Project Synopsis
In
power distribution systems, the power grid station gets supply from different
feeder stations like a thermal power station, a wind power station, a solar
power station etc. For feasible transmission, the frequency and voltage of the
AC supply should be within the limits as decided by the grid, depending upon
the demand of the power supply. In case these limits are exceeded and the
demand for power is more than the demand for supply, it results in grid
failure. In such situations, the feeder unit is completely disconnected from
the grid, causing islanding situation. Thus synchronization is needed between
the grid and the feeder unit.
This
project defines a way to detect the variations in frequency and voltage of the
power supply from the feeder unit to determine the synchronization failure.
Here a frequency variation detection system and a voltage variation detection
system are used. The frequency variation is achieved using an external timer
and detection is achieved using the internal timer of the microcontroller. Incase
of the frequency limits exceeding or being too low, the lamp is accordingly
switched on. The voltage variation is achieved using a VARIAC and detection is achieved
using a comparator arrangement and any variation in voltage is detected by the
microcontroller. Incase of any voltage variations, the lamp is switched on.
Block Diagram Explanation
The
DC power supply section consists of the step down transformer to step down the
AC supply voltage, the bridge rectifier to convert AC to pulsating DC, the
regulator IC to produce a regulating output.
The
voltage detection section consists of an OPAMP IC with two comparators to sense
any change in low or high voltage. For both the comparators, the varied input
voltage is compared with a threshold reference indicating low and high limit of
the voltage respectively.
The
frequency detection section consists of a zero voltage detector which gives the
input at the normal frequency, a timer which gives the input at varied
frequency.
The
control section consists of a microcontroller which receives input from the
voltage sensing or the current sensing circuit and accordingly operates the
relay driver, which is a transistor in this case.
The
output section consists of the relay driven by the relay driver which in turn
switches on or off the lamp, which is the load in this case.
Circuit Explanation
The
DC power supply part consists of the AC supply being stepped down using a step
down transformer. A bridge rectifier consisting of diodes converts this AC to
pulsating DC voltage, which is regulated using a regulator IC. This DC supply
is then given to the Microcontroller and the other parts of the circuit.
Voltage
Sensing Part
The
microcontroller is connected to the zero voltage sensing circuit to ensure the
frequency of the supply is at normal frequency of 50Hz. A VARIAC is used to get
variable voltage.
Initially
both the presets are adjusted such that both the output pins of the OPAMP IC
are at normal low and normal high level. The VARIAC is adjusted so as to get
the input AC voltage more than the normal value. Now the normally high pin of
the OPAMP IC will go low, giving an interruption pulse to the pin of the
microcontroller. The microcontroller accordingly sends a high logic pulse to
switch on the relay driver, which in turn energizes the relay and the lamp
glows as it gets the AC power supply. Similarly when the VARIAC is adjusted so
as to get input AC voltage less than the normal value, at some point, the
normally low pin of the OPAMP IC goes high and the microcontroller on receiving
this interruption, sends a high logic signal to the relay driver to switch on
the relay and hence the lamp which starts glowing.
Frequency
Sensing Part
The
VARIAC is adjusted such that the AC input voltage is at its normal value. The
microcontroller pin is connected to the output of the timer through a PNP
transistor. The timer works in astable mode to produce signals at frequencies
which can be adjusted using the variable frequency. This output is connected to
the internal timer of the microcontroller which accordingly calculates the frequency
of the pulses and when the frequency of the pulses goes beyond the normal
frequency or less than the normal frequency, the relay driver is triggered,
which in turn energizes the relay and the AC supply is given to the lamp which
starts to glow.
Output Video Explanation
Initially
the normal frequency input is given to the microcontroller pin through a zero
voltage detector circuit. There are two presets- one representing the low
voltage set point (LVSP) and other representing the high voltage set point (HVSP).
When the LVSP preset is adjusted such that the input voltage is less than the
threshold low voltage, the lamp starts glowing. Similarly when the HVSP preset
is adjusted so that the input voltage is higher than the threshold upper
voltage, the lamp starts glowing. In both the cases, the glowing of lamp
indicates the voltage limits have been exceeded or going beyond.
Now
the voltage is set at stable point and the slide switch is connected such that
the microcontroller now receives input through a timer. The variable resistor
of the timer is adjusted such that the frequency of the output signals from the
timer changes. Incase the frequency deviates from the normal limit, the lamp
starts glowing, which indicates islanding or isolating of the grid from the
feeder point has taken place due to deviation in frequency.
Conclusion
This
project gives a demonstration of the system to detect the power grid
synchronization failure. Any deviation in the supply voltage from the lower and
threshold limits or any changes from the set frequency is detected and
accordingly the grid is disconnected from the supply source, which is indicated
by the glowing of lamp in this case.
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