio (TTR) test equipment, the test equipment used sometimes experiences technical and non-technical problems so a comparative test method is needed to get a good analysis. From the problems above, it is necessary to understand how to analyze the test results both calculations, using TTR test equipment and using alternating low voltage sources directly.

A. Transformer
A transformer is an electrical device that can convert or transfer alternating electrical energy (AC) from one voltage level to another based on the principle of electromagnetic induction (EMF Induction) that occurs between 2 (two) or more inductors (coils), but does not change the input source frequency or transformer output frequency [7]. The transformer can only work on alternating current (AC). In general, transformers are divided into 2 (two) types, namely step-up transformers and step-down transformers [8], [9]. A transformer has two or more coils that are inductive. The coils are connected magnetically. When one of the coils (primary) is connected to an alternating voltage source, a magnetic flux occurs in the laminated iron core, because the coil forms a closed network, current will flow in the primary coil resulting in flux in the coil and self-induction occurs) and induction will occur on the other side of the coil (secondary), from this induction produces a current that flows in the secondary coil when the secondary circuit is loaded, so that the overall electrical energy is channeled by magnetization. If the magnetic = it will produce a flux that induces a voltage in the primary winding of:

B. Transformer Ratio
Comparison of transformer windings there are 3 kinds of them [11]: 1. TNR (Transformer Nameplate Ratio) is the ratio data contained on the transformer name plate.
2. TVR (Transformer Voltage Ratio) is the ratio data obtained from measuring the voltage between the primary and secondary sides. Where for the group vector having a neutral terminal must be included in the measurement.
Deviation or difference in TTR or TVR is the allowable tolerance value in the form of %. according to IEC standard 60076-1, the maximum allowable % deviation value is ± 0.5% [12], [13].

C. Transformer Vector Group
In the vector group transformers there are several symbols that indicate the relationship of each winding. First symbol: HV winding connection. Second symbol: LV winding connection. Third symbol: phase shift expressed as the number of hours [14].
In the vector groups of transformers there is the term clock number, which is the difference in the phase angle between the low-voltage coil side and the high-voltage coil. Each hour digit is multiplied by 30°. For example: 0 = 0°, 1 = 30°, 5 = 150°, 6 = 180°, 7 = 210° (-150°), 11 = 330° (-30°). Based on the value of the phase angle, the vector groups can be grouped as in table (1)  According to IEC 60076-1, the vector group notation is HV-LV respectively. For example, a step up transformer with a delta connected secondary and a wye connected primary, is written as 'Dy1'. This indicates that the LV winding lags behind the HV by 30°. Transformer connection notation is very important to know the connection of the high voltage coil and low voltage coil in the main transformer tank (main tank) so that it can make it easier to identify the type of connection used.

D. Research Methodology and Data Collection Technique
The research methodology used is the Observational Research and Literature study, which is observing the transformer to be studied by testing TTR and group vectors in two ways, namely using test equipment (TTR) and using a 3-phase low voltage source, the test results are compared with the calculated data (theoretical). The analysis is based on the standard IEC 60076-1 or SPLN D3.00:2007 where the maximum deviation is ± 0.5% and reads the group vector based on the high voltage winding against the voltage winding with the angle difference indicated by the clock. The following is a flow chart of the research used. Data retrieval is done by observing the transformer name plate, the data as reference data for analysis. Further data collection by testing using test equipment using either TTR 100 or manual testing, namely using a low-voltage AC source directly by performing two different tests, namely the ratio test and the vector group test. This data retrieval is carried out as a comparison between reference data and primary data which includes test or measurement data.
In testing the vector group using a 3-phase 200 VAC low voltage source, it is done by connecting 2 (two) terminals with the same letter (marking), between the high voltage terminal side and the low voltage terminal side, then on the high-voltage terminal, it is connected to a lowvoltage source of 200 VAC, while the low-voltage terminal is open (Figure 3). The condition in this experiment is that the 1U terminal on the high voltage side and 2u on the low voltage side is connected using an additional cable, then voltage measurements are made on the other terminals (1V,1W,2v and 2w). Vector group testing using an AC low voltage source needs to pay attention to several points that must be met, namely: The transformer has a good (balanced) winding ratio for all phases (normal transformer) and the low voltage source used must be good (balanced) and stable.

III. RESULTS AND DISCUSSION
In this research, the samples used were 2 units of distribution transformer 20 kV/400 V, vector group Dyn5 with a capacity of 400 kVA (sample 1) and 630 kVA (sample 2). The standard used is the international standard IEC 60076-1 and the national standard SPLN D3:2007. For writing or reading vector groups starting from a higher voltage to a lower voltage and the clock number according to the phase angle shift between the primary and secondary coils, the test data uses the "Megger" TTR100 test tool and a low voltage source of 200 VAC as follows.
A. Test results using the TTR 100 test tool "Megger"    From the results of the TVR test measurements (Table  5) and (Table 6), to get the value of the voltage comparison, a calculation is carried out using equation (4) because the vector group of the sample transformer is Dyn5. And to find out the deviation or difference in the ratio of the results of the TVR test using equation (6), the value of the ratio and deviation of each transformer is in accordance with tables (7) and (8).   (7) the deviation obtained is below 0.5%. With a total average deviation of 0.3% for each tap, and for transformer sample 2 (630 kVA) as listed in tables (4) and (8), the deviation obtained for the U and V phases has a normal ratio value, but for the W phase is not normal, which is above the permissible tolerance, with a total average deviation of each Tap by 47%.
Based on vector group testing using a low voltage source of 200 VAC as shown in tables (9) and (10) transformer sample 1 (400 kVA) shows the transformer clock value is 5. While for transformer sample 2 (630 kVA) it does not match any clock transformer based on table (2) due to winding abnormal W phase.
IV. CONCLUSION Based on the test results obtained several conclusions, among others: A. Test the turns ratio and vector group using the TTR 100 "Megger" test kit: 1. Transformer sample 1 (400 kVA) from all tested Taps obtained a deviation value below 0.5%, and for the average deviation of each Tap it was 0.3%. The vector group is Dyn5, so this transformer is in normal condition. 2. Transformer sample 2 (630 kVA) from all tested Taps found abnormalities in the W phase winding with a deviation of more than 0.5% and for a total average deviation of 47%. B. Tests using a low voltage source 3 Phase 200 VAC obtained the same results as testing using the TTR 100 test equipment, namely: 1. Transformer sample 1 (400 kVA) under normal conditions with a deviation value below 0.5% with an average deviation of 0.29% and for group vector testing by connecting terminals 1U and 2u the results are 1V-2v = 1W-2w = 1V-2w > 1W-2v, which indicates the clock transformer is 5, so the group vector is the same as the data on the installed nameplate, namely Dyn5. 2. Transformer sample 2 (630 kVA) has an abnormality in the W phase winding with a deviation of above 0.5% with an average deviation of 23.86%, for group vector testing by connecting the 1U and 2u terminals, the results are 1V-2v = 1W-2w = 1V-2w > 1W-2v which does not show the transformer clock = 5, because there is a winding damage in the W phase.