Power Flow Case Studies

A. What are some of the benefits of a high voltage electric transmission system? B. Why is transmission capacity in the U.S. decreasing? C. How has transmission planning changed since the mid 1990s? D. How is the power flow used in the transmission planning process? DESIGN PROJECT 1: A NEW WIND FARM

You've just been hired as a new power engineer with Kyle and Weber Wind (KWW), one of the country's leading wind energy developers. KWW has identified the rolling hills to the northwest of the Metropolis urban area as an ideal location for a new 200 MW wind farm. The local utility, Metropolis Light and Power (MLP), seems amenable to this new generation development taking place within their service territory. However, they are also quite adamant that any of the costs associated with transmission system upgrades necessary to site this new generation be funded by KWW. Therefore, your supervisor at KWW has requested that you do a preliminary transmission planning assessment to determine the least cost design.

Hence, your job is to make recommendations on the least cost design for the construction of new lines and transformers to ensure that the transmission system in the MLP system is adequate for any base case or first contingency loading situation when the KWW wind farm is installed and operating at its maximum output of 200 MW. Since the wind farm will be built with Type 3 DFAG wind turbines, you can model the wind farm in the power flow as a single, equivalent traditional PV bus generator with an output of 200 MW, a voltage setpoint of 1.05 per unit, and with reactive power limits of _100 Mvar.

In keeping with KWW tradition, the wind interconnection point will be at 69 kV, and for reliability purposes your supervisor requests that there be two separate feeds into the interconnection substation.

The following table shows the available right-of-way distances for the construction of new 69 kV and /or new 138 kV lines. All existing 69 kV only substations are large enough to accommodate 138 kV as well.

Design Procedure

1. Load DesignCase1 into PowerWorld Simulator. This case contains the initial system power flow case, and the disconnected KWW generator and its interconnection bus. Perform an initial power-flow solution to determine the initial system operating point. From this solution you should find that all the line flows and bus voltage magnitudes are within their limits. Assume all line MVA flows must be at or below 100% of their limit values, and all voltages must be between 0.95 and 1.10 per unit.

2. Repeat the above analysis considering the impact of any single transmission line or transformer outage. This is known as n-1 contingency analysis. To simplify this analysis, PowerWorld Simulator has the ability to automatically perform a contingency analysis study. Select Tools, Contingency Analysis to show the Contingency Analysis display. Note that the 57 single line/transformer contingencies are already defined. Select Start Run (toward the bottom right corner of the display) to automatically see the impact of re moving any single element. Without the KWW generation the sys tem has no contingency (n-1) violations.

3. Using the available rights-of-ways and the transmission line parameters/costs given in the table, iteratively determine the least expensive system additions so that the base case and all the contingences result in reliable operation points with the KWW generation connected with an output of 200 MW. The parameters of the new transmission lines(s) need to be derived using the tower configurations and conductor types provided by the instructor. In addition, the transmission changes you propose will modify the total system losses, indicated by the yellow field on the one-line. While the system losses are not KWW's responsibility, your supervisor has asked you to consider the impact your de sign changes will have on the total system losses assuming the system operates in the studied condition for the next 5 years. Hence, you should minimize the total construction costs minus the savings associated with any decrease in system losses over the next 5 years.

4. Write a detailed report including the justification for your final recommendation.

Simplifying Assumptions

To simplify the analysis, several assumptions are made:

1. You need only consider the base case loading level given in Design Case1. In a real design, typically a number of different operating points/loading levels must be considered.

2. You should consider all the generator real power outputs, including that of the new KWW generation, as fixed values. The change in the total system generation due to the addition of the 200 MW in KWW generation and any changes in the system losses are always picked up by the system slack.

3. You should not modify the status of the capacitors or the transformer taps.

4. You should assume that the system losses remain constant over the 5-year period, and you need only consider the impact and new de sign has on the base case losses. The price for losses can be assumed to be $50/MWh.

5. You don't need to consider contingencies involving the new trans mission lines and possibly any transformers you may be adding.

6. While an appropriate control response to a contingency might be to decrease the KWW wind farm output (by changing the pitch on the wind turbine blades), your supervisor has specifically asked you not to consider this possibility. Therefore the KWW generator should always be assumed to have a 200 MW output.

