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How to Prepare Power Systems for GATE Exam

Hi Guys,

Today I am here with a post that all of you have been waiting for as to how to study Power Systems for GATE. In my personal opinion, this is a subject which has the widest course and different topics have different flavors like the approach in Fault Analysis is entirely different from the approach in Transmission Lines. The weightage of this subject in GATE exam varies between 8 to 12 marks. The weightage is generally less than Electrical Machines but it is very important from GATE as well as future point of view. In general, it is expected from Electrical Engineer that he must have a vast knowledge of this subject. In this subject, there are a lot of chapters and each chapter is actually is itself a field of research but a good thing about this subject is none of the chapters is too much in-depth. So here you have to focus on the width and not on the depth of the subject.
The major topics in Power Systems can be listed as follows:
  • Transmission and Distribution

The first topic under this unit is overhead transmission lines and in this topic first, you need to memorize the expressions for Inductance and Capacitance for both Single Phase and Three Phase Lines. So you should practice some problems based on this which involve the computation of GMD and GMR. Also, you should read about the construction of Transmission Lines such as Bundled Conductors and Stranded Conductors. This will increase your understanding of Power Systems.

The next topic that you should cover is modeling of Short and Medium Transmission Lines as Long Transmission Lines is not much important in Steady State Analysis and Short Transmission Line is most important. In short Transmission Lines, most number of questions can be framed and you need to remember the circuit of Short Transmission Line. For medium Transmission Line, you can memorize the ABCD parameters of Nominal-pi Model and Nominal-T Model. For long Transmission Line, small questions can be framed based on the concepts of Lossless Line parameters as well as wave parameters such as wavelength, the speed of wave etc.

The next thing that you need to study is the power expressions at both Sending and the Receiving end in terms of ABCD Parameters. This will help you in finding the power for any type of Transmission Line once you have the knowledge of its two-port parameters.

The next topic under this unit is Surges and in that topic, you need to memorize the Thevenin equivalent circuit for analysis of surges and by that, you can easily calculate the Transmission and Reflection Coefficient for Voltage and the coefficients for currents are not important.

The other important topic is the concept of Voltage Control and Power Factor Correction. In this topic, there are various Compensation techniques such as Shunt Capacitor, Series Capacitor, and Shunt Inductor. Conceptually, you need to understand the applications of each of these components. But there is a general method involved in finding the rating of compensating equipment required. If you know this method, you won’t need anything else for numerical purposes. In power factor correction as well, you need to know the formula for computing the rating of compensating equipment required to bring the power factor to the desired value.

The next topic is UG Cables and in UG Cables there are two important concepts, one is the insulation resistance and other is the capacitance model for 3 phase  3 core belted cable which has core capacitance and sheath capacitance but the question is rare from this topic.

The other small topic under this unit is Overhead Insulator where only a single problem based on voltage distribution across different discs can be asked and that you can easily derive using KCL or remember the expression up to 3 discs.

The last topic is the Distribution Systems which also has a single type of problem which involves just applying KVL and finding the currents in different branches.

HVDC and FACTS are not included in GATE course and can be left.

  • Load Flow Studies

In this unit, you need to study about the construction of Bus Admittance Matrix using Direct Inspection Method and algorithm of modification of Bus Impedance Matrix. Also, we need to study the fault Analysis by the use of Bus Impedance Matrix which involves knowing that diagonal elements of the matrix represent Thevenin impedances at different buses.

The next thing that we need to remember is the order of Jacobian Matrix in Newton-Raphson Load Flow Solution.

Also, there are some simple question where we need to find voltages and their angles at different buses and those are the simplest questions as they just involve using KVL between different buses and using power expression for Short Transmission Line and if possible remember the method of iteration for Gauss-Seidel Method because the iteration is generally small and questions up to one iteration can be asked.

 

  • Economic Load Dispatch

In this topic, you just need to study the transmission loss coefficients and penalty factor and then how to distribute load across various generating units based on Incremental Cost and Penalty Factor. This topic is now removed from GATE course now so you can leave it but it is very important for other exams so one look at this topic is not harmful.

How to Prepare Power Systems for GATE?

  • Fault Analysis

Here we need to study about Symmetrical and Unsymmetrical faults and first thing in case of Symmetrical faults is the transient and steady-state component of fault current and there you just need to memorize a single expression by which you can easily derive the maximum momentary current as well.

Then in symmetrical fault analysis, you need to solve problems where we find the symmetrical SC currents by drawing the equivalent circuits and also problems where we need to calculate the current limiting reactance.
One thing that you need to take care of is units as the answer may be given in pu or in ohms so you need to take care else you can mark the incorrect answer.

The last thing about symmetrical faults is calculating the ratings of Circuit Breakers based on Symmetrical fault Analysis like Momentary Current, Breaking Current and Making Current.

Before going to Unsymmetrical Fault Analysis you need to study about Symmetrical Components which involves just a small transformation matrix and about Sequence Networks in which zero sequence network for Transformers in most important and can be asked directly in the exam.

In case of Unsymmetrical Fault Analysis, we need to remember the equivalent circuits for all three types of faults LG, LL and LLG Faults and how to calculate fault current from sequence components.

 

  • Power System Stability

This is the smallest topic in Power Systems but still, it is very important. Here Steady State Stability is not Important but transient stability is and you need to remember the two inertia constants “H” and “M” for that and the swing equation.

The next thing is that you need to remember the equivalent M and H when there are two machines which are swinging coherently or incoherently.

The last and most important topic is the equal area criteria for calculating the critical clearing angle and I would recommend that you understand the method involved instead of memorizing the expressions involved as that will reduce the chances of error.

Also, nowadays questions are being framed on finding the acceleration, speed of Machine when a disturbance is introduced. These questions are based on Swing Equation and if you remember how to apply the Swing Equation then these problems can be solved easily. But the main problem occurs in units so you should be very clear about units.

 

  • Power System Protection

This is most rarely asked topic and is widest of all the units but nowadays questions are being asked from topics that are generally ignored by the students so there is a chance that question may be asked from this topic.

The only numerical part in this unit is concentrated around Plug Setting Multiplier(PSM), Distance Relay and the differential relay so you need to be clear on both the aspects and practice a few problems based on these two concepts.

Other than that for theoretical purposes, you can remember the usage of some general relays like Distance Relays types and which type is used where and also the types of relays used in Transformers-like Buchholz Relay.

If you guys have any query you can join our free doubt solving Facebook group. Also if you guys liked this post please share it with your co-aspirants so that all can benefit and see that power systems is actually not so scary.
Happy Learning!

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