Question

Voltage regulating transformers (load voltage control). This question 4.3 explores the use ofregulating transformers to control voltages at bulk supply load- buses An important application of voltage regulating transformers is to control the voltage on the low-voltage (LV) side of transformers in bulk supply substations. Such sub- stations receive power from the transmission system (e.g. at voltages at 275 kV and higher) and transform it to sub-transmission / distribution system voltages (e.g. 66 kV or 33 kV) for distribution to consumers. The transformers in these sub-stations typically employ on-load tap-changing transformers (OLTCS) to regulate the volt- age on the LV side of the transformer. By doing so the power and reactive power drawn from the LV winding is approximately constant in the steady-state The objective of this question is to obtain insight into the main factors that deter- mine the off-nominal tap position required to achieve a specified voltage at the LV side of the transformer The University of Adelaide, 2019 Electric Power Systems Page: 4 of 6 Consider the following circuit Regulating Transformer E-E/8 VH=VNOM Figure 2: Problem 4.3 - Regulating transformer supplying a load Show that the source voltage phasor E is: (a) (?x È = E(cos&+jsin8) (3) -X = Although it is possible to obtain an accurate closed form solution ofthe above equa- tion for the off-nominal tap-position twe will instead seek an approximate solution in order to provide conceptual insight. Assume that the transmission angle ô is small and that consequently cos6 -1. Under this assumption it follows that