Faculty of Engineering and Information Technology
Subject: 48583 Power Systems Operation and Protection
Assessment Title: Protection Problems
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Assignment – Protection Problems
Q1(10 marks). In Chapter 2 of the textbook Fundamentals of Power System Protection, there is a discussion on the application of IDMT relay on a radial distribution feeder for the following system:
The first step in the protection design is to decide CT ratios and plug settings for relays A and B. Then the second step is to determine the time multiplier settings for the two relays by the following criterion:
As commented in the above textbook, the above design principle can be extended to an n-bus system with the aid of a computer algorithm. Consider a radial feeder consists of n buses and (n-1) IDMT relays shown as below:
Please write a computer program to do the corresponding IDMT relay designs following the steps given in the textbook. The following requirements must be met.
i) Use Matlab m file to write your code.
ii) Input to your program will include at least: minimum and maximum fault currents at each of the buses; available CT ratios.
iii) Output of your program must be the CT ratios, plug settings, and time multiplier settings for each of the n-1 relays.
iv) You must test your program by the example on the textbook (i.e. the system in Q1.1), and test it further by an example of a 4-bus radial system (use assumed input parameters). Testing details (e.g. input) and results (i.e. output) must be presented in the report.
v) The computer code (m file) must be enclosed in the report, and necessary comments are needed to make the code readable.
Q2 (7 marks). The diagram shows part of a 132/11/3.3 kV network supplying a 200
1 O/H TL15 km 2 6.2?
30 MVA 3 2cable km 4
22.5% 0.28 ?
200 A 100 A
4 MVA 5 Load
100 A 100 A
• The source impedance is 0.0029 pu on a 10 MVA base. Convert all impedances to a 10 MVA base.
• Determine the fault current and MVA seen by each CB for 3-phase faults at each busbar and on the load side of the 200 A fuse.
• Calculate the full load current seen by each breaker.
Determine the setting of each relay.
Choose suitable 200 A fuse and relay characteristics.
Q3 (13 marks) Capacitor banks are usually connected at the low-voltage bus of high-voltage substations to maintain the voltage of the system. This configuration is shown in Figure 1.
Figure 1 Single-line diagram of capacitor connection
Figure 2 Capacitor bank in the substation
Part A (4 marks)
By referring to the power system literature:
1) Name and explain briefly the three important protective relays which are usually used for the protection of capacitor banks in the substations.
2) During the capacitor switching, a high transient current called the inrush current flows into the circuit. Explain briefly this inrush current.
Part B (9 marks)
The inrush current is usually limited by installing an inductance in series with the capacitor bank as shown in Figure 3. In this figure, the capacitor bank is shown by C. ?????? and CB represent the equivalent inductance of the network and the circuit breaker, respectively. The needed inductance to limit the inrush current is shown by L.
Parameters of the above figure are:
????,3?? = capacity of the capacitor bank = 20 ??????
????-?? = phase to phase voltage = 63 ????
??????,3?? = Equivalent short circuit current at the capacitor location = 40 ????
????????,???? = Maximum current capability of each circuit breaker = 8 ????
?? = Frequqency of the network = 50 ????
1) By using ??????,3??, calculate the value of ?????? (H). By using ????,3??, calculate the equivalent capacitor on each phase.
2) At first, the circuit breaker is open. Calculate the inrush current when the capacitor bank is switched on (????1 is closed). Is a series inductance (L) necessary in this case? If yes, calculate its corresponding value. (Note: for calculations, consider the worst case scenario. It means that CB is switched on when the source voltage is at its maximum value.)
3) Usually, more than one capacitor bank are used in parallel in the substations as shown in Figure 1 and Figure 4. Calculate the inrush current when the second capacitor is switched on while the first capacitor bank is already energized. Is a series inductance (L) necessary in this configuration? If yes, calculate the corresponding value. (Note: in this case, you can neglect the current from the upstream network.)