Percentage Differential Relay for Protection of Three Phase Transformer

In this Instructable, I will show you how to make Percentage Differential Relay using Arduino, which is a very common microcontroller board. Power transformer is the most important equipment to transfer power in power system.

The cost to repair a damaged transformer is very high (millions of dollars). That’s why protective relays are used to protect power transformer from being damaged. It ease to fix a relay rather than to a transformer. So, Differential relay is used to protect transformer from internal fault. In some cases it fails to operate or mal-operation due to MI currents, stationary over excitation of core, external faults in the presence of CT saturation, power transformer ratio mismatch, operation due to high second harmonic component. In this scenario percentage differential protection, harmonic-restrained differential protection is used, respectively.

Simulation is done on software MATLB Simulink Figure shows simulation diagram of system in which transformer is protected by percentage differential relay. Simulation parameters are as follow:

Simulation Parameters:

Primary voltage phase to phase rms..................400V

Secondary voltage phase to phase rms.............220V

Source voltage...................................................400V

Source Frequency..............................................50Hz

Transformer Rating............................................1.5KVA

Transformer Configuration.................................Δ/Y

Resistance........................................................300 Ohm

Figure shows simulation model of designed differential relay. This relay takes primary and secondary currents of power transformer as input parameter and give logical output in form of Boolean variable.

Relay output is used as input parameter for circuit breaker on source side. Circuit breaker is normally close and opens when it receives logical 0 input.

Hardware required for Differential Relay Trainer are as follow:

• 3×Power Transformer (440VA - Single Phase)
• Arduino MEGA328
• 16x4 LCD
• 6×ACS712 Current Sensors
• Connecting Wires
• 3×5V Relay Module
• Indicators

Everything is assembled according to simulation diagram.

“Differential protection based on the principle of that power input to the transformer under normal condition is equal to power out”

In this protection scheme spill (differential) current is not compared to constant value but it varies as input current varies. Although, It's compared with fraction of line current. As current increases, fractional value of current also increases. Starting inrush magnetizing current is although very high but it is controlled by percentage differential relay. Because when input current increases, specific percentage of line current also increases and relay withstands input transient response of transformer.

There are two fault analysis:

1. Internal Fault
2. External Fault

Case 1 (Internal Fault):
t<0.5 --> Relay Logic = 1 --> I = Max

t>0.5 --> Relay Logic = 0 --> I = Zero

Case 2 (External Fault):

t<0.5 --> Relay Logic = 1 --> I = Max
t>0.5 --> Relay Logic = 1 --> I = Infinity

Now its time for the main thing- coding our Relay...

Final Thesis for more detail is attached below.