Troubleshooting Power Flows That Fail to Solve

All power flow equations are non-linear and require an iterative technique to solve. A wide range of data input values that increase the chance of numerical round off are inherent in many systems. For these reasons, some power flows may fail to converge. The following are some helpful hints in troubleshooting a failed power flow case.

• Often, trouble can be traced to generator control voltages being too high or too low. Typical values should be 1.0-1.05 with 1.01 as a good starter. Don't be afraid to vary the values to get a solution. You can then gradually increase the voltage to the desired level.
• If a voltage controlled load tap changing transformer is electrically close to a PV generator, they will fight each other to control the voltage. They need not be trying to control the same bus--just being electrically close may cause problems, since one bus voltage will affect the other's. Try blocking the LTC, turning it off, or changing it to MVAR control. You can also change the generator type to PQG (constant power, constant var). Using the same reasoning, the voltage that the LTC is trying to control may be influenced to a larger degree by a nearby generator or utility supply.
• Transformer LTCs do not have the same amount of control over system voltages as large generators. If you are used to controlling your system with generation, don't be surprised if LTC control is not sufficient.
• If the load is too large, convergence may be difficult. This is especially true if high impedance lines are present. Try converting the load to constant impedance. If the case solves, you know that is the problem.
• If a generator is trying to control voltages that are too electrically remote, the solution may diverge. Try controlling the generator bus first. If you get a good solution, gradually work your way to the remote bus. Remember the effect other controlling factors might have such as utility supplies and LTCs. Even though they may appear far away on the one-line, they can be electrically close. Check your input data.
• Isolated or separated systems are not allowed in the power flow. This is because each subsystem must have a Swing bus in order to solve. Be very careful when opening or closing breakers.
• A great deal of opening and closing breakers in the Power Flow focus may add to the possibility of round off in some systems, which can eventually cause the solution to diverge. If you suspect this is the problem, open the Database Edit focus by clicking Database Edit (toolbar menu: Analysis > Database Edit). Then, re-enter the Power Flow focus and re-open or close the breakers that you want to be different than their normal states (normally open or normally closed). Alternatively, while you are in the Database Edit focus, you can change the normal states directly.
• If you get power flow divergence, check the Mismatch report in the Power Flow Summary report. Find the bus that has the largest mismatch over a period of several iterations. Go to that bus in the one-line and look for problems. Some areas to look for include:

  • Loading which is too high. Check for decimal place errors, remembering that EasyPower input data is in MVA not kVA.

    Transformer taps or motor kV's which may be off by a decimal point. A transformer MVA may also be entered too low, or the motor HP too high.

    Look for these problems in the area (within two buses) of the bus with the greatest mismatch. After you feel you have checked this area thoroughly, move onto the bus with the next-highest mismatch on the list, and so forth.

• Another problem area is overloading of the complete system. Typical user input is to place all loads at a 100 percent scaling factor. Typical scaling factors for process plants are in the 40-70 percent range. A good place to start is to scale all group loads at 40-50 percent and all large individual motor loads to 70 percent. A quick and easy way to tell if this is the problem is to go to the database and turn off a large selection of loads. Then re-run the power flow and check for results.

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