2. Objective :-
The main intention of this project is to design a
microcontroller based harmonic restrained differential
relay protection system that can be used in power
transformer protection.
Normal differential relay can fail due to magnetizing
inrush current. The magnetizing inrush current
waveform normally consists of several harmonics while
the internal fault current consists of the fundamental
component.
To solve the problem of inrush current the harmonic
restraint differential relay will be used.
3. Numerical Differential Protection of Power Transformer
The tripping condition of percentage differential relay
is as follows:
There may be false tripping of the relay due to
magnetizing inrush current (MIC) and over excitation
inrush current.
4. Magnetizing inrush current:
This is the maximum instantaneous current that
a transformer draws when it is first switched on.
Power transformers can draw as high as 8-30
times its rated current depending on its
resistance.
For MIC second harmonic inrush condition -
𝑖1𝑠 − 𝑖2𝑠 2
≥ 0.15 𝑖1𝑠 − 𝑖2𝑠 1
5. Over excitation inrush:
Over excitation when referring to a transformer
means an increase in magnetic flux in the core
above allowable/normal levels. This causes the
magnetizing current to increase.
For over excitation fifth harmonic inrush
condition
𝑖1𝑠 − 𝑖2𝑠 5≥ 0.08 𝑖1𝑠 − 𝑖2𝑠 1
6. The magnetizing inrush current is rich in second
harmonics.
Over excitation causes and increase in inrush current
with a large increase in the odd harmonic component,
especially the third and fifth harmonic components .
final restraining (blocking) criterion is given by
𝑖1𝑠 − 𝑖2𝑠 2≥ 0.15 𝑖1𝑠 − 𝑖2𝑠 1
𝑖1𝑠 − 𝑖2𝑠 5≥ 0.08 𝑖1𝑠 − 𝑖2𝑠 1
7. Discrete Fourier Transform(DFT):
The discrete fourier transform (DFT) is used
to evaluate the fourier coefficients from N
samples of x(t) taken at time t=0,Ts,2Ts,
….(N-1) Ts, where Ts =T/N is the sampling
interval .
The input to the DFT is a sequence of
samples(numbers) rather than a
continuous function of time x(t).
The DFT equation is obtained from
8. For the microprocessor implementation of DFT the
equations can be written as follows:-
ak =
bk =
Where K=0,1,2, ……N-1
9. The amplitude of the kth component is given by
ck =
For 1st Harmonic over formula will be-
12. Devices:
I to V converter ( ACS755XCB-050) :
The detected signal can be used for measuring the
amount of current in the conductor. Since the
microcontroller can take a maximum of 5V DC input.
An ACS75x series sensor converts the current to a
maximum of 5V.
13. Sample and hold circuit :
Used to keep the instantaneous value of the rapid
varying analog signal constant during conversion
period.
It is basically an operational amplifier which charges
a capacitor during the sample mode and retains the
value of the charge of the capacitor during the hold
mode.
14. Analog Multiplexer:
-An analog multiplexer selects one out if the multiple
inputs and transfers it to common output. In our case
we have two inputs I1s and I2s and we have to select
one at a time using mutiplexer.
Analog to Digital Converter: (ADC0801)
15. Bipolar to Unipolar Converter :-
• The bipolar to unipolar converter converts both
positive and negative values of voltage to only
positive values having maximum values of +5V.
• The output of the summing circuit is given by
• The output of the inverter is given by
16. Bipolar to unipolar converter
The first part of the circuit is a summing point and
the second part of the circuit is an inverter. Here
we are using LM741 as bipolar to unipolar
converter.
17. Relay Control:
An electromagnetic relay has been employed as a
switch to isolate the transformer from the power
system in case a fault occurs.
The relay sends signals to the contactor which in
turn disconnects the circuit and isolates the
transformer from the power system.
