Lecture34 – ECE504PS – Spring 2007

Goals of this lecture are to:

  1. Project reminder -- project will be due Friday, April 27th for on-campus students and Friday, May 11th for off-campus students.
  2. Review L31HW
  3. Improving stability
  4. Integrate new Load and resource – What will it cost ?.

The nomogram from the L31 HW should look something like:

The limiting contingencies are:

·  #13 and #14 (limit the Area 3 to Area 4 transfer to about 390 MW).

·  #7 & #8 (limit the Area 1 to Area 2 transfer to about 450 MW).

·  #3 # #4 (limit the combined export from area to about 450 MW).

Identify these contingencies on the system below; the form of these limits should make some sense. (Note that the dashed lines are assumed are out of service in the simulation and assumed to not currently exist in our simulation.

The colored lines were found from several simulations and are the HW solution. Once this first cut a nomogram is found, it is a good idea to build base cases for the corner points. I only used the transient stability index for the screening, but included the transient voltage criteria for the corner points. (Note that the voltage dip moved one corner in).

Generally, what might improve system stability? Think swing equation:

Pmax = V1V2/X & (2H/ωs) (d2/dt2) d=Pm-Pe

During the fault bus voltages are depressed and Pe can become quite small.

  1. Make H larger (not likely!)
  2. Decrease Pm (at least initially for units near fault)
  3. Make V1 and V2 larger
  4. Make X smaller
  5. Limit time of fault

What ways are to improve transient stability:

  1. The inertia of the unit isn’t likely tobe something you can change
  2. Decrease Pm
  3. Improve governor response (e.g. improved turbine valve control)
  4. Shed unit(s)
  5. Brake
  6. Note that I think of entered the turbine models to match those discussed in Kundar section 11.1.4 and compared graphically in Figure 11.21 shown below. It would be nice to hook these models individually to an infinite bus to match this figure. Similar results can be acquired by making step changes to the power reference in TSAT.

  1. Make V1 and V2 larger
  2. Maintain voltage schedules on transmission
  3. Be sure units are not bucking during normal operation by appropriate control of switched devices
  4. Have well tuned excitation systems (note that very hot excitation systems that are not tuned can introduce a negative synchronizing torque – This is what I’ve done for our three area system. The exciters respond quite nicely to a step change in voltage reference; however, they are significantly reducing the stability of the simulations. This is why I’ve pulled the excitation model for the nomograms. Hopefully, it can all be sorted out in time and tied into small-signal stability discussions (and PSS – Power System Stabilizer discussions).
  5. Make X smaller
  6. The best way to make X smaller is to add transmission! (Good luck)
  7. Upgrade voltage (if I were to build the three area example for a future class, then Area 3 would be a 115kV system with 115/230 step up transformers at Station A. We would be able to look at the system improvement of moving the transformers to the hydro station to Station A and rebuilding the line to 230kV.
  8. We can also add series compensation (SSR may be a hazard)
  9. Single pole switching (less line lost for many faults).
  10. Add midway switching stations to very long parallel lines (only half as much line is lost). Sta A to Sta 3 9 would be a consideration in our three area example.
  11. Creative topology changes. Contingencies #3 and #4 are significant in our three are example. These are contingencies on very short (2 mile lines). Two miles of new 230kV transmission probably cost less than $1M; however, full double breaker double terminations on both ends could almost triple the cost. Why add terminations? Look at the solution on next page (Double circuit to make even more robust):

Reduce the time of the fault. Zone 2 clearing can be 3 or 4 time slower than zone one clearing. The cost of communication is relatively small. In our three area system we are already down to 5 cycles (this isn’t likely to be improve significantly).

Next thing to think about: A new 400 MW industrial load wants to locate in our service territory. (It will create jobs, bring in additional tax revenue, make the mayor happy, etc). The new load wants have control of its energy costs so plans to build six 70 MW gas turbines to supply its load. The industrial hopes to buy from the market most of the time, but use it units as supplemental capacity or for peaking. The new load will be located off of Station 3 9, and the generation can be located midway between Station A and Station39 or near Area 3 Load.

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