EPPM563: Power System Protection Assignment 1

Assignment 1 CT Analysis

Current Transformer (CT) is used to transform higher primary current to smaller secondary current so that it can be used by the relay. The objectives of this assignment are:

• Study the saturation characteristics of a CT.
• Relate the parameters of a saturable transformer in Simscape to the available CT standards Students are required to perform the following tasks: A simple single phase system will be used to set a saturable transformer model to fit a 10VA, 100:5 10P20 CT. The single phase system is shown below:

1. From the given CT specification, calculate the standard burden of the CT and set it as the resistance parameter on the resistor labelled Burden in the system shown in Figure 1. Show clearly your calculations and how the parameter was set for the Burden.
2. Set the winding voltages for the block labelled CT in Figure 1 according to the specification of the given CT. Explain your answer and clearly show how the parameter was changed in the parameter dialog box.
3. Set the system at 10KV with a frequency of 50Hz, and the Load and Load 1 set at unity power factor. The Load real power should be set such that the rated primary current of the CT will flow through the primary windings of the CT before B1 closes. The Load 1 should be set such that when B1 closes at 0.5s, the ALF multiplied by the standard primary current of the CT will flow in the primary of the CT. With a line length of 2km and default values for impedances. The saturation characteristics of the CT is to be set using the following parameters. Set the values of a to 2 and c to 1.017 in the matlab workspace. Plot the difference of current flowing through the Line and the current through the burden multiplied by the nominal CT ratio of the CT. Prepare a 1s plot. Calculate the error of the CT from the RMS values of the currents. What is the CT error at this point? Has it violated the specification given(10P20)?. d. Adjust the value of c in the saturation characteristics of the CT as shown below from 1.01 till 1.1 at steps of 0.001. 
4. Plot the CT error versus the value of c when the load is set to allow the rated current multiplied by the ALF in the primary windings. From the plot determine the minimum value of c that will provide a CT error that full fills the given specification. Provide an explanation using the saturable transformer model of why the CT error trend is as obtained in the plot.
5. With the saturable characteristics obtained in c, perform a parameter sweep of the system by varying the burden impedance starting from 100% the standard burden to 100 times the standard burden value. Plot the CT error versus the burden value for this sweep. Comment on how the CT error changes with the burden value by analysing the plot.

Brief Summary of Assessment Requirements

The assignment focuses on analyzing the saturation characteristics of a Current Transformer (CT) and understanding how CT parameters relate to standard CT specifications using MATLAB Simscape. The key tasks include:

1. CT Burden Calculation:

   • Compute the standard burden of a 10VA, 100:5 10P20 CT and set it as the resistance in the Simscape model.

2. Winding Voltage Setup:

   • Configure primary and secondary winding voltages in Simscape according to CT specifications.

3. System Setup:

   • Model a 10kV, 50Hz single-phase system with two loads.
   • Ensure rated primary current flows through the CT and simulate the scenario where ALF (Accuracy Limiting Factor) multiplied current flows when B1 closes at 0.5s.

4. Saturation Characteristic Analysis:

   • Use parameters a=2 and c=1.017 in MATLAB workspace.
   • Plot the difference between line current and secondary current scaled by nominal CT ratio.
   • Calculate CT error and check compliance with 10P20 standard.

5. Parameter Sweep for Saturation Constant c:

   • Vary c from 1.01 to 1.1 in steps of 0.001.
   • Plot CT error versus c and determine the minimum c meeting CT specification.
   • Explain the trend using the saturable transformer model.

6. Burden Impedance Sweep:

   • Sweep burden impedance from 100% to 100× standard value.
   • Plot CT error versus burden and analyze how burden affects CT accuracy.

Step-by-Step Approach Guided by Academic Mentor

The assignment was approached systematically under the mentor’s guidance as follows:

1. Understanding CT Specifications:

   • The mentor helped the student interpret the 10VA, 100:5 10P20 CT parameters and identify the standard burden from the CT rating.
   • Formulae for calculating burden resistance were explained, and the student was guided to input it correctly in Simscape.

2. Configuring Winding Voltages:

   • The mentor demonstrated setting primary and secondary voltages in the CT block of Simscape.
   • The student learned how voltage parameters directly influence CT response and accuracy.

3. Setting Up the System in Simscape:

   • The mentor explained modeling a single-phase system with two loads, showing how to simulate rated current and ALF conditions.
   • Step-by-step guidance ensured the student correctly modeled 10kV, 50Hz system, with correct load values to replicate CT operating conditions.

4. Plotting and Analyzing Saturation Characteristics:

   • Using initial values a=2 and c=1.017, the student plotted the difference between primary and secondary current and calculated CT error.
   • The mentor explained how to check against 10P20 CT accuracy limits.

5. Parameter Sweep for Saturation Constant c:

   • The mentor guided the student on how to vary c in steps of 0.001, plot CT error, and determine the minimum value of c meeting specification.
   • The student learned to interpret trends using the saturable transformer model, linking physics of core saturation with observed CT error.

6. Burden Impedance Sweep Analysis:

   • Under guidance, the student performed a burden sweep (100% to 100× standard) and plotted CT error versus burden.
   • Mentor explained how CT error changes with burden impedance and the importance of selecting appropriate burden in real systems.

Outcome Achieved

• A Simscape CT model was successfully configured with correct burden, winding voltages, and system settings.
• CT saturation characteristics were analyzed, and error calculations verified compliance with 10P20 standard.
• Parameter sweeps helped determine optimal saturation constant c and illustrate burden effects on CT error.
• All plots and analyses were completed, and explanations were provided linking CT behavior to physical principles.

Learning Objectives Covered

1. Apply CT specifications to a practical simulation model.
2. Analyze CT saturation characteristics and interpret error trends.
3. Understand the effect of system parameters (burden, ALF, winding voltage) on CT accuracy.
4. Develop skills in MATLAB Simscape for modeling electrical equipment.
5. Relate theoretical CT concepts to practical outcomes and compliance with standard specifications.

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