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Real-time path rating

The real-time path rating application is designed to evaluate path rating limits for a system. The path rating calculation first determines a provisional path rating based on a series of powerflow contingency analysis calculations. It then refines this calculation by verifying that the same set of contingencies are secure based on dynamic simulation. The details of the algorithm are described in the section at the end of this write-up. Details on running the calulation are given in the next section.

Input

The input deck is of the form

<?xml version="1.0" encoding="utf-8"?>
<Configuration>
  <RealTimePathRating>
    <printCalcFiles> false </printCalcFiles>
    <sourceArea> 1 </sourceArea>
    <sourceZone> 1 </sourceZone>
    <destinationArea> 1 </destinationArea>
    <destinationZone> 2 </destinationZone>
    <calculateGeneratorContingencies>true</calculateGeneratorContingencies>
    <calculateLineContingencies>true</calculateLineContingencies>
    <contingencyList>contingencies_14.xml</contingencyList>
    <useBranchRatingB>false</useBranchRatingB>
    <maxVoltage>1.1</maxVoltage>
    <minVoltage>0.9</minVoltage>
    <checkQLimit>false</checkQLimit>
    <monitorGenerators> true </monitorGenerators>
    <frequencyMaximum> 61.8 </frequencyMaximum>
    <contingencyDSStart> 1.0 </contingencyDSStart>
    <contingencyDSEnd> 1.015</contingencyDSEnd>
    <contingencyDSTimeStep> 0.005 </contingencyDSTimeStep>
    <!--
    <tieLines>
      <tieLine>
        <Branch> 2 5 </Branch>
        <Tag> BL </Tag>
      </tieLine>
      <tieLine>
        <Branch> 9 14 </Branch>
        <Tag> BL </Tag>
      </tieLine>
    </tieLines>
    -->
  </RealTimePathRating>
  <Powerflow>
    <networkConfiguration> IEEE14_ca_mod_rate6.raw </networkConfiguration>
    <maxIteration>50</maxIteration>
    <tolerance>1.0e-3</tolerance>
    <LinearSolver>
      <PETScOptions>
        -ksp_type richardson
        -pc_type lu
        -pc_factor_mat_solver_package superlu_dist
        -ksp_max_it 1
      </PETScOptions>
    </LinearSolver>
  </Powerflow>
  <Dynamic_simulation>
    <generatorParameters>IEEE14.dyr</generatorParameters>
    <simulationTime>10.0</simulationTime>
    <timeStep>0.005</timeStep>
    <faultEvents>
      <faultEvent>
        <beginFault> 1.0</beginFault>
        <endFault>   1.03</endFault>
        <faultBranch>6 7</faultBranch>
        <timeStep>   0.005</timeStep>
      </faultEvent>
    </faultEvents>
    <generatorWatch>
      <generator>
       <busID> 1 </busID>
       <generatorID> 1 </generatorID>
      </generator>
    </generatorWatch>
    <generatorWatchFrequency> 1 </generatorWatchFrequency>
    <generatorWatchFileName> gen_watch.csv </generatorWatchFileName>
    <LinearMatrixSolver>
      <!--
                   These options are used if SuperLU was built into PETSc 
      -->
      <Ordering>nd</Ordering>
      <Package>superlu_dist</Package>
      <Iterations>1</Iterations>
      <Fill>5</Fill>
    </LinearMatrixSolver>
  </Dynamic_simulation>
</Configuration>

This file contains three major blocks, the RealTimePathRating block, the Powerflow block and the Dynamic_simulation block. The Powerflow and Dynamic_simulation blocks are described elsewhere and are only mentioned here to note that the PSS\E raw file describing the system is specified in the Powerflow networkConfiguration field and the PSS\E dyr file describing the generator properties is specified in the Dynamic_simulation generatorParameters field. The fields in the RealTimePathRating block are described below.

  • printCalcFiles: This flag indicates whether a file is printed describing each power flow calculation for each contingency for each value of the rating parameter. For a reasonable sized system, this will likely result and enormous number of files so it should be set to false in most cases.

  • sourceArea,sourceZone: Specifies the zone within an area that represents the generation source in the calculation. If no zone is specified, then the entire area is assumed to be the source.

  • destinationArea,destinationZone: Specifies the zone within an area that represents the destination load in the calculation. If no zone is specified, then the entire area is assumed to be the destination.

  • calculateGeneratorContingencies, calculateLineContingencies: if true, automatically generate a list of generator and line contingencies by assuming that all active generators and/or lines contribute a contingency. If both of these are false or not specified, assume that contingencies are specified in and external contingency file.

  • contingencyList: specifies an external contingency file that contains a listing of all contingencies that are used in the calculation. This file uses the same format as the contingency application. If either the calculateGeneratorContingencies or calculateLineContingencies are set to true, then this file is ignored.

  • useBranchRatingB: use the RATEB parameter to evaluate line contingencies instead of RATEA.

  • minVoltage,maxVoltage: minimum and maximum values of the voltage used in evaluating power flow contingencies.

  • checkQLimit: perform the Q-limit test, convert PV buses to PQ buses if the test fails and rerun power flow calculation.

  • monitorGenerators: monitor generators for frequency violations. If this parameter is false then the dynamic simulation part of the path rating calculation is skipped. Only generators in the source zone or area are checked.

  • frequencyMaximum: maximum allowable frequency for monitored generators. The default is 61.8 Hz.

  • contingencyDSStart,contingencyDSEnd,contingencyDSTimeStep: Start time, end time and time step for faults in the dynamic simulation contingecies. These apply to all contingencies evaluated in the dynamic simulation phase of the path rating calculation.

