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Doc # 1ZUA003101-AAB NON-IDEAL BIAS WINDING APPLICATION FOR LOAD TAP CHANGER TYPE-USB INTRODUCTION: In some cases, it is not practical to design transformers such that the ideal arrangement of tap sections is obtained. Two such cases will be addressed here. They are: 1. The case of one turn tap sections. 2. The case of a mixture of differing numbers of turns per tap section. In cases where the total number of active turns in the RV winding is greater than 16 but not evenly divisible by 16, it is necessary for some tap sections to have more turns than others. In these cases, the LTC will produce nonlinear ratio steps. This is true for all LTC’s (reactance and resistance) but is often applicable and produces results within the 0.5% standard allowed by IEEE C57.12.00. An Example of “mixed” tap sections: THE CASE OF ONE TURN TAP SECTIONS: As can be seen from Diagram 1, the bias winding is an isolated tap section having the same number of turns and volts as in the tap sections of the regulating winding (RV). This winding is used to excite a small 2:1 autotransformer which produces a ½ tap voltage for the bias circuit. As the tap changer operates, the diverter is connected to the same tap for two operations, one with the bias circuit and one without the bias winding. This produces two output voltages for each tap of the RV winding. Diagram 1 shows that the bias winding is not in the circuit when the diverter is on position “v” with load current through selector contact “b” . Bias circuit insertion would occur when the diverter moves to position “x” and current is being drawn through contact “a”. THE CASE OF DIFFERING NUMBERS OF TURNS PER TAP SECTION: Diagram 1 Sequence Chart Transformer Rating: OLTC Applied: V2: 138/34.5 kV 34.5 kV 19,919 V=34.5/3 kV If this transformer design were to be optimized at 100 V per turn, let us assume, then the following would apply: Desired Volts/Turn: RV: RV active turns: 100 V/t 1992 = V2 X 10% 20t = RV/100V Since 20 turns does not divide evenly into 8 active sections, the following arrangement could be used (see Diagram 2): Turns/Tap: 4 w/ 2 turns 4 w/ 3 turns Extra Tap for Reversing: 1 w/ 2 turns Turn Arrangement: 2,3,2,3,2,3,2,3,2 turns Bias Winding Turns: 1 turn Max. Calculated Error: 0.368%(Pos 20 or 3R) Diagram 2 Turn Arrangement Graph 1 Graph 1 shows the output voltage of this non-ideal arrangement compared to the ideal values and those calculated using a reactance LTC. Note, that there are only (4) positions that have greater error with the bias winding scheme and (24) positions that produce the exact same voltage as with reactance LTC. All positions produce ratios well within the 0.5% standard allowance (IEEE C57.12.00). Bias Winding – An isolated tap section used to produce ½ size steps in voltage. Glossary: Effective Sections – Max. number of tap sections that could be loaded at one time. Regulating winding (RV) – The total collection of all tap sections. Effective Turns – Vector sum of turns carrying load current at a given moment. LTC - On Load Tap Changer Tap - A connection to a sectionalized winding Tap Section – A group of one or more turns between taps. Tap Voltage – Volts per turn times turns per tap section. Step Voltage – The change in the output voltage with each LTC operation. V2 – Line to ground voltage of transformer secondary. Diagram 3 – Single Phase Diagram Applying One Turn Sections Notes: