Automatic Load Sharing of Transformers using Microcontroller
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ABSTRACT: The transformer is a static device, which converts power from one level to another level. The aim of the
project is to protect the transformer under overload condition by load sharing. Due to overload on transformer, the
efficiency drops and windings get overheated and may get burnt. Thus by sharing load on transformer, the transformer
is protected. This will be done by connecting another transformer in parallel through a micro-controller. The micro
controller compares the load on the first transformer with a reference value. When the load exceeds the reference value,
the second transformer will share the extra load. Therefore, the two transformer work efficiently and damage is
prevented. In this project three modules are used to control the load currents. The first module is a sensing unit, which
is used to sense the current of the load and the second module is a control unit. The last module is micro-controller unit
and it will read the analogue signal and perform some calculation and finally gives control signal to a relay. A GSM
modem is also used to inform the control station about switching. The advantages of the project are transformer
protection, uninterrupted power supply, and short circuit protection. When designing low-voltage power system to the
supply large load currents, paralleled lower-current modules are often preferred over a single, large power converter for
several reasons. These include the efficiency of designing and manufacturing standard modular converters which can
be combined in any number necessary to meet a given load requirement and the enhanced reliability gained through
Transformer is a static device which converts energy at one voltage level to another voltage level. It is an electrically
isolated inductively coupled device which changes voltage level without change in frequency. Transformer transfers ac
voltage from one electrical circuit to another by the principle of mutual induction. Distribution transformers are one of
the most important equipment in power system and are also known as the heart of the power system. The reliable
operation of a power system depends upon the effective functioning of the distribution transformer. Therefore
monitoring and controlling of key parameters like voltage and current are necessary for evaluating the performance of
the distribution transformer. Thus it helps in avoiding or reducing the disruption due to the sudden unexpected failure.
Transformers being one of the most significant equipment in the electric power system, needs protection as a part of the
general system protection approach. Moreover the increasing population and their unavoidable demands have led to an
increasing demand on electrical power. With this increased needs, the existing systems have become overloaded. The
overloading at the consumer end appears at the transformer terminals which can affect its efficiency and protection
systems. Due to overload on the transformer, the efficiency drops and the windings gets over heated and may get burnt.
It takes a lot of time to repair and involves a lot of expenditure. Transformers are occasionally loaded beyond
nameplate ratings because of existing possible contingencies on the transmission lines, any failure or fault in power
systems, or economic considerations. One of the reported damage or tripping of the distribution transformer is due to
thermal overload. To eliminate the damaging of transformers due to overloading from consumer end, it involves the
control against over current tripping of distribution transformer. Rise in operating temperature of the transformer due to
overloading has an influence on ageing of transformers. The accelerated aging is one of the main consequences of
overloading power transformers. Thus load limitations must be implemented to operate the transformers within safe
limits. Moreover on overloading the transformers voltage regulation may increase and power factor drops. The project
is all about protecting the transformer under overload condition. This can be done by connecting another transformer in
parallel through a microcontroller and a relay which shares the excess load of the first transformer. The transformers
are switched alternatively to avoid thermal overloading. Therefore, two transformers work efficiently under overload
condition and damage can be prevented. If there is a further increase in load beyond the capacity of two transformers
there will be a priority based load shedding of consumers which will provide un-interrupted power supply for the
hospitals, industries etc.
II. RELATED WORK
In the power system transformers may be loaded beyond their nameplate ratings due to a fault or some emergency
conditions. This type of overloading can cause either short term failures or long term failures. Increase in hot spot
temperature would also lead to the accelerated ageing of the transformers resulting in transformer overload. So in order
to keep the body temperature of the transformer within its nameplate rating, the transformer must be loaded efficiently
in a controlled manner.
Our system aims at load sharing of transformers and priority based load shedding. The procedure of load sharing is
1. Loads are supplied from a single transformer under normal condition and a standby transformer is connected in
parallel through a circuit breaker.
2. A current transformer measures the load current continuously and feeds it to the controller by converting it to a
corresponding D.C value in order to compare with the reference value set by the user.
3. Whenever the load current exceeds reference value, the controller sends a high signal to the relay which
energises the relay coil. The relay coil thus sends a tripping signal to the circuit breaker of the standby
4. Thus the load is shared by the transformers equally as the transformers are identical. The current transformer still
measures the load current and compares it with the reference value.