Available New Rights-of-Ways for Design Case 1

Right-of-Way/Substation Right-of-Way Mileage(km)

KWW to PAI 9.66 KWW to PETE 11.91 KKWW to DEMAR 19.31 KKWW to GROSS 7.24 KKWW to HISKY 18.02 KKWW to TIM 20.92 KKWW to RAY 24.14 KWW to ZEB 17.7

DESIGN PROJECT 2: SYSTEM PLANNING FOR GENERATION RETIREMENT

After more than 70 years of supplying downtown Metropolis with electricity it's time to retire the ToledoEdison power plant. The city's downtown revitalization plan, coupled with a desire for more green space, make it impossible to build new generation in the downtown area. At the same time, a booming local economy means that the city-wide electric demand is still as high as ever, so this impending plant retirement is going to have some ad verse impacts on the electric grid. As a planning engineer for the local utility, Metropolis Light and Power (MLP), your job is to make recommendations on the construction of new lines and transformers to ensure that the transmission system in the MLP system is adequate for any base case or first contingency loading situation. The below table shows the right-of-way distances that are available for the construction of new 69 kV and /or new 138 kV lines.

All existing 69 kV only substations are large enough to accommodate 138 kV as well.

Design Procedure

1. Load into PowerWorld Simulator which contains the system dispatch without the ToledoEdison generator. Perform an initial power flow solution to determine the initial system operating point. From this solution you should find that all the line flows and bus voltage magnitudes are within their limits. Assume all line MVA flows must be at or below 100% of their limit values, and all voltages must be between 0.95 and 1.10 per unit.

2. Repeat the above analysis considering the impact of any single transmission line or transformer outage. This is known as n-1 contingency analysis. To simplify this analysis, PowerWorld Simulator has the ability to automatically perform a contingency analysis study. Select Tools, Contingency Analysis to show the Contingency Analysis display. Note that the 57 single line/transformer contingencies are already defined. Select Start Run (toward the bottom right corner of the display) to automatically see the impact of re moving any single element. Without the ToledoEdison generation this system is insecure for several contingencies, including at least one that has nothing to do with the power plant retirement (but it still needs to be fixed).

3. Using the rights-of-way and the transmission line parameters/costs given in the table, iteratively determine the least expensive system additions so that the base case and all the contingences result in se cure operation points. The parameters of the new transmission lines(s) need to be derived using the tower configurations and conductor types provided by the instructor. The total cost of an addition is defined as the construction costs minus the savings associated with any decrease in system losses over the next 5 years.

4. Write a detailed report discussing the initial system problems, your approach to optimally solving the system problems and the justification for your final recommendation.

Simplifying Assumptions

To simplify the analysis, several assumptions are made:

1. You need only consider the base case loading level given in Design Case2. In a real design, typically a number of different operating points/loading levels must be considered.

2. You should consider the generator outputs as fixed values; any changes in the losses are always picked up by the system slack.

3. You should not modify the status of the capacitors or the transformer taps.

4. You should assume that the system losses remain constant over the 5-year period and need only consider the impact and new design has on the base case losses. The price for losses can be assumed to be $50/MWh.

Available New Rights-of-Ways

Right-of-Way/Substation Right-of-Way Mileage (km)

BOB to SCOT 13.68 BOB to WOLEN 7.72 FERNA to RAY 9.66 LYNN to SCOT 19.31 LYNN to WOLEN 24.14 SANDER to SCOTT 9.66 SLACK to WOLEN 18.51 JO to SCOT 24.14

DESIGN PROJECTS 1 AND 2: SAMPLE TRANSMISSION SYSTEM DESIGN COSTS

Transmission lines (69 kV and 138 kV) New transmission lines include a fixed cost and a variable cost. The fixed cost is for the design work, the purchase/ installation of the three-phase circuit breakers, associated relays, and changes to the substation bus structure. The fixed costs are $200,000 for a 138-kV line and $125,000 for a 69-kV line.

The variable costs depend on the type of conductor and the length of the line. The assumed cost in $/km are given here.

Lined impedance data and MVA ratings are determined based on the conductor type and tower configuration. The conductor characteristics are given in Table A.4 of the book. For these design problems assume a symmetric tower configurations with the spacing between the conductors student specific.

To find your specific value consult the table at the end of this design project.

Transformers (138 kV/69 kV) Transformer costs include associated circuit breakers, relaying and installation.

101 MVA $950,000 187 MVA $1,200,000 Assume any new 138/69 kV transformer has 0.0025 per unit resistance and 0.04 per unit reactance on a 100-MVA base.

Bus work Upgrade 69-kV substation to 138/69 kV $200,000

DESIGN PROJECT 3: SYSTEM PLANNING

Time given: 11 weeks

Approximate time required: 40 hours Additional references:

Ill. 22 shows a single-line diagram of four interconnected power systems identified by different graphic bus designations. The following data are given:

1. There are 31 buses, 21 lines, and 13 transformers.

2. Generation is present at buses 1, 16, 17, 22, and 23.

3. Total load of the four systems is 400 MW.

4. Bus 1 is the swing bus.

5. The system base is 100 MVA.

6. Additional information on transformers and transmission lines is provided in [10, 11].

Based on the data given:

1. Allocate the total 400-MW system load among the four systems.

2. For each system, allocate the load to buses that you want to represent as load buses. Select reasonable load power factors.

