Power Transfer Distribution Factors

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Power Transfer Distribution Factors (PTDF) indicate the incremental change in real power that occurs on transmission lines due to real power transfers between two regions. These regions can be defined by areas, zones, super areas, single buses, injection groups or the system slack. These values provide a linearized approximation of how the flow on the transmission lines and interfaces change in response to a transaction between the Seller (source) and the Buyer (sink). These values can then be visualized on the onelines using animated flows (see below for details).

When considering areas, zones, or super areas as part of the transaction, the transaction for which the PTDFs are calculated is modeled by effectively scaling the output of all generators on AGC in the source and sink areas in proportion to their relative participation factors. Generators in the source area effectively increase their output, while generators in the sink area effectively decrease their output.

When including injection groups as part of the transaction, the AGC status of any participating generators does not matter. All generators in the injection group with non-zero participation factors will be included in the transaction. When loads are included in injection groups, loads in the Seller would be effectively decreased in order to effect a transaction and loads in the Buyer would be effectively increased in order to effect a transaction. Buses in the injection group are also allowed to participate in the transaction even if the bus has no generation or load. Buses are modeled as an increase in injection if in the seller and a decrease in injection if in the buyer.

Of course, the PTDF calculation is a linear calculation and no changes are actually made to the output of generators, loads, or buses to study the impact of the transaction regardless of the type of seller or buyer.

An important aspect to consider in calculating the PTDF is how the losses associated with the transfer are allocated. Simulator assumes that the Seller increases its injection by 100% of the transfer amount (generators increase and loads decrease), while the Buyer decreases its injection (generators decrease and loads increase) by 100% of the transaction minus any change in system losses. In other words, the Buyer accounts for the entire change in the system losses. Of course it is possible that a transfer may result in decreased system losses; for that case, the Buyer’s change in injection will be greater than 100% of the transfer.

To Calculate the Power Transfer Distribution Factors:

The animated flows that appear on the oneline diagram may represent either actual flows or PTDF values. To specify that the display should show distribution factors, click the button labeled Visualize PTDFs on the Power Transfer Distribution Factors dialog. Once this button is clicked, the flow arrows on all open onelines will represent distribution factors, any transmission line pie charts will show PTDF values, and the caption of the button will change to Visualize Actual Power Flows. Click the button again to visualize actual power flows instead of distribution factors.

Note that when calculating PTDF values for interfaces that include contingent elements, the PTDF values reported are actually what are referred to as an Outage Transfer Distribution Factor (OTDF). See Line Outage Distribution Factors (LODFs) for more information.