ES3E7 Power Flow Assignment Resit: 9 Bus Systems
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ES3E7 Power Flow Assignment Resit: 9 Bus Systems
2021/22
Introduction
In this assignment, you will be finding the parameters of the components in the 9-bus, 50 Hz power system whose single line diagram is shown in Figure 1. Once the parameters are found you will carry out power flow studies on the system under light and heavy load conditions, with multiple assumptions concerning the impedances in the system, under a transmission line outage condition, and to find the required reactive power compensation needed to maintain operation of the power system within its operating conditions.
Along with this document and the briefing sheet (giving details about the write-up, hand-in date and assignment weighting), you are provided with an Excel spreadsheet called “Power System Data” which includes data on the transformers, transmission lines and generators needed to complete the project.
To generate the admittance matrices, modify the admittance matrix and perform the power flow studies for this assignment, you will need to use the Matlab code provided for you throughout the course. All of this is available on Moodle in the section for the appropriate week, and videos detailing how to use the code are on the lecture capture area in Moodle.
Each task details what should be provided in the final report.
The power system operating conditions that should be maintained are:
1. The voltage on all transformers must remain within ±0.05pu of 1pu (based on the corresponding rated voltages).
2. All generators and transformers must not exceed their rated operating conditions
3. All transmission lines must remain below their rated current capacity (ampacity).
Task 1: Transmission Line Parameters
15 Marks
It can be seen in Figure 1 that there are 6 transmission lines in the power system, labelled TL1 to TL6. Information for these transmission lines can be found in the Power System Data Excel spreadsheet. Use this data to find the equivalent circuit parameters for each of the transmission lines. Indicate what type of model would be used when simulating these transmission lines.
Present the calculations for transmission line TL1, then show the results for the other five transmission lines. Note that some of the transmission lines are the same. Values should not be given in their per unit form. Inductance and capacitance values can be given in H and F, respectively, or they can each be expressed as a reactance in Ohms.
Task 2: Transformer Parameters
15 Marks
Figure 1 shows that there are 3 transformers (Y-Y connected) in the power system, labelled TF1 to TF3. Details of the transformers, along with results of open and short circuit tests of the transformer on a single-phase equivalent can be found in the Power System Data Excel spreadsheet. In the short circuit test, the secondary windings are shorted, and in the open circuit test, the primary windings are left open.
Present the calculations for transformer TF1, and then present the final results for the other 2 transformers. Parameters should be referred to the 275kV side, and shunt branch values given in terms of Siemens.
Task 3: Convert to Per Unit Values
10 Marks
In Tasks 1 and 2, you found the ohmic values of the parameters of the system. To perform a power flow study, these parameters must be in per unit form. Show the equations needed to do this, and then show the final results. The three-phase base power used throughout the system is:
You may present the transformers’ per unit parameters referred to the transformer base power, but note they must be referred to the power system’s base in the actual power flow study.
Compare the shunt admittance values of the transmission lines and the transformers. Discuss why it may be justifiable to omit the transformers shunt admittances in the power flow study.
Task 4: Light Load Power System Definition
5 Marks
Figure 2 shows the power system under light load conditions.
Produce a table showing the following information for each bus:
· The bus type (i.e. slack, PV or PQ).
· All known parameters.
· The initial conditions for the unknown voltages.
Bus 2 is a PV bus, and yet has a known reactive power load of j0.15 pu (i.e a per unit reactive power of 0.15 us being removed from the system). However, the code provided to carry out a power flow study will ignore any value of reactive power for a PV bus. Considering that the voltage magnitude is known and fixed, show how to incorporate the reactive load on bus 2 into the Admittance Matrix. Show the calculation of the per unit reactance used.
Figure 2: Power System in Light Load Conditions
Task 5: Light Load Power Flow Study
15 Marks
Three power flow studies will be performed on the system under the load conditions shown in Figure 2. Each will compare the results when ignoring certain elements that form the Admittance Matrix. The three cases are
1. Perform a power flow study when only the line inductive reactance of the system is taken into account.
2. Perform a power flow study when only the line inductive reactance and resistance of the system are considered.
3. Perform a power flow study when the line inductive reactance and resistance, and the shunt values of the system are included (ignore the transformer shunt values if you feel it its justified).
For all cases, the reactive power load on bus 2 should be included.
Tabulate the results for all three cases showing, for each bus, the voltage magnitude (pu), the voltage angle (radians), the real power (pu) and reactive power (pu).
Explain the results by addressing the following points:
1. Explain why the slack bus must only needs to provide 1.45 pu of real power in case 1, but more in cases 2 and 3
2. Explain why, when compared to cases 1 and 2, the generators in case 3 are required to provide less reactive power.
3. Explain why the voltage dropped across the transmission lines is more significant in case 2 when compared to case 1.
4. Compared to case 2, the voltage dropped across the transmission lines in case 3 is less severe. Give a reason for this.
5. Do any of the generators, transformers or transmission lines operate outside of their rated values in any of the cases?
Task 6: Heavy Load Power Flow Study
10 Marks
Figure 3 shows the power system in heavy load conditions. Note that the loads and generation values are higher. Repeat the power flow study for each case described in Task 5 for the power system in heavy load conditions. Show Tabulated results for all three cases showing, for each bus, the voltage magnitude (pu), the voltage angle (radians), the real power (pu) and reactive power (pu).
Explain the results by addressing the following points:
1. How do the results compare to the light load study?
2. Do any of the generators, transformers or transmission lines operate outside of their rated values in any of the cases?
Figure 3: Power System in Heavy Load Conditions
Task 7: Transmission Line Outage
10 Marks
For this analysis, set up the power flow study for heavy load (Figure 3), in case 3 conditions (i.e take resistance, line reactance and shunt admittance into account).
A fault occurs meaning that transmission line TL1 is removed from the system. Modify the Y_Bus to take this into account and perform a power flow study on the subsequent system.
Tabulate the results showing, for each bus, the voltage magnitude (pu), the voltage angle (radians), the real power (pu) and reactive power (pu).
Describe and give reasons for the effects of the transmission line outage on:
1. The voltage magnitudes across the system.
2. The reactive power supplied by the generator connected to bus 2.
Task 8: Compensation Equipment for Worst Case Scenario
10 Marks
The transmission line outage at heavy load represented a worst-case scenario condition for this power system. To mitigate the effects of such an outage, compensation equipment providing reactive power can be installed on PQ buses 4, 5, 6, 7, 8 and 9. Experimentally determine suitable reactive power values for 1 or more of the available buses, which mean that the power system operates within its operating conditions under heavy load with transmission line TL1 removed. No single bus should provide more that 2.0 pu of compensating reactive power. The lower the reactive power provided for compensation, the smaller the compensator needs to be, reducing costs.
Show the results of your power flow study with the compensation reactive power, demonstrating that the system is operating within its operating conditions.
Explain why the value of the reactive power compensation needs to be controlled, rather than always providing reactive power with static equipment.
2022-08-26