3. Taking into consideration the load you allocated above, select appropriate transmission-line voltage ratings, MVA ratings, and distances necessary to supply these loads. Then determine per-unit transmission-line impedances for the lines shown on the single-line diagram (show your calculations).

4. Also select appropriate transformer voltage and MVA ratings, and determine per-unit transformer leakage impedances for the transformers shown on the single-line diagram.

5. Develop a generation schedule for the 5 generator buses.

6. Show on a copy of the single-line diagram per-unit line impedances, transformer impedances, generator outputs, and loads that you selected above.

7. Using PowerWorld Simulator, run a base case power flow. In addition to the printed input/output data ?les, show on a separate copy of the single-line diagram per-unit bus voltages as well as real and reactive line flows, generator outputs, and loads. Flag any high/low bus voltages for which 0.95 V 1.05 per unit and any line or transformer flows that exceed normal ratings.

8. If the base case shows any high/low voltages or ratings exceeded, then correct the base case by making changes. Explain the changes you have made.

9. Repeat (7). Rerun the power-flow program and show your changes on a separate copy of the single-line diagram.

10. Provide a typed summary of your results along with your above calculations, printed power-flow input/output data files, and copies of the single-line diagram.

DESIGN PROJECT 4: POWER FLOW/SHORT CIRCUITS

Time given: 3 weeks

Approximate time required: 15 hours

Each student is assigned one of the single-line diagrams shown in Figures 6.23 and 6.24. Also, the length of line 2 in these figures is varied for each student.

Assignment 1: Power-Flow Preparation For the single-line diagram that you have been assigned (Ill. 23 or 6.24), convert all positive-sequence impedance, load, and voltage data to per unit using the given system base quantities. Then using PowerWorld Simulator, create three input data files: bus input data, line input data, and transformer input data. Note that bus 1 is the swing bus. Your output for this assignment consists of three power-flow input data files.

The purpose of this assignment is to get started and to correct errors before going to the next assignment. It requires knowledge of the per-unit system, which was covered in Section 3, but may need review.

Assignment 2: Power Flow Case 1. Run the power flow program and obtain the bus, line, and transformer input/output data files that you prepared in Assignment 1.

Case 2. Suggest one method of increasing the voltage magnitude at bus 4 by 5%. Demonstrate the effectiveness of your method by making appropriate changes to the input data of case 1 and by running the power flow program.

Your output for this assignment consists of 12 data files, 3 input and 3 output data ?les for each case, along with a one-paragraph explanation of your method for increasing the voltage at bus 4 by 5%.

During this assignment, course material contains voltage control methods, including use of generator excitation control, tap changing and regulating trans formers, static capacitors, static var systems, and parallel transmission lines.

DESIGN PROJECT 5: POWER FLOW

Time given: 4 weeks Approximate time required: 25 hours Ill. 25 shows the single-line diagram of a 10-bus power system with 7 generating units, 2 345-kV lines, 7 230-kV lines, and 5 transformers. Per-unit transformer leakage reactances, transmission-line series impedances and shunt susceptances, real power generation, and real and reactive loads during heavy load periods, all on a 100-MVA system base, are given on the diagram.

Fixed transformer tap settings are also shown. During light load periods, the real and reactive loads (and generation) are 25% of those shown. Note that bus 1 is the swing bus.

Design Procedure Using PowerWorld Simulator (convergence can be achieved by changing load buses to constant voltage magnitude buses with wide var limits), deter mine:

1. The amount of shunt compensation required at 230- and 345-kV buses such that the voltage magnitude 0.99 V 1.02 per unit at all buses during both light and heavy loads. Find two settings for the compensation, one for light and one for heavy loads.

2. The amount of series compensation required during heavy loads on each 345-kV line such that there is a maximum of 40 deg.angular displacement between bus 4 and bus 10. Assume that one 345-kV line is out of service. Also assume that the series compensation is effectively distributed such that the net series reactance of each 345-kV line is reduced by the percentage compensation. Determine the percentage series compensation to within +/- 10%.

Note: To prepeare this article, we used: PowerWorld Simulator: an interactive power systems simulation package designed to simulate high voltage power systems operation on a time frame ranging from several minutes to several days. The software contains a highly effective power flow analysis package capable of efficiently solving systems with up to 100,000 buses.