  • tieLines: This field is used to define user-specified tie-lines. In the event that this field is not specified, the RTPR will calculate tie-lines automatically by choosing all active lines between the source and destination areas. Each tie-line is specified by the tieLine sub-block, which contains two additional fields. The Branch field contains two integers, representing the indices of the bus at each end of the line and the Tag field contains a one or two character string representing the line ID.

Output

Output for the real-time path rating calculation is fairly compact. If printCalcFiles parameter is set to false, then the main result is the path rating values in standard output. If standard out is redirected to a file (recommended) then the rating values can be found by searching for Rating in the output. This will typically yield something like

% grep Rating file.out
Final Power Flow Rating: 1.850000
Final Dynamic Simulation Rating: 1.640000

If the system hits an upper or lower bound when performing the rating calculations, some additional lines may appear in the output (these can also be found by searching on Rating).

In addition to the rating values, the calculation exports a set of files with the names line_flt_cnt_XXX.txt, where XXX stands for a value of the rating parameter. These are summaries of each of the power flow contingency calculations describing how many times each line exhibited a fault for each value of the rating parameter that was tested in the simulation. Note that the rating calculation itself only checks tie-lines while these files list faults on all lines in the system.

Fundamental algorithm

The goal of this application is to determine the real-time line rating (RTPR) for the tie-lines between two areas or zones. The real-time line rating is determined through steady-state and dynamic analysis and will yield a maximum MVA flow on the tie-lines without causing any steady-state security (line flow violations, voltage violations) or dynamic-security (generator instability) violations. This implementation considers a simple model that only considers tie lines between two areas or zones. The only violation considered is MVA flow violations on tie-lines for the power flow calculation and frequency violations for the dynamic simulation. The implementation consists of the following steps:

  1. The system is divided into a source region and a destination region. The destination region is characterized by the loads in that region and the source region is characterized by the generators in that region. The destination and source regions can be characterized by either an area or a zone within an area.

  2. Tie-lines between the source and destination regions can be specified in one of two ways. The first is that the user can manually specify each of the lines that they want to consider as tie-lines. The second method is to have the RTPR application itself identify the tie-lines. In this case, any active line directly connecting the source and destination areas will be considered a tie-line.

  3. The RTPR calculation proceeds by determing a scaling factor that is used to simultaneously increase or decrease the loads in the destination region while simultaneously matching the change in load with a change in generation in the source region. The path rating is the maximum value of the scaling parameter that allows operation with no overloads on the tie-lines and no large deviations in frequency from the generators.

  4. For a given value of the rating parameter, the active loads in the destination region are scaled by the rating parameter. This will yield an absolute increase or decrease in the power consumption in the destination region. The change in power consumption is matched by a corresponding change in the power generation in the source region. The change in power generation is evaluated by the following algorithm:

    1. Calculate the total real power generation in the source region for the active generators for the unperturbed system.
    2. Calculate that margin for the generators in the source region. If the rating parameter is greater than 1, then calculate the total real power generation based on the maximum allowable generation. If the rating parameter is less than 1, then calculate the minimum power generation based in the minimum allowable generation for each generator. The difference between this number and the original power generation is the margin.
    3. If the change in load is greater than the available margin then stop the path rating calculation and calculate the value of the rating parameter based on the available margin. If the change in load is less than the margin, then calculate the fraction of the margin that will match the change in load and then change the generation for the individual generators by adding (or subtracting) the previously determined fraction times the individual margin for the generator from the unperturbed generation value.

  5. The path rating is determined by evaluating the system against a series of N-1 contingencies. These contingencies can be specified either explicitly by the user or by having the RTPR application determine them automatically. For automatic generation, it is possible to choose only generator faults, only line faults or both. For automatically created generator faults, a list of all active generators in the source regions is determined and each of these generators is associated with a fault. For line faults, all active lines connecting two buses that are both located in the source area or zone are associated with a fault.

  6. A complete set of power flow calculations containing both the base case (no contingency) and the complete set of contingencies is run at a given value of the rating parameter, starting with a rating of 1.0. For both the base case and each contingency, the tie lines are checked to see if there is an overflow violation. The overflow violation can be based on the Rating A or Rating B parameter. The default for the RTPR calculation is Rating B. If there is a violation for any contingency or the base case, then the rating parameter is decreased until there are no violations. If there is no violation initially, then the rating parameter is increased until there is a violation on the tie-lines. The final value of the rating parameter is the power flow rating for the system. The calculation proceeds by changing the path rating in units of 0.05 (5%). When a limit is found, the calculation reverts to the previous value of the rating and start changing in increments of 0.01 (1%). The use of two different increments is designed to speed up the calculation for final values of the rating parameter that are significantly different from the starting value of 1.0.

  7. After the power flow rating is completed, the system is then checked using dynamical simulation. The dynamic simulation rating is assumed to be more stringent than the power flow rating, so the power flow rating is considered to be an upper bound on the dynamic simulation value. The dynamic simulation rating will only check for lower values of the rating.

  8. Each of the contingencies created for the power flow rating is converted to a fault for dynamic simulation. These faults must be supplemented by information on when the fault occurs and how long it lasts. The dynamic simulation path rating defines a start time for the fault, an end time for the fault and a simulation length that is applied to all faults. Simulations are run for each of the faults and the frequency of all generators in the system is checked to see if any frequencies exceed a threshold. This threshold is configurable but is currently set to 61.8 Hz. If there is a frequency violation for any fault, the rating parameter is considered to generate a violation.

  9. The first dynamic simulation path rating calculation starts with the power flow rating value. If there are no frequency violations, then the calculation stops and the dynamic simulation path rating is the same as the power flow path rating. If there is a violation, then the rating is decreased until there is no violation. This value is then the dynamic simulation path rating. The rating parameter for dynamic simulation is used to scale the source and destination areas or zones in the same way as for the power flow ratings.