Case Study - Importance of Accurate Data while doing Power System Study

First of all Apologies, as i am unable to remember whether this case study has been shared or not by me or by any of my team member

Power System Study, Relay Co-ordination and Short Circuit studies are being done by many industries for smooth operation of their plants. These studies are helpful in avoiding mal-operation tripping of plants and bring out loopholes in Electrical Protection system.

First step of such study is Data collection from Plants. In coming paragraphs, we will see why data collection is necessary for Power System Study and its importance in Power System Study Process!

Data collection involves the process of collecting data in streamlined manner related to electrical system of plant. It is the backbone for entire Power System Study process. Whether it is qualitative or quantitative data collection, ACCURATE collection ascertains the accurate conclusions!! It is important to be accurate while collecting data on site and data needs to be based on upto date system modification after installation. One cannot always refer existing system drawings for present system study of any plant.

Background

Power System Study involves some of following studies as below:

 What data we need for Study?

·      Plant electrical Single Line Diagram

·      System Voltage

·      Utility data (From Electricity Board)

·      Source data

·      Load data (Connected loads/ Running loads/ Maximum load)

·      Switchgear data (Breaker Type/Make/Rating/Model number/Fuse rating/Relay Model and make/CT Ratio)

·      Power and Distribution Transformer Data

·      Plant operating philosophies

·      Motor/Generator/VFD/DG manufacturer datasheet

·      Plant Future expansion details

·      Bus-bar ratings

·      HT & LT Cable details

·      Capacitor bank details

·      System grounding details

 Study also requires one dedicated site authorized person to accompany engineer during data collection and allow access to all Equipments, switchgear and rooms (Having permit). All the data collected should be documented in precise manner as it serves as Encyclopaedia for the plant.

Significance of Accurate data:

There are many tools available in market for Power System Study and Analysis i.e. ETAP, EMTP-RV, PSCAD, EasyPower, SKM Power Tool, MATLAB and many others. However, output of these tools totally dependent on data used to create the system model. Incorrect data can lead to poor results. It is always a good idea to verify all the data in the field/site and that is the reason System Protection always sent our engineer to sit for data verification.

Following are some examples how inaccurate data leads to inaccurate results:

1.    Transformer Impedance data:

We can observe impact of different % impedance on fault current value.

2.    Cable data:

Major difference in cable length can result into higher or lower fault current and voltage drop.

3.    Motor data:

Assumed motor starting time, Hot/Cold withstand time can lead to inaccurate protection settings for Motor Relay. Motor starting current data is also necessary to calculate Phase Overcurrent protection and avoid nuisance tripping during motor starting.

4.    Generator Impedance data:

Here we can see, how different impedance data for Generator impact on fault current value. Wrong fault current value can lead to wrong Relay setting calculation and arc flash hazards.

From a above results, you can understand the importance of accurate data. However, we also understand that in old plants, all such accurate data may not be available, so in that case few data must be assumed. it is understood that all data must be verified on site and with its documentation and drawings. This practice will lead to near to accurate results in any Power system study tool.

Conclusion & Precaution:

This article shows how Data collection for Power System Study stands as backbone for the entire process of Power System Study.

 This is why soft copy and hard copy of plant equipment data and its data sheet must be preserved.

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