FUNCTIONS PERFORMED BY
THE RELAY MADE
1.OVER VOLTAGE PROTECTION
Power System Protection
Modern Power Systems use different types of relays
for the protection of the total power system during
faults, the modern relay which is used for the
protection of power systems are the microcontroller
The power equipments which are used in a Power
station are very costly and hence to protect these
equipments from the faults generated within them
i.e. the ground faults we need an earth fault relay. An
earth fault relay detects the ground fault or earth
fault where the phase is shorted to the earth or
ground; it monitors the amount of earth current and
accordingly follows the characteristics which are
defined to it and trips the circuit.
In selecting an earth fault relay the grounding of
the system is considered. For e.g. if the grounding
is solid grounding then a fast acting relay i.e. the
speed of the relay has to be very fast in detecting
and tripping of the circuit, if the grounding is high
impedance grounding then the sensitivity of the
relay has to be increased to detect the earth fault
in the system.
In this project an overvoltage and overcurrent
relay which is made using a microcontroller .The
microcontroller is programmed in such a way that
it will perform its action whenever there is
abnormal conditions such as overvoltage or
A relay is an electrically operated switch. Many
relays use an electromagnet to operate a switching
mechanism mechanically, but other operating
principles are also used. Relays are 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 first relays
were used in long distance telegraph circuits,
repeating the signal coming in from one circuit and
re-transmitting it to another. Relays were used
extensively in telephone exchanges and early
computers to perform logical operations.".
A type of relay that can handle the high power
required to directly drive an electric motor is called
a contactor. Solid-state relays control power circuits
with no moving parts, instead using a semiconductor
device to perform switching. Relays with calibrated
operating characteristics and sometimes multiple
operating coils are used to protect electrical circuits
from overload or faults; in modern electric power
systems these functions are performed by digital
instruments still called "protective relays .
A simple electromagnetic relay consists of a coil of
wire surrounding a soft iron core, an iron yoke which
provides a low reluctance path for magnetic flux, a
movable iron armature, and one or more sets of
contacts (there are two in the relay pictured). The
armature is hinged to the yoke and mechanically
linked to one or more sets of moving contacts. It is
held in place by a spring so that when the relay is deenergized there is an air gap in the magnetic circuit.
In this condition, one of the two sets of contacts in
the relay pictured is closed, and the other set is open.
Other relays may have more or fewer sets of contacts
depending on their function.
When an electric current is passed through the coil it
generates a magnetic field that attracts the armature,
and the consequent movement of the movable
contact(s) either makes or breaks (depending upon
construction) a connection with a fixed contact. If
the set of contacts was closed when the relay was deenergized, then the movement opens the contacts and
breaks the connection, and vice versa if the contacts
were open. When the current to the coil is switched
off, the armature is returned by a force,
approximately half as strong as the magnetic force,
to its relaxed position.
A microcontroller -based system can be used for detecting faults
in the Power system. The real time data monitoring of various
electrical parameters in the Power system helps us in detecting
electrical faults. In this system the abnormal conditions are
detected by the microcontroller and necessary initiation of the
trip signal to the circuit breaker is given. For this process to
happen, real time monitoring of the data is required. Since the
microcontroller understands only binary language we need to
convert our analog signal to digital by using ADC.
After getting this data based upon the programming in the
memory (ROM/RAM) the microcontroller takes the decision of
the tripping of electrical system i.e. it detects faults based on the
conditions of the program written.
A microcontroller is a multipurpose, programmable,
clock-driven register based electronic device that reads
binary instructions from a storage device called memory,
accepts binary data as input and processes data according
to those instructions, and provides results as output.
A typical programming machine is used for performing a
specific function/ task. It can be represented with three
components 895C1 microcontroller , memory, and
input/output. These three components work together or
interact with each other to perform a given task, thus they
comprise a system. The physical components are called
hardware. A set of instructions written for the
microcontroller to perform a task is called a program and a
group of programs is called software.
Previous figure represents a block diagram of
proposed microcontroller-based over current relay.
The limited dc voltage output of the measuring unit
is proportional to the operating current, i.e.,
The voltage is fed to a microcomputer, whose
function is to perform the following jobs sequentially
through an appropriate programming
1) analog-to-digital conversion;
2) fault detection;
3) function generation;
4) automatic variable time delay achievement;
5) pulse generation.
The microcomputer first converts the dc analog input voltage
into a digital equivalent in terms of the hexadecimal system,
This digital value is tested. If there is a fault, this means that
tested value exceeds a stored digital pick-up value, then it will
be processed through a software function generator, which
consequently determines the type of the resulting overcurrent
The operation time of the relay is determined by a specified
time delay subroutine depending upon the value of (Y)h,
i.e.,t=K3(Y)h …. (4)
To make the microcontroller work as an over voltage/under
voltage relay, another set of three ADC’s are required, which
have addresses different from that of current Converting
Here the voltage is stepped down by a P.T and the output of
the P.T is directly fed to the bridge rectifier such that it
converts the sinusoidal signal into positive half cycles, this
positive cycle signal is fed to the ADC’s. The circuit is
designed in such a way that the rated voltage of the line is
represented by +4v positive half cycles. The output +4v
positive half cycles of the three phases are fed to an
ADC0801 IC’s with addresses 1000H, 0800H, 0400H. To
implement the microcontroller as an over voltage and under
voltage relay the program above is modified.
Circuit for detecting Over voltage/Under
Microcontroller-based relays offer many advantages and
benefits over electromechanical relays they are:
I. Reduced installation costs
2. Reduced maintenance cost
3. Application flexibility
4. Improved monitoring and control functions
The use of microcontroller-based relays has become very
common. Many utilities are taking advantage of the new
features and innovations offered in these relays.
New developments in microcontroller based relays offer
added benefits by further reducing costs and by improving
the relay functions and features.
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