Differential Protection of Transformer Using Arduino

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Volume 3, Issue 7, 2016

7 Available online at www.ijiere.com

International Journal of Innovative and Emerging

Research in Engineering

e-ISSN: 2394 - 3343 p-ISSN: 2394 - 5494

Differential Protection of Transformer Using Arduino

Ishant Sharma, Tarak Patel


, Prof. Dhaval Tailor


aB.E. Student, Electrical Department, ADIT, New Vidhyanagar bAssistant Professor, Electrical Department, ADIT, New Vidhyanagar


The title of project is “differential Protection of transformer using Arduino”. In this project differential protection is employed for protection of transformer and this is done using Arduino as brains, the current sensor measure current entering and leaving the transformer and Arduino decide when to disconnect transformer according to ratio difference and average of current

Keywords: Differential scheme, Arduino, current sensor


The transformers are main part of power system network. They do stepping up voltage and stepping down of voltage. The stepping of voltage is done at power generating station and then power is transmitted at high voltage to reduced transmission loss.

Transformers are used in a wide variety of applications, from small distribution transformers serving one or more users to very large units that are an integral part of the bulk power system . In the design of electrical power transmission and distribution system, there are various factors that need to be considered in the quest to satisfy the needs of electricity consumers. Electrical power systems experience faults at various times due to various reasons. These faults must be foreseen and safety precautions applied to the power system. The power systems engineer must include in his design, safety measures in order to avert any destructive occurrences that the system may undergo at any given time. Power system protection is very essential and necessary for a dependable electrical power supply [11].

For decades, fuse, circuit breakers and electromechanical relays were used for the protection of power systems. The traditional protective fuses and electrometrical relays present several draw backs. [12].

There are various ways are employed for protection of transformer like overcurrent, differential, etc. Each is employed for different purpose. The type of protection for the transformers varies depending on the application and the importance of the transformer. Transformers are protected primarily against external as well as internal fault and overload. The type of protection scheme used should minimize time for clearing fault and must discriminate between fault conditions and overload condition. Here for internal fault differential protection scheme is used. Here in this scheme we used microcontroller instead of CTs and mechanical relay. Here by using microcontroller instead of conventional relay gives many advantages like fast fault detection and clearing as well as cost reduction as in comparison with electromechanical and solid-state relays, microcontroller based relay perform real time computation which leads to enhancement in relay performance, facilitating faster, more secure protection for power transformer.


Differential Protection



Volume 3, Issue 7, 2016

8 differential relay [2]. Here one thing is for sure that the CTs must be connected in such a way that difference between incoming and outgoing current flows through O.C and another one is that the CT ratio must be chosen such that same current flows in sound condition. Due to internal fault currents on both secondary of current transformer differs for some value, then only this differential current will flow through the operating coil. [2]. If this difference is more than the peak up value of the relay, then it will energize relay for C.B and C.B will isolate transformer. The differential relay is used for protection of electrical equipment against internal fault, like inter turn and earth fault due to damage of insulation. And here it should be also noted that it can create an avalanche effect type of situation. [3]

Here in differential protection of transformer primary and secondary currents are first stepped down by Current transformer of specific ratio are used such that in no fault condition their current is same and their secondary is connected in additive polarity and a coil (operating coil) is connected as shown in below figure.1.

Figure.1 Differential protection scheme [2]

The connection is such that difference between current of secondary of two of CT flows through operating coil. Here we face a most common problem of inrush current, magnetizing current, mismatch of CT and CT saturation. For example, CT saturation leads to inaccurate current measurement and, therefore, may cause mal-operation of differential relays. There is a huge possibility of mismatch of CT. This results in spill current in operating coils which results in unwanted tripping which is undesirable.

B. Percentage Differential Protection:

Here restraining coil is use in addition of operating coil. The restraining coil is placed in series with both CT’s secondary and produces opposite torque than operating coil i.e. it produces restraining torque. Under normal condition the operating torque is less than restraining torque and torque is proportional to average of both the current or we can say it is proportional to I1+I2/2 and torque produced by operating coil is proportional to I1-I2, as this current flow through operating coil. Thus, total torque depends on ratio of difference and average. [4] Thus, these values decide tripping action. The below Figure.2 shows percentage differential protection scheme


Volume 3, Issue 7, 2016

9 Now, ideally for ‘through fault’ (external fault), the CT secondary on both the side of transformer will have the same value of current, but practically due to the above mentioned problems, the current will no longer be same. Rather, the CT error will aggravate as the primary current increases. And thus the spill current builds up as the external fault current goes on increasing. Figure. below shows characteristics of differential relay [8].

In detail The operating element of a differential relay is an overcurrent element. Because the ratios of CTs provided at the different terminals cannot be perfectly matched for all operating conditions, restraining elements are also included in differential relays. The restraining elements are intended to ensure that the relay does not operate during external faults. Restraint windings provide stability from CT mismatch and CT output differences due to CT saturation. Such relays, also known as percentage differential relays, operate if the differential current is above a set percentage of restraint current. [9]

fig. 3 Characteristics of differential relay [9]

Disadvantages of traditional scheme:

1) Difference in lengths of pilot wires:

This results at unequal potential at operating coil which results in spill current and may cause mal operation. This can be overcome by putting resistor in pilot wire. But it crates problem of accuracy.

2) CT ratio error:

If CT turns ratio are not selected according to turns ratio of transformer, then very high spill current flows through operating coil. This results in mal operation. And if transformer tapping are change according to load then it is difficult to correct it.

3) Magnetizing inrush current appears

Wherever a transformer is energized on its primary side producing harmonics then no current will be seen by the secondary. CT’s as there is no load in the circuit. This difference in current will actuate the differential relay. A harmonic restraining unit is added to the relay which will block it when the transformer is energized.