5. Whenever the load current falls below reference value one transformer is shut down and this is done in an
alternative manner to avoid thermal overloading.
6. If the load value increases further beyond the capacity of two transformers, load will be cut-off from the main
supply based on the priority level set by the user. This is done to provide un-interrupted power supply to higher
7. Each of the process is informed to the controller by a GSM and the load parameters are continuously displayed
in the LCD.
In the block diagram circuit breakers are used to make and break the connections to the transformers. A relay is used to
send a tripping signal to the circuit breakers and they are energised on receiving a signal from the microcontroller. The
current transformer is used for measurement purpose.
A circuit breaker is used to isolate the faulty point of the power system in case of abnormal conditions such as faults. It is
a protective device which energizes and de-energizes a circuit and provides over-current protection. Circuit breakers
operate on receiving a signal from relay.
A transformer is an electrical device that transfers electrical energy between two or more circuits through
electromagnetic induction Transformers convert AC voltage from one level to another level with a little loss of power. A
transformer operates on the principals of “electromagnetic induction”, in the form of mutual induction. The transformer
used here is a step-down transformer so that it can be directly fed to the measuring devices by rectification.
The microcontroller is used to compare the load current with reference value. Atmega328 is the controller used for this
purpose and it also provides a provision for GSM module and in built ADC.
Relays are components which allow low power circuit to operate high current application circuits. It is an electrically
operated switch and is used where it is necessary to control a circuit by a low-power signal with complete electrical
isolation between control and controlled circuits, or where several circuits must be controlled by one signal. The relay
used here is of electromagnetic type.
The Current Transformer is a type of “instrument transformer” that is designed to produce an alternating current in its
secondary winding which is proportional to the current being measured in its primary. Current transformers reduce high
voltage currents to a much lower value and provide a convenient way of safely monitoring the actual electrical current
flowing in an AC transmission line using a standard ammeter. The principle of operation of a current transformer is
same as that of an ordinary transformer.
A GSM modem is a specialized type of modem which accepts a SIM card, and operates over a subscription to a mobile
operator, just like a mobile phone. Here the purpose of GSM modem is to send the monitoring parameters values and
faults of transformer to authorized person’s number in control room.
It is a class of wireless modem devices that is designed for communication of a computer with the GSM and GPRS
network. It requires a SIM card to send the message.
IV. SYSTEM DESCRIPTION
The system consists of microcontroller, transformers, circuit breakers, relay, Current Transformer (CT), GSM modem
and LCD display. The transformers are step down transformers in which only one transformer is operating under
normal condition. The input to the transformer is fed through a circuit breaker to which a relay is connected. The circuit
breaker is in closed position for transformer which is operating. Here the transformer feeds five loads which are
provided with individual circuit breakers for protection. A stand by transformer is connected in parallel to the main
transformer through a circuit breaker. In order to measure the current through the transformer a current transformer is
connected to the secondary of the operating transformer. The current transformer measures the load current
continuously and is fed to the microcontroller through a rectifier circuit. The output from the current transformer can
also be fed directly to inbuilt ADC pins of controller, instead of using a rectifier. A GSM connected to the controller
enables communication between the system and control room. The maximum load limit is entered to the controller
through a keypad and a LCD display gives an indication of the same. The microcontroller continuously compares the
CT value with the maximum limit entered. Whenever the current exceeds the maximum limit, the main transformer
gets overloaded and the second transformer shares the total load equally. At the same time the GSM sends a message to
the control room. When there is a further increase in load beyond the rated capacity of two transformers,
microcontroller will give control signal to the circuit breaker of respective load to open, based on the priority level set
by the user. When the load decreases and comes to normal value which is less than the maximum limit, the first
transformer will shut down automatically. This type of alternative switching method avoids the possibility of thermal
overloading by providing enough time for the transformer to cool naturally. Each time the transformer is overloaded or
switched a message is sent to the control room about the mode of operation. Thus it enables efficient operation of
existing transformer and provides un-interrupted power supply to hospitals, industries and other important areas.
In the proposed system, only one transformer is operating to feed the loads. A standby transformer is connected in
parallel through a circuit breaker and relay. The current transformer continuously measures the load current and feeds it
to the microcontroller ADC pins. The reference value or the maximum load limit is entered by the user and priority
level of the load is also set by the user or concerned authority. As the load demand increase during peak hours, a single
transformer would not be able feed all the load. During this condition, when the load demand exceeds the reference
value, the microcontroller will give a control signal to energize the relay coil. Thus the standby transformer will be
connected in parallel and will share the load equally since the transformers are of the same ratings. Thus all the loads
are fed efficiently providing un-interrupted power supply. The GSM modem will send a message to the control room
about the load sharing and a display will be shown in the LCD display. When the load increases further to a value which is greater than the capacity of the two transformers, priority based load shedding will be implemented. The loads
which have the lowest priority will be shut down by opening the respective circuit breakers. This message is also sent
to the control room. When the load decreases, and comes to normal working condition, first transformer will be shut
down in order to avoid thermal overloading. This is done because the first transformer operates for a longer time
interval than standby transformer and its body temperature rises. By providing alternative switching, the transformers
can be cooled by natural methods. Each time the GSM will send message about the active transformer thus making load
sharing and load shedding efficient.
Transformers work only when the relays are latched. The transformer used here is step down transformer which
converts 230V to 12V. The controller and LCD require a DC operating voltage of 5V while the relay and GSM operate
at a DC voltage ranging from 9V to12V. A power supply circuit is provided to get 12V and 5V DC from the 230V
mains by a full wave bridge rectifier. A 7805 regulator ensures a regulated 5V supply to the LCD and controller. Here
five inductive loads are connected to the transformer each provided with an individual relay. A current transformer
continuously measures the load current of the transformer and feeds to the controller. The current transformer is
connected to a zener diode in order to measure quick response. Under normal working condition relay 1 is latched and
supply passes to the load through a manual switch and a relay contact. Normally the relay will be latched initially and
on receiving a low signal from microcontroller the relay will be de-energized and becomes open thus interrupting the
supply. The controller is programmed in such a way to send a low signal to relay when the transformers are overloaded.
Thus the load cannot be operated even if the manual switch is closed. At the same time the GSM sends a message to the
control room about the transformer operation. Here as the fifth load is switched on both the transformers are overloaded
and in order to maintain the supply priority based load shedding is also included.
Whenever the load decreases and reaches a normal level, transformers are switched alternatively to avoid thermal
overloading. The main advantage of this type of switching is that, it allows the transformer to cool by natural methods
thus increasing its lifespan.
The controller used is ATmega 328 which is 8 bit controller and has 20 I/O pins of which there are 14 digital I/O pins
and 6 analog pins. The keypad is configured as rows and columns of which either row or column is set as input. Thus
the controller reads the value by matrix arrangement. Crystal oscillator is connected to pin no. 9 and 10 of the controller
which generates clock signals. Digital pins PD0 to PD7 are configured as output and all the output devices like LCD
and GSM are interfaced to this port. The row pins of the keypad and the relay is connected to the port B while the
column pins of keypad and CT is connected to port C of the controller. All the loads are connected to the port B
through a relay which is supplied with a 12V supply. Relay contacts are shown named as RLY 1, RLY 2, and RLY 3.
Of these two relay contacts corresponds to two transformers and the third relay corresponds to priority based load
shedding. A crystal oscillator is used to generate clock signals to the controller. It is also possible to use internal
oscillator of the controller unless the ADC pins are used. A CT is connected to ADC pin 5 of the controller and a
variable resistor is used to vary the value of CT. A GSM is connected to digital pins 0 and 1for communication
purpose. In order to view the message displayed by GSM, a virtual terminal is used. A 4x3 keypad is used to set the
maximum load limit and the LCD gives a visual indication of CT value and maximum load limit. For LCD display four
bits of address lines are connected to the controller. RESET pin is connected to the power supply so that the controller
works continuously. The controller can be reset by applying a logical low signal to RESET pin. Relays are connected
with transistors which act as switches. Whenever a high signal is applied to the base of the transistor, the transistor act
as a closed switch and energizes the coil of the relay which in turn latches the relay contactors. A freewheeling diode or
a flywheel diode is connected across the relay coil to protect the collector terminal of transistor from the back emf of
the relay coil.