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Power System Study Relay co-ordination case study

Introduction

Recently, power system studies were conducted for our client, a 10 MW hydro-generation plant located in Karnataka. The client has been experiencing frequent electrical equipment failures in the plant, resulting in significant financial losses.

case study 1

Background:

The site is equipped with a single 66kV grid incomer and three step-up power transformers, each rated at 6.3 MVA, along with three alternators, each with a capacity of 4 MVA. Additionally, there are two auxiliary transformers dedicated to supplying power to plant loads. The site generates power and also exports electricity to the common grid. Operations at the plant are primarily seasonal, focusing mainly on the rainy season.

During a shunt fault at the HT side, the client experienced multiple failures at the LT side. One incident involved observing sparks from the transformer wheels. The client has been unable to identify and resolve this issue for almost a decade. Consequently, the client authorized Bigeta to investigate and offer a solution to the problem.

Scenario:

The Bigeta team visited the site with the instruments. Our analysis began with collecting the updated Single Line Diagram (SLD), various plant operation scenarios, and load patterns. We also obtained breaker settings, models for both LV and HV sides, as well as the relay settings under which the plant was operating.

We observed fee technical issues:

  • The MV and LV panels were operating without instantaneous settings.
  • The site utilizes electro-mechanical relays for backup protection, including overcurrent and earth fault relays.
  • Earthing inspections at the site had not been conducted for several years.
  • The power transformers at the site were equipped with only one body and neutral earthing.

Analysis:

Power measurements were done with Class-A meter in HT side to check load pattern and any abnormality at HT side.

Our analysis started with by drawing the entire plant SLD in ETAP. Initially load flow and short circuit studies were done to understand the plant load pattern, fault current level.

After that relay co-ordination study was performed. It was observed during fault condition the breaker is taking more time to trip almost 2.3 seconds.

The TCC curves overlapped with transformer withstand capacity.

We also observed that relay to relay operating time (time-discrimination) is more.

The main-incomer distance relay was faulty.

Recommendations:

Following recommendations were suggested based on our analysis:

  • The main incomer distance relay should be replaced. The new settings were suggested by us.
  • We suggested new settings for all the relay with proper time discrimination for all the relays.
  • We suggested to install instantaneous relay with proper settings for all the relays.
  • We recommended that the client install an additional earth pit and establish interconnection according to IEEE 80, which mandates two earthing points interconnected for transformers.