Introduction to Industrial Automation

control panels with a two-wire interconnection.

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  Introduction to Industrial Automation

3.8.2   Operation Control of a Power Transformer

Every three-phase power transformer contains a device for the level control of oil, which is denoted 

as a Buchholz device, and a temperature sensor for controlling the temperature of the oil. Both 

of these devices have two output contacts SPST configured in two levels, one warning and one 

final level. This means that if the level of the oil is below a specific level, the SPST contact will 

be energized, which subsequently will be properly utilized for signaling an alarm. If the level of 

oil continuously decreases, or a bubble flow is being created, then the second SPST contact is 

energized, which will be utilized for achieving an operation STOP. A similar functionality can 

be considered for the other two contacts of the thermometer. As presented in Figure 3.33, the 

power transformer is connected from one side to the high voltage network through the utiliza-

tion of a medium voltage disconnector switch and the automatic switch (oil or air based). From 

the lower voltage side, the power transformer is connected to the low voltage main switch. The 

medium voltage automatic switch, as well as the low voltage main switch is equipped with a trip 

coil through which the transformer can be set out of operation, which means that each one breaks 

the corresponding circuit. The trip coil is activated and causes the breaking when the voltage is 

applied on its terminals.

At this point, we would like to design a proper automation circuit suitable for the indication 

of operational faults as well as for the breaking operation in critical errors. More specifically, the 

whole automation is desired to satisfy the following specifications:

  1. For the four kinds of error types, a separate light indication should exist.

  2. For the two alert signaling errors (SPST contacts) of the Buchholz device and the thermo-

meter, a sound signaling (horn) should exist.

N

C

50 Hz 230 V

R

START/

STOP

START/

STOP

START/

STOP

e

d

2

Control

panel 1 

Control

panel 2 

Control

panel n 

d

1

d

1

d

2

C