4) CT saturation:

If any CT is at verge of saturation, then on external or overloading this can produce high amount of error and relay may mal operate. [4]

The above all problem except (3) is solved by percentage differential protection.

C. Protection based on microcontroller:

Here Arduino is use as brains for decision-making. Here CTs are bypassed and instead we use current sensor, ACS712 hall-effect sensor. They are used for converting current signal to voltage signal. As Arduino only measure voltage at ADC port so we need current sensor to convert current value to proportional voltage value. The output of current sensor is feed to A0 and A1 port. And value of voltage measured at ADC is converted into digital form and value of maximum value of voltage is determined and maximum value of current is determined.

D. About current sensor:


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10 Figure.3 Current sensor [7]

Therefore, we need to compensate this offset inside the program of Arduino. As current to measure is in AC form thus the output voltage signal is in AC offset form at 2.5 volt. So we need rectifier to convert this AC signal to proportional DC signal and then output is given to Arduino or we can determine maximum value of voltage and maximum current is determined. And RMS value of current is determined.


A microprocessor based transformer relay scheme consists of several subsystems, such as, analog processing, analog to digital (A/D) conversion, digital processor, relay output, and power supply subsystems. And here current signal is converted to voltage signal via current sensor and then it is digitalized by ADC. [10].

Here CTs are not used instead of them we use current sensor which gives or converts current signal to voltage signal. In conventional method we used CT for stepping down current such that current at both secondary’s are same in sound condition. But by using current sensor and giving its output to ADC port of Arduino, we get value of voltage at current sensor, but here problem is that the output of current sensor varies sinusoidal and have offset of 2.5 volt. So this problem can be solved either by using rectifier or by writing program in such a way that it will measure maximum voltage and thus we can have maximum value of current.

Now here there no problem of spill current or error can be compensated easily by proper programming e.g. we can subtract value of magnetizing current form primary side current and also we can use high rated current senor for large transformer.

After determining RMS value of current of both primary and secondary the current at secondary is converted to current referred to primary. And both values are compared and current difference and average is determined. And then ratio is taken and action is taken according to ratio.


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11 Figure.5 schematic diagram of differential protection scheme

Figure.6 Hardware circuit


Here as we know that rectifier is not used instead of we are determining maximum value of current. This is possible by program. Here maximum value of voltage is determined and then maximum value of current is determined by using relation of 66mV/A. And then RMS value of both current is calculated.


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With CT:

Sr No.

Voltage During External Fault During Internal Fault

(V) Pri Side (Panel)

Pri CT

sec side (panel)

sec CT

Residual Resi arduino Pri side(Panel) Pri CT Resi DMM Resi Arduino

1 60 1.4 0.9 3.5 1.3 0.112 0.18 1.4 0.7 0.68 0.75

2 100 1.8 1 4 1.4 0.134 0.18 1.85 1 0.92 0.96

3 150 2.7 1.4 6 1.6 0.186 0.23 2.7 1.4 1.342 1.37

4 200 3.7 1.9 8 2 0.251 0.29 3.7 1.9 1.82 1.79

5 230 4.25 2.2 9.3 2.15 0.305 0.29 4.3 2.2 2.103 2.1

Table 2 Reading of current in both the winding with CT

CT Bypassed :

Sr No.

Voltage During External Fault During Internal Fault Biased setting ext Biased setting Int

(V) Pri Side (Panel)

Pri Side (Arduino) Sec Side(Panel) Sec Side(Arduino) Pri side(Panel)

Pri Side (Arduino)

Difference Ratio Difference Ratio

1 60 1.4 1.43 3.5 4 1.5 1.5 0.21 0.06 2.04 1.86

2 100 1.8 1.79 4 3.91 1.8 1.79 0.23 0.06 2.48 1.88

3 150 2.15 2.57 5.9 5.83 2.7 2.62 0.21 0.03 3.66 1.92

4 200 3.6 3.5 7.9 7.7 3.6 3.5 0.43 0.05 4.63 1.94

5 230 4.1 3.97 8.6 8.84 4.2 4 0.42 0.05 5.56 1.95


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15 at serial monitor:

Figure 9.


From this project we have concluded that at internal fault current at both winding differ by a huge value which can be measured accurately by current sensor. And we have derived very reliable protection as Current transformer is bypassed and it is only for small size transformer. For bigger size either current sensor with high rating is used or simply current is stepped down by CT.

References [1] wikipedia. wikipedia. [Online] www.wikipedia.org.

[2] electrical4u. electrical4u.com. [Online] 2011. http://www.electrical4u.com/differential-relay/. [3] electrical-engineering-portal.com . [Online] electrical-engineering-portal.com .

[4] N.G.C. Protection and Switchgear. [book auth.] N.G.Chothani. Protection and Switchgear. s.l. : Oxford Pub. [5] SUNROM.COM. SUNROM.COM. [Online] SUNROM.COM.

[6] [Online] www.slideshare.net/jagajyotijagannathjena/pptmajor2015.


Volume 3, Issue 7, 2016



Biased Differential Protection Scheme for Transformer Protection Through Micro-Controller Based Relay












pacworld [online] http:// www.pacw.org/issue/june_2010_issue/cover_story/

[10]O. OZGONENEL and C. AKUNER “A Study on magnetizing Inrush Current of Different Core Material” ELECO. International Conference on Electrical and Electronics Engineering. pp 36-39. Bursa, Turkey 2005.


Protection of power transformer by using PIC microcontroller



fig. 3 Characteristics of differential relay [9]
fig. 3 Characteristics of differential relay [9] p.3
Table 2 Reading of current with ct bypassed

Table 2

Reading of current with ct bypassed p.8
 Figure 9.
Figure 9. p.9