26MVA Transformer HV Over Current SEL 751A Calculations

Dear All,

Overcurrent protection of a transformer is a type of protective measure that is implemented to prevent excessive current from flowing through the transformer, which can cause damage or even failure to the transformer and the connected equipment.

Overcurrent protection is typically achieved through the use of protective relays that detect and respond to abnormal current levels. These relays are designed to sense the magnitude and duration of the current, and if it exceeds a pre-set threshold, they act to isolate the transformer from the power source by opening circuit breakers or other disconnecting devices.

There are different types of overcurrent protection schemes that can be used for a transformer, including primary and secondary protection, time-overcurrent protection, instantaneous overcurrent protection, and directional overcurrent protection. The type of protection used depends on the specific application and the level of protection required.

Overall, overcurrent protection is an essential feature for ensuring the safe and reliable operation of transformers and their connected equipment, and it plays a critical role in preventing damage or failure due to excessive current.

Transformer over current protection for the larger unit is used as backup protection, main protection is transformer differential protection that has zero time delay. In case of differential protection fail, backup over current protection will operate.

In this course, the relay setting of a real 26MVA transformer that is installed and commissioned already.  So these are actual relay settings.

In this training, the following topics are covered,

• Basics of over-current protection

• Types of over-current protection

• Definite time and inverse time over current protection

• Types of curves of over-current protection

• How to calculate the operating time of the relay in case of a fault

• What are pickup and set values?

• What is plug setting & current settings, what is the difference between them?

• What is the time delay and time delay multiplier?

• What is disk emulation and how it is calculated?

• Impact of disk emulation over pick current and time coordination

• How earlier electromagnetic relays do look like and how they work

• Induction disc relays

• Introduction to SEL 751A over current protection relay

• Understand the front interface of the relay

• Latch and unlatch Leds

• Pickup and reset of relay

• Introduction to 26MVA transformer nameplate

• How to calculate the full load current of HV and LV sides

• What is percentage impedance and how to calculate the fault MVA with the percentage impedance of the transformer?

• What is the short circuit current and how to calculate HV & LV short circuit current?

• How to calculate short circuit current at HV & LV using full load current and percentage impedance?

• Calculate the HV over current settings.

Best Regards