LAB SHEET POWER STATIONS

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Introduction

For economic considerations, small-three-phase power transformers (less than 100 kVA) are usually protected with fuses. The main disadvantage of using fuses to protect power transformers is their long operating time under low overcurrent conditions. This delays transformer disconnection, and in certain cases, provides little protection to the transformer and rather protects the system disconnecting the faulty transformer after the fault has reached an advanced stage.

Design of Protection Scheme

A simplified diagram of an overcurrent and earth fault protection system is shown in Fig. 1.
Assume that the current set point of the Three-Phase Overcurrent Relay is approximately 200% of the nominal full-load current of the power transformers. Based on the above information, compute the transformation ratio of the current transformers.
Design the values of the load resistances to be connected in star for full load current of the transformer. Complete the design and fill up all the details in the simplified diagram given.


Equipment to be used for testing the protection system

Mobile Workstation, Protective Relay Control Station, Transmission Grid “A”, Interconnection Module, Universal Fault Module, Power Supply, Faultable Transformer, Resistive Loads, Current Transformers, AC Ammeter, AC Voltmeter, Three-phase Overcurrent Relay, AC/DC Current Sensitive Relay, and Connection Leads.


Testing procedure and observations

1. Ensure that the Protective Relay Control Station is connected to a three-phase power source. Make sure that the DC Power Supply to the Protective Relay Control Station is turned off. Also, make sure that all fault switches on the Three-phase overcurrent relay and AC/DC current sensitive relay are set to O (off) position.