A New Approach to Real-Time Line Thermal Limit Monitoring

California Energy Commission Engineering Data Management Colorado State University Southwest Research Institute Electrical Power Research Institute

Power Line Systems, Engineering Data Management, EPRI, the Southwest Research Institute and Colorado State University are part of a team developing real-time rating technology for the California Energy Commission project.

On March 2nd 2000, Andy Stewart of Engineering Data Management and Eric Peyrot of Power Line Systems gave a presentation on "A New Approach to Real-Time Line Thermal Limit Monitoring" at the Power Engineers "Line Design 2000" Conference. During this presentation a prototype PLS-CADD specially modified for real-time rating applications was demonstrated. The demonstration used both data logged from a prototype instrument in service at Public Service of New Mexico and real time data received from a test span set up in the presentation room.

Below you will find an outline describing the theory and implementation used by Power Line Systems in the prototype real-time rating version of PLS-CADD.

Goal: Instrument spans to determine the following in real-time

Remaining clearance margin
Maximum real-time static rating
Maximum emergency rating

Methods:

Real-time clearance monitors
Real-time line modeling software
Sag-tension calculations
Ground and obstacle modeling
IEEE std. 738

Calculating conductor catenary constants and temperatures:

Instrumented spans:
- Get wire elevation at a specified target position in span
  May not always be practical to install target at most critical point in the span
- Calculate catenary constant for curve through target and known span ends
- Calculate effective wire temperature using sag-tension routines
  Actual conductor temperature can vary throughout conductor cross section
Non-instrumented spans:
- Assume same effective wire temperature as in closest instrumented span
  Limits of validity for this assumption:
  - For adjacent spans within same tension section
    Usually a good approximation. Same as the ruling span approximation (constant horizontal component of tension) used in most sag-tension programs. Can use multi-span sag-tension program to get feel for accuracy.
  - For other spans:
    Accuracy questionable.

Calculating clearances and critical temperature:

Given conductor position and model for ground and obstacles can calculate clearances at all points along span
Use sag-tension results to determine critical temperature at which clearances are violated

Calculating real-time ratings:

IEEE Std. 738 requires following inputs to calculate conductor temperature:
- Conductor temperature (from sag-tension calculation)
- Ambient temperature (from instrument or weather station)
- Solar radiation (from instrument, weather station or maybe even calculated)
- Current (from instrument or SCADA)
- Conductor properties (from test or manufacturer)
- Wind speed and direction:
  Rating very sensitive to even small changes in wind (see figures below)
  Rating sensitive to direction (parallel wind only 40% as effective as perpendicular)
  Measuring instruments expensive and may require regular service
  Accurate point measurements of wind are of questionable value since wind can vary along span due to shielding effects of hills and trees.
  Since we already have calculated conductor temperature and know all the inputs except for wind we can calculate an effective wind. This is in effect using the entire span as an anemometer.
  
  Curve #1: Base line 104 deg. F, 11:00 sun, 2 ft./sec wind (1.4 mph)
  Curve #2: #1 with ambient temp. changed to 94 deg. F
  Curve #3: #1 with no sun
  Curve #4: #1 with wind changed to 7.5 ft./sec (5mph)
Rating calculations:
- Real-time rating: Current causing conductor to reach critical temperature in steady state
- Real-time emergency rating: Current causing conductor to reach critical temperature in a certain amount of time

Implementation:

Chose to add functions to existing PLS-CADD program
- Already have sag-tension, IEEE std. 738 and line modeling capabilities
- Have functions for uprating line should real-time ratings prove insufficient:
  Re-tensioning, structure modifications, cutting out conductor, reclipping insulators...
New functions:
- Real-time rating screens
  Graphics and text displays based on data from log files or updated in real-time as new data becomes available
  
  The screen above shows the remaining clearance and real-time rating for each monitored circuit. Clicking in the last column opens the screen below which provides details on the individual spans.
  
  The screen above shows the remaining clearance and the real-time ratings for all of the monitored spans in a circuit. It also shows the weather and clearance measurements reported by the real-time instrumentation.
  
  This screen shows plan, profile, and 3-d views for a selected span with sags as measured in real-time. If the real-time instrumentation is capable of sending a live picture of the span then the picture on the bottom right is also updated in real-time.
- Calculate critical temperature for each span
  
  This graph shows the temperatures at which clearance problems arise in each individual span. It can be used to select the spans to be instrumented. It is also useful in determining which spans need to be upgraded for higher temperature operation